KINETIC MODELING OF GASIFICATION REACTIONS FOR LIGNITE ...

i

KINETIC MODELING OF GASIFICATION

REACTIONS FOR LIGNITE COAL UNDER

HIGH PRESSURE

by

Imran Nazir Unar

Thesis submitted to MUET for the degree of

Doctor of Philosophy

in

Chemical Engineering

Directorate of Postgraduate Studies

Faculty of Engineering

Mehran University of Engineering and Technology, Jamshoro.

December 2018

ii

DEDICATION

To my beloved

FATHER and MOTHER

Who taught me the path of TRUTH

that I followed for my success

&

To my

WIFE & dauthers ABEEL and AIZA

Who helped me to follow the path of

TRUTH

iii

Certificate of Approval

This is to certify that the research work presented in this thesis, entitled "Kinetic

Modeling of Gasification Reactions for Lignite Coal under High Pressure" was

conducted by Engr. Imran Nazir Unar under the supervision of Prof. Dr. Abdul Ghani

Pathan, and approved by all the members of the thesis committee.

No part of this thesis has been submitted anywhere else for other degree. This thesis is

submitted to Department of Chemical Engineering in partial fulfillment of the

requirements for the degree of Doctor of Philosophy in the Field of Chemical

Engineering, and accepted by the Dean Faculty of Engineering of Mehran University

of Engineering and Technology.

Student Name: Engr. Imran Nazir Unar Signature:__________________

1. External Examiner: Dr. Abdul Waheed Bhutto Signature: _________________

Dean, Faculty of Engineering, Dawood UET, Karachi

2. Internal Examiner: Dr. Khadija Qureshi Signature: _________________

Professor, Chemical Engineering Department, MUET

3. Supervisor: Dr. Abdul Ghani Pathan Signature: _________________

Professor, Chemical Engineering Department, MUET

4. Co-Supervisor: Dr. Muhammad Aslam Uqaili Signature:_________________

Professor, Electrical Engineering Department &

Vice Chancellor of MUET Jamshoro

5. Co-Supervisor: Dr. Rasool Bux Mahar Signature: _________________

Co-Director, USPCAS-W, MUET, Jamshoro

6. Director: Prof. Dr. Khanji Harijan Signature:_________________

Postgraduate Studies, MUET, Jamshoro

7. Dean: Dr. Muhammad Moazam Baloch Signature: _________________

Faculty of Engineering, MUET, Jamshoro

Date: ___________________

iv

Author’s Declaration

I Engr. Imran Nazir Unar hereby state that my thesis titled "Kinetic Modeling of

Gasification Reactions for Lignite Coal under High Pressure" is my own work and has

not been submitted previously by me for taking any degree from Mehran University of

Engineering and Technology or any other Degree awarding institute and to the best of

my knowledge has not been submitted by any other scholar for the same purpose

anywhere else in the country/world.

At any time if my statement is found to be incorrect even after my graduation, Mehran

University of Engineering and Technology has the right to withdraw my PhD degree.

Name of Student: Engr. Imran Nazir Unar

Date: ______________________________

v

Plagiarism undertaking by the Scholar

I solemnly declare that research work presented in the thesis titled "Kinetic Modeling

of Gasification Reactions for Lignite Coal under High Pressure" is my research work

with no significant contributions from any other person. Small contribution/help

whenever taken has been duly acknowledged.

I understand the zero-tolerance policy of the Higher Education Commission and

Mehran University of Engineering and Technology towards plagiarism. Therefore, as

an author of the above titled thesis declare that no portion of my thesis has been

plagiarized and any material used as reference is properly referred /cited.

I undertake that if I am found guilty of any formal plagiarism in the above titled thesis

even after award of PhD degree, the university reserves the rights to withdraw/revoke

my PhD degree and that HEC and MUET has the right to publish my name on the

HEC/MUET website on which names of students are placed who submitted plagiarized

thesis.

Student Signature: __________________

Name: Engr. Imran Nazir Unar

vi

Copyright

© Copyright, 2018

Mehran University

of

Engineering and Technology

ALL RIGHTS RESERVED

vii

Acknowledgment

The work carried out was not possible without the support and guidance of many

individuals and organizations. First of all, I pay my sincere gratitude to Prof. Dr. Abdul

Ghni Pathan, my supervisor, who not only motivated me to initiate the work but always

a source of light, whenever I found myself in dark. I am thankful to my co-supervisor,

Prof. Dr. Muhammad Aslam Uqaili who remained a source of light and guidance

throughout the work and always put his all efforts to provide me the facilities. I would

also like to my heartiest thanks to Prof. Dr. Rasool Bux Mahar (co-supervisor), who

always encouraged me and give me new paths to find my way. I will not forget the

support and affection from all the faculty members in the department of chemical

engineering, especially Prof. Dr. Suhail Ahmed Soomro, Prof. Dr. Shaheen Aziz, Prof.

Dr. Khadija Qureshi, Engr. Zulfiqar Ali Bhatti and Engr. Zulfiqar Ali Solangi who

always supported and share their experiences.

I also pay thanks to Prof. Dr. Muhammad Aslam Uqalili, Vice Chancellor, Mehran

University of Engineering and Technology for his untiring efforts to provide research

faculties to the researchers. Prof. Dr. Khanji Harijan, Director, Post Graduate studies

was also a source of inspiration and much more friend who always motivated to

conclude my work. I would also appreciate the guidance provided by Prof. Dr. Wang

and Prof. Dr. Rudon Li for their continuous support during my stay in China.

It will be unfair if I will not pay tribute to my family for their sacrifice during my

research work and supported me throughout the period. Last but not the least; I am

thankful to all of my friends and well-wishers who not only supported me during my

work but always wished to complete work timely.

Finally, I acknowledge, Higher Education Commission, British Council, Shenyang

Aerospace University China and Mehran University of Engineering and Technology

for providing financial resources and research facilities during the research work.

viii

TABLE OF CONTENTS

Title Page i

DEDICATION ii

Certificate of Approval iii

Author’s Declaration iv

Plagiarism undertaking by the Scholar v

Copyright vi

Acknowledgements vii

Table of Content viii

List of Abbreviations

List of Figures

List of Notations

List of Tables

Abstract

CHAPTER 1 INTRODUCTION 1

1.1 NATIONAL AND STRATEGIC IMPORTANCE OF COAL 1

1.2 COAL GASIFICATION 2

1.2.1 Introduction 3

1.2.2 Thermochemistry of Gasification 3

1.2.3 Types of Gasifiers 5

1.3 ADVANCEMENT IN GASIFICATION SYSTEMS 8

1.4 IMPORTANCE OF COAL GASIFICATION IN RELATION TO THAR

COAL DEVELOPMENT 10

1.5 PROBLEM STATEMENT 12

1.6 OBJECTIVES 13

1.7 SCOPE AND OVERVIEW OF THE RESEARCH WORK 13

1.7 THESIS STRUCTURE 14

CHAPTER 2 KINETIC MODELING FOR COMBUSTION AND

GASIFICATION REACTIONS 16

2.1 GENERAL OVERVIEW 16

2.2 INTRODUCTION TO KINETIC MODELING 17

ix

2.3 LAWS FOR RATE DETERMINATION OF CHAR O2 (COMBUSTION)

REACTION AND MECHANISM 18

2.4 LAWS FOR RATE DETERMINATION OF CHAR-CO2 (GASIFICATION)


2.5 LAWS FOR RATE DETERMINATION OF CHAR-H2O (GASIFICATION)


2.6 KINETIC MODELS 22

2.6.1 Volumetric Model (VM) 24

2.6.2 Modified Volumetric Model (MVM) 24

2.6.3 Grain Model (GM) or Shrinking Core Model (SCM) 25

2.6.4 Random Pore Model (RPM) 25

2.7 REVIEW FOR WORK CONDUCTED ON KINETIC MODELING FOR

GASIFICATION AND COMBUSTION REACTIONS 26

2.8 SUMMARY 38

CHAPTER 3 LITERATURE REVIEW ON GASIFICATION TECHNOLOGIES

AND MODELING STUDIES

39

3.1 GENERAL DISCUSSION 39

3.2 HISTORICAL DEVELOPMENT OF GASIFICATION 39

3.3 COMMERCIAL GASIFICATION TECHNOLOGIES 42

3.4 FIXED-BED OR MOVING-BED PROCESSES 42

3.4.1 The Sasol-Lurgi dry bottom processes 43

3.4.2 British Gas Lurgi (BGL) 45

3.4.3 Multipurpose Gasifier (MPG) 45

3.5 FLUIDIZED BED PROCESSES 46

3.5.1 High-Temperature Winkler (HTW) Gasifier 47

3.5.2 HRL Process 48

3.5.3 BHEL Gasifier 49

3.5.4 Circulating fluidized-bed (CFB) processes 49

3.5.5 Kellogg Brown and Root (KBR) transport gasifier 51

3.5.6 U-Gas Process Gasifier 52

3.6 ENTRAINED FLOW PROCESSES 52

3.6.1 The Koppers-Totzek atmospheric process 53

3.6.2 Shell Coal Gasification Process (SCGP) 55

3.6.3 PRENFLO™ Gasifier/Boiler (PSG) 55

x

3.6.4 Siemens Gasifier 56

3.6.5 GE Energy Gasifier 57

3.6.6 ConocoPhillips E-Gas Gasifier 58

3.6.7 Mitsubishi Heavy Industries (MHI) gasifier 59

3.6.8 The EAGLE Gasifier 59

3.6.9 ICCT Opposite Multiple Burner (OMB) Process 60

3.7 CURRENT EXPERIMENTAL PRACTICES ON COMBUSTION &

GASIFICATION 61

3.7.1 Experimental work on Gasification Systems 61

3.7.2 Experimental work on Combustion related to Gasification Studies 66

3.7.3 Experimental work on Gasification Studies on Thar Lignite 67

3.8 MODELING AND SIMULATION WORK 67

3.9 RESEARCH ON FLAMELESS COMBUSTION/ GASIFICATION 83

3.10 SUMMARY 90

CHAPTER 4 EXPERIMENTAL WORK 92

4.1 GENERAL DESCRIPTION OF EXPERIMENTAL WORK 92

4.1.1 Sample Collection and Preparation 92

4.1.2 Proximate and Ultimate Analysis 93

4.1.3

TGA Analysis (Moisture removal, devolatization and Combustion

Study) 93

4.1.4 PTGA Analysis (Char Gasification Study) 95

4.1.5 Data analysis method 96

4.2 RESULTS AND DISCUSSION FOR EXPERIMENTAL WORK 101

4.3 RESULTS OF PROXIMATE AND ULTIMATE ANALYSIS 101

4.4 RESULTS FOR MOISTURE REMOVAL AND DEVOLATIZATION

KINETICS 103

4.4.1 Least square regression analysis for moisture removal 104

4.4.2 Least square regression analysis for devolatization 105

4.4.3 Rate constant “k” for drying and devolatization steps 112

4.5 RESULTS FOR COMBUSTION KINETICS 114

4.5.1 Rate constant “k” for the combustion reaction 118

4.6 RESULTS FOR COAL GASIFICATION KINETICS AT ATMOSPHERIC

AND ELEVATED PRESSURE 119

xi

4.6.1 Least square regression analysis for Char+CO2 reactions 121

4.6.2 Least square regression analysis for Char+H2O reaction 124

4.6.3 Rate constant “k” for gasification reactions 129

4.7 EFFECT OF PRESSURE ON GSIFICAITON KINETIC PARAMETERS 131

4.8 SUMMARY OF EXPERIMENTAL RESULTS 133

CHAPTER 5 CFD MODELING AND SIMULATION 134

5.1 CFD MODELING AND SIMULATION 134

5.2 CFD MODELING OF CHINESE COAL GASIFIER 135

5.2.1 Description of Physical system for Chinese Coal Gasifier 135

5.2.2 Development of computational domain for Chinese gasifier 136

5.2.3 Computational Models 137

5.2.4 Combustion/ Gasification Model 139

5.2.5 Boundary Conditions and Calculation Methods 142

5.3 CFD MODELING OF NEWLY DESIGNED COAL GASIFIER 142

5.3.1 Description of Physical system for Newly Designed Gasifier 142

5.3.2 Development of Computational Domain for Proposed Geometry 143

5.3.3 Probability Density Function (PDF) approach 144

5.4 MODEL DEVELOPMENT IN ASPEN PLUS®V10 FOR VALIDATION OF

MODIFIED GEOMETRY RESULTS 144

5.4.1 Coal Pyrolysis 146

5.4.2 Volatile combustion 147

5.4.3 Char gasification 147

5.5 MODELING AND SIMULATION RESULTS FOR CHINESE LIGNITE 148

5.5.1 Identification of Best Reaction Mechanism for Lignite Coal and

Validation of the CFD model 149

5.5.2 Effects of Coal/Oxygen Distribution on syngas composition 154

5.5.3 Effects on Char Conversion 155

5.5.4 Effects on Syngas Exit Temperature and Maximum inside

Temperature 156

5.5.5 Effects of coal distribution on particle trajectories 160

5.5.6 Effects on Turbulent Intensity 160

5.5.7 Heat generation and consumption analysis 161

5.6 MODELING AND SIMULATION RESULTS FOR THAR LIGNITE 163

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5.6.1 Effects of different models and O/C ratio 167

5.6.2 Effect of pressure on syngas composition and char conversion 168

5.6.3 Streamlines-Flow analysis for Multi-Opposite Burners 170

5.7 VALIDATION OF CFD RESULTS OF MODIFIED GEOMETRY WITH

ASPEN PLUS MODEL RESULTS 170

5.8 COMPARATIVE STUDY FOR NEWLY DESIGNED GASIFIER 172

5.9 THE FINAL PROPOSED SYSTEM 176

5.10 SUMMARY FOR CFD MODELING AND SIMULATION WORK 177

CHAPTER 6 CONCLUSION 178

6.1 CONCLUSIONS 178

6.1.1 Concluding remarks for Drying, Devolatization and Combustion

Steps 179

6.1.2 Concluding remarks for Gasification Reactions 179

6.1.3 Concluding remarks for CFD Modeling and Simulation Work 180

6.2 RECOMMENDATIONS FOR FUTURE WORK 181

REFERENCES 183

APPENDICES 211

A.1 TGA Model SDT Q600 211

A.2 Quartz fixed-bed reactor for char production 211

A.3 Thermax500 PTGA 211

A.4 List of Publications 212

xiii

List of Abbreviations

3D Three Dimensional

CFB Circulating Fluidized Bed

CFD Computational Fluid Dynamics

DDGS Dried Distiller’s Grains with Solubles

DPM Discrete Phase Model

EFR Entrained Flow Reactor

FCCCD Face Centered Central Composite Design

FVM Finite Volume Method

GM Grain Model

HAFTC High Ash Fusion Temperature Coal

IGCC Integrated Gasification Combined Cycle

LES Large Eddy Simulation

MILD Moderate and Intense Low-Oxygen Dilution

MSPV Multi Solids Progress Variables

MVM Modified Volumetric Model

NOx Nitrogen Oxides

O/C Oxygen to Carbon Ratio

OMB Opposite Multi Burner

PDF Probability Density Function

xiv

PHTER Pressurized High Temperature Entrained Flow

Reactor

PSDF Power Systems Development Facility

PTGA Pressurized Thermogravimetric Analysis

R&D Research and Development

ROM Reduced Order Model

RPM Random Pore Model

RSM Response Surface Methodology

SCM Shrinking Core Model

SOx Sulfur Oxides

TDL Tunable Diode Laser

TGA Thermogravimetric Analysis

TRF Turbulent Reacting Flow

UCG Underground Coal Gasification

UDF User Defined Function

VM Volumetric Model

xv

List of Figures

Fig 1.1: The moving-bed gasifier with various inlet and outlets 06

Fig 1.2: The fluidized-bed gasifier with various inlets and outlets 07

Fig 1.3: Fundamental Entrained flow gasifier with various inlets and outlets 08

Fig 1.4: The flow diagram of research activities 14

Fig 2.1: Directions of Literature Survey 16

Fig 3.1: Sasol-Lurgi dry bottom gasifier 44

Fig 3.2: British Gas Lurgi Gasifier 45

Fig 3.3: Lurgi Multipurpose Gasifier 46

Fig 3.4: High-Temperature Winkler Gasifier 48

Fig 3.5: IDGCC process of drying and Gasification developed by HRL 49

Fig 3.6: Lurgi circulating fluid-bed gasifier 50

Fig 3.7: KBR Transport Gasifier 51

Fig 3.8: U-Gas Process Gasifier 52

Fig 3.9: Koppers-Totzek gasifier 54

Fig 3.10: Shell Gasifier 55

Fig 3.11: PRENFLOTM 56

Fig 3.12: Siemens Gasifier 57

Fig 3.13: GE Energy Gasifier 58

Fig 3.14: Conoco Philips E-Gas Gasifier 58

Fig 3.15: MHI Gasifier 59

Fig 3.16: The EAGLE Gasifier 60

Fig 3.17: The ICCT Opposed Multiple Burner gasifier 61

Fig 4.1: Steps for Experimental Work 92

xvi

Fig 4.2: Schematic diagram of Thermax500 PTGA 96

Fig 4.3: Moisture removal at different heating rates 103

Fig 4.4: Volatiles removal at different heating rates 104

Fig 4.5: Linearity of Volumetric and Grain models for Moisture Removal at different

Heating Rate for Sample GT-01-443

106

Fig 4.6: Linearity of Volumetric and Grain models for Moisture Removal at different

Heating Rate for Sample GT-01-493

107

Fig 4.7: Linearity of Volumetric and Grain models for Devolatization at Low

Temperature with different Heating Rate for Sample GT-01-443

108

Fig 4.8: Linearity of Volumetric and Grain models for Devolatization at Low


109

Fig 4.9: Linearity of Volumetric and Grain models for Devolatization at High


110

Fig 4.10: Linearity of Volumetric and Grain models for Devolatization at High


111

Fig 4.11: Rate constant (k) for drying step with (a) VM and (b) GM 113

Fig 4.12: Rate constant (k) for devolatization step with (a) VM-Low Temp_500°C

(b) VM-High Temp_900°C (c) GM-Low Temp_500°C (d) GM-High

Temp_900°C

114

Fig 4.13: Conversion of Char Samples from GT-01-443 in Oxygen environment at

different heating rates

115

Fig 4.14: Conversion of Char Samples from GT-01-493 in Oxygen environment at

different heating rates

115

Fig 4.15: Linearity of Volumetric and Grain models for Combustion of with different

Heating Rate for Coal Sample GT-01-443

116

Fig 4.16: Linearity of Volumetric and Grain models for Combustion of with different

Heating Rate for Coal Sample GT-01-493

117

Fig 4.17: Rate constant (k) for combustion step with (a) VM and (b) GM 119

Fig 4.18: Conversion of Char samples against Temperature at different pressures for

CO2 and H2O reacting gases

120

Fig 4.19: Least square regression analysis of Volumetric and Grain Models for

Char+CO2 reactions of sample S1

122



122



123

xvii


Char+H2O reactions of sample S1

125



126



127

Fig 4.25: Comparison of experimental and predicted conversion for Char-CO2/H2O

reactions of sample S1

129

Fig 4.26: Rate constant (k) for Char +CO2 reaction with (a) VM and (b) GM 130

Fig 4.27: Rate constant (k) for Char +H2O reaction with (a) VM and (b) GM 131

Fig 4.28: Effect of pressure on frequency factor (A) and activation energy (E) 132

Fig 5.1: (Left) Geometry of Gasifier with main inlets and outlets. (Right) The

Sections of gasifier at AA’ and BB’ level

136

Fig 5.2: Meshed computational domain of Chinese Coal Gasifier (Geometry-A) 137

Fig 5.3: Different views of newly designed gasifier operating conditions 143

Fig 5.4: Meshed computational domain of Proposed Gasifier (Geometry-B) 143

Fig 5.5: The process flow sheet for the simulation model of Entrained Flow Gasifier 146

Fig 5.6: The comparison of various preliminary simulation results with experimental

values for CO, H2 and CO2 mole fraction in Syngas (a) at O/C ratio=0.9, (b)

at O/C ratio=1.0 and (c) at O/C ratio=1.1.

150

Fig 5.7: The temperature profile for various preliminary simulated cases and

experimental work along central axis of gasifier (a) at O/C ratio=0.9, (b) at

O/C ratio=1.0 and (c) at O/C ratio=1.1

151

Fig 5.8: The velocity Vectors at the Sectional Planes of the gasifier (Case E) with a

close view of AA' and BB' planes.

153

Fig 5.9: The particles residence time at the Sectional Planes of the gasifier (Case E)

(for clarity only 200 particle tracks are shown with particle end time limited

at 0.8 sec)

153

Fig 5.10: The Mole % for CO, H2 and CO2 with variation in total Coal/ Oxygen % at

AA' Level

155

Fig 5.11: The Char Conversion with variation in Coal/Oxygen % at AA' Level 156

Fig 5.12: Syngas Average Exit Temperature (K) 157

Fig 5.13: Radial Temperature profiles for various simulation cases at different heights

of gasifier

158

Fig 5.14: Temperature Contours for Sectional Planes at AA' Level and BB' Level 159

Fig 5.15: The particles residence time at the Sectional Planes of the gasifier; Cases:

C30_O50, C50_O50 and C70_O50

160

xviii

Fig 5.16: Turbulent Intensity (%) Contours for selected cases 161

Fig 5.17: Net heat generated due to reactions (W) for selected cases 162

Fig 5.18: Heat consumption in different ways 162

Fig 5.19: The mole % of CO, CO2, and H2 at the exit of gasifier at different O/C

Ratios

168

Fig 5.20: The devolatization and Char conversion at different O/C Ratios 168

Fig 5.21: Effect of Pressure on the composition of CO, CO2 and H2 in syngas 169

Fig 5.22: Effect of Pressure on char conversion and exit gas temperature 169

Fig 5.23: Streamlines –Flow analysis for both the geometries 170

Fig 5.24: Comparison of CFD model results with Aspen Plus®V10 model results for

(a) CO, (b) CO2, (c) H2 and (d) Volatiles

171

Fig 5.25: Comparison of CFD model results with Aspen Plus®V10 model results for

(a) Devolatization (b) Char Conversion and (c) Syngas Exit Temperature

172

Fig 5.26: Comparison of temperature contours from both geometries 174

Fig 5.27: Comparison between the Original Geometry (A) and Modified Geometry

(B) through contours of important syngas components

175

Fig 5.28 Proposed system of coal gasification system with newly designed gasifier 176

Fig 5.29 Overall material balance of newly designed gasifier 176

xix

List of Notations

a Absorption coefficient

C Coefficient of Function for Linear-Anisotropic Phase

𝜌, 𝜌𝑝 Density, Density of particles (Kg/m3)

Dt Diffusion Coefficient for Turbulence (m2/sec)

𝐷𝑖 Diffusivity (m2/sec)

ε Dissipation Rate of Turbulence (m2/sec3)

DF Drag Force (Kg-m/sec2)

μ Dynamic Viscosity (N-sec/m2)

εw Emissivity

xF Force vector component along x-axis (Kg-m/sec2)

qr Heat Flux for Radiation Heat (J/m2-sec)

G Incident Radiation

k Kinetic Energy for Turbulence (m2/sec2)

Gk Mean Velocity Gradients

[C] Molar concentration of specie (Kmol/m3)

jC Mole fraction of species j

Mj Molecular weight of specie j

𝑤𝑗,𝑟 Net Production Rate of Species i through Chemical Reaction (Kmol/m3 s)

Prt Prandtl number for Turbulence

' Rate exponent for product reactant specie

'' Rate exponent for product specie

s Scattering coefficient (m-1)

Sct Schmidt number for Turbulence

mS , jS , hS , rS Source terms for mass, momentum, energy, and species

pc specific heat at constant pressure (J/kg-K)

σ Stefan–Boltzmann Constant

𝑣′′𝑗,𝑟 Stoichiometric Coefficient for Product j in Reaction r

𝑣′𝑖,𝑟 Stoichiometric Coefficient for Reactant i in Reaction r

xx

ij symmetric stress tensor

T Temperature (K)

μt Turbulence Viscosity

𝜆 Turbulent Thermal Conductivity (W/m-K)

u , pu Velocity, the velocity of particles (m/s)

Cμ Viscosity Constant

xxi

List of Tables

Table 1.1: World Energy Consumption by Fuel 01

Table 2.1 : Review of Kinetic Modeling of Carbon-based materials with various

techniques

27

Table 3.1: Sizes and capacities of Sasol-Lurgi dry bottom gasifiers 44

Table 3.2: Characteristics of Important Entrained Flow Gasifiers 53

Table 4.1 : Samples collected from Block-IX Thar Coal Field 93

Table 4.2: Experimental conditions in TGA for Moisture Removal and Devolatization 94

Table 4.3 : Experimental conditions in TGA for Combustion Reaction 95

Table 4.4: Proximate Analysis and Heating Value of Coal Samples 102

Table 4.5: Ultimate Analysis of Coal Samples 102

Table 4.6: Calculated kinetic parameters for moisture removal and devolatization steps 112

Table 4.7: Calculated kinetic parameters for char combustion step 118

Table 4.8: Arrhenius Parameters for Char+CO2 124

Table 4.9: Arrhenius Parameters for Char+H2O 128

Table 5.1: Properties of Chinese Coal 135

Table 5.2: Various preliminary cases to optimize the best reaction plan 141

Table 5.3: Selected kinetic parameters for Devolatization and Gasification/ Combustion

Reactions

141

Table 5.4: The chemical species used in the model 145

Table 5.5: Functions of each block used in Aspen Plus®V10 Model 147

Table 5.6: Operated parameters for Simulated Cases 149

Table 5.7: Simulation Cases in Block 1 (PDF model used to calculate the species) 163

Table 5.8: Simulation Cases in Block 2 (Species Model used with finite rate chemistry)

– Optimization of Oxygen-to-Carbon (O/C) Ratio

164

Table 5.9: Simulation Cases in Block 3 (Different Pressures at Optimized O/C ratio) 164

Table 5.10: Simulation Cases in Block 4 (Different feed rates with varying Pressures at

fixed Optimized O/C ratio) – Optimization of Feed Rate.

165

Table 5.11: Results for Simulation Cases in Block 1 (PDF model used) 166

Table 5.12: Results for Simulation Cases in Block 2 (Species model used) 166

Table 5.13: Results for Simulation Cases in Block 3 (Effect of Pressure on gasification) 166

xxii

Table 5.14: Results for Simulation Cases in Block 4 (Effect of Pressure on gasification) 167

Table 5.15: Comparative study for Original and Modified geometry of gasifier with two

different feedstocks

173

xxiii

Abstract

Pakistan is considered a coal-rich country after the discovery of Thar coalfield with the

total lignite reserves are estimated more than 175 billion tonnes. On the other side coal

contributes only about 7% of the total energy mix in Pakistan. The fundamental issues

of non-utilization of abundant coal reserves are lacks of financial resources, modern

technology, and expertise. A multi-pronged strategy, for the utilization of an indigenous

coal, is required to overcome the energy crisis in the country. Coal gasification is one

of the potential options available for the development of Thar coal. Lots of research has

carried out in rest of the world and researchers concluded that coal characteristics play

a vital role in gasification and hence the development of technology is based on the

characteristics of coal which will be used as feedstock in the technology. To design the

efficient gasification system, the kinetics of combustion and gasification reactions is an

essential part of the fundamental study.

In gasification process, solid or liquid hydrocarbon feedstock is converted into a

synthetic fuel gas that has the capability to produce electricity or could be utilized to

synthesize a raw material for the manufacture of chemicals, hydrogen, or transportation

fuels. The primary constituents of syngas are carbon monoxide and hydrogen. The

gasification process begins with a rapid removal of volatiles known as devolatilization

(or pyrolysis), leaving behind a char that is mainly composed of fixed carbon and ash.

The next step is the heterogeneous reactions of char with O2, CO2, and H2O (steam) to

produce the syngas. The reaction of char with oxygen (combustion reaction) is kept

controlled by having limited oxygen to produce sufficient heat for the endothermic

gasification reactions of char with CO2 and steam. The char reactivity with CO2 and

steam and their reaction kinetics is playing an important role and hence considered

essential for mathematical, physical process modeling, optimization and economical

operation of the gasifier.

The core objective of present research is to determine the kinetic parameters for the

overall gasification reactions including drying, devolatization, combustion and

reactivity of Thar chars with CO2 and H2O (steam) at atmospheric as well as elevated

xxiv

pressures, using TGA and PTGA. The present study also aims to find a suitable kinetic

model to explain the variation of char reactivity and its changes with the progress of the

CO2 and H2O gasification reactions individually. Key kinetic parameters like frequency

factor (or pre-exponential factor) and activation energy, are calculated, by means of two

standard kinetic models i.e. Volumetric Model and Grain Model and a comparative

study has been done on the basis of observed results.

After the experimental work, for a further better understanding of coal gasification with

indigenous coal parameters, the CFD modeling of entrained flow gasifier was done.

Initially, the Chinese coal gasifier with multi-opposite burners was modeled using

Chinese coal data. The Euler-Lagrangian mechanism was used. Various reaction

mechanisms were tested and the model results are validated on published experimental

results. After successful validation of the model, the geometry of the gasifier was

modified by introducing a neck section for Thar Lignite and simulations were carried

out using validated reaction mechanisms. Optimization was carried out for various

parameters like O/C ratio, pressure, and coal and oxidant feed rates.

Finally, it was concluded that the temperature and kinetics of gasification reactions

(reactions of char to CO2 and H2O) can be controlled with the optimized coal and

oxidant distribution between the two stages. These parameters are critical to the overall

performance of the gasifier, so coal and oxygen feedings must be optimized between

the two stages of the gasifier to get the optimized performance. Species Transport

approach shown good results as compared to Probability Density Function (PDF) in

terms of CO and H2 production. Geometry–A (original) shown good results with

Chinese coal whereas Geometry –B (modified with neck) was found best for Thar coal.

The maximum CO and H2 were observed 24.19% and 32.35% at O/C ratio of 1.881. At

maximum CO and H2 generation, the moisture removal was observed 97.7% whereas

char conversion was observed 98.24%. Lower and Higher (LHV) heating values of

syngas produced from modified geometry were calculated using Aspen HYSYS

software. It was observed that the syngas produced from Geometry-B has 12.27 MJ/Kg

lower heating value. The calculated ratio of LHV/coal heating value for Thar coal was

0.815 which was greater than Chinese coal with geometry-A (i.e 0.212).

1

CHAPTER 1

INTRODUCTION

1.1 NATIONAL AND STRATEGIC IMPORTANCE OF COAL

Pakistan is dependent majorly on fossil fuels for fulfilling its energy needs but

currently, the country is facing a massive challenge to overcome the serious energy

crisis. Social and industrial activities are severally affected due to huge electricity

shortfall and continuous depleting natural gas reservoirs. Coal is considered an

economic energy source and widely utilized throughout the world for generating power,

cement and in other process industries. Fuel wise distribution of world energy

consumption scenario is presented in Table 1.1 and discloses that coal is still sharing

major contribution (37%) for producing electricity in the world and it is on second place

(30.3%) for other primary energy utilization.

Table 1.1: Fuel wise distribution for Energy consumption in the World

(Petroleum, 2016)

Fuel/ Resource Primary Energy Consumption Electricity Consumption

Coal 30.3% 37%

Gas 23.7% 16%

Oil 33.1% 9%

Renewable 8.0% 21%

Nuclear 4.9% 17%

Sindh province of Pakistan is bestowed with huge lignite coal deposits i.e. more than

184 billion tons, which constitutes 99% of total coal deposits of the Country. Field wise

coal reserves of Sindh are as follows (Private Power & Infrastructure Board, 2008,

Petroleum, 2016):

2

a) Thar Coal Field = 175.60 billion tons

b) Lakhra Coal Field = 1.64 billion tons

c) Thatta – Sonda = 7.46 billion tons

Total = 184.70 billion tons

Thar coalfield, with the total reserves of more than 175 billion tons of lignite, has the

potential to fulfill the country’s energy requirements for more than a century.

It has been observed on practical backgrounds that despite economic development stage

of any coal-rich nations, their indigenous coal has been utilized to boost the electricity

sector of that nation. Pakistan spends upwards of US$ 2.00 billion annually of hard-

earned foreign exchange on the importation of fuel oil to support its current inventory

of oil-fired, electricity – generation installations. The dependency of imported oil for

fulfilling energy needs for the country like Pakistan who has proven potential of

generating electricity from indigenous coal for more than 100 years seems

inappropriate.

Since the time of discovery of Thar coalfield, its development has been a dream for the

Pakistani nation. Several efforts have made by Pakistan so far but unfortunately could

not achieve any remarkable result due to diverse reasons, including geopolitical,

technical, financial and law & order. Non-availability of fresh water at Thar, the

presence of highly saline groundwater, poor strata conditions, and high stripping ratio,

have made Thar coal projects less attractive for investors. Coal gasification through

gasifier is a potential and viable option for the development of Thar coal.

1.2 COAL GASIFICATION

Coal gasification is important process in which low-economic solid or liquid feedstocks

are converted into syngas. The details about gasification are discussed in subsequent

sub-sections.

3

1.2.1 Introduction

Gasification of coal is a complex chemical process in which flammable gases like CO

and H2 are being produced from coal or any other carbonaceous material in the presence

of oxygen/air and steam, at high temperature and pressures. The gas produced during

gasification is known as “syngas” which is rich in CO and H2 and can be utilized as

town gas for the household purpose, to generated electricity, to make methanol, liquid

fuels through the Fischer-Tropsch process, ammonia, urea etc. Air separation plant is

used to recover 95% pure oxygen which is used in the gasifier. Recovered nitrogen can

be used as a dilution in a gas turbine combustor and as feedstock for ammonia synthesis

process. The separated CO2 from syngas could be utilized as raw material for the

synthesis process of urea. Coal gasification is considered one of the environment

friendly technologies due to its capability of achieving extremely low NOx and SOx,

along with particulate emissions. The extraction of Sulphur in coal is possible during

the gasification process in the form of solid which then can be sold commercially. There

are 385 gasifiers of various designs currently operational worldwide. Total worldwide

gasifier capacity was expected around 75,000 MWth by the end of 2010 .

1.2.2 Thermochemistry for Gasification

Gasification is based on principal concept partially oxidization of carbonaceous

material like coal at high pressure and temperature with the limited amount of oxidant

like oxygen or air which triggers a number of complex elementary reactions producing

a mixture of gases primarily contain carbon mono oxide (CO) and hydrogen (H2) in

larger fractions. Apart from production of carbon monoxide and hydrogen, high

fractions of carbon dioxide (CO2) and water vapors (H2O) are also available in syngas

produced from gasification along with nitrogen (N2) and traces of methane (CH4)

depending on the process conditions. Few other chemical species like oxides of sulfur

and nitrogen (SO2, SO3, NO, and NO2), ammonia (NH3), hydrogen sulfide (H2S), and

hydrogen chloride (HCl) could also be available in the product syngas. The syngas also

contains Ash in addition to all gaseous products and the composition of ash dependent

on the nature of feedstock, conditions of the reactor and other parameters.

4

As per gasification theory, the occurrence of gasification is subdivided into different

zones or stages. These stages or zones may be the regions spatially localized inside of

the gasifier reactor vessel, as in moving-bed gasifiers, or may occur all the reactions in

each stage of the process throughout the entire volume of the reactor vessel, as happens

in fluidized-bed gasifiers. These stages, with their associated chemistry, can be

summarized as follows (Probstein and Hicks, 2006, Higman and Van Der Burgt, 2003,

Gautam et al., 2010).

1) Removal of moisture or Drying – The raw material of gasification or feedstock

undergoes drying process in which inherent moisture of feedstock evaporates due to

increasing temperature. The dry-feed is then ready for the upcoming process of

pyrolysis.

2) Pyrolysis or Devolatilization – The chemical bonds in the feedstock material starts

breaking due to heat produced during combustion reactions which result in the

generation of compounds with different molecular weights. The vaporization of lighter

compounds occurs from which some them remain in gaseous forms whereas rest of

compounds form a thick slurry known as tar. The char is produced from heavier

compounds.

3) Combustion – The flammable of combustible components like carbon, carbon

monoxide, hydrogen etc. react with oxygen, known as combustion process. In this

process, heat is released which act as a driving force for the entire gasification process.

COOC →+ 221 (ΔH= –111 KJ/mol) (1.1)

2221 COOCO →+ (ΔH= –283 KJ/mol) (1.2)

22 COOC →+ (ΔH= –394 KJ/mol) (1.3)

OHOH 222 21 →+ (ΔH= –242 KJ/mol) (1.4)

4) Gasification – During this, the carbon dioxide and steam react with the unburnt char

in the presence of heat to form carbon monoxide and hydrogen. This process is

endothermic in nature and requires high temperature to occur.

5

Water-Gas Reaction

22 HCOOHC ++ (ΔH= +131 KJ/mol) (1.5)

Boudouard Reaction

COCOC 22 →+ (ΔH= +172 KJ/mol) (1.6)

In addition, the other important reactions for gasification process are written as under:

Methane Combustion:

OHCOOCH 2224 22 +→+ (ΔH= –803 KJ/mol) (1.7)

Methanation (or Hydrogasification) Reaction:

422 CHHC + (ΔH= –75 KJ/mol) (1.8)

Methane Reforming Reaction:

224 3HCOOHCH ++ (ΔH= +206 KJ/mol) (1.9)

Water-Gas Shift Reaction:

222 HCOOHCO ++ (ΔH= –41 KJ/mol) (1.10)

Other Reactions:

222 22 HCOOHC ++ (ΔH= +90.1 KJ/mol) (1.11)

422 21

21 CHCOOHC ++ (ΔH= +7.7 KJ/mol) (1.12)

224 22

1 HCOOCH +→+ (ΔH= –36 KJ/mol) (1.13)

Boudouard, water-gas shift and methanation reactions where shown “fundamental”

reactions for the gasification process in previous work (Perry and Green, 1999).

1.2.3 Types of Gasifiers

Different types of gasifier are discussed as under.

1.2.3.1 Moving Bed

Fig.1.1 is showing the diagram of a generic moving bed gasifier. The flow arrangement

of feedstock and oxidant is countercurrent in moving bed gasifiers where coal is fed

from the top of the reactor and oxygen or air is blown from the bottom of the reactor.

The gasification of coal occurs in its traveling from top to bottom and remaining ash is

6

taken out from the bottom of the gasifier. The heat generated from combustion reactions

is utilized as pre-heating source of coal before its entering in gasification zone due to

countercurrent flow arrangement. Therefore, the temperature of outgoing syngas from

the gasifier is significantly lower as compared to the temperature required for complete

conversion of coal. The moving bed gasifier pertaining the coal residence time in the

order of hours within the gasifier.

Few salient features of moving bed gasifiers are as follows:

• It requires less oxidant;

• It produces the product syngas with a relatively higher fraction of methane;

• It produces hydrocarbon liquids, like the production of tars and other oils;

• It shows higher “cold gas” thermal efficiency including the heating values of the

hydrocarbon liquids;

• It has poor fines handing ability; and

• It requires special arrangements for managing caking of coal issues.

Fig. 1.1: The moving-bed gasifier with various inlet and outlets (Source: Higman

and Van Der Burgt (2008))

1.2.3.2 Fluidized Bed

Fig. 1.2 shows a generic fluidized bed gasifier diagram. A fluidized bed gasifier is a

well-stirred or back-mixed type of reactor in which the fresh coal particles are

continuously mixed with older, hot, partially and fully gasified particles. The uniform

temperature is maintained throughout the fluidized bed due to mixing. The flow of

7

steam, oxidant or recycled gas into the reactor must be maintained in such a range so

that coal particles remain in floating conditions within the bed and could not settle down

or skip out from the bed. However, the particles become smaller and lighter as they are

completely gasified and eventually entrained out of the gasifier. To avoid the

agglomeration of a particle within the bed, the temperatures of the bed should be well

below to the initial ash fusion temperature of coal. Usually, the larger particles that are

escaped out from the gasifier are captured through a cyclone downstream and fed back

into the bed as a recycled stream. Overall, fluidized bed pertains shorter residence time

of coal particles as compared with the residence time of moving bed gasifier.

The fundamental characteristics of fluidized bed gasifiers are as follows:

• It has extensive recycling of solid particles;

• It has a moderate temperature which is uniform throughout the bed; and

• Adequate requirements of steam and oxygen

Fig. 1.2: The fluidized-bed gasifier with various inlets and outlets (Source:

Higman and Van Der Burgt (2008))

1.2.3.3 Entrained Flow

Fig. 1.3 shows a diagram of generic entrained flow gasifier. The fine particles of coal

are inserted into the reactor co-currently with an oxidant. The rapid heat is transferred

from reactor environment into coal particles and reaction with oxidant started

immediately. The reaction occurs with a very short residence time of order a few

8

seconds within the entrained flow gasifier. The operating temperature of entrained flow

gasifier must be kept at higher ranges due to short residence time for achieving good

carbon conversion efficiency. Therefore, oxygen is used instead of air in most of the

entrained flow gasifiers and they are operated at the temperature which should be above

to the coal slagging point. Few salient features of entrained flow gasifiers are given

below:

• It is operated at high temperature, above to the coal slagging point;

• Raw syngas contained content of molten slag;

• It requires relatively high oxidant;

• Raw syngas contains a high amount of sensible heat; and

• It has the capability of gasifying all coal regardless of the amount of fines, caking

characteristics, and rank of coal.

Fig. 1.3: Fundamental Entrained flow gasifier with various inlets and outlets

(Source: Higman and Van Der Burgt (2008))

From various generic designs of gasifiers discussed above, entrained flow gasifier

shows the highest conversion rate in the range of 95-99% with large efficiency.

1.3 ADVANCEMENT IN GASIFICATION SYSTEMS

Moderate and Intense Low-Oxygen Dilution (MILD) combustion or more commonly

known as Flameless combustion is a novel and efficient combustion technology which

emits fewer oxides of nitrogen (NOx) and soot (though in some cases air is preheated

at very high temperatures) (Cavaliere and De Joannon, 2004). Currently, researchers

9

have deviated their focus to apply this latest novel technology for the combustion and

gasification of pulverized coal (Stadler et al., 2009b). This technology is based on state

of the art design of gas burners in the industries where heat treatment occurs. In this

technology, the emissions of NOx are in the lowest limit of detection. The reaction zone

in flameless combustion or gasification is prolonged throughout the large volume of

reactor or furnace. Therefore the inherent peaks of high temperature disappear which

cause the flame formation and produce a homogeneous distributions of species and

temperature fields. Flameless combustion or gasification occurs by significantly

diluting the reactants in the primary reaction zone through rapid and high recirculation

of exhaust (flue) gases. Experimental verifications have already been made for

reduction of NOx from coal combustion or gasification with modification and

advancement in burner designs (Ristic et al., 2008, Stadler et al., 2009a).

During the oxidization of most of the chars, the content of oxygen in the reaction zone

is in the higher range in formation of conventional flames. This oxidation or combustion

reaction is much faster in terms of magnitude than the gasification reactions of char

with water vapor and carbon dioxide. Therefore, during numerical investigations, those

gasification reactions are often neglected (Backreedy et al., 2006, Fiveland and Latham,

1993, Kim et al., 2007, Molina et al., 2000, Williams et al., 2002).

The fast dilution of oxygen (O2) in the primary reaction zone occurs during flameless

combustion through extensive recirculation of hot flue gases (combustion products).

Therefore the char oxidation follows a diverse parallel path due to higher partial

pressures of H2O and CO2 than O2 in the reaction zone. The distinct advantages of

flameless gasification over traditional combustion and gasification are (1) emitting

lower pollutants (2) saving of energy sources (3) producing lower noise (4) enhanced

thermal efficiency of combustion/gasification and (5) reduced cost of equipment due to

a reduction in the size of equipment (Tang et al., 2007).

10

1.4 IMPORTANCE OF COAL GASIFICATION IN RELATION TO THAR

COAL DEVELOPMENT

There are three potential options available for Thar coal development, viz. (a)

Integrated open pit mine and coal-fired power generation, (b) Underground coal

gasification (UCG) and (c) Open-pit coal mine and gasification through gasifier. Option

(a) requires a conventional strip mining method for the production of coal and its use

in the coal-fired boiler to run the steam turbine for power generation. Strata conditions

are very much favorable for the strip mining operation, but dewatering of three ground

aquifers is necessary around the mining operation to keep the pit dry. For which 25

million m3 of groundwater is to be pumped out per year for a pit having a production

capacity of 6 million tons. Since the groundwater is highly saline, so it can’t be used

for power plant without desalination. The additional cost of desalination plant will push

the power generation cost to a prohibitive limit. The government of Sindh has prepared

a scheme to bring 90 million m3 per year of fresh water from Nara canal to Thar

coalfield for power plants and mining projects, which will cost Rs. 27 billion.

Additional pumping and clarification cost of canal water is estimated as Rs. 125 million

per year. This option seems to be difficult to materialize.

In option (b) coal is converted in-situ to combustible gases (i.e. CO, H2, and CH4)

through complex reactions at high temperatures and pressures (say 5 MPa, 900oC)

where auto-thermal chemical equilibrium (ACE) conditions are approached. This is a

condition at which the heat value of the product gases and the conversion efficiency of

the gasified coal is a maximum, but carbon oxidation reactions dominate at lower

temperatures and pressures leading to a high CO2 content in the product gases and a

low heat value. Favorable hydro-geological conditions including strong roof and floor

rocks, deep coal seams (>300m), moisture content <40% and no water aquifer in the

vicinity of the coal seam, are essential for the underground coal gasification process.

Thar coalfield does not meet these conditions where the rock strata are mostly

composed of very weak clay-stone, siltstone, and sand. Rock mechanics investigations

carried out at Mehran University reveal that the average value of uniaxial compressive

strength of rock strata and coal is 2.25 MPa and 3.2 MPa respectively which shows that

the rock formation at Thar is even weaker than the coal. There are two major water

11

aquifers in the vicinity of coal seams, one is within the coal zone while the other is

below the coal seams. The bottom aquifer is 47m thick and is under pressure. When the

coal is ignited for UCG at Thar, the water from both the aquifers will rush into the

cavity and will not allow the temperature to rise thus producing a high concentration of

CO2. Due to a very weak rock formation above and below the coal seams, there will be

a substantial amount of gas and heat losses. There is a great chance of surface

subsidence as a result of a big cavity produced in a coal zone due to underground coal

conversion. Due to severe fracturing of overburden strata, the shallow water aquifer,

which is presently used by the local people, will be badly contaminated and there is also

a chance of disappearance of the aquifer around the area of UCG activity. On the basis

of technical reasons as discussed above, it can be concluded that underground coal

gasification is not a viable option for Thar coal.

In option (c) coal is first mined by open pit operation and then fed into a gasifier for

gasification. There are 385 gasifiers of various designs currently operational

worldwide. Total worldwide gasifier capacity is around 45,000 MWth and is expected

to some 75,000 MWth in near future. Various technologies of gasifiers are available

around the world but staged entrained flow gasifier has the highest efficiency with the

production of CO+H2 >80%. Syngas is used to make methanol, ammonia, oxo-alcohols,

liquid fuels via the Fischer-Tropsch process and power generation. IGCC technology is

mostly used to produce electricity from syngas. IGCC technology has a number of

advantages over conventional technologies. Its efficiency is 50% whereas that of the

sub-critical coal-fired power plant is 34%. Pollutant emissions are also significantly

reduced-even compared with advanced conventional technologies, with 33% less NOx,

75% fewer Sox, almost no particulate emissions and up to 90% of mercury emissions

can be captured. IGCC also uses 30-40% less water than a conventional plant. One of

the main barriers to the widespread uptake of IGCC has been its cost. The capital cost

of IGCC is $1850 per KW and operation & maintenance cost is $40 per KW whereas

the capital cost of the conventional coal-fired power plant is $1250 per KW for the sub-

critical system and $1300 per KW for super-critical. Operational & maintenance cost

of both sub-critical and super-critical is $24 per KW. A viable option for power

generation from syngas is through the combined cycle (gas turbine + steam turbine). It

12

requires only 25% of water as compared to the steam turbine. The waste heat of the

combined cycle operations can be utilized for drying of coal and desalination of

groundwater which will be available as a result of the mining operation at Thar coalfield

as mentioned above and we will not require to bring water from Nara Canal. The capital

cost of the gas turbine is $905/KW which is 27% less than a sub-critical steam turbine,

30% less than super-critical and 51% less than IGCC. Operation and maintenance cost

of the gas turbine is about $18 per KW. In order to make this program a real success,

strong R&D activity is required to support surface coal gasification and subsequent use

of syngas for the making of liquid fuels, methanol, ammonia etc. and power generation.

1.5 PROBLEM STATEMENT

As discussed in the previous section that surface coal gasification is the best option for

economic utilization of Thar lignite but the real challenge is the selection and proper

design of a gasifier which suites the characteristics of indigenous Thar lignite coal.

Selection of appropriate technology for any coal requires great knowledge and practical

experience but as per research Entrained Flow Gasifiers have shown great applications

for all types of feedstocks ranging from low-grade coals to biomass and solid waste and

also valid for all types of feeding shape of feedstocks like slurry or dry. But to design

an efficient technology for Thar lignite, strong research and development activity is

required for coal gasification and subsequent use of syngas for making of liquid fuels,

methanol, ammonia etc and power generation. As per previous research understanding

of the kinetics of the gasification reaction is essential for designing an efficient gasifier

(Bermúdez et al., 2011). Coal gasification is based on two fundamental reactions i.e.,

the reaction of char with CO2 and reaction of char with steam (H2O). These reactions

are highly dependent on the type and nature of coal. Lots of work have reviewed from

different parts of the world in which researchers have utilized basic techniques to

extract the kinetic parameters of those gasification reactions with specific to their types

of fuel. But very scant literature is available for the gasification of Pakistani coal

particularly Thar lignite gasification. So the aim of this research is to highlight

fundamental technical issues with Thar lignite gasification and extract the kinetic

parameters for gasification reactions at atmospheric and higher pressures.

13

1.6 OBJECTIVES

Looking at the versatility of coal gasification, Mehran University has devised a strategy

to establish a strong R&D base for the development of indigenous technology suitable

for local coal with the following objectives:

1. To examine the issues involved in the design of high-pressure double-stage

entrained flow coal gasifier under excess enthalpy combustion conditions

(flameless) using CFD simulations.

2. To conduct proximate and ultimate analysis of indigenous lignite coal.

3. Kinetic study of drying, devolatization and gasification reactions for indigenous

lignite using Atmospheric and Pressurized Thermogravimetric Analyzers (TGA

and PTGA).

4. CFD modeling of lignite gasification process under flameless combustion

conditions, using the data obtained from proximate and ultimate analysis and

kinetic study and its validation.

5. To optimize the performance of high-pressure double-stage entrained-flow

gasifier with numerical simulations for various important design parameters like

coal/oxygen distribution between the stages, Oxygen/Carbon ratio, the particle

size of coal, amount of steam, temperatures of feed streams etc.

6. Detailed design of a gasifier.

1.7 SCOPE AND OVERVIEW OF THE RESEARCH WORK

The present research study is on the development of kinetic models of indigenous coal

gasification reactions at high pressure. The working of the research project is divided

broadly into two parts. The first part is Experimental Work in which the samples of coal

collected from Thar Coal Field were analyzed for its proximate and ultimate analysis.

Then the thermogravimetric analysis (TGA) was made in different environments at

atmospheric and pressurized conditions. The TGA results were utilized to extract the

kinetic parameters of various intermediate gasification processes like drying,

devolatization, combustion, and gasification reactions.

14

The second part is regarding the computational fluid dynamics (CFD) modeling work.

In this part, the 3D CFD model was developed and simulated with Chinese coal. During

the simulations, the conditions were set as per published experimental work on double

stage entrained flow gasifier. The model was validated against published results. The

publications were made in the international conference and journal (Unar et al., 2014).

After this, the gasifier was modified and tuned for indigenous coal characteristics

through numerical simulations. Finally, the optimized designed double stage entrained

flow gasifier with multiple opposite burners was fabricated and experimentally verified

the applicability of kinetic models. The whole process is explained in Fig. 1.4.

Fig. 1.4: The flow diagram of research activities

1.8 THESIS STRUCTURE

A brief introduction regarding the Coal Gasification, its mechanism, various

technologies of gasification, Energy scenario in Pakistan and importance of coal

PhD Research Activities

Experimental Work CFD Modeling/ Simulation

Coal Sample Collection

Sample Preparation

Characterization

TGA Analysis

PTGA Analysis

Development of 3D Computational Domain

Selection of Governing Equations and Boundary Conditions

Development of CFD Model

Simulation and Validation

Modification of Geometry and Input Data

Optimization of modified CFD model on indigenous coal data

Proposed a New Optimized Designed of Gasifier

Proposed Process of Gasification system

Development of Kinetic Models

15

gasification research in Pakistan perspectives and objectives has discussed in present

Chapter.

Chapter 2 introduces the general aspects of kinetic modeling and various fundamental

kinetic models which have been used by numerous researchers. The rate determination

techniques of Char-CO2, Char-Steam, and Char-O2 are reviewed.

In Chapter 3, the history of gasification is discussed along with different commercial

designs of gasifiers. Further, the experimental and simulation & modeling work on

gasification are reviewed. The latest trends in gasification particularly the work

published in the area of flameless gasification is also reviewed.

Chapter 4, basically divided into two sections. The first section discusses the

methodology of the experimental work. The details of experimental work including

basic characterization of coal, TGA and PTGA experimental methodologies are

discussed. The second part discusses the results and discussion of experimental work.

The proximate and ultimate analysis of selected samples has tabulated. The kinetic

modeling parameters are extracted using volumetric and grain models at atmospheric

and pressurized conditions.

Similarly, Chapter 5 is divided into two parts. The first part explains the CFD modeling

strategies and methods used. The validation mechanism has also explained. Whereas

the second part discussed the results and discussion of modeling and simulation work.

The 3D CFD modeling results in the forms of contours, plots, and tables are also

presented and discussed in detailed.

The conclusion is given in Chapter 6 on the basis of the discussed results. Finally, the

future recommendations are presented.

16

CHAPTER 2

KINETIC MODELING FOR COMBUSTION AND

GASIFICATION REACTIONS

2.1 GENERAL OVERVIEW

An extensive literature review was conducted in different directions as shown in Fig.

2.1. The work conducted on the Kinetic Modeling of different gasification steps is

reviewed in this Chapter. Whereas the experimental and modeling work conducted on

current and advanced gasification systems like Flameless Combustion/ Gasification

will be reviewed in the next chapter.

Fig. 2.1: Directions of Literature Survey

Literature Survey

Kinetic Modeling

Experimental Work

Modeling and

Simulation

Flameless Combustion/Gasification

17

2.2 INTRODUCTION TO KINETIC MODELING

Reactivity of coal plays a significant role in understanding the coal gasification and

other coal conversion processes like carbonization, combustion, and liquefaction (Çakal

et al., 2007). A series of physical and chemical changes are taking place in coal on

heating it in an inert or oxidizing environment. Re-solidification, softening, the porosity

of the solid particle, surface morphology belongs to physical changes whereas

recombination and breakage of bonds come in the category of chemical changes

(Tromp et al., 1989, Elbeyli and Pişkin, 2006).

In coal combustion, moisture removal is the first step. After the removal of moisture,

coal converted into semi-coke and volatiles (gas and oil) in the pyrolysis process.

Pyrolysis is an important process for coal conversion and generated char

characterization. Ignition behavior and flame stability of coal are also affected by

Pyrolysis. The Combustion process takes place after char production in pyrolysis,

where char reacted with oxygen to produced combustion products like carbon dioxide,

water vapors, oxides of nitrogen and sulfur etc. Economic utilization of coal for power

generation needs an in-depth understanding of combustion characteristics of coal

(Azhagurajan and Nagaraj, 2009).

In gasification process, solid or liquid hydrocarbon feedstock (with low economic

value) is converted into a synthetic fuel gas that has the capability to produce electricity

or can be used as a raw material for the manufacture of chemicals, hydrogen, or

transportation fuels (Gary and Russell, 2011). The primary constituents of syngas are

carbon monoxide and hydrogen. Coal gasification technology significantly fulfills the

environmental control regulations (Us).

The gasification process begins with a rapid removal of volatiles known as

devolatilization (or pyrolysis), leaving behind a char that is mainly composed of fixed

carbon and ash. The next step is the heterogeneous reactions of char with O2, CO2, and

H2O (steam) to produce the syngas. The reaction of char with oxygen (combustion

reaction) is kept controlled by having limited oxygen to produce sufficient heat for the

endothermic gasification reactions of char with CO2 and steam. Therefore, these

18

gasification reactions have a strong influence on the char conversion efficiency under

specific process conditions (Wu et al., 2007a, Li et al., 2009).

2.3 LAWS FOR RATE DETERMINATION OF CHAR-O2 (COMBUSTION)

REACTION AND MECHANISM

The char combustion or the reaction of char with O2 is the fastest reaction among all

other reactions of char with gases occurring in a gasifier or combustor. Usually, the

reaction occurs at the external surface of char particles and reaction controlled by the

diffusion offered by a layer of ash formed on the surface. However, the reaction may

be controlled by gas-film diffusion at a higher temperature and/or increased particle

size. Moreover, chemical reaction controlled regime would be reached for the reaction

at considerable low temperature ranges and/or particle size, and the internal pore

surfaces of the particles are utilized for the occurrence of uniform reaction over there.

In the combustors where pulverized coal is used, the combustion is controlled through

chemical reaction kinetics for particles below 50 μm, whereas diffusion is dominated

during combustion for particles larger than 100 μm (Irfan et al., 2011).

For the reaction of carbon-oxygen, several mechanisms have been proposed during

research conducted over past forty years; however, Walker et al. (Walker et al., 1967)

rejected most of the proposed reaction mechanisms. For the combustion reaction, Elliott

(Elliott, 1981) predicted that at high temperature it shows a first-order mechanism due

to adsorption control of oxygen, and at low temperature, the combustion reaction

behaves like zero order reaction due to the desorption control of the product gases CO2

and CO. The extensive investigations were made for those carbon-oxygen reactions for

coal char (Laurendeau, 1978, Bews et al., 2001, Smith, 1982, Williams et al., 2001) but

still, poor understanding is there for lots of aspects. However, Hurt and Calo (Hurt and

Calo, 2001) summarized the mechanisms for combustion of coal char and is its

universal applications is assumed for the fuels originated from lignocellulosic materials.

The most extensively applied treatment is based on a global reaction of simple nature:

22 COOC →+ (2.1)

The power rate law form of this equation is as follows:

19

n

Oc PRTEAr2

)/exp(−= (2.2)

Where2OP =Partial Pressure of O2, A is the Arrhenius constant more commonly known

as Pre-exponential factor and E is known as the activation energy.

Actually, char-oxygen (C–O2) reaction comprises of a series of elementary reactions

based on adsorption and desorption processes, which includes following elementary

reactions:

)(221

2 OCOCk

f →+ (2.3)

COOCk2

)( → (2.4)

f

k

CCOOC +→ 2

3

)(2 (2.5)

)(/)( 22

4

OCCOCOOOCk

+→+ (2.6)

)(5

2 OCCOCOCk

f +→+ (2.7)

Where rate constants of Arrhenius nature are termed as k1 – k5. The oxygen

chemisorption on active sites are represented by reaction (2.3) and CO is formed by

desorption through reaction (2.4) (similarly described by the mechanisms of

gasification reactions, Cf represents available or active site). CO2 is formed either by

surface reaction (2.5) or through interaction gaseous oxygen with surface complexes

(2.6) through which new complex C(O) may or may not be generated on the product

side. The rate of reaction of carbon with CO2 (2.7) is much smaller than the rate of

reaction of carbon with O2. Therefore, at the time of carbon-oxygen reaction

consideration the reaction (2.7) is significantly excluded on usual basis.

Langmuir–Hinshelwood (L–H) form have been modeled as Semi-global mechanisms

of kinetic laws. For the reactions (2.3) and (2.4), its simplest forms is as under:

20

21

21

2

2

kPk

Pkkr

O

O

c+

= (2.8)

2.4 LAWS FOR RATE DETERMINATION OF CHAR-CO2

(GASIFICATION) REACTION AND MECHANISM

The reaction of char with CO2 (or more simply Char-CO2) is frequently used to check

the reactivities of various kinds of char produced from diverse parent coal through

different processes. This reaction shows a relatively slow rate of reaction, reactivity is

easy to measure and it is similar to the reaction of char with steam. The consumption

of CO2 is not industrial as much as steam in the activation or gasification processes but

at the laboratory level, it is used as a preferred agent due to high importance of C–CO2

reaction. Around the temperature of 723°C, the rate of C-CO2 reaction is slower and

gives better process control of the gasification and provides ease in the analysis of the

various important variables (Çakal et al., 2007). Various researchers (Nozaki et al.,

1992, Sha et al., 1990, Mühlen et al., 1985, Adánez et al., 1985, Kajitani et al., 2006,

Liu et al., 2000, Shufen and Ruizheng, 1994, Blackwood and Ingeme, 1960) extensively

investigated the pressure effects on gasification reaction of char with CO2 and proposed

the modern theories at the conditions of atmospheric pressure (Kapteijn et al., 1992,

Chen et al., 1993). The char-CO2 reaction usually occurs uniformly throughout the inner

surfaces of char particles and controlled by the rate of chemical reaction at lower

temperatures (less than 1000°C) with smaller sizes of char particles (<300 μm) (Sha et

al., 1990, Shufen and Ruizheng, 1994). However, diffusion through pore becomes

significant and important at high temperatures usually more than 1100°C and with

pulverized char having particle size less than 100μm (Liu et al., 2000). This also shows

that the reactivity of char is greatly influenced by temperature. The reactivity data of

high-pressure gasification at low temperature is extrapolated to high-temperature

conditions by Liu et al. (Liu et al., 2000) and a model was developed to estimate the

performance of entrained flow gasifier which is operated at high temperatures.

Kinetic data for coal and char gasification is necessary for the designing of coal

gasifiers. Various investigations have been conducted to determine the kinetics data for

21

coal gasification. The gasification of char with CO2 without a catalyst is a reaction with

endothermic nature:

mol

KJHCOCOC 7.15922 =+ (2.9)

The above reaction is often utilized to measure the rate of gasification due to its slower

rate and ease in measurement. The following mechanism of oxygen-exchange is used

to interpret the reaction [178]:

)(1

2 OCOCOi

+→ (2.10)

)()(3

OCCOOCj

f +→+ (2.11)

)(2

2

COCOi

j→ (2.12)

Where available active sites are shown by Cf and occupied sites are represented by

C(O). The reactivity, Rc is calculated through the following equation:

)1/(222 321 COCOCOCO PkPkPkR ++= (3.13)

Where rate constants are designated as k1, k2 and k3 and PCO2 and PCO are the partial

pressures of CO2 and CO respectively. The rate is approximate to first order at low-

pressure conditions with respect to CO2 concentration ignoring the CO delaying effect

but at a pressure above 1.52 MPa, it approaches to zero order (Dutta et al., 1977).

2.5 LAWS FOR RATE DETERMINATION OF CHAR-H2O

(GASIFICATION) REACTION AND MECHANISM

The mechanism of gasification reaction of char with stream is extensively investigated

through Langmuir–Hinshelwood kinetics in which influences of pressure are expressed

on the reactions of adsorption and desorption throughout gasification of char

(Woycenko et al., 1992, Ye et al., 1998, Beamish et al., 1998, Çakal et al., 2007,

Messenböck et al., 2000, Ochoa et al., 2001, Nozaki et al., 1992). The similar

22

mechanisms have been considered for C–CO2 reaction and C–H2O reaction (Woycenko

et al., 1992, Liu and Niksa, 2004). The proposed elementary reactions of adsorption-

desorption for Char-H2O are as follows:

)(() 22

4

4

OCHOHCk

k++

− (2.14)

())(5

CCOOCk

+ (2.15)

Where active sites of char are represented by C(). The rate equation for C-H2O reaction

has the following form:

OHH

OH

OHPkkPkk

PkR

22

2

2

5454

4

//1 ++=

−

(2.16)

2.6 KINETIC MODELS

The methods conventionally utilized for gasification rates measurement could roughly

be divided into two groups; (1) rapid single-point measurements, and (2) slow real-time

measurements. In rapid measurement technique, reactors are used with high rates of

mass and heat transfer to react the feedstock (coal or biomass) for a specified amount

of time, most of the order of 1 second to several minutes. Then the sample is removed

from the reactor and efforts are made to calculate the degree of conversion. Wire mesh

reactors (Messenböck et al., 1999) and drop tubes (Bryan Woodruff and Weimer, 2013,

Matsumoto et al., 2009, Simone et al., 2009, Biagini et al., 2005) are few examples of

such types of reactors. On one side this technique is efficient and effective to measure

the reactions taking place in the shorter time period but on the other side this technique

is limited for the only single extent of conversion per experiment and provides

difficulties for conducting extensive analysis over a range of gas concentrations,

temperatures, and conversion degrees.

Fixed beds (Smoliński et al., 2010, Luo et al., 2009, Ahmed and Gupta, 2011, Lussier

et al., 1998, Hüttinger and Merdes, 1992) and thermogravimetric analysis (TGA)

(Huang et al., 2010, Mühlen et al., 1985, Alevanau et al., 2011) are few examples of

systems for real-time measurement which have wide applications for gasification

23

studies of coal and biomass. These techniques are conventionally applied for reactions

occurring over 5 min to an hour.

In TGA measurements, usually, there are two methods for studying the kinetics of

gasification reactions. On in which the sample in the chamber heated at some higher

temperature in an inert environment (N2) and then keeping that temperature constant

the reacting gas is shifted. This method is called Isothermal Technique. While in

another method, the reacting gas is introduced from a lower temperature (usually room

temperature) and then at a constant heating rate, the sample is heated. This is called a

non-isothermal technique. In both the methods the weight loss in char due to the

reaction of reacting agent (O2, CO2 or Steam) is recorded. The ash-free basis conversion

(X) versus time profiles are usually calculated from the weight loss versus time data

obtained from the experiments using the following equation.

ashmm

mmX

−

−=

0

0 (2.17)

Where m shows the sample’s instantaneous mass, initial mass is represented by m0 is

the initial mass, and mash is the unconverted mass of sample (remaining mass) showing

the ash content. The derivative of conversion degree with respect to time, usually

detonated as dX/dt, is used to calculate the apparent rate of reaction. The standard

kinetic expression for calculating the overall rate of gas-solid reactions (like in

combustion/ gasification) is given by the following expression (Gil et al., 2012, Lu and

Do, 1994):

)(),( XfTPkdt

dXg= (2.18)

Where k is the apparent rate of combustion or gasification reaction including the effects

of the partial pressure of reacting gas (Pg) and temperature (T), and f(X) designates the

changes in the chemical and physical properties of the sample as the reaction of

combustion and/or gasification proceeds. With the assumption of the constant partial

pressure of reactive gas, the apparent rate of combustion or gasification reaction will

24

be dependent on temperature only and expression becomes the Arrhenius kinetic

equation, written as follows:

RTEAek /−= (2.19)

Where A is Arrhenius constant more commonly known as pre-exponential factor and E

is the activation energy. To describe the term f(x) in Eq. (2.18), various researchers

tested and proposed a number of kinetic models. Few of them are the most fundamentals

and are discussed briefly in subsequent paragraphs.

2.6.1 Volumetric Model (VM)

The volumetric model (VM) is the simplest model. It is assumed in this model that the

gas is diffused uniformly within the entire particle and consequently, reaction is

simplified through the assumption of homogeneous gas reaction with the char (Molina

and Mondragon, 1998, Fermoso et al., 2009, Murillo et al., 2004, Mianowski et al.,

2012). The Volumetric Model (VM) equation is given as under:

)1( Xkdt

dXVM −= (2.20)

The Eq. (2.20) is integrated and following integral form is achieved:

tkX VM=−− )1ln( , Where kVM is model-corresponding kinetic constant.

2.6.2 Modified Volumetric Model (MVM)

Kasaoka et al. (Kasaoka et al., 1985) introduced first the Modified Volumetric Model

(MVM) through modifying VM. However, the changing rate constant is assumed with

conversion of solid (X) during the proceeding of reaction (Wu et al., 2007a, Murillo et

al., 2004, Nowicki et al., 2011). The MVM equation is given as:

)1)(( XXkdt

dXMVM −= (2.21)

The Eq. (2.21) is integrated and converged in the following form:

25

batX =−− )1ln( (2.22)

Where kinetic constant corresponding to the model is represented by kMVM(X),

Meaningless empirical parameters are represented by a & b. Equation for kMVM(X) is:

b

bb

MVM XbaXk1

1

)1ln()(−

−−= (2.23)

The integration of Eq. (3.23) gives the mean value of kMVM(X) as follows:

=1

0

)()( dXXkXk MVMMVM (2.24)

2.6.3 Grain Model (GM) or Shrinking Core Model (SCM)

The Grain Model (GM) or more commonly known as Shrinking Core Model (SCM)

assumes that the outer surface of char particle is utilized for the occurrence of reaction

(Molina and Mondragon, 1998, Fermoso et al., 2009, Nowicki et al., 2011, Zou et al.,

2007, Ochoa et al., 2001). Reaction progressively travels inside the char particle leaving

the ash layer behind. The Shrinking core of unreacted solid phase is observed at the

particles’ intermediate conversion which reduces as reaction moves further. The

mathematical expression of GM can be written as follows:

3

2

)1( Xkdt

dXGM −= (2.25)

The integral form of Eq. (3.25) is:

( ) tkX GM=

−− 3

1

113 (2.26)

2.6.4 Random Pore Model (RPM)

Bhatia and Perlmutter (Bhatia and Perlmutter, 1980) developed the Random Pore

Model (RPM) by assuming the random overlapping of the pore surface during the

course of the reaction (Molina and Mondragon, 1998, Fermoso et al., 2009, Murillo et

al., 2004, Ochoa et al., 2001, Bhatia and Perlmutter, 1980, Liu et al., 2003, Wu et al.,

26

2009a). Hence, the reaction occurs on continuously changing surface area. Structural

parameter ‘ψ’, a characteristic of the model, is used to represent those changes. The

RPM expressed mathematically as follows:

)1ln(1)1( XXdt

dX−−−= (2.27)

And its integral form is:

( ) tkX RPM=−−− 1)1ln(12

(3.28)

To calculate the structural parameter, ψ, following equation is used:

2

0

00 )1(4

S

L

−= (3.29)

The estimation of ψ (at maximum X) can be given as:

1)1ln(2

2

max +−=

X (3.30)

2.7 WORK CONDUCTED ON KINETIC MODELING FOR

GASIFICATION AND COMBUSTION REACTIONS

A number of researcher has worked on the kinetic modeling of combustion and

gasification reactions. The most of the feed is carbonaceous materials like coal,

biomass, solid waste etc. Various types of equipment have used for getting the basic

time or temperature based data like weight loss. Thermogravimetric Analysis (TGA) is

the most common among all of those. The important operating parameters, model

equations used and calculated values of pre-exponential factor (A) and activation

energy (E) by various authors are reviewed in Table 2.1.

Table 2.1: Review of Kinetic Modeling of Carbon-based materials with various techniques

Sr.

No. Feedstock

Equipment

Used Objective of Research

Short Description for

Experimental Conditions

Rate Law/ Kinetic Model Used for

Calculation of A and E A E Ref.

1

New Mexico

bituminous coal,

Washington

subbituminous coal,

and North Dakota

lignite

TGA

Reactivities and surface areas

of coals which were gasified

in steam at high temperatures

were determined.

1 atm. 100 mesh size. 76 mole %

steam. Wt of Sample: 100 mg.

Constant heating Rate

Apparent Gasification Rate (Rc)

dtdWWo

dtdXRc −== /1

(24 to 9300

min-1)

(36.51 to

98.52

KJ/mol)

(Lee, 1

98

7)

2

Burner-jet

fine-grained coke

derived from Irsha-

Borodinsky and

Kuznetsky (II) coal

Fixed-Bed

Reactor

The chemical reactivity and

kinetics of nine Canadian

coal samples ranking from

lignite to

Semi-anthracite and one

wood sample were examined

in a fixed gasifier in the

presence of air and steam at

950-1000

1 atm. 50g sample. 6mm dia

opening wire,

Air+Steam=(2.0dm3/min

+3g/min-water rate). Temp.

Range=950-1000°C.

Gasification run=30 min,

Cooled with N2= 45cm3/min

dt

dc

WR =

1

Shrinking Core Model

( ) 3/111 X

tt −−=

t* is a complete conversion n

so KPrCt /=

Not Mentioned Not

Mentioned

(Fu

ng

and

Kim

, 19

90

)

3

Brown coal samples

(from Victoria) and

Bituminous coal (New

South Wales)

Rigaku-

Denki,

Type 8085

(Thermoflex)

TG-DTA

The pyrolysis of a suite of

brown coal samples and

bituminous coal maceral

concentrates is investigated

by non-isothermal TGA

Atmosphere: N2; Flow rate: 0.1

dm3/min; Sample size: 15 mg;

Heating rate: 10°C/min; Temp.

range: 20 to 950~ crucible:

platinum; TG range: 10 rag;

DTG range: 5 rag-rain -1. All

samples were air-dried and

pulverized to pass an 80 mesh

3.26 (Non-Isothermal)

1)1[(2

)1(33/1

3/1

−−

−=

−a

ak

dt

da

1.36×10-3 to

2.87×107 sec-1

(18.47to

168.56

KJ/mol)

(Ma et al., 1

99

1)

4 Four kinds of Chinese

coals TGA

The coal devolatization

process of different coals was

studied by means of TGA

method

Non-Isothermal; Heating Rates:

10 and 20°C/min; Enviro:N2,

Particle Size:74μm to 100μm;

Max: temp; 800°C

)/exp()( RTEKVVd

dV−−=

−−=−

P

P

RT

E

E

RTK

nVVV exp

1exp

2

2.27×10-8 to

3.26×10-8

s-1

165 to 253

KJ/mol

(Qiu

and

Liu

,

19

94)

27

5

Coal samples from

Soma, Tuncbilek and

Afsin Elbistan regions

PL-1500

TGA

TG/DTG was used to

determine the kinetic

parameters

of raw and cleaned coal

samples

Sample Size: 10mg;

Non-Isothermal;

Heating Rate:10°C/min;

Temp Range:20-900°C;

Air Flow:50ml/min;

Particle Size:<60mesh

nkdt

d

=

Coats and Redfern integral method

−

−=

−

−− −

RT

EERT

E

AR

nT

n

)/21(ln)1(

)1(1ln

2

1

Not calculated 2.8 to 45.7

KJ/mol

(Ozb

as et al.,

20

02)

6

5 coals (Alaska,

Cyprus, Drayton,

CNCIEC and

Denisovsky)

Pressurized

drop tube

furnace

(PDTF)

PTGA

The reactivity of char-CO2

gasification was investigated

with a PTGA in the temp.

range 850-1000° C and the

total pressure range 0.5-2.0

MPa

Sample Size: 10mg;

Non-Isothermal;

Heating Rate: 20°C/min;

Temp.Range:850-1000°C;

Env.CO2 balanced with N2

Shrinking Particle Model

3/2)1( xkdt

dxg −= , 3)

3

11(1 tkx g−−=

−=

RT

EAPkg n

CO exp2

5.17×104

to

1.0×108

min-1 MPa-1

149 to 223

KJ/mol

(Park

and

Ah

n, 2

00

7)

7 Chinese Binxian coal

Self-made

thermal

balance

(TGA)

The gasification activities of

three kinds of Binxian chars

with CO2 were studied at

1000–1300 °C and under

atmospheric pressure in self-

made thermal balance.

Sample Size: Not mentioned;

Isothermal; 3 chars prepared and

used in TGA; Temp: 1000,

1050,1100, 1150, 1200; 1250;

1300 C, CO2 rate: 200 ml/min

Random Pore Model

)1ln(1)1( xxss o −−−=

2

00 /)1(4 SLo −=

Not Calc. 160 to 180

KJ/mol

(Liu

et al.,

20

08)

8

Typical Chinese

bituminous coal from

Shenmu

Cahn

Thermax 500

PTGA

The gasification rates of

Shenmu coal chars with CO2

were experimentally studied

with a PTGA

Sample Size: 10.5 mg, Heating

Rate: 25°C/min (Room Temp to

750° C) and 2°C/min (from 750

to 1000 °C); Pressure: 0.1, 0.6,

1.6,3.1 MPa; Non-isothermal;

Environment CO2

dt

daR = ,

n

COPaRT

EaA

dt

da2

3/2)1(exp −

−=

2.89×1011 285.5

KJ/mol

(Wan

g et al.,

20

08)

9 Coal, Biomass and a

Pet-coke

Setaram TAG

24

The reactivity in the steam of

five different types of solid

fuels (two coals, two types of

biomass and a petcoke) has

been studied

Sample size: 6mg;

Isothermal Conditions; Temp.

Range: 998-1323 K;

Environment: Steam (at423K,

10-40Vol % with N2); flow rate

of reactive gas :150 cm3/min

Vol. Model: )1( Xkdt

dXVM −=

Grain Mod. or Shrinking Core Model:

3/2)1( Xkdt

dXGM −=

Random Pore Model:

)1ln(1)1( XXKdt

dXRPM −−−=

VM:2.97×103

to 2.07×107

GM: 2.22×103

to 1.64×107

RPM: 1.83×103

to 1.93×107

VM: 171-

238

GM: 171-

239

RPM: 171-

238

KJ/mol

(Ferm

oso

et al., 20

08

) 28

10 Biomass (Cynara

cardunculus) in

Down

flowing

Entrained

Flow Reactor

(EFR),

A complete set of

devolatilization and

combustion experiments

performed with pulverized

(∼500 μm) biomass in an

entrained flow reactor under

realistic combustion

conditions are presented.


realistic conditions; fuel

transported with air; Feed flow

40 g/h; heating rate104 K/s:

Combustion Conditions:1040,

1175 and1300°C (O2=4% molar)

Devolatization Conditons:800,

930, 1040, 1175°C.

One-step Devolatization Law:

Vkdt

dVv−=

Heterogeneous oxidation:

n

sOspc PkdNdt

dCm ,

2

2−==

•

)/exp( pvvv RTEAk −=

)/exp( pccs RTEAk −=

Av=47.17s-1

Ac=0.46

g/(m2.s.Pa)

Ev=11

KJ/mol

Ec=63

KJ/mol

(Jimén

ez et al., 200

8)

11

2 typical Chinese

coals,

Petroleum coke

and pitch coke

SETARAM

TG-

DTG/DSC

(TGA)

Investigation of physical

properties and gasification

reactivity of coal char and

petroleum coke

Sample Size: Not Mentioned;

Particle Size: <73μm;

Isothermal Method;

Environment: CO2 ashWWdt

dW

dt

dXR

−−==

0

1 Not Calculated

Not

Calculated

(Wu

et

al.,

20

09b

)

12 Australian coal

SETARAM

Setsys

Evolution

model

TGA

The reactivity of four

pulverized Australian coals

was measured under

simulated air (O2/N2) and

oxy-fuel

(O2/CO2) environments

Sample Size: 2-3mg;

Non-Isothermal, Temp. Range:

303-1474K; Heating Rate:

25K/min; Gas Flow rate: 20

ml/min; O2 conc. In N2/CO2: 2-

50%v/v basis

Char Reactivity:

dt

dm

mmR

ashco

cm)(

1

,

,−

−= s-1

Coal Reactivity: dt

dm

mR

o

pm)(

1,

−=

Not Calculated Not

Calculated

(Rath

nam

et al.,

20

09)

13

Woody Biomass

(Small chips of

Douglas-fir)

Pressurized

Thermobalan

ce ALVAC

9600

The effect of pyrolysis

conditions on char reactivity

has been studied using

Raman spectroscopy


Isothermal Conditions; Temp:

700, 800, 900, 1000 and

1100°C; Atmospheric

Conditions; Environment CO2

)(XfKdt

dXP=

Random Pore Model:

)1ln(1)1( XXKdt

dXP −−−=

Not Calculated Not

Calculated

(Ok

um

ura et

al., 20

09

)

14

Biomass char (a wood

portion of Japanese

cedar, Japanese cedar

bark, a

a mixture of hardwood,

and Japanese

lawngrass)

drop tube

furnace

(DTF)

The purpose of this study

was to investigate the

gasification kinetics of

biomass char, such as the

wood

Sample Size: 1-10g/h;

Residence Time: 0.5-3s;

Temp. Range: 900-1200°C;

Pressure:0.4MPa

Environment: H2O OR CO2 with

N2

Particle Size: 70-80 μm

)(XfKdt

dXR rg ==

−=

g

aa

n

grRT

EAPK exp

Random Pore Model:

)1ln(1)1( XXKdt

dXR rg −−−==

H2O

Gasification:

9.99×104

CO2

Gasification:

2.24×103

H2O

Gasification:

136 KJ/mol

CO2

Gasification:

93.9 KJ/mol

n=0.22

(Matsu

mo

to et al.,

20

09)

29

15 Wood Particle

(Biomass)

Thermo-

Balance

Reactor

A simple expression for the

apparent reaction rate of

large wood char gasification

with steam is proposed

Sample size: 500 mg

Particle size: Min.100μm, Large

particle size-14.3, and 21.1mm.

Reacting gas velocity: 5 m/s;

Temp.850-900°C; The partial

pressure of steam: 0.02-0.06

MPa.

)exp()(RT

EpXAg

dt

dXr n

A −==

Overlapped grain model

3/21

0

1

0

1

0

]])1(1ln[

)(ln1][)1(1[)(

X

XXg

−

−−

−−

+−−=

170.8 s-1 MPa-

0.35

103.3

KJ/Mol

n=0.35

εo=0.5

(Um

eki et al., 2

01

0)

16 Biomass (Pinus

densiflora)

Fixed Bed

Reactor

Carbon conversion was

calculated during CO2

gasification

Sample size: 1 gm;

Isothermal, Temp. Conditions:

850, 900, 950, 1000, 1050 °C,

CO2, flow rate of CO2: 0.5

lit/min; Particle size:250-300

μm.

)(),(2

XfpTkdt

dxCO=

Volume Reaction Model (VRM)

)1( xkdt

dxVRM −=

Shrinking Core Model (SCM)

3/2)1( xkdt

dxSCM −=

Random Pore Model (RPM)

)1ln(1)1( Xxkdt

dxRPM −−−=

VRM

9.03E+5

SCM

3.75E+7

RPM

1.51E+4

[min-1]

VRM

172 KJ/mol

SCM

142 KJ/mol

RPM

134 KJ/mol

(Seo

et al., 201

0)

17

PET from post-

consumer soft-drink

bottles

Thermobalan

ce (Setaram

TAG24)

The reactivity of the PET char

in CO2 was determined by

isothermal TGA at different

temperatures.

Sample Size: 5 gm; Particle

Size: 0.5-1mm; Isothermal

conditions; Environment: CO2;

Flow of CO2: 50 N ml/min;

Temp. Conditions: 925, 975,

1025, 1075 and 1125°C;

Atmospheric pressure.

)(Xkfdt

dX=


2/1)]1ln(1)[1( Xxkdt

dx−−−=

2

00 /)1(4 SLo −=

Not Mentioned Not

Mentioned

(Gil et al., 2

010

)

18 Food Waste Chamber

Reactor

Characteristics of syngas

from the pyrolysis and

gasification of food waste

have been investigated.

Sample Size: 35 g;

Isothermal Condition; Temp.

750, 800, 850, 900° C; steam

flow: 8 g/min; Inert Gas flow:

6.4 g of Ar.

dt

dm

mr

1−=

RTEkAer /−=

5028 to 67778

min-1

111 – 125

KJ/mol

(Ah

med

and

Gu

pta,

20

10)

30

19 Char from Lignite Coal

Pressurized

Thermogravi

metric

Analyzer

(PTGA,

Thermax

500)

Char–CO2 gasification

reactions in the presence of

CO and char–steam

gasification reactions in the

presence of H2 were studied

at the atmospheric condition

using a TGA

Sample Size:10 mg;

Isothermal Study; Temp.

Conditions: 1148, 1173 and

1198 K;

Environment: Mixture of CO2

and H2; Atmospheric

Conditions; Particle Size:

200μm

Langmuir–Hinshelwood (L–H) Model

For Char-CO2

2

2

)/()/(1 3132

1

COCO

CO

pkkpkk

pkr

++=

2

2

65

4

1 COCO

CO

pkpk

pkr

++=

For Char-Steam-CO2

222

22

6532

41

1 COCOOHH

COOH

pkpkpkpk

pkpkr

++++

+=

2

2

22

2

65

4

32

1

11 COCO

CO

OHH

OH

pkpk

pk

pkpk

pkr

+++

++=

k1=2.827E9

k2=1.598E-5

k3=2.25E4

k4=3.485E4

k5=3.458E-9

k6=1.38E-3

[bar/sec]

E1= 216

E2= -146

E3= 74

E4= 143

E5= -227

E6= -74

[KJ/mol]

(Hu

ang

et al., 201

0)

20

Carbonaceous waste

materials (Sewage and

industrial sludge, Fluff,

and Scrap Tire

Powder)

TGA

(TG, Netzsch

STA 409)

Synthesis gas production by

steam-gasification of

carbonaceous waste materials

was kinetically examined.

Sample Size: 53-161 mg;

Particle Size Range: 0.1-4.5

mm; Reactive Gas: H2O (8.5%

Vol in Ar); Heating Rate: 20 and

10K/min; Non-Isothermal from

room temp to 1476 K and then

Isothermal.

=

=N

i

iT XX1

, =

−=N

i

iiT Xk

dt

dX

1

' )1(

−=

)(exp

,

,0

'

tRT

Ekk

ia

ii

For Isothermal phase:

)1('

ii

i Xkdt

dX−= ,

=

= it

ii

C

tTCX

,

),(

0.1E+3

to20.2E9

SeWc-1

65.7E+3 to

185.4E+3

KJ/mol

(Piatk

ow

ski an

d

Stein

feld, 2

010

)

21

Two bio-oils produced

from

birch and aspen forest

and wood

TGA (TA

instruments

Q600)

The char produced by

pyrolysis of an 80 wt% bio-

oil/20 wt% char mixture at

heating rates of 100–10,000

°C/s was subjected to steam

gasification in a TGA

Sample size: 2 mg;

Drying at 110°C for 5 min;

Steam+N2 flow=200 ml/min;

Particle Size:0-250μm

Char Reactivity: dt

dX

XXr

−=

1

1)(

Langmuir-Hinshelwood kinetic Model

)1/(222 OHHOH bPaPkPr ++=

n

OHkPr2

= ,Where,

−=

RT

Ekk exp0

Wood:

9×104–5×106

Bio-Oil

4×104–3×106

[s-1Pa-1]

Wood:

235 KJ/mol

Bio-oil:

219 KJ/mol

(Sak

agu

chi et al.,

20

10)

31

22

Steam activated carbon

and Spectroscopically

pure graphite

TGA

(Setaram Co.,

France)

Non-isothermal TGA data

were used to evaluate the A

and E for the uncatalyzed

gasification by CO2 of two

carbons

Sample size: 8 mg; Non-

isothermal; Temp. Range: 383 K

to 1523 K; Heating Rates: 5, 10,

15, 20 K/min; Reacting Gas =

CO2; Reacting Gas

Flow=40ml/min; particle size:

40μm.

)(exp

)()(

fRT

EA

TkfdT

d

dt

d

−=

==

KAS/Vyazovkin linear method

RT

E

Eg

AR

T−=

)(lnln

2

Carbon:

7.06×105 to

2.83×106

Graphite:

2.51×105 to

1.78×106

[min-1]

Carbon:

145-155

Graphite:

153-169

[KJ/mol]

(Liu

et al., 20

11

)

23 Argentinean coal

TGA

(Model 2000,

Cahn

Instruments

Inc.)

The kinetics of Argentinean

asphaltite char gasification

using carbon dioxide as a

gasifying agent was studied

between 1048 and 1223 K

Sample Size:10 mg; Non-

isothermal, Heating Rate:

4°C/min, CO2/Ar Flow=15 lit/h;

PCO2=80KPa; Particle Size: 20-

150μm

dt

d

dt

dm

mmRate

c

=

−−=

0

1

)().().(2COPFTKG

dt

d

=

The Iso-conversional method

X

CO

a PRT

EBk

dt

d2

exp.)1( 0

3/2

−−=

1.1×105 185 KJ/mol

(Fo

ug

a et al., 20

11

)

24

Oil shale (from the

El-Lujjin deposit in

Jordan)

TGA

(Simultaneou

s Thermal

Analyzer

SDT Q600)

Thermogravimetric (TG)

data of oil shale obtained at

MI (Waste to Energy

laboratory) were studied to

evaluate the kinetic

parameters for El-Lujjun oil

shale samples

Sample Size: 8-15 g;

Non-isothermal; Heating rate: 5,

10, 15 and 20°C/min, Temp.

820°C; Particle Size: 50μm.

)(. XfKdt

dX=

Volumetric Model:

)1.( XKdt

dX−=

Drying:

1.6E-3 to

3.97E-1

Devolati:

2.6E+1 to

3.02E+2

CO2 Gasi:

4.2E+3 to

9.2E+6 [s-1]

Drying:

4.7 - 17

Devolati:

56.2 - 68

CO2 Gasi:

122.2-182.2

[KJ/mol]

(Sy

ed et al., 2

01

1)

25 Wood Chips (Yellow

pines)

Chamber

Reactor

Kinetics of woodchips char

gasification has been

examined through steam and

CO2 as gasifying

agents

Sample Size: 35 g;

Isothermal Condition;

Temp.900° C; Gasifying agent:

4.42 g/min of H2O (steam) or 5.4

g/min CO2. Gasifying agent

partial pressure: 1.5, 1.2, 0.9 and

0.6 bars. Total Pressure=2 Bar

dt

dXr =


)1ln(1)1( Xxkdt

dx−−−=

Not calculated Not

Calculated

(Ah

med

and

Gu

pta, 2

01

1)

32

26 Sewage Sludge Char

Thermobalan

ce (Netzsch

STA 409 PG,

Germany).

Gasification of char derived

from sewage sludge was

studied under different

oxidizing atmospheres

containing CO2, O2 or H2O

Sample size: 20 mg; Isothermal

conditions; Total flow rate of

gasifying agent (O2 or CO2

mixed with H2O)=50

m3/min(with stram it is higher);

Particle size: 70μm; Different

Environments:

10% O2+Ar [450, 500, 550°],

16% H2O+Ar [750, 800, 850°C],

50% CO2+Ar [800, 850, 900° C]

)().,(

sg rTykdt

d=

First-order kinetics (Vol.Model)

)1(

−= vkdt

d

tkv=−− )1ln(

Shrinking Core Model

tks=−−= 3/1)1(1)(

O2:

1.16×104

H2O:

5.96×106

CO2:

1.09×106

[sec-1]

O2:

114 KJ/mol

H2O:

227KJ/mol

CO2:

193 KJ/mol

(No

wick

i et al., 201

1)

27

Solid

Waste Recovered

Fuels (SRF)

TGA

The gasification reactivity

data of some solid

waste recovered fuels (SRF)

obtained from

thermogravimetric

analysis (TGA) experiments

is presented


Isothermal; Temp. 700, 750, 800

and 850°C; Gasifying agent: H2O

and CO2 separately; atmospheric

conditions;

22

2

2

3

1

3

1

1

1 Hb

OH

f

OHf

OHC

Pk

kP

k

k

PkR

++

=−

COb

CO

f

COf

COC

Pk

kP

k

k

PkR

3

1

3

1

1

2

2

2

1 ++

=−

( )2

,modexp, −=N

j

CC jeljRRL

Steam

K1f:6.49E7

K1b:95.3

K3:1.64E9

CO2

K1f:1.64E7

K1b:4.59E2

K3:8.83E7

[bar-1s-1]

Steam

E1f:204

E1b:54.32

E3:243

CO2

E1f:188

E1b:88.27

K3:225

[KJ/mol]

(Ko

nttin

en et al., 2

012

)

28

Empty Fruit Bunches

(EFB)

[Seri Ulu Langat Palm

Oil

Mill, Dengkil,

Selangor, Malaysia]

TGA

(Model

Mettler

Toledo, TGA/

SDTA851,

USA)

Empty fruit bunches (EFBs),

a waste material from the

palm oil industry, were

subjected to pyrolysis and

gasification

Sample Size: 10 mg; Non-

isothermal; Heating rates: 10, 20

and 30°C/min; Max. Temp.

1000°C; Gasifying agent :Air;

flow: 100mL/min; Particle Size:

0.3-1 mm.

)()(

fTkdt

d=

n

iif )1()( −=

Cellulose

4E+3-6.5E+6

Lignin and

Char Comb:

2.95-16.62

[Sec-1]

Cellulose:

61.2-73.7

Lignin and

Char Comb:

40-48

[KJ/mol]

(Mo

ham

med

et al., 20

12

)

29 Thar Coal (Block V of

Thar coalfield) TGA

The concept of weighted

mean activation energy has

been used to assess the

reactivity of Thar coal in

terms of pyrolytic and

combustion behavior using

non-isothermal TGA

Non-isothermal study; For

Pyrolysis: N2 flow 30 ml/min

from 110-800°C@10°C/min. For

Combustion: O2 flow 20ml/min

from 110 to 1000°C @16°C/min;

particle size: 60 mesh

)(.exp xfRT

EA

dt

dx a

=−

First Order Kinetics (Volumetric

Model)

)1()( xxf −=

Not mentioned

Pyrolysis:

19.20–63.55

Combustion:

23.68–54.49

[KJ/mol]

(Sarw

ar et al.,

20

11)

33

30

Hard coal from the

“Janina” coal from

Poland

TGA

[Netzsch

STA 409PG

Luxx]

The kinetics of polish hard

coal and its char gasification

using CO2 as a gasifying

agent was

studied at atmospheric

pressure between 293 and

1223 K at different linear

heating rates.

Sample Size: 5 mg; Non-iso

conditions; (for Coal) Heating

Rate: 1, 3 and 10 K/min; Temp.

Range: 293-1273 K; Gaifying

Agent: N2 (10ml/min) CO2 (50

ml/min); (for Char) Heating

Rate: 1.25, 5 and 10 K/min;

Temp. Range: 293-1273 K;

Gaifying Agent: N2 (25 ml/min)

CO2 (50 ml/min);

)()(

fTkdt

d=

F1: First Order Kinetics

−= 1)(f

F2: Second Order Kinetics 2)1()( −=f

A2: Accidental Nucleation, Avrami-

Erofeev equation 2/1)]1ln()[1(2)( −−−=f

A3: Accidental Nucleation, Avrami-

Erofeev equation 3/2)]1ln()[1(3)( −−−=f

F1: 4.9

F2: 6.1

A2: 4.2

A3: 4.3

[LogA, 1/s]

F1: 172

F2: 193

A2: 158

A3: 160

[KJ/mol]

(Łab

ojk

o et al., 2

01

2)

31

Bituminous Coal (from

China) and Palm Shells

(from Malaysia)

TGA (Perkin

Elmer Pyris

STA 6000)

Kinetics of bituminous coal

and palm shells were

evaluated using TGA under

different environments

(N2/CO2/O2)

Sample Size: 10mg; particle size:

100-300 μm; Non-isothermal;

Heating rate: 10 K/min; Temp.

Room Temp to 1273 K;

Environments: O2/N2 and

O2/CO2; gas flow: 50 cm3/min

)/exp()1(

RTEA

dT

d−

−=

Integral form: )()1ln( xpR

AE

−=−

Doyle’s Approximation for P(x):

−−

=−

RT

E

R

AE052.133.5ln)]1ln[ln(

Coats-Redfern’s Approximation for

P(x): RT

E

R

AE

T−

=

−−

ln

)1ln(ln

2

Doyle’s:

1.57×10-7 to

6.13×10-4

Coats-

Redfern’s:

-1.82×10-5 to -

2.09×10-8

[sec-1]

Doyle’s:

32.11 –

82.84

Coats-

Redfern’s:

44.95 –

99.88

[KJ/mol]

(Irfan et al., 2

01

2)

32 Douglas fir chips

[Biomass]

TGA (TGD-

9600;

ALVAC)

The CO2 gasification

behavior of biomass chars

derived at 800 °C under

N2/CO2/O2 atmospheres was

investigated using the RPM

model at gasification temp.

of 800–1000 °C

Sample size: not mentioned;

Non-isothermal; Heating Rate:

10°C/min [upto 100°C with Ar]

and then 2°C/min from 800-

1000 °C; Environment: CO2,

Particle Size: 2-4mm.

)()( XfTkdt

dX=


5.0)]1ln(1)[1( XXkdt

dXp −−−=

Not mentioned Not

mentioned

(Han

aok

a et al.,

20

12)

34

33

4 coals: a semi-

anthracite,

a medium-volatile

bituminous coal and

two high-volatile

Bituminous coals

Thermobalan

ce (Setaram

TAG24)

The thermal reactivity and

kinetics of four coal chars in

an oxy-fuel combustion

atmosphere (30%O2–

70%CO2) were studied using

a thermobalance

Sample Size: 5 mg; Isothermal

Conditions; 400, 450, 500, 550,

600° C; Gasifying agent (30%

O2, 70% CO2); Flow of gas: 50

Nml/min; Particle size: 75-150

μm.

)()( XfTkdt

dX=

Vol. Mod: )1( Xkdt

dXVM −=

Grain or Shrinking Core Model:

3/2)1( Xkdt

dXGM −=

Random Pore Model:

2/1)]1ln(1)[1( XXkdt

dXRPM −−−=

VM

3.55E4 to

4.09E5

GM

2.51E4 to

2.66E5

RPM

8.1E3 to

2.82E5

[s-1]

VM

118-128

GM

118-128

RPM

117-127

[KJ/mol]

(Gil et al., 2

012

)

34

Three types of Chinese

coals from Mulei,

Shenmu, and Shigouyi

TGA

(Netzsch

STA-449F3)

Comparative study on the

gasification reactivity

of the 3 types of Chinese coal

chars with steam and

CO2 at 850–1050° C was

conducted by isothermal

TGA


conditions; Temp. 850, 900, 950,

1000, 1050° C; gasifying agent:

CO2 or Steam; Composition and

flow of gasifying gas: 3g/h

steam+100mL/min N2, 130

mL/min CO2

nth order DAEM model n

RTE

v

vvek

dt

vvd

−= −

*

**)/( /

0

RT

Ek

nt

n 1ln

1

)1(1lnln 0

1

+−

−

−−=

−

Not mentioned

Steam:

40-160

KJ/mol

CO2:

110-180

KJ/mol

(Fan

et al., 201

2)

35

Switchgrass (from

Colorado State

University)

[Biomass]

Downflow

fixed bed

reactor

A novel kinetics

measurement technique has

discussed based on a

modified fixed bed and data

collected solely

from a gas flow meter

Sample Size: 50mg and 150mg;

Isothermal; Temp. 1000°;

Gasifying Agent: Steam

'

tV

V

dt

dXr

•

==

Steam-char reaction

Langmuir–Hinshelwood Model

22

2

32

1

01 HOH

OH

pKpK

pKk

++=

k1: 2.51E3

k2: 6.74E-2

k3: 3.04E-1

[bars-1s-1]

E1: 112.6

E2: -37.3

E3: -36.6

[KJ/mol]

(Bry

an W

oo

dru

ff

and

Weim

er, 201

3)

36

Algal biomass

(Chlorella sp.)

And woody biomass

(commercial wood

mix)

TGA

(Model STA

449F3

Jupiter)

The gasification reactivity

measurement of an algal

biomass (Chlorella sp.) and

woody biomass (commercial

wood mix) char was

performed in a TGA at 3

different temperatures

Sample size: 4-5 mg; Isothermal

conditions; Temperatures: 800,

950, 1100° C; Environments:

CO2 and Steam; Gas

compositions: 20 mL/min

H2O/CO2 + 80mL/min N2;

ii

idt

dw

wR

−=

1 Not calculated

Not

calculated

(Kirtan

ia et al.,

20

14)

35

37 A Chinese lignite Coal

TGA

(Customer

Designed)

The chars were gasified

with CO2, H2O and their

mixtures in a TGA system to

investigate gasification

kinetics and derive the rate

expression


Conditions; Temp; 1273 K;

Environments: CO2 and H2O

(Steam); Gas mixture flow:

3NL/min; Atmospheric

Conditions.

21 /1)/(1/122

kPkr COCO +=

43 /1)/(1/122

kPkr OHOH +=

Shrinking Core Model And Langmuir–

Hinshelwood Expression

1.564E-4 to

9.723E-4

[s-1]

Not

calculated

(Ch

en et al.,

20

13

a)

38 Coal (Lignite and

Bituminous)

TGA (Linseis

STA PT

1600) and

PTGA

(Linseis

GmbH in

Selb,

Germany)

The devolatilization rate of

two coals are measured in a

pressurized high temperature

entrained flow reactor at up

to 1600°C and 4.0 MPa and

in a wire mesh reactor

Sample size: 40-60 mg;

Isothermal; Temp. 1000° C; )1( xdt

dx

mdt

dmr

−=

=

Pyrolysis:

293 s-1

Char

Deactivation

23.4 s-1

Pyrolysis:

51 KJ/mol

Char

Deactivation

117 KJ/mol

(Trem

el and S

plieth

off,

20

13

a, Trem

el and

Sp

lietho

ff, 20

13b

)

39 Wood, Miscanthus,

and Straw [Biomass]

TGA (Hi-Res

TGA 2950;

TA

Instruments,

New Castle,

DE, USA)

The CO2 gasification kinetics

for each biomass sample was

established in the temp.

range from 800 °C to 1300

°C by the combination of

TGA and a novel aerosol-

based method

Sample size: 2.5 mg; Isothermal

Conditions; Temp. 800 to 1300°

C; Initially heated at 50°C/min

till temp with N2 then switch the

gas to CO2(33%) +N2 mixture;

Gas flow: 150 ml/min;

0

)(),(

S

XSTCK

dt

dXgas =

)/exp(),( RTEAaTCK gas −=

Random Pore Model

)1ln(1)1( XXkdt

dXRPM −−−=

Wood:

2.23×108

Miscanthus:

3.24×105

Straw:

4.48×104

[sec-1]

Mean Values

Wood:

240.5

Miscanthus:

187

Straw:

164.5

[KJ/mo]

Mean Values

(Lin

and S

trand

,

20

13)

40

Brown Coal

from Loy Yang

(Victoria, Australia)

TGA (TG-

DTA2000S,

MAC

Science)

The mechanisms and kinetics

model of the char

gasification and volatile–char

interactions were discussed

to describe quantitatively the

inhibition of char gasification

by volatiles

Sample size: Not mentioned;

Isothermal conditions; Temp:

800° C; Environment: Steam

(H2O).

cncchar kk

dt

dX+=

Non-catalytic Gasification

HncHncOHnc

OHnc

ncPKPKPK

PKk

432

1

22

2

1 +++=

Catalytic Gasification

HcHcOHc

OHc

cPKPKPK

PKk

432

1

22

2

1 +++=

Non-catalytic

Knc1=4.3E-4

Knc2=2.8

Knc4=200

Catalytic

Kc1=4.5E-3

Kc2=4.5

Kc4=45

[Diff. units]

Not

calculated

(Kajitan

i et al., 20

13

) 36

41

3 coals

(Spanish

anthracite, Spanish

lignite, and Russian

bituminous coal)

Entrained

Flow Gasifier

(EFR) at

LITEC’s

The results of an

experimental study on the

gasification kinetics of three

coals of different ranks under

realistic conditions in an EFR

are presented

Sample size: Not mentioned;

Particle size: 53-63 μm;

Environment: CO2, O2, and N2

with the different composition;

Temp. Range: 800-1450°C.

VRTEAVkdt

dVpvvv )/exp(−=−=

)/exp(, p

n

siii RTEPAk i −=

Devol:

9.2E4-1.85E8

Oxy:

1.03E-4-0.095

Gasification:

0.038-7.55 [s-1]

Devol:

92.3-184

Oxy: 58-108

Gasification:

100-148.5

[KJ/mol]

(Go

nzalo

-

Tirad

o et al.,

20

13)

42 Typical Chinese Coal

TGA

(NETZSCH

STA 409C,

Germany)

Char surface active sites

were first measured with the

help of the chemisorption

process of CO2 at 300 °C,

using a TGA

Sample size: 20 mg; Non-

isothermal; Heating rate: 5, 10,

20 K/min; Temp. 950-1300°C;

Environment: CO2

)()./exp( xgRTEAdt

dx−=

Ozawa Method:

)(log)/log(

315.2)/(4567.0log

m

m

xGRAE

RTE

−

+−−=

=x

xgdxxG0

)(/)(

Not calculated

Slow-

Pyrolysis

192 to 310

Rapid-

Pyrolysis

212 and 243

[KJ/mol]

(Xu

et al., 20

13

)

43

Beechwood chips

(Biomass) provided by

SPPS Company,

(France)

Macro TGA

Gasification reactivity of

high-heating-rate chars in

steam, CO2 and their

mixtures was investigated in

a new macro-TG

experimental device

Sample size: 0.1-1 g (for

pyrolysis); Isothermal

Conditions: Temps. 800, 850,

900, 950° C; Environment: H2O,

CO2, and Their Mixture

dt

dX

Xdt

dm

MR

t

t

t

t

X

)(

)(

)(

)(

)(1

11

−=−=

)()50( ),(),,( XFRRii PTXPT =

n

iTPT PkRi

= )(),()50(

Char+H2O

26.3×103

Char+CO2

55.18×103

[Sec-1bar-1]

Char+H2O

139 KJ/mol

Char+CO2

154 KJ/mol

(Gu

izani et

al., 20

13

)

44

3 coal samples:

lignite

coal from ‘‘Turów’’

mine, and 2 sub-

bituminous coals

from ‘‘Piast’’ and

‘‘Wieczorek’’ mines (

Poland)

TGA

(Netzsch

STA 409 PG

Luxx)

The gasification reactivities

of three char samples from

Poland toward CO2 were

investigated isothermally

using TGA


Conditions; Temp. 900, 950,

1000°C; Particle Size: 200 μm;

Environment: CO2, Flow of CO2

)()( XfTkdt

dX=

Vol. Mod. (VM) )1( Xkdt

dXVM −=

Mod. VM.(MVM) )1( XXkdt

dXMVM −=

GM. 3

2

)1( Xkdt

dXGM −=


)1ln(1)1( XXkdt

dXRPM −−−=

VM

6.87E7-1.22E9

MVM

1.76E7-6.1E8

GM

1.11E7-2.17E7

RPM

1.88E6-1.74E7

[min-1]

VM

199-246

MVM

187-240

GM

187-208

RMP

184-209

[KJ/mol]

(To

maszew

icz et al., 201

3)

37

38

2.8 SUMMARY

The gasification is a complex phenomenon in terms of elementary reactions and their

mechanism. Overall the whole gasification process has divided into four broad steps.

The first step is known as drying in which moisture is removed from the coal on the

increase of temperature. The second step is called devolatization or pyrolysis in which

volatiles are removed from the parent coal in the absence of oxygen. The third step is

char and volatiles combustion with releasing of CO2, H2O, and heat in the reactor.

Finally, the unburnt char is reacted with CO2 and/or H2O at a higher temperature to

produce syngas components like CO and H2. The fundamental kinetic mechanisms,

associated mathematical correlations (kinetic models), and experimental work

conducted to extract concerned kinetic parameters were reviewed in this chapter.

Among various kinetic models, Volumetric Model and Shrinking Core model (or more

commonly known as Grain Model) were proposed as most simple, accurate and easy in

calculations in the research. The solution of these models was carried out by either

direct plot method or integral method as per their simplicity and getting good accuracy.

A number of published literature is reviewed regarding the experimental work

conducted in the quest of kinetic modeling. In this regard, various types of feedstocks

like coal, biomass, waste, tires, etc. were used. Most promising devices for kinetics

modeling of these fuels were lab-scale reactors, wire-mesh reactors, drop-tube furnaces,

and thermogravimetric analyzers. Among these TGA has most significant applications

in this field and provides best and accurate results with all types of feedstocks. The

prime objective of most of the research was to calculate the important kinetic

parameters like activation energy (E) and pre-exponential factor (A). From the review,

it was concluded that each material has distinct behavior towards the kinetics of

combustion and gasification reactions. Keeping in view the importance of kinetics of

coal gasification, the literature on kinetic modeling for indigenous coal was surveyed

but few published literature specific to combustion was found. No literature for

indigenous coal regarding gasification kinetics has searched. So there is the great

necessity of conducting research on indigenous lignite coal for extracting kinetic

mechanism for the whole gasification process.

39

CHAPTER 3

LITERATURE REVIEW ON GASIFICATION TECHNOLOGIES

AND MODELING STUDIES

3.1 GENERAL DISCUSSION

The kinetic modeling of various gasification processes is reviewed in detail in the

previous chapter. The standard kinetic models and extraction of kinetic parameters

through experiments were also discussed. In this chapter, the literature survey is mainly

focused on the Gasification Technologies and general practices for experiments on

gasifiers and combustors along with simulation and modeling strategies. Apart from

this, the latest advancement in gasification system will also be reviewed.

3.2 HISTORICAL DEVELOPMENT OF GASIFICATION

Humans used wood as a fuel at first, and the practice of using wood as a fuel is still

continued by a huge population of the world for heating the homes and cooking meals.

But wood is also used for producing furniture, in the construction of buildings and in

industrial processes as a raw material in the form of charcoal like reduction of ore.

Hence the increasing demand of wood made the shortage of this fuel and this shortage

led to investigate some other fuel instead of wood and found ‘Coal’ as a suitable

alternative (Higman and Van Der Burgt, 2008).

Coal production and utilizing are not new in the human civilization history but its

production reached at peak level in the second half of the 18th century with the industrial

revolution in England. The development of coke oven was an initial stage to fulfill the

requirements of metallurgical industry for providing coke as an alternative to costly

charcoal. Gas was produced from coal through pyrolysis process up to the end of 18th

century on a slightly larger scale. Finally, gasification turns into the commercial process

with the foundation of London Gas, Light and Coke Company in 1812. Since after,

gasification pertains major role in the development of various industrial processes.

40

Town gas was used as the most significant gaseous fuel during the first century of

industrial development. Two processes i.e., pyrolysis and water gas process were used

to produce town gas. In pyrolysis, coke along with gas of relatively high heating value

(20,000–23,000 kJ/m³) is produced in ovens operated in batch mode. Whereas coke is

converted into a mixture of hydrogen and carbon monoxide in water gas process which

is also batch mode operation. The gas produced in water gas process categorizes as

medium BTU gas with is approximately 12,000 kJ/m³ heating value.

Illumination was the first application of industrial gas produced from coal. The scope

of applications of gas further expanded for heating, utilization in the chemical industry

as raw material and recently generation of electric power. Initially cooking and heating

were the only applications of town gas due to its expensive production from

gasification. For these applications, town gas had advantages over the alternatives

options like coal and candles. But with the advent of electric bulbs around 1900 gas lost

its worth for lightening purpose its only utilization remained for space heating.

Producer gas and blast furnace gas were the only gases produced through continuous

process till the end of the 1920s. Coke’s partial oxidation with humid air was the key

process to produce Producer gas. However, the application of both gases was restricted

in the vicinity of their production due to their low heating values (3500–6000 kJ/m³).

The partial oxidation of solid fuels for the production of gases has advantages of ease

in handling of gases and obtaining a fraction of hydrogen which remained a clean and

high heating value gas. Though the start of gasification was based on producing gas for

lighting and heating purpose, but from 1900 and onward gasification become an

important source for producing raw material for other chemical industries due to an

equal amount of hydrogen and carbon monoxide with water gas process.

After the availability of pure oxygen from Carl von Linde commercialized cryogenic

air separation process during the 1920s, synthesis gas along with hydrogen was

produced from gasification processes in complete continuous mode with an oxygen

blast. At that time some most important processes were developed which became the

precursors of several important processes of the current era. Like in 1926 Winkler fluid-

bed process was developed, in 1931 Lurgi moving-bed pressurized gasification process

41

was invented and Koppers-Totzek entrained-flow process came into existence in 1940.

These processes became the foundation for the technological progress for the

gasification of solid fuels in the upcoming 40 years. However, the capacity of the

gasifier was the basic need in the quest of gasification technology innovation due to

synthetic fuels program of Germany during wartime and worldwide expansion of the

ammonia development industry.

South African Coal, Oil, and Gas Corporation, nowadays more commonly known as

Sasol was also established at that time. The Fischer-Tropsch synthesis is the foundation

of this process to produce synfuels complex from coal gasification. Sasol becomes the

world’s biggest gasification hub after its extensions at the end of 1970.

The importance of coal gasification along with its technological progress declined in

the 1950s due to the advent of plentiful quantities of natural gas and naphtha. But the

scope of syngas applications got enhanced with the increasing demand for ammonia for

the expansion of the fertilizer industry. The increased demand could only be met with

the steam reforming of naphtha and natural gas. The shell and Taxaco (formally known

as GE) processes for gasification of oil were developed in the 1950s. They fulfilled the

demand of ammonia production from syngas at the locations with a shortage of naphtha

or natural gas.

The real gasification importance revived for generating liquid and gaseous fuel in the

early 1970s due to first oil crisis along with the enhanced cost of natural gas. The

investigations started for developing new technologies with appreciable investments.

Initially, efforts were made for developing coal hydrogenation via either direct

liquefaction or through hydro-gasification in which coal is converted directly into

methane and known as a substitute natural gas (SNG). Several processes for hydro-

gasification were investigated through demonstration units but the process could not

get commercial viability due to the thermodynamic constraint of high pressure (Speich,

1981). In reality, moving-bed gasification technology based plant of SNG with oxygen-

blown mode was to be built for providing synthesis gas as an alternative step of

methanation (Dittus and Johnson, 2001).

42

Few older processes were developed further with from fuel research programs by

various investors. Existing technology of Lurgi was modified in a slagging version with

the partnership between British Gas and Lurgi (BGL) (Brooks et al., 1984). Koppers-

Totzek gasifier was modified in its pressurized versions by a joint venture of Shell and

Koppers and produced Shell Coal Gasification Processes (SCGP) and Prenflo

technologies (Van Der Burgt and Kraayveld, 1978). A modified version of Winkler

fluidized-bed process was developed by Rheinbraun known as High Temperature

Winkler (HTW) process (Speich, 1981), and coal slurry feeding system was introduced

in to oil gasification process by Texaco (Schlinger 1984). However, the importance of

coal gasification and liquefaction and its research decreased in the 1980s with a new

oversupply of oil.

3.3 COMMERCIAL GASIFICATION TECHNOLOGIES

A wide range of reactor types is in use since long for carrying gasification processes

considering practical constraints. Those types of reactors are broadly grouped into one

of three renowned classifications: (1) fixed-bed or moving-bed gasifiers, (2) fluid-bed

gasifiers, and (3) entrained-flow gasifiers. The classification is based on certain

characteristics of gasifiers fall in each class that differentiates the gasifier from one

class to other. The classification of commercial gasifiers based on these three broad

types is discussed in subsequent sections.

3.4 FIXED-BED OR MOVING-BED PROCESSES

In fixed-bed gasifiers (sometimes called moving-bed gasifiers) a fixed bed of coal

moves slowly downward direction due to gravity during its gasification by a blast

mostly counter-current to the coal flowing direction. The downward moving coal is

pyrolyzed and preheated by hot synthesis gas, produced in the gasification zone during

counter-current flow. This method is economical in the consumption of oxygen, but

produced synthesis gas is contaminated with pyrolysis products. The exiting syngas is

usually at lower temperatures but even with high temperature, the core of the bed

reached hardly at slagging temperatures. A lump of coal is used in moving-bed

processes. The coal caking property with the presence of excess fines produces

43

blockage of passing syngas in an upward direction. Few commercially developed

technologies are discussed as under.

3.4.1 The Sasol-Lurgi dry bottom processes

The Lurgi dry bottom process was patented in 1927 which also known as “coal pressure

gasification”, later in 1931, the existing technology was modified by Lurgi with the

cooperation of Technical University in Berlin, in a pressurized version with oxygen

blast for lignite coal gasification (Gmbh, 1970, Küffner, 1997). The first modification

was commercialized in 1936 (Rudolf, 1984). Two large-scale plants, Brüx (Czech

Republic) and Böhlen (Saxony) came into existence in 1944 for the production of town

gas. The technological developments like automation, scale-up, and optimization of

operation for only pressurized Lurgi dry bottom gasifier were continued for several

years. A joint venture between Lurgi and biggest technology tycoon Sasol further

developed an advanced version of gasifier known today as Sasol-Lurgi Dry Bottom

Gasifier and made efforts for its marketing.

The reactor is the heart of the Lurgi process where down coming coal feedstock meets

with upflowing blast and syngas counter-currently (Fig. 3.1). The coal is inserted into

the reactor in a cyclic manner using a lock-hopper mechanism for maintaining the

pressure of reactor vessel. The boiling water is filled in an annular space between the

two walls of the dual walled jacked reactor vessel. The exothermic heat from the reactor

core is transferred into the boiling water and converts it into steam. Even distribution

of coal from the lock-hopper in the reactor is maintained through a mechanical

distribution device. The slowly down-flowing coal undergoes the processes of moisture

removal (drying), devolatilization, combustion, and gasification. The rotating grate is

used to remove the ash from the reactor through ash lock-hopper. The precooling of ash

to 300–400°C is taking place at grate zone through entering blast of steam and oxygen.

The grate is also used for even distribution of entering blast in the reactor bed. The hot

ash preheats the upward flowing blast which reaches at combustion zone and the

reaction of char with oxygen produces CO2 and heat and then gasification reactions

start in the presence of steam, coal, and CO2 producing CO, H2 and fraction of CH4.

Table 3.1 summarizes the standard sizes of reactors.

44

Fig. 3.1: Sasol-Lurgi dry bottom gasifier (source: (Gmbh, 1970)).

Table 3.1: Sizes and capacities of Sasol-Lurgi dry bottom gasifiers (Rudolf, 1984,

Higman and Van Der Burgt, 2008)

Type

Nominal Vessel

Diameter

(cm)

Coal feeding rate

(tons / hour)

Production of Dry Gas

(103 Nm3/day)

MK-III 300 20 1000

MK-IV 400 40 1750

MK-V 500 60 2750

The biggest complex for gasification in the world is installed at South Africa operated

by Sasol synthetic fuel company which produces 55 million Nm3/day syngas from 13

MK-III, 83 MK-IV, and 1 MK-V reactors. About 170 000 bbl/day F-T liquid fuels are

being produced from the generated synthesis gas (Erasmus and Van Nierop, 2002).

45

3.4.2 British Gas Lurgi (BGL)

The modified version of Lurgi gasifier with slagging capabilities, known as British Gas

Lurgi (BGL) gasifier (Fig. 3.3), was developed by British Gas from the period of 1958

to 1965 in the Research Station at Gas Council Midlands. The feeding capacity of 13ft

developed BGL gasifier was 100 t/day (Seed et al., 2007). The BGL gasifier contains

refractory-lined walls with dry feeding and oxygen-blowing characteristics. This

gasifier shows good performance with different types of coals along with the capability

of acceptance for blends of various solid fuels like wood waste, RDF and tires. The

demonstration plant of this gasifier is operated from 1986 to 1990 in North America

with 500 TPD capacity. Its first commercial version has been operated with lignite from

2000 – 2005 at Schwarze Pumpe Power Station, Germany.

Fig. 3.2: British Gas Lurgi Gasifier (Source: (Higman and Van Der Burgt, 2008))

3.4.3 Multipurpose Gasifier (MPG)

The Multipurpose Gasifier (MPG) technology was developed by Lurgi on the basis

gasification process with fixed-bed configuration. It is refractory-lined gasifier with

down fired and oxygen-blown mechanisms and considered a good choice for a wide

46

range of raw materials used as feedstocks including solid waste, coal slurries, and

petroleum. Coal and/or petcoke are used in this with a quench configuration. The

demonstration plant by Lurgi is in operation since 1968 at Schwarze Pumpe Power

Station, Germany.

Fig.3.3: Lurgi Multipurpose Gasifier (Source: (Breault, 2010))

3.5 FLUIDIZED BED PROCESSES

In Fluid-bed gasifiers, the mass and heat transfer are promoted with excellent mixing

between oxidant and solid feed. The wastage of unreacted fuel with ash removing could

be minimized with material’s even distribution in the bed and hence carbon conversion

is limited on this even distribution characteristic in the operation of the fluid-bed

gasifier. As ash slagging creates hindrances in the fluidization process so the

temperature of fluidized bed gasifier usually restricted below the ash softening point

during its operation. Investigative tries were made to operate the gasifier at controlled

and limited ash softening zone for improving carbon conversion efficiency. Particle

size is the critical parameter in the fluidized bed operation as fine particles could be

entrained with syngas and become wastage of fuel. Cyclones are used to reduce the

wastage of unreacted fuel in the product gases. Biomass and low-rank coals are suitable

47

for fluidized bed gasifiers as it is usually operated at low temperatures with limited

conversion efficiencies. Few commercial technologies are discussed as under.

3.5.1 High-Temperature Winkler (HTW) Gasifier

The first oxygen-blown modern continuous gasification process was Winkler

atmospheric fluid-bed process. The patent of the process was granted in 1922 whereas

the first unit was constructed in 1925. More than 70 reactors since then are

commercialized till to date with about 20 million Nm³/d of total capacity (Bögner and

Wintrup, 1984).

Any kind of fuel can be gasified with the Winkler process. Bituminous, Sub-

bituminous and brown coal along with coke have been used in commercial plants. Less

than 10 mm particle size is usually required with no drying of coal containing less 10%

moisture. The screw conveyor is used to feeding the fuel into the gasifier. The blast

enters from the conical shaped grate from the base which maintains the fluidization of

bed. To reduce the tar content in the syngas and recover the entrained fuel particles,

feeding of an additional quantity of blast is maintained at above the bed level. The

refractory is used on the reactor walls. Commercial gasifiers are operated at the

temperature range of 950 - 1050°C which well below to ash melting point. Gas velocity

is maintained at 5 m/s for catering maximum load in Winkler Gasifier (Higman and

Van Der Burgt, 2008).

The “High-Temperature Winkler” is the modification of the original Winkler process

developed by Rheinbraun. The phrase “High-Temperature” in the name is actually a

misnomer as the modification mainly in the increase of pressure from the original

design and a demonstration unit has operated 30 bar. Fig. 3.4 shows a High-

Temperature Winkler Gasifier with fluidized bed configuration. It could be operated

either with air or oxygen blown mode, utilizes dry feed and producing dry bottom ash.

The development of technology was based on lignite coal but later a wide range of

feedstocks could be gasified efficiently. A demonstration plant installed in 1977,

operated for 20 years for producing about 800,000 metric tons methanol by consuming

1.6 million metric tons of dry-lignite (Higman and Van Der Burgt, 2008).

48

Fig. 3.4: High-Temperature Winkler Gasifier (Source:(Higman and Van Der

Burgt, 2008))

3.5.2 HRL Process

The HRL Process based on the fluid-bed configuration shown in Fig. 3.5 was initially

developed by the State Electricity Commission of Victoria and then modified by HRL

Limited during the time period of 1989–1998. The process was specifically designed

for the brown coal found in Victoria, Australia containing high-moisture. The most

vibrant characteristic of HRL process is the utilization of syngas coming out from

circulation fluidized bed gasifier at high temperature as a drying medium for high

moisture (about 60-67%) feed coal. The demonstration unit of 24t/d capacity of brown

coal has been operated for about 1200 hours. Typhoon gas turbine of 5 MW capacity

has been energized from the syngas generated at 25 bar pressure from the gasification

plant at Morwell, in Latrobe Valley coal fields of Victoria. A demonstration unit of 400

MW has also been commissioned at brownfield site of Loy Yang, near Morwell in 2010

with a joint venture between Harbin Power Engineering and HRL (Anon, 2007).

49

Fig. 3.5: IDGCC process of drying and Gasification developed by HRL (Source:

(Johnson, 2001))

3.5.3 BHEL Gasifier

Bharat Heavy Electricals Limited (BHEL) developed a pressurized gasifier with fluid-

bed configuration based on the local coal characteristics and conditions. India possesses

huge coal reserves with high ash content ranges in 40% except for few lignite coal fields

in the south, hence system with non-slagging features could be a better choice for

India’s high ash coal reserves. IGCC test facility of 6.2 MWe was developed at initial

stage by BHEL using moving bed gasifier at Tiruchirapalli site and tested the operation

of IGCC using 18t/d Indian sub-bituminous coal with high ash content and then the unit

was modified into air-blown fluidized be gasification system of 165 t/d capacity. The

unit was tested for 980-1050°C temperature and in the range of 3-10 bar pressure

(Viswanathan et al., 2006). A 125 MWe IGCC unit has also been commissioned by

BHELat Auraiya in Uttar Pradesh.

3.5.4 Circulating fluidized-bed (CFB) processes

Several advantages of the transport reactor and fluidized bed system are merged in the

circulating fluidized bed (CFB) process. Excellent mixing is achieved through high slip

velocities and that promotes promote tremendous mass and heat transfer. Particles with

smaller size are either converted 100% in one go or separated from the product gas and

50

recycled back into the reactor. Internal recycling occurs again and again of larger

particles which take some time before converted into a smaller size and then recycled

from the outgoing product gas. The rate of circulation in CFB is much higher than a

classical stationary bed system hence a higher rate of heat transfer has been observed

from entering particles of feed. The tar formation is reduced during the process of

heating from that rapid heat transfer. The salient features of circulation fluid bed

systems are high conversion rate, high heat and mass transfer and less sensitive to the

feedstock nature of feed particle size. That’s why this technology is equally popular

among gasification of solid waste, biomass, and low or high-rank coals.

The circulating fluid-bed gasifier developed by Lurgi is shown in Fig. 3.6. The system

of CFB contains a chamber of the reactor, an interconnected cyclone for recycling and

a seal pot. Most of the heavier particles are entrained with the high velocities (usually

from 5-8 m/s) of gas and evacuated from top of the reactor chamber. The recycling of

separated solids from the cyclone is carried out from seal pot. Air is used usually as

gasifying agent inserted into the reactor at separate nozzles categorized as primary and

secondary air. The particle size for biomass materials should be reduced up to 25–50

mm (Greil et al., 2002).

Fig. 3.6: Lurgi circulating fluid-bed gasifier (Source: (Greil and Hirschfelder,

1998))

51

3.5.5 Kellogg Brown and Root (KBR) transport gasifier

Kellogg Brown and Root or shortly KBR transport gasifier, shown in Fig. 3.7, is

classified as high-velocity regime of fluid-bed configuration with air or oxygen-blown

arrangements. The gas velocities in the rise section of KBR gasifier are usually reported

in the range of 11–18 m/s (Smith et al., 2002). The modification of in the shape of KBR

in conventional circulating fluidized bed arrangement was primarily done in higher

rates of circulations, riser densities and increased velocities which gave better

throughput and enhanced heat transfer and mixing rates. The design of KBR became

reliable based on the experience of years for construction and designing FCC units of

the petroleum industry. The KBR gasifier is designed for feeding coarse low rank coals,

without burners and with non-slagging characteristics. Currently, Mississippi Power

Company is the owner of a 560 MWe IGCC in Kemper County.

Fig. 3.7: KBR Transport Gasifier (Source: (Smith et al., 2002, Higman and Van

Der Burgt, 2008))

52

3.5.6 U-Gas Process Gasifier

Fig. 3.8 shows a dry-feed U-Gas gasification process based on fluidized bed

configuration. All types of coal and blends of biomass with coals can be fed in this unit.

It is a non-slagging gasifier produces dry bottom ash and work efficiently with both

either oxygen or air-blown configuration. Synthesis Energy Systems (SES) currently

hold license agreement of 30 years. The systems are installed and working for more

than 20 years in Finland, Shanghai, and Hawaii. Presently 520 MWth syngas is being

produced from two plants (Higman and Van Der Burgt, 2008).

Fig. 3.8: U-Gas Process Gasifier (Source: (Breault, 2010, Higman and Van Der

Burgt, 2008))

3.6 ENTRAINED FLOW PROCESSES

Entrained flow processes are beneficial for handling any type of coal or biomass as

feedstock, producing tar-free clean syngas and frit or inner slag form of ash. These

benefits of the entrained-flow process are being enjoyed on the compromise of high

consumption of oxygen, especially with high moisture or ash content feedstocks or

slurry feedings along with the additional feed preparation steps.

53

Different internal configurations are designed for achieving maximum contact of the

gasifying agent with feedstock based on entrained flow process and are summarized in

Table 3.2, in which characteristics and features of important processes based on

Entrained-flow are outlined. Slagging gasifiers with entrained-flow configurations

remained most successful systems for coal gasification, developed after 1950 which are

being operated at high temperatures usually above 1400°C and at the pressures ranges

between 20–70 bar (Higman and Van Der Burgt, 2008). Most commercial IGCC plants

are equipped with entrained-flow gasifiers in which hard coals are being preferred. The

technologies mentioned in Table 3.2 are discussed briefly in subsequent paragraphs.

Table 3.2: Characteristics of Important Entrained Flow Gasifiers (Source:

(Higman and Van Der Burgt, 2008))

Type of

Process

No. of

Stages

Nature

of Feed

Flow

direction

Type of

Reactor

Wall

Cooling System of

Syngas

Type of

Oxidant

Koppers-

Totzek 1 Dry Up Jacket Syngas Cooler Oxygen

Shell SCGP 1 Dry Up Membrane Gas quench and

syngas cooler Oxygen

Prenflo 1 Dry Up Membrane Gas quench and

syngas cooler Oxygen

Siemens 1 Dry Down Membrane Water quench

and/or syngas cooler Oxygen

GE Energy 1 Slurry Down Refractory Water quench

and/or syngas cooler Oxygen

E-Gas 2 Slurry Up Refractory Two-stage

gasification Oxygen

MHI 2 Dry Up Membrane Two-stage

gasification Air

Eagle 2 Dry Up Membrane Two-stage

gasification Oxygen

OMB

Process 2 Dry Down Refractory

Two-stage

gasification Oxygen

3.6.1 The Koppers-Totzek atmospheric process

The first slagging gasification process based on entrained-flow configuration was

operated at atmospheric pressure, similar to the start of fixed-bed and fluid-bed systems.

54

The development of Koppers-Totzek (KT) process operated at atmospheric pressure

was carried out in the 1940s, and its commercial units were installed mostly for

manufacturing ammonia at Greece, Finland, Zambia, Turkey, South Africa, India, etc.

About 95% conversion of feedstock is reported for the South African unit (Krupp-

Koppers, 1996). Installation of new units of this type of gasifier is seized in present

years.

In the KT reactor, burners are mounted on sides for the oxygen and coal feeding, the

gas outlet is at top and slag outlet is available at the bottom as shown in Fig. 3.9. The

initial units were designed with 2 opposed burners along with the total capacity of 5000

Nm³/h whereas the later revised units were improved in the capacity up to 32,000 Nm³/h

using four burners. The temperature of the exiting gas from the top is decreased from

1500ºC to 900°C through quenching with water near the reactor’s top to condense the

slag and then it is used for the production of steam through syngas cooler with water

tubes (Higman and Van Der Burgt, 2008).

Fig. 3.9: Koppers-Totzek gasifier (Source: (Higman and Van Der Burgt, 2008))

55

3.6.2 Shell Coal Gasification Process (SCGP)

The development of Shell gasifier has started in 1956 and its first demonstration unit

came into existence in1974 (Mark, 2009). Dry feeding of crushed coal is used as

feedstock in Shell gasification process. Water wall, oxygen-blown shell gasifier is

shown in Fig. 3.10. The issues regarding the durability of refractory have been

eliminated with water all lining in the gasifier. This is suitable for various types of

feedstocks, like all types of coals from low rank to high rank, pet coke biomass, waste

etc. The alliance of Shell, Black & Veatch, and Uhde work together on commercial

terms in which technology of gasification is being provided by shell whereas EPC is

carried out by the rest of two companies. About 8500 MWth syngas is being produced

from 26 plants up to 2010 (Higman and Van Der Burgt, 2008).

Fig. 3.10: Shell Gasifier (Breault, 2010)

3.6.3 PRENFLO™ Gasifier/Boiler (PSG)

Uhde marketed the technology of pressurized entrained flow gasifier along with

generation of steam, known as PRENFLO™ Gasifier-cum-Boiler shown in Fig. 3.11.

It is a membrane wall, oxygen-blown, dry feed gasifier in which extensive variety of

solid fuels including anthracite, lignite, hard coal, refinery residue, biomass etc., could

be gasified. The gasification at 48 TPD rate is being carried out in demonstration unit

in Fürstenhausen, Germany. Biggest IGCC Plant of the world based on solid-feedstock

at Puertollano, Spain is equipped with this technology.

56

Fig. 3.11: PRENFLOTM (Breault, 2010)

3.6.4. Siemens Gasifier

Deutsches Brennstoffinstitut developed Siemens gasifier for the gasification of solid

waste and low-rank coal in 1975 at Freiberg, Germany. Its first demonstration unit of

200 MW thermal capacity was installed in 1984 at Schwarze Pumpe (Higman and Van

Der Burgt, 2008). Noell grouped initially marketed the technology with the name GPS

and then name of Future Energy but later in 2006, it was purchased by Siemens. Fig

3.12 shows the Siemens gasifier which is an oxygen-blown, dry feed, a top fired reactor

equipped with water wall screens. Feedstocks of a wide variety can be gasified in this

gasifier ranging from low-rank to bituminous coals. A power block with Gasification

Island is provided by Siemens. China granted a contract of $39 million to Siemens for

installation of two gasifiers with 500MW capacity of each for their Shenhua DME

Project (Higman and Van Der Burgt, 2008). Currently, 787 MWth syngas is being

produced from one plant operation.

57

Fig. 3.12: Siemens Gasifier (Breault, 2010, Higman and Van Der Burgt, 2008)

3.6.5 GE Energy Gasifier

Taxaco developed a gasifier initially which was named The Chevron-Texaco gasifier

after the merging of both companies. GE purchased the technology and named GE

Energy gasifier as shown in Fig.3.13. The gasifier is oxygen-blown slagging gasifier

with entrained flow configuration, fed with a slurry of coal and water along with

refractory-lined reactor. The company offers two versions of the gasifier: gasifier

equipped with a radiant cooler and a gasifier with full quench arrangements. Pet coke,

bituminous coal or blends of low/rank coals with pet coke are suitable feedstocks for

this type of gasifier. GE Energy works in a commercial alliance with Bechtel where

gasification technology would be provided by GE whereas EPC for IGCC Plant would

be tackled by Bechtel. More than 15000 MWth syngas is being produced by 64

operational plants in 2010 (Breault, 2010).

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3.6.6 ConocoPhillips E-Gas Gasifier

DOW Chemicals developed Conoco Philips E-gas Gasifier as shown in Fig.3.14 its

demonstration was carried from 1987 to 1995 at Louisiana Gasification Technology

Inc. (LGTI). It is a double-stage gasifier in which feed’s 80% is inserted from the first

stage located at a lower level of the gasifier. The oxygen-blown gasifier is being

injected with coal-water slurry and lined with refectory. It is also equipped with the

continuous systems for removal of slag and dry particulate matter. A wide range of

coals including bituminous, PRB, petcoke and their blending are suitable for

gasification through the E-Gas process. Gasification technology is being provided by

ConocoPhillips on commercial basis whereas EPC alliances are to be made with other

organizations for the development of combined power plant cycles. A plant of 590

MWth syngas production capacity is in operation whereas planning of 6 more plants is

in progress (Higman and Van Der Burgt, 2008).

Fig. 3.13: GE Energy Gasifier

(Breault, 2010)

Fig. 3.14: Conoco Philips E-Gas

Gasifier (Breault, 2010)

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3.6.7 Mitsubishi Heavy Industries (MHI) gasifier

The Combustion Engineering and Mitsubishi Heavy Industries (MHI) mutually

developed a slagging, air-blown based on Combustion Engineering as shown in Fig.

3.15. A system of dry feed is available in this gasifier and low-rank coals containing

high moisture are found suitable for this technology. It is double-stage entrained flow

gasifier equipped with the water-wall membrane. 250 MWe power is being produced

from a single demonstration unit installed in 2007 at Nakoso, Japan (Higman and Van

Der Burgt, 2008).

Fig. 3.15: MHI Gasifier (Breault, 2010)

3.6.8 The EAGLE Gasifier

Electric Power Development Company developed a two-stage, oxygen-blown, dry feed

reactor in Japan, known as The EAGLE gasifier as shown in Fig. 3.16. Its commercial

facility was started in March 2002 after successful trails using 150 t/d pilot plant

(Tajima and Tsunoda, 2002).

The high oxygen-to-fuel ratio is maintained at first stage with about 1600°C

temperature. In the second stage, the temperature is reduced up to 1150°C from

endothermic reactions of char and coal with carbon dioxide in lean oxygen conditions.

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The tangential direction of firing is maintained to increase the residence time for

particles of coal within the reactor. Equal quantities of total coal are being injected from

both stages whereas the oxygen rates are used to control the reactor.

Fig. 3.16: The EAGLE Gasifier (Tajima and Tsunoda, 2002)

3.6.9 ICCT Opposite Multiple Burner (OMB) Process

In 1995, East China University of Science and Technology at Shanghai started the

development of a new concept for the gasification process in its Institute of Clean Coal

Technology (ICCT). Initially, the design was based on opposed multiple burners

(OMB) with coal-water slurry feeding arrangements in the reactor vessel lined with

refractory as shown in Fig. 3.17. A successful operation of the pilot plant with 22 t/d

capacity was carried out in 2000. The OMB gasifier of 750 t/d capacity was operated

at 65 bar on a commercial scale in 2004 at Dezhou. In 2005 two more units of 1150 t/d

capacity were commissioned in the power plant of Yankuang near Lunan. Both plants

gave 98% conversion of carbon. Further modifications were made in the design to cater

dry feed with carrier gases like CO2 or N2 by ICCT and membrane walls with water

cooling systems (Yu et al., 2007, Zhou et al., 2006). Licenses for another 7 plants were

granted in 2007 (Yu et al., 2007).

The conventional horizontal ball mill has used for preparing slurry and membrane

piston pump is used for feeding. The down-flow OMB reactor possesses a distinctive

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feature of having four burners mounted on opposite sides, which promotes high

efficiency for carbon conversion. The efficient quenching design of the reactor for

cooling the syngas consumes less water. The quench’s water bath is used to collect the

slag and lock-hopper is used to discharge the collected slag (see Fig. 3.17). Depending

on the pressure of operation, the gas from quench water system is cooled up to the range

of 220–250°C. Finally, a cyclone, a jet mixer, and water scrubber are used for the

removal of particulate matter.

Fig. 3.17: The ICCT Opposed Multiple Burner gasifiers (Zhou et al., 2006)

3.7 CURRENT EXPERIMENTAL PRACTICES ON COMBUSTION AND

GASIFICATION

Various researchers worked on lab-scale or pilot scale combustors/ gasifiers for a better

understanding of the processes. A selected work has been reviewed here.

3.7.1 Experimental work on Gasification Systems

Choi et al. (2001c) conducted experiments on an entrained flow gasifier with a capacity

of 1 ton/day feeding of coal-water slurry (at 65% concentration) with oxygen blown

conditions at above 1300 °C temperature. The slag formation and its control were

investigated in the temperature range of gasification reactions by characterizing the

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fusion temperature of ash through CaO addition as flux material. The effects for the

ratio of O2 to coal feed rate on the composition of product gas, syngas calorific value,

cold-gas efficiency and temperature of gasifier were assessed to characterize the

gasification performance. Guo et al. (2007) investigated the performance of the pilot

scale unit of a pulverized coal fed entrained-flow gasifier operated at 1-3 MPa and 30-

45 tons/day capacity. Coal was inserted into the gasifier using pneumatic conveying

mechanism with the help of CO2 and N2 as carrier gases. The effects of steam-to-carbon

and Oxygen-to-carbon ratios on gasification performance were evaluated. Choi et al.

(2007) investigated the performance of the gasification process of vacuum residue (VR)

using entrained flow gasifier at the Korea Institute of Energy Research (KIER) with

oxygen-blown conditions. The temperature and pressure of reaction were maintained

in the range of 1200-1250°C and 1.0 Kg/cm2 respectively during experiments along

with 0.4-0.7 steam-to-fuel ratio and 0.8-1.2 oxygen-to-fuel ratio. As per results, the

syngas was produced with a combined fraction of CO and H2 in the range of 77-88%

along with its heating value found in the range of 2300-2600 kcal/Nm3. The cod gas

and carbon conversion efficiencies were found in the ranges of 68-72% and 95-99%,

respective.

Yun and Chung (2007) utilized pilot-scale of fluidized bed pressurized gasifier with a

dry-feeding system for gasification of subbituminous coal from Indonesia. A syngas

was produced with CO, H2, and CO2 in the ranges of 36-38%, 14-16%, and 5-8%

respectively. Metal filers were used to remove particulates from syngas up to 99.8% at

200-250°C temperature. Desulfurization of syngas was carried out up to 0.5% using Fe

chelate for removing compounds containing sulfur like COS and H2S. Niu et al. (2008)

performed the atmospheric gasification of diesel oil on lab-scale entrained flow gasifier

with two impinging burners installed on opposite sides. The composition of important

constituents of syngas (Like CO, H2, CO2, O2, CH4) was monitored at various O/C

ratios ranging from 1.48 to 2.36 using mass spectrometry. The results utilized to clarify

the reaction and mixing characteristics within the gasifier, understanding the

combustion and gasification mechanism and provide a foundation for mathematical

modeling.

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The gasification characteristics of high ash fusion temperature coal (HAFTC) was

investigated by Wu et al. (2008a) in a lab-scale gasifier with the downward flow for

designing new technology of entrained flow gasifier for using HAFTC as feedstock in

it with dry slag. According to the results, the high conversion of carbon was achieved

with high temperature with higher fractions of H2O and CO2 due to a higher amount of

oxygen. The optimized temperature range was found from 1300° to 1350°C as beyond

this upper-temperature limit the decrease in cold gas efficiency was observed. Font et

al. (2010) investigated the effects of feed conditions (blend of pet-coke with coal with

equal ratio and addition of limestone) and separation of trace elements on the

performance of entrained flow gasifier installed at Puertollano IGCC power plant

(Spain) having a capacity of 335 MW. Liang et al. (2011) studied the fuel conveying

characteristics at 4 MPa pressure in lap-scale entrained flow gasifier and effects of

moisture content on those characteristics of conveying. Lignite and soft coal were used

for the experiments with similar particle size and density. The decrease of mass flow

rate was observed for lignite on increasing moisture from 3.24% to 8.18%, but with

similar operating conditions, the mass flow rate of soft coal was first increased and then

decreased with the increase of moisture from 0.4% to 6.18%. The flowing

characteristics of lignite were found better than soft coal.

Gonzalo-Tirado et al. (2012) investigated the devolatization, oxidation, and gasification

of pulverized sub-bituminous coal from Indonesia with CO2 through entrained flow

reactor. The kinetic parameters were derived using obtained data from experiments for

all three mentioned processes. Li and Whitty (2012) studied char-slag transition

phenomena during gasification for three types of coals using entrained flow-reactor

operated with laminar flow. Particles with different conversions were oxidized partially

at above ash fusion temperatures. Finally, physical characterization like particle size,

density, morphology, and internal surface area, was carried out for prepared particles

of char and slag. Preliminary lab-based characterization of Australian coals’ suite was

carried out by Roberts et al. (2012a) (2012b) to assess the performance under the

conditions of practical entrained flow gasifier and then tested on a pilot scale entrained

flow gasifier having a capacity of 5MWth. The data generated from laboratory and pilot

scale experiments on a suite of coals provides a fundamental understanding and

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correlations between the measurements taken in laboratory gasification experiments

and behavior of coal gasification under realistic practical conditions.

Pyrolysis and gasification behavior of German anthracite, bituminous and lignite from

Rhenish was analyzed by Tremel et al. (2012) at the conditions of commercial-scale

entrained flow gasifier. A pressurized high temperature entrained flow reactor (PiTER)

along with wire mesh reactor were used to quantify the yield of volatiles at high

pressure and high temperature. Sun et al. (2013) used first time the tunable diode laser

(TDL) absorption technique for measuring the temperature of gas and concentration of

species during the experiments on pilot-scale entrained-flow slagging, oxygen-blown

high-pressure coal gasifier. The operating parameters were 18 atm pressure and 1800

K temperature. Continuous solid feed fitted high-pressure gasifier with fixed bed

configuration was used by Fermoso et al. (2010) to perform coal gasification

experiments using steam and oxygen as agents for gasification. Coal gasification at high

pressure was assessed on the basis of important operating parameters including the

concentration of steam and oxygen and temperature.

Bläsing and Müller (2011) provided details of the effect of pressure on the release of

key chemical species, e.g. sodium, potassium, sulfur, and chlorine during coal

gasification. A total of 19 different coals were investigated in lab-scale gasification

experiments in an electrically heated pressurized furnace at absolute pressures of 2, 4,

and 6 bar in an atmosphere of He/7.5v% O2 at 1325 °C. Mass spectroscopy with

molecular beam was used for analysis of hot gases. Butterman and Castaldi (2011)

characterized the pore structure of char produced during thermogravimetric analysis of

lignin with both N2/H2O and CO2 environments using SEM (Hitachi 4700). The

complete conversion of lignin into volatiles was found CO2 environment during thermal

processing as compared to a N2/H2O gasification medium due to more porous surface

and intricate channel structure by CO2 thermal treatment during pyrolysis. Fushimi et

al. (2011) investigated the effect of hydrogen and tar on the reaction rate of woody

biomass char in steam gasification through varying the concentrations in a rapid-heating

thermo-balance reactor. It was observed that the gasification of biomass char through

steam could be separated into two periods.

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Zhang et al. (2011) investigated the influences of interactions between char and

volatiles on the promotion of char structure during the steam gasification of brown coal

from Victoria. An innovative single-stage fixed-bed/fluidized-bed quartz reactor was

used to conduct the experiments in the absence and presence of interaction between

volatile and char. As per results, features of char structure were significantly influenced

by the interactions between volatile and char. Biomass char was used by Yang et al.

(2012) produced during the pyrolysis of Dunaliella salina feedstock at a temperature of

500°C. The fixed-bed reactor was used to conduct the reaction study between biomass

char and CO2 for investigating the effect of steam and temperature on the yield of CO,

conversion of CO2 and composition of syngas. According to the results, the yield of CO

observed 61.84% and conversion of CO2 was found to be 0.99 mol/(mol of CO2) at

800°C without using catalyst and steam. High conversion of CO2 could be achieved via

high temperature and Steam.

Gao et al. (2013) pyrolyzed the Huolinhe lignite at a fast heating rate in the CO2

atmosphere using fixed bed reactor attached with thermo-balance and investigated the

influence of CO2 on the behavior of pyrolysis. The gas composition, yield of char and

its physical properties including FT-IR spectra, surface structure, element etc. were

analyzed. As per results, the gasification of nascent char with CO2 increase the H

radical generation. Li (2013) provided an overview of the research work conducted on

Volatile-Char interaction so far and examined the importance and mechanisms for the

interaction of char with volatiles during the gasification process of fuels categorized as

low-ranked fuels. Stelzner et al. (2013) aimed to determine the best fuel dilution

configuration for studying the partial oxidation reactions through calculating

experimentally the impacts of various fuel dilutions on exit temperature, kinetics, and

elementary combustion. Moreover numerical and experimental investigations were also

conducted to evaluate the phenomena of thermochemical reactions in the flame and

post-flame zones. Tay et al. (2013) investigated the variations in the reactivity and

structure of char during the gasification process of brown coal from Victoria.

Gasification of brown coal from Loy Yang was carried out in an innovative

fixed/fluidized bed reactor at 800 °C with different gasification atmospheres. TGA was

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used to measure intrinsic kinetics of char reacted with air at low temperatures ranges

from 380 or 400 °C.

3.7.2 Experimental work on Combustion related to Gasification Studies

The combustion reaction is important during the gasification process as it gives the

primary heat for the occurrence of endothermic char reactions with CO2 and Oxygen.

A hefty research has already been conducted on combustion of coal with different

operating conditions. Among those, few are discussed here which are important for

gasification scenario.

Bejarano and Levendis (2008) conducted a fundamental investigation on the single

particle combustion of various ranked coals (like bituminous or lignite) along with

synthetic chars with different particle sizes at varying mole fractions of O2 balanced

either by CO2 or N2 gases. Drop-tube furnace was used in laboratory and the

experiments were conducted in the temperatures range from 1400 to1600 K. O2/N2

environment was found favorable and gave higher combustion reaction rate during

combustion reactions as compared to an O2/CO2 environment with a same mole fraction

of O2. Dhaneswar and Pisupati (2012) investigated the characteristics of the rank of

coal during combustion in the O2/CO2 environment using four suites of coals. Drop-

tube reactor was used to conduct the combustion experiments in oxy-fuel and air

atmospheres. TGA was used to measure the intrinsic kinetic rate parameters.

Combustion characteristics were assessed by Khatami et al. (2012) for single particles

(75-90 μm particle size) of lignite, high-volatile bituminous, sub-bituminous and

sugarcane bagasse in O2/CO2 and O2/N2 environments with 20-100% mole fraction of

oxygen. The experiments were conducted in electrically-heated, transparent, bench-

scale drop-tube furnace at 1400 K temperature. The time history of burnout and

temperature were measured high-resolution, high-speed cinematography whereas

combustion was monitored through optical pyrometry of three colors.

Karlström et al. (2013) examined the oxygen concentration effects on the rate of

oxidation within the temperature range of 1223–1673 K for 5 types of char produced

from anthracite coals. Plug flow reactor (4 meters long) was used in experiments with

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isothermal conditions and fast heating rate as per realistic situation (104-105K/s) for

the determination of burnout profiles. Finally, the multivariable optimization method

was used to determine the kinetic parameter and order of reaction.

3.7.3 Experimental work on Gasification Studies on Thar Lignite

Thar lignite has explored about two decades ago and lots of fundamental studies like

its chemical and physical analysis has carried out by several researchers (Choudry et

al., 2010). But scant literature is available on the kinetic studies of char combustion and

gasification reactions for Thar coal. For instance, Anila S. et al (Sarwar et al., 2011,

Sarwar et al., 2014) studied the kinetics of pyrolysis and combustion of Thar coal using

Thermo-gravimetry. Reactivity of catalytic gasification of Thar lignite chars with steam

at atmospheric pressures was studied by Jaffri and Zhang (Jaffri and Zhang, 2009). But

no study is available for the kinetic modeling of Thar coal with CO2 and steam at high

pressure.

3.8 MODELING AND SIMULATION WORK

The kinetic modeling of reactors and its applications for designing the commercial

gasification reactors are limited today due to the involvement of extreme mathematical

complications in current chemical reactors like plug flow reactors (PFR) or continuous

stirred tank reactors (CSTR). However, the development of sophisticated models is

possible with a deep understanding of chemical and physical processes regarding coal

combustion and gasification along with the availability of powerful computers and

high-performance computing machines (Williams et al., 2002). In this regard,

computational fluid dynamics (CFD) has proved its robust applied benefits for several

emerging technologies of this era. Among those technologies, coal combustion

remained the most important applications which have taken the advanced shape after

the utilization of complex coal combustion kinetics through CFD. Though CFD has

provided an advanced platform in designing of coal combustion devices like furnaces

or combustors, demand has increased for getting quantitative analysis rather than results

with qualitative results (Williams et al., 2002).

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Since the past two decades, the focus is more diverted towards the application of CFD

for the advancement of gasification of coal and other fuels (like biomass and waste).

CFD has provided great benefit in the development of gasification technology like

utilizing CFD computations directly to industrial scale avoiding the scale-up issues

from laboratory data. Some previous work on CFD modeling of different types of

gasification systems has been reviewed here.

Korchevoi et al. (1996) developed mathematical models, algorithms and programs for

the thermochemical conversion of single particles of coal with high-ash at raised

pressures. The installation and operation issues of circulating fluidized bed were

investigated. Fletcher et al. (2000) used the Lagrangian approach in the development of

a detailed CFD model on CFX package for simulating the reaction and flow in a

biomass-fed entrained flow gasifier. Chen et al. (2000) developed a 3D CFD model for

entrained-flow coal gasifiers consisted of conventional combustion sub-models for coal

in pulverized form. Multi Solids Progress Variables (MSPV) method was used as an

extension in coal-gas mixture fraction model to simulate the gasification reactions and

product mixing. The turbulence impacts on the properties of gases were calculated via

PDF model clipped with the function of Gaussian distribution.

Choi et al. (2001a) predicted the process of coal gasification of an entrained-flow

gasifier with a slurry feeding system through numerical computations. The numerical

model for coal gasification was developed by dividing the process of complicated coal

gasification into various simplified steps like evaporation of slurry, volatiles removal

from coal (or devolatilization), turbulent flow carrying homogeneous and

heterogeneous reactions and heat transfer occurrence between both phases. Gas phase

turbulence was embedded via the k-ε turbulence model while the behavior of

homogeneous and heterogeneous reactions was calculated using Random-Trajectory

model. Choi et al. (2001b) numerically simulated the gas flow behavior in an entrained-

flow gasifier for describing the process of coal gasification and investigated the effects

of diameters and angel of gas injecting nozzles, the velocity of incoming gas, extension

in the length of burner and geometry of gasifier. Turbulence was calculated with the

standard k-e turbulence model whereas velocity and pressure were coupled with a

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SIMPLE algorithm. The results depicted that parabolic distribution of flow pattern is

insensitive with respect to variation in selected parameters.

Bockelie et al. (2002) have presented initial results for various entrained-flow coal

gasifiers, illustrating, for example, performance variations when using different

feedstocks. This work was the part of a program to investigate generic improvements

for operation and design of such gasifiers. Vicente et al. (2003) used a numerical model,

based on the Euler-Euler framework, for the simulation of coal gasification process in

an entrained flow gasifier. Separate continuity equations were solved for both

particulate and gas phases with the Eulerian mechanism. Wang et al. (2004) performed

three-dimensional numerical simulations for non-premixed combustion turbulent

mixing processing in the high temperature combustion furnace. The flow, combustion,

heat transfer and NOx turbulent formation were simulated. The distributions of mixture

fraction and its turbulent fluctuation were predicted under three different inlet air

temperature conditions in the combustion furnace. The comparison between simulation

and experimental results are in good agreement.

The performance of entrained flow gasification was evaluated by Wu et al. (2004)

through mathematical calculations for various gasification parameters. The studied

gasification parameters included gasifier species and its ratio to gasifying medium,

gasification temperature and pressure and residence time. As per results O2/H2O and

O2/ CO2 as gasifying medium separately have their own advantages. Gasification

temperature was a key factor on the gasification process and syngas composition, while

the pressure in gasification has significant impact on gasification process but it possess

a small influence on syngas composition near equilibrium. Bockelie et al. (2005)

described a modeling approach using CFD for simulating oxygen-blown, pressurized

entrained flow coal gasifiers. The coal gasification reaction kinetics at high fuel loading

rate, high pressure and slagging walls were calculated through sub-models. The

comparison of results obtained from CFD computations and experimental work by

other research shown satisfied agreement. Though the model was developed on oxygen-

blown conditions but air-blown conditions could also be simulated through developed

model.

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Shi et al. (2005) and (2006) developed a CFD model of an oxygen-blown, coal-slurry,

double-stage entrained-flow gasifier for performing simulations of advanced power

plant. The flow of coal slurry was simulated by discrete phase model (DPM). The user-

defined functions (UDFs) were used to calculate the chemical and physical processes

during coal slurry gasification. The predicted syngas composition was compared with

the model developed in Aspn Plus® platform based on equilibrium reactor assumptions

and found good similarity. 100% char conversion was observed from first stage whereas

86% char conversion was achieved from second stage. Zitney and Guenther (2005)

developed CFD models of two commercial-scale gasifiers for simulating power plant

of advanced nature. The first gasifier was the pressurized, oxygen-blown, coal-slurry

two-stage entrained-flow gasifier, modeled in FLUENT. Two phases were modeled

through Eulerian-Lagrangian framework where solid phase was modeled as discrete

phase using discrete phase model (DPM) and gas phase was treated as continuous

phase. The scale-up of the transport gasifier in advanced power plant at Alabama was

the second gasifier. The CFD multiphase code based on transient Eulerian-Euerian

MFIX model was used to model the complicated processes of gasification and solid-

gas hydrodynamic.

Hla et al. (2006) used FLUENT as CFD platform to develop the relation for

determination of rate of gasification at high pressure and temperature as a function of

various physical and reactor parameters. The “effectiveness facror” was applied in the

development of correlation. Watanabe and Otaka (2006) predicted the performance of

gasification in an entrained flow gasifier and modeled the reactions occur in the process

of coal gasification. The research was mainly aimed to develop a technique based on

numerical simulation for evaluating the design performance and optimization of

gasification system, and to adopt the best model validation method. Hla et al. (2007)

developed a model for coal gasification conversion that could be used as predictive tool

for assessing the gasifier performance with various types of feedstock compositions and

operating conditions. Intrinsic data for chemical kinetics for char-gas reactions were

measured at lower pressure and temperature levels as compared to realistic conditions

entrained flow combustors/gasifiers.

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A novel concept of pressurized gasifier based on fluidized bed configuration known as

Power High-Temperature Winkler gasifier (PHTW gasifier) was modeled by Gräbner

et al. (2007). A 4800 t/day with 1000 MW power capacity gasifier in a power plant with

oxygen/steam blown and fueled by lignite coal was simulated numerically at 33 bar

pressure. Wu et al. (2007b) used FLUENT software for simulating the Texaco coal

gasifier with a robust model including various sub-models. PDF model was used to

define the chemical process by defining coal-slurry as fuel stream and oxygen as an

oxidizer stream. Turbulence in flow was calculated by Realizable k-ε model whereas

radiative heat transfer was calculated through the P-1 model. User-defined functions

were used to incorporate the heterogeneous reactions. Dai et al. (2008) developed a

novel type of technology for pressurized entrained flow gasifier with the pulverized

coal feeding system. The special feature of the gasifier was the installation of symmetric

four nozzles attached on the upper portion of the gasifier. The model of gasifier was

based on the principle of Gibbs energy minimization technique. The results obtained

from simulations were compared with pilot trial data and found good agreement.

Entrained flow gasifier could be designed, assessed and improved through this model.

The oxidization zone of the double-stage gasifier with downdraft mechanism was

investigated by Gerun et al. (2008) through the CFD model with 2D axisymmetric

dimensions. Sharma (2008) presented kinetic and thermodynamic modeling for

reduction reactions of char in a biomass-fed, downdraft gasifier. Gas composition along

with its heating value, un-reacted char and fuel conversion efficiency, the temperature

of leaving gas and output power of gasifier were predicted by coupling of energy and

mass balances with parameters of kinetic rate and equilibrium relations. Validation was

carried out for the model results by comparing those with experimental data. The effects

of temperature of reduction zone reactions and length of char bed were investigated on

the sensitive parameters of gasification. Tinaut et al. (2008) presented a stationary one-

dimensional model for downdraft fixed-bed biomass gasifier. Sub-processes of biomass

gasification like drying of biomass, devolatization, volatile and char oxidation, char

gasification reactions and reforming of hydrocarbon were incorporated in the model.

Experimental validations were conducted for Fuel/air equivalence ratios, combustion

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rate of biomass and temperature field predictions and found good agreement between

predicted and experimental results.

Silaen and Wang (2008) used FLUENT software to perform numerical simulations for

the oxygen-blown generic entrained-flow gasifier. Navier-stokes equations along with

transport model equations for seven species were solved. Finite rate chemistry was used

to solve three heterogeneous reactions whereas the minimum value between eddy-

breakup combustion model and the finite rate was used as a solution of two

homogenous reactions. Xiu et al. (2008) experimentally validated the fast pyrolysis

kinetic parameters of corn stalk for the prediction of the behavior of its gasification in

the horizontal entrained-flow reactor (HER). Experiments were conducted with a

uniform feeding rate of 0.3 Kg/h, and in the temperature range from 792-1031 K.

Particle image velocimetry (PIV) was used to validate the CFD modeling results and

then prediction of corn stalk pyrolysis in HER was carried out using previous fast

pyrolysis kinetic data for corn stalk. The comparison of pyrolysis results obtained from

model and experiments shown good similarity and found capable to predict the

laboratory-scale, pilot-scale or full commercial scale units. Ajilkumar et al. (2009)

performed numerical simulations in FLUENT software to evaluate the performance of

an air-blown entrained flow gasifier at laboratory scale. An Eulerian-Lagrangian

modeling approach was used for simulating gas and particle phases. The gas-phase

turbulence was calculated using k–ε model along with stochastic tracking model was

used to predict the particle dispersion in the gas phase. Devolatization, combustion of

char and volatiles and gasification reactions were incorporated in the model developed

for coal gasification. The performance of gasification was evaluated on varying air

ratios along with varying inlet steam and air temperatures. Steam/air pre-heating found

directly proportional relation to the gasifier inside temperature and thus its increase

raise the rate of various reactions to occur in the gasifier.

Barranco et al. (2009) formulated a novel kinetic model of char combustion for coal in

pulverized form. The rate of nth–order global chemical reaction was considered as a

function of the mass of fuel and intrinsic reactivity of coal. Rayleigh method of

dimensional analysis was used to develop the rate of reaction equation for the

combustion of char. The reactivity of char was found dependent on various parameters

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related with coal like the composition of minerals in coal, apparent density and surface

area of char, activation energy, the temperature of reaction and time taken for

devolatilization. The empirical validation has described by Rojas et al. (2012) in its

part-2 series. Chui et al. (2009a) developed a CFD model of a pressurized pilot-scale

entrained flow coal gasification system. The simulations were carried out keeping the

aim for advancement in the coal gasification technology at commercial scale. The

performance of gasifier and IGCC system at various operating conditions were

investigated by Gnanapragasam et al. (2009) through gasifier’s sensitivity analysis.

Variations in the oxygen/coal and steam/coal ratios, thermal conditions during

operation of the IGCC system, and type of fuel were investigated during the sensitivity

analysis.

Jaojaruek and Kumar (2009) established an important model specifically for the

pyrolysis zone of a downdraft gasifier based on finite computation method. Implicit

finite difference method was used to solve the mass and energy conserving equations.

Heat transfer considered convection, conduction, and the influence of solid radiation

components. Chemical kinetics concept was also adopted to simultaneously solve the

temperature profile and feedstock consumption rate on the pyrolysis zone.

Experimental and numerical investigations were carried out for Opposed Multi-Burner

(OMB) gasifier having an internal diameter of 1 m at high pressure and temperature for

studying the flow field of the particle-gas regime by Ni et al. (2009). The behavior of

particle-gas flow was simulated using an Eulerian-Lagrangian framework based 3-D

numerical model. The dispersion of ash/slag particles due to their collision was

predicted. Experimental data was used to validate the simulated results and found good

similarity between both. According to the results, the section height above to burner

increased the residence time of material and the flux of deposition raised with the

velocity of inlet streams.

Papadikis et al. (2009) developed the CFD model based on the Eulerian-Lagrangian

framework for fast biomass pyrolysis in an entrained flow reactor. The discrete particle

of biomass is thermally degraded into biomass char and releasing tar and gases as per

formation of the model. Semi-global model with two stages was used to define the

chemical reactions. The composition of gases produced during the pyrolysis process in

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the radial direction was predicted along with their influence on the properties of

particles. Simone et al. (2009) coupled experimental investigations with CFD modeling

for extraction of important global chemical kinetic parameters that could be used in the

biomass applications like co-combustion and gasification. The experiments were

conducted in an advanced lab-scale drop tube furnace at varying heating rates, particle

size within the temperature range of 400 –800°C. CFD was used as a tool for diagnosis

and prediction using extracted global kinetic parameters experimentally. Wang et al.

(2009) developed a 3D comprehensive CFD model for simulating fluidized bed gasifier

for gasification of coal. Homogeneous/heterogeneous chemical reactions along with

solid-gas flow were considered in the development of the model.

Numerical simulations were carried out by Chyou et al. (2010) in FLUENT software

for the process of coal gasification in an oxygen-blown cross-type double-stage gasifier

(like E-Gas) for getting more insight regarding gasification mechanism in such type of

gasifiers. Mixing of the species at the molecular level was assumed. Species transport

equations with an eddy-breakup model for reaction were solved along with 3-D Navier-

Stokes governing equations. The effects of the concentration of coal-to-water ratio in

coal-slurry and O2/coal ratio on the performance of gasification were investigated.

Dong et al. (2010) developed a CFD model of PC boiler of 600 MWe, fueled by co-

firing of coal with syngas produced from gasification of biomass. Emun et al. (2010)

developed a model in Aspen Plust® software for IGCC containing a Texaco gasifier

along with its related process units. The model was used to enhance the efficiency of

IGGC process along with the performance of environmental constraints through

utilizing Pinch analysis techniques, Process integration and parametric studies. The

varying parameters were considered in the air separation unit, coal preparation, cleaning

of gas cleaning, gas turbine, recovery of sulfur, steam turbine and recovery of heat.

Gerber et al. (2010) discussed the approach for multiphase modeling through Eulerian

framework for wood gasification occurs in fluidized bed taking. The gasification model

was based on sub-processes like pyrolysis of wood, gasification of char and

homogeneous reactions occurring gas phase. Montagnaro and Salatino (2010) analyzed

the conversion and trajectory of the carbon particles in the slagging regime of

gasification with entrained-flow configuration. The concept for the segregation

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framework of carbon particle was used to develop the simplified one-dimensional

model of a gasifier with entrained-flow configuration. The influence of various patterns

for segregation of carbon particle on the extent and conversion of carbon particle.

Nguyen et al. (2010a) (2010b) used FLUENT for the development of a three

dimensional CFD model for predicting coal gasification performance in an entrained-

flow gasifier. The model consisting sub-models of devolatization (pyrolysis),

gasification of char and homogeneous reactions in the gas phase. The particulate phase

was modeled using discrete phase model (DPM) whereas the combustion and

gasification of carbon/char were calculated using Multiple Surfaces Reaction (MSR)

model. The parameters studied were temperature, flow field, and distribution of species

composition inside the gasifier. The results from published literature were used for

comparing with the CFD model results. Sadhukhan et al. (2010) analyzed the

combustion of a high-ash single-particle coal char at high pressure. The combustion of

coal char with high ash was characterized by a complete dynamic shrinking core model

including simplified kinetics. The kinetics of the reaction, mass and heat transfer

phenomena along with details of intra-particle forces were included in the model. The

transient governing differential equations of the model were solved through the finite

volume method (FVM). Time taken for complete conversion of char particle and

weight-loss profile could be predicted from the combustion model of char at varying

concentration of oxygen and temperature.

Silaen and Wang (2010) numerically simulated the generic oxygen-blown entrained-

flow gasifier. The Navier-Stokes equations along with dynamics of particles were

solved by Eulerian-Lagrangian technique. Equations for species transport along with

homogeneous and heterogeneous reactions were incorporated in the model. The

heterogeneous reactions were solved by finite rates method. Slezak et al. (2010)

numerically simulated commercial-scale single-stage down flow and double-stage

upward flow entrained-flow gasifiers for investigations the influence of the size of

particle and density on the overall performance of gasifier. Zhang et al. (2010) carried

gasification of carbon and coal char by CO2 in the presence of potassium and calcium

catalysts and usual theoretical models for the kinetics of heterogeneous reactions where

reviewed. The conversion based reactivities calculated from random pore model (RPM)

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and those extracted from experiments were compared and found significant deviations

at high or low levels of conversion as theoretical predictions.

Álvarez et al. (2011) investigated the combustion of coal contain different volatiles

composition, at varying atmospheres of O2/CO2 in a reactor equipped with entrained

flow configurations. The predictions of rates of drying, profiles of temperature, time

for complete conversion and major species composition like CO, CO2, and O2 were

carried out and compared with literature, found a satisfactory resemblance. Chejne et

al. (2011) developed a mathematical model for coal gasification with the capability to

predict conversion, temperature, composition of produced gas, velocity, distribution of

solid particle size and other fluid-dynamic related parameters in pressurized fluidized

bed gasifier. Model validation was done from published experimental results. Cornejo

and Farías (2011) developed a 3D-CFD model of fluidized bed coal gasifier using

FLUENT, consisting of moisture removal (drying), pyrolysis (devolatilization),

combustion and gasification sub-processes. The euler-lagrangian framework was used

to describe the gas phase and particulate phase. Homogenous and heterogeneous

reactions were also incorporated.

Gordillo and Belghit (2011) developed a numerical model of biomass-fed solar

downdraft gasifier utilized steam kinetics. The model possesses the capability for

predicting steady state and dynamic profiles of species concentration and temperature,

based on the balances of heat and mass. The radiative energy transfer into the bed was

calculated by the Rosseland equation. The varying variable was char reactivity factor

(CFR). The effects of dynamic heat transfer in the bed along with the velocity of steam

were investigated. The comparison between model predictions and experimental results

from literature confirmed the validity of the model. Kempf et al. (2011) illustrated

stability problems, the need for consistent modeling in premixed and non-premixed

combustion, and showed how RANS models that would have frequently been applied

in an LES context can lead to strong conceptual errors. The application of the error

landscape approach to a complex non-premixed flame was outlined and investigated

several error indicators that could have been developed for situations where no

experimental reference data was available.

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The generation of electricity and production of fuel were thermally improved by Kiso

and Matsuo (2011) through gasification with dry feeding of coal. They proposed a new

process in which water was injected at the outlet of the gasifier and was vaporized to

enhance the extent of the shift reaction. This process utilized the high temperature of

the syngas, which was sufficient for the shift reaction to occur without a catalyst. A

model was developed that incorporated the shift reaction velocity to evaluate the

proposed process. Nayak and Mewada (2011) simulated the coal gasification process

through Aspen Plus for presenting its overview. The influence of flow rate of oxygen

and steam-to-coal ratio on the composition of produced gas was investigated. Seo et al.

(2011) investigated computationally the operating parameters like transporting gas/coal

ratio, O/C ratio, reaction pressure and temperature for the gasification of Adaro coal

and the results were compared with experiments. 82.19% cold gas efficiency was

estimated from the optimized parametric conditions. Unar (2011) performed the CFD

simulations on downdraft gasifier with entrained-flow configuration for evaluating the

gasification process of Thar lignite. The FLUENT software was used for the

simulations in which effects of coal compositions and form of coal (dry and slurry)

were investigated. Navier-Stokes equations were solved for nine species and

homogeneous reactions were treated with eddy-dissipation combustion model.

Xu et al. (2011a) and (2011b) conducted the experiments for steam gasification of

biomass and coal and observed the fundamental difference between the gasification of

those two fuels based on the differences of microstructures in the fuels. A char

gasification model was developed on the basis of reaction kinetics and steam and gas

transportation mechanisms. Álvarez et al. (2012) investigated the emissions of NO from

gasification of high volatile bituminous and anthracite coals in an entrained flow reactor

with oxygen and air conditions through experiments and numerical simulations. A CFD

model was used with three assumptions (1) all of the nitrogen in the fuel had been

converted to HCN (2) all of the nitrogen in the volatile would evolve as HCN, and (3)

a conversion factor used to calculate the No formed by char–N reaction. Botero et al.

(2012) and (2013). studied numerically (using 1-D RO model) the influence of slurry

feeding of CO2 on the kinetics of Illinois coal gasification and eventually on conversion

of carbon along with consumption of oxygen in a single-stage gasifier with the

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entrained-flow configuration at high pressure. According to results, CO2 slurry feeding

system has great influence on the gasification of coal and produces almost double CO

as compared to conventional oxidizing agents. On further investigations, it was

observed that the slurry of coal with water decreases the rate of gasification up to 60%.

Chen et al. (2012) predicted the gasification of coal in an entrained flow gasifier using

a numerical method. The effects of injection patterns along with the ratio of steam-to-

coal were investigated on the production of syngas and composition of H2 in the syngas.

The developed numerical method predicted the composition of syngas with satisfaction.

Lee et al. (2012) investigated different angles for oxygen injectors and various types of

burners for gasification in a lab-scale entrained flow gasifier. The experiments were

conducted to determine the impacts of the amount of oxygen, coal burner’s angel and

location of oxygen in the gasifier on the composition of syngas and reactor temperature.

The optimization of operating parameters was carried out using simulations in Aspen

Plus software by inserting the data obtained from experiments. The impinging zone for

industrial scaled Opposed Multi-Burner (OMB) gasifier was modeled in 3D by Li et al.

(2012). The eulerian-lagrangian framework was used to treat the particulate and gas

phases and the realizable k-ε model was used to calculate the turbulence in the gas

phase. Assuming particles as the hard spheres, the modified Nanbu method and

Simulation Monte Carlo (DSMC) method were used to calculate the collisions of

particles. The laboratory experiments conducted on an equipment with two jets on

opposite side were used to validate the model.

Meng et al. (2012) developed a 3D model of thermogravimetric (TG) furnace using

COMSOL Multiphysics Software for a better understanding of velocity and

temperature profiles inside the furnace. The model results were compared with

experiments and found a good agreement. Monaghan and Ghoniem (2012a) described

the development of a dynamic reduced order models (ROM) model for entrained flow

gasifier in the first part while second part presented its validation for four designs of

entrained flow gasifiers and conducted the sensitivity analysis (Monaghan and

Ghoniem, 2012b). Murgia et al. (2012) developed a CFD model of updraft air-blown

coal Wellman-Galusha gasifier and simulated gasification process through the

developed model. The Euler-Euler framework was used to simulate the multiphase

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regime due to the presence of a high volume fraction of solid phase. No-continuous

feeding of coal was considered along with the extraction of ash and aimed to

characterize the behavior of process based on time and space.

Abani and Ghoniem (2013) investigated reacting multi-phase flow in a lab-scaled

entrained flow gasifier of axial-flow type fed with coal using Reynolds-average Navier

Stokes (RANS) and large-eddy simulations (LES) models. The multiphase scenario

was modeled using the Euler-Lagrangian Framework. RANS and LES models were

compared on the basis of performance and found LES better for capturing the structure

of unsteady flow within the gasifier. Chen et al. (2013b) established a 3D CFD model

of slag for conducting the study of slag/ash behavior like burning of char at the wall,

deposition of ash or char, the flow of molten slag, the formation of a layer of solid slag

on the walls of the reactor. The pilot-scale combustion unit for coal slagging was

simulated using the developed model. Chen et al. (2013c) evaluated numerically and

compared the gasification potential of raw and terrified bamboo along with bituminous

coal containing high volatile in an oxygen-blown entrained-flow gasifier. More than

90% carbon conversion was achieved with all the cases. Franchetti et al. (2013) applied

Large Eddy Simulation (LES) to study the jet flame produced by pulverized coal. The

LES framework under investigations consisted a set of models like combustion of coal,

transport of particles through the Lagrangian approach and heat transfer by radiations.

Non-reactive and reactive experimental results were used to validate the model and

found satisfactory results.

Holkar and Hebbal (2013) used FLUENT software to compare the two models for

radiative heat transfer, i.e., P1 Model and Discrete ordinates (DO) Model. The furnace

of boiler operated by pulverized coal and over-fire air (OFA) ports were evaluated for

fitting temperature profile. The accuracy of DO and P1 models was checked by

comparing numerical results with experimental investigations. Janajreh and Al Shrah

(2013) developed a CFD model for gasification of wood based on their experimental

setup of an air blown, lab-scale downdraft gasifier. The particle phase was model

through Lagrangian framework whereas gas phase turbulence was calculated through

the k-ε model. The computed composition of species and temperature profile based on

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equilibrium and zero-dimension approach was compared with experimental results and

found a reasonable match.

Janajreh et al. (2013) developed conventional gasification method with air and plasma

gasification method for gasifying numerous feedstock like coal, waste tire, plywood,

oil shale, pine needles, algae, municipal solid waste (MSW) and untreated/treated

wood. Both methods were founded on the Gibbs energy minimization approach. Steam

or air was used in plasma gasification whereas only air was used in conventional

gasification as an oxidizing agent. The efficiency of process and composition of syngas

were the performance evaluation parameters. As per results, the gasification of waste

could be possible through plasma gasification. Kumar and Ghoniem (2013)

implemented a validated model of entrained flow gasifier for investigating the influence

of particle size on the overall performance of GE and MHI design of gasifiers. The

limitations from either boundary layer or intrinsic kinetic were observed for carbon

conversion due to variation in particle size. Lu and Wang (2013a) concentrated their

focus on rates of water gas shift (WGS) reaction in the presence and absence of

catalysts. Initially, three published WGS reaction rates were modified to reduce the

difference in prediction as compared to experimental values. Later in their second part

of study (Lu and Wang, 2013b) simulations were performed for Japanese research

gasifier (CRIEPI) with published rates of WGS reactions and modified rates as per

findings of part 1. 3D Navier-Stokes equations were solved in the developed CFD

model along with species transport model equations for selected species. The

devolatization of fuel was described by Chemical Percolation Devolatilization (CPD)

model.

Luan et al. (2013) developed a 3D CFD model of oxygen blown, pressurized E-Gas

entrained flow gasifier. 3D Navier-Stokes equations were solved with Eulerian-

Lagrangian Framework and chemical reactions were calculated through Finite-

Rate/Eddy-Dissipation Model. The investigations proved that the chemical reactions

are affected through finite-rate chemistry. The performance of gasifier was successfully

evaluated through CFD simulations. CFD simulations were carried out by Singer et al.

(2013) for the combustion of lignite coal in an oxyfuel pilot-scale test facility with a

feed containing 29% oxygen for investigating concerned regions of char burning flame.

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Trajectories of coal particles were examined to calculate typical transient boundary

conditions at the time of exposure of char particles along the traveling in the furnace.

Char conversion then evaluated based on the local environment using those accurate

boundary conditions in the model of single particle combustion. Stöllinger et al. (2013)

simulated the combustion process of pulverized coal in a partially industrial scale

furnace using established model probability density function (PDF). The results

obtained from simulations were found sensitive to the selected model due to the transfer

of mass from solid to gas phase through combustion of char and devolatilization.

A framework of the model was presented by Tremel and Spliethoff (2013c) that

described the kinetics for reactions occurring during gasification in an entrained flow

gasifier. Numerous sub-models were incorporated in the model like devolatilization of

fuel, gasification of char, evolution of surface, pore diffusion, cooling of char, diffusion

through boundary layer, and particle density and size variations. Experimental

verifications were made from the date of devolatization of fuel and kinetics of lignite

gasification. Xiangdong et al. (2013a) developed an equilibrium model in Aspen Plus

software for gasification of coal in the Texaco type coal gasifiers to calculate the

product gas composition, conversion extent of carbon and temperature of the gasifier.

The gasification process was divided into the three stages in the model: (1) combustion

and pyrolysis stage, (2) Heterogeneous reactions (char-gas) stage and (2) Homogeneous

reactions (gas phase) stage. The results obtained from simulations were in good

agreement from experimental values obtained from the literature. A fluidized bed

gasifier was numerically simulated by Xie et al. (2013) through a comprehensive 3D

model. The multiphase particle-in-cell model (MP-PIC) was used in the model where

fluid phase was described by Eulerian method and particle phase was calculated

through discrete particle method. Heat and mass transfer, the flow of dense particulate

matter, chemistry for heterogeneous and homogeneous within the mixture of fluid were

considered. The particle distribution function (PDF) transport equation was solved to

calculate the particle dynamics. The flow pattern formation, particle species profile, and

composition of the gas, reaction rates distributions and carbon consumption were

examined at various conditions of operations. The predicted composition of product gas

was compared with experimental data and found in good agreement.

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Vascellari et al. (2013a) developed a CFD model of entrained flow gasifier using sub-

models of advanced nature for conversion of coal and results were compared with data

extracted from experiments. The investigations were mainly focused to model pyrolysis

process more accurately. A technique based on iterative solution was proposed and

validated to reduce the gap between calculations from various pyrolysis models like

FG-DVC, FLASHCHAIN, CPD and single-or multisite empirical kinetic models

available in CFD. The simplified Single Nth-Order Reaction (SNOR) kinetic model

was calibrated through CBK/G and CBK/E models and found appropriate for CFD

calculations (Vascellari et al., 2014). Unar et al. (2014) developed an Euler-Lagrangian

framework based 3D CFD model for two-stage entrained flow dry feed coal gasifier

with multi opposite burners (MOB) equipped with tangential and impinging nozzles.

Investigations were made through numerous numerical simulations for studying the

effects of distributions for fuel and oxidant between the two gasifier’s stages. Finite

rate/eddy dissipation model was used to define heterogeneous and homogeneous

chemical reactions. Published kinetic data was used in the reaction. Turbulence was

incorporated thorough realizable k–ε turbulent model. Reaction mechanisms were

created with different reaction schemes and validated through published experimental

results.

Bi et al. (2015) developed a heat transfer with slag flow model coupled with a capturing

of particles via sub-model in a 3-D gasifier model for describing the characteristics of

slag and process of gasification in an entrained flow gasifier. Halama and Spliethoff

(2015) presented a 3D-CFD model of a pressurized entrained flow gasifier for

gasification of Rhenish lignite. The model was validated against experimental data

obtained from a pilot-scale unit. A good correlation was obtained between simulations

and experimental data. Labbafan and Ghassemi (2016), simulated numerically a three-

dimensional oxygen-blown, two-stage E-Gas entrained flow gasifier. A coal containing

high ash was characterized for gasification at high pressure.

Wang et al. (2017) numerically simulated a 2D double-stage entrained flow gasifier

with dry feed. The performance of gasifier was investigated at various oxygen to carbon

ratios. Moreover, the different reaction schemes were validated from date published in

the literature. As per results, The molar composition of syngas particularly CO and H2

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increased and optimized at 0.8 O/C ratio whereas temperature gradually decreases from

2nd injection level due to the water-shift reaction. Gao et al. (2018) developed Euler-

Lagrangian based biomass gasification model in an Entrained flow gasifier. The

submodels of char-gas reactions were inserted with intrinsic biomass kinetic data for

calculating the rate of heterogeneous reactions. The simulations were carried out at

different temperatures and equivalence ratios (ERs).

3.9 RESEARCH ON FLAMELESS COMBUSTION/GASIFICATION

Excess enthalpy combustion, known as FLAMELESS COMBUSTION, is, technology

has popularized and developed rapidly since the 1990s (Tang et al., 2007, Runge, 1993).

Since then various countries have developed their own technologies characterized, few

examples described, like, HiTAC (Japan), MILD-Moderate and Intensive Low

Oxidation Dilution (Italy), FLOX-Flameless Oxidation (Germany), and LNI-Low NOx

Injection (America) (Qi et al., 2003).

Firstly, Weinberg (1971) brought an idea of excess enthalpy combustion in 1971. In

this technique, the heat of flue gases is recycled, which was originally used to preheat

reactants through the heat exchanger, regenerative heater or others. Many researchers

have numerically analyzed MILD combustion/Flameless combustion, both at industrial

scale (Galletti et al., 2007, Parente et al., 2011, Mancini et al., 2007) and lab-scale

(Galletti et al., 2009, Coelho and Peters, 2001, Christo and Dally, 2005, Christo and

Dally, 2004, Kim et al., 2005, Frassoldati et al., 2010, Ihme and See, 2011, Mardani

and Tabejamaat, 2010, Mardani et al., 2010) furnaces. To define and develop suited

CFD sub-models, some effective efforts were made for this kind of combustion regime

using a huge variety of sub-models in CFD. Few examples are given, like, schemes of

global kinetics ranging from (Galletti et al., 2007) and detailed and reduced (Galletti et

al., 2009, Christo and Dally, 2005, Frassoldati et al., 2010, Mardani and Tabejamaat,

2010, Mardani et al., 2010) that were employed. A lot of combustion models were taken

into an account similarly and were investigated the interaction of the turbulence

chemistry. Some researchers also worked on Flameless gasification system utilizing the

excess enthalpy combustion (Flameless combustion) mechanism (Kobayashi et al.,

1999, Yashikawa, 1999, Tingyu et al., 1998, Tingyu et al., 1999a, Tingyu and Yang,

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1999, Tingyu et al., 1999b, Li and Jiazhou, 2008). A selected literature review is

presented here.

Jiang et al. (2000) successfully developed the honeycomb ceramic regenerator that was

cheap and can recover waste heat effectively, which could reduce the production cost

of high -temperature air, promotes the development of new technology. Principle and

critical technology of high-temperature air, high-temperature air flameless combustion,

and high-temperature air gasification were analyzed to create conditions for developing

high-temperature air resources and applying new type combustion and gasification

technology. Golovitchev and Jarnicki (2001) provided the understandings of the

processes chemically and physical fundamentally that occurred in high temperature

during combustion in air or exhaust gases with low usage of oxygen content using

approach chemistry in the detailed. O'neal (2004) aimed to develop in a project was

continuous advanced MILD gasification products and processes for processes

upgrading that will be eventually capable of commercialization the system. The

program core task was to a bench-scale investigated data to generate the design of mild

gasification reactor for a larger scale and study of a bench-scale of upgrading of char

products to add the value.

Gupta (2006) demonstrated the benefits and applications of flameless oxidation. It was

described that the flameless oxidation of fuels or high-temperature combustion is new

and innovative means for the conversion of chemical energy to the thermal energy of

fuels. Flameless or colorless oxidation of fuels could be obtained using high-

temperature combustion air at low oxygen concentration (with heat and gas

recirculation) incorporated in the basic design principles of High-Temperature Air

Combustion (HiTAC) technology. Gasification using HiTAC offers clean

transformation of solid and liquid fuels into clean syngas that can be used for cleaner

combustion. Murer et al. (2006) characterized the natural gas experimentally with

flameless combustion and with FLUENT by CFD modeling. And in a laboratory scale

performed the measurements furnace are used as boundary conditions and validation

data for the various models tested. Turbulence and combustion were being modeled

respectively the Eddy-Dissipation model and with k-ε standard model, and model of

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Finite Rate was combined. And according to the chamber temperature, the combustion

of furnace presents 2 regimes, numerically and experimentally.

Onwudili and Williams (2008) discussed that in water about major two organic

reactions under high temperatures and high pressures, hydrothermal oxidation, and

gasification. These methods possess great potential towards the thermochemical

treatment of moisture content present highly in wastes of organic with sources and

varieties differently. The application of these hydrothermal processes to a range of

model compounds and biodegradable waste samples were described to illustrate the

potential of these novel organic waste treatment technologies i.e. flameless oxidation.

Water-cooled sampling tube and gas purification analytic system were used by Guo et

al. (2008) in a hot-state experimental study of the gas concentration distribution of a

nozzle plane in a gasification furnace. Through an image processing, the flame image

was divided into three portions along the direction of a gas sampling tube: namely, a

flame impinging zone, a transition zone, and a flameless zone. Test results showed that

the gas constituents of the flame impinging plane in the gasification furnace are closely

related with the flame shape, and the gas concentration in the transition zone has the

greatest changes.

Wei et al. (2008) summarized the development of coal MILD gasification. The catalyst

type, the mechanism and reaction kinetics of catalytic gasification were discussed. The

effect of the factors on gasification reactivity and the product were analyzed, such as

coal structure, catalyst, gasification temperature, and atmosphere. Numerical

simulations for different working condition of MILD combustion were executed by

(Calchetti et al., 2009) using solid fuel and slurry form in FLUENT. The combustion

was modeled with eddy dissipation conceptual model and adopted the P1 radiation

model for radiative heat transfer. The realizable k-ε model was used to calculate

turbulence in the flow. A special and effective mixing nozzle was developed by

Masashi et al. (2009) for creating a homogeneous mixture of air and fuel by mixing

rapidly, thus this rapid-mixing nozzle was thereafter applied to a burner of Bunsen-type

to observe characteristics of combustion of the rapid-mixture. Finally, in conclusion,

the rapid-mixture combustion of exhibited the structure of same flame and

characteristics of combustion prepared perfectly flame was premixed, for such purpose

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rapid-mixing nozzle the mixing time was short extremely such as a few milliseconds.

In this paper, the rapid-mixing nozzle was used for creating premixed preheated flames

and also there mixing time was less than that of the fuel, the delay ignition time.

The IFRF experiments were analyzed by Schaffel et al. (2009) used the mathematical

model that is CFD-based. Both models such as one is the Chemical-Percolation-

Devolatilization (CPD) model and another one is model with the char combustion

intrinsic reactivity were adapted for combusted of Guasare coal. As well as, the flow-

field of the temperature and the fields of the oxygen were predicted accurately by the

model that is CFD-based. The temperature predicted and composition of the gas fields

was uniformed and to demonstrate which slowdown combustion occurred volume of

the whole furnace. And the predictions on CFD-based was highlighted the reduction

potential of NOx combustion of MILD according to the following mechanism in the

system. A numerical study was presented by Stadler et al. (2009b) on the importance

of char reaction with CO2 and H2O formation on pollutant in the flameless combustion.

And the models numerically designed that was being used against experimental data

for validation. The wall temperature by varying and of air ratio the excess of the burner

and investigated other various cases in which considered the impact of gasification and

was assessed on the NO formation prediction. Conclusively, the investigated within

ranges given parameters the char fraction increased up to 35% when that was being

gasified.

Szego et al. (2009) described characteristics its stability and performance of a parallel

jet MILD, Intense or Moderate Low Oxygen Dilution, the system of combustion of

burner in a laboratory-scale furnace and on the same wall the exhaust and reactants

ports all were mounted. The measurement of thermal field for cases was presented

without and with combustion air preheat, additionally measurements for a range of

equivalence ratio to global emission and temperature, extraction of heat, fuel dilution

and preheating of air levels. Caprariis et al. (2010) analyzed the coal devolatilization

process that took place in an oxy-combustion reactor working under pressure and in

flameless condition. The work was focused on the coal devolatilization study and

research, and the combustion process was entirely influenced. The devolatilization

process analysis was performed FG-DVC (Functional Group-Depolymerization

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Vaporization Cross-Linking model) know as dedicated scientific software and

experimental tests supported with TGA. And the models like Kinetic models were

developed of the pyrolysis and a CFD code (FLUENT) was implemented into that and

analyzed the behavior of the burner. Ecological and technical aspects of High-

Temperature Air Combustion was implemented to power station boilers fired with

pulverized coal was considered by Schaffel-Mancini et al. (2010). Numerical

simulations CFD-based were performed for determination of its dimensions and the

boiler shape, both the distance between locations block of the burner and burners was

optimized. Finally, that HTAC technology was concluded that for pulverized coal-fired

boilers it could be a realizable, effective, clean and efficient technology.

Tang et al. (2010a) (2010b) (2010d) analyzed a novel dry pulverized coal gasifier based

on of formation mechanism, realization conditions, and implementation was

approached to the new technology of flameless oxidation, which was applicable to

double-high coal. A test system for the pressurized coal gasifier was described, and the

study processes and results of the test were given. The feasibility of flameless

gasification inside the gasifier was validated from the gasification reaction images,

which accord with the characteristic of flameless oxidation. Tang et al. (2010c) studied

visually gasification flow characteristics of a new gasifier put forward which was based

on technology flameless oxidation. Numerical simulation 3D for high ash was

conducted of the gasifier melting point of the pulverized coal. And results shown in the

gasifier, that the temperature, inner velocity, and species concentrations, it was able to

investigate that in the gasifier temperature field became much possible an increase in

average temperature, the intensity of the gasification would be accordingly boosted up,

based on flameless oxidation spatial gasification reactions in the gasifier able to

achieve.

Numerical study results were presented by Danon et al. (2011) in a furnace for four

type of burner configurations equipped with three pairs in burners of flameless

combustion firing Dutch natural gas. And results of the simulations were also validated

in this research work against experimental work published previously. The CFD was

used through utilizing its commercial code Fluent 6.30. (EDC), Eddy Dissipation

Concept, for turbulence model was used for interaction of the chemistry in combination

88

with the realizable k-ε model. And also due to relatively of the low Reynolds numbers

it was found that in the flow of cooling air in the annulus of cooling tubes and it was

predicted that rates of extraction of heat were improved of these cooling tubes as

laminar by treating the cooling tubes flow. Khoshhal et al. (2011) numerically

investigated fuel temperature influence on the formation of NOx. Emission of NOx was

developed CFD modeling an experimentally equipped furnace with high-temperature

system combustion of air (HiTAC). And results were shown better when compared

experimental values with the predicted results, in which the suitability was suggested

of combustion of adoption and formation of NOx models to predict the flow

characteristics, heat transfer, combustion, and emissions of NOx in the chamber of

HiTAC. Li et al. (2011) worked for the research since many years and development of

combustion of intense or moderate low oxygen dilution (MILD). The combustion of

MILD and its requirements for establishing were observed than conventional systems

was more relaxed. Also, it was revealed that of different type of combustion, i.e.,

partially premixed, fully premixed, and non-premixed by firing various fuels could be

achieved (i.e., liquid fuels, solid fuels, and gaseous fuels). The significant expansion of

MILD combustion could be predicted by analyzing its commercial applications.

Investigations were made systematic numerically by Mi et al. (2011) for the influence

of studying conditions of the initial injection of reactants on characteristics of flame

from a parallel multi-jet burner in a laboratory-scale furnace. Particularly,

characteristics varying from the visible flame to invisible combustion of Intense or

Moderate Low-oxygen Dilution (MILD) was explored. Parameters were examined like

different initial separation of fuel and air streams (S), fuel nozzle diameter (Df), air

nozzle diameter (Da), and air preheat temperature (Ta). Qualitatively the present

simulations agree with experimental measurements work. A modeling study was

reported of Reynolds averaged Navier-Stokes (RANS) by Wang et al. (2011) that was

investigated the effects circulating of heat extracted (Qout/Qin) through the furnace wall

and the recirculation rate of internal exhaust gas on the premixed combustion

performance. And suggested the results that the ratio Qout/Qin and recirculation rate of

the exhaust gas played remarkable roles in the combustion of premixed flameless while

establishing the system. Aminian et al. (2012) numerically studied the burner of jet-in-

89

hot co-flow (JHC) emulating MILD conditions of combustion in the 2D axisymmetric

domain by solving the Reynolds Averaged Navier-Stokes equations. And for the

turbulence-chemistry interaction treatment, the Eddy Dissipation Concept (EDC) was

used. And systematic methodology to analyze sources all possible of discrepancies was

used and observed the difference between numerical and experimental data, and tried a

lot when shaded light of specific models on the suitability for combustion of MILD.

De Joannon et al. (2012) characterized in a steady laminar the reactive structures and

on a dense grid unidimensional mixing layer of parameters conditions in MILD

combustion with diluted and hot fuel. Regarding in terms of the temperature were

studied the structures and profiles of heat release for various ranges in a mixture fraction

space of rates of stretch and for two reference pressures, ranging from 1.00 and 10.00

bar, and used another standard code and also kinetic scheme with the standard.

Vascellari and Cau (2012) deeply investigated the turbulence-chemistry influence

interaction on pulverized coal combustion of MILD and to reproduce the process

discussed the accuracy and suitability of models. Two models were analyzed of

turbulence-chemistry interaction models, such as finite rate chemistry Eddy Dissipation

Concept and fast chemistry Eddy Dissipation Model. While advanced turbulence-

chemistry models resulted of comparison that with numerical results used with complex

kinetic mechanisms given the best agreement results.

Experiments and numerical simulations were conducted by Cao et al. (2012) of staged

entrained-flow gasifier for dry pulverized coal which was proposed based on flameless

oxidation technology. The influence of changing mole ratio of O to C and feeding

condition on syngas components concentration of CO, H2, and CO2, the calorific value

of syngas, carbon conversion were analyzed. Simulations results were in good

agreement with the experimentally measured results. Vascellari et al. (2013b)

investigated a newly developed sub-model application and analyzed for gasification

and char particle combustion. For this model, the distinguishing feature representation

was detailed of convection processes and the diffusion as well as reactions with the

homogeneous nature around the char particle in the boundary layer. Ansys FLUENT

was used as commercial CFD code. Coupled solver was used for simulating the IFRF

full scale pulverized of coal combustion MILD furnace, and experimental data was

90

detailed and for evaluation of the model were available. A good and reliable agreement

was given for the new model as compared to the standard modeling approach with

measured data.

3.10 SUMMARY

The historical development of gasification process, commercial gasification

technologies and experimental and modeling work on gasification technologies have

reviewed in this chapter. Gasification remained an old technique which was initially

utilized to produce a syngas for further synthesis of chemicals. Later the syngas being

used as the source of power generation. There are several gasification technologies have

developed considering different types of feedstocks. All those technologies are grouped

in three configurations i.e., Fixed Bed or Moving Bed, Fluidized Bed and Entrained

Bed. Among these, entrained bed configuration gave maximum efficiency and versatile

nature of feed capacity. Wide experimental work was conducted on all three types of

configurations. The main focus of all the studies was to design an efficient gasifier for

a particular type of feedstock like low, moderate or high-grade coals, the biomass of

different types, solid waste including hospital waste, waste tires, a slurry form of feed

etc. Numerous types of oxidizers have tested like oxygen, air, steam, CO2, and mixture

of these. Various operating parameters were optimized for individual configurations

like fuel feeding rate, oxidizer feeding rate, oxygen-to-fuel ratio, oxygen-to-carbon

ratio, the pressure of gasifier, the temperature of feed streams, fuel composition etc.

The performance of gasifiers were evaluated mostly on the basis of the quantity of

syngas produced, the composition of syngas, the heating value of syngas, fuel

conversion efficiency, the temperature of exiting syngas, the temperature at inside the

gasifier, the efficiency with whole IGCC system, slagging or ash fusion issues etc.

Among various types of entrained flow configurations, entrained flow gasifier with

opposite multi-burners (OMB) got special attention by the researchers in recent years.

The opposite injectors for fuel or oxidizer produce impinging flows inside the gasifier

and flameless combustion conditions are being produced which reduces the NOx

formation and enhance the system efficiency. OMB has shown good performance for

91

low-grade coals due to workability at low temperatures. The slag formation and their

concern issues could be avoided in this type of gasifier.

Computational fluid dynamics (CFD) proved a simple and robust tool to design

complex chemical systems like reactors, gasifier etc. The gasification technology

emerged due to CFD modeling and simulation as a number of researchers developed

various models of all types. Usually, multiphase flow modeling is being carried out

with either Euler-Euler framework considering both phases as the continuous phase or

Euler-Lagrangian framework in which solid phase is being treated with discrete phase

model (DPM) whereas gaseous phase is being calculated through continuity equations.

The finite rate chemistry has proved a better option in CFD modeling of the gasification

process. Turbulent flows are being captured by a k-ε turbulence model. As per an

extensive literature review, it is concluded that CFD plays a vital role in designing a

gasifier based on local coal chemistry and characteristics. Hence in this research, the

CFD has used as a basic tool to develop an indigenous gasification technology for local

coal based on entrained flow gasifier with OMB.

92

CHAPTER 4

EXPERIMENTAL WORK

4.1 GENERAL DESCRIPTION OF EXPERIMENTAL WORK

Main purpose of the experimental work is to determine the kinetic parameters pre-

exponential factor ‘A’ and activation energy ‘E’ for Thar lignite at atmospheric and

elevated pressures. Overall experimental work is divided into four steps as shown in

Fig.4.1. In the first step, coal samples were collected and prepared for the upcoming

analysis. After the sample preparation, proximate and ultimate analysis tests were

conducted, then the experiments were performed on TGA for extraction of kinetic

parameters of coal drying, devolatization, and combustion at atmospheric pressure.

Then PTGA experiments were conducted on prepared char for studying coal

gasification kinetics with CO2 and steam (H2O). These three steps are further described

in more details in the next sections.

Fig. 4.1: Steps for Experimental Work

4.1.1 Sample collection and preparation

The samples of Thar lignite were collected from available drill holes GT-01 and GT-

02 of Block IX of Thar coalfield. The details about the sample ID, the thickness of the

geotechnical layer, depth of sample location etc., are given in Table 4.1.

After the collection of samples, the samples were properly preserved by waxing. Then

they were brought to the laboratory where they were taken out from the waxed layered

shell. The samples were ground and converted into powder form in conventional lab

scale grinder. The coal powder was passed through 100 mesh size sieve (150μm).

Minus 100 mesh size sieve powder was collected and stored in plastic bags for

experimental work.

Sample Collection and

Preparation

Proximate and Ultimate Analysis

Experiments on TGA

Experiments on PTGA

93

4.1.2 Proximate and Ultimate Analysis

The prepared samples were used for conducting proximate and ultimate analysis tests.

The ASTM standard methods were used for these tests. For Proximate Analysis, ASTM

D 3172 method was used. Moisture was measured using ASTM D3173-03, Volatiles

were determined with ASTM D3175-02, Ash was analyzed with ASTM D3174-02,

Fixed Carbon was determined using ASTM D3174-02 and Ultimate Analysis was

conducted by ASTM D3176. Finally, the Gross Calorific Value was calculated using

ASTM D5865 standard.

Table 4.1: Samples collected from Block-IX Thar Coal Field

Sr# Sample ID From (m) To (m) Thickness (m)

01 KTN-GT01-123 229.66 229.83 0.17

02 KTN-GT01-138 249.43 249.77 0.34

03 KTN-GT01-139 250.33 250.57 0.24

04 KTN-GT01-140 250.57 250.76 0.19

05 KTN-GT-02-627 203.42 203.53 0.11

06 KTN-GT-02-632 204.26 204.47 0.21

07 KTN-GT-02-642 205.82 205.92 0.10

08 KTN-GT-02-644 206.11 206.33 0.22

09 KTN-GT-02-691 215.17 215.33 0.16

10 KTN-GT-02-697 216.17 216.34 0.17

11 KTN-GT01-443 229.43 229.66 0.23

12 KTN-GT01-493 249.89 250.33 0.44

4.1.3 TGA Analysis (Moisture removal, devolatization, and Combustion Study)

Thermogravimetric analyzer (TGA) SDT Q600 (Appendix A.1) was used to study the

kinetic of moisture removal, devolatization, and combustion of collected samples. The

non-isothermal technique was used in TGA analysis. The individual samples weighing

30-40 mg was heated from room temperature to 110° C at different heating rates for

moisture removal study. The sample kept in that condition for 5 minutes and then again

94

heated from 110°C to 900°C at three different heating rates for devolatization study and

kept there for 5 minutes. The inert environment was maintained in the combustion

chamber of TGA by flowing N2 at 100 ml/min. The char prepared after devolatization

was cooled with N2 below 100°C and then again heated from 200°C to 900°C in an

Oxygen environment at three different heating rates. All the experimental conditions of

moisture removal, devolatization, and combustion analysis are given in Table 4.2 and

Table 4.3. The weight loss was recorded with time and temperature and then recorded

weight loss data was utilized in the extraction of kinetic parameters of these processes.

Table 4.2: Experimental conditions in TGA for Moisture Removal and

Devolatization

Experimental Conditions Values/Information

Technique Non-Isothermal (Dynamic heating rate)

Weight of Sample 30 - 40 mg

The particle size of Sample 150 μm (100-mesh)

Method Set in TGA

1 Select gas N2

2 Ramp 10°C/min, 20°C/min, 30°C/min to 110.00 °C

3 Equilibrate at 110.00 °

4 Isothermal for 5.00 min

5 Ramp 20°C/min , 30°C/min, 40°C/min to 900.00 °C

6 Equilibrate at 900.00 °C


8 Cooling

Flow-rate of N2 100mL/min

Signals Weight Loss, time, Temperature.

95

Table 4.3: Experimental conditions in TGA for Combustion Reaction

Experimental Conditions Values/Information

Technique Non-Isothermal (Dynamic heating rate)

Weight of char sample 10 - 20 mg

The particle size of Sample 150 μm (100-mesh)

Method Set in TGA

1 Select gas N2

2 Ramp 20 °C/min, 30 °C/min, 40 °C/min to 200.00 °C

3 Equilibrate at 200.00 °


5 Select gas 2 (O2)

6 Ramp 20°C/min , 30°C/min, 40°C/min to 900.00 °C

7 Equilibrate at 900.00 °C


9 Mark end of cycle 0

Flow-rate of N2 ,O2 100mL/min

Signals Weight Loss, time, Temperature.

4.1.4 PTGA Analysis (Char Gasification Study)

Prior to the gasification experiments, coal char was prepared in a quartz fixed-bed

reactor in a nitrogen environment at an atmospheric pressure (Appendix A.2). The

lignite sample, weighing 10±0.05 gram, was heated from room temperature to 900° C

at the rate of 20° C min−1 and kept for 20 minutes. Then the devolatilized char was

cooled with nitrogen to room temperature and was ground to -100 µm size.

Prepared char was used for kinetic study using Cahn Thermax500 Pressurized

Thermogravimetric Analyzer (PTGA) (Appendix A.3). Fig. 4.2 shows the schematic

diagram of Thermax500 PTGA, which consists of three parts: pressurized balance,

pressurized furnace, and control system. A ceramic extension wire, hanging on one arm

of a balance, extends into the quartz reaction tube. A platinum sample pan was hung on

96

the end of the extension wire. A set of thermocouples is installed about 8 mm below

the sample pan to measure the temperature of the reaction zone. The reactant gas flows

through the sample pan in an upward direction with the controlled flow rate. A manual

back pressure regulator was applied to minimize the fluctuation of pressure.

Three lignite char samples having IDs KTN-GT01-123, KTN-GT01-140 and KTN-GT-

02-627 (As per list is given in Table 4.1) were tested in PTGA (Thermax 500) at three

pressures viz: 1 atm, 5 atm, and 10 atm. Each sample, weighing 10±0.5 mg, was loaded

into the sample pan, and then the system was pressurized with the reacting gas either

CO2 or H2O (steam) in a pure state to the set pressure. Then the system was heated from

room temperature to 1000° C, at a constant heating rate of 10° Cmin−1. The reaction

was completed before the temperature reaches to 1000° C. At each pressure, trial

experiments were done to minimize the external diffusion resistances by adjusting the

flow rate of reacting gas.

Fig. 4.2: Schematic diagram of Thermax500 PTGA

4.1.5 Data analysis method

The conversion of moisture removal, devolatization, combustion, and char-gasification

was calculated from the recorded weight loss curves of individual steps using the

relationship:

97

1000

0 −

−=

WW

WWX (4.1)

Where W is the instantaneous coal/char weight, W∞ is the weight of residual ash and

W0 is the weight of initial coal/char. The apparent reaction rate was defined as the

change rate of conversion X. A general kinetic expression for the overall reaction rate

may be written as follows (Lu and Do, 1994).

( ) )(, XfTCkdt

dXg= (4.2)

where k is the apparent gasification reaction rate, which is a function of temperature (T)

and the effect of the gasifying agent concentration (Cg) and f(X) describes the changes

in physical or chemical properties of the sample as the gasification proceeds. Assuming

that the concentration of the gasifying agent remains constant during the process, the

apparent gasification reaction rate is dependent on the temperature and can be expressed

as follows, using the Arrhenius Equation with the reaction order (n) which represents

the effect of partial pressure of reactant gas:

RT

E

neAPk−

= (4.3)

Where, A and E are the pre-exponential factor (Arrhenius constant) and activation

energy, respectively. In the present study, n was taken as 0.1.

In this work, two models, Volumetric Model (VM) and Grain Model (GM), have been

used to describe the reactivity of the chars. These models give different formulations of

the term f(X). The VM assumes a homogeneous reaction throughout the particle and a

linearly decreasing reaction surface area with conversion (Ishida et al., 1971). The

overall reaction rate is expressed by:

( )Xkdt

dX−= 1 (4.4)

The Grain Model or Shrinking Core Model, proposed by Szekely and Evans (1971),

assumes that a porous particle consists of an assembly of uniform nonporous grains and

98

the reaction takes place on the surface of these grains. The space between the grains

constitutes the porous network. The shrinking core behavior applies to each of these

grains during the reaction. In the regime of chemical kinetic control and assuming the

grains have a spherical shape, the overall reaction rate is expressed in these models as:

( )3

2

1 Xkdt

dX−= (4.5)

This model predicts a monotonically decreasing reaction rate and surface area because

the surface area of each grain is receding during the reaction.

For constant heating rate (β), a linear relationship between time and temperature can be

written as,

dt

dT= (4.6)

Now, substituting Eqs. 4.6 and 4.3 in Eqs. 4.4 and 4.5, rearranging gives:

dTeAP

X

dXRT

En−

=− )1(

(4.7)

( )dTe

AP

X

dXRT

En−

=

− 3

2

1

(4.8)

The above equations can be used to evaluate E and A at constant heating rate using TG

data. Unfortunately, the right-hand side of Eq. 4.7 and Eq. 4.8 has no definite integrals

which make it difficult to find the exact solution. Therefore, several procedures are

devised to estimate the value of the temperature integral. In the following sections, two

different methods are discussed which can be used to evaluate the value of Arrhenius

parameters using Eqs. 4.7 and 4.8.

4.1.5.1 Integral method

This method is based on the approximation of temperature integral made by Coats and

Redfern (1964), and it is widely used and accepted for the calculation of kinetic

99

parameters. For each discrete event of gasification, Eqs. 4.7 and 4.8 are integrated with

temperature limits from T0 to T while the conversion factor (X) has the limits 0 to X:

For Volumetric Model: −

=−

T

T

RT

EnX

dTeAP

X

dX

001

(4.9)

For Grain Model:

( )

−

=

−

T

T

RT

EnX

dTeAP

X

dX

00 3

2

1

(4.10)

Integration of left side of Eqs. 4.9 and 4.10 are:

)1ln(1

0

XX

dXX

−−=−

(4.11)

( )

−−=

− 3

1

0 3

2)1(13

1

X

X

dXX

(4.12)

As the right-hand side of Eq. 4.9 and 4.10 have the same expression with no definite

integral, so assumptions are made to solve this integral. The first assumption is to take

T0 = 0, as no reaction is taking place at T0 and also X is zero at T0. Therefore the right

hand side of Eq. 4.8 becomes:

−−

=

T

RT

EnT

T

RT

En

dTeAP

dTeAP

00

(4.13)

Coats and Redfern (1964) has used series of expansion method to solve the right-hand

side of the above integral which results in the following:

RT

EnT

T

RT

En

eE

RT

E

RAPTdTe

AP −−

−=

212

0

(4.14)

Substituting Eqs. (4.11) and (4.12) (for left side integrals) and Eq. (4.14) (for right side

integrals) in Eqs. (4.9) and (4.10), re-arranging and taking natural log on both sides

gives:

100

For Volumetric Model: RT

E

E

RT

E

RAP

T

X n

−

−=

−− 21ln

)1ln(ln

2

For Grain Model: RT

E

E

RT

E

RAP

T

Xn

−

−=

−−

21ln

)1(13

ln2

3

1

The above equations can be more simplified by assuming E » 2RT. Which is a

reasonable assumption, as the value of activation energy (E) is normally 5–10 order of

magnitude more than the product 2RT.

For Volumetric Model: E

RAP

RT

E

T

X n

ln

)1ln(ln

2+−=

−− (4.15)

For Grain Model:

E

RAP

RT

E

T

Xn

ln

)1(13

ln2

3

1

+−=

−−

(4.16)

The above equations represent the equations of a straight line having a slope of -E/R

and an intercept of ln(APnR/βE). Using the values of X and T from TGA and PTGA

experiments, graphs of left-hand sides of Eqs. (4.15) and (4.16) can be plotted against

1/T. The plots should result in a series of data points close to a straight line. Regression

analysis with the least square fitting method is used to find the equation of the straight

line and plot it to evaluate the values of E and A.

4.1.5.2 Direct Arrhenius plot method

Eqs. (4.7) and (4.8) can be modified by taking natural log on both sides, and

rearranging:

For Volumetric Model:

nAP

TR

E

dT

dX

Xln

1

1

1ln +

−=

− (4.17)

For Grain Model:

( )

nAP

TR

E

dT

dX

X

ln1

1

1ln

3

2+

−=

−

(4.18)

101

Where 12

12

TT

XX

dT

dX TT

−

−=

The value of the above differential can be taken from the DTG curve. Putting the above

value in Eqs. (4.17) and (4.18),

For Volumetric Model:

nTT AP

TR

E

TT

XX

Xln

1

1

1ln

12

12 +

−=

−

−

− (4.19)

For Grain Model:

( )

nTT AP

TR

E

TT

XX

X

ln1

1

1ln

123

2

12 +

−=

−

−

−

(4.20)

Similar to the Integral method, plots of the left-hand sides of Eqs. (4.19) and (4.20) are

obtained and least square regression analysis is used to find E and A for gasification

reactions.

4.2 RESULTS AND DISCUSSION FOR EXPERIMENTAL WORK

As mentioned earlier that lignite samples of main coal seam were obtained, at different

depths, from drill holes GT-01 and GT-02 of Block-IX at Thar coalfield. These samples

were prepared in the laboratory and proximate and ultimate analysis tests were

conducted as per ASTM standard methods. Then experiments were conducted on TGA

and PTGA for kinetic analysis. The results are discussed in the subsequent sections.

4.3 RESULTS OF PROXIMATE AND ULTIMATE ANALYSIS

The results of the proximate and ultimate analysis are given in Table 4.4 and Table 4.5

respectively. It was observed from the proximate analysis of Thar lignite that moisture

content is ranging from 14 to 45 % which confirms the previous studies finding

(Choudry et al., 2010). It was also noticed that the fixed carbon content is less than the

volatiles which shows the basic lignite characteristics. The amount of sulfur is up to 2%

which indicates the good characteristics in terms of less SOX production during

combustion or gasification. The heating value of the collected samples was found in the

range of 5156.54 to 6476.23 Btu/lb. The values were compared with the values obtained

102

by other researchers (Jaffri and Zhang, 2009, Choudry et al., 2010, Sarwar et al.,

2014)and found in similar ranges.

Table 4.4: Proximate Analysis and Heating Value of Coal Samples

Sr.

# Sample ID

Proximate Analysis Heating Value

(Btul/lb) Moisture

(%)

Volatiles

(%)

Ash

(%)

Fixed Carbon

(%)

1 KTN-GT01-123 14.18 38.27 20.54 27.01 6339.56

2 KTN-GT01-138 35.65 33.8 4.24 26.31 6093.99

3 KTN-GT01-139 39.16 31.12 4.87 24.84 5417.51

4 KTN-GT01-140 30.04 38.31 9.78 21.87 5876.11

5 KTN-GT-02-627 41.61 30.91 3.52 23.95 5286.04

6 KTN-GT-02-632 45.88 28.4 3.3 22.43 5156.54

7 KTN-GT-02-642 26.58 36.86 5.23 31.32 6352.16

8 KTN-GT-02-644 20.77 49.87 4.23 25.12 6387.24

9 KTN-GT-02-691 29.82 40.01 6.34 23.82 6476.23

10 KTN-GT-02-697 27.49 42.14 10.16 20.21 6084.62

Average 31.12 36.97 7.22 24.69 5947

Standard Deviation 9.68 6.33 5.27 3.1 492.13

Table 4.5: Ultimate Analysis of Coal Samples

Sr. # Sample ID

Ultimate Analysis

C

(%)

H

(%)

N

(%)

O

(%)

Total S

(%)

1 KTN-GT01-123 32.34 6.8 0.32 58.44 2.1

2 KTN-GT01-138 30.32 9.55 0.34 58.29 1.5

3 KTN-GT01-139 34.24 6.76 0.62 57.65 0.73

4 KTN-GT01-140 32.12 6.54 0.32 59.61 1.41

5 KTN-GT-02-627 35.32 6.17 0.35 56.43 1.73

6 KTN-GT-02-632 34.21 7.12 0.76 57.12 0.79

7 KTN-GT-02-642 31.23 6.87 0.43 60.23 1.24

8 KTN-GT-02-644 30.87 7.23 0.88 59.93 1.09

9 KTN-GT-02-691 32.65 8.54 0.73 57.43 0.65

10 KTN-GT-02-697 33.21 7.12 0.43 58.46 0.78

Average 32.65 7.27 0.52 58.36 1.20

Standard Deviation 1.61 1.01 0.21 1.25 0.48

103

4.4 RESULTS FOR MOISTURE REMOVAL AND DEVOLATIZATION

KINETICS

Coal gasification comprises four fundamental processes, (1) drying or removal of

moisture (2) devolatization or removal of volatiles (3) char combustion and (4) char

gasification (means reaction with CO2 or H2O). One of the fundamental objectives of

the present research was to conduct the kinetic modeling of coal gasification processes.

In this regard, the TGA analysis was carried out and the data were evaluated for

extracting the kinetics of each process. First, the coal samples were heated under N2

environment from room temperature to 110°C with different heating rates. Then the

dried coal was further heated, at fixed heating rates, to 900°C under the same inert

environment for removal of volatiles. The detailed experimental conditions are already

explained in Table 4.2. Two prepared coal samples (i.e., GT-01-443 and GT-01-493)

were run on TGA for moisture and volatiles removal. The results of moisture and

volatiles removal were calculated using Eq. 4.1 and shown in Fig. 4.3 and 4.4. It is

observed from these figures that the removal of moisture or volatiles is delayed with

the decrease in heating rate. At higher heating rates i.e. 30°C / minute for moisture

removal study and 40°C / minute for devolatization, the removal occurs fast enough

and all moisture and volatiles are removed before 40 minutes of heating. The similar

trend was obtained from research conducted by other researchers (Sarwar et al., 2011,

Sarwar et al., 2014).

(a) Sample: GT-01-443 (b) Sample: GT-01-493

Fig. 4.3: Moisture removal at different heating rates

104

(a) Sample: GT-01-443 (b) Sample: GT-01-493

Fig. 4.4: Volatiles removal at different heating rates

It was further observed from the devolatization removal trends of Fig. 4.4 that the

removal of volatiles is fast enough in the lower ranges (from 110 to 500°C) but it

becomes slower in higher ranges of temperature (>500°C) as explained by (Sarwar et

al., 2011). This is due to the removal of lighter volatiles at lower temperature whereas

the heavier volatiles at higher temperatures. This is the reason that the kinetic modeling

of devolatization step was carried out in two steps categorized with lower and higher

temperature ranges.

The kinetic modeling and Arrhenius parameters (A and E) calculations for removal of

moisture and devolatization were carried out using two standard models i.e.,

Volumetric Model (VM) and Grain Model (GM). The mathematical description has

already been discussed in the previous chapter. Integral solution method was used for

both models.

4.4.1 Least square regression analysis for moisture removal

The TGA data has been used to conduct the least square regression analysis for Eqs.

4.15, and 4.16. These equations are based on the integral method strategy for solving

Volumetric and Grain Model differential equations and are standard forms of straight

line with ln(-ln(1-X)/T2) as an ordinate for Volumetric Model whereas ln(3(1-(1-

X)(1/3))/T2) as an ordinate for Grain Model and 1/T as abscissa. The TGA experiments

produced the weight-loss data against temperature ‘T’ which is used in Eq. 4.1 to

105

calculate corresponding conversion ‘X’ against ‘T’. The calculated ordinates for VM

and GM were then plotted against 1/T as per Eq.4.15 and Eq. 4.16. The graphs are

shown in Fig. 4.5 (a-f) and 4.6 (a-f) for the moisture removal of sample GT-01-443 and

GT-01-493 respectively. The R2 was found in the range of 0.97-0.99 which is

considered as good linearity. Hence it is summarized that volumetric and grain models

showed good results with all hearing rates.

4.4.2 Least square regression analysis for devolatization

The removal of volatiles was studied using TGA within the temperature range of 110°C

to 900°C with three different constant heating rates. During this study, it was noticed

that the volatiles removal followed dual kinetic rates due to the presence of heavy and

lighter volatiles, as observed by Sarwar et al. (2011). So the kinetics study was divided

into low and high-temperature ranges. The linear graphs for devolatization at low-

temperature ranges are shown in Figs. 4.7 and 4.8, whereas devolatization for high-

temperature ranges are shown in Fig. 4.9 and 4.10 for samples GT-01-443 and GT-01-

493. Linear regression for best fit straight was carried out in all graphs. It was observed

that at high or low temperature ranges both the models (Volumetric and Grain model)

show a good linearity with R2 in the range of 0.92-0.99.

Overall it is summarized that volumetric and grain models showed good results with all

hearing rates. The calculated Arrhenius parameters for moisture removal and

devolatization are tabulated in Table 4.6. The values of moisture removal and

devolatization kinetics are in accordance with low-grade coals or low-calorific fuels

from previous research (Jiménez et al., 2008, Okumura et al., 2009, Piatkowski and

Steinfeld, 2010).

106

(a) Heating Rate 10°C/min, Vol. Model (d) Heating Rate 10°C/min, Grain Model

(b) Heating Rate 20°C/min, Vol. Model (e) Heating Rate 20°C/min, Grain Model

(c) Heating Rate 30°C/min, Vol. Model (f) Heating Rate 30°C/min, Grain Model

Fig. 4.5: Linearity of Volumetric and Grain models for Moisture Removal at

different Heating Rate for Sample GT-01-443

y = -5821.5x + 4.1686

R² = 0.9967

-14.5-14

-13.5-13

-12.5-12

-11.5-11

-10.5-10

0.0

02

6

0.0

02

7

0.0

02

8

0.0

02

9

0.0

03

0.0

03

1

ln(-

ln(1

-x)/

T2)

1/T

y = -5997.9x + 3.9804

R² = 0.9952

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

ln(-

ln(1

-x)/

T2)

1/T

y = -7482.9x + 13.326

R² = 0.9691

-9.5-9

-8.5-8

-7.5-7

-6.5-6

-5.5-5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

ln(-

ln(1

-x)/

T)

1/T

y = -5125.1x + 2.0674

R² = 0.9936

-14.5-14

-13.5-13

-12.5-12

-11.5-11

-10.5-10

0.0

02

6

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

0.0

03

1

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -5617.8x + 2.8483

R² = 0.997

-14.5

-14

-13.5

-13

-12.5

-12

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -6654x + 10.852

R² = 0.9819

-9.5-9

-8.5-8

-7.5-7

-6.5-6

-5.5-5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

107




Fig. 4.6: Linearity of Volumetric and Grain models for Moisture Removal at

different Heating Rate for Sample GT-01-493

y = -7335.4x + 14.395

R² = 0.9998

-10

-9

-8

-7

-6

-5

0.0

02

7

0.0

02

8

0.0

02

9

0.0

03

0.0

03

1

0.0

03

2

0.0

03

3

ln(-

ln(1

-x)/

T2)

1/T

y = -7102.7x + 12.544

R² = 0.9858

-10-9.5

-9-8.5

-8-7.5

-7-6.5

-6-5.5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

0.0

03

1

0.0

03

15

0.0

03

2

ln(-

ln(1

-x)/

T2)

1/T

y = -6912.5x + 11.802R² = 0.9754

-10-9.5

-9-8.5

-8-7.5

-7-6.5

-6-5.5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

0.0

03

1

0.0

03

15

ln(-

ln(1

-x)/

T2)

1/T

y = -6668.1x + 12.298

R² = 0.9993

-10

-9

-8

-7

-6

-5

0.0

02

7

0.0

02

8

0.0

02

9

0.0

03

0.0

03

1

0.0

03

2

0.0

03

3

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -6481.5x + 10.641

R² = 0.9932

-10-9.5

-9-8.5

-8-7.5

-7-6.5

-6-5.5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

0.0

03

1

0.0

03

15

0.0

03

2

ln(3

[1-(

1-X

)(1

/3) ]

/T2)

1/T

y = -6319.3x + 9.9918

R² = 0.9858

-10

-9.5

-9

-8.5

-8

-7.5

-7

-6.5

0.0

02

65

0.0

02

7

0.0

02

75

0.0

02

8

0.0

02

85

0.0

02

9

0.0

02

95

0.0

03

0.0

03

05

0.0

03

1

0.0

03

15

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

108




Fig. 4.7: Linearity of Volumetric and Grain models for Devolatization at Low


y = -3284.9x - 8.9313

R² = 0.993

-15

-14.5

-14

-13.5

-13

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

ln(-

ln(1

-x)/

T2)

1/T

y = -2965.1x - 9.4983

R² = 0.9874

-15.2

-14.8

-14.4

-14

-13.6

-13.2

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -3568.9x - 8.5644

R² = 0.9976

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

ln(-

ln(1

-x)/

T2)

1/T

y = -3247.6x - 9.1282

R² = 0.9956

-15.2

-14.8

-14.4

-14

-13.6

-13.2

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -3814.5x - 1.61

R² = 0.9222

-10

-9

-8

-7

-6

-5

-4

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

0.0

02

0.0

02

1

0.0

02

2

ln(-

ln(1

-x)/

T2)

1/T

y = -3631.6x - 1.9823

R² = 0.9265

-10.5

-9.5

-8.5

-7.5

-6.5

-5.5

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

0.0

02

0.0

02

1

0.0

02

2

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

109




Fig. 4.8: Linearity of Volumetric and Grain models for Devolatization at Low


y = -5895.1x + 2.0292R² = 0.9774

-10

-9

-8

-7

-6

-5

-4

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

0.0

02

ln(-

ln(1

-x)/

T2)

1/T

y = -5636.5x + 1.5594R² = 0.98

-10

-9

-8

-7

-6

-5

-4

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

0.0

01

9

0.0

02

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -6214.1x + 2.0309R² = 0.9733

-9

-8

-7

-6

-5

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

ln(-

ln(1

-x)/

T2)

1/T

y = -5882.5x + 1.4496R² = 0.9761

-9

-8

-7

-6

-50

.00

12

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

0.0

01

7

0.0

01

8

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -523.17x - 7.8065

R² = 0.9687

-9.2

-9.1

-9

-8.9

-8.8

-8.7

0.0

01

9

0.0

02

0.0

02

1

0.0

02

2

0.0

02

3

0.0

02

4

0.0

02

5

0.0

02

6

0.0

02

7

ln(-

ln(1

-x)/

T2)

1/T

y = -512.99x - 7.8525

R² = 0.9689

-9.3

-9.2

-9.1

-9

-8.9

-8.8

-8.7

0.0

01

9

0.0

02

0.0

02

1

0.0

02

2

0.0

02

3

0.0

02

4

0.0

02

5

0.0

02

6

0.0

02

7

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

110




Fig. 4.9: Linearity of Volumetric and Grain models for Devolatization at High


y = -981.63x - 12.167

R² = 0.9912

-13.18

-13.16

-13.14

-13.12

-13.1

-13.080

.00

09

3

0.0

00

95

0.0

00

97

0.0

00

99

0.0

01

01

0.0

01

03

0.0

01

05

ln(-

ln(1

-x)/

T2)

1/T

y = -125.69x - 13.326

R² = 0.995

-13.456

-13.455-13.454

-13.453

-13.452

-13.451

-13.45

0.0

00

99

0.0

00

99

5

0.0

01

0.0

01

00

5

0.0

01

01

0.0

01

01

5

0.0

01

02

0.0

01

02

5

0.0

01

03

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -869.65x - 12.267R² = 0.9871

-13.18

-13.16

-13.14

-13.12

-13.1

-13.08

0.0

00

94

0.0

00

95

0.0

00

96

0.0

00

97

0.0

00

98

0.0

00

99

0.0

01

0.0

01

01

0.0

01

02

0.0

01

03

0.0

01

04

ln(-

ln(1

-x)/

T2)

1/T

y = -432.22x - 12.861

R² = 0.9716

-13.425

-13.42

-13.415

-13.41

-13.405

-13.4

-13.395

0.0

01

24

0.0

01

25

0.0

01

26

0.0

01

27

0.0

01

28

0.0

01

29

0.0

01

3

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -4368.1x - 0.7394

R² = 0.9987

-5.25-5.2

-5.15-5.1

-5.05-5

-4.95-4.9

-4.85

0.0

00

94

0.0

00

95

0.0

00

96

0.0

00

97

0.0

00

98

0.0

00

99

0.0

01

0.0

01

01

0.0

01

02

0.0

01

03

0.0

01

04

ln(-

ln(1

-x)/

T2)

1/T

y = -3567.1x - 1.8566

R² = 0.9991

-5.55

-5.5

-5.45

-5.4

-5.35

-5.3

-5.25

-5.2

0.0

00

94

0.0

00

95

0.0

00

96

0.0

00

97

0.0

00

98

0.0

00

99

0.0

01

0.0

01

01

0.0

01

02

0.0

01

03

0.0

01

04

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

111




Fig. 4.10: Linearity of Volumetric and Grain models for Devolatization at High


y = -3982x - 0.2997

R² = 0.9942

-6

-5.5

-5

-4.5

-4

-3.5

-3

0.0

00

8

0.0

00

9

0.0

01

0.0

01

1

0.0

01

2

0.0

01

3

0.0

01

4

ln(-

ln(1

-x)/

T2)

1/T

y = -3468x - 1.1136

R² = 0.9963

-6

-5.5

-5

-4.5

-4

0.0

00

8

0.0

00

9

0.0

01

0.0

01

1

0.0

01

2

0.0

01

3

0.0

01

4

ln(3

[1-(

1-X

)(1

/3) ]

/T2)

1/T

y = -3877.4x - 0.7973

R² = 0.9975

-6

-5.5

-5

-4.5

-4

0.0

00

9

0.0

00

95

0.0

01

0.0

01

05

0.0

01

1

0.0

01

15

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

ln(-

ln(1

-x)/

T2)

1/T

y = -3345.4x - 1.6564

R² = 0.9986

-6.5

-6

-5.5

-5

-4.5

-4

0.0

00

9

0.0

00

95

0.0

01

0.0

01

05

0.0

01

1

0.0

01

15

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -3714x - 1.1373

R² = 0.9972

-6.5

-6

-5.5

-5

-4.5

-4

0.0

00

9

0.0

00

95

0.0

01

0.0

01

05

0.0

01

1

0.0

01

15

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

ln(-

ln(1

-x)/

T2)

1/T

y = -3221.6x - 1.9425

R² = 0.998

-6.5

-6

-5.5

-5

-4.5

-4

0.0

009

0.0

009

5

0.0

01

0.0

010

5

0.0

011

0.0

011

5

0.0

012

0.0

012

5

0.0

013

0.0

013

5

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

112

Table 4.6: Calculated kinetic parameters for moisture removal and

devolatization steps

Step Model Heating Rate A E (KJ/Mol)

GT-01-443 GT-01-493 GT-01-443 GT-01-493

Moisture

Volumetric

10°C/min 3.762×103 1.309×108 48.4 60.99

20°C/min 6.422×103 3.983×107 49.87 59.05

30°C/min 1.376×108 2.769×107 62.21 57.47

Grain

10°C/min 4.051×102 1.462×107 42.61 55.44

20°C/min 1.939×103 5.420×106 46.71 53.89

30°C/min 1.031×107 4.142×106 55.32 52.54

Devolatization

(Low Temp)

Volumetric

20°C/min 8.684×103 8.970×102 27.31 49.01

30°C/min 2.043×102 1.421×103 29.67 51.66

40°C/min 3.050×101 8.519×103 31.71 4.35

Grain

20°C/min 4.446×103 5.361×102 24.65 46.86

30°C/min 1.058×102 7.520×102 27.00 48.91

40°C/min 2.001×101 7.978×103 30.19 4.26

Devolatization

(High Temp)

Volumetric

20°C/min 1.021×104 5.902×101 8.16 33.11

30°C/min 1.227×104 5.241×101 7.23 32.24

40°C/min 8.341×101 4.764×101 36.32 30.88

Grain

20°C/min 4.101×106 2.278×101 1.045 28.83

30°C/min 3.368×105 1.915×101 3.59 27.81

40°C/min 2.229×101 1.847×101 29.66 26.78

4.4.3 Rate constant “k” for drying and devolatization steps

The rate constant “k” as explained by Eq. 4.3 was calculated using extracted kinetic

parameters (activation energy E and pre-exponential factor A) for drying and

devolatization steps. Due to the exponential nature of values, the values of ln(k) are

plotted for selected samples with different heating rates. Fig. 4.11 shows the ln(k)

values for drying step whereas Fig. 4.12 shows the ln(k) values for devolatization step.

113

For drying, 100°C temperature was taken as constant temperature. Rate constant was

observed increasing with the increase in the heating rate for sample GT-01-443 as per

observation of Tremel and Spliethoff (2013a) but it is slightly decreased on increasing

heating rate for another sample i.e., GT-01-493. The probable reason is the difference

of moisture quantity with the in the coal as explained by Ma et al. (1991). An almost

similar trend was observed with both the models but VM showed the little higher

prediction of k as compared to GM.

The devolatization step is further sub-divided into low (500°C) and high temperature

(900°C) volatiles. The reverse trend as compared to drying was observed for

devolatization rate constant. Again the reason is the amount of volatiles available in

each sample (Qiu and Liu, 1994).

(a) Volumetric Model @100°C (a) Grain Model @100°C

Fig. 4.11: Rate constant (k) for drying step with (a) VM and (b) GM

02468

101214161820

10°C/min 20°C/min 30°C/min

Ln

(k)

Heating Rate

GT-01-443

GT-01-493

02468

1012141618


Ln

(k)

Heating Rate

GT-01-443

GT-01-493

114

(a) Volumetric Model (Low Temp_500°C) (c) Grain Model (Low Temp_500°C)

(b) Volumetric Model (High Temp_900°C) (d) Grain Model (High Temp_900°C)

Fig. 4.12: Rate constant (k) for devolatization step with (a) VM-Low

Temp_500°C (b) VM-High Temp_900°C (c) GM-Low Temp_500°C (d) GM-

High Temp_900°C

4.5 RESULTS FOR COMBUSTION KINETICS

Combustion is an important step in the gasification process (Rathnam et al., 2009).

After the removal of moisture and volatiles, the char is formed which contains the fixed

carbon and ash of the coal. The char reacts with available limited oxygen and

combustion takes place. This combustion produces the necessary heat for the

occurrence of endothermic reactions of gasification (char reactions with CO2 and H2O).

During TGA analysis, after the drying and devolatization step, the car was formed and

that was cooled in N2 environment down to room temperature. Then re-heating of the

chamber was done with three different constant heating rates i.e., 20°C, 40°C and 50°C

with the O2 environment. The conversion of the char samples, obtained from GT-01-

443 and GT-01-493, with O2 was calculated using Eq. 4.1 and shown in Fig. 4.13 and

4.14 respectively.

0

2

4

6

8

10


Ln

(k)

Heating Rate

GT-01-443

GT-01_493

0

2

4

6

8

10


Ln

(k)

Heating Rate

GT-01-443

GT-01_493

0

2

4

6

8

10


Ln

(k)

Heating Rate

GT-01-443

GT-01_493

0

5

10

15

20


Ln

(k)

Heating Rate

GT-01-443

GT-01_493

115

Fig. 4.13: Conversion of Char Samples

obtained from GT-01-443, in Oxygen

environment at different heating rates

Fig. 4.14: Conversion of Char Samples

obtained from GT-01-493, in Oxygen

environment at different heating rates

It was observed that the combustion of char started after 300°C and it showed a slower

conversion rate with higher heating rates as observed by other researchers (Rathnam et

al., 2009, Irfan et al., 2012). After the complete combustion, a drop in temperature was

noticed due to the little cooling by the incoming cool oxygen in the chamber.

The least square regression analysis was done in a similar way as carried out for

moisture removal and devolatization steps. Both Volumetric and Grain models fit well

for combustion of char for all three tested heating rates, as shown in Fig. 4.15 and Fig.

4.16 for char samples obtained from GT-01-443 and GT-01-493 respectively. The

calculated kinetic parameters for combustion reaction are tabulated in Table 4.7. The

data is in good agreement as compared with previous studies (Irfan et al., 2012). It is

observed from this Table that the Arrhenius parameter (A) is higher for the sample GT-

01-443 than the sample GT-01-493. It means that the combustion reaction of GT-01-

443 is faster than other sample. Activation energy (E) of sample GT-01-443 is also

higher which affects the combustion start-up and this is also proven from the conversion

curves shown in Fig. 4.13 and 4.14, in which the sample GT-01-443 shows start-up of

combustion after 350°C whereas the sample GT-01-493 was ignited just near at 300°C.

Moreover, it was also noticed that the heating rate is inversely proportional to the

kinetic parameters for the combustion as explained by Sakaguchi et al. (2010).

116




Fig. 4.15: Linearity of Volumetric and Grain models for Combustion of with

different Heating Rate for char sample obtained from GT-01-443

y = -16669x + 9.2334

R² = 0.948

-16.5

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(-

ln(1

-x)/

T2)

1/T

y = -14815x + 6.54

R² = 0.9704

-16.5

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -13644x + 4.5642

R² = 0.9957

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(-

ln(1

-x)/

T2)

1/T

y = -12033x + 2.2587

R² = 0.9985

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -11739x + 1.6927

R² = 0.9972

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

15

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(-

ln(1

-x)/

T2)

1/T

y = -10294x - 0.3552

R² = 0.992

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

15

0.0

01

2

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

117




Fig. 4.16: Linearity of Volumetric and Grain models for Combustion of with

different Heating Rate for char sample obtained from GT-01-493

y = -11708x + 3.9525

R² = 0.9672

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

0.0

01

6

0.0

01

65

0.0

01

7

ln(-

ln(1

-x)/

T2)

1/T

y = -10512x + 2.028

R² = 0.9765

-16.5

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

0.0

01

6

0.0

01

65

0.0

01

7

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -6956.4x - 5.6591

R² = 0.9868

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

0.0

01

1

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

ln(-

ln(1

-x)/

T2)

1/T

y = -6199.9x - 6.6993

R² = 0.9956

-16.5

-16

-15.5

-15

-14.5

-14

-13.5

-13

0.0

01

0.0

01

1

0.0

01

2

0.0

01

3

0.0

01

4

0.0

01

5

0.0

01

6

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

y = -9251.9x - 0.6908

R² = 0.9952

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

0.0

01

6

0.0

01

65

ln(-

ln(1

-x)/

T2)

1/T

y = -8191.7x - 2.3196

R² = 0.997

-16.5-16

-15.5-15

-14.5-14

-13.5-13

-12.5-12

0.0

01

25

0.0

01

3

0.0

01

35

0.0

01

4

0.0

01

45

0.0

01

5

0.0

01

55

0.0

01

6

0.0

01

65

ln(3

[1-(

1-X

)(1/3

) ]/T

2)

1/T

118

Table 4.7: Calculated kinetic parameters for char combustion step

Sample ID Model Heating Rate A E (KJ/Mol)

GT-01-443

Volumetric

20°C/min 3.412×106 138.59

30°C/min 3.929×104 113.44

40°C/min 2.552×103 97.60

Grain

20°C/min 2.051×105 123.17

30°C/min 3.455×103 100.04

40°C/min 2.887×102 85.58

GT-01-493

Volumetric

20°C/min 1.219×104 97.34

30°C/min 7.274×101 57.84

40°C/min 1.855×102 76.92

Grain

20°C/min 1.913×103 87.40

30°C/min 2.291×101 51.55

40°C/min 3.221×101 68.11

4.5.1 Rate constant “k” for the combustion reaction

The rate constant “k” was calculated from Eq. 4.3 for the combustion step at different

heating rates from both VM and GM models at constant temperature i.e., 800°C, as

shown in Fig. 4.17 From the figure, it was revealed that the rate constant is decreasing

with increasing heating rate for both the samples. GT-01-443 shows higher rate constant

as compared to GT-01-493 and a probable reason is of higher carbon and less moisture

content in that sample (Konttinen et al., 2012). The heating rate has a vital impact on

rate constant values as lower heating rate means high residence time and hence higher

rate constant similar to findings of Sarwar et al. (2014).

119

(a) Volumetric Model @800°C (a) Grain Model @800°C

Fig. 4.17: Rate constant (k) for combustion step with (a) VM and (b) GM

4.6 RESULTS FOR COAL GASIFICATION KINETICS AT

ATMOSPHERIC AND ELEVATED PRESSURE

As mentioned earlier in this chapter that three lignite char samples having IDs KTN-

GT01-123, KTN-GT01-140 and KTN-GT-02-627 (As per list is given in Table 4.1)

were tested in PTGA (Thermax 500) at three pressures 1 atm, 5 atm, and 10 atm

respectively. For ease in writing the samples, IDs were marked as S1, S2, and S3 for

GT01-123, KTN-GT01-140, and KTN-GT-02-627 respectively. The external diffusion

due to high pressures as explained by Wang et al. (2008) was minimized by testing the

different flow rates of CO2 and H2O at 5 atm and 10 atm. The coal char conversion has

calculated, using Eq. 4.1 and the results are plotted in Fig. 4.18. It is observed that the

conversion of char started at from 750 to 800°C for all the samples with both reacting

gases i.e. CO2 and H2O at 1 atm pressure. This range is similar to other studies

(Tomaszewicz et al., 2013, Lin and Strand, 2013, Chen et al., 2013a). At higher

pressures, the char conversion is little earlier than atmospheric pressure following the

findings by Wang et al. (2008). Fig. 4.18 shows that the increasing pressure slightly

increases the apparent reaction rate which confirms the observations of earlier studies

(Park and Ahn, 2007, Wang et al., 2008, Fermoso et al., 2009, Botero et al., 2013). The

fundamental reason for this behavior is the increase of density of reacting gases at

0

2

4

6

8

10

12

14

16


Ln

(k)

Heating Rate

GT-01-443

0

2

4

6

8

10

12

14


Ln

(k)

Heating Rate

GT-01-443

120

higher pressures due to which it diffuses faster into the solid char particle and enhance

the reactivity with it.

(a) S1_CO2 (b) S1_H2O

(c) S2_CO2 (d) S2_H2O

(e) S3_CO2

(f) S3_H2O

Fig. 4.18: Conversion of Char samples against Temperature at different

pressures for CO2 and H2O reacting gases

121

It is observed from Fig. 4.18 (a – f) that the slope of the curve, after the initiation of

reactions, at atmospheric pressure is less than the slopes of curves at higher pressures

(5 atm and 10 atm). It means the overall rate of reaction, at higher pressure, is higher

and starts little earlier than atmospheric pressure which confirms the previous findings

(Wang et al., 2008, Park and Ahn, 2007).

The kinetic modeling and Arrhenius parameters (A and E) calculations were carried out

by the same standard models i.e., Volumetric Model (VM) and Grain Model (VM), as

already applied for drying, devolatization, and combustion steps. Here for the

gasification reactions, two solution strategies, Integral Method and Direct Plot Method

were used for both models.

4.6.1 Least square regression analysis for Char+CO2 reactions

Char reactions with CO2 have been evaluated on the basis of volumetric and grain

model using equations 4.15, 4.16, 4.19 and 4.20. The representative data for selected

samples are presented in Fig. 4.19, 4.20 and 4.21 (a-d) for the Char-CO2 reactions with

respect to 1/T. Linear regression analysis has been done through data points in the range

of 10-90% conversion, as shown in Fig. 4.19 to 4.21. It is observed from these graphs

that at atmospheric pressure both the models with each method show a good linearity

with R2>0.99. At higher pressures (5 atm and 10 atm) the direct method gives linearity

in satisfactory limits, whereas, the integral method shows a good linearity. All the

samples show good linearity with either model at any pressure but Volumetric Model

with the integral method shown best performance with R2>0.999.

(a) CO2_Volumetric_Direct

(b) CO2_Volumetric_Integral

122

(c) CO2_Grain_Direct

(d) CO2__Grain_Integral

Fig. 4.19: Least square regression analysis of Volumetric and Grain Models for








123







From the above discussion, it can be concluded that the integral method gives better

results for both the models i.e., Volumetric and Grain Models at atmospheric as well

higher pressures. The calculated Arrhenius parameters are summarized in Table 4.8

which is in good agreements with previous studies (Blackwood and Ingeme, 1960, Sha

et al., 1990, Yun and Lee, 1999, Park and Ahn, 2007, Wang et al., 2008, Long et al.,

2012).

124

Table 4.8: Arrhenius Parameters for Char+CO2

Samples Pressure Arrhenius

Parameters

Volumetric Model Grain Model

Direct Integral Direct Integral

S1

1 atm

A 2.181×109 2.729×106 3.027×106 1.078×105

E (KJ/mol) 251.9308 274.0045 189.9 244.6395

R2 0.9945 0.9982 0.9648 0.9927

5 atm

A 2.657×109 4.329×107 1.116×106 9.859×105

E (KJ/mol) 267.47 294.997 195.396 261.101

R2 0.9895 0.9999 0.9382 0.9974

10 atm

A 8.677×109 7.849×108 2.134×107 3.992×107

E (KJ/mol) 269.581 311.999 215.84 286.135

R2 0.996 0.9999 0.9795 0.9991

S2

1 atm

A 1.134×1011 1.906×109 1.069×107 2.174×107

E (KJ/mol) 272.816 317.6031 189.343 278.5356

R2 0.9724 0.9964 0.8703 0.9884

5 atm

A 9.202 ×1010 3.145×1010 1.936×106 1.832×108

E (KJ/mol) 282.917 336.451 187.747 292.254

R2 0.9936 0.9999 0.9008 0.9963

10 atm

A 1.385×1012 9.939×1011 3.923×107 6.509×109

E (KJ/mol) 298.19 356.671 208.399 314.652

R2 0.995 0.999 0.9272 0.9971

S3

1 atm

A 3.951×1021 7.99×107 9.848×1017 1.44×106

E (KJ/mol) 488.7884 280.9134 416.4316 247.0339

R2 0.9904 0.9959 0.9968 0.9895

5 atm

A 2.701×1010 1.399×109 1.077×107 3.083×107

E (KJ/mol) 264.701 300.235 197.956 268.675

R2 0.9919 0.9999 0.9484 0.9979

10 atm

A 9.671×1010 4.527×1010 7.232×106 4.674×108

E (KJ/mol) 284.854 341.339 200.933 302.048

R2 0.9597 0.9999 0.8648 0.997

4.6.2 Least square regression analysis for Char+H2O reaction

Char reactions with H2O have been evaluated on the basis of Volumetric and Grain

Models using equations 4.15, 4.16, 4.19 and 4.20. Evaluated values of char reactions

are plotted against 1/T, as shown in Fig. 4.22, 5.23 and 5.24 (a-d) for selected samples.

Best fit straight lines are also plotted, through the data points, in all graphs.

125

(a) H2O_Volumetric_Direct

(b) H2O _Volumetric_Integral

(c) H2O _Grain_Direct

(d) H2O _Grain_Integral



126







127







It is observed from the analysis, that the reactions of char with steam (H2O), for both

the models, with each solution method, show a satisfactory linearity at all pressures in

all samples. As discussed earlier the Integral method has given good linearity with all

the cases. Volumetric Model with integral method proved best for the reactivity

calculations as this model R2>0.999.

From the above discussion, it can be concluded that the integral technique yields better

results for both the models i.e, volumetric and grain models at atmospheric as well

higher pressures. The calculated Arrhenius Parameters for char+H2O are summarized

in Table 4.9 which are in good resemblance from literature (Hecht et al., 2012, Bryan

Woodruff and Weimer, 2013, Kajitani et al., 2013). The calculated values of A and E

for steam gasification of char are in similar range as calculated by Jaffri and Zhang

(Jaffri and Zhang, 2009) for Thar lignite.

128

Fig. 4.25 (a to f) shows a representative comparison of experimental data for sample 1

with calculated data from volumetric and grain models using the integral method. Good

predictions of char conversion have observed at all pressures for both models. Hence

the validity of the volumetric model and grain model confirms for the samples selected

in this research.

Table 4.9: Arrhenius Parameters for Char+H2O

Samples Pressure Arrhenius

Parameters

Volumetric Model Grain Model

Direct Integral Direct Integral

S1

1 atm

A 2.134×107 4.172×104 2.792×104 1.626×103

E (KJ/mol) 204.0588 231.4701 142.3773 202.5872

R2 0.9919 0.9963 0.9306 0.9867

5 atm

A 1.918×1010 7.555×108 8.304×106 1.824×107

E (KJ/mol) 284.488452 320.388304 213.985732 287.032536

R2 0.998 0.9999 0.9651 0.9979

10 atm

A 7.228×1012 1.540×1012 9.819×108 2.069×1010

E (KJ/mol) 339.244456 390.999106 256.977426 352.164412

R2 0.9972 0.9999 0.9574 0.9977

S2

1 atm

A 8.024×1010 8.301×1011 2.112×106 4.467×109

E (KJ/mol) 287.881 375.9923 192.2031 329.542

R2 0.9628 0.9403 0.9628 0.9139

5 atm

A 3.015×1013 5.897×1013 5.510×108 3.188×1011

E (KJ/mol) 327.438576 395.339014 232.966594 351.191674

R2 0.9893 0.9999 0.9186 0.9969

10 atm

A 5.047×1014 1.156×1015 2.696×109 3.485×1012

E (KJ/mol) 357.950956 428.345594 250.783496 378.220488

R2 0.9856 0.9999 0.9247 0.997

S3

1 atm

A 2.274×1010 4.567×105 1.014×108 3.139×104

E (KJ/mol) 275.56753 240.2247 228.186 217.6522

R2 0.979 0.9871 0.9316 0.9913

5 atm

A 1.775×109 1.337×107 2.892×106 3.334×105

E (KJ/mol) 264.401828 284.613162 205.098066 251.565012

R2 0.9904 0.9999 0.9628 0.9971

10 atm

A 6.935×1011 2.130×1010 1.273×108 3.302×108

E (KJ/mol) 294.56502 325.343448 220.853096 290.624184

R2 0.997 0.9515 0.8992 0.9202

129

(a) S1_CO2_1 atm

(b) S1_H2O_1 atm

(c) S1_CO2_5 atm

(d) S1_H2O_5 atm

(e) S1_CO2_10 atm

(f) S1_H2O_10 atm

Fig. 4.25: Comparison of experimental and predicted conversion for Char-

CO2/H2O reactions of sample S1

4.6.3 Rate constant “k” for gasification reactions

Similar to drying, devolatization and combustion steps, the rate constant “k” was

calculated for gasification reactions i.e., reactions of char with CO2 and H2O. The

130

values of A and E calculated through integral method (see Table 4.8 and 4.9) were used

in the calculation of rate constant (k). The calculations were made at a fixed temperature

of 950°C. Fig. 4.26 shows the calculated values of ln(k) for Char+CO2 reaction with all

three samples i.e., S1, S2, and S3 at 1, 5 and 10 atm pressures. Similarly Fig. 4.27 shows

the calculated values of ln(k) for Char+H2O reaction with all three samples i.e., S1, S2,

and S3 at 1, 5 and 10 atm pressures. From these figures, it was observed that with

increasing pressure the rate constant is increasing, with both the models. The amount

of increase is highly dependent on the nature of the sample due to variation in its

composition as explained by (Fan et al., 2012). Overall it was observed that the rate

constant of the Char+CO2 reaction is higher than Char+H2O reaction at the respective

sample and pressure conditions in accordance with previous studies (Fan et al., 2012,

Kajitani et al., 2013, Kirtania et al., 2014).

(a) Volumetric Model (Integral) @950°C (b) Grain Model (Integral) @950°C

Fig. 4.26: Rate constant (k) for Char +CO2 reaction with (a) VM and (b) GM

(a) Volumetric Model (Integral) @950°C (b) Grain Model (Integral) @950°C

Fig. 4.27: Rate constant (k) for Char +H2O reaction with (a) VM and (b) GM

0

5

10

15

20

25

30

S1 S2 S3

Ln

(k)

Different Samples

1 atm 5 atm10 atm

0

5

10

15

20

25

S1 S2 S3

Ln

(k)

Different Samples

1 atm 5 atm

0

5

10

15

20

25

30

35

40

S1 S2 S3

Ln (

k)

Different Samples

1 atm 5 atm 10 atm

0

5

10

15

20

25

30

35

S1 S2 S3

Ln

(k)

Different Samples

1 atm 5 atm

131

4.7 EFFECT OF PRESSURE ON GASIFICATION KINETIC

PARAMETERS

Reaction study of Thar lignite char with CO2 and H2O reveals that the frequency factor

‘A’ and activation energy ‘E’ increase on increasing the pressure as shown in Fig. 4.28

(a-c). This is in accordance with earlier work (Wang et al., 2008, Park and Ahn, 2007).

Increase in frequency factor indicates the higher rate of reaction and this can be verified

from the increasing trend in the slope of reactivity curves at higher pressures (Fig. 4.18).

High Activation Energy means the reactions require more heat energy to initiate and it

can be verified from the reactivity curves, which clearly indicate that the reaction of

char with CO2 and H2O starts at higher temperatures as pressure is increased but due to

higher frequency factor at that pressure the conversion archives faster than lower

pressure.

132

(a) Sample S1

(b) Sample S2

(c) Sample S3

Fig. 4.28: Effect of pressure on the frequency factor (A) and activation energy

(E)

133

4.8 SUMMARY OF EXPERIMENTAL RESULTS

• The proximate and ultimate analysis of collected samples confirms the composition

of Thar lignite as reported in the literature.

• The Volumetric Model and Grain Model fit well for Moisture Removal,

Devolatization, Combustion and Gasification Processes.

• The Devolatization step was broken into low and high-temperature ranges to

increase the linearization of the models. The low-temperature range was selected

from room temp. to 500°C whereas high-temperature range was from 501 - 900°C.

• At atmospheric pressure, the inherent kinetics of Char-CO2 or Char-H2O reactions

is dominant and drives the overall reaction. At high pressures, the diffusion of

reacting gas in solid particle plays an important role and diffusion rate is dominant

over inherent kinetics of those heterogeneous reactions.

• Volumetric model and grain model have shown satisfactory results in the reactivity

study of Char-CO2 or Char-H2O reactions at atmospheric or high pressures with the

integral method. Among both Volumetric model remained best for Thar lignite

samples at all the pressures.

• Increasing pressures affect the overall kinetics of the heterogeneous reactions in

terms of increase in frequency factor and activation energy.

• Overall it was observed that the rate constant of the Char+CO2 reaction is higher

than Char+H2O reaction at the respective sample and pressure conditions

• The frequency factor (A) and activation energy (E) for moisture removal,

devolatization and combustion, and gasification steps were calculated from

Volumetric Model and Grain Model. This is important data and could be utilized as

a base for future work.

134

CHAPTER 5

CFD MODELING AND SIMULATION

5.1 INTRODUCTION

For a successful and efficient coal gasification, a deep understanding of all physical and

chemical changes involved in the process is required. Computational Fluid Dynamics

(CFD) provides an easy, cost-effective and reliable way, to study the effect of various

controlling parameters, like coal composition, oxidant to fuel ratio, residence time of

fuel particles in the system, temperature and pressure of the gasifier, rate of chemical

reactions, etc. on the gasification process. Since last 20 years, the CFD has been an

effective tool to simulate and visualize the gasification process (Chen et al., 2001,

Tominaga et al., 2000, Wu et al., 2008b). But unfortunately, this robust tool has not

been utilized for designing a gasification technology which suits the characteristics of

indigenous coal. Hence in the present research, an attempt is made to develop a generic

gasification technology using CFD that gives maximum efficiency with local coal. It is

observed from literature that Entrained flow gasifiers with multi-opposite burners are

efficient even for low-grade coals. So this type of gasifier is selected for our case.

In this study, there are two phases of CFD modeling and simulation. In the first phase,

the CFD model was developed for lab scale double stage entrained flow coal gasifier

with multiple opposite burners. The numerical simulations were performed on the

Chinese coal data and results were verified from published experimental work. In the

second phase, the modified geometry was modeled using CFD techniques for Thar coal

data. The kinetic parameters for Thar Coal Gasification were taken from the

experimental work on TGA and PTGA as discussed in Chapter 4. The commercial CFD

code ANSYS FLUENT®14.0 was used for all computations. The validation for CFD

modeling results with Thar Coal and modified geometry was carried out with the model

developed in AspnPlus®V10 software following the strategy described by Xiangdong

et al. (2013b). The individual geometries (computational domains) and numerical

setup along with boundary conditions are discussed in the subsequent sections.

135

5.2 CFD MODELING OF CHINESE COAL GASIFIER

To understand the strategies of modeling of an entrained flow gasifier, the Chinese

double stage entrained flow gasifier was selected to simulate numerically. The physical

description of the system and mathematical models used are described as under.

5.2.1 Description of Physical system for Chinese Coal Gasifier

An oxygen-blown, entrained flow coal gasifier for Chinese high-ash lignite coal is used

in this study as shown Fig. 5.1. The proximate, ultimate analysis of coal used in the

model is listed in Table 5.1. The particle diameters are fitted to the Rosin-Rammler

distribution (minimum dia is 4μm, maximum dia is 125μm, and mean dia 45.6 μm).

The dimensions and coal feeding rate (7.2kg coal/hour) are taken from earlier research

work (Tang et al., 2010c). The inner radius of the experimental gasifier is 0.32 m and

its height is 0.55 m. Four nozzles are installed at two levels of gasifier chamber named

as AA’ Level (Top) and BB’ Level (Bottom). Two opposite nozzles, exactly at the

central axis line for each level, are impinging in nature whereas the other two nozzles

are slightly at a side of the axis and produced a swirl in the flow.

Table 5.1: Properties Of Chinese Coal (Tang et al., 2010c)

Proximate Analysis (w/%) Ultimate Analysis (war/%)

Moisture 1.16 C 63.51

Ash 23.10 H 4.19

Volatile 27.03 O 6.39

Fixed Carbon 48.21 N 1.02

S 0.63

Heating Value 32.91 MJ/Kg

136

Fig. 5.1: (Left) Geometry of Gasifier with main inlets and outlets. (Right) The

Sections of gasifier at AA’ and BB’ level

5.2.2 Development of computational domain for Chinese gasifier

A 3D computational domain was developed in Ansys Meshing®14.0 with a total of

135934 tetrahedral cells. Near the burners, the high density of the grid was applied due

to high turbulence and combustion zones. Fig. 5.2 shows the developed grid and

zoomed view of nozzles where a high density of grid was applied. The minimum

orthogonal quality of the grid was 0.64.

137

Fig. 5.2: Meshed computational domain of Chinese Coal Gasifier (Geometry-A)

5.2.3 Computational Models

In present work, a numerical study was carried out with 3D, steady and incompressible

turbulence flow with heterogeneous and homogeneous reactions. Therefore, time-

averaged- steady-state Navier–Stokes, mass momentum and energy and species

equations have solved. The governing equations for the system are as follows (Silaen

and Wang, 2010).

mij

i

Sux

=

)(

jjiij

ii

jji

i

Suuxx

Pguu

x+−

+

−=

)()(

_____''

hip

ii

ip

i

STucx

T

xTuc

x++

−

=

_____' ')(

rji

i

j

i

i

ji

i

SCux

CD

xCu

x+

−

=

_____' ')(

where ij is the symmetric stress tensor and

_____''

jiuu is the Reynolds stress. The

realizable k –ε turbulence model is also employed to solve the turbulent flow. The

turbulence kinematic viscosity is calculated by:

(5.1)

(5.3)

(5.2)

(5.4)

138

/2kCt =

Where, C is the viscosity constant, k the turbulence kinetic energy and ε the turbulence

dissipation rate. K and ε are calculated from the following transport equations (Jones

and Launder, 1972, Launder and Spalding, 1974):

−+

+

=

k

ik

t

i

i

i

Gx

k

xku

x)(

kGC

kGC

xxu

xkk

i

t

i

i

i

2

21)(

−+

+

=

where Gk is the generation of turbulence kinetic energy due to mean velocity gradients.

The turbulence heat conductivity(λ) and diffusion coefficient(D) in Eqs. (5.3) and (5.4)

are given by:

it

tp

i

ipx

Tc

x

TTuc

=

−=

Pr'

_____'

i

j

t

t

i

j

ijix

C

Scx

CDCu

−=

−=

_____' '

Where, Prt (=0.85) is the turbulence Prandtl number and Sct (=0.7) is the turbulence

Schmidt number.

The Lagrangian approach has been used to calculate the motion of particles by using

the Discrete Phase Model (DPM). The trajectories of coal particles are predicted in

DPM by integrating the force balance on the coal particle when they move through the

continuous phase of the fluid (Silaen and Wang, 2010). This force balance equates the

coal particle inertia with the forces acting on the coal particle and can be written (for

the x-direction in Cartesian coordinates) as.

x

p

p

xpD

pFguuF

dt

du+

−+−=

)(

(5.5)

(5.6)

(5.8)

(5.9)

(5.10)

(5.7)

139

The interaction between the discrete phase and the continuous phase is also taken into

account by treating the heat and mass losses of the particles as the source terms in the

governing equations. The P-1 model has adapted to calculate the radiant heat in the

gasifier (Wu et al., 2008b, Gerun et al., 2008). In the P1 model, the local radiation

intensity is calculated by the equation

44 TaGaGqr −=−

Where

GCa

qss

r −+

−= )(3

1

Where a is the absorption coefficient, σs is the scattering coefficient, G is the incident

radiation, C is the linear anisotropic phase function coefficient, and σ is the Stefan-

Boltzmann constant.

5.2.4 Combustion/ Gasification Model

Species transport model (Eq. 5.4) is used for the gasification reactions chemistry. This

modeling approach gives an option to define the important reactions and set their kinetic

parameters. During the high-temperature environment of coal gasification, the coal will

be decomposed into volatiles, char, and ash (Chen et al., 2007). The compositions

released from the coal can be expressed by the following equilibrium equation (Wen

and Chaung, 1979).

Coal ➔ α1Volatiles + α2 H2O + α3Char + α4Ash

After undergoing fast heating, the hot flow around coal particles will trigger a number

of physical and chemical reactions (Du et al., 2007). The reactions include the

devolatilization of coal, the combustion of volatiles and unburned char as well as the

gasification of the char. In present work, the volatiles are lumped into one volatile gas

species C1.37H4.58O0.44 and are calculated from proximate and ultimate analysis of coal.

The volatiles release is described by a two-step devolatilization model (Du and Chen,

2006) and it is given as follows:

VolatileYCharYCoal lll

kl

+−→ )1( (for low temperature)

(5.11)

(5.12)

(5.13)

(5.14)

140

VolatileYCharYCoal hhh

kh

+−→ )1( (for high temperature)

Where Y is the stoichiometric coefficient. At low-temperature Eq. (5.14) is dominated

whereas Eq. (5.15) shows a higher rate at high temperature. The reaction kinetic

equations are as follows:

CoalYkYkdt

dVhhl )( 1 +=

)/exp( plll RTEAk −=

)/exp( phhh RTEAk −=

Where V denotes the mass fraction of volatiles, k is the reaction rate constant, A the pre-

exponential factor, TP the coal particle temperature and E the activation energy of the

reaction. The values of Yl, kl, Yh, kh, El and Eh are obtained from previous studies (Du

and Chen, 2006, Ubhayakar et al., 1977) and they are listed in Table 5.2.

When char is produced from coal devolatilization, CO and H2 can be generated from

char gasification. There are various reactions selected by different researchers to define

the gasification reaction mechanism (Silaen and Wang, 2010, Vicente et al., 2003,

Gerun et al., 2008, Bouma et al., 1999, Choi et al., 2001a, Watanabe and Otaka, 2006,

Fletcher et al., 2000, Ajilkumar et al., 2009, Silaen and Wang, 2012, Chui et al., 2009b).

First, the preliminary simulations were carried out to find the best reaction-plan among

different reactions. The details of those cases are tabulated in Table 5.2.

The various reaction mechanisms are modeled to involve the following chemical

species: C(s), O2, N2, CO, CO2, H2O, H2, and Volatiles. Finite rate/Eddy dissipation rate

model has been used to calculate the rate of formation for each species and update the

source term rS in Eq. (5. 4) by the following expression.

=

=N

j

rjjr wMS1

,

( )

−−=

==

rr N

ieq

N

i

frjrjrj CK

Ckvvw11

'

,

''

,,

''''

][1

][

)/( RTEB

faeATk

−=

(5.16)

(5.17)

(5.18)

(5.15)

(5.19)

(5.20)

(5.21)

141

In the above equation, the forward reaction rate constant fk is established based on

Arrhenius law; A is the pre-exponential factor, B the temperature exponent and Ea the

activation energy of the reaction. The values of A, B, and Ea for various reactions are

obtained from earlier studies and given in Table 5.3.

Table 5.2: Various preliminary cases to optimize the best reaction plan

Reactions Simulation Cases

A B C D E F Vol + 2.295 O2 → 1.37 CO2 + 2.29 H2O

(Volatiles complete Combustion)

Vol + 1.61 O2 → 1.37 CO + 2.29 H2O

(Volatiles partial Combustion)

C<s> + 0.5 O2 → CO (Char partial combustion)

C<s> + O2 → CO2 (Char complete combustion)

C<s> + CO2 → 2CO (Gasification, Boudourad reaction)

C<s> + H2O → CO + H2 (Gasification)

CO + 0.5 O2 → CO2 (CO combustion)

H2 + 0.5 O2 → H2O (H2 combustion)

CO + H2O ↔ CO2 + H2 (Watershif Reaction)

Table 5.3: Selected kinetic parameters for Devolatization and

Gasification/Combustion Reactions

Devolatization (Du and Chen, 2006, Ubhayakar et al., 1977) Yl ; Yh 0.3; 1

kl ; kh (s-1) 2 ×105; 1.3 ×107

El; Eh (KJ mol-1) 104.6; 167.4

Combustion/Gasification Reactions (Chen et al., 2012) A B Ea(J Kmol-1)

Heterogeneous Reactions (Solid-Gas Phase) C(s) + O2 → CO2 0.002 0 7.9×107

C(s) + 0.5O2 → CO 0.052 0 6.1×107

C(s) + CO2 → 2CO 242 0 2.75×108

C(s) + H2O → CO + H2 426 0 3.16×108

Homogeneous Reaction (Gas Phase) CO + 0.5O2 → CO2 2.239×1012 0 1.7×108

H2 + 0.5 O2 → H2O 6.8×1015 0 1.68×108

CO +H2O ↔ CO2+ H2 (WGS Reaction) f 2.75×1010 0 8.38×107

b 2.65×10-2 0 3.96×103

C1.37H4.58O0.44(Volatile)+1.61 O2→1.37 CO+2.29 H2O

(Volatile Partial Combustion)

2.119×1011 0 2.027×108

C1.37H4.58O0.44(Volatile)+2.295 O2→1.37 CO2+2.29 H2O

(Volatile Complete Combustion)

2.119×1011 0 2.199×1011

142

5.2.5 Boundary Conditions and Calculation Methods

The mass-flow inlet and pressure-outlet boundary conditions were used for all

input/output streams. Buoyancy force has considered in the present model. Water

cooled walls were assumed to be at constant temperature at 800 K. No-slip state (zero

velocity) is applied on the surfaces of walls. Steady-state simulations were carried out

with an implicit pressure-correction scheme (pressure-based solver) by decoupling

energy and momentum equations. For coupling the velocity and pressure, a SIMPLE

algorithm was followed. Convective terms were spatially discredited by the second-

order-upwind scheme. The values for temperature dependent properties were calculated

using piecewise-polynomial equations for all gas and solid species. Convergence of the

solution was achieved when the mass, turbulent kinetic energy and momentum

residuals satisfied at 10-3 and residuals for energy and radiation at 10-6. Parallel

processing was used for computation. The cold flow simulations were converged first

in all the cases then the reacting flows were solved by activating all reactions along

with the injection of coal with an ignition temperature of 2000 K.

5.3 CFD MODELING OF NEWLY DESIGNED COAL GASIFIER

After the simulations on Chinese coal gasifier with Chinese coal data, the geometry of

the gasifier was modified. The details of geometry and other physical features are

discussed in the following section.

5.3.1 Description of Physical system for Newly Designed Gasifier

The geometry of oxygen-blown, entrained flow coal gasifier with multiple opposite

burners (shown in Fig. 5.1) was modified by implying a throat section. The basic reason

for doing this is to increase the residence time of the coal particle in the upper section.

Fig. 5.3 shows the 3 views of new geometry along with its dimension. The proximate,

ultimate analysis of Thar coal was used in the model and given in previous Chapter

(Table 4.4 and 4.5). The particle diameters are fitted to the Rosin-Rammler distribution

(minimum dia is 4μm, maximum dia is 125μm, and mean dia 45.6 μm).

143

Fig. 5.3: Different views of newly designed gasifier operating conditions

5.3.2 Development of Computational Domain for Proposed Geometry

3D computational domain was developed in Ansys Meshing®14.0 with a total of

126275 tetrahedral cells. Similar to the previous geometry case, near the burners the

high density of grid was applied due to high turbulence and combustion zones. Fig. 5.4

shows the developed grid and zoomed view of nozzles where a high density of grid was

applied. The minimum orthogonal quality of grid was achieved 0.53 which usually

come in excellent criteria.

Fig. 5.4: Meshed computational domain of Proposed Gasifier (Geometry-B)

144

5.3.3 Probability Density Function (PDF) approach

Under species modeling approach as discussed in section 5.2.4, there are few other

options available to calculate the species chemistry apart from the chemical reactions.

Among those Non-premixed modeling is prominent which utilize Probability Density

Function (PDF) approach. Non-premixed modeling involves the solution of transport

equations for one or two conserved scalars (the mixture fractions). Equations for

individual species are not solved. Instead, species concentrations are derived from the

predicted mixture fraction fields. The thermochemistry calculations are preprocessed

and then tabulated for look-up in ANSYS FLUENT. Interaction of turbulence and

chemistry is accounted for with an assumed-shape Probability Density Function (PDF)

(Ansys, 2011).

The basis of the non-premixed modeling approach is that under a certain set of

simplifying assumptions, the instantaneous thermochemical state of the fluid is related

to a conserved scalar quantity known as the mixture fraction, f. The mixture fraction

can be written in terms of the atomic mass fraction as (Sivathanu and Faeth, 1990)

oxifueli

oxii

ZZ

ZZf

,,

,

−

−=

Where Zi is the elemental mass fraction for the element, i. The subscript ‘ox’ denotes

the value at the oxidizer stream inlet and the subscript ‘fuel’ denotes the value at the

fuel stream inlet. In this research, few simulations are carried out using the PDF

approach with geometry B. Then the results are compared with species transport

approach.

5.4 MODEL DEVELOPMENT IN ASPEN PLUS®V10 FOR VALIDATION

OF MODIFIED GEOMETRY RESULTS

The CFD modeling results with modified geometry and Thar coal feedstock were

verified and validated by conducting a comparative study between CFD results and the

results extracted from the model developed in Aspen Plus®V10 software. The modeled

entrained flow gasifier in Aspen Plus was initially developed by Aspen (2010) and then

(5.22)

145

modified by Xiangdong et al. (2013b). The following are modified from previous work

(Aspen, 2010, Xiangdong et al., 2013b).

i. The properties of components like the composition of feed coal along with its

calorific value.

ii. The composition and quantity of pyrolysis gases, char, and tars produced during

devolatilization stage.

iii. Gasification operating and design parameters like flowrates of coal, oxygen and

steam, temperature and pressure, the diameter of gasifier and height of gasifier.

iv. Indigenous kinetics of reactions was inserted through FORTRAN coding.

The chemical species present in the process are tabulated in Table 5.4. In the list C6H6

represents tar, Char 1 represents the pyrolyzed coal at 1 atm whereas the Char2 is the

corrected solid phase of coal. The composition of Thar lignite (calculate from this

research) was inserted through the Coal stream.

Table 5.4: The chemical species used in the model

Symbol Category Title

CO2 Conventional Carbon Dioxide

CO Conventional Carbon monoxide

H2 Conventional Hydrogen

O2 Conventional Oxygen

N2 Conventional Nitrogen

H2S Conventional Hydrogen sulfide

H2O Conventional Water

CH4 Conventional Methane

C6H6* Conventional Benzene

C Solid Carbon graphite

S Solid Sulfur

COAL Non-Conventional -----

CHAR1* Non-Conventional -----

CHAR2* Non-Conventional -----

ASH Non-Conventional -----

Fig. 5.5 displays the model of the gasification process in terms of a process flow

diagram created in Aspen Plus®V10. The quenching system of hot gases released from

146

the gasifier is not included in the developed model. Table 5.5 highlights important

blocks used in the model along with its core function. The process of coal pyrolysis is

simulated through PYROLYS and PRESCORR blocks. The combustion process of

volatiles is modeled by COMBUST block. The block of GASIFIER is used to simulate

the gasification process of char. Rest of blocks are helping aid in the construction of

whole simulation process.

Fig. 5.5: The process flow sheet for the simulation model of Entrained Flow

Gasifier (Aspen, 2010, Xiangdong et al., 2013b)

As per general facts, gasification process is based on three fundamental stages, so

accordingly during modeling of the gasification process in Aspen Plus, these three

stages are simulated via three kinds of unit operations. These are explained as follows:

5.4.1 Pyrolysis of Coal

PRESCORR and PYROLYS blocks are two RYield reactors which are used in the

current model to simulate the process of coal pyrolysis. The PYROLYS block being

first RYield reactor is simulating the pyrolysis of coal at 1 atm based on devolatization

experimental work. Then PRECORR block (second RYield reactor) is correcting the

obtained yield of individual species released during pyrolysis on the basis of operating

pressure of gasifier. A user-defined function USPRES is automatically correct the yield

calculations within the model.

147

5.4.2 Combustion of Volatiles

The combustion kinetics of volatiles is not considered in the model as the time-scale of

gas combustion is too short and all the combustible gasses converted in a shorter time.

The combustion of volatiles is simulated through the COMBUST block based on

RStoic reactor tool available in Aspen Plus by setting fractional conversions at 1.0 for

all combustible gases.

5.4.3 Char gasification

The process of char gasification is modeled using RPlug reactor and the block named

as GASIFIER. This block performs the gasification reaction calculations based on two

important factors i.e., gasification reaction kinetics (which has been calculated in during

experimental work in present research) and residence time of char within the gasifier.

Based on these two factors, the composition and yield of syngas are calculated.

TABLE 5.5: Functions of each block used in Aspen Plus®V10 Model

Model/Tool Block Function of Block

RYield PYROLYS The pyrolysis of coal is simulated based on experimental

work for devolatization at atmospheric conditions (1 atm).

RYield PRESCORR Pressure corrections are being made in this block for the

yield and composition of volatiles produced during

pyrolysis from 1atm to gasifier’s operating pressure.

Sep2 SEPSG Solid char and gases are being separated in this block

RStoic COMBUST The combustion of violates is simulated through this block

RStoic SEPELEM Char species is decomposed into its elementary

components (like C, H2, O2, N2, S, and ash) for defining

heterogeneous reactions easily in GASIFIER block.

Mixer MIXER The feedstock is being mixed for inserting in the

GASIFIER block.

RPlug GASIFIER Model the char gasification process

Calculator SPELMCAL Stoichiometric coefficients of C, O2, H2, S, N2, and ash are

determined for solving reaction in SEPELEM block.

Calculator GASIFCAL Calculate the residence time of solid char which is then

used in GASIFIER block.

148

5.5 MODELING AND SIMULATION RESULTS FOR CHINESE LIGNITE

After the ending of experimental investigations, the second phase of research was

started in which the performance of entrained flow gasifier with multiple burners

(injectors) was evaluated through numerical simulations using commercial CFD code

ANSYS FLUENT®14.0. The reaction and fluid dynamics was calculated with the help

of numerical computations. For making best CFD modeling strategy, initially, a

published experimental work was selected on a working entrained flow coal gasifier

with multiple burners at China whose detailed physical description along with geometry

is given in section 5.2 (Fig. 5.1). The Chinese coal was used as feedstock of the gasifier.

The properties of Chinese coal are given in Table 5.1. The gasifier is a double stage,

where the upper stage is referred as AA’ level and the lower level is referred as BB’

level (Fig. 5.1).

Various cases were simulated by varying the mass fraction (%) of total coal or total

oxidant at both injection levels. The effects of coal and oxidant distributions for two

stages were investigated by maintaining the overall oxygen/coal ratio at a constant

value of 1 as per available experimental condition (Tang et al., 2010c). The names of

simulation cases referred to the percentages of coal and oxygen at top level (AA'-

Level). Table 5.6 describes parameters of simulated conditions of all cases.

149

Table 5.6: Operated parameters for Simulated Cases

Case

Name

1st Level (Upper Level, AA'-Level) 2nd Level (Down Level, BB'-Level)

Fraction

of Total

Coal

Mass

Flow of

Coal

Fraction

of Total

Oxygen

Mass

Flow of

Oxygen

Fraction

of Total

Coal

Mass

Flow of

Coal

% of

Total

Oxygen

Fraction

of Total

Oxygen

% kg/sec % kg/sec % kg/sec % kg/sec

C30_O40 30 0.0006 40 0.0008 70 0.0014 60 0.0012

C40_O40 40 0.0008 40 0.0008 60 0.0012 60 0.0012

C50_O40 50 0.001 40 0.0008 50 0.001 60 0.0012

C60_O40 60 0.0012 40 0.0008 40 0.0008 60 0.0012

C70_O40 70 0.0014 40 0.0008 30 0.0006 60 0.0012

C30_O50 30 0.0006 50 0.001 70 0.0014 50 0.001

C40_O50 40 0.0008 50 0.001 60 0.0012 50 0.001

C50_O50 50 0.001 50 0.001 50 0.001 50 0.001

C60_O50 60 0.0012 50 0.001 40 0.0008 50 0.001

C70_O50 70 0.0014 50 0.001 30 0.0006 50 0.001

C30_O60 30 0.0006 60 0.0012 70 0.0014 40 0.0008

C40_O60 40 0.0008 60 0.0012 60 0.0012 40 0.0008

C50_O60 50 0.001 60 0.0012 50 0.001 40 0.0008

C60_O60 60 0.0012 60 0.0012 40 0.0008 40 0.0008

C70_O60 70 0.0014 60 0.0012 30 0.0006 40 0.0008

Note: The highlighted case (C50_O60) is the case at original experimental conditions (Tang et al., 2010c)

and the results of that case will be used to validate the model.

5.5.1 Identification of Best Reaction Mechanism for Lignite Coal and Validation

of the CFD model

The reaction mechanism is an important aspect to understand gasification modeling.

Researchers proposed various mechanisms of gasification among which few most

commonly applied mechanisms are highlighted in Table 5.2. Fig.5.6 shows the

comparison between the experimental results (Tang et al., 2010c) and various

preliminary simulation cases from A to F mentioned in Table 5.2. In all those cases the

coal or oxygen distribution between the two stages is kept constant as per experimental

conditions (Coal 50% (of total) at AA' level and O2 60% (of total) at AA' level). The

oxygen/carbon (O/C) ratio was taken 0.9, 1.0 and 1.1. The temperature profiles along

the axis of the gasifier for all simulated cases and experimental observations are shown

in Fig. 5.7 at different O/C ratios.

150

(a) (b)

(c)

Fig. 5.6. The comparison of various preliminary simulation results with

experimental values (Tang et al., 2010c) for CO, H2 and CO2 mole fraction in

Syngas (a) at O/C ratio=0.9, (b) at O/C ratio=1.0 and (c) at O/C ratio=1.1.

151

(a) at O/C ratio=0.9 (b) at O/C ratio=1.0

(c) at O/C ratio=1.1

Fig. 5.7: The temperature profile for various preliminary simulated cases and

experimental work along central axis of gasifier (a) at O/C ratio=0.9, (b) at O/C

ratio=1.0 and (c) at O/C ratio=1.1

It is clear from Fig. 5.8 for all O/C ratio cases, that there is a great impact for reaction

sets on the overall composition of syngas. Case A follows the most common reaction

sets i.e. the partial combustion of char and volatiles and the involvement of CO

combustion in the bulk gas phase. But from Fig. 5.8 (a to c), it shows that it predicts

quite less CO and high CO2 as compared to experimental values for this particular type

of gasifier with different arrangements of injecting nozzles. The possible cause is that

the well-mixed pattern of particles with gas due to impinging and tangential injection

152

patters of fuel and oxygen. The velocity vectors and particles are shown in Fig. 5.8 and

Fig. 5.9 respectively that gives a clear idea of the flow behavior of gas inside the gasifier

along with the 250 collisions of particles. The Fig. 5.9 shows the particles' residence

time for 200 tracks only with limited end time i.e. 0.8sec. Fig. 5.9 confirms the idea that

there is a greater chance to have the particle combustion as compared to CO combustion

due to homogeneous temperature mixing but in case A the false unrealistic prediction

is visible due to rapid CO combustion and increasing the temperature of overall gas

mix. The excess temperature rise can also be verified in Fig. 5.9 for that case. Rest of

the cases (from B to F) shows a good agreement of H2 mole % but there are significant

differences for CO and CO2 mole %. In this regard, Case E shows the best results

compared with experimental values for prediction of syngas composition and

temperature at different O/C conditions. The reaction mechanism of case E contains the

complete combustion of volatiles and char and ignoring the CO combustion reaction.

The values of CO and H2 mole % for this case are in good agreement and show less

than 1% error while the prediction of CO2 is moderate but good as compared to other

cases. The one reason for not predicting the good CO2 is the assuming no formation of

other species like CH4 etc. Temperature prediction through case E is also in satisfactory

limits (less than 1% error) so the Case E has been validated and the solution strategies

and reaction mechanism of Case E has followed to conduct the rest of studies.

153

Fig. 5.8. The velocity Vectors at the Sectional Planes of the gasifier (Case E) with

a close view of AA' and BB' planes.

Fig. 5.9. The particles residence time at the Sectional Planes of the gasifier (Case

E) (for clarity only 200 particle tracks are shown with particle end time limited

at 0.8 sec).

154

5.5.2 Effects of Coal/Oxygen Distribution on syngas composition

15 different cases were simulated with coal/oxygen distribution variations at both the

levels as per Table 5.5. The effects on the mole % of CO, CO2, and H2 by varying the

total coal % at AA' level are shown in Fig. 5.10. As per Fig. 5.10(a), the mol% of CO

shows first increasing and then decreasing trend by increasing coal % at AA' level with

40 and 50% oxygen at the same level. This behavior is slightly changed with 60%

oxygen case where it is almost of increasing order. The optimized conditions for

maximum CO production (52.59%) was found at 50% Coal and oxygen at AA' level

(C50_O50 case). The minimum CO mol % was observed 50.123% in C30_O60 case.

Exactly inverse trends can be seen for CO2 mol % with respective cases (Fig. 5.10(b)).

A plausible cause for these trends is the variation of Oxygen/Coal ratio at local levels

and the rate of mixing due to vortex formation with the tangential nozzles (either at AA'

level or BB' level) as explained by Seo et al (Seo et al., 2011). The maximum and

minimum for CO2 mol % were observed 19.98% and 18.39% in C30_O60 and

C50_O40 cases respectively.

The production for H2 was found in the range of 27 to 28% in all the cases as per Fig.

5.10(c). In this narrow range (27-28%) it shows a bit increasing and then decreasing

trend with increasing coal at AA' level for 50% oxygen; a decreasing trend with 40%

oxygen and almost increasing behavior with 60% at AA' level. The reason for this

behavior of H2 production in these various cases is the non-availability of abundant

water because all the gasification simulations were carried out without steam. Further,

there is also less moisture (1.16%) present in the coal as per its proximate analysis (refer

to Table 5.1). The less presence of water has a great influence on the water-shift reaction

as per earlier studies (Lu and Wang, 2013a, Lu and Wang, 2013b).

155

(a) CO Mol % (b) H2 Mol %

(c) CO2 Mol %

Fig. 5.10: The Mole % for CO, H2, and CO2 with variation in total Coal/Oxygen

% at AA' Level

5.5.3 Effects on Char Conversion

The coal/oxygen variation at both distributions has an impact on its local ratio

distribution and that has also impact on the overall char conversion. This impact is

shown in Fig. 5.11 for various simulated cases. In general, an increase can be seen in

conversion by increasing the coal at the top level (from 30% to 50%) with all oxygen

cases. But conversion decreases by increasing coal from 50% to 70% at the top level

(AA') with 40% oxygen at that level. The other two conditions (50% and 60% Oxygen

at the top) show a continual increase in conversion by increasing coal from 50% to

70%. This behavior is due to greater residence time of the coal injected at top-level

(AA' level) as compared to the coal injected at the bottom level (BB' level) according

156

to the conclusions of earlier studies (Guo et al., 2007, Watanabe and Otaka, 2006). So

the higher amount of coal injected at AA' level with sufficient oxygen has enough

residence time to reach at maximum conversion, usually more than 99%. The minimum

char conversion i.e., 95.45% was found with 30% coal and 40% oxygen at top level

case (C30_O40) whereas the maximum char conversion was found 99.7% in C60_O50

and C70_O60 both the cases.

Fig. 5.11: The Char Conversion with variation in Coal/Oxygen % at AA' Level

5.5.4 Effects on Syngas Exit Temperature and Maximum inside Temperature

The temperature plays an important role in the gasification system. The variations in

the coal and oxygen at both the injection levels actually affect local oxygen/coal ratio

as discussed in previous sections and hence the defined combustion/gasification

reactions occurred with different rates. Because of the difference of those exothermic

and endothermic reaction rates, there is an increase or decrease in the overall

temperatures. Fig. 5.12 shows an overall picture for the syngas exit temperatures for

simulated cases. It is clear that in all the cases the exit temperature for syngas is in the

range from 1250 K to 1450 K. Chao Li (Li et al., 2012) observed that in the center of

impinging zone there is an increase of particle cohesion and agglomeration by resulting

a high particle concentration region. The rapid deceleration of particles near the center

157

of impinging zone improves the performance of gasifier in terms of heat and mass

transfer.

Fig. 5.12: Syngas Average Exit Temperature (K)

The radial temperature profiles from the center can be seen in Fig. 5.13 for three coal

distribution scenarios. It is evident that there is a great impact for the local temperature

distribution due to variation in coal or oxidant between two stages. Further, the results

can be verified by examining the temperature profile contours of the top and bottom

injection sectional planes as given in Fig. 5.14.

It is apparent from those temperature contours that the inside temperature increases by

increasing the amount of coal at any level with appropriate amount of oxygen. The

maximum temperature was observed 2027 K at AA' level with 70% of total Coal and

60% of total oxygen (Case: C70_O60, Fig.5.30(c)).

158

Fig. 5.13: Radial Temperature profiles for various simulation cases at different

heights of gasifier

159

(a) 40% Oxygen at AA' Level

(b) 50% Oxygen at AA' Level

(c) 60% Oxygen at AA' Level

Fig. 5.14: Temperature Contours for Sectional Planes at AA' Level and BB'

Level

160

5.5.5 Effects of coal distribution on particle trajectories

The particle residence time is key parameters for the conversion of char and occurrence

of different reactions with their specific rates, as explained in an earlier section. The

effects on the particle residence time can be depicted from Fig. 5.15, where particles

trajectories are shown for variation of coal distribution with fixed 50 % oxygen

distribution at both the levels. For clarity, the trajectories are limited to 20 particle

streams with maximum of 0.2 seconds. The variation in coal at any injection level with

fixed oxygen actually impacted on the local particle/gas volume ratio and hence

ultimately impacted on the particle trajectories. It is observed that the less particle/gas

ratio increases the particles residence time and confirms the previous studies (Steiler et

al., 1996).

Fig. 5.15: The particles residence time at the Sectional Planes of the gasifier;

Cases: C30_O50, C50_O50, and C70_O50

(Note: For clarity, only 20 particle tracks are shown with particle end time limited at 0.2 sec).

5.5.6 Effects on Turbulent Intensity

Turbulent intensity has predicted by solving the equations of the reliable k-ɛ model from

Eq. 5.5 to 5.9. Due to impinging and tangential nozzles at AA' and BB' levels, the

major turbulence has observed on the sectional planes on these levels. The effects of

coal and oxygen distribution on Turbulent Intensity can be visualized from Fig. 5.16.

161

The turbulent intensity of 108% to 300 % was observed at the main reaction zone on

both the planes as per previous observation (Vascellari and Cau, 2012). The obvious

reason is the high mixing rate at these sectional planes for injectors.

(a) at constant 50% coal at both injection levels

(b) at constant 50% Oxygen at both injection levels

Fig. 5.16: Turbulent Intensity (%) Contours for selected cases

5.5.7 Heat generation and consumption analysis

The overall energy balances accounted through Eq. (5.3). The primary source for

generation of heat energy is from exothermic reactions. Then the heat is consumed by

endothermic reactions and remaining heat is either absorbed by walls which are

assumed to be at a constant temperature of 800 K (water cooled system) or taken out

with syngas. A fraction of heat is also consumed by the solid fuel particles to raise their

temperature for drying, devolatization and then reactions. The net heat generated

162

through reactions has observed in the range of 3000 W and shown in Fig. 17 for a few

selected cases. The maximum heat consumed by the solid fuel particles and observed

in the range of 18700 W (Fig. 5.17). Heat lost through walls is found in the range of

9000 W whereas the heat carried away by syngas is found in 2000 to 26000 W (Fig.

5.18).

Fig. 5.17: Net heat generated due to reactions (W) for selected cases

Fig. 5.18: Heat consumption in different ways

163

5.6 MODELING AND SIMULATION RESULTS FOR THAR LIGNITE

After the successful validation of CFD modeling and simulation results from available

Chinese coal data, the geometry of the gasifier was partially modified by introducing a

throat section (with shorter diameter) between two stages. The geometry details are

given in section 5.3 and Fig. 5.3.

Various cases were simulated using modified geometry and then compared the results

with the previous geometry results. All the simulation cases are grouped into four major

blocks as described below:

Block 1: Simulations with Probability Density Function (PDF) Method for optimization

of O/C ratio.

Block 2: Simulations with Species Transport Model (Kinetic Approach) for

optimization of O/C ratio.

Block 3: Simulations at different Pressures with Optimized Kinetic Model and O/C

ratio.

Block 4: Simulations at different Pressures with Increased Feed Flow.

The case information is group-wise tabulated in Table 5.7 to 5.10.

Table 5.7: Simulation Cases in Block 1 (PDF model used to calculate the species)

Sr.

No. Case Name Geometry

Type

of

Coal

Pressure

Coal

Feed

Rate

O2 Feed

Rate O/Coal

Ratio

O/C

Ratio

atm (Kg/sec) (Kg/sec)

3 A_2.963OC_PDF Previous

Geometry

Thar

Coal 1 0.0028 0.003 1.0714 2.963


Geometry

Thar

Coal 1 0.0028 0.0024 0.8571 2.53


Geometry

Thar

Coal 1 0.028 0.03 1.0714 2.963

6 B_2.963OC_PDF

Modified

Geometry

with Neck

Thar

Coal 1 0.0028 0.003 1.0714 2.963

7 B_2.819OC_PDF

Modified

Geometry

with Neck

Thar

Coal 1 0.002 0.002 1 2.819

164

Table 5.8: Simulation Cases in Block 2 (Species Model used with finite rate

chemistry) – Optimization of Oxygen-to-Carbon (O/C) Ratio

Sr. No.

Case Name Geometry Type

of Coal

Pressure Coal Feed

Rate O2 Feed

Rate O/Coal Ratio

O/C Ratio

Atm (Kg/sec) (Kg/sec)

1 A_2.097OC_Kin Previous Geometry Thar Coal

1 0.0028 0.0018 0.6429 2.097

2 B_2.097OC_Kin Modified Geometry

with Neck Thar Coal

1 0.0028 0.0018 0.6429 2.097


with Neck Thar Coal

1 0.0028 0.0012 0.4286 1.665


with Neck Thar Coal

1 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

1 0.0028 0.0021 0.75 2.314


with Neck Thar Coal

1 0.002 0.00108 0.54 1.89

Table 5.9: Simulation Cases in Block 3 (Different Pressures at Optimized O/C

ratio)

Sr. No.


of Coal

Pressure

Coal Feed Rate

O2 Feed Rate

O/Coal Ratio

O/C Ratio


1 B_1.881OC_Kin_P2 Modified Geometry

with Neck Thar Coal

2 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

3 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

4 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

5 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

7 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

10 0.0028 0.0015 0.5357 1.881


with Neck Thar Coal

20 0.0028 0.0015 0.5357 1.881

165

Table 5.10: Simulation Cases in Block 4 (Different feed rates with varying

Pressures at fixed Optimized O/C ratio) – Optimization of Feed Rate.

Sr. No.


of Coal

Pressure Coal Feed

Rate O2 Feed

Rate O/Coal Ratio

O/C Ratio


1 B_1.88OC_Kin_P5_2 Modified Geometry with Neck

Thar Coal

5 0.0056 0.003 0.5357 1.8809


Thar Coal

5 0.0028 0.0015 0.5357 1.8809


Thar Coal

10 0.0028 0.0015 0.5357 1.8809


Thar Coal

1 0.0056 0.003 0.5357 1.8809


Thar Coal

10 0.0056 0.003 0.5357 1.8809


Thar Coal

5 0.0056 0.003 0.5357 1.8809

7 B_1.88OC_Kin_P10_2

@ 10 atm kin

Modified Geometry with Neck

Thar Coal

10 0.0056 0.003 0.5357 1.8809


Thar Coal

1 0.0084 0.0045 0.5357 1.8809


Thar Coal

5 0.0084 0.0045 0.5357 1.8809


Thar Coal

10 0.0084 0.0045 0.5357 1.8809

11* B_1.88OC_Kin_P5_3 Modified Geometry with Neck

Thar Coal

10 0.0084 0.0045 0.5357 1.8809

The above-mentioned cases were simulated using the validated numerical schemes and

reaction mechanism. After getting the converged solution of each simulated case, the

results regarding the volatile conversion, char conversion, syngas composition, and

outlet temperature are tabulated from Table 5.10 to 5.13.

166

Table 5.11: Results for Simulation Cases in Block 1 (PDF model used)

Sr. No.

Case Name Devolatization

Char Conv.

Syngas Analysis (mol %) Exit

Temp

% % CO CO2 H2 H2O O2 Vol (K)

1 A_2.963OC_PDF 100 58.69 6.651 37.48 1.834 53.3 0 - 2196

2 A_2.53OC_PDF 100 34.56 9.982 34.29 3.96 51.5 0 - 1823

3 A_2.963OC_PDF 100 56.34 5.299 39 1.742 53.8 0 - 1910

4 B_2.963OC_PDF 100 60.03 8.561 35.5 2.378 52.6 0 - 2262

5 B_2.819OC_PDF 100 44.6 7.388 36.85 2.181 53.2 0 - 2105

Table 5.12: Results for Simulation Cases in Block 2 (Species model used)

Sr. No.

Case Name Devolatization

Char Conv.

Syngas Analysis (mol %) Exit

Temp


1 A_2.097OC_Kin 100 99.67 13.6 37.51 35.85 0 0 12.43 1400

2 B_2.097OC_Kin 100 100 13.96 37.17 35.9 0 0 12.8 1322

3 B_1.665OC_Kin 90.27 74.19 17.57 30.85 33.01 0 0 18.4 974

4 B_1.881OC_Kin 97.7 98.24 24.19 27.54 32.35 0 0 15.76 1065

5 B_2.314OC_Kin 100 100 1.5 48.22 38.92 0.98 0 10.25 1589

6 B_1.89OC_Kin 90.25 92.12 18.98 31.62 34.24 0 0 15.09 996.6

Table 5.13: Results for Simulation Cases in Block 3 (Effect of Pressure on

gasification)

Sr.

No. Case Name

Devolatization Char

Conv. Syngas Analysis (mol %)

Exit

Temp


1 B_1.881OC_Kin_P2 98.25 93.75 22.85 28.39 33.26 0 0 15.38 999.9

2 B_1.881OC_Kin_P3 96.35 88.56 20.67 30.31 33.7 0 0 15.23 995.1

3 B_1.881OC_Kin_P4 95.82 87.74 19.43 31.25 34.25 0 0 14.95 962.8

4 B_1.881OC_Kin_P5 95.34 86.57 18.81 31.65 34.54 0 0 14.8 946.4

5 B_1.881OC_Kin_P7 94.26 84.71 14.8 34.8 35.14 0 0 14.9 938.7

6 B_1.881OC_Kin_P10 94.77 84.52 6.67 29.2 27.6 0 0 11.53 931.2

7 B_1.881OC_Kin_P20 92.21 83.11 5.5 36.5 23.4 0 0 12.2 923

167

Table 5.14: Results for Simulation Cases in Block 4 (Effect of Pressure on

gasification)

Sr.

No. Case Name

Devolatization Char

Conv. Syngas Analysis (mol %)

Exit

Temp


1 B_1.88OC_Kin_P5_2 100 99.99 25.8 26.2 32.6 0 0 15.3 1079

2 B_1.88OC_Kin_P5_1 96.58 89.39 17 33 35.4 0 0 14.5 954.7

3 B_1.88OC_Kin_P10_1 98.57 92.32 7.77 31.1 28.9 0 0 12.36 1087

4 B_1.88OC_Kin_P1_2 99.83 98.99 24.76 27 32.3 0 0 15.7 1244

5 B_1.88OC_Kin_P10_2 98.55 95.33 23.12 28.1 33.9 0 0 14.7 1003

6 B_1.88OC_Kin_P5_2 99.97 99.9 24.58 27.12 33.14 0 0 14.8 1086

7 B_1.88OC_Kin_P10_2

@ 10 atm kin 95.67 85.15 24.9 26.5 32.7 0 0 15.7 994

8 B_1.88OC_Kin_P1_3 99.71 98.78 26.4 25.7 32.6 0 0 15.2 1276

9 B_1.88OC_Kin_P5_3 100 100 25.6 26.6 32.5 0 0 15.3 1183

10 B_1.88OC_Kin_P10_3 100 100 25.2 26.7 32.7 0 0 15.1 1129

11* B_1.88OC_Kin_P5_3 99.95 99.8 26 26.02 32.6 0 0 15 1112

5.6.1 Effects of different models and O/C ratio

From the results, it was observed that PDF model predicts less molar composition of

syngas components (Table 5.11) as compared to the species transport model (Table

5.12) due to the involvement true kinetic expression during the computations. CO is

coming 6-7% whereas less than 2% H2 is achieved through PDF model with modified

geometry.

The results regarding the mole % of CO, CO2, and H2 at the exit of gasifier during the

simulations for various O/C are plotted in Fig. 5.19 whereas the devolatization and char

conversion against O/C ratios are plotted in Fig. 5.38. From Fig. 5.19 it can be seen that

the increase in O/C ratio initially increases the CO mole % but then decreases. Its

optimized value achieved about 24% at 1.881 O/C ratio. At this O/C ratio, the minimum

CO2 was coming out from the gasifier. There is a minute increase can be seen in H2 but

168

that is not very much significant. Further, the char conversion was also achieved

maximum at this ratio (Fig. 5.20) so 1.881 O/C ratio is considered as the optimized ratio

for further simulations.

Fig. 5.19: The mole % of CO, CO2, and H2 at the exit of gasifier at different O/C

Ratios

Fig. 5.20: The devolatization and Char conversion at different O/C Ratios

5.6.2 Effect of pressure on syngas composition and char conversion

The simulations were carried out at various pressure from 1 atm to 20 atm as per Table

5.13. The variations in the syngas composition (CO, CO2, and H2) are shown in Fig.

5.21 whereas the char conversion along with exit temperature are shown in Fig. 5.22

169

with respect to pressure. It is evident from these figures that pressure has inverse effects

on the syngas composition. An increase of pressure decreases the percentage of CO and

H2 components in the syngas whereas CO2 content increase with increase of pressure.

This is due to the kinetics effects already discussed in the kinetic experimental

discussion at high pressure. The same behavior is witnessed from the Char conversion

history at various pressures (Fig. 5.22). Char conversion decreases with increasing

pressure due to diffusional resistances increases on the char particles. The exit gas

temperature decreases as less amount of char converted in the chamber.

Fig. 5.21: Effect of Pressure on the composition of CO, CO2, and H2 in the syngas

Fig. 5.22: Effect of Pressure on char conversion and exit gas temperature

170

5.6.3 Streamlines-Flow analysis for Multi-Opposite Burners

The designed gasifier is entrained flow gasifier with multi-opposite burners. The

Chinese gasifier was modified for meeting the Thar lignite requirements by introducing

a neck section between the two stages. The streamlines-flow analysis after simulation

was conducted for both the geometries and compared as shown in Fig. 5.23. It is seen

that the without throat (Geometry-A) there is less residence time and flow is smooth

going down. This is good for the coal with high fixed carbon and less moisture but with

high moisture, there must be some delay for complete drying between 1st and 2nd stage.

This constraint is removed by having a throat which restricts the smoothness of the flow

and creates some positive disturbance. One can observed this turbulence through stream

flow analysis of Geometry-B (modified with neck section).

(a) Geometry–A (b) Geometry – B

Fig. 5.23: Streamlines –Flow analysis for both the geometries

5.7 VALIDATION OF CFD RESULTS OF MODIFIED GEOMETRY WITH

ASPEN PLUS MODEL RESULTS

The CFD modeling of modified geometry with Thar coal feedstock was carried out and

the results discussed in detailed in previous sections. Due to absence of physical model,

the CFD results were validated with Aspen Plus®V10 model results. The model of

171

entrained flow gasifier was developed as per guidelines are given in previous work

(Aspen, 2010) and then simulations were carried out at similar O/C ratios as per CFD

analysis. The results of important syngas components like CO, CO2, H2 and Volatiles,

Devolatization, char conversion, and syngas temperature were compared from both

modeling approaches. The Fig. 5.24 (a-d) shows the comparison of important

components’ composition in outlet syngas. The comparison of devolatization, char

conversion, and syngas temperature are shown in Fig. 5.25 (a-c). From the comparison,

it can be observed that CFD results showing good agreement with Aspen Plus model

results. An insignificant error has observed in the CO, CO2, H2 and Volatile percentages

in the syngas. Moreover, less than 5% error is being observed in devolatization, char

conversion, and syngas temperature. The similar situation can be seen in previous work

conducted by Xiangdong et al., (2013) (Xiangdong et al., 2013b). Hence the CFD

model is validated and could be used for further analysis.

(a) CO (b) CO2

(c) H2 (d) Volatiles

Fig. 5.24: Comparison of CFD model results with Aspen Plus®V10 model results

for (a) CO, (b) CO2, (c) H2 and (d) Volatiles

0

5

10

15

20

25

30

1.665 1.881 1.89 2.097 2.314

Mol

%

O/C Ratio

CO - CFD Model

CO - Aspen Model

0

10

20

30

40

50

60

1.665 1.881 1.89 2.097 2.314

Mol

%

O/C Ratio

CO2 - CFD Model

CO2 - Aspen Model

0

5

10

15

20

25

30

35

40

45

1.665 1.881 1.89 2.097 2.314

Mo

l%

O/C Ratio

H2 - CFD Model

H2 - Aspen Model

0

2

4

6

8

10

12

14

16

18

20

1.665 1.881 1.89 2.097 2.314

Mol

%

O/C Ratio

Volatiles - CFD Model

Volatiles - Aspen Model

172

(a) Devolatization (b) Char Conversion

(c) Syngas Exit Temperature

Fig. 5.25: Comparison of CFD model results with Aspen Plus®V10 model results

for (a) Devolatization (b) Char Conversion and (c) Syngas Exit Temperature

5.8 COMPARATIVE STUDY FOR NEWLY DESIGNED GASIFIER

The newly designed gasifier was tested with Thar lignite as feedstock and then its

performance was evaluated and discussed in detail in the above sections. Finally, both

the gasifier were evaluated on the basis of the heating value of syngas produced and

other parameters tabulated in Table 5.15. The syngas heating value was calculated using

standard processing modeling tool Aspen HYSYS®V10.

From the information given in Table 5.15, it is verified that the Chinese coal is good in

heating value i.e. 32.91 MJ/Kg whereas Thar lignite is much below to this value and

lies only the range of 15.06 MJ/Kg. But after gasification, the syngas heating value is

almost in equal range that of produced from Chinese coal. The lower heating value

(LHV) of syngas produced from Thar lignite with modified geometry is 12.27 MJ/Kg

84

86

88

90

92

94

96

98

100

102

1.665 1.881 1.89 2.097 2.314

%

O/C Ratio

Devolat-CFD Model

Devolat-Aspen Model

0

20

40

60

80

100

120

1.665 1.881 1.89 2.097 2.314

%

O/C Ratio

CharConv.-CFD Model

CharConv.-Aspen Model

0

500

1000

1500

2000

1.665 1.881 1.89 2.097 2.314

Tem

p. (K

)

O/C Ratio

Temp-CFD Model

Temp-Aspen Model

173

(with considering volatiles) and 7.506 MJ/Kg (without volatiles inclusion) whereas the

Chinese coal gives syngas of 8.96MJ/Kg LHV. Similarly, the higher heating value

(HHV) of syngas produced from Thar lignite with modified geometry is 14.43 MJ/Kg

(With volatiles) and 8.77 MJ/Kg (without volatiles) is in comparison with 10.08 MJ/Kg

HHV of Chinese coal syngas. The ratio of LHV to coal heating value or HHV to coal

heating value for Thar lignite is 0.815 and 0.958 respectively which is much greater

than 0.212 and 0.306 for Chinese coal. The Char conversion is also in satisfactory limits

of above 98%. The coal feeding rate 10.08 KG/hr is also higher for modified geometry

as there is less fixed carbon in Thar lignite.

Table 5.15: Comparative study for Original and Modified geometry of gasifier

with two different feedstocks

Sr.

No. Parameter

Modified Geometry with Thar

Coal Feedstock

Original Geometry with

China Coal Feedstock

1 Coal Heating Value 15.06 MJ/Kg 32.91 MJ/Kg

2 Lower Heating

Value of Syngas

12.27 MJ / Kg (With Volatiles)

7.506 MJ / Kg (Without Volatiles) 8.95 MJ / Kg

3 Higher Heating

Value of Syngas

14.43 MJ / Kg (With Vol.)

8.777 MJ / Kg (Without Vol.) 10.08 MJ / Kg

4 Ratio - LHV/Coal

Heating Value

0.815 (With Vol.)

0.498 (Without Vol.) 0.212

5 Ratio - HHV/Coal

Heating Value

0.958 (With Vol.)

0.583 (Without Vol.) 0.306

6

Devolatization

(Removal of

Volatiles from

Coal)

97.7% 100%

7 Fixed Carbon

Conversion 98.24% 98.6%

8 Geometry

Difference With Throat Without Throat

9 Coal Feeding Rate 10.08 Kg/hr 7.2 Kg/hr

174

The comparison of simulation results based on Temperature and mole fraction of CO,

CO2, and H2 from both geometries is carried out by the development of contours of

those variables. For comparison, the optimized cases were selected from simulated

cases conducted on both geometries. The temperature contours at sectional planes from

both the geometries are shown in Fig. 5.26 whereas the contours of mole fractions of

important syngas components like CO, CO2, and H2 are shown in Fig. 5.27. It is evident

from the figures that the temperature from geometry A is higher with Chinese coal

feeding as compared to the temperature obtained from Thar lignite with geometry-B.

The fundamental reason is the higher carbon percentage in Chinese coal as compared

to Thar lignite. Similarly, the less CO is being observed from Thar lignite due to same

reason but a good amount of H2 is produced from Thar lignite due to higher moisture

content.

Fig. 5.26: Comparison of temperature contours from both geometries

175

(a) Original Geometry (Geometry-A)

(b) Modified Geometry (Geometry-B)

Fig. 5.27: Comparison between the Original Geometry (A) and Modified

Geometry (B) through contours of important syngas components

176

5.9 THE FINAL PROPOSED SYSTEM

The complete system including the coal feeding system, validated new gasifier, ash

collection system, and syngas cleaning system is proposed and shown in Fig. 5.28. The

overall material balance for gasifier is shown in Fig. 5.29. According to overall material

balance, the design gasifier possesses the maximum capacity of 0.0028 Kg/sec (10 Kg/

hr) and can produce up to 0.00356 Kg/sec syngas (12.816 Kg/hr) containing 24% CO

and 32% H2.

Fig. 5.28: Proposed system of coal gasification system with newly designed

gasifier

Fig. 5.29: Overall material balance of newly designed gasifier

177

5.10 SUMMARY FOR CFD MODELING AND SIMULATION WORK

CFD model was developed for lab scale double stage entrained flow coal gasifier with

multiple opposite burners available at China which is referred here as Geometry A. The

numerical simulations were performed on the Chinese coal data and results were

verified from published experimental work. After this, the modified geometry (referred

as Geometry-B) was modeled using CFD techniques for Thar coal data. The kinetic

parameters for Thar Coal Gasification were taken from the experimental work on TGA

and PTGA. The commercial CFD code ANSYS FLUENT®14.0 was used for all

computations. The validation for CFD modeling results with Thar Coal and modified

geometry was carried out with the model developed in AspnPlus®V10. The

summarized points are as under:

• Geometry A showed good results with Chinese Coal whereas for Thar Coal the

Geometry –B (with neck) was found best.

• The maximum CO and H2 were observed 24.19% and H2 was 32.35% at O/C ratio

of 1.881 with Geometry B with Thar Coal Data.

• With Geometry B and Thar Coal at maximum CO and H2 generation, the moisture

removal was observed 97.7% whereas char conversion was observed 98.24%.

• Lower and Higher (LHV) heating values of syngas produced from modified

geometry were calculated using Aspen HYSYS software. From that, it was

observed that the syngas produced from Geometry-B has 12.27 MJ/Kg lower

heating value.

• The calculated ratio of LHV/coal heating value for Thar coal is 0.815 which is

greater than Chinese Coal with geometry-A (i.e., 0.212).

• Less than 1 % different was observed in syngas composition and char conversion

with and without Sulfur inclusion for CFD modeling.

• Good agreement was found between the results of the CFD model and Aspen Plus

model at different O/C ratios.

• Detailed design of optimized gasifier was proposed along with complete

gasification system. Overall material balance of the proposed gasification system

was also presented.

178

CHAPTER 6

CONCLUSION

6.1 CONCLUSION

Pakistan is considered among coal-rich countries but at the same time facing a major

energy crisis. The fundamental reason for not utilizing indigenous coal is the

unavailability of technology and basic coal characteristics which are essential to design

the advanced energy harvesting systems like gasification. Kinetics of gasification

reactions is one important aspect which is a basic hurdle to utilize the indigenous lignite

coal. The prime aim of this research to extract the kinetic parameters of fundamental

steps involved in the gasification process and then evaluate those kinetic parameters

using CFD modeling and Simulation. In this regard, the thermogravimetric analysis

(TGA) was utilized to study thermal degradation of Pakistani lignite under different

environments and then the data is used to calculate the Arrhenius parameters for various

steps of gasification processes.

The sample characterization started from the proximate and ultimate analysis. The

proximate and ultimate analysis was carried out for few samples to see the composition

of Thar lignite. The moisture lies in the range of 14 to 46%, volatiles in 28% to 50%,

Ash in 3 to 20% whereas fixed carbon lies in the range of 20 to 31% for selected

samples. After this, the reactivity study was conducted for Thar Coal Samples for

Moisture removal, Devolatization, Combustion and Gasification Reactions. At the first

stage, the moisture removal and devolatization studies were carried out at different

heating rates on TGA. Then Gasification reactions of Thar Chars were studied with

CO2 and H2O reacting gases at three pressures i.e, 1, 5 and 10 atm. Pressurized

Thermogravimetric Analyzer (PTGA) was used for this study. All the experiments

were conducted at non-isothermal conditions. Two standard kinetic models i.e,

Volumetric Model and Grain Model were used to evaluate the experimental data and

values of Arrhenius parameters (A and E) were calculated by adopting the direct plot

method and integral method. The least square regression technique was used to analyze

the models.

179

6.1.1 Concluding remarks for Drying, Devolatization and Combustion Steps

The Volumetric Model and Grain Model fit well for drying, devolatization, combustion

and Gasification Processes. The Devolatization step was broken into low and high-

temperature ranges to increase the linearization of the models. The low-temperature

range was selected from room temperature to 500°C whereas high-temperature range

was from 501 - 900°C. The range of calculated values of frequency factor (A) and

activation energy (E) for moisture, devolatization, and combustion are as follows:

Step Model A E (KJ/mol)

Min Max Min Max

Moisture Removal Vol. Model 3.6×103 1.38×108 48.4 62.2

Gr. Model 4.05×102 1.46×107 42.6 55.3

Devolatization

(Low Temp)

Vol. Model 8.52×103 8.68×103 27.7 31.7

Gr. Model 4.45×103 7.98×103 24.7 30.2

Devolatization

(High Temp)

Vol. Model 4.76×101 1.02×104 7.23 36.3

Gr. Model 1.28×101 4.10×106 1.04 2.97

Combustion Vol. Model 1.855×102 3.412×106 57.83 138.58

Gr. Model 3.221×101 2.051×105 51.5 123.17

6.1.2 Concluding remarks for Gasification Reactions

At atmospheric pressure, the inherent kinetics of Char-CO2 or Char-H2O reactions is

dominant and drives the overall reaction. At high pressures, the diffusion of reacting

gas in solid particle plays an important role and diffusion rate is dominant over inherent

kinetics of those heterogeneous reactions. The high pressure of reacting gas increases

the overall rate of reaction of gasification. Volumetric model and grain model are

showing satisfactory results in the reactivity study of Char-CO2 or Char-H2O reactions

at atmospheric or high pressures with the integral method. The direct method is showing

good results at atmospheric pressure but it is invalid to be used at high pressures.

Increasing pressures affect the overall kinetics of the heterogeneous reactions in terms

of increase in frequency factor and activation energy.

180

The calculated values of frequency factor (A) for Char-CO2 reaction lies in the range

of 2.729×106 to 7.99×107 and 1.078×105 to 2.174×107 from the volumetric model and

grain model respectively at atmospheric pressure using the integral method. The range

lies between 7.849×108 to 9.939×1011 and 3.992×107 to 6.509×109 from the volumetric

model and grain model respectively at 10 atm using the integral method. Similarly, the

calculated values of frequency factor (A) for char-H2O reaction lies in the range of

4.172×104 to 8.301×1011 and 1.626×103 to 4.467×109 from the volumetric model and

grain model respectively at atmospheric pressure using the integral method. The range

lies between 2.13×1010 to 1.156×1015 and 3.302×108 to 3.485×1012 from the volumetric

model and grain model respectively at 10 atm using the integral method.

The activation energy (E) calculated in the range of 244.64 - 317.6 KJ/mol at

atmospheric pressure whereas it is lying in the range of 286.13 – 356.67 KJ/mol at 10

atm pressure for Char-CO2 reaction. Similarly, it is found 202.58 – 376 KJ/mol and

290.6 – 428.34 KJ/mol at atmospheric and at 10 atm respectively for char-H2O reaction.

6.1.3 Concluding remarks for CFD Modeling and Simulation Work

Finally, the CFD modeling was done for developing Entrained Flow Gasification

System with Multi-opposite Burners. The complete combustion of char and volatiles

reaction mechanism was used in the modeling which could predict the mol% of CO,

H2, and CO2 with less than 1% error. Kinetic Parameters of devolatization and

gasification reactions played a vital role in the simulation of real gasification process.

The variation of oxygen or coal at any injection level basically impacted on the local

oxygen/coal ratio and the gas-solid mixing pattern. This impact is transferred in the

variation of char consumption rates with combustion and gasification reactions and that

plays a key role in the variation of syngas components and temperature. The production

for H2 was found in the range of 27–28 mol% in all the cases. [Maximum CO mol% =

52.59% for 50% coal and 50% oxygen at upper injection level whereas the minimum

CO mol% = 22.37% for 30% coal and 70% oxygen at upper level]. The exit temperature

for syngas was found in the range from 1250 K to 1450 K. The maximum temperature

was observed 2027 K at AA’ level with 30% of total coal and 70% of total oxygen at

the upper level.

181

The maximum char conversion was found 99.79% with coal 60% and oxygen 50% of

the total at AA’ level. The minimum char conversion was observed 95.45% at 30% coal

with 40% oxygen at AA’ level. In general with oxygen/coal above or equal to 50% of

total at upper injection level has shown an optimized performance. Overall it was

concluded that the coal and oxygen distribution has great effect on the syngas

composition, char conversion, and exit syngas temperature.

The temperature and kinetics of gasification reactions (reactions of char to CO2 and

H2O) can be controlled with the optimized coal and oxidant distribution between the

two stages. These parameters are critical to the overall performance of the gasifier, so

coal and oxygen feedings must be optimized between the two stages of the gasifier to

get the optimized performance. Species Transport approach shows good results as

compared to Probability Density Function (PDF) in terms of CO and H2 production.

Geometry–A shows good results with Chinese Coal whereas for Thar Coal the

Geometry –B (with neck) was found best. The maximum CO and H2 were observed

24.19% and H2 was 32.35% at O/C ratio of 1.881. At maximum CO and H2 generation,

the moisture removal was observed 97.7% whereas char conversion was observed

98.24%. Lower and Higher (LHV) heating values of syngas produced from modified

geometry were calculated using Aspen HYSYS software. From that, it was observed

that the syngas produced from Geometry-B has 12.27 MJ/Kg lower heating value. The

calculated ratio of LHV/coal heating value for Thar coal is 0.815 which is greater than

Chinese Coal with geometry-A (i.e 0.212).

Further, comparison of CFD results with Aspen Plus®V10 Model results confirmed a

good agreement between both the strategies. The insignificant error was observed

between the results obtained from the CFD model and Aspen Plus model.

6.2 RECOMMENDATIONS FOR FUTURE WORK

The work could be extended in the following directions for further investigations.

• Detailed kinetic modeling would be recommended for a large number of samples

for Pakistani lignite with further advanced standard kinetic models like Random

Pore Model.

182

• The Online syngas analyzer should be used during the gasification experiments

during TGA for better understanding the behavior of reactions.

• Drop Tube Furnace would be used for gasification studies to see the intrinsic

kinetics of feedstocks during actual gasification environment. The exit gas analysis

should be mandatory in those experiments.

• Transient CFD modeling of a gasification system with extracted kinetic data will

be recommended in future.

• The validation of CFD model with the actual system will give high understanding

and confidence about the modeling results.

183

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Zou, J. H.; Zhou, Z. J.; Wang, F. C.; Zhang, W.; Dai, Z. H.; Liu, H. F. and Yu, Z. H.

(2007) Modeling Reaction Kinetics of Petroleum Coke Gasification with CO2.

Chemical Engineering and Processing: Process Intensification, 46, 630-636.

211

APPENDICES

A.1 TGA Model SDT Q600

A.2 Quartz fixed-bed reactor for char

production

A.3 Thermax500 PTGA

212

A.4 LIST OF PUBLICATIONS

• Unar, I. N., Wang, L., Pathan, A. G., Mahar, R. B., Li, R., & Uqaili, M. A.

(2014). “Numerical simulations for the coal/oxidant distribution effects

between two-stages for multi opposite burners (MOB) gasifier”. Energy

Conversion and Management, Elsevier (IF=2.77) Vol. 86, pp. 670-682 .

• Wang, L., Jia, Y., Kumar, S., Li, R., Mahar, R.B., Ali, M., Unar, I.N., Sultan,

U. and Memon, K., 2017. Numerical analysis on the influential factors of coal

gasification performance in two-stage entrained flow gasifier. Applied Thermal

Engineering, 112, pp.1601-1611.

• A.G. Pathan, M.A. Uqaili, F.I. Siddiqui and I.N. Unar, (2015). “Sustainable

Development of Thar Coal, Pakistan”, Presented at The 7th International

Conference on Mining Science and Technology (ICMST), China, April 2015.

• Imran Nazir Unar, Lijun Wang, Abdul Ghani Pathan, Rasool Bux Mehar,

Rundong Li and M. Aslam Uqaili (2013) "Study the Coal/Oxidant Distribution

Effects in a Two-stage Dry-Feed Coal Gasifier with Numerical Simulations",

Presented at 1st International Coal Conference, held and organized by MUET

Jamshoro under INSPIRE Project of HEC from November 7-9 November 2013.

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