CFD Modeling of Soot Formation from Asphaltene Gasification

Vinoj Kurian1, Andre Bader2, Petr Nikrityuk1, Rajender Gupta1

1Department of CME, University of Alberta, Canada

2IEC, TU Bergakademie Freiberg, Germany

Outline

o Introduction

o Motivation & Objectives

o Experimental setup

o CFD Modeling of Soot Formation

o Conclusions

2

Gasification is the process where any carbonaceous material is converted into synthesis gas (syngas) by partial oxidation with air, O2 and /or steam.

Combustion

C + O2 CO2 ∆H= -405.9 kJ/mol

Heterogeneous Gasification Reactions

C + ½ O2 CO ∆H= -123.1 kJ/mol

C + CO2 ↔ 2CO ∆H= +159.7 kJ/mol

C + H2O ↔ CO + H2 ∆H= +118.9 kJ/mol

C + 2H2 ↔ CH4 ∆H= -87.4 kJ/mol

Homogenous reactions

CO + H2O ↔ H2 + CO2 ∆H= -40.9 kJ/mol

CO + 3H2 ↔ CH4 + H2O ∆H= -206.3 kJ/mol

3

Gasification

4

Soot The term soot is given to the particulates formed during combustion of carbonaceous fuels under sub-stoichiometric conditions.

o Soot usually forms at 1000 to 2500 °C.

o Fundamental unit: Spherules (10–50 nm)

o Shape: Clusters or chains of spherules

o Decrease thermal NOx

o More difficult to gasify than coal char & harder to burn

o Major contributor to global warming after CO2

o Environmental concerns on pollutant emission

o Many PAHs are carcinogenic & mutagenic.

o Can be breathed into the lungs and cause substantial

Stanmore, B. R. et al., Carbon, 2001, 39, 2247 –2268

5

Proximate Analysis

Volatiles 63

Fixed carbon 36.3

Ash 0.6

Moisture 0.1

Athabascan Asphaltene Ultimate Analysis

Carbon 82.68

Hydrogen 8.34

Sulfur 7.75

Nitrogen 1.2

Oxygen 0.03

Vanadium 1763 ppm

Nickel 671 ppm

o H/C = 1: 1.10 to 1.20

o Heaviest and most polar molecular component

o Most Inorganic matter in bitumen is concentrated in asphaltenes.

Sheremata, J.M. et al., Energy&Fuels,18(5),1377-1384

6

Asphaltene Gasification

o Metals like V & Ni liberate during gasification and pass through downstream of gasifier.

o If soot formed during gasification encapsulate the metals, the fouling and erosion can be avoided.

o Understanding the soot formation during gasification is important.

http://www.escet.urjc.es/~sop/alumnos/proyectos/descargas/propuesta18.pdf

Pyrolysis

Chhiti et al., Energy Fuels, 2011, 25, 345–351

Fuel

Tar

Primary Gas

Primary Char

Soot

Zhang, H. Ph.D. Thesis, NITROGEN EVOLUTION AND SOOT FORMATION DURING SECONDARY COAL PYROLYSIS, Brigham Young University, 2001.

Su

rface

gro

wth

7

Soot Formation Mechanism

Soot Formation Mechanism

8

Nucleation of soot particle (inception and growth of PAHs)

Particle coagulation

Particle surface reactions (Growth and surface oxidation)

Particle agglomeration

J. Warnatz, U. Maas, R. Dibble, Combustion: physical and chemical fundamentals, modeling and simulation, experiments, pollutant formation, Springer, 2006. Sarofim et al. , Springer Series in Chemical Physics , 1994, 59, 485-99

Objectives

Model soot formation during asphaltene pyrolysis and gasification in a drop tube furnace. Validation of the model by comparing with experimental results.

9

Process Parameters Value

Feed particle size 207 µm

Pyrolysis Temperature

1000 °C

1200 °C

1400 °C

Partial Oxidation @ 1200oC

Lambda = 0.13

Lambda = 0.22

Lambda = 0.31

10

Experimental Set up

Collection of soot on substrate aluminium foil

1

4 5 6

2 3

Entrained Flow Reactor Mullite tube 2.5 in. ID × 5 ft. L Max Temperature: 1500 oC Atmospheric pressure

Dekati low pressure impactor user manual ver. 3.4, 2007.

CFD model

o Ansys Fluent 16.0 o 2D-axisymmetric geometry o k-ω-SST turbulence model o P-1 radiation model o Incompressible ideal gas o Fuel injection : DPM o Devolatilization : Single rate o Soot formation : Moss-Brookes-Hall o Soot Oxidation model : Lee model

Simulation Methodology

11

Volatiles during pyrolysis

C2H2

C6H6

CH4

CO

N2

Assumptions o Soot precursor : C2H2 and C6H6

o Soot surface growth from : C2H2

o Soot-Radiation interaction enabled o Detailed gas phase mechanism DRM-22 is used o Total 26 species and 108 reactions

o 71 % of feed asphaltene – volatiles; rest is char. H2S is accounted as N2.

The Moss-Brookes Model

Solves transport equations for normalized radical nuclei concentration and soot mass fraction :

The instantaneous production rate of soot particles

The source term for soot mass concentration

The model assumes that the hydroxyl radical is the dominant oxidizing agent

soot inception due to acetylene or benzene

12

The Moss-Brookes-Hall Model

It is based on a soot inception rate due to two-ringed and three-ringed aromatics, as opposed to the Moss-Brookes assumption of a soot inception due to acetylene or benzene (for higher hydrocarbons)

Hall et al. proposed a soot inception rate based on the formation rates of two-ringed and three-ringed aromatics , from acetylene, benzene, and the phenyl radical based on the following mechanisms:

The inception rate of soot particles as given to be eight times the formation rate of above species

Both the coagulation term and the surface growth term were formulated similar to those used by Brookes and Moss.

Oxidation due to O2 was added, in addition to the soot oxidation due to the hydroxyl radical. 13

Drop Tube Furnace Meshing Carrier gas +DPM

Gasifying agent

Outflow BC

Wall Temp.

Mesh quantity 76530 quadrilateral cells

14

Boundary Conditions

Inlet inner – Mass flow inlet - N2

Inlet Outer- Mass flow inlet - Air/N2

Outlet – Outflow

Velocity and Temperature profile

15

16

1200oC

1400oC

Pyrolysis @ 1000oC

Devolatilization – C6H6

17

Soot Mass Fraction

Pyrolysis @ 1000oC

1400oC

1200oC

18

0

0,2

0,4

0,6

0,8

1

1000 1200 1400

Soo

t m

ass

frac

tio

n

Temperature oC

Experiment

Model

0

0,1

0,2

0,3

0,4

0,5

13 31

Soo

t m

ass

frac

tio

n

Stoichiometric Oxygen %

Experiment

Model

Validation

Soot mass fraction increases with increase in pyrolysis temperature.

Soot mass fraction decreases with increase in stoichiometric oxygen %.

Pyrolysis-Nucleation, Coagulation, Surface Growth

19

0

0,2

0,4

0,6

0,8

1

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

So

ot

mas

s fr

acti

on

Axial distance (m)

1000 deg C

1200 deg C

1400 deg C

0

1000

2000

3000

4000

5000

6000

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Rate

of

nu

clea

tion

(1e+

15

part

icle

s/m

3-s

)

Axial distance (m)

1000 deg C

1200 deg C

1400 deg C

0

1000

2000

3000

4000

5000

6000

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Rate

of

coagu

lati

on

(1e+

15

part

icle

s/m

3-s

)

Axial distance (m)

1000 deg C

1200 deg C

1400 deg C

0

0,02

0,04

0,06

0,08

0,1

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Ra

te o

f su

rface

gro

wth

(kg/m

3-s

)

Axial distance (m)

1000 deg C1200 deg C1400 deg C

20

0

0,1

0,2

0,3

0,4

0,5

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

So

ot

mas

s fr

acti

on

Axial distance (m)

Lambda=0.13

Lambda=0.22

Lambda=0.31

0

2000

4000

6000

8000

10000

12000

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Rate

of

nu

clea

tion

(1e+

15

part

icle

s/m

3-s

)

Axial distance (m)

Lambda=0.13

Lambda=0.22

Lambda=0.31

0

2000

4000

6000

8000

10000

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6Rate

of

coa

gu

lati

on

(1

e+1

5

pa

rtic

les/

m3-s

)

Axial distance (m)

Lambda=0.13

Lambda=0.22

Lambda=0.31

0

0,02

0,04

0,06

0,08

0,1

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Rate

of

surf

ace

gro

wth

(kg

/m3-s

)

Axial distance (m)

Lambda=0.13

Lambda=0.22

Lambda=0.31

Partial Oxidation @ 1200oC -Nucleation, Coagulation, Surface Growth

Conclusions

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Soot mass fraction: Increases with increase in pyrolysis temperature

Decreases with increase in stoichiometric oxygen %

The effect of nucleation, coagulation and surface growth of soot particles can be

analyzed in detail using the model.

Very simple CFD model predicted the effect of Temperature and stoichiometric

ratio on soot formation and qualitatively in agreement with experimental results.

Future Work

Fine tune the present model to predict the soot formation more accurately.

Implement different devolatilization and soot oxidation mechanisms.

Acknowledgements

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