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Modeling Biomass Gasification Surface Reactions: The Effect of Hydrogen Inhibition By Daniel Ivan Pineda A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in Engineering – Mechanical Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Jyh-Yuan Chen, Chair Professor Robert W. Dibble Professor Fotini K. Chow Fall 2014
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Page 1: Surface kinetics modeling of gasification reactions

Modeling Biomass Gasification Surface Reactions:The Effect of Hydrogen Inhibition

By

Daniel Ivan Pineda

A thesis submitted in partial satisfaction of the

requirements for the degree of

Master of Science

in

Engineering – Mechanical Engineering

in the

Graduate Division

of the

University of California, Berkeley

Committee in charge:

Professor Jyh-Yuan Chen, ChairProfessor Robert W. Dibble

Professor Fotini K. Chow

Fall 2014

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Modeling biomass gasification surface reactions: the effect ofhydrogen inhibition

Copyright © 2014

by

Daniel Ivan Pineda

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Abstract

Modeling Biomass Gasification Surface Reactions: The Effect of Hydrogen Inhibitionby

Daniel Ivan PinedaMaster of Science in Engineering – Mechanical Engineering

University of California, BerkeleyProfessor Jyh-Yuan Chen, Chair

The gasification of carbonaceous char into more versatile gaseous species such as carbonmonoxide and hydrogen presents a potentially less carbon-intensive thermochemical conver-sion pathway of both fossil fuel-based and biomass-based solid fuels into energy for humanutilization. Observations of product gases of gasification reactions—particularly hydrogen—inhibiting the reaction itself have been documented for decades. These have so far beenquantified in Langmuir-Hinshelwood-type relations for overall surface gasification rates. De-tailed surface kinetic reaction mechanisms that can be applied to a variety of reactor modelsand conditions are needed, particularly those which describe gasification inhibition by thepresence of hydrogen. To determine the effects of hydrogen inhibition reactions in gasi-fication conditions, existing surface kinetic mechanisms were employed in simulations andcompared with experimental data also available in the literature. Where appropriate, theseexperiments were simulated as perfectly stirred reactors with surface reactions using thesesurface reaction mechanisms. Preliminary results reveal the inability of current models inpredicting experimentally observed inhibition trends. Additional reactions were appendedto the surface mechanism based on a combination of theoretical and empirical considerationsto improve the model. Reasonable agreement with experimental data is found with regard tothe kinetics of carbon gasification reactions in the inhibiting presence of hydrogen. Thus, amodified surface kinetic reaction mechanism is proposed utilizing sticking coefficients. Simu-lations of a hypothetical simplified char gasification reactor are conducted using this modifiedmechanism. Temperature, residence time, and surface area were varied to determine theireffects on the formation of product gases. This investigation represents a promising stepin the direction of a new surface kinetic mechanism for a generalized carbon gasificationreaction that can be used in a variety of reactor models.

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Dedication

To my family:

My grandparents, Luciano Lozano Guerra & Antelia de Leon Garza,Juan Inocenio Pineda & Graciela Carolina Martinez; from humblebeginnings you have shown me that perseverance will take me a longway.

My Aunt Noelia, for your unwavering support in my educationalendeavors; from the hand-laminated NASA Voyager spacecraft pho-tographs you gave me before I can remember, you have always strivento expand my horizons.

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Contents

List of Figures iv

List of Tables vi

1 Thermal Energy Conversion in the World 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Social Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.2 Economic Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.1 Biomass Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.2 Thermal Conversion Pathways . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Hypothesis and Research Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Surface Kinetics, Pyrolysis, Gasification, Combustion Overview 72.1 Surface Chemistry Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Langmuir Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.2 Langmuir-Hinshelwood Model . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.1 General Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3.1 General Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.3.2 Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Relevant Experiments and Models in the Literature 153.1 Investigation of Carbon Oxidation . . . . . . . . . . . . . . . . . . . . . . . . 153.2 Investigation of the H2 inhibition effect . . . . . . . . . . . . . . . . . . . . . 16

4 Numerical Simulation Setup 184.1 Chemkin™ and Surface Chemkin™ . . . . . . . . . . . . . . . . . . . . . 184.2 Surface Perfectly Stirred Reactor (SPSR) . . . . . . . . . . . . . . . . . . . . 19

4.2.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.2.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.3 Numerical Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.3.1 Initial Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.3.2 Modified Damped Newton’s Method . . . . . . . . . . . . . . . . . . 22

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4.3.3 Time Stepping Method . . . . . . . . . . . . . . . . . . . . . . . . . . 234.4 Surface Kinetic Mechanism Considered . . . . . . . . . . . . . . . . . . . . . 23

4.4.1 Proposed Additional Reactions for H2 Inhibition . . . . . . . . . . . . 244.5 Experimental data comparisons with calculations . . . . . . . . . . . . . . . 28

4.5.1 Steam Gasification in a Small Scale Reactor . . . . . . . . . . . . . . 284.5.2 Steam Gasification in a Larger Reactor . . . . . . . . . . . . . . . . . 314.5.3 Carbon Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.6 Analysis of Proposed Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 344.6.1 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.6.2 Alternative Possibilities for Proposed Reactions . . . . . . . . . . . . 354.6.3 Possible compounding inhibition by CO . . . . . . . . . . . . . . . . 364.6.4 Water-Gas Shift Considerations . . . . . . . . . . . . . . . . . . . . . 37

5 Reactor Simulation Results 385.1 Variation in Residence Time . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.2 Variation in Reactor Temperature . . . . . . . . . . . . . . . . . . . . . . . . 405.3 The Effect of Surface Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.4 Potential Implications for Real Reactors . . . . . . . . . . . . . . . . . . . . 43

6 Conclusions 446.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.1.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.2 Simulation Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6.2.1 Limitations Imposed by Model Assumptions . . . . . . . . . . . . . . 456.2.2 Limitations Imposed by Numerical Considerations . . . . . . . . . . . 456.2.3 Surface Mechanism Intentions vs Application . . . . . . . . . . . . . . 46

6.3 Broader Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

A Additional Reference Material 47A.1 Surface Chemistry Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

A.1.1 Unimolecular Reaction Dynamics . . . . . . . . . . . . . . . . . . . . 47A.1.2 Heterogeneous Catalysis and Gas Surface Reactions . . . . . . . . . . 47

B Computational Parameters 48B.1 Surface Chemkin input file . . . . . . . . . . . . . . . . . . . . . . . . . . 48

C Bibliography 50

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List of Figures

1.1 Diagram depicting a spectrum of high temperature solid fuel reactions . . . . 41.2 Reaction sequence diagram for biomass gasification reproduced from Basu [20] 5

2.1 Example of adsorption and desorption for a carbon surface reacting with awater molecule. In (b), bulk carbon replaces surface carbon. . . . . . . . . . 7

2.2 C-H-O ternary diagram reproduced from Basu [55] . . . . . . . . . . . . . . 10

4.1 Diagram of Surface PSR reproduced from Meeks et al. [96] . . . . . . . . . . 194.2 Gasifiation rate as a function of variation of H2 in the inlet gas. Comparison

of the model proposed by Huttinger and Merdes backed up by experimentaldata (dashed-dashed lines) with the mechanism in Table 4.1 (square mark-ers) and the proposed mechanism in Table 4.3 (circle markers). Red colorindicates simulations and experiments with 1 bar of H2O, blue color indicatessimulations and experiments with 0.55 bar of H2O, and green color indicatessimulations and experiments with 2.2 bar of H2O. Experimental and simu-lated temperature is 1273 K. . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.3 Variation of Cf site density used with the mechanism in Table 4.3 and resultinginitial zero-H2 rates of gasification (solid lines with squares) compared withthe experimental gasification rate data reported by Gadsby et al. [22] (dashedlines) at 1073 K for the different partial pressures of H2O. The intersectionsof simulations and experiments are marked with dashed black lines. . . . . . 31

4.4 Rate of CO2 produced by steam gasfication of carbon at 1073 K as a functionof variation of H2 in the inlet gas; effect of using multiple PSRs in series. Solidlines are from SPSR simulations using the surface kinetics mechanism in Table4.3 with 8 PSRs, dashed-dashed lines are using 4 PSRs, and dashed-dot linesare using 1 PSR. Surface site density is held constant at 1.0× 10−10 mol/cm2.X Markers are the corresponding experimental data from Gadsby et al. [22]. 33

4.5 Comparison of the CO2/CO ratios obtained with existing models and data inthe literature. The colored solid lines with markers indicate models for whichexperimental data are available, while the dashed colored lines indicate thecorresponding models for which there are no experimental data with which toverify. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.6 Relative sensitivity of the parameters of interest for the SPSR Simulation ofthe experiments by Huttinger and Merdes. . . . . . . . . . . . . . . . . . . . 35

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4.7 Relative sensitivity of the parameters of interest for the SPSR simulation ofthe experiments by Gadsby et al. . . . . . . . . . . . . . . . . . . . . . . . . 36

5.1 Exit Mole Fractions (gas phase) and steady state site fractions (surface phase)as a function of reactor residence time. Solid lines represent simulations witha surface area of 1.0×106 cm2, dashed-dashed lines 1.0×105 cm2, and dashed-dot lines 1.0× 104 cm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.2 Exit Mole Fractions (gas phase) and steady state site fractions (surface phase)as a function of reactor temperature. Solid lines represent simulations with asurface area of 1.0× 106 cm2, dashed-dashed lines 1.0× 105 cm2, and dashed-dot lines 1.0× 104 cm2. Residence time is 0.1 seconds. . . . . . . . . . . . . 41

5.3 Detailed Surface Species Molar Balance rates as a function of reactor tem-perature. CO production is the black line in the positive region, while Cb

consumption is the black line in the negative region. Solid lines represent sim-ulations with a surface area of 1.0×106 cm2, dashed-dashed lines 1.0×105 cm2,and dashed-dot lines 1.0× 104 cm2. Residence time is 0.1 seconds. . . . . . . 42

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List of Tables

3.1 Rate constants from Huttinger and Merdes [72] used in Equation 3.2. . . . . 17

4.1 Surface Mechanism used by Hecht et al. [70], where C(X) indicates a surfacecomplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.2 Proposed Surface Mechanism with suggested H2 inhibition reactions, whereC(X) indicates a surface complex . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3 Proposed Surface Mechanism with H2 inhibition reactions, where C(X) indi-cates a surface complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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List of reactions

Reaction {2.1}: Heterogeneous Reaction - Simple Decomposition Adsorption Step 8Reaction {2.2}: Heterogeneous Reaction - Simple Decomposition Desorption Step 8Reaction {2.3}: Heterogeneous Reaction - Langmuir-Hinshelwood Mechanism Step 1 8Reaction {2.4}: Heterogeneous Reaction - Langmuir-Hinshelwood Mechanism Step 2 8Reaction {2.5}: Heterogeneous Reaction - Langmuir-Hinshelwood Mechanism Step 3 9Reaction {2.6}: Carbon Reaction: Boudouard Reaction . . . . . . . . . . . . . . . 11Reaction {2.7}: Carbon Reaction: Water-Gas Reaction . . . . . . . . . . . . . . . 11Reaction {2.8}: Carbon Reaction: Hydrogasification Reaction . . . . . . . . . . . 11Reaction {2.9}: Carbon Reaction: Partial Oxidation . . . . . . . . . . . . . . . . 11Reaction {2.10}: Oxidation Reaction: Carbon . . . . . . . . . . . . . . . . . . . . 12Reaction {2.11}: Oxidation Reaction: CO . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.12}: Oxidation Reaction: CH4 . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.13}: Oxidation Reaction: H2 . . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.14}: Shift Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.15}: Methanation Reaction 1 . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.16}: Methanation Reaction 2 . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.17}: Methanation Reaction 3 . . . . . . . . . . . . . . . . . . . . . . 12Reaction {2.18}: Steam Reforming Reaction 1 . . . . . . . . . . . . . . . . . . . . 12Reaction {2.19}: Steam Reforming Reaction 2 . . . . . . . . . . . . . . . . . . . . 12Reaction {2.20}: Boudouard Reaction: Step 1 . . . . . . . . . . . . . . . . . . . . 12Reaction {2.21}: Boudouard Reaction: Step 3 . . . . . . . . . . . . . . . . . . . . 12Reaction {2.22}: Boudouard Reaction: Step 3 . . . . . . . . . . . . . . . . . . . . 12Reaction {2.23}: Water-Gas Reaction: Step 1 . . . . . . . . . . . . . . . . . . . . 13Reaction {2.24}: Water-Gas Reaction: Step 2 . . . . . . . . . . . . . . . . . . . . 13Reaction {2.25}: Water-Gas Reaction: Step 3 . . . . . . . . . . . . . . . . . . . . 13Reaction {2.26}: Hydrogen Inhibition: Model 1 . . . . . . . . . . . . . . . . . . . 13Reaction {2.27}: Hydrogen Inhibition: Model 2 . . . . . . . . . . . . . . . . . . . 13Reaction {2.28}: Carbon Surface Oxidation: Step 1 . . . . . . . . . . . . . . . . . 13Reaction {2.29}: Carbon Surface Oxidation: Step 2 . . . . . . . . . . . . . . . . . 13Reaction {2.30}: Carbon Surface Oxidation: Step 3 . . . . . . . . . . . . . . . . . 13Reaction {A.31}: Unimolecular Reaction - Simple Decomposition Step . . . . . . 47Reaction {A.32}: Heterogeneous Reaction - Eley-Rideal Mechanism Step 1 . . . . 47Reaction {A.33}: Heterogeneous Reaction - Eley-Rideal Mechanism Step 2 . . . . 47

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Acknowledgments

I thank Professor Chen for his guidance, and thank MaryAnne Petersfor her reliable support. Special thanks to Colin H. Smith, for readingand editing this thesis, and for continued mentorship in my graduatestudies.

The investigation was supported by the National Science Foundation,Grant No. DGE 1106400. Additionally, funding for the computer sys-tem used for the modeling in this study was in part provided by FY13Committee on Research - Faculty Research Grant, sponsored by theBerkeley Division of the Academic Senate, University of California.

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Chapter 1

Thermal Energy Conversion in theWorld

1.1 Motivation

For most of the 20th century, the widespread use of petroleum has fueled the world’s economyand formed the backbone of global trade. In the latter half of the twentieth century, the priceof oil began to rise and continues to do so. This is partly due to the fact that greater energyinvestments are required to extract remaining fossil fuel resources as the readily obtainablefractions of Earth’s resources continue to decline [1]. In addition, it has been recognized asfar back as the 1970s that fossil fuel emissions not only have the potential to contribute toglobal climate change, but are already doing so. It is the broad scientific consensus thathuman activities are driving climate change [2] and that average global temperatures willnot stabilize if humans continue to consume fossil fuels at current rates [3]. The result-ing climactic change has already impacted all geographical areas of the United States andwill continue to intensify in the next few decades [4]. In a carbon-constrained world, thetransition to less carbon intensive energy sources will not happen quickly and is not imme-diately feasible for all current energy requirements. In particular, high energy density is anindispensable feature of liquid fuels when it comes to energy usage in transportation. Thethermochemical conversion of biomass to energy is an active research area, in part becausebiomass can be processed into high energy density liquid and compressible gaseous fuels viadifferent chemical and thermal pathways while remaining a carbon-neutral energy resource[5], although scalability concerns motivate further research. Biomass—which includes agri-cultural and forestry residues, wood, and crops specifically grown for energy use—is alreadythe fourth most used energy resource behind coal, oil, and natural gas [6]. The gasificationof biomass is a potentially useful thermal conversion pathway which can produce higher-energy density liquid and gaseous fuels while indirectly mitigating anthropogenic climacticchange. These benefits can be realized by providing a bridge between the dominant energyconsuming processes of today with the carbon-constrained and fossil-fuel-constrained worldof the future.

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1.1.1 Social Concerns

Most of the energy available to humans on earth comes in the form biomass; indeed, it isthe most widely used ‘renewable’ energy resource, ahead of hydroelectric power [7]. Thehigh usage of carbon-neutral sources of energy such as biomass is due to the raw burningof biomass for the purpose of cooking and heating in developing nations [8]. This rawcombustion results in high levels of airborne pollutants [8–10] which cause significant healthhazards to women and young children [11]. These populations are most vulnerable to thehealth hazards because of the time they spend at home when food is being cooked in stoveswhich may not be very efficient [12]. It is clear that refinement of biomass into safer andcleaner biofuels is a critical aspect of the biomass to energy movement and must be includedin the international dialogue regarding current and future clean energy initiatives. Moreover,there are concerns that focusing on food crops for biofuels—such as corn for ethanol in theUnited States—is not in the best interest of countries for which food security is a problem,and it is suggested that more focus should be placed on crops which grow quickly and onland which would not be suitable for agriculture anyway [13]. Such crops are far morelignocellulosic and are more difficult to break down with thermochemical methods, makingthis an area of active research.

1.1.2 Economic Concerns

Transportation demand for liquid fuels does not vary significantly under scenarios in whichthe price of oil is increased [1]. Likewise, the advancement of fuel cell technologies is also in-creasing the demand for methods of generating hydrogen and other synthensis gas (syngas)compounds, which are mixtures of hydrogen, carbon monoxide, and other organic com-pounds. While biochemical conversion methods of biomass to fuel such as ethanol distil-lation and algae utilization [14] are available, thermal reformation of biomass fuel sourcesinto fuels and/or syngas remains a viable option for regions that lack the resources to hostchemical refining capabilities. In addition, there is research focus on small scale reactorsin these regions because it is more expensive to transport bulk raw biomass than processedbio oil. It is more efficient to process biomass on-site to bio oil or compressed syngas andthen transport for further refinement elsewhere [15]. Non-catalytic partial oxidation—thatis, a thermochemical conversion that does not require catalysts—of existing liquid fuels hasbeen demonstrated as a potentially locally accessible method of energy conversion [16] forin situ syngas production. Gasification of solid fuels such as biomass can provide additionalpathways for localized syngas production and consumption.

1.2 Process Overview

The motivation of this study is based on the idea that biomass-to-energy processes standto benefit from an increased understanding of inhibition reactions. This investigation usesreaction kinetics and draws on experiments studying the combustion and gasification of coalchar. The gasification of biomass char rather than coal char presents some complications inthe overall process, which are acknowledged in this and other sections.

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1.2.1 Biomass Constituents

Biomass is primarily composed of three constituents: hemicelluloses, cellulose, and lignin[17], but there are other constituents to biomass than these main three. The composition ofdrywood is given by Borman and Ragland [18] as being composed of cellulose, hemicellulose,lignin, resins (extractives), and ash forming minerals. Cellulose (C6H10O5) is a condensedpolymer of glucose (C6H12O6), and is the primary ingredient in the fiber walls of drywood,representing 30 to 40% of the dry weight of wood. Hemicellulose is a named given tosubstances which consist of various sugars other than glucose that encase cellulose fibersand represent about 20 to 35% of the dry weight of wood. Lignin (C40H44O6) is a nonsugarpolymer that gives strength to the wood fiber, representing about 15 to 30% of the dryweight. Wood extractives include oils, resins, gums, fats, waxes, and some other heavyorganic compounds that do not exceed a few percent of the overall dry weight, except inthe bark of wood, where they can represent as much as 20 to 40% of the dry weight. Ash,the remaining constituents, compose about 0.2 to 1% by weight of dry wood, and consistmainly of potassium (K), Manganese (Mn), Magnesium (Mg), and sodium oxides (NaOx),with lesser amounts of other oxides of metals such as Iron (Fe) and Aluminum (Al). The ashcontent of bark is around 1 to 3%. Ash is an important consideration in biomass pyrolysisand gasification processes since the metals can agglomerate and cause significant problemsfor industrial gasifiers [15].

1.2.2 Thermal Conversion Pathways

Pyrolysis, gasification, and combustion of solid fuels all lie in a spectrum of high temperaturechemical kinetics, the distinctions at the industrial scale typically being the amount of oxygenthat is present during the high temperature process. However, oxygen is not necessary forgasification to occur. Gasification of a solid can occur when a gas—for example, water vaporor carbon dioxide—reacts with the solid surface to form another gas (hydrogen and/or carbonmonoxide, respectively). Within each group of processes, other distinctions are present, suchas high temperature vs low temperature reactions, fast or slow reactions, and homogeneousor heterogeneous reactions. It is important to note that all of these processes may besimultaneously occurring in the same reactor, in different stages or areas of a reactor. Ageneral diagram which demonstrates the typical major distinctions is given in Figure 1.1.Depending on the configuration of a given reactor, the processes can happen in any orderand the output of any process can be recirculated into the others. A generic updraft gasifierwill be used as a primary guide for the analysis in this study. In this type of gasifier,solid feed enters from the top, while hot process gases enter from below, react as they flowthrough the solids, and flow out of the top of the reactor as producer gas or syngas [19].This configuration is convenient for modeling purposes because each of the processes canbe relatively isolated for independent simulation and analysis. A reaction diagram adaptedfrom Basu [20] is given below in Figure 1.2, further dividing the processes into individualreactions and products. The focus of this investigation is explicitly to study the surfacereaction kinetics of solid char gasification reactions; however, brief discussions of the otherprocesses are given here for the sake of completeness and to provide context.

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Figure 1.1: Diagram depicting a spectrum of high temperature solid fuel reactions

Pyrolysis

Pyrolysis reactions are the first chemical reactions which happen from the perspective ofbiomass feed entering the system. According to Demirba and Arin [21], “Pyrolysis is thethermal degradation of biomass by heat in the absence of oxygen, which results in theproduction of charcoal (solid), bio-oil (liquid), and fuel gas products.” Upon examiningFigure 1.2, it is reasonable to conclude that the pyrolysis reactions are complex and varieddepending on the types of biomass under consideration. The process of pyrolysis breaksbiomass into gases which include CO, H2, CH4, and H2O; liquids including tar, oil, andnaphtha; oxygenated compounds including phenols and acids; and solid char. Each of thesegroups of pyrolysis products undergoes reactions; char undergoing char gasification reactionswhile the other groups undergo gas phase reactions that can include cracking reactions,reforming reactions, combustion reactions, and shift reactions.

Gasification

Gasification, like pyrolysis, is a high temperature chemical reaction of solid organic com-pounds into liquid and mostly gaseous products [20]. Unlike pyrolysis, however, gasificationprocesses can include a limited amount of oxygen, which oxidizes the carbon in the solid fuelinto CO. CO can also be formed by CO2 or H2O being reduced at the solid carbon surface.Gasification in industrial processes is typically preceded by pyrolysis, such that inputs tothe gasification process are mostly char-like organic materials, for which chemistry is betterunderstood than that of virgin biomass going into a pyrolysis zone. Gases interact with thesolid char interface to extract carbon off of the surface into gases such as CH4, CO, andCO2. Gaseous H2O can interact with the char surface to be reduced into gaseous H2. Thesereactions have several steps which are discussed in more detail in Section 2.3.2.

Combustion and Oxidation Reactions

The combustion process can be a critical aspect of converting solid biomass into productgases due to the endothermic nature of many pyrolysis and gasification reactions, though it

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Figure 1.2: Reaction sequence diagram for biomass gasification reproduced from Basu [20]

is not employed in some reactors. In some industrial scenarios, it is feasible for high heatfluxes to be available to reactors without consuming any of the fuel inside of the gasifier, sothat the gasification, pyrolysis, and drying processes can be sustained, despite the presenceof endothermic reactions. However—as mentioned—owing to the fact that the net energyreturn on biomass can be greatly diminished with even minor transportation costs, andsince a substantial portion of both biomass and coal is available far away from industrializedgeographies, it is suggested that smaller reactors should be favored, which need to providetheir own sources of heat [15]. It is in these scenarios where combustion reactions whichconsume a portion of the fuel provide the required heat to sustain the gasification andpyrolysis reactions which produce the syngas products that can be further refined elsewherewith a relatively lower transportation expense. In an updraft gasifier, most of the combustionreactions take place in the form of char or carbon combustion reactions, since the initial feedby that point has already been pyrolyzed and gasified into particles primarily composed ofcarbonaceous char [20].

1.3 Hypothesis and Research Goals

This investigation hypothesizes that inhibiting effects of hydrogen gas (H2) on gasificationrates can be quantified in a finite rate surface kinetic reaction mechanism. The primaryconcern for this hypothesis is understanding the reactions of carbon with oxygen and water.Gasification of biomass is an overall lower temperature process than gasification and com-bustion of coal owing to the relatively lower caloric value of the fuel. The aforementionedinhibiting effects of the presence of CO and H2 on the surface kinetics of carbon gasification

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reactions are documented in experimental literature. It is possible that operators of gasifica-tion reactors in both large industrial plants and small rural generators may want to increasethe temperatures in the reactor by recycling produced syngas into the reactant stream. It isalso possible that the longer required residence times of biomass in gasification reactors givethe product gases more time to inhibit the reactions at the surface. While the inhibitioneffects are documented, there is a lack of detailed surface kinetic mechanisms that describethe interactions of the reactions with the carbon surface. Understanding more details ofthese effects at the surface molecular level will assist with the design of biomass gasifiers, inaddition to other technologies which convert carbonaceous char into syngas.

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Chapter 2

Surface Kinetics, Pyrolysis,Gasification, Combustion Overview

2.1 Surface Chemistry Overview

Surface reactions of carbon with gaseous species in high temperature processes have longbeen discussed [22–26] as fundamental considerations of industrial applications, in no smallpart due to the widespread use of coal over the last two centuries. A basic understandingand review of surface chemistry and surface reaction kinetics is necessary to continue withunderstanding the following sections. In general, surface reactions are complicated processesthat include a few critical steps: (1) diffusion of reactants from the bulk fluid onto thesurface, (2) adsorption of the reactants onto the surface (Figure 2.1a), (3) reaction on thesurface, (4) desorption of the products from the surface (Figure 2.1b), and (5) diffusion ofthe products from the surface back to the bulk fluid. It is important to note that in step (4),it is possible (and for this study, necessary) for reactants originally part of the bulk surfaceto be carried into the bulk fluid. A more comprehensive overview of some different surfacechemistry models is given in Appendix A.1.

(a) Gaseous H2O deposits an oxygen atomonto the carbon surface to form gaseousH2 while leaving behind a surface complex,C(O).

(b) The surface complex C(O) undergoesdesorption and is carried into the bath gasas gaseous CO.

Figure 2.1: Example of adsorption and desorption for a carbon surface reacting with a watermolecule. In (b), bulk carbon replaces surface carbon.

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Page 19: Surface kinetics modeling of gasification reactions

2.1.1 Langmuir Adsorption

An investigation into surface reactions in gasification and pyrolysis can benefit from lookinginto surface reactions involved in heterogeneous catalysis. A generalized surface-catalyzedreaction of gaseous species A with at a localized site S on a surface follows as [27]:

A + S −−⇀↽−− A · S {2.1}

The rate of this reaction depends on the magnitude of the adsorption coefficient Kads, eq,which is the equilibrium constant for the reaction (Reaction 2.1 is assumed to be in equilib-rium), defined by the ratio of the rate constant of adsorption to rate constant of desorptionkabs/kdes. The rate of surface reactions is also limited by the number of available surface sitesS0. More discussion is provided by Steinfeld et al. [27], but ultimately, we can express theconcentration of adsorbed species A · S in terms of the dimensionless quantity of coverage,θ, given by Equation 2.1:

θ =S

S0

=Kads, eq [A]

1 +Kads, eq [A]=

bp

1 + bp(2.1)

The physical meaning of θ is the ratio of available surface sites which are occupied by theadsorbed species; as the surface becomes saturated, θ approaches unity. The right hand sideequality follows as a consequence if adsorption is from a low pressure gas that obeys theideal gas law, where b = Kads, eq/kBT and p is the pressure of gas A. Equation 2.1 is knownas the Langmuir adsorption isotherm. Reaction 2.1 is followed by:

A · S k2−→ Products {2.2}

And the total rate of conversion, combining Equation 2.1 and Reaction 2.2, is

R = −d [A]

dt= k2θS0 =

k2S0Kads, eq [A]

1 +Kads, eq [A](2.2)

It should be noted by inspection in Equation 2.2 that at low [A], Kads, eq[A] � 1, so therate is proportional to the product of the partial pressure of A and the total amount ofcatalyst, while at large [A], Kads, eq[A] � 1, the rate becomes independent of pressure, andthe catalyst is considered saturated.

2.1.2 Langmuir-Hinshelwood Model

Steinfeld et al. [27] discuss a critical possible mechanism for biomolecular surface-catalyzedreactions. In Reaction 2.3, gas-phase species A reacts with surface-phase species S to form asurface complex A · S. Likewise, in Reaction 2.4, gas-phase species B reacts with S to formB · S.

A + S −−⇀↽−− A · S governed by: K(A)

ads, eq {2.3}

B + S −−⇀↽−− B · S governed by: K(B)

ads, eq {2.4}

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The surface complexes could desorb from the surface, or they can undergo further reactions,such as in Reaction 2.5, to form other products.

A · S + B · Sk′2−→ Products {2.5}

Reactions 2.3 and 2.4 are assumed to happen very fast compared to reaction 2.5. The overallrate of reaction is simplified as Equation 2.3, where θ is the degree of surface coverage forthe surface sites.

R = k′2θS02 =

k′2K(A)ads, eq[A]K

(B)ads, eq[B]S0

2{1 +K

(A)ads, eq[A] +K

(B)ads, eq[B]

}2 (2.3)

Reactions 2.3 through 2.5 and Equation 2.3 are part of the Langmuir-Hinshelwood model.Other basic surface models and reactions are discussed in Appendix A.1, but the Langmuir-Hinshelwood model is the one most often referred to in gasification literature.

2.2 Pyrolysis

Many physical and chemical models must be employed to accurately model pyrolysis pro-cesses, which is essential both for gasification and combustion of solid biomass. In additionto the chemical kinetics, heat transfer must be considered; from the conduction inside thesolid particles, to the diffusion inside of the particle pores, to the convection and radiationfrom the surface of the pellet. There has been much research into kinetic mechanisms forbiomass pyrolysis over the years [6, 17, 28]. There are many proposed models [17], someof which take into account the varying compositions of biomass based on its levels of cel-lulose, hemicellulose, and/or lignin [29–32] some studies have specifically investigated thepyrolysis kinetics of cellulose [33–38], but most studies have evaluated mass-based “semi-global” reaction kinetics [39–46]. There has been little progress in the way of a chemicalkinetic mechanism composed of elementary reactions. Indeed, it has been acknowledged inthe literature that such mechanisms are scarce [17].

2.2.1 General Chemistry

From a molecular perspective, pyrolysis of a solid fuel or a large hydrocarbon molecule occursthrough thermal decomposition reactions [47], which can involve either third-body collisionsor the vibrational destruction of a molecular bond. The chemistry of pyrolysis is varied overthe course of the process, and can depend on environmental conditions, including but notlimited to the presence of catalysts. Shorter residence times and higher temperatures lead toinitial cracking that can break off longer chained hydrocarbons from the solid mass, whichcan later condense into liquids and heavy hydrocarbons, while longer residence times andlower temperatures allow these long chained hydrocarbons to undergo secondary crackinginto smaller chained hydrocarbons and gases [21]. Much existing knowledge regarding thekinetics of the pyrolysis of solid fuels is drawn from observations of the pyrolysis of plastics[48–52], whose predictable structures form a generally known collection of products. Thefire science community in particular has provided significant contributions to the state of

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knowledge regarding the pyrolysis and combustion of solid plastics [44, 53, 54] , for thepurposes of fire protection engineering. These investigations into the thermal degradation ofpolymers can yield valuable insights to the breakdown of biomass-based organic materials.

2.3 Gasification

In general, gasification is the oxidation of solid fuels, although the degree of oxidation andthe temperatures are both lower than those of combustion reactions. A typical gasificationprocess from virgin biomass to products proceeds as follows: 1 ) drying; 2) thermal decom-position or pyrolysis; 3) partial combustion of some gases, vapors, char; and 4) gasificationof decomposition products. The medium in which gasification occurs can be oxygen, steam,or air, or limited amounts of some of these at the expense of others. The products of gasi-fication can vary depending on the overall atomic composition in the reactor, as well as thetemperature and residence time. In Figure 2.2, the area bounded by the dotted lines typi-

Figure 2.2: C-H-O ternary diagram reproduced from Basu [55]

cally represents the products of gasification. More oxygen in the overall reactions would leadto combustion, while less oxygen would result in reactions more like pyrolysis to producechar.

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2.3.1 General Practice

Solid fuels undergoing gasification require an oxidizing agent, which can provide the oxygenatoms that eventually end up in carbon monoxide in the product gas. Frequently usedoxidation agents include air, pure oxygen, steam, carbon dioxide, or a mixture of these.Due to its availability, air is frequently used as an oxidation agent, although the presenceof nitrogen can reduce the total effective conversion of solid fuel into a gaseous fuel, andat extremely high temperatures (combustion temperatures) will lead to the production ofnitrogen oxides (NOx). While pure oxygen tends to increase the heating value of the productgas [56], the economic cost of pure oxygen can be prohibitive. It has been observed in othernon-catalytic fuel reforming practices that the presence of H2O in the reformed fuel canboost the H2 content of the product gas [57]. Using steam can increase the heating value ofthe gas, but due to the endothermic nature of the reactions which will be discussed in thefollowing subsection, requires higher temperatures (above 800 °C) if no catalysts are present[56]. As mentioned previously, water is inherently a constituent of biomass, and because ofthis, steam will always be present in some quantities during the gasification process.

2.3.2 Chemistry

Basu [20] notes some important differences between the char produced from biomass andother char in the industry; specifically, that char produced through pyrolysis of biomass isnot pure carbon; it contains some oxygen and hydrogen. Biomass char is more porous andreactive than coke—its porosity is around 40% to 50%, whereas the porosity of coal char isabout 2% to 18%. The pores are larger (about 20 to 30 µm) compared to those of coal char(around 5 Angstroms). The reactivity of biomass char increases as the char is converted(rather than decreases) because the higher amounts of alkali metals present in biomass characts as a catalyst for the gasification reactions. Often in studies where a biomass gasificationreactor is modeled incorporating computational fluid dynamics, the chemistry of pyrolysisis simplified into a single pyro-oxidation step [58–61], and then subsequent reactions withchar follow Langmuir-Hinshelwood kinetics [58, 61, 62], which are discussed in Section 2.1.There has been much investigation into the detailed surface kinetics of char gasification[6, 56, 63, 64], and a few studies have incorporated these mechanisms into simulations [65–70], although mostly in the context of coal char gasification and/or combustion.

Overall Relevant Char Gasification Reaction Groups

According to Basu [20], the following are typical gasification reactions at 25 °C, with theirrespective heats of reaction given in units of (kJ/mol).

Carbon Reactions:

C + CO2−−⇀↽−− 2 CO +172 kJ/mol {2.6}

C + H2O −−⇀↽−− CO + H2 +131 kJ/mol {2.7}C + 2 H2

−−⇀↽−− CH4 −74.8 kJ/mol {2.8}C + 1

2O2 −→ CO −111 kJ/mol {2.9}

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Oxidation Reactions:

C + O2 −→ CO2 −394 kJ/mol {2.10}CO + 1

2O2 −→ CO2 −284 kJ/mol {2.11}

CH4 + 2 O2−−⇀↽−− CO2 + 2 H2O −803 kJ/mol {2.12}

H2 + 12

O2 −→ H2O −242 kJ/mol {2.13}

Shift Reaction:

CO + H2O −−⇀↽−− CO2 + H2 −41.2 kJ/mol {2.14}

Methanation Reactions:

2 CO + 2 H2 −→ CH4 + CO2 −247 kJ/mol {2.15}CO + 3 H2

−−⇀↽−− CH4 + H2O −206 kJ/mol {2.16}CO2 + 4 H2 −→ CH4 + 2 H2O −165 kJ/mol {2.17}

Steam Reforming Reactions:

CH4 + H2O −−⇀↽−− CO + 3 H2 +206 kJ/mol {2.18}CH4 + 1

2O2 −→ CO + 2 H2 −36 kJ/mol {2.19}

It is obvious that most of these reactions are not elementary reactions, and as such do notstrictly represent reactions at the molecular level. Reactions 2.6 through 2.19 are overallthermodynamic reactions which can be broken down into both heterogeneous and homoge-neous elementary reactions.

Heterogeneous Reactions

The carbon reactions 2.6 through 2.9 and oxidation reaction 2.10 are or involve reactions atthe carbon surface, rather than inside a carbonaceous volume. Indeed, transmission electronmicroscopy has revealed hexagon-shaped etch pits on carbon surfaces following gasificationreactions [71]. A good discussion of the reactions can be found in a review by Laurendau[63], investigating coal char gasification reactions. Reaction 2.6 is known as the Boudouardreaction, and its overall behavior can be described by the following surface kinetic mechanism[64] :

Step 1: Cfas + CO2

kb1−→ C(O) + CO {2.20}

Step 2: C(O) + COkb2−→ Cfas + CO2 {2.21}

Step 3: C(O)kb3−→ CO {2.22}

where Cfas is a Carbon Free Active Site and C(O) is a carbon-oxygen surface complex. Thesereaction kinetics can be approximated by the Langmuir-Hinshelwood rate model (mentionedearlier) to express the apparent gasification rate, rb:

rb =kb1PCO2

1 + (kb2/kb3)PCO + (kb1/kb3)PCO2

(2.4)

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where PCO2and PCO are the partial pressure of CO2 and CO, respectively. Reaction 2.7,

more relevant when the char is being gasified in steam, is known as the water-gas reaction,and can be broken down into the following steps:

Step 1: Cfas + H2Okw1−−→ C(O) + H2 {2.23}

Step 2: C(O) + H2kw2−−→ Cfas + H2O {2.24}

Step 3: C(O)kw3−−→ CO {2.25}

While Reactions 2.23 and 2.24 are thought to encompass hydrogen inhibition (known as theoxygen exchange model), hydrogen has been suggested to inhibit the reaction [72] throughthe following steps involving hydrogen adsorption that may also be taken into account:

Step 4: Cfas + H2−−⇀↽−− C(H2) {2.26}

Step 5: Cfas + 12

H2−−⇀↽−− C(H) {2.27}

While the oxygen exchange model has been preferred [63], it is not possible to determinewhich mechanism actually dominates hydrogen inhibition since the set of reactions 2.23 and2.24, and reaction 2.26 result in a Langmuir-Hinshelwood expression in Equation 2.5 whichis identical in both cases [72]:

rw =kw1PH2O

1 + (kw1/kw3)PH2O+ (kw2/kw3)PH2

(2.5)

Use of Reaction 2.27 would result in a P1/2H2

dependence in the denominator instead of aPH2

dependence in Equation 2.5. It is possible that all of the reactions occur simultane-ously, with one or another being preferred at higher temperatures or pressures, so all will beacknowledged here. While the effect of H2 inhibition has been quantified in studies and ele-mentary reactions have been proposed [56], rates for those elementary surface reactions 2.26and 2.27 have not yet been proposed or verified based on the available literature. Reaction2.8, known as the hydrogasification reaction, is the slowest reaction of the carbon reactionsin gasification, and occurs in a hydrogen environment. The only special oxidation reactionat the surface to be considered here is Reaction 2.10, and to some extent Reaction 2.9 sinceit involves the combustion of solid char. Both of these reactions are overall reactions. Whensolid carbon meets oxygen, both reactions 2.9 and 2.10 are possible, but the preferred path-ways depend on the temperature of the reaction. It is suggested that the reactions can bebroken down into the steps in Reactions 2.28 through 2.30 [6, 73].

Step 1: 2 Cf + O2ko1−−→ 2 C(O) {2.28}

Step 2: Cf + C(O) + O2ko2−−→ CO2 + C(O) {2.29}

Step 3: C(O)ko3−−→ CO {2.30}

Here, surface complexes form, and both CO and CO2 are produced via separate reactions.This is a semi-global reaction mechanism; while these reaction steps are simpler, they arenot necessarily elementary reactions, and may contain additional steps. The steady state

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expression [73] for the overall gasification rate can be formed from the above in Equation2.6.

rg =ko1ko2P

2O2

+ ko1ko3PO2

ko1PO2+ ko3/2

(2.6)

A common method employed in the literature [20] is to combine reactions 2.9 and 2.10 andwrite them as:

βC + O2 −→ 2(β − 1)CO + (2− β)CO2 (2.7)

where β is between 1 and 2 and depends on temperature according to Arthur [23]:

β =[CO]

[CO2]= 103.4exp

(−12400 cal/mol

RT

)(2.8)

where T is the temperature of the char and R is the universal gas constant. This methodwith these reactions, however, does not describe elementary heterogeneous reactions with thecarbon surface, and will not be employed in this document except for comparison purposes.Reaction 2.14, the so-called “water gas shift reaction” is the exothermic reaction of H2Owith CO to produce H2 and CO2. This reaction, usually assisted by a catalyst (and as suchcan be considered a heterogeneous reaction), is of great importance to many industrial fuelreforming processes [74].

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Chapter 3

Relevant Experiments and Models inthe Literature

Other researchers have carried out experiments on carbonaceous char gasification processes,and there have been some attempts to formulate reaction rate models taking into accountinhibition by product gases. Langmuir-Hinshelwood models have been applied to calculatethese rates and compare them with experimental data, and some elementary heterogeneousreaction mechanisms have been proposed. These relevant works will be discussed and someof them will be used as a basis of comparison for the mechanisms and models proposed inthis investigation.

3.1 Investigation of Carbon Oxidation

Ratios of CO2/CO production have been investigated by Tognotti et al. [75], and Shaddixet al. [76] recently reviewed this study—among others—numerically and confirmed its empir-ical correlation (Given in Equation 3.1) is credible to 1250 K, remarking that the CO2/COratio is particularly important since the heat released by each oxidation reaction is dramati-cally different (394 kJ/mol for CO2, 110 kJ/mol for CO) [77]. The authors propose that theCO2/CO ratios can be summarized by the relation reproduced in Equation 3.1.

[CO2]

[CO]= AoP

nO2

exp

(B

T

)(3.1)

In Equation 3.1, Ao = 0.02, PO2is the O2 partial pressure in atmospheres, T is the tem-

perature in Kelvin, and B and n are empirically determined parameters that vary based onexperimental conditions. Mitchell et al. [65] used Surface Chemkin to model the conver-sion of CO to CO2 in the boundary layers of char particles at high temperatures. This studyderived an 18-step surface kinetics mechanism from the reviews of Laurendau [63] and Es-senhigh [78] and includes reactions of radicals with the surface. Lee et al. [66] used a 5-stepsurface kinetics scheme based on the work of Bradley et al. [79] that did not include theformation of CO2 at the surface since the investigation was at higher temperatures, wheresurface production of CO2 can sometimes be neglected. Chelliah et al. [80] used a semi-global surface reaction mechanism also based on the work of Bradley et al. [79] to model

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the oxidation of non-porous graphite rods in cross flow with various oxidizer streams. A dif-ferent mechanism of carbon oxidation and gasification has emerged over the last few years,starting with Molina et al. [67], who chose the kinetic parameters to satisfy the CO2/COratio described by Tognotti et al. in Equation 3.1 in a heterogeneous mechanism modelingthe conversion of char-N to NO, N2O, and HCN in fluidized bed combustion conditions.This mechanism was simplified in a later paper by some of the same authors [68] (Reducedfrom 16 steps to 12 steps) when investigating conversion of char nitrogen to nitric oxidefurther. The mechanism was further refined by Hecht et al. in subsequent investigations ongasification reactions in coal combustion [69, 70, 81]. This mechanism is given in Table 4.1and will be the basis of surface chemistry based numerical simulations detailed in Section4.4. Because the mechanism used in this investigation is ultimately based on the experimen-tal work of Tognotti et al. [75], Equation 3.1 will be a useful basis of comparison in latersections. In particular, the use of ratios of products rather than specific rates of reactionmake comparison among different experimental setups more straightforward.

3.2 Investigation of the H2 inhibition effect

It has been observed since as far back as 1946 [22] that the presence of H2 inhibits steamgasification reactions from taking place, represented in Reactions 2.26 and 2.27. Gadsbyet al. [22] experimentally investigated steam-carbon reactions at 800 °C at atmospheric pres-sure in a porous bed, altering the inlet concentrations of H2, concluding that H2 inhibitsthe steam gasification reactions and also that the presence of CO inhibits CO2 gasificationreactions, and modeled the rates using Langmuir-Hinshelwood kinetics, though the term hadnot yet entered the widespread use that it enjoys today. Nearly all of the investigations sincehave modeled the gasification rates this way. Long and Sykes [82] continued the work ofGadsby et al., studying the inhibition effect of hydrogen on the steam gasification reactionsof coconut shell charcoal at total atmospheric pressure while varying the partial pressuresof the inlet gases between 10 and 760 mmHg in temperatures ranging from 680 to 800 °C.The authors concluded that only about 2% of the total surface actually takes part in thereaction, and also suggest that the reacting H2O dissociates to oxygen and hydrogen speciesseparately. The kinetics of the steam carbon reaction in porous graphite was investigatedby Johnstone et al. [83] using a tubular reactor at temperatures between 860 and 938 °C,which has the advantage of more accurately controlling temperature and flow patterns thana porous bed. Blackwood and McGrory [84] investigated the carbon-steam reaction at up to50 atm between 750 and 830 °C, and observed that at high steam and hydrogen pressuresand low temperatures, a significant fraction of the carbon is gasified into methane, whileacknowledging that H2 inhibits the H2O-carbon reaction. Biederman et al. [26] conductedCO2 gasification experiments on graphite at low pressure and determined that inhibition iscaused by dissociative chemisorption of molecular hydrogen on active surface sites. Muhlenet al. [85] reported on the effect of H2 on the reactivity of H2O gasification at high pressure(40 bar) and temperature on coal char. The authors reported that increasing H2 had aninhibition effect on steam gasification rates, and that increasing CO had an inhibition effecton CO2 gasification rates. Huttinger and Merdes [72] experimentally investigated the gasifi-cation of carbon with various concentrations of H2O and H2 at elevated pressures at 10 bar

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and 1273 K, with the partial pressure of H2O varying between 0.55 bar and 4.3 bar. Theauthors discuss the different mechanisms in which H2 may inhibit other gasification reactionsat the carbon surface, particularly reactions 2.26 and 2.27. The authors express the rate ofreaction at the surface in Equation 3.2, which is similar in form to Equation 2.5.

rs =ck1pH2O

1 + k1k2pH2O

+(K3 + k−1

k2

)pH2

+K3k−1

k2p2H2

(3.2)

The rate constants for Equation 3.2 and their respective units are given in Table 3.1.

ck1 = (12.82± 0.28) mmol mol−1 min−1 bar−1

k1/k2 = (0.167± 0.009) bar−1

k−1/k2 = (0.225± 0.007) bar−1

K3 = (6.83± 0.2) bar−1

Table 3.1: Rate constants from Huttinger and Merdes [72] used in Equation 3.2.

High pressure experiments on the gasification of peat char also demonstrated a significantinhibition effect by the product gases H2 and CO in steam and CO2 gasification, respectively[86]. Experiments at atmospheric pressure have also demonstrated an H2 inhibition effecton the gasification of wood char [56]. Espinal et al. [87] investigated the reactions of H2Owith carbon surfaces using density functional theory, and did so with surfaces that wereclean, oxidized, and hydrogenated. It was determined that while the interaction of H2Owith an oxidized surface was in some cases exothermic, in general, the reactions of H2O withthe carbon surface are endothermic, and that the presence of hydrogen inhibits gasificationreactions by blocking active sites. Similar to the premise of the work in this investigation,Qiao et al. [88] investigated the multiphysics modeling of carbon gasification processes in awell-stirred reactor with detailed homogeneous chemistry, and the effect of H2 addition wasinvestigated; however, this study did not utilize a surface kinetic mechanism that accountedfor intermediate adsorbed species. The model was primarily focused on particles, and alsoincorporated radiation and particle porosity. The experimental investigation with the mostinformation regarding its experimental procedure is the one by Huttinger and Merdes [72].In addition, the experimental samples are of fairly small size with a good resolution of massloss, which simplifies modeling efforts in regards to perfectly stirred reactor approximations.As such, Equation 3.2 using the experimentally obtained values in Table 3.1 will be used asa benchmark with which to compare the proposed mechanism of this investigation.

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Chapter 4

Numerical Simulation Setup

Simulations are performed to determine the effects of hydrogen inhibition reactions in gasifi-cation conditions. Existing surface reaction mechanisms available in the literature are appliedto both models and experimental data obtained by other researchers. These experiments aresimulated as perfectly stirred reactors with surface reactions as described in Section 4.2 us-ing the surface reaction mechanisms described in Section 4.4. The outcome of the numericalsimulations are compared with the existing models and data in the literature. Additionalproposed reactions are appended to the mechanism and compared with experimental data.

4.1 Chemkin™ and Surface Chemkin™Chemkin™ is a suite of applications originally developed by Sandia National Laboratoriesto solve chemical kinetic problems. While there is much documentation available regard-ing the Chemkin suite of applications [89], a brief overview is given here for the sake ofcompleteness. The applications utilizing Chemkin-II™ used in this document are the Sur-face Chemkin™ application [90] and the Surface Perfectly Stirred Reactor application [91].These models have been used in applications such as thermal chemical vapor deposition(CVD) systems and for catalysis studies [92]. Documentation for the Surface Chemkincomputer program has been developed throughout the years [90, 93, 94], and the develop-mental formulations of the program routines are available in the literature [95]. SurfaceChemkin does not consider reactions within bulk solids; but rather, computes reaction ratesand thermochemical properties at solid-gas interfaces. In these simulations, the solid sur-face is considered to be undergoing etching, that is, losing solid material into the gas phase.The BLK species are species (in this case, carbon atoms) in the bulk of the solid do notreact but replace the carbon free active surface site once it is carried away by a reactioninto the bath gas. The solutions assume that the solid surface is infinitely deep, and anyBLK species which become active sites and then which get carried away are immediatelyreplenished. As such, this study cannot account for the transient effects of particles whichchange shape or size, and merely seeks to establish meaningful understanding of the surfacereaction kinetics. In addition, the effects of transport properties, such as diffusion or heattransfer—particularly radiative heat transfer—are not examined, and are assumed to notaffect the chemistry of the reactions which are prescribed in these simulations at constant

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temperature.

4.2 Surface Perfectly Stirred Reactor (SPSR)

A Perfectly Stirred Reactor (PSR), also known as a continuously stirred tank reactor, is achemical engineering concept intended to simulate a zero-dimensional reactor (either tran-sient or steady-state) where the inlet substances are mixed homogeneously instantaneouslyupon entry into the reactor [47]. The reactants are given a residence time to proceed throughreactions governed by chemical kinetics, and are often accompanied by simulated tempera-ture control and heat transfer parameters. Surface reactions have been incorporated into thismodel, originally written for application to thermal chemical vapor deposition systems [91].The model takes into account finite-rate elementary chemical reactions both in the gas phaseand on the surface. A simplified diagram of the model is given in Figure 4.1. The programruns in tandem with the Chemkin-II and Surface Chemkin routines, which manage thechemical gas phase and surface reaction mechanisms. SPSR simulations have an advantagein that they are computationally inexpensive relative to multidimensional CFD codes.

Figure 4.1: Diagram of Surface PSR reproduced from Meeks et al. [96]

4.2.1 Assumptions

The SPSR code assumes a homogeneous mixture and that temperature is spatially uniform.Mixing rates are assumed to be high (infinite) relative to chemical reaction rates — thereactor is chemically kinetically limited. This is a good assumption for low pressure reactors,which will be the subject of focus for this study (10 bar or less). Mass transport to the walls is

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assumed infinitely fast, so the relative importance of surface reactions to gas-phase reactionsis determined only by the surface to volume ratios of each material and the relative reactionrates. The flow through the reactor is constant and characterized by a residence time, τ ,which can either be specified in this analysis instead of a flow rate, or can be derived froma specified flow rate. For the simulations that follow, the temperature of the reactor isprescribed and the energy equation is not solved, so only the reaction chemistry will beconsidered.

4.2.2 Governing Equations

Perfectly stirred reactor calculations generally assume that the mass inflow is equal to themass outflow; however, in a system where there are surface reactions, the possibility oretching mass from the surface or deposition of mass to the surface must be considered. Thegeneral conservation of mass equation for the system is given in Equation 4.1, where theterms in the summations are the mass changes due to reactions with the reactor surface, m∗

is the mass flow that is entering the reactor, and m is the mass flow that is leaving the reactor.Of the surface terms, Am is the surface area for the mth material of M total materials thatmake up a surface. From each mth material surface, the molar rate of production sk,m ofeach kth gas phase species is multiplied by Am and the molecular weight Wk of the kthspecies (with Kg total gas species) to yield its mass contribution to the equation.

d(ρV )

dt= m∗ − m+

M∑m=1

Am

Kg∑k=1

sk,mWk (4.1)

In the notation used in this investigation, variables that indicate states at the entrance ofthe reactor are marked with an asterisk. For each gas phase species, the conservation ofmass can be rewritten as:

dYkdt

=1

τ(Y ∗k − Yk) +

ωkWk

ρ+

1

ρV

M∑m=1

Am

(sk,mWk − Yk

Kg∑k=1

sk,mWk

)k = 1, . . . , Kg

(4.2)

where Yk is the mass fraction of the kth species, ωk is that species’ gaseous molar productionrate, and ρ is the density of the gas phase in the reactor, while V is the reactor volume. τ ,the residence time of the gas in the reactor, is given by:

τ =ρV

m(4.3)

The molar production rates of gaseous species (ωk) and surface species (sk,m) are equal tothe sum (over all the reactions) of the rate constant times the molar concentration(s) ofreactants, further discussed in Section 4.4.1. For the surface phase species, the conservationequation applied to every species in each surface phase n contained on each surface materialm is defined as

d

dt(AmckWk) = AmWksk k = Kf

s (m), . . . , K ls(m); m = 1, . . . ,M (4.4)

20

Page 32: Surface kinetics modeling of gasification reactions

where ck is the molar concentration in (mol/cm2) of the kth surface species. ck can beexpressed in terms of other variables:

ck =ρn,mZkσk

(4.5)

where Zk is the site fraction of the kth species, σk is a species coverage factor (how manysites a single species of k occupies), and ρn,m is the site density of the surface phase k. Theexpression for the evolution of Zk, after making some assumptions about the surface arearemaining constant and some numerical considerations, is given by:

dZkdt

= σksk,mρn,m

− Zkρn,m

dρn,mdt

+Zkτ

1−Kl

s(n,m)∑l=Kf

s (n,m)

Zl

k = Kfs (m), . . . , K l

s(m); m = 1, . . . ,M

(4.6)

For the bulk phase species, there is the possibility of deposition or etching of the materials inthe reactor; however, for cases where there is only one bulk species such as this investigation,the bulk species mole fraction is defined as one. For the sake of completeness, the governingequation regarding the conservation of the kth bulk phase in the mth material is given by:

dXbk

dt=Xb0k −Xb

k

τ+Xbk

τ

1−Kl

b(n,m)∑l=Kf

b (n,m)

Xbl

(4.7)

More details of the conservation equations with regard to the bulk phases can be found in theuser manuals of the Surface Chemkin programs [96]. The energy conservation equation,while not solved for in this investigation, is given by:

cpdT

dt=

1

τ

Kg∑k=1

Y ∗k (h∗k − hk)−Kg∑k=1

hkωkWk

ρ− 1

ρV

M∑m=1

Am

Kg∑k=1

hksk,mWk −Q

ρV(4.8)

Here, cp is the specific heat at constant pressure for the gas mixture in the reactor, h∗k andhk are the enthalpies of the gas mixtures entering and exiting the reactor respectively, andQ is the rate of heat loss from the reactor walls. These governing equations make up theSPSR model.

4.3 Numerical Method

The numerical solution methods for the perfectly stirred reactor application can be foundin documentation developed over the years at both Sandia National Laboratories and Re-action Design [91, 96–98]. An overview is reproduced here for completeness. The governingequations in Section 4.2.2 result in the required solution of systems of nonlinear algebraicequations. The SPSR program incorporates a subprogram called Twopnt [98], which usesa hybrid Newton/time-integration procedure. Originally developed for premixed flame mod-eling applications, Twopnt solves the system of algebraic equations by first applying a

21

Page 33: Surface kinetics modeling of gasification reactions

damped modified Newton algorithm to the set of N nonlinear algebraic equations representedby the steady state versions of Equations 4.2, 4.6, 4.7, and 4.8. If the Newton algorithmfails to converge, the code automatically conditions the solution estimate by integrating thetime dependent versions of the equations over a fixed number of time steps, specified by theuser. This provides a new starting estimate for the Newton algorithm which is closer to thesteady state solution, and this process repeats until a solution is converged upon.

4.3.1 Initial Estimation

The user has the option to specify the estimated species mole fractions in the solution.Otherwise, the first estimated solution in SPSR is an exit molar composition based onthermodynamic equilibrium calculations with the STANJAN program [99], which minimizesthe Gibbs free energy function. If the user is solving more than one problem by using theCNTN continuation problem command, then the starting estimate for the problem is thesolution to the previous problem.

4.3.2 Modified Damped Newton’s Method

Newton’s method is used to calculate approximate solution vectors, φ, for the species and en-ergy conservation in Equations 4.2, 4.6, 4.7, and 4.8. Thus, the vector φ for this investigationcan be described:

φ = [T, Y1, Y2, . . . , YKg , ZKfs, . . . , ZKl

s]T (4.9)

The approximate solution vectors are then substituted into the equations, which will not beequal to zero if they are not true solution vectors; this results in a residual vector, F . Thus,the goal is to find φ so that F (φ) = 0. The Newton algorithm, Equation 4.10, requires theevaluation of the Jacobian matrices Jn = ∂F/∂φ, which can be computationally expensive.

φn+1 = φn −(∂F

∂φ

)−1φnF (φn) (4.10)

Twopnt tries to contain computational overhead by keeping the same Jacobian throughmany iterations, such that Jn may be based on a solution φ that is less than n iterations inthe past. In addition, Twopnt employs a damping factor λn between 0 and 1 to advanceφn to φn+1, so that Equation 4.10 becomes Equation 4.11.

φn+1 = φn − λn (Jn)−1 F (φn) (4.11)

If convergence with an old Jacobian does not succeed, Twopnt generates a new one. Theinverse of Jn is not computed; instead, Twopnt solves the system of linear equations,Equation 4.12, for the undamped vector ∆φn.

Jn∆φn = F (φn) (4.12)

Twopnt adjusts the damping parameter to prevent the solution from evolving to non-physical outcomes, such as negative temperature or mole fractions, by halving λn or solvingfor a new Jacobian if the undamped steps do not decrease in magnitude. The Newtoniterations continue until convergence criteria are met, specified by the user.

22

Page 34: Surface kinetics modeling of gasification reactions

The Jacobian Matrix

The Jacobian Matrix is a dense matrix, the size of which is N ×N . Instead of analyticallyderiving and evaluating expressions for theN2 elements, they are formed numerically throughfinite difference perturbations. This approach is satisfactory for the problems being solved,since accuracy on the order of the working precision of a computer is not necessary for themodified Newton method. It is for this reason that ‘old’ Jacobians can be successfully useddespite being technically inaccurate. The Jacobian is evaluated from a one-sided differenceformula:

Ji,j =Fi (φj + δ)− Fi (φj)

δ(4.13)

In Equation 4.13, δ = r · φj + a, where r and a are the relative and absolute perturbation,respectively, which by default are the square root of the computer’s unit roundoff error.

4.3.3 Time Stepping Method

If the damping doesn’t compute a satisfactory solution vector, Twopnt computes a newJacobian. If this also does not work, then Twopnt will take a specified number of time stepsusing the unsteady form of the governing equations to compute transient solution vectorswhich can be used as new starting estimates for the Newton iterations. The transient versionof the system of governing ordinary differential equations are solved using the backward-Eulermethod, for which an example is given in Equation 4.14, where n is the time step index and∆t is the time step.

dT

dt=T n+1j − T nj

∆t(4.14)

At each time step, the system of equations is solved using the same Newton method as inthe steady state version of the problem. After the prescribed number of time steps (whichcan be altered by the user), the program uses the new solution vectors in the steady stateversion of the Newton method. Provided that sufficiently reasonable time steps are specified,the solution will eventually reach convergence.

4.4 Surface Kinetic Mechanism Considered

As discussed, many of the gasification reactions take place in heterogeneous reactions onthe surface of carbonaceous char particles, often in a variety of oxidizing environments. Re-cent work by Hecht et al. covers carbon surface kinetics in gasification reactions in regardsto coal char particle combustion [69, 70]. Hecht et al. utilized Chemkin-II™ and Sur-face Chemkin™ routines to carry out an investigation of simulated gasification reactionson pulverized coal char particles, utilizing a more simplified surface kinetics mechanism thatdescribes Reaction 2.6 via Reactions 2.20 through 2.22, Reaction 2.7, Reaction 2.9, and Re-action 2.10. The reaction mechanism (with Arrhenius parameters) is reproduced in Table4.1. The authors who used the mechanism in Table 4.1 note that if all elementary reac-tions were to be considered, the mechanism would be more detailed. Unfortunately, limitedknowledge of the intrinsic reaction rates necessitates the use of overall gasification stepsformulated based on the best estimates available of activation energy and studies regarding

23

Page 35: Surface kinetics modeling of gasification reactions

Reaction A(mol/cm2s) Ea (kJ/mol)

1. Cb + Cf + O2 −→ CO + C(O) 3.3× 1015 167.42. Cf + O2 −→ C(O2) 9.5× 1013 142.33. Cf + CO2 −→ CO + C(O) 3.6× 1015 251.04. Cf + H2O −→ H2 + C(O) 4.4× 1014 222.05. C(O) + Cb −→ CO + Cf 1.0× 1008 0.06. C(O2) + Cb −→ CO2 + Cf 1.0× 1008 0.0

Table 4.1: Surface Mechanism used by Hecht et al. [70], where C(X) indicates a surface complex

relative rates of oxidation versus gasification [81]. In these studies, Surface Chemkin™was used because it provides a more realistic approximation to the process of char oxidationdue to its basis on active sites and active site conservation [67]. It is for this reason thatthis investigation will also use Surface Chemkin™ applications to model gasification reac-tions. A Chemkin-friendly format of the surface kinetics input file, surf.inp, is providedin the Appendix. In a desorption reaction (such as Surface Reaction 6), the bulk carbon Cb

replenishes the lost surface complex as carbon free active site Cf. For the present study, thethermodynamic properties of the surface complexes (i.e. C(O), C(O2), and Cf) were esti-mated by using the properties of similar species adsorbed on a platinum surface [92]. Thethermodynamic properties of the bulk carbon, Cb, were estimated using the thermodynamicproperties of solid carbon, C(S). For the gas phase reaction mechanism, GRI-MECH 3.0was used [100]. It is not expected that homogeneous reactions will play a significant role inthe results at the simulated reactor outlet, due to the high values of surface area to volumeratio. Because of this, the selection of the homogeneous reaction mechanism is not critical.

4.4.1 Proposed Additional Reactions for H2 Inhibition

Outlined in Table 4.2 is a surface kinetics mechanism based on the mechanism used byHecht et al. [70], with four additional reactions appended to account for the effect of hydro-gen inhibition. The suggested reactions are not listed with defined pre-exponential factors,activation energies, or sticking coefficients. Unless otherwise noted (asterisks), the reactionsfollow Arrhenius rate expressions, in the form given by Equation 4.15.

ki = AiTβie−Ei/RT (4.15)

In Equation 4.15, ki is the rate constant, Ai is the pre-exponential factor, determined viacollision theory or otherwise, T is the absolute temperature of the reaction, R is the universalgas constant, and Ei is the activation energy for the reaction. Some reactions have anadditional temperature dependence via βi, though for all of the surface reactions consideredin this investigation, the value of β is zero.

24

Page 36: Surface kinetics modeling of gasification reactions

Reaction A(mol/cm2s) Ea (kJ/mol)

1. Cb + Cf + O2 −→ CO + C(O) 3.3× 1015 167.42. Cf + O2 −→ C(O2) 9.5× 1013 142.33. Cf + CO2 −→ CO + C(O) 3.6× 1015 251.04. Cf + H2O −→ H2 + C(O) 4.4× 1014 222.05. C(O) + Cb −→ CO + Cf 1.0× 1008 0.06. C(O2) + Cb −→ CO2 + Cf 1.0× 1008 0.07. Cf + H2 −→ C(H2) γ7* 0.08. C(H2) −→ Cf + H2 A8 Ea,89. 2 Cf + H2 −→ 2 C(H) γ9* 0.010. 2 C(H) −→ 2 Cf + H2 A10 Ea,10

*Sticking Coefficient Reaction

Table 4.2: Proposed Surface Mechanism with suggested H2 inhibition reactions, where C(X) indi-cates a surface complex

Sticking Coefficients

In addition to traditional Arrhenius rate expressions, Surface Chemkin allows for theuse of sticking coefficients in reaction mechanisms. A sticking coefficient is the probabilitythat a surface reaction will occur when there is a “collision” between a gas-phase reactantspecies and a surface phase reactant species. Typically, sticking coefficients are utilized whenthere is limited understanding of a reaction under investigation. Since the inhibition effectof H2 is not well quantified, this study will incorporate sticking coefficient reactions into themechanism in Table 4.1. As an initial investigation into the surface kinetics of hydrogeninhibition on carbon surfaces in gasification, sticking coefficients were utilized in Reactions7 and 9. The functional form of a sticking coefficient for Surface Chemkin is given inEquation 4.16.

γi = aiTbie−ci/RT (4.16)

In Equation 4.16, the user provides ai, bi, and ci for every reaction i, and Surface Chemkinhandles the conversion of the sticking coefficient γi to the usual mass-action kinetic rateconstant ki,γ [95] through Equation 4.17 [47].

ki,γ =γi

(Γtot)m

√RT

2πW(4.17)

In Equation 4.17, γi is the sticking coefficient for reaction i, and W is the molecular weightof the gas species colliding with the surface. Γ is the total molar site density on the surface,which remains constant for this investigation. m is the sum of all of the stoichiometriccoefficients of the surface reactants for the reaction i under consideration. Thus, m = 1 forReaction 7 and m = 2 for Reaction 9.

25

Page 37: Surface kinetics modeling of gasification reactions

Parameters Considered

There are a few uncertainties when appending reactions to an existing reaction mechanism,and in this case there are seven main parameters under investigation. First, the stickingcoefficients γ for Reactions 7 and 9 are uncertain. The sticking coefficient expresses itselfin the kinetic rate constant for Reactions 7 and 9 through the relation in Equation 4.17.Once on the reaction surface site, the H2 must be granted the ability to desorb throughan additional reaction in the mechanism, in this case through Reactions 8 and 10. Two ofthe uncertain parameters are the activation energies Ea for these desorption reactions. Twoadditional parameters arise for these reactions in the form of the pre-exponential coefficientsA. Lastly, it has been experimentally observed that the “active” surface area of carbona-ceous char can vary significantly based on char thermochemical pretreatment [101], creatingmodeling difficulties since the “active” surface area is not easily measured experimentally.This parameter is modeled through the molar site density variable, Γ. Note from Equation4.17 that this parameter also influences the kinetic rate constants for the reactions utilizingsticking coefficients.

Steady State Formulation

The experimental data regarding the steam gasification of char reported by Huttinger andMerdes [72] will be used as a basis of comparison for the development of the inhibitionreactions that will be considered in Table 4.2. For steam gasification without any oxygen orcarbon dioxide in the reactant gas, only Reactions 4, 5, 7, 8, 9, and 10 are relevant. Theonly reaction of these that consumes the bulk carbon and thus determines the gasificationrate is Reaction 5, which is written in rate form in Equation 4.18.

d[Cb]

dt= −k5[C(O)] (4.18)

The rate of reaction is dependent on a rate constant for the reaction as well as the surfacemolar concentration of C(O) on the surface of the char. C(O) is, in turn, dependent onthe concentrations of the other site fractions as well as the concentration of species in thegaseous phase above the surface. The perfectly stirred reactor model computes a steadystate solution. Thus, all of the surface phase species can be assumed to be in steady state,and the rates of change of these species is assumed to be zero. The rate equations for thesurface species can be written as follows:

d[C(O)]

dt= −k5[C(O)] + k4[Cf ][H2O] (4.19)

d[C(H2)]

dt= k7[Cf ][H2]− k8[C(H2)] (4.20)

d[C(H)]

dt= k9[Cf ]

2[H2]− k10[C(H)]2 (4.21)

While a rate equation can also be written for Cf, due to its involvement in several of thereactions, it is difficult to solve for directly. However, since these are the only surface species,we can write that the sum of their site fractions is equal to one in Equation 4.22.

ZCf+ ZC(O) + ZC(H2)

+ ZC(H) = 1 (4.22)

26

Page 38: Surface kinetics modeling of gasification reactions

In this case where only one surface phase is considered, the surface molar concentration ofspecies is related to its surface site fraction via Equation 4.23 (Note that this is simply adifferent representation of Equation 4.5):

[Xk] =ZkΓ

σk(4.23)

In Equation 4.23, Γ is the density of sites on the surface phase under consideration, andσk is the number of sites that each species k occupies. In this case, each surface speciescomplex only occupies a single site, so σ is trivially equal to one in all cases consideredhere. Substituting the relation provided in Equation 4.23 into Equation 4.22 yields a siteconservation expression in terms of the surface molar concentrations:

[Cf ] + [C(O)] + [C(H2)] + [C(H)] = Γ (4.24)

The steady state assumption is easily applied to rate equations 4.19 through 4.21 to yieldthe following steady state site fractions:

[C(O)]ss =k4k5

[Cf ][H2O] (4.25)

[C(H2)]ss =k7k8

[Cf ][H2] (4.26)

[C(H)]ss =

√k9k10

[H2][Cf ] (4.27)

Note that each of the steady state concentrations in Equations 4.25 through 4.27 is dependenton the steady-state concentration of Cf. Equation 4.25 can be substituted into Equation 4.18to yield an expression for the rate of change in bulk carbon Cb in terms of gaseous H2O molefraction and the surface site fraction of Cf, in Equation 4.28:

d[Cb]

dt= −k4[Cf ][H2O] (4.28)

In Equation 4.28 it becomes apparent that the rate of carbon gasification by steam is directlyproportional to the concentration of H2O in the gas phase, which is intuitive based on ageneral understanding of gasification processes. Substituting Equations 4.25 through 4.27into Equation 4.24 and solving for [Cf] yields the steady state surface site fraction of Cf inEquation 4.29:

[Cf ]ss =Γ

1 + k4k5

[H2O] + k7k8

[H2] +(k9k10

[H2])1/2 (4.29)

Equation 4.29 can be substituted into Equation 4.28 to yield the total molar rate of bulkcarbon gasification by steam:

d[Cb]

dt= rg =

−k4Γ[H2O]

1 + k4k5

[H2O] + k7k8

[H2] +(k9k10

[H2])1/2 (4.30)

27

Page 39: Surface kinetics modeling of gasification reactions

It should be noted that the expression in Equation 4.30 is similar to Equation 3.2 proposedby Huttinger and Merdes; this similarity will be used in the next section to estimate themolar site density Γ. The form of gasification reaction rate expressed in Equation 4.30 is nowin a form suitable for sensitivity analysis in terms of the parameters of this investigation.A similar equation can be derived for the case where all of the reactions in the surfacemechanism are included. The result is presented in Equation 4.31, but is not considered incomparison to experiments which only consider the reactant gases H2O and H2.

d[Cb]

dt=

−Γ (k2[O2] + k3[CO2] + k4[H2O])

1 +(k1k5

+ k2k6

)[O2] + k3

k5[CO2] + k4

k5[H2O] + k7

k8[H2] +

(k9k10

[H2])1/2 (4.31)

4.5 Experimental data comparisons with calculations

The model and the mechanism must be compared with experimental observations to demon-strate two things: 1) SPSR simulations are a valid model to use when investigating thesurface kinetics studies of interest, and 2) the mechanism must account for observed experi-mental trends otherwise not accounted for.

4.5.1 Steam Gasification in a Small Scale Reactor

Experimental parameters such as flow rate, surface area, and temperature were adjustedin the SPSR calculations to match the experimental conditions in the study by Huttingerand Merdes [72] as closely as possible. The flow rate of gasification agent was specified as1333 standard cubic centimeters per minute, and the temperature was specified at 1273 Kwith the reactor being adiabatic. The pressure of the reactor was specified at 10 bar total,with different partial pressures of H2O remaining constant while H2 partial pressures werevaried, with Ar as the balance gas. The surface area of the char samples was determined byHuttinger and Merdes by Brunauer-Emmett-Teller (BET) measurements to be 8900 cm2/g,and the samples used were 0.2 grams each, so the simulated surface area was 1780 cm2.While the reactor volume was not specified by Huttinger and Merdes in regards to theirexperimental apparatus, the prescribed SPSR volume did not alter the results significantlywhen changed, and so the internal pore volume for coal char of 0.12 cm3/g reported byLaurendau [63] was used for the simulations—0.024 cm3 for a 0.2 gram sample. Whilethe surface area of a char sample can be measured experimentally on a per weight or permass basis using a variety of different techniques, the actual concentration of active reactionsites can vary depending on a variety of factors, including but not limited to the thermalhistory and the degree of carbon conversion of the sample. The samples used by Huttingerand Merdes were polyvinylchloride (PVC) coke, which had been prepared by the pyrolysisof PVC at 600 °C in an N2 atmosphere, followed by heat treatment in Ar at 1000 °C for2 hours. Differences in char reactivity have been observed to span up to three orders ofmagnitude by varying pyrolysis and pretreatment conditions [102]. While the site densityused by Hecht et al. [70] is given as Γ = 1.7×10−9 mol/cm2, the rate expression ck1 in Table3.1 reported by Huttinger and Merdes includes the surface site concentration in the variablec. Comparing the rate constants k1 from the mechanism in Table 4.1 multiplied by Γ (as in

28

Page 40: Surface kinetics modeling of gasification reactions

Reaction A(mol/cm2s) Ea (kJ/mol)

1. Cb + Cf + O2 −→ CO + C(O) 3.3× 1015 167.42. Cf + O2 −→ C(O2) 9.5× 1013 142.33. Cf + CO2 −→ CO + C(O) 3.6× 1015 251.04. Cf + H2O −→ H2 + C(O) 4.4× 1014 222.05. C(O) + Cb −→ CO + Cf 1.0× 1008 0.06. C(O2) + Cb −→ CO2 + Cf 1.0× 1008 0.07. Cf + H2 −→ C(H2) 0.12* 0.08. C(H2) −→ Cf + H2 1.5× 1008 19.6

*Sticking Coefficient Reaction

Table 4.3: Proposed Surface Mechanism with H2 inhibition reactions, where C(X) indicates asurface complex

Equation 4.30) and the reported value of ck1 in Table 3.1, the molar surface site density Γcan be calculated to be 6.19× 10−10 mol/cm2.

H2 reaction addition to mechanism

To keep the mechanism simple for potential use in CFD codes, the only reactions consid-ered for addition from Table 4.2 were Reactions 7 and 8 involving molecular hydrogen thatdoes not dissociate into smaller molecules. Owing to the lack of detailed kinetic informationregarding the inhibition of hydrogen, sticking coefficients for the adsorption of H2 were es-timated. The desorption reaction as estimated utilized the activation energy, 19.63 kJ/mol,obtained from studies of H2 absorption and desorption on carbon nanotubes [103]. The pre-exponential factor was estimated based on an order of magnitude estimate from transitionstate and hard sphere reaction models by Baetzold and Somorjai [104]. While desorptionreactions are typically on the order of 1013 s−1, the inhibition mechanism proposed is under-stood to be a simplification of the reactions actually happening, and that this pair of addedreactions likely encompasses surface diffusion or second order surface reactions, which canbe rate-limiting on the order of 107 to 109 s−1. These additional reactions are included in aproposed mechanism in Table 4.3. Using both the original and proposed mechanism, simula-tions were run with increasing H2 partial pressure to determine the effect on the gasificationrates. As is documented in Figure 4.2, no effect was found with the original mechanism inTable 4.1, contrary to the experimental data of Huttinger and Merdes [72], and the rate ofgasification without any added H2 is demonstrably in worse agreement as the partial pressureof H2O increases. It is clear from Figure 4.2 that the additional proposed reactions predictgreater agreement with the experimentally derived results of Huttinger and Merdes, althoughthe good agreement at higher partial pressure of H2 comes at the expense of agreement in0% H2 conditions.

29

Page 41: Surface kinetics modeling of gasification reactions

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

20

25

30

35Simulation of Gas Formation Rates using Equation by Hüttinger vs SPSR simulations

pH

2

, bar

Gasific

ation r

ate

of C

arb

on, m

mol m

in−

1 m

ol−

1

Hüttinger Exp, pH

2O

= 0.55 bar

Hüttinger Exp, pH

2O

= 1 bar

Hüttinger Exp, pH

2O

= 2.2 bar

Unaltered Mech, pH

2O

= 0.55 bar

Unaltered Mech, pH

2O

= 1 bar

Unaltered Mech, pH

2O

= 2.2 bar

Proposed Mech, pH

2O

= 0.55 bar

Proposed Mech, pH

2O

= 1 bar

Proposed Mech, pH

2O

= 2.2 bar

Figure 4.2: Gasifiation rate as a function of variation of H2 in the inlet gas. Comparison of themodel proposed by Huttinger and Merdes backed up by experimental data (dashed-dashed lines)with the mechanism in Table 4.1 (square markers) and the proposed mechanism in Table 4.3 (circlemarkers). Red color indicates simulations and experiments with 1 bar of H2O, blue color indicatessimulations and experiments with 0.55 bar of H2O, and green color indicates simulations andexperiments with 2.2 bar of H2O. Experimental and simulated temperature is 1273 K.

30

Page 42: Surface kinetics modeling of gasification reactions

4.5.2 Steam Gasification in a Larger Reactor

Contributing to the good agreement in Figure 4.2 is the small sample size and short residencetime used in the experiments, since the presence of H2 in the product stream can significantlyinfluence the calculated kinetic rate constants. These factors make the assumptions regard-ing the use of a Perfectly Stirred Reactor model with surface reactions more valid, which—asmentioned—does not account for the effects of transport. However, the mechanism beingtuned to one particular experiment does not necessarily make it valid in describing otherexperiments. It may be the case that the SPSR assumptions cease to be valid in larger scalereactors, where the effects of transport, thermal radiation, and char particle interaction aremore important.

For this reason, the SPSR model and proposed mechanism were applied to a different ex-periment by Gadsby et al. [22] under slightly different conditions. The experiment is larger(30 grams of char instead of 0.2 grams), which means that the total surface area specifiedby the reactor model must increase substantially. Gadsby et al. [22] report a surface area onthe order of 1.0×106 cm2/gram, which is in agreement with the specific surface area alreadyused in this investigation. This results in a total surface area in the reactor of 3.0× 107 cm2.The site density was ultimately determined to be 1.0 × 10−10 mol/cm2 (Figure 4.3), aftercalculating an average of the optimal values where the results intersected for each partialpressure of H2O, weighted towards to the lower partial pressures.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 10−10

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

SPSR Results vs Gadsby Exp − Site Density Variation, no H2 addition

Cfas

Site Density, mol/cm2

rate

(g

. C

O2 /

min

)

pH

2O

= 190 mmHg

pH

2O

= 380 mmHg

pH

2O

= 633 mmHg

Figure 4.3: Variation of Cf site density used with the mechanism in Table 4.3 and resulting initialzero-H2 rates of gasification (solid lines with squares) compared with the experimental gasificationrate data reported by Gadsby et al. [22] (dashed lines) at 1073 K for the different partial pressuresof H2O. The intersections of simulations and experiments are marked with dashed black lines.

31

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The volume of the cylindrical reactor is calculated from the dimensions reported by theauthors to be approximately 75 cm3, though since much of this space is occupied by the charparticles, the actual volume of the surrounding gas is lower. A packing efficiency of approx-imately 65% was calculated from equations reported by Scott and Kilgour [105], resultingin a simulated volume of 26.27 cm3. The rates of reaction were measured in the originalexperiment by measuring the mass of CO2 at the outlet of the experiment after oxidizing allof the CO, so that the rate of gasification of carbon on the surface could be back-calculated.However, owing to the longer residence time of the experiment and the larger surface area,the authors took steps to account for the H2 produced by the gasification reactions inhibit-ing the reactions further downstream in the reactor (a transient process), to calculate theinstantaneous rates of gasification.

The SPSR model—a steady-state problem—reports rates in terms of steady state molarbalances from inlet to outlet. However, it is still possible to directly compare species com-position at the outlet, since these are reported by the authors. The SPSR code provides anoption to use multiple PSRs in series, with the outlet of one PSR being assumed as the inletof the next. Since the simulation of this particular experiment is of a relatively large reactorrelative to what could safely be assumed as a perfectly stirred reactor, it was decided todivide the reactor model into smaller reactors in series. This way, the surface site fractionsand the species mole fractions can change throughout the reactor instead of being limited toa single average for the whole reactor. The reactor was first split into four, and then eightreactors in series. It was observed that the profiles of the CO2 production at the outlet ofthe last reactor changed shape slightly, and that the surface density had to be adjusted to1.0 × 10−10 mol/cm2 per the method described by Figure 4.3 in order to reach a modestagreement in the no-H2 cases to account for the active site variability in the char. This is, asbefore, about an order of magnitude lower than the values used by Hecht et al., but owing tothe large variation in site density depending on pretreatment, and the presence of impuritiesin the char reported by the authors, it is a reasonable value. The results using eight PSRs inseries and the effect of increasing the number of PSRs is demonstrated in Figure 4.4, usingthe same surface site density for each case.

That the results are more significantly altered (increased) in the regions where there isonly some added H2 to the inlet stream demonstrates that the ‘averages’ obtained usingonly one PSR in the series are not necessarily symmetrical in the reactor, meaning thatmost of the CO and/or CO2 production occurs in the earlier stages of the reactor. Whetherthe differences can be attributed to reaction processes not accounted for by the surfacemechanism—or attributed to transport processes not accounted for by SPSR—is uncertain.It is noted that the well-mixed assumption used for the SPSR model cannot accurately applyto a porous media reactor, since the mixture is not homogeneous as the gas phase speciestravel downstream and is in reality limited by convection and diffusion.

32

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0 10 20 30 40 50 60 70 80 90 1000

0.05

0.1

0.15SPSR Simulation of Gas Formation Rates vs Experimental Data by Gadsby et al.

pH

2

, mmHg

rate

, (g

. C

O2 /

min

)

Proposed Mech, 633 mmHg H2O

Proposed Mech, 380 mmHg H2O

Proposed Mech, 190 mmHg H2O

Exp Data, 633 mmHg H2O

Exp Data, 380 mmHg H2O

Exp Data, 190 mmHg H2O

Figure 4.4: Rate of CO2 produced by steam gasfication of carbon at 1073 K as a function ofvariation of H2 in the inlet gas; effect of using multiple PSRs in series. Solid lines are from SPSRsimulations using the surface kinetics mechanism in Table 4.3 with 8 PSRs, dashed-dashed linesare using 4 PSRs, and dashed-dot lines are using 1 PSR. Surface site density is held constant at1.0× 10−10 mol/cm2. X Markers are the corresponding experimental data from Gadsby et al. [22].

4.5.3 Carbon Oxidation

The addition of hydrogen inhibition to the mechanism should not alter results obtainedfrom simulations of reactions in air or pure oxygen. To confirm this, and to address thevalidity of using perfectly stirred reactor calculations to model experimental reactions ofinterest, simulations were run by using conditions as close to those of Tognotti et al. [75]as possible. The representative sample size of the particles that Tognotti et al. tested in anelectrodynamic balance were determined to be approximately 4.5 × 10−6 grams per samplebased on the measured density and diameter values reported. Using the measured BETsurface area of 860 m2/g, this resulted in a surface area of 38.7 cm2 per sample. The reactorvolume was reported to be 10 cm3. The site density was modeled as 1.7 × 10−9 mol/cm2,using the values reported by Hecht et al. [70]. In the experiments, the surrounding gas wasalso considered to be at ambient temperature, since the sample was heated through lasers,and the sample temperature was determined by 2-color infrared pyrometry with an errorof approximately +/− 50K. This was addressed in the SPSR simulations by specifying thesurface temperature separately from the gas temperature. Residence times varied depending

33

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on the surface temperature of the sample, since the experiments by Tognotti et al. were rununtil 50% of the carbon sample had been converted; these were replicated as accurately aspossible in the SPSR simulations. The results are documented in Figure 4.5. Tognotti et al.also cited the work of others in the investigation and provided the corresponding empiricalparameters, so these have also been included for completeness [23, 25, 77]. Error barsrepresenting the reported estimated error in temperature have been applied to the curves ofTognotti et al. for which experimental data is available. It can be seen in Figure 4.5 that

750 800 850 900 950 1000 1050 1100 1150 1200

10−1

100

Reactor Temperature, K

Ratio o

f X

CO

2/X

CO

SPSR − Exit Mole Fraction CO2/CO Ratio vs. Temperature − Literature Comparison

This Study

Arthur (1951)

Otterbein (1968)

Du (1991)

Tognotti (1991)

Figure 4.5: Comparison of the CO2/CO ratios obtained with existing models and data in theliterature. The colored solid lines with markers indicate models for which experimental data areavailable, while the dashed colored lines indicate the corresponding models for which there are noexperimental data with which to verify.

the SPSR simulations with the proposed surface kinetics mechanism replicates the CO2/COratios reported in the literature quite well, despite not accounting for radiation or mixingeffects.

4.6 Analysis of Proposed Mechanism

4.6.1 Sensitivity Analysis

A brute force sensitivity analysis was conducted varying parameters of interest in Equation4.30 considering the reactions in Table 4.3 for conditions similar to those found in the reactor

34

Page 46: Surface kinetics modeling of gasification reactions

used by Huttinger and Merdes. Again, for simplicity, the mechanism used in the simulationsfrom Table 4.3 does not include Reactions 9 and 10 from Table 4.2, so Equation 4.30 becomesEquation 4.32 below:

d[Cb]

dt= rg =

−k4Γ[H2O]

1 + k4k5

[H2O] + k7k8

[H2](4.32)

The relative sensitivity of rg for each parameter xi was calculated using Equation 4.33:

Erelxj

=xj,ref

rg(xj,ref)

∆rg∆xj

(4.33)

In Equation 4.33, ∆xj = 1.5xj − 0.5xj and ∆rg = rg(1.5xj) − rg(0.5xj). An example ofthe results is shown in Figure 4.6; in this case, the gaseous mole fraction of H2O is 0.22while that of H2 is 0.09. It is observed that the overall rate of gasification is largely most

Figure 4.6: Relative sensitivity of the parameters of interest for the SPSR Simulation of the exper-iments by Huttinger and Merdes.

sensitive to the molar site density, Γ. The remaining parameters are at least three orders ofmagnitude smaller than these. As in the last experimental comparison, an example of therelative sensitivity of the parameters under these different conditions is given in Figure 4.7.The adjusted site density Γ and the reduced temperature have slightly altered the results,but otherwise no significant differences can be seen.

4.6.2 Alternative Possibilities for Proposed Reactions

For the proposed mechanism in Table 4.3, only two reactions have been appended (Reactions7 and 8), and only the additional formation of C(H2) on the surface is considered. For this

35

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Figure 4.7: Relative sensitivity of the parameters of interest for the SPSR simulation of the exper-iments by Gadsby et al.

reaction mechanism, the steady state analysis in Section 4.4.1 shows that the formation ofC(H2) on the surface is the primary reason that gasification rates decrease, since C(H2) isformed at the expense of free active sites Cf, and as per Equation 4.28, the overall gasificationrate (at steady state) is directly proportional to [Cf]. Reactions 7 and 8 show up only inthe k7

k8[H2] term of Equation 4.32, indicating that a greater concentration of H2 will lower

the value of rg. However, the extent of this inhibition in the steady state solution otherwisedepends only on the ratio k7/k8—if this ratio is kept constant, the values of k7 and k8 could bechanged, which would alter the proposed mechanism in Table 4.3 but yield the same results.It is important to confirm that the individual rate constants resulting from the proposedmechanism are realistic at the conditions simulated (T = 1273 K, Γ = 6.19×10−10 mol/cm2).Using the relation between a rate constant and a sticking coefficient in Equation 4.17 andthe Arrhenius expression in Equation 4.15, the value of k7 is calculated to be approximately5.6× 109 cm3/mol·s, and the value of k8 is calculated to be approximately 2.4× 107 s−1. Asmentioned in Section 4.4.1, this is within a reasonable range for surface reactions which maybe rate-limited by surface diffusion or second-order surface reactions, albeit the adsorptionrate calculated from the sticking coefficient is somewhat large (a possible explanation followsbelow regarding potential compounding inhibition). For comparison, the existing steamgasification reactions 4 and 5 similarly yield k4 = 3.4×105 cm3/mol·s and k5 = 1.0×108 s−1.

4.6.3 Possible compounding inhibition by CO

It is known that CO molecules can also inhibit gasification reactions on the surface of char.It is possible that, in the experiments with which this model was compared, the effects of CO

36

Page 48: Surface kinetics modeling of gasification reactions

inhibition in addition to that of H2 inhibition contribute to the observed trends in gasificationrates, since CO is also a product of H2O gasification and is present in the product streams.There is currently no implementation of CO inhibition in the surface mechanism in Table4.3, so these effects are not included in the model. Work remains to be done to explore ifthese compounding effects mean that the sticking coefficient proposed in the mechanism inTable 4.3 should actually be lower, and to see if the oxidation reactions should be adjustedas well. Altering the existing oxidation reactions was not considered in this investigation dueto the existing good agreement with the results of the experiments conducted by Tognottiet al. [75].

4.6.4 Water-Gas Shift Considerations

The experiments of both Huttinger and Merdes and Gadsby et al. in the preceding compar-isons either did not propose a model for or did not measure the formation of CO2 separatelyfrom the formation of CO in the steam gasification reactions. It is, however, documentedthat CO2 can be formed in the Water-Gas Shift Reaction (WGSR) in Reaction 2.14. Therelative importance of this reaction in carbonaceous char gasification processes varies signif-icantly depending on the char involved; differences in the sources of biomass char have inparticular indicated varying effects on the product species such as increasing the amount ofCO2 at the expense of CO [106]. The presence of impurities such as metal oxides has beensuggested as the culprit behind the varying relative importance of Reaction 2.14 [107]. Thesurface reaction mechanism in Table 4.3 does not provide a pathway for CO2 to be producedfrom the gasification of H2O at the surface, though the WGSR is the slowest of the surfacereactions generally considered in the gasification processes of carbonaceous char. Until moreresearch can be done on the surface kinetics of the WGSR, the present mechanism in Table4.3 is sufficient for determining the gasification rates of a carbon surface reacting with steamand air, but not for determining individual formation rates of CO2 and CO in conditionswhere WGSR is relevant (long residence times, low temperatures, high ash-content fuels).

37

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Chapter 5

Reactor Simulation Results

This chapter discusses the results of the perfectly stirred reactor simulations of the gasifi-cation of a 1 gram sample of carbonaceous char. The effects of residence time and reactortemperature on exit mole fractions and surface site fractions of the reactor are examined byvarying residence times at constant reactor temperature and varying temperature at con-stant residence time. Using the surface mechanism in Table 4.3, simulations were computedwith an an inlet gas consisting of an equimolar mixture of air (21% O2 / 79% N2) and water(H2O) at 298 K, with a fixed reactor temperature of 1100 K for the varying residence timeruns and a fixed residence time of 0.1 seconds for the varying temperature runs. In thesesimulations, the reactor temperature is prescribed and the surface and gas temperatures arethe same. The reactor volume was modeled as 0.12 cm3, the surface area was varied from1.0× 104 cm2 to 1.0× 106 cm2 based on mass-specific values reported by Laurendau [63] forcoal char particles. It is noted that the effective reactive surface area of a particle can changedepending on the temperature and the stage of the particle’s reaction with the surroundinggases. For example, porosity can decrease as temperatures increase, since the graphitizationof the particle surface can substantially reduce pore size, and thus, effective surface area.This variation in surface area is expected to capture this variety in char porosity. The sitedensity was based on the values used by Hecht et al. [70] to be 1.7× 10−9 moles/cm2.

5.1 Variation in Residence Time

Figure 5.1 shows the influence of reactor residence time on the exit mole fractions and thesurface site fractions of the species of interest. The mole fractions of both CO and CO2

rise and seem to plateau before changing concentrations at higher residence times. It is atthis point that the carbon in the surface is converted into both CO and CO2 in air. At theprescribed temperature of 1100 K, the reactor produces a higher concentration of CO overCO2 for all residence times. At longer residence times, the concentration of CO increases atthe expense of CO2, although for smaller values of surface area, the mole fractions of CO2

in the exit species increases slightly. Of the reactant gases at 1100 K, O2 is consumed atresidence times much shorter than those at which H2O is consumed. The time when O2

is consumed is about four orders of magnitude of the time when H2O is consumed for allsimulated surface area values. The carbon active site, Cfas, remains unity in steady-state as

38

Page 50: Surface kinetics modeling of gasification reactions

10−8

10−6

10−4

10−2

100

102

0

0.1

0.2

0.3

0.4

0.5

Residence Time, τ, [s]

Mole

Fra

ction o

f S

pecie

s i, X

iExit Mole Fractions vs. Reactor Residence Time

XCO

XCO2

XO2

XH2

XH2O

10−8

10−6

10−4

10−2

100

102

0

0.2

0.4

0.6

0.8

1

Residence Time, τ, [s]

Site F

raction o

f S

pecie

s i, Z

i

Surface Site Fractions vs. Reactor Residence Time

ZCf

ZC (H2)

Figure 5.1: Exit Mole Fractions (gas phase) and steady state site fractions (surface phase) as afunction of reactor residence time. Solid lines represent simulations with a surface area of 1.0 ×106 cm2, dashed-dashed lines 1.0× 105 cm2, and dashed-dot lines 1.0× 104 cm2

39

Page 51: Surface kinetics modeling of gasification reactions

Cf for the shortest residence times, when no H2 is produced from the surface. However, asH2 is produced beginning at around τ = 10−4 seconds for the largest simulated surface area,it begins to occupy the active sites as C(H2), eventually occupying up to 55% of the availableactive sites at the longest residence times simulated. It appears that as the steady state exitmole fractions of H2 level off above residence times above 1 second, the steady state sitefractions of C(H2) also levels off, suggesting that the modeled active site occupation is notinfluenced by increased residence time so much as it is influenced by the concentration of H2

in the gas phase. While C(O) and C(O2) also occupy a finite number of sites in the steadystate, their surface site fractions are almost negligible compared to those of Cf and C(H2),so they are not included in Figure 5.1 or in Figure 5.2.

5.2 Variation in Reactor Temperature

Figure 5.2 shows the exit mole fractions and the steady state surface site fractions as in-fluenced by reactor temperature at a constant residence time of 0.1 second. Most of theoxygen is consumed at temperatures less than 900 K, while H2O decreases slightly from 600to 900 K. At temperatures between 800 K and 1200 K, the mole fractions of H2 and COincrease considerably as H2O decreases; it is in this same range that the consumption of bulkcarbon (Cb) increases, corresponding to the consumption of H2O at the surface, indicated inFigure 5.3. The surface site fractions as a function of temperature in the lower plot of Figure5.2 show an interesting trend that may be considered insightful. As with the residence timevariation in Figure 5.1, the surface site fraction of C(H2) tracks with the steady state exitgaseous mole fractions of H2 from 800 to about 1150 K. However, for the largest simulatedsurface area, at temperatures above 1150 K, the surface site fraction of C(H2) decreases eventhough the exit gasesous mole fraction of H2 remains constant. This is also the case with theother simulated surface areas, although the maximum site fractions of C(H2) are shifted tohigher temperatures and are also lower in magnitude. The surface site fractions converge athigher temperatures for all simulated surface areas. This is possibly an indication that highertemperatures drive greater desorption rates from the surface, while adsorption is apparentlyprimarily influenced by the gaseous concentration of H2, which is not unexpected consideringthe formalism behind the use of sticking coefficients in the mechanism. However, it is alsopossible that the site occupation decreases since at higher temperatures, the density of thegas decreases while residence time remains constant; further explanation accompanies thediscussion regarding Figure 5.3. With such a high surface area to volume ratio, the surfacereactions dominate the exhaust mixture composition even at high temperatures, indicatedby Figure 5.3. As expected, the production rates increase as the temperature increases,although for CO they reach a peak at around 950–1100 K and then start to taper off, whilefor CO2, the peak at around 600–800 K and then fall. Interestingly, the rate of bulk carbonremoval seems to peak in the range of 600–1000 K, before slowly reversing its trend as thetemperature increases. The consumption rate of O2 at the surface also peaks at around600–800 K and then tapers off. This suggests that the decrease in production of CO2 andthe increase in production of CO in the range of 600–1000 K is due to the surface increasingits uptake of gaseous CO2 at these temperatures and converting it to gaseous CO and thesurface complex C(O), which inevitably leaves the surface as gaseous CO, being replaced

40

Page 52: Surface kinetics modeling of gasification reactions

400 600 800 1000 1200 1400 1600 18000

0.1

0.2

0.3

0.4

0.5

Reactor Temperature, [K]

Mole

Fra

ctio

n o

f S

pe

cie

s i, X

i

Exit Mole Fractions vs. Reactor Temperature

XCO

XCO2

XO2

XH2

XH2O

400 600 800 1000 1200 1400 1600 18000

0.2

0.4

0.6

0.8

1

Reactor Temperature, [K]

Site F

raction o

f S

pecie

s i, Z

i

Surface Site Fractions vs. Reactor Temperature

ZCf

ZC (H2)

Figure 5.2: Exit Mole Fractions (gas phase) and steady state site fractions (surface phase) as afunction of reactor temperature. Solid lines represent simulations with a surface area of 1.0 ×106 cm2, dashed-dashed lines 1.0× 105 cm2, and dashed-dot lines 1.0× 104 cm2. Residence time is0.1 seconds.

41

Page 53: Surface kinetics modeling of gasification reactions

400 600 800 1000 1200 1400 1600 1800−8

−6

−4

−2

0

2

4

6

8x 10

−6

Reactor Temperature, [K]

Surf

ace N

et P

roduction R

ate

, m

ol/s

Surface Rates vs. Reactor Temperature

sCb

sCO

sCO2

sO2

sH2

sH2O

Figure 5.3: Detailed Surface Species Molar Balance rates as a function of reactor temperature. COproduction is the black line in the positive region, while Cb consumption is the black line in thenegative region. Solid lines represent simulations with a surface area of 1.0×106 cm2, dashed-dashedlines 1.0× 105 cm2, and dashed-dot lines 1.0× 104 cm2. Residence time is 0.1 seconds.

42

Page 54: Surface kinetics modeling of gasification reactions

by the bulk carbon Cb at the surface to form a new active site Cf. The apparent decreaseof reaction rate with temperature is a consequence of holding the residence time constantfor these simulations—as the temperature increases, the density of the gas decreases, butis constrained to spend the same amount of time in the reactor, which also has a constantvolume. Thus, the mass flow is decreasing with temperature, and so the surface interactswith fewer gas molecules, decreasing the overall gasification rate.

5.3 The Effect of Surface Area

It was observed that an increase in surface area by an order of magnitude roughly shiftsthe exit mole fraction curves in the residence time variation plot of Figure 5.1 by about anorder of magnitude. This is expected based on a simple dimensional analysis of the units ofthe surface molar production rate (mol/cm2s). The variations in temperature in Figures 5.2and 5.3 are more interesting, especially considering the emergence of a second peak in CO2

mole fraction at around 1100 K with the lowest simulated surface area. The rise in the molefractions of the products—as well as the decline in the mole fractions of the reactants—beginto ‘stiffen’ as the surface reacts with the gas at an overall faster rate, indicated by Figure5.3. However, at the highest temperatures, the mole fractions (gaseous and surface phase)and the surface molar surface production rates for all simulated surface areas collapse ontoa single curve.

5.4 Potential Implications for Real Reactors

The peak production of CO and peak consumption of Cb at 1000 K suggests that syngasproduction of CO with air inlet at higher temperatures at a given residence time may be im-practical, since the increased energy costs associated with raising the reactor to above 1000 Kwould not yield producer gases with any greater heating value. As previously mentioned inSection 2.3.1, gasification agents other than air, such as steam, are used, typically to promoteproduction of desired constituents in the product gas, in this case hydrogen. However, in thesurface kinetic model of gasification reactions, there are a limited number of reaction siteson the carbonaceous surface. Thus, reactions with water vapor may occupy sites that wouldotherwise be occupied by oxygen being converted to CO or CO2. If temperatures increasebeyond 900–1000 K, then the steam gasification reactions are favored and H2 productionincreases, also boosting CO production. Such reactions are endothermic, and future workwill need to be explored with the possibility of injecting steam into a region of the reactordownstream of the oxidation reactions. Such work will require a more advanced model thatincludes the effects of transport, though a one dimensional model may suffice for a porousbed. For now, modeling results indicate that syngas production from the gasification of charby an equimolar mixture of air and steam is promising under well mixed conditions.

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Chapter 6

Conclusions

6.1 Summary

In this investigation, laboratory-scale char gasification reactors from the literature were mod-eled as zero-dimensional perfectly stirred reactors (or continuously stirred reactors) with asimplified carbon oxidation surface reaction mechanism that included the effects of gasifica-tion reactions by both water and carbon dioxide. The experimental data from the literaturewere compared with the simulation data, and it was concluded that the mechanism did notreasonably account for inhibition reactions by hydrogen gas, which itself is a product ofcarbon gasification by steam. Based on theoretical and experimentally observed chemicalkinetic data—such as activation energy—about reactions of molecular hydrogen with car-bon, two new reactions have been proposed to be appended to a surface mechanism fromliterature. The modified mechanism employed in a perfectly stirred reactor model agreesreasonably well with the experimental data from several different investigations spanningseveral decades of research. The model does not account for the effect of transport in asimulated reactor, and the agreement with experimental setups utilizing larger samples andlonger residence times deteriorates, while still predicting observed trends.

6.1.1 Observations

Hydrogen has been shown both experimentally and theoretically to inhibit carbon gasifica-tion reactions by occupying the active sites on the surface of carbonaceous char. For a givenresidence time in a reactor, site occupation by hydrogen increases with temperature as theconcentration of hydrogen rises in the product gas, and then decreases after reaching a highenough temperature. Agreement with experimental rate-of-reaction data is highly depen-dent on the reactivity of the char—which has been adjusted in this model by changing thesite density—which can vary significantly across different kinds of char depending on ther-mochemical pretreatment history or biological origin. This underscores the importance ofdeveloping more accessible methods of experimentally determining the specific active surfacearea or the reactivity of char, rather than solely the specific total surface area.

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6.2 Simulation Limitations

Perfectly Stirred Reactors can be useful models for investigating the surface kinetics of gasi-fication reactions, provided that the laboratory experiments with which they are comparedis conducted with 1) relatively small sample sizes of char (less than 1 gram), 2) precisemass loss and exhaust species measurements, and 3) short residence times (on the order of0.1 seconds or less). It is clear that the use of a model is not without its limitations how-ever, owing to several different factors. These include limitations presented by the model’sgoverning assumptions and resulting equations, the numerical capabilities of the solver, andthe uncertainties in the surface chemistry model itself, which can be difficult to reduce sinceintrinsic kinetic measurements are extremely difficult to obtain in these conditions.

6.2.1 Limitations Imposed by Model Assumptions

In the preceding simulations, reactor temperature was prescribed, and the energy equationwas not solved, due to a lack of accurate thermodynamic data for the surface species inthe mechanisms used. More detailed and complete thermodynamic data on surface complexspecies are needed to model reactors in more realistic situations with heat loss in futureresearch. In Perfectly Stirred Reactor calculations, the effects of thermal radiation andmolecular diffusion are neglected. It is important to consider that these simulations, there-fore, cannot determine the accuracy of the proposed surface reaction mechanism in regardsto diffusion to and from the surface, and they cannot be considered to model char particleswhich are at a much higher temperature than their immediate surrounding environment.Because the SPSR simulations do not consider changing porosity as a function of burn off orcarbon conversion, they cannot be taken to accurately model reactors which are composedof transient gasification processes. Regardless, an understanding of steady state conditionsstill provides important insight to formulate reaction mechanisms.

6.2.2 Limitations Imposed by Numerical Considerations

It was observed during the course of this study that appending a sticking-coefficient reactionto the surface kinetic mechanism dramatically increased the computational cost in seekingconvergence for a steady state solution, particularly for simulated reactors with large surfaceareas or long residence times. This increased computational cost can be mitigated with goodstarting estimates provided by the user for now, but future work will need to undertakento investigate the more detailed chemistry regarding the adsorption of hydrogen onto thecarbon active sites. Future studies should focus on the pre-exponential factor and activationenergy for the reaction so that the use of sticking coefficients can be avoided and solutionscan be converged upon more rapidly.

Selection of Gas Phase Reaction Mechanism

The selection of an appropriate gas phase mechanism can reduce the computational cost,since the size of the Jacobian, as discussed in Section 4.3, scales as N2. Since gas phasereactions did not have a significant influence in these SPSR simulations—for which the

45

Page 57: Surface kinetics modeling of gasification reactions

surface area to volume ratios often exceeded 106—a reduced gas phase mechanism maybe more appropriate than GRI-MECH 3.0, which has 53 species (and thus 53 additionalequations to solve).

6.2.3 Surface Mechanism Intentions vs Application

In this investigation, an existing mechanism used in the literature for one application—oxy-combustion of char particles—has been adjusted based on experiments from a differentapplication—steam gasification of char—and used in a still different application—gasificationof char by an equimolar mixture of air and steam. This raises modeling concerns; in par-ticular, the mechanism used by Hecht et al. was originally written as adsorption-limited,such that surface complexes to not accumulate on the char active sites. The reactions thathave been added to the mechanism are not written this way, because the available literaturestrongly suggests the gasification inhibition by product species specifically occurs by blockingactive sites, which fundamentally challenges the original intentions with which the mecha-nism was written. This mechanism is also intended for use in simulations of experimentsthat were conducted at atmospheric pressure. While H2 can have an inhibiting effect on therates of carbon gasification at atmospheric pressure, at high pressures (above 10 bar), it ispossible for gaseous hydrogen to gasify the carbon surface into methane via the hydrogasifi-cation reaction [108]. While it is noted that efforts were made to keep the surface mechanismsimple for potential use in CFD codes, more complicated surface mechanisms in the futuremay include the formation of C(H) on the surface along with other radical species, such asC(OH).

6.3 Broader Impacts

The gasification of solid fuels provides a potentially viable thermochemical pathway to con-vert existing carbon-intensive energy sources into cleaner fuels, or at least into precursorswhich can be refined into cleaner heavier fuels. The initial gasification processes can becarried out at lower temperatures than traditional combustion reactions, which can aid inthe reduction of pollutants such as nitrogen oxides. In addition, large deposits of availablecoal around the world ensure that coal will continue to have an impact in global energyproduction policies for the foreseeable future. A better understanding of how hydrogen caninhibit gasification reactions is important to the development of more efficient gasificationtechnologies which can be applied to both coal and biomass energy conversion. For example,future studies of how product gases can best be kept away from the surfaces of carbona-ceous char once formed can aid in these developments. While the breakdown of gasificationprocesses into more elementary chemical steps for broader use in more generalized models isboth challenging and intensive from a modeling perspective, incremental progress is slowlyrevealing more details regarding the mechanisms behind these reactions.

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Appendix A

Additional Reference Material

A.1 Surface Chemistry Reference

Included in this Appendix is more detailed information about some different surface chem-istry reaction models, referenced in Section 2.1.

A.1.1 Unimolecular Reaction Dynamics

Unimolecular reaction dynamics might become important in terms of the thermal dissociationof some molecules, particularly cracking of higher hydrocarbons. According to Steinfeld et al.[27], a unimolecular reaction generally goes as follows:

A∗ −→ Products {A.31}Where the ∗ means that A has enough vibrational energy to decompose. These reactionscan include isomerization reactions, where there exists a significant energy barrier betweenone isomer of a molecule and another isomer of that same molecule. These unimolecularreactions can also include unimolecular dissociation reactions where there is an activationenergy for recombination, as well as unimolecular dissociation reactions where there is noactivation energy for recombination.

A.1.2 Heterogeneous Catalysis and Gas Surface Reactions

While the Langmuir-Hinshelwood model is discussed in Section 2.1, another biomolecularheterogeneous reaction is the Eley-Rideal process.

A + S −−⇀↽−− A · S governed by: K(A)

ads, eq {A.32}

A · S + Bk′′2−→ Products {A.33}

R = k′′2θAS0pB =k′′2K

(A)ads, eq[A]pBS0

1 +K(A)ads, eq[A]

(A.1)

Reactions A.32 and A.33 and Equation A.1 describe the Eley-Rideal Process, where pB isthe partial pressure of gas B in the system. Reaction A.32 is assumed to happen very fastcompared to reaction A.33.

47

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Appendix B

Computational Parameters

This section provides technical details that are too lengthy to put in the main document,but are still useful to the reader as reference material, especially for reproducible purposes.

B.1 Surface Chemkin input file

The following is a section of surf.inp, the Surface Chemkin input file. Included is aChemkin-friendly format of the thermodynamic data used for the SPSR simulations, and aChemkin-friendly surface reaction mechanism described in Table 4.3.

SITE/CFAS/ SDEN/1.7E-09/

C(*)

CO(*)

CO2(*)

END

BULK C(B) /0.500/

END

THERMO ALL

300. 1000. 3000.

! not sure whether to specify C(S) or C(F); C(S) has same properties as C(B)

! C(F) from JY’s Pt catalyst spsr files, C(S)/C(B) from thermdat

C(F) 71496C 1 I 300.00 3000.00 1000.00 1

0.13942949E+01 0.23809458E-02-0.14461439E-05 0.40382496E-09-0.42764283E-13 2

-0.37513079E+04-0.86746618E+01-0.25750111E+01 0.18247293E-01-0.26176132E-04 3

0.18141982E-07-0.49479751E-11-0.29253435E+04 0.10556563E+02 4

C(S) 121286C 1 S 0300.00 5000.00 1000.00 1

0.01490166E+02 0.01662126E-01-0.06687204E-05 0.01290880E-08-0.09205334E-13 2

-0.07074019E+04-0.08717785E+02-0.06705661E+01 0.07181500E-01-0.05632921E-04 3

0.02142299E-07-0.04168562E-11-0.07339498E+03 0.02601596E+02 4

C(B) 121286C 1 S 0300.00 5000.00 1000.00 1

0.01490166E+02 0.01662126E-01-0.06687204E-05 0.01290880E-08-0.09205334E-13 2

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-0.07074019E+04-0.08717785E+02-0.06705661E+01 0.07181500E-01-0.05632921E-04 3

0.02142299E-07-0.04168562E-11-0.07339498E+03 0.02601596E+02 4

! The following (*) values I got from JY’s Pt catalyst spsr files:

C(O) 71596C 1O 1 I 300.00 3000.00 1000.00 1

0.27885898E+01 0.47618916E-02-0.28922878E-05 0.80764992E-09-0.85528566E-13 2

-0.32301440E+05-0.17349324E+02-0.51500222E+01 0.36494587E-01-0.52352264E-04 3

0.36283964E-07-0.98959502E-11-0.30649511E+05 0.21113126E+02 4

! Right now CO2(*) is the same as CO(*) for testing purposes

C(O2) 71596C 1O 2 I 300.00 3000.00 1000.00 1

0.27885898E+01 0.47618916E-02-0.28922878E-05 0.80764992E-09-0.85528566E-13 2

-0.32301440E+05-0.17349324E+02-0.51500222E+01 0.36494587E-01-0.52352264E-04 3

0.36283964E-07-0.98959502E-11-0.30649511E+05 0.21113126E+02 4

C(H2) 71496C 1H 2 I 300.00 3000.00 1000.00 1

0.43280953E+00 0.81685976E-02-0.49483621E-05 0.13831503E-08-0.14685304E-12 2

-0.12604655E+05-0.50810048E+01-0.29375691E+01 0.15544317E-01-0.69790189E-05 3

-0.32011799E-08 0.24627931E-11-0.11621097E+05 0.12716053E+02 4

END

REACTIONS KJOULES/MOLE

! Surface mechanism from Hecht et al. 2013

C(B) + C(F) + O2 => CO + C(O) 3.3E15 0.0 167.4

C(F) + O2 => C(O2) 9.5E13 0.0 142.3

C(F) + CO2 => CO + C(O) 3.6E15 0.0 251.0

C(F) + H2O => H2 + C(O) 4.4E14 0.0 222.0

C(O) + C(B) => CO + C(F) 1.0E8 0.0 0.0

C(O2) + C(B) => CO2 + C(F) 1.0E8 0.0 0.0

! Additional Reactions Added

C(F) + H2 => C(H2) 0.12 0.0 0.0

STICK

C(H2) => C(F) + H2 1.5E8 0.0 19.6

END

49

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Appendix C

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