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The Pennsylvania State University

The Graduate School

Department of Energy and Mineral Engineering

STRUCTURAL CHARACTERISTICS AND CO2 REACTIVITY OF PARTIALLY

GASIFIED PITTSBURGH NO.8 COAL CHARS GENERATED IN A HIGH-PRESSURE,

HIGH-TEMPERATURE FLOW REACTOR

A Dissertation in

Energy and Mineral Engineering

Vijayaragavan Krishnamoorthy

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

August 2018

The dissertation of Vijayaragavan Krishnamoorthy was reviewed and approved* by the following:

Sarma V. Pisupati Professor of Energy and Mineral Engineering Dissertation Advisor Chair of the committee

Jonathan P. Mathews Professor of Energy and Mineral Engineering

Mark S. Klima Professor of Energy and Mineral Engineering

Anil K. Kulkarni Professor of Mechanical and Nuclear Engineering

Luis H. Ayala Professor of Petroleum and Natural Gas Engineering Associate Department Head of Graduate Education

* Signatures are on file in the Graduate School

ABSTRACT

Integrated gasification combined cycle (IGCC) is an advanced power generation technology based on gasification of coal or solid fuels. Despite many commercial operations, the knowledge of char gasification rates at high pressures and temperatures, crucial to the design and troubleshooting of the gasifiers, are relatively unknown. While many kinetic studies have been performed at atmospheric pressure and low heating rates, there are few studies that examined the reactivity of chars generated at high temperatures and elevated pressures

Gasification rate of chars in entrained-flow gasifiers is dependent on both intrinsic reactivity and the gas diffusion rate of reactants into pores. Therefore, the knowledge of intrinsic reaction rate and the structural features of the char are necessary for developing a kinetic model. The aim of the thesis is to determine the intrinsic reactivity and the structural features of the chars generated at elevated pressures and temperatures pertinent to conditions of the entrained-flow gasifiers.

A series of interrelated studies were conducted to characterize the gasification behavior of a widely used Pittsburgh No,8 coal. To generate chars under conditions similar to that of the gasifier, a 20 kW high-pressure, high-temperature flow reactor (HPHTFR) was designed to operate up to

1650°C and 30 bar. The chars obtained at various temperatures, pressures, and pyrolysis atmospheres were characterized for physical and chemical structure using surface area analyzer,

XRD, Raman, and morphological analysis. The CO2 kinetics on chars were obtained using a high pressure thermogravimetric analyzer (HPTGA).

The structural properties and intrinsic kinetics of chars widely reported in the literature were generated in inert atmospheres. However, the pyrolysis of feedstock occurs in the presence of reaction gas. This difference can affect char structural properties and intrinsic reactivity. To determine the role of pyrolysis atmosphere, chars were generated in three different atomspheres-

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CO2/N2, Ar and N2- at 1100°C and 6.2 bar. The chars generated in the CO2/N2 atmosphere showed higher conversion compared to that of chars generated in N2 and Ar atmospheres. The increased conversion in the CO2/N2 atmosphere was attributed to increased gasification of tar/soot. While the volatile yield showed some difference, char properties such as surface area, swelling ratio, defects to graphitic band ratio and crystallite sizes showed no difference. The kinetic parameters of the chars were obtained using the nth order model. The activation energy was found to be independent of pyrolysis atmospheres. The order of reaction was found to be significantly affected by the pyrolysis atmosphere. The order of reaction followed the trend: CO2/N2> N2 ≈Ar. The order of the reaction was found to correlate with surface area evolution.

Gasification of coal can be impacted by the organic and inorganic compositional heterogeneity, which further impact the char morphology, and the intrinsic reactivity. To account for the compositional heterogeneity, chars were generated from various size fractions (-106+75,

-150+106, -212+150, -420+212 µm at 1300°C and 11.3 bar) and density fractions (<1.3 g/cc, 1.3-

1.6 g/cc, >1.6g/cc of -106+75 µm at 1300°C and 11.3 bar). Chars were also generated over a range of temperatures (1100, 1300, and 1400°C at 11.3 bar for the -150+106 µm fraction), pressures (3.4,

6.2, 11.3, 15.5, and 21.7 bar at 1300°C for -150+106 µm fraction) to study the effect of temperature and pressure on char structures and reactivity. Chars were characterized for morphology, pore structure (i.e. surface area and pore volume), reflectance, and reactivity using oil immersion microscopy, N2 adsorption technique, reflectance microscopy, and thermogravimetric analyzer, respectively. The results were statistically analyzed to determine the effects of the four parameters on conversion, structural characteristics, and intrinsic reactivity. The results showed that the conversion was most affected by temperature, and followed by feed particle size, pressure, and feed particle density. Maceral differences played a significant role in affecting the group-I

concentration and swelling ratio. Feed particle density significantly affected group-I concentration, while both feed particle size and feed particle density affected swelling ratio. In the case of intrinsic reactivity, particle density showed the largest effect, followed by temperature, particle size, and pressure.

The intrinsic gasification rate is an important parameter for designing a kinetic model. Chars were obtained by partially gasifying Pittsburgh No.8 coal in CO2 atmosphere at 1300°C and over a range of pressures (3.4, 6.2, 11.3, 15.5, and 21.7 bar) in the HPHTFR. The intrinsic reaction rate of those chars with CO2 was obtained at the char generation pressure using the HPTGA. The kinetic parameters were obtained using the nth order model. The intrinsic reaction rate, and activation energy were found to be independent of the char generation pressure. The order of reaction was obtained by varying CO2 partial pressures. The order of reaction decreased with increase in char generation pressure. The comparison of initial char with the char obtained at ~20% conversion in the HPTGA for surface area and pore volume showed that the reaction primarily occurs in microporous regions. The order of reaction also closely followed the surface area during gasification in the HPTGA.

Through this research, a comprehensive assessment of the entrained-flow gasification behavior of

Pittsburgh No.8 coal has been performed using proven experimental techniques under conditions of industrial interest. The structural features and kinetics were obtained. The generated data provide optimum, and trends that can be used as direct inputs to kinetic modelling and gasifier design applications.

TABLE OF CONTENTS

List of Figures ...... vii List of Tables ...... x Nomenclature ...... xii Acknowledgements ...... xv Introduction...... 1 Literature Review ...... 8 Research Objectives ...... 28

Effects of Pyrolysis Atmosphere on Volatile Yield and CO2 Reactivity of the Char Samples Generated in a Higher Pressure High Temperature Flow reactor ...... 30 Effects of Temperature, Pressure, Feed Particle Size, and Feed Particle Density on Structural Characteristics and Reactivity of Chars ...... 60 Effect of Pressure on Intrinsic Kinetics of Chars Generated at High Temperature and High Pressures in a High-Pressure, High-Temperature Flow Reactor ...... 94 ...... 112 References………………………………………………………………………………………116 Appendix A Reactor Description ...... 127 Appendix B Particle Velocity Calculation ...... 135 Appendix C Tar and Soot Separation ...... 137 Appendix D High Pressure TGA Data Processing ...... 140 Appendix E Effectiveness Factor Calculations ...... 141 Appendix F Uncertainty Analysis ...... 144 Appendix G Feed Particle Size Distribution to a Slurry-Fed Gasifier...... 145 Appendix H Specific Contributions Toward Designing and Construction of the HPHTFR ...... 146

List of Figures Figure Page

1.1 A typical IGCC process flow diagram 2

2.1 Transformation of coal during gasification in a slurry fed two-stage 9 gasifier

2.2 The overall reaction scheme of a heterogeneous gasification reaction 13

2.3 The change in reaction rate of a porous carbon with temperature. 15 Where j is the true reaction order; n being the apparent reaction order

2.4 Effect of heat treatment temperature on the reactivity of carbonaceous 18 materials. R1000 is the reaction rate of the corresponding coal char heat treated at 1000 °C and R/R1000 is the reactivity ratio

2.5 Structural rearrangement leading to crystallinity in carbon 18

2.6 Effect of residence time on oxygen reactivity of chars generated at 5 21 bar pressure in a high pressure entrained-flow reactor

4.1 Schematic of the high-pressure, high-temperature flow reactor 34

4.2 Volatile yield for chars generated in different pyrolysis atmospheres 42

4.3 Polished cross section of the char generated in the N2 atmosphere 43

4.4 Conversion profiles of chars with CO2 in HPTGA 47

4.5 The ratio of D band intensity to G band intensity 48

4.6 XRD pattern of chars generated in different pyrolysis atmosphere 48

4.7 Pore size distribution of chars 49

4.8 Apparent reactivity of chars before and after washing with THF (at 52 875°C)

4.9 CO2 reactivity of the reference char and the THF washed reference 52 char

4.10 GC-MS chromatograms of THF soluble tars obtained by washing 53 chars generated in CO2/N2 and N2 atmospheres (only qualitative analysis)

vii

4.11 Instantaneous reaction rates of chars with conversion (before and after 57 washing with THF)

4.12 Arrhenius plots for chars generated in different atmospheres 58

4.13 Effectiveness factor for char-CO2 reaction at 6.2 bar 59

5.1 Methodology 64

5.2 Coal conversion at elevated pressures 77

5.3 Swelling ratio of chars 78

5.4 Char morphology (obtained on chars before washing with THF) 79

5.5 SEM images of chars generated at 6.2 bar (left) and 21.7 bar (right) 80

5.6 Char pore structure 81

5.7 Intrinsic and apparent reactivities of chars; a.) effect of pressure b.) 82 effect of inorganic matter c.) effect of temperature d.) effect of particle size

5.8 Role of soot, tar, and volatiles on the intrinsic reactivity of chars 83 generated over a range of pressures

5.9 HRTEM pictures of soot: a) <1.3 g/cc char (Left) b) 1.3-1.6 g/cc char 85 (Right)

6.1 Reactivity of HPHTFR chars with CO2 in HPTGA obtained at 875°C 99 and 20% conversion. (SA is initial N2 surface area and SA20% is the surface area measured at ~20% conversion)

6.2 The ratio of defects to graphite band (Id/Ig) for the HPHTFR chars 100 (before washing with THF) obtained using Raman spectroscopy

6.3 HRTEM pictures of soot generated at different pressures: Left: 6.2 101 bar; right: 21.7 bar

6.4 XRD pattern of chars generated at different pressures 101

6.5 Conversions profiles of chars with CO2 generated at the same 104 pressures as it was generated in HPHTFR. (for 21.7 bar char, the conversion profile was obtained at 20 bar CO2 pressure)

viii

6.6 CO2 reactivity versus conversion profiles of chars generated at various 106 pressures

6.7 Pore size distribution of initial char and char after 20% conversion in 110 HPTGA

A-1 High pressure feeder (shown in left) and the reactor (shown in right) 128

A-2 Water filter (left) and the copper rods connecting the heating element 133 (shown in right) A-3 Downstream section of the reactor 134

C-1 A photograph of a char sample generated from -150+106 µm cut at 138 1100°C before THF washing (left) and the THF soluble tars (right).

D-1 Data processing for char data generated at 875°C and 6.2 bar in 140 HPTGA (Char generated from -150+106 µm feed cut in HPHTFR at 1100°C and 6.2 bar in CO2/N2) E-1 Effectiveness factor variations with temperature for 3.4 and 6.2 bar 143 chars H-1 A side view picture of the feeder 147

H-2 A) Gas dosing system (left) B) Gas flow controllers (Right) 147

H-3 A) Fresh refractory castable (left) B) Refractory castable with cracks 148 after drying

List of Tables Table Page

1.1 Summary of the major types of gasifier 5

2.1 Important reactions that occur in gasifiers 11

2.2 Effect of temperature and pressure on the equilibrium trends of 12 components in the gasifier

2.3 Influence of operating parameters and char characteristics on 15 reaction regime

2.4 Coal gasification studies conducted at high heating rates and elevated 26 pressures

4.1 Coal properties 33

4.2 Experimental conditions 35

4.3 Proximate analysis of the chars generated in different pyrolysis 42 atmospheres

4.4 Thermal swelling ratio of chars 44

4.5 Values of La and Lc for chars 49

4.6 Compounds identified by the GC-MS 53

4.7 Kinetic parameters for the chars generated in various pyrolysis 59 atmospheres (at 10% conversion)

5.1 Compositional analysis of feed samples 65

5.2 Properties of chars analyzed 74

5.3 Effect of various parameters on conversion 92

5.4 Effects of various parameters on group-I char concentration and 92 swelling ratio

5.5 Effect of various parameters on intrinsic reactivity 92

6.1 Proximate analysis of chars used in this study 96

6.2 Kinetic parameters of chars generated at various pressures 107

6.3 Changes in pore volume and surface area for the 20% converted char 110 in HPTGA in comparison to the initial char

7.1 Effects of pressure, temperature, particle density, and particle size on 114 conversion, structural features, and intrinsic reactivity

B-1 Particle velocity calculation parameters 136

B-2 Flow rates and velocities of particle and gas 136

C-1 Proximate analysis of chars before and after washing with THF 138

C-2 Proximate analysis of chars washed by the above procedure and by 139 Soxhlet apparatus procedure. F-1 Kinetic parameters of the char sample from three runs 144

G-1 Representative yield of narrow size fractions covering the feed particle 145 size distribution to a slurry-fed gasifier

Nomenclature

UPPER CASE LETTERS

-2 -1 Aint Intrinsic pre-exponential factor (gm s )

-1 -1 -n A0 Specific pre-exponential factor (gg s bar )

Ba and Bc Full width half maximum of (100) and (002) peaks in XRD spectrum

Cf Active site

CD Drag coefficient

DA,B Binary molecular diffusivity of A and B (cm2s-1)

2 -1 Deff Effective diffusivity (cm s )

2 -1 Dk Knudsen diffusivity (cm s )

D Diameter of the reaction tube (m)

Ea Activation energy (kJ/mol)

FC Fixed carbon (%)

La Crystallite width (Å)

Lc Stacking height (Å)

MA Molecular mass of component A (g mol-1)

-1 MW Average molecular mass (g mol )

-1 MC Molecular mass of carbon (g mol )

-1 NAv Avogadro’s number (mol )

P Total pressure (bar)

Pr Reactant pressure (bar)

R Gas constant (Jmol-1 K-1)

-2 -1 -2 -1 Rint Intrinsic reactivity (gm min or gm s )

-1 -1 -1 -1 Rapp Apparent reactivity (gg min or min or s )

xii

Re Reynolds number

2 SA N2 Surface area (m /g)

2 SA20% conv N2 surface area at 20% conversion (m /g)

Tg Gas temperature (K)

T Particle temperature (K)

VA and VB Diffusion volumes of component A and B (FSG equation)

Vg Gas velocity (m/s)

VM Volatile matter (%)

Vp Particle velocity (m/s)

X Conversion (%)

LOWER CASE LETTERS

a Molar ratio of reactant gas to carbon

b Molar ratio of product gases to carbon

do Feed particle diameter (m)

d Char particle diameter (m)

g Acceleration due to gravity (m s-2)

j True reaction order

k Rate constant (gg-1s-1 bar-n)

-2 -1 -n kint Intrinsic rate constant (gm s bar )

m Instantaneous mass (dry ash free, g)

mo Initial mass (dry ash free, g)

n Apparent reaction order

slpm Standard liters per minute

GREEK SYMBOLS

α Ratio of Knudsen to molecular diffusivity

xiii

ε Porosity

η Effectiveness factor

λ X-ray wavelength (Å)

µ Fluid viscosity (Pa s)

-3 ρg Gas density (kgm )

-3 ρp Particle density (kgm )

-3 ρpo Initial particle density (kgm )

θA and θC Scattering angles corresponding to (100) and (002) peaks in XRD spectrum

σ Standard deviation (in uncertainty analysis)

-3 σT True density (kgm )

τ Tortuosity

ϕ Thiele modulus

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Acknowledgements

I express my gratitude to my advisor, Dr. Sarma Pisupati, for his continued support throughout this project and for giving me complete freedom to conduct my research and providing me with valuable advice.

I also thank my committee members, Dr. Jonathan Mathews, Dr. Mark Klima and Dr. Anil

Kulkarni for their encouraging words and sharing their valuable inputs and guidance. Special thanks to Dr. Mathews for helping me understand the issues with various characterization techniques and imparting me with the fundamentals of coal utilization through his EGEE 597G course.

I appreciate the US Department of Energy and Reliance Industries for their financial support. I like to thank Nandakumar Krishnamoorthy for spending time in making me understand various design issues during building of the high-pressure reactor. I also like to thank Aime Tchapda, Jim Kasab,

Nari Soundarrajan, Brad Maben, Ron Wasco, Ron Wincek, David Johnson, Bruce Miller, and

Sharon Miller for their help in building the high-pressure flow reactor. I acknowledge the support of Gary, Julie, Max, and Nicole for their assistance with reflectance microscopy, SEM, Raman and

XRD. Special thanks to Fei Feng for extending his help in operating the reactor.

I thank my parents, and my brothers for their support, and for providing intelligent and thoughtful conversation on various topics. The balance and grounding provided to me were essential and integral to Ph.D. life.

Introduction

1. Introduction Coal has been a dominant source of energy for generating electricity throughout the world and will continue to be the primary source of energy moving into the future [1]. However, increasing environmental and health concerns directed toward coal utilization have led to stringent environmental regulations [2]. The emergence of clean coal technologies, particularly integrated gasification combined cycle (IGCC), provides a more promising alternative to conventional coal- fired units to reduce coal’s environmental footprint by converting pollutants and CO2 into by- products. Figure 1-1 shows the process flow diagram of a typical IGCC process. A brief description of the IGCC process is as follows: The IGCC plant constitutes three major components: gasifier, gas processing unit, and the combined cycle unit (i.e., gas and steam turbines) [3]. In the gasifier unit, the organic portion of the fuel, such as coal, biomass, petcoke, and other solid carbonaceous fuels, are converted into synthetic gas (i.e., CO and H2). The inorganic portion of the feed is removed as molten slag or ash [4]. The ash-laden hot synthesis gas exiting the gasifier is then processed. The processing involves cooling the syngas, and removal of the ash and gaseous pollutants from the syngas. High purity syngas exiting the gas processing unit is combusted in the gas turbine connected with a heat recovery unit for generating steam for a steam turbine. The combined steam and gas turbines make the process efficient compared to a conventional pulverized coal plant.

The heart of the IGCC process is its gasifier. There are three basic gasifier designs used for coal gasification: a. Fixed-bed gasifier (sometimes referred to as a moving-bed gasifier) b. Fluidized-

bed gasifier and c. Entrained-flow gasifier [5]. Among these gasifiers, an entrained-flow gasifier is

Syngas processing unit

Combined cycle unit

Figure 1-1: A typical IGCC process flow diagram [6]

predominantly used for large-scale commercial applications [3]. The entrained-flow gasifiers typically operate under high temperatures (1200-1600°C) and high pressures (20-70 bar) [7]. This

type of gasifier uses a finer particle size distribution (<600 µm) compared to the fixed-bed (5-50 mm) and fluidized-bed gasifiers (0.5-5 mm) [4, 8]. Higher temperatures and finer particle size distribution mean a shorter residence time to convert coal into syngas, and consequently higher throughput for this type of gasifier. Higher operating pressure means a smaller volume of the gasifier for a given residence time. There are different types of entrained-flow gasifiers and they differ in feed system, usage of oxidant, type of flow, number of stages, and the gasifier lining [7,

9, 10].

The two commonly used feed systems for entrained-flow gasifiers are a dry lock hopper system and a coal-water slurry feed system. The dry feeding system is economical when the gasifier is operated at lower pressures (<30 bar). At higher pressures (>30 bar), the cost of pressurization and depressurization of the lock hopper becomes too expensive. For higher pressures (>30 bar), the coal-water slurry system is more reliable and economical [10]. The solids loading in the slurry range between 55-65 wt%. Unlike the dry feed system where pulverized coal (70% <75 µm) is used, the particle size distribution required for the coal-water slurry system is much wider as it is needed to produce a stable slurry [10]. Coarser feed particle size distribution in the feed means longer residence time required for the slurry feed gasifiers to achieve complete conversion of the feed to syngas [10]. Another complication is that an enormous amount of energy is spent in the vaporization of water making the gasifier with coal-water slurry system less efficient than a dry- feed gasifier.

Entrained-flow gasifiers use either oxygen or air as an oxidizing agent [3]. The oxidant is necessary to generate sufficient heat to drive the endothermic gasification reactions. If oxygen is used, energy is spent in separating oxygen from nitrogen. If air is used, the syngas is diluted by nitrogen leading

to lower total heating value of the syngas. The lower heating value of the syngas then reduces the overall efficiency of the IGCC process.

Entrained-flow gasifiers also differ in the gas flow and the number of stages. Most gasifiers are designed to operate with updraft. This allows the bottom of the gasifier to be extremely hot for melting the slag [11]. Some gasification technologies use two stages to improve thermal efficiency and lower the oxygen requirements; however, this adds to the complexity of the process.

The refractory lining used in gasifiers varies with design. In general, the refractory linings can be classified as air-cooled or water-cooled. The water-cooled refractory lining is also referred to as a membrane-wall design with stainless steel tubes to carry water/steam covered with a thin layer of Al2O3-SiC refractory [4]. On the other hand, an air-cooled refractory lined design is traditionally alumina based, and/or zirconia-based refractory for the hot face and has a thick multi layered refractory to reduce heat loss. This type of design is relatively inexpensive compared to water-cooled design. The downside of the design is that the lifetime is extremely short, ranging from between four and 24 months as compared to 10 years for a water-cooled lining [4]. The summary of various gasifier designs is shown in Table 1-1.

Although substantial operating experience has been able to resolve many of the operating-related issues, the state of the knowledge of gasification reactions at high pressures is relatively low when compared with traditional combustion technologies. Because of the limited knowledge, the design of the gasifiers and their operating conditions have traditionally been based on heuristics [12]. This approach has yielded poorly designed gasifiers operating far below the maximum achievable reliability and efficiency. Moreover, the gasifiers are expected to use different fuels over their life time. Incorporating the experimentally determined gasification behavior of various fuels in

arriving at the design parameters is necessary for the successful development of a reliable and a highly efficient gasifier.

Table 1-1: Summary of the major types of gasifier [6, 10, 11]

Gasifier Gasifier Feed Oxidant Stages Gas Lining Max. Example technology supplier system flow Pressure (bar) Shell coal Shell Dry O2 1 Up Water- 40 Buggenum, gasification lock cooled Netherlands process hopper membrane (Now wall dismantled) Prenflo Shell Dry O2 1 Up Water- 40 Puetollunno, (Formerly lock cooled Spain (now Krup hopper membrane dismantled) Uhde) wall Siemens Siemens Dry or O2 1 Down Air cooled 20-30 Shenhua Slurry Ningxia Coal Group GE Gasifier GE Slurry O2 1 Down Air cooled 60 Tampa (formerly Electric Texaco) E-Gas CBI Slurry O2 2 Up Air cooled ~60 Wabash Lummus (now (Formerly closed). Conoco) Reliance industries limited Clean Coal Mitsubishi Dry Air 2 Up Water- 20-30 Nokoso, Power Heavy cooled Japan Gasifier Industries membrane (MHI) wall The The Dry O2 2 Up Water- 40 Tianjin Huaneng Huaneng cooled IGCC power Clean Clean membrane plant Energy Energy wall Research Research Institute Institute (HCERI)

To effectively design a gasifier, there must be better understanding of gasification behavior of coal, including 1) volatile yield, composition, and combustion; 2) heterogeneous char-gas kinetics;

3) homogeneous gas phase kinetics; 4) syngas composition and yield; 5) fate of inorganic species; and 6) slagging and fouling behavior [7, 13]. Some of these such as gas phase reaction (or

homogeneous reaction) kinetics is largely fuel independent and the kinetics can be obtained from the literature. Models like Chemical Percolation Devolatilization (CPD) and Flash Chain can be useful in predicting the volatile composition for a given coal [14-16]. However, other processes like heterogeneous gas-solid reaction kinetics, syngas composition yield, fate of inorganic species, and slagging and fouling behavior are all dependent on the feedstock and must be experimentally evaluated. Among all the parameters, char gasification kinetics is the most important parameter in the design and troubleshooting of the gasifiers. For example, the reaction time required for complete conversion of the feed stock can be obtained from the heterogeneous char gasification rate. The reaction time also determines the dimensions of the gasifier. For a given dimension, the throughput can be determined. The reaction rate is also necessary to determine the optimum temperature and pressure. Temperature in the gasifier should be high enough that the inorganics are converted into molten slag and the organics are completely converted into syngas. Operating the gasifier at higher temperature than the ash melting temperature means more oxygen is required to maintain the temperature. More oxygen means more parasitic power (i.e., amount of power consumed by the process), which lowers the efficiency of the process. The knowledge of gasification kinetics at higher pressures is therefore necessary to design and operate the entrained- flow gasifiers efficiently.

Gasification rate in an entrained-flow gasifier is affected by both the rate of diffusion of the reactant gas into the pores and the intrinsic reaction rate (i.e., reaction rate free of mass transfer diffusion) [17]. Therefore, the knowledge of char pore structure and the intrinsic reaction rate are also important. Much of the gasification reactivity data in the literature have been measured using chars generated at atmospheric pressure and low heating rates. While the data certainly provide valuable insights into reaction processes, it may not be useful for predicting the kinetics in

entrained-flow gasifiers as char-generation conditions (i.e., temperature, pressure, reactants and initial particle heating rate) affect pore structure and gasification reactivity. Therefore, it is imperative that the chars are generated under conditions that match as many variables of the entrained-flow gasifier as possible. This makes the char structure more realistic and useful for the model development. Part of the research is to determine the effect of pressure on the structural characteristics of the char generated at high initial particle heating rate and temperature, and the consequent effect on CO2 reactivity. Additionally, reactivity of chars with CO2 under kinetic- controlled regime is also evaluated.

Literature Review

2. Fundamentals of the Gasification Process

Gasification is a process by which the organic portion of the solid fuels, through partial oxidation, is transformed into useful gaseous products termed as synthesis gas (primarily CO and H2).

Gasification of the organic matter is a three-step process: pyrolysis, combustion, and gasification.

These steps may or may not occur distinctively or sequentially [10]. Figure 2-1 shows the three- step process as a solid fuel transforms into the gas phase. The first step of the process is the pyrolysis also known as devolatilization. When a solid fuel is exposed to high temperatures, it loses volatiles (i.e., CO, H2, CH4, light hydrocarbons and tars) instantaneously to form a carbon- rich solid residue called char. The second step is the combustion step, where the volatiles and a portion of the char react with O2 to generate CO2, H2O, and CO. This step provides the necessary heat to drive the endothermic gasification reactions [10] and create the atmosphere required to facilitate the gasification reactions. The last step being the endothermic gasification reactions, where char heterogeneously reacts with CO2 and H2O to form CO and H2 at temperatures above

800°C. The char gasification step is the slowest step and therefore determines the overall rate of conversion of the feedstock in the gasifier [10]. Between the two reactants, the char-H2O reaction rate is faster than the char-CO2 reaction [18, 19]. The CO and H2 thus formed also undergo various homogeneous reactions by interacting with the volatiles and reactants to form a final gasifier outlet syngas composition. The ash that is left after gasification of char is entrained out of the gasifier or deposited on the refractory wall and captured at the bottom of the gasifier as a molten slag.

Pyrolysis Volatiles Char

O Stage-I CO/CO 2

CO CO2

Stage-II

H O H2 2

Ash formation/deposition

Slag flow

Figure 2-1: Transformation of coal during gasification in a slurry fed two-stage gasifier (adapted from Harris [20])

2.1. The Chemistry of the Coal Gasification Process

Regardless of the gasifier, most of the heterogeneous and homogenous reactions operating within the gasifier are universal in nature. To model gasification kinetics, it is necessary to comprehend the important reactions and their kinetics. There are number of reviews reported on the important reactions that occur in a gasifier [8, 21, 22]. Important heterogeneous and homogeneous reactions and their enthalpies are shown in Table 2-1. The extent to which each reaction occurs within the gasifier is dependent on the residence time of the gas and the particle, temperature, pressure, and

reactor configuration. The combustion and oxidation of char (C-O2) are the fastest of all the heterogeneous reactions and are exothermic in nature. These two reactions reach completion within the gasifier and create the atmosphere for gasification reactions. The leftover char reacts with other reactants such as H2, CO2 and H2O. These reactions are endothermic in nature and occur only above 700°C [8]. The reaction rate of char with these reactants follow the order:

Reaction rate: char-O2>> char-H2O> char-CO2>> char-H2

5 The char-O2 reaction is about 10 times faster than the char-CO2 reaction [23, 24]. The char-H2O reaction is about 1-3 times faster than the char-CO2 reaction [18, 19, 23], while the char-CO2 reaction is at least 100 times faster than the char-H2 reaction [25]. The slow nature of char-H2 reaction means it does not play a significant role except at high pressures.

The final syngas composition is very much dependent on the homogeneous reactions. Many of these reactions achieve equilibrium within the gasifier [8]. The oxidation and combustion reactions provide necessary heat to drive the endothermic gasification reactions. The water-gas shift reaction determines the CO:H2 ratio of the syngas exiting the gasifier. The Sabatier reaction, which is dominant at higher pressures, increases the methane concentration and consequently increases the calorific value of the syngas. The final syngas composition is based on the operating temperature and pressure of the gasifier. An increase in gasifier temperature increases H2 and CO, while it decreases CO2, H2O and CH4. In the case of pressure, H2O and CH4 increase with pressure and

CO and H2 decrease with pressure. The syngas composition within the gasifier is also important as it can interfere in the heterogeneous gasification reactions by preferentially adsorbing onto the char surface thereby slowing down the heterogeneous reaction rate of the gasification reactions.

Table 2-1: Important reactions that occur in gasifiers [8, 18, 21, 22]

-1 Reaction ΔHrxn (kJ mol ) Reaction type Heterogeneous reactions 퐶 + 0.5푂 ⟶ 퐶푂 -111 Partial oxidation 퐶 + 푂 ⟶ 퐶푂 -394 Combustion 퐶 + 퐶푂 ⟶ 2퐶푂 172 Boudouard reaction 퐶 + 퐻푂 ⇋ 퐶푂 + 퐻 131 Gasification 퐶 + 2퐻 ⟶ 퐶퐻 -75 Gasification Homogeneous reactions 퐶푂 + 0.5푂 → 퐶푂 -283 Oxidation of CO 퐻 + 0.5푂 → 퐻푂 -242 Combustion of H2 퐶푂 + 퐻푂 ⇋ 퐶푂 + 퐻 -41 Water-gas shift 퐶퐻 + 퐻푂 ⇋ 퐶푂 + 3퐻 206 Methane steam reforming 퐶퐻 + 퐶푂 ⇋ 2퐶푂 + 2퐻 247 Methane dry reforming 퐶푂 + 4퐻 ⇋ 퐶퐻 + 2퐻푂 73 The Sabatier reaction

In designing a gasification process, it is necessary to understand the effect of temperature and pressure on the equilibrium positions of various reactions. The effects of temperature and pressure on equilibrium of a C-H-O system is shown in Table 2-2.

2.2.Reaction Regimes

The heterogeneous gas-solid reaction is an intricate coupling of transport phenomenon and chemical kinetics [26]. The overall reaction scheme that happens with a char particle can be described by the following series of steps (shown in Figure 2-2):

1. The reactant is transported to the particle outer surface

2. The reactant diffuses into the char pore

3. The reactant gets adsorbed onto the active site

4. The reactant dissociates forming reaction intermediate(s) (eg. carbon-oxygen complex)

5. The product desorbs from the char surface

6. The product diffuses through the char pore structure

7. The product gets transported from the char outer surface to the bulk gas phase

Table 2-2: Effect of temperature and pressure on the equilibrium trends of components in the gasifier (taken from Kristiansen [8])

Gas components Increase in Temperature Increase in Pressure

H2O Decreases Increases

CO2 Decreases Constant

H2 Increases Decreases CO Increases Decreases

CH4 Decreases Increases

These steps show that the solid conversion depends on chemical reaction and the ability of the reactant to diffuse into the porous char structure (i.e., mass transfer). As char properties also influence the reaction rate, it is important to understand the effect of those properties such as active sites, char pore structure, and char morphology on reactivity.

Active sites: These sites can form bonds with the reactants and subsequently form products [27,

28]. These sites are defects and dislocations on the char surface, edges of the carbon crystallites, and heteroatoms.

Ash: Ash creates more dislocations and consequently increase the active site concentration. Some ash particles such as iron can also catalyze the gasification reactions. However, at high temperatures, ash can melt and reduce the active site concentration.

Char pore structure and morphology: The char pore structure determines the surface area available for the reactant to interact with the char structure. Additionally, it influences the diffusion of the reactant within the pores, determines the concentration of the reactant within the pores and consequently the overall rate [11].

Char

Char surface

Figure 2-2: The overall reaction scheme of a heterogeneous gasification reaction (Modified after Jurtz et al. [29]) Based on the char properties and operating parameters, the heterogeneous reactions can occur in one of the three regimes [18]. Figure 2-3 shows the effect of temperature on the reaction rate over the three reaction regimes:

1. Regime 1 or chemical reaction control regime: This regime occurs at low temperatures

(<1000°C) and is dominant when the rate of reaction is much slower than that of diffusion

of gases. Because the reaction rate is slower, the gas can diffuse throughout the particle

and the concentration of the reactant within the pores is same as the bulk gas concentration.

The reaction rate is function of total accessible surface area and the concentration of active

sites. The activation energy (Ea) and the order of the reaction (n) measured in this regime

are the true activation energy and the true order of the reaction.

2. Regime 2 or pore diffusion limited regime: This regime occurs at sufficiently high

temperatures where the rate of diffusion of gases into pores is comparable with the

chemical reaction rate. High reaction rate means the gas phase reactant is consumed as it

penetrates through the pores and this results in a significant concentration gradient between

the bulk gas phase and the pores. Predicting the gasification rate in this regime requires the

knowledge of char pore structure and the chemical reaction rate. The activation energy is

almost half of the true activation energy because of the influence of the diffusion. However,

the order of the reaction in this regime is higher than the true order of the reaction as the

reaction becomes more sensitive to reactant concentration.

3. Regime 3 or mass transfer-controlled regime: This regime is dominant at very high

temperatures where the chemical reaction rate is so high that the reactant is consumed on

the particle surface. As a result, there is a huge concentration gradient established between

the bulk gas phase and the particle surface. The concentration of the reactant within the

particle is zero. Except for particle size, no other char properties are required for arriving

at the kinetics. The measured activation energy is almost zero and the order of reaction

approaches unity.

The controlling mechanism is dependent on the relative influence of operating parameters

(temperature and pressure) and the char characteristics (particle diameter, char porosity, and active site concentration). Low temperature, low pressure, fewer active sites, smaller particle size and higher porosity all favor reaction regime 1, while the opposite favors regime 3. Between the two extremes is regime 2 where both mass transfer and reaction rate play a role in influencing the

overall gasification rate. Most gasifiers operate under regime 2, although the controlling regime may change within the gasifier depending upon the operating parameters and char characteristics.

Table 2-3 summarizes the influence of operating parameters and char characteristics on reaction regime.

Regime-3

Regime-1

Figure 2-3: The change in reaction rate of a porous carbon with temperature (Adapted from Walker et al. [18]) where j is the true reaction order; n being the apparent reaction order.

Table 2-3: Influence of operating parameters and char characteristics on reaction regime

T P Ct d Porosity

Regime 1 L L L L H

Regime 2 M M M M M

Regime 3 H H H H L

L is low; M is medium; H is high (Regime I occurs when the temperature (T), pressure (P), active site concentration (Ct) and particle diameter(d) are small in values, while the porosity of the particle is large)

2.3. Char-CO2 Reaction Mechanisms

It is well known that the slowest reactions control the rate of conversion of carbon within the gasifier. The two major reactions are char-H2O(g) and char-CO2 reactions. The reaction mechanism discussion is restricted to the char-CO2 reaction as it is the prime focus of the dissertation. Reviews by Walker et al. and others shed light on the various mechanisms proposed for the char-CO2 reaction [18, 26, 30]. One of the widely accepted mechanisms for the char-CO2 reaction is the two-step process proposed by Ergun [31], as shown in Equations 2.1 and 2.2. The first step is the dissociation of the carbon dioxide molecule by interacting with a carbon active site in which a CO molecule is released, and a carbon-oxygen complex [C(O)] is formed

퐶 + 퐶푂 ⇋ 퐶(푂) + 퐶푂 --2-1 The second step is the desorption of the carbon-oxygen complex leading to formation of another carbon monoxide molecule.

퐶(푂) → 퐶푂 + 퐶 --2-2 It is accepted that the desorption of the carbon-oxygen complex is the rate limiting step [18]. The review presented by Roberts and Harris shows that CO can inhibit the char-CO2 reaction by preferably adsorbing on to the active site [32].

2.4. Effect of Pyrolysis Conditions on Reactivity

Pyrolysis conditions affect the char properties and consequently the reactivity. This section briefly describes the role of temperature, residence time, pressure, feed particle density, feed particle size, and heating rate on char reactivity.

2.4.1. Effect of Temperature on Char Reactivity

It is generally accepted that the reactivity of char decreases with increase in pyrolysis temperature.

Fermoso et al. pyrolyzed a biomass sample at 1000°C and 1400°C in an atmospheric drop tube furnace [33]. The char reactivity was measured in a pressurized thermogravimetric analyzer

(PTGA) in CO2 at 0.1 MPa and 1.0 MPa. The study showed that the reactivity of char generated at 1000°C was more reactive than the char generated at 1400°C. The difference in reactivity was suggested to be due to increased structural ordering for the char generated at 1400°C. Feng et al. summarized the decrease in reactivity with increase in pyrolysis temperature for a biomass and two coal samples (shown in Figure 2-4) [34]. The low-rank coal and biomass were shown to be more sensitive to pyrolysis temperature than the bituminous coal. The reason for the drastic decline in reactivity was attributed to thermal deactivation of char, which is a combination of loss of surface area and active sites. The characteristic time for deactivation of char at higher temperatures is comparable to the char-CO2 reaction and hence thermal deactivation is more pronounced during gasification [34]. Besides thermal deactivation, the other reasons for loss of reactivity with pyrolysis temperature include loss of catalytic effect, loss of micropore structure and active sites due to ash melting above ash fusion temperature, structural ordering leading to graphite-like reorganization. The increased structural ordering with temperature is pictorially shown in Figure

2-5.

Figure 2-4: Effect of heat treatment temperature on the reactivity of carbonaceous materials. R1000 is the reaction rate of the corresponding coal char heat treated at 1000 °C and R/R1000 is the reactivity ratio (taken from Feng et al.[34])

Figure 2-5: Structural rearrangement leading to crystallinity in carbon

2.4.2. Effect of Initial Particle Heating Rate

Initial particle heating rate is an important factor that is known to affect char reactivity. Studies have shown that increase in particle heating rate increases char reactivity [35, 36]. Cai et al. varied the heating rates (5 to 5000 °C/s) in a wire mesh reactor to pyrolyze coals of various ranks [35].

The combustion reactivity of those chars was measured in a TGA at 500°C. The increase in

reactivity was more pronounced for low-rank coals than for high-rank coals. In every case, the reactivity increased with increase in heating rate up to 1000°C/s and levelled off between 1000°C/s and 5000°C/s [36]. It was suggested that increase in heating rate increases the feeder pores and surface area and consequently increase the reactivity. Beyond 1000°C/s, the feeder pore concentration reached the upper limit beyond which increase in heating rate does not increase reactivity. Similarly, Zhou et al. also observed char reactivity to increase with increase in heating rate when the residence time of the char at the peak temperature is zero [36]. For longer residence times, the effect of heating rate significantly decreases. It must be emphasized here that the initial particle heating rate in an entrained-flow gasifier is >104 °C/s. Although the above studies are useful in bringing out the fundamentals of the effect of heating rate, low heating rate experiments reported in these two studies may not directly explain the reactivity of char in an entrained-flow reactor. Roberts and Harris compared the reactivity of chars generated from a pressurized entrained-flow reactor (PEFR) and a horizontal tube furnace (HTF) at 1100°C [23]. The authors found that the O2, CO2, and H2O(g) reactivities of the char generated in the PEFR were about 20-

30 times higher than that of the reactivities of the char generated in the HTF. The increase in reactivity with heating rate, based on a biomass sample, is explained by DiBlasi [37]:

“While, for slow heating rates, volatile pyrolysis products are released through the natural porosity and no major change takes place in the particle morphology, for fast heating rates the original cellular structure is lost as a consequence of melting phenomena. Fast volatile release produces substantial internal overpressure and coalescence of the smaller pores, leading to large internal cavities and a more open structure of both wood and lignin. Hence, for pyrolysis carried out at atmospheric pressure, chars produced at slow heating rates mainly consist of a micropore structure whereas those obtained with high heating rates mainly present macropores. An analysis

of the literature review on coal chars leads to the conclusion that the surface area developed by mesopores and macropores is a better indicator for the reactive surface than the total surface area also including the contribution of micropores (below 2 nm) which probably do not participate in the reaction. Therefore, the increase of the char reactivity with the heating rate during pyrolysis can be explained by the occurrence of gasification reactions mainly on the surface of large pores which may also be associated with a higher total surface area and/or a higher concentration of active site.”

2.4.3. Effect of Residence Time

The residence time of char during pyrolysis can affect char reactivity. Zhou et al. showed that increasing the residence time during pyrolysis can drastically reduce char reactivity [36]. The study was conducted in a wire mesh reactor, which is not reflective of the conditions in an entrained- flow gasifier. Tremel et al. pyrolyzed chars in a PEFR over a range of residence times at 1200°C and 1400°C [38]. The chars pyrolyzed at 1200°C showed significant decrease in O2 reactivity with residence time. The decrease in reactivity with residence time was suggested to be the result of destruction of active sites with residence time. The results of that study are shown in Figure 2-6.

Figure 2-6: Effect of residence time on oxygen reactivity of chars generated at 5 bar pressure in a high pressure entrained-flow reactor (Taken from Tremel et al.[38])

2.4.4. Effect of Feed Particle Density

Coal is a heterogeneous mixture of organic and inorganic matter. Each particle may have significant variation in maceral constituents and inorganic matter. These differences result in different char structures [39-41], which in turn affect the char reactivity. In general, particles containing predominantly vitrinite and liptinite develop a lot of fluidity during pyrolysis and evolve into a highly porous chars, while those containing inertinites do not develop fluidity and generate highly dense low-porosity chars [40]. Formation of highly porous chars can be explained based on the "explosive ejection" mechanism proposed by Gray [42]. When a pulverized vitrinite particle from a caking coal is subjected to high heating rate the whole particle becomes fluid with gas bubbles. As the gas bubbles coalesce, particle expands due to swelling. Because of the high heating rate, the outer layer of the particle hardens before all the gas bubbles coalesce. Hardening of outer shell results in build-up of pressure within the particle. Consequently, the pressure within

the particle is released through multiple explosions resulting in formation of cenospherical (high porosity) char particle.

Bailey et al. classified the char structure into three major groups: group-I char (>70% porosity, and <5 µm wall thickness), group-II char (40-70% porosity, and >5 µm wall thickness), and group-

III char (<40% porosity, and variable wall thickness) [43]. Gilfillan examined the structure of chars generated from six different coals. The morphological analysis showed that low-density fractions generated more thin-walled cenospheres (a large central void surrounded by thin-wall), while the proportion decreased with increase in particle density [41]. Yu et al. conducted a systematic study to determine the morphology of chars generated from various density fractions of coal during pyrolysis of an Australian sub bituminous and bituminous coals in a drop tube furnace [44]. The authors concluded that the char morphology is closely related to the maceral composition and the group-I type decreases with increase in feed particle density. Although these studies focused on char morphology, there is no systematic study conducted on the reactivity behavior of the chars generated from various density fractions.

2.4.5. Effect of Feed Particle Size

The particle size is another factor that is known to affect char morphology, and consequently the reactivity. Yu et al. studied the effect of particle size on the swelling ratio during pyrolysis of

Australian coals in a drop tube furnace. The authors observed that the swelling ratio decreases with increase in particle size. The particle size distribution used was representative of the feed to the dry-fed gasifier (<150 µm) [44]. It is important to recognize that the particle size distribution to a slurry-fed gasifier (95 % of the feed <600 µm) covers a wider range than to a dry-fed gasifier (90% of the feed <100 µm) [10]. The larger particle size distribution is necessary for obtaining a stable

slurry to a slurry-fed gasifier. No investigation has been conducted to look at the effect of particle size, representative of the feed to the slurry-fed gasifier, on char structure and reactivity.

2.4.6. Effect of Pressure

High pressure studies, relevant to that of an entrained-flow gasifier, are very limited in the literature. A summary of the high-pressure studies is shown in Table 2-4. The summary shows that very few higher-pressure data are available in the literature. This is due to difficulties in designing and operating a lab-scale high pressure reactor. Even fewer kinetic studies are available. A brief discussion of the kinetic studies is presented here:

Lee et al. studied the reactivity of char generated from Illinois No. 6 coal at 916°C and 1–38 bar in N2 atmosphere using an entrained-flow reactor [45]. The apparent reaction rate of char in air (at

410°C) increased for char generated at 7 bar than those of chars generated at atmospheric pressure or 21.3 bar pressure at residence time <1 s. This was attributed to increased fluidity at 7 bar, generating additional active sites, and the increase in char hydrogen content with devolatilization pressure. Above 7 bar, the reactivity of chars decreased due to increased structural ordering.

Benfell investigated the effect of pyrolysis pressure on combustion reactivity of two Australian coals [46]. The chars were produced from a pressurized drop tube reactor at three different pressures: 5, 10, and 15 bar. The combustion reactivity of the chars was obtained in a HPTGA at

375°C in 50% O2/N2 atmosphere. The overall effect of pyrolysis pressure on apparent reactivity was inconsistent, while the intrinsic reactivity, obtained by normalizing with CO2 surface area, was observed to be unaffected by the pyrolysis pressure.

Roberts et al. prepared chars from three different coals in a PEFR at 1100°C and at ~5, 10, and 15 bar in N2 atmosphere. Reaction rates in CO2, H2O, and O2 were measured in a PTGA [23]. Chars from two coals that were in the middle and later stages of conversion in the PEFR generally showed

an increase in CO2 reactivity with increasing pyrolysis pressure. Interestingly, the CO2 reactivity of the char in the early stages of conversion showed less sensitivity to pyrolysis pressure. In the

O2 atmosphere, the effect of pyrolysis pressure on reactivity was less obvious. However, the intrinsic rate showed little variation with pyrolysis pressure for reaction with all the gases. The authors concluded that the difference in reactivity with pressure was a result of changes to the microporous structure and not due to variations in chemical structure.

Zeng and Fletcher generated Pittsburgh No.8 coal chars in a high pressure flat flame burner [47].

The apparent reaction rate was determined on those chars in a HPTGA with O2 as the reaction gas.

The general trend was that the apparent reactivity decreased with increase in pyrolysis pressure, while the intrinsic reactivity, obtained by normalizing with CO2 surface area, remained constant irrespective of the pyrolysis pressure.

More recently, Tremel et al. generated chars at two different pressures—5 and 25 bar— and at two different temperatures at 1200°C and 1400°C in a PEFR [38]. The apparent reactivity was measured in a HPTGA with O2 as the reaction gas. For shorter residence time (≤2 s) chars, the apparent reactivity increased with increase in pyrolysis pressure irrespective of the pyrolysis temperature. When the apparent reactivity was normalized with CO2 surface area, effect of pyrolysis pressure diminished.

In conclusion, the effect of pyrolysis pressure on apparent reactivity is not clear, though all the studies agree that the intrinsic reactivity, obtained by normalizing with CO2 surface area, showed no change with pressure. It is important to emphasize that all the data were obtained either for O2 reactivity or for CO2 reactivity for selected coals. Additionally, none of the studies listed in Table

2-4 looked at the effect of particle size, and inorganic matter on structural features and the reactivity of chars. For feedstock such as coal, which is highly heterogeneous, there is a need to

determine the role of pressure on the structural features for a wide range of particle sizes and density fractions. The limited data in the literature warrants more extensive investigation of the effect of char generation pressure at high heating rates (>104 °C/s) on intrinsic kinetics and char structure.

Table 2-4: Coal gasification studies conducted at high heating rates and elevated pressures

Reference Coal Particle size (Reactor) Reaction Low temperature Comments (µm) conditions reactivity conditions

o Lee et al. [45, Illinois #6 Bituminous 53-75 (HPEFR)1189 C/1-38 bar /N2 Air at 410°C Char morphology, swelling ratio, 48] and reactivity

Wu et al. [49, Australian Bituminous -90 +63 (DTF/PDTF) 1300oC/1-15 Not applicable Char morphology, swelling ratio, 50] bar/N2 and ash formation

Liu et al. [51] Australian Bituminous -90 +63 (PDTF) 1100oC/5-15 bar 385-416°C (in air) Char morphology, Reactivity

850-950°C (in CO2)

Benfell et al. Australian Bituminous 63-90 (PDTF) 1300°C/5.1-15.2 bar/N2 375°C/O2 Char morphology, and reactivity [46] coals

Ahn et al. [52] Indonesian 45-64 (PDTF) 900-1400°C/5-15 Not applicable Effect of pressure and temperature Subbituminous coal bar/CO2 and N2 on kinetics and modeling development

Kajitani et al. Australian NL 26-39 (PDTF) Pyrolysis: 1400°C for CO2: ≥850°C Reactivity at high temperature and [53] Bituminous and pyrolysis in N2 atmosphere low temperature conditions were H2O:≥850°C obtained using random pore model Chinese S Gasification in CO2 or Bitumininous H2O:1100°C-1500°C/0.2-2 MPa O2: ≥425°C

o Roberts et al. Australian thermal NA (PEFR)1100 C/5-15 bar/N2 50%O2/375°C; CO2/900°C; Reactivity, surface area, Char [23] coals H2O/800°C in HPTGA structure (XRD)

Yu et al. [54] Australian thermal 45-180 (PEFR) 1100 and 1400oC/ 20 Char morphology and swelling coals (SubB and bar/O2, CO2 and H2O ratio Bituminous) 63-95 o (PDTF)1300 C/5-15 bar/N2

o Zeng and American Bituminous 44-90 (HPFFB) 1300 C/2-15 bar/ N2 O2/ T= unknown Swelling ratio, reactivity, SEM Fletcher [47] coals and lignite /105 °C/s images, surface area

Harris et al. 14 Thermal coals 45-180 (Heated grid)1100 °C /15.5 CO2 reactivity at 900°C/H2O Specific reactivity, gas [55] bar/103 °C/s reactivity at 800°C composition, carbon conversion

Roberts et al. Australian thermal 45-180 (PEFR) Varying CO2 /N2 CO2 reactivity at atmospheric Reactivity at high temperature and [56] coals pressure fluidized bed reactor low temperature conditions with T:1000-1400°C; P: 10-20 bar and TGA at 900°C CO2 Hodge et al. [17]

o Tremel et al. Lignite, bituminous 80-160 (PEFR) 1200-1400 C/5 and 25 CO2 and H2O reactivity in Volatile yield and gasification [38, 57] and anthracite bar/H2 and N2 HPTGA kinetics in CO2 atmosphere

Shurtz and American coals 45-75/53-66 (HPFFB) upto 1727°C/ P: upto NA CO2 reactivity Fletcher [58] 15.5 bar

Lewis et al. Bituminous coals 45-75 (HPFFB) ~1527°C (10.1, NA CO2 reactivity global model [59] 12.6,15.1 bar), 1338°C (15.1 bar)/ (O2, H2O, CO2, CO, N2)

PDTF: Pressurized drop tube furnace; PEFR: Pressurized entrained-flow reactor; HPFFB: High pressure flat flame burner; NA: Not applicable

Research Objectives

The literature review has clearly identified that the state of knowledge of high-pressure reaction kinetics for chars generated at elevated pressures, high heating rates and temperatures is not sufficient to design and optimize the operation of the commercial entrained-flow gasifiers. With that in perspective, the objective of this research was to characterize char gasification at conditions pertinent to commercial entrained-flow gasifiers for a widely used coal. The work mainly focuses on the measurement of intrinsic gasification rates and structural features of chars generated at conditions of high initial particle heating rates at elevated temperature and pressures. This research will aid in the design and efficient operation of commercial entrained-flow gasifiers, as well as provide data to develop comprehensive kinetics models applicable over a wide range of temperatures and pressures at high heating-rate conditions. The specific contributions of the research are:

1. The pyrolysis atmosphere can affect char properties which in turn affect the reactivity.

Choosing the appropriate atmosphere for char generation is essential in arriving at a more

realistic kinetic model. It is therefore necessary to determine the role of reaction

atmosphere on char properties and consequent effect on reactivity.

2. Most of the studies reported in the literature generated char from one feed particle size

fraction and studied structural characteristics and reactivity on that char. However, coal

being a heterogeneous combination of organic and inorganic matter, the structural

characteristics of the resultant char would vary based on the amount of inorganic matter

present in the coal particle as well as variation in macerals. The structural characteristics

in turn affect the char reactivity. The objective is to quantify the effect of particle size

fraction, particle density, temperature, and pressure on structural characteristics and O2

reactivity of char obtained from one coal.

3. The intrinsic reaction rate of char with CO2 is important in developing a kinetic model.

There are only a few data sets in the literature that analyzed CO2 reactivity on coal char

generated at high pressure and temperature. This study sheds light on the effect of char

generation pressure on intrinsic reactivity of chars with CO2.

Effects of Pyrolysis Atmosphere on Volatile Yield and CO2 Reactivity of the Char Samples Generated in a High-Pressure, High-Temperature Flow Reactor

4. Abstract

The effects of pyrolysis atmosphere on volatile yield, structural characteristics, and CO2 reactivity have been examined on chars generated from Pittsburgh No.8 coal at 6.2 bar pressure and 1100°C in a high-pressure, high-temperature flow reactor (HPHTFR) in Ar, N2, 50 (vol)% CO2 and N2

(i.e., CO2/N2) atmospheres. The chars were characterized for volatile yield, thermal swelling ratio, surface area, pore size distribution, crystallite structure, and defects to graphitic intensity ratio using proximate analysis, tap density technique, surface area analyzer, XRD, and Raman spectroscopy. The char-CO2 reactivity was measured at 6.2 bar using a high pressure thermogravimetric analyzer (HPTGA). Coal pyrolyzed in CO2/N2 showed higher volatile yield

(27%) compared to coal pyrolyzed in argon (~16%) and nitrogen (~19%). Except for volatile yield, there was no significant difference in structural properties for chars generated in different pyrolysis atmospheres. The difference in volatile yield was found to be due to presence of unconverted tetrahydrofuran (THF) soluble tar/soot covering the char surface as well as collected separately in the filter. The results also showed that the apparent and intrinsic reactivities were highest for the char generated in N2 atmosphere. The kinetic parameters (activation energy and pre-exponential factor) for the char-CO2 reaction were ascertained using Arrhenius equation. The activation energies did not differ significantly among the chars generated in different pyrolysis atmospheres.

The order of reaction was found to follow: CO2/N2 char> N2 char≈ Ar char. The order of reaction correlates well with the surface area evolution of the char.

4.1. Introduction

Integrated gasification combined cycle is one of the advanced technologies that has potential to reduce coal’s carbon footprint. Despite many commercial operations, the char gasification rates at high pressures and temperatures, crucial to the design and troubleshooting of the gasifiers, are relatively unknown. There are few studies that reported high pressure reactivity of chars. Those chars were obtained either in an inert atmosphere or in the presence of reacting gases [23, 47, 57,

58]. Several researchers have studied the effect of reaction gas atmosphere on volatile yield, char characteristics, and reactivity in atmospheric pressure conditions. Rathnam et al. [60] examined the volatile yield of four pulverized coals (i.e., 63–90 μm) during pyrolysis in CO2 and N2 atmospheres. The experiments showed that the volatile yields in CO2 atmosphere were 4-24% higher compared to N2 atmosphere. The authors attributed the higher volatile yield in CO2 atmosphere to gasification. This observation contradicts the findings of Borrego and Alvarez where the volatile yield of two bituminous coals decreased by about 60% and 30% when N2 was replaced by CO2 during experiments in a drop tube reactor at 1300°C [61]. Brix et al. attributed this unusual observation to high volumetric flow rate of cold gas reducing the particle heating rate when using higher thermal capacity CO2 as compared to lower thermal capacity N2. Brix et al. studied the effect of gas atmosphere on the char morphology, surface area, and volatile yield of a bituminous coal during pyrolysis in CO2 and N2 atmospheres at 1400°C in a drop tube reactor [62].

Interestingly, the authors observed no effect of pyrolysis atmosphere on char morphology, volatile yield, and surface area. More recently, Wang et al. studied the effect of simulated pyrolysis gas

(containing CO2, CO, H2, CH4), and N2 on char reactivity and characteristics of a Chinese bituminous coal using a bubbling fluidized bed reactor [63]. The temperatures used in the study were between 450 to 600°C and the residence time of the particles was kept at 15 minutes. Under

these conditions, the authors observed that the pyrolysis atmosphere had no significant effect on char morphology but noted the pyrolysis atmospheres affected the char reactivity in CO2 and H2O atmospheres. It must be recognized that in this temperature range, the heating rate would be less than 100°C/s and that does not reflect the heating rate and temperature experienced by the particles in an entrained-flow gasifier. However, from the short review, it is apparent, the effects of pyrolysis gas atmosphere on volatile yield and CO2 reactivity are not conclusive. The objective of this investigation is to: 1) determine the volatile yield (or conversion) of chars generated in various pyrolysis atmospheres at elevated pressure; and 2) determine the CO2 reactivity of chars generated in various gas atmospheres.

4.2. Experimental Section

4.2.1. Coal Preparation

A widely used bituminous coal, Pittsburgh No.8 coal, was used for this study. The coal was dry ground in an industrial rod mill. The ground coal was sieved between 100 and 140 ASTM meshes and stored in an Ar filled aluminum bag. The particle size fraction of -150+106 µm was chosen as it covers about 10% of the feed to the slurry feed entrained-flow gasifier. The properties of the coal are shown in .

4.2.2. Char Preparation

All the experiments were conducted in an electrically heated HPHTFR. The reactor is capable of reaching a maximum temperature of 1650°C and 30 bar pressure. The schematic of the reactor is shown in Figure 4-1. The same reactor, used at atmospheric pressure, is described in detail elsewhere [64, 65]. The reactor consists of a feeder, a high temperature furnace, a sample collection section, and a gas analysis section. The high temperature furnace section consists of a high alumina ceramic (reaction) tube (65 mm i.d × 700 mm long) with a mullite flow straightener (honeycomb)

placed at the top surrounded by six super Kanthal heating elements, all encased in a refractory- casted, lined carbon steel pressure vessel. The concentric section between the refractory wall and the outer surface of the reaction tube was used to preheat secondary gas. The secondary gas constitute the majority of the gas (~90%) flowing into the reactor. Inert gas was used as the primary gas to transport particles from the feeder to the reaction zone. The gases were supplied by gas cylinders, and the flow rates were controlled by calibrated mass flow controllers. The reactor temperature was continuously monitored by a thermocouple placed close to the outer surface of the ceramic tube around the mid-section. Once the desired temperature was achieved, the reactor was purged with the reaction gas for few minutes. After which the reactor pressure was increased using a back-pressure regulator. The gas was allowed to flow for about 10-15 minutes at the desired pressure until the gas composition exiting the reactor, monitored using a calibrated micro gas chromatography reached the desired gas composition.

Table 4-1: Coal properties

Coal Pittsburgh No.8 coal Particle size fraction -150+106 µm Proximate analysis (dry basis weight %) Volatile Matter 38.0 Fixed Carbon 54.7 Ash 7.3 Ultimate analysis (dry basis) Carbon 82.2 Hydrogen 5.4 Nitrogen 1.6 Sulfur 1.5 Oxygen 2.0 Calorific Value, MJ/kg 34.0

Figure 4-1: Schematic of the high-pressure, high-temperature flow reactor After reaching the desired gas composition, a sample of about 30 to 45 g was fed into the reactor at a stable feed rate of 3.00±0.02 g/min. To ensure the flow was laminar in the reactor, a flow straightener placed at the top of the ceramic tube. The values of Reynolds number ranged from

480 to 730. As the critical number for transitioning into the turbulent region is 2300, it is safe to say that all the experiments were conducted in the laminar flow regime. The ceramic tube wall temperature was maintained at 1100°C, which is on the lower side of the operating temperature of a typical slurry-fed entrained- flow gasifier. The lower temperature was chosen to avoid significant difference in conversion between chars generated in inert atmosphere and reaction gas atmosphere.

The summary of experimental conditions maintained in the reactor to generate chars is shown in

Table 4-2.

Table 4-2: Experimental conditions

Pyrolysis gas Primary stream Secondary stream Maximum reactor atmosphere in the gas/ flow rate /flow rate tube wall reactor temperature/Pressure* N2 N2/6 slpm N2/48 slpm 1100 ±7 °C/6.2 bar Ar Ar/6 slpm Ar/48 slpm 1100 ±7 °C/6.2 bar 50 vol.% CO2 and N2/6 slpm CO2 (27 slpm) and 1100 ±7 °C/6.2 bar 50% N2 or (CO2/N2) N2 (21 slpm) * absolute pressure; slpm is standard liters per minute Upon injection of the sample into the reactor, the particles devolatilize/react as they pass through a 0.60 m reaction zone to form char-ash particles. The particle residence time was determined by solving discrete phase momentum equation for particle velocity assuming uniform gas velocity.

The particle residence time for a 125 µm particle was calculated to be ~2.3 s. The char-ash particles collected through the water-cooled probe were captured by a series of filters with the last filter having a mesh opening of 1µm. The resulting product gas exited the filter at room temperature and was further cooled in a heat exchanger to remove any remaining moisture before analyzed for components via micro gas chromatography. The char sample, contaminated with soot, was collected in the filter and used for further analysis.

4.2.3. Char Characterization

4.2.3.1. Proximate analysis

Proximate analysis of chars was carried out in a Leco MAC 400 analyzer. Approximately 0.1-0.2 g of sample was crushed in a pestle and mortar before analysis. Moisture, volatile matter, ash, and fixed carbon were obtained. The volatile yield (for pyrolysis) or conversion (during gasification in

CO2/N2) was determined using ash as the tracer. The term volatile yield will be used in the text to refer to conversion of the char generated in the CO2/N2 atmosphere.

4.2.3.2. Thermal swelling ratio

The thermal swelling ratio of the chars was determined by the procedure explained by Fletcher et al. [47] The apparent density of the chars and coal sample was determined using the tap density technique assuming the packing factor of coal sample and char to be the same. The apparent

density ratio ( ) and weight loss data were then related to the swelling ratio (d/do) by:

= × -- 4-1

Where, d/do represents swelling ratio. The subscript “o” refer to parent coal and without it refer to char. The mass ratio m/mo in the equation is expressed on an as-received basis. The density ratio

ρp/ρp0 refers to the ratio of apparent densities, where apparent density is defined as the mass of a particle divided by the total volume enclosed by the outer surface of the particle (assumed to be spherical). The bulk or bed density (ρp for the char and ρpo for the coal) is measured using the tap technique.

4.2.3.3. Surface area analysis and pore size distribution

The chars were degassed at 150°C for a period of 4-10 hours in vacuum. Total surface area and pore size distribution of chars were determined using N2 adsorption-desorption isotherms at 77 K.

The isotherms were analyzed using the Brunauer-Emmett-Teller (BET) equation to calculate total surface area, and Barrett-Joyner-Halenda (BJH) analysis to calculate pore size distribution.

4.2.3.4. Raman spectra

Raman spectra of the chars was obtained with an excitation laser at 532 nm using Horiba Confocal

Raman microscopy. Particles of each sample were placed on a slide and were focused using a 50X objective lens. The laser power on the char surface was controlled approximately at 5 mW. Scans from 800 cm−1 to 2300 cm−1 were performed on each sample. Resolution was approximately 1 cm−1, and the acquisition time for each spot was 60 s. The laser spot diameter reaching the sample was 5 μm. The spectra of at least 10 different spots were obtained for every sample. The ratio of defects to graphite band was obtained, averaged, and the error bar was determined using t- distribution with 95% confidence interval.

4.2.3.5. X-ray diffraction

X-ray diffraction analyses were performed using a PANyltical instrument equipped with a 45-kW high-intensity rotating anode (Cu Kα radiation, 1.5406Å, 0.02°/step, 225 s/step). The analyses were performed on finely ground thin powder samples of the coal chars mounted on a zero-background quartz holder. Measurements were recorded from a start angle 2θ =10° to an end angle of 70°. The

XRD patterns were analyzed for the Full Width Half Maximum (FWHM) using JADE software also accounting for instrumental broadening. From the FWHM, the structural parameters were obtained using conventional Scherrer equations given by:

. 퐿 = -- 4-2

. 퐿 = -- 4-3

where λ is the wavelength of the radiation used, Ba and Bc are the FWHM of the (100) and (002) peaks, respectively, and θa and θc are the corresponding scattering angles. The FWHM is a strong function of the background curve fitting. In order ensure the validity of the data, the FWHM was obtained on three different days and an error bar was obtained using the t-distribution.

4.2.3.6. Tar/soot removal and analysis

The removal of tar/soot from char was done by washing the sample with tetrahydrofuran (THF).

THF can be used to remove tar [66] and is known to provide greater extractability of coal tar without altering the characteristics [67]. To extract the tar/soot from char, a small amount (~0.5 g) of char was added to 100 ml of tetrahydrofuran and allowed to stand for about 15 minutes followed by heating it to 66°C. The boiling slurry was filtered to remove the char sample, while the tar/soot dissolved in THF was concentrated by vaporizing THF in vacuum. The THF soluble tar/soot was analyzed in an Agilent 7890A Gas Chromatograph (GC) and 5975C Mass Spectrometer (MS) mounted with HP-5ms (length 30 m, dia 250 μm, film 0.25 μm) column. Sample was injected at

280 °C with a gas flow of 1 ml/min (STP). The transfer line between the GC and the MS was held at 300 °C. MS spectra were acquired in scan mode in the range 40–500 uma. The THF washed char sample was dried in a vacuum oven at 60°C and 25 in Hg vacuum for over 12 hours. d

4.2.3.7. Char reactivity

Reactivity of chars, obtained from the HPHTFR, was measured using a HPTGA. All the TGA experiments were conducted at 6.2 bar. A small amount of char sample (~35 mg) was heated to

900°C and 6.2 bar in Ar atmosphere and held at that temperature for 15 minutes to remove any volatiles. In all cases, the weight loss curve reached an asymptote before 15 minutes. The system temperature was brought down in Ar atmosphere (i.e., 825-875°C) at which the gas atmosphere was switched to CO2 and the conversion was measured by weight loss. The flow rates of the

reaction gas during the isothermal step as well as the inert gas (i.e., Ar) during temperature ramp- up step were maintained at 0.41 slpm. The typical data obtained from the HPTGA takes the form of weight loss versus time. The data were smoothed using regression in Microsoft excel as described in the Appendix D. The isothermal gasification step was then normalized assuming the char was at 0% conversion and conversion percent (X) on dry ash-free basis versus time was obtained by:

푋 = × 100 -- 4.4

Where mo and mt are the mass of dry ash-free char present at the beginning of the isothermal gasification step and at time t, respectively.

From the weight loss curves, the apparent reaction rate (Rapp) was calculated by:

-1 푅 = ×( ) s -- 4.5 where mi is the sample mass (dry ash-free basis) remaining at reaction time t.

Apparent reaction rate is a function of intrinsic char reactivity and reactive surface area. The intrinsic reaction rate is the rate measured free of mass transfer restrictions (under regime I conditions), expressed as gm-2s-1. The intrinsic reactivity was calculated by normalizing the apparent reactivity with BET surface area by

-2 -1 푅,% = g m s -- 4.6 % Expressing the rates as intrinsic reaction rates allows comparison of reaction rate measurements of physically different samples having different surface areas.

The reaction rate was related to temperature and partial pressure at specified conversion using equation 4.7:

푅 = 퐴 푒푥푝 푃 -- 4.7

-1 -1 - Where, Rapp is the measured reaction rate (g g s ), Ea is the measured activation energy (kJ mol

1 -1 -1 ), R is the universal gas constant (J K mol ), Tp is the particle temperature (K), PCO2 is the partial pressure of CO2 (in bar), and n is the reaction order.

The activation energy is obtained from the slope of the plot of ln Rapp vs 1/TP. The intercept of

-1 -1 -1 the line represents the pre-exponential factor (A= A0푃 ), expressed as gg s or s . Reaction orders with respect to CO2 were determined by measuring reaction rates at various CO2 partial pressures (25, 50, and 100% of the total pressure).

Reaction rates measured at 10% conversion, instead of 0% conversion, were used in calculating kinetic parameters. This was done to avoid fluctuations in temperature and gas composition during transition from Ar to CO2. It is also important to emphasize that only about 65-80% of the volatiles reported by the proximate analyzer was removed during the ramp-up stage of the analysis for chars before washing with THF. To avoid any residual tar or soot affecting the char reactivity, 10% conversion was chosen for calculating reactivity.

For high pressure chars, it is not reasonable to assume that the surface area of the chars at 10% conversion is same as the initial char surface area. Therefore, the surface area was obtained at

10±2% conversion of char during the isothermal step in the HPTGA. The surface area measurement requires about 100 mg of char. Therefore about 75 mg of char, obtained from

HPHTFR, was gasified in the HPTGA for the sake of obtaining sufficient quantity of char. The reaction was stopped at 10±2% conversion and the procedure was repeated to obtain the necessary amount of char. There is no difference in reactivity when 75 mg of char was gasified compared to

when gasifying 35 mg of char. For the sake of clarity in discussion, the char obtained at 10±2% conversion in the HPTGA will be discussed as 10% converted char in the rest of the text.

4.3. Results and Discussions

4.3.1. Volatile yield

The effect of pyrolysis atmosphere on volatile yield was ascertained using ash as the tie component. The volatile yields are low ranging from ~16% for the char pyrolyzed in Ar atmosphere to ~27% for the char obtained in the CO2/N2 atmosphere (shown in Figure 4-2). The difference in volatile yield is significant given the standard deviation for the volatile yield in

CO2/N2 was ±1.8%. An atmospheric pressure study also observed higher volatile yield during pyrolysis in CO2 atmosphere compared with N2 atmosphere and attributed this increased volatile yield to gasification [60]. However, at elevated pressures, the amount of volatiles released decreases due to increased mass transfer resistance [47]. The increased mass transfer resistance leads to deposition of tar and soot on the char particles. Additionally, the tar and soot released from the char particles within the reactor, particularly in the inert atmosphere, that are not completely decomposed, can also deposit on the char particles collected in the filter. Higher volatiles in the char, shown in Table 4-3, indicates the presence of tar and soot. The difference in conversion could be due to the amount of tar/soot settled on the char particles. The char samples collected from the reactor showed presence of low density agglomerates, which are much larger than the char particles. To confirm the presence of tar/soot, microscopic analysis was performed on the polished cross-section of the char sample generated in N2 atmosphere. From Figure 4-3, it appears that tar/soot present on the char surface was formed within the reactor and a portion of that was collected and mixed with char particles in the filter. The presence of tar and soot with the char particles generated at elevated pressures were also reported by Shurtz and Fletcher [58]. The extent

of tar/soot present in each char sample was determined by washing the sample in THF. The conversion was also determined after chars were washed in THF. The conversion for the char generated in the CO2/N2 atmosphere increased marginally from 27 to 29%, while the conversion for the chars generated in N2 and Ar atmospheres increased from 19% to 29% and 16% to 28%, respectively. These observations confirm that the difference in conversion is the result of increased rate of gasification of THF soluble components in the presence of CO2.

35 Before THF After THF 30 25 20 15 10 Conversion Conversion (%) 5 0 Ar CO2/N2 N2 Pyrolysis atmosphere

Figure 4-2: Volatile yield for chars generated in different pyrolysis atmospheres

Table 4-3: Proximate analysis of the chars generated in different pyrolysis atmospheres

Pyrolysis atmosphere VMd, % FCd, % Ashd, % Before washing with THF Argon 23.5 67.9 8.6 Carbon dioxide/Nitrogen 19.3 71.0 9.7 Nitrogen 25.7 65.4 8.9 After washing with THF Argon 20.9 69.1 9.8 Carbon dioxide/Nitrogen 20.4 69.7 10.0 Nitrogen 20.0 70.1 9.9 Subscript “d” represents dry basis; VM: Volatile matter; FC: Fixed carbon

Tar/soot

Thin-walled cenosphere Figure 4-3: Polished cross section of the char generated in the N2 atmosphere

4.3.2. Surface area, pore size distribution and swelling ratio

Although the primary focus of this study is to measure the volatile yield during pyrolysis, and to determine the kinetics of chars generated in different atmospheres, the results of surface area, pore size distribution, and swelling ratio are also presented to determine the difference in structural features.

The chars were analyzed for BET surface area and BJH pore size distribution measurements and the results are as follows: The surface area of chars generated in CO2/N2, N2, and Ar were 9.5±0.2,

6.5, and 6.3 m2/g. The error around the mean surface area, which was calculated based on t- distribution with 95% confidence interval, was 0.2 m2/g. The uncertainty analysis confirmed that the difference in surface area is statistically significant. After washing with THF, the surface areas

2 of chars pyrolyzed in CO2/N2 and N2 were almost the same at 8.5 and 8.7 m /g, respectively. All these show that pyrolysis atmosphere does not affect the initial surface area of the chars.

The other factor that can affect the kinetics is the morphology of the chars. A char that underwent higher swelling is more likely to fragment during gasification [68]. Increased fragmentation affects the char particle size distribution in the gasifier and that affects the kinetics and particle residence time. From that perspective, swelling ratio of the chars was examined by measuring the apparent

density using a tap density technique. The swelling ratios of chars generated in the CO2/N2, N2, and Ar atmospheres were found to be 1.70, 1.74 and 1.72, respectively (shown in Table 4-4).

Table 4-4: Thermal swelling ratio of chars

Gas atmosphere Before THF washing After THF washing CO2/N2 1.70±0.006 1.69 N2 1.74±0.004 1.63 Ar 1.72±0.005 1.65

Similarities in swelling ratio can also be due to the presence of tar and soot with char. The soot collected from the reactor was large agglomerates of extremely low in density. Presence of soot and tar with char can lower the density of a char sample and consequently increase the swelling ratio. Therefore, the swelling ratio of the THF washed chars was also determined. The char generated in CO2/N2 showed slightly higher swelling ratio of 1.69 compared to 1.65 and 1.63 for chars generated in N2 and Ar atmospheres, respectively. The substantial decrease in swelling ratio for the chars generated in N2 and Ar can be attributed to increase in volatile yield or due to removal of tar/soot.

4.3.3. Char Reactivity

It must be emphasized that the reactivity measured is not just char reactivity but also includes soot reactivity. There was difficulty in quantifying and separating the soot from char. Although the reactivity measured is a combination of soot and char, only char reactivity will be referred henceforth in the text for sake of clarity in the discussion.

Figure 4-4 shows the conversion profiles of the chars generated in different pyrolysis atmosphere with CO2 at 6.2 bar in the HPTGA. The temperature effect on conversion is straightforward where increase in temperature increased conversion (in daf basis) for all the chars. The apparent reaction rate was calculated and found to increase with increase in temperature for all the chars. Among the

chars, the apparent reaction rate at 10% conversion at 875°C followed the order: N2 pyrolyzed char

-1 -1 -1 (0.0153 min )> Ar pyrolyzed char (0.0136 min )> CO2/N2 pyrolzed char (0.0129 min ). The apparent reaction rate is affected by a combination of parameters such as physical properties (i.e., surface area and morphology) and chemical structure (i.e., crystallite structure) [69]. As discussed earlier, the surface area (ranging between 6-10 m2/g) and the morphology (determined through the swelling ratio) of the chars were not too different. With the surface area being not too different, the reactivity difference must be due to chemical structural difference. The chars were analyzed in

Raman spectroscopy for defects and graphitic regions to ascertain structural differences. A char with higher intensity of defects compared to graphite is expected to structurally more reactive [70].

Interestingly, the ratios of the intensity of defects to graphite were also similar for chars generated in different atmospheres (shown in Figure 4-5). These results also do not explain the difference in reactivity of chars generated in different pyrolysis atmospheres. Since the Raman spectra is surface technique, the soot contamination might have affected the analysis. Therefore, the chars were characterized for the structural ordering of carbon using XRD. The XRD patterns are shown in

Figure 4-6 The lateral size of the crystallite La, and the stacking height of the crystallite Lc were obtained using (100) and (002) peaks, respectively. The average La values for the chars ranged between 32-34 Å, with the lowest being ~32 Å for the char pyrolyzed in N2 to the highest value of

~34 Å for the char generated in Ar. The range is far narrower for the stacking height (Lc) of the crystals and was between 13.1-14.2 Å (shown in Table 4-5). The error bar calculated based on t- distribution showed that the difference in the crystal parameters was statistically insignificant to conclude that pyrolysis atmosphere affected the lattice structure. The ratios of the intensity of defects to graphite were also similar for chars generated in different atmospheres. These results

also do not explain the difference in reactivity of chars generated in different pyrolysis atmospheres.

The other possibility for the difference in reactivity could be due to physical and chemical transformations that occurred either during the pyrolysis stage (i.e., the ramp-up stage in the

HPTGA) or the gasification stage (i.e., the isothermal stage) in the HPTGA. To ascertain the transformations during the pyrolysis stage, the chars generated in the N2 and CO2/N2 atmospheres were heated to 875°C in 6.2 bar pressure. The chars were maintained at 875°C for ~15 minutes and cooled down. The surface area of the chars pyrolyzed in N2 and CO2/N2 after the ramp-up stage was 19.8 and 22.3 m2/g, respectively. This difference in surface area also does not warrant significant difference in reactivity. This was followed by analyzing the surface area of the chars gasified in CO2 atmosphere at 875°C and 6.2 bar in the high pressure TGA. The reaction was stopped at ~10% conversion and the chars were analyzed for surface area. Interestingly, the char

2 generated in CO2/N2 showed the highest surface area of 155±7 m /g, followed by the chars

2 2 generated in N2 and Ar atmospheres at 113±6 m /g and 104 m /g, respectively. Higher surface area for the CO2/N2 char can be attributed to increased generation of finer pores (pore width <30 Å).

This was reflected in ~30% lower pore volume for the char generated in N2 compared to the char generated in CO2/N2 (shown in Figure 4-7). Although the difference in surface area is significant, the increased surface area for the char pyrolyzed in CO2/N2 did not result in higher reactivity. This suggests that the structure of the char generated in CO2/N2 is intrinsically less reactive compared to the char generated in inert atmospheres. XRD data (showed in Table 4-5) on the 10% converted char did not show significant difference in lattice parameters. This further confirms that the techniques (i.e., Raman spectroscopy and XRD) used to determine the structural differences are not adequate.

80 80 825°C_CO2/N2 825°C_CO2/N2_THFwashed 850°C_CO2/N2 850°C_CO2/N2_THFwashed 60 60 %) 875°C_CO2/N2 875°C_CO2/N2_THFwashed (

40 40 Conversion 20 Conversion (%) 20

0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 Time (min) Time (min)

80 60 825°C_N2 825°C_N2_THFwashed 850°C_N2 850°C_N2_THFwashed

60 %) ( %) 875°C_N2 40 875°C_N2_THFwashed (

40 20

20 Conversion Conversion

0 0 0 20 40 60 80 100 120 0 20 40 60 Time (min) Time, min

100 30 825°C_Ar 825°C_Ar_THFwashed 850°C_Ar 850°C_Ar_THFwashed 80 875°C_Ar 25 875°C_Ar_THFwashed 20 60 15 40 10 Conversion Conversion (%) Conversion(%) 20 5

0 0 0 20 40 60 80 100 0 10 20 30 Time (min) Time (min)

Figure 4-4: Conversion profiles of chars with CO2 in HPTGA

1.15

1.10

1.05 Id/Ig

1.00

0.95 Argon Carbon dioxide/Nitrogen Nitrogen Pyrolysis atmosphere

Figure 4-5: The ratio of D band intensity to G band intensity (before washing with THF)

Carbondioxide/Nitrogen 002 peak Nitrogen 20000 Argon

15000 100 peak Intensity 10000

5000

10 20 30 40 50 60 70 2 Theta

Figure 4-6: XRD pattern of chars generated in different pyrolysis atmosphere

Table 4-5: Values of La and Lc for chars

Pyrolysis Initial char 10% converted char atmosphere in HPHTR La (Å) Lc(Å) La (Å) Lc(Å) CO2/N2 33.1±3.1 13.7±2.8 37.3 14.5 N2 32.1±2.2 13.1±1.0 37.6 13.8 Ar 33.8±1.5 14.2±1.5 40.2 14.2 * Error bar was calculated based on t-distribution

0.0060 0.0060

CO2/N2_Initial char N2_Initial char CO2/N2_10% conv N2_10% conv 0.0045 CO2/N2_10% conv_Repeat 0.0045 N2_10% conv_Repeat

0.0030 0.0030 Pore volume (cc/g) volume Pore 0.0015 volumePore (cc/g) 0.0015

0.0000 0.0000 10 100 1000 10 100 1000 Pore size distribution(Å) Pore size distribution(Å)

0.0060

Ar_Initial char Ar_10% conv 0.0045

0.0030

Pore volumePore (cc/g) 0.0015

0.0000 10 100 1000 Pore size distribution (Å)

Figure 4-7:Pore size distribution of chars

It can be hypothesized that higher reactivity for the char pyrolyzed in N2 and Ar may be due to products formed from the highly reactive tar components on the char surface, which were not released during the temperature ramp-up stage in the HPTGA. The hypothesis was based on the observation that only about 65-80% of the volatiles, as determined by the proximate analysis, was

released during the pyrolysis stage in the HPTGA. To remove the effects of tar/soot captured on the particle surface, the THF washed chars were analyzed for CO2 reactivity. Interestingly, the

CO2 reactivity of the sample pyrolyzed in N2, after washing with THF, decreased by ~25% at

875°C, while the reactivity of the sample pyrolyzed in Ar and CO2/N2 decreased by ~11% each at the corresponding temperature (shown in Figure 4-8). The difference in apparent reactivity for the three chars washed in THF is also far narrower ranging from 0.0115 min-1 for the THF washed

-1 CO2/N2 char to 0.0122 min for the THF washed N2 char. To further confirm if this decrease in reactivity was only due to removal of tar/soot and not due to THF, a calcined coke was washed in the THF and dried using the same procedure employed to wash coal chars. The CO2 reactivities of calcined petcoke and THF washed calcined coke were ascertained at 875°C and 6.2 bar. The reactivities showed no significant difference confirming that the THF had no effect on these samples (shown in Figure 4-9). All these point to the role of tar/soot components affecting the reactivity and not the THF. Despite washing with THF, the reactivity of the char generated in the

N2 atmosphere showed slightly higher reactivity than the THF washed char generated in CO2/N2.

It is adequately shown that the structural difference due to pyrolysis atmosphere, based on the techniques used, proved to be inconclusive. Ironically, at 10% conversion in the HPTGA, the char generated in CO2/N2 yielded higher surface area, higher pore volume, and almost similar reaction rate as the 10% converted char generated in the N2 atmosphere. This implies that the intrinsic reactivity was much higher for the char generated in the N2 atmosphere at 10% conversion than the char generated in the CO2/N2 atmosphere at similar conversion. One possibility for reduced intrinsic reactivity for char generated in CO2/N2 could be that the soot formed is highly ordered compared to the soot formed in inert atmospheres. Hurt et al. suggested that CO2 can accelerate

the transition of soot to coke [71]. It appears that the difference in reactivity for chars generated in different atmospheres could be due to difference in crystallinity of the soot.

To determine the composition of THF soluble tar/soot that contributed to increased reactivity, the

THF soluble was analyzed in the GC-MS. The GC-MS spectrograms and the compounds speciation obtained from chars pyrolyzed in N2 and CO2/N2 are shown in Figure 4-10 and Table

4-6. Qualitative analysis of the tar/soot showed the presence of polyaromatic hydrocarbons (PAH) for chars generated in N2 and CO2/N2. Interestingly, naphthalene was not observed in the THF soluble tar, while it was one of the major tar components obtained during pyrolysis of Pittsburgh

No.8 coal char in CO2 in atmosphere at 1300°C and 1 bar [72]. Higher concentration of polyaromatic hydrocarbons including a 6-member ring compound (i.e., benzo(ghi)perylene) and absence of naphthalene suggest increased retrogressive reactions at higher pressure leading to polyaromatic condensation. These PAHs must have acted as precursors to the product that contributed to differing reactivities for the chars generated in inert atmospheres. To determine the products from the tar/soot, the char generated in N2 atmosphere was heated to 875°C at 10°C/min in Ar atmosphere and was cooled in the HPTGA. The sample (~50 mg) was suspended in 35ml of

THF for 5 hours and heated to 66°C. The filtrate was analyzed in the GC-MS. The filtrate showed no PAH confirming that tar compounds were completely decomposed to form higher molecular weight soot. The higher molecular weight soot must have been more reactive than the char itself.

0.016 Before THF After THF

) 0.012 -1

0.008 Reactivity Reactivity (min 0.004

0 Argon Carbondioxide/Nitrogen Nitrogen

Figure 4-8: Apparent reactivity of chars before and after washing with THF (at 875°C)

0.003

0.0025 ) -1 0.002

0.0015

Reactivity (minReactivity 0.001 Reference char THF washed reference char

0.0005

0 0 5 10 15 20 Conversion (%)

Figure 4-9: CO2 reactivity of the reference char and the THF washed reference char

400000

CO2/N2 N2

300000

200000 Intensity

100000

0 8.0 10.5 13.0 15.5 18.0 20.5 Retention time (min)

Figure 4-10: GC-MS chromatograms of THF soluble tars obtained by washing chars generated in CO2/N2 and N2 atmospheres (only qualitative analysis)

Table 4-6: Compounds identified by the GC-MS

Retention time Compounds Area % (min) CO2/N2 N2 10.70 1.4 Benzene diamine, N,N’-bis (1-methyl 4.4 2.3 ethyl) 10.95 2,5, cyclohexadiene-1,4-dione, 2,6-bis (1,1, 12.9 6.3 dimethyl ethyl) 11.04 1 H cyclopropa(a)napthalene 17.6 2.5 13.52 Phenanthrene 7.6 3.0 15.44 Fluoranthene 14.5 17.6 15.59 1,8, Anthracene diamine 0.0 2.5 15.69 Phenaleno (1,9-bc) thiophene 0 1.7 15.79 Pyrene 29.0 27.3 17.43 and 17.73 Cyclopenta (cd) pyrene 9.4 14.5 19.38/19.75 Benzo(k)flouranthene 1.5 5.8 19.81 Benz(e)acephenanthrylene 1.3 5.7 21.71 Benzo(ghi)perylene 1.8 10.8 Total area under the curve (%) 100 100 (Other peaks were either the stabilizer in THF (i.e, BHT) or contaminants present in THF)

4.3.4. Kinetic Parameters

The reaction rates versus conversion are shown in Figure 4-11. The kinetic parameters of the chars were obtained using the apparent reaction rate at 10% conversion. This was to avoid the fluctuation in temperature as well as allowing the TGA to reach equilibrium in terms of reaction gas atmosphere. The reactivity of chars was obtained from the Arrhenius plots of ln k vs T-1 as shown in Figure 4-12. Higher regression coefficients for all the plots confirm that the reaction was in the kinetic controlled regime. The kinetic parameters and the correlation coefficient were determined and are listed in Table 4-7. To further confirm that the reaction was in the kinetic-controlled regime, an internal effectiveness factor was calculated assuming all particles are Cenospherical in shape with a wall thickness of 5 µm. The wall thickness of 5 µm was assumed due to higher swelling ratio. The calculations showed that the effectiveness factor is 1 confirming that the reaction was in Regime-1 for all the chars below 900°C (effectiveness factor lower than 1 is an indication of mass transfer limitation and the kinetics will not be true intrinsic kinetics). The variation of effectiveness factor with temperature at 6.2 bar is shown in Figure 4-13.

The activation energy and pre-exponential factor, shown in Table 4-7, are independent of the pyrolysis gas atmosphere. Similarly, the pre-exponential factor normalized with surface area also was not affected by the pyrolysis atmospheres. These similarities could have been the result of tar and soot covering the char surface. To confirm the hypothesis, the chars were washed with THF and the kinetic parameters were obtained. Interestingly, the activation energy and the pre- exponential factor remained unaffected with pyrolysis atmosphere. All these point to pyrolysis gas atmosphere not affecting the activation energy. It is important to emphasize that the reactivity measured here is not just the reactivity of char. In the absence of clear evidence on the

concentration of soot before and after THF washing and at 10% conversion, it is safe to say that the kinetic parameters represent the reactivity of soot and char.

4.3.5. Order of Reaction

The order of reaction was calculated by varying the CO2 concentration from 25% (1.55 bar) to

100% (6.2 bar). The order of reaction follows the trend: CO2/N2 char>Ar char≈N2 char. The order of reaction is an indication of the influence of the reactant gas concentration on apparent reactivity

[19]. The significance of the order of reaction can be understood from the char-CO2 reaction mechanism. One of the widely accepted mechanisms postulated by Ergun [31] for char-CO2 reaction is shown in Equations 2-1 and 2-2 [26, 73]. According to the mechanism, the overall rate of the reaction is proportional to the number of active complexes (C(O)) formed and desorbed during the reaction. At lower CO2 partial pressure, not all active sites would form carbon-oxygen complexes. With increase in partial pressure, many active sites form the carbon-oxygen complexes. The extent to which the carbon-oxygen complexes form is the order of reaction. Higher order of reaction for the char pyrolyzed in CO2/N2 implies that higher rate of formation of carbon- oxygen complexes as the partial pressure was increased from 1.55 to 6.2 bar CO2 pressure. In other words, the reaction rate of the char is highly sensitive to CO2 concentration. The higher surface area at 10% conversion for the char generated in CO2/N2 indicates that not all active sites are saturated with surface complexes. For the chars generated in inert atmospheres (i.e., N2 and Ar), lower order of reaction can be linked to lower surface area and possibly due to residual products of tar components. The hypothesis that the tar components or the products of tar components contributing to increased reactivity is further confirmed by the significant increase in order of reaction for the THF washed chars. The order of reaction increased considerably by 120%, 83%

and 41% for the chars pyrolyzed in N2, CO2/ N2, and Ar, respectively. The correlation between order of the reaction and reactivity confirms that the order of reaction is a function of surface area.

4.4. Conclusions

Pyrolysis of a bituminous coal was studied in N2 and CO2/N2 and Ar-based atmospheres in a high pressure, high temperature flow reactor. Chars were obtained at 1100°C and 6.2 bar with sampling residence time of ~2.3 s. The volatile yield was highest for the char generated in the CO2/N2 atmosphere compared to inert atmospheres, while there was no noticeable difference in volatile yield observed between N2- and Ar-based environments. The difference in volatile yield with pyrolysis atmosphere was found to be due to gasification of tar and soot by CO2. Except for the volatile yield, there were no differences in the N2- BET surface areas, lattice parameters of the crystallite carbon, swelling ratio, and the defects to graphite band ratio with pyrolysis atmospheres.

The lack of differences with swelling ratio can be attributed to tar/soot deposition. The decomposed product from the tar/soot present on the char surface contributed to increased reactivity. The intrinsic reactivity was found to be highest for the char generated in N2 atmosphere and lowest for the char generated in CO2/N2 atmosphere. Both Raman and XRD techniques did not adequately described the structural differences. The kinetic parameters of the chars were obtained using nth order model. The activation energies were found to be independent of the pyrolysis atmospheres. The order of reaction correlated well with the N2 surface area.

0.016 0.016 ) ) -1 -1 0.012 0.012

0.008 0.008

825°C_CO2/N2_THFwashed

Reactivity Reactivity (min 0.004 825°C_CO2/N2 Reactivity (min 0.004 850°C_CO2/N2_THFwashed 850°C_CO2/N2 875°C_CO2/N2 875°C_CO2/N2_THFwashed 0 0 0 10 20 30 40 50 60 70 80 0 20 40 60 Conversion (%) Conversion (%)

0.02 0.02 825°C_N2_THFwashed 850°C_N2_THFwashed ) ) -1 -1 0.016 0.016 875°C_N2_THFwashed

0.012 0.012

0.008 0.008 Reactivity Reactivity (min Reactivity Reactivity (min 0.004 825°C_N2 0.004 850°C_N2 875°C_N2 0 0 0 10 20 30 40 50 60 70 80 0 20 40 60 Conversion (%) Conversion (%)

0.03 0.03 825°C_Ar

) 825°C_Ar_THFwashed 0.025 ) -1 0.025

850°C_Ar -1 850°C_Ar_THFwashed 0.02 875°C_Ar 0.02 875°C_Ar_THFwashed 0.015 0.015

0.01 0.01 Reactivity Reactivity (min 0.005 Reactivity (min 0.005

0 0 0 20 40 60 80 0 10 20 30 Conversion (%) Conversion (%)

Figure 4-11: Instantaneous reaction rates of chars with conversion (before and after washing with THF)

T-1 (K-1) T-1 (K-1) -8 -8.4 0.00086 0.00088 0.0009 0.00092 0.00086 0.00088 0.0009 0.00092 -8.6 -8.4 CO /N _THF washed char CO2/N2 2 2 ) 1

- -8.8

-8.8 ) -1

(s -9 ln ln k (s -9.2 ln ln k -9.2 y = -27738x + 15.659 -9.6 R² = 0.9733 -9.4 y = -24671x + 12.934 R² = 0.9998 -10 -9.6

-1 -1 T-1 (K-1) T (K ) -8 -8.4 0.00086 0.00088 0.0009 0.00092 0.00086 0.00088 0.0009 0.00092 -8.2 -8.6 N2 -8.4 N2_THF washed char )

1 -8.8 - )

-8.6 -1 -9

ln ln k (s -8.8 ln ln k (s -9.2 y = -26000x + 14.152 -9 R² = 0.9986 -9.4 -9.2 y = -27011x + 15.265 R² = 0.996 -9.4 -9.6

T-1 (K-1) T-1 (K-1) -8.2 -8.4 0.00086 0.00088 0.0009 0.00092 0.00086 0.00088 0.0009 0.00092 -8.4 -8.6 -8.6 Ar_THF washed char -8.8

) Ar

-8.8 ) -1 -1 -9 -9 ln ln k(s

nk(s ln k -9.2 -9.2 y = -25536x + 13.771 R² = 0.9908 -9.4 y = -27607x + 15.65 -9.4 R² = 0.9998 -9.6 -9.6

Figure 4-12: Arrhenius plots for chars generated in different atmospheres

Table 4-7: Kinetic parameters for the chars generated in various pyrolysis atmospheres (at 10% conversion)

Pyrolysis CO2/N2 N2 Ar atmosphere Before After Before After THF Before After THF THF THF THF THF Ea (kJ/mol) 231 205 225 216 230 212 A (s-1) 6.3x106 0.4x106 4.3x106 1.4x106 6.2x106 1x106 -1 -n 6 6 6 6 6 6 A0 (s bar ) 2.8x10 0.13x10 2.6x10 0.6x10 3.9x10 0.4x10 n 0.46 0.65 0.25 0.55 0.29 0.53 -2 -1 3 3 3 3 3 Aint (gm s ) 40x10 3x10 40x10 13.7x10 60x10 ND 2 SA10% conv (m /g) 155 152 113 102 104 ND R2 0.973 1.000 0.996 0.999 1.000 0.991 * Error bar with 95% confidence interval around activation energy and pre-exponential factor are ±8.7 kJ/mol and 5.9x106 s-1; R2: Regression coefficient

1.0

0.8

0.6

0.4 Argon Carbondioxide/Nitrogen Nitrogen Effectiveness0.2 factor, η

0.0 800 900 1000 1100 1200 1300 1400 1500 Temperature (°C)

Figure 4-13: Effectiveness factor for char-CO2 reaction at 6.2 bar

Effects of Temperature, Pressure, Feed Particle Size, and Feed Particle Density on Structural Characteristics and Reactivity of Chars Generated During Gasification of Pittsburgh No.8 coal in a High-Pressure, High- Temperature Flow Reactor

5. Abstract

Effects of feed particle size, feed particle density, temperature, and pressure on char porous structure, morphology, reflectance, and reactivity under conditions relevant to entrained-flow gasification were investigated. The chars were generated over a range of temperatures (1100, 1300, and 1400°C at 11.3 bar for the -150+106 µm fraction), pressures (3.4, 6.2, 11.3, 15.5, and 21.7 bar at 1300°C for the -150+106 µm fraction), for various size fractions (-106+75, -150+106, -212+150,

-420+212 µm at 1300°C and 11.3 bar) and density fractions (<1.3, 1.3-1.6, >1.6g/cc for the -

106+75 µm at 1300°C and 11.3 bar) of Pittsburgh No.8 bituminous coal using a high-pressure, high-temperature flow reactor (HPHTFR) in a equimolar mixture of CO2 and N2. Chars were characterized for conversion, morphology, pore structure (i.e., surface area and pore volume), reflectance, and reactivity using proximate analysis, oil immersion microscopy, N2 adsorption technique, reflectance microscopy, and a thermogravimetric analyzer, respectively. The results were statistically analyzed to determine the effects of the four parameters on conversion, structural characteristics, and intrinsic reactivity. The results showed that the conversion was most affected by temperature, and followed by feed particle size, pressure, and feed particle density. Maceral differences played a significant role in affecting the group-I concentration and swelling ratio. Feed particle density significantly affected group-I concentration, while both feed particle size and feed particle density affected swelling ratio. In the case of intrinsic reactivity, particle density showed the largest effect, followed by temperature, particle size, and pressure.

5.1. Introduction

The gasification process is one of the ways to utilize coal efficiently in a carbon-constrained world.

However, there are some challenges associated with this technology [4, 10]. One of the challenges is the poor understanding of the behavior of coal under high temperatures and high pressures. The behavior of coal under high-temperature and high-pressure conditions is not well understood due to difficulty in designing and operating a reliable lab-scale reactor at high pressures and temperatures. This understanding is important from the perspective of designing high efficiency gasifiers and optimizing and troubleshooting existing gasifiers.

Gasification of chars in entrained-flow gasifiers occurs in regime-2 conditions where the reaction is affected by both the intrinsic rate and the diffusion of reactants into the char pore structure [56].

Development of a robust kinetic model requires data on pore structure, morphology, and reactivity of chars [17, 56]. Several studies have reported the effects of pressure and temperature on structural features and, in some cases, the consequent effect on the reactivity of the chars [38, 47, 54, 74].

These studies specifically focused on the pyrolysis or gasification behavior of narrow particle size distributions over a range of pressures or temperatures. It is important to recognize that the particle size distribution of the feed to slurry gasifiers covers a wide range [75]. Feed particle size is known to affect swelling ratio. Besides particle size, heterogeneity of coal macerals, composition, and concentration of inorganic matter can all affect char morphology [44, 75]. Assuming that pore structure, morphology, and kinetics are uniform for the entire range of coal particle sizes can lead to inaccuracies of the kinetic model. Critical examination of the literature revealed that not a single investigation was conducted to determine the effect of particle size and density on char morphology, pore structure, structural ordering, and the consequent effect on reactivity of chars generated at high pressures and temperatures relevant to that of entrained-flow gasification. This

investigation focuses on the effects of feed particle size distributions, inorganic matter (or density fractions), operating temperature, and pressure on structural properties and reactivity of chars generated during high pressure gasification. This understanding is important in developing a robust kinetic model.

5.2. Objectives

The principal aim of this investigation is to determine the effects of pressure, temperature, particle size and feed particle density on the structural characteristics and reactivity of the chars generated at elevated pressures. As mentioned earlier, knowledge of pore structure and morphology (i.e., structural features), along with intrinsic reactivity, are essential in developing a char kinetic model.

Pressure, temperature, particle size and inorganic matter all can affect structural characteristics of the char, which in turn can affect intrinsic reactivity. Therefore, it is imperative that the structural features of the char must be generated under conditions similar to those of the chars generated in an entrained-flow gasifier. Additionally, the impact of various structural features on reactivity can give a broad understanding of the gasification behavior of coal under elevated pressures.

The specific research objectives of the investigation are:

1.) To determine the effects of temperature, pressure, particle size, and inorganic matter on the

structural features and the reactivity of the chars generated from a high-pressure, high-

temperature, flow reactor.

2.) To determine the relationship between the structural features and the intrinsic and apparent

reactivities of the chars.

5.3. Experimental

5.3.1. Samples

Bituminous coals are the preferred feedstock for entrained-flow gasifiers. Therefore, a widely used bituminous coal, Pittsburgh No. 8 seam coal, was used for this study. The coal was dry ground in a pilot scale rod mill to obtain a particle size distribution (PSD) very similar to the PSD of a coal fed into a commercial pressurized entrained-flow gasifier. The yield of a narrow size fractions of the dry ground raw coal covering the feed particle size distribution to a slurry-fed gasifier is shown in Appendix G. A representative sample of the ground coal was sieved to obtain four particle size fractions: -106+75, -150+106, -212+150, -420+212 µm, to study the impact of particle size fractions on char characteristics and reactivity. These particle sizes were chosen as they together constitute ~60% of the feed to a commercial entrained-flow gasifier [75]. Among these particle sizes, -150+106 µm was used to study the impact of pressure and temperature.

Another objective of the study was to investigate the effect of ash yield on char reactivity. To obtain coal fractions of varying ash yield, the whole quantity of coal of one particle size (-106+75

µm) was further separated into three density fractions (SG) (<1.3, 1.3-1.6, and >1.6 g/cc) by the float-sink method described elsewhere [64], using mixtures of liquids of different densities

(toluene/perchloroethylene). The methodology employed in this investigation is shown in Figure

5-1. Proximate analysis of the different coal density fractions showed that the <1.3 g/cc fraction had the lowest ash yield (3.3 wt%), the 1.3-1.6 g/cc fraction with intermediate ash yield (7.5 wt%), and the >1.6 g/cc fraction with the highest ash yield (65.3 wt%). Computer-controlled scanning electron microscopy (CCSEM) analysis on different density fractions reported elsewhere [75] showed that the inorganic matter occurs primarily as included matter (inorganic particles embedded in the organic matrix) for the <1.3 and 1.3-1.6 g/cc fractions, while the inorganic matter

predominantly occurs as discrete particles for the >1.6 g/cc fraction. The proximate and ultimate analyses of all the samples used in this study are provided in Table 5-1.

Ground in a rod mill rod mill

Size separation

-106+75 µm -150+106 µm -212+150 µm -425+212 µm

Separated into three density fractions i.e., <1.3g/cc, 1.3- 1.6g/cc, >1.6g/cc

Chars were generated at high pressures and temperatures in HPHTFR

Chars were characterized for Devolatilize at 950°C in a porosity, morphology, and proximate analyzer to swelling ratio remove soot

N2 surface area

Apparent and intrinsic reactivities in a TGA

Figure 5-1: Methodology

Table 5-1: Compositional analysis of feed samples

Particle size Proximate analysis (wt%, db) Ultimate analysis (wt%, db) fraction (µm) VM FC Ash C H N S O

-425+212 40.4 50.5 9.1 81.2 5.2 1.3 2.2 1.0

-212+150 36.6 52.6 10.8 79.9 5.0 1.1 2.1 1.1

-150+106 38.0 54.7 7.3 82.2 5.4 1.6 1.5 2.0

-106+75 38.2 53.5 8.3 82.0 5.1 1.5 1.6 1.5

<1.3g/cc of 38.1 58.6 3.3 85.6 5.6 1.5 1.2 2.8 -106+75

1.3-1.6 g/cc of 35.8 56.7 7.5 82.1 5.0 1.4 1.7 2.3 -106+75

>1.6g/cc of 16.2 18.5 65.3 23.9 1.6 0.4 8.4 0.4 -106+75 db: dry basis; VM: volatile matter; FC: Fixed carbon;

5.3.2. High-Pressure, High-Temperature Flow Reactor

All the experiments were conducted in an electrically heated HPHTFR operated under laminar flow conditions. The schematic of the reactor is shown in Figure 4-1. The details of the reactor, used at atmospheric pressure, are described elsewhere [64].

A sample of about 30 to 45 g was dry fed into the reactor at a stable feed rate of 3.00±0.02 g/min for all particle sizes. Each sample was entrained into the reactor through a water-cooled feed injector using N2 as the primary gas. Primary gas constitutes about 10% of the total flow rate through the reactor. The bulk of the flow rate (~90%) through the reactor comes from secondary gas, which was composed of CO2 and N2. The secondary gas was preheated to the temperature of the reactor in the annular space between the reactor tube and the furnace wall. To ensure the flow was laminar in the reactor, a flow straightener was placed at the top of the ceramic tube. The gas

flow rates used for these experiments ranged between 33 standard liters per minute (slpm) at 3.4 bar to 150 slpm at 21.7 bar. Oxygen was not used in any of the experiments as the study was aimed at simulating the second stage of the slurry-fed gasifier. The gas composition in the reactor was maintained at 50 vol% CO2, with the rest being N2. The gas composition was constantly monitored using the micro gas chromatograph (micro-GC). The particle residence time was determined by solving the discrete phase momentum equation for particle velocity. The residence time, taking particle density in to account, was calculated to be between 2.0 and 2.8 s.

Upon injection of the sample into the reactor, the particles react with the reaction gas as they pass through the 0.60 m reaction zone to form char-ash particles. The char-ash particles collected through the water-cooled probe were captured by a series of filters with the last filter having a mesh opening of 1µm. The resulting product gas exited the filter at room temperature and was further cooled in a heat exchanger to remove any remaining moisture before analyzed for components via micro-GC. The char sample collected in the filter was contaminated with tar and soot. All the char samples were washed with THF to remove any tar on the char surface. The THF washed samples were analyzed for ash, surface area, swelling ratio and reactivity as described in the following section.

5.3. Char Analysis

5.3.1. Proximate Analysis

Proximate analysis of THF washed chars was carried out in a LECO MAC 400 analyzer.

Approximately 0.15 g of sample was crushed to pass through a 150 µm screen before analysis.

The analysis was duplicated to obtain average volatile matter, ash, and fixed carbon. Total conversion was calculated using ash as the tie component, assuming inorganic components did not volatilize during the gasification process and all ash particles were collected from the reactor.

5.3.2. Surface Area Analysis and Pore Size Distribution

Total surface area and pore size distribution of THF washed chars were determined using N2 adsorption-desorption isotherms at 77 K. The isotherms were analyzed using the Brunauer-

Emmett-Teller (BET) equation to calculate total surface area, and BJH analysis to calculate pore size distribution.

5.3.3. Density and Swelling Ratio Measurements

The tap density technique, as described by Fletcher [76], was used to find the ratio of apparent density of the char sample to that of the parent coal sample. A known amount of the sample was filled in a graduated cylinder and tapped until coal/char settles and the mass per unit volume was determined. The density ratio was determined assuming the packing factor for coal and the char to be same. From the extent of mass release and apparent density ratio, swelling factor was calculated by [76]:

= × -- 5-1

5.3.4. Morphology and Petrographic Analysis

Morphology and petrographic analysis were performed on the char samples prior to washing the char with THF. The analysis was performed on a polished cross section of the sample that was prepared by mixing the char particles with cold setting epoxy. After hardening, the epoxy was cut longitudinally to expose the particle gradation, remounted in epoxy, and polished for optical microscopy. Polishing of all specimens involved grinding the surfaces using 400 and 600 grit papers and then carrying out two polishing steps, i.e., 0.3 µm alumina slurry on a high nap cloth, and 0.05 µm alumina slurry on silk.

Point count analysis was performed using the Zeiss Universal research microscope at 625X magnification with polarized light. While there are different ways of classifying observed char structures [43, 77, 78], the classification system proposed by Bailey et al. [43] was employed in this study. Morphology of the chars was manually quantified using point-count readings as described in Gilfillan et al. [41]. A minimum of 85 morphological points was identified. The char types identified include tenuisphere, tenuinetwork, crassisphere, crassinetwork, solids, fusinoid, and fragments. The differentiation between tenuisphere (wall thickness <5 µm) and crassisphere

(> 5 µm) was made based on wall thickness. The uncertainty in the categorization was determined by analyzing the two sections of <1.3 g/cc of -106+75 µm char sample generated at 1300°C two times each with an overall point counting ranging from 372 to 429 readings. The overall standard deviation for group-I char (>70% porosity and <5 µm), group-II type (40-70% porosity, >5 µm), group-III type (<40% porosity and variable thickness), and fragments was 6.8%, 3.5%, 2.7% and

4.4%, respectively. The variation in char morphology in most cases is primarily due to group-I char and fragments. Although care was taken while handling the chars, some fragmentation was a result of handling the fragile group-I char particles.

Random reflectance of samples was determined using reflectance microscopy. A mean random reflectance was obtained on a vitrinite-derived char cross section over the polished surface. The mean random reflectance was reported based on an average of a minimum 20 readings. The overall variation in the readings was quantified assuming t-distribution and reflectance was found to vary between 0.19-0.45%.

5.3.5. HRTEM Analysis

The samples were prepared by crushing under ethanol in a mortar. A suspension of the crushed particles was then deposited on a carbon film. The high-resolution transmission electron

microscope images (Structural resolution limit =0.24 nm) were obtained using a Talos F200X was operated at 200 kV with EDS detector for determining elemental composition.

5.3.6. Image Analysis

Char samples were prepared by mounting a small amount of sample on a 10 mm diameter stub.

The external morphology of chars was obtained using FEI Quanta 200 scanning electron microscope operated in a backscattered mode at 20 kV in a high vacuum with a spot size of 7.

5.3.7. Char Reactivity

Char (i.e., char+soot) reactivity was measured using a Rubotherm thermogravimetric analyzer at atmospheric pressure. A small amount of char sample (~10 mg) was heated to 400°C at a heating rate of 10°C/min. A stream of Ar (100 ml/min) was used to sweep the char sample during the heat- up period. After thermal equilibration at the desired temperature for few minutes, the isothermal step was initiated by switching Ar with O2 at the same flow rate. The reactivity measured at 400°C includes volatiles, soot and tar. To determine the effect of soot and remaining volatiles on char reactivity, char samples were devolatilized in a proximate analyzer at 950°C and 1.01 bar for seven minutes. The apparent and intrinsic reactivities were determined on the devolatilized chars at

400°C in O2 atmosphere as described above.

Apparent reactivity was calculated as the rate of weight loss with respect to the instantaneous char mass by:

푅 = × -- 5-2 .

Apparent reactivity is a combination of effects of char surface activity and reactive surface area.

By normalizing the apparent reactivity with the surface area, intrinsic reactivity was determined by:

푅 = -- 5-3 .

Where N2 surface area is used here as the normalizing parameter. The apparent reactivity at ~5% conversion, instead of 0% conversion, was used in determining intrinsic reactivity. The reason that went into choosing 5% conversion for reactivity was to allow some time for O2 concentration to equilibrate in the TGA after transitioning from Ar atmosphere. It was also assumed that the surface area of the char at 5% conversion may not be different from the char fed into the TGA.

5.3.8. Uncertainty Analysis

Uncertainty in conversion was calculated by generating chars from the particle size fraction of

-150+106 µm at 1300°C and 6.2 bar by repeating four times. The average ash content of each of the THF washed char samples was determined based on the duplicates in the proximate analyzer.

The average ash content was determined through root mean square (RMS) of average ash content measured from the proximate analyzer from the duplicates and the. variation in ash content between duplicates in the proximate analyzer. The RMS ash content (σ) was determined as follows:

휎 = 휎 + 휎 -- 5-4 Similarly, the RMS ash content was determined for other chars. Similarly, the RMS ash content was determined for other chars. With the RMS ash content, the conversion for each char was calculated using ash as the tie component. The uncertainty around the average conversion was determined using t-distribution with 95% confidence interval. The standard error for conversion was calculated to be 8.2%. High uncertainty of 8.2% around the mean conversion value can be attributed to fluctuations in temperature, pressure, and gas flow rate. Another important reason for the scatter can be attributed to the presence of soot. Although the char samples were washed with

THF and in some cases both with toluene and THF, soot was not completely removed from the char surface. The difference in soot among char samples seemed to have contributed to the higher uncertainty.

Similarly, the uncertainty in the average apparent reactivity with 95% confidence interval was determined by repeating the -150+106 µm particle size fraction char sample obtained at 1300°C

-1 and 11.3 bar for O2 reactivity four times. The error was found to be 0.0009 min .

5.4.Results and Discussion

5.4.1. Effect of Pressure

Effect of pressure on conversion for -150+106 µm particle size fraction at 1300°C is shown in

Figure 5-2. The total conversion varied between ~43% at 3.4 bar to ~59% for 21.7 bar pressure.

Lower conversion for 3.4 and 6.2 bar char can be attributed to a lower particle residence time of

~2.0 s, while at other pressures, the particle residence time was calculated to be 2.3-2.4 s. Even though there is a huge difference in conversion, the uncertainty of 8.2% around average conversion means the pressure has no significant effect on conversion. This result agrees with a previous study in which conversion range of the Pittsburgh No.8 coal pyrolyzed in N2 and small amount of oxygen over a range of pressures varied between 48-60% on dry ash free basis [47]. It is noteworthy that the conversion range for the chars obtained in the gasification environment (CO2/N2) in the current work compares well with the conversion range obtained in the pyrolysis atmosphere (N2 and post flame gases) reported by Zeng and Fletcher [47]. It is highly likely that the chars obtained in the current work was partially gasified by CO2. The similarities in conversion range despite difference in the gas atmosphere can be attributed to differing experimental methodologies where a different particle size fraction (-90+63 µm), and a different apparatus (High pressure flat flame burner) were used to generate chars.

Char morphology is a key parameter for modeling char-CO2 reactivity in an entrained-flow gasifier

[17]. Figure 5-3 and 5-4 show the swelling behavior and morphology of the chars generated at different pressures. The pressure has limited effect on group-I char formation as the concentration marginally increased from 48 vol. % at 3.4 bar to 55 vol.% at 11.3 bar. Correspondingly, group-II char formation decreased from ~18 vol.% at 3.4 bar to ~5 vol.% at 11.3 bar. The significant drop in group-II char concentration happened between 3.4 and 6.2 bar where the concentration of group-

II char dropped to ~3%. Transformation of group-II char to group-I char can be attributed to increased fluidity with pressure [79]. However, further increase in pressure to 11.3 bar did not change the concentration of group-I char significantly. The caking coals like that of Pittsburgh

No.8 generate a lot of fluidity [79, 80] and quickly transform to high porosity group-I char at lower pressures (<5 bar), and further increase in pressure does not significantly alter the group-I char concentration.

Although group-I char concentration did not change much at elevated pressures, swelling ratio increased from 1.39 to 1.76 even as pressure was increased from 3.4 to 15.5 bar. Decrease in swelling ratio above 15.5 bar suggests a pressure effect. Earlier studies have indicated that the maximum swelling (both at low heating rate and high heating rate) for caking coals (or fractions) occurs in the 8-20 bar pressure range (external pressure) and then decreases with further increase in pressure [39, 47, 74, 80]. Increase in swelling ratio with pressure can be explained based on the mechanism proposed by Yu et al. [81]. During the devolatilization process, a considerable amount of volatiles is transported in the form of bubbles. Small bubbles are created during the initial stage of pyrolysis. At lower pressures, the bubbles merge to form larger bubbles, move to the exterior surface, and burst open (shown in Figure 5-5). This occurs due to limited mass transfer resistance.

As the pressure increases, the movement of volatiles becomes difficult due to increased mass

transfer resistance. The volatiles trapped within the particles expand to form thinner bubble walls until achieving a force balance with the exterior reactor pressure. The expansion with increase in pressure resulted in the generation of highly porous chars with increased swelling ratio. Above certain pressure, the external force becomes more than the internal pressure resulting in reduction in swelling. It appears that pressure seemed to have contributed to reduction in swelling for 21.7 bar char. The surface area showed a huge increase from 109 to 332 m2/g as the pressure was increased from 3.4 to 11.3 bar (shown in Table 5-2). With further increase in pressure to 15.5 bar, the surface area decreased to 289 m2/g and reduced further to 170 m2/g at 21.7 bar. Interestingly, the surface areas for chars generated over a range of pressures, reported in this study were much higher than the surface area reported by Zeng and Fletcher [47]. Gale et al. [82] suggested that at very high heating rates (~7x104 °C/s) experienced by the particles in a high-pressure flat flame burner used by Zeng and Fletcher [47], the rate of release of volatiles is faster than the relaxation time available for particles to swell. Due to limited relaxation time, the particles fragment leading to reduced swelling. This may explain the difference in swelling ratio, surface area, and porosity.

Like surface area, a similar trend was also observed for pore volume, where it increased from 0.14 to 0.30 cc/g as pressure was increased from 3.4 to 11.3 bar and reduced to 0.19 cc/g with further increase in pressure up to 21.7 bar (shown in Table 5-2 and Figure 5-6). The rapid increase in surface area and pore volume with increase in pressure to 11.3 bar can be attributed to the opening of pores of size <50 Å. Analysis of pore size distribution shows that the pore volume in the size

<50 Å increased by ~192% as the pressure was increased from 3.4 bar (0.042 cc/g) to 11.3 bar

(0.123 cc/g). Correspondingly, the volume in the pores of size >100 Å increased marginally from

0.078 cc/g to 0.116 cc/g. With further increase in pressure to 21.7 bar, the volume of the pores of size <50 Å decreased to 0.065 cc/g, while the volume of the pores of size >100 Å remained similar

Table 5-2: Properties of chars analyzed (after washing with THF)

* Feed particle Experimental Proximate analysis Surface Pore Rr size/Density conditions (wt.%, db) area volume (m2/g) (cc/g) (%)

VM FC Ash

Effect of Temperature

1100°C/11.3 bar 21.3 68. 6 10.1 7 0.02 6.86±0.23 -150+106 µm 1300°C/11.3 bar 19.6 64.7 15.7 332 0.30 8.03±0.45 1400°C/11.3 bar 17.0 62.8 20.2 136 0.14 8.16±0.41 Effect of Pressure 3.4 bar/1300°C 17.8 70.0 12.2 109 0.14 7.20±0.19 6.2 bar/1300°C 19.2 68.1 12.7 232 0.18 7.32±0.22 -150+106 µm 11.3 bar/1300°C 19.6 64.7 15.7 332 0.30 8.03±0.45 15.5 bar/1300°C 19.1 66.7 14.2 289 0.20 NA 21.7 bar/1300°C 18.2 65.9 15.9 170 0.19 NA Effect of Initial Particle Size Distribution -425+212 µm 17.4 68.8 13.8 119 0.08 7.08±0.37 -212+150 µm 1300°C/11.3 bar 20.0 62.3 17.7 196 0.12 7.17±0.31 -150+106 µm 19.6 64.7 15.7 332 0.30 8.03±0.45 -106+75 µm 21.4 58.0 20.6 333 0.30 7.98±0.36 Effect of Inorganic Matter <1.3 g/cca 24.8 66.5 8.7 327 0.25 8.05±0.42 1.3-1.6 g/cca 1300°C/11.3 bar 21.6 59.2 19.2 261 0.21 7.85±0.44 >1.6 g/cca* 5.8 2.7 91.5 11 0.02 ND db: dry basis, % Rr= % random reflectance, *Analysis done before washing with THF, a -106+75 µm feed particle size fraction was used. (i.e., 0.064 cc/g) to that of the char generated at 11.3 bar. The reduction in total surface area at 21.7 bar can be attributed to significant reduction in pores of size <50 Å. The fragmentation was measured based on mass% of char below 53 µm. It was found out that the fragments <53 µm increased from ~5 mass% for 11.3 bar char to ~7 mass% for 21.7 bar char.

The effect of pressure on char-O2 reactivity is shown in Figure 5-7. The reactivity was obtained on chars after heating it to 950°C to remove any soot and tar. The apparent reactivity was highest for the char generated at 6.2 bar (0.0188 min-1), while the chars generated at other pressures showed similar reactivities ranging between 0.0115 min-1 (for 3.4 bar char) and 0.0142 min-1 (21.7

bar char). The biggest difference in reactivity between 3.4 and 6.2 bar char was 63%. Normalizing the apparent reactivity with the N2 surface area (i.e., intrinsic reactivity) showed that the intrinsic reactivity ranged between 3.0x10-5 and 5.3x10-5 gm-2 min-1. The result shows that the pressure has limited effect on intrinsic reactivity. The small difference cannot be completely due to soot as the chars were heated to 950°C in a proximate analyzer before measuring the O2 reactivity in the TGA.

The surface area of the high-pressure chars analyzed again after heating it to 950°C was used in determining the intrinsic reactivity. The difference in intrinsic reactivity reported in this work is larger than the difference in intrinsic reactivity reported for chars generated at higher pressures in other studies [38, 46, 47]. The other investigations used CO2 surface area to determine the intrinsic reactivity. A very high CO2 surface area means the difference in intrinsic reactivity becomes negligible.

Efforts were also made to determine the role of tar and soot in affecting the intrinsic reactivity. It is known that soot formation is complex and varies with operating conditions [83]. To determine the role of tar and soot in affecting the reactivity, the intrinsic reactivity of the char generated at elevated pressure was heated to 950°C and was compared with the intrinsic reactivity of the char without heating it to 950°C. The intrinsic reactivities are shown in Figure 5-8. The tar and soot generated at 3.4 bar seems to be more reactive than the tar and soot generated at higher pressures

(>3.4 bar). It appears that pressure seem to have increased the structural ordering of soot leading to decrease in reactivity.

5.4.1. Effect of Inorganic Matter

The effect of inorganic matter was studied using three density fractions of -106+75 µm sample

(i.e., <1.3, 1.3-1.6, and >1.6 g/cc). The conversion on dry-ash free basis of different feed density fractions at 1300°C and 11.3 bar is shown in Figure 5-2. Interestingly, the conversion was highest

for the >1.6 g/cc fraction, while the conversion for the 1.3-1.6 and >1.6 g/cc fractions were similar.

This is surprising, as inorganic matter was expected to generate meltphase at 1300°C and cover carbonaceous matter. An experimental investigation with the >1.6 g/cc fraction at 1500°C and 1 bar, reported elsewhere [65], showed significant agglomerate formation (~80% of char-ash particles were agglomerates). In this case, however, the agglomerates in the char-ash from the >1.6 g/cc fraction were a mere 9 wt. %. It is possible that the particles were agglomerated even before entering the reactor. As this material passed through the hot zone, the melt phase generated by the inorganics cemented the agglomerates. Despite agglomeration, the high-density fraction showed higher conversion compared to other density fractions. One of the possibilities is the contamination of chars from low and medium-density fractions by soot which affected their gasification reactivity. The HRTEM pictures presented in Figure 5-9 confirmed the presence of soot for low and medium density fractions. Not surprisingly, no soot was observed for the char generated from the high-density fraction. It appears that the reduced vitrinite content (~50 vol%) in the high- density fraction seemed to have resulted in limited soot formation consequently higher accessibility of the char surface by CO2 leading to higher gasification.

100 100 -106+75 µm/1300°C/11.3 B -150+106 µm/1300°C 80 80

60 60

40 40

20 20 Conversion (daf weight (daf Conversion %) Conversion (daf weight Conversion %) 0 0 3.4 Bar 6.2 Bar 11.3 Bar 15.5 Bar 21.7 Bar Whole coal <1.3 g/cc 1.3-1.6 g/cc >1.6 g/cc Reactor pressure Feed particle density

100 100 -150+106 µm/11.3 B 1300°C/11.3 B 80 80

60 60

40 40

20 20 Conversionweight(daf %) 0 Conversionweight%) (daf 0 1100°C 1300°C 1400°C -425+212 µm -212+150 µm -150+106 µm -106+75 µm Temperature Feed particle size cut

Figure 5-2: Coal conversion at elevated pressures

2.0 2.0

1.5 1.5

1.0 1.0 Thermalswelling ratio (d/do) 0.5 Thermalswelling ratio (d/do) 0.5 3.4 Bar 6.2 Bar 11.3 Bar 15.5 Bar 21.7 Bar <1.3 g/cc 1.3-1.6 g/cc >1.6 g/cc Reactor pressure Feed particle density

2.0 2.0

1.5 1.5

1.0 1.0

Thermalswelling ratio (d/do) 0.5 Thermalswelling ratio (d/do) 0.5 1100°C 1300°C 1400°C -425+212 µm -212+150 µm -150+106 µm -106+75 µm Temperature Feed particle size cut

Figure 5-3: Swelling ratio of chars

60 60

50 50

40 40 30 30 20 Volume (%)

Volume(%) 20 10 10 0 0 -I II II ts -I II II ts -I II II ts p - I n p - I n p - I n I I I s I I I s I I I s u p - e u p - e u p - e - I I t - I I t - I I t o u p o u p o u p p - -I n p - -I n p - -I n r o u m r o u m r o u m u p p e u p p e u p p e r o g r o g r o g o u u m o u u m o u u m G G r ra G G r ra G G r ra r ro o g r ro o g r ro o g G F G F G F G G r ra G G r ra G G r ra G F G F G F c c c /c /c /c r r r g g g a a a .3 .6 .6 B B B 1 -1 1 4 2 3 < 3 > 3. 6. 1. . 1 1 Reactor pressure Feed particle density

60 60 50 50

40 40

30 30

20 20 Volume (%) Volume(%)

10 10

0 0 -I II II ts -I II II ts -I II II ts -I II II ts -I II II ts -I II II ts -I II II ts p - I n p - I n p - I n p - I n p - -I n p - -I n p - -I n u p - e u p - e u p - e u p - e u p p e u p p e u p p e o u p o u p o u p o u p o u u m o u u m o u u m r o u gm r o u gm r o u gm r o u gm r ro o g r ro o g r ro o g G r ro a G r ro a G r ro a G r ro a G G r ra G G r ra G G r ra G G r G G r G G r G G r G F G F G F F F F F

m m m m C C C µ µ µ µ 0° 0° 0° 0 0 0 2 0 6 5 1 3 4 21 15 10 7 1 1 1 + + + 6+ 5 2 0 0 2 1 5 -1 Temperature -4 -2 -1 Feed particle size cut Figure 5-4: Char morphology (obtained on chars before washing with THF)

Figure 5-5: SEM images of chars generated at 6.2 bar (left) and 21.7 bar (right)

Effect of reactor pressure Effect of feed particle density 0.025 0.025 3.4 B <1.3 g/cc 6.2 B 1.3-1.6 g/cc 11.3 B >1.6 g/cc 0.020 15.5 B 0.020 21.7 B 0.015 0.015

0.010 0.010 Pore volume Pore (cc/g)

Pore volume Pore (cc/g) 0.005 0.005

0.000 0.000 10 100 1000 10 100 1000 Pore size distribution (Å) Pore size distribution (Å)

Effect of temperature Effect of feed particle size cut 0.025 0.025 1100°C -425+212 µm 1300°C -212+150 µm 0.020 1400°C 0.020 -150+106 µm -106+75 µm

0.015 0.015

0.010 0.010 Pore volume(cc/g) Pore 0.005 volumePore (cc/g) 0.005

0.000 0.000 10 100 1000 10 100 1000 Pore size distribution (Å) Pore size distribution (Å)

Figure 5-6: Char pore structure

Figure 5-7: Intrinsic and apparent reactivities of chars; a.) effect of pressure b.) effect of inorganic matter c.) effect of temperature d.) effect of particle size (Intrinsic reactivity was determined by normalizing the apparent reactivity with the surface area of chars obtained after heating it to 950°C). Intrinsic reactivity for the char from >1.6 g/cc fraction was determined at 400°C in O2 without heating it to 950°C.

0.00010 Initial char Char after heating it to 950°C/1.01 Bar 0.00008

0.00006

0.00004

0.00002 Intrinsic Reactivity (g/m².min) Reactivity Intrinsic

0.00000 3.4 6.2 11.3 15.5 21.7 Char Generation Pressure (Bar)

Figure 5-8: Role of soot, tar, and volatiles on the intrinsic reactivity of chars generated over a range of pressures

Table 5-2, and Figure 5-3, 5-4, and 5-6 show the swelling ratio, morphology, surface area, and pore size distribution of chars for the <1.3, 1.3-1.6, and >1.6 g/cc fractions. Increase in feed particle density decreased the swelling ratio of the char from 1.61 to 1.00, surface area from 327 to 11 m2/g, group-I char concentration from 55 to 0%, and pore volume from 0.25 to 0.02 cc/g. Some of these results are in agreement with the observations made by Yu et al [44], who also showed that both porosity and group-I char concentration decreased with increase in particle density. Variation in maceral content and inorganic matter concentration over various density fractions was shown to affect swelling [40]. Analysis of the density fractions of the Pittsburgh no.8 coal for maceral composition, reported elsewhere [75], revealed that the highest density fraction had about 50 vol.% inertinite, while the low and medium-density fractions were rich (>75 vol.%) in liptinite and vitrinite combined. Higher vitrinite content in the low and medium-density fractions implies higher fluidity, and consequently higher group-I char formation, surface area, and pore volume. Unlike vitrinite, inertinite does not develop much fluidity and hence does not generate high-porosity and high surface area char [40]. Hence, the high-density fraction, with ~50% inertinite, generated about

50% group-III char with no group-I char. Besides maceral content, the inorganic matter increased with feed particle density. It was suggested that ash grains in the char reduce the fluidity of the particle and thereby preventing the entire particle from swelling [39, 40, 84]. Absence of swelling means there is no substantial surface area or pore volume generated during gasification of the highest density fraction.

Figure 5-7 shows the apparent and intrinsic reactivities of chars. The apparent reactivity was highest for the char generated from the >1.6 g/cc fraction, while the chars generated from low and medium-density fractions were relatively less reactive. It is important to recognize that the char surface area and pore volume decreased with increase in feed particle density. This implies that

the chars from the high-density fraction is structurally more reactive than that of the char from the low and medium-density fractions. The lower reactivity for the char from the low-density fraction could be due to increased structural ordering. The vitrinite content, which is known to cause fluidity and eventually to structural ordering, increased with decrease in feed particle density. The reflectance microscopy analysis showed that the inertinite-derived section of the char was qualitatively found to be isotropic in nature with random reflectance of <5%, while the vitrinite- derived section of the char showed reflectance of >6.5%. Some particles showed the presence of pyrolytic carbon. The higher reflectance is indicative of higher structural ordering. The random reflectance of the high-density fraction was not reported due to the limited amount of vitrinite- derived portions in the char. The high degree of structural ordering seemed to have contributed to lower intrinsic reactivity for the char derived from the <1.3 g/cc fraction.

Figure 5-9: HRTEM pictures of soot: a) <1.3 g/cc char (Left) b) 1.3-1.6 g/cc char (Right)

5.4.2. Effect of Temperature

The effect of temperature on conversion was investigated by generating chars from the -150+106

µm fraction at 11.3 bar and three different temperatures (1100±13, 1300±15, and 1400±10 °C).

These temperatures were chosen to cover the temperature range typically found in the second stage of a slurry-fed gasifier [85]. As Figure 5-2 shows, the increase in temperature from 1100 to

1400°C increased the conversion from 30 to 70%. Notably, the char generated at 1100°C is in the early gasification stage (<50% conversion), while the chars generated at 1300 and 1400°C were in the middle (50-70% conversion) and late gasification (>70% conversion) stages, respectively.

Proximate analysis of the chars revealed that, despite difference in conversion, the amount of volatiles retained in the char was more or less similar irrespective of temperature. Similar amount of volatiles is an indicative of chars contaminated with soot.

Figure 5-3 and Figure 5-4 show the effect of temperature on swelling ratio and morphology. As the temperature was increased, the swelling ratio decreased marginally from 1.68 for 1100°C char to 1.65 for 1300°C char. Interestingly, the slight drop in swelling ratio corresponds with an increase in the group-I char concentration from 46 to 55 vol.% for 1100°C char and 1300°C char, respectively. The swelling ratio is inversely proportional to the apparent density ratio of char to coal, and mass loss (shown in equation 5.1). It can also be seen from Figure 5-4 that fragmentation increased with corresponding increase in temperature. Gale et al. suggested that fragmentation can reduce swelling ratio [82]. As fragmentation increases, the packing factor also increases and consequently the apparent density ratio of char to feed. Secondly, the conversion for 1300°C char

(~58%) was much higher than that of 1100°C char (~30%). Higher conversion and fragmentation seemed to have offset the contribution of increase in group-I char (i.e., highly porous low-density chars) concentration towards swelling ratio. However, with further increase in temperature to

1400°C, the swelling ratio dropped to 1.38. The decrease in swelling ratio above 1300°C can be attributed to increased fragmentation leading to higher packing ratio and consequently higher apparent density ratio of char to feed. Increased fragmentation with temperature can be attributed

to pore enlargement and pore overlapping due to internal reaction with conversion causing thinning and ultimately breaking of the wall structure [79]. Besides fragmentation, higher conversion also contributed to lower swelling ratio.

Unlike swelling ratio, temperature showed a significant effect on surface area and pore volume

(shown in Table 5-2 and Figure 5-6). The surface area and pore volume increased dramatically by a factor of 46 and 14, respectively, as temperature was increased from 1100°C to 1300°C.

However, with further increase in temperature to 1400°C, the surface area and pore volume decreased by ~60% and ~53% of 1300°C char, respectively. It is important to emphasize that the conversion for the char generated at 1400°C was above 70%. It is known that beyond a certain conversion (>60%), the pores of the chars collapse and merge, leading to lowered surface area and pore volume [86]. Higher conversion and higher fragmentation for 1400°C char seemed to have reduced the surface area and the pore volume.

Char generation temperature showed significant effect on char reactivity, with apparent reactivity decreased by ~59% from 23.7x10-3 min-1 for 1100°C char to 9.6x10-3 min-1 for 1400°C char

(shown in Figure 5-7). Normalizing the apparent reactivity with the surface area also showed that there was a significant drop in intrinsic reactivity from 7.8x10-5 gm-2min-1 for 1100°C char to

3.0x10-5 gm-2min-1 for 1300°C char, and the intrinsic reactivity slightly increased to 3.7x10-5 gm-

2min-1 at 1400°C. Previous studies also observed similar trends where the intrinsic reactivity decreased with increase in char generation temperature [33, 87]. In this investigation, however, the intrinsic reactivities of chars generated at 1300°C and 1400°C are not very different. Tremel et al. showed that above 2 s residence time, temperature does not significantly affect the intrinsic reactivity [38].

The significant drop in reactivity at higher temperatures could be due to increased structural ordering. The chars were characterized for structural ordering through random reflectance. The random reflectance showed a significant increase from 6.86±0.23% for 1100°C char to

8.03±0.45% for 1300°C. However, further increase in temperature to 1400°C did not increase the reflectance significantly (i.e., 8.16±0.41%). Reflectance analysis showed that some of the particles, from 1300 and 1400°C chars, had pyrolytic carbon with very high reflectance (>10%).

Presence of pyrolytic carbon is an indication of graphitization. All these point to increased structural ordering and graphitization for the decline in reactivity for chars generated at 1300 and

1400°C chars.

5.4.3. Effect of Feed Particle Size Cut

The effect of initial coal particle size on conversion was studied on chars produced from four particle size fractions— -106+75, -150+106, -212+150, and -425+212 µm—generated at 1300°C and 11.3 bar. The conversion decreased from 65 to 38% as particle size increased from -106+75 to 212-425 µm (shown in Figure 5-2). The reduction in conversion with particle size can be attributed to increased diffusional resistance for reaction gas into char particles. Interestingly, the amount of volatiles left in the char did not vary much with increase in feed particle size fraction.

Higher amount of volatiles left in the char from the finest particle size fraction, despite higher conversion, suggests that the higher fluidity generated during pyrolysis quickly transformed to soot.

Figure 5-3 and Table 5-2 show the effect of initial particle size on swelling ratio, surface area, and pore volume. As the feed particle size fraction increased from -106+75 to -425+212 µm, the char surface area and pore volume decreased from 333 to 119 m2/g, and 0.30 cc/g to 0.08 cc/g, respectively. The difference in surface area and pore volume can be attributed to difference in

organic composition. A study conducted by Hower showed that brittle microlithotypes, such as vitrite, are likely to partition to finer sizes, while hard-to-grind microlithotypes rich in inertinite and liptinite are likely to concentrate in the coarse-sized fractions (>150 µm) [88]. The presence of higher concentration of non-caking inertinite as a part of multimaceral microlithotype in a larger coal particle may have led to reduced fluidity and consequently reduction in group-I char concentration, surface area and pore volume for the coarse-sized fractions. Interestingly, the particle size marginally affected the char swelling ratio with swelling ratio increasing from 1.58 for the char from -425+212 µm to 1.65 for the char from -150+106 µm fraction. Earlier studies under pyrolysis observed that swelling (or porosity) decreased with an increase in coal particle size [44, 89, 90]. It is important to recognize that the chars from coarser size fractions (i.e., -

212+150 µm and -425+212 µm) are in early stages of char conversion (<50%), while the chars from the finer size fractions (i.e., -106+75 µm and -150+106 µm) are in middle stages of conversion (50-70%). Moreover, the extent of fragmentation of chars from the finer size fractions are much higher than the chars from the coarser size fractions. Fragmentation is known to increase packing factor and consequently reduce swelling ratio. A plausible reason for this observation is that the lower conversion and lower fragmentation might have resulted in higher swelling ratio for chars generated from the coarse-size fractions. However, for a given conversion, particle size inversely affects the swelling ratio.

The effects of initial feed particle size on apparent and intrinsic reactivities are shown in Figure

5-7. The apparent reactivity increased with particle size. The notable observation is that the reactivities of chars generated from the fine size fractions were closer compared with the reactivities of chars from the coarse size fractions. This trend is also visible in the pore size distribution (shown in Figure 5-6 and Table 5-2). It appears that the organic composition (i.e.,

maceral composition) among finer sized fractions are more similar compared to the maceral composition of the coarser-size fractions. Secondly, the total conversion for chars from the coarser size fractions are similar compared to the conversion for chars from the finer size distributions.

Lower surface areas and higher apparent reactivities for chars from the coarser size distributions mean the chars are structurally more reactive. For the chars from the fine-sized fractions, there is a predominant effect of structural ordering leading to reorganization of carbon atoms. The reorganization of carbonaceous material would only occur with the vitrinite-derived portion of the feed. Higher concentration of inertinite in the char from the coarse-sized particle size distributions means lower structural reordering. This explanation is supported by the lower random reflectance for chars (shown in Table 5-2) derived from coarse-sized cuts (~7% for chars from -212+150 µm, and -425+212 µm size fractions) than that of the chars from the fine-sized cuts (~8% for chars from -106+75 µm and -150+106 µm size fractions).

5.4.4. Statistical Analysis

The effects of temperature, pressure, feed particle size, and particle density on structural characteristics (i.e., pore volume, surface area, swelling ratio and group-I char) and the intrinsic reactivity were statistically evaluated by fitting regression model in Minitab software. The null hypothesis assumes none of the parameters affect the structural characteristics and the intrinsic reactivity. The null hypothesis must be rejected when p-value is <0.025 and this is based on 95% confidence interval. The effects of various parameters (i.e., temperature, pressure, feed particle size, and feed particle density) on conversion were statistically determined and reported in Table

5-3. From the p values, the conversion is most effected by temperature followed by particle size.

Pressure and particle density showed no significant effect on conversion due to large uncertainty.

Similarly, the effects of these parameters on group-I char concentration and swelling ratio were

statistically determined and reported in Table 5-4. Particle density is the only factor that was found to statistically affect the group-I char formation. The insignificant effect of other parameters on group-I char concentration can be attributed to the nature of the coal and high uncertainty in data.

Thermoplastic coals like Pittsburgh no.8 has tendency to generate a lot of fluidity at higher temperatures and heating rates leading to formation of highly porous chars irrespective of pressure, and particle size. The thermoplastic behavior may have led to this observation. Second, the high uncertainty of 6.8% on group-I concentration also contributed to insignificant effect of parameters such as particle size, pressure, and temperature on group-I char concentration.

In the case of swelling ratio, both particle density and pressure showed significant effects. It is interesting to note that the pressure affects swelling ratio, while it has insignificant effect on group-

I char concentration. This is because Pittsburgh no.8 coal forms higher amount of fluidity at lower pressures to form large concentrations of highly porous char (or group-I char). Further increase in pressure increases the swelling ratio without affecting the group-I char concentration.

The effects of various parameters on intrinsic reactivity are presented in Table 5-5. Feed particle density has the highest effect on the intrinsic reactivity, followed by temperature, and feed particle size. Variation in maceral content (vitrinite content) with particle size and particle density affected the char structure and consequently the intrinsic reactivity. In the case of temperature, higher conversion for chars generated higher pressures have affected the reactivity. For the char generated at 1100°C, the intrinsic reactivity was much higher (>100%) compared to chars generated at 1300 and 1400°C.

Table 5-3: Effect of various parameters on conversion

Predictor F value P value Pressure 7.1 0.033 Particle density 2.7 0.133 Temperature 43.0 0.001 Particle size 13.3 0.008

Table 5-4: Effects of various parameters on group-I char concentration and swelling ratio

Predictor Group-I char concentration Swelling ratio F value P value F value P value Pressure 0.1 0.730 8.4 0.023 Particle density 12.4 0.003 11.81 0.003 Temperature 0.7 0.439 4.41 0.074 Particle size 3.1 0.120 0.0 0.910

Table 5-5: Effect of various parameters on intrinsic reactivity

Predictor Intrinsic reactivity F value P value Pressure 0.01 0.932 Particle density 7795 0.000 Temperature 12.51 0.012 Particle size 9.52 0.022

5.5. Summary and Conclusions

The effects of particle size, inorganic matter, temperature and pressure on structural properties

and reactivities of Pittsburgh No. 8 coal were studied under reducing conditions. The char-ash

particles were analyzed for conversion, reactivity, group-I char concentration, and swelling

ratio. Based on the investigations, the following summary and conclusions were drawn:

 Among the parameters tested, temperature has the maximum effect on coal conversion,

followed by particle size, pressure, and feed particle density.

 Feed particle density showed the highest effect on the group-I char formation, with low-

density particles generating higher concentration of highly porous group-I chars. Group-I

char concentration was found to correlate with vitrinite content. The other factors such as

particle size, temperature, and pressure were not found to affect the group-I char

concentration.

 Swelling ratio is affected by both pressure, and feed particle density. Other factors such as

temperature and particle size showed no statistically significant on swelling ratio. Lack of

clear effect of temperature and particle size is attributed to difference in conversion levels

of the chars.

 Intrinsic reactivity is affected by all parameters except for pressure. Pressure affects the

structural features such as swelling ratio and surface area and not the chemical reactivity

of the char. The effect of the four parameters on intrinsic reactivity follows the order: feed

particle density>temperature> feed particle size>pressure.

Effect of Pressure on Intrinsic Kinetics of Chars Generated at High Temperature and High Pressures in a High-Pressure, High-Temperature Flow Reactor

6. Abstract

The knowledge of intrinsic gasification rates on chars generated at high temperatures and elevated pressures are crucial to the design and optimization of gasifiers. The intrinsic kinetic data on

American coals is non-existent in the open literature. Therefore, a widely used Pittsburgh No.8 coal was partially gasified in CO2 atmosphere at 1300°C over a range of pressures (3.4, 6.2, 11.3,

15.5, and 21.7 bar) in a high-pressure, high-temperature flow reactor. The intrinsic reaction rate of those chars with CO2 was obtained using a high pressure thermogravimetric analyzer. The kinetic parameters were obtained using an nth order model. The intrinsic reaction rate, and activation energy were less affected over the range of char generation pressure (3.4-21.7 bar). The order of reaction was determined by varying CO2 partial pressures. The order of reaction decreased with increase in char generation pressure. The comparison of initial char with the char obtained at ~20% conversion in the HPTGA for surface area and pore volume showed that the reaction primarily occurred in microporous regions. The order of reaction also closely followed the evolution of surface area during the gasification.

6.1. Introduction

Integrated gasification combined cycle is one of the advanced technologies that has potential to reduce coal’s carbon footprint. Despite many commercial operations, the knowledge of char gasification rates at high pressures and temperatures, crucial to the design and troubleshooting of the gasifiers, is lacking. Unlike atmospheric pressure combustion data, only limited high pressure

experimental gasification data have been reported and that is in part due to practical difficulty in designing and operating the reactors at high pressures and temperatures.

The general approach to predict the gasification rate in the entrained-flow gasifiers is to develop kinetic models from the kinetic data obtained from lab-scale reactors. Within the gasifier, the ability of the reaction gas to penetrate within the char pore structure and the intrinsic reaction kinetics control the reaction rate of the char. The intrinsic reaction kinetics is obtained under conditions free of diffusion or mass transfer limitations. The mass transfer characteristics can be modelled using char surface area, porosity, and char morphology. By combining mass transfer and heat transfer effects with intrinsic reactivity, gasification rates in a commercial gasifier can be predicted. The evolution of char surface area, porosity, and char morphology during gasification have been investigated and reported in the previous chapter. The intrinsic kinetics, being another important parameter, requires attention. Intrinsic reactivity is dependent on char preparation conditions and the parent fuel. Rosenberg et al. showed that chars that are produced above 1300

°C in a flow reactor were similar to the chars from the industrial-scale application [91]. However, the intrinsic reactivities, often reported in the literature, were obtained on chars generated at low heating rates [92, 93] or low pressures [19, 33, 94], which are not representative of commercial gasifier conditions. While there are few studies in the literature that have reported the intrinsic reactivities of chars generated under conditions relevant to that of the entrained-flow gasifier [53,

56, 57], more data are needed to develop a better understanding of reaction pressure on reactivity and consequently can be used in optimizing gasifier operating conditions.

The objectives of this investigation include examining the reactivity of the chars generated over a range of reactor pressures under conditions similar to that of entrained-flow gasifiers in a PTGA to obtain kinetic parameters. The char characteristics (surface area and chemical structure) are

related to char reactivity to examine the role of reactor pressure on intrinsic kinetics. Besides the kinetic parameters, the effect of CO2 pressure on the reactivity of a highly reactive and less reactive chars are also examined.

6.2. Experimental Section

6.2.1. Sample Preparation

Five chars generated at different pressures were used in this work. All the chars were generated from Pittsburgh No.8 coal at 1300°C in the HPHTFR as described in the chapter 4. The chars were washed with THF to remove tar and soot affecting the CO2 reactivity. The THF washed char sample was dried in a vacuum oven at 60°C and 25 in Hg vacuum for over 12 hours. The chars were immediately analyzed for proximate analyses and N2 surface area. The properties of chars are presented in Table 6-1.

Table 6-1: Proximate analysis of chars used in this study

HPHTFR pressure Proximate analysis (dry wt%) Initial char surface area (bar) VM FC Ash (m2/g of dry char) 3.4 17.8 70.0 12.2 109 6.2 19.2 68.1 12.7 232 11.3 19.6 64.7 15.7 332 15.5 19.1 66.7 14.2 289 21.7 18.2 65.9 15.9 170

6.2.2. Low-Temperature Rate Measurements

Char reactivity with CO2 was measured at the same pressure as it was generated in the HPHTFR under regime-I conditions using the HPTGA. The apparent reaction rate and the intrinsic reaction rate, explained previously, were obtained on a small amount of sample: ~35 mg for 3.4 and 6.2 bar pressures and ~70 mg for other pressures. To avoid any effect of volatiles on reactivity, the chars

were treated to remove any residual volatile by heating each to 950°C and at the same pressure at which the char was generated. For 21.7 bar char, the sample was heated at 20 bar due to instrument limitation. The sample was held at 950°C for about 15 minutes and cooled down to the temperature

(i.e., between 825-925°C) where isothermal gasification experiments were conducted. From the weight loss curves, the instantaneous reaction rates were calculated. The kinetic parameters, activation energy and pre-exponential factor, were obtained using an nth order rate equation. The reaction rate was related to temperature and partial pressure at specified conversion using equation

4.5. The apparent reaction rate was obtained at 20% conversion during the isothermal step. This was because fluctuations were high in the early stages (<10% conversion) of the data collection.

6.2.3. Surface Area

The chars were dried at 150°C in vacuum for 10 hours before measuring the surface area. Total surface area and pore size distribution of the chars (from the HPHTFR) and 20±3 % converted chars in the HPTGA were determined using N2 adsorption-desorption isotherms at 77 K. The isotherms were analyzed using the Brunauer-Emmett-Teller (BET) equation to calculate total surface area, and Barrett, Joyner, and Halenda (BJH) analysis to calculate pore size distribution.

6.2.4. Microstructural Analysis

The microstructural analysis of the chars was obtained using XRD and Raman spectroscopy. The procedure employed in obtaining the data was covered in previous chapters.

6.2.5. HRTEM Analysis

The samples were prepared by crushing under ethanol in a mortar. A suspension of the crushed particles was then deposited on a carbon film. The high-resolution transmission electron

microscope images (Structural resolution limit =0.12 nm) were obtained using a Talos F200X was operated at 200 kV with EDS detector for determining elemental composition.

6.3. Results and Discussions

6.3.1. Effect of Reactor Pressure on Char Reactivity

The apparent reactivities were measured for chars that were generated over a range of pressures in the HPHTFR using the HPTGA at 875°C are shown in Figure 6-1. The reactivity of the 21.7 bar char was measured at 20 bar pressure in the HPTGA due to instrument limitation. Except for the

21.7 bar char, the intrinsic reactivity was measured at the same pressure as it was generated in the

HPHTFR. The apparent reactivity reached a maximum for the char generated at 6.2 bar and decreased with further increase in pressure to 11.3 bar and decreased marginally at 21.7 bar. Since the apparent reactivity is a function of surface area and the intrinsic reactivity, the intrinsic reactivity was obtained by normalizing with initial char surface area. Normalizing the apparent reactivity with surface area of the HPHTFR char, showed a significant drop in intrinsic reactivity with char generation pressure. The intrinsic reactivity is dependent on the number of active sites per N2 surface area. Lower intrinsic reactivity at higher pressures could be due to deactivation of chars. The loss of active sites could be due to higher fluidity with pressure resulting in increased structural ordering. Senneca showed that above 900°C, char gasification rate with CO2 and thermal annealing occur at the same rate [95].

The increased ordering could be due to combination of high pressure and temperature. To verify the extent of structural ordering, the chars were analyzed using confocal Raman spectroscopy.

Raman spectroscopy is useful in obtaining defects to graphitic ratio (Id/Ig) [96]. The reactivity increases as the disordered (or amorphous) nature of char increases, while the reactivity decreases with graphite band in the Raman spectra. Counter intuitively, the average defects to graphite band

ratio of 21.7 bar char was higher than the average ratio for the 3.4 bar char, even though 3.4 bar char has much higher intrinsic reactivity than 21.7 bar char (shown in Figure 6-2). Higher average of defects to graphite band ratio for 21.7 bar char suggests a possible presence of soot, as defects band (representing >6 membered aromatic rings but less than graphite [97]) also takes soot into account. Previous studies have also reported presence of soot on chars generated at higher pressures [58, 98]. However, the difference in overall defects to graphite bands ratio is less than the error bar indicating not much variation in structure. The other studies have shown that the

Raman analysis can become insensitive to structural changes for chars generated at higher temperatures (>1200°C) and longer residence times [70, 99].

0.016 0.00016 t

Apparent reactivity r

Intrinsic reactivity (Nomalized with SA) s

Intrinsic reactivity (Normalized with SA20%)

r Intrinsic reactivity (Normalized with SA20% and pressure) e

0.012 0.00012 i

(

/ 0.008 0.00008 m

)

(

0.004 0.00004 ²

/ Apparentreactivity (1/min)

0.000 0.00000 ) 5 10 15 20 Char generation pressure (Bar)

Figure 6-1: Reactivity of HPHTFR chars with CO2 in HPTGA obtained at 875°C and 20% conversion. (SA is initial N2 surface area and SA20% is the surface area measured at ~20% conversion)

1.15

1.10

1.05

1.00

Id/Ig 0.95

0.90

0.85

0.80 3.4 Bar 6.2 Bar 11.3 Bar 21.7 bar Reactor Pressure

Figure 6-2: The ratio of defects to graphite band (Id/Ig) for the HPHTFR chars (before washing with THF) obtained using Raman spectroscopy With confocal Raman spectroscopy being a surface technique, soot contamination seemed to have affected the analysis. The presence of soot in the char can be construed by the higher amount of volatiles left in the HPHTFR char. High resolution transmission electron microscopy (HRTEM) images on the chars further confirmed the presence of soot (shown in Figure 6-3). Chars washed with THF also did not show substantial difference in the volatile content (shown in Appendix C).

Therefore, it can be concluded that Raman spectra may not yield the true structure of the char but rather the char contaminated with soot. Therefore, chars were analyzed for crystallite features using XRD. Interestingly, the XRD pattern also showed no significant difference in crystallite size (shown in Figure 6-4). All these point to evolution of char physical structure and surface area during course of gasification as the reason for difference in reactivity. This result is in line with the observation made by the earlier study that also observed that crystallite sizes were not affected by the reactor pressure [23].

100

Figure 6-3: HRTEM pictures of soot generated at different pressures: Left: 6.2 bar; right: 21.7 bar

35000 50 50 3.4 bar Stacking height, Lc Lateral size, La 30000 (002) 6.2 bar S

t 11.3 bar 40 40 a )

c 21.7 bar k

25000 (Å

n a

L 30 30

20000 ,

e e

i z

g (100) i

s h

15000 l t

, a

Intensity 20 20

L r e

c t

( 10000 a

Å L 10 10 ) 5000

0 0 0 10 20 30 40 50 60 70 5 10 15 20 2 Theta (Deg) Reactor Pressure (Bar)

Figure 6-4: XRD pattern of chars generated at different pressures It is clear that char physical structural changes contributed to differences in reactivities. Therefore, the surface area of the chars was measured around the conversion at which the apparent reactivity was measured. In this case, the char surface area was obtained at 20±3% conversion in the HPTGA.

The apparent reactivity was obtained at 20% conversion to ensure all the remaining volatiles left after the heat-up stage was consumed and only char reactivity was obtained. Interestingly, the intrinsic reactivity at 20% conversion obtained by normalizing with N2 surface area at 20% showed

101

pressure has limited effect on the reactivity. This confirms the observations made from the Raman and XRD analysis that the chemical structure is not affected by the HPHTFR pressure. Similar observations were made by other researchers regarding the effect of pyrolysis pressure on the intrinsic reactivity of chars with O2 or CO2 or H2O normalized using CO2 surface area [23, 38, 46,

47].

6.3.2. Kinetic parameters

The conversion profiles of the chars generated over a range of pressures are shown in Figure 6-5.

Not surprisingly the conversion increased with increase in temperature. The instantaneous reaction rate was obtained (shown in Figure 6-6) from the weight loss curves obtained from the HPTGA.

From the instantaneous reaction rates obtained at 20% conversion, activation energy, pre- exponential factor, and order of reaction were calculated using the nth order equation (shown in equation 4.5). Except for 6.2 bar and 21.7 bar chars, the activation energy (Ea) and pre-exponential factor (A) of other chars were obtained in the temperature range of 850-900°C. The activation energy for 6.2 bar was obtained at slightly lower temperature range (i.e., 825-875°C) to avoid diffusion issues. As the temperature range was increased to 850-900°C for 6.2 bar char, the activation energy decreased from 215 kJ/mol to 185 kJ/mol. Lowering of activation energy indicates that the reaction is not completely free of mass transfer limitation. Since the objective was to obtain the intrinsic rate under regime I conditions, the reactivity for 6.2 bar char was obtained in the temperature range of 825-875°C.

102

100 100 3.4 bar char_850°C 6.2 bar char_825°C 3.4 bar char_875°C 6.2 bar char_850°C 80 3.4 bar char_900°C 80 6.2 bar char_875°C

60 60

40 40 Conversion (%) Conversion (%) Conversion 20 20

0 0 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 Time (min) Time (min)

100 100 11.3 bar char_850°C/11.3 bar 15.5 bar char_850°C/15.5 bar 11.3 bar char_875°C/11.3 bar 15.5 bar char_875°C/15.5 bar 80 11.3 bar char_900°C/11.3 bar 80 15.5 bar char_900°C/15.5 bar

60 60

40 40 Conversion (%) Conversion Conversion (%) Conversion 20 20

0 0 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 Time (min) Time (min)

103

100 21.7 bar char_875°C 21.7 bar char_900°C 80 21.7 bar char_925°C

40 Conversion (%)

0 0 20 40 60 80 100 120 Time (min)

Figure 6-5: Conversions profiles of chars with CO2 generated at the same pressures as it was generated in HPHTFR. (for 21.7 bar char, the conversion profile was obtained at 20 bar CO2 pressure)

104

0.030 0.030 3.4 bar char_850°C 3.4 bar char_875°C 6.2 bar char_825°C 0.024 3.4 bar char_900°C 0.024 6.2 bar char_850°C 6.2 bar char_875°C

0.018 0.018

0.012 0.012 Reactivity (1/min) Reactivity Reactivity (1/min) Reactivity 0.006 0.006

0.000 0.000 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Conversion (%) Conversion (%)

0.030 0.030

11.3 bar char_850°C 15.5 bar char_850°C 11.3 bar char_875°C 0.025 0.025 15.5 bar char_875°C 11.3 bar char_900°C 15.5 bar char_900°C 0.020 0.020

0.015 0.015

0.010 0.010 Reactivity (1/min) Reactivity Reactivity (1/min) Reactivity

0.005 0.005

0.000 0.000 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Conversion (%) Conversion (%)

105

0.030 21.7 bar char_875°C 0.025 21.7 bar char_900°C 21.7 bar char_925°C

0.020

0.015

0.010 Reactivity (1/min) Reactivity

0.005

0.000 0 10 20 30 40 50 60 70 80 90 Conversion (%)

Figure 6-6: CO2 reactivity versus conversion profiles of chars generated at various pressures

106

In the case of 21.7 bar char, the instantaneous rates were obtained at a slightly higher temperature range to increase the reaction rate. The results of the experiments are presented in Table 6-2. It can be seen from Table 6-2 that the char generation pressure did not significantly affect the activation energy and pre-exponential factor. The activation energy for the chars generated at

1100°C and 6.2 bar reported in the earlier chapter were also in a similar range of 200-220 kJ/mol.

It was observed in an earlier study that the activation energy was independent of the char generation pressure and temperature [92]. Like activation energy, the frequency factor when normalized with surface area also showed that the pyrolysis pressure had less significance on active site generation.

Table 6-2: Kinetic parameters of chars generated at various pressures

HPHTFR HPTGA conditions Ea A A0 Aint Aint n SA20% Pressure Pressure T range kJ/mol (s-1) (s-1 bar-n) (gm-2s-1) (gm-2s- (m2/g) (bar) (bar) (°C) 1bar-n) 3.4 3.4 850-900 237 6.0x106 3.7x106 2.4x104 1.5x104 0.40 298 6.2 6.2 825-875 239 15x106 4.9x106 3.6x104 1.2x104 0.61 409 11.3 11.3 850-900 223 0.7x106 0.5x106 0.2x104 0.2x104 0.09 368 15.5 15.5 850-900 249 13x106 6.5x106 3.4x104 1.7x104 0.25 393 21.7 20.0 875-925 229 1x106 0.5x106 0.4x104 0.2x104 0.21 265 * Surface area (SA20%) was measured at 20±3% converted char in HPTGA operated at either at 850°C or at 875°C 6.3.3. Effect of Pressure on the Order of Reaction

The order of reaction was obtained by varying the CO2 concentration from 25% to 100% of the reaction gas in the HPTGA. The general trend for the order of reaction is that it decreases with increase in pressure (shown in Table 6-2). The results are consistent with the literature data where the reaction order was determined at pressures above atmospheric pressure. Dutta showed that the char-CO2 reaction order tends to zero at pressures above 15 bar [100], while the reaction order is close to one at atmospheric pressure. Similarly, Roberts also observed a decrease in reaction order with increase in pressure [101]. The order of reaction also seems to correlate with the surface area,

107

although the exceptions to the trend being the order of reaction obtained for 3.4 and 11.3 bar chars.

As explained in the earlier chapter, the order of reaction is a function of active sites available for the formation of carbon-oxygen complexes. At lower pressures, not all active sites are occupied and an increase in pressure increases the formation of active site complexes and consequently results in an increased rate of reaction and order of reaction. At higher pressures, most of the active sites are saturated with the carbon-oxygen complexes. Any change in pressure does not significantly increase the rate of formation of carbon-oxygen complexes resulting in a drop in reaction rate and order of reaction. For chars generated at lower pressures (3.4 and 6.2 bar), lower partial pressure of CO2 at 25% and 50% seemed to have not completely saturated the active sites.

As a result, an increase in partial pressure increased the reaction rate and the order of reaction for those two chars. This is also related to the increased rate of surface area evolution for chars generated at 3.4 and 6.2 bar. A closer look at the new surface area and pore size generation, shown in Figure 6-7 and Table 6-3, reveal that there was an increase of 173 % and 76% in surface area for chars generated at 3.4 and 6.2 bar at 20% conversion, respectively. In the case of higher pressure chars, the surface area increase was 11%, 36 % and 56% for 11.3 bar char, 15.5 bar char and 21.7 bar char, respectively. It is noteworthy that most of the new surface area came from the microporous regions, while the mesoporous regions either declined or increased marginally. This could be due to the higher reaction rate in the microporous regions.

With the order of reaction, pre-exponential factor was determined by normalizing with both surface area and pressure. It was found that the pre-exponential factors were similar orders of magnitude. This shows pressure has limited effect on pre-exponential factor.

108

0.030 0.030

3.4 bar HPHFTR char 0.025 0.025 6.2 bar HPHTFR char 3.4 bar HPHTFR char_20% con 6.2 bar HPHTFR char_20% con

0.020 0.020

0.015 0.015

0.010 0.010 Pore volume Pore (cc/g) Pore volume Pore (cc/g)

0.005 0.005

0.000 0.000 10 100 1000 10000 10 100 1000 10000 Pore size distribution (Å) Pore size distribution (Å)

0.030 0.030 11.3 bar HPHTFR char 15.5 bar HPHTFR char 11.3 bar HPHFTR char_20% con 0.025 0.025 15.5 bar HPHTFR char_20% con

0.020 0.020

0.015 0.015

0.010 0.010 Pore volume Pore (cc/g) Pore volumePore (cc/g)

0.005 0.005

0.000 0.000 10 100 1000 10000 10 100 1000 10000 Pore size distribution (Å) Pore size distribution (Å)

109

0.030 21.7 bar HPHTFR char 21.7 bar HPHTFR char_20% con 0.024

0.018

0.012 Pore volume Pore (cc/g) 0.006

0.000 10 100 1000 10000 Pore size distribution (Å)

Figure 6-7: Pore size distribution of initial char and char after 20% conversion in HPTGA

Table 6-3: Changes in pore volume and surface area for the 20% converted char in HPTGA in comparison to the initial char

% Change Pore size distribution 3.4 bar 6.2 bar 11.3 bar 15.5 bar 21.7 bar Surface area 173 76 11 36 48 <30Å 73 30 3 25 14 30-500Å 17 -23 -5 12 4 >500Å 22 17 1 16 -2

110

6.4. Conclusions

The intinsic reactivity of soot and char generated from Pittsburgh No.8 coal over a range of pressures upto 21.7 bar in the HPHTFR was measured in the HPTGA. Here are the following conclusions:

 The intrinsic reactivity, obtained by normalizing with the surface area, showed less

dependence on char generation pressure. Lack of variation in microstrcutural features,

determined using XRD and Raman, confirms that the differences are due to changes in char

phsyical features.

 The activation energy and pre-exponential factor were determined using an nth order rate

equation. The activation energy was found to be insensitive to char generation pressure.

 The order of the reaction was obtained by varying the CO2 concentration from 25% to

100% of the total pressure. The order of reaction decreased with increased in pressure. The

order of reaction closely followed the surface area evolution

111

Summary of Major Findings and Recommendations

Chars have been generated from Pittsburgh No,8 coal at high temperatures and elevated pressures using a high-pressure, high-temperature flow reactor. The experimental conditions were designed to simulate an entrained-flow gasifier. The partially converted chars, contaminated with soot, were collected and analyzed for structural features and intrinsic reactivity. The intrinsic reactivity with

CO2 was determined for the chars generated over a range of pressures and for chars generated in various pyrolysis atmospheres. These data will be useful in the modelling and optimization of entrained-flow gasifier systems. The summary of the findings and the recommendations for future work are presented in the following sections:

7.1. Objective 1: Determine the effect of pyrolysis atmosphere on volatile yield and CO2 reactivity

of the char samples

Chars were obtained in three different pyrolysis atmospheres—Ar, CO2/N2 and N2—at 1100°C and 6.2 bar. The chars were analyzed for volatile yield (or conversion), CO2 reactivity, swelling ratio, chemical structural properties using Raman and XRD, and pore features using a surface area analyzer. The volatile yield was found to be highest for char generated in the CO2/N2 atmosphere, while there was no noticeable difference in volatile yield observed between the N2- and Ar-based environments. The conversions were found to be similar after washing the chars with THF. The difference in volatile yield with pyrolysis atmospheres was found to be due to tar or soot collected with the char. Except for the volatile yield, there were not much of differences in the N2 surface area, lattice parameters of the crystallite carbon, swelling ratio, and the defects to the graphite band ratio with pyrolysis atmospheres. The chars, before and after washing with THF, were heated to

900°C and 6.2 bar before kinetics were measured with CO2. Washing with THF showed 10-25% decline in reactivity when compared with reactivity obtained on chars before washing with THF.

112

The highest decline of 25% of reactivity was observed for N2. The intrinsic reactivity was determined by normalizing with the surface area of the char obtained at 10% conversion. The char generated in the CO2/N2 was found to be less reactive than the char generated in inert atmospheres.

Although there were differences in the intrinsic reactivities, the activation energies calculated from these chars were found to be independent of the pyrolysis atmosphere. The same result was observed for chars after washing with THF. The parameter that was found to be affected by the pyrolysis gas atmosphere was the order of reaction. The order of reaction followed the order:

CO2/N2> N2 ≈Ar. The order of the reaction was correlated with the surface area evolution of the char.

7.2. Objective 2: Determine the effect of reactor pressure, temperature, inorganic matter, and

particle size fraction on the structural features and the intrinsic reactivity of chars

No attempt has been made in the open literature to study the effects of pressure, temperature, particle density, and particle size relevant to that of the slurry-fed entrained-flow gasifier. This investigation explored the effects of these variables on the conversion, char structure and consequent effect on intrinsic reactivity. A summary is listed in Table 7-1.

7.3.Objective 3: Determine the intrinsic kinetics of chars generated at various pressures

The intinsic reactivity of char is an important parameters needed to develop a kinetic model and in combination with structural features of chars. The chars were generated over a range of pressures in the HPHTFR. The intrinsic kinetics of the chars were determined in the HPTGA. The reactor pressure showed no effect on the microstrcutural features of the char as determined by XRD and

Raman. The reactor pressure affects char porosity and pore volume which in turn affect the apparent reactivity. The char generated at 6.2 bar consistently showed the highest reactivity.

However, when normalized with N2 surface area, the pressure effect becomes less significant. The

113

activation energy and pre-exponential factor were determined using nth order rate equation. The activation energy was found to be insensitive to char generation pressure. The order of the reaction was obtained by varying the CO2 concentration from 25% to 100% of the total pressure. The order of reaction decreased with increase in pressure. The order of reaction closely followed the surface area.

Table 7-1: Effects of pressure, temperature, particle density, and particle size on conversion, structural features, and intrinsic reactivity

Predicting Response variable variable With increase Conversion Swelling ratio Surface area Group-I Intrinsic in char reactivity Pressure Not affected Increase up to Increase up to Increases up Unaffected 15.5 bar 11.3 bar and to 11.3 bar decreases Temperature Increase Decrease Reaches Reaches Decrease maximum at maximum at 11.3 bar and 11.3 bar and decrease decrease Density Unaffected for Decrease Decrease Decrease Increase low and middle density fractions. Increased for highest density fraction Particle size Decrease Unaffected Decrease Decrease Increase (With increase in particle size, conversion, surface area, and group-I char decreased, while the intrinsic reactivity increased. Swelling ratio was unaffected by particle size)

7.4. Recommendations for future work

Based on this study, the recommendations for future work are the following:

 The activation energies were not affected by pressure or by temperature (by comparing

kinetics of chars generated at 1100°C char and 1300°C at 6.2 bar). Gasification of a wide

suite of coals under different conditions can help establish this conclusion.

114

 No attempt has been made in the open literature to determine the high pressure kinetics of

chars generated from various density fractions. Although chars were generated as a part of

this work, the high pressure intrinsic kinetics was not obtained. The intrinsic kinetics of

chars with CO2 and H2O can be combined with char structural features reported in this

dissertation can be used to develop a comprehensive kinetic model.

 The chars obtained from different particle size fractions can be separated into narrow

particle size fractions to study the effect of char particle size distributions on high pressure

reactivity.

 The char structure data and kinetics can be combined to predict the char kinetics and the

data must be validated in an industrial or in a pilot-scale gasifier.

115

References

[1] H.W. Schiffer, World energy resources, in: World energy council report, 2016.

[2] A. Franco, A.R. Diaz, The future challenges for “clean coal technologies”: Joining efficiency increase and pollutant emission control, Energy, 34 (2009) 348-354.

[3] N. Holt, Gasification Technology Status-December 2006, in: EPRI and the Gasification users association report, Palo Alto, CA, USA, 2006.

[4] V. Krishnamoorthy, S. Pisupati, A Critical Review of Mineral Matter Related Issues during

Gasification of Coal in Fixed, Fluidized, and Entrained Flow Gasifiers, Energies, 8 (2015)

10430-10463.

[5] C. Higman, M. van der Burgt, Chapter 5 - Gasification Processes, in: Gasification (Second

Edition), Gulf Professional Publishing, Burlington, 2008, pp. 91-191.

[6] NETL-USDOE, Typical IGCC configuration,Retrieved on: 28th March, 2018, from: https://www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/igcc-config.

[7] A.G. Collot, Matching gasification technologies to coal properties, International Journal of

Coal Geology, 65 (2006) 191-212.

[8] A. Kristiansen, Understanding Coal Gasification, in, IEA Coal Research, London, 1996.

[9] C. Higman, Integrated Gasification Combined Cycle (IGCC) design considerations for high availability-‑ Vol1: Lessons from existing operations, in: EPRI report, Palo Alto, CA, USA,

2007.

[10] S.V. Pisupati, V. Krishnamoorthy, 2 - Utilization of coal in IGCC systems, in: Integrated

Gasification Combined Cycle (IGCC) Technologies, Woodhead Publishing, 2017, pp. 83-120.

[11] E.M. Hodge, The coal char-CO2 reaction at high temperature and high pressure., PhD thesis, in Chemical Engineering, University of New South Wales, 2009.

116

[12] D.A. Bell, B.F. Towler, M. Fan, Chapter 3 - Gasification Fundamentals, in: Coal

Gasification and Its Applications, William Andrew Publishing, Boston, 2011, pp. 35-71.

[13] A. Tremel, H. Spliethoff, Gasification kinetics during entrained flow gasification – Part III:

Modelling and optimisation of entrained flow gasifiers, Fuel, 107 (2013) 170-182.

[14] T.H. Fletcher, A.R. Kerstein, R.J. Pugmire, D.M. Grant, Chemical percolation model for devolatilization. 2. Temperature and heating rate effects on product yields, Energy & Fuels, 4

(1990) 54-60.

[15] D.M. Grant, R.J. Pugmire, T.H. Fletcher, A.R. Kerstein, Chemical model of coal devolatilization using percolation lattice statistics, Energy & Fuels, 3 (1989) 175-186.

[16] S. Niksa, A.R. Kerstein, FLASHCHAIN theory for rapid coal devolatilization kinetics. 1.

Formulation, Energy & Fuels, 5 (1991) 647-665.

[17] E.M. Hodge, D.G. Roberts, D.J. Harris, J.F. Stubington, The Significance of Char

Morphology to the Analysis of High-Temperature Char−CO2 Reaction Rates, Energy & Fuels,

24 (2010) 100-107.

[18] P.L. Walker, F. Rusinko, L.G. Austin, Gas Reactions of Carbon, in: D.D. Eley, P.W.

Selwood, P.B. Weisz (Eds.) Advances in Catalysis, Academic Press, 1959, pp. 133-221.

[19] J. Tanner, S. Bhattacharya, Kinetics of CO2 and steam gasification of Victorian brown coal chars, Chemical Engineering Journal, 285 (2016) 331-340.

[20] D.J. Harris, Char gasification reactivity: Measurement and application, in: Pittsburgh coal conference, Johannesburg, South Africa, 2007.

[21] H.D. Schilling, Coal Gasification: Existing Processes & New Developments, Springer

Netherlands, 1981.

117

[22] C. Higman, M. van der Burgt, Chapter 2 - The Thermodynamics of Gasification, in:

Gasification (Second Edition), Gulf Professional Publishing, Burlington, 2008, pp. 11-31.

[23] D.G. Roberts, D.J. Harris, T.F. Wall, On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity, Energy & Fuels, 17 (2003) 887-895.

[24] D.J. Harris, I.W. Smith, Intrinsic reactivity of petroleum coke and brown coal char to carbon dioxide, steam and oxygen, Symposium (International) on Combustion, 23 (1991) 1185-1190.

[25] J.L. Johnson, Fundamentals of coal gasification, in: M.A. Elliott (Ed.) Chemistry of coal utilization, Wiley, New York, 1981.

[26] N.M. Laurendeau, Heterogeneous kinetics of coal char gasification and combustion,

Progress in Energy and Combustion Science, 4 (1978) 221-270.

[27] N.R. Laine, F.J. Vastola, P.L. Walker, The importance of active surface area in the carbon- oxygen reaction, The Journal of Physical Chemistry, 67 (1963) 2030-2034.

[28] L.R. Radović, P.L. Walker, R.G. Jenkins, Importance of carbon active sites in the gasification of coal chars, Fuel, 62 (1983) 849-856.

[29] N. Jurtz, M. Kraume, D. Wehinger Gregor, Advances in fixed-bed reactor modeling using particle-resolved computational fluid dynamics (CFD), in: Reviews in Chemical Engineering,

2018.

[30] F. Kapteijn, R. Meijer, J.A. Moulijn, Transient kinetic techniques for detailed insight in gas- solid reactions, Energy & Fuels, 6 (1992) 494-497.

[31] S. Ergun, Kinetics of the Reaction of Carbon with Carbon Dioxide, The Journal of Physical

Chemistry, 60 (1956) 480-485.

[32] D.G. Roberts, D.J. Harris, High-Pressure Char Gasification Kinetics: CO Inhibition of the

C–CO2 Reaction, Energy & Fuels, 26 (2012) 176-184.

118

[33] J. Fermoso, C. Stevanov, B. Moghtaderi, B. Arias, C. Pevida, M.G. Plaza, F. Rubiera, J.J.

Pis, High-pressure gasification reactivity of biomass chars produced at different temperatures,

Journal of Analytical and Applied Pyrolysis, 85 (2009) 287-293.

[34] B. Feng, A. Jensen, S.K. Bhatia, K. Dam-Johansen, Activation Energy Distribution of

Thermal Annealing of a Bituminous Coal, Energy & Fuels, 17 (2003) 399-404.

[35] H.Y. Cai, A.J. Güell, I.N. Chatzakis, J.Y. Lim, D.R. Dugwell, R. Kandiyoti, Combustion reactivity and morphological change in coal chars: Effect of pyrolysis temperature, heating rate and pressure, Fuel, 75 (1996) 15-24.

[36] Y. Zhuo, R. Messenböck, A.G. Collot, A. Megaritis, N. Paterson, D.R. Dugwell, R.

Kandiyoti, Conversion of coal particles in pyrolysis and gasification: comparison of conversions in a pilot-scale gasifier and bench-scale test equipment, Fuel, 79 (2000) 793-802.

[37] C. Di Blasi, Combustion and gasification rates of lignocellulosic chars, Progress in Energy and Combustion Science, 35 (2009) 121-140.

[38] A. Tremel, T. Haselsteiner, M. Nakonz, H. Spliethoff, Coal and char properties in high temperature entrained flow gasification, Energy, 45 (2012) 176-182.

[39] V. Strezov, J.A. Lucas, T.F. Wall, Effect of pressure on the swelling of density separated coal particles, Fuel, 84 (2005) 1238-1245.

[40] J. Yu, J.A. Lucas, T.F. Wall, Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: A review, Progress in Energy and Combustion Science,

33 (2007) 135-170.

[41] A. Gilfillan, E. Lester, M. Cloke, C. Snape, The structure and reactivity of density separated coal fractions, Fuel, 78 (1999) 1639-1644.

119

[42] V.R. Gray, The role of explosive ejection in the pyrolysis of coal, Fuel, 67 (1988) 1298-

1304.

[43] J.G. Bailey, A. Tate, C.F.K. Diessel, T.F. Wall, A char morphology system with applications to coal combustion, Fuel, 69 (1990) 225-239.

[44] J. Yu, J. Lucas, V. Strezov, T. Wall, Swelling and Char Structures from Density Fractions of

Pulverized Coal, Energy & Fuels, 17 (2003) 1160-1174.

[45] C.W. Lee, R.G. Jenkins, H.H. Schobert, Structure and reactivity of char from elevated pressure pyrolysis of Illinois No. 6 bituminous coal, Energy & Fuels, 6 (1992) 40-47.

[46] K.E. Benfell, G.-S. Liu, D.G. Roberts, D.J. Harris, J.A. Lucas, J.G. Bailey, T.F. Wall,

Modeling char combustion: The influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char, Proceedings of the Combustion Institute, 28

(2000) 2233-2241.

[47] D. Zeng, T.H. Fletcher, Effects of Pressure on Coal Pyrolysis and Char Morphology, Energy

& Fuels, 19 (2005) 1828-1838.

[48] C.W. Lee, R.G. Jenkins, H.H. Schobert, Mechanisms and kinetics of rapid, elevated pressure pyrolysis of Illinois No. 6 bituminous coal, Energy & Fuels, 5 (1991) 547-555.

[49] H. Wu, G. Bryant, K. Benfell, T. Wall, An Experimental Study on the Effect of System

Pressure on Char Structure of an Australian Bituminous Coal, Energy & Fuels, 14 (2000) 282-

290.

[50] H. Wu, G. Bryant, T. Wall, The Effect of Pressure on Ash Formation during Pulverized

Coal Combustion, Energy & Fuels, 14 (2000) 745-750.

120

[51] G. Liu, P. Benyon, K.E. Benfell, G.W. Bryant, A.G. Tate, R.K. Boyd, D.J. Harris, T.F.

Wall, The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions, Fuel, 79 (2000) 617-626.

[52] D.H. Ahn, B.M. Gibbs, K.H. Ko, J.J. Kim, Gasification kinetics of an Indonesian subbituminous coal-char with CO2 at elevated pressure, Fuel, 80 (2001) 1651-1658.

[53] S. Kajitani, S. Hara, H. Matsuda, Gasification rate analysis of coal char with a pressurized drop tube furnace, Fuel, 81 (2002) 539-546.

[54] J. Yu, D. Harris, J. Lucas, D. Roberts, H. Wu, T. Wall, Effect of Pressure on Char

Formation during Pyrolysis of Pulverized Coal, Energy & Fuels, 18 (2004) 1346-1353.

[55] D.J. Harris, D.G. Roberts, D.G. Henderson, Gasification behaviour of Australian coals at high temperature and pressure, Fuel, 85 (2006) 134-142.

[56] D.G. Roberts, E.M. Hodge, D.J. Harris, J.F. Stubington, Kinetics of Char Gasification with

CO2 under Regime II Conditions: Effects of Temperature, Reactant, and Total Pressure, Energy

& Fuels, 24 (2010) 5300-5308.

[57] A. Tremel, H. Spliethoff, Gasification kinetics during entrained flow gasification – Part II:

Intrinsic char reaction rate and surface area development, Fuel, 107 (2013) 653-661.

[58] R.C. Shurtz, T.H. Fletcher, Coal Char-CO2 Gasification Measurements and Modeling in a

Pressurized Flat-Flame Burner, Energy & Fuels, 27 (2013) 3022-3038.

[59] A.D. Lewis, E.G. Fletcher, T.H. Fletcher, CO2 Gasification Rates of Petroleum Coke in a

Pressurized Flat-Flame Burner Entrained-Flow Reactor, Energy & Fuels, 28 (2014) 4447-4457.

[60] R.K. Rathnam, L.K. Elliott, T.F. Wall, Y. Liu, B. Moghtaderi, Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions, Fuel Processing Technology,

90 (2009) 797-802.

121

[61] A.G. Borrego, D. Alvarez, Comparison of Chars Obtained under Oxy-Fuel and

Conventional Pulverized Coal Combustion Atmospheres, Energy & Fuels, 21 (2007) 3171-3179.

[62] J. Brix, P.A. Jensen, A.D. Jensen, Coal devolatilization and char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres, Fuel, 89 (2010) 3373-3380.

[63] Q. Wang, R. Zhang, Z. Luo, M. Fang, K. Cen, Effects of Pyrolysis Atmosphere and

Temperature on Coal Char Characteristics and Gasification Reactivity, Energy Technology, 4

(2016) 543-550.

[64] V. Krishnamoorthy, S.V. Pisupati, Fate of Sulfur during Entrained-Flow Gasification of

Pittsburgh No. 8 Coal: Influence of Particle Size, Sulfur Forms, and Temperature, Energy &

Fuels, 30 (2016) 3241-3250.

[65] V. Krishnamoorthy, A.H. Tchapda, S.V. Pisupati, A study on fragmentation behavior, inorganic melt phase formation, and carbon loss during high temperature gasification of mineral matter rich fraction of Pittsburgh No. 8 coal, Fuel, 208 (2017) 247-259.

[66] E. Louw, Structure and combustion reactivity of inertinite-rich and vitrinite rich South

African coal chars: quantification of the structural factors contributing to reactivity differences,

Ph.D. Thesis, in Department of Energy and Mineral Engineering, The Pennsylvania State

University, University Park, Pa, 2013.

[67] M.D. Guillen, J. Blanco, J.S. Canga, C.G. Blanco, Study of the effectiveness of 27 organic solvents in the extraction of coal tar pitches, Energy & Fuels, 5 (1991) 188-192.

[68] G. Liu, H. Wu, R.P. Gupta, J.A. Lucas, A.G. Tate, T.F. Wall, Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion, Fuel, 79 (2000) 627-633.

[69] B. Feng, S.K. Bhatia, J.C. Barry, Variation of the Crystalline Structure of Coal Char during

Gasification, Energy & Fuels, 17 (2003) 744-754.

122

[70] C. Sheng, Char structure characterised by Raman spectroscopy and its correlations with combustion reactivity, Fuel, 86 (2007) 2316-2324.

[71] R.H. Hurt, A.F. Sarofim, J.P. Longwell, Gasification-induced densification of carbons:

From soot to form coke, Combustion and Flame, 95 (1993) 430-432.

[72] A.H. Tchapda, V. Krishnamoorthy, Y.D. Yeboah, S.V. Pisupati, Analysis of tars formed during co-pyrolysis of coal and biomass at high temperature in carbon dioxide atmosphere,

Journal of Analytical and Applied Pyrolysis, 128 (2017) 379-396.

[73] B. Feng, S.K. Bhatia, On the validity of thermogravimetric determination of carbon gasification kinetics, Chemical Engineering Science, 57 (2002) 2907-2920.

[74] C.W. Lee, A.W. Scaroni, R.G. Jenkins, Effect of pressure on the devolatilization and swelling behaviour of a softening coal during rapid heating, Fuel, 70 (1991) 957-965.

[75] N. Soundarrajan, N. Krishnamurthy, S. Pisupati, Characterization of Size and Density

Separated Fractions of a Bituminous Coal as a Feedstock for Entrained Slagging Gasification,

International Journal of Clean Coal and Energy, 2 (2013) 58-67.

[76] T.H. Fletcher, Swelling properties of coal chars during rapid pyrolysis and combustion,

Fuel, 72 (1993) 1485-1495.

[77] E. Lester, D. Alvarez, A.G. Borrego, B. Valentim, D. Flores, D.A. Clift, P. Rosenberg, B.

Kwiecinska, R. Barranco, H.I. Petersen, M. Mastalerz, K.S. Milenkova, C. Panaitescu, M.M.

Marques, A. Thompson, D. Watts, S. Hanson, G. Predeanu, M. Misz, T. Wu, The procedure used to develop a coal char classification—Commission III Combustion Working Group of the

International Committee for Coal and Organic Petrology, International Journal of Coal Geology,

81 (2010) 333-342.

123

[78] D. Alvarez, A.G. Borrego, R. Menéndez, Unbiased methods for the morphological description of char structures, Fuel, 76 (1997) 1241-1248.

[79] T.F. Wall, G.S. Liu, H.-w. Wu, D.G. Roberts, K.E. Benfell, S. Gupta, J.A. Lucas, D.J.

Harris, The effects of pressure on coal reactions during pulverised coal combustion and gasification, Progress in Energy and Combustion Science, 28 (2002) 405-433.

[80] M.R. Khan, R.G. Jenkins, Swelling and plastic properties of coal devolatilized at elevated pressures: an examination of the influences of coal type, Fuel, 65 (1986) 725-731.

[81] J.L. Yu, V. Strezov, J. Lucas, G.S. Liu, T. Wall, A mechanistic study on char structure evolution during coal devolatilization—Experiments and model predictions, Proceedings of the

Combustion Institute, 29 (2002) 467-473.

[82] T.K. Gale, C.H. Bartholomew, T.H. Fletcher, Decreases in the swelling and porosity of bituminous coals during devolatilization at high heating rates, Combustion and Flame, 100

(1995) 94-100.

[83] W. Mühlbauer, C. Zöllner, S. Lehmann, S. Lorenz, D. Brüggemann, Correlations between physicochemical properties of emitted diesel particulate matter and its reactivity, Combustion and Flame, 167 (2016) 39-51.

[84] J. Yu, V. Strezov, J. Lucas, T. Wall, Swelling behaviour of individual coal particles in the single particle reactor, Fuel, 82 (2003) 1977-1987.

[85] J. Ma, S.E. Zitney, Computational Fluid Dynamic Modeling of Entrained-Flow Gasifiers with Improved Physical and Chemical Submodels, Energy & Fuels, 26 (2012) 7195-7219.

[86] S.K. Bhatia, D.D. Perlmutter, A random pore model for fluid-solid reactions: I. Isothermal, kinetic control, AIChE Journal, 26 (1980) 379-386.

124

[87] T.K. Gale, C.H. Bartholomew, T.H. Fletcher, Effects of Pyrolysis Heating Rate on Intrinsic

Reactivities of Coal Chars, Energy & Fuels, 10 (1996) 766-775.

[88] J.C. Hower, Maceral/ microlithotype partitioning with particle size of pulverized coal:

Examples from power plants burning Central Appalachian and Illinois basin coals, International

Journal of Coal Geology, 73 (2008) 213-218.

[89] H. Gao, S. Murata, M. Nomura, M. Ishigaki, M. Qu, M. Tokuda, Experimental Observation and Image Analysis for Evaluation of Swelling and Fluidity of Single Coal Particles Heated with

CO2 Laser, Energy & Fuels, 11 (1997) 730-738.

[90] D. Yu, M. Xu, Y. Yu, X. Liu, Swelling Behavior of a Chinese Bituminous Coal at Different

Pyrolysis Temperatures, Energy & Fuels, 19 (2005) 2488-2494.

[91] P. Rosenberg, H.I. Petersen, E. Thomsen, Combustion char morphology related to combustion temperature and coal petrography, Fuel, 75 (1996) 1071-1082.

[92] D.G. Roberts, D.J. Harris, Char Gasification with O2, CO2, and H2O: Effects of Pressure on Intrinsic Reaction Kinetics, Energy & Fuels, 14 (2000) 483-489.

[93] J. Fermoso, B. Arias, C. Pevida, M.G. Plaza, F. Rubiera, J.J. Pis, Kinetic models comparison for steam gasification of different nature fuel chars, Journal of Thermal Analysis and

Calorimetry, 91 (2008) 779-786.

[94] K.B. Kabir, A. Tahmasebi, S. Bhattacharya, J. Yu, Intrinsic kinetics of CO2 gasification of a

Victorian coal char, Journal of Thermal Analysis and Calorimetry, 123 (2016) 1685-1694.

[95] O. Senneca, P. Russo, P. Salatino, S. Masi, The relevance of thermal annealing to the evolution of coal char gasification reactivity, Carbon, 35 (1997) 141-151.

125

[96] S. Potgieter‐Vermaak, N. Maledi, N. Wagner, J.H.P.V. Heerden, R.V. Grieken, J.H.

Potgieter, Raman spectroscopy for the analysis of coal: a review, Journal of Raman

Spectroscopy, 42 (2011) 123-129.

[97] X. Li, J. Hayashi, C.-Z. Li, FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal, Fuel, 85 (2006) 1700-1707.

[98] A. Tremel, H. Spliethoff, Gasification kinetics during entrained flow gasification – Part I;

Devolatilisation and char deactivation, Fuel, 103 (2013) 663-671.

[99] E. Bar-Ziv, A. Zaida, P. Salatino, O. Senneca, Diagnostics of carbon gasification by Raman microprobe spectroscopy, Proceedings of the Combustion Institute, 28 (2000) 2369-2374.

[100] S. Dutta, C.Y. Wen, R.J. Belt, Reactivity of Coal and Char. 1. In Carbon Dioxide

Atmosphere, Industrial & Engineering Chemistry Process Design and Development, 16 (1977)

20-30.

[101] D.G. Roberts, Intrinsic Reaction Kinetics of Coal Chars with Oxygen, Carbon Dioxide and

Steam at Elevated Pressures, PhD Thesis, in Department of Chemical Engineering, University of

Newcastle, New South Wales, Australia, 2000.

[102] J.P.G. Kehoe, R. Aris, Communications on the theory of diffusion and reaction — IX.

Internal pressure and forced flow for reactions with volume change, Chemical Engineering

Science, 28 (1973) 2094-2098.

[103] A.C. Beath, Mathematical Modelling of Entrained Flow Coal Gasification, Ph.D. Thesis, in Department of Chemical Engineering, University of Newcastle, New South Wales, Australia,

1996.

[104] E.N. Fuller, P.D. Schettler, J.C. Giddings, New method for prediction of binary gas-phase diffusion coefficients, Industrial & Engineering Chemistry, 58 (1966) 18-27.

126

Appendix A Reactor Description

The reactor used in the generation of high pressure chars is briefly explained in various publications. The reactor that was operated at atmospheric pressure was modified to operate at pressures of upto 21.7 bar and temperatures of up to 1500°C. The flow reactor has three sections: feeder section, reactor section, and char collection and gas analysis section. Figure 4.1 shows the process flow diagram of the reactor.

The feeding section comprises of the feeder that can separated from the main reactor so that it can be pressurized and depressurized to facilitate the cleaning and replenishing of the samples to be tested. The feeder is a novel design that can feed up to 30g/min of coal. The feeder hopper can store up to ~5 kg of coal in it. The feeder is designed in such a way that the sample can be replenished even when the feeder is under pressure using the lock column shown in Figure A-1.

The solid sample is entrained into the reactor using transport gas such as N2 or Ar.

The reactor consists of a ~58 mm diameter ceramic tube (also called the reaction tube) heated by

6 Kanthal Super RA electrical heating elements surrounded by refractory all encased in a water- cooled carbon steel pressure vessel. The total length of the heated reaction zone is 0.70 m. The secondary gas is preheated in the section between the reaction tube and the refractory. The gas temperature in the reaction tube is not measured directly. Instead, the B-type thermocouple continuously monitors the temperature of the wall of the reaction tube. The thermocouple is placed

1 cm away from the wall of the external surface of the reaction tube. In other words, the reactor temperature is the wall temperature. Besides the reaction tube, the reactor also has a movable water-cooled collection probe that can take samples of gas, tar and char at any height within the reaction tube and thereby allowing chars to be collected at various residence times.

127

The sampled gas and char are collected isokinetically, which is confirmed by the proportion of the gas flows through the collection probe (i.e., ~28% of gas flows through the probe corresponding to the cross-sectional area) and the bulk line (i.e., ~72% of volumetric gas flows through the bulk line). The sample is collected in a series of filters with the last filter having the mesh opening of either 1µm or 5 µm. The remaining gas and char, which is not removed by the sampling probe, passes through similar set of filters and is collected and stored separately. The pressure inside the reactor is maintained through the back-pressure regulator. The flow rate of the gas exiting the back-pressure regulator is constantly monitored using a rotameter. A portion of the gas exiting the back-pressure regulator is analyzed in the micro-Gas Chromatography (micro-GC). The gas is analyzed using two columns in the micro-GC—molecular sieve and plot U. The cycle time of the

GC was set about 90s for analysis.

Figure A-1: High pressure feeder (shown in left) and the reactor (shown in right)

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Steps to be followed before starting the reactor:

Pre-startup check list:

 The water to the reactor and the heating elements is critical without which the reactor

should not be operated at high temperatures and pressures. There are two chiller coolers

feeding cold water to the heating elements and for external surface cooling of the reactor.

 The first step is to ensure the chiller coolers are clean, and in safe working condition. Check

the status of the two chiller coolers by checking the refrigerant head pressure gauges. The

refrigerant head pressure for the larger and smaller chiller coolers should be around 300

psi, and 200 psi, respectively.

 Clean the tanks of the chiller coolers and water lines by flushing the deionized or distilled

water through the water lines and draining by running the chiller cooler for three times.

 The tanks of the chiller coolers may still have corroded products, which can be cleaned

with a rag soaked in dilute sulfuric acid (0.5 N) solution. The water filters must also be

cleaned while the tanks of the chiller coolers are cleaned.

 After cleaning the tank, distilled or deionized water is filled in the tanks and must be used

to flush and drain the water from the entire system. Keep repeating the process until the

filters are clean. The filter must be cleaned every time the water is drained out of the circuit.

Use only deionized/distilled water

 After cleaning the water system, the tanks are filled with deionized or distilled water and

the water must allowed to run through the system in closed circuit for about 30 minutes.

 Check the temperatures of the copper rods attached to the heating elements (see Figure A-

2) using an IR thermometer gun. The temperature of the collet through which water enters

the copper rods must be constantly observed until the temperature drops below 15°C.

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Higher temperature is an indication of water not flowing through the copper rod. If the

temperature is well above 20°C, move the collet forward or backward to see if the

temperature drops. This should fix the issue.

 If the temperature still does not drop, remove the water hoses connected to the heating

element rods. Flush the rods with diluted vinegar (1 gallon of vinegar in 3 gallons of

deionized water) to remove the corroded products. After flushing with vinegar, the copper

rod must be flushed with water for about 10 minutes. This should fix the issue.

 The next step is checking for leaks in the reactor upstream, reactor, and reactor

downstream. The reactor section is isolated from the upstream and downstream. This is

done by closing the ball valves at the inlet of the secondary line and to the injector. The

outlet of the reactor is the collection probe which is closed with a cap fitting. Make sure

the collection probe and the bulk line are tightly fastened to the holding bar.

 A leak in the reactor is determined by pressurizing the reactor to 500 psi. The reactor is

connected to the compressor from the preheater side bypassing the mass flow controllers.

Increase the pressure of the reactor at 500psi with a 100psi step and check for leaks after

each step.

 After pressure testing the reactor, calibrate the mass flow controllers with the reaction

gases. Get the calibration equation for each gas in each mass flow controller.

 Check the leak at the reactor upstream side by pressuring it to 500 psi. Similarly, pressurize

the downstream side to check for leaks.

 The other important task is to operate the reactor isokinetically. This is done by allowing

28% of gas through the sample line (also known as collection probe line) and the remaining

through the bulk line. The isokinetic flow rate must be checked with the layout to be used

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during experimentation. The flow rate through the sample line and the bulk line fitted with

rotameters can be checked by operating the reactor at low pressures (2-3 bar). The flow

rates can be controlled using a needle valve in one of the two lines. The needle valves once

set must not be changed throughout the experiment campaign. These tasks together

constitute pre-startup tasks.

During start-up

 Reactor start-up task begins with heating up the reactor to the desired temperature. It is

recommended to heat up the reactor manually.

 The reactor has a thick refractory castable and should be heated slowly. The heating rate

should never exceed 50°C/hr (or 5% increase in power every hour).

 Once the desired temperature was reached, the feed sample should be dropped in a column

connected to the hopper. The sample is not directly dropped to the hopper because it was

found that a small amount of sample gets fed into the reactor during pressurization of the

feeder.

 Ramp up the pressure in the reactor slowly by allowing secondary gas and primary gas. It

takes about 30-35 minutes to reach 300 psi at 1300°C. After the desired pressure is reached,

the gas must be allowed to flow through at the desired flow rate for 10-15 minutes. This

is done to remove oxygen that is present before the pressurization.

 The gas exiting the reactor must be analyzed for the gas composition using micro GC. If

the desired composition is reached after 10 minutes, the sample is fed into the hopper and

feeding can be started.

 The feeder is turned on to feed the sample into the reactor at the rate of ~3g/min.

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 When operating the reactor manually, the reactor temperature is monitored throughout the

gasification reaction. Due to endothermic nature of the reaction, the temperature of the

reactor drops by ~10-20 °C. The temperature where the reactor reaches the stable value is

assumed to be at steady state. When the feeding stops, the temperature of the reactor climbs

by ~20 °C. The increase in temperature is assumed to be the end of the reaction.

 At this point, the inlet gas mixture to the reactor is turned off. The reactor is depressurized

until it reaches atmospheric pressure.

Emergency procedure

Inadequate water supply to the reactor and the heating elements, and unexpected power shutdown can be detrimental to the reactor. Besides these, there can be other unexpected events that can cause emergency situations. So far, water supply disruption and unexpected power shutdowns were the causes of the emergency situations. During these events, the following emergency procedures must be performed:

 When the water supply to the reactor is disrupted due to the chiller cooler issue or due to

a water leak in one of the lines, the water supply must be shifted from the chiller cooler

supply to the domestic water supply. There are 15 ball valves that must be opened or

closed during the process as the water supply is shifted from the chiller cooler to the

domestic water supply. All the ball valves are clearly labelled and must be opened or

closed as per the number. It is necessary that an emergency drill simulating such a

situation is conducted before start-up of the reactor to feel comfortable about this task as

it should be done in less than 10 minutes.

 After the emergency is brought under control, the temperature of the reactor must be

slowly brought down to room temperature. Do not proceed with the experiments.

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 The other emergency is the unexpected and scheduled shutdowns. If the unexpected

shutdown happens, stop the experiment immediately. Bring down the temperature and

check for the reactor tube condition. It is likely the reactor tube has to be changed.

 It is imperative that the Energy Institute personals (Ronnie and Brad are the contact

persons) are intimated about the experimental plan so that scheduled power shutdown

does not interfere during experiments.

Collet

Figure A-2: Water filter (left) and the copper rods connecting the heating element (shown in right)

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Bulk line

Collection probe/Sample line

Particulate filter

Figure A-3: Downstream section of the reactor

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Appendix B Particle Velocity Calculation

Particle velocity was calculated to determine the particle residence time and determine if the reactor was operated as a drop tube or entrained-flow reactor. The particle velocity was calculated using a simple force balance among gravity, drag, and buoyancy. The force balance is given by:

푚 푎 = 휋푟휌 = 휋푟휌 − 휌 푔 + 휋푟휌 푉 − 푉 퐶 --B.1

Where a is acceleration and is a derivation of particle velocity (Vp) with respect to time. The mass of the particle is expressed as the product of volume of the particle and the particle density

(ρp). ρg is the density of the gas. Vg is the velocity of gas. The drag coefficient (CD) in B.1 is given by:

퐶 = + 0.5407 --B.2

The drag coefficient is valid for spheres with Reynolds number (Re) <6000. The Reynolds number is calculated by:

푅푒 = --B.3

Where µ is the gas viscosity in Pa.s, d is the diameter of the particle in m, and ρg is the gas density used in B1 and B3 was calculated by:

휌 = --B.4

135

Where P is the gas pressure in Pa, Mw is the molecular weight of the gas (kg mol-1), R is the gas

-1 -1 constant (8.314 J mol K ), and Tg is the gas temperature in K.

The particle velocity was calculated using equation B1 in MATLAB with ode45 solver. The initial particle velocity was assumed same as the velocity of the gas.

Table B-1: Particle velocity calculation parameters

Input parameters Values Pyrolysis gas N2 CO2/N2 Ar Particle Diameter 0.000125 m 0.000125 m 0.000125 m Peak temperature 1373 K 1373 K 1373 K Pressure 6.2 bar 6.2 bar 6.2 bar Gas density 1.52 kg/m3 1.96 2.17 Gas flow rate 54 slpm 54 slpm 54 slpm Gas velocity in the 0.276 m/s 0.276 m/s 0.276 m/s reactor Viscosity 4.78x10-5 Pa.s 4.78x10-5 Pa.s 4.78x10-5 Pa.s Calculated particle 0.258 m/s 0.259 m/s 0.258 m/s velocity (from Matlab)

Table B-2: Flow rates and velocities of particle and gas

Gas flow Velocity Temp(°C)/Pres. Particle Transport Secondary Total Vg Vp (bar) size (µm) (slpm) (slpm) (slpm) (m/s) (m/s) 1300/3.4 -150+106 6 27 33 0.310 0.290 1300/6.2 -150+106 7 45 52 0.304 0.292 1300/11.3 -150+106 10 74 84 0.270 0.258 1300/15.5 -150+106 15 92 107 0.251 0.244* 1300/21.7 -150+106 15 135 150 0.251 0.242*

1100/11.3 -150+106 13 87 100 0.281 0.265 1300/11.3 -150+106 10 75 84 0.270 0.258 1400/11.3 -150+106 10 71 81 0.271 0.263

1300/11.3 -425+212 10 77 87 0.281 0.264 1300/11.3 -212+150 10 77 87 0.277 0.252 1300/11.3 -106+75 10 77 87 0.277 0.27 * reaction zone:0.58 m; Vg and Vp are gas and particle velocities

136

Appendix C Tar and Soot Separation

The char samples collected from the reactor also contained extremely large, low density soot/tar agglomerates. The soot and tar agglomerates were extremely low in density compared to the char particles. The amount of these agglomerates formed increased with increase in temperature. The agglomerates were formed as the experiments were carried out in the absence of oxygen and the filters were not heated. These two factors resulted in the collection of substantial tar and soot along with char particles. Figure C-1 shows the large agglomerates of soot and tar along with char particles. To separate tar and soot from char particles, each char sample was washed in

Tetrahydrofuran (THF). The procedure used in removing soot and tar from char particles is as follows:

1. A small amount of char sample (<0.5g) was added to a beaker along with 125 ml of THF.

2. The slurry was allowed to stand for about 15 minutes until the liquid becomes clear.

3. The slurry was slowly heated to its boiling point in a hot plate. To avoid breaking the char

particles, the slurry was not physically stirred. However, boiling of the liquid ensured

sufficient turbulence within the beaker.

4. The slurry was made to boil for about 2-3 minutes before filtering the particulates on

Whatman filter paper. Filtering the particulates while the slurry was hot was done to ensure

no soot/tar settle either on the filter paper or on the particles. Figure C-1 shows the filtrate

after washing a char sample. The color change clearly indicates dissolution of tar or soot

into the THF.

137

Figure C-1: A photograph of a sample generated from the -150+106 µm fraction at 1100°C Soot agglomerates (left) and the THF soluble tars (right). The chars were analyzed using a proximate analyzer before and after washing with THF. The proximate analysis is shown in Table C-1. An increase in ash content in char indicates removal of tar and soot.

Table C-1: Proximate analysis of chars before and after washing with THF

HPHTFR Before washing with THF After washing with THF Pressure VMd FCd Ashd Surface VMd FCd Ashd Surface (bar) (%) (%) (%) area (%) (%) (%) area (m2/g) (m2/g) 3.4 15.4 73.2 11.4 137 17.8 70.0 12.2 109 6.2 19.3 69.7 11.0 263 19.2 68.1 12.7 232 11.3 23.6 63.8 12.6 352 19.6 64.7 15.7 332 15.5 21.5 66.5 12.0 307 19.1 66.7 14.2 289 21.7 22.1 64.0 13.9 131 18.2 65.9 15.9 170 Subscript d stands for dry basis

Louw used the Soxhlet apparatus for removing tar from the char samples [66]. This was done by washing the sample with THF for 24 hours. However, in this case, only about 15 minutes was used to remove the residual tar. To compare the extractability between the two techniques, a 21.7 bar

138

char sample washed with THF in the Soxhlet apparatus and the proximate analysis data were determined. The results presented in Table C-2 show minimal difference between the procedures.

Table C-2: Proximate analysis of chars washed by the above procedure and by Soxhlet apparatus

THF washing VMd (%) FCd Ashd (%) method (%) Current procedure 18.2 65.9 15.9 Soxhlet appartus 18.3 64.2 17.5 .

139

Appendix D High Pressure TGA Data Processing

The weight loss data obtained from the HPTGA was not smooth. The conversion profile calculated using the weight loss data for 6.2 bar char is shown in Figure D-1. To obtain meaningful data, the conversion profile was smoothened using regression analysis in Microsoft excel. The isothermal gasification step was then normalized assuming the char was at 0% conversion at the beginning of the step and conversion on an ash-free basis versus time was obtained.

80 80

y = -0.0048x2 + 1.2355x 60 60 R² = 0.9992 (%)

40 40

20 20 ovrin(%) Conversion Conversion

0 0 0 20 40 60 80 100 0 20 40 60 80 100 Time (min) Time (min)

Figure D-1: Data processing for char data generated at 875°C and 6.2 bar in HPTGA (Coal sample: -150+106 µm generated in the HPHTFR at 1100°C and 6.2 bar in CO2/N2)

140

Appendix E Effectiveness Factor Calculation

The effectiveness factor (η) can be defined as the ratio of the measured reaction rate to the chemical reaction rate (i.e., reaction rate without diffusion control) at a given temperature. In other words, it is the ratio of surface involved in the reaction to the total surface available for the reaction. In the Regime-I conditions (chemical reaction rate control), the effectiveness factor is 1. This ratio decreases to 0.5 and 0 when Regime-II (pore diffusion control) and Regime-III (mass transfer rate control) controls the reaction rate, respectively. The effectiveness factor is calculated using the

Thiele modulus. The Thiele modulus is a function of particle shape factor, reaction gas concentration, bulk gas diffusivity, reaction rate, and the order of the reaction. The Thiele modulus modelled by Kehoe and Aries [102], modified by Beath [103] , and tabulated by Hodge et al. [17] is used in calculation. The effectiveness factor was calculated assuming the flat-plate model due to high swelling ratio of char particles.

The Thiele modulus for the flat-plate geometry is given by:

√() ∅ = --E-1

Where, R is the gas constant, Tp is the particle temperature, k is the intrinsic reactivity, P is the reactant gas pressure, n is the order of the reaction, S is the internal mass specific area, σT is the true density of the particle and assumed to be 2000 kg/m3, a is the molar ratio of reactant gas to

carbon (i.e., 1 for C-CO2 reaction), α is the ratio of Knudsen to molecular diffusivity (훼 = )

141

M is the molecular weight of carbon, b is the molar ratio of product gas to carbon, L is the thickness of the wall and assumed to be 7.5 µm

The effectiveness factor is calculated based on the equation:

휂 = --E-2

Where Deff is the effective diffusivity:

퐷 =( + ) --E-3

Where Dk is the Knudsen diffusion coefficient calculated using:

퐷 = --E-4

d is the diameter of the pore (in m), Tg is the temperature of the gas (K), MA is the molecular weight of CO2.

DAB is the molecular diffusion coefficient for CO2-CO system calculated using Fuller, Schettler and Giddings equation:

.. . --E-5 퐷 = × +

142

MA is the molecular weight of CO2, MB is the molecular weight of CO, VA is the diffusion volume of CO2, VB is the diffusion volume of CO. Diffusion volumes can be obtained from the publication of Fuller et al. [104]. ε is the porosity and is assumed to be 0.8, τ is the tortuosity and assumed to be 1.414.

The effectiveness factor calculations for 3.4 and 6.2 bar chars generated at 1300°C is shown in

Figure E-1.

1.0 3.4 bar 6.2 bar

0.9

0.8

0.7

0.6

0.5 Effectivenessfactor 0.4

0.3

0.2 900 1000 1100 1200 1300 1400 1500 Temperature (°C)

Figure E-1: Effectiveness factor variations with temperature for 3.4 and 6.2 bar chars

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Appendix F Uncertainty Analysis

Uncertainty in the kinetic parameters is determined by repeating the entire set of experiments at least three times. The error (E) associated with the mean of the activation energy and pre- exponential factor with a 95% confidence is determined using following equation based on the t- distribution:

퐸 =±푡( ) --F-1 √

Example: Kinetic parameters of char-CO2 reaction was measured for the sample pyrolyzed in N2 at 1100°C and 6.2 bar in the high pressure TGA.

Table F-1: Kinetic parameters of the char sample from three runs

Set-1 Set-2 Set-3 Temperature Range 825-875°C 825-875°C 825-875°C Sample mass ~35mg ~35mg ~35mg Ea (kJ/mol) 231 225 231 A (s-1) 7.6x106 4.3x106 8.9x106

Degrees of freedom: 2 (i.e, Sample size-1=3-1); t95%=4.303 (obtained from the two-tailed t- distribution table)

Average Ea = (231+225+231)/3= 229 kJ/mol

Standard deviation for Ea = 3.5 kJ/mol

Error = (4.303*3.5)/1.732 = ±8.7 kJ/mol

The mean activation energy with 95% confidence interval is 229±8.7 kJ/mol

Similarly, the mean pre-exponential factor with 95% confidence interval is 6.8x106±5.9x106 s-1

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Appendix G Feed Particle Size Distribution to a Slurry-Fed Gasifier

A representative feed particle size distribution to a commercial slurry-fed gasifier is reported by

Nari et al. [75]. To get the feed particle size distribution, the Pittsburgh No.8 coal was dry-ground in a pilot-scale rod mill. The feed distribution is shown in Table G-1.

Table G-1: Representative yield of narrow size fractions covering the feed particle size distribution to a slurry-fed gasifier

Particle size fraction -75 -106+75 -150+106 -212+150 -425+212 -600+425 +600 (µm)

Yield (wt. %) 28.9 8.9 9.5 11.2 26.2 10.2 5.1

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Appendix H Specific Contributions Toward Designing and Construction of the High- Pressure, High-Temperature Flow Reactor

The design and construction of the HPHTFR is not possible without the contributions of Dr. Aime

Tchapda and Dr. Nandakumar. While their contributions are significant, my specific contributions also ensured that the reactor operates in a reliable and consistent manner.

Feeder

The initial design of the feeder did not provide a consistent feeding of the sample. This was a result of turbulence within the feeder creating a fluidizing environment in the hopper. A series of experiments conducted on the feeder revealed that the turbulence was a result of step around the transport gas route in the feeder. The step in the feeder was removed based on the recommendation.

The feeder provided a consistent feeding after the removal of the step.

While running the atmospheric pressure experiments, it was found that a small amount of sample was fed into the reactor in an uncontrollable manner immediately after the start of the transport gas. The turbulence was suspected due to large volumetric flow of transport gas through the feeder.

To avoid the uncontrollable flow during the early stages, a column (shown in Figure H-1) was added to the feeder. This allows sample to be fed into the hopper minutes before start of the feeding. No uncontrollable flow was observed when the sample was dropped in the hopper after stabilization of flow.

Gas Dosing System

The gas dosing system is an important component to the HPHTFR. The entire gas dosing system and the interlocks were designed based on the necessary flow rates required to achieve entrained flow conditions. The picture of the gas dosing system is shown in Figure H-2.

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Column

Load cell Figure H-1: A side view picture of the feeder

Figure H-2: A) Gas dosing system (left) B) Gas flow controllers (Right)

Curing the refractory

One of the major issues faced while building the reactor was curing of the refractory. There were several blocks of refractory were casted. A thick block of refractory means predisposition for crack propagation. The crack propagation in a small refractory castable is shown in Figure H-3. The

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crack propagation was controlled by cooling the entire section of the refractory castable with a mixture of ice and water.

Figure H-3: A) Fresh refractory castable (left) B) Refractory castable with cracks after drying

Water-cooling system design

The reactor shell temperature reached above 90°C when the reactor was operated at 1300°C. In certain pockets of the reactor, the temperature breached 100°C. A very high shell temperature can affect the longevity of the shell and can lead to other issues. Based on the heat transfer calculations, the water cooling system was designed. With the new water-cooling system in place, the shell temperature did not exceed 20°C.

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VITA

EDUCATION

DOCTOR OF PHILOSOPHY IN ENERGY AND MINERAL ENGINEERING (2011-2018) The Pennsylvania State University, University Park, USA

MASTER OF SCIENCE IN ENERGY AND MINERAL ENGINEERING (2008-2010) The Pennsylvania State University, University Park, USA

BACHELOR OF ENGINEERING IN CHEMICAL ENGINEERING (2002-2006) S.V. National Institute of Technology, Surat, India

HONORS AND AWARDS

 Outstanding graduate teaching assistant award (2017)  Charles B. Darrow award in fuel science (2017)  Frank and Lucy Rusinko graduate fellowship (2017)  Outstanding paper award at the 69th Indian Chemical Engineering Congress, Chennai, India (2016)  Delivered a guest lecture entitled “Ash formation during combustion and gasification” at Bharath Heavy Electricals Limited, Trichy, India (2012)

SELECTED PUBLICATIONS AND PRESENTATIONS

 A.H. Tchapda, V. Krishnamoorthy et al., Analysis of tars formed during co-pyrolysis of coal and biomass at high temperature in carbon dioxide atmosphere, Journal of Analytical and Applied Pyrolysis, 2017, 128, 379  V. Krishnamoorthy, A.H. Tchapda, S.V. Pisupati, A study on fragmentation behavior, melt phase formation, and carbon loss during high temperature gasification of mineral-matter rich fraction of Pittsburgh no.8 coal, Fuel, 2017, 208, 247  S.V. Pisupati, V. Krishnamoorthy, Utilization of coal in IGCC systems, in Integrated gasification combined cycle technologies (Editors: Ting Wang and Gary J. Stiegel). Woodhead publishing, 2017  V. Krishnamoorthy, S.V. Pisupati, Fate of sulfur during entrained-flow gasification of Pittsburgh no. 8 coal: Influence of particle size, sulfur forms, and temperature. Energy & Fuels 2016, 30, 3241  V. Krishnamoorthy, S.V. Pisupati, A critical review of mineral matter related issues during gasification of coal in fixed, fluidized, and entrained flow gasifiers. Energies 2015, 8, 10430  V. Krishnamoorthy, S.V. Pisupati, Effect of initial particle sizes on the transformations of mineral matter rich fractions of coal during entrained flow gasification. 7th International Freiberg Conference on IGCC & XtL Technologies, Huhhot, China, 2015  V. Krishnamoorthy, S.V. Pisupati, Sulfur release behavior during entrained flow gasification of coal. 7th IEA CCC conference on clean coal technologies, Krakow, Poland, 2015