Potassium Catalyst Mobility and its Effect on Co-Gasification Kinetics

by

Jill Lam

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Jill Lam 2016

Potassium Catalyst Mobility and its Effect on Co-Gasification Kinetics

Jill Lam

Master of Applied Science

Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto

2016

Abstract

The mobility of potassium catalyst in various gasification environments was investigated. Tools

and methods were developed for mobility studies: (1) intra-particle and inter-particle mobility of

potassium was observed with imaging Time-of-Flight – Secondary Ion Mass Spectrometry, (2) a

versatile, bench-scale, flow system was designed to study the effect of reaction conditions, feed

injection, and pretreatment protocols on the gasification kinetics. Mixed feeds of non-catalyzed

and catalyzed feedstocks showed a variety of behaviour attributed to inter-particle mobility.

Synergistic behaviour, where total gasification profile improved over the combined separate

reactivity, was observed. In other cases, a decrease in combined performance (antagonism) was

attributed to catalyst contact with aluminosilicate poisons in certain feedstocks. Preheating of

chars, allowing pre-dispersion of potassium, produced higher reactivity than sudden injection.

The latter better simulates the reaction history of particles in fluidized-bed gasifiers and indicates

that catalyst mobility kinetics will have an effect on gasifier performance.

ii

Acknowledgments I wish to express my sincerest gratitude to my supervisor, Professor Charles Mims, for providing

the opportunity of this fantastic learning experience. Your guidance, knowledge, and

encouragement were invaluable throughout this project.

In addition, I would like to thank our numerous collaborators and colleagues at the University of

Calgary, University of British Columbia, Nexterra Energy Corp., and University of Toronto for

their extensive knowledge and mutual support in this subject. Also, I extend my appreciation to

Peter Brodersen and SI Ontario for their contributions in the ToF-SIMS study.

I would thank my defense committee, Professor Cathy Chin and Professor Bradley Saville, for

their time and insight regarding this project.

Finally, I would like to thank my family and friends for their continual love and support in

everything I do.

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Table of Contents Acknowledgments .......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................. vi

List of Figures ............................................................................................................................... vii

Chapter 1 Introduction .................................................................................................................... 1

1.1. Thesis Overview .................................................................................................................. 1

1.2. Statement of Objectives ...................................................................................................... 2

Chapter 2 Background & Literature Review .................................................................................. 4

2.1. Gasification ......................................................................................................................... 4

2.1.1. Process ...................................................................................................................... 4

2.1.2. Catalysts .................................................................................................................... 7

2.2. Reaction Mechanism & Kinetics ........................................................................................ 9

2.3. Carbon Substrates & Feedstocks ....................................................................................... 11

2.3.1. Model Carbons ........................................................................................................ 11

2.3.2. Coal ......................................................................................................................... 11

2.3.3. Biomass ................................................................................................................... 14

2.4. Gasification of Binary Feedstocks .................................................................................... 16

Chapter 3 Experimental Design & Protocols ................................................................................ 18

3.1. Materials ............................................................................................................................ 18

3.1.1. Precursors ................................................................................................................ 18

3.1.2. Char and Carbon Preparation .................................................................................. 18

3.2. System ............................................................................................................................... 20

3.2.1. Steam Generator ...................................................................................................... 23

3.2.2. Drop Mechanism ..................................................................................................... 23

iv

3.2.3. Reactor Configurations ........................................................................................... 24

3.3. Protocols ............................................................................................................................ 26

3.3.1. Mode I: Intra- and Inter-Particle Potassium Mobility ............................................. 26

3.3.2. Mode II: Gasification Kinetics in a Flow System ................................................... 26

Chapter 4 ToF-SIMS Surface Analysis of Potassium Mobility .................................................... 28

4.1. Intra-Particle Mobility ....................................................................................................... 28

4.2. Inter-Particle Mobility ....................................................................................................... 30

Chapter 5 Kinetic Characterization of Individual Carbon Substrates ........................................... 33

5.1. Feedstock Characterization ............................................................................................... 33

5.2. Kinetic Analysis of Steam Gasification ............................................................................ 34

Chapter 6 Kinetics of Binary Blends: Admixtures of Carbon Substrates ..................................... 38

6.1. Direct Introduction to Steam, Gasification Atmosphere ................................................... 38

6.2. Thermal Conditioning in Inert, Argon Atmosphere .......................................................... 42

6.2.1. Switchgrass Char with Model Spherocarbon .......................................................... 43

6.2.2. Switchgrass Char with Coal Char ........................................................................... 45

6.3. Extent of Potassium Mobility on Synergy ........................................................................ 50

Chapter 7 Conclusions & Recommendations ............................................................................... 51

7.1. Conclusions ....................................................................................................................... 51

7.2. Recommendations ............................................................................................................. 52

References ..................................................................................................................................... 54

v

List of Tables Table 2-1: Chemical Reactions in a Gasification process .............................................................. 6

Table 2-2: Classification of Coal (adapted from Kabe, Ishihara, & Qian, 2004) ......................... 12

Table 2-3: Mineral Matter Constituents of Coal (reproduced from Laurendeau, 1978) .............. 13

Table 2-4: Structural Constituents of Selected Biomass (reproduced from McKendry, 2002) .... 15

Table 5-1: Proximate, Ultimate, and Ash Analysis of Switchgrass and Coal (received from

Intertek and Loring) ...................................................................................................................... 33

vi

List of Figures Figure 2-1: Reaction Sequence for Gasification (reproduced from Weiland, Means, & Morreale,

2012) ............................................................................................................................................... 4

Figure 3-1: Schematic of Pyrolysis System .................................................................................. 19

Figure 3-2: Schematic of Bench-Scale, Flow System .................................................................. 20

Figure 3-3: Bench-Scale Gasification Apparatus (Top-Left: Overview of Apparatus, Top-Right:

2-Way Switching Valve Flow Tubing, Bottom-Left: Mass Flow Controllers, Bottom-Right:

Water Traps and Sampling Lines) ................................................................................................ 22

Figure 3-4: Platinum-Catalyzed Steam Generator ........................................................................ 23

Figure 3-5: Drop Mechanism for Rapid Introduction of Carbon Samples into the Reactor ......... 24

Figure 3-6: Multiple Configurations of Reactor System .............................................................. 25

Figure 4-1: ToF-SIMS Surface Analysis of Intra-Particle Potassium Mobility on Poly(furfuryl

Alcohol) Coke ............................................................................................................................... 29

Figure 4-2: ToF-SIMS SEM Surface Analysis of Inter-Particle Potassium Mobility of Potassium

Carbonate Impregnated Spherocarbon (SCK) with Poly(furfuryl Alcohol) Coke (PC), Panel (a)-

(b), and Switchgrass Char (SGC) with Spherocarbon (SC), Panel (c)-(d) ................................... 31

Figure 5-1: Switchgrass Char, Coal Char, and Spherocarbon Conversion versus Time during

Steam Gasification at 850°C ......................................................................................................... 35

Figure 5-2: Switchgrass Char, Coal Char, and Spherocarbon Reaction Rate versus Conversion

during Steam Gasification at 850°C ............................................................................................. 36

Figure 6-1: Switchgrass Char, Spherocarbon, and 1:1 Switchgrass Char-Spherocarbon

Conversion versus Time during Steam Gasification at 850°C ..................................................... 39

vii

Figure 6-2: Switchgrass Char, Coal Char, 1:1 Switchgrass Char-Coal Char, Panel (a), and 3:1

Switchgrass Char-Coal Char Conversion, Panel (b), versus Time during Steam Gasification at

850°C ............................................................................................................................................ 40

Figure 6-3: Switchgrass Char, Spherocarbon, 1:1 Switchgrass Char-Spherocabon Direct Drop,

and 1:1 Switchgrass Char-Spherocarbon Heat Soak Conversion versus Time during Steam

Gasification at 850°C .................................................................................................................... 43

Figure 6-4: Switchgrass Char, Spherocarbon, 1:1 Switchgrass Char-Spherocarbon Direct Drop,

and 1:1 Switchgrass Char-Spherocarbon Heat Soak Reaction Rate versus Conversion during

Steam Gasification at 850°C ......................................................................................................... 44

Figure 6-6: Switchgrass Char, Coal Char, 3:1 Switchgrass Char-Coal Char Direct Drop, and 3:1

Switchgrass Char-Coal Char Heat Soak Conversion versus Time during Steam Gasification at

850°C ............................................................................................................................................ 46

Figure 6-7: Switchgrass Char, Coal Char, 1:1 Switchgrass Char-Coal Char Direct Drop, and 1:1

Switchgrass Char-Coal Char Heat Soak Reaction Rate versus Conversion during Steam

Gasification at 850°C .................................................................................................................... 48

Figure 6-8: Switchgrass Char, Coal Char, 3:1 Switchgrass Char-Coal Char Direct Drop, and 3:1

Switchgrass Char-Coal Char Heat Soak Reaction Rate versus Conversion during Steam

Gasification at 850°C .................................................................................................................... 49

viii

1

Chapter 1

Introduction

1.1. Thesis Overview

Shifting global trends emphasizes the need to address resource scarcity, energy security,

sustainability, and environmental impact. With recoverable reserves estimated at 6.6 and 233

billion tonnes within Canada and the United States, respectively, coal has the potential and

capacity to be a major energy resource within North America (National Energy Board [NEB],

2013; U.S. Energy Information Administration [EIA], 2013). Presently, coal is primarily utilized

in steam, turbine-generated electricity (International Energy Agency [IEA], 2012). Coal-fired

electricity encounters limitations in efficiency and diversity with significant environmental

impact, in regards to carbon emissions and air pollution, which has motivated government

strategies to retire coal-fired power plants (EIA, 2014; NEB, 2013).

Gasification is an alternative strategy for utilizing coal that has long been the focus of research,

development, and technological innovation. Gasification is the thermo-chemical conversion of

coal or other carbonaceous solids to a gaseous form of hydrogen, carbon monoxide, and

methane. This product gas can be transported with existing infrastructure, converted to heat or

electricity, transformed to liquid fuels via Fischer-Tropsch process, or separated to its individual

components for specific applications (Higman & van der Burgt, 2008). Furthermore, the

inclusion of renewable biomass as a feedstock presents opportunities to reduce the net carbon

footprint of the process.

A significant technological limitation of gasification is the high operating temperatures in excess

of 500°C. The thermodynamics of synthesis gas (CO and H2) production requires a high

operating temperature (800°C+) to allow a high reaction conversion, but even at these

temperatures, the kinetics of gasification are limiting (National Research Council (U.S.) &

Lowry, 1963). This operating parameter presents issues with energy input, carbon emissions,

material construction, and safety considerations. Addition of catalysts can increase the economic

feasibility of some feedstocks, as it can reduce this operating temperature while maintaining

comparable reaction rates. Furthermore, methane as a gasification product is thermodynamically

2

feasible at lower temperatures and its production increases the heating value of the gas product

(Brown, Liu, & Norton, 2000; Gangwal & Truesdale, 1980; Lang & Neavel, 1982).

Alkali metals, in particular potassium, exhibit a catalytic effect on both carbon gasification rates

and the production of methane in a gasifier. The mobility of the alkali catalyst is a key feature

that allows the formation of new active sites as the carbon is gasified (Lobo, 2013). Previous

studies have indicated substantial catalyst mobility that may be kinetically important under

certain conditions (Baker, 1979; Coates, Evans, Cabrera, Somorjai, & Heinemann, 1983; Lobo,

2013; Mims, Chludzinski, Pabst, & Baker, 1984). Mobility allows the formation of new active

sites within a particle (intra-particle mobility), which determines the burn-off profile of

individual particles (Mims & Pabst, 1980). Inter-particle mobility allows catalyst to move

between particles thereby allowing a catalyzed portion of gasifier feed to effectively catalyze the

entire char bed. Alkali metals are naturally present in some biomass feedstocks, such as

switchgrass, wood waste, and co-gasification of such feedstocks with other renewable and non-

renewable feedstocks was a main target of this work (Brown et al., 2000). In this and other

works, co-gasification of biomass and coal has exhibited both synergistic and inhibition effects

attributed to catalyst mobility (Brown et al., 2000; Collot, Zhuo, Dugwell, & Kandiyoti, 1999;

Habibi et al., 2013b; Sjöström, Chen, Brage, & Rosén, 1999).

1.2. Statement of Objectives

This research at the University of Toronto was funded as part of a collaboration amongst

industry and two other universities, University of Calgary and University of British Columbia.

Work regarding the co-feeding of renewable resources (switchgrass, wood waste, biosolids, etc.)

was funded by an NSERC strategic grant and involved close collaboration with Nexterra who

operate gasifiers in many locations, including the campus of the University of British Columbia.

Similar collaborative work on fossil resources was also funded in part by Carbon Management

Canada, with a mandate to lower the carbon footprint of fossil fuel usage and matching CO2

sequestration schemes. This research has resulted in data presented in two publications. The

body of this thesis describes the main efforts here at the University of Toronto.

3

The co-gasification of biomass with coal presents a promising development in gasification

technology. The inclusion of renewable biomass reduces the net carbon emissions of the process

and contributes catalytic species, such as potassium and calcium, that are inherent in biomass

mineral matter and do not require recovery. It has been suggested that mobility of alkali metal

catalysts, particularly potassium, may be the mode of contact for enhanced reactivity (Lobo,

2013). Catalyst mobility, dispersion, and migration rates have a broad implication on potential

synergy and kinetics of co-gasification, where the intimate contact and application of catalytic

species may be limited by the materials and techniques employed. Understanding of the inherent

mobility kinetics from intra-particle, inter-particle, and macroscopic perspectives may translate

into practical and technological improvements for continued innovation of gasification.

In order to study the catalyst mobility of potassium and its effect on co-gasification, the

following objectives were defined for this research:

1. Design and implement a bench-scale, flow system with a range of parameters for the

gasification of carbonaceous materials.

2. Examine intra-particle and inter-particle mobility of potassium catalyst with a Time-of-

Flight Secondary Ion Mass Spectrometer.

3. Determine switchgrass char, coal char, and model carbon characteristics and steam

gasification kinetics to establish baselines for comparison with binary blends.

4. Investigate the effect of potassium catalyst mobility on the kinetics of binary blends of

switchgrass char with coal char/model carbon during steam gasification.

4

Chapter 2

Background & Literature Review

Gasification is a process to convert solid fuel into gaseous form, which has been employed for

over a century. The process is capable of converting a myriad of feedstocks into a versatile gas

product for energy, transport, and/or chemicals. Individual feedstocks present technological

advantages and limitations depending on the application and process units employed.

Incorporation of catalysts can provide fundamental improvement to the process, particularly in

regards to reducing operating temperatures.

2.1. Gasification

Gasification is the conversion of solid carbonaceous material with a gasifying agent to produce a

fuel-rich gas consisting of primarily of hydrogen (H2) and carbon monoxide (CO) with carbon

dioxide (CO2), methane (CH4). This resulting gas mixture is designated as synthesis gas

(syngas). Oxidizing agents of oxygen (O2), steam (H2O), and CO2 and mixtures thereof are

employed in this process.

2.1.1. Process

The process of gasifying solid carbonaceous material involves drying, pyrolysis, and

gasification, as seen in Figure 2-1. These stages are not necessarily in series and boundaries may

overlap in any given system.

Figure 2-1: Reaction Sequence for Gasification (reproduced from Weiland, Means, &

Morreale, 2012)

5

Drying occurs to remove moisture from the feedstock material and preheat the material for

subsequent stages. The removal of feedstock moisture content is of key importance in balancing

the thermal requirements of the process, particularly for biomass.

H2O(l) → H2O(g) ΔH°vap = 43.99 kJ/gmol 2-1

Pyrolysis involves the thermal decomposition of the carbonaceous material releasing volatiles

consisting of low-molecular-weight gases and high-molecular-weight, condensable

hydrocarbons, i.e. tars. Depending on the pyrolysis conditions, these high-molecular-weight

volatiles can undergo further cracking, reforming, and/or combustion in the gas phase. Changes

in particle morphology and molecular structure results from pyrolysis and are dependent on the

constituents of the material and thermal conditioning.

carbonaceous material→ char(s) + volatiles(g) ΔH°rxn = positive, variable 2-2

Char gasification is the rate-controlling step and dictates the overall kinetics of the gasification

process. Walker, Rusinko, & Austin (1959) estimated the relative rates for the gas-forming

reactions to be C+O2 » C+H2O › C+CO2 » C+H2, as detailed in Table 2-1.

char(s) + gasifying agent(g)→ syngas(g) + ash(s) ΔH°rxn = variable 2-3

The principle chemical reactions involved in carbon gasification are summarized in Table 2-1. In

order to maximize yield of syngas, the endothermic reactions, Reaction 2-2 and 2-3, are required

and proceed only at high temperatures. These gas-forming reactions are usually thermally

balanced by the exothermic oxygen reactions, Reaction 2-1 and 2-6. The resulting gas phase

species interact in accordance with the remaining reactions to produce syngas of varying

composition and heating values. The thermal balancing effect of the methane producing

reactions (2-10 to 2-12) is obvious from their exothermic reaction enthalpies.

6

Table 2-1: Chemical Reactions in a Gasification process

Reaction ΔH°rxn (kJ/mol)

C(s) + ½O2(g) → CO(g) -111 partial oxidation 2-1

C(s) + CO2(g) ↔ 2CO(g) +172 reverse Boudouard 2-2

C(s) + H2O(g) ↔ CO(g) + H2(g) +131 water-gas 2-3

C(s) + 2H2(g) ↔ CH4(g) -74.8 hydrogasification 2-4

H2O(g) + CO(g) ↔ H2(g) + CO2(g) -41.2 water-gas shift 2-5

Oxidation Reactions

C(s) + O2(g) → CO2(g) -394 2-6

CO(g) + ½O2(g) ↔ CO2(g) -284 2-7

H2(g) + ½ O2(g) → H2O(g) -242 2-8

CH4(g) + 2O2(g) ↔ CO2(g) + 2 H2O(g) -803 2-9

Methanation Reactions

2CO(g) + 2H2(g) → CH4(g) + CO2(g) -247 2-10

CO(g) + 3H2(g) ↔ CH4(g) + H2O(g) -206 2-11

CO2(g) + 4H2(g) → CH4(g) + 2H2O(g) -165 2-12

The number of simultaneous and competing reactions in this process emphasizes the significance

of pressure and temperature operating parameters in regards to thermodynamic equilibrium

conditions. Reaction 2-2 and 2-3 favour higher temperatures and lower pressures. Conversely,

Reaction 2-4 favours lower temperatures and higher pressures. The water-gas shift, Reaction 2-5,

plays a key role in shifting product equilibriums to the desired composition selectivity.

Following downstream processing, the resulting product gas can be transported with existing

infrastructure, utilized in energy generation, and/or converted to liquid fuels via the Fischer-

Tropsch process.

7

2.1.2. Catalysts

The presence of inorganic impurities or additives in carbonaceous solids has been observed to

both enhance and inhibit the kinetics of gasification (Walker et al., 1959; Wood & Sancier,

1984). An early, systematic study conducted by Taylor and Neville (1921) involved the

evaluation of potassium, sodium, lithium, calcium, and barium carbonates, sodium chloride,

ferric oxide, and nickel as catalysts for steam and carbon dioxide gasification of coconut-shell

charcoal, kelp char, coke, sugar charcoal, DuPont charcoal. Further studies established alkali and

alkaline earth metals (AAEM) as popular additives to promote the reaction rate of gasification.

The relative activity for alkali metal carbonates decreases with atomic weight: Cs › K › Na › Li

(Wood & Sancier, 1984; Gangwal & Truesdale, 1980). Investigations have concluded that iron

and nickel compounds to be less effective compared to AAEM (Walker, Shelef, & Anderson,

1968). In regards to anion effects, alkali metal carbonates and hydroxides have the greatest

catalytic effect whereas halides have been found to inhibit catalytic effects. Alkali metal

sulphates, oxides, nitrates, and acetates occupy an intermediate range of activity (Wood &

Sancier, 1984)

Potassium has been the focus of many catalytic gasification studies. It promotes the reaction

more effectively than sodium, which counters its comparatively lower abundance. . Most studies

utilized potassium salts, such as potassium carbonate (K2CO3) and potassium hydroxide (KOH),

to a variety of carbons, such as graphite, coals of all ranks, and petroleum coke, at various stages

of the gasification process in order to evaluate reactivity, selectivity, and product gas heating

value. The dispersion of potassium catalyst on carbon prior to gasification has been found to be

irrespective of application techniques, such as admixing, ion exchange, and incipient wetness

impregnation (Wood & Sancier, 1984). Studies suggest that the catalyst exhibits mobility across

the carbon substrate upon heating and is capable of high dispersion and intimate contact (Mims

& Pabst, 1980).

Baker (1979) utilized controlled atmosphere electron microscopy to study catalyst particle

behaviour on graphite in an oxygen atmosphere. It was concluded that catalyst mobility

coincided with the Tammann temperature, resulting in pitting of imperfect basal plane regions

and/or the recession and channelling of edges or step regions. Coates et al. (1983) observed

potassium catalyst dispersed as 0.1-0.5 µm particles in a graphite-steam system at 500°C,

8

developing channels from the crystal edges with a hexagonal morphology at the particle-graphite

interface. At 600°C, the ~1 µm catalyst particles exhibited wider dispersion with enlargement of

pits and widespread channeling from edges and pits, which suggest wetting of the catalyst. Mims

et al. (1983) observed only wetting of the potassium catalyst in a graphite-steam system,

resulting in edge recession with hexagonal faceting at rates of 0.2 to 5 nm/s in the temperature

range of 550°C to 800°C. In an oxygen atmosphere, hexagonal faceting of edge recession did not

develop. In a carbon dioxide atmosphere, higher catalyst loading resulted in the observation of

particles on the basal plane of the graphite crystals, but did not participate in gasification. Spiro,

McKee, Kosky, and Lamby (1984) utilized hot stage microscopy to study a carbon dioxide

system at ~900°C. The formation and migration of potassium catalyst particles on graphite was

observed in situ, followed by quenching and further imaging with scanning electron microscopy

(SEM). Channelling, attributed to smaller catalyst particles, and pitting, of hexagonal and

rounded morphology, was observed. In situ, hot stage microscopy of potassium carbonate, on

anthracite coal char, exhibited melting of and disappearance into the pores in less than two

minutes, suggesting wetting and migration into the bulk carbon. SEM images of quench samples

indicated penetration of catalyst and internal channelling. The mobility and dispersion of alkali

metal catalysts, particularly potassium, is a unique feature to be utilized in gasification processes.

9

2.2. Reaction Mechanism & Kinetics

Aside from cases of mass transport and diffusion limitations, the reaction kinetics of gasification

is controlled by the intrinsic reaction rate wherein carbon is converted from a solid to a gas. The

number of active carbon sites available for conversion dictates this intrinsic reaction rate. From a

molecular perspective, a selected proportion of the total carbon atoms in the solid structure are

thermodynamically favoured for conversion. As seen in studies with graphite, researchers have

found the “edge” carbons, rather than the ordered, hexagonal configuration of the basal plane

carbon atoms, to be the sites of gasification attack. Since the structure of char carbons are

microcrystalline, disorganized analogs of the graphite structure, the “edge” carbons in chars are

certainly the reactive sites in char gasification (Franklin, 1951). Studies of graphite have shown

preferences for one or the other of the graphite “edge”, i.e. the so-called ‹1010› “armchair”

versus ‹1120› “zigzag”, depending on reaction conditions. In graphite studies of potassium

catalysis, it appeared that the ‹1010› “armchair” edge is more active (Mims et al., 1983; Walker,

1978). At the molecular level, the non-catalyzed reaction proceeds via formation of functional

groups at the “edge” carbons (carbonyls, lactones, phenolic, etc.), which then decompose to form

CO or CO2 thereby removing a carbon atom from the lattice. In the case of alkali catalysis, the

active site was proposed to be a surface “salt” where potassium adducts to surface anionic

groups, such as phenoxides, which then release CO in a redox reaction with steam or CO2 (Mims

& Pabst, 1983). Methane formation is also catalyzed by potassium (Coates et al., 1983; Mims

and Krajewski, 1986).The availability of active carbon sites can depend on the thermal

conditioning of the carbon substrate and the development of the carbon matrix throughout

conversion. Furthermore, if an inorganic catalytic species is involved, it must be mobile enough

to form a new active site following the gasification of its surface carbon atom partner.

The incorporation of catalysts improves the reaction kinetics of these active carbon sites that

introduce complex dynamics as the carbon matrix acts as both substrate and support for the

catalyst. As the carbon is converted, a catalyst limited system can experience an increase in

reactivity, as catalyst concentration increases, and/or a decrease in reactivity, as the catalyst

agglomerates or deactivates. Contributions from catalyzed and non-catalyzed active sites can

exhibit transient kinetics resulting in dynamic systems. The availability and development of

carbon active sites play a crucial role in the application of catalysts. A linear correlation between

10

catalyst-carbon molar ratios and reactivity up to a saturation limit was observed by Mims and

Pabst (1980). The saturation limit was dependent on the carbon substrate and the pretreatment

employed. A system limited by active carbon sites requires catalyst loading optimization.

11

2.3. Carbon Substrates & Feedstocks

The carbonaceous materials employed in a gasification process are organic in origin and may be

differentiated as fossil fuels or biomass. Fossil fuels, such as coal, petroleum, and natural gas, are

distinguished as organic material that has been transformed over millions of years with heat and

pressure within geological formations. Biomass involves organic material that is a relatively

recent participant in the carbon cycle and therefore renewable, such as plant, animal and

microbial matter and solid wastes. The following sections detail coal and plant biomass as the

main focus for this investigation. Understanding the components of heterogeneous carbon

substrates can provide insight on their gasification behaviour and reactivity, while model carbon

substrates can provide relative comparisons.

2.3.1. Model Carbons

In this investigation, a model carbon is designated as a carbon substrate that embodies well-

defined characteristics of particular interest for experimental study. Model carbons, such as

graphite, have been utilized in gasification studies in order to elucidate the intrinsic reaction

mechanism and rates from a particular facet of interest (Walker et al., 1968). While models are

useful in providing idealized mechanistic and kinetic information, readily available feedstocks

deviate significantly from these defined qualities.

2.3.2. Coal

Coal is a sedimentary organic rock of a heterogeneous nature. The formation of coal,

coalification, involves biochemical and geochemical transformation of organic masses. The

extent of coalification determines the degree to which the original material approaches the

structure of pure, graphitic carbon, designated as coal rank. The four classes of coal, in order of

increasing rank, are lignite, sub-bituminous, bituminous, and anthracite. Coal ranking is

designated from the proximate and ultimate analysis as well as calorific value, as seen in Table

2-2.

12

Table 2-2: Classification of Coal (adapted from Kabe, Ishihara, & Qian, 2004)

Coal Class Volatiles (%) C (wt%) H (wt%) O (wt%) Heating Value (MJ/kg)

Anthracite Meta 1.8 94.4 2.0 2.0 34.4 Anthracite 5.2 91.0 2.9 2.3 35.0 Semi 9.9 91.0 3.9 2.8 35.7 Bituminous Low-volatile 19.1 89.9 4.7 2.6 36.2 Med-volatile 26.9 88.4 5.2 4.2 35.9 High-volatile A 38.8 83.0 5.5 7.3 34.7 High-volatile B 43.6 80.7 5.6 10.8 33.3 High-volatile C 44.6 77.7 4.4 13.5 31.9 Sub-bituminous Sub-bituminous A 44.7 76.0 5.3 16.4 30.7 Sub-bituminous B 42.7 76.1 5.2 16.6 30.4 Sub-bituminous C 44.2 73.9 5.1 19.2 26.1 Lignite Lignite A 46.7 71.2 4.9 21.9 28.3

Petrography is an alternative classification of coal based on visual observation of reflected or

transmitted light through thin coal sections (van Krevelen, 1961). Petrographic classification of

coal involves the microscopic distinction between fundamental constituents, which are

designated macerals. Laurendau (1978) characterizes the three maceral groups are as follows:

Vitrinite is the principal component of coal, constituting 60-90 wt%. It originates from

the woody tissue of plants. It has a plasticizing nature that ultimately forms the coke

portion of the coal structure.

Liptinite (formerly exinite) originates from plant spores, cuticles, and resins. It comprises

of a high proportion of hydrogen. It fluidizes readily during thermal decompositions to

form volatiles consisting of tars and light gases.

Inertinite is composed of fossil charcoal and highly degraded plant biomass with a dull,

granular consistency. It does not plasticize or devolatilize, thereby forming the char

fraction of coal.

Carbon content increases and atomic H/C ratio decrease in the order: liptinite, vitrinite, inertinite

(Laurendau, 1978). Difference in maceral composition is less discernable for high rank coals that

approach the structural uniformity of graphite.

13

Coal can be composed of 10-30 wt% mineral matter in addition to 20-30 trace metals distributed

through the coal structure (Laurendau, 1978). Four major types of mineral matter are outlined in

Table 2-3 with major constituents. During a thermal degradation process, such as pyrolysis,

gasification, or combustion, mineral matter is converted to the ash constituents of SiO2 (20-

60%), Al2O3 (10-35%), Fe2O3 (5-35%), CaO (1-20%), and MgO (1-5%), as seen in most ash

analyses (Laurendau, 1978). At temperatures greater than 1300-1900 K, the mineral matter may

melt and ooze through char pores to the surface of coal (Laurendeau, 1978).

Table 2-3: Mineral Matter Constituents of Coal (reproduced from Laurendeau, 1978)

Type (wt%)

Aluminosilicates (clays)

Kaolinite Al2Si2O5(OH)4 50

Illite KAl3Si3O10(OH)2

Oxides

Silica SiO2 15

Hematite Fe2O3

Carbonates

Calcite CaCO3

10 Siderite FeCO3

Dolomite CaCO3·MgCO3

Ankerite 2CaCO3·MgCO3·FeCO3

Sulfide/Sulfates

Pyrite FeS2 25

Gypsum CaSO4·2H2O

The reactivity of different coal feedstocks depends on the porosity, such as inner structure,

surface area, and carbon active sites, the crystal structure of the fixed carbon, and catalytic

effects of ash components (Higman & van der Burgt, 2008). In general, lower rank coals have

higher surface area and thereby higher reactivity, while higher rank coals have lower surface area

and reactivity (Walker, 1978; Higman & van der Burgt, 2008). The higher surface area of low

rank coals during gasification can be attributed to the higher volatile content establishing the

14

pore structure and surface area for gas-solid contact. The lower surface area of high rank coals is

attributed carbon content and structure with fewer dislocations and heteroatoms to provide active

sites. The heterogeneity of coal and its transformation during the gasification process is complex.

Extensive studies to elucidate reactivity trends based on these many factors continue to be

elusive in most cases.

Indigenous mineral material in coal can also deactivate an added catalyst. AAEM compounds

react with alumina- and silica-bearing minerals to form stable and water-insoluble

aluminosilicates that are unable to exhibit catalytic activity. For coals with high ash contents,

observations of complex variations in gasification kinetics during conversion have been

attributed to reactions between the catalyst and the mineral constituents (Wood & Sancier, 1984).

In the case of ash deactivation, a catalyst loading beyond a threshold value is required to observe

improvements in reactivity. The mineral matter analysis of the specific feedstocks utilized in this

study are presented in Chapter 5.

2.3.3. Biomass

The term biomass refers to non-fossilized and biodegradable organic material originating from

plants, animals, and microorganisms derived from biological sources. Biomass includes

products, by-products, residues, and waste from agriculture, forestry and related industries, as

well as the non-fossilized and biodegradable organic fractions of industrial and municipal solid

wastes. Biomass is considered a renewable resource as it actively participates in the carbon cycle

in a relatively current time frame, as opposed to fossilized materials. (Demirbas, 2009)

Biomass is a complex mixture of organic materials consisting of:

Extractives include proteins, lipids, sugars, and starch.

Structural material composed primarily of cellulose, hemicellulose, and lignin. Cellulose

is a linear chain polysaccharide solely consisting of anhydrous glucose. Hemicellulose

encompasses a variety of amorphous polysaccharides. In comparison to cellulose,

hemicellulose involves various sugar monomers, excluding anhydrous glucose, in a

matrix polysaccharide (heteropolymer) consisting of short and/or branched chains. Lignin

is a highly cross-linked, high molecular weight polymer (Demirbas, 2009).

15

Ash is composed of inorganic, mineral components that are variable among plant type

and soil conditions. Potassium, calcium, and magnesium are common constituents that

are known to catalyze the gasification reaction (Vassilev, Baxter, Andersen, & Vassileva,

2010)

Table 2-4: Structural Constituents of Selected Biomass (reproduced from McKendry, 2002)

Biomass Cellulose (wt%) Hemicellulose (wt%) Lignin (wt%)

Softwood 35-40 25-30 27-30

Hardwood 45-50 20-25 20-25

Wheat Straw 33-40 20-25 33-40

Switchgrass 30-50 10-40 5-20

The diversity of biomass feedstocks provides insight on heterogeneity as governing characteristic

of biomass materials. Rapid pyrolysis heating and quenching produces intermediate pyrolysis

condensates of high-molecular-weight species. Higher pyrolysis temperatures induce cracking of

these species to form permanent, low-molecular weight gases. Cellulose and hemicellulose form

mainly volatile products on heating due to the thermal cleavage of the sugar units. Hemicellulose

tends to yield more gases and less tar than cellulose (Basu, 2010). As seen in Table 2-4, cellulose

and hemicellulose constitute a significant fraction in biomass correlating with typical proximate

analysis volatiles of ~60-80% (Emami-Taba, Irfan, Daud, & Chakrabarti, 2013). The greater

porosity and reactivity or resulting biomass chars correlates to the volume remaining after

cellulose and hemicellulose devolatilization. Lignin mainly forms char as it is not readily cleaved

to lower molecular weight fragments. Relative rates of thermal decomposition for structural

components of biomass are: hemicellulose › cellulose » lignin (Demibras, 2009).

16

2.4. Gasification of Binary Feedstocks

The gasification of binary feedstock blends, co-gasification, presents a promising avenue to

catalytic gasification. The addition of AAEM catalyst improves the process economics and

presents the additional consideration of catalyst recovery. The combination of biomass, with low

heating value and high levels of inherent AAEM, with coal, with its high heating value and low

levels of inherent catalyst, may address the shortcomings of the individual feedstock. The

incorporation of biomass also reduces the net carbon footprint of the process. The multitude of

factors that affect the reactivity of individual feedstocks are to be evaluated with these binary

blends of varying proportions.

Brown et al. (2000) observed an enhancement in reactivity in a TGA, CO2-char gasification

study blending coal char with switchgrass char or ash in the range of 750-980°C. Relative

reactivity was determined by comparing the slope of the initial linear decrease of the sample

weight to that of pure coal char. Switchgrass ash blends exhibited greater enhancement than

switchgrass char blends due to the higher loading of potassium. A nearly eight-fold increase in

coal char gasification rate was observed at 895°C with a 10:90 coal char and switchgrass ash

mixture.

Parenti (2009) reproduced the findings of Brown’s study of the TGA, CO2-char gasification

study of a coal char and switchgrass char blend in addition to other coal-biomass blends. A 10:90

switchgrass char to Illinois No. 6 char mixture exhibited a synergistic effect, which was

enhanced with a 30:70 blend ratio. A synergistic effect was observed for a corn stover char and

Powder River Basin (sub-bituminous) coal char. An inhibition effect was observed for a North

Dakota Lignite char and mixed hardwood char blend.

Habibi et al. (2013b) observed both synergistic and inhibition effects in a TGA, CO2-char

gasification study blending sub-bituminous coal char and fluid coke with switchgrass char or ash.

Relative reactivity was determined by comparing conversion over time and reaction rate over

conversion to the non-interacting weighted average of the 50:50 mixtures.

Coal char and switchgrass char mixtures exhibited reactivity lower than the non-interacting and

pure coal char profiles. This inhibition was attributed to the deactivation of the potassium

17

catalyst. Deactivation resulted from the interaction of potassium with alumina and silica

constituents of coal ash to form aluminosilicates as suggested by XRD analysis of 50%

converted samples. Synergistic effects were observed with coal char and switchgrass ash

mixtures. It was concluded that a K/Al molar ratio greater than 1 can saturate constituents that

deactivate potassium. In comparison, the low-ash content of Illinois No. 6 coal (~9 wt%) utilized

in Brown’s (2000) study the sub-bituminous coal in this study had 30.5 wt% ash, which explains

the lack of agreement between the switchgrass char and coal char findings. Increasing

gasification temperatures reduced the synergistic and inhibition effects, indicating the

comparatively greater influence of temperature.

A fluid coke of less than 2 wt% ash was mixed with switchgrass char and ash and subjected to

the same conditions. Synergistic enhancement to reactivity was observed with lower ash and

alumina/silica constituents. During the initial 50% conversion, fluid coke char and switchgrass

char mixtures had higher reactivity, attributed to the conversion of the switchgrass char,

compared to fluid coke char-switchgrass ash mixtures. In the final 50% conversion, a reverse

trend was observed with higher reactivity of fluid coke char-switchgrass ash mixture. XRD

analysis detected no presence of potassium aluminosilicates in the fluid coke and switchgrass

char/ash mixtures. An increase in gasification temperature from 850°C to 950°C resulted in the

same trends.

Synergistic effects have been observed for coal-biomass blends with minimal discussion

regarding the mode of transport for these catalytic species. Inhibition effects have also been

observed without any explanation regarding the mode of deactivation for the catalysts. In order

to apply these fossil fuel and biomass blends, an enriched understanding of how these

heterogeneous materials interact must be investigated to better utilize existing gasifier

technology, systems, and units.

18

Chapter 3

Experimental Design & Protocols

Utilization of biomass and coal of Canadian origin provided a local and national focus to this

study. In particular, switchgrass of high inherent potassium catalyst content was selected as

biomass. Model carbons were also utilized in order to contrast potential behavioural differences

from catalyst deactivation resulting from the ash content of the sub-bituminous coal. The design

and construction of an unconventional, bench-scale, flow system was undertaken to study the

transport mode and kinetics of potassium-catalyzed gasification. Time-of-Flight – Secondary Ion

Mass Spectrometry (ToF-SIMS) was utilized in order to study intra-particle and inter-particle

potassium mobility from a qualitative and semi-quantitative perspective.

3.1. Materials

3.1.1. Precursors

Model carbon substrates studied were spherical activated carbon, i.e. spherocarbon, (Sigma-

Aldrich) and microporous, glassy coke derived from poly(furfuryl alcohol) (PFA, CedarLane).

Switchgrass (Manitoba, Canada) and sub-bituminous coal (Genesee, Alberta, Canada) were

selected as locally-sourced, technically-relevant feedstocks.

3.1.2. Char and Carbon Preparation

Carbon substrates from the precursor samples were prepared with the pyrolysis system, as

illustrated in Figure 3-1. The system consists of a rotameter, quartz reactor, and scrubber

contained within a fume hood. The rotameter was calibrated to 40 mL/min of nitrogen (Linde,

99.9999%). The nitrogen was fed to a horizontal quartz tube (1” ID, 3’ L) sealed with ultra-torr

fittings and enclosed by an electrically-heated furnace. The outlet gas flowed through two

impinger bottles, isopropyl alcohol followed by water, enclosed in an ice bath. The pyrolysis

system operated at atmospheric pressure.

Precursor samples were contained in a quartz boat centrally positioned in the quartz tube. A

thermocouple was positioned above the quartz boat to maintain the furnace set-point via a

19

proportional-integral-derivative (PID) controller. The reactor was heated at 25°C/min to the

desired set-point, held for 2 hr, and allowed to cool overnight. Any volatiles were condensed

from the outlet gas via the scrubber prior to fume hood removal. The pyrolysis system converts

to a combustion system via a 3-way-valve connected to compressed air in order to produce ash

from feedstocks.

Figure 3-1: Schematic of Pyrolysis System

Spherocarbon was utilized as received with no further pretreatment. Switchgrass and coal

precursors were subjected to the aforementioned pyrolysis process in 3-5 g batches. The resulting

char samples were collected from the system to be ground by mortar and pestle, sieved to 140-

177µm (80-100 mesh), and stored. The liquid PFA precursor was poured into the quartz boat for

preliminary thermosetting within a muffle furnace at 5°C/min to an 80°C set-point followed by

the pyrolysis process. The resulting coke samples were collected from the system to be ground

by mortar and pestle, sieved to 140-177µm (80-100 mesh), and stored or cut into wafers with a

microtome.

Catalyzed carbon substrates were fabricated from the aforementioned PFA carbon wafer and

spherocarbon for intra-particle and inter-particle studies, respectively. Application of potassium

carbonate (K2CO3, Sigma-Aldrich, 99.99%, anhydrous powder) onto the PFA carbon wafer

involved contacting a moistened K2CO3 particle to a discrete area of the flat carbon surface.

Spherocarbon was loaded with K2CO3 at 20 wt% via incipient wetness impregnation.

20

3.2. System

A bench-scale, flow system was designed and constructed to study gasification kinetics, as

detailed in Figure 3-2. Key features incorporated were a steam generator, drop mechanism, and

multiple reactor configurations in order to address limitations of systems that are typically

employed in literature.

Figure 3-2: Schematic of Bench-Scale, Flow System

Studies of (steam) gasification kinetics conducted in bench-scale systems typically utilize fixed-

bed reactors, thermogravimetric analyzers (TGAs), or horizontal boat reactors (Gangwal &

Truesdale, 1980). As there is no direct convective flow through the sample in the latter two

configurations, a fixed bed reactor configuration was designed to reduce/address diffusion

limitations/diffusional effects.

Furthermore, it is common for these systems to be conducted with simultaneous heating of the

sample within the reactor under a flow of reactant or inert gas at varying heating rates. As the

thermal conditioning of carbon materials can affect the resulting structure and kinetics,

particularly during devolatilization, a drop mechanism was implemented to better simulate the

rapid heating of material injected into an isothermal fluidized-bed gasifier. Simulation of fixed-

21

bed gasifier zones following sample injection is also possible with the programmable heating

rates of the PID temperature controller.

Steam generators utilized in literature employ liquid syringe pumps into heated lines with carrier

gas to convey reactant gas to the reactor. A gas phase, catalytic steam generator was

implemented for the precise flow of steam into the reactor with all flow lines encased in a

hotbox. The utilization of this steam generator allows for ease in varying steam-hydrogen ratios,

oxygen-steam ratios, and dilution with inert gas. Variability in steam-hydrogen is essential for

systematic study of product inhibition on reaction kinetics (Holstein, 1983). The steam generator

may also be adapted to a CO/CO2 reactant gas with downstream modification of the gas

analyzer.

22

Figure 3-3: Bench-Scale Gasification Apparatus (Top-Left: Overview of Apparatus, Top-

Right: 2-Way Switching Valve Flow Tubing, Bottom-Left: Mass Flow Controllers, Bottom-

Right: Water Traps and Sampling Lines)

23

3.2.1. Steam Generator

The gasification reactant gas is produced by a steam generator consisting of a 2.2 g of 5 wt%

platinum catalyst supported on alumina (Sigma-Aldrich) and 0.8 g of activated alumina (Sigma-

Aldrich). The 2” steam generator contains 150-425 µm particles of H:D ratio of five.

Stoichiometric hydrogen (H2, Linde, 99.9995%) and oxygen (O2, Linde, 99.999%) flows are

reacted at 275°C to produce the corresponding ratio of steam-hydrogen.

Figure 3-4: Platinum-Catalyzed Steam Generator

3.2.2. Drop Mechanism

A drop mechanism positioned directly above the reactor allows for samples to drop into the

reactor hot zone and gas reactant atmosphere within seconds, to simulate the thermal

conditioning of gasifier material feeding. In contrast, samples studied in TGAs undergo a steady

heating profile, typically 10-20°C/hr, in an inert gas atmosphere for a relatively extended period

before introduction of reactant gas. The drop mechanism consists of a blind bore rod encased

within a cross fitting sample chamber sealed with o-rings.

24

Figure 3-5: Drop Mechanism for Rapid Introduction of Carbon Samples into the Reactor

3.2.3. Reactor Configurations

The reactor component of the system consists of an inner quartz reactor nested within a stainless

steel outer reactor. The inner reactor is 26 cm in length with an inner diameter of 7 mm and

bottom frit of 40-90 µm nominal maximum pore size. The stainless steel outer reactor is 40.6 cm

in length with an inner diameter of 12.5 mm. As seen in Figure 3-3, this design allows for three

configurations via a 2-way switching valve: downdraft fixed-bed, updraft fixed-bed, and

fluidized-bed.

25

Figure 3-6: Multiple Configurations of Reactor System

26

3.3. Protocols

3.3.1. Mode I: Intra- and Inter-Particle Potassium Mobility

Carbon wafers were fabricated from PFA coke and loaded with potassium carbonate in

accordance to the Section 3.2 protocol. The carbon sample was secured on a heated stage and

loaded into the high vacuum chamber of the ToF-SIMS. An initial baseline scan was performed

at 25°C followed by commencing stage heating. Subsequent scans were performed at 100°C,

200°C, 300°C, 400°C, 425°C, 450°C, 475°C, and held at 500°C.

Admixed blends of catalytic and non-catalytic particles were prepared for gasification within the

bench-scale system. Samples were subjected to a steam gasification environment at 700°C for 2

hr, followed by quenching with an inert, argon flow and removal of the quartz reactor from the

furnace in less than 2 min. The sample was allowed to cool under a continued flow of argon prior

to analysis in the ToF-SIMS.

This versatile collection of materials, techniques, and analyses presents new methods in

evaluating gasification reactivity, particularly from the perspective of potassium mobility.

Following studies explore semi-quantitative and qualitative analyses of potassium mobility and

the effect of heat treatment and gas atmospheres on apparent reaction rates. Many of the

capabilities are not showcased, but future research would be readily adapted to study activation

energies, product inhibition effects in Langmuir-Hinshelwood kinetics, effect of volatiles from

precursor samples.

3.3.2. Mode II: Gasification Kinetics in a Flow System

The bench-scale, flow system is equipped with a SRI-8610C gas chromatograph (GC) that splits

the sample stream to an online, total carbon and injection, composition stream for analysis. The

online, total carbon stream consists of methanizer to convert carbon monoxide and carbon

dioxide to methane with a nickel catalyst to a flame ionization detector. The injection,

composition stream consists of a 1 µL sample conveyed with a helium (He, Linde, BIP) carrier

gas to a 6’ Haysep-D column followed by second methanizer and flame ionization detector set.

The resulting detector signals are collected by PeakSimple software (ver. 4.32) every 0.2 sec and

27

3.0 min for the total carbon and compositions streams, respectively. This system configuration

allows for high data resolution of total, gaseous, carbon species and corresponding composition

at intervals.

Following system assembly and leak check, the furnace, steam generator, and hotbox heating

was initiated to 850°C, 275°C, and 170°C set-points, respectively, with argon, purge gas (Ar,

Linde, BIP) through the quartz and stainless steel reactors (1 mLN/min Ar, 1 mLN/min Ar).

System back-pressure of 2.5 atm and GC detector calibrations were performed prior to each

experiment. A 3:1 steam-hydrogen generation was initiated (30 mLN/min H2O, 10 mLN/min H2)

and confirmed by a ~25°C increase in the steam generator. Samples of 5 mg fixed carbon (5-8

mg char) were loaded into the drop mechanism. A sweeping gas of argon through the drop

mechanism prevented any steam migration with condensation and/or sample agglomeration.

Inert (40 mLN/min Ar, 20 mLN/min Ar-carrier) and steam baselines were established for 30 min

each, confirming initial system steady state.

Following the commencement of data logging with PeakSimple software, the samples were

dropped, at the 3.0 min time-point, into the initial reactant gas atmosphere. Switching to an

alternate gas atmosphere was accomplished through a micro-electric, Vici-Valco switching

valve. The reaction proceeded until the initial baseline was reached. Confirmation of any carbon

remnants and mass balance was performed by introducing 1.25% O2 (0.5 mLN/min O2, 40

mLN/min Ar) stream into the reactor.

28

Chapter 4

ToF-SIMS Surface Analysis of Potassium Mobility

This study investigates high temperature, potassium mobility on carbon substrates by utilizing a

Time-of-Flight – Secondary Ion Mass Spectrometer (ToF-SIMS) for qualitative and semi-

quantitative, surface analysis. Intra-particle potassium mobility was investigated in situ on a

heated stage within the ToF-SIMS. Inter-particle potassium mobility was investigated with two

types of partially converted carbon substrates: spherocarbon impregnated with potassium

carbonate and switchgrass char with inherently high potassium content, analyzed with the ToF-

SIMS.

4.1. Intra-Particle Mobility

The intra-particle mobility study utilized a carbon wafer, derived from poly(furfuryl) alcohol

coke (PC), with discretely applied potassium carbonate, as outlined in Section 3.1.2. This carbon

sample was subjected to the experimental protocol outlined in Section 3.3.1. Figure 4-1 exhibits

the time-lapse, progression of the carbon surface over the temperature range of 25°C to 500°C.

The bright areas of the 500 by 500 µm, field-of-view correspond to the presence of potassium by

secondary, positive, ion count, as indicated by the logarithmic scale. The discrete application

area of potassium carbonate is located in the top-right corner, with correlating K/C count ratios

located in the bottom-left corner.

29

Figure 4-1: ToF-SIMS Surface Analysis of Intra-Particle Potassium Mobility on

Poly(furfuryl Alcohol) Coke

Scans indicate no mobility of the applied potassium at temperatures below 300°C, as concluded

by the distinct boundary for the application area and relatively steady K/C count ratio shown in

Figure 4-1 (a)-(c). At temperatures above 300°C, the application boundary began to lose

distinction and develop a gradient. This series of scans are within the range of the potassium

carbonate Tammann temperature where bulk, lattice re-crystallization occurs. At 475°C, the K/C

count ratio decreased to less than 5% of the original loading with the application boundary

indiscernible.

The radial expansion of the application boundary from approximately 100 to 200 µm can be

attributed to intra-particle potassium dispersion and mobility. Since ToF-SIMS is a surface

sensitive technique, potassium below the top few atomic layers would be invisible. Migration out

of the surface analysis zone and into the bulk of the carbon sample can explain the observation of

30

decreasing K/C count ration in Figure 4-1. This unaccounted potassium can also be due to low

potassium carbon loading. Evaporation of potassium within the high vacuum of the analytical

chamber could cause the decrease in surface potassium measurements, but at these modest

temperatures, this process is not thought to be active.

Mims et al. (1983) utilized controlled atmosphere electron microscopy to study the effect of

potassium salt on graphite-oxygen and graphite-steam reactions. Up to temperature of 550°C in a

steam atmosphere, potassium catalyst particles were observed mobilizing to active edge sites

exhibiting liquid-like characteristics suggestive of wetting interactions with no nucleation on the

basal plane surface. Above 550°C, these particles disappeared but their presence was perceived

by edge recession of a hexagonal facetted appearance. It is suggested that effective dispersion is

achieved by the interfacial interaction of potassium catalyst with active carbon edges.

The observed dispersion to double the application radius, coupled with the findings of McKee

and Chatterji (1975) and Mims et al. (1983), supports intra-particle, potassium mobility and

dispersion on carbon well below typical gasification temperatures. The PFA-derived carbon

wafer in this study is significantly more disordered compared to the graphite materials utilized by

these previous studies and can be considered as a three-dimensional. The comparatively greater

number of carbon active sites may allow for potassium to disperse within the carbon bulk

invisible to the ToF-SIMS technique.

4.2. Inter-Particle Mobility

Catalyst and non-catalyst containing carbon substrates were selected and blended to study inter-

particle mobility during steam gasification. Extending beyond intra-particle potassium mobility

in Section 4.1, a model carbon blend of potassium carbonate impregnated spherocarbon and

poly(furfuryl alcohol) coke, designated SCK-PC, was investigated to establish inter-particle

mobility. A blend of switchgrass char, a biomass feedstock of high, inherent potassium content,

and spherocarbon, designated SGC-SC, was also studied for comparison with SCK-PC. The

experimental protocol from Section 3.3.1 was employed for partial gasification of the carbon

materials followed by ToF-SIMS secondary electron microscopy (SEM) imaging and analysis.

31

Figure 4-2: ToF-SIMS SEM Surface Analysis of Inter-Particle Potassium Mobility of

Potassium Carbonate Impregnated Spherocarbon (SCK) with Poly(furfuryl Alcohol) Coke

(PC), Panel (a)-(b), and Switchgrass Char (SGC) with Spherocarbon (SC), Panel (c)-(d)

Figure 4-2 exhibits the initial blended samples and blended samples partially converted in a

steam atmosphere. Individual particles are highlighted with their correlating K/C count ratios.

Panel (a) is a selected result of potassium carbonate impregnated spherocarbon (SCK), spherical

particles, blended with poly(furfuryl alcohol) coke (PC), angular particles prior to gasification

treatment. The selected poly(furfuryl alcohol) coke and catalyst impregnated spherocarbon

particles have K/C count ratios of 1.38 and 122, respectively. Panel (b) is a selected result

following steam gasification treatment at 700°C for 2 hr. The K/C count ratios for selected post-

32

treatment poly(furfuryl alcohol) coke and catalyst impregnated spherocarbon particles are 450

and 148, respectively. Panel (c) is a selected result of switchgrass char (SGC), rectangular

particles that inherently contain potassium (Table 5-1), blended with spherocarbon (SC),

spherical particles, prior to gasification treatment. The selected spherocarbon and catalyst

containing switchgrass char particles have K/C count ratios of 1.56 and 486, respectively. Panel

(d) is a selected result following steam gasification at 700°C for 2 hr. The K/C count ratios of

selected post-treatment spherocarbon and catalyst containing switchgrass char particles are 39.4

and 261, respectively. Evidence of inter-particle potassium mobility is shown by the large

increase in K/C count ratio of the initially non-catalyzed particles, poly(furfuryl alcohol) coke

from panel (a)-(b) and spherocarbon from panel (c)-(d).

The increase in K/C count ratios for the non-catalyst containing particles, PC and SC, clearly

demonstrates that inter-particle mobility of potassium occurs during steam gasification. The

increase in K/C count ratios of catalyst containing particles, SCK and SGC, may be due to the

partial conversion of carbon substrate. Furthermore, potassium dispersion primarily on the

surface of PC, which has a comparatively lower surface area than SCK, registered higher

potassium ion counts due to the surface analytical technique employed. Accordingly, the higher

surface area and internal porosity of the SC and SCK particles only allows for the surface

fraction of the potassium to be detected. Inter-particle mobility of potassium with simple

physical contact of the carbon particles is clearly supported by this ToF-SIMS SEM surface

analysis. A more complete multi-particle analysis to study extent and uniformity of the mobility

as well as the effects of various reaction conditions protocol is reserved for future studies. The

surface analysis specificity of the ToF-SIMS technique does not account for all of the potassium.

Karimi, Semagina, and Gray (2011) conducted ToF-SIMS depth profiling analysis of potassium

carbonate loaded bitumen coke subjected to a nitrogen atmosphere at 600°C for 15 min. Surface

potassium ion counts sharply declined followed by a steady potassium ion count for the

remaining depth of ~2 µm. There are indications that the extent of intra- and inter-particle

potassium dispersion can be non-uniform which may have indeterminate implications on

intrinsic reaction mechanisms and apparent kinetics.

33

Chapter 5

Kinetic Characterization of Individual Carbon Substrates

This study investigates the individual carbon substrates to establish preliminary characterization

and kinetic limits, prior to binary blending. The switchgrass and coal feedstocks are

characterized by proximate, ultimate, and ash analyses. Steam gasification of switchgrass char,

coal char, and spherocarbon are compared on the basis of conversion versus time and reaction

rate versus conversion to establish relative reactivity.

5.1. Feedstock Characterization

Table 5-1: Proximate, Ultimate, and Ash Analysis of Switchgrass and Coal (received from

Intertek and Loring)

Switchgrass Coal Proximate Analysis ash 6.3 30.5 volatiles 76.9 31.3 fixed carbon 16.8 38.3 Ultimate Analysis C 47.9 73.1 H 6.2 4.3 N 0.8 1.0 S 0.1 0.4 O 45.0 21.2 Ash Analysis SiO2 52.5 57.6 Al2O3 2.1 23.6 TiO2 0.02 0.5 Fe2O3 0.3 2.8 CaO 6.4 5.6 MgO 6.5 1.3 Na2O 1.6 2.6 K2O 20.3 0.8 P2O5 5.0 0.1 SO3 2.6 2.3

34

The proximate, ultimate, and ash analyses of switchgrass and coal are summarized in Table 4-1.

Coal has a greater fraction of fixed carbon and ash than switchgrass. Conversely, switchgrass

consists primarily of volatiles, as is the case for most biomass materials. The ash content of coal

is predominantly composed of catalyst deactivating species: silica and alumina. Marginal

fractions of catalytic species, Fe2O3, CaO, MgO, Na2O, and K2O, total 4.00 wt% of coal or 5.81

wt% of coal char. These individual species enable variable enhancements in carbon reactivity

and should be considered in the observation of kinetic improvements. The ash content of

switchgrass has comparable levels of silica, but much less alumina. The catalytic species in the

ash content of switchgrass total 2.21 wt% of switchgrass or 9.57 wt% of switchgrass char.

Potassium oxide is a majority contributor at 57.9%, which affirms its selection as a catalyst

containing feedstock. Upper limits of initial K/C molar ratios, based on ash analysis and fixed

carbon, for switchgrass char and coal char are 0.0193 and 0.00154, respectively.

5.2. Kinetic Analysis of Steam Gasification

Switchgrass char (SGC), coal char (CC), and spherocarbon (SC) were selected as individual

carbon substrates based on the properties summarized in Section 3.1.2, Table 3-1. Switchgrass

char and coal char were prepared as outlined in Section 3.1.2. These three materials were

subjected to gasification in a 3:1-steam:hydrogen atmosphere at 850°C as detailed in Section

3.3.2. The resulting reactivity metrics of conversion versus time and reaction rate versus

conversion are discussed below.

35

Figure 5-1: Switchgrass Char, Coal Char, and Spherocarbon Conversion versus Time

during Steam Gasification at 850°C

Figure 5-1 compares the resulting conversion profiles of switchgrass char, coal char, and

spherocarbon. Switchgrass char conversion was faster than coal char and spherocarbon on a time

basis to attain 50% conversion in 28.7 min, as seen in Figure 5-1. Coal char and spherocarbon

required 63.3 min and 416 min to attain 50% conversion, respectively. Ultimately, switchgrass

char, coal char, and spherocarbon attained 90% conversion in 3.54 hr, 42.0 hr, and 39.6 hr,

respectively.

Figure 5-2 compares the rates of reaction of switchgrass char, coal char, and spherocarbon over

conversion. Maximum rates of reaction of switchgrass char, coal char, and spherocarbon were

0.0544, 0.0479, and 0.0314 min-1, respectively. There is an interval of approximately 10-35%

conversion where the reaction rate of coal char is greater than switchgrass char. Following this

interval, the coal char reaction rate decreases and is comparable to spherocarbon beyond 50%

conversion, as seen in the inset of Figure 5-2.

36

Figure 5-2: Switchgrass Char, Coal Char, and Spherocarbon Reaction Rate versus

Conversion during Steam Gasification at 850°C

The attainment of maximum reaction rates for switchgrass char, coal char, and spherocarbon

within the initial 20% conversion may be associated with the conversion of adsorbed surface

species and rapid introduction to the reactor via drop mechanism. The lowest reaction rate

throughout conversion for spherocarbon is as expected for a carbon substrate with no inherent

catalyst species. The relatively high reaction rate for switchgrass char may be attributed to the

catalytic enhancement by the inherent potassium content. The maximum reaction rate at 7.65%

conversion followed by decreasing reaction rate over conversion is indicative of a system where

carbon active sites and their development over conversion are rate-controlling compared to

catalyst availability.

The reaction rate of coal char exhibits a bimodal profile with an approximate inflection at 50%

conversion. As discussed in Section 5.1, coal char is composed of a significant fraction of ash

that consists of various catalytic species, which may have enhanced the initial reaction rates. The

subsequent decline in reaction rate and catalytic enhancement may be due to catalyst

agglomeration as the reduction in the carbon matrix progressed with conversion. A concomitant

37

consideration is the effect of thermal conditioning on the morphological qualities of the coal char

throughout conversion.

Ashu (1977) conducted a study to evaluate the effect of thermal conditioning, such as

temperature, heating rate, and heat soak, on the reactivity of coal char. Four coal chars were

subjected to slow (10°C/min) and rapid (8000°C/min) heating rates to a maximum temperature of

800°C and compared. The rapid heating rates resulted in increased reactivity for all chars upon

air gasification at 500°C in TGA. Surface area measurements, N2 and CO2, also increased with

rapid heating rates, with N2 surface area achieving greater gains than CO2 surface area. Decrease

in reactivity and surface area was correlated with increasing maximum temperature and heat

soak time. It has been suggested that active carbon sites are present predominantly in macropores

while micropores do not participate in the reaction (Mermoud, Salvador, Van de Steen, &

Golfier, 2006; Radovic, Walker, & Jenkins, 1983).

It is possible the initially high reaction rate of coal char prior to 50% conversion can be attributed

to the relatively rapid heating rate of the sample drop and reactivity enhancement by inherent

catalytic species. The increase in surface area in tandem with the steam gasification atmosphere

quickly converted active carbon sites. The subsequent decrease in reactivity could be the result

of catalyst agglomeration, stabilization of the carbon matrix and pore structures, and depletion of

readily available macropore active sites with only inaccessible micropores remaining.

38

Chapter 6

Kinetics of Binary Blends: Admixtures of Carbon Substrates

This study investigates the co-gasification of switchgrass char blended with carbon substrates by

utilizing a drop tube furnace for kinetic analysis. Based on the indications of inter-particle

mobility in Section 4.2, the switchgrass char and spherocarbon blend was complimented with a

switchgrass char and coal char blend. Kinetic analysis of these binary blends involved the

evaluation of conversion over time and reaction rate over conversion.

6.1. Direct Introduction to Steam, Gasification Atmosphere

A 1:1 fixed carbon basis was selected for the blend of switchgrass char and spherocarbon,

designated 1:1-SGC:SC. A 1:1 and 3:1 fixed carbon basis was selected for the blend of

switchgrass char and coal char, designated 1:1-SGC:CC and 3:1-SGC:CC, respectively. The

resulting blends contained approximately 5 mg total of fixed carbon based on the proximate

analysis of Table 4-1. The switchgrass char and spherocarbon/coal char were admixed with no

further treatment to promote particle contact or dispersion of potassium catalyst. These three

samples were directly introduced to a 3:1-steam:hydrogen, gasification atmosphere at 850°C,

according to the experimental protocol detailed in Section 3.3.2.

The resulting reactivity metric of conversion versus time is compared to a theoretical conversion

profile and evaluated. A theoretical, non-interacting conversion profile is the weighted sum of

the catalyzed carbon, switchgrass char, and non-catalyzed carbon, spherocarbon or coal char, as

seen in Equation 5-1. This theoretical conversion approximates independent and simultaneous

gasification of catalyzed and non-catalyzed portions of the sample. A profile that is greater or

less than theoretical is designated as synergy or inhibition, respectively.

Xtheoretical = f·XSGC + (1 – f)·XSC/CC 5-1

39

Figure 6-1: Switchgrass Char, Spherocarbon, and 1:1 Switchgrass Char-Spherocarbon

Conversion versus Time during Steam Gasification at 850°C

Figure 6-1 depicts the results from the 1:1 admixture of switchgrass char, containing inherent

potassium catalyst, and non-catalyzed spherocarbon in comparison to its switchgrass char and

spherocarbon constituents and theoretical non-interaction conversion. The theoretical conversion

profile fits to the empirical 1:1 switchgrass char-spherocarbon within ±2.53%, as seen in Figure

6-1. The 1:1 switchgrass char-spherocarbon sample attained 50% and 90% conversion at 86.7

min and 25.9 hr, respectively. A small positive deviation over the mid-range of the theoretical

conversion profile is observed. This is consistent with a small amount of synergy due to catalyst

mobilization from the switchgrass char to a portion of the spherocarbon, but overall, the sample

gasifies as if each constituent was gasifying independently.

40

Figure 6-2: Switchgrass Char, Coal Char, 1:1 Switchgrass Char-Coal Char, Panel (a), and

3:1 Switchgrass Char-Coal Char Conversion, Panel (b), versus Time during Steam

Gasification at 850°C

41

Figure 6-2 depicts the results from two different ratios of switchgrass char, containing inherent

potassium catalyst, and coal char in comparison to its switchgrass char and coal char

constituents. The 1:1 and 3:2 switchgrass char-coal char admixtures, with the corresponding

theoretical non-interaction conversion profile, are shown in panel (a) and (b), respectively.

Similar to the 1:1 switchgrass char-spherocarbon results, the resulting conversion profiles are

close to the theoretical non-interaction conversion profile, where the switchgrass char and coal

char constituents convert independently. The 1:1 switchgrass char-coal char mixture fits within

±3.74%, as seen in panel (a), and the 3:1 switchgrass char-coal char fits within ±4.91%, as seen

in panel (b). The 1:1 switchgrass char-coal char and 3:1 switchgrass char-coal char sample

attained 50% conversion at 38.6 min and 37.0 min, respectively. Ultimately, the 1:1 switchgrass

char-coal char and 3:1 switchgrass char-coal char sample attained 90% conversion at 19.4 hr and

11.3 hr, respectively.

In contrast to the results in Figure 6-1, there is evidence of a small negative deviation of both

admixtures from the theoretical in the mid-range of the conversion profiles. This is consistent

with a very modest inhibition of the catalyst, but overall, there is no substantial effect. While

strong synergy and inhibition effects were observed in studies by Brown et al. (2000), Parenti

(2009), and Habibi et al. (2013b), none of the studies encountered a case where non-interaction

was observed. As shown in Section 4.1, the onset of intra-particle mobility below typical

gasification temperatures would predict adequate mobility between particles in good contact. It is

likely that the sample introduction, via drop mechanism, into a static fixed bed has produced a

largely non-contacting mixture. A possible mechanism for segregation would arise from the

different aerodynamics of the elongated, low-density switchgrass char particles and the

comparatively more dense and compact coal char and spherocarbon particles. A couple of studies

have implied the significance of the mode and kinetics of potassium mobility. The study by

Collot et al. (1999) regarding the co-gasification silver birch, wood char and Daw Mill, coal char

in a fixed- and fluidized-bed observed a relatively lower catalytic effect in rates in fluidized-bed,

attributing this result due to limited or reduced contact between particles during fluidization.

Kajita, Kimura, Norinaga, Li, and Hayashi (2010) conducted a 850°C steam gasification study

with cedar char admixed with nanoporous γ-alumina where initial catalytic kinetics were

followed by deactivation of mobile potassium on the alumina resulting in kinetics similar to non-

catalyzed, acid-washed cedar char. The thermal conditioning, contact efficiency, and resulting

42

dispersion and mobility of potassium catalyst on carbon may be kinetically significant thereby

affecting apparent reaction rates, as demonstrated in the following experiment.

6.2. Thermal Conditioning in Inert, Argon Atmosphere

The following study aims to evaluate the kinetics of potassium mobility and its effect on

apparent reaction kinetics for the admixture of switchgrass char and spherocarbon/coal char

blends. Thermal conditioning in the form of an inert heat soak was applied prior to steam

gasification. These 1:1 switchgrass char and spherocarbon, 1:1 switchgrass char and coal char,

and 3:1 switchgrass char and coal char blends that are heat soaked are designated HS-1:1-

SGC:SC, HS-1:1-SGC:CC, and HS-3:1-SGC:CC, respectively.

The three blends were directly introduced to an inert, argon atmosphere at 850°C, according to

the experimental protocol detailed in Section 3.3.2. Following 30 min of this thermal

conditioning, a 3:1-steam:hydrogen gas stream was introduced to the reactor via a switching

valve in order to commence gasification. The resulting reactivity metrics of conversion versus

time and reaction rate versus conversion are compared to theoretical conversion profiles and

evaluated.

43

6.2.1. Switchgrass Char with Model Spherocarbon

Figure 6-3: Switchgrass Char, Spherocarbon, 1:1 Switchgrass Char-Spherocabon Direct

Drop, and 1:1 Switchgrass Char-Spherocarbon Heat Soak Conversion versus Time during

Steam Gasification at 850°C

Figure 6-3 compares the conversion profiles of 1:1 switchgrass char-spherocarbon admixtures

with a direct drop in a gasifying atmosphere, as detailed in Section 6.1, to a sample conditioned

with an inert heat soak followed by a gasifying atmosphere. The individual switchgrass char and

spherocarbon conversion profiles are also depicted in Figure 6-3. The 1:1 switchgrass char-

spherocarbon heat soak conversion profile shows a strong synergism and exceeds the 1:1

switchgrass char-spherocarbon direct drop by 15.4-24.5% and is within 69.2-92.6% of

switchgrass char from 20% conversion or 12.1 min onward, as seen in Figure 6-3. Ultimately,

the 1:1 switchgrass char-spherocarbon sample attained 50% and 90% conversion at 49.3 min and

10.7 hr, respectively, which is approximately half the time of 1:1 switchgrass char-spherocarbon

direct drop.

44

Figure 6-4: Switchgrass Char, Spherocarbon, 1:1 Switchgrass Char-Spherocarbon Direct

Drop, and 1:1 Switchgrass Char-Spherocarbon Heat Soak Reaction Rate versus

Conversion during Steam Gasification at 850°C

Figure 6-4 depicts the reaction rates of the heat soak and direct drop 1:1 switchgrass char-

spherocarbon admixtures and its individual constituents over conversion. Maximum reaction

rates for 1:1 switchgrass char-spherocarbon direct drop and 1:1 switchgrass char-spherocarbon

heat soak were 0.0298 and 0.0212 min-1, respectively. The reaction rate profile for 1:1

switchgrass char-spherocarbon direct drop exhibits a lower than theoretical maximum in the

initial 10% of conversion. Subsequently, 1:1 switchgrass char-spherocarbon heat soak exceeds

the 1:1 switchgrass char-spherocarbon direct drop and theoretical profiles and is within 39.6-

59.4% of switchgrass char, in the range of 20-80% conversion.

These observations suggest that a synergistic effect beyond the weighted sum was facilitated by

the inert atmosphere heat soak. The reduction of the reaction rate profile to approximately half of

switchgrass char coincides to an equivalent dilution of K/C ratio in the case of 1:1 switchgrass

char-spherocarbon heat soak. This profile correlation supports the Section 4.2 observations of

dispersed potassium species on spherocarbon structure, originally non-catalyzed, to enhance

45

reaction kinetics. The initially lower reaction rate within the initial 10% conversion of 6.07 min

may be attributed to a transient state such as oxidation of potassium catalyst sites from the inert

heat soak reduced state.

6.2.2. Switchgrass Char with Coal Char

Figure 6-5: Switchgrass Char, Coal Char, 1:1 Switchgrass Char-Coal Char Direct Drop,

and 1:1 Switchgrass Char-Coal Char Heat Soak Conversion versus Time during Steam

Gasification at 850°C

Figure 6-5 compares the conversion profiles of 1:1 switchgrass char-coal char admixtures with a

direct drop in a gasifying atmosphere, as detailed in Section 6.1, to a sample conditioned with an

inert heat soak followed by a gasifying atmosphere. The individual switchgrass char and coal

char conversion profiles are also depicted in Figure 6-5. The 1:1 switchgrass char-coal char heat

soak conversion profile is within the 1:1 switchgrass char-coal char direct drop by ±1.45%, as

seen in Figure 6-5. Ultimately, the 1:1 switchgrass char-coal char heat soak sample attained 50%

and 90% conversion at 37.6 min and 17.9 hr, respectively, which is comparable to 1:1

switchgrass char-coal char direct drop. The 3:1 switchgrass char-coal char heat soak conversion

46

profile exceeds the 3:1 switchgrass char-coal char direct drop by 6.83-14.8% and is within 96.7-

108% of switchgrass char from 20% conversion or 7.37 min onward, as seen in Figure 6-6.

Ultimately, the 3:1 switchgrass char-coal char heat soak sample attained 50% and 90%

conversion at 26.8 min and 4.61 hr, respectively, which is comparable to that of switchgrass

char.

Figure 6-6: Switchgrass Char, Coal Char, 3:1 Switchgrass Char-Coal Char Direct Drop,

and 3:1 Switchgrass Char-Coal Char Heat Soak Conversion versus Time during Steam

Gasification at 850°C

Figure 6-6 compares the conversion profiles of 3:1 switchgrass char-coal char admixtures with a

direct drop in a gasifying atmosphere, as detailed in Section 6.1, to a sample conditioned with an

inert heat soak followed by a gasifying atmosphere. The individual switchgrass char and coal

char conversion profiles are also depicted in Figure 6-6. The 1:1 switchgrass char-coal char heat

soak conversion profile suggest non-interaction between switchgrass char and coal char and

synergy is suggested by the 3:1 switchgrass char-coal char heat soak conversion profile. As

discussed in Section 5.1, coal char has a significant proportion of ash that contains potassium

deactivating species. In contrast to the synergy observed with the 1:1 switchgrass char-

47

spherocarbon heat soak blend, it is possible this reversion to a non-interacting behaviour for 1:1

switchgrass char-coal char heat soak is due to the deactivation of mobile potassium by the ash

components of coal char. The K/Al ratio for 1:1 switchgrass char-coal char heat soak is 0.359,

due to the inclusion of coal ash components in this blend, resulted in non-interaction that

contrasts the inhibition observed by Habibi et al. (2013b). The non-interaction profile may be

within the saturation range of the K/Al ratio. In such a case, the coal char portion deactivates

mobile potassium and continues to exhibit individual reactivity. Accordingly the switchgrass

char portion has reduced levels of catalyst due to potassium mobility, but maintains individual

reactivity due to rate-controlling, active carbon site limitations. The K/Al ratio for 3:1

switchgrass char-coal char heat soak is 1.29, nearly three times greater than 1:1 switchgrass char-

coal char heat soak, due to the increase in potassium catalyst species and decrease in deactivating

ash species. The observed synergy of 3:1 switchgrass char-coal char heat soak may be the result

of the saturation of deactivating species and excess potassium to enhance reactivity, as seen by

Habibi et al. (2013b) in switchgrass ash and coal char blends.

48

Figure 6-7: Switchgrass Char, Coal Char, 1:1 Switchgrass Char-Coal Char Direct Drop,

and 1:1 Switchgrass Char-Coal Char Heat Soak Reaction Rate versus Conversion during

Steam Gasification at 850°C

Figure 6-7 depicts the reaction rates of the heat soak and direct drop 1:1 switchgrass char-coal

char admixtures and its individual constituents over conversion. Maximum reaction rates for 1:1

switchgrass char-coal char direct drop and 1:1 switchgrass char-coal char heat soak were 0.0346

and 0.0340min-1, respectively. The aforementioned enhancement in coal char reactivity

attributed to rapid thermal conditioning and macropore active sites within the initial 20%

conversion is not observed. Both 1:1 switchgrass char-coal char direct drop and 1:1 switchgrass

char-coal char heat soak coincide with that of theoretical, non-interacting switchgrass char-coal

char from 20% conversion onward. Figure 6-8 depicts the reaction rates of the heat soak and

direct drop 3:1 switchgrass char-coal char admixtures and its individual constituents over

conversion. Maximum reaction rates for 3:1 switchgrass char-coal char direct drop and 3:1

switchgrass char-coal char heat soak were 0.0517 and 0.0346 min-1, respectively. 3:1 switchgrass

char-coal char heat soak exhibited an enhanced profile shortly after its maximum at 10%

conversion that exceeds or equals that of switchgrass char.

49

Figure 6-8: Switchgrass Char, Coal Char, 3:1 Switchgrass Char-Coal Char Direct Drop,

and 3:1 Switchgrass Char-Coal Char Heat Soak Reaction Rate versus Conversion during

Steam Gasification at 850°C

The reaction rates of the 1:1 switchgrass char-coal char heat soak and 3:1 switchgrass char-coal

char heat soak blends are comparable for the initial 30% conversion, attaining similar reaction

rates. Following 30% conversion, 1:1 switchgrass char-coal char heat soak exhibits profile

features similar to that of non-catalyzed coal char, whereas 3:1 switchgrass char-coal char heat

soak exhibits profile features comparable to switchgrass char. Contributions of individual blend

components to particular profile features is limited. Habibi (2013a) observed enhanced, coal char

reactivity greater than switchgrass char when coal and switchgrass ash to were co-pyrolyzed.

This may indicate that switchgrass char reactivity is limited by active carbon sites compared to

coal char, given a high concentration and uniform dispersion of potassium catalyst. An

implication of such dissimilar carbon matrices and structures in the case of 3:1 switchgrass char-

coal char heat soak is that transfer of potassium catalyst species allow for a significant increase

50

in the reaction rate of the coal char component, which results in a total reaction rate greater than

individual switchgrass char.

6.3. Extent of Potassium Mobility on Synergy

This study has established that potassium mobility may result in synergistic effects on apparent

reaction kinetics, but under a thermal conditioning of an inert heat soak. This specific case

introduces implications regarding the extent of catalyst mobility on the kinetics of the reaction

mechanism.

The rate of potassium mobility on a carbon substrate is undetermined. Furthermore, the effect of

gas atmosphere on potassium mobility should be considered, as the non-interacting behaviour

persisted during the conversion of direct drop, steam gasification blends that spanned hours. In

contrast, a synergistic effect resulted from a relatively brief, 30 min, inert heat soak. It is unlikely

this contrast is due to potassium immobility in steam atmospheres because intra-particle mobility

in a steam atmosphere above the mobility onset temperature was observed ex situ, with the

extent and uniformity of potassium dispersion is inconclusive. The interaction and impact of

thermal conditioning, gas atmosphere, and potassium mobility on synergistic kinetics require

further investigation.

The development of the carbon structure and matrix during conversion and its effect on the

catalyzed reaction mechanism is undetermined. The variations in carbon structure and matrix are

known to be substrate specific and impact the reactivity. A catalyst-limited system would exhibit

an increase in reaction rate as the K/C ratio increased with conversion. The general decline of the

reaction rate profiles observed in this study may imply that the catalytic effect of potassium is

limited by the availability of carbon active sites, which is governed by the inherent substrate

properties. Furthermore, dispersion over dissimilar carbon substrates, via potassium mobility,

may manifest into indeterminate, transient effects and profile features. The reduced maximums

for the inert heat soak, reaction rate profiles may be the result of the re-distribution to a carbon

substrate with a lower fraction of active carbon sites. The dispersion and distribution of mobile

potassium among active and inactive carbon sites of dissimilar, carbon substrates and its impact

on synergistic kinetics requires further investigation.

51

Chapter 7 Conclusions & Recommendations

7.1. Conclusions

The progression of in-situ, intra-particle mobility of potassium on a model carbon wafer was

observed with a Time-of-Flight – Secondary Ion Mass Spectrometer (ToF-SIMS) under high

vacuum. The onset of potassium mobility occurred at approximately 300°C, which is below

typical gasification temperatures. The rate of mobility was not determined because the potassium

dispersed into the bulk of the model carbon wafer and is invisible to the surface sensitive ToF-

SIMS analytical technique.

Inter-particle mobility of potassium was observed on ex situ, partially converted, carbon

substrates with ToF-SIMS surface analysis. Potassium catalyst originating from spherocarbon

impregnated with potassium carbonate and mineral component of switchgrass char transferred to

non-catalyzed, model carbon particles subjected to a steam gasification atmosphere of 700°C for

2 hr. Selection of switchgrass as a source of mobile potassium catalyst was confirmed.

Switchgrass char, coal char, and spherocarbon were selected and characterized for binary blend

co-gasification. Spherocarbon was observed to be the least reactive as expected of a model

carbon with no ash component containing catalytic species. Switchgrass was composed primarily

of volatiles, to produce char with low proportion of ash. Coal was composed relatively equal

amounts of ash, volatiles, and fixed carbon, to produce char with high proportion of ash.

Switchgrass ash contained a high proportion of potassium catalyst, whereas coal ash contained

low amounts of a variety of catalytic species with a high proportion of catalyst deactivating

alumina. Switchgrass char was relatively more reactive than coal char during steam gasification,

which was attributed to it inherent potassium catalyst. Coal char exhibited initially high

reactivity, which was attributed to the impact of rapid thermal conditioning on carbon active site

and structure development and inherent catalyst species. The following decline in reactivity was

attributed to carbon matrix stabilization and agglomeration of immobile catalyst species.

52

Steam gasification of binary blends of switchgrass char with coal char/spherocarbon subjected to

rapid thermal conditioning exhibited largely non-interacting behaviour in contrast to previous

reports of synergy and inhibition. A thermal pretreatment involving an inert argon heat soak was

implemented, to pre-disperse the potassium. Synergy was observed following this pretreatment

for binary blends of 1:1 switchgrass char with spherocarbon. Non-interaction of 1:1 switchgrass

char with coal char was attributed to the introduction of catalyst deactivating species in the ash

component of coal. Synergy was again observed with 3:1 switchgrass char with coal char due to

the increase in potassium catalyst and decrease in coal ash.

7.2. Recommendations

This investigation has established intra-particle and inter-particle mobility of potassium catalyst

as the mode for enhanced reactivity and synergistic co-gasification of binary blends. Further

studies are required to maximize potassium mobility and dispersion and to determine sub-

optimal conditions where potassium mobility is kinetically limited and binary blends exhibit

non-interaction.

Quantification of intra-particle and inter-particle potassium mobility will allow for optimization

of thermal conditions for effective catalyst dispersion. Intra-particle migration of potassium

beyond the detectable range of the ToF-SIMS technique may be addressed by the refinement of

the sample preparation protocol by increasing potassium loading and reducing carbon wafer

thickness. Inter-particle mobility rates may be quantified by the development of a statistical

method for particle sample counts imaged by the ToF-SIMS.

Variation of gas atmospheres, such as inert, reactant, and product, and thermal conditioning will

determine optimal process parameters for enhanced reaction kinetics. A specific case to consider

is direct introduction of the sample to a steam gasification atmosphere, followed by a switch to

inert argon, and then reversion to a steam atmosphere in order to determine if thermal

conditioning or gas atmosphere has greater effect on synergy.

Incorporation of a systematic study of carbon materials, catalysts, and ash for co-gasification will

allow for the analysis of the interaction between catalyst and active carbon site. Blends of coal

char with potassium carbonate and/or switchgrass ash may elucidate the saturation limit of

53

catalyst deactivating species in the coal ash. Blends of switchgrass ash with potassium carbonate

and/or switchgrass ash may determine if switchgrass char is catalyst or carbon active site limited.

Integration of partially converted sample extraction for ToF-SIMS analysis with macroscopic

kinetic analysis of co-gasification will provide insight into the progress of the individual

components of the binary blends during co-gasification. Imaging of partially converted samples

may explain ambiguous features observed in conversion and reaction rate profiles. In

conjunction with a refined statistical method of particle count samples imaged by the ToF-SIMS

technique, the effect of particle contact on potassium mobility in a fixed bed may also be

elucidated.

54

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