The Pennsylvania State University

The Graduate School

College of Earth and Mineral Sciences

EFFECT OF COAL RANK DURING OXY-FUEL COMBUSTION: ROLE OF CHAR-

CO2 REACTION

A Thesis in

Energy and Mineral Engineering

by

Sakthivale Roshan Dhaneswar

2011 Sakthivale Roshan Dhaneswar

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

August 2011

The thesis of Sakthivale Roshan Dhaneswar was reviewed and approved* by the following:

Sarma V. Pisupati

Associate Professor of Energy and Mineral Engineering

Thesis Advisor

Sharon F. Miller

Research Associate at EMS Energy Institute

Graduate Faculty, Department of Energy and Mineral Engineering

Yaw D. Yeboah

Professor of Energy and Mineral Engineering

Head of the Department of Energy and Mineral Engineering

*Signatures are on file in the Graduate School

iii

ABSTRACT

The role of coal rank in oxy-fuel combustion is investigated using a suite of four coals: a

low volatile bituminous coal, a high volatile bituminous coal, a subbituminous coal and a lignite.

The study was conducted using the -170+200 mesh fraction coal in a drop-tube reactor (DTR) at

1,873 K to generate chars at three residence times under air, 21% O2/79% CO2 (oxy-fuel

combustion) and 30%O2/70%CO2 (enhanced oxy-fuel combustion) atmospheres. It was observed

that oxy-fuel combustion produced a higher carbon conversion compared to that in air for the

low-rank subbituminous coal and lignite and also for the high volatile bituminous coal at longer

residence times. This effect was even more pronounced at shorter residence times than at longer

residence times for all ranks of coal. For the high-rank low volatile coal, air combustion produced

a higher carbon conversion than oxy-fuel combustion. Also, as the fraction of O2 was increased in

the mixture of O2/CO2, the carbon conversion was found to increase. The higher carbon

conversion in low-rank coals under oxy-fuel conditions was attributed to the char-CO2

gasification reactions occurring at high temperatures. The activation energy of the char-CO2

reaction for low-rank coals was more sensitive to temperature than high-rank coals, and the effect

was more pronounced in enhancing the carbon conversion. Investigation of the catalytic activity

(specifically in catalyzing the char-CO2 reaction) of ion-exchangeable cations in lower rank coal

showed that there was a marked decrease in the isothermal reactivity at lower temperatures in a

TGA. However, the same effect was not noticed in the drop tube reactor at higher temperatures.

The lack of catalytic activity at higher temperatures (> 1,000 K) was attributed sintering of CaO

at high temperatures The BET surface area of the chars was found to decrease and the average

pore diameter was found to increase with decrease in coal rank showing the higher concentration

of macropores in low-rank coals, leading to higher reactivity. Modified burning and reactivity

profiles were generated using a thermogravimetric analyzer. The theoretically extrapolated

iv

reaction rates corroborated well with the combustion test results with the lower rank coals

showing higher char-O2 and char-CO2 reactivity. The effectiveness factors for the char-O2 and

char-CO2 reactions indicated a higher degree of diffusion control for the char-O2 reaction. At

similar temperature, CO2 molecules are able to access a higher percentage of the internal surface

area compared O2 molecules due to relatively lower reactivity of char-CO2 reaction. The results

also indicated that carbon conversion was a function of both overall reactivity and effectiveness

factor. The influence of the char-CO2 reaction on particle temperature was theoretically modeled

and the particles were at much higher temperatures compared to the gas at lower temperatures

due to absence of the char-CO2 reaction. However, the particles approached the gas temperature

at higher temperatures due to the endothermic char-CO2 reaction.

v

TABLE OF CONTENTS

LIST OF FIGURES..................................................................................................................viii

LIST OF TABLES......................................................................................................................xi

ACKNOWLEDGEMENTS.......................................................................................................xii

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

Chapter 2 Literature Review .................................................................................................... ....4

2.1 Gas and Particle Temperatures..................................................................................4

2.2 Flame Characteristics................................................................................................5

2.3 Ignition Characteristics.............................................................................................6

2.4 Effect of Pressure......................................................................................................7

2.5 Char Morphology......................................................................................................7

2.6 Char Conversion and Reactivity...............................................................................8

2.7 Char Conversion rate Parameters Estimation.........................................................22

2.8 Process Feasibility and Economics.........................................................................25

2.9 Summary.................................................................................................................26

Chapter 3 Problem Statement .................................................................................................. .28

3.1Methodology............................................................................................................28

3.2Hypothesis...............................................................................................................29

Chapter 4 Experimental Details ............................................................................................... 31

4.1 Coal Sample Preparation........................................................................................31

4.2 Drop Tube Reactor.................................................................................................31

4.2.1 Combustion Test Conditions........................................................................33

4.2.2 Devolatilization Test Conditions..................................................................34

vi

4.3 Microproximate Analysis.......................................................................................35

4.4 Determination of Char Reactivity..........................................................................36

4.5 Modified burning Profiles......................................................................................37

4.6 Compositional Analysis..........................................................................................38

4.6.1 CHN Analysis...............................................................................................38

4.6.2 Sulfur Analysis..............................................................................................38

4.7 Surface Area Analysis.............................................................................................39

4.8 Major/Minor Oxide Analysis of Ash......................................................................39

4.9 Removal of ion-exchangeable cations....................................................................40

Chapter 5 Results and Discussion ............................................................................................ .42

5.1 Combustion Test Results - Parent coals..................................................................42

5.2 Devolatilization and Isothermal Reactivity Studies................................................48

5.3 Theoretical char-O2 and char-CO2 reaction rate study............................................58

5.4 Theoretical char particle temperature model...........................................................67

5.5 Combustion Test Results - AAW coals..... .............................................................73

5.6 Thermogravimetric modified burning profiles under O2/N2 and O2/CO2

atmospheres...................................................................................................................75

Chapter 6 Summary, Conclusions and Recommendations ...................................................... .78

6.1 Summary and Conclusions.....................................................................................78

6.2 Future Recommendations.......................................................................................80

Appendix A Calculation of extrapolated rate at combustion condition using intrinsic

model ........................................................................................................................ .81

Appendix B Example of char conversion calculation..... ................................................ .84

Appendix C Sample calculation to extract intrinsic rate parameters from an

isothermal char reactivity plot .................................................................................. .85 Appendix D Proximate and Ultimate Analysis of DTR chars..........................................86 Appendix E Plots of Rd, Rs, Rac and R as a function of temperature for the four

vii

coals...........................................................................................................................90

Appendix F Sample char particle temperature calculation ............................................. .94

References..................................................................................................................................96

viii

LIST OF FIGURES

Figure 1-1. General flow process of oxy-fuel combustion..........................................................2

Figure 2-1. Plot of reaction rate vs. inverse temperature depicting three zones..........................9

Figure 2-2. Schematic diagram depicting a shrinking particle with an unreacted shrinking

core, and concentration profile of gas reactant..........................................................................10

Figure 2-3. General shape of the effectiveness factor vs. temperature curve............................19

Figure 2-4. Comparison of char reaction with O2 (oxidation) and CO2 (gasification)..............21

Figure 2-5. Low temperature TGA kinetic data extrapolated to high temperatures

(spheres: O2, rectangles: steam, triangles: CO2)........................................................................22

Figure 2-6. Reactivity profiles in air at 673 K for chars obtained under different pyrolysis

conditions...............................................................................................................24

Figure 4-1. Schematic of Drop tube reactor...............................................................................32

Figure 5-1. Carbon conversion (%) vs. residence time (s) for Pocahontas coal........................46

Figure 5-2. Carbon conversion (%) vs. residence time (s) for Pittsburgh coal..........................46

Figure 5-3. Carbon conversion (%) vs. residence time (s) for Dietz coal..................................47

Figure 5-4. Carbon conversion (%) vs. residence time (s) for Beulah lignite...........................47

Figure 5-5. Char-O2 reactivity profiles for Pocahontas coal......................................................52

Figure 5-6. Char-O2 reactivity profiles for Pittsburgh coal........................................................53

Figure 5-7. Char-O2 reactivity profiles for Dietz coal...............................................................53

Figure 5-8. Char-O2 reactivity profiles for Beulah lignite.........................................................54

Figure 5-9 Char-CO2 reactivity profiles for Pocahontas coal....................................................54

Figure 5-10. Char-CO2 reactivity profiles for Pittsburgh coal...................................................55

Figure 5-11. Char-CO2 reactivity profiles for Dietz coal...........................................................55

Figure 5-12. Char-CO2 reactivity profiles for Beulah lignite....................................................56

ix

Figure 5-13. Extrapolated oxidation rates for the four coals used in the study.........................60

Figure 5-14. Char-O2 reaction effectiveness factors for the four coals used in the

study...........................................................................................................................................60

Figure 5-15. Extrapolated gasification rates for the four coals used in the study......................61

Figure 5-16. Char-CO2 reaction effectiveness factors for the four coals used in the

study...........................................................................................................................................61

Figure 5-17. Variation of effectiveness factor and intrinsic reaction rate for char-O2

reaction of Pittsburgh coal.........................................................................................................62

Figure 5-18. Variation of effectiveness factor and intrinsic reaction rate for char-CO2

reaction of Pittsburgh coal.........................................................................................................62

Figure 5-19. Extrapolated oxidation Rac and Rd for the four coals used in the study................64

Figure 5-20. Extrapolated gasification Rac and Rd for the four coals used in the study.............64

Figure 5-21. Char-O2 effectiveness factors for two particle sizes of Dietz coal........................65

Figure 5-22. Char-O2 effectiveness factors for two particle sizes of Dietz coal........................65

Figure 5-23. Computed char particle temperature (K) vs. oxygen concentration(%) at a

gas temperature of 1,373 K under oxy-fuel conditions..............................................................70

Figure 5-24. Computed char particle temperature (K) vs. oxygen concentration(%) at a

gas temperature of 1,573 K under oxy-fuel conditions..............................................................70

Figure 5-25. Computed char particle temperature (K) vs. oxygen concentration(%) at a

gas temperature of 1,673 K under oxy-fuel conditions..............................................................71

Figure 5-26. Computed char particle temperature (K) vs. oxygen concentration(%) at a

gas temperature of 1,773 K under oxy-fuel conditions..............................................................71

Figure 5-27. Computed char particle temperature (K) vs. oxygen concentration(%) at a

gas temperature of 1,873 K under oxy-fuel conditions..............................................................72

Figure 5-28. Simulated temperature (oC) profiles for an oxy-fuel PC boiler.............................72

x

Figure 5-29. Carbon conversion (%) vs. residence time (s) for Dietz and AAW Dietz

coal.............................................................................................................................................73

Figure 5-30. Carbon conversion (%) vs. residence time (s) for Beulah and AAW

Beulah lignite.............................................................................................................................75

Figure 5-31. Thermogravimetric burning profile generated under air for the four coals

and a switchgrass sample used in the study...............................................................................76

Figure 5-32. Thermogravimetric modified burning profile generated under 21%O2/79%CO2

for the four coals and a switchgrass sample used in the study...................................................76

Figure C-1. Plot of ln (k) vs. 1/T for Beulah char-O2 reactivity................................................85

Figure E-1. Extrapolated oxidation Rs, Rac, Rd and R for Pocahontas coal...............................90

Figure E-2. Extrapolated gasification Rs, Rac, Rd and R for Pocahontas coal............................90

Figure E-3. Extrapolated oxidation Rs, Rac, Rd and R for Pittsburgh coal.................................91

Figure E-4. Extrapolated gasification Rs, Rac, Rd and R for Pittsburgh coal..............................91

Figure E-5. Extrapolated oxidation Rs, Rac, Rd and R for Dietz coal.........................................92

Figure E-6. Extrapolated gasification Rs, Rac, Rd and R for Dietz coal.....................................92

Figure E-7. Extrapolated oxidation Rs, Rac, Rd and R for Beulah lignite...................................93

Figure E-8. Extrapolated gasification Rs, Rac, Rd and R for Beulah lignite...............................93

xi

LIST OF TABLES

Table 1-1. Typical flue gas composition......................................................................................3

Table 2-1. Char-CO2 reaction activation energy values reported in literature...........................23

Table 4-1. Typical PC boiler residence times............................................................................34

Table 4-2. DTR combustion test parameters.............................................................................34

Table 5-1. Analysis of coal and switchgrass samples................................................................44

Table 5-2. Major and minor elemental oxide analysis of the ash reported as oxides (%) for the

coals used in the study...............................................................................................................45

Table 5-3. Reproducibility of Combustion tests........................................................................48

Table 5-4. Analysis of char samples from DTR........................................................................49

Table 5-5. Surface area analysis of devolatilization char samples.............................................50

Table 5-6. Char-O2 reactivity for the four coals........................................................................57

Table 5-7. Char-CO2 reactivity for the four coals......................................................................57

Table 5-8. Oxidation and gasification rate parameters for the four coals..................................57

Table 5-9. Rate Parameters used in the theoretical particle temperature model........................68

Table 5-10. Characteristic temperatures under air and oxy-fuel atmospheres...........................77

Table D-1. Proximate and Ultimate Analysis of Pocahontas combustion test chars.................86

Table D-2. Proximate and Ultimate Analysis of Pittsburgh combustion test chars...................86

Table D-3. Proximate and Ultimate Analysis of Dietz combustion test chars..........................87

Table D-4. Proximate and Ultimate Analysis of Beulah combustion test chars........................88

Table D-5. Proximate and Ultimate Analysis of AAW Dietz and Beulah combustion test

chars...........................................................................................................................................88

xii

ACKNOWLEDGEMENTS

First, I wish to express my deep sense of gratitude to my advisor, Dr. Sarma V. Pisupati

for his constant support, guidance and motivation. The reason I have been able to complete this

work successfully is because of the essential skills of critical thinking and analysis and technical

writing he has helped me imbibe over the course of my research. Secondly, I thank the

Department of Energy and Mineral Engineering and Dr. Yaw D. Yeboah for providing me

support by means of Teaching Assistantship throughout my Masters. I am grateful to my

committee members, Dr. Yaw D. Yeboah and Dr. Sharon Miller for their inputs in refining my

thesis. Next, I thank the Energy Institute for permitting me to use their facilities and equipment. I

acknowledge the support and cooperation of Ron Wincek, Keith Miska, Ron Wasco, Tom Motel,

Magda Salama and Henry Gong for helping me perform my analysis and experiments and

troubleshooting problems.

I also acknowledge the contribution of research group members, Vijay, Nari and Aime

and Research Associate, Dr. Nandakumar Krishnamurthy towards my research my means of

evoking interesting technical discussions. I am grateful to Dr. Prabhat Naredi, previous research

group member whose thesis was also based on oxy-fuel combustion. His thesis helped me get

started with my research, and in gaining a deeper understanding of the technology. I owe a special

thanks to my group of friends particularly my roommates, Githin and Vivek Raja for providing

me a life away from my lab and making me feel at home. And finally I thank my parents,

Sakthivale and Vijayalakshmi and sister, Priyankari for their constant love and support and belief

in me.

Chapter 1

Introduction

During the 1990's, the need for reducing greenhouse gases was recognized. Among the

greenhouse gases, CO2 emitted into the atmosphere from burning fossil fuels, contributes about

82% to "global warming"[1]. Carbon dioxide capture and storage (CCS) is now included in most

OECD countries' energy policies and R&D programs as one of the strategies to mitigate carbon

dioxide emissions from large emitters[2].

The power generation industry accounts for nearly 40% of the CO2 emissions[3].

Alternatives to conventional air-blown pulverized coal (PC) combustion, such as coal

gasification, integrated gasification combined cycle (IGCC) and pressurized combustion are

being investigated to enhance energy efficiency, reduce greenhouse gas and pollutant emissions,

and minimize the size and capital cost of future coal-based power plants. However, one of the

most promising near-term alternatives to conventional PC combustion is oxy-fuel combustion

(atmospheric pressure PC combustion in mixtures of oxygen and recirculated flue gas)[4] and is

being considered as one of the leading carbon capture technologies for the power generation

industry[2, 5-7].

Conventional pulverized fuel coal-fired boilers use air for combustion in which nitrogen

in air (approximately 79% by volume) dilutes the CO2 concentration in the flue gas. The capture

of CO2 from such dilute mixtures by stripping using chemicals such as amines is relatively

expensive. During oxy-fuel combustion, a combination of oxygen (typically greater than 95%

purity)[8] and recycled flue gas (RFG) is used for combustion of the fuel. By recycling the flue

gas, a gas consisting mainly of CO2 (~90%[9]) and water vapor is generated, which can be

sequestered (or stored as solid CO2 in gas hydrates[10]) without stripping of the CO2 from the gas

2

stream[11]. The water vapor is usually removed before compression and storage of CO2 so that it

does not condense and form acids. Alternatively, CO2 can be used for methane recovery from un-

mineable coal seams, enhanced oil recovery (EOR) from depleted oil wells or even for production

of value added chemicals.

A general flow process of oxy-fuel combustion is shown in Figure 1-1. An oxy-fuel PC

fired power plant consists of four main units: oxygen generation, oxy-fuel (O2/CO2) combustion,

flue gas treatment and CO2 recovery/disposal. Wet (hot) gas recycling rather than dry gas (cold)

recycling has been shown to improve economy[12]. The H2O concentration in the flue gas stream

is generally between 10-40% depending on fuel moisture content and whether it is wet or dry

recycling[9]. The recycled flue gas is used to control flame temperature and to make up the

volume of missing N2 to ensure that there is enough gas to carry the heat through the boiler[11].

Figure 1-1. General flow process for oxy-fuel combustion[11]

*ASU - Air separation unit

To produce high purity oxygen, air is separated into oxygen using four methods of

separation: cryogenic separation method, adsorption method, membrane separation method and

chemical separation method. Of these four methods, cryogenic separation yields the best

results[12] for large quantities. Burdyny and Struchtrup[13] have recently proposed a new

method wherein air is drawn into an O2/N2 separation membrane using vacuum to produce

Cold RFG Hot RFG

3

oxygen enriched air and then using cryogenic distillation to produce high purity oxygen, thereby

reducing total energy requirements.

The advantages of oxy-fuel combustion include: (1) no need to separate CO2 from the

flue gases, (2) improved boiler efficiency, (3) low power consumption due to small amount of

flue gas is involved (20% lesser than conventional air combustion[9]), and (4) no denitrification

and desulphurization requirement[14]. Although the SO2 concentration is found to be 3-4 times

higher for oxy-fuel combustion due to accumulated effect of recycled flue gas and reduced gas

volumes, the mass emission rate (lb/MMBtu) is significantly lower than that in conventional air

combustion. The NOx concentration also increases due to reduced gas volume but the mass

emission rate is significantly lower than that in conventional air combustion. Furthermore, most

of the NOx is reduced due to recycle in the flame region of the furnace[9, 15]. Table 1-1 shows

the typical flue gas composition for a conventional PC system and an oxy-fuel system.

Table 1-1. Typical flue gas composition[16]

Parameter Conventional PC

Combustion

Oxy-fuel PC Combustion

O2 (%) 3.2 3.1

CO2 (%) 14.65 69

H2O (%) 5.85 27.5

NOx (ppm) 154 82

Another major advantage of oxy-fuel combustion is that the same furnace that is used for

conventional combustion may be used for oxy-fuel combustion with an appropriate exhaust gas

recycle ratio[17, 18]. Thus, with rising global need to control CO2 emissions, oxy-fuel

combustion offers a quick and economic means to retrofit existing plants. This research attempts

to understand the effect of coal rank on carbon conversion.

4

Chapter 2

Literature Review

The major difference between oxy-fuel combustion and conventional air combustion is

the replacement of N2 with recycled flue gas (primarily CO2). This leads to changes in flame

propagation speed, flame stability and flame temperatures[19]. Also, CO2 has a higher specific

heat[8] and emissivity[15] compared to N2 which means that CO2 can absorb more heat and

radiation as compared to N2 and affect the heat transfer characteristics within the boiler.

2.1 Gas and Particle Temperatures

The gas temperatures are lower in O2/CO2 combustion than in O2/N2 combustion with

similar O2 concentrations though higher O2/CO2 ratios produce similar temperature profiles as

combustion in air. It has been shown that a 30%O2/CO2 mixture can produce matching gas

temperature profiles to those of combustion in air[8]. Particles burn at lower temperatures and

require longer combustion times in O2/CO2 compared to combustion in O2/N2[20] and this is due

to the lower gas temperatures in an oxy-fuel environment caused by the higher specific heat of

CO2 (1.6 times that of N2)[21]. The particle temperatures are also reported to be lower compared

to those in air combustion. Since CO2 is a reactive gas, Hampartsoumian et al.[22] attributed the

reduction in particle temperatures to endothermic char-CO2 reaction taking place in oxy-fuel

combustion. An increase in furnace temperature by 200 K, results in an increase of over 100 K in

both gas and char temperatures[20]. Many studies [8, 15, 20, 23] also have reported that char

particle temperatures are similar in air and 30% O2/CO2 mixture. As the oxygen mole fraction in

5

the combustion medium is increased, char particle temperatures increase and combustion times

decrease in both atmospheres. Single particle studies have shown an increase in particle

temperature from 1,850 K at an O2 mole fraction of 0.2 to 3,200 K at an O2 mole fraction of 1.0.

The same study also has shown that peak char particle temperatures for lower rank coals like

lignites (2700 K) were similar, if not slightly higher than those for bituminous coals (2650

K)[20].

While char-CO2 reaction consumes carbon and increases carbon conversion, the reaction

being endothermic, also reduces the temperature and therefore the reaction rate. Hence, the net

effect depends on the balance between the two. The effect of this when burning different ranks of

coal is important.

2.2 Flame Characteristics

The flame is less bright in oxy-fuel combustion due to CO2 molecule's ability to absorb

IR radiation[15] and the volatile flame temperature is reduced by as much as 200 K under oxy-

fuel conditions as compared to air[20]. Hjartstam et al.[24] and Smart et al.[25] have shown that

with reduced amount of recycled flue gas, more intense combustion and higher flame

temperatures could be achieved. A higher concentration of O2 in CO2 (35%O2/CO2 mixture) has

been shown to produce flame characteristics similar to those of combustion in air[15, 19] and this

is because higher O2 concentrations lead to higher flame temperatures[4]. Andersson et al.[26]

observed no significance difference in total radiation (sum of particle and gas radiations) between

oxy-fuel and air firing. They concluded from their study that the particle radiation constitutes a

significant fraction of the total radiation and that the total intensities become similar for air and

oxy-fuel firing as long as the gas temperatures are similar due to similar particle radiation in each

case. Smart et al.[27] conducted radiation and convective heat transfer studies in a pilot-scale test

facility and have shown that radiative heat flux is inversely related to the recycle ratio though

6

convective heat flux increases with increasing recycle ratio and that it was possible to have a

working range of recycle ratios where both convective and radiative heat fluxes were comparable

to air.

2.3 Ignition Characteristics

Ignition is delayed in oxy-fuel combustion as compared to air combustion of similar

oxygen concentrations[28-31]. To study the ignition characteristics in a CO2 rich atmosphere,

Kiga et al.[17] measured the flame propagation speed of a pulverized-coal cloud in O2/CO2,

O2/N2 and O2/Ar atmospheres using a microgravity combustion chamber. They found the flame

propagation speeds in an O2/CO2 atmosphere were markedly lower compared to that in O2/N2

and O2/Ar which they attributed to the difference in specific heat (CO2 has the highest specific

heat among all the gases) since the difference in thermal conductivity of each gas mixture was not

large. Shaddix and Molina[28] reported that a higher O2 concentration in oxy-fuel combustion

can produce matching ignition times to air combustion. Particle devolatilization is delayed in oxy-

fuel combustion[28] though it has been shown that the volatile yield is higher[32, 33] as

compared to air combustion. In thermogravimetric studies, Rathnam et al.[32] attributed the

higher volatile yield in a CO2 environment to the onset of the char-CO2 gasification reaction at

1,030 K, which significantly increased the mass loss. Zhang et al.[34] said that coal particle

ignition occurs in oxy-fuel environments as a volatile cloud due to the accumulation of unburnt

volatiles rather than as individual particles in air combustion. Qiao et al.[31] claim that the

endothermic char-CO2 reaction is responsible for the higher particle ignition temperature in air

compared to oxy-fuel combustion.

7

2.4 Effect of Pressure

Pressurized oxy-fuel combustion has been shown to increase the net efficiency during

oxy-fuel combustion due to lesser fuel requirements, lesser pollutants and CO2 emissions. Higher

pressure reduces the distance of the volatile flame from the particle and increases the temperature

of the flame. As a result, heat transfer from the flame to particle and the devolatilization rate and

extent are increased. The pressure also affects the rate of gas phase reactions (combustion of CO

to CO2) in the boundary layer in the char combustion stage. The combustion in the boundary

layer also increases heat transfer to the particle, increasing the char reactivity[35]. Hong et al.[36]

and Deng et al.[37] compared cases operating at atmospheric pressure and pressurized conditions

and found a 3% increase in power plant net efficiency in the latter case. They performed a

thermodynamic analysis to identify the performance difference in an ambient and pressurized

boiler process and found that the net efficiency improvement of 3% was due to an increase of

4.14% in boiler efficiency, a decrease of 2.73% in steam cycle efficiency, and an increase of

1.61% in efficiency from auxiliary load decrease. They further stated that a pressurized system

helps to recover more heat from the flue gas and aids purification and compression of a

concentrated CO2 stream, thus increasing the efficiency.

2.5 Char Morphology

Borrego and Alvarez[19] characterized chars obtained under both air and oxy-fuel

environments. They found more vesiculated particles in oxy-fuel chars compared to air

combustion chars while network structures and voids distributed throughout the particle's surface

are more common in air combustion chars indicating a lower capacity of the bubbles (large voids)

to coalesce. Li et al.[38] found similar characteristics in the chars generated under an oxy-fuel

environment and air with oxy-fuel chars having thicker surfaces and compact pores, contributing

to a reduction in fragmentation. Also chars obtained under air were found to be round in shape

8

with a limited amount of secondary devolatilization within the voids and hence were considered

to be more isotropic compared to oxy-fuel combustion chars. In terms of reflectance, vitrinite

reflectance increases for chars compared to the parent coal reflectance value due to volatile

release in the furnace and carbon enrichment. Reflectance values are higher for oxy-fuel chars

compared to air combustion chars. However, in higher O2 concentrations in oxy-fuel

environments, the values for both oxy-fuel chars and air combustion chars were found to be

similar[19]. A similar trend was noticed for inertinite reflectance with oxy-fuel inertinites having

higher values compared to air combustion inertinites. Reflectance values showed a drop with

increasing O2 concentration. An increase in reflectance values indicates an ordering of char

structure and carbonization which could have a negative impact on carbon conversion. In general,

it was found that for low O2 concentrations, the chars had intact walls and granular appearance

while more extensively burned chars had large coalesced voids[19]. Since this study looks into

the effect of coal rank, the above results may not be valid for every rank of coal due to difference

in conversion rates in air and oxy-fuel atmospheres.

2.6 Char Conversion and Reactivity

Char reactions can occur in three zones as shown in Figure 2-1. At low temperatures, the

chemical reaction rate is slow compared to diffusion and the chemical reaction is the rate

determining step (zone I). In zone II, the overall reaction rate is controlled by both chemical

reaction and pore diffusion and this occurs at moderate temperatures. In zone III, mass transfer

limitations in the boundary layer of the particle control the overall reaction rate and this occurs at

high temperatures.

9

Figure 2-1. Plot of reaction rate vs. inverse temperature depicting three zones[39]

The unreacted shrinking core model shown in Figure 2-2 is the most simple and best

macroscopic approach to model gas-solid reactions[39]. Reactions take place on the surface of the

particle and the reaction front recedes towards the center of the particle with time as shown in the

figure. As the reaction takes place by consuming carbon, an ash layer forms. The reactant gas has

to diffuse to the particle and product gases have to diffuse away from the particle.

10

Figure 2-2. Schematic diagram depicting a shrinking particle with an unreacted shrinking core,

and concentration profile of gas reactant[40] (*CAc is the concentration of gaseous reactant at the

center of the particle, CAs is the concentration at the surface and CAb is the bulk concentration)

11

This sequential operation involves three basic processes: gas film diffusion, ash layer diffusion

and surface reaction of the central core. The overall rate coefficient depends on these three basic

resistances, any of which can be rate controlling[39].

Char reacts heterogeneously with oxidizer gas (O2/CO2/H2O) and the reactions are given

by[41],

C + 1/2O2 CO Ho = -9.2*10

6 J/kg 2.1

C + CO2 2CO Ho = +1.439*10

7 J/kg 2.2

C + 2H2O 2CO + H2 Ho = +1.096*10

7 J/kg 2.3

The char reaction rate varies with reactant media having the following order, Rair >> RH2O ~ RCO2

> RH2. The catalytic effect of inorganic impurities (discussed in detail later in this chapter) and

surface area have also been shown to play an important role[42]. Radovic et al.[43] stated that

active surface area and not total surface area played a role in determining the reactivity and that

oxygen chemisorption capacity of chars at 375 K and 0.1 MPa was a good indicator of the

concentration of carbon active sites. In coal chars, active sites would include sites bonded to

heteroatoms (principally H), nascent sites (created during pyrolysis and gasification), dangling

carbon atoms (single bonded), edge carbon atoms (double bonded) and trigonally bonded basal

carbon atoms. Temperature has the highest influence on reactivity[44] with rate increasing with

increase in temperature. The reactivity of chars decreases with increasing severity of pyrolysis

conditions and residence times[23, 45, 46] due to a decrease in active sites (i.e. feeder pores[47])

either by thermal annealing (T ~ 973 - 1,373 K) (the char becomes structurally ordered), a

reduction in catalytic activity caused by species like CaO sintering or due to the loss of hydrogen

and oxygen[39, 43]. Gale et al.[46] attributed the decrease in reactivity to the flattening,

smoothing, or ordering of carbon-layered planes during the depletion of non-aromatic

components in the char matrix, thereby increasing the relative concentration of aromatic

compounds as mass is released. This is further proved by the fact that up to 1,473 K, heat

12

treatment of chars that are treated with pulses of oxygen have higher combustion reactivity than

chars prepared under inert atmospheres. The mechanism has been attributed to chemisorption and

formation of intercalation compounds, limiting the extent of graphene layers stacking and

rearrangement upon heat treatment[48]. Walker et al.[49] stated that the reactivity of US coals

may be attributed to three primary factors: catalysis of gasification, active site concentration, and

the accessibility of reacting gas to the active sites. They also stated that the presence of active

catalysts and the possession of a high concentration of active sites are necessary but not sufficient

conditions for the high reactivity of coal chars. Overall reactivity decreases[20, 42, 50] and

characteristic temperatures (initial temperature, peak temperature and burnout temperature)

increase[51] with an increase in coal rank with the reactivity of lignites being of the order 20-40

times that of anthracites[52] due to a higher quantity of feeder pores and reactive sites on the

surface[47].

The rate of the gasification reaction is enhanced by the presence of alkali and alkaline-

earth metal salts or oxides[52-56] and their importance increases for lower rank of coal (C <

80%)[53]. Ye[56] studied the gasification of Bowmans coal with CO2 and steam and found the

reaction to be strongly catalyzed by Na, K and Ca. The effectiveness of catalysis of the CO2

gasification reaction by alkaline-earth elements is in the following order: Be Mg < Ba Sr <

Ca[54]. Kapteijn et al.[54] have indicated that the oxide (CaO) is the active species in the CO2

gasification reaction. Radovic et al.[52] and Walker et al.[49] attributed the relatively high

gasification reactivity of lignites primarily to the catalytic activity of highly dispersed CaO on the

char surface. They also found that the char reactivity decreases with increasing pyrolysis

residence time, caused by CaO sintering and subsequently decreasing its dispersion. The high

initial dispersion of inherent catalysts on the char surface is due to the presence of abundant

exchangeable cations (mainly Ca2+

) on the carboxylic groups[43]. The initial reactivity of Ca

around 1,000 K has been found to be equal to that of potassium, a well known gasification

13

catalyst. However with increasing carbon burn-off, it shows severe deactivation due to

sintering[54]. Chen[57] studied the sintering behavior by varying the percentage of CaO (between

5-10%) in CaO-MgO-Al2O3-SiO2 glass-ceramics and showed the samples to sinter at

temperatures around 1173 K. Miura et al.[53] have said that the decrease in reactivity is caused

by a decrease in number of active carbon sites and in catalytic activity. Prinsloo et al. [55]

conducted CO2 gasification studies on bituminous coals and found the rate of catalyzed

gasification reactions to first increase with carbon conversion and then decrease at higher carbon

conversion which was attributed to the collapse of particle structure, pore plugging, cation loss

due to migration of Na into the pores or Ca sintering or reaction of alkali metals with mineral

matter. Other possibilities include formation of intercalated compounds or stable aluminosilicates

and vaporization[53]. The catalytic activity of CaO can be explained by the following set of

reactions[54]:

CaO + CO2 CaO . O + CO 2.4

CaO . O + C CaO + C[O] 2.5

C[O] CO 2.6

Kapteijn et al. [54] proposed that the rate determining step in the CO2 gasification of carbon is the

release of CO from the carbon structure. The increase in site density can be achieved by the

formation of more CO groups by an oxygen transfer mechanism. A more detailed mechanism

represented in the work of Miura et al.[53] shows the importance of the edge carbon atoms as

compared to the basal carbon atoms in oxygen adsorption on carbonaceous materials. The

activation energy of the reaction does not change due to catalytic effect, which suggests that the

catalyst only increases the number of active sites on the char surface[54].

The two types of reactions basically contributing to char conversion in oxy-fuel

combustion are char-O2 and char-CO2[41] reactions. In general, it has been shown that higher

amount of O2 in CO2 than in N2 is required for similar burnout[19] despite contribution from

14

char-CO2 reaction and this has been attributed to lower particle temperatures[21]. Char burnout

times are similar for air and 30%O2/CO2 mixture with burnout increasing, higher the O2

concentration[4, 41, 58]. Burnout efficiency is the best under 30% O2/CO2 mixture followed by

air and lastly 21%O2/CO2 mixture. Li et al.[38] and Li et al.[59] conducted their studies in a drop-

tube furnace and found that the lower char reactivity under oxy-fuel environments as compared to

air was the reason for lower burnout. Brix et al.[60] studied char combustion at high temperatures

(~1,673K) and low oxygen concentration (3.1-3.7%) and obtained lower conversions under an

O2/CO2 environment as compared to O2/N2 environment, which they attributed to the lower

molecular diffusion of O2 in CO2 as compared to N2. Borrego and Alvarez[19] attributed the

lower conversion in oxy-fuel environments to CO2 forming cross-links on the surface of the char

particle.

Studies conducted by Varhegyi et al.[61, 62] showed that coal combustion in O2/CO2

atmosphere, the char-CO2 reaction has a much lower rate than the char-O2 reaction and that the

net reaction rate was proportional to the partial pressure of O2 (PO2). They also stated that CO2

does not participate in the elementary reactions of oxidation and that any changes in ambient CO2

concentration do not affect the concentration of evolved CO2 at the molecular level. The CO2

only influences secondary reactions of the CO and CO2 formed (a detailed mechanism is shown

later in the chapter). Maximum reaction rates are reached when increased concentrations of O2 are

used. Increased O2 concentrations also lead to increased devolatilization rates due to closer

proximity of the volatiles flame to the coal particle and the increased temperature of the volatiles

flame[4]. Murphy and Shaddix[4] said that the best fit to their combustion results from an

entrained flow reactor were shown by the n-th order Langmuir-Hinshelwood equation given by,

2.7

15

where k1 and k2 are constants dependent on temperature. The apparent reaction order, n was found

to vary between 0.1 for near-diffusion-limit oxygen-depleted conditions to 0.5 for oxygen-

enriched conditions.

The dependence of reaction rate on temperature and fractional burnoff is given by,

dj/dt = Aj exp(-Ej/RT)g(PO2,PCO2)fj(j)

where Aj and Ej are pre-exponential and activation energy, respectively; g and f are empirical

functions; and g(PO2,PCO2) is proportional to PO2 and does not depend on PCO2[61].

The studies by Varhegyi et al.[61, 62] were conducted at high pressure and moderate

temperatures (~1,223 K) and they have reported no influence of the char-CO2 reaction as stated

above. Liu et al.[44] modeled the char-CO2 gasification reaction at high temperature and pressure

by extrapolating char reactivity data obtained at moderate temperatures. They showed that for

low-rank coal char at 1,123 K, both the apparent and intrinsic reactivity increased with CO2

partial pressure. They also showed that the apparent reaction rate increased two orders of

magnitude as the coal rank decreased and that the activation energy of the char-CO2 gasification

reaction generally decreased as the coal rank decreased. The char-CO2 reaction is composed of

the following reactions:

Cf + CO2 C(O) + CO 2.9.1

C(O) CO 2.9.2

CO + Cf C(CO) 2.9.3

CO2 + C(CO) 2CO + C(O) 2.9.4

CO + C(CO) CO2 + 2Cf 2.9.5

The intrinsic reaction rate in the form of the Langmuir-Hinshelwood expression is given by,

2.8

2.10

16

where k1, k2, k3 and k4 are temperature dependent rate constants which can be represented by an

Arrhenius type equation,

ki = Ai e-Ei/RT

where Ai is the pre-exponential factor and Ei is the activation energy.

Rathnam et al.[32] conducted reactivity studies in a thermogravimetric analyzer after

generating char in a drop-tube reactor. They also showed an increase in weight loss at

temperatures exceeding about 1,030 K due to the char-CO2 reaction. The authors also conducted

combustion tests in the drop-tube reactor on four coals but the drawback of this study was that the

authors did not provide answers as to why two of their selected coals showed higher conversion

in an oxy-fuel environment as compared to air. The selected coals did not represent a wide range

in rank thus not giving a clear rank effect.

Saastamoinen et al.[35] conducted pressurized oxy-fuel combustion under different

concentrations of O2 and CO2. They reported that when the gas oxygen concentration is low and

the carbon dioxide concentration is high, both char oxidation and gasification may be important

in the char mass reduction. This is more significant at high temperatures when the char oxidation

rate is limited by transport of O2 to the particle surface (diffusion limitations). Under these

conditions, CO oxidation (in the boundary layer of the particle) is faster, which further consumes

the remaining small O2 content on the surface and the char oxidation rate increases the CO2

concentration on the surface of the char particle. They also claim that under certain conditions

(high CO2 concentration, high gas temperature, high gas pressure, large particle size (diffusion

control) and porous char particles (large internal area open to CO2)), the char gasification reaction

may become important compared to the char oxidation reaction. However the char-O2 reaction is

many orders of magnitude faster than the char-CO2 reaction, when the O2 concentration at the

particle's surface is substantially reduced by CO oxidation at the boundary layer, the gasification

reaction can compete favorably with the char-O2 reaction in the overall heterogeneous reaction

2.11

17

rate. Shaddix and Murphy (as referenced by Buhre et al. [11]) found that in oxygen enriched

combustion, CO2 gasification of char becomes important at practical temperatures. The authors

measured particle burning rates vs. temperature and saw little difference in a O2/CO2 or O2/N2

atmosphere. They also stated that a decrease in burning rate in an O2/CO2 environment was due to

a decrease in O2 diffusion through the particle boundary layer though the authors did not provide

any conclusive evidence that this was the reason.

Zhang et al.[34] investigated the combustion of brown coal in O2/N2 and O2/CO2

mixtures. They found that up to 25% of the nascent char may undergo gasification to yield CO to

improve the reactivity of the local fuel/O2 mixture. The subsequent homogeneous oxidation of

CO released extra heat for the oxidation of both volatiles and char. The authors did not provide

any concrete experimental or theoretical evidence as to how they concluded that 25% of the

nascent char undergoes gasification.

Li et al.[59] produced chars under a CO2 environment in a drop-tube reactor at 1673 K

and gasified surfaces were analyzed using an SEM. They also reported that under oxy-fuel

conditions when the oxygen partial pressure is low in the later stages of combustion, the

gasification of unburned char would have a significant effect on the char conversion. The authors

conducted combustion tests on three Indonesian low-rank coals and all three coals showed higher

conversions in air as compared to 21%O2/79%CO2. They do not provide an explanation for this

though they showed the occurrence of the gasification reaction through SEM images.

Naredi[21] has shown theoretically that there is an exponential increase in the rate of the

char-CO2 reaction beyond 1,800 K with very little influence below that temperature. Also he

claimed that at higher temperatures, an increase in rate caused by the char-CO2 reaction possibly

compensates for a decrease in O2 availability and lower particle temperatures during oxy-fuel

combustion.

18

When the temperature exceeds 1,700 K under entrained flow conditions, the char

reactions are limited by mass transfer into the porous structure of the char particles[44]. An

effectiveness factor , which is the ratio of the actual rate per unit internal surface area to the rate

attainable if no pore diffusion resistance existed should be used. The apparent reaction rate is then

calculated by,

Rapp = RinS

where S is the internal surface area. The effectiveness factor primarily depends on particle

temperature and size and can be calculated by the well known Thiele modulus approach (sample

calculations shown in Appendix A)[44]. The Thiele modulus depends on char properties, reaction

conditions and particle shape. The basic form of the Thiele modulus, derived for solid-gas

catalytic reactions, includes reactant concentration, rate constant, the effective diffusivity and a

shape factor (F) which determines the geometry of the particles, given as[63],

The effective factor is then calculated as[21],

The effectiveness factor is 1 in zone I, and ranges from 1 to 0 in zone II and is equal to 0 in zone

III[63]. It has been shown for the char-CO2 reaction, when the temperature is raised from 1,400K

to 2,000 K, the calculated effectiveness factor decreases from unity to 0.13 indicating that the

reaction transfers from chemical reaction limited to pore diffusion limited. Figure 2-3 shows the

general trend of effectiveness factor as a function of temperature.

2.12

2.13

2.14

19

Figure 2-3. General shape of the effectiveness factor vs. temperature curve[63]

An effective diffusivity coefficient, Deff is significant for the calculation of the effectiveness

factor and is strongly dependent on the pore size within the particle. The total porosity of the char

particle is written as the sum of the macro-, meso- and micro-porosity, which is written as,

T = a + e + i

where, T, a, e, and i are total, macro-, meso- and micro-porosity respectively[44].

Hodge[63] has shown that at high temperatures (zone II conditions), the activation energy

of the char-CO2 reaction is half that measured under zone I conditions. Based on the results of the

study, the author also stated that the transition from zone I to zone II depended on char

morphology and reactivity but it could be safely assumed that under entrained flow conditions, it

occurred at temperatures >1,473 K.

Tree et al.[64] modeled both char-O2 and char-CO2 reactions in their study on oxy-fuel

combustion. They obtained the oxidation and gasification rate parameters from various literature

2.15

20

sources[65-67]. They included the gasification reactions in their model although gasification

reactions are much slower than oxidation reactions because of the high concentration of CO2 in

oxy-fuel combustion. Figure 2-4 shows a comparison of oxidation and gasification reactions for

various ranks of coal and in general it is claimed that rates increase with decreasing rank. Other

studies have shown a similar effect of an increase in rate with CO2 partial pressure[21, 63, 68]

and decrease in coal rank with the reaction order being around 0.7-0.8[63]. Goetz et al.[66]

conducted char gasification studies on four coals: a lignite, a subbituminous coal, a high volatile

A coal and a high volatile C coal. They found both combustion and gasification reactivity of the

chars to follow the order: lignite > subbituminous > hvCb > hvAb and that the gasification

reactivity of the least reactive char was lower by a factor of 10 compared to the most reactive

char. They also found that the combustion and gasification rates were a strong function of pore

structure, temperature and reactant gas concentration. Out of all the coals they studied, only the

hvAb coal swelled though it showed the least gasification reactivity. Figure 2-5 shows results

from a study by Roberts and Harris[69] where low temperature kinetics data from char-O2, char-

CO2 and char-H2O reactions have been extrapolated to high temperatures approaching mass

transfer limitations. The exponential increase in rate of the char-CO2 and char-H2O as

temperature increase can be clearly seen.

21

Figure 2-4. Comparison of char reaction rates with O2 (oxidation) and CO2 (gasification)[64]

A computational study conducted by Mann and Kent[70] for full scale boilers showed

that with the incorporation of the char-CO2 and char-H2O reactions in their model, the accuracy

of the burnout predictions improved and this was thought to be due to the significant role played

by these species in oxygen deficient regions. Also, they found that the higher concentration of

CO2 in the furnace in comparison to H2O led to greater mass loss from the char-CO2 reaction in

comparison to the char-H2O reaction. An important conclusion of this study was that the high

temperatures in the furnace were sufficient to make the transfer rates the important controlling

factor but chemical kinetics remained important for the char-CO2 and char-H2O reactions. The

drawback of the results of this study was that the conclusion was inferred based on the model

results and there was no substantial evidence provided to prove that chemical kinetics was an

important factor at high temperatures for the char-CO2 and char-H2O reactions.

22

Figure 2-5. Low temperature TGA kinetic data extrapolated to high temperatures

( : O2, : steam, : CO2)[69]

2.7 Char conversion Rate Parameters Estimation

Le Manquais et al.[71] compared chars obtained at high heating rate (drop-tube reactor)

and low heating rate (thermogravimetric analyzer (TGA)) for their reactivities. They stated that

conflicting trends have been reported when TGA rate parameters were applied to pulverized coal

combustion. They found that the drop-tube reactor chars showed an increased burnout propensity

while moving from zone II to zone III. Char morphologies were different for both types of chars,

with the TGA chars resembling the raw coal with an undeveloped pore network and the drop-tube

chars being highly porous, swollen and with a high surface area. The drop-tube chars had an order

23

of magnitude higher reactivity as compared to the TGA chars as their higher porosity reduced

mass transfer limitations and would more closely resemble pulverized coal combustion.

There are a number of ways to extract rate parameters from experimental data leading to

a wide scatter in pre-exponential factor and activation energy even for the same rank of coal[21].

Table 2-1 shows how the rate parameters can vary based on the method used.

Table 2-1.Char-CO2 reaction activation energy values reported in literature

Jenkins et al.[47] used the maximum value (shown in Figure 2-6) in the rate profile to

compare char reactivity. Scaroni et al.[75] determined rate parameters using averaged rate values

during different extents of burnoff given by,

where Ru = (dW/dt)/W is the reactivity determined on the basis of unburnt carbon. They also felt

that determining rate parameters using the maximum value was not appropriate because of an

extremely short rectilinear region. Fletcher et al.[46] and Dugwell et al. [45] determined rate

Ref. Method Operating

Temperature

Coal Rank Activation Energy (kJ/mol)

[72] X=0, X=0.5 1273-1673 K bituminous 62

subbituminous 82

lignite 98

[73] Shrinking core 973-1173 K lignite 146

subbituminous 151

high-volatile

bituminous

155

lignite 79

[74] Random pore

model

1173-1333 K subbituminous 147

high-volatile

bituminous

180

2.16

24

based on fixed value of burnoff. Naredi and Pisupati[23], noticed a variation in activation energy

over the 10-30% burnoff range indicating that the char burning is not always under kinetic

control. They attributed the initial part of the rate curve up to the maximum to intraparticle

diffusion limitations and also due to lower partial pressure of the reaction gas and so they

estimated the rate parameters from the slope of the region after the occurrence of the maximum.

They also noticed that the rate parameters determined using this method closely matched those

determined using the maximum and hence concluded that rate parameters could be extracted at

any conversion level after the occurrence of the maximum. Figure 2-6 shows a typical rate profile

from the study of Naredi and Pisupati[23] depicting the occurrence of the maximum. It was

decided to utilize with the method of estimation of rate parameters adopted by Naredi and

Pisupati based on their results showing that the initial part of the curve up to the maximum was

subject to diffusion limitations.

Figure 2-6. Reactivity profiles in air at 673 K for chars obtained under different pyrolysis

conditions[23]

maximum

25

2.8 Process Feasibility and Economics

There have been many recent studies[7, 16, 36, 37, 76-80] conducted on evaluating oxy-

fuel combustion on the utility scale. Pak et al.[76] theoretically compared an oxy-fuel combined

cycle system with CO2 liquefaction and utilizing low pressure steam to a conventional steam

turbine power generation system utilizing low pressure steam. They found that the oxy-fuel

system could generate 2.03 times greater electric power than the conventional system with a net

CO2 reduction of 180 kilotons/year and an exergy efficiency of 54.2%. The system was evaluated

to be economically feasible with it surpassing the conventional system if a CO2 credit of $30/ton

was applied to captured CO2. Xiong et al.[78] also showed that the cost of electricity for an oxy-

fuel plant would be greater (1.5-1.7 times) than a conventional PC combustion plant though, a

CO2 sale price of $17-22/ton would level the costs. However, the system has a 2.41% degradation

of net power generation efficiency. Liszka and Ziebik[77] also theoretically compared an oxy-

fuel coal-fired power unit to a conventional unit and saw an efficiency drop of 10.89% due to

CO2 compression and purification. The said that the high impact of CO2 compression system on

the overall efficiency could be limited by sub-critical liquefaction and better heat regeneration

systems. Zhou et al.[81] computed process requirements for a boiler retrofit with minimal impact

on thermal and emission performance and found that an optimal wet flue gas recycle ratio

depended on the type of coal and exit oxygen concentration and was typically around 0.7-0.75.

They also said that dry flue gas recycle could be used to enhance flame temperatures in oxy-fuel

combustion. Their computational fluid dynamics model showed oxy-fuel combustion and air

combustion to be very similar in terms of thermal characteristics. Hjartstam et al[24] showed in

their modeling study that with appropriate adjustment of the recycle rate, desired combustion

stability and structure of coal-fired oxy-fuel flames may be achieved without any significant

impact on the emission level.

26

2.9 Summary

It has conventionally been thought that oxy-fuel combustion produced lower

conversions[19, 38, 59, 60] compared to air combustion due to lower particle reactivity, particle

temperatures and heating rate. The char-CO2 reaction has been the focus of many studies[21, 30,

32, 35, 44, 59, 61-63, 66, 70]. There is a lot of scatter in literature regarding the contribution of

CO2 to char conversion in an oxy-fuel environment. Certain studies[61, 62] show no effect of

CO2 on char conversion due to the gasification rate being much slower than the oxidation rate.

There are recent studies[32, 34, 59] that have observed the char-CO2 reaction though have not

provided substantial evidence to prove the same. Both the char-O2 and char-CO2 reactions are

influenced by temperature and intrinsic reactivity of the coal and hence, there is expected to be an

effect of coal rank and temperature on conversion. The oxidation and gasification reactivities

increase with decrease in rank of coal and this has been shown theoretically and experimentally

by Tree et al.[64] have and Goetz et al.[66], respectively. Hodge[63] has shown that the

activation energy of the char-CO2 reaction under zone II conditions is half that under zone I

conditions and Liu et al.[44] have stated that the activation energy of the char-CO2 reaction

generally decreases with increase in coal rank. Roberts et al.[69] have shown the reactivity of the

char-CO2 and char-H2O reaction to increase exponentially with an increase in temperature as

compared to the char-O2 reaction. From the literature review, it has been shown that under certain

conditions of high temperature, high CO2 partial pressure, large particle size and porous char

particles, the gasification reactions can compete favorably with the oxidation reactions in the

overall heterogeneous rate[35]. Mann and Kent[70] have shown the importance of the char-CO2

reaction in oxygen deficient regions of the boiler and how computational model predictions

improve on incorporation of the char-CO2 reaction. Naredi[21] has shown numerically that higher

conversions can be obtained by burning coals more reactive to CO2 and also that there is an

27

exponential increase in the rate of the char-CO2 reaction at temperatures beyond 1800 K. Based

on the above discussion, some of the questions that this thesis tries to address are:

Would there be any difference in using high furnace temperatures? Particles generally

reach higher temperatures compared to the gas temperature and using higher furnace

temperatures, leads to higher particle temperatures. It is believed that other studies[61,

62] did not report any influence of char-CO2 reaction because of the low furnace

temperature (

28

Chapter 3

Problem Statement

3.1 Methodology

The aim of this work was to provide an experimental and theoretical analysis of the effect

of coal rank and char-CO2 reaction under oxy-fuel conditions. To investigate the contribution of

the char-CO2 reaction in increasing overall conversion under oxy-fuel conditions, combustion

tests were conducted in a lab-scale drop-tube reactor with four coals (of different ranks) at three

different residence times at 1873 K. The chars generated under air and oxy-fuel environments

were analyzed for carbon conversion. Chars at conversion approximately equal to the ASTM

volatile content of the parent coal were also generated in a reactive environment (21% O2/79%

CO2). This was done to ensure maximum removal of volatile matter, to obtain representative coal

char samples. In order to quantify the result of the combustion tests and to obtain a more

fundamental understanding of char-O2 and char-CO2 reactivity for the four coals, modified

burning profiles were obtained on the parent coals using a bench-scale thermogravimetric

analyzer (TGA). Isothermal reactivity profiles were also obtained on the char samples. The

generated intrinsic char-O2 and char-CO2 rate parameters were extrapolated to high temperatures

taking into consideration diffusion limitations using a theoretical model (Appendix A). The char

samples were characterized by using proximate analysis, ultimate analysis and surface area

analysis to identify differences in structure and composition. Since catalytic activity due to ion-

exchangeable cations is well recognized in low-rank coals, samples were washed with ammonium

29

acetate to remove the ion-exchangeable cations and to compare the effect of coal rank on a

catalytic activity-free basis. Catalytic activity is generally high at lower temperatures in a TGA

but in an actual boiler at high temperatures, its influence is lower. To investigate their importance

at high temperatures on reactivity, combustion tests were performed on ammonium acetate-

washed coal samples and the carbon conversions were compared with those obtained for the

parent coals in a DTR. Also, the char particle temperature in oxy-fuel atmospheres was

theoretically modeled. The effect of varying various parameters such as reactive gas partial

pressure, gas temperature and coal rank on particle temperature was also studied. In summary, the

main objectives of this study were:

To include the char-CO2 reaction at high temperatures in computing the carbon

conversion, specifically in an oxy-fuel environment.

To study the role of coal rank and the char-CO2 reaction under simulated boiler

conditions (high temperature, high heating rate, and similar residence times) in air and

oxy-fuel environments.

To determine the role of catalytic activity of alkali and alkaline-earth metal salts and

oxides under TGA and at high temperatures.

To gain a better understanding of the primary factor contributing to the difference in

carbon conversion in air and oxy-fuel environments and to theoretically model the

phenomenon.

3.2 Hypothesis

At high temperatures (~1,873 K) in a drop-tube reactor, combustion in oxy-fuel

environments produces higher carbon conversion than that in air for low-rank coals. This is due to

the increasing contribution of the char-CO2 reaction at high temperatures along with the char-O2

30

reaction thereby, increasing the overall carbon conversion. The reactions take place under zone II

conditions and due to high temperatures and high CO2 partial pressure in oxy-fuel combustion,

the gasification reactions become important and compete favorably with the oxidation reactions.

For low-rank coals, the rate of the char-CO2 gasification reaction is much higher than that for

higher rank coals. The activation energy of the char-CO2 reaction is higher than the char-O2

reaction and hence, it is more sensitive to an increase in temperature with the rate increasing

exponentially at high temperatures. Hence, oxy-fuel combustion may be more suitable for low-

rank coals in decreasing the unburnt carbon because of their increased reactivity in an O2/CO2

environment.

31

Chapter 4

Experimental Details

This chapter describes the various lab equipment used and the procedures followed.

4.1 Coal Sample Preparation

A suite of four coals of different ranks was selected from the Penn State Coal Sample

Bank: a low volatile, Pocahontas #3 Seam (DECS-19) bituminous coal; a high volatile, Pittsburgh

Seam (DECS-34) bituminous coal; a subbituminous, Dietz Seam (DECS-38) coal and a Beulah

Seam lignite (DECS-11). The coals were dried in an air drying oven at 333 K until the surface

moisture weight loss was less than 0.1%/hr. The coals were then ground in a ball mill and sieved

to obtain particles in the size range -170+200 mesh (74 - 88) that were used in the drop-tube

reactor for pyrolysis and combustion studies. The reason for selecting a narrow size range was to

limit the effect of particle size as reactivity varies greatly with particle size.

4.2 Drop Tube Reactor

The drop-tube reactor (DTR) used in this study was a laminar flow furnace with

controlled wall temperatures and particle residence times, that simulated flow conditions, high

heating rate and temperatures of an actual PC boiler. The particle residence time is a function of

gas flow rates, particle size and insertion length of the probe. Figure 4-1 shows the schematic of

the DTR.

32

The reactor is a single zone, electrically heated furnace capable of being operated at a

maximum temperature of 1,873 K. The reactor tube is made of high purity alumina refractory

material positioned vertically. Six U-shaped Kanthal-super heating elements are attached on the

wall. A rotary type feeder, Acrison GMC-60 supplies coal into an entrained flow of a primary gas

through a water-cooled injection tube. The position of the tip of the injector is at the level of the

bottom of a mulliteTM

flow-straightener which supplies the preheated secondary gas to the

Figure 4-1. Schematic of Drop tube reactor[21]

33

furnace. Secondary gas from two inlets enters the furnace through the top of the preheater and

exits the flow straightener along with the coal-laden primary gas. A char collection probe is

inserted from the bottom of the furnace and positioned at a desired height. Char particles are

collected isokinetically through the water cooled collection probe by using a vacuum pump. The

particles are collected on a Whatman 541 ashless filter paper and the gas is then passed through a

condenser to remove water vapor before being sent to a continuous emission monitoring (CEM)

system.

Calculations were performed to ensure laminar flow condition in the DTR and the

Reynolds number was observed to vary between 127 and 1,774 for a mean gas velocity between

0.36 m/s to 5.08 m/s assuming air as the gas.

4.2.1 Combustion Test Conditions

Three gas compositions (by volume) were used: 21%O2/79%N2 (air), 21%O2/79%CO2

(oxy-fuel combustion) and 30%O2/70%CO2 (enhanced oxy-fuel combustion). The combustion

tests were conducted to measure carbon conversion of the coals at three residence times. These

were the first set of tests to be conducted on the coals to investigate the rank effect. The residence

time was varied by changing the location of the collection probe and, keeping the gas flowrates

constant. The gas residence times investigated were: 0.8 s (0.254 m), 1.2 s (0.406 m) and 1.7 s

(0.559 m). The residence time was calculated from the volumetric flow rate of the gas taking into

account gas expansion at high temperature. The length is the distance of the probe from the top of

the furnace. The typical residence times of PC boilers are shown in Table 4-1 and the results

obtained in this study would be practically applicable to boilers in the size range 60-210 MW.

The other parameters used in the combustion tests are given in Table 4-2.

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Table 4-1. Typical PC boiler residence times*[82]

Boiler size (MW) Residence time (s)

60 MW ~0.8

110 MW ~1.2

210 MW ~2-2.5

500 MW ~3.5

*mid-burner region to furnace outlet

Table 4-2. DTR combustion test parameters

Coal feedrate (g/min) 0.4

Total gas flowrate (l/min) 10

Primary gas flowrate (l/min) 4

Secondary gas flowrate (l/min) 6

Pre-heater temperature (K) 873

Furnace temperature (K) 1,873

4.2.2 Devolatilization Test Conditions

To measure the reactivity of the coals used in the study in a thermogravimetric analyzer

(TGA), chars were generated in the DTR under an oxy-fuel atmosphere (21%O2/79%CO2) to

simulate conditions such as high heating rate and particle temperature of a real boiler. The

conditions used were the same as those shown in Table 4-2 except for the residence time. A series

of tests at short residence times were conducted for each coal sample to obtain a char sample that

had a carbon conversion (%) approximately equal to the ASTM volatile matter of the coal.

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4.3 Microproximate Analysis

A Perkin-Elmer 7 thermogravimetric analyzer (TGA) was used to conduct

microproximate analysis on the DTR feed coal and chars generated to determine the char

conversion. This was done because of the limited amount of sample generated and sample

requirements for further analysis. The procedure adopted is similar to that followed by Man et

al.[83] in their study. About 4-5 mg of char was placed in the TGA pan for analysis. The furnace

was held at room temperature in an inert atmosphere (N2 at 100 cc/min) for 1 hour. The sample

was then heated to 380 K at the rate of 5 K/min and held at that temperature for 2 hours to drive

off the moisture. It was then heated to 1,223 K at the rate of 50 K/min and was held at that

temperature for 7 minutes to expel the volatile matter. The sample temperature was then

decreased to 873 K and the gas was switched to air to initiate oxidation. It was then heated at the

rate of 10 K/min to 1,023 K and held for 2 hours to determine the ash yield.

The carbon conversion of the chars generated was calculated by the ash tracer technique

using the formula[84]:

3.3.1

where W is the carbon conversion (%), A0 is the ash content of feed coal (%, dry basis) and A1 is

the ash content of the collected char (%, dry basis). A sample calculation is shown in Appendix

B. The underlying assumptions of the ash tracer technique are that coal ash is conserved within

the char and that the ash fraction in the coal is not affected by temperature-time history of the

particle. This requires that the ash species are not volatilized, or at least the extent of

volatilization which occurs in the experiment is the same as that which occurs under the

conditions of the standard ash test so that carbon conversion may be calculated from an ash

balance of the coal[84].

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4.4 Determination of Char Reactivity

Char-O2 and char-CO2 reactivity tests were conducted in the TGA. These tests were

carried out to try to explain the difference in carbon conversions obtained during the DTR

combustion tests in air and oxy-fuel atmospheres for the various coal samples. The procedure was

adapted from the work of Naredi[21] who performed several preliminary tests (on the same TGA)

to make sure that the operating parameters eliminated mass transfer limitations. About 4-5 mg of

sample was placed in a platinum crucible for analysis. The furnace was held at room temperature

in an inert atmosphere (N2 at 100 cc/min) for 30 min. The sample was then heated to 383 K at the

rate of 10 K/min and held there for 1 hour to drive off moisture. It was then heated to 1,223 K at

the rate of 50 K/min and held there for 7 minutes to drive out the residual volatiles. The sample

was then cooled down to the required reaction temperature at a rate of 20 K/min and held there

for 10 min to establish a thermal equilibrium. The gas was then switched from N2 to O2 or CO2 to

initiate char oxidation or/gasification. The weight of the sample was recorded every 30 s until

~90% (dry and ash free (daf)) weight loss was achieved. After each experimental run,

conversion-time data was converted into rate of weight loss normalized with respect to initial

mass (m0 (daf)) to construct reactivity plots with the rate calculated as,

3.4.1

For the char-O2 reactivity tests, reaction temperatures of 673K, 698K and 723K were used and for

the char-CO2 reactivity tests, reaction temperatures of 1,123K, 1,148K and 1,173K were used.

Plots of ln k vs. 1/T were generated and rate parameters were extracted after the occurrence of the

maximum in the reactivity plots. The slope of the line gives (-E/R) and the intercept gives ln A. A

and E are the pre-exponential factor and activation energy, respectively. A example calculation is

shown in Appendix C.

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4.5 Modified Burning Profiles

Burning profiles of fuel samples indicate relative combustion rates and heat release

profiles in a boiler when samples are heated non isothermally at a constant rate from room

temperature to about 1,273 K. The procedure was developed by Wagoner and Duzy[85] to

determine the relative rate of combustion and heat release profiles. The burning profiles were

generated using the TGA based on the procedure outlined by Pisupati[51]. The term 'modified' is

used as the existing procedure which is performed under air has been adopted for runs under oxy-

fuel environments. About 5 mg of sample was placed in the crucible for analysis. The furnace

was then raised to enclose the sample which was held at room temperature in an inert atmosphere

(N2 at 100 cc/min) for 30 min. The gas was then switched to air or oxy-fuel atmosphere

(21%O2/CO2) and the sample was heated to 1,273 K at the rate of 10 K/min. While performing

tests under oxy-fuel conditions, a special blended gas (21%O2/CO2) cylinder was used to supply

the gas. Data was recorded every 60 s and then the burning profile was generated by plotting

derivative weight % change (%/min) vs. temperature (K). From these profiles the initial

temperature (IT), peak temperature (PT) and burnout temperature (BT) were obtained to compare

various coals. The initial temperature (IT), is arbitrarily defined as the temperature at which the

weight loss exceeds 0.1 %/min after the initial moisture peak. The peak temperature (PT) is

defined as the temperature at which the weight loss is maximum. Burnout temperature is

arbitrarily defined as the temperature at which the rate of weight loss decreases to 1.0 %/min

towards the end of conversion. Dmax is the value of derivative weight % at the peak temperature

(PT). The relative peak mass loss rates and the temperature range provide the relative rates of

combustion of various fuels.

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4.6 Compositional Analysis

4.6.1 CHN Analysis

The carbon, hydrogen and nitrogen content of the feed coals and char samples were

measured using an analyzer, LECO TruSpec CHN. First, blank calibration of the instrument was

performed by running three blank samples. Then, a coal CHN calibration standard, AR 1706 was

run 3-5 times to calibrate the instrument. A furnace temperature of 1,223 K and afterburner

temperature of 1,123 K was maintained. Also, a system check and leak check were performed to

ensure proper functioning of the instrument. About 0.1-0.2 g of sample was weighed into a tin

foil which was compressed into a ball and pushed into the furnace pneumatically. After complete

combustion, the flue gases were passed through various detectors to measure carbon, hydrogen

and nitrogen contents and vented out. The system was purged and the baseline was established

before each run.

4.6.2 Sulfur Analysis

The total sulfur content of the feed coals and the char samples were measured using a

sulfur analyzer, LECO SC 132. A few blank runs were performed using a random coal sample to

saturate the pores of the sulfur adsorbent. Then, the calibration sample was run about 3-5 times

until the value stabilized and a new calibration curve was generated. About 0.1-0.2 g of sample

was placed in the sample boat for analysis. The system was purged and the baseline was

established before each run. The sample boat was then pushed into the furnace to combust the

sample in an atmosphere of enriched air at 1,673 K. The flue gases were then passed through a

series of adsorbents and particle filters before passing through the sulfur detectors to measure the

total sulfur content.

The oxygen content was then determined by difference (100 - %C - %H - %N - %S). The

higher heating value (HHV) for the coal samples was provided by the Penn State Coal Sample

Bank.

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4.7 Surface Area Analysis

This analysis was performed on the DTR generated four coal char samples and one

ammonium acetate-washed sample so as to use the pore area analysis as an empirical tool to

explain the TGA char reactivity profiles. The surface area of the sample was determined using

Micrometrics ASAP 2020 surface area analyzer and the analysis was performed by personnel at

the Materials Research Laboratory (MRL), Penn State. About 0.4-0.5 g of sample was outgassed

for a minimum of 12 hours at 383 K. The sample temperature was decreased to 77K and nitrogen

gas was then introduced in controlled increments. After each d