Char-slag transition during pulverized coal gasification

In coal gasification the char-slag transition is a process in which porous char transforms into molten slag at temperatures above ash fluid temperature. It is associated with physical changes of the char particle, such as particle density, size, porous structure and mineral-carbon association. Despite the large number of investigations on coal gasification, the physical phenomena during char-slag transition have not been well studied. In addition, little data regarding ash deposition behavior on gasifier walls during char-slag transition have been reported. This study aims to clarify the physical changes of char particles and ash deposition behavior on gasifier walls during char-slag transition in pulverized coal gasification. To achieve these objectives, two types of experiments were carried out using a laminar entrained-flow reactor: (1) char and ash formation experiments and (2) ash deposition experiments. In the first type of experiment, char and ash particles with different conversions were prepared using two bituminous coals and a subbituminous coal. The prepared char and ash samples were characterized using various techniques to obtain information on particle density, size, porous structure and mineral-carbon association. These data were used to identify the point of the char-slag transition for different coals. Results show that during the transition: (1) particle size decreases, which is caused by shrinkage in the initial stage and by fragmentation in the later stage; (2) particle density increases due to particle size reduction; and (3) particle internal surface area decreases because of ash melting induced pore blockage. In the second type of experiment, particle collection efficiency was measured for a bituminous coal at various conversions. Such information was used to derive the variation of particle stickiness during the char-slag transition. Results indicate that the particle stickiness increases dramatically during the transition. This dramatic increase is attributed to exposure of included minerals on the particle surface, which is caused by particle shrinkage and fragmentation. An empirical model is developed for the prediction of the char-slag transition by considering the ash content of the parent coal, which can be determined by proximate analysis. A hypothetical mechanism is proposed to describe the particle fates upon impaction on gasifier walls during char-slag transition. A simple correlation is established for characterizing the evolution of the particle stickiness during the transition.

CHAR-SLAG TRANSITION DURING PULVERIZED COAL GASIFICATION by Suhui Li A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemical Engineering The University of Utah May 2010 Copyright © Suhui Li 2010 All Rights Reserved STATEMENT OF DISSERTATION APPROVAL The dissertation of Suhui Li has been approved by the following supervisory committee members: Kevin J. Whitty , Chair March 9, 2010 Date Approved Thomas H. Fletcher , Member March 9, 2010 Date Approved Eric G. Eddings , Member March 9, 2010 Date Approved Terry A. Ring , Member March 9, 2010 Date Approved Milind D. Deo , Member March 9, 2010 Date Approved and by JoAnn S. Lighty , Chair of the Department of Chemical Engineering and by Charles A. Wight, Dean of The Graduate School. ABSTRACT In coal gasification the char-slag transition is a process in which porous char transforms into molten slag at temperatures above ash fluid temperature. It is associated with physical changes of the char particle, such as particle density, size, porous structure and mineral-carbon association. Despite the large number of investigations on coal gasification, the physical phenomena during char-slag transition have not been well studied. In addition, little data regarding ash deposition behavior on gasifier walls during char-slag transition have been reported. This study aims to clarify the physical changes of char particles and ash deposition behavior on gasifier walls during char-slag transition in pulverized coal gasification. To achieve these objectives, two types of experiments were carried out using a laminar entrained-flow reactor: (1) char and ash formation experiments and (2) ash deposition experiments. In the first type of experiment, char and ash particles with different conversions were prepared using two bituminous coals and a subbituminous coal. The prepared char and ash samples were characterized using various techniques to obtain information on particle density, size, porous structure and mineral-carbon association. These data were used to identify the point of the char-slag transition for different coals. Results show that during the transition: (1) particle size decreases, which is caused by shrinkage in the initial stage and by fragmentation in the later stage; (2) particle density increases due to iv particle size reduction; and (3) particle internal surface area decreases because of ash melting induced pore blockage. In the second type of experiment, particle collection efficiency was measured for a bituminous coal at various conversions. Such information was used to derive the variation of particle stickiness during the char-slag transition. Results indicate that the particle stickiness increases dramatically during the transition. This dramatic increase is attributed to exposure of included minerals on the particle surface, which is caused by particle shrinkage and fragmentation. An empirical model is developed for the prediction of the char-slag transition by considering the ash content of the parent coal, which can be determined by proximate analysis. A hypothetical mechanism is proposed to describe the particle fates upon impaction on gasifier walls during char-slag transition. A simple correlation is established for characterizing the evolution of the particle stickiness during the transition. TABLE OF CONTENTS ABSTRACT ....................................................................................................................... iii LIST OF TABLES ............................................................................................................ vii LIST OF NOMENCLATURE ......................................................................................... viii ACKNOWLEDGMENTS ................................................................................................. xi Chapter 1. INTRODUCTION ....................................................................................................... 1 1.1 Background and Motivation ................................................................................. 1 1.2 Outline of This Thesis ........................................................................................... 6 2. LITERATURE REVIEW ............................................................................................ 7 2.1 Porous Structure of Coal Char .............................................................................. 7 2.2 Ash Characteristics ............................................................................................. 18 2.3 Ash Deposition and Slagging Behavior .............................................................. 24 2.4 Concluding Remarks ........................................................................................... 40 3. OBJECTIVES AND APPROACHES ....................................................................... 43 4. EXPERIMENTAL DETAILS ................................................................................... 45 4.1 Overview ............................................................................................................. 45 4.2 Experimental Setup ............................................................................................. 46 4.3 Experimental Procedures .................................................................................... 60 4.4 Experimental Conditions .................................................................................... 61 4.5 Coal, Char and Ash Analyses .............................................................................. 66 5. RESULTS: CHAR-SLAG TRANSITION ............................................................... 72 5.1 Char Burnout Behavior ....................................................................................... 73 5.2 Particle Density and Size .................................................................................... 80 5.3 Particle Internal Surface Area ............................................................................. 92 5.4 Particle Morphology ........................................................................................... 96 vi 5.5 Identification and Modeling of the Char-Slag Transition ................................ 103 6. RESULTS: ASH DEPOSITION .............................................................................. 109 6.1 Particle Collection Efficiency ............................................................................ 111 6.2 Impaction Efficiency Calculation ......................................................................115 6.3 Particle Capture Efficiency ................................................................................119 6.4 Hypothetical Particle Fates ............................................................................... 125 6.5 Modeling of the Particle Stickiness .................................................................. 126 6.6 Concluding Remarks ......................................................................................... 129 7. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ............ 131 7.1 Summary of Results .......................................................................................... 131 7.2 Implications for Industrial Gasifiers ................................................................. 133 7.3 Recommendations for Future Work .................................................................. 133 Appendices A. COOLING RATE CALCULATION IN THE COLLECTION PROBE ................. 135 B. DESIGN OF THE CYCLONE COLLECTOR ....................................................... 137 C. WATER COOLING SYSTEM ................................................................................ 141 D. GAS SUPPLY UNIT ............................................................................................... 144 E. EXPERIMENTAL PARAMETERS ........................................................................ 145 F. FLUENT SIMULATION PARAMETERS ............................................................. 149 G. SEM-EDS ANALYSIS OF THE ASH DEPOSIT .................................................. 150 REFERENCES ............................................................................................................... 155 LIST OF TABLES 1. Definition of ash fusion temperatures ........................................................................... 22 2. Summary of engineering indices of slagging potential ................................................. 37 3. Experimental conditions for different coals .................................................................. 62 4. Proximate and ultimate analyses of the coals used in this work ................................... 67 5. Ash chemistry of the coals used in this work ................................................................ 67 6. Ash fusion temperatures of the coals used in this work ................................................ 68 7. Char and coal properties relating to the char-slag transition ...................................... 106 8. Characteristic geometries of the cyclone .................................................................... 139 9. Experimental parameters for the Illinois #6 coal at 1400 °C ...................................... 147 10. Experimental parameters for the Illinois #6 coal at 1500 °C .................................... 147 11. Experimental parameters for the Black Thunder coal at 1400 °C ............................ 148 12. Experimental parameters for the Pittsburgh #8 coal at 1500 °C ............................... 148 13. Input and output parameters of the FLUENT simulation at 1400 °C ....................... 149 LIST OF NOMENCLATURE 𝑆.………………...Internal Surface Area (Ash Free) of the Partially Converted Char, m2/g 𝑆0……………………………….Internal Surface Area (Ash Free) of the Fresh Char, m2/g 𝜓……………………………………………………. Structural Parameter, Dimensionless 𝑋…………………………………………………………Coal Conversion, Dimensionless 𝐿0………………………………………………...Pore Length (Ash Free) of the Char, m/g 𝜌0………………………………………………..Density of the Fresh Char Particle, g/cm3 𝜖……………………………………………………………..Char Porosity, Dimensionless 𝜖0……………………………………………………...Fresh Char Porosity, Dimensionless X……………………………………………………….Porosity Parameter, Dimensionless 𝜂𝑒…………………..Collection Efficiency of the Carbon-Containing Ash, Dimensionless 𝜂𝑎𝑠𝑕 ………………………..Collection Efficiency of the Carbon-Free Ash, Dimensionless 𝑋𝑐……………………………Carbon Burnout of the Char or Ash Particle, Dimensionless 𝑅…………………………………………………………...Slagging Index, Dimensionless 𝑅𝑒………………………………………………………Reynolds Number, Dimensionless 𝜌𝑔………………………………………………………………………Gas Density, kg/m3 𝜐𝑔………………………………………………………………….Mean Gas Velocity, m/s 𝐷𝑡………………………………………………………………………..Tube Diameter, m 𝜇𝑔………………………………………………………………………Gas Viscosity, Pa·s 𝜐𝑡…………………………………………………………..Particle Terminal Velocity, m/s ix 𝑔………………………………………………………….Acceleration of Gravity, 9.8m/s2 𝑑𝑝……………………………………………………………………..Particle Diameter, m 𝜌𝑝…………………………………………………………………..Particle Density, kg/m3 𝑆𝑡…………………………………………………………..Stokes Number, Dimensionless 𝜐𝑝……………………………………………………………………..Particle Velocity, m/s φ………………………………………..Non-Stokesian Correction Factor, Dimensionless 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎 𝑟𝑏𝑜𝑛 ………………………………….Carbon Content in the Char or Ash Particle, wt% 𝑚𝑐𝑕𝑎𝑟 ……………………………..Mass of the Char Sample Used for the LOI Analysis, g 𝑚𝑟………………………………..Mass of the Burnout Residual after the LOI Analysis, g 𝜂………………………………………………………….Particle Collection Efficiency, % 𝑚𝑑…………………………………………Mass of the Deposit on the Deposition Plate, g 𝑚𝑡…………………………Mass of the Total Particle Approached the Deposition Plate, g 𝑓………………Feeding Rate of the Coal Particle Approaching the Deposition Plate, g/hr 𝑡…………………………….Elapsed time of a single run of the deposition experiment, hr 𝐶𝑐𝑜𝑎𝑙 𝑎𝑠𝑕 …………...Ash Content of the Parent Coal Determined by Proximate Analysis, wt% 𝜌𝑎……………………………………Apparent (Bulk) Density of the Char Particle, g/cm3 𝑚𝑐𝑝…………………………………………Mass of the Cylinder loaded with Particles, g 𝑚𝑐…………………………………………………………………Mass of the Cylinder, g 𝑉……………………………………………..Volume of the Particles in the Cylinder, cm3 𝜌𝑝……………………………………………………………Density of the Particle, g/cm3 𝜙………………………………………...Packing Voidage of the Particles, Dimensionless 𝐶𝑐𝑜𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 …….Carbon Content of the Parent Coal Determined by Proximate Analysis, wt% x 𝑚𝑝……………………………………………………..Mass of the Char or Ash Particle, g 𝑚0……………………………………………………...Mass of the Parent Coal Particle, g 𝑑0………………………………………………..Diameter of the Parent Coal Particle, μm 𝜌0……………………………………………….Density of the Parent Coal Particle, g/cm3 𝑛……………………………………...Number of Fragments Formed from a Char Particle 𝑑1………….Particle Diameter Calculated with Density Assuming No Fragmentation, μm 𝑑2……………………………………Particle Diameter Measured with a Microscope, μm 𝐶𝑐𝑕𝑎𝑟 𝑎𝑠𝑕 ……………………………………………….Ash Content of the Char Particle, wt% 𝑚……………………………………Slope Constant of a Linear Equation, Dimensionless 𝑏…………………………………Intercept Constant of a Linear Equation, Dimensionless 𝜂…………………………………………….Particle Collection Efficiency, Dimensionless 𝐼……………………………………………..Particle Impaction Efficiency, Dimensionless 𝐺………………………………………………Particle Capture Efficiency, Dimensionless 𝜁………………………………………………………..Particle Stickiness, Dimensionless 𝜀……………………...Fractional Coverage of the Particle by Molten Ash, Dimensionless 𝑐𝑖…………..Initial Ash Content when Ash Starts Appearing on the Particle Surface, wt% 𝑐𝑓…………………………Final Ash Content when Ash Covers the Particle Surface, wt% 𝐴………………………………………………………...Constant in eq 30, Dimensionless 𝐵………………………………………………………...Constant in eq 30, Dimensionless 𝐶………………………………………………………...Constant in eq 30, Dimensionless ACKNOWLEDGMENTS I would like to thank the following people: Professor Kevin Whitty, my advisor. Without his support, guidance and encouragement during this dissertation research, I could not have finished my Ph. D. study. His critical scientific thinking, enthusiastic research interest, and serious working attitude have been and will be a continuous standard and inspiration for my study and work. Professor Tom Fletcher, Professor Eric Eddings, Professor Terry Ring, Professor Milind Deo, my supervisory committee members. Their valuable suggestions helped improve this dissertation to a new level. Mr. Dana Overacker, for his continuous help in my experimental work as well as the advice in the design and construction of the laminar entrained-flow reactor. He has always been encouraging and supportive during my study as a foreign student. Dr. Scott Sinquefield, for his important suggestions in the design of the laminar entrained¬-flow reactor, which saved me a lot of time. Professor Jan Miller, for generously providing the instrument for measuring the particle surface area. Professor Wu Yuxin, for his instruction on using FLUENT software to calculate the particle impaction efficiency and meaningful discussions on my experimental results. Special gratitude goes to Professor JoAnn Lighty, who provided me the xii opportunity of studying at the University of Utah. Thanks extend to my parents for their continuous love and support on my study. Financial support of this work was provided by the U.S. Department of Energy's National Energy Technology Laboratory under Award Number FC26-08NT0005015. CHAPTER 1 INTRODUCTION 1.1 Background and Motivation Coal is the most abundant fossil fuel on the earth, comprising about 75% of the world's total resources of fossil fuels (1). It is the second largest part (about 24%) of the world's energy supply and maintains the largest share (about 39%) of the electricity generation in the world (2). Coal will continue to be used as a major energy resource and dominant fuel for electrical power production in the foreseeable future. The utilization of coal, however, is limited because of its disadvantages, including the requirement of costly pollution control systems, high ash content, not directly applicable in transport systems. In addition, coal-fired power plants are the biggest contributor to CO2 emissions (3), which is a major greenhouse gas. Although controversial, recent research on global warming has raised increasing concern on CO2 emissions from coal-fired power plants because coal contains more carbon than other fossil fuels. Consequently, extensive efforts have been devoted to the development of technologies for the clean and efficient utilization of coal. Integrated gasification combined cycle (IGCC) with CO2 capture has been identified as one of the most promising solutions because of its advantages, such as ultra-low emissions of air pollutants and greenhouse gas, high efficiency in power generation, flexible feedstocks and a wide variety of end products (3). The heart of an IGCC power plant is the gasifier which converts coal and other 2 solid fuels into synthesis gas (syngas). In commercial IGCC applications, the predominant type of coal gasifier is the entrained-flow slagging gasifier (3, 4) operating under high pressures (20-70 bar) and at high temperatures (1300-1500 °C). Figure 1 shows a schematic of a typical entrained-flow coal gasifier. Coal slurry is gasified by oxygen in co-current flow. Steam is fed into the gasifier for adjusting the CO/H2 ratio in the syngas. Ash is removed as a form of slag at the bottom of the gasifier. Figure 1. Schematic of a Texaco entrained-flow coal gasifier. 3 In an entrained-flow slagging gasifier, coal particles usually undergo two conversion stages that take place almost simultaneously because of the high heating rate: coal pyrolysis (devolatilization) and char gasification. This conversion process is schematically illustrated in Figure 2. Coal pyrolysis produces a variety of volatiles, including tars, hydrocarbon liquids and gases species such as carbon monoxide, water, hydrogen, methane, and other organic compounds (5). The volatiles immediately react with the oxidants surrounding the coal particle. The coal particle then transforms into a swollen, porous, reactive char particle which contains mainly carbon and inorganic matter. Char gasification involves heterogeneous (gas-solid) reactions in which carbon is converted into syngas (carbon monoxide and hydrogen) by oxidants such as oxygen and steam. The term gasification refers to partial-oxidation under substoichoimetric conditions, which constitutes the major difference compared with combustion. The inorganic matter in the coal transforms into ash or slag. Char gasification is the key step in this process because (1) it is the rate-limiting step that determines the overall coal conversion and reaction rate, and (2) it involves the char-slag transition that is associated with ash formation and ash deposition. Figure 2. Conversion process of coal particles in an entrained-flow slagging gasifier. 4 Because of the short residence time (a few seconds) of coal particles in entrained-flow gasifiers, high temperatures are required to achieve high conversion. The high temperatures help break down the tars and oils but create challenges of handling ash melting and slagging. Operation of an entrained-flow slagging coal gasifier is under the essential condition that ash formed in the gasifier can be continuously removed as a liquid slag flow (3, 6). Build-up of slag on the gasifier wall causes erosion and corrosion of the refractory, thus creating problems in the operation of gasifiers and syngas coolers such as excessive maintenance and unscheduled shut downs. Therefore, ash deposition related issues are usually a major concern in the design and operation of entrained-flow coal gasifiers. In particular, deposition of a particle on the gasifier wall during the char-slag transition significantly affects its burnout behavior by increasing its residence time on the wall. For example, in the EAGLE (Coal Energy Application for Gas, Liquid & Electricity) project, a special design makes the large coal particles tend to deposit on the gasifier wall and flow downwards with the liquid slag (3). This design ensures that large coal particles have a long residence time and achieve a high conversion. On the other hand, computational fluid dynamics (CFD) modeling (Figure 3) of a Texaco entrained-flow coal gasifier indicates that char particles strike the gasifier wall at different positions with various impaction angles (7). Upon impacting the gasifier wall, the char particles might rebound or adhere on the wall surface depending on the kinetic energy and stickiness of the particle and the wall. Particles during the char-slag transition have unique properties that affect the deposition behavior. 5 Figure 3. CFD simulation of particle trajectories in an entrained-flow coal gasifier. (Reprinted with permission from Wu (7). Copyright 2008 Tsinghua University.) The char-slag transition largely determines the overall coal conversion and the ash formation and deposition behavior. However, the specific nature of the char-slag transition has not been well studied despite the large number of investigations on char gasification and ash formation and deposition. Little attention has been paid to characterizing the physical phenomena associated with the char-slag transition, particularly at high temperatures. These physical changes include particle structure, particle morphology, and mineral-carbon association. The effect of these physical phenomena on ash deposition behavior during the char-slag transition is not well understood and needs to be considered in developing ash deposition models. This research was motivated by: (1) the lack of understanding on the physical changes of char particles during the char-slag transition, and (2) the need to clarify the 6 ash deposition behavior during the char-slag transition. This dissertation presents a lab-scale experimental study on the char-slag transition in pulverized coal gasification. 1.2 Outline of This Thesis After an introduction to the background and motivation of this work in Chapter 1, a literature review of the related field follows as Chapter 2. Chapter 3 outlines the objectives of this research and the approaches used to complete these objectives. Chapter 4 describes in detail the experimental setup and materials used to perform the studies. The results of experiments are presented and discussed in Chapters 5 and 6. Chapter 5 addresses the physical phenomena in char-slag transition and Chapter 6 deals with ash deposition behavior during char-slag transition. Conclusions of this work and recommendations for future work are summarized in Chapter 7. Supplemental information is included in Appendices A-G. CHAPTER 2 LITERATURE REVIEW Coal in a pulverized coal combustion or gasification system undergoes two major conversion steps: pyrolysis and char oxidation. In the first step, volatile matter is released and porous, reactive char particles are formed. In the second step, the organic matter (mainly carbon and hydrogen) in char particles are converted into carbon monoxide, carbon dioxide and hydrogen in heterogeneous reactions with various oxidants, including oxygen and water steam. The inorganic matter (minerals) in the coal transforms into ash, which may contain a small amount of residual carbon. Char oxidation is the rate-limiting step and determines the carbon conversion and the ash formation. Therefore, it is the most important step in pulverized coal combustion or gasification processes and has been studied extensively. In particular, the transition from porous, reactive char to nonporous, low-reactive slag occurs in the later stage of char oxidation. The char-slag transition involves the variation of porous structure of char, the transformation of mineral-carbon association and the formation of ash particles. 2.1 Porous Structure of Coal Char 2.1.1 Characterization of Porous Structure Extensive efforts (8-12) have been made to characterize porous structure of coal char. These characteristics include morphology, dimension, wall thickness, sphericity, 8 porosity, crystallinity, surface area and pore size distribution. Among these features, surface area and pore size distribution are most widely used as the macroscopic measurements reflecting the microscopic characteristics in the porous structure. Therefore, surface area and pore size distribution received the most attention in the studies of the porous structure evolution (13). The International Union of Pure and Applied Chemistry (IUPAC) document of Harber (14) defines the surface area and the pore size distribution of particles. The surface area of a particle includes the external and internal surface area. The external surface is regarded as the envelope surrounding the discrete particle or agglomerates. The internal surface is designated as the surface of the walls of the pores and connections inside the particle. The surface area of particles mentioned in this dissertation is generally regarded as internal surface area because the external surface area is usually negligible compared to the internal surface area. The pore size distribution is defined as the distribution of pore volume with respect to pore size. The pores inside the particle can be classified as three groups according to their sizes: micropores, mesopores and macropores. Harber (14) also recommended size boundaries for the classification: 2 nm as the upper limit width for the micropore and 50 nm as the upper limit for the mesopore. The internal surface area and the pore size distribution are usually measured by gas physical adsorption methods. The most commonly used standard procedure to determine the internal surface area is the Brunauer-Emmett-Teller (BET) analysis using nitrogen as adsorptive, which is described in another IUPAC document (15). The mesopore size distribution can be calculated by the Barrett-Joyner-Halenda (BJH) method (16) and the micropore size distribution can be calculated by the Horvath-Kawazoe (HW) method 9 (17). However, nitrogen has the drawback of very slow diffusion in micropores. Jagiello and Thommes (18) measured the adsorption isotherms of activated carbon using N2, Ar and CO2 as adsorptive. Although calculated pore size distributions based on N2, Ar and CO2 adsorptions were consistent in both micropore and mesopore ranges, the N2 adsorption took much more time to reach equilibrium than the CO2 adsorption did in the micropore analysis. They concluded that CO2 adsorption is preferred for faster measurements in the micropore range. 2.1.2 Role of Porous Structure The variation in the porous structure of coal char is one of the most important subjects in studying coal char conversion. It has been recognized that the porous structure and its evolution have major influence in the conversion mechanism of coal char gasification (13). The porous structure of coal char has a major impact in determining its reactivity during gasification (9, 19). Koranyi (20) studied the relationship between the porosity and reactivity during CO2 gasification of three British bituminous coal chars. In his study, the porosity of char was measured by gravimetric adsorption of CO2 at 195 K. The results clearly showed a linear correlation between reactivity and microporosity. He suggested that this is because the active surface area is related to the total surface area. Hurt et al. (21) investigated the role of microporous surface area in the CO2 gasification of a subbituminous coal char in the temperature range of 800-900 °C using a thermogravimetric analyzer (TGA). They found that reactions mainly took place outside the microporous network on the surface of larger pores. The evidence was that the gasification rate was insensitive to the large changes of total microporous surface area of 10 coal char caused by heat treatment. Therefore, they suggested that gasification reactions mainly occur at active sites that represent crystallite edge groups or reactive edges that are chemically associated with catalyst particles. In a parallel study (22), they investigated the role of microporous surface area in CO2 gasification of synthetic carbon, i.e., uncatalyzed gasification. In contrast to the gasification of coal char, reactions mainly occurred within the micropores of synthetic carbon. The evidence was the micropore widening and the increase of average micropore size, which was determined from surface area measurements. The micropore surface area measured from CO2 adsorption remained constant while the mesopore surface area measured from N2 adsorption increased is an indication of pore widening. The increase of the Dubinin gradient with conversion is an indication of increase in average pore size. Hurt (23) also showed that kinetically-limited carbon gasification does not take place at constant particle diameter, but is accompanied by reaction-induced atomic rearrangements, which leads to particle densification and shrinkage. The porous structure affects the mass transfer and heat transfer in the char particle, which subsequently influences the reaction rate and conversion. Hampartsoumian et al. (24) studied the effect of porous structure of char on the gasification rate by investigating the relationship between the effectiveness factor of diffusivity and physical properties of char (porosity and density) during CO2 gasification. The overall reaction rates of two U.K. coal derived chars were measured using a TGA and the progressive changes in pore structure provide implications for the reaction mechanism. Results showed that in the CO2 gasification of coal chars, the two-stage oxygen exchange mechanism holds and the intraparticle diffusional limitation becomes significant at temperature above 1173 K. In 11 the experimental study of Kawahata and Walker (25), the surface area of char was observed to decrease due to the increase in the diffusional resistance at elevated temperature. Thermal conductivity reflects the structural change of char particles during reaction and is a measure of the heat transfer. Weiss et al. (26) and Zhang et al. (27) proposed a method to measure the thermal conductivity of synthetic char during oxidation. In brief, a single Spherocarb particle was levitated in an electrodynamic balance (EDB) chamber and was laser heated. The natural convection drag and the photophoretic force were measured as a function of carbon conversion and temperature. Thermal conductivity was inferred from the photophoretic force. Using this method, Bar-Ziv and Kantorovich (28) studied the role of porous structure in char oxidation. For the thermal measured conductivity, a decrease of five times was observed for 0-30% burnout, followed by a constant up to 80% burnout and an increase of 2 times up to complete burnout. The dramatic change in thermal conductivity during the burnout process indicates that the porous structure seriously affects the heat transfer in the particle. They pointed out that thermal conductivity is determined by the porosity, the dimension of pores and microcrystals, and the connectivity of the solid portions of the char particle. Among these characteristics, the connectivity is the most important factor. These results suggest that the change in thermal conductivity can be used to evaluate the small changes in the porous structure of char during oxidation, and hence provides insight into the heat transfer property of porous chars. The porous structure of coal char has significant influence on the formation and characteristics of ash and slag during char conversion. Zhang et al. (29) investigated the role of porous structure in the shrinkage and fragmentation behavior of highly porous 12 synthetic char particles during kinetically controlled oxidation. Synthetic char particles were suspended in an electrodynamic chamber for oxidation at 700-1000 K. Meanwhile, in-situ measurements on the particle mass, density, size and shape were performed continuously. No fragmentation but shrinkage was observed at conversions up to 80%. To understand this phenomenon, they examined the fine structure of particles burned in a TGA using high resolution transmission electron microscopy (HRTEM). The HRTEM images showed increased ordering of the microporous structure of the particle with the conversion, which accounts for the shrinkage and integrity of the particle. Wu and colleagues (30-33) studied ash liberation of included minerals during pulverized coal combustion. They used a drop tube furnace to combust coal particles to different burnout levels at 1300 ºC and 1-15 atmospheric pressures. Results showed that ash liberation is determined by the char structure at different conversions. Fragmentation of porous char results in fine ash particles in the early stages of coal combustion, while coalescence of included minerals leads to the formation of coarse ash particles in the later stages of combustion. They proposed a mechanism (Figure 4) describing the effect of char morphology and structure on char fragmentation and ash liberation. The effect of char structure on ash formation during pulverized coal combustion was also studied by Kang et al. (34). Two kinds of prepared char samples were combusted with a laminar flow reactor at 1650 K in a 1/1 (v/v) mixture of O2 and N2. Results showed that cenospheric char particles produced by rapid heating yielded more fine ash particles than noncenospheric chars produced under slow pyrolysis. 13 Figure 4. Char fragmentation and ash formation mechanism proposed by Wu et al. (32). Bar-Ziv and Kantorovich (13) reviewed the experimental and modeling efforts concerning the role of porous structure in char oxidation. This review was focused on the shrinkage, fragmentation and thermal conductivity of char particles during oxidation. They demonstrated that there is a general behavior connected with the evolution of the porous structure and concluded that this evolution is controlling most of the physicochemical changes of highly porous chars during oxidation. Shrinkage reflects the changes in the external shape and the decrease in the dimension of char particles. It affects the gasification rates, effective diffusivities and fragmentation behavior of the char particles in kinetically controlled char oxidation. The 14 shrinkage phenomenon was observed by Hurt et al. (35) in the study of kinetically controlled gasification of porous carbons. They used a TGA to gasify both synthetic char and a variety of coal chars at low temperatures (723-1273 K). The SEM pictures of the gasified chars showed that homogeneous shrinkage occurred. They concluded that the shrinkage was not caused by the reaction on the external surface of the char particle, but rather the reaction on the microporous solid phase-the microcrystals. The experimental data did not agree well with the numerical solutions of the Gavalas random pore model, which did not consider shrinkage. Most of the previous pore models were based on pore size distribution, because experimental measurements on physical adsorption can only provide information on the size distribution and surface area of pores. In contrast, shrinkage is caused by the change in dimension and shape of pore edges at the intersections, which cannot be measured by gas adsorption and was neglected by the overlapping pore model. Fragmentation occurs at a threshold porosity depending on the structure geometry (13). Kerstein and Niksa (36) predicted this threshold porosity to be around 70%. None of the experiments carried out under chemically controlled conditions resulted in fragmentation at any stage until the completion of burn out. Zhang et al. (29) studied the structural changes of char particles during chemically controlled oxidation to identify the factors governing fragmentation. Results showed that little fragmentation occurred even on particles with porosity over 70%. This was explained by the bimodal pore size distribution theory, which requires that both the macroporosity and the microporosity exceed a certain value. HRTEM images also showed an increase in microporosity, which is responsible for the shrinkage of particles. Actually, it is the shrinkage that keeps the 15 macroporosity constant. Zhang et al. (29) also studied the fragmentation of highly porous char burning in the chemically controlled regime. They characterized the fragmentation behavior as follows: 1) the char particle shrinks monotonically until a hole is formed at around 80% conversion; 2) the external diameter of the char particle decreases and the diameter of the hole increases until these two diameters converge at about 97% conversion. Bar-Ziv et al. (13) concluded that fragmentation of oxidizing char particles under chemically controlled conditions is determined by the porosity of the large pores in the microcrystal. 2.1.3 Pore Models Various models have been developed to describe the porous structure evolution during oxidation and were reviewed by Bar-Ziv et al. (13). They pointed out that the models best suited for describing the surface features of the particle are the continuum models based on a random pore structure, which can be divided into two categories: randomly overlapping pores (37-39) and randomly intersecting nonoverlapping pores (40, 41). The randomly overlapping pore model, developed by Bhatia, Perlmutter and Gavalas (37-39), regards the solid particle as composed of overlapping pores with random distribution, i.e., the position and orientation of pores are independent of each other. Pores are usually treated as capillary, cylinder or slit for simplification in calculation. This model is a simple approach describing the porous structure development during reaction. It predicts the surface area evolution as S = S0 1 − ψ ln 1 − X (1) 16 where S and S0 are surface areas at conversion X and 0 per unit mass of residual carbon, respectively. ψ is a dimensionless structural parameter defined as ψ = 4πL0 ρ0S0 2 (2) in which L0 and ρ0 represent the pore length per unit mass of residual carbon and true density of the particle at conversion 0, respectively. X is defined as X = 1−ϵ 1−ϵ0 (3) where ϵ and ϵ0 are porosity at conversion X and 0, respectively. Therefore, the relative surface area S/S0 is dependent on two parameters: conversion X and structural parameter 𝜓 . Because 𝜓 is dependent on the initial characteristics of the particle, the relative surface area is only a function of conversion X. Eq 1 can be expressed as (S/S0)2 = 1 + ψ − ln 1 − X (4) Eq 4 indicates that the plot of the square of the relative surface area against - ln(1 − X) is linear for a specific particle, and the slope is ψ. This plot can be used to validate the randomly overlapping pore model. Some of the previous experimental data agreed well with this model in the relative surface area evolution during gasification (13), whereas disagreement between experimental data and modeling results was also reported. Morinoto et al. (42) studied the 17 development of porous structure of coal chars during CO2 gasification. In their study, three coal samples were gasified to various conversions by a TGA and surface areas of char samples were measured by gas adsorption analysis. The measured surface areas were significantly larger than those calculated from the random pore model, especially at high conversion. Further calculation also showed that the structural parameter 𝜓 was not a constant throughout the gasification process. They attributed this phenomenon to the widening of narrow micropores (submicropores). These submicropores, which were inaccessible to N2 adsorption, were widened into micropores as conversion proceeded, and became accessible to N2 adsorption. The increase of the number of micropores that were accessible to N2 adsorption increased the measured surface area. However, the randomly overlapping pore model did not include the formation of new micropores. Nor did it predict shrinkage and fragmentation of the particle, which were observed by many researchers. Consequently, Kantorovich and Bar-Ziv (28, 40, 41) developed a randomly intersecting nonoverlapping pore model to incorporate shrinkage and fragmentation. This model utilized a -subskeleton‖ mechanism (Figure 5) that included the following features: (1) Oxidation prefers to occur at edges of microcrystal and causes break-restoration of the microcrystal network. (2) The subskeleton of large microcrystals does not change while the fine structure of small ones changes with respect to conversion. (3) Coalescence takes place for small microcrystals. In good agreement with experimental data, this model is able to connect physical changes in the microcrystal structure directly to reactivity. 18 Figure 5. "Subskeleton" mechanism. (Reprinted with permission from Bar-Ziv and Kantorovich (28). Copyright 1994 Elsevier) A model that can express the gasification behavior of char particles with complicated structures was proposed by Yamashita et al. (43). This model treats char particles before reaction as three-dimensional cubes, which consist of randomly arranged small lattices. These lattices can be classified as char, ash or macropores depending upon the proximate analysis of char particles. The numerical results based on this model showed that wall thickness plays an extremely important factor in determining the transition temperature between the kinetically controlled regime and the pore-diffusion limited regime. It also showed that the fragmentation behavior of char particles is dependent upon the reaction regime. The fragmentation occurred at late stage of burnout in the chemical reaction regime, but it shifted to initial stage in the pore-diffusion regime. 2.2 Ash Characteristics Mineral matter in pulverized coal that contributes to ash formation and deposition is classified into two categories according to the association between minerals and the carbon matrix: excluded minerals and included minerals (44-46). Excluded minerals are discrete mineral grains that are not associated with the coal particle. Included minerals 19 are the mineral matter that is embedded within or organically bonded with the carbon matrix in the coal particle. Due to the complex composition of mineral matter, ash characteristics vary over a wide range from coal to coal. For bituminous coal (high-rank coal), the included mineral matter is mainly in the form of embedded minerals (47). Part of the alkali and alkaline earth metals in low-rank subbituminous coals are chemically bonded to carboxylic and phenolic groups in the coal (48). Van Dyk et al. (49) summarized the coal ash characteristics and the analytical tools used to determine these characteristics. 2.2.1 Ash Chemistry and Mineralogy The American Society for Testing and Materials (ASTM) (50) defines the standard procedure to perform elemental analysis on ash. Pulverized coal is burned in an oxidizing atmosphere at 972-1016 K. The elements in the coal are quantitatively measured by a series of spectroscopic techniques, including atomic absorption spectroscopy, inductively coupled plasma-atomic emission spectroscopy and X-ray spectroscopy. The elements present in coal ash are mainly silicon, aluminum, iron and calcium with small amounts of magnesium, titanium, sodium and potassium, which are reported in the form of their oxides. Depending on its elemental composition, coal ash is divided into two categories: lignitic ash and bituminous ash. Lignitic ash contains more CaO and MgO than Fe2O3, whereas bituminous ash contains more Fe2O3 than CaO and MgO. These elements can be classified as basic or acidic. The basic elements are mainly iron, calcium, magnesium, sodium and potassium, and the acidic elements are mainly silicon, aluminum and titanium. However, this kind of ash elemental analysis is performed under laboratory 20 conditions that do not represent the true environment in a practical combustion or gasification system (49). Scanning electron microscopy with energy dispersive X-ray spectrometer (SEM-EDS) is able to obtain the elemental composition and grain size of inorganic minerals in the coal char particle, as well as directly view the size, morphology and structure of a single particle at microscopic level. In addition, the SEM-EDS is capable of providing information on the association type of minerals in the coal: excluded or included (embedded within and organically associated). Because of its powerful features, this technique has been applied to study the transformations of inorganic minerals during pulverized coal combustion (48-53) and the ash deposit formation mechanism (54-56). Recently, computer-controlled scanning electron microscopy (CCSEM) has been developed to statistically analyze ash chemistry and physical aspects. This technique has advantages over the traditional SEM-EDS technique in that it automatically locates individuals in a number of coal, char and ash particles and determines the size, shape and mineralogy (57, 58). However, it also has its limitations, such as complex data interpretation and the fact that one element can only be assigned to one mineral category (59, 60). Matjie and Van Alphen (61) successfully analyzed a Sasol (South African Coal, Oil, and Gas Corporation) gasification ash using CCSEM and X-ray diffraction (XRD). Detailed mineralogical and chemical information was obtained to identify a number of potentially viable byproducts from the bulk ash. Vuthaluru and French (62, 63) conducted a systematic investigation on the ash chemistry and mineralogy of an Indonesian coal during combustion in both laboratory and pilot scale furnaces. For the laboratory scale, ash formation experiments using raw 21 coal, washed coal, raw coal and a bauxite mixture were carried out using a drop tube furnace at 1473 and 1673 K, and the ash deposition experiments were performed under 1023 K with a rotating alumina probe oriented perpendicular to the particle laden gas flow. Ash samples were characterized using XRD and QEMSCAN. The QWMSCAN is an automated technique that can provide mineralogical data of samples. It combines features from SEM, EDS and electron probe microanalyzer (EPMA). They found that the ash that rebounded from the deposition probe had a lower glass content and higher crystalline phase (quartz and mullite) than that adhered on the probe. Of the three coal samples, the raw coal-bauxite ash has the lowest glass content with high corundum, which indicates a low ash deposition propensity. The pilot scale experiments substantiated the findings made in the laboratory scale experiments and suggested that a 3% bauxite additive offers the best reduction in slagging and fouling propensities compared to raw coal alone. 2.2.2 Ash Fusibility Ash fusibility provides an indicator concerning ash melting and slagging behavior in coal combustion and gasification. ASTM (64) describes in detail the standard method to measure ash fusion temperatures. In brief, an ash sample is prepared by burning coal in an oxidizing atmosphere at 972-1016 K. This ash is pressed into a mold to form a cone shape. The cone is heated in an either oxidizing or reducing atmosphere at a heating rate of 8 K/min. Cone deformation is visually observed. Temperatures associated with specific cone deformation are recorded. Ash fusibility is characterized by four temperatures, which are listed in Table 1. A schematic diagram illustrating the deformation of the cone in the ash fusibility analysis is presented in Figure 6. 22 Table 1. Definition of ash fusion temperatures Characteristic Temperature Definition Initial deformation temperature (IT) The cone begins to deform Softening temperature (ST) The cone has deformed to a spherical shape Hemispherical temperature (HT) The cone has fused to a hemispherical lump Fluid temperature (FT) The cone has melted to a nearly flat layer Figure 6. Cone deformation at different ash fusion temperatures. The composition of elements in coal ash has a strong influence on the ash fusibility (44, 49, 65). The mineral matter in the ash exists as higher-oxidized forms in an oxidizing atmosphere, whereas it exists as reduced or lesser-oxidized forms in a reducing environment. Because the melting temperatures of these forms are different, the ash fusion temperatures measured under oxidizing condition and reducing condition are different. For example, bituminous ash usually has a high content of iron, which can exist as reduced or lesser-oxidized forms (Fe, FeS2 and FeO) in reducing environment and higher-oxidized forms (Fe2O3 and Fe3O4) in oxidizing environment (65). The melting temperatures are usually higher for the higher-oxidized forms than for the reduced and lesser-oxidized forms. Therefore, the presence of a large amount of iron in bituminous 23 ash significantly influences its fusion temperatures. As the amount of iron in coal ash increases, the difference between oxidizing fusion temperatures and reducing fusion temperatures increases. However, lignitic ash usually contains a low content of iron and a high content of calcium and magnesium. The oxidized forms of calcium and magnesium have lower melting temperatures than their reduced forms. Consequently, the oxidizing fusion temperature of lignitic ash may be lower than the reducing fusion temperature. As can be expected, the higher content of calcium and magnesium the coal ash contains, this effect will be more prominent. The base to acid ratio also provides an indication on the melting and viscosity properties of coal ash. Bases and acids in coal ash can form compounds that have lower melting temperatures than the original bases and acids. When the base to acid ratio approaches 1, the melting temperature of coal ash reaches a minimum value. When the base to acid ratio deviates largely from 1, the melting temperature of coal ash reaches a maximum value. Although ash fusion analysis has been widely used in predicting ash melting behavior, it is not satisfactory in many practical applications because of its limitations (49, 59). For example, ash used in ash fusibility tests is produced under laboratory conditions that differ greatly from practical combustion and gasification conditions. Moreover, measurement of ash fusion temperatures is based on visual observations, which are subjective and have a shortcoming of poor repeatability and reproducibility. A deviation of 200 ºC in ash flow temperature has been reported by different labs on the same coal ash (66). 24 2.3 Ash Deposition and Slagging Behavior Ash deposition and slagging is one of the major issues in the design and operation of pulverized coal combustion and gasification systems. It is essential to clarify the mechanism of ash deposition and establish a model to predict it. Investigations concerning the mechanism and models for predicting ash deposition and slagging under both combustion and gasification conditions are presented in this section. Although combustion is different from gasification, many of the research results about the ash deposition and slagging under combustion conditions can provide indication to research under gasification conditions. 2.3.1 Mechanisms of Ash Deposition and Slagging Generally, there are four steps (67) involved in ash deposition or slagging: 1) ash formation; 2) transfer of ash particles to the wall surface; 3) sticking or rebounding; 4) deposit build-up or slagging. 2.3.1.1 Ash formation. Ash is formed from inorganic species in the coal during coal burnout process. Ash formation is therefore strongly influenced by the coal burnout process. Ash generated by coal combustion typically has a bimodal size distribution: two peaks above and below 2 μm (13). Ash particles larger than 2 μm (coarse ash) result from char fragmentation, coalescence or agglomeration of fine particles. Ash particles less than 2 μm (fine ash) are formed from vaporization, condensation, aggregation of mineral matter released during combustion. In pulverized coal combustion and gasification, different minerals undergo different physical-chemical transformations because of the variation in their association with coal matrix, resulting in different contributions to ash formation (45, 46, 65, 68). 25 Generally, excluded minerals are in equilibrium with the bulk gas environment at the gas temperature in the combustor or gasifier, whereas included minerals are in equilibrium with the local atmosphere at the local temperature within the char particles (65). Liu et al. (69) studied ash formation from excluded minerals considering mineral-mineral association. In their work, three size-graded Australian coals were burned in a drop tube furnace at 1673 K. Both the coals and ash were analyzed by QEMSCAN to reveal the transformation in the morphology of different minerals. Results show that illite, ankerite and siderite change spherical shape after combustion, whereas other minerals do not have significant changes in morphology. Wu et al. (31) investigated the influence of char structure and burnout on ash liberation from included minerals. In their work, size-selected (63-90 μm) coal particles were combusted in a drop tube furnace at 1573 K to five burnout levels. Experimental data showed that char structure determined the ash liberation at different burnout levels. Highly porous char had a tendency to fragment and to release fine ash particles, while low-porosity char had less fragmentation and formed coarse ash particles in the late stage of burnout by coalescence of the mineral matter. Wu et al. (32) also studied the effect of pressure on ash formation using a pressurized drop tube furnace. Results showed that ash generated at high pressure is much finer than that generated at low pressure. This difference is attributed to the structure difference of chars generated at different pressures. These results confirm previous claims that ash formation is related to the structure and morphology of char. Quann and Sarofim (48) investigated the transformation of organically bound minerals during lignite combustion using scanning electron microscopy. Char particles with different conversions were obtained by burning coal 26 particles in a laminar flow DTF at different residence times. SEM micrographs of the char particles showed that the atomically dispersed alkaline earth metals formed submicron mineral grains on the char surface at low burnout. The submicron minerals coalesced into ash droplets in a size range of 1-10 μm. Many small ash particles in the size range of 1-8 μm were also formed by shedding from the char surface. Kang et al. (34) studied the effect of char structure on ash formation during pulverized coal combustion. In their study, char samples with two kinds of porous structures were prepared by different heating rates. Cenospheric char was generated by introducing coal particle into a laminar flow reactor heated to 1650 K, and noncenospheric char was generated in a pyrolysis oven heated at 0.1 K/s. Experimental data showed that cenospheric char yielded more fine ash particles than noncenospheric char. They concluded that the macropore structure influences the ash formation by inducing fragmentation and controlling the extent of ash coalescence. 2.3.1.2 Transfer to the wall. Wall (70) summarized the mechanisms of ash transferring to the wall surface in a pulverized coal-fired boiler. On the basis of the size distribution of ash particles, three modes were identified: (1) Ash particles larger than 10-15 μm are transferred to the wall surface by inertial impaction. 2) Fine ash (less than 1 μm or 10 μm) are transferred by thermophoresis and eddy diffusion. 3) Vapors and gases are transferred by molecular diffusion and condensation. Baxter and colleagues (71, 72) laid the foundation for ash deposition by proposing five mechanisms of ash particles transferring to the wall surface: inertial impaction, thermophoresis, condensation, chemical reaction and eddy impaction. Inertial impaction has been identified as the dominating mechanism in ash deposition (73). The other four mechanisms, which are 27 called near wall effects, were shown to be insignificant compared to inertial impaction (74, 75). Details of these mechanisms will be presented in the following sections. Inertial impaction refers to the process in which particles are transported to the target surface by gas flow and impact on the surface. A conceptual illustration of inertial impaction is shown in Figure 7. Large or heavy particles with high kinetic energy tend to traverse the streamlines and hit the obstacle. Small or light particles with low kinetic energy are prone to bypassing the obstacle by following the gas streamlines. The impacted particle might rebound or adhere depending upon the particle and impaction surface properties. Figure 7. Schematic illustration of inertial impaction on a cylinder in cross flow: gray blobs represent ash particles. 28 The rate of inertial impaction (impaction efficiency) depends on the impaction surface geometry, particle size and density and gas flow properties. For example, the impaction efficiency increases as the Stokes number of the particle increases. Calculations of impaction efficiency as a function of Stokes number can be found elsewhere (76-79). Another factor that affects the inertial deposition rate is the particle capture efficiency, which is a function of ash particle chemistry and viscosity, the deposit surface composition, morphology and viscosity (80). It can be estimated from empirical correlations based on the parameters above. There are large variations in capture efficiencies of different chemical components. The tendency of each mineral component to deposit on the surface is directly proportional to its chemical composition (72). The product of the inertial impaction efficiency and the capture efficiency yields collection efficiency. Collection efficiency is a measure of the ratio of ash particles deposit on the surface to the ash particles impact the surface. In general, the rates of inertial impaction on cylinders in cross flow, i.e., ash particles approaching heat transfer tubes, have been well established, whereas the rates on walls in parallel flow (resembling particles impacting gasifier walls) are less well established (72). The particle capture efficiency, which was shown to be representative of the intrinsic tendency of ash particles to deposit (81), is far from being well understood. Thermophoresis is the movement of particles caused by local temperature gradient. Usually large or heavy particles move along the direction of gradient, whereas small or light particles exhibit opposite behavior. In most cases thermophoretic deposition is negligible, but in some cases it makes a major contribution to the deposition of submicron ash particles. Thermophoretic deposition rate usually decreases as the 29 deposition layer builds up, which increases the surface temperature of the deposition layer and thus reduces the temperature gradient. Condensation is the process that released inorganic vapor deposits on a surface cooler than the vapor phase. The condensed vapor on the deposition surface influences the overall ash deposition behavior by forming a sticky surface on the deposition target. Usually low-rank coals release more inorganic vapor than high-rank coals. Therefore, it is not a major contribution to ash deposition for high-rank coals. Chemical reactions may occur between gas phase and the ash deposit. These reactions can be divided into three main categories: sulfation, alkali absorption and oxidation. Eddy impaction involves only fine (submicron) ash particles. These fine particles are too small to impact the gasifier wall by inertial impaction based on Stokes number calculated with average stream velocities. Turbulent eddies add momentum to these particles and disrupt steady streamlines so that these particles have enough momentum to impact the gasifier wall. Since turbulent eddies are difficult to describe, this process is less understood than any of the processes discussed in this section. Its description is related mainly to empirical coefficients. 2.3.1.3 Sticking and rebounding. Upon transferring to a target surface, an ash particle either sticks to the surface or rebounds from the surface depending on the overall effective stickiness (73). The overall effective stickiness is a function of ash particle stickiness and impaction surface stickiness. The stickiness of ash particles upon impaction on a surface is a function of surface tension, kinetic energy and viscosity of the particle. The stickiness of the impaction surface is determined by the surface property. 30 Issak et al. (82) found that for synthetic ash the stickiness criterion is 10-20% weight fraction liquid phase in the particle. Experiments and thermodynamic calculations (83) using synthetic ash particles confirmed that a weight fraction of 15-70% melting phase is required for an ash particle to be sticky, when the ash is alkali-rich. That is, the threshold stickiness criterion for alkali-rich ash is 15% weight fraction of molten phase. However, for silica-rich ash, the threshold stickiness criterion is a viscosity range of 105-108 Pa·s for a particle with kinetic energy typical of coal-fired boilers (80). Below this critical viscosity, ash particles adhere to the surface, whereas above this critical viscosity, particles rebound. The stickiness was shown to be a function of temperature because the viscosity of the ash is a function of temperature. A sticky ash may rebound from the target surface if the kinetic energy of ash particle is too high, i.e., the impacting velocity is too high (73). If the kinetic energy is too low, the ash particle will follow the gas stream around the target surface, rather than colliding with the surface. Walsh et al. (84) found that the there is a narrow size range (large enough to impact and small enough to stick) for coal ash particles to stick on the target surface. It was also found that there is an optimal velocity for coal ash particle to stick on the surface (85). These sticking criteria, however, were derived from properties of synthetic ash (pure inorganic minerals) and did not consider the residual carbon in the ash particle. For an ash particle formed in a boiler or a gasifier, there is always residual carbon although its content decreases as the burnout increases. Consequently, many researchers studied the deposition behavior of ash particles containing residual carbon, i.e., during the burnout process of coal. McCollor (86) investigated ash deposit initiation in a simulated fouling regime. Five coals of different ranks were combusted in a laminar flow reactor with 31 various oxygen concentrations. Ash deposit was collected on a water-cooled probe that was maintained at 773 K. The gas temperature at the coal injection point and deposition point were 1773 and 1473 K, respectively. SEM analyses on the ash deposit and fly ash sample showed that: (1) Oxygen concentration plays a minor effect on deposit initiation. 2) The deposit initiation layers feature characteristic components with critical mass and viscosity. 3) The propensity for initial ash deposition can be roughly related to the fraction of ash particles in the bulk fly ash possessing these characteristic components. Srinivasachar et al. (80) investigated the inertial deposition of ash generated from combustion of a Texas lignite coal to validate the stickiness criterion of 107 Pa·s. Vuthaluru and French (62) conducted ash deposition experiments during the combustion process of an Indonesian coal and the coal with bauxite additive using a drop tube furnace at a probe temperature of 750 ºC. QEMSCAN analyses on samples from burning pure coal showed that the deposit sample has high silica, iron and moderate aluminum elements with lower glass content, compared with the ash formation sample. In contrast, the ash deposit sample from coal with bauxite additive has low silica, iron and high alumina contents. Results also showed that ash particles in this deposit are distributed sparsely, suggesting the lack of a deposit initiation layer. Russell et al. (87) performed ash deposition experiments of a Spanish anthracite to study the effects of mineral distributions on slagging propensity. Slags were prepared by burning different fractions of density-separated coals (original coal, fraction containing mainly excluded minerals and fraction containing mainly included minerals) in an entrained flow reactor (EFR) under combustion conditions. CCSEM characterization of the slag samples showed that coal with excluded minerals produces a slag with similar nature and chemistry to the 32 original coal. However, coal with included minerals produces a vitreous, iron-rich ash deposit. Wall (70) reviewed transformations of excluded and included minerals and their roles in ash deposition in pulverized coal combustion. Koyama et al. (88) studied the ash deposits on the wall of a 50 t/day two-stage entrained-bed coal gasifier. This study relates the property of ash deposits to the slagging behavior of the coal gasifier. The gasifier was operated at 1270-1870 K under 3 MPa for 218 hours. Morphological characterization using SEM, EDS and XRD classified the ash deposits into three groups: powder, lump and slag. The powder and lump adhered to the gasifier wall very weakly whereas the slag adhered to the wall very strongly. The powder contained char particles which served as a dispersive material preventing sintering on the wall. The presence of char particles increased the sintering temperature because carbon has a higher melting temperature than inorganic minerals. This dispersive effect decreased as the carbon content in the char particles decreased. The carbon content was affected by the oxygen to coal ratio in the feed of the gasifier. Bool and Johnson (89) studied the effect of residual carbon on ash deposition behavior during the reducing stage of two-stage coal combustion. In their work, two sets of experiments were performed using an entrained flow reactor (EFR) to evaluate the ash stickiness at various degrees of char burnout. In the first set of experiments, a Pittsburgh #8 coal was fed into the EFR under fuel lean conditions and at 1573 K. Specific char conversions were achieved by varying the residence times of coal particles in the reactor. Two deposition probes were placed perpendicular to the flow direction for ash deposit collection. Results showed that the collection efficiency increased dramatically until approximately 70% carbon content and then remained essentially unchanged. This 33 observation suggests that there is a critical char burnout for the ash stickiness of this particular coal. Control experiments using four bituminous coals showed that the ash stickiness increases as increasing the stoichiometric ratio in the gas flow, because of the decrease of residual carbon content in the ash. A simple model was developed to correlate the effective ash stickiness with carbon burnout: 𝜂𝑒 = 𝜂𝑎𝑠𝑕 ⋅ 𝑋𝑐 (5) where 𝜂𝑒 is the collection efficiency of the carbon-containing ash, 𝜂𝑎𝑠𝑕 is the collection efficiency of the pure ash, and 𝑋𝑐 is the carbon burnout. This model is in good qualitative agreement with the experimental data for the washed Pittsburgh #8 and the Black Thunder coals. However, it underpredicts the collection efficiency at low carbon conversion for the high-ash run-of-mine Pittsburgh #8 and the Silverdale coals. This underestimation may be caused by the excluded ash in the coal, which is not considered by the model. The excluded ash exists as mineral grains in the coal and forms sticky ash particles during combustion, even at low carbon conversion. The model also overpredicts the collection efficiency at low carbon conversion for the low-ash cleaned Pittsburgh #8 coal. This overestimation may be due to the atomically dispersed inorganic materials in the cleaned coal. The atomically dispersed inorganic material has a different releasing mechanism than the extraneous ash minerals. The exposure of the atomically dispersed inorganic material requires higher carbon burnout than the excluded ash minerals. All the ash deposition experiments mentioned above focused on combustion conditions, featuring a traditional experimental setup that utilizes a cylindrical deposition probe (rotating or non-rotating) perpendicular to the particle laden gas stream at the 34 bottom of an entrained-flow reactor (or a drop-tube furnace). The deposition probe was usually gas cooled to a temperature much lower than that in the reactor. This kind of configuration was designed to simulate ash deposition caused by inertial impaction on cylinders in cross flow, i.e., ash particles approaching heat exchanger tubes. Deposit build-up and removal. As stated in the previous section, a weight fraction of 15-70% molten phase or a viscosity of 105-108 Pa·s is the threshold criterion for ash to be sticky. Both of the two stickiness criteria depend on the temperature for a specific ash. As temperature increases, the melting phase fraction increases and the ash viscosity decreases. Roughly, the ash melting behavior can be inferred from the ash fusion temperatures. Because of the large variation of the composition of the ash, the fusion temperatures vary a lot: below, within and above the typical operating temperatures of coal gasifiers. If the initial deformation temperature is above the temperature in the gasifier, ash particles most likely exist as nonsticky solid particles (containing less than 15% molten phase). These solid particles will bounce off upon impacting the gasifier wall or at worst deposit as dust particles that are easily removed. If the initial deformation temperature is below the temperature in the gasifier, ash particles tend to melt and form molten slag or at least soften to a plastic state (containing more than 15% molten phase). The plastic matter or molten slag will deposit on the gasifier wall, which is called slagging. The slagging behavior is dependent on both the ash fusion temperature and the gasifier wall temperature. If the gasifier wall temperature is higher than the ash fluid temperature, the slag (containing more than 70% molten phase) is prone to flow down the wall and is continuously removed. The thickness of the slagging layer is limited, which does not affect the operation of gasifier. However, if the wall 35 temperature is between the initial deformation temperature and the hemispherical temperature, the ash will be too viscous to flow and the deposit will build up on the wall surface. In this case, the formed slag layer will significantly affect the operation of the gasifier causing unscheduled shut down and excessive maintenance. Unfortunately, most coals have a wide ash fusion temperature range (IT to HT) which covers the typical operating temperature of modern coal gasifiers. Therefore, slagging can be a serious concern for many coal gasifiers. For the continuous operation of a coal gasifier, a stable removal of slag is very important. Otaka et al. (6) developed a numerical model simulating the molten slag flow in a coal gasifier. This model was used to calculate the heat transfer in molten slag flow with free surface and phase change (solidification) and provided the basis for a new method evaluating of the discharging performance of a coal gasifier. By performing calculations with three types of coals, they concluded that: (1) the surface level of slag at the bottom of the gasifier rose as temperature decreased and this phenomenon can be accelerated by the formation of a solidification layer, and (2) the temperature at which overflow of molten slag occurs can be predicted by the simulation tool and this temperature can serve as an important index in operating the gasifier. Rawers et al. (90) studied the initial interaction of coal slag with refractory materials. They compacted gasifier slag on the surface of a new series of high-chromia alumina sesquioxide refractories and increased the temperature to that of typical commercial gasifiers in an Ar-CO reducing atmosphere. Upon melting, the slag contact angle, slag spread along the interface and the slag penetration were monitored by a camera. The slag wetting (contact angle less than 90º) did not occur until 100 K above the 36 melting temperature. The researchers suggested that a single factor played a controlling role in these phenomena. The activation energy associated with this factor was determined to be 226.8±4.2 kJ. They concluded that the slag infusion or wicking into the refractory played a more important role than the change in surface wetting in the slag-refractory wetting mechanism. They also concluded that the slag infusion rate was dependent on the slag composition: the lower the iron content in the slag, the greater the wicking action. 2.3.2 Prediction and Modeling of Ash Deposition and Slagging Various levels of models for prediction of ash deposition and slag removal have been developed. These models employ slagging indices, ash deposition mechanisms, thermodynamic calculations and computational fluid dynamics (CFD). Traditional models apply slagging indices based on ash composition, ash fusion temperatures and ash viscosity to predict slagging potential of coal ash (44). These engineering indices were developed for specific coal types as an industrial standard. A summary of these indices is presented in Table 2. Improved models are based on ash deposition mechanisms described in section 2.3.1. Baxter and co-workers (71, 72) developed a mechanistic model describing ash deposition during pulverized coal combustion on the basis of the transformation of mineral matter during transport of ash particles to target surface. Contributions from inertial impaction, thermophoresis, condensation and chemical reaction are included in this model. The predicted deposition rate, morphology and strength of deposit are consistent with the observations in a 600 MW utility boiler. 37 Table 2. Summary of engineering indices of slagging potential Index Required Analysis Applicable Coal Slagging Potential Low Medium High Severe 𝑅 = 𝐵𝑎𝑠𝑒 𝐴𝑐𝑖𝑑 a Ash chemistry Bituminous <0.6 0.6-2.0 2.0-2.6 >2.6 𝑅 = 𝐻𝑇+4𝐼𝑇 5 b Ash Fusibility Lignite >1343 K 1343-1505 K 1422-1505 K < 1422 K 𝑅 = 𝑇250 𝑜𝑥𝑖𝑑 −𝑇10000 𝑟𝑒𝑑 97.5 c Viscosity Bituminous Lignite <0.5 0.5-1.0 1.0-2.0 >2.0 a𝐵𝑎𝑠𝑒 𝐴𝑐𝑖𝑑 = 𝐶𝑎𝑂+𝑀𝑔𝑂+𝐹𝑒2𝑂3+𝑁𝑎2𝑂+𝐾2𝑂 𝑆𝑖 𝑂2+𝐴𝑙2𝑂3+𝑇𝑖𝑂2 , 𝑆 = 𝑆𝑢𝑙𝑓𝑢𝑟, weight percent, dry based. b𝐻𝑇, hemispherical temperature; 𝐼𝑇, initial deformation temperature. c𝑇250 𝑜𝑥𝑖𝑑 is the temperature at which the viscosity of ash is 250 poise in an oxidizing atmosphere and 𝑇10000 𝑟𝑒𝑑 is the temperature corresponding to a viscosity of 10000 poise in a reducing atmosphere. Other improved models use thermodynamic equilibrium calculations to evaluate the properties, transformation and reaction of ash, which exists as multicomponent and multiphase during combustion and gasification. These models are based on the principle of minimization of free Gibbs energy and thermodynamic data of ash components, with an assumption that equilibrium can be reached in the system (70). Some commercial software packages were developed, such as FactSage and Mingtsys. The thermodynamic equilibrium can be combined with fuels analysis to calculate the temperatures at which ash particle reach stickiness criterion. Mueller et al. (91) used thermodynamic equilibrium analysis with chemical fractionation to predict the melting behavior of biomass ash in a fluidized bed boiler. On the basis of the melting behavior as a function 38 of temperature, the stickiness criterion was determined. Van Dyk et al. (92) studied mineral matter transformation during Sasol-Lurgi fixed bed dry bottom gasification using high temperature XRD and FactSage modeling. The FactSage modeling result confirmed the high temperature XRD observation and provided insight into specific mineral matter transformation and reactions including organic and inorganic matter. However, the complexity of the combustion system, great variation of coal composition and uncertainty of some thermodynamic data may bring error in thermodynamic calculations. Carling et al. (93) studied the complexity of applying thermodynamic equilibrium calculations to study mineral matter during combustion. They found that the uncertainty can be several orders of magnitude when the concentrations are low. Wang and Harb (94) summarized ash deposition models based on ash chemistry and deposition mechanisms. They pointed out that these models failed to predict local deposition rates and the relationship between operating conditions and ash deposition during coal combustion because they did not incorporate the combustion conditions. They suggested that a comprehensive model considering the aerodynamics and other operating conditions of the combustor needs to be developed. The CFD based numerical simulation technology makes it feasible to incorporate the furnace design and operating conditions into ash deposition models. Sophisticated ash deposition models combining CFD and advanced ash analysis are capable of predicting local ash deposition growth and heat transfer change through the furnace wall. Lee and Lockwood (95) developed an ash deposition model combining the CCSEM fly ash data and CFD. It takes into account the burner geometry, operating conditions and ash properties. This model was validated in predicting the slagging 39 propensity of three U.K. coals in a pilot scale burner. The predicted heat flux agreed well with the measured data (<8% error). The predicted slagging propensity and deposition pattern were in qualitative agreement with observations. The model also successfully predicted chemical partitioning in initial deposit layers. Ma et al. (96) developed a comprehensive slagging and fouling prediction tool, AshPro, for coal-fired boilers. This model integrates CFD simulation with ash behavior models. It is able to predict localized ash deposition characteristics including deposit thickness, chemical composition, physical properties and heat transfer and their effect on boiler operation. Validation of AshPro was performed on a 512 MW tangentially-fired boiler. Wang et al. (97) developed a slagging model for a coal-fired combustor. This model considers capture of char particles on the molten slag wall surface and the combustion of these particles. It combines a wall burning model with the conventional slag model. A comparison with pilot scale experimental results showed that the wall burning effect should be taken into account in modeling a slagging combustor. Shimizu and Tominaga (98) developed a model of char capture by molten slag surface under high temperature gasification conditions. The char particle was assumed to be captured if impacting the molten slag surface, whereas it was assumed to rebound if impacting the char particle adhered to the slag surface. The modeling results agree well with the experimental results performed in an EFR. Coda et al. (99) investigated the slagging behavior of wood ash under entrained-flow gasification conditions with both experimental and simulation approaches. Experiments were performed in both atmospheric and pressurized EFRs, and the simulation was carried out with thermodynamic equilibrium calculations. Both of the reactors feature a multistage premixed flat flame gas burner to provide the fast heating 40 rate for devolatilization and the reaction gas for subsequent gasification. The downstream reactor tube is electrically heated to ensure a constant temperature for gasification. In their experiments, the flame temperature was set to be about 2323 K and the reactor tube temperature was kept at around 1723 K. The pressurized reactor was operated at 1 MPa. The ash deposition and slagging behaviors of three wood samples were characterized by a deposition probe, equipped with an alumina deposition plate. The SEM images of the deposition plate showed that wood ash was difficult to melt at conditions typical of coal gasifiers (1573-1773 K) for slagging operation. The elemental analysis of the ash deposit indicated that the high content of CaO increased the melting temperature of wood ash, which could be decreased by Ca silicates. It was also observed that the ash deposition and slagging behavior were not significantly affected by the pressure inside the reactor. Thermodynamic equilibrium modeling showed that there is an optimum addition of flux which can lower the ash melting temperature to a minimum point. The modeling work also showed that the previous formulas for coal slag characterization could not be applied directly to predict the characteristics of wood slag. 2.4 Concluding Remarks Because of the large body of literature concerning coal gasification, this review is focused on the porous structure of coal char and the formation mechanism and deposition behavior of coal ash. Most of the research results are consistent, whereas discrepancies do exist in many cases partially because of the complex properties of the coals and large variations of experimental techniques used by individual studies. For example, there is a general agreement that the porous structure plays an important role in determining the reactivity of char and the formation of ash during pulverized coal combustion and 41 gasification. The evolution of surface area and pore size distribution has been widely applied as macroscopic measurements reflecting microscopic changes in the porous structure of the char particle, despite the controversy whether the total surface area or the active surface area should be used to predict the char reactivity. Various pore models have been developed to describe the porous structure of char and correlate char reaction rate with surface area. Although every modeling result was shown to have reasonable agreement with measured rates in its specific experiment, none of the models can be applied to predict all the experimental data. Most of the investigations on ash formation mechanisms and deposition behavior were performed under conditions of coal combustion rather than gasification. However, these studies can also help improve the understanding on the ash formation and deposition under coal gasification conditions. Ash deposition mechanisms for different ashes have been well studied. Coarse ash is mainly transported to the target surface by inertial impaction, and fine ash is mainly transported by thermophoresis, condensation, diffusion and eddy impaction. Inertial impaction has been identified as the most important mechanism in ash deposition. The ash composition determines if an ash particle adheres to the target surface upon collision. The fraction of molten phase plays a controlling role for alkali-rich ash to deposit on the impaction surface, whereas the viscosity plays a key role for silicate-rich ash. In addition, ash composition determines the ash fusibility and viscosity. Despite the large amount of research conducted in the area of char porous structure and ash deposition, many issues remain unaddressed. A few of them are listed below. Few of the previous experimental studies on the porous structure were carried out at high 42 temperatures (>1400 ºC) particularly in the late stage of conversion (>90%). Little attention has been paid to characterizing the physical aspects associated with the transformation from porous char to molten slag. Although the ash deposition caused by inertial impaction on cylinders in cross flow has been well studied, the inertial impaction on walls in parallel flow is not well understood. The variation of ash particle stickiness during the char-slag transition has not been quantified. The stickiness criteria based on synthetic ash need to be corrected for ash particles containing residual carbon, which is common in practical pulverized coal gasification. The corrected ash stickiness criteria need to be incorporated into advanced CFD models for enhancing the accuracy in predicting ash deposition behavior. These problems are the motivation of this research. CHAPTER 3 OBJECTIVES AND APPROACHES This research is aimed at investigating the physical phenomena associated with the transition from porous char to molten slag and ash deposition behavior during the transition under gasification conditions. On the basis of the literature review in the last chapter, five tasks were established: 1. Characterize physical changes of char particles in the late stages of gasification, including particle density, size, internal surface area and mineral-carbon association. 2. Use the physical changes to identify the critical conversion at which the char-slag transition occurs. 3. Study ash deposition behavior on gasifier walls during the char-slag transition under simulated gasification conditions. 4. Quantify the particle stickiness during the char-slag transition. 5. Develop an empirical correlation to describe the particle stickiness as a function of conversion or residual carbon. In order to finish these tasks, both experimental and modeling approaches were undertaken. These approaches are briefly summarized as follows: 1. A laminar entrained-flow reactor (LEFR) which is capable of working at up to 1500 °C was designed and built. Using this reactor, two types of experiments 44 were performed: (1) char and ash formation experiments and (2) ash deposition experiments. 2. In char and ash formation experiments, char and ash particles with various conversions were prepared. These particles were characterized to identify the changes in particle density, size, surface area and morphology. These data were used to identify the char-slag transition. 3. In ash deposition experiments, particle collection efficiencies at different conversions were measured. This information was used to derive the variation of particle stickiness during the char-slag transition. 4. The particle stickiness was quantified as a function of conversion or residual carbon of the particle. This information was used to develop a simple expression for describing the evolution of particle stickiness during the char-slag transition. CHAPTER 4 EXPERIMENTAL DETAILS This chapter presents in detail the experimental setup used for investigating the physical phenomena associated with the char-slag transition and the ash deposition behavior during the transition. It consists of three sections. The first section provides an overview of the experiments, the second section describes the experimental apparatus and procedures, and the last section presents the properties of the coal samples and introduces the techniques employed to analyze the resulting char, ash and ash deposit samples. 4.1 Overview The objective of the experiments was to investigate physical changes of coal char during the char-slag transition and to provide data for an ash deposition model that can be incorporated into CFD modeling of entrained-flow slagging coal gasifiers. Therefore, experiments were mainly focused on the ash deposition behavior as well as the changes in physical characteristics of char particles during the char-slag transition. Three coals were selected for the experiments: an Illinois #6, a Pittsburgh #8 and a Black Thunder from the Power River Basin (PRB). Two types of experiments were carried out: char and ash formation experiments and ash deposition experiments. The purpose of the char and ash formation experiments was threefold: (1) prepare and collect char and ash particles with different conversions that cover the range of the char-slag 46 transition, (2) evaluate the physical changes of char and ash particles and the transformations of mineral-carbon association in the char-slag transition, and (3) identify an indicator for the char-slag transition. The goal of ash deposition experiments was to assess the intrinsic propensity of particle deposition during the char-slag transition. All the experiments were conducted in a high temperature laminar entrained-flow reactor (LEFR). The LEFR has been widely used for studying coal conversion and ash deposition behavior because it can provide well controlled experimental conditions while closely representing the environment in a practical combustion or gasification system. The char and ash formation experiments were performed using the three coals, whereas the ash deposition experiments were conducted using the Illinois #6 coal and the Black Thunder coal. 4.2 Experimental Setup The LEFR used for performing char and ash formation experiments is shown schematically in Figure 8. The experimental setup for conducting ash deposition experiments is essentially the same as that for char and ash formation experiments except the manner of sample collection, which will be described later. The LEFR comprises five components: an electrically-heated furnace, a coal feeder, a water-cooling loop, a gas supply unit and a sample collector. Details of the five components are presented in the following section. 47 Figure 8. Schematic diagram of the experimental setup for performing char and ash formation experiments. 48 4.2.1 Design of the Reactor The design of the LEFR followed the principles established in the classic work of Flaxman and Hallett (100) and also took into account previous designs of entrained-flow reactor (101, 102) and drop tube furnace (103). 4.2.1.1 Furnace. The furnace is the heart of the LEFR. It is a single zone vertical furnace (Carbolite, STF 16/610). The furnace is electrically heated by six rod-shaped SiC heating elements. A type R thermocouple is installed at the axial midpoint of the furnace to provide a temperature measurement signal for the temperature controller. The maximum operating temperature is 1600 ºC. The heated length of the furnace is 610 mm. A section view of the furnace is shown in Figure 8. Two coaxial alumina tubes (CoorsTek, 99.8% Al2O3) were installed inside the furnace. The inner tube serves as a plug flow reactor. The annulus between the inner and outer tubes is used to preheat the reaction gas before flowing into the inner tube. The outer tube is 8.89 cm o.d. × 7.94 cm i.d. × 137 cm length. The inner tube is 5.72 cm o.d. × 5.08 cm i.d. × 102 cm length. Porous insulation material with ultra-low thermal conductivity is used to fill the gap between the furnace wall and the outer tube to minimize heat loss and to dampen thermal shock at high temperatures. Reaction gas is injected through three ports on the bottom flange of the furnace and is preheated while flowing upwards in the annulus between the two coaxial alumina tubes. The gas then makes a 180° turn flowing through an alumina honeycomb flow straightener and enters the inner reactor tube. As shown in Figure 8, the honeycomb sits on the top of the inner alumina tube and is flush with the bottom of the injection probe, which is the end of the injecting section and the start of the reacting 49 section. The o.d. of the honeycomb flow straightener fits the i.d. of the outer alumina tube and the i.d. of the flow straightener fits the o.d. of the injection probe. A groove is cut on the bottom of the flow straightener so that it can center the inner alumina tube. The honeycomb has a configuration of 16 cell/cm2, an open frontal area of 72%, a hydraulic diameter of 0.216 cm and a height of 5.08 cm. This configuration provides sufficient pressure drop for generating a laminar flow, which is essential for the entrained particles traveling along the centerline of the reaction tube to undergo identical reaction conditions. Both of the ends of the two alumina tubes sit on aluminum flanges and are sealed with silicone o-rings. The flanges hang on a triangle bracket. 4.2.1.2 Coal feeder. A schematic diagram of the coal feeder is presented in Figure 9. The coal feeder consists of an infusion syringe pump (Harvard Apparatus, 552222) and a coal container. The coal container is made of an Acrylic tube (2.54 cm o.d. × 1.90 i.d. × 25.40 cm length). The bottom of the coal container is attached to the syringe holder of the infusion pump. The coal container is connected to the injection probe via a stainless steel feeding tube. The bottom of the feeding tube is mounted on the injection probe and the top of the tube flushes with the coal bed in the container. A loose Swagelok fitting with Teflon ferrules is used to connect the feeding tube and the coal container to allow the coal container sliding along the feeding tube. When the pump infuses, the coal container is pushed upwards so that the coal bed is raised above the feeding tube and those particles near the tube fall into it. Meanwhile, the coal container is vibrated by an engraver to make the coal bed level. Coal particles that jump into the feeding tube are entrained into the injection probe by a carrier gas. The feeding rate is controlled by the travel rate of the syringe pump, which is in the range of 1-10 ml/hr. 50 Figure 9. Schematic diagram of the coal feeder. Coal particles are pneumatically transported into the reaction zone of the reactor via a water-cooled injection probe, which connects the feeder and the reaction zone through a hole in the upper flange of the reactor. The injection probe is made of three coaxial stainless steel tubes, which form two annuluses. Cooling water flows through the two annuluses to prevent the coal particles from being devolatilized before entering the reaction zone. The probe has a dimension of 1.27 cm o.d. × 0.22 cm i.d. × 86 cm length. Indeed, the selection of the cross sectional area of the annulus is a compromise between cooling capacity and gas flow stability. Although a larger annulus can provide higher cooling capacity, the annulus must be as thin as possible to minimize heat loss from the reactor and to prevent turbulence at the gas inlet of the reaction zone. An insulation plug 51 is installed on the cooling jacket of the injection probe to avoid cooling the preheated reaction gas. 4.2.1.3 Sample collection. In the ash formation experiments, the reacted gas-solid mixture exited the reactor through a water-cooled collection probe. A section view of the collection probe is shown in Figure 8. The probe is made of three co-axial stainless steel tubes with an inverted cone-shaped head. The collection probe is 3.18 cm o.d. × 1.52 cm i.d. × 96.52 cm long. The cone has an angle of 45º with respect to its axis and an outside diameter of 4.762 cm on the top. It covers about 88% of the cross sectional area of the reactor tube, which increases the collection efficiency. A porous stainless steel tube is installed inside the collection probe to form an annulus. Nitrogen gas is introduced into the annulus and permeates through the micropores of the porous tube. This gas flow quenches the reacting stream and reduces the thermophoresis deposit of the solid particles on the probe surface. Calculation of the quenching rate is included in Appendix A. Most of the solid particles are collected in a cyclone. The tar and fine particles are captured on a Teflon filter installed after the cyclone. A schematic diagram of the cyclone collector is presented in Figure 10. The cyclone consists of a particle-laden gas inlet, a gas outlet, a cylindrical body, a cone section and a particle outlet. The particle-laden gas exiting the collection probe enters the cyclone via the gas inlet. Particles with desired diameters fall out of the cyclone through the particle outlet at the bottom of the cyclone.. Fine particles and gas leave the cyclone via the gas outlet on top of the cyclone. The cyclone is made of glass allowing the determination of the start of the experiment by observing the particles entering the cyclone. 52 Figure 10. Schematic diagram of the cyclone used for collecting char and ash particles. Cyclones have been widely used for gas-solid separation. The basic principle of cyclonic separation is centrifugal force. In brief, a high speed vortex flow is formed when the particle-laden gas flows into the cyclone. Large or heavy particles with high inertia cannot follow the flow stream and strike the wall of the cyclone, falling to the bottom of the cyclone and being collected in a container. The gas stream with small or light particles exits from the top of the cyclone. The most important parameter of the cyclone is the cut diameter. It is defined as the size of the particle that can be removed from the gas flow with 50% efficiency. Design of the cyclone based on the desired cut diameter has been well established (104). The cyclone used in this study has a cut diameter of 2-4 53 μm depending on the gas flow rates in the LEFR. Detailed design of this cyclone and the calculation of cut diameter are presented in Appendix B. In the ash deposition experiments, the collection probe was replaced by a specially designed deposition probe. The overall experimental setup and the section view of the deposition probe are shown in Figures 11 and 12, respectively. The deposition probe consists of an alumina tube and a clean alumina plate. A 45º (with respect to the cross-sectional plane of the probe) V-shaped groove was cut on top of the alumina tube for housing the alumina plate. This configuration allows simulation of inertial impaction on refractory walls because: (1) the alumina plate has a ceramic surface similar to the fresh refractory wall, and (2) the uncooled plate has a surface temperature roughly the same as that of the uncooled refractory wall. A clean alumina plate was used because this research was focused on the intrinsic particle surface stickiness and the initial stage when the slagging layer starts building up. Reacted particles struck the plate when the particle-laden gas stream approached the plate at a 45º angle. Upon impaction, particles with sufficient stickiness adhered on the deposition plate and were designated as deposit sample. Ash particles that did not adhere onto the plate were received by a cyclone at the bottom of the deposition probe. The deposition plate is removable from the deposition probe so that it can be weighed before and after the deposition experiment. The weight difference is the weight of particle that deposit on the plate. The 45° incident angle was randomly chosen because this study was focused on the particle stickiness and the incident angle only affects the rebound velocity, i.e., does not change the particle stickiness. 54 Figure 11. Schematic diagram of the LEFR configuration for conducting deposition experiments. 55 Figure 12. Section view of the deposition probe. • • • • • • • • • • • • • • • • • • • Ash Particles Alumina Plate Alumina Tube 56 4.2.1.4 Water cooling system. Cooling water circulates through the injection probe and the collection probe to protect them against high temperature. The cooled injection probe protects the coal particles from being devolatilized before reaching the reaction zone. The cooled collection probe helps quench the reacting particle-gas mixture exiting the reaction zone. A closed-loop water circulation system was designed and built to provide sufficient, reliable cooling capacity while saving water. Detailed information concerning the water circulation system is presented in Appendix C. 4.2.1.5 Gas supply unit. The function of the gas supply unit is to provide reaction gas, carrier gas and quenching gas for the reactor. It consists of two nitrogen gas cylinders, one air gas cylinder, gas regulators, rotameters, valves and gas tubes. The rotameters have an accuracy of 5% of the maximum flow rate. Detailed configuration of the gas supply unit is presented in Appendix D. 4.2.2 Operation of the Reactor 4.2.2.1 Temperature control. A temperature controller is used to provide programmed heat-up and cool-down for the reactor. Under normal operating conditions, a ramp rate of 4 K/min is used in both heat-up and cool-down to reduce thermal shock to the alumina tubes. It usually takes 5 hours to reach 1473 K from room temperature and 6 hours to cool down to the room temperature. During heat-up and cool-down, a small amount of nitrogen gas flow is supplied to the reactor protecting the alumina tubes and the honeycomb flow straightener. During normal down times (weekends and nights), the temperature was kept at 1000 K to extend the life of heating elements and alumina tubes. 4.2.2.2 Gas supply and reactor pressure. The flow rates of gases are controlled by rotameters (Cole Parmer, direct reading), which have a maximum error of 5% 57 of their ranges. Pressure regulators are installed between gas cylinders and rotameters to provide stable gas flow. Rotameters are calibrated by bubbling flowmeters before running experiments. The pressure inside the furnace is monitored by a Magnehelic differential pressure gauge (Dwyer, 2320) that is connected to the reactor through a hole on the top flange via a 3.3 mm Teflon tube. The reactor is usually maintained at atmospheric pressure by adjusting the power of the vacuum pump. 4.2.2.3 Residence time. The residence time of particles in the reactor is determined by the length of reacting pathway and the gas velocity inside it. The length of the reaction pathway is 610 mm. The gas velocity inside the reactor can be adjusted by the flow rate of the reaction gas. For gasification experiments, a long residence (about 5 s) is desirable. For a 610 mm reacting length and a 1400 ºC operating temperature, a gas velocity of 0.12 m/s is required at the axis of the reactor to achieve this residence time. The corresponding reaction gas flow rate is around 2.5 standard liter per minute (SLPM) and the corresponding Reynolds number of the gas flow is 28. The velocity of the carrier gas can be several times of that of the reaction gas while still keeping a stable laminar flow (100). 4.2.3 Characterization of the Reactor 4.2.3.1 Feeding rate. For a steady state gasification experiment, a constant feeding rate is necessary. The feeding rate of the coal feeder is determined by the infusion rate (ml/hr) of the syringe pump and the tap density of the coal particles. The feeding rate was calibrated at various infusion rates of the syringe pump with a coal bulk density of 0.7 g/cm3. The calibrated result is shown in Figure 13. 58 Figure 13. Feeding rate calibration of the coal feeder. The measured feeding rate was determined by dividing the weight loss of the coal feeder by the run time. The nominal rate was calculated by the product of the coal bulk density and the syringe pump infusion rate. This calibration curve was used for calculating actual feeding rate of coal particles in the experiments. 4.2.3.2 Collection efficiency. The collection efficiency of the collection probe is defined as the ratio of the particles collected in the cyclone to the total particles flowing down in the reaction tube just above the collection probe. The collection efficiency was tested by feeding and collecting coal particles at room temperature. All the other conditions were the same as when operating the reactor at high temperature. For a 610 mm reacting length, the collection efficiencies at various feeding rates are presented in Figure 14. At about 1.75 g/hr feeding rate, a collection efficiency of 90% was achieved. The other 10% of the particles deposit on the inner wall of the collection probe. 59 Figure 14. Measured collection efficiency of the collection probe at various feeding rates. 4.2.3.3 Temperature calibration. The temperature profile at the centerline of the furnace was measured to compare with the temperature controller settings. Five points equally spaced at the axis of the reaction tube were selected for temperature measurement using a type K thermocouple (3.3 mm diameter). Each measurement was taken with a nitrogen gas flow of 3 SLPM, which is a typical flow rate used in the experiments. The temperature profiles that were measured at furnace set point temperatures of 800, 1000 and 1200 ºC, respectively. The measured temperature profile and the set points are plotted in Figure 15 for comparison. This temperature calibration curve provides the basis for setting the temperature controller when running experiments. For example, for an experimental temperature of 1200 ºC, the temperature controller was set at 1225 ºC for compensating the difference between the actual temperature and the temperature controller signal. 60 Figure 15. Axial temperature profile of the reactor. 4.3 Experimental Procedures Experimental procedures were basically the same for the two types of experiments. In brief, coal particles were injected from the top of the reactor into a premixed, preheated air-nitrogen gas stream and were partially converted at various residence times. In the ash formation experiments, resulting particles exit the reactor through a water-cooled, nitrogen-quenched collection probe and were collected in a cyclone. Each experiment was run for 2 hours to collect sufficient particles for further analyses. In the ash deposition experiments, char and ash particles with different degrees of conversion approached the deposition plate before exiting the reactor. Upon impaction, particles with sufficient stickiness adhered on the deposition plate and were designated as deposit sample. Ash particles that did not adhere on the plate were received by a cyclone at the bottom of the deposition probe. After the ash deposition experiment, nitrogen was fed into the reactor to provide an inert environment for the deposit on the plate until the 61 reactor cooled down. The deposition probe was then taken out of the reactor and the deposition plate was removed for weight measurement. The weight of deposit was determined by the weight difference of the deposition plate before and after the deposition experiment. Each deposition experiment was run for 2 hours to collect enough deposit to minimize the weighing error. 4.4 Experimental Conditions The experimental conditions were identical for char and ash formation experiments and ash deposition experiments. The pressure inside the reactor was maintained at ambient pressure, 0.85 bar (the altitude of Salt Lake City is about 1350 m). The furnace temperature was set to either 1400 or 1500 ºC by using the temperature calibration curve in Figure 15. The temperature was chosen to be above the ash flow temperature of the specific coal ash. The feeding rate of coal particles was 30 mg/min to avoid particle agglomeration in the injection probe. The flow rate of air in the reaction gas mixture was varied for different coals to keep a stoichiometric ratio (oxidant/fuel, molar basis) of 0.7, which provided an overall reducing atmosphere in the reactor. The term oxidant is defined as the oxygen in the air and coal. The term fuel refers to all the combustible elements (carbon, sulfur and hydrogen) in the coal. The experimental run for preparing fresh chars by devolatilization used pure nitrogen. The residence time of the coal particles in the reactor was varied from 1 to 6 s in 1 s increment. The use of a long residence time was due to the low oxygen content (0.7%-4.6%) in the reaction gas in accordance with the low feeding rate of coal. Experimental conditions for different coals are summarized in Table 3. Detailed experimental parameters for achieving these experimental conditions are presented in Appendix E. 62 Table 3. Experimental conditions for different coals Coal Experimental Conditions Stoichiometric Ratio (O2/C)a Temperature (°C) Residence Time (s) Illinois #6 0, 0.7 1400, 1500 1-6 Black Thunder 0, 0.7 1400 1-5 Pittsburgh #8 0, 0.7 1500 1-6 aMolar basis. The Reynolds number of the gas flow inside the reactor was calculated by 𝑅𝑒 = 𝜌𝑔 𝜐𝑔 𝐷𝑡 𝜇 𝑔 (5) where 𝜌𝑔 is the gas density (kg/m3), 𝜐𝑔 is the mean gas velocity (m/s), 𝐷𝑡 is the tube diameter (m) and 𝜇𝑔 is the gas viscosity (Pa·s). Reynolds numbers of the gas under typical experimental conditions (1400-1500 °C, 1-6 s residence times) are presented in Figure 16. The maximum Reynolds number in the experiments is below 50, which indicates a laminar flow, in which the particles travel along the centerline. The laminar flow can be assumed to be a plug flow, in which the mass transfer of oxidants in the radial direction (from the near-wall region to the centerline of the reactor) of the bulk phase is dominated by molecular diffusion. Molecular diffusion is much slower than convection. Although an overall stoichiometric ratio of 0.7 guarantees insufficient oxygen for complete combustion of the coal, oxygen molecules might be present in the near-wall region because coal particles travel along the centerline of the reactor. 63 Figure 16. Reynolds number of the gas flow in the reactor under typical experimental conditions. As a matter of fact, oxygen, carbon monoxide and carbon dioxide were detected coexisting in the reaction gas exiting the reactor using a gas chromatograph (Varian, 490-GC). Therefore, the gas environment in the reactor might be reducing around the centerline of the reactor while oxidizing near the reactor wall. Because coal particles travel along the centerline of the reactor, the atmosphere in the vicinity of the coal particles can still be considered as reducing (gasification). The residence time of coal particles in the reactor was assumed to be the same as the residence time of gas flow in the reactor. This assumption was validated by determining the terminal (settling) velocity and the Stokes number of coal particles. Particle terminal velocity is the particle-to-fluid relative velocity at which the particle experiences zero acceleration. It is affected by the external force (usually gravity), the buoyant force from the fluid and the drag force by the fluid. The Stokes number of a 64 particle is defined as the ratio of the stopping distance of a particle to the characteristic dimension of an obstacle. A particle with low Stokes number has small inertia to resist the external force exerted by the fluid. For Stokes number much less than 1, a particle will closely follow the fluid streamline when passing an obstacle. Therefore, a particle with low Stokes number and terminal velocity much lower than the gas flow velocity will follow the streamline of the gas flow and have approximately the same residence time as the gas flow. The particle terminal velocity is calculated by 𝜐𝑡 = 𝑔𝑑𝑝 2 (𝜌𝑝 −𝜌𝑔 ) 18𝜇 𝑔 (6) where 𝑔 is the acceleration of gravity (9.8 m/s2), 𝑑𝑝 is the particle diameter (m), 𝜌𝑝 is the particle density (kg/m3), 𝜌𝑔 is the gas density (kg/m3) and 𝜇𝑔 is the gas viscosity (Pa·s). The particle terminal velocity calculated using eq 6 and the reaction gas velocities in the center line of the reactor at typical experimental conditions (1400 °C, 1-6 s residence times) are presented in Figure 17 for comparison. The gas flow profile in the reaction tube is assumed to be parabolic (laminar flow). Therefore, the centerline gas velocity is twice as the average gas velocity in the reaction tube. Data in Figure 17 indicates that the particle terminal velocity is much lower than the gas velocity. Therefore, taking into account the time required to reach the settling velocity, the particle velocity can be assumed to be the same as the gas flow velocity providing the condition that the particle Stokes number is much less than 1. 65 Figure 17. Comparison of the particle terminal velocity and the gas velocity at typical experimental conditions assuming a particle density of 1 g/cm3. The particle Stokes number is calculated by 𝑆𝑡 = 𝜌𝑝 𝑑𝑝 2 𝜐𝑝 9𝜇 𝑔 𝑑𝑐 φ (7) where 𝜌𝑝 is the particle density (kg/m3), 𝑑𝑝 is the particle diameter (m), 𝜐𝑝 is the particle velocity (m/s), 𝜇𝑔 is the gas viscosity (Pa·s) and 𝑑𝑐 is the characteristic dimension (m) of the obstacle. φ is a non-Stokesian correction factor that is only important when particles do not obey Stokes law, i.e., large particles with high velocities relative to the gas (𝑅𝑒𝑝 1/2 ≫ 1). In this dissertation, the particle velocity was replaced by gas velocity because the particle velocity was insignificant compared to the gas velocity as shown in Figure 17. Stokes numbers of particles under typical experimental conditions (1400 °C, 1-6 s residence times) are presented in Figure 18. 66 Figure 18. Stokes number of particles under typical experimental conditions assuming a particle density of 1 g/cm3. Figure 18 indicates that the particle Stokes number is much less than 1, i.e., the particles follow the streamline of the gas flow in the reactor. Consequently, the particle residence time can be approximately determined by assuming that the particle velocity is equal to the gas velocity in the reactor. 4.5 Coal, Char and Ash Analyses 4.5.1 Coal Preparation and Properties Coal rank plays an important role in the reaction behavior during gasification process. Three pulverized coals of different ranks were used for the experiments: Illinois #6, Pittsburgh #8 and Black Thunder (Powder River Basin). These coals are typical feedstocks in entrained-flow coal gasifiers. The Illinois #6 is a high-volatile C bituminous coal, the Pittsburgh #8 is a high-volatile A bituminous coal, and the Black Thunder is a subbituminous coal. 67 All the coals were sieved to a size range of 43-63 μm to minimize the effect of particle size distribution on char conversion and ash deposition. Before sieving, the coal samples were dried in a muffle furnace at 104 °C for 24 hours to remove the moisture according to an ASTM method (105). The properties of the coals and the ashes were determined by Wyoming Analytical Laboratories. The proximate and ultimate analyses, the ash chemistry and the ash fusion temperatures are listed in Tables 4-6, respectively. Table 4. Proximate and ultimate analyses of the coals used in this work Coal Proximate Analysis (wt%, mf)a Ultimate Analysis (wt%, maf)b Moisturec Ash Volatiles Fixed Carbon C H N S O Illinois #6 3.63 10.89 36.42 52.69 74.52 4.96 1.48 4.66 14.38 Pittsburgh #8 1.08 9.00 38.22 52.64 84.07 5.58 1.53 3.86 4.96 Black Thunder 24.59 6.82 49.07 44.11 77.91 3.63 1.18 0.35 16.93 aMoisture free, method: ASTM D5142. bMoisture ash free, method: ASTM D5142/5373. cmoisture free. Table 5. Ash chemistry of the coals used in this work Coal Ash Chemistry (wt%, oxide)a Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 TiO2 SO3 Illinois #6 17.75 5.23 18.99 2.06 0.89 0.05 1.67 0.16 46.58 0.88 4.59 Pittsburgh #8 19.68 4.54 27.79 1.20 0.85 0.02 0.90 0.34 39.66 0.84 4.18 Black Thunder 16.84 21.61 5.86 0.50 5.06 0.02 1.69 1.00 36.04 1.32 9.06 aMethod: ASTM D4326 (XRF). 68 Table 6. Ash fusion temperatures of the coals used in this work Coal Ash Fusion Temperature (oxidizing, °C) Ash Fusion Temperature (reducing, °C) IT ST HT FT IT ST HT FT Illinois #6 1244 1254 1286 1343 1104 1116 1139 1246 Pittsburgh #8 1308 1342 1369 1394 1085 1104 1137 1229 Black Thunder 1184 1188 1193 1213 1142 1150 1160 1191 aMethod: ASTM D1857. 4.5.2 Char, Ash and Ash Deposit Characterization Carbon contents of the collected char and ash particles (ash formation sample) were determined using a hot foil loss-on-ignition (LOI) instrument (FERCO, HF400). About 0.01 g of sample was completely burned using this apparatus. The sample was weighed before and after the LOI analysis. By assuming that all the mass loss is carbon, the carbon content 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 was calculated by 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 = 𝑚𝑐𝑕 𝑎𝑟 −𝑚𝑟 𝑚𝑐𝑕 𝑎𝑟 × 100% (9) where 𝑚𝑐𝑕𝑎𝑟 is the mass of the char sample before the LOI analysis and 𝑚𝑟 is the mass of the burnout residual (assuming purely ash) after the LOI analysis. For the deposition experiments, the particle collection efficiency 𝜂 is defined as 𝜂 = 𝑚𝑑 𝑚𝑡 × 100% (10) where 𝑚𝑑 is the mass (g) of particles that deposited on the deposition plate and 𝑚𝑡 is 69 the total mass (g) of particles that approached the deposition plate in the deposition experiment. 𝑚𝑡 was calculated by 𝑚𝑡 = 𝑓 𝑡𝐶𝑐𝑜𝑎𝑙 𝑎𝑠 𝑕 1−𝑐𝑐𝑕 𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 (11) where 𝑓 is the feeding rate (g/hr) of coal particles approaching the deposition plate, 𝑡 is the elapsed time (hr) of the deposition experiment, 𝐶𝑐𝑜𝑎𝑙 𝑎𝑠𝑕 is the weight fraction of ash in the coal and 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 is the weight fraction of carbon in the char or ash particles that impacted the deposition plate. 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 was determined by eq 9 using samples collected in ash formation experiments. Because the reaction conditions and sampling position of collecting particles in ash formation experiments were identical to those of ash deposition experiments, it is reasonable to assume that the carbon content of the ash formation sample is the same as the particles approaching the deposition plate. The particle collection efficiency calculated by eqs 10 and 11 is an averaged value of all the particles that impacted the deposition plate. The apparent powder (bulk) density of char and ash particles was measured with the method that was used by Tsai and Scaroni (106). In brief, a graduated cylinder was filled with the sample and then tapped gently for uniformly packing to the minimum volume. The mass of the cylinder was measured before and after being filled with the particles. Assuming the same packing factor, the bulk density of the particles was calculated as 𝜌𝑎 = 𝑚𝑐𝑝 −𝑚𝑐 𝑉 (11) 70 where 𝜌𝑎 is the bulk density of the particles, 𝑚𝑐𝑝 is the mass of the cylinder containing particles, 𝑚𝑐 the mass of the empty cylinder and 𝑉 is the volume the powders occupy in the cylinder including the voids between particles. The effective particle density was calculated as 𝜌𝑝 = 𝜌𝑎 1−𝜙 (12) where 𝜌𝑝 is the effective particle density and 𝜙 is the packing voidage, which was assumed to be 0.5 according to previous research (106, 107). The use of a constant voidage in the calculation of particle density is valid only on conditions that particles have a uniform size at the same conversion and that particles have a constant size at different conversions. In general, the voidage of a packed bed increases with decreasing particle size. The error associated with the use of a constant voidage was estimated to be in the range of 10-20% (106). The diameter of the particles was statistically determined using an Olympus optical microscope and Image J software. Images of a number of particles (20-100) were taken using the microscope. The Image J software automatically locates the individual particles in the images and calculates the projected area of the individual particles. By assuming the particles are spherical, the mean particle size was determined from the averaged diameter of the examined particles. Internal surface areas of the char and ash particles (ash formation sample) were measured by isothermal gas adsorption using a surface area and porosimetry analyzer (Micromeritics, Tristar II 3020) with N2 as adsorptive gas at 77 K (liquid nitrogen bath). Each sample was degassed under 523 K with a N2 gas flow for 2 hours in order to remove 71 the moisture and other adsorbed gases before analysis. The internal surface area was calculated using the BET method. Microimages of the char and ash particles were captured using a scanning electron microscope (FEI Nova nano) equipped with an Everhart-Thornley detector under high vacuum mode. The accelerating voltage was 10-15 kV and the working distance was 5 mm. Particles were affixed to the sample holder using carbon tape as conductive base. The cross section of the ash deposit on the deposition plate was obtained by breaking the deposition plate using a hammer. The broken pieces were then examined using the SEM to obtain the microimages of the cross section of the deposit. X-ray microanalysis signals of the samples were collected using the SEM-EDS under high vacuum mode. The accelerating voltage was 15 kV and the working distance was 5 mm. The X-ray microanalysis signals were processed by the Genesis software (EDAX) to quantitatively determine the elemental compositions of the ash particles and deposits. The X-ray microanalysis has a spatial resolution of 5 μm and an accuracy of 1-2%. These analyses show the fingerprint of mineral-carbon association and elemental distribution of the ash and deposit samples. CHAPTER 5 RESULTS: CHAR-SLAG TRANSITION Char-slag transition involves significant changes in the physical properties of the particle including density, size, porous structure and morphology. The particle density increases when the particle becomes mineral-rich from carbon-rich. The particle size decreases due to shrinkage and/or fragmentation. The porosity decreases when the particle transforms from porous char to molten slag and the carbon in the particle depletes. This chapter presents the physical changes of particles during the char-slag transition and the identification of the char-slag transition using these properties. The particle density was estimated from the apparent (bulk) density of the char and ash samples. The particle size was measured from the image of a number of particles by assuming spherical shape of the particles. As stated in section 2.1.1, particle surface area and pore size distribution are most widely used as the macroscopic measurements reflecting the microscopic characteristics in the porous structure (13). Therefore, the evolution of the porous structure is characterized by the surface area of the char and ash particles including the mesopore and micropore surface area. The particle morphology was characterized by SEM images. All of these properties were evaluated as a function of coal conversion. The critical conversion corresponding to the char-slag transition was determined using the changes in the particle density, size and internal surface area. For example, when the particle density started increasing, the conversion of this data point is 73 designated as the critical conversion. When there was ambiguity in identifying the critical conversion, the three critical conversions determined with the three specific property changes (particle density, size and internal surface area) were compared to ascertain a single point. This single point was confirmed by the morphological changes shown on the SEM images The char-slag transition is strongly associated with the physical transformation of mineral matter in the particle. In pulverized coal combustion and gasification, different minerals undergo different physical-chemical transformations resulting in different contributions to ash formation. Excluded minerals are melted into liquid slag particles at temperatures above the ash fluid temperature, which have relatively low internal surface area. Included minerals can coalesce (32, 108) within hot char particle or can be liberated from the carbon matrix and form ash particles by char fragmentation (106, 109, 110) and shedding (48, 52, 111) of minerals from char particles in the burnout process. 5.1 Char Burnout Behavior The conversion of coal that was used in char and ash formation experiments was determined with a method used in previous studies (106, 112). This method uses ash as a tie component (ash tracer). The coal conversion 𝑋 was calculated as 𝑋 = 1 − 𝐶𝑐𝑕 𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 𝐶𝑐𝑜𝑎𝑙 𝑎𝑠 𝑕 1−𝐶𝑐𝑕 𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 𝐶𝑐𝑜𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 × 100% (13) where 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 is the weight fraction of residual carbon in the collected char and ash particles, 𝐶𝑐𝑜𝑎𝑙 𝑎𝑠𝑕 is the weight fraction (moisture free) of ash in the parent coal, and 𝐶𝑐𝑜𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 is the weight fraction (moisture free) of carbon and other combustible matter in 74 the parent coal. 𝐶𝑐𝑕𝑎𝑟 𝑐𝑎𝑟𝑏𝑜𝑛 was determined by the LOI analysis described in section 4.5.2. 𝐶𝑐𝑜𝑎𝑙 𝑎𝑠𝑕 was determined by the proximate analysis presented in section 4.5.1. 𝐶𝑐𝑜𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 was determined by subtracting the sum of coal ash content (moisture free) and coal oxygen content (moisture free) from 100%. The coal oxygen content can be calculated from the ultimate analysis in section 4.5.1. The use of ash tracer in calculating coal conversion in eq 13 is based on three assumptions: (1) the conservation of ash before and after the reaction, (2) identical ash composition for the samples prepared in the LEFR and generated in proximate ash analysis and (3) the collected samples consisting of only carbon and ash. None of these assumptions are accurate. For example, vaporization of certain ash components is expected to occur because the mineral matter in the ash tends to exist as reduced or lesser-oxidized forms in a gasification environment particularly for the alkali and alkali-earth metals. The vaporized ash in the reaction gas can deposit on the collection probe when subjected to a quenching environment. Furthermore, the reaction conditions in the LEFR are different from those in proximate analysis, which can result in different mineral matter transformation and different ash composition. On the other hand, fine ash particles (submicron) liberated from char fragmentation are hard to be collected in the cyclone, thus reducing the ash content of the char particle determined using the LOI analysis. Borrego and Alvarez (112) discussed the error of calculating the coal conversion using ash tracer method qualitatively. They concluded that the ash tracer method is acceptable for mainly comparative purposes between experiments performed at a single temperature varying only the reaction atmosphere. 75 5.1.1 Illinois #6 Coal The carbon content and coal conversion of the Illinois #6 char and ash particles prepared at 1400 and 1500 °C are presented as a function of residence time in Figures 19 and 20, respectively. To test the reproducibility of the data, the experiment at the temperature of 1400 °C was performed three times. Error bars were calculated using Student's t-test with a confidence interval of 90%. The error was mainly due to the variation in controlling the flow rate of the reaction gas using rotameters and was partially introduced by determining the