Advanced Coal Characterization: A Review - American Chemical

Energy & Fuels 2007, 21, 451-460

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Advanced Coal Characterization: A Review† Rajender Gupta Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada

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ReceiVed August 20, 2006. ReVised Manuscript ReceiVed NoVember 29, 2006

Coal is highly heterogeneous in nature, and for this reason, several analytical techniques are needed for its characterization so as to accurately predict its behavior during conversion processes such as combustion, gasification, or liquefaction. Conventional analyses such as proximate analysis, ash analysis, and ash fusion temperatures assume coal as a homogeneous material and provide only bulk properties. The performance correlations based on these analyses are unable to describe adequately the impact of coal quality on conversion efficiencies and plant performance. A number of advanced bulk analytical techniques, such as FTIR and 13C NMR, provide information on the organic structure of coal. Chemical fractionation technique provides information on the inorganic matter present in coal in a form other than mineral grains. Bulk analysis techniques such as XRD and SIROQUANT provide information on the types of minerals present in coal. Thermomechanical analysis (TMA)san advanced bulk analytical techniquesprovides detailed thermal behavior of ash relevant to power-plant operations. Several advanced characterization techniques have emerged recently which consider pulverized coal as a heterogeneous material made up of individual particles and are able to examine these coal particles in much greater detail. An automated reflectogram (AR) technique provides a variation of reflectivitys a measure of heterogeneity in the organic part. A computer-controlled scanning electron microscopy (CCSEM) analysis technique has been developed over the last 25 years to provide much more detailed information on mineral matter in coal and mineral-coal associations in pulverized coal. The paper discusses the details of these techniques and how the analysis from these techniques is used in modeling procedures to provide a better understanding of coal conversion behavior.

Introduction There has been an ever-increasing focus on a cleaner environment and more efficient use of fuel resources. With coal being a major source of fuel in most parts of the world, there is a continued interest in the efficient use of coal and the development of clean coal technologies. This requires a detailed understanding of the fundamental properties of coal, thus making the area of coal characterization of paramount importance. Coal is highly complex in nature. The organic matter itself is highly heterogeneous and consists of several maceral types and minerals with varying physical and chemical structure, as shown in Figure 1. In this figure, the darker part of the coal particle with low reflectivity known as vitrinite maceral corresponds to a higher hydrogen content, whereas the shiny part of high reflectivity known as inertinite maceral corresponds to a higher carbon content. Some very bright parts of the particle may also correspond to mineral grains in coal particles. The inorganic matter is dispersed in coal in a random manner in the form of mineral inclusions and dissolved salts and is chemically associated with the organic structure. On grinding, the coal breaks down into particles of quite a variable natures pure maceral particle, pure mineral grains, and coal particles containing inorganic matter, as in Figure 2. The heterogeneous nature of this fuel encounters difficulties in characterization and correlation of the structure to its processing and conversion characteristics. The characterization of such a complex material, therefore, necessitates more than one analytical technique to † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * E-mail: [email protected].

Figure 1. Heterogeneous nature of organic matter in pulverized coal sample (63-90 µm).

accurately predict its behavior during conversion processes such as combustion, gasification, coking, and liquefaction. The need for understanding the implication of the fundamental properties of coal upon its utilization as a fuel has led to the development of advanced techniques for characterizing its heterogeneity. A multidimensional approach is often used these days to characterize various properties of coal. The characterization approach also depends on the type of application, including ash issues and combustion behavior in different energy systems. The paper initially discusses major issues in coal utilization followed by conventional analytical techniques. A brief discussion on the number of advanced bulk analytical techniques is

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There are three major issues in coal utilization: (i) conversion issues, (ii) operational issues, and (iii) environmental issues. Conversion Issues. The various steps involved in conversion are preparation, conversion of coal to char/ash, and, eventually, char combustion or gasification. The issues pertinent to the preparation require information on the physical properties of coal such as density, hardness, and other mechanical properties of coal. The research on reactivity and conversion of coal has primarily been limited to experimentation on drop-tube furnaces and thermogravimetric analysis (TGA). The conversion issues are usually broken down into two steps: pyrolysis and char reactivity. Pyrolysis (Devolatilization). Coals of different types exhibit wide variations in their devolatilization behavior because of different extents of coalification. The degree of aromatization in the coal structure increases with the increase in the rank of the coal. The information on maceral composition of coal is of paramount importance on the devolatilization and char conversion issues. The primary physical changes that occur when any particular coal is heated depend on the melting and decomposition behavior of coal. Variables that influence devolatilization rates include temperature, residence time, pressure, particle size, and coal type, and final temperature is possibly the most important one.1 Char Formation and Combustion. The physical structure of coal, including pore structure, surface area, particle size, etc., is important in understanding and modeling combustion and char oxidation process. The conversion characteristics such as the calorific value, volatile and ash content, and other physical property values provided by the bulk analysis of coal are required for a better design of a combustion system. Char character has been extensively studied because of its importance in char combustion and gasification processes.2,3 Char burnout

is critical in assessing the overall efficiency of the conversion process. The char burnout during the coal conversion process largely depends on the reactivity of char, and therefore, the accurate prediction of char behavior is of paramount importance. Pore-size distribution within char is probably one of the most important aspects of char character because the conversion of char takes place by the diffusion of gas through the pore into the char. According to their morphology, the chars can be categorized into cenosphere, honeycomb, and unfused chars. Each char type has a different char structure and, consequently, a different reactivity. The petrography (maceral composition) and presence of mineral matter in coal have a significant influence on the character of the resulting char or ash. The most important issues related to char/ash are their movement through the boiler/gasifier and their deposition on various surfaces. These characteristics of char/ash depend on the structure of char/ash and its thermal and mechanical properties, which in turn are strong functions of the maceral composition and the mineral matter present in the parent coal. Operational Issues. The flame stability, erosion, slagging, and fouling characteristics are important aspects in the determination of the efficiency of the conversion process. Flame instability and slagging/fouling may lead to enormous downtimes and, therefore, operational losses. Fouling and slagging occur because of the deposition of ash on boiler surfaces. The molten ash then sinters and forms deposits that are difficult to remove. Elemental information and mineral interactions of the parent coal provide important information regarding the propensity of a particular coal to form deposits. Analysis such as ash fusion temperature (AFT) is conducted to understand the process of slagging/fouling. Another technique, thermomechanical analysis, discussed in a later section, has been recommended as a better technique to assess thermal behavior of coal ash in boilers. Instability in the flame can also lead to nonuniform heat flux and incomplete conversion. The volatile matter in coal is used as a design parameter of the boiler to obtain a continuous flame. The volatile matter is estimated by conventional analysis using a muffle furnace at low heating rates at 915 °C. However, the volatile matter determined in this manner is not the same as is evolved in a boiler. The estimate of volatile matter in a coalfired boiler at high flame temperatures is obtained by experiments in a drop-tube furnace. There are models, such as the coal percolation devolatilization model from Brigham Young University,4 available to estimate the volatile matter for a given time-temperature history based on some coal composition parameters. These methods are based on the bulk properties of coal. Environmental Issues. The environmental issues in coal utilization can be categorized into gaseous emissions such as oxides of nitrogen and oxides, fine particulates, and greenhouse gas emissions. The greenhouse gas emission is more related to the efficient use of coal rather than the properties of coal. The issue of oxides of nitrogen and sulfur is well-understood, and methods of control are in place. The current paper is related to the formation of fine particles as it is also linked with the emission of trace elements. With the increasing concern about the environmental impact of potentially hazardous trace elements from coal combustion, attention has been focused to the levels of these trace elements present in waste products released into

(1) Saxena, S. C. Prog. Energy Combust. Sci. 1990, 16, 55-94. (2) Jones, R. B.; McCourt, C. B.; Morley, C. Fuel 1985, 64, 14601466.

(3) Simons, G. A. Prog. Energy Combust. Sci. 1983, 9, 227. (4) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992, 6 (4), 414-431.

Figure 2. SEM from QEMSCAN of a pulverized coal sample (6390 µm). Black color shows coal, whereas other colors/shades correspond to different minerals.

followed by two analytical techniques that characterize the heterogeneous nature of coal. The paper finally discusses a number of applications describing the implications of coal heterogeneity on these issues. Major Issues in Coal Utilization

AdVanced Coal Characterization: A ReView

the environment. Most recently, amendments to the U.S. Clean Air Act (1990) identified 11 trace elements commonly found in coal as potentially hazardous air pollutants, and much of the current research is focused at the effect of these trace elements upon the environment. The present state of research is aimed to target the respective quantities of these trace elements in various waste streams, viz. air, water, and solid residue, and to relate it back to the nature of the elemental composition of the parent coal. The mineral matter and the maceral composition of coal also have a significant influence on the fine particle emissions from the combustion of coal. The proportion and size distribution of fine and submicron particles influence the distribution of these trace elements into fly ash, coarse ash, and those escaping into the environment. The formation of fine particles depends both on the inherent size distribution, the types of minerals, and the char character. It has been shown by Gupta et al.5 that a vitrinite rich coal results in a highly porous cenospheric char structure that leads to a higher proportion of fine ash during combustion. Thus, an accurate determination of mineral and maceral composition of coal is required to predict the overall impact of coal combustion on the environment. Conventional Analytical Techniques Proximate and Ultimate Analysis. Proximate and ultimate analysis of coal provides important information regarding the overall characteristic of a particular coal. Proximate analysis includes moisture, ash, volatile matter, and fixed carbon in coal estimated from standard methods. It must be noted that there is no ash present in coal; however, the inorganic matter in coal is expressed in the form of ash. It allows an approximate prediction of the coal behavior during its preparation and conversion. For example, the fuel ratio (fixed carbon/volatile matter) has been correlated with char reactivity and NOx formation.6,7 In a similar way, the conventional ash analysis has been used to assess erosion, slagging, and fouling potentials of a coal.8 The ultimate analysis includes elemental analysis of coal and has been used to assess its combustion characteristics and to estimate the maximum emission of sulfur and nitrogen oxides. The detailed description of these analyses can be found in a number of references.9,10 Ash Fusion Temperature (AFT). The fusibility of ash is important in understanding the process of slagging and fouling inside the boilers during the conversion of coal and coal blends. Ash fusion temperatures give an indication of the softening and melting behavior of fuel ash and, therefore, an estimation of the variability in fusibility characteristics among different coals. Ash fusion temperatures are also able to provide an indication of the progressive melting of coal ash to slag. Ash fusion temperatures are widely cited in fuel specifications for boilers despite a relatively poor record of correlating with slagging or (5) Gupta, R. P.; Yan, L.; Gupta, S. K.; Wall, T. F. Effect of Minerals and Macerals on Thermal Performance of Boilers. Presented at Engineering Foundation Conference on Effects of Coal Quality on Power Generation, Park City, UT, May 2000. (6) Pershing, D. W.; Wendt, J. O. L. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 1976; Vol. 16, p 389. (7) Pershing, D. W.; Wendt, J. O. L. Ind. Eng. Chem. Process Des. DeV. 1979, 18, 60. (8) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corporation: New York, 1985. (9) Sharkey, A. G.; McCartney, J. T. Physical Properties of Coal and its Products. In Chemistry of Coal Utilisation; Elliott, M. A., Ed.; John Wiley and Sons: New York, 1981. (10) Montgomery, W. J. Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1978; Vol. 1, pp 192-246.

Energy & Fuels, Vol. 21, No. 2, 2007 453 Table 1. Physical Properties of Coal and Respective Analysis properties chemical properties physical properties mechanical properties thermal properties electrical properties

analysis proximate analysis, ultimate analysis, and ash analysis density, specific gravity, pore structure, surface area, reflectivity hardness/abrasiveness friability, grindability, dustiness index calorific value, heat capacity, thermal conductivity, plastic, agglomerating index, free-swelling index electrical resistivity, dielectric constant, magnetic susceptibility

fouling behavior. Reasons for the poor predictive behavior include the following: (1) Fusion temperatures are based on fuel ash, whereas deposits commonly are enriched and depleted in several elements relative to the fuel. (2) Fusion temperatures are measured over short time periods while heating ash at a rate of 8 ( 3 °C (15 ( 5 °F) per minute, whereas ash deposits typically accumulate for hours and are formed during cooling relative to the bulk gas temperature. (3) Fusion temperatures do not account for either boiler design or boiler operation, both of which strongly influence slagging and fouling behavior. (4) Fusion behavior changes when samples are allowed to stand at a given temperature. Fusion temperatures generally significantly decrease if the samples equilibrate at a given temperature for an hour or so. Fusion temperatures at one time were also quite subjectively measured, but this criticism has been addressed by the development of automated techniques, such as thermomechanical analysis (TMA), for performing the measurements that require no intervention by the operator. Despite the shortcomings, fusion temperatures are valuable guides to the high-temperature behavior of the fuel inorganic material. The ash fusion temperature has been correlated with the mineral and chemical composition of coal ash.11 Petrographic Analysis. The rank of a coal is directly related to its carbon content, but unfortunately, it does not tell much about the chemical structure or the reactivity of the coal during its conversion. The advent of microscopic analysis in coal research has resulted in the identification of different maceral components that make up coal and the effect which the petrographic composition has on the conversion properties and the reactivity of coal. The maceral components in coal are combined into three principal categories: vitrinite, exinite, and inertinite. Exinite is characterized by the highest hydrogen content, volatile matter content, and heating value, whereas inertinite displays the least of these properties.12 Inertinite has the highest density and the greatest degree of aromaticity. The proportion of these maceral groups in a coal determines its combustion properties significantly. Several advances in characterization techniques such as reflectance microscopy, NMR techniques, Fourier transform infrared (FTIR) spectroscopy, and X-ray techniques have also been applied to the study of organic matter in coal. The following table (Table 1) provides other important bulk properties of coal that influence its conversion and analysis (11) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Fuel Process. Technol. 1995, 45 (1), 27-51. (12) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydro-pyrolysis. In Chemistry of Coal Utilisation; Elliott, M. A., Ed.; John Wiley and Sons: New York, 1981.


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required for the estimation of these properties. These analytical methods have been described in the literature.10 Advanced Bulk Analytical Techniques The conventional analyses for coal discussed in the previous section are most commonly used for describing a coal for general purposes. A number of advanced bulk analytical techniques discussed in this section provide information on the average structure of organic and inorganic matter in coal and are used for better understanding of the organic and inorganic nature of coal. The analytical techniques such as differential thermal analysis, differential scanning calorimetry, thermomechanical analysis, chemical fractionation, X-ray fluorescence, and X-ray diffraction (XRD) are used to understand the nature of minerals and inorganic matter in coal and their thermal behavior. These analyses are usually performed on minerals in coal (obtained from low-temperature ashing of coal) or on an ash sample. The techniques such as TGA, FTIR, and NMR are used to understand the organic structure of coal. XRD has also been used to measure the graphitic structure in the char formed. The drawback of these conventional analyses, however, is that they measure the bulk properties of the sample assuming that coal is a homogeneous material. Differential Thermal Analysis (DTA). Differential thermal analysis (DTA) conducted under controlled atmospheric and heating conditions provides a method of mineral identification in coal.9,13 The method offers advantages in that the samples can be used directly for analysis without any pretreatment. The technique is based on heating an unknown sample with a reference sample under controlled conditions and measuring the temperature of the samples with respect to the temperature of the furnace. The difference in these temperature displays peaks according to exothermicity or endothermicity of the reactions occurring inside the sample. The characteristic peaks of various mineral groups have been identified, and by a comparison with the database, the peaks obtained from the unknown sample can be identified as specific mineral groups. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) is based on the heat needed or released during a phase change. It measures energy necessary to establish a nearly zero temperature difference between a substance and an inert reference material, as the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate. There are two types of DSC systems in common use. In power compensation DSC, the temperatures of the sample and reference are controlled independently using separate, identical furnaces. The temperatures of the sample and reference are made identical by varying the power input to the two furnaces; the energy required to do this is a measure of the enthalpy or heat capacity changes in the sample relative to the reference. In heat flux DSC, the sample and reference are connected by a low-resistance heat flow path. The assembly is enclosed in a single furnace. Enthalpy or heat capacity changes in the sample cause a difference in its temperature relative to the reference; the resulting heat flow is small compared with that in differential thermal analysis (DTA) because the sample and reference are in good thermal contact. The temperature difference is recorded and related to enthalpy change in the sample (13) Warne, S. S. J. Differential Thermal Analysis of Coal Minerals. In Analytical Methods for Coal and Coal Products; Elliott, M. A., Ed.; John Wiley and Sons: New York, 1979; Vol. III, pp 447-476.

Figure 3. Typical TMA trace of a coal ash and its derivative.

using calibration experiments. Hansen et al.14 have used DSC to assess the thermal behavior of ash. Thermomechanical Analysis (TMA). Slagging and formation of ash deposits inside furnaces is a major problem faced during the coal conversion process. Thermomechanical analysis (TMA) is a technique used to characterize the melting and slagging properties of coal ash.15-20 It allows the comparison of the tendency of coal particles to form significant deposits and provides information on the fusibility, melting, and sintering behavior as well as information such as deposit strength, and its influence on heat transfer can be derived. The TMA technique involves heating of an ash sample under load and measuring the penetration of a ram into the sample. The results are obtained in the form of increase in % penetration with increasing temperature. The TMA results provide an indication of the extent of melting in an ash sample. Figure 3 shows a typical TMA curve for an ash sample. The sharp peaks in the derivative curve in this figure represent distinct phase changes during heating. A scanning electron microscopy (SEM) examination of a sample and comparison with TMA data show an increase in melting with the increase in % penetration of the samples. Chemical Fractionation. Chemical fractionation is a technique that provides species-specific information using selective extraction of elements based on solubility that reflects their association in the coal. The process consists of three successive extractions: (i) using water to remove water-soluble salts containing elements such as sodium; (ii) using ammonium acetate to remove elements such as sodium, calcium, and magnesium that are ion exchangeable; and (iii) using hydrochloric acid to remove acid-soluble species such as alkaline earth sulfates, carbonates, etc. The residual material typically consists of silicates, oxides, and sulfides. This wet-chemistry technique is not ideal in that it can easily be biased by incomplete penetration of the sample by the solutions, resulting in partially soluble compounds. However, when carefully performed, it is found that it is both accurate (14) Hansen, et al. Impacts of Mineral Impurities in Solid Fuel Combustion; Gupta et al., Eds.; Kluwer Academic/Plenum Press: Norwell, MA, 1999; pp 341-356. (15) Saxby, J. D.; Chatfield, S. P. Presented at AIE 7thAustralian Coal Science Conference, 1996; pp 391-398. (16) Saxby, J. D.; Chatfield, S. P. Presented at AIE 8th Australian Coal Science Conference, 1998. (17) Bryant, G. W.; Lucas, J. A.; Gupta, S. K.; Wall, T. F. Energy Fuels 1998, 12 (2), 257-261. (18) Gupta, S. K. The Ash Fusibility Characteristics of Thermal Coals. Ph.D. Thesis, University of Newcastle, Newcastle, U.K., 1998. (19) Gupta, S. K.; Gupta, R. P.; Bryant, G. W.; Juniper, L.; Wall, T. F. Impact of Mineral Matter in Solid Fuel; Gupta et al., Eds.; Kluwer Academic/Plenum Press: Norwell, MA, 1999; pp 155-170. (20) Bryant, G. W.; Browning, G. J.; Gupta, S. K.; Lucas, J. A.; Gupta, R. P.; Wall, T. F. Energy and Fuels 2000, 14, 326-335.


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and precise, nearly within the limits of the elemental analysis procedures themselves. Many of the mineral species commonly found in coal can be distinguished by chemical fractionation. The key to doing so is to compare the elemental composition of the materials extracted at each stage of the process, based on their solubility in the three solutions.21-23 In general, the solubility for each of the elements can be used as a matrix to determine the species composition of the material. The results of chemical fractionation analyses can be used to estimate the overall species composition of both minerals and nonmineral inorganic components in coal. This technique is particularly most suitable for lignite and biomass fuels. Chemical fractionation has also been used to study the effect of minerals based on elements such as Na and Cl in coal on slagging and fouling characteristics and also to study the formation of fine particles in ash from combustion. X-ray Fluorescence Analysis (XRF). X-ray fluorescence established itself in the area of trace element analysis because of its ability to do rapid simultaneous quantitative analyses of a large number of elements. It offers several advantages over conventional techniques of trace element analysis, such as atomic absorption spectroscopy, including automated routine analyses and consolidating three independent analyses: sulfur content, ash content, and yield of liquefied coal. It has also been used in the analysis of effluents from coal conversion processes.24 Mo1 ssbauer Spectroscopy. Mo¨ssbauer spectroscopy is used in the analysis of iron bearing minerals in coal. It is well-known that the iron bearing minerals display a significant importance in slag formation in a coal combustion system. Huggins and Huffman25 have provided comprehensive information on the application of Mo¨ssbauer spectroscopy in coal characterization. Electron Probe Microanalysis. Electron probe microanalysis (EPM) is based on the principle that a sample will produce X-rays characteristic of the elements present in the sample when exposed to a finely focused electron beam. The electron column and basic instrumentation for EPM are very similar to those from scanning electron microscopy (SEM). The major difference is that, while SEM describes the surface morphology by visual magnification of the surface, EPM is able to describe the chemical composition of the sample.26 A scanning electron microscope, when equipped with an X-ray spectrometer, can carry out the same analysis as that of an EPM. Modern scanning electron microscopes are commonly equipped with energy dispersive spectrometers (EDS) that allow qualitative and quantitative chemical analysis. EPM can be used as a powerful analytical tool in coal research if used in conjunction with other analytical techniques such as SEM, transmission electron microscopy (TEM) and optical microscopy, X-ray diffraction, and Mo¨ssbauer spectroscopy.

X-ray Diffraction for Analysis of Mineral Matter in Coal. X-ray diffraction (XRD) of coal reveals important qualitative and quantitative information on the mineral matter composition of coal and the interaction of mineral matter during conversion processes. CSIRO Australia has developed a program called SIROQUANT that estimates the minerals quantitatively27,28 as well as a technique that is used to validate the advanced characterization of minerals in coalsCCSEM. High-temperature XRD is widely used for the investigation of the mineral reactions that occur during coal combustion in power-plant boilers. In the past, the nature of these reactions has been deduced either from a study of the chemistry of the reaction products or from phase-equilibria modeling. The sample is heated under controlled conditions to simulate combustion processes; the mineral reactions can be studied dynamically, and the effects of parameters such as variations in heating rates and variable gas atmospheres can be evaluated. This technique offers a particular advantage to the power-generation industry in that it provides knowledge of the melt phase, both the temperature at which it first formed and the abundance, because first appearance and nature of a melt phase is of critical importance in the formation of slagging and fouling deposits. Dynamic high-temperature XRD can also be used to study the mineral reactions that occur in slagging gasifiers in which the fluidity of the ash is of critical importance. Fluxes are often added to achieve a desired fluidity, and high-temperature XRD can be used to identify the changing mineral reactions that occur when a flux is added and also to quantify the effect on mineral and melt abundance of adding a particular flux. XRD has been useful in obtaining the structural information of coal and chars.29,30 The changes in crystallite structure of coal during conversion can be observed by an analysis of XRD spectra of the char/ash. X-ray diffraction analysis of the chars can also reveal significant changes in crystallite size for samples produced at different temperatures in a wide temperature range (∼900-1500 °C).31 Figure 4 shows the estimation of the proportion of amorphous material in a carbonaceous sample by analysis of X-ray diffraction background. Thermogravimetric Analysis (TGA). A small sample of coal is heated in a controlled environment at a specified heating rate in thermogravimetric analysis, and it is commonly performed to understand the pyrolysis behavior of coal. The rate of mass loss in TGA has been widely used to determine the reactivity and kinetic parameters of char combustion or gasification.32 There are two modes of TGA analysis for estimating the reactivity of char: isothermal and non-isothermal.33 13C and 1H NMR Spectroscopy. 13C NMR spectroscopy is a powerful method for identifying structural parameters in coals and coal chars as a nondestructive analytical tool for probing the microscopic environment surrounding nuclear spins.34,35 The

(21) Benson, S. A.; Holm, P. L. Ind. Eng. Chem. Prod. Res. DeV. 1985, 24, 145. (22) Korbee, R. Am. Chem. Soc., DiV. Fuel Chem. 2000, 45 (3), 556559. (23) Steel, K. M.; Besida, J.; O’Donnel, T. A.; Wood, D. G. Fuel Process. Technol. 2001, 70, 171-192. (24) Fruchter, J. S.; Petersen, M. R. Environmental Characterisation of Products and Effluents from Coal Conversion Processes. In Analytical Methods for Coal and Coal Products; Academic Press: New York, 1979; Vol. III, pp 247-275. (25) Huggins, F. E.; Huffman, G. P. Moessbauer analysis of ironcontaining phases in Coal, Coke, and Ash. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1979; Vol. III, pp 372-422. (26) Raymond, R.; Gooley, R. Electron Probe Microanalyzer in Coal Research. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1979; Vol. III, pp 337-355.

(27) Taylor, J. C. Powder Diffr. 1991, 6, 2-9. (28) Ward, C. R., et al. Int. J. Coal Geol. 1999, 40, 281-308. (29) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587. (30) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39 (12), 1821-1833. (31) Lu, L.; Sahajwalla, V.; Harris, D. Coal Structure and its Influence on High Temperature Coal/Gas Reactions, CRC for Black Coal Utilisation; Project 2.4; The University of New South Wales: Australia, 1998. (32) Roberts, D. Intrinsic Reaction Kinetics of Coal Chars with O2, CO2 and H2O at elevated pressures. Ph.D. Thesis, University of Newcastle, Newcastle, U.K., 2000. (33) Tang, L. G.; Gupta, R. P.; Sheng, C. D.; Wall, T. F. Fuel 2005, 84 (2-3), 127-134. (34) Gupta, R. P.; Wall, T. F. Workshop on Coal Characterization for Existing and Emerging Technologies; The University of Newcastle: Australia, 1996; pp B22-B27.


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Figure 4. XRD analysis showing the estimate of crystallinity of a typical char.31

Figure 5. Predicted and actual values for the volatile matter of coal samples using 1H NMR.37

carbon NMR spectra are derived from the resonance of the C-13 isotope of carbon, which is the only carbon isotope with a magnetic moment. Coal is thought to consist of a matrix of aromatic clusters connected by aliphatic bridging groups, aliphatic and carbonyl side chains attached to the aromatic cluster, and solvent-extractable components referred to as the “mobile phase”.36 13C NMR has been used to quantify the average carbon skeletal structure of a given coal with 12 parameters that describe the aromatic and aliphatic regions of the coal matrix. From these structural parameters, combined with an empirical relationship between bridgehead carbons and aromatic carbon per cluster, a description of the lattice structural of coal can be attained. Useful structural parameters determined from these analyses include the number of carbons per cluster, the number of attachments per cluster, the number of bridges and loops, the ratio of bridge to total attachments, the average aromatic cluster molecular weight, and the average side chain molecular weight.36 A recent development is the use of proton NMR (1H NMR) spectroscopy for coal characterization. The sensitivity of hydrogen atoms to magnetic resonance, the ease in measuring the spectra, and fast data acquisition on relatively cheap low(35) Genetti, D.; Fletcher, T. H.; Pugmire, R. J. Energy Fuels 1999, 13 (1), 60-68. (36) Smith, K. L., et al. The structure and reaction processes of coal; Plenum Press: New York, 1994.

field NMR instruments makes this characterization technique particularly useful. Research conducted on Australian coals has produced promising relationships between NMR measurements and coal properties.37 FTIR Spectroscopy. FTIR is a powerful tool for probing the functional groups in coal and chars. As a nondestructive analytical tool, it identifies molecular vibration, both stretching and bending, due to the absorption of infrared radiation. A sample exposed to continuously changing wavelengths of infrared radiation absorbs light when the incident radiation corresponds to the energy of a particular molecular vibration. Energies of the stretching vibrations correspond to infrared radiation with wave numbers between 1200 and 4000 cm- 1, while the bending vibration is in the range of 500-1200 cm-1. This part of the infrared spectrum is particularly useful for detecting the presence of functional groups, because these groups have characteristic and invariant absorption peaks at these wavelengths. FTIR has been extensively used in the identification of the chemical structure of coal.38,39 FTIR and other chromatographic instruments such as GC/MS have been coupled to TGA (thermogravimetric analyzer) or other similar furnaces, providing controlled conditions to study pyrolysis or combustion byproducts. Chromatographic Techniques. Analytical pyrolysis techniques such as gas chromatography/mass spectrometry (GC/ MS), along with multivariate data analysis techniques, are powerful analytical tools for simultaneously investigating coal structure and reactivity of organic materials. During pyrolysis, complex mixtures are released from coal because of distillation, desorption, and thermal degradation. The chemical nature of these products depends on both the chemical composition and the structure of the coal and on the heating condition. Consequently, the result of a chemical analysis of these devolatilized coal products provides information on the origin structure of the coal. The chromatographic techniques have contributed significantly to the environmental impact of combustion and pyrolysis by providing excellent emission-monitoring tools for qualitative and quantitative analysis. Scanning Electron Microscopy. Morphology and cross section of coal and char/ash have been studied widely using (37) Harmer, J.; Callcott, G.; Maeder, M.; Smith, B. E. Fuel 2000, 80 (9), 1341-1349. (38) Cloke, M.; Gilfillan, A.; Lester, E. Fuel 1997, 76, 1289. (39) Gilfillan, A.; Lester, E.; Cloke, M.; Snape, C. Fuel 1999, 78 (14), 1639-1644.



Figure 6. Figure showing (a) morphology and (b) cross section of char particles.

It has been stated earlier that coal residue obtained after its conversion may contain various quantities and nature of mineral matter, and it is necessary to assess the beneficial and detrimental effects that the mineral matter may have both on the process in which it is involved and on its ultimate disposal. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been extensively utilized41-43 to identify maceral composition and mineral matter in coal and to explore the relationships between coal minerals and certain maceral types. Advanced Microscopic Analytical Techniques

Figure 7. Automated reflectogram showing the maceral composition of two coal samples.

Figure 8. CCSEM analysis showing major mineral types in coal, included (locked in coal matrix) and excluded (liberated from the coal matrix).

SEM, and the analysis of the SEM image can provide valuable information such as particle size, swelling of the particles during conversion, and the structure of char/ash. This information can be correlated to the fundamental properties of parent coal and the process parameters. Figure 6 displays typical SEM images for the morphology and cross section of a char particle. Image analysis on these SEM images has been used to estimate the porosity of char particles.40

Recent advances in analytical techniques have resulted in a multidimensional approach to the characterization of coals. The correlation of the extensive coal structure data from these approaches provides a good database to model the behavior of coal during its conversion. This section discusses mainly two techniques that quantify the heterogeneity of coal in general and of pulverized coal in particular. Automated reflectogram (AR) provides the reflectivity distribution in coal, or the heterogeneity in organic matter. Computer-controlled scanning electron microscopy (CCSEM) analysis provides the distribution of minerals in coal at the individual particle level, or the heterogeneity in inorganic matter. These characterization techniques have been used extensively in the CRC for Coal in Sustainable Development for assessing the performance of coals in power generation. The bulk properties of the coal are able to provide important information on coal, but they are coal-specific; therefore, current research is aimed at obtaining relationships between the bulk properties of coal and the fundamental properties of individual particles. This will enable the establishment of universally applicable correlations regardless of coal type or rank. These analytical techniques assume coal to be a mixture several types of coal particles and provide a mechanistic approach in modeling coal conversion at the individual particle level. In pulverized (40) Tang, L.; Sheng, C.; Gupta, R. P.; Wall, T. F. Fuel 2005, 84 (10), 1268-1276. (41) Gluskoter, H. J.; Lindahl, P. C. Science 1973, 181, 264-266. (42) Strehlow, R. A.; Harris, L. A.; Yust, C. S. Fuel 1978, 57, 185186. (43) Wall, T. F.; Gupta, R. P.; Bryant, G.; Gupta, S. K.; Yan, L. Presented at International Conference on Ash Behaviour Control in Energy Conversion Systems, Japan, 1998; pp 127-134.


Gupta

Table 2. Effect of Particle Reflectivity and Mineral Distribution on Char Morphology coal particle reflectivity

char obtained

ash obtained

attributes

F ) 0.9 F ) 1.5

cenospheric char mesospheric char

fine ash fine/medium coarseness ash

F ) 2.2

solid char

coarse ash

highly porous char (few included minerals) char with intermediate particle size and porosity (several included minerals) forms coarser and solid char (mostly composed of included minerals)

coal combustion, an individual coal particle or mineral grain behaves independently. For this reason, the analyses from these techniques are very powerful in assessing the performance of coal blends.44 Automated Reflectogram. Coal petrography is a standard tool for the characterization of the organic (maceral) and inorganic (mineral) constituents of coal. Two types of data are derived from this method: rank (defined by vitrinite reflectance) and composition (maceral composition). Both of these can be related to the chemical composition and will control the physical behavior of coal during processing and utilization. The overall maceral composition of coal is valuable in predicting conversion behavior of coal, but the models based on such predictions have limitations of accuracy; therefore, there is a need to characterize the coal heterogeneity based on a more fundamental property of coal so that the model predictions can be improved. Automated reflectogram provides the possibility of characterizing coal based on single-particle composition and modeling their conversion behavior more accurately. In order to reduce operator subjectivity and improve the accuracy of petrographic analysis, a method was developed for fully automated coal petrographic analysis of single-seam coal. Full maceral reflectograms (FMRs) were collected using image techniques and then processed to produce a vitrinite and inertinite group abundance, a combined minerals plus liptinite abundance, and random vitrinite reflectance information. The technique involves obtaining a number of 256 gray-scale images of a polished surface of coal particles. For a given sample, ∼2 million pixels were taken. Then the gray scale is calibrated with the samples of standard reflectance, and finally, these images are processed to give a distribution of reflectance as shown in Figure 7. Within the FMR, inflection points in the cumulative frequency format coinciding with the commencement and end of the vitrinite proportion can be used to determine maceral group proportions.45 This form of analysis may be used to “fingerprint” coals by producing a reflectance signature of the bulk constituents in a representative sample. The full signature exhibits inflections that coincide theoretically with the boundary between the different maceral and mineral groups. The vitrinite content is given by the area under the vitrinite peak in Figure 7. The material with reflectivity values less than this range may be mineral matter or liptinite or microporosity, whereas the material with reflectivity values greater than this are either inertinite maceral or brighter mineral matter. CCSEM Analysis. The computer-controlled scanning electron microscopy technique is used to determine the size, quantity, distribution, and semiquantitative composition and association of mineral grains in pulverized coal, as described by Skorupska and Carpenter46 and Gupta and co-workers.47,48 The CCSEM instrument consists of a scanning electron micro(44) Rushdi, A.; Gupta, R. P.; Sharma, A.; Holcombe, D. Fuel 2005, 84 (10), 1246-1258. (45) O’Brien, G.; Jenkins, B.; Esterle, J.; Beath, H. Fuel 2003, 82, 1067. (46) Skorupska, N. M.; Carpenter, A. M. Computer-Controlled Scanning Electron Microscpoy of Minerals in Coal; IEAPER/07; IEA Coal Research: London, 1993; p 31. (47) Gupta, R. P.; Wall, T. F.; Kajigaya, I.; Miyamae, S.; Tsumita, Y. Prog. Energy Combust. Sci. 1998, 24, pp 523-543.

scope that is interfaced with a computer-controlled acquisition and data-collection system. The CCSEM method was originally developed ∼15 years ago and has been refined, enhanced, and continuously developed.49 The system measures at least 1 000 different mineral particles, which appear randomly placed in a polished section of the coal embedded in epoxy. In the scanning electron microscope, the electron beam is under the control of the computer and is stepped across the field-of-view in a relatively coarse grid (256 × 256) in order to locate mineral particles in the coal/epoxy matrix. The back-scattered electron intensity is used to discriminate between mineral particles and the background of coal macerals and epoxy mounting medium. Once a mineral particle is located, the grid spacing is greatly reduced (4096 × 4096). The crosssectional area of the particle is measured by extending eight diagonals through the center of the particle to the edge of the particle and summing the areas of the triangles so produced. After the area is measured in this manner, the electron beam is positioned at the center of the particle and an energy dispersive X-ray (EDX) spectrum is collected for ∼3-4 s. The chemical information contained in this spectrum is then used as a fingerprint to identify the mineral. The raw data on each particle, which includes information about its size, chemistry, and location, are stored in the computer memory. The mineral recognition program and general data handling have been extensively developed over recent years. By analyzing at least 1 000 particles in the coal section in this manner, a reasonably quantitative description of the coal mineralogy can be obtained. The mineral composition obtained from CCSEM analysis of a typical coal sample is shown in Figure 8. There have been several modifications to the CCSEM technique over the years. One of the improvements was to move from single elemental analysis at a central point to an average elemental analysis derived from several analyses obtained at a number of points on a grid imposed on each particle. This accounted for variation in the mineral composition within a mineral grain. This was further advanced in QEMSCAN,50 where analysis at an individual point was converted into a corresponding mineral to obtain coal-mineral and mineralmineral associations as shown in Figure 2. Implications on Predicting Coal Quality Impacts There are several implications on the assessment of the performance of coal during combustion or gasification. Some of these are discussed in this section. Erosion and Abrasion. Abrasion of mills is associated with minerals such as pyrites and quartz, and erosion is mostly associated with quartz. However, in reality, the abrasion and erosion are strongly influenced by the grain size as well. CCSEM or an equivalent technique only can provide such detailed information. (48) Gupta, R. P.; Yan, L.; Gupta, S. K.; Wall, T. F. Int. J. Energy Clean Air 2005, 6, 157-170. (49) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P.; Lee, R. J. Scanning Electron Microsc. 1980, 1, 531-540. (50) Liu, Y.; Gupta, R. P.; Sharma, A.; Wall, T. Fuel 2005, 84 (10), 1259-1267.



Figure 9. Plot showing the relationship between the reflectivity of coal and the porosity of resulting char.60

Figure 10. Comparison of porosity distribution from correlation with reflectogram with experimental distribution from image analysis.

Ash Formation and Deposition. Several studies in the past have addressed the issue of ash deposition on furnace surfaces, and models have been proposed for predicting ash character and deposition performances in combustion furnaces.51-54 An ash deposition phenomenon occurs because of the impaction of individual particles on the hot deposition surface. The deposition depends on the physical environment inside the furnace as well as the chemistry of the individual coal particle. The chemistry of each particle is decided by the ash structure/morphology and the mineral distribution inside the ash particle. This information is provided by a detailed CCSEM and petrographic and microscopic analyses of individual coal particles. The CCSEM analysis of coal provides vital information on the distribution of mineral based on individual coal particles and, therefore, is able to more accurately predict phenomena such as char fragmentation and ash deposition during conversion.48 Char fragmentation depends largely on the structure of char and the mineral distribution in the individual char particles. The char derived from a low-reflectivity coal particle usually forms a cenospheric char particle, which fragments easily and produces finer char, and the coal particles of high reflectivity will form a solid char with larger size and mineral grains that come closer during diffusion-controlled combustion, coalesce, and may produce larger ash particles. Yan et al.55-57 and Gupta and co-workers48 have developed a detailed ash formation based on detailed information from CCSEM. Fine Particle Formation. Buhre et al.58 investigated the formation of fine particles from five well-characterized coals in a drop-tube furnace. They concluded that fine ash particles are formed by a combination of mechanisms: included mineral coalescence, excluded mineral fragmentation, char fragmentation, and vaporization and subsequent condensation of inorganic matter. The amounts of PM2.5 formed are expected to correlate

with the amount of char particles displaying swelling behavior, expressed as char group I particles. Thin-walled cenospheric char particles can fragment during combustion, resulting in the release of fine included minerals originally present in these char particles, which transform to ash particles of a few micrometers in size. This expectation could not be confirmed conclusively from the experiments. More experiments using different coals and maceral types could confirm this expectation. Char Formation and Burnout. Previous modeling attempts at predicting behavior of coal have used average properties of coal provided by the conventional approaches to characterization. The models, such as the CPD model4 for predicting volatile release from coal and the CBK model59 for predicting char combustion, make use of the bulk analyses and some advanced characterization techniques such as NMR analysis. These approaches have assumed that the coal is a homogeneous material, and therefore, these models often result in an inaccurate prediction of coal behavior. The inherent heterogeneity of coal elicits the need for understanding the effect of the fundamental properties of individual coal particles that constitute the entire fuel mixture and their impact on the behavior of coal in various conversion technologies. Future models that will be based on such properties as reflectivity of individual particles will require highly advanced techniques of characterization. The char produced from a coal particle with low reflectivity (higher vitrinite content) will be a highly porous char, whereas a coal with high reflectivity and a higher inertinite content will form a less porous char. The prediction of carbon burnout has been an area of intense research focus in recent years. The devolatilization occurs quickly, and the burnout is determined by char combustion. There are several char combustion models available, most of which rely on the bulk properties of coal. A recent study has taken into consideration the heterogeneity of coal by distributing the coal particles into two bins of different reflectivity. It has been observed that a small change in reflectivity has a pronounced effect on apparent reactivity of coal particles. Therefore, the current approach of modeling combustion behavior of coal involves distribution of coal particles into several bins of increasing reflectivity and modeling the char character and reactivity distribution of the char particles.33 Automatic reflectogram provides the required reflectivity distribution in a coal sample, and therefore, it becomes indispensable in such an approach for modeling coal combustion behavior.

(51) Wang, H.; Harb, J. N. Prog. Energy Combust. Sci. 1997, 23, 267282. (52) Wigley, F.; Williamson, J. Prog. Energy Combust. Sci. 1998, 24, 337-343. (53) Wilemski, G.; Srinivasachar, S.; Sarofim, A. F. Proceedings of Engineering Foundation Conferences on Inorganic Transformations and Ash Deposition during Combustion; Benson, S., Ed.; 1991; pp 545-564. (54) Richards, G. H. Investigations of Mechanisms for the Formation of Fly Ash and Ash Deposits for Two Powder River Basin Coals. Ph.D. Thesis, Brigham Young University, Provo, UT, 1994. (55) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2002, 81, 337-344. (56) Yan, L.; Gupta, R. P.; Wall, T. F. Energy Fuels 2001, 15, 389394. (57) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2001, 80, 1333-1340. (58) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F. Coal Quality and Fine Ash Formation during Combustion. Presented at Pittsburgh Coal Science Conference, Osaka, Japan, Sept 2004.

(59) Hurt, R.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113, 181197.


The overall character of a coal can be approximated, in many cases, by the additive nature of the properties such as reflectivity and mineral matter associated with individual particles. This makes the modeling of coal behavior simple and precise. Several difficult problems such as the prediction of the effect of coal blending on the conversion process can also be addressed more accurately by using models considering the heterogeneity of coal and the information obtained by conducting individual particle analysis (see Table 2). There have been several attempts already in estimating the distribution of char porosity from detailed reflectivity distribution of coal from automated reflectogram.40,60 Figure 9 shows the relationship between char porosity distribution and coal automated reflectogram. This correlation was applied to a number of coals for estimating the char porosity from reflectivity distribution. Figure 10 presents a typical comparison of the predicted porosity distribution for a coal with the experimental distribution obtained from image analysis of SEM images. The modeling of unburnt carbon can now be based on the heterogeneous nature of coal. The pulverized coal can be assumed to be a mixture of coal particles of different reflectivity. Each reflectivity bin produces char of different porosity corresponding to the reflectivity as per Figure 9. The combustion of individual char particles can be integrated to give unburnt carbon as a function of residence time. (60) Gupta, R. P., et al. Char Morphology from the Automated Reflectogram of a Coal and its Implications on Burnout and Ash Formation. Proceedings of the International Conference on Power Engineering, Kobe, Japan, Nov 2003.

Gupta

Conclusions The current state of the art in coal research depends on coal conversion models that consider coal as a homogeneous material. Recent advances in coal characterization discussed in this paper have shown that individual pulverized coal particles differ significantly in their composition and, therefore, in their behavior during conversion. These advanced techniques have provided important insights into the study of coal particles on a microlevel. The automated reflectogram can divide coal particles into different reflectivity bins that can be treated individually in the boiler to assess the combustion characteristics of coal. Similarly, CCSEM-based techniques have led to an advanced ash formation model, giving details of ash character in terms of particle-size distribution and chemistry distribution. This then can be used to assess the ash impacts on erosion or ash deposition. It is clear that several properties of coal are additive in nature and can be estimated by simple addition of the respective property of individual particles. The strength of the models based on these techniques lies in the assessment of coal blends. Acknowledgment. The author is thankful to the contribution of Dr. Yan Li, Dr. Sushil Gupta, Dr. Tang Lianguang, Dr. Atul Bhargawa, Dr. Liu Yinghui, and Professor Terry Wall from University of Newcastle, Australia. The author is also thankful to Mike Mason and Kevin Kerrison of Pacific Power for the development of CCSEM; to Dr. Paul Gottelib and Dr. Alan Butcher for the development of QEMSCAN for coal; and to Dr. Graham O’Brien of CSIRO Australia for developing the automated reflectogram. EF060411M

Advanced Coal Characterization: A Review - American Chemical

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