Observations of Heterogeneity in Large Pulverized Coal Particles

Copyright © 1999 American Chemical Society ... The heterogeneity of the 125−212 μm size cuts from four pulverized coal samples from the United Kin...
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Energy & Fuels 1999, 13, 592-601

Observations of Heterogeneity in Large Pulverized Coal Particles J. R. Gibbins,*,† T. J. Beeley,‡ J. C. Crelling,§ A. C. Scott,| N. M. Skorupska,‡ and J. Williamson⊥ Mechanical Engineering Imperial College, London SW7 2BX, U.K., Geology Department, Southern Illinois University, Carbondale, Illinois 62901, Geology Department, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, U.K., National Power PLC, 8 Marylebone Passage, London W1N 7HE, U.K., and Department of Materials, Imperial College, London SW7 2BX, U.K. Received July 27, 1998

The heterogeneity of the 125-212 µm size cuts from four pulverized coal samples from the United Kingdom, United States, and South Africa has been examined in detail by means of density separation and optical and scanning electron microscopy. While average properties of the overall size cut samples are similar to those of the parent coals, a high degree of heterogeneity exists between average compositions for the different density cuts within each sample, between different particles within the same density cuts, and, in many cases, within the particles themselves. Conventional analyses are unsuited for describing this heterogeneity and an alternative descriptive method based on maceral associations has been used, together with illustrative micrographs. The implications of coal heterogeneity for particle behavior during grinding, pulverized coal combustion and coal characterization for modeling purposes are discussed.

Introduction An understanding of the heterogeneous nature of pulverized coals is essential for the development of improved conceptual and, hence, quantitative models for pulverized coal processing and combustion. In particular, char burnout phenomena are dependent on the nature of individual particles which, in the critical later stages of combustion, react virtually independently of each other. Conventional sampling and test methods, however, concentrate almost exclusively on obtaining average coal property values. With limited information on property ranges thus available, there is a tendency to treat coal as being homogeneous and to forget its true heterogeneous nature. Coal heterogeneity arises because coal is formed from a mixture of different biomass precursors, which themselves have undergone a range of biochemical and geological processes, together with a wide range of minerals deposited during and after coal deposition.1 Stopes2 showed that the organic components can be subclassified into different coal maceral groups, but maceral composition, as generally used at present, has limited value in relation to pulverized coal combustion for the following reasons: (a) only a limited number of maceral subgroups, typically vitrinite, liptinite, and * Author to whom correspondence should be addressed. † Mechanical Engineering Department, Imperial College. ‡ National Power PLC. § Southern Illinois University. | University of London. ⊥ Department of Materials, Imperial College. (1) Teichmuller, M. Int. J. Coal Geol. 1989, 12, 1-87. (2) Stopes, M. C. Proc. R. Soc. 1919, B90, 470-487.

inertinite, are generally reported from routine characterization tests, yet macerals (unlike minerals for which they were analogously named) do not have a fixed composition but vary within a coal and between coals (coals of different ages from different geographical regions are formed from widely different plants3 which may vary considerably in their basic chemistry4,5), (b) macerals are classified by appearance rather than by relevant physical properties, and (c) perhaps most seriously of all, standard maceral test procedures still give only an average distribution for the whole coal sample. The need to move beyond average property values in pulverized coal combustion arises because, although the overall combustion behavior is the aggregate result of the combustion of all the individual pulverized coal particles, the averaged behavior of an array of different particles clearly may not always be the same as the behavior of an homogeneous array of particles having the same average composition. Average coal properties are obviously entirely appropriate for overall plant mass and energy balances. The total rapid-heating volatile yield for homogeneous gas-phase combustion in the near-burner region will also probably be well-predicted (3) Collinson, M. E.; Scott, A. C. In Coal and Coal-Bearing Strata: Recent Advances, Scott, A. C., Ed.; Geology Society Special Publication 32; 1987; pp 67-85. (4) Collinson, M. E.; Van Bergen, P. F.; Scott, A. C.; De Leeuw, J. W. In Coal and coal-bearing strata as oil-prone source rocks?; Scott, A. C., Fleet, A. J. Eds.; Geology Society of London Special Publication 77; 1994; pp 31-70. (5) Van Bergen, P. F.; Collinson, M. E.; Briggs, D. E. G.; De Leeuw, J. W.; Scott, A. C.; Evershed, R. P.; Finch, P. Acta Bot. Neerl. 1995, 44, 319-342.

10.1021/ef980161c CCC: $18.00 © 1999 American Chemical Society Published on Web 03/13/1999

Heterogeneity in Large Pulverized Coal Particles

by suitable tests (e.g., high-temperature wire-mesh or drop tube furnace studies) yielding an average value, since essentially all of the coal feed can be expected to react within a relatively short period and the volatiles themselves mix. Even then the detailed pattern of volatile release will depend partly on individual particle size distributions and compositions. Individual particle properties become much more important in heterogeneous gas/solid char combustion, in particular in relation to the amount of unburnt char leaving a boiler. While the bulk of the char remaining after devolatilization burns to completion relatively rapidly, a small fraction of the coal particles will be large enough (e.g., >100 µm mean diameter) to have intrinsically long combustion times. A level of not more than 5% C in ash is targeted in most utility plants to allow the fly ash to be used in cement manufacture6 and to ensure satisfactory electrostatic precipitator performance. This corresponds to approximately 99% burnout for a coal with 20% ash. Average coal properties can thus be seen to be entirely appropriate for describing the bulk of the coal which does burn, but not necessarily describe the few percent of the coal, termed the least likely to burn (LLB) fraction,7 which by burning or not is sufficient to determine whether burnout is acceptable or unacceptable for a utility plant operator. Individual particle densities will also be important in determining whether particles are retained in the aerodynamic classifiers incorporated in most large-scale coal mills and recycled for further grinding, which in turn may affect overall burnout. In this case, an unusual abundance of low-density particles, which would be able to leave the mill at relatively larger diameters, might indicate a propensity to give poor burnout due to the greater residence times required to complete combustion of these particles. This paper presents comparative observations of a large-diameter size cut (125-212 µm), likely to be typical of the LLB fraction, from PF prepared from a range of commercially important bituminous coals. Four levels of heterogeneity are examined: (a) differences between average values for the large size cuts and the whole coal, (b) differences between individual density fractions within the size cut, (c) differences between particles within a single density fraction, (d) the internal composition of particles. The main emphasis of the study has been to show, by means of optical and scanning electron microscopy, the range of particle types that can be encountered in the larger pulverized coal size cuts even within limited ranges for particle density. This combination of observation methods in conjunction with density separation and their application to 125-212 µm size fractions illustrates important aspects of the nature these particles that could not have been revealed in most previous studies. Conventionally coals are ground to 70% vitrinite and SA1 >70% inertinite and also to provide a contrast between Laurasian (Herrin No. 6 and Gascoigne Wood) and Gondwanan coals (SA1 and SA2). Table 1 provides proximate and elemental analysis and shows the variation in petrographic composition for the coals and size cuts. Sample Preparation. The Herrin No. 6 sample was supplied ground and sized by Sandia National Laboratories from a batch obtained from the Penn State coal bank. SA1 and SA2 and Gascoigne Wood (all supplied as SA2 > SA1. The range of different maceral/mineral associations present in selected density fractions, together with variations in the relative abundances of different associations with density, is shown in the top sections of Figures 1-4. Predictably, liptinite-rich associations predominate at lower densities for all coals, but liptinite associations also occur in some particles across all of the density range examined (approximately 1.2-1.5 SG). At low and median densities, some pure vitrinite particles were observed for the Herrin No. 6 and, to a lesser extent, for the Gascoigne Wood and SA2 coals (although there is an inevitable bias toward underestimating the complexity of associations from observations of coal particle sections since not all of the (13) Scott, A. C. Int. J. Coal. Geol. 1989, 12, 443-475. (14) Bartram, K. In Coal and Coal-Bearing Strata: Recent Advances; Scott, A. C., Ed.; Geology Society Special Publication 32; 1987; pp 187199. (15) Beeley, T. J. et al., Proceedings of the 26th Symposium (International) on Combustion The Combustion Institute: Pittsburgh, 1996; pp 3103-3110.

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Figure 11. Photomicrograph (upper left) and SEM image (lower left) with distributions of silicon and aluminum for the 1.26 density cut of the SA1 sample. Note the correspondence of the Si and Al with the dark areas of the photomicrograph and the light areas of the SEM image indicating the presence of clay minerals. Figure 10. Photomicrograph (upper left) and SEM image (upper right) with distributions of silicon and aluminum and sulfur for the 1.29 density cut of the Herrin No. 6 sample. Note the correspondence of the Si and Al with the dark areas of the photomicrograph and the light areas of the SEM image indicating the presence of clay minerals. Note also that the sulfur distribution is ubiquitous and is most likely due to the presence of organic sulfur; this illustrates the difficulty of sulfur removal from this coal by mechanical means alone.

components present in a particle may be visible). No pure vitrinite particles were observed for the highinertinite, high-ash South African coal SA1. Although predictably more common at higher particle densities, semi-fusinite associations were seen across the range for both South African coals and the U.K. Gascoigne Wood coal. It is interesting to note that PF from a U.K., Laurasian, coal can contain approximately as wide a variety of particle maceral associations as the inertinite-rich Gondwanan coals, even though at very different abundances. It is debatable, however (and beyond the scope of this paper), whether this also implies a similar range of particle behavior during combustion, given the large differences in coal-forming flora and deposition conditions between these coals. Only for the high-vitrinite (>70%) Herrin No. 6 sample were semi-fusinite associations observed only at higher densities. It is also notable that some vitrinite was present in virtually all of the (large) PF particles from this coal. Overall, density separation was found to give a reasonably homogeneous range of particle compositions at lower densities, since, as noted earlier, low densities can only be achieved when particles are vitrinite-rich or contained enough liptinite to offset denser inclusions (i.e., inertinite, minerals). As liptinite contents in all the coals examined were relatively low, the abundance of the latter class of particles was limited. At higher

particle densities, the high density of minerals compared to macerals allows even relatively small mineral inclusions to give higher average particle densities. Thus, although broad trends still exist for increasing inertinite and mineral content with increasing particle density, the range of particle compositions within a density cut also tended to increase. Internal Composition of Particles. In addition to differences between average compositions for individual particles, even within a narrow density range, inspection of Figures 6-9 shows that the internal distribution of macerals and minerals also varies between particles. Macerals can be found in more or less discrete zones (e.g., 7a-I and II, 7b-I, 8a-I) or intimately mixed (e.g., 8a-II and III). The former distribution might be expected to encourage thermally induced fragmentation during initial heating or devolatilization, due to the different maceral properties. In better-mixed particles, however, some synergistic effects can probably be expected. The principal consequence for char burnout in PF combustion would probably be a wider extent of particle softening during devolatilization than might be expected from the average maceral composition, with hydrogenrich vitrinite and liptinite components fluxing the naturally less-fluid semi-fusinite and fusinite fractions. Additional information on mineral distributions can be obtained from SEM. Figures 9 and 10 show two optical, SEM and EDAX, views of particles from the Herrin No. 6 coal. The particle in Figure 9 contains a mineral-rich vein (principally aluminosilicate clays), although this vein still contains some organic material. The mineral matter is more finely distributed in Figure 10, associated with inertinite in a vitrinite matrix. Sulfur is distributed widely in both particles of this high-sulfur coal, in addition to grains of pyrite (visible as bright spots in both reflected light and SEM backscattered images).

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Figure 12. Photomicrograph (upper left) and SEM image (lower left) with distributions of silicon and aluminum for the 1.54 density cut of the SA1 sample. Note the correspondence of the Si and Al with the dark areas of the photomicrograph and the light areas of the SEM image indicating the presence of clay minerals.

Both South African coals tend to contain finely divided mineral matter intimately mixed with organic material (Figures 11-14) and distributed in a number of bands. UV fluorescence has also been used to highlight liptinite inclusions in Figures 13 and 14. Small amounts are distributed as fine veins in both particles, demonstrating the high degree of mineral/maceral heterogeneity that can occur in Gondwanan coals. Distributed liptinite veins are also present in the Gascoigne Wood particles shown in Figure 15, but in addition, much thicker bands of liptinite are prominent. The latter are probably megaspores from the very different vegetation forming this Carboniferous coal, which was dominated by arborescent lycopsids (e.g., the spore-producing, tree-like clubmoss Lepidodendron) as opposed to the South African Permian coals dominated by gymnospermous glossopterids (extinct nonflowering, seed-bearing trees). The liptinite macerals such as cutinite occur in thin sheets, whereas the sporinite maceral comprises both small microspores or pollen (usually less than 50 µm diameter) and megaspores (in the range 200 µm to 5 mm). The Barnsley Seam source vegetation is known to have been dominated by plants producing large megaspores.14 Conclusions While the average petrographic composition of the large-diameter particle size cuts examined in this study was close to that of the whole coals, compositions were found to vary widely between individual particles. Relatively few pure monomaceral particles were observed, and individual macerals were generally well-

Gibbins et al.

Figure 13. Photomicrograph in reflected light (upper left) and fluorescent light (upper right) and SEM image (lower left) with distributions of silicon and aluminum for the 1.44 density cut of the SA2 sample. Note the correspondence of the Si and Al with the dark areas of the photomicrograph and the light areas of the SEM image indicating the presence of clay minerals. The light areas in the fluorescence photomicrograph indicate liptinite macerals and are not associated with mineral matter.

mixed in heterogeneous particles. This suggests that some synergy during devolatilization is likely, which may explain the low frequency of unfused particles observed in unburnt carbon residues from utility boilers. The ability of studies using relatively pure macerals (from hand-picking or obtained by micronizing and density separation) to represent actual PF combustion behavior may also be limited. Average density cut properties showed clear trends with changing density. This reflects relatively homogeneous classifications of particles in lower density cuts, but the presence of significant amounts of mineral matter in many particles causes wider variations in the maceral composition between individual particles within higher density cuts. While the wide range of particle types observed implies a range of behavior during combustion, the fact that inertinite group macerals are frequently found in mixed-maceral particles suggests that the proportion of unfused and, hence, less reactive char particles produced during coal devolatilization cannot simply be inferred from the inertinite content of a coal. Since it has also been found in a related study that the intrinsic reactivity of chars from inertinite-rich samples can approach or even exceed the reactivity of vitrinite chars when prepared at combustion temperatures (1800 °C)15, the influence of a coal’s average inertinite content on its burnout potential during PF combustion appears less clear than has generally been considered.

Heterogeneity in Large Pulverized Coal Particles

Figure 14. Photomicrograph in reflected light (upper left) and fluorescent light (upper right) and SEM image (lower left) with distributions of silicon and aluminum for the 1.51 density cut of the SA2 sample. Note the correspondence of the Si and Al with the dark areas of the photomicrograph and the light areas of the SEM image indicating the presence of clay minerals. The light areas in the fluorescence photomicrograph indicate liptinite macerals and are not associated with mineral matter.

A wide range of particle mineral contents was also observed. The minerals were generally well-dispersed and also frequently associated with the inertinite macerals. It is possible that the well-dispersed minerals act as a catalyst for char oxidation (although ash particles usually do not appear to wet the surface of residual chars), but the measurements of overall char intrinsic reactivity mentioned above have shown that the net effect of any catalytic activity, together with the thermally induced structural changes, cannot be expected to result in any large increases in char conversion rates. Some reduction in particle combustion rates during the later stages of char burnout can probably be expected, given the quantities of mineral matter observed and the frequent admixture with organic material as a result of increased resistance to oxygen transport and/or increased heat losses from the greater surface area for a given amount of reacting char.

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Figure 15. Photomicrographs in white light (upper) and fluorescent light (lower) of the 1.21 g/mL fraction of the Gascoigne wood sample. The dominance of liptinite in this fraction is shown by the abundance of bright liptinite material in the fluorescence image.

It can therefore be argued that increased inertinite content in a coal may indicate poor burnout potential because it is associated with problem-causing mineral matter distributions rather than only because the inertinite itself gives rise to large char particles with lower volatile yields or which are less reactive. If this is the case, although some degree of correlation could be expected between (average) coal inertinite content and char burnout measurements, it would be preferable to replace this coincidental indicator with a more direct measurement of coal particle mineral/maceral heterogeneity. Acknowledgment. The authors are grateful for financial and technical support for this work provided by the U.K. Department of Trade and Industry through ETSU Contract C77 and National Power PLC and for the assistance from members of the Electron Microscopy Unit at Royal Holloway. EF980161C