Energy & Fuels 2001, 15, 1319-1321
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An Approach toward a Combined Scheme for the Petrographic Classification of Fly Ash James C. Hower*,† and Maria Mastalerz‡ University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511, and Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, Indiana 47405-2208 Received June 29, 2001. Revised Manuscript Received August 7, 2001 In recent years, two distinct systems for describing fly ash petrography have been used in the literature. In a system devised by Bailey et al.1 and modified by Lester et al.2 and Alvarez et al.,3 among others, emphasis was placed on the textural description of the char structure. This system is the base of a char classification considered by the International Committee for Coal and Organic Petrology. In the nomenclature used at the University of Kentucky Center for Applied Energy Research4-12 the emphasis has been on the forms of the carbon, as well as the type of inorganic matter, in combustion fly ash, and not the overall texture. The latter authors have been able to make correlations, however tentative, between the form of the carbon and the surface properties of the fly ash. This has been applied to the study of air entrainment in cement8,9 and the capture of mercury from flue gas streams in power stations.6-7,12 Specifically, Maroto-Valer et al.11 and Hower et al.,7 in a study of density-gradient concentrates of a high-carbon fraction of a fly ash, found that surface area of fly ash carbon increased from inertinite through isotropic carbon (char) to anisotropic carbon (char). Baltrus et al.,13 on the basis of their study of a series of class F fly ashes, reserved the right to disagree with the latter conclusions. As they point out... “One reason for the discrepancy may simply be trying to compare small differences in mercury adsorption and surface area in materials that themselves have small mercury adsorption capacities and surface areas (p 462).” In either case, a * Corresponding author. Tel: 859-257-0261. E-mail: hower@ caer.uky.edu. † University of Kentucky Center for Applied Energy Research. ‡ Indiana Geological Survey, Indiana University. (1) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225-239. (2) Lester, E.; Cloke, M.; Allen, M. Energy Fuels 1997, 10, 6696-6703. (3) Alvarez, D.; Borego, A. G.; Mene´ndez, R. Fuel 1997, 76, 12411248. (4) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. International Coal Testing Conference 11th, Lexington, KY, May 10-12, 1995; pp 49-54. (5) Hower, J. C.; Rathbone, R. F.; Robl, T. L.; Thomas, G. A.; Haeberlin, B. O.; Trimble, A. S. Waste Manage. 1997, 17, 219-229. (6) Hower, J. C.; Finkelman, R. B.; Rathbone, R. F.; Goodman, J. Energy Fuels 2000, 14, 212-216. (7) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitkakphon, T. Energy Fuels 2000, 14, 224-226. (8) Hill, R. L.; Sarkar, S. L.; Rathbone, R. F.; Hower, J. C. Cement Concrete Res. 1997, 27, 193-204. (9) Hill, R. L.; Rathbone, R. F.; Hower, J. C. Cement Concrete Res. 1998, 28, 1479-1488. (10) Maroto-Valer, M. M.; Taulbee, D. N.; Hower, J. C. Energy Fuels 1999, 13, 947-953. (11) Maroto-Valer, M. M.; Taulbee, D. N.; Hower, J. C. Fuel 2001, 80, 795-800. (12) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Energy Fuels 2000, 14, 727-733.
Figure 1. Examples of glass surrounded by carbon. In these cases the bulk analysis of the fly ash will not properly represent the proportion of components actually at the surface. Images are about 330 microns on long axis.
textural classification alone would not provide the information necessary to make the correlation between coke/ char type and surface area. Likely, the discrepancies in interpretation also indicate that component classifications by themselves do not provide a sufficient description. In an attempt to gain more information from the petrographic analysis of fly ash, we are proposing a simple combination of the two systems. In this combined scheme, information on the carbon forms, the inorganic constituents (the latter generally being more abundant in most fly ashes), and the textural features of the char are included. In part, this is based on the same philosophy (13) Baltrus, J. P.; Wells, A. W.; Fauth, D. J.; Diehl, J. R.; White, C. M. Energy Fuels 2001, 15, 455-462.
10.1021/ef010146z CCC: $20.00 © 2001 American Chemical Society Published on Web 08/31/2001
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Energy & Fuels, Vol. 15, No. 5, 2001
Figure 2. (A) Spinel crystals in glassy matrix. Image is about 400 microns on long axis. (B) Spinel (bright crystals) and mullite (bladed and tabular dark crystals) in glass. Image is about 330 microns on long axis.
that underlies a combined maceral/microlithotype analysis. The first part of the descriptor is the type of material, and the second term is a descriptor encompassing the overall association and texture. Both are important, and the proposed scheme gives equal weight to both of the primary systems of description. Alonso et al.14 also gave some consideration to both the optical properties of the carbon and the carbon texture in their investigation of the pyrolysis of a series of coals. An approach to the acquisition of data in the combined system could be the entry of a two-digit number in a spreadsheet. The first number describes the fly ash component,4-12 as follows: 1 2 3 4 5 6 7 8 9 12-
isotropic carbon anisotropic carbon inertinite glass mullite spinel quartz sulfide oxides (Ca, Fe) uncombusted coal petroleum coke
Uncombusted coal and petroleum coke generally do not occur in combination with other components, therefore, they are recorded as single numbers, with no second digit. Other one-digit entries could also be accommodated such (14) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D.; Parra, J. B.; Mene´ndez, R. J. Anal. Appl. Pyrolysis 2001, 58-59, 887-909.
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Figure 3. Examples of neoformed carbons (isotropic and anisotropic char/carbon) and inertinite in the same fly ash particle. Images are about 330 microns on long axis.
as a category for other minerals. The second number describes the texture,1-3 as follows: 1 2 3 4 5 6 7 8 9 0
tenuisphere crassisphere tenuinetwork crassinetwork mixed porous mixed dense inertoid fusinoid solid mineroid
The derived information is, of course, directly translatable into the amount of a certain component within a textural type. The numerical code, therefore, is simply the means to assess the component/texture systematics. As an example of the use of the code, an isotropic char/ coke in a crassinetwork would be assigned a 14 code. The combined classification system yields comprehensive information on the contribution of a fly ash component and its textural arrangement from a single petrographic analysis. This way, compositional characteristics of various char types as well as textural information of the char hosting the individual component are obtained. This, in turn, provides some information that can be related to the surface area of the fly ash (from the contribution of individual components) and also some genetic information inferred from a char type. One potential weakness of the combined system, also a weakness of both individual systems, is the lack of
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information on the position of the phase relative to the surface. A carbon form surrounded by other, potentially different, materials does not contribute to the surface properties of the fly ash. The opposite situation, glass spheres surrounded (in two dimensions) by carbon, is illustrated in Figure 1. Of course, in a polished section, we can only deal in the exposed surface, and a form enclosed in one plane could be at the surface in another plane. To address this problem, a third digit could be added as a descriptor of the apparent contribution to surface properties. The combined system offers limited information on the associations of the inorganic phases. For example, mullite and spinel are often associated with glass phases. The codes, “50” and “60,” respectively, do not tell anything about the other, if any, associated inorganic phases. For example, Figure 2a illustrates a fly ash sphere composed of spinel (primarily magnetite) in a glassy matrix. Figure 2b shows spinel and mullite in a glassy matrix. The latter argument can also be extended to the carbon phases. The “14” code noted above for an isotropic char/carbon in a crassinetwork does not have the ability to describe the complexity of the crassinet-
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work. Is it simply isotropic char, or does it include anisotropic char/carbon and inertinite? Figure 3 illustrates two examples of a mix of carbon types in the same particle. Possibly, we can assume that, during a 500 count petrographic analysis, such problems would be addressed through the course of counting different portions of similar particles. And if a particle is so rare that it is encountered only once or twice out of 500 counts, it is not a major contributor to the surface properties. Fly ash is a complex material and knowledge of its properties is important in the utilization of the material. Although many previous studies have emphasized the carbon components in fly ash, for commercial purposes an optimum fly ash would have a very low carbon content. A classification system which accounts for both the carbon and inorganic portions of the fly ash and accounts for the associations of components within individual particles has a better chance of predicting the behavior of fly ash in industrial applications than do classification schemes describing only the components or only the texture. EF010146Z
