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Communications An Approach Toward a Combined Scheme for the Petrographic Classification of Fly Ash: Revision and Clarification James C. Hower,*,† Isabel Sua´rez-Ruiz,‡ and Maria Mastalerz§ University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511, Instituto Nacional del Carbo´ nsINCAR (CSIC), Ap. Co., 73, 33080 Oviedo, Spain, and Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, Indiana 47405-2208 Received August 3, 2004. Revised Manuscript Received November 3, 2004 Earlier, Hower and Mastalerz1 proposed a classification scheme for fly ash, incorporating the genetic classification that was previously in use at the University of Kentucky Center for Applied Energy Research2 and the char textural classification that was developed by Bailey et al.,3 Lester et al.,4 and Alvarez et al.5 The system has not yet been widely adopted, in part because of the detailed description that is required. For example, studies by Mardon and Hower6 and Mastalerz et al.7 used the genetic classification, and studies by Vassilev and co-workers8-10 used the textural classification. Fly ash petrography, although not a fundamental part of many studies of coal combustion byproducts, does have the potential to enhance the understanding of the utilization or disposal properties of the ash. For example, in fly ash used as a replacement for Portland cement, a char encasing glassy particles will effectively increase the amount of carbon (loss on ignition) in the mix, because the glass particles are blinded from potential cementitious reactions. To make the system widely acceptable, some shortcomings of the Hower and Mastalerz1 system must be addressed. First, they did not explore char origins from higher-rank coals (specifically anthracites). Second, they did not sufficiently address the origin of char types, with respect to the maceral precursors.
Table 1. Fly Ash Classificationa identification Genetic Classification isotropic carbon/char (derived from vitrinite or inertinite) anisotropic carbon/char (derived from vitrinite or inertinite) inertinite glass mullite spinel quartz sulfide (rare, but will occur with uncombusted coal) sulfate oxides and hydroxides (Ca, Fe) uncombusted coal (partially melted/ partially oxidized vitrinite and/or liptinite and inertinite in association with the latter association) petroleum coke other organic other inorganic
1 2 3 4 5 6 7 8 9 0 1234-
1 2 3 4 5 a
* Author to whom correspondence should be addressed. Telephone: 859-257-0261. E-mail address:
[email protected]. † University of Kentucky Center for Applied Energy Research. ‡ Instituto Nacional del Carbo ´ n-INCAR. § Indiana University. (1) Hower, J. C.; Mastalerz, M. Energy Fuels 2001, 15, 1319-1321. (2) Hower, J. C.; Rathbone, R. F.; Graham, U. M.; Groppo, J. G.; Brooks, S. M.; Robl, T. L.; Medina, S. S. In International Coal Testing Conference, 11th, Lexington, KY, May 10-12, 1995; pp 49-54. (3) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225-239. (4) Lester, E.; Cloke, M.; Allen, M. Energy Fuels 1996, 10, 696-703. (5) Alvarez, D.; Borego, A. G.; Mene´ndez, R. Fuel 1997, 76, 12411248. (6) Mardon, S. M.; Hower, J. C. Int. J. Coal Geol. 2004, 59, 153-169. (7) Mastalerz, M.; Hower, J. C.; Drobniak, A.; Mardon, S. M.; Lis, G. Int. J. Coal Geol. 2004, 59, 171-192. (8) Vassilev, S. V.; Menendez, R.; Borrego, A. G.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Fuel 2004, 83, 1563-1583. (9) Vassilev, S. V.; Vassileva, C. G.; Karayigit, A. I.; Bulut, Y.; Alastuey, A.; Querol, X. Int. J. Coal Geol. 2005, 61, 35-63. (10) Vassilev, S. V.; Vassileva, C. G.; Karayigit, A. I.; Bulut, Y.; Alastuey, A.; Querol, X. Int. J. Coal Geol. 2005, 61, 65-85.
identification
identification
Textural Classification tenuisphere 6 crassisphere 7 tenuinetwork 8 crassinetwork 9 mixed porous 0
mixed dense inertoid fusinoid solid mineroid
Modified after ref 1.
Certain fly ashes are the result of the combustion of a wide range of coal ranks. Hower and Mastalerz1 only focused their classification on low- and medium-rank coals, because the coal ranks used throughout the midwestern United States range from subbituminous to low volatile bituminous. Medium volatile and low volatile bituminous coals are not common components of combustion blends in the Ohio River Valley, meaning that fly ashes in the region from which they drew their examples are primarily from subbituminous and high volatile bituminous ranks. In contrast, combustion blends in Spain, for example, are frequently a combination of anthracite and bituminous coals, both imported and locally mined. The purpose of this note is to enhance the classification scheme (Table 1), to take into account other criteria.
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Figure 1. (a) Anthracitic vitrinite-derived anisotropic char. (b) Semi-anthracitic vitrinite-derived anisotropic char. (c) Semi-anthracitic vitrinite-derived anisotropic char (center), with included inertinite, with anthracitic vitrinite-derived anisotropic char (upper left and bottom center). (d) Anthracitic inertinite-derived anisotropic char. All photomicrographs were taken under the following conditions: reflected light, oil immersion, crossed polars, λ plate inserted, and picture width of 350 µm along the long axis.
For ashes derived from anthracitic coals and blends of varying coal ranks, the fly ash classification requires revision. It is not sufficiently open to include all types of unburned chars or fly ashes generated by all types of coals. The inclusion of anthracite poses special problems in identifying the transition between the coal and the fly ash char and in identifying the source macerals. For example, anisotropic chars (Figure 1a-c) are attributed to anthracitic vitrinite sources. Further confounding expectations, inertinite-derived anisotropic chars are noted among fly ash chars (Figure 1d). Anthracite-derived chars can take the form of both isotropic and anisotropic chars, as also seen in fly ash chars from bituminous coals. Even though anthracite-rank coals would not be expected to display much, if any, thermoplastic properties, some vesiculated chars have been observed in chars from anthracites. Indeed, porous chars have been noted in ashes from the combustion of meta-anthracites, quite contrary to the expected behavior of high-rank coal. The Hower and Mastalerz1 classification system must be clarified, to reflect the derivation of isotropic and anisotropic chars from a wider rank range than originally anticipated. At one level, this is not really a problem, because the genetic classification does not necessarily rely on knowing the original coal rank, just the final form of char in the fly ash. On the other hand, the presence of anthracite-derived thermoplastic chars does inhibit the use of the classification system for determining the rank origin of fly ash chars (for example, deciding whether one rank in a mixed-rank blend is a bigger contributor to the unburned char). One approach to the resolution would be the study of a series of ashes from blends of known ratios of bituminous/anthracites blends, with tight control of the source, as done by Mardon and Hower6 and Sakulpitakphon and co-workers11,12 for plants burning single-source high volatile A bituminous coals. In this manner, if the different ratios of feed coals produce different mixes of fly ash chars, we could begin to understand the differentiation between chars from different rank sources. There are also problems with the fate of inertinite. In the genetic classification, it might be inferred that isotropic and anisotropic chars are derived from vitrinite and the fly ash inertinite is derived from the coal (11) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Energy Fuels 2000, 14, 727-733. (12) Sakulpitakphon, T.; Hower, J. C.; Schram, W. H.; Ward, C. R. Int. J. Coal Geol. 2004, 57, 127-141. (13) Nandi, B. N.; Brown, T. D.; Lee, G. K. Fuel 1977, 56, 125-130. (14) Shibaoka, M. Fuel 1985, 64, 263-269. (15) Vleeskens, J. M.; Mene´ndez, R. M.; Roos, C. M.; Thomas, C. G. Fuel Process. Technol. 1993, 36, 91-99.
inertinite. Although the latter is true, it is not fully descriptive of all inertinite. In fly ash from high volatile bituminous coals, isotropic fused/unfused particles can be derived from inertinite. Nandi et al.13 noted (on p 126) that “... it has been observed by the authors that the so-called inert macerals either do not burn or burn less readily than the reactive macerals.” Shibaoka14 found that, although some inertinite does not form vesicles, vesicular char is formed in a plastic state from vitrinite and some inertinite (as demonstrated in some inertinite-rich coals), possibly from lowreflectance inertinite (semifusinite). Fusinite formed a dense char. Vleeskens et al.15 tested inertinite-rich coals in the 1-MW combustor at KEMA, which is located in The Netherlands. They found that some inertinite-derived chars are dense, in contrast to porous vitrinite- and reactive-semifusinite-derived chars. If inertinite-derived chars have a slower burnoff than vitrinite-derived chars, then their percentage would increase in the later stages of burnoff. Experiments demonstrated that semifusinite- and fusinite-derived dense chars have similar burn-out rates, up to 95% burnout, at which point fusinite-derived chars burn off at a slower rate. The percentage of unburnt char is not consistent between coals of similar inertinite content. Indeed, the concentration of inertinite-type chars in fly ash is not a function of combustion efficiency; rather, particle size and coal rank are also factors, potentially confused with the percentage of inertinite. In consideration of the latter studies, we must note that, although fly ash inertinite is likely derived from inertinite in the coal, it cannot be said that isotropic and anisotropic chars are derived strictly from vitrinite. Certainly, some inertinitessmost likely those among the semifusinite populationscontribute to the neoformed chars. Overall, we must consider this topic to be open and flexible. Only minor numerical changes to the previous system are required, because of the caveats discussed previously. What has been amended is the understanding of the breadth of the source materials that can potentially form the fly ash entities. Proper consideration has now been given to the potential role of anthracite in the development of isotropic and anisotropic chars. The role of low-reflectance inertinite is producing vesicular chars is also noted. Noncoal chars in the fuel can potentially produce chars that have the potential to stretch the limits of the classification. For example, with care, it is possible to classify certain biomass chars as being distinct from coal-derived chars. Acknowledgment. A portion of this work was performed with financial support from the Spanish Minis-
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terio de Ciencia y Tecnologı´a PN I+D+I (Project PPQ 2001-2359 CO2-02).
1a refers to anisotropic char, rather than isotropic char.) The corrected paper was posted 2/11/2005.
Note Added after ASAP Publication. The version published on the Web 1/15/2005 contained errors. (Figure
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