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Energy & Fuels 1993, 7, 1030-1038
Effect of the Occurrence and Composition of Iron Compounds on Ash Formation, Composition, and Size in Pilot-Scale Combustion of Pulverized Coal and Coal-Water Slurry Fuels Sharon Falcone Miller and Harold H. Schobert' Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 4, 1992. Revised Manuscript Received July 12,1993@
Two coals, Beulah (North Dakota) lignite and Elk Creek (West Virginia) high-volatile A bituminous, were burned in both pulverized coal and coal-water slurry fuel forms to study the effect of the modes of occurrence and composition of iron compounds in the coal on the particle size distribution and composition of ash. The slurry preparation process appeared to cause a significant reduction in the particle size distribution of pyrite in the lignite, relative to the pulverized coal. This in turn caused a change in the dominant mechanism of ash formation. In the Beulah pulverized coal, pyrite fragmentation is the major process, forming submicrometer- or micrometer-sized iron oxide particles. In the slurry, however, coalescence and agglomeration, facilitated by the fluxing action of iron incorporated into aluminosilicates, dominate ash formation. This behavior could not have been predicted by relying solely on elemental composition data without the supplementary information on mineral matter particle size distribution in the fuels. The Elk Creek fuels provide a useful contrast. In this case both the composition and particle size distributions of the mineral matter in the two fuels are quite similar, and the same ash formation mechanism, coalescence, dominates in both cases. The principal cause of the differences in ash particle size distribution is the formation of a coal particle agglomerate during atomization of the Elk Creek slurry.
Introduction In order to understand ash formation during the combustion of coal, it is necessary to characterize the way in which inorganic species occur in the coal. Inorganics can be incorporated in several ways: as ion-exchangeable cations, as coordination complexes, and as discrete minerals.' The way in which a particular element reacts depends on whether it is present as a cation or in mineral matter. The complexity of ash formation processes is increased because a given element can exhibit several types of behavior or can occur in several forms. Calcium, for example, can be found both as ion-exchangeable cations and in mineral matter. Therefore, it is important to know the distribution of inorganics in the coal to understand their role in ash formation. Mineral matter inclusions usually account for the formation of 1-30-pm ash particles.2 Larger ash particles form due to coalescence of mineral particles on the receding char surface. Cations are primarily responsible for the formation of submicron particles, by vaporization and condensation or nucleation of vaporized cations. Sulfation of particles may also occur. The major processes that determine the particle size distribution (PSD) of the ash are nucleation and coalescence of submicron particles within the boundary layer surrounding the burning char; coalescence of mineral matter near the burning char surface, and shedding of fused ash inclusions and char fragmentation.3 Abstract published in Aduance ACS Abstracts, September 15,1993. (1)Falcone, S. K.;Schobert, H. H. InMineraZMatter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Societv: Washineton, DC. 1986: Chapter 9. (2) Neville, M.; Sarofim, A. F. Fuel 1985, 64, 384. (3) Helble, J. J.; Neville, M.; Sarofim, A. F. Symp. (Int.) Combust., [Roc.], 21 1986, 411.
The PSD of ash is an important parameter in determining its behavior in the combustion process. The size of an ash particle will determine, in part, how it passes through, or is deposited in, a combustor. We have shown previously that the PSD of mineral matter originally present in the coal has a role in determining the PSD of the resulting ash.4 However, the development of a complete understanding of ash formation processescannot rely on a knowledgeonly of the PSD of the mineral matter, because both the PSD and composition of the ash can be affected by chemical interactions among the various inorganic components. The PSDs of ashes from combustion of Beulah (North Dakota) lignite as pulverized coal and as a coal-water slurry fuel (CWSF) were not determined solely by the PSDs of the mineral matter in the two fuels; although the PSD of the mineral matter in the CWSF was finer than that of the pulverized coal, the PSD of the CWSF ash was coarser than the pulverizedcoal ash.4 This suggests that chemical, as well as physical, processes determine the PSD of ash and led us to examine the chemical behavior of several of the major inorganic components as it relates to ash PSD and composition. Although ash deposition is not within the scope of the present study, it is important to bear in mind that deposition is governed not only by the ash PSD but also by the composition of the ash and chemical interactions among ash components. The elements occurring in coal as organically bound cations are primarily, but not exclusively,the alkali metals and alkaline earth metals, associated with carboxylate groups in the coal structure. Minerals identified in coals (4) Miller,
S. F.; Schobert, H. H. Energy Fuels 1993, 7, 520.
0887-0624/93/2507-1030$04.00/00 1993 American Chemical Society
Composition of Iron Compounds include silicates, oxides, hydrated oxides, carbonates, sulfides, sulfates, phosphates, and chlorides; at least 50 different minerals have been identified.& In the present paper we focus on the behavior of iron compounds in ash formation processes. Iron is perhaps best known as being present in coals as pyrite, FeSz, and the carbonate mineral siderite, FeC03. In addition, some iron can occur on ionexchange sites or coordinated to heteroatoms in the coal structure. The behavior of mineral matter during combustion depends on whether it is inherent or extraneous in nature. Inherent mineral matter is intimately mixed with the coal such that its thermal history during the combustion process is determined by the coal particle combustion. Generally, organically bound inorganics and finely disseminated minerals that are not removed by pulverizing to a "utility grind" (70% 40 pm during bench scale combustion of the Beulah lignite has been reported.10 Increased pyrite fragmentation was observed with increased oxygen levels in the gas stream. Fragmentation of pyrite in Illinois No. 6 bituminous coal in a laminar flow reactor reduced the average particle diameter by a factor of 1.8 after 47 ms.24 In this study, pyrite fragmentation also produced iron-containing particles with diameters approximately 1.8 times smaller than in the original coal when the Beulah pulverized coal was fired at 19.1 kg/h. Results from this study verify the application of observed mineral behavior in small-scale combustors to larger scale facilities. Some larger inherent pyrite particles also fragment during oxidation as they transform to p y r r h ~ t i t e . ~In that work, iron was subsequently released as 0.02-0.2-pm iron oxide particles. Finely disseminated (
