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

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.