Effect of the Occurrence and Modes of Incorporation of Alkalis, Alkaline Earth Elements, and Sulfur on Ash Formation in Pilot-Scale Combustion of Beulah Pulverized Coal and Coal‒Water Slurry Fuel

Beulah (North Dakota) lignite was fired in both pulverized and coal-water slurry form to study the effect of fuel form and rank on ash formation processes. The fuels were burned in a down-dired pilot-scale combustor at 316 MJ/h. In both of the Beulah fuels, organically bound sodium was important in the formation of micrometer and submicrometer sodium sulfate particles and coatings on larger silicate and aluminosilicate particles. The presence of sulfates indicates the importance of sulfur fixation by alkaline elements during combustion. Organically bound calcium is highly reactive within the char particle, participating in the formation of mixed aluminosilicates during char burnout; calcium is not associated with sulfur and does not appear to react outside of the char particle. The form in which the Beulah coal was fired, i.e., pulverized or as a CWSF, had no appreciable effect on the behavior of the alkalis and sulfur during combustion.