Aggregate formation from vaporized ash during pulverized coal combustion

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The mechanisms governing the formation of inorganic aerosols during coal combustion have been investigated by studying the aerosol produced during the combustion of coals and synthetic chars in a laminar flow laboratory furnace. Three fundamental processes which determine the final ash particle size distribution have been identified. (1) The nucleation and coalescence of submicron primary ash particles occurs entirely within a boundary region surrounding the burning char particle. This is demonstrated by the combustion of a mixture of carbon particles impregnated with either iron or magnesium salts, which produced ash particles consisting entirely of either pure iron oxide or pure magnesium oxide. (2) Large clusters (0.3–0.6 μm aerodynamic diameter) of these submicron primary particles are formed in a high concentration region near the burning char surface. This is explainable by a convective diffusion model which accounts for reduction in ash particle diffusivity as the size increases. (3) from 200 to 500 micrometer sized ash particles are generated per coal particle, as a consequence of both shedding of fused ash inclusions and char fragmentation during combustion.