Effect of Fuel Particle and Droplet Size Distribution on Particle Size Distribution of Char and Ash during Pilot-Scale Combustion of Pulverized Coal and Coal-Water Slurry Fuels

The objective of this study was to investigate the effect of fuel form—pulverized coal and coal-water slurry fuel—on the particle size distribution (PSD) of char and ash formed during combustion. Two coals, Elk Creek (West Virginia) high volatile A bituminous coal and Beulah (North Dakota) lignite, were fired as a standard utility grind pulverized fuel and a coal-water slurry (CWSF) in a down-fired combustor at 316 MJ/h in 20% excess oxygen. The relationships of the PSDs of particles in the pulverized coal and CWSF and of the size distribution of the CWSF droplets to the char and ash samples vary between the Beulah and Elk Creek fuels. For Beulah pulverized coal and CWSF, fuel particle and droplet size alone does not determine the PSDs of the ashes. The CWSF ash PSD is coarser than the pulverized coal ash PSD even though both fuels have similar PSDs. The CWSF ash shows extensive agglomeration and coalescence while the pulverized coal ash exhibited fragmentation. The d₅₀ value of the inorganic phases in the Beulah pulverized coal is approximately 4 times larger than the d₅₀ identified in the CWSF. However, the coarser PSD of the inorganics in the pulverized coal did not result in a coarser ash PSD, while the finer PSD of the CWSF inorganic phases resulted in a coarser ash PSD. The difference in ash PSDs between the two fuels is attributed to differences in the PSDs and behavior of the mineral matter in the fuels during combustion. For Elk Creek pulverized coal and CWSF, the ash PSDs are related to the original pulverized coal and CWSF droplet size distributions. The Elk Creek CWSF droplet size distribution is coarser than that of the pulverized coal. In turn, the Elk Creek CWSF ash is coarser than the pulverized coal ash. For the Elk Creek CWSF, the ash PSD is related to the coarser droplet size distribution rather than to the original CWSF particle size. The coarser PSD of the CWSF ash is due to the increased number of mineral matter particles, to other mineral particles, and to the carbonaceous portion of the fuel. This would increase the extent to which the mineral matter interacts and increases the local temperature experienced by the inherent mineral matter particles.