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 distr ibution (PSD) of char and ash formed during combustion. Two coals, Elk Creek (West Virginia) high volatile A bituminous coal and Beulah (Nor th Dakota) lignite, were fired as a standard utility grind pulverized fuel and a coal-water slurry (CWSF) in a down-fired combustor at 316 M J/h in 20% excess oxygen. The relationships of the PSDs of particles i n the pulverized coal and CWSF and of the size distribution of the CWS F droplets to the char and ash samples vary between the Beulah and Elk Creek fuels. For Beulah pulverized coal and CWSF, fuel particle and d roplet size alone does not determine the PSDs of the ashes. The CWSF a sh PSD is coarser than the pulverized coal ash PSD even though both fu els 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 ap proximately 4 times larger than the d50 identified in the CWSF. Howeve r, the coarser PSD of the inorganics in the pulverized coal did not re sult in a coarser ash PSD, while the finer PSD of the CWSF inorganic p hases resulted in a coarser ash PSD. The difference in ash PSDs betwee n the two fuels is attributed to differences in the PSDs and behavior of the mineral matter in the fuels during combustion. For Elk Creek pu lverized coal and CWSF, the ash PSDs are related to the original pulve rized coal and CWSF droplet size distributions. The Elk Creek CWSF dro plet 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 mine ral matter interacts and increases the local temperature experienced b y the inherent mineral matter particles.