EFFECT OF MINERAL MATTER PARTICLE-SIZE ON ASH PARTICLE-SIZE DISTRIBUTION DURING PILOT-SCALE COMBUSTION OF PULVERIZED COAL AND COAL-WATER SLURRY FUELS

The objective of this work was to investigate the effect of the partic le size distribution (PSD) of mineral matter in coal on the particle s ize distribution of ash produced during firing of two coals in pulveri zed coal and coal-water slurry forms. The coals used in this work were Beulah (North Dakota) lignite and Elk Creek (West Virginia) high vola tile A bituminous coal. Combustion experiments were performed in a pil ot-scale 316 MJ/h down-fired unit with 20% excess air. The dominant me chanism of ash formation in the Beulah pulverized coal was fragmentati on of mineral particles, specifically pyrite, resulting in a finer ash particle size distribution than that of the original mineral matter ( 62% reduction in d50 of mineral matter). By contrast, the main mechani sm for determining the ash particle size in the Beulah coal-water slur ry fuel (CWSF) was coalescence and agglomeration of the inorganic port ion of the fuel (225% increase in d50 of mineral matter). The size dis tribution and occurrence of inorganic matter in the fuels were the mos t important factors in determining ash size. Differences in pyrite PSD and occurrence between the two fuels were significant in determining the dominant mechanism for ash formation. The CWSF preparation process resulted in a significant reduction in the pyrite PSD and removal of organically bound sodium from the CWSF. The reduction in sodium in the CWSF did not significantly reduce the coalescence of ash particles du ring combustion. The PSDs of ashes from both pulverized and slurried E lk Creek coal are coarser than the original mineral matter, due to coa lescence of inherent aluminosilicates and silicates during combustion. The particle size of the Elk Creek coal-water slurry fuel ash is slig htly coarser than that from pulverized coal, due to the larger agglome rate formed upon atomization of the Elk Creek slurry. Atomization qual ity was the most important factor in determining the particle size of the ash. Subsequent papers will discuss the chemical interactions amon g the inorganic components during combustion of these fuels.