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 particle size distribution (PSD) of mineral matter in coal on the particle size distribution of ash produced during firing of two coals in pulverized coal and coal-water slurry forms. The coals used in this work were Beulah (North Dakota) lignite and Elk Creek (West Virginia) high volatile A bituminous coal. Combustion experiments were performed in a pilot-scale 316 MJ/h down-fired unit with 20% excess air. The dominant mechanism of ash formation in the Beulah pulverized coal was fragmentation 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 mechanism for determining the ash particle size in the Beulah coal-water slurry fuel (CWSF) was coalescence and agglomeration of the inorganic portion of the fuel (225% increase in d₅₀ of mineral matter). The size distribution and occurrence of inorganic matter in the fuels were the most 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 during combustion. The PSDs of ashes from both pulverized and slurried Elk Creek coal are coarser than the original mineral matter, due to coalescence of inherent aluminosilicates and silicates during combustion. The particle size of the Elk Creek coal-water slurry fuel ash is slightly coarser than that from pulverized coal, due to the larger agglomerate formed upon atomization of the Elk Creek slurry. Atomization quality was the most important factor in determining the particle size of the ash. Subsequent papers will discuss the chemical interactions among the inorganic components during combustion of these fuels.