Influence of char fragmentation on ash particle size distributions

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The influence of char fragmentation on the size distribution of combustion-generated ash has been investigated in a detailed laboratory study utilizing both coals and synthetic chars. Combustion of three different coal types resulted in production of ash particles in the 1–10-μm size range at all conditions considered (1500 and 1750 K, 8% O 2 and above). The amount of ash in the 1–5-μm range was relatively invariant with combustion conditions, and typically represented 10%–20% of the total ash mass, which greatly exceeds the mass predicted from a model assuming that one ash particle is produced per char particle. The macroporosity of the char was found to be the important variable in determining fragmentation behavior, as evidenced by results obtained from combustion of synthetic chars. Macroporous Spherocarb doped with sodium silicate yielded 75 ash particles greater than 1 μm in diameter per char particle, whereas nonmacroporous sucrose carbon black chars doped with sodium silicate yielded only 1 ash particle per char particle. A site percolation model was developed to simulate the process of fly ash formation during pulverized coal combustion. Modeling results, utilizing a monodisperse mineral grain size distribution for the cases of 0% and 10% porosity, qualitatively predicted the features of experimentally observed ash particle size distributions obtained from combustion of synthetic chars.