Characterization of submicron fly-ash in cyclone-fired boilers

Abstract

This study contains information concerning the submicron fraction of fly-ash produced by cyclone-fired boilers. Experimental activities involved size fractionation and chemical analyses of separated ash particles. Approximately 10% of the fly-ash under investigation consists of particles in the submicron range. The chemical composition of the submicron fraction is considerably different from that of the bulk. In particular, silicon and sulfur are enriched while iron and aluminum are depleted. Calcium shows slight possible enrichment trends while sodium, potassium and magnesium exhibit constant concentrations for all size ranges. Silicon enrichment is consistent with equilibrium predictions which indicate significant vaporization of this element under reducing conditions. The sulfur trends are attributed to the surface adsorption of gaseous sulfur-containing species. Enrichment of iron is also predicted whereas the opposite is found experimentally. The morphology of submicron ash was examined extensively with the aid of the scanning electron microscope. Submicron ash can be approximately visualized as aggregates, each containing an average of 10 or so primary particles. The primary particles themselves are about 0.06 micron in diameter. This study, intended as an aid to the theoretical development, has provided basic data on the particle size versus enrichment characteristics of fly-ash in a cyclone-fired boiler. Inmore » general, the vaporization condensation mechanism is further confirmed as a major source of submicron fly-ash.« less

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