Effect of the interaction between sodium and soot on fine particle formation in the early stage of coal combustion

Mineral transformation and soot formation are the two governing mechanisms for fine particle formation in the early stage of coal combustion. The interaction between the evaporative alkali species and soot, which can have significant effect on fine particle formation, however, is not fully understood. To investigate the interaction between Na and soot in the early stage of coal combustion, we characterized the particles derived from four coals samples, including acid-washed and Na-loaded samples, under both the pyrolytic and oxidizing atmospheres. It was found that Na can reduce the soot yield and particle size under the pyrolysis condition. The Na-doped soot was formed by flame synthesis of vaporized minerals and soot precursors, and the Na content had a strong effect on soot oxidation activity. The Na-soot interaction was found to reduce the fine particle yield in the early combustion stage, as the high Na concentration and strong sooting tendency of a sodium doped bituminous (YK-Na) resulted in a lower fine particle yield. A schematic of the Na-soot interaction pathways is proposed based on the experimental results. Catalyzing soot oxidation and eliminating ultrafine mineral particles were found to be the two principal ways by which Na-soot interaction affects the formation and evolution of fine particles.