Effects of Na and Fe on the formation of coal-derived soot in a two-stage flat-flame burner

Abstract

Coal-combustion-generated soot can be a source of particulate emissions and significantly affects the temperature profile inside the practical boiler. In this work, we experimentally investigated the effects of sodium (Na) and iron (Fe) on soot formation in pulverized coal combustion. Two kinds of coal samples with similar properties were used, with one rich in Na while the other rich in Fe. The water washed sample for each coal was ultilized to further explore the mineral effects. A novel two-stage flat-flame burner was designed to flexibly simulate the reducing-to-oxidizing atmosphere which a real coal particle stream undergoes in the boiler. The ex-situ sampling indicates the existence of mineral species, including Na and Fe, doped in the mature soot agglomerates. In-situ Laser-Induced Incandescence (LII) diagnostics, incorporated with on-line measurements of the particle size distribution (PSD), gives quantitative concentrations of primary soot particles. The experiment results show a reduced soot yield after water washing for both coal samples, elucidating a promoting effect of Na on soot formation in pulverized coal combustion. A similar effect is inferred for Fe, since the water washed Fe-rich coal produces more soot than the water washed Na-rich coal even though their residual Na contents in ash are close. It is postulated that both Na and Fe form nano-sized particles at the early stage of coal combustion and serve as effective nuclei for the soot precursor, the tar. Besides, the catalytic effect of Na on soot oxidation is confirmed because of the significant soot reduction in the combustion of Na-rich coal when the oxygen mole fraction in the inner region increases.