Formation of Soot Precursors in Diffusion Flames

Abstract

—Soot inception in pyrolysis and combustion occurs via high temperature molecular growth processes. While the subsequent steps of soot surface growth, agglomeration, aggregation and oxidation have been extensively investigated, relatively little knowledge of the soot inception chemistry exists. This kind of molecular level understanding is needed to provide a basis for intervening to mitigate the soot formation process. The present study focused on the molecular precursors of soot in a variety of hydrocarbon/oxygen/nitrogen flames. A Wolfhard-Parker type two-slot burner was used to establish stable vertical laminar diffusion flames. Methane, ethane, ethylene, acetylene and 1,3-butadiene were used as fuels, alone or in mixtures. Quenched probe sampling of the pyrolysis zone on the fuel side of these flames for gas chromatographic analysis shows that the principal stable products are ethane, ethylene, acetylene, methylacetylene, diacetylene, vinylacetylene, butadiene and benzene. The relative amounts of these species vary with the type of fuel molecule used. For example, ethane is a significant intermediate in methane diffusion flames, because of the recombination of methyl radicals present in high concentration. The methyl radicals are also responsible for the relatively high concentrations of methylacetylene observed in methane flames, but not with other fuels. With diffusion flames containing unsaturated molecules in the fuel, sharp increases in the concentration of. acetylenes was observed relative to methane flames. Probing normal to the vertical flame front across steep concentration and temperature gradients, the first visually observed onset of soot formation is at the location of sharp increases in the concentration of molecular intermediates. Methane flames doped with acetylene, 1,3-butadiene and mixtures or these unsaturates show that both are essential for the formation of benzene, the first aromatic soot precursor. These studies, combined with detailed flame structure studies of soot and soot precursor formation should lead to an improved understanding of the chemistry of soot inception.