Dehydrogenation and growth of soot in premixed flames

Abstract

Dehydrogenation of soot formed in premixed sooting flames burning methane and ethylene in similar equivalence ratio and temperature conditions was studied in detail by measuring the type of C–H bonds, as derived from a newly developed FT-IR quantitative method, and the soot absorption coefficient. The measurement of the aliphatic and aromatic hydrogen content of soot has shown to give useful insights in the different dehydrogenation and soot mass growth processes observed for the methane and ethylene flame. In the ethylene flame, aliphatic hydrogen is preferentially and almost completely removed in the early soot formation region and the reduction of soot formation rate was observed after the aliphatic hydrogen decrease. This can be attributed to the decrease of radical sites formation and the consequent loss of surface reactivity toward further carbon addition. Downstream of the soot formation region, coagulation and thermal annealing of the ethylene soot particles, accompanied by a negligible dehydrogenation, become the predominant phenomena. The corresponding steep rise of the absorption coefficient is a signature of the increase of aromatic structures interconnected by conjugated sp2 bonds, marking the end of soot formation and growth process. In the methane flame, the decrease of aliphatic hydrogen was found to occur to a lower extent and delayed in respect to the maximum soot formation rate. The reduced soot dehydrogenation observed in the methane flame was attributed to the lower [H]/[H2] ratio, as testified by the larger presence of molecular hydrogen, which reduces the number of radical sites necessary for soot dehydrogenation and aromatic growth. In this condition, the later phases of coagulation and internal structural carbon rearrangements did not significantly occur, as demonstrated by the low value and steadiness of the methane soot absorption coefficient.