Decomposition and hydrocarbon growth processes for hexadienes in nonpremixed flames

Abstract

Alkadienes are formed during the decomposition of alkanes and play a key role in the formation of aromatics due to their degree of unsaturation. The experiments in this paper examined the decomposition and hydrocarbon growth mechanisms of a wide range of hexadiene isomers in soot-forming nonpremixed flames. Specifically, C3 to C12 hydrocarbon concentrations were measured on the centerlines of atmospheric-pressure methane/air coflowing nonpremixed flames doped with 2000 ppm of 1,3-, 1,4-, 1,5-, and 2,4-hexadiene and 2-methyl-1,3-, 3-methyl-1,3-, 2-methyl-1,4-, 3-methyl-1,4-pentadiene, and 2,3-dimethyl-1,3-butadiene. The hexadiene decomposition rates and hydrocarbon product concentrations showed that the primary decomposition mechanism was unimolecular fission of C C single bonds, whose fission produced allyl and other resonantly stabilized products. The one isomer that does not contain any of these bonds, 2,4-hexadiene, isomerized by a six-center mechanism to 1,3-hexadiene. These decomposition pathways differ from those that have been observed previously for propadiene and 1,3-butadiene, and these differences affect aromatic hydrocarbon formation. 1,5-Hexadiene and 2,3-dimethyl-1,3-butadiene produced significantly more C 3H 4 and C 4H 4 than the other isomers, but less benzene, which suggests that benzene formation pathways other than the conventional C3 + C3 and C4 + C2 pathways were important in most of the hexadiene-doped flames. The most likely additional mechanism is cyclization of highly unsaturated C5 decomposition products, followed by methyl addition to cyclopentadienyl radicals.