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 3 H 4 and C 4 H 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.

Application of an enhanced PAH growth model to soot formation in a laminar coflow ethylene/air diffusion flame

“…Those first two works were extended to include soot modeling [23,24,25]. Results obtained in [21] agreed well with the experimental data sets for two different ethylene/air flames; one studied first by McEnally and Pfefferle [26] and one by Santoro et al [27]. However, despite overprediction of acetylene and benzene in [23] and "soot precursors" in [24] in the central region of the flame, soot formation was underpredicted by their model in the central region of the flames as well.…”

Sooting tendencies of terpenes and hydrogenated terpenes as sustainable transportation biofuels

“…These radicals only contain five carbon atoms, so they must undergo addition reactions with other species to form benzenoid aromatics, which causes soot production to be slower than in b-OM and other terpenes. This hypothesis is also consistent with our hexadiene study [48], where 1,5-hexadiene produced relatively low concentrations of aromatics but high concentrations of C3 species. It is also consistent with the flame-flame molecular-beam MS (MBMS) study of Bierkandt et al [49], where MC produced relatively large amounts of C5 species.…”

Comprehensive reaction mechanism for n-butanol pyrolysis and combustion

“…The McEnally and Pfefferle studies on doped methane flames, Refs. [4,37,38] among others, have demonstrated that for the small amount of dopant used, the centerline temperature profile exhibits no significant change between the undoped-and doped-flame (Fig. 5).…”