Fuel decomposition and hydrocarbon growth processes for substituted cyclohexanes and for alkenes in nonpremixed flames

Abstract

Cycloalkanes are major constituents of petroleum-based fuels. In the experiments reported here, centerline profiles of C3–C12 hydrocarbons were measured in coflowing methane/air flames whose fuel was separately doped with 2000 ppm each of nine substituted cyclohexanes, cycloheptane, eight isomers of heptene, and three isomers of octene. The decomposition rates of the dopants increased in the following order: cycloalkanes with saturated side-chains < cycloalkanes with unsaturated side-chains ≈ non-cyclic alkanes < alkenes. This order and the absolute magnitude of the decomposition rates could be reproduced with the assumption that unimolecular dissociation was the dominant decomposition process for all of the dopants. The observed hydrocarbon products indicated that the specific dissociation pathways were allylic C–C fission plus six-center elimination for the alkenes, and ring-opening isomerization to alkenes for the cycloalkanes. All of the cycloalkanes produced similar distributions of aromatic hydrocarbons, which shows that they formed aromatics by addition reactions between their aliphatic decomposition products, not by direct dehydrogenation of their alkyl rings. Consequently, methylcycloheptane and cycloheptane produced the same concentrations of aromatics as 1-heptene and 2-heptene, which are their immediate decomposition products. Dimethylcyclohexanes produced more benzene than methylcyclohexane, and cyclohexanes with unsaturated side-chains (vinylcyclohexane, ethynylcyclohexane, and ethylidenecyclohexane) produced even more. The benzene production from methylcyclohexane was comparable to that from the most branched heptane isomer (2,3,3-trimethylbutane), which indicates that the cycloalkane portion of practical fuels will have a proportionately greater effect on soot production than the non-cyclic alkane portion.