Fuel decomposition and hydrocarbon growth processes for oxygenated hydrocarbons: butyl alcohols

Society is shifting towards the utilization of biomass fuels, so the mechanisms that produce toxic byproducts and molecular soot precursors from oxygenated hydrocarbons are of great interest. In the experiments reported here, temperature, C1 to C12 hydrocarbons, and major species were measured in coflowing methane/air flames whose fuel was separately doped with 3500 ppm of each of the four isomers of butanol and the two isomers of butane. The relative decomposition rates of the dopants, two sets of simple kinetic calculations, and the dependence of the observed products on fuel composition all indicate that the dominant decomposition process was ummolecular dissociation, not H-atom abstraction. Two dissociation pathways were important: four-center elimination of H2O to produce butene, and C-C fission followed by beta scission of the resulting radicals to produce alkenes, aldehydes, and ketones. Four-center elimination was dominant for tert-butanol, while C-C fission was dominant for the other butanols and the butanes. The butanols produced much higher concentrations of aldehydes and ketones than the butanes, which suggests that emissions of toxic byproducts may be an issue with oxygenated fuels. All of the dopants increased the benzene concentration, in the order iso-butane similar to tert-butanol > iso-butanol > 2-butanol > 1-butanol similar to n-butane; thus, the presence of a branch in the carbon backbone affected benzene formation much more than the presence of a hydroxyl group. The branched dopants produced more benzene because their dissociations led to more propene and butene relative to ethylene than did the linear dopants. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.