Effects of C₁-C₃ hydrocarbon blending on aromatics formation in 1-butene counterflow flames

We present a combined experimental and numerical analysis to provide insights into the chemical mechanisms that are responsible for the formation of one- and two-ring aromatic species under non-premixed conditions. To this end, sampling experiments in a 1-butene counterflow flame were conducted via time-of-flight molecular beam mass-spectrometry. The experimental dataset, which consists of 26mole fraction profiles of C₁ to C₁₂ hydrocarbons, was used to trace the formation of aromatic structures starting from the early-fuel decomposition stages. In additional experiments, the underlying fuel-dependent molecular-weight growth chemistry was identified by enhancing specific radical-radical and radical-molecule reactions via the partial substitution of 1-butene with methane, acetylene, and propene. Support for the interpretation of the experimental results was provided by the analysis of the formation pathways of aromatic species predicted by two established chemical mechanisms. By critically combining the information obtained from the measured relative changes in species concentrations and numerical simulations, the relevance of different reaction pathways was determined. Together with propargyl, the allyl radical was found to play a significant role in benzene formation and a direct dependence of toluene, ethylbenzene, and styrene on the benzene amount was observed. The less-studied propargyl addition to benzyl radicals was found to be the main contribution to naphthalene production under the investigated conditions. While species such as phenylacetylene (C₈H₆) and the −C₂H substituted naphthalene (C₁₂H₈) showed an expected increase upon the addition of acetylene, also indene formation exhibits a dependence on acetylene chemistry. Formation pathways involving sequences of acetylene and methyl addition to aromatic radicals were identified as the responsible reactions for these trends. While the influence of the specific fuel chemistry on the formation of the second aromatic ring is confirmed by the analysis of these new measurements, the final inclusion of additional datasets acquired in previous works provides indications that such a dependence might progressively decay for three- and four-ring aromatics.