Exploring formation pathways of aromatic compounds in laboratory-based model flames of aliphatic fuels

This presentation summarizes our recent experimental and flame modeling studies focusing on understanding of the formation of small aromatic species, which potentially grow to polycyclic aromatic hydrocarbons (PAHs) and soot. In particular, we study premixed flames, which are stabilized on a flat-flame burner under a reduced pressure of ≈15–30 torr, to unravel the important chemical pathways to aromatics formation in flames fueled by small C₃–C₆ hydrocarbons. Flames of allene, propyne, 1,3-butadiene, cyclopentene, and C₆H₁₂ isomers 1-hexene, cyclohexane, 3,3-dimethyl-1-butene, and methylcyclopentane are analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry. Isomer-specific experimental data and detailed modeling results reveal the dominant fuel-destruction pathways and the influence of different fuel structures on the formation of aromatic compounds and their commonly considered precursors. As a specific aspect, the role of resonance-stabilized free radical reactions is addressed for this large number of similar flames of structurally different fuels. While propargyl and allyl radicals dominate aromatics formation in most flames, contributions from reactions involving other resonance-stabilized radicals like i-C₄H₅ and C₅H₅ are revealed in flames of 1,3-butadiene, 3,3-dimethyl-1-butene, and methylcyclopentane. Dehydrogenation processes of the fuel are found to be important benzene formation steps in the cyclohexane flame and are likely to also contribute in methylcyclopentane flames.