Exploring Old and New Benzene Formation Pathways in Low- Pressure Premixed Flames of Aliphatic Fuels

A modeling study of benzene and phenyl radical formation was performed for three low-pressure premixed laminar flat flames having an unsaturated C2 or C3 hydrocarbon fuel (acetylene, ethylene, and propene). Predictions using three published detailed elementary-step chemical kinetics mechanisms were tested against molecular beam mass spectrometry (MBMS) species profile data for all three flames. The differences between the predictive capabilities of the three mechanisms were explored, with an emphasis on benzene formation pathways. A new chemical kinetics mechanism was created combining features of all three published mechanisms. Included in the mechanism were several novel benzene formation reactions involving combinations of radicals such as C2H C4H5, C2H3 C4H3, and C5H3 CH3. Reactions forming fulvene (a benzene isomer) were included, such as C3H3(propargyl) C3H5(allyl), as well as fulvene to benzene reactions. Predictions using the new mechanism showed virtually all of the benzene and phenyl radical to be formed by reactions of either C3H3 C3H3 or C3H3 C3H5, with the relative importance being strongly dependent on the fuel. C5H3 CH3 played a minor role in fulvene formation in the acetylene flame. The C2Hx C4Hx reactions did not contribute noticeably to benzene or phenyl radical formation in these flames, sometimes being a major decomposition channel for either fulvene or phenyl radical. The formation pathways for C3H3 and C3H5 were delineated for the three flames; although the key reactions differed from flame to flame, CH2 C2H2 ⇔ C3H3 H was important for all three flames.