Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flamesy

ion via reaction (647) is dominant, due to its low activation energy, up to 0.4 cm above the burner while reaction (644) becomes dominant farther from the burner. Reaction with OH to give phenol, i.e., reaction (652), does not contribute significantly to benzene depletion while the reverse reaction even leads to benzene formation close to the burner. Benzene formation in the acetylene flame results mainly from propargyl recombination. However, the reverse reaction is a minor consumption pathway at the end of the reaction zone with a maximum rate at 6–7 mm above the burner. Details of benzene formation chemistry in both acetylene and ethylene flames is discussed below. The analysis of benzene net production rates showed that phenyl and to some extent phenoxy radicals are key intermediates in benzene depletion. The chemistry of formation and subsequently depletion of phenoxy and phenol is discussed below. Phenyl: A key intermediate in benzene depletion Phenyl represents a bifurcation between further oxidation, i.e., decay to smaller species and growth to PAH. The complexity of its chemistry has proven to be challenging to model accurately. The contributions of individual reactions to the net phenyl production rate in the premixed benzene flame show that phenyl is formed nearly entirely from benzene via the hydrogen abstraction reactions (647) and (644), discussed above. The reverse of unimolecular hydrogen loss from phenyl to give benzyne, C6H5 Ð benzyneþH; ð636Þ is a minor phenyl formation pathway below 0.65 cm above the burner and beyond 1.1 cm while the contribution of this reaction to phenyl consumption exhibits a maxium at 0.8 cm. Unimolecular decay of phenyl was studied extensively by Madden et al. in an ab initioMO study followed by a RRKM treatment. Their rate constants for the formation of benzyne and linear C6H4 (C6H4 , 1,5-hexadiyn-3-ene) were used in the present work. Hydrogen loss to give linear C6H4 , i.e., C6H5 Ð C6H4 þH; ð637Þ was found to be a major phenyl consumption pathway with a maxium at 0.85 cm from the burner. Phenyl isomerization to 1,3-dien-6-yne radicals, i.e., ring opening, C6H5ðLÞ Ð C6H5; ð631Þ also identified on the potential energy surface by Madden et al., is the most important phenyl consumption reaction in the later part of the reaction zone, i.e., between 0.8 and 1.3 cm from the burner. Its production rates show that linear C6H5 (1,3-dien-6-yne radicals) decays quickly to linear C6H4 (reaction (624)y). The peak mole fraction of linear C6H5 was predicted to be more than three orders of magnitude smaller than that of phenyl. Model predictions compared with the C6H4 mole fraction profile of species measured by MBMS 27 show good agreement in shape and location but the peak value is overpredicted 2.5 fold if the C6H4 is linear or threefold if it is benzyne. The predicted 1,3,5-hexatriyne (triacetylene, C6H2) peak is 80% higher than the corresponding experimental value. Analysis of production rates shows 1,3,5-hexatriyne to be formed by sequential hydrogen release and abstraction beginning with linear C6H4 while C4H2 þ C2H Ð C6H2 þH ð613Þ was identified as major consumption pathway. A similar analysis in the acetylene flame, shows reaction (613) to be a major 1,3,5-hexatriyne formation pathway while sequential hydrogen release/abstraction from linear C6H4 is also significant. Other, minor, phenyl consumption pathways in the later part of the reaction zone of the benzene flame are the reverse