Aromatic hydrocarbon formation in nonpremixed flames doped with diacetylene, vinylacetylene, and other hydrocarbons: Evidence for pathways involving C4 species

We have studied flames doped with diacetylene and vinylacetylene to determi ne directly by experiment whether C4 + C2 reactions can be important source s of aromatics in combustion systems. Measurements were made of gas tempera tures, major species mole fractions, C1 to C12 hydrocarbon mole fractions, and soot volume fractions in methane/air coflowing nonpremixed flames whose fuel was separately doped with diacetylene, vinylacetylene, propadiene, an d acetylene. The dopant concentrations in the unreacted fuel were 5000 ppm or less, which was sufficiently low that the dopants did not affect the fla me temperatures, flame size and shape, or H-atom concentrations; thus chang es in aromatic hydrocarbon concentrations could be directly attributed to r eactions of the dopants and their decomposition products. Diacetylene and v inylacetylene produced larger increases in aromatic hydrocarbon concentrati ons and soot volume fraction than did propadiene or acetylene. Furthermore, the phenylacetylene to benzene mole fraction ratios were higher in the dia cetylene-doped and vinylacetylene-doped flames than in the others. These ob servations and the profiles of individual hydrocarbons indicate that n-C4H3 addition to acetylene and to diacetylene can be important sources of aroma tics in flames where a source of n-C4H3 is readily available. (C) 2000 by T he Combustion Institute.