SPECIES AND SOOT CONCENTRATION MEASUREMENTS IN A METHANE AIR NONPREMIXED FLAME DOPED WITH C4 HYDROCARBONS/

The fuel in a co-flowing methane/air nonpremixed flame was doped with 7400 ppm of 1,3-buradiene, l-butene, isobutene, and n-butane. Temperat ure, mole fractions of 24 stable C1 to C12 hydrocarbons, and soot volu me fraction were measured along the centerline of the five resulting f lames. The additives significantly increased the concentrations of aro matic species and of soot. The specific changes could be reasonably ex plained by conversion of the additives to aromatic ring precursors, bu t nor by effects of the additives on temperature and H atom concentrat ions. Isobutene and 1-butene generated the highest concentrations of C 3 products, while 1,3-butadiene produced the highest concentrations of C4 products. However, several pathways readily interconvert C3 and C4 species, such that increases in both C3 and C4 species were observed for almost every additive. Recombination of propargyl radicals (C3H3) appears to be responsible for most aromatic ring formation on the cent erline of these flames. Reactions of acetylene with n-C4H5 and/or n-C4 H3 may also contribute in the 1,3-butadiene-doped flame. Benzene appea rs to be in partial equilibrium with acetylene in the upper half of th e flames. The kinetic processes that establish this equilibrium are mo st likely ring fragmentation reactions in one direction and acetylene addition to n-C,H, in the other. The probable source of the n-C4H3 is acetylene addition to C2H, with the strong temperature-dependence of C 2H concentrations responsible for limiting this process to the upper h alf of the flames. Production of one-ring aromatic species is an impor tant rate-limiting step to soot formation in these flames. (C) 1998 by The Combustion Institute.