The use of carbon-13-labeled fuel dopants for identifying naphthalene formation pathways in non-premixed flames

Centerline profiles of C3 to C12 hydrocarbons were measured in a nitrogen-diluted methane/air cof-lowing non-premixed flame whose fuel was separately doped with 1700 ppm of carbon-13-labeled styrene. toluene, and benzene. The outer carbon on the vinyl side chain was labeled in the added styrene. Large concentrations of singly labeled phenylacetylene and naphthalene were observed in the styrene-doped flame, providing direct experimental evidence under real flame conditions for the conversion of styrene to phenylacetylene by dehydrogenation of the side chain, and of phenylacetylene to naphthalelle by thr H-abstraction/carbon-addition (HACA) mechanism. The methyl carbon in the added toluene was labeled. Large concentrations of singly labeled naphthalene were observed in the toluene-doped flame and were greatly out of proportion to the increase in singly labeled phenylacetylene, providing direct experimental evidence for an additional naphthalene formation pathway involving reactions of die benzyl radical. All six carbons in the added benzene were labeled. Large concentrations of hextuply labeled naphthalene were observed in the benzene-doped flame, consistent with the HACA and benzyl naphthalene formation mechanisms. No 10-fold labeled naphthalene was observed, indicating that conversion of die added benzene to cyclopentadienyl radical followed by cyclopentadienyl self-reaction was not a significant source of naphthalene production in that flame.