Mechanistic Studies of Toluene Destruction in Diffusion Flames

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Abstract
The concentrations of stable species in an atmospheric-pressure, laminar diffusion flame burning a fuel mixture of methane and toluene (1 mole percent) were measured using a microprobe gas sampling/mass spectrometer system. These measurements are compared to results from a flame burning pure methane under identical temperature and flow conditions. In each of the flames studied little or no change is observed in the peak concentrations of a number of major chemical species such as water, carbon monoxide, and carbon dioxide. Small increases occurred in the peak concentrations of hydrogen, acetylene, methylacetylene, vinylacetylene, diacetylene, and butadiene. The presence of increased amounts of these products suggests limited decomposition of the benzyl or phenyl radicals which are likely initial intermediates of aromatic pyrolysis. Relatively larger increases occurred in the peak concentrations of a number or aromatic compounds including phenylacetylene, styrene, and naphthalene. For the toluene flame, the largest absolute differences in peak concentrations were observed for benzene and ethylbenzene. However, a steady state analysis for the concentration of benzyl radical leads to the conclusion that ethylbenzene is most likely formed from the reaction of benzyl and methyl radicals in a highly reversible process. Thus, the dominant toluene destruction pathway appears to be via benzene production. Additional experiments were performed in which the toluene was replaced with benzene, styrene, ethylbenzene or deuterated toluene. Increases in the amount of soot formed in the toluene doped flame are qualitatively observed, A calculation of mass balance for the carbon added to the flame as toluene suggests high conversion efficiency for toluene into soot at heights above 10 mm from the burner surface. The production of soot early in this flame suggests the rapid conversion of aromatic radicals, formed in the initial steps of toluene pyrolysis, into species directly along the growth pathway leading to multiple ring structures and eventually soot particles.