Experimental study of naphthalene formation pathways in non-premixed methane flames doped with alkylbenzenes

To study the naphthalene formation pathways of monoalkylbenzenes that are often present in practical fuels, a nitrogen-diluted non-premixed methane flame was separately doped with approximate to 500 ppm of toluene, ethylbenzene, and the structural isomers of propylbenzene and butylbenzene. Centerline profiles of temperature, stable Cl to C12 hydrocarbons, soot volume fraction, and broadband laser-induced fluorescence were measured in each flame. A significant fraction of the added aromatic rings remained intact and thus provided a foundation for second ring formation. The additives decomposed along two principal routes, which determined the predominant naphthalene formation pathways: when the carbon attached to the phenyl group was secondary (with respect to carbon-carbon bonds), the alkylbenzene rapidly decomposed to form benzyl radical; when the carbon attached to the phenyl group was tertiary or quartary, the decomposition, thermally or through H abstraction, and subsequent a scission resulted in the formation of styrene or methylstyrene. The styrene and methylstyrene further decomposed to phenylacetylene. Naphthalene was formed via two main routes, the H abstraction/C2H2 addition (HACA) route and an alternate route through benzyl radical. The relative importance of these routes was strongly dependent on the primary decomposition products of the additives. For instance, the route through benzyl was found to be of major importance for the normal-alkylbenzenes, all of which form large amounts of benzyl radical. The naphthalene correlated closely with the soot volume fractions and broadband laser induced fluorescence signals. These results confirm that second ring formation is an important rate-limiting step in soot formation in flames fueled with alkylbenzenes.