Sensitizing effect of acetylene and 1,3-butadiene on toluene oxidation in laminar flames

Fuel surrogates are composed of a limited number of different hydrocarbons and designed to mimic the combustion properties of real-world fuels. Toluene is often a major component of such surrogates. Consequently, the main oxidation pathways of toluene have been investigated intensively and many reaction mechanisms are available in the literature. Here, major intermediates of toluene consumption in low-pressure laminar flames are identified by comparison with mass-resolved threshold photoelectron spectra measured by photo electron photoion coincidence spectroscopy at the Swiss Light Source. The measurements confirm the main consumption pathways of toluene in the literature. In fuel surrogates, the oxidation chemistry of toluene interacts with the oxidation chemistry of other hydrocarbons in the mixture. The interaction is particularly important in the flame front where the fuel is consumed at low temperatures and fuel conversion is initiated. In toluene flames doped with 1,3-butadiene or acetylene investigated here, an acceleration of toluene conversion by the low-temperature reactions of the additive is observed experimentally for both flames. The observations are explained by simulations of the flame. For 1,3-butadiene, typical low-temperature oxidation reactions, e.g., the formation of peroxide intermediates, form small flame radicals that play an important role for the acceleration. For acetylene, hydrogen-addition to the fuel leads to the formation of vinyl radicals and the subsequent oxidation sequences of the vinyl radicals release further small radicals. The oxidation of acetylene also increases the temperature and accelerates toluene consumption at an earlier stage in the flame than without acetylene. Remarkable and counterintuitive for the design of fuel surrogates is the fact that the acceleration mechanism of acetylene seems to be more effective than that of 1,3-butadiene, which is reflected in faster toluene consumption in the acetylene/toluene flame front than in the 1,3-butadiene/toluene flame front, while a neat toluene flame shows the slowest consumption of toluene.