Soot formation in shock-tube pyrolysis of toluene, toluene-methanol, toluene-ethanol, and toluene-oxygen mixtures

Soot formation during the pyrolysis of argon diluted mixtures of toluene and binary mixtures of toluene-methanol and toluene-ethanol, and during the oxidation of toluene has been studied in a reflected shock tube. Soot induction times and rates of soot formation were measured at 632.8 and 1152.0 nm by a laser beam attenuation method and these showed an Arrhenius dependence on shock temperature. Soot yields and soot amounts were also measured. The soot yield and amount were found to decrease with the addition of methanol and ethanol to toluene, with more pronounced effects for the methanol addition. The addition of oxygen to toluene strongly suppressed soot with a shift of the soot yield to lower temperatures. This last effect was not found during alcohol addition to the toluene and therefore an alternative route to the soot formation at lower temperatures is suggested. A kinetic model was used to interpret the experimental trends and reasonably reproduced the experimental observations. However, the lack of good quantitative agreement emphasised the urgent need in establishing reliable kinetic data and reaction pathways on the oxidation of the benzyl radical and PAH species.