Experimental and modelling studies of the effects of methanol and ethanol addition on the laminar premixed low-pressure n-heptane/toluene flames

摘要

The effects of methanol and ethanol on a laminar premixed low-pressure (30 Torr) n-heptane/toluene flame were investigated by using synchrotron photoionization and molecular-beam mass spectrometry techniques. The overall carbon flux was maintained constant (0.6 g/min) and the equivalence ratio was kept at 2.0 for all the tested flames. Mole fraction profiles of major and intermediate species including most aromatic species were derived and compared among the flames. Simulations were performed based a multi-fuels kinetic mechanism, and the predicted concentrations of most flame species agree well with the measured results. The addition of alcohol showed negligible chemical effect on the hydrocarbon fuels decomposition. Meanwhile the formation of aromatic hydrocarbon intermediates, particularly of those that formed at the later stages of reaction zone, was significantly inhibited. The kinetic study shows that massive HO_2 and OH radicals are induced by alcohol addition at the intermediate-temperature region. Thus, the rate of toluene consumption through hydrogen abstraction increases at this region. For this reason as well as that toluene is partially replaced by alcohol fuels, the toluene consumption rate decreases in the followed high-temperature polycyclic aromatic hydrocarbons (PAHs) formation region, which leads to the more significant decrease in PAHs production rates due to the amplification effect of combination reactions in PAHs formation. Such phenomenon indicates that the introduction of alcohol could significantly restrain the formation of aromatic hydrocarbons.