Kinetic Effects of Methanol Addition on the Formation and Consumption of Formaldehyde and Benzene in Premixed n-Heptane/Air Flames

A numerical investigation was conducted to explore the kinetic effects of methanol addition on the formation and consumption of formaldehyde and benzene in premixed stoichiometric n-heptane/air flames at atmospheric pressure. The flame modeling was performed by solving the premixed flame model with a comprehensive kinetic scheme of hydrocarbon fuels. We studied the species distributions, formation temperatures, temperature sensitivities, reaction contributions, and the rates of production and consumption for formaldehyde and benzene. Results showed that formaldehyde and benzene were produced in two temperature zones and the accumulation effect in the low-temperature zone was the most important factor for the peak concentrations of them in flames. When methanol was added into n-heptane/air flames, cross-reactions were hardly found in the formation routes of formaldehyde and benzene. Both the increased peak concentration and the decreased formation temperature of formaldehyde were primarily attributed to the fact that CH₃O (+M) <=>CH₂O + H (+M) and CH₂OH + O₂<=>CH₂O + HO₂ were promoted in low-temperature zone. Methanol addition decreased the rates of production and consumption of benzene proportionally, and served as a diluent fuel in benzene formation and consumption. CH₃, CH₃O, CH₂OH, C₃H₃, and A-C₃H₅ were the most important precursors for the formation of formaldehyde and benzene. The conversion rates of these species into formaldehyde and benzene were explored as well. Results showed that methanol addition suppressed the conversion of C₃ species into benzene, but it hardly showed obvious effect on the conversion of CH₃, CH₃O, and CH₂OH into formaldehyde.