A modeling study of the effect of surface reactions on methanol–air oxidation at low temperatures

A detailed modeling study was performed to investigate the inhibitory effect of the interior surface of a reactor in the process of low-temperature oxidation of methanol–air mixture. By using the reduction technique, which is for the gas-phase kinetic mechanism, a modified nine-step skeletal mechanism for the low-temperature methanol oxidation was developed. Important quenching species as well as surface reactions which have great influences on gas-phase branching reactions were identified. The adsorption of the species such as hydroxyl, hydroperoxyl, hydrogen peroxide and formaldehyde and the desorption of stable molecules generated by the recombination of the above adsorbents have constituted the surface kinetic model for methanol–air mixture. Computations based on the established heterogeneous radical quenching mechanism show excellent agreement with the experimental results from literatures, demonstrating that the radical quenching not only has significant inhibitory impact on slow oxidation, but also enables the retardation of ignition delay time. For the purpose of evaluating the inhibitory influence of the surface reactions on auto-ignition, a more in-depth five-step reduced mechanism combining the gas-phase reactions and the surface reactions was developed. Based on this mechanism, an explicit expression for the auto-ignition prediction was derived with the activation-energy asymptotic treatment. The formula can predict the ignition delay time with sufficient accuracy for both inert and high-reactive surfaces.