Reaction of Hydroperoxy Radicals with Primary C_1-5 Alcohols: A Profound Effect on Ignition Delay Times

Ignition delay times of primary alcohols often display a noticeable sensitivity to their initial reactions with HO₂ radicals. In view of the transient nature of HO₂ radicals, kinetic models on combustion of alcohols utilize theoretically obtained constant parameters for the abstraction HO₂ + alcohols reactions. Rate constants for the title reactions in pertinent kinetic models are often extrapolated from analogous computed values for either alkanes + HO₂ or n-butane + HO₂ reactions. Even for the simplest alcohol, methanol, literature values for the reaction rate constants considerably vary within one order of magnitude. Herein, we compute reaction rate constants for H abstraction from the weakest sites in primary C_1-5 alcohols by HO₂ (methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, t-butanol, n-pentanol, and i-pentanol). In most cases, our reaction rate coefficients tend to slightly exceed corresponding values deployed in pertinent kinetic models. We have thoroughly assessed the predictive performance of literature kinetic models in computing ignition delay times of these alcohols based on the updated rate constants for HO₂-abstraction reactions. In the case of methanol, updating kinetic parameters for the reaction CH₃OH + HO₂ → CH₂OH + H₂O₂ improves prediction of ignition delay times at lower temperatures in reference to original literature kinetic models. Likewise, a modified kinetic model for n-butane and t-butanol affords better agreement with experimental values of ignition delay times at low temperatures and high pressures. Kinetic parameters presented herein will be useful to accurately account for salient oxidation features of alcohols in real combustion engines.