Reactions of HO2 radicals in combustion chemistry

Abstract

Recent attempts to model alkane oxidation with comprehensive mechanisms have shown that HO 2 /H 2 O 2 chemistry is very important in determining auto-ignition behavior between 600 and 1100 K, particularly as the pressure is raised above 1 atmosphere. Unfortunately, unlike the situation with OH radical reactions, very few kinetic data exist for reactions of HO 2 radicals at combustion temperatures. For this reason, recent experimentally determined rate constants for Rxns. (B) and (C) are presented in the paper. Accurate Arrhenius parameters are available for HO 2 + C 2 H 6 and for HO 2 + tetramethyl-butane, and single temperature rate constants have been obtained for HO 2 attack on methane, isobutane and 2,3-dimethylbutane. Based on thermochemical arguments, Arrhenius parameters are suggested for HO 2 attack at primary, secondary and tertiary C-H sites in alkanes which provide a data base for HO 2 + alkanes in the temperature range 600–1100 K. Outside these temperatures, the HO 2 reactions are unlikely to be important. Reaction (C) is an important route for the conversion of HO 2 radicals into OH, and Arrhenius parameters are available for a number of alkenes. Both k c and E c are very structure dependent, the reaction becoming faster as the ionisation energy of the alkene is reduced. It is argued that the Arrhenius parameters do refer to the initial HO 2 addition step. Most of the kinetic data for Rxns (B) and (C) have been obtained by use of the decomposition of tetramethylbutane in the presence of O 2 as a novel source of HO 2 over the temperature range 650–800 K. The system is discussed in outline in the paper. Reaction (2) plays a key role in determining ignition delay times at higher pressures between 600 and 1000 K, and is also used as a reference reaction in the tetramethylbutane decomposition studies. Unfortunately, only one experimental determination of k 2 has been made above about 550 K. The low-temperature data have been re-interpreted and Arrhenius parameters are suggested for use over the range 600–1000 K HO 2 + HO 2 → H 2 O 2 + O 2 (2)