Bimolecular Peroxy Radical (RO₂) Reactions and Their Relevance in Radical Initiated Oxidation of Hydrocarbons

Abstract

The kinetics of peroxy radical (RO 2 ) reactions have been of long-standing interest in atmospheric and combustion chemistry. Nevertheless, the lack of kinetic studies at higher temperatures for their reactions with other radicals such as OH has precluded the inclusion of this class of reactions in detailed kinetics models developed for combustion applications. In this work, guided by the limited roomtemperature experimental studies on selected alkyl-peroxy radicals and literature theoretical kinetics on the prototypical CH 3 O 2 + OH system, we have performed parametric studies on the effect of uncertainties in the rate coefficients and branching ratios to potential product channels for RO 2 + OH reactions at higher temperatures. Literature kinetics models were used to simulate autoignition delays, laminar flame speeds, and speciation profiles in flow and stirred reactors for a variety of common combustion-relevant fuels. Inclusion of RO 2 + OH reactions was found to retard autoignition in fuel-lean (φ = 0.5) mixtures of ethane and dimethyl ether in air. The observed effects were noticeably more pronounced in ozone-enriched combustion of ethane and dimethyl ether. The simulations also examined the influence of ozone doping levels, pressures, and equivalence ratios for both ethane and dimethyl ether oxidation. Sensitivity and flux analyses revealed that the RO 2 + OH reaction is a significant sink of RO 2 radicals at the early stage of autoignition, affecting fuel oxidation through RO 2 ↔ QOOH, RO 2 ↔ alkene + HO 2 , or RO 2 + HO 2 ↔ ROOH + O 2 . Additionally, the kinetic stability of the trioxide formed from RO 2 + OH reactions was investigated using master equation analyses. Last, we discuss other bimolecular reactions that are missing in literature kinetics models but are relevant to hydrocarbon oxidation initiated by external radical sources (plasma-enhanced, ozone-enriched combustion, etc.). The present simulations provide a strong motivation for better characterizing the bimolecular kinetics of peroxy radicals.

Combustion in a Sustainable World: From Molecules to Processes

“…Temperature- and pressure-dependent rate coefficients have also been investigated for a prototypical fuel radical reaction with molecular oxygen, C 2 H 5 + O 2 , combining experimental results from laser photolysis and photoionization mass spectrometry with master equation simulations . In addition, a parametric study of Cho et al examined the role of bimolecular reactions of peroxy radicals, RO 2 , with other radicals (e.g., OH) in radical-initiated oxidation of hydrocarbons . These often neglected reactions were found to significantly influence autoignition and species formation, particularly in radical rich systems such as ozone-enriched combustion .…”

“…8 In addition, a parametric study of Cho et al examined the role of bimolecular reactions of peroxy radicals, RO 2 , with other radicals (e.g., OH) in radical-initiated oxidation of hydrocarbons. 9 These often neglected reactions were found to significantly influence autoignition and species formation, particularly in radical rich systems such as ozone-enriched combustion. 9 Cyclic ethers provide an important marker of the flux through QOOH radicals, which are the central transient intermediate in low temperature chain branching.…”

“…9 These often neglected reactions were found to significantly influence autoignition and species formation, particularly in radical rich systems such as ozone-enriched combustion. 9 Cyclic ethers provide an important marker of the flux through QOOH radicals, which are the central transient intermediate in low temperature chain branching. Doner et al report a stereochemically specific ab initio master equation theoretical study of the sequence of decomposition reactions for the five radicals derived from the prototypical cyclic ether, 2,4-dimethyloxetane.…”