posted on 2022-12-23, 16:08authored byJaeyoung Cho, Clayton R. Mulvihill, Stephen J. Klippenstein, Raghu Sivaramakrishnan
The kinetics of peroxy radical (RO2) 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 room-temperature
experimental studies on selected alkyl-peroxy radicals and literature
theoretical kinetics on the prototypical CH3O2 + OH system, we have performed parametric studies on the effect
of uncertainties in the rate coefficients and branching ratios to
potential product channels for RO2 + 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 RO2 + 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 RO2 + OH reaction is a significant
sink of RO2 radicals at the early stage of autoignition,
affecting fuel oxidation through RO2 ↔ QOOH, RO2 ↔ alkene + HO2, or RO2 + HO2 ↔ ROOH + O2. Additionally, the kinetic
stability of the trioxide formed from RO2 + 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.