Simulating Low Temperature Combustion: Thermochemistry, Computational Kinetics and Detailed Reaction Mechanisms

Abstract

Detailed chemical kinetic models are important to the understanding and prediction of combustion properties. Better estimations require an accurate description of thermochemistry and kinetic rate parameters. This study identifies important reaction pathways at the low temperature chemistry of branched conventional and alternative fuels. Rate constants and branching ratios for important reactions are provided and important phenomena are investigated. The thermochemistry and kinetics of the 2-methylhexane model, an important component in gasoline surrogate, is updated using recent group values and rate rules from the literature. New reactions, such as hydroperoxyalkylperoxy (OOQOOH) alternative isomerization, are also added to the model. The results show that both conventional and alternative isomerization of OOQOOH radicals significantly affect the model reactivity.

The kinetics of a biofuel; iso-butanol, is also investigated in this study to understand alcohol combustion chemistry and identify sensitive reactions that require more attention. The results indicate that iso-butanol is sensitive to the chain propagation reaction of α-RO2 radical and the water elimination of γ-QOOH. Because both reactions decrease model reactivity, accurate rate constants are needed to correctly determine fuel reactivity.

In light of the above mentioned kinetic modeling studies, high levels computational chemistry calculations were performed to provide site-specific rates rules for OOQOOH conventional isomerization considering all possible reaction sites. This is also one of the first studies to investigate the effect of chirality on calculated rate constants. Results indicate that chirality is important when two chiral centers exist in the reactant.

OOQOOH alternative isomerization rate constants are usually assigned in analogy to the isomerization of an alkylperoxy (RO2) radical which may introduce some uncertainty. To test the validity of using analogous rates, this study calculates the rate constants for selected alternative isomerization reactions. The effect of intramolecular hydrogen bonding in the calculated energies and rate constants for different reaction pathways is investigated. The result shows that alternative isomerization is a competing pathway only when it proceeds via a less strained transition state relative to the conventional isomerization transition state. A detailed analysis of the hydrogen bonding effect helped to identify cases where assigning rates in analogy may not be valid.