Revisiting the Kinetics and Thermodynamics of the Low-Temperature Oxidation Pathways of Alkanes: A Case Study of the Three Pentane Isomers.

This paper describes our developing understanding of low-temperature oxidation kinetics. We have investigated the ignition of the three pentane isomers in a rapid compression machine over a wide range of temperatures and pressures, including conditions of negative temperature coefficient behavior. The pentane isomers are small alkanes, yet have structures that are complex enough to allow for the application of their kinetic and thermochemical rules to larger molecules. Updates to the thermochemistry of the species important in the low-temperature oxidation of hydrocarbons have been made based on a thorough literature review. An evaluation of recent quantum-chemically derived rate coefficients from the literature pertinent to important low-temperature oxidation reaction classes has been performed, and new rate rules are recommended for these classes. Several reaction classes have also been included to determine their importance with regard to simulation results, and we have found that they should be included when developing future chemical kinetic mechanisms. A comparison of the model simulations with pressure-time histories from experiments in a rapid compression machine shows very good agreement for both ignition delay time and pressure rise for both the first- and second-stage ignition events. We show that revisions to both the thermochemistry and the kinetics are required in order to replicate experiments well. A broader validation of the models with ignition delay times from shock tubes and a rapid compression machine is presented in an accompanying paper. The results of this study enhance our understanding of the combustion of straight- and branched-chained alkanes.

Detailed chemical kinetic modeling of cyclohexane oxidation.

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Rules for reaction rate constants are developed for the low-temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Because cyclohexane produces only one type of cyclohexyl radical, much of the low-temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical with O2 through five-, six-, and seven-membered-ring transition states. The direct elimination of cyclohexene and HO2 from RO2 is included in the treatment using a modified rate constant of Cavallotti et al. (Proc. Combust. Inst. 2007, 31, 201). Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data, are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies on cyclohexane. Calculations indicated that the production of 1,2-epoxycyclohexane observed in the experiments cannot be simulated according to the current understanding of low-temperature chemistry. Possible "alternative" H-atom isomerizations leading to different products from the parent O2QOOH radical were included in the low-temperature chemical kinetic mechanism and were found to play a significant role.