Pressure-dependent kinetics on the C4H7 potential energy surface and its effect on combustion model predictions

Butene (C4H8), the smallest alkene possessing both branched and straight-chain isomers, is usually an important intermediate with relatively high concentration in the combustion of hydrocarbons and oxygenated fuels. In the present study, the kinetics of thermal isomerization, decomposition and chemical activation reactions of three typical butenyl isomers, i.e. nC(4)H(7) (CH2=CH-CH2-CH2), saxC(4)H(7) (CH2=CH-CH-CH3) and iC(4)H(7) (Cliz=gal(3))-CH2), were systematically investigated by theoretical calculations. High-level ab initio calculations coupled with the BMW/master equation method were used to compute the temperature and pressure dependent rate coefficients. The results show that the existence of vinylic C-C bond in iC(4)H(7) largely impedes its decomposition rate. The transformation from iC(4)H(7) to straight-chain C4H7 is kinetically unfavorable due to the high strain energy of the 3-membered ring structure of the isomerization transition state. Furthermore, the calculated rate coefficients were incorporated into USC Mech II and Aramco Mech 2.0 to examine the impact of our computed pressure-dependent kinetics on model predictions. Although the simulation results demonstrate limited improvement on ignition delay time and laminar flame speed of butene, substantial changes are observed for the mole fractions of important intermediate species, e.g., allene and 1,3-butediene, in butene pyrolysis. Modified Arrhenius representations of the calculated rate constants are given and should be used in combustion modeling. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.