Comprehensive mechanism for the gas-phase oxidation of propene

This paper presents a first test of the extension of our computer code EXGAS to the generation of detailed mechanisms for the oxidation and combustion of alkenes. In the first part, an analysis of the elementary reactions from the literature allowed us to define new specific generic reactions involving alkenes and their free radicals, as well as correlations to estimate the related rate constants. The corresponding generic rules were then implemented in the EXGAS code. The second part, a mechanism for the oxidation of propene involving 262 species and including 1295 reactions was generated by EXGAS. The predictions of this mechanism were compared, without any change of the best available kinetic data, with two sets of experimental measurements: the first obtained in a static vessel between 580 K and 740 K; the second used a jet-stirred reactor between 900 K and 1200 K. If one takes into account that no fitting of individual rate constants was done, the mechanism reproduces correctly both the negative temperature coefficient (NTC) observed at approximate to 630 K and the variations of the concentrations with residence time of C3H6, CO, CO2, CH4, C2H2, C2H4, C2H2, C3H4, HCHO, CH3CHO, C2H3CHO, and cyclic ethers (C3H6O), especially the general shape of these curves and their minima, maxima, and inflection points. Flux and sensitivity analyses were performed to get insight into the kinetic structure of the mechanism explaining the observed characteristics, such as the NTC or the autocatalytic behavior of the reaction. At low temperatures, these analyses showed that the NTC is mainly due to the reversibility of the addition to oxygen of the adducts, .C3H6OH, which via a mechanism similar to that of alkyl radicals and involving two additions to oxygen, yields degenerate branching agents. At high temperatures, in both kind of reactor, the determining role of termination reactions involving the very abundant allyl radicals has been emphasized, especially the recombination of allyl and hydroperoxyalkyl radicals, which is the main source of acrolein. (C) 2001 by The Combustion Institute.