Construction and simplification of a model for the oxidation of alkanes

This paper analyzes some of the chemical and kinetic principles that rule t he automatic generation of reaction mechanisms by the system EXGAS for the oxidation of alkanes. The systematic inclusion in the mechanism of all poss ible reaction pathways, in comparison with other models published, permits one to discuss the role of the different classes of reactions and to deduce some methods for simplifying a priori these mechanisms from a kinetic basi s. The first technique is based on an analysis of the reactivity of free ra dicals and of the relative rates of generic reactions versus temperature. T his analysis of reaction rates allows us to discuss the temperature ranges where it is necessary to consider one or two successive additions of an oxy gen molecule to a hydrocarbon radical when modeling the oxidation of alkane s. A method to decrease the number of species and reactions deriving from t he second addition of oxygen is also reported. Substantial reductions of th e number of reactions and species included in the primary mechanism can be attained by these techniques. The validity of the simplified mechanisms obt ained by these methods is illustrated with some examples derived from the m odeling of the oxidation of n-heptane and n-decane. (C) 2000 by The Combust ion Institute.