REDUCED MECHANISMS FOR PROPANE PYROLYSIS

Reduced chemical mechanisms describing the pyrolysis of pure propane in a cracking tube with a uniform firebox temperature are presented. The full chemistry is taken from the model of Dente and Ranzi and consists of 422 reactions involving 48 species. By use of techniques for the identification of redundant species and the principal component analysis of the local rate sensitivity matrix, the mechanism is reduced to 122 reactions. The concentration profiles along the cracking tube calculated using this mechanism are within 1-5% of the profiles calculated using the full chemistry. The use of overall sensitivities and approximations involving fast reversible reactions enables the selection of a further 72 redundant reactions in the mechanism. The resulting 50-step scheme reproduces the concentrations of the major products and the main reactor features, although some minor products are poorly represented. The number of species in the model is further reduced by the application of the quasi-steady-state approximation (QSSA). The calculation of the instantaneous error induced by the QSSA and the species lifetime reveals a set of 12 QSSA species, and explicit analytical expressions for a subset of these species are presented. The substitution of these expressions results in a model containing only 12 coupled differential equations. The errors induced in the concentrations of non-steady-state species by the application of the QSSA are shown to be minimal.