Modeling of hydrocarbons pyrolysis at low temperature. Automatic generation of free radicals mechanisms

Abstract This paper describes a system which permits the computer aided design of comprehensive primary mechanisms and simplified secondary mechanisms coupled with the relevant thermochemical and kinetic data in the case of alkanes pyrolysis at low temperature (below 400 °C). The mechanism is generated by writing systematically all the generic reactions: initiations, isomerizations, decompositions by s-scission, metatheses, additions and terminations. The mechanism generated can be presented as a file to be used to run simulations by means of CHEMKIN II codes. The system generates a primary mechanism, that is to say a mechanism in which only the initial organic molecule or the organic molecules contained in the initial mixture are considered as reactants. A secondary mechanism, including reactions whose reactants are the molecular products formed by the primary mechanism, can be created following diverse rules. Specific heats, standard enthalpies of formation and entropies of molecules and radicals and kinetics data for each reaction are calculated automatically using various softwares or correlations. Concerning the rate parameters, we take the approximation that all reactions in a class have the same kinetic values because they undergo the same transformation. This system has been tested by modeling the pyrolysis of hexane and tetradecane at low temperature, around 400 °C. With the mechanism generated, we obtain a good agreement between the experimental and simulated values both for reactants and products formed.

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