Computer aided generation of kinetic mechanism – Application to the oxidation of large alkenes at low-temperature S. Touchard 1 , R. Fournet 1 , P.A. Glaude 1* , V. Warth 1 , F. Battin-Leclerc 1 , M. Ribaucour 2 , R. Minetti 2 1 Département de Chimie-Physique des Réactions, UMR n°7630 CNRS, INPL-ENSIC, 1 rue Grandville, BP 451, 54001 NANCY Cedex, France 2 PhysicoChimie des Processus de Combustion et de l'Atmosphère, UMR n°8522 CNRS, Université des Sciences et Technologies de Lille, Bâtiment C11 59655 Villeneuve d'Ascq Cedex (France) Abstract This paper presents a new model of the oxidation of 1-pentene generated using the system EXGAS, developed in our laboratory, and details the improvements which were needed in the definition of the specific generic reactions involving alkenes and their free radicals, as well as for the correlations to estimate the related rate constants. This mechanism has been validated using experimental data obtained in a rapid compression machine between 700 and 900 K and in a plug flow reactor between 654 and 716 K. Results reveal an acceptable agreement between simulated and experimental data for auto-ignition delays and for the distribution of the products. The analysis of the mechanism generated for 1-pentene shows the importance of some new reaction pathways specific to long chain alkenes. If this study confirms the great role played by the reaction of addition of small radicals on the double bond and by the specific reactivity of the allylic radical for the auto-ignition delays. It also put in evidence the huge role played by the reaction of allylic radical with O 2 giving a diene and has allowed us refine the kinetic value for this generic reaction. * Corresponding author: pierre-alexandre.glaude@ensic.inpl-nancy.fr Associated Web site: http://www.ensic.inpl-nancy.fr/DCPR/ Proceedings of the European Combustion Meeting 2003 Introduction Modeling the combustion of organic compounds requires to take in consideration several thousands of reactions, especially in the case of phenomena observed in spark ignited engines. Consequently an automatic procedure can be a convenient and rigorous way to write such large mechanisms. Several attempts have been made to develop computer tools for the automatic generation of mechanisms, but, until now, none of them has produced a system which is rapidly adaptable to different kinds of compounds or which correctly describes the macroscopic behaviour of the fuels (for instance induction period and conversion) and the distribution of the products formed. The system developed in our laboratory tends to reach this target. Its structure includes a complex mechanism generator named EXGAS, which has allowed the generation of mechanisms for the oxidation of alkanes, ethers, and their mixtures. The mechanisms generated by EXGAS have been validated by modeling the oxidation of several compounds (e.g. n-butane [1], n-decane [2], n-hexadecane [3], mixtures of n- heptane/iso-octane [4]). Recently, the development of EXGAS has been started towards the modeling of the oxidation of alkenes. They are included in LPGs (up to 40 %) and in gasoline (up to 20 %). Moreover, alkenes are primary products formed during the oxidation of alkanes. More generally, it is thought that unsaturated compounds play a significant role in the kinetics of combustion. Convenient first tests of this extension of EXGAS to alkenes were the oxidation of propene [5,6] and 1- butene [6]. Our work on the oxidation of propene at low temperature [5] has shown the determining role of the •C 3 H 6 OH adducts, via a mechanism similar to that of alkyl radicals involving two additions to oxygen and yielding degenerate branching agents. The main interest of this development of EXGAS towards the modeling of the oxidation of alkenes is to be able to model larger alkenes more representative of those included in fuels, such as 1-pentene, which is included in gasoline up to 1 % and has a research octane number of 90.9. The oxidation of 1-pentene has been experimentally studied by Ribaucour et al. [7] in a rapid compression machine between 600 and 900 K and by Prabhu et al. [8] in a plug flow reactor between 654 and 716 K. Ribaucour et al. [7] have proposed a detailed mechanism able to reproduce their measured ignition delays but which did not contain all the possible reactions that could be envisaged for the oxidation of an alkene. For instance, the abstractions of alkylic H-atoms, the isomerizations of the peroxyradicals deriving from the successive additions of hydroxyl radicals and oxygen molecules or the formations of unsaturated cycloethers have not been considered.