Modelling Study of the Low-Temperature Oxidation of Large Methyl Esters

This study focuses on the automatic generation by the software EXGAS of kinetic models for the oxidation of large methyl esters using a single set of kinetic parameters. The obtained models allow to well reproduce the oxidation of a n-decane / methyl palmitate mixture in a jet-stirred reactor. This paper also presents the construction and a comparison of models for methyl esters from C7 up to C17 in terms of conversion in a jet-stirred reactor and of ignition delay time in a shock tube. This comparison study showed that methyl esters larger than methyl octanoate behave similarly and have very close reactivities. * Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2009 Introduction Due to the depletion of fossil fuels, the development of renewable energy is more vital than ever [1,2]. The production of biofuels such as methyl esters is encouraged by the European Union in order to incorporate them in existing diesel fuels. Molecules produced from vegetable oils are methyl esters with long carbon chain as methyl palmitate (C17H34O2). Due to the large size of the related kinetic mechanism (over 20000 reactions), it is necessary to determine the optimal model in terms of size and performance which could serve as a good surrogate model to simulate the oxidation of these large methyl esters and to investigate the difference with the combustion of alkanes induced by the ester function. Methyl butanoate (C5H10O2) has been the subject of several modelling studies. Fisher et al. [3] proposed a first detailed kinetic mechanism for the oxidation of this species in 2000. This mechanism was validated against experimental pressure data obtained in closed vessels [4]. While the experimental studies have mainly been performed under conditions corresponding to the high temperature region, this mechanism, written by analogy with n-alkanes, included all relevant pathways for both low and high temperature regions. More recently Metcalfe et al. [5] proposed a new version of the methyl butanoate mechanism based on that developed by Fisher et al. [3]. This model was validated against shock tube data. Gaïl et al. [6] proposed another version of the Fischer et al. mechanism [3] with validation using jetstirred reactor data, variable pressure flow reactor data and opposed flow diffusion flame data. These studies led to a better understanding of the specific chemistry due to the presence of the ester group but they also demonstrated that methyl butanoate is not a good surrogate for biodiesels because the alkylic chain is too short and the influence of the ester group on the chemistry is emphasized. Herbinet et al. [7] developed a model for the oxidation of a larger methyl ester, methyl decanoate, including all pertinent reactions to low and high temperature regions. The model has been validated against limited available data. One feature of this model is its ability to reproduce the early production of carbon monoxide dioxide observed at low temperature by Dagaut et al. [8] during the experiments. The model predicts ignition delay times very close to those observed for n-decane in a shock tube and suggests that large methyl esters and alkanes of similar size have very similar reactivity. This large model was reduced and used to model extinction and ignition of laminar non premixed flames containing methyl decanoate [9]. The purpose of this paper is to present detailed kinetic mechanisms for the oxidation of several large methyl esters which have been automatically generated with the software EXGAS using a single set of kinetic parameters. The rules used for the automatic generation of these mechanisms are described in this paper and a comparison of models for methyl esters from C9 up to C17 in terms of conversion in a jet-stirred reactor and of ignition delay times at low temperature (700<T<1100K) is then performed. Description of the mechanisms generated with EXGAS The detailed kinetic mechanisms used in this study have been automatically generated by the computer package, EXGAS. This software has already been used for generating mechanisms in the case of alkanes [1012], ethers [13] and alkenes [14]. We will recall here very shortly its main features which have already been much described and we will present the improvements of the reactions and rate constants needed to well represent the behavior of methyl esters. General features of EXGAS The system provides reaction mechanisms made of three parts: ● A comprehensive primary mechanism, where the only molecular reactants considered are the initial organic compounds and oxygen. According to the choices of the user, the reactant and the primary radicals can be systematically submitted to the different types of following elementary steps: ha l-0 03 77 70 7, v er si on 1 22 A pr 2 00 9 Author manuscript, published in "European Combustion Meeting 2009, Vienne : Austria (2009)"