Modeling of Soot Precursors in a Rich Premixed Ethylene Flame by using Original and Reduced Mechanisms V. Dias, J. Vandooren ∗ Université catholique de Louvain – Belgium Abstract A kinetic model containing 402 reactions and 78 chemical species has been built in order to determine the key processes governing the formation of soot precursors in a premixed rich ethylene/oxygen/argon flame (φ = 2.50). This mechanism reduced to 212 reactions and 72 species does not affect the kinetic pathways. Benzene and phenyl radicals are important precursors for the formation of other aromatics like phenol, toluene, phenylacetylene and styrene. Cyclopentadiene plays also an important role in the formation of heavy hydrocarbons: it can loose an hydrogen atom to produce cyclopentadienyl radicals which react between themselves to form naphthalene. The indene is produced from indenyl radical and by reaction between cyclopentadiene and vinyl radicals. Both original and reduced mechanisms reproduce very well experimental data. ∗ Corresponding author: vandooren@chim.ucl.ac.be Proceedings of the European Combustion Meeting 2007 Introduction Polycyclic Aromatic Hydrocarbons (PAHs) are formed in most practical combustion systems, particularly those operating under fuel-rich conditions and in diffusion flames. Many PAHs play an important role in the formation of soot during combustion. During the past decade, the formation of PAHs has been investigated extensively and several mechanisms have been postulated for the formation of simple PAHs molecules [1-7]. It is now widely accepted that the formation of benzene and phenyl radicals constitutes the first step in the growth process that leads to PAHs and ultimately to soot particles. However, despite the extensive work devoted to identify the elementary reactions leading to the first aromatic ring, the dominant benzene formation pathway is not yet well cleared out. Two different sets of reactions have been proposed, the C 4 + C 2 pathways and the C 3 + C 3 channels. Several authors have proposed some values for the rate constant of the propargyl radicals recombination, like Pope and Miller [8], Scherer et al. [9], Appel et al. [10], Miller and Klippenstein [11], Rasmussen et al. [12],… However, modeling rich ethylene and acetylene flames, Wang and Frenklach [2] concluded that the propargyl recombination reaction does not appear to be the unique channel of benzene formation. Depending on flame conditions, the contribution of reactions between n- C 4 H 5 , or n-C 4 H 3 with C 2 H 2 to produce benzene may be as significant as the propargyl recombination. As a conclusion regarding the current knowledge of the formation of the first aromatic ring, relative contributions of pathways involving C 4 or/and C 3 species depend on experimental conditions, in particular temperature, pressure, mixing ratio, and, most important, the kind of fuel. Finally, many kinetic models published in the literature have been built to simulate flame parameters of various fuels burning at different equivalence ratios. However, none reproduces correctly and simultaneously concentrations profiles of C 1 to C 10 species in flames with a large equivalence ratio. Firstly, the aim of this work is to elaborate an original kinetic mechanism by using a combination of some already existing in the literature. However, such a mechanism appears to be too large to be directly used for industrial simulations, where important parameters coming from the fluid mechanics must be taken into account. The second part of the work is to reduce this original mechanism by considering only main reaction pathways, but keeping the same degree of validity compared to experimental results. In this paper, we report these original and reduced reaction mechanisms established for a rich flat premixed flame of ethylene/oxygen/argon, by describing the main pathways leading to the benzene formation and its interaction with other soot precursors. A laminar premixed ethylene/oxygen/argon flame burning at 50 mbar with an equivalence ratio of 2.50 (33 % C 2 H 4 , 40 % O 2 and 27 % Ar) previously investigated by using MBMS technique [13] has served as the reference system. In that flame, besides common compounds, C 1 to C 10 species were monitored. Finally, the understanding of the reaction mechanism in such ethylene rich flame will improve the knowledge of kinetics governing the combustion of alkanes, in general. Modelling Before building a novel mechanism, several kinetic models have been tested and the corresponding numerical simulations compared with the experimental results: the Miller and Melius’s [14], the Marinov et al.’s [3], the Appel et al.’s [10], the Richter and Howard’s [15] and the Rasmussen et al.’s mechanisms [12]. It has been evaluated the degree of validity of each mechanism and therefore it has helped to elaborate a novel one from the combination of some of them. The detailed kinetic model involving 78 chemical species