Soot formation in ﬂames of model biodiesel fuels Qiyao Feng a , Aydin Jalali b , Adam M. Fincham b , Yang Lee Wang b , Theodore T. Tsotsis a , Fokion N. Egolfopoulos b,⇑ a Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1211, USA b Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, USA article info Article history: Received 5 October 2011 Received in revised form 21 November 2011 Accepted 7 January 2012 Available online 30 January 2012 Keywords: Laminar ﬂames Non-premixed ﬂames Soot Biodiesel Methyl esters abstract The sooting propensities of non-premixed ﬂames of a class of model biodiesel fuels, namely fatty acid esters, were studied systematically. Soot volume fractions were measured using the laser extinction method in the counter-ﬂow conﬁguration, for different fuel/N 2 molar ratios and atmospheric pressure. The experimental data were compared against those obtained in ﬂames of n-alkanes with similar carbon numbers and a ﬂame of a surrogate diesel fuel. For all cases considered, it was determined that the soot volume fraction increases with the fuel concentration, as expected. Furthermore, the model biodiesel fuels were shown to produce signiﬁcantly less soot compared to the corresponding n-alkanes. Additional experimental studies were carried as well, in order to assess the effects of carbon number, type of ester group (methyl or ethyl), and extent of saturation on the sooting propensity of ﬂames of these model bio- diesel fuels. Three recently developed chemical kinetic models were utilized to model the ﬂames and thus investigate the kinetic pathways controlling the formation of C 2 H 4 and two key soot precursors, namely C 2 H 2 and C 3 H 3 , aiming to provide insight into the experimentally observed differences in the sooting pro- pensity among the ﬂames of the various fuels that were considered. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction The use of biodiesel is attracting substantial attention currently as a potentially effective and environmentally friendly solution to the world’s dependence on conventional petroleum resources. A typical biodiesel is a multicomponent mixture of long-chain fatty acid mono-alkyl esters (FAME) derived from vegetable oils or ani- mal fats. As with conventional diesel, concerns exist with the use of biodiesel in terms of its emissions, especially particulate matter (PM) due to its potential health effects. Increasingly stringent envi- ronmental regulations relating to such emissions are likely to be- come the critical factor in the development of new generation, fuel-ﬂexible diesel engines. Thus, the number of studies on PM resulting from the combustion of biodiesel and its blends in diesel engines has been increasing (e.g., [1–10]). Graboski and McCormick [11], Lapuerta et al. [12], and Xue et al. [13] have reviewed past studies on emissions when using biodie- sel-based fuels in engines instead of conventional diesel. In most of these studies, it has been demonstrated that PM emissions de- crease notably as the biodiesel content in the fuel-blend increases, and attempts have been made to provide insight into these obser- vations. Frijters and Baert [14] have reported, for example, that PM emission reductions, when using oxygenated fuels, correlate strongly to the fuel’s oxygen content. Knothe et al. [15], who ob- served 73–83% reductions in PM emissions when using three pure methyl esters (methyl-oleate, methyl-palmitate, and methyl-lau- rate) instead of conventional diesel fuel, attributed this behavior to the aromatic compounds that are present in regular diesel but are absent in biodiesel fuels. Desjardins et al. [16] have studied the effect of oxygenated additives on the sooting propensities of hydrocarbon fuels. It was determined that oxygenated functional groups, such as those in alcohols, esters, and ethers, reduce the sooting propensity of the base fuel, and that the extent of the reduction depends strongly on the functional group type. Westbrook et al. [17] have modeled the effect of the suppression of the sooting propensity in diesel en- gines by the addition of oxygenates into the fuel. It was observed that soot formation is initiated by fuel-rich premixed ﬂame igni- tion followed by quenching due to the lack of oxygen. The presence of oxygenated additives reduces the production of soot precursor species during the fuel-rich premixed ignition due to the additional oxygen present in the fuel’s structure. Violi and coworkers [18,19] used kinetic modeling to explain the ‘‘early’’ formation of CO 2 during methyl ester combustion, the decrease in soot production and the slight increase in NO x forma- tion during biodiesel combustion, and to investigate differences in the oxidation pathways of methyl-butanoate and n-butane. It 0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustﬂame.2012.01.003 ⇑ Corresponding author. Fax: +1 213 740 8071. E-mail address: egolfopo@usc.edu (F.N. Egolfopoulos). Combustion and Flame 159 (2012) 1876–1893 Contents lists available at SciVerse ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustﬂame