Soot formation in flames of model biodiesel fuels

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/N2 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 biodiesel fuels. Three recently developed chemical kinetic models were utilized to model the flames and thus investigate the kinetic pathways controlling the formation of C2H4 and two key soot precursors, namely C2H2 and C3H3, aiming to provide insight into the experimentally observed differences in the sooting propensity among the flames of the various fuels that were considered.