Impacts of ester's carbon chain length and concentration on sooting propensities and soot oxidative reactivity: Application to Diesel and Biodiesel surrogates

This study is devoted to the evaluation of the impact of both esters' carbon chain length and concentration on sooting propensities of Diesel and Biodiesel surrogates. Soot particles were produced using a steady atmospheric axis-symmetric co-flow diffusion flame burner. Concentrations ranging from 3 to 30% (on molar basis) of methylbutanoate (MB), methyloctanoate (MO) or methyldecanoate (MD) were added to a surrogate Diesel made up of a binary mixture of 70% n-decane and 30% a-methylnaphthalene (alpha-MN). For every considered mixture, the sooting propensity was measured in terms of Yield Sooting Index (YSI) in a methane diffusion flame doped with 3.5% vapor of the mixture. Maps of soot volume fraction in the flame were extracted using a light extinction method (LEM). Soot samples produced along the combustion of surrogate Diesel and Biodiesel fuels were then collected and characterized using physico-chemical techniques, i.e. elemental analysis (CHONS), Thermogravimetric Analysis (TGA), Raman spectroscopy, and Temperature Programmed Oxidation (TPO). Results evidenced that ester functions contained in Biodiesel surrogates reduce soot production. This decrease was more pronounced when the concentration of the oxygenated additive investigated was higher. However, it has been surprisingly determined that YSI decreases when the aliphatic carbon chain of the ester additive is longer. On the other hand, physico-chemical characterizations of the generated model soot revealed that oxygen and soluble organic fraction (SOF) content decreases when the amount of biodiesel surrogate in the fuel in-creases. Nevertheless, no important dissimilarities have been registered in the graphitic structures of the different soot. Finally, the behavior towards oxidation reactivity indicated that the Biodiesel-derived soot were less reactive than the Diesel-derived one.