Study of soot formation during the combustion of Diesel, rapeseed methyl ester and their surrogates in turbulent spray flames

Effects induced by the use of rapeseed methyl ester (RME) as additive or Diesel substitute on the soot formation process have been studied in turbulent spray flames. Investigations have been carried out by coupling Laser-Induced Incandescence and Fluorescence (LII/LIF) at 1064, 532 and 266 nm. LII and LIF profiles obtained with fuels containing various amounts of ester (from 10 to 100 vol.%) showed that the addition of RME to a European low-sulphur Diesel or to a Diesel surrogate (a n-decane/1-methylnaphthalene blend derived from the 'IDEA' fuel) induces significant reductions of the quantities of soot and soot precursors (including high-number ring aromatic species and light soot precursors). The study of different RME surrogates (n-decane, n-hexadecane, 1-octadecene and methyl oleate) also revealed that the details of the oxidation of biodiesels could be mimicked only using large methyl esters as surrogates. N-alkanes and n-alkenes were found to be unable to reproduce the soot formation process occurring during the combustion of large fatty acid methyl esters (FAME) such as those contained in RME. The analysis of the correlation existing between the threshold soot index (TSI) and the peak soot volume fraction measured in flames of m-IDEA/RME blends containing up to 80% of ester allowed the identification of the different effects involved in the soot reduction (i.e. the dilution and the ester functional group effects). The estimation of their relative contribution has also been investigated. Finally, the LII fluence curves and time decays obtained at different heights above the burner in flames burning Diesel and a Diesel/RME mixture have been compared. By this way, it has been demonstrated that biodiesel soot are bigger than Diesel ones in the soot formation region. On the other hand, particles oxidize much faster when RME is added to Diesel. (c) 2012 Elsevier Ltd. All rights reserved.