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

Significance Statement

    The use of oxygenated biofuels appears to be an interesting means to reduce our energetic dependence on petroleum as a fuel source. Among the wide variety of alternative fuels currently studied, fatty acid methyl esters (FAME) also called biodiesels have received a particular attention since these monoalkyl esters of animal or vegetable derived long chain fatty acids are quite similar to conventional Diesel in its main characteristics. Studies are still necessary, however, to better understand the effects associated to the use of FAME on Diesel engine performances and pollutant emissions. In this work, 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 generated using a lab-scale hybrid flat flame burner. Investigations have been carried out by coupling Laser-Induced Incandescence and Fluorescence (LII/LIF) at 1064, 532 and 266 nm to obtain mappings and concentration profiles of soot and its precursors (including light soot precursors and high-number ring aromatic species). 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 (80 vol. %) / 1-methylnaphthalene (20 vol. %) blend called m-IDEA fuel) induces significant reductions of the quantities of soot and its 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. The influence of the oxygen moiety on the sooting propensity has been analyzed through the study of the correlation existing between the peak soot volume fraction measured in flames of m-IDEA/RME blends containing up to 80% of ester and the threshold soot index (TSI) of these fuels. Such a methodology allowed the identification of the effects involved in the soot reduction (i.e. the dilution and the oxygenated 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 while particles oxidize faster when RME is added to Diesel.    

Diapositive 1

Fuel, Volume 107, 2013, Pages 147-161.

R. Lemaire, S. Bejaoui, E. Therssen.

Université Lille Nord de France, F-59000 Lille, France and

Mines Douai, EI, F-59500 Douai, France and

Laboratoire PC2A, UMR CNRS 8522, F-59655 Villeneuve d’Ascq, France

Abstract

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 its 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.

 

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