Analysis of the sooting propensity of C-4 and C-5 oxygenates: Comparison of sooting indexes issued from laser-based experiments and group additivity approaches

Abstract

Oxygenated biofuels have received a particular attention during the last few years due to their ability to reduce soot emissions. Numerous experimental and numerical studies focusing on the sooting propensity of such alternative fuels especially revealed that the ability of oxygenates to limit the soot production was strongly related to their molecular structure in addition to their oxygen content. The objective of the present work is thus to analyze and quantify the potential of soot reduction of several C-4 and C-5 oxygenated molecules used as additives in a Diesel surrogate (the IDEA fuel composed of 70vol% n-decane and 30vol% α-methylnaphthalene). To do so, soot concentration measurements have been carried out by Laser-Induced Incandescence (LII) in 33 turbulent spray flames burning mixtures of IDEA with 12.5, 25 and 50vol% of various oxygenated additives including alcohols, aldehydes, ketones and esters. The different oxygenates have been selected to assess the impact of the position of the oxygenated functional group, the number of oxygen atoms and the type of CO link (single or double) on the production of soot. Based on the peak soot volume fractions measured in standardized jet flames, a Fuel Equivalent Sooting Index (FESI) has been inferred for each considered oxygenated additive and compared with corresponding Yield Sooting Index (YSI), Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) values measured and/or estimated based on structural group contribution methods. Obtained results show that all the tested oxygenates reduce the sooting propensity of the base fuel in which they are added. For a given number of carbon atom in the molecule, the sooting tendency of oxygenated compounds increases in the following order: esters<aldehydes<ketones<alcohols. Reasons explaining such trends have been discussed considering the different effects that may impact the soot formation (i.e. the chemical effect induced by the presence of oxygenated functional groups in the fuel mixtures and the dilution of the base fuel by the additives which remains the most important factor for all the considered molecules). Finally, FESI values derived from a previous database obtained by analyzing flames burning gasoline-surrogate/ethanol blends [1] have been confronted with recently published OESI values proposed in [2] for gasoline/ethanol blends.