Group additivity in soot formation for the example of C-5 oxygenated hydrocarbon fuels

Sooting tendencies have been measured for 29 compounds with 5 carbon atoms and different oxygen contents, along with 12 additional oxygenated pure compounds and 10 blends of commercial fuels. The test compounds include alcohols, ethers, aldehydes, ketones, acids, esters, keto-esters and hydroxy-esters. The Threshold Sooting Index (TSI), as defined and used in the literature, was primarily used to quantify the sooting tendency of the compounds. It is shown that this index does not account for the presence of oxygen atoms in the fuel molecules, and the impact of fuel oxygen on the stoichiometric air requirement. The application of TSI to oxygenated fuels leads to sooting tendencies inconsistent with those reported in the literature. A new sooting index is proposed for oxygenated and non-oxygenated compounds, which considers the relation between the height of the flame tip and the volumetric stoichiometric air requirement of the flame. This new index is denoted as the Oxygen Extended Sooting Index (OESI). The results obtained by this new index corroborate that not only the oxygen content but also the molecular structure has an influence on the sooting tendency of fuels. Different functional groups, not only those including oxygen atoms, impart different sooting tendencies. A structural group contribution approach based on group additivity is proposed to interpret experimental observations on the effect of oxygen functional groups on the sooting tendency of fuels using the example of C-5 oxygenated fuels. Groups with a higher fraction of carbon–carbon bonds showed a higher contribution to the sooting tendency than those with a higher concentration of carbon–hydrogen bonds. Among the C-5 mono-oxygenated compounds, the sooting tendency increased in this order: aldehydes < alcohols < ketones < ethers < n-alkanes and for the C-5 di-oxygenated compounds, the sooting tendency order was: acids < esters < di-ethers. In general, both unsaturated and branched compounds showed slight increases in sooting tendency with respect to their saturated and linear counterparts. Finally, the new sooting index was applied to commercial biofuel blends (denatured ethanol–gasoline and soybean-derived biodiesel–diesel) and empirical correlations were obtained.