Sooting tendencies of unsaturated esters in nonpremixed flames

Biodiesel fuels form an important constituent of the renewable energy resources landscape. These fuels are produced through transesterification of fatty acids with methanol or ethanol to yield long-chain methyl or ethyl esters of 16–18 carbon atoms, frequently with one or more CC double bonds. The CC double bonds are expected to play a major role in the combustion chemistry of these biodiesel fuels, especially in the area of soot emission. To better understand the effect of these double bonds on the sooting properties of esters, we have measured the sooting tendencies of twenty C4 to C7 unsaturated esters and report their Yield Sooting Indices (YSI). The CC double bond was found to have a significant effect on the sooting tendency. In most cases, the unsaturated ester was more sooting than its saturated counterpart, although the increase in sooting tendency was strongly influenced by other structural features of the compound. Esters with the CC double bond nearest the carbonyl group had lower increases in the sooting tendency from the parent saturated ester than compounds where the CC double bond was further along the carbon backbone. Unsaturated methyl esters had larger increases in YSI (compared to their parent saturated ester) than unsaturated ethyl or propyl esters. The strong dependence of the sooting tendencies of these esters on their chemical structure indicates the complexity of the chemical processes involved, and is an important area for further studies of these compounds.