The sooting behavior of lactones as sustainable fuels

Lactones are an interesting category of sustainable fuels since they have the same carbon backbones as sugars but are liquids at room temperature. Engine studies have shown that lactones can reduce soot emissions as well as net carbon dioxide emissions. In this study quantitative sooting tendencies were measured for 10 lactones with a wide range of molecular structures. They included compounds with ring sizes varying from three to six carbons, unsubstituted compounds, substituted compounds with side chain lengths ranging from one to seven carbons, and one compound with a double bond in the ring. Two alkenoic acids were also tested since they are possible isomerization products of lactones. The sooting tendencies were characterized by yield sooting index (YSI), which is based on the soot yield when a methane/air nonpremixed flame is doped with 1000 μmol mol⁻¹ of the test fuel. The results show that the lactones have lower sooting tendencies than conventional gasoline, diesel fuel, and Jet A aviation fuel, even when accounting for their lower heats of combustion. However, the sooting tendencies depend strongly on molecular structure, so the right lactones must be chosen to maximize the emissions benefits. The measured sooting tendencies are generally larger than those predicted with a group contribution method, which indicates that the lactones have high sooting tendencies given the set of atoms they contain. To explain this observation, reactive molecular dynamics simulations and quantum chemistry calculations were performed. The results show that the lactones tend to decompose directly to CO₂, so the oxygen atoms are being used inefficiently to sequester only one carbon atom.