Bio-derived lactones – Combustion and exhaust emissions of a new class of renewable fuels

Abstract

The use of bioderived drop-in fuels is an essential step in the reduction in fossil fuel usage. While ethanol and biodiesel are known quantities, the use of novel biomass that does not compete with food for its production could be vital in ensuring a reliable supply. Lactones are a class of chemicals that can also be sourced from 2nd generation biomass and possess molecular attributes that, from previous investigations within the group, are believed to be effective in reducing particulate emissions relative to diesel, while maintaining high ignition propensity. In this study, a systematic investigation of the combustion and exhaust emissions of a series of lactone fuels in a compression ignition engine was undertaken. The results indicated that blended C6 lactones- ε-caprolactone, δ-hexalactone and γ-caprolactone- displayed promising ignition qualities, relative to butanol, an alcohol often employed in diesel blending experiments. Ignition delay decreased as the length of the lactone side chain increased, but combustion was seemingly more stable in both ε-caprolactone and γ-caprolactone, compared to the methyl branched δ-hexalactone, which possessed the highest CO emissions and particle number. The C10 lactone, γ-decalactone, within the diesel-butanol blend, possessed excellent ignition quality, while also reducing particle mass significantly. All blends produced lower nitrogen oxides (NOx) emissions and particle mass than unblended fossil diesel. Subsequently, two C10 and two C12 lactones were employed- based on their potential to be derived from biomass- as pure fuels in the engine. Despite significant differences in physical properties compared to diesel, stable combustion was observed, with ignition delay approximately equivalent to that of base diesel. The high carbon number and long alkyl chains of the C12 lactones meant that particulate emissions were comparable to diesel. C10 lactones showed greater alleviation of particulate emissions, likely due to a combination of lower carbon number and extended ignition delay relative to diesel, while all pure lactone fuels significantly reduced NOx emissions, suggested to be due to combustion phasing.