Impact of Isomerization and Carbon Number on Blending Limits of Higher Alcohols in Diesel Fuel

Higher alcohols are attractive sustainable blendstocks for diesel fuel as they may be produced through biological processes or the upgrade of abundant bioethanol. To use alcohols as blendstocks for diesel fuel, the properties of the resulting blends must comply with the standard specifications for diesel fuel. By understanding the effects of alcohol molecular structure on relevant fuel properties, processes producing higher alcohols can be designed to elicit the desired structure. In this study, blends of five pure higher alcohols (1-butanol, 1-hexanol, 1-octanol, 2-ethyl-1-butanol, and 2-ethyl-1-hexanol) with conventional diesel fuel were characterized to evaluate the effects of both carbon number and branching on the suitability of alcohols as a diesel fuel blendstock. In addition to properties regulated by ASTM D975, combustion performance was also studied through the analysis of the spray autoignition process and sooting tendency. Unbranched alcohols of higher molecular weight better preserved the flash point, lubricity, and derived cetane number when blended with diesel fuel. Blending 1-butanol into diesel fuel at any volume fraction resulted in a flash point less than 40 °C. All other alcohols considered in this study remained in compliance with ASTM D975 at a 20% blending volume fraction. None of the alcohols significantly compromised the cloud point, but pure 1-octanol had a cloud point about 10 °C greater than diesel fuel. While branched isomers resulted in lower cetane numbers than normal alcohols, isomerization preserved the cold flow performance for heavy alcohol blendstocks. It was also found that the sooting tendency was uniformly reduced by normal alcohol blending, independent of alcohol molecular weight. Blends with branched alcohols resulted in a 25% smaller reduction in the sooting tendency compared to normal isomers. These results can inform the design of higher-alcohol-based blendstocks for diesel fuel across a variety of applications.