Biodiesel *

Biodiesel is defined by ASTM International as a fuel comprised of monoalkyl esters of long-chain fatty acids (FAs) derived from vegetable oils or animal fats, designated B100, and meeting the requirements of ASTM D6751. The European biodiesel standard, EN 14214, de facto defines biodiesel as FA methyl esters (FAMEs). Consequently, alternative fuels such as neat vegetable oils, hydrocarbons prepared therefrom via hydrotreatment, or Fischer–Tropsch diesel produced by gasification of biomass do not technically qualify as biodiesel, despite their occasional reference as such. As the name implies, biodiesel is a substitute or blend component for middle distillate fuels (petrodiesel) for combustion in diesel engines and generators and is thus not suited for spark-ignition (gasoline) engines. The primary reason lipids are converted to biodiesel as opposed to their direct use as fuel is their excessive kinematic viscosity (KV) that is approximately an order of magnitude higher than petrodiesel. For example, KVs (40°C) of soybean oil, soybean oil methyl esters (SMEs) and ultra-low-sulfur (< 15 parts per million S) diesel (ULSD) fuel are 31.49, 4.12, and 2.30 mm²/s, respectively (Moser and Vaughn, 2010). High KV results in poor fuel atomization in the combustion chamber, which causes operational problems such as engine deposits (Knothe, 2005).