Improving the low-temperature properties of biodiesel: Methods and consequences

Biodiesel is widely accepted as an additive for fossil derived diesel in compression ignition engines. It offers many advantages including: higher cetane number; reduced emissions of particulates, NOx, SOx, CO, and hydrocarbons; reduced toxicity; improved safety; and lower lifecycle CO2 emissions. A characteristic of biodiesel limiting its application is its relatively poor low-temperature flow properties, which are primarily a consequence of the fatty acid make-up of the oil feedstock. Attempts to influence the fatty acid profile of either the oil feedstock or the biodiesel product include winterisation and fractionation which reduce the fraction of saturated fatty acids and result in large reductions in yield. A reduction in saturated fatty acids reduces ignition quality of the fuel, while an increase in unsaturation reduces oxidation stability. Additives designed for petroleum diesel have been used with limited success and specific additives for biodiesel remain in their infancy. The addition of branched moieties either to the alkyl head-group of the ester or as a side-chain to the tail-group can reduce the cloud point. Specifically, the removal of the double bonds on the ester group and the addition of a side-chain may provide a benefit in terms of low-temperature properties and offer improved oxidation stability. However, a negative impact on ignition quality and viscosity may result.