Branched-Chain Fatty Acid Methyl Esters as Cold Flow Improvers for Biodiesel

Biodiesel is an alternative diesel fuel derived mainly from the transesterification of plant oils with methanol or ethanol. This fuel is generally made from commodity oils such as canola, palm or soybean and has a number of properties that make it compatible in compression-ignition engines. Despite its many advantages, biodiesel has poor cold flow properties that may impact its deployment during cooler months in moderate temperature climates. This work is a study on the use of skeletally branched-chain-fatty acid methyl esters (BC-FAME) as additives and diluents to decrease the cloud point (CP) and pour point (PP) of biodiesel. Two BC-FAME, methyl iso-oleate and methyl iso-stearate isomers (Me iso-C18:1 and Me iso-C18:0), were tested in mixtures with fatty acid methyl esters (FAME) of canola, palm and soybean oil (CaME, PME and SME). Results showed that mixing linear FAME with up to 2 mass% BC-FAME did not greatly affect CP, PP or kinematic viscosity (ν) relative to the unmixed biodiesel fuels. In contrast, higher concentrations of BC-FAME, namely between 17 and 39 mass%, significantly improved CP and PP without raising ν in excess of limits in the biodiesel fuel standard specification ASTM D 6751. Furthermore, it is shown that biodiesel/Me iso-C18:0 mixtures matched or exceeded the performance of biodiesel/Me iso-C18:1 mixtures in terms of decreasing CP and PP under certain conditions. This was taken as evidence that additives or diluents with chemical structures based on long-chain saturated chains may be more effective at reducing the cold flow properties of mixtures with biodiesel than structures based on long-chain unsaturated chains.