Volume 6, Issue 10 (November 2009)
The Impact of Fischer–Tropsch Diesel on the Oxidation Stability of Biodiesel (FAME) Blends
Fischer–Tropsch (FT) distillates and first generation biodiesel are alternative fuels that can both be used directly as neat fuels in an internal combustion diesel engine or as blend stocks with conventional diesel fuel. Biodiesel is generally produced by the catalytic trans-esterification of triglycerides (vegetable oil) with methanol to form fatty acid methyl esters (FAME) and glycerol as a by-product. The FT process (or FT synthesis) is a catalyzed chemical reaction in which synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons of various compositions. The oxidation and thermal stability of diesel fuel are very important properties since unstable diesel fuels can form soluble gums and or insoluble organic particulates that can clog the fuel filters and fuels system components. In the quest for increased efficiency of internal combustion compression ignition engines, the engines are increasingly downsized and turbo charged. These developments, together with the latest demands from modern injector systems, place a greater demand on the oxidative and thermal stability performance of a fuel due to the exposure of fuels to higher temperatures in comparison to historic engine and injector designs. Recirculation of heated fuel can also cause injector deposits originating from either gum or particulate formation. Biodiesel (FAME) and FT diesel have shown to be fully compatible as blend components with conventional crude derived diesel. FT diesel, produced from the low temperature FT (LTFT) process, generally has a lower density and viscosity and good cold flow properties. It is also a very stable fuel, which compensates for the high viscosity and density, poor cold flow properties, and unstable nature associated with biodiesel (FAME). This paper discusses the results obtained from a comparative study of the oxidation stability (using the Rancimat methodology) of FT (high temperature FT and LTFT) and petroleum diesel fuels, blended with up to 20 vol % of soya and rapeseed derived biodiesel (FAME).