Investigation of biodiesel thermal stability under simulated in-use conditions

Biodiesel is an alternative diesel fuel produced by transesterification of vegetable oils or animal fats. While biodiesel products numerous environmental benefits such as reduced exhaust emissions, it is more prone to oxidation than petroleum-based diesel fuel and this can alter its properties. When oxidation occurs at ordinary temperatures, the initial products are hydroperoxides. As the oxidation continues, the peroxides may split and form aldehydes, ketones, and short chain acids that produce unpleasant odors. Sediment and gums are formed through polymerization of the peroxides and can cause fuel filter plugging. The objective of this study was to relate the chemical and physical processes associated with biodiesel oxidation to the conditions that affect diesel fuel system performance. A relationship was sought between the test that is used by the engine industry to define engine fuel stability requirements (ASTM D2274) and the tests used by the fats and oils industry to characterize oxidation (Peroxide Value and Acid Value). It was found during the course of this study that the ASTM fuel stability method is not suitable for biodiesel. While oxidation causes the fuel viscosity to increase, fuel filter plugging was not necessarily a natural consequence of biodiesel oxidation even when the fuel was oxidized to a level beyond what would be observed in practice. The effect of fuel temperature and blending with diesel fuel on the oxidation was investigated and the interrelationship between the fuel's acid value and viscosity is shown.