Oxidative stability of biodiesel: recent insights

Biodiesel, the fatty acid methyl ester (FAME) of vegetable and animal oil, is now used extensively worldwide, with blends of up to 7% common. The blending level is still somewhat limited due to a perceived susceptibility of these fuels to oxidation. Oxidation follows a number of pathways, with the primary mechanism being auto-oxidation, a radical process that results in the production of a range of oxygenated components. These eventually increase the viscosity of the fuel and form deposits detrimental to operation. Further fuel properties are also heavily reliant on the level of oxidation. As such, one of the main challenges in the use of biodiesel is its long-term instability when stored. Typically synthetic anti-oxidants have been used to address this issue; however, these systems can also add to the formation of deposits, as well as hazardous emissions, on combustion. Recently, research has focused on novel antioxidant development mainly from plant extracts, although there are a number of other routes for improved performance, including the commercialization of hydrogenated vegetable oil (HVO), a prominent alternative to FAME-based biodiesel due to its higher stability, straight chain paraffin composition, and better cold flow properties. In this review, the factors that promote this oxidation are presented, including molecular composition, metal contamination, temperature and light exposure, as well as the latest findings on the inclusion of HVO, the current state-of-the-art analytical techniques employed, and the impact of higher pressure injection systems on vehicles that demonstrate deposit formation is not solely due to the unsaturated biodiesel components.