Biodiesel is subject to oxidative breakdown which is related to the double bond con­tent of the fatty acid methyl esters, and oxidation can lead to increased acidity, forma­tion of shorter fatty acids and the production of gums. This is an important feature, as stability is needed if biodiesel is to survive long-term storage, particularly for biodiesel from waste oils that have lost their natural antioxidants.

During oxidation the fatty acid methyl ester form a radical next to the double bond which binds oxygen forming a peroxide radical. The peroxide radical reacts with a fatty acid forming an acid releasing the radical and thus forming an autocata­lytic cycle.

The first study on the oxidative stability of biodiesel was by du Plessis et al. (1985) using sunflower-derived biodiesel stored at different temperature for 90 days. Other studies by Bondioli et al. (2004) and Mittelbach and Gangl (2001) on the stor­age of rapeseed-derived biodiesel were run over 1 year and 200 days, respectively. In the study by Mittelbach and Gangl (2001), rapeseed biodiesel was stored in poly­ethylene bottles at 20-22°C open or closed, in the light and in the dark. Samples were removed at intervals and the oxidative stability measured using the Rancimat system (Fig. 7.18). In the Rancimat system, the sample is heated to 100°C and air is passed

Fig. 7.18. The effect of storage conditions on the stability of biodiesel. (Redrawn from Mittelbach and Gangl, 2001.)

through the liquid. The air is run into a conductivity cell filled with water. After a few hours the conductivity in the cell increases rapidly as a result of volatile organic acid compounds produced by the oxidative breakdown of the sample collecting in the cell. It is the time required for the induction of the increase in conductivity that is taken as a measure of oxidative stability.

Bondioli et al. (2003) studied the long-term storage, 1 year, of 11 different biodie­sel preparations, some containing antioxidants. The list of samples and the results after 12 months’ storage are given in Table 7.16. After 1 year’s storage, a number of the parameters measured did not change but oxidative stability as revealed by the Rancimat data did change. Those samples with the lowest value changed less, such as the distilled sunflower biodiesel. The Rancimat values decreased with time and were dependent on both the state of the sample and the storage conditions. The addition of two antioxidants TBHQ and pyrogallol had little effect on stability in this case.

To combat the oxidation of biodiesel, natural and synthetic antioxidants have been added. Crude and distilled palm oil methyl esters were tested for their oxidative stability and these were 25.7 and 3.52 h in the Rancimat system (Liang et al., 2006). The crude palm oil methyl ester mixture contained 644 ppm vitamin E (a-tocopherol) and 711 ppm P-carotene, whereas the distilled version contained very little of these compounds. Various quantities of three antioxidants a-tocopherol, butylated hydro — xytoluene (BHT) and tert-butyl hydroquinone (TBHQ) were added to the distilled methyl ester mixture. The two synthetic antioxidants were better than the natural antioxidants and 50 ppm was sufficient to achieve the EN 14214 Rancimat standard. Dunn (2005) has also determined the effect of different antioxidants on soybean biodiesel and the NREL report indicates that antioxidants will be required for extended storage.

Another approach has been to blend Jatropha and palm oil biodiesel. Palm oil biodiesel contains high levels of palmitic (C16:0) and oleic (C18:1) acids which are

Peroxide value Viscosity Rancimat induction

(meqO2/kg) (mm2/s) time (h)

resistant to oxidation, whereas Jatropha methyl esters contain mainly oleic (C18:1) and linoleic (C18:2) acids. Thus, a mixture of the two esters will be more resistant to oxidation (Sarin et al., 2007).