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The European biodiesel standard EN-14214 calls for determining the oxidation stability (OS) at 110 °C with a minimum induction time of 6 h by the Rancimat method (EN-14112). The ASTM standard D-6751 has recently introduced a minimum induction period of 3 h. Palm methyl ester (PME) has been successfully evaluated as a diesel substitute in summer and with an additive in winter due to its poor cold-flow properties. Neat PME exhibited an OS of 9.24 h; thus, it was highly stable. Research was conducted to investigate the effect of the presence of transition metals, likely to be present in the metallurgy of storage tanks and barrels, on the highly stable PME. It was found that the influence of metal was detrimental and catalytic even for stable PME. Small concentrations of metal contaminants showed nearly the same influence on OS as large amounts. Copper showed the strongest detrimental and catalytic effect. Antioxidants, namely, tert-butylated hydroxytoluene (TBHT), tert-butylated phenol derivative (TBP), octylated butylated diphenyl amine (OBPA), and tert-butylhydroxquinone (TBHQ) were doped to improve the OS of metal-contaminated PME. It was found that the antioxidant TBHQ was most effective among all of the antioxidants used.

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To minimize use of biodiesels synthesized from edible oils, such as palm oil, because of the raising food versus fuel issue, palm biodiesel (PBD) was blended in different weight ratios with biodiesels synthesized from tree-borne non-edible oil seeds, jatropha and pongamia, to examine the effects on the cold filter plugging point (CFPP) of PBD. The CFPP of PBD improved significantly after blending with jatropha biodiesel (JBD) and pongamia biodiesel (PoBD). The dependence of CFPP upon the esters of fatty acid composition was also examined. Good correlations between the CFPP and palmitic acid methyl ester (PAME) were obtained. A correlation between CFPP and total unsaturated fatty acid methyl ester (X) was also determined. Using these two correlations, the CFPP of different biodiesel blends can be determined.