Impact Of Some Natural Antioxidants On The Oxidative Stability Of Biodiesel

Biodiesel is a mixture of fatty acid methyl esters or ethyl esters, and it's usually made from vegetable oils, animal fats, used frying oils and so on. Oxidative stability is an important specification for biodiesel quality. In this work, the impact of natural antioxidants on the oxidative stabilities of biodiesel was investigated, and the results were compared to those, in which synthetic antioxidants were added. The effects of some metal ions on the oxidative stability of biodiesel were discussed as well.With the addition of 500ppm sesamol, fat-soluble tea polyphenols, ferulic acid, caffeic acid orα-tocopherol, the induction periods (IP) of soybean oil based biodiesel (SOBD) are 5.3h, 4.6h, 3.3h, 6.4h and 3.2h, respectively. For comparsion, three commercialized synthetic antioxidants were applied into SOBD. The IP values of SOBD are 4.3h, 4.1h and 7.8h, respectively, with the addition of 500ppm BHA, BHT or TBHQ.For used oil based biodiesel (UOBD), the IP values of UOBD are 4.0h, 0.25h, 1.6h, 4.2h and 1.6h, respectively, at 1000ppm of sesamol, fat-soluble tea polyphenols, ferulic acid, caffeic acid orα-tocopherol. As 1000ppm BHA, BHT or TBHQ was applied, IP values of UOBD are 4.1h, 3.6h, 3.0h, respectively. Those results implied that sesamol and fat-soluble tea polyphenols have a similar antioxidative ability as to BHA and BHT.Metal ions experimental results show that the impact of metal ions on oxidative stability of biodiesel is in the sequence of Fe3+> Cu²⁺> Co²⁺> Mn²⁺> Ni²⁺.The impact of antioxidants on the acid value of biodiesel was tested by titration method. the acid value of the blank samples of SOBD and UOBD are 0.41 mg/g KOH and 0.62 mg/g KOH, respectively. The acid value of antioxidants modified biodiesel increased, but less than 0.1 mg/g KOH, with the addition of 1000ppm sesamol, BHA, BHT or TBHQ. It was found that fat-soluble tea polyphenols had little effect on the acid value of SOBD (0.49 mg/g KOH), but much more effect on the acid value of UOBD (0.92 mg/g KOH). Ferulic acid and caffeic acid had huge impacts on the acid value of biodiesel owing to carboxyl group in the molecule.To test the storage stability, the IP value of biodiesel, which was placed both indoor and outdoor, was measured after a certain period. The results show that the oxidation of biodiesel is a continuously changing process .