Oxidation Stability Of Biodiesel And Its Antioxidant

The oxidative stability of biodiesel refers to the ability to maintain its nature quality when biodiesel contacts with air or oxygen. As an important index to evaluate the quality of biodiesel, the oxidative stability has a direct influence on the application of biodiesel. Biodiesel can be oxidized easily in the process of production and storage, then a series of aldehyde, ketone, carboxylic acid and polymer are generated. Those substances increase the acid value of biodisel, cause a blockage to the diesel fuel filter and fuel injector, which seriously affects the quailty and performance of biodiesel. The national standard requires the oxidative stability of biodiesel no less than 6h. This paper discussed the influence factors and effect rules of biodiesel oxidation stability, investigated the oxidation kinetics and mechanism, and selected the proper antioxidants. The specific work and results were as follows:Firstly, the research focused on the influence of raw material(biodiesel), the content of water, glycerol, methanol in biodiesel, the atmosphere in the storage process, the environment temperature, the metal ion, the light and ventilation conditions on the oxidation induction time(OIT)of biodiesel. The experimental results showed that the OIT increase with the decrease of the carbon chain length and the unsaturation degree, and decrease with the increase of the content of water, methanol in biodiesel, and the content of water, methanol should be limitted to 0.05%, 0.3% respectively. The OIT declined exponentially with temperature, and decreased linearly with the increase of oxygen at the same temperature. The metal ions can promote the degradation of biodiesel in the sequence of Cu2+, Co2+, Fe3+, Mn2+, Ni+. The effect order of storage condition to the OIT:light and ventilation> light with no ventilation > dark and ventilation>dark with no ventilation. Biodiesel should be stored at a nontransparent and sealed steel container containing as little metal ion as possible, the storage time of biodiesel should be no more than 15 days.Secondly, the biodiesel oxidation experiments were done according to the standard method(EN15751:2009), the concentration of fatty acid methyl ester(FAMEs) were measured with GC, and the relationship of the concentration of FAMEs and oxidation time under different temperatures were obtained, and were investigated. The experimental results showed that the oxidative degradation of biodiesel could be regarded as pseudo first-order reaction. The reaction rate constant decreases in the sequence of methyl linolenate, methyl linoleate, methyl palmitate、methyl oleate. The corresponding activation energy was 75.49, 80.52, 82.15, 85.31 k J/mol respectively. The reaction rate constant increases with the decrease of the carbon chains and the increase of double bonds. The OIT of biodiesel varies linearly with the reciprocal of the reaction rate constant.Thirdly, the degradation products of biodiesel were analysed with GC-MS. The degradation products were mainly acid, aldehyde, ketone, alcohol, epoxy compounds. The amount of alkyl aldehyde was the largest. The oxidation products associated with smell were aldehydes, ketones, furans. The substances generated in the early period of the reaction were mainly volatile substances, the non-volatile substances were mainly produced in the later period.Fourthly, the antioxidant ability of various antioxidants was investigated. Sesamol was the best antioxidant among the four kinds of natural antioxidants: the ferulic acid, tea polyphenol, sesamol and VE. TBHQ was the best antioxidant among the four synthetic antioxidant: TBHQ、BHA、BHT、1010. BHA, BHT and 1010 had close antioxidation effect, but TBHQ, BHA, BHT could influence the acid value of biodiesel. The composition of BHT and 1010 as 1:1 could improve the antioxidant effects and had negligible effect on the acid value. Furthermore, the antioxidation effect of the composition of different antioxidants was investigated. TBHQ / citric acid / propylene glycol, BHA / citric acid/ propylene glycol, o-phenylenediamine/gallic acid present a positive cooperative relationship.Fifthly, The OIT of biodesel which put for a long time was obviously under the national standard. This paper investigated the methods to improve the oxidation stability, such as adding antioxidants, distillation, mixing with the new biodiesel. The effect of antioxidants addition, distillation was little on oxidation stability of biodiesel placing for a long time, while the mixing with new biodiesel was a feasible way to improve its shelflife.