Oxidation stability of methyl esters studied by differential thermal analysis and Rancimat

The oxidation stability of methyl esters derived from fresh rapeseed oil and waste frying oil, used as alternative biodiesel fuels, both distilled and undistilled, unstabilized and stabilized by pyrogallol and BHT, was studied by differential thermal analysis (DTA) under nonisothermal conditions at various heating rates and by the Rancimat test under isothermal conditions at 110 degrees C. The results obtained by both techniques are compared. Both techniques show that oxidation stability increases considerably with the addition of antioxidants and that pyrogallol is very efficient. Distillation of the methyl esters prepared from rapeseed oil decreases their oxidation stability, obviously owing to the removal of natural antioxidants. The stability of methyl esters prepared from the waste frying oil is determined mainly by the history of the oil. From the DTA measurements, the kinetic parameters of an Arrhenius-like equation describing the temperature dependence of the oxidation induction period were obtained. The parameters enable one to assess the protective factor of antioxidants for temperatures outside the measuring region, estimate the residual stability, and model the process of biodiesel oxidation under nonisothermal conditions.