Submitted to: Fuel Processing Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/22/2004
Publication Date: 1/15/2005
Citation: Dunn, R.O. 2005. Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel). Fuel Processing Technology. 82(5):381-387. Interpretive Summary: Biodiesel, an alternative fuel made from soybean oil, is very reactive to oxygen when placed in contact with air. Maintaining fuel quality against the effects of oxidative degradation during long-term storage is vital to the widespread commercialization of biodiesel. This work evaluates several oxidation inhibitors (antioxidants) commonly used for protecting edible fats and vegetable oils for applicability to biodiesel made from soybean oil (methyl soyate). Pressurized-differential scanning calorimetry (P-DSC) results were analyzed to rank four synthetic and one natural antioxidant with respect to ability to enhance resistance to oxidation. P-DSC results showed that small concentrations (less than 0.1 percent) of antioxidant were necessary to significantly improve the resistance of biodiesel. Some antioxidants tested were not physically compatible when added to blends of biodiesel and petroleum-based diesel fuels. This work demonstrated that P-DSC is a rapid and accurate technique for screening the activity of various antioxidants with respect to protecting biodiesel against the effects of oxidative degradation. Results from this study will benefit biodiesel fuel producers, distributors and customers interested in maintaining fuel quality during long-term storage.
Technical Abstract: Biodiesel, an alternative diesel fuel derived from transesterification of vegetable oils or animal fats, is composed of saturated and unsaturated long-chain fatty acid alkyl esters. When exposed to air during storage, autoxidation of biodiesel can cause degradation of fuel quality by adversely affecting properties such as kinematic viscosity, acid value and peroxide value. One approach for increasing resistance of fatty derivatives against autoxidation is to treat them with oxidation inhibitors (antioxidants). This study examines the effectiveness of five such antioxidants, tert-butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PrG) and a-tocopherol in mixtures with soybean oil fatty acid methyl esters (SME). Antioxidant activity in terms of increasing oxidation onset temperature (OT) determined by non-isothermal pressurized-differential scanning calorimetry (P-DSC). Analyses were conducted in static (zero gas flow) and dynamic (positive gas flow) mode under 2000 kPa (290 psig) pressure and 5 deg C/min heating scan rate. Results showed that PrG, BHT and BHA were most effective and a-tocopherol least effective in increasing OT. Increasing antioxidant loading (concentration) showed sharp increases in activity for loadings up to 1000 ppm followed by smaller increases in activity at higher loadings. Phase equilibrium studies were also conducted to test physical compatibility of antioxidants in SME-No. 2 diesel fuel (D2) blends. Overall, this study recommends BHA or TBHQ (loadings up to 3000 ppm) for safeguarding biodiesel from effects of autoxidation during storage. BHT is also suitable at relatively low loadings (210 ppm after blending). PrG showed some compatibility problems and may not be readily soluble in blends with larger SME ratios. Although a-tocopherol showed very good compatibility in blends, it was significantly less effective than the synthetic antioxidants screened in this work.