Oxidation Stability of Rapeseed Biodiesel/Petroleum Diesel Blends

The effects of fuels, including contaminants such as fuel oxidation products, on vehicle fuel system materials are important for vehicle durability and operation. Fuel oxidation is accelerated at the elevated temperatures and pressures of vehicle fuel systems. An extended time-course study of the oxidation of a biodiesel fuel blend consisting of 30% (v/v) rapeseed methyl ester in petroleum diesel (B30) was conducted at 70 and 90 degrees C with three aeration rates. Oxidation rates increased with increasing temperature as indicated by decreases in induction period (Rancimat), concentrations of unsaturated fatty acid methyl esters, and the time required to form hydroperoxides and acids. Peroxide values (PVs) rose to peak values in the range of 250-400 mequiv of O2/kg at 90 degrees C and 375-450 mequiv of O2/kg at 70 degrees C and then declined. Total acid number (TAN) increased rapidly as peroxides were formed after which the rate of TAN increase slowed, reaching values of up to 32 mg of KOH/g at 90 degrees C and 20 mg of KOH/g at 70 degrees C. The timing and maximum values of PV and TAN were somewhat dependent on aeration rate. Fuel mass decreased initially due to fuel volatilization, then increased as oxygen was incorporated, and then decreased reflecting volatilization of fuel and volatile oxidation products. Peroxide concentration showed a peak that coincided with the most rapid rate of oxygen incorporation, acid formation, and polyunsaturated FAME degradation. Net oxygen incorporation exhibited a plateau at approximately 5-6 wt % O above the 3.5 wt % O initially present in the FAMEs. This exceeds (by approximately 50%) the amount needed for incorporation of a single oxygen atom at every carbon atom allylic to carbon-carbon double bonds in the FAMEs that were oxidized.