Oxidation Stability of Diesel/Biodiesel Fuels Measured by a PetroOxy Device and Characterization of Oxidation Products

In the present work, the oxidation stability of diesel, rapeseed (RME), and soybean (SME) fatty acid methyl esters (FAME) and a blend of diesel with 10% (v/v) RME (B10-RME) was studied. Fuel samples were aged in the PetroOxy test device from 383 to 423 K at 7 bar. Experiments were conducted in oxygen excess, and the global kinetic constants were determined. The global kinetic constants for diesel, B10-RME, and RME at 383 K were 7.92 x 10(-6), 2.78 x 10(-5), and 8.87 x 10(-5) s(-1), respectively. The oxidation products formed at different stages of the oxidation were monitored by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis-differential thermal analysis (TGA-DTA), and gas chromatography/mass spectrometry (GC/MS). The impact of the FAME nature and level of blending on the kinetic rate constant and the oxidation products was investigated. Results show that RME oxidation forms C-19 epoxy as the main oxidation product, in addition to a methyl ester FAME derivative and short-chain oxidation products, such as alkane, alkene, aldehydes, ketones, alcohols, and acids with a carbon number up to C-11. The overall amount of oxidation products increases with a higher degradation time. The DTA profile suggests that higher molecular weight products are formed at an advanced level of oxidation. For all highly oxidized fuels, a similar DTA peak was obtained at a temperature of around 573 K, which may suggest the formation of products having similar molecular weights for both diesel and FAME.