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

Abstract

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 × 10–6, 2.78 × 10–5, and 8.87 × 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 C19 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 C11. 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.