OXIDATION KINETICS OF BIODIESEL BY NON-ISOTHERMAL PRESSURIZED-DIFFERENTIAL SCANNING CALORIMETRY

Biodiesel is a renewable fuel that can be made from transesterification of plant oils, waste greases, animal fats, or microalgal oils with methanol. During long-term storage, autoxidation can adversely affect the viscosity and ignition quality of biodiesel. The objective of this work was to investigate the kinetics of oxidation of fatty acid methyl esters (FAME) made from canola, palm, and soybean oils (CaME, PME, and SME) plus methyl oleate and stearate (MeC18: 1 and MeC18:2). The FAME were analyzed by non-isothermal pressurized-differential scanning calorimetry (PDSC) in dynamic mode (positive airflow) at heating scan rates (beta) of 2, 5, 10, 15, and 20 degrees C min(-1). Results were analyzed to infer oxidation rate constants (k(T)). First order and two autocatalytic kinetic models were applied to calculate isothermal oxidation induction times (Delta t) for each FAME. While Delta t values varied between the models, ranking the FAME in descending order of Delta t was the same for each model regardless of temperature. In contrast, ranking the FAME by the oxidation induction period (IPR) obtained from a Rancimat instrument depended greatly on temperature. Shelf-life data for each FAME were calculated by extrapolating IPR measured at high temperatures to 25 degrees C (SL1). Ranking the FAME by SL1 yielded trends that were unpredictable based on the IPR data. On the other hand, Delta t values calculated at 25 degrees C (SL2) from the PDSC results were more consistent. These findings suggest that estimating the shelf-life of biodiesel from dynamic mode non-isothermal PDSC data yielded more reliable results than extrapolation of IPR data measured at high temperatures.