An experimental study on thermal stability of biodiesel fuel

Biodiesel fuel, as renewable energy, has been used in conventional diesel engines in pure form or as biodiesel/diesel blends for many years. However, thermal stability of biodiesel and biodiesel/diesel blends has been minimally explored. Aimed to shorten this gap, thermal stability of biodiesel is investigated at high temperatures.;In this study, batch thermal stressing experiments of biodiesel fuel were performed in stainless steel coils at specific temperature and residence time range from 250 to 425 °C and 3 to 63 minutes, respectively.;Evidence of different pathways of biodiesel fuel degradation is demonstrated chromatographically. It was found that biodiesel was stable at 275 °C for a residence time of 8 minutes or below, but the cis-trans isomerization reaction was observed at 28 minutes. Along with isomerization, polymerization also took place at 300 °C at 63 minutes. Small molecular weight products were detected at 350 °C at 33 minutes resulting from pyrolysis reactions and at 360 °C for 33 minutes or above, gaseous products were produced. The formed isomers and dimers were not stable, further decomposition of these compounds was observed at high temperatures.;These three main reactions and the temperature ranges in which they occurred are: isomerization, 275--400 °C; polymerization (Diels-Alder reaction), 300--425 °C; pyrolysis reaction, ≥350 °C.;The longer residence time and higher temperature resulted in greater decomposition. As the temperature increased to 425 °C, the colorless biodiesel became brownish. After 8 minutes, almost 84% of the original fatty acid methyl esters (FAMEs) disappeared, indicating significant fuel decomposition.;A kinetic study was also carried out subsequently to gain better insight into the biodiesel thermal decomposition. A three-lump model was proposed to describe the decomposition mechanism. Based on this mechanism, a reversible first-order reaction kinetic model for the global biodiesel decomposition was shown to adequately describe the experimental data points of the concentrations or the decomposition percentage as a function of time. The forward and reverse rate constants were determined at each temperature for the model. The Arrhenius pre-exponential factors A for k1 and k2 obtained were 1.50 x 109 and 257 min-1, and the energies of activation Ea were 126.0 and 46.0 KJ/mol, respective. The high linearity of the Arrhenius plots (R2 > 98%) further validated the rationality of the assumed reversible first-order kinetics to represent the overall biodiesel decomposition.;Moreover, a Van't Hoff plot was established, the reaction enthalpy DeltaHo for biodiesel thermal decomposition is 80.0 KJ/mol, indicating the overall decomposition is an endothermic reaction.