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,
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 × 10^9 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 ΔHo for biodiesel thermal decomposition is 80.0 KJ/mol, indicating the overall decomposition is an endothermic reaction.