AN EXPERIMENTAL STUDY ON THERMAL STABILITY OF FAEE BIODIESEL FUEL WITH ETHANOL

Biodiesel fuel is usually synthesized from vegetable oil and animal fat, and now it has been widely used in tractors, trucks, buses and ships. Unlike traditional fossil fuels, biodiesel is more environment-friendly and it is renewable, which can help people solve the energy crisis to some extent. Nowadays, supercritical esterification has become a novel way to produce biodiesel, but high temperature may cause the decomposition of biodiesel. Researchers mainly focus on studying the properties of fatty acid methyl ester (FAME), however, fatty acid ethyl ester (FAEE) biodiesel has only been slightly explored. So in this study, ethanol was used to synthesize biodiesel instead of methanol. Also, the thermal decomposition of FAEE biodiesel with excess ethanol was also studied. In the synthesis studies of FAEE biodiesel, the conditions evaluated were: molar ratio of ethanol to oil of 9:1, potassium hydroxide is chosen as the catalyst, and the reaction is kept at 75°C (±1°C) and 1 atm for 2 hours. In order to study the decomposition of biodiesel, the thermal decomposition experiments were performed in stainless steel coils at the temperature from 250°C to 425°C for residence time from 3 to 63 minutes. When ethanol was added, the volume ratio of ethanol to biodiesel was 1:1. All the products were analyzed by GC-FID and GC-MS. There are mainly three kinds of reactions observed in the thermal decomposition of biodiesel: isomerization, polymerization and pyrolysis. They occurred at the following temperature range respectively: ≥ 275 °C, ≥ 300 °C and ≥ 350 °C. When the temperature is below 275 °C, the decomposition ratio is less than 5%, which suggests that FAEE biodiesel is stable below 275 °C. It is also found that at a given temperature, longer residence time results in higher decomposition ratio. A first-order reversible reaction model was proposed to represent the thermal decomposition of FAEE biodiesel, this model fits the experimental data very well from 250 °C to 400 °C. At 425 °C, the first-order irreversible reaction model is better, which suggests that the main reaction is pyrolysis at 425 °C. The reaction rate constants were calculated using these two models. Kinetic analysis was also conducted. The Arrhenius equation and van\u27t Hoff equation were used to define the kinetic parameters. For the forward reaction, the pre-exponential factor (A) is 2.54×109 min-1 and the activation energy (Ea) is 128.2 kJ/mol. For the reverse reaction, A is 7.98 min-1 and Ea is 29.6 kJ/mol. For the entire reaction, the standard enthalpy (∆H0) is 98.6 kJ/mol. In fitting the data, the high coefficient of determination of higher than 0.97 was obtained, which supports the hypothesis that the first-order reversible reaction model was reasonable over the range of conditions studied