In-depth study of the transesterification reaction of Pongamia pinnata oil for biodiesel production using catalyst-free supercritical methanol process

[Display omitted] •Biodiesel from Pongamia pinnata oil in a free-catalyst one-step process.•Maximum FAME yield of approximately 81% at 300°C, 90min and 43:1 methanol/oil.•Thermal decomposition observed at the temperature of 325°C.•Maximum thermal decomposition degree of 38% at 350°C and 90min. Non-edible vegetable oils are promising substitutes for traditional edible food crops used in the synthesis of biodiesel. Among them, Pongamia pinnata oil, also known as Karanja oil, is considered as a good candidate with potential availability of more than 135Mtpa. The present work offers an in-depth study of the transesterification reaction of Karanja oil in supercritical methanol in one-step catalyst-free process. Triglyceride (TG) conversion and the yield of fatty acid methyl esters (FAMEs) are analyzed in the temperature and reaction time ranges of 250–350°C (12–43MPa) and 15–90min, respectively, at an alcohol-to-oil molar ratio of 43:1. This study also covers the evolution of intermediate products such as monoglycerides (MG) and diglycerides (DG) and the thermal decomposition of fatty acid chains for the stated reaction conditions. Optimal reactions conditions were found at 300°C and 90min reaction with almost complete triglyceride conversion. Significant thermal decomposition was observed from 325°C, mainly caused by the degradation of polyunsaturated fatty acid methyl esters. Maximum degree of thermal decomposition of 38% was determined at 350°C after 90min reaction time.