Thermal decomposition and stability of fatty acid methyl esters in supercritical methanol

In recent years, non-catalytic supercritical processes for biodiesel production have been proposed as alternative environmentally friendly technologies. However, conditions of high temperature and pressure that occur while biodiesel is in supercritical fluid can cause fuel degradation, resulting in low yield. In this study, we performed the thermal decomposition of fatty acid methyl esters (FAMEs) in supercritical methanol at temperatures ranging from 325 °C to 420 °C and pressure of 23 MPa to investigate the degradation characteristics and thermal stability of biodiesel. The primary reactions we observed were isomerization, hydrogenation, and pyrolysis of FAMEs. The main pathway of degradation was deduced by analyzing the contents of degradation products. We found that if FAME has shorter chain length or is more saturated, it has higher thermal stability in supercritical methanol. All FAMEs remained stable at 325 °C or below. Based on these results, we recommend that transesterification reactions in supercritical methanol should be carried out below 325 °C (at 23 MPa) and 20 min, the temperature at which thermal decomposition of FAMEs begins to occur, to optimize high-yield biodiesel production.