Thermogravimetric assessment of the thermal degradation during combustion of crude and pure glycerol

According to the United States Energy Information Administration (US EIA), the annual pure biodiesel production in the USA was 6.88 million cubic meters in 2020. Similarly, significant biodiesel production was reported across the world with countries like Brazil, Germany, and Indonesia estimated to have produced 5.9, 3.2, and 6.2 million cubic meters in 2019. This enormous biodiesel production, which is widespread globally, has led to huge amount of waste in the form of glycerol as a product of transesterification. It was estimated that crude glycerol production was 189.27 million cubic meters in 2021. Thermochemical conversion processes, such as combustion, gasification, and pyrolysis, are viable ways to utilize the waste glycerol. In this study, the thermogravimetric analyzer was used to investigate and compare the characteristics of crude and pure glycerol combustion. In particular, the kinetic and thermodynamic parameters as well as the ignition and burnout temperatures were evaluated for the two fuels. The glycerol samples were subjected to a temperature range of 50–700 °C under dissimilar heating rates, i.e., 5, 10, 15, and 20 °C/min. Results of the thermal decomposition process indicate that while there is a single stage in pure glycerol, the crude glycerol is characterized with three decomposition stages. In addition, the experimental results showed that the main combustion process in both samples occurred at about 150–(325 ± 25) °C. The effect of heating rate on TG and DTG curves showed that at higher heating rates, the degradation curves shifts to higher temperature values. The crude glycerol ignition temperature is in the range of 195–218 °C compared to 180–212 °C for pure glycerol, and their burnout temperatures were 463–508 °C and 238–276 °C, respectively. The activation energies were evaluated with Kissinger method and found to be 75 kJ/mol for the devolatilization event in the crude glycerol and 79.6 kJ/mol for the pure glycerol.