A Kinetic Study of the Thermal Degradation of Fir and Aspen Ethanol Lignins

Thermogravimetric (TG/DTG) methods have been used to study the thermal destruction of fir and aspen ethanol lignins at different heating rates in an inert (argon) medium. For each of the lignins, the main stages of thermal degradation were found under conditions of programmed heating of samples from 298 to 1173 K at heating rates of 5, 10, and 20 K/min. It was found that with an increase in the heating rate, the ranges of the main degradation of lignins expand. Shifts toward higher temperatures were 13.8 K for fir lignin and 21.1 K for aspen lignin. At the same temperatures, ethanol lignin from aspen showed a greater weight loss than fir lignin. Using the Ozawa–Flynn–Wall, Broido, and Redfern–Coats kinetic models, the results of the thermogravimetric analysis of the studied lignins are analyzed and the values ​​of the activation energy are calculated. The most reliable values of ​​the activation energy were obtained by the Ozawa–Flynn–Wall isoconversion method for the main temperature range of degradation (from ~569 to ~725 K): 113 kJ/mol for fir ethanol lignin and 106 kJ/mol for aspen ethanol lignin.