Kinetic Analysis of the Generation of Active Sites During the Low-Temperature Pyrolysis of Coal

Decomposition of oxygen-containing functional groups simultaneously produces CO and CO2, accompanied by the generation of active sites. Two different methods were applied in the study to speculate the kinetics of gas generation and the formation of active sites in coal pyrolysis. The evolution of CO and CO2 gas generation during isothermal pyrolysis under different temperatures was first analyzed, and the generation of CO and CO2 in the pyrolysis process was divided into three stages: the rapid decline stage, slow decline stage, and stabilization stage. In the early stage of the pyrolysis reaction, functional groups exhibited a rapid decomposition rate and a high conversion rate (a), which rapidly decreased the gas concentrations. With increasing pyrolysis time, the decomposition of functional groups declined with a low conversion rate (a) and showed a steady reaction rate in the later stage. On this basis, a method to calculate the activation energy during pyrolysis based on the data of gases generated in a steady period with a continuous cooling temperature was proposed. Activation energies of 70.12–148.39 kJ/mol for the CO formation and 64.47–127.38 kJ/mol for the CO2 formation were obtained from four different coals. In addition, a thermogravimetric analyzer was used to explore the apparent activation energies in the decomposition process via Coats-Redfern (C-R) method; activation energies of 39.33–126.65 kJ/mol and pre-exponential factors of 1.31–5.15E+08 s−1 were obtained for different coals. The results revealed that the CO had a greater generation activation energy than CO2, and a higher metamorphism degree implies a higher decomposition activation energy, which indicates that the decomposition of carboxyl groups and low-rank coals result in higher active sites.