Pyrolysis kinetic analysis of sequential extract residues from Hefeng sub-bituminous coal based on Coats-Redfern method

Sequential extract residues (Ri, i=1, 2, 3, 4, 5) were obtained from Hefeng acid-washing coal (HFAC) by petroleum ether, carbon disulfide, methanol, acetone and isometric carbon disulfide/acetone mixture, sequentially. Pyrolysis behavior of the samples was carried out using thermogravimetry analysis. Coats-Redfern method with different reaction order was used to analyze the pyrolysis kinetic of each sample, and the kinetic parameters, including correlation coefficient (R2), activation energy (E), pre-exponential factor (A), were calculated. Results showed that the weight loss of extract residues was higher than HFAC, and pyrolysis behavior varies greatly for residues, which may be due to unstable structure after extraction. From conversion-temperature (α-T) curves, pyrolysis process was divided into three stages: low-temperature stage (150-350 oC), medium temperature stage (350-550 oC) and high temperature stage (550-950 oC). And the medium temperature stage made great contribution to the process of pyrolysis, which was dominated by depolymerization and decomposition reaction, and the effect of kinetic fitting to this stage is better, with R2 higher than 0.95. Relationship between kinetic parameters and reaction order showed that swelling effect might be an important reason for the discrepancy of E for each sample in the process of pyrolysis. And Ln(A)-E relationship has a great significance to predict E and the A under higher reaction order.

Effects of Chemical Properties and Inherent Mineral Matters on Pyrolysis Kinetics of Low-Rank Coals

The kinetics during the pyrolysis process depend on both chemical structure and inherent mineral matters in coal, but normally, only one of these components is investigated in literature. In the present work, four low-rank coals were pyrolyzed in a thermogravimetric analyzer at a heating rate of 10 K/min in a constant nitrogen stream at a temperature up to 900 °C to study the pyrolysis behavior and kinetics. Two of the samples were raw coal (R-YL) and clean coal (C-YL) obtained through the washing process. The results showed that the coal-washing process mainly affected the inorganic part (mineral matters) and structure in coal, which did not largely change the chemical properties. The pyrolysis behavior in primary stage (before 550 °C) was mainly affected by the chemical properties of coal, while the pyrolysis behavior in higher temperature also depended on inherent mineral matters. The kinetics of four coals were obtained using the Coats–Redfern (CR) method with five theoretical models. The difference of E value was almost negligible for R-YL and C-YL, also showing that the coal-washing process did not largely change the chemical properties. The higher frequency factor A for clean coal C-YL showed a more porous structure due to the coal-washing process. The apparent activation energy E in the third stage was affected by the thermodynamic property of inherent minerals.