Investigation of kinetic and thermodynamic parameters of coal pyrolysis with model-free fitting methods

[Display omitted] •Non-isothermal pyrolysis of low-medium rank coals were conducted with TGA.•Pyrolysis kinetics were analyzed with four model-free fitting methods.•KAS, OFW and Starink were more accurate than the Friedman method.•Thermodynamic parameters were obtained and correlated with pyrolysis process. Kinetic analyses are important means for understanding the complicated coal pyrolysis process. This work conducted non-isothermal pyrolysis of four low-medium rank coals with thermogravimetric analyzer at different heating rates (5, 10 and 20 K/min). Four model-free fitting approaches including KAS, OFW, Starink and Friedman methods were used to analyze pyrolysis kinetics of the coals. Thermodynamic parameters including enthalpy (ΔH) Gibbs free energy (ΔG) and entropy (ΔS) were calculated with the activation values (E) obtained with KAS method. For the kinetic analysis, the KAS, OFW and Starink methods give close E values over the whole conversion range, but the Friedman method shows more deviant results. The thermodynamic favorability of coal pyrolysis is discussed, both ΔH and ΔG increase with conversion consistently with the variation of E, revealing that pyrolysis is endothermic and thermodynamic favorability is reduced with conversion rises. The minus ΔS implies that coal structures evolve from disordered to organized ones as universally accepted. These results are expected to deepen our understanding and to optimize the conditions of coal pyrolysis.