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

Highlights

• 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.

Abstract

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 ($\mathrm{\Delta }\mathrm{H}$) Gibbs free energy ($\mathrm{\Delta }\mathrm{G}$) and entropy ($\mathrm{\Delta }\mathrm{S}$) were calculated with the activation values ($\mathrm{E}$) obtained with KAS method. For the kinetic analysis, the KAS, OFW and Starink methods give close $\mathrm{E}$ values over the whole conversion range, but the Friedman method shows more deviant results. The thermodynamic favorability of coal pyrolysis is discussed, both $\mathrm{\Delta }\mathrm{H}$ and $\mathrm{\Delta }\mathrm{G}$ increase with conversion consistently with the variation of E, revealing that pyrolysis is endothermic and thermodynamic favorability is reduced with conversion rises. The minus $\mathrm{\Delta }\mathrm{S}$ 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.