Kinetic and thermodynamic studies on the mechanism of low-temperature oxidation of coal: A case study of Shendong coal (China)

Due to the heterogeneous characteristics of coal constituents, it is difficult to directly apply conventional methods for calculating the kinetic and thermodynamic characteristics of coal oxidation at low temperatures. In this work, the complex macromolecular matrix of coal was divided into elements C, H, O, S, and N, which are all involved in oxidation reactions. Based on the changes in element occurrence during low-temperature oxidation of coal, the kinetic and thermodynamic characteristics of coal oxidation were studied at temperatures below 200 °C. A kinetic study revealed that the changes in element occurrence during coal oxidation at low temperature followed pseudo-first-order kinetics. The activation energies for the changes in element occurrence obtained by using the pseudo-first order kinetic have been found to be very close to those calculated by applying the Coats and Redfern's equation. At a particular temperature, the release of element H showed the highest rate constant (K) and lowest activation energy (Ea) compared with those same values for C and N. A kinetic compensation effect between K and Ea was also observed for the changes in element occurrence. Negative enthalpy (ΔH) values indicated that the changes in S and O occurrence produced heat, while the changes in C, H, and N occurrence were endothermic, having positive ΔH values. The low values of the rate constants and frequency factors suggested the non-spontaneous nature of changes in element occurrence, which was further supported by the negative entropy (ΔS) values and positive Gibb's free energy (ΔG) values associated with the changes in element occurrence. The enthalpies of formation for CO2, CO, and H2O were calculated, and the exothermic nature for the formation of CO2 and H2O was evident given their negative ΔH values. Based on the kinetic and thermodynamic characteristics of low-temperature coal oxidation, the mechanism of coal self-heating was also explored.