Solid fossil fuels thermal decomposition features in air and argon

- Coals oxidation is described by TGA data processing.
- TGA experiments were held in inert and oxidizing environments.
- Sample set contains coals of different nature and ranks.
- Two groups of coals are found in context of their interaction with oxygen.
- New kinetic model is proposed to mathematically process TGA data.

Investigation and characterization of coals oxidation is one of the most current problems due to the fact that it may lead to quality loss and spontaneous combustion hazards at mining, storage and utilization. A detailed analysis was performed for thermogravimetric analysis of selection of coals with a wide rank range, different structural characteristics, petrographic and elements composition and origins during experiments in inert and oxidizing environments. It was found that for both the considered environments, the coals thermogravimetric curves retain their space disposition order. Values of the temperatures corresponding to maximal decomposition rates correlate with coal rank as for inert, as for oxidizing environments. For bituminous coals and anthracite, temperatures corresponding to maximal decomposition rate drifted to the zone of higher temperatures in case of experiments in air in comparison with tests in inert. For lignites, they moved to lower temperature intervals. Values of the maximal thermal decomposition rate for bituminous coals well correlate with aromaticity degree. For lignites, the maximal decomposition rates in air and argon have comparable values, whereas for high-rank coals and anthracite they grow in oxidizing environment in comparison with inert one. Kinetic parameters were evaluated for the most common stages of coals mass change during thermogravimetric experiments in air and argon. Kinetic parameters of coals pyrolysis and combustion correlate not only with rank but also with aromaticity degree. For bituminous coals at high temperature intervals at experiments in inert environment, there was found a decrease of activation energy values with rank and aromaticity degree growth. As for corresponding values for bituminous coals tested in oxidizing environment, there was an increase of activation energy with rank and aromaticity degree. Two groups were allocated for coals in context of their interaction with oxygen. The first group included lignites and peat and is characterized by enhanced volatiles release at temperatures over 100 °C. The second one contains coals that are prone to oxygen adsorption at low-temperature intervals (up to 300-400 °C) (bituminous coals and anthracite). New parameter was proposed for preliminary describing coals group affiliation. This parameter denotes mass gain or decrease for coals tested in oxidizing environment. The complex of kinetic parameters may serve as an additional but informative tool to view at coals oxidation mechanisms. This, presumably, could be performed at simultaneous characterization of thermogravimetric data with heat flow and gas analysis.