Non-isothermal kinetic study of bituminous coal and lignite conversion in air and in argon/air mixtures

The exact characteristics of the pyrolysis and combustion kinetics for Siberian bituminous coals and lignites were studied for better understanding of the fuel conversion processes. The bituminous coal samples of Kuznetskiy deposit and lignite samples of Kansko-Achinsk deposit were investigated in argon and in argon/air gas mixtures. The pyrolysis and oxidation experiments were executed at four heating rates (5, 10, 20 and 30 °C/min) under TG/DSC analyses coupled with mass-spectrometry. The activation energy for bituminous coal and lignite samples was decreased with conversion degree during oxidation. The combustion products of highest oxidation degree (CO₂ and H₂O) were dominated in gases released in oxidizing atmosphere. Contrariwise, activation energy was increased during pyrolysis with high content of CO, CO₂, H₂O, CH₄, and H₂ in released gases. The two isoconversional models were applied to determine the activation energy dependence on fuel samples conversion: Starink model and Ozawa iterative procedure. The mean arithmetic values of the resulted activation energy were 60 kJ/mole and 400 kJ/mole for oxidation and pyrolysis processes, respectively. These values are in good agreement with the results, presented previously for the other coals. The Starink model showed higher accuracy and lower activation energy values. The heating rate by non-isothermal oxidation and pyrolysis had the significant influence on the reaction rate because of evolution processes of the reactive surface and pore structure of the coal samples.