Effect of atmosphere reaction and heating rate on the devolatilization of a Colombian sub-bituminous coal

• It analyses the impact of heating rate and environments on coal pyrolysis.
• Find differences in the behavior during thermochemical transformation.
• Devolatilization kinetic in oxy-combustion differs from the under a combustion.
• Differences between thermophysical properties are responsible of discrepancy.
• The CO by gasification and temperature, has relationship with heating rate.

This work studies the combined effect of heating rate and reaction atmosphere on the devolatilization of sub-bituminous type A coal (SubbA coal). Non-isothermal thermogravimetric analysis (TGA) was performed on SubbA coal undergoing pyrolysis. Mass loss dataset was fit to a lumped kinetic model using two different model-free fitting methods (FWO and Vyazovkin). Meaningful differences were obtained in devolatilization kinetics under N2 (traditional combustion) and CO2 (oxy-combustion process with gas recirculation). Through TGA it was found that at low heating rates, the pyrolysis mass loss rate is higher in N2 than in CO2. Based on the activation energy distribution, CHN analysis on residual char, and tracking the evolution of products in gaseous phase during devolatilization in CO2, it was demonstrated that at temperatures higher than 660 °C, CO2 reacts with the formed char. This gasification reaction becomes a controlling stage of the thermochemical transformation in CO2. Additional pyrolysis experiments were conducted in a fast-heating hot plate reactor (HPR) with 2 mm particles in N2 and CO2 using heating rates between 50 °C/s and 1200 °C/s. These higher heating rates are similar to those of fluidized beds. In the HPR experiments, devolatilization is more intense for CO2 than N2. Differences arising from the two environments are likely caused by the different thermal and diffusive properties of both gases.