A study on the effects of catalysts on pyrolysis and combustion characteristics of Turkish lignite in oxy-fuel conditions

The catalytic pyrolysis and combustion characteristics of low calorific value Turkish lignite in various ambient conditions were explored and the evolution of gases during pyrolysis tests was examined using a Thermogravimetric Analyzer coupled with a Fourier Transform Infrared spectrometer (TGA–FTIR). Potassium carbonate (K2CO3), calcium hydroxide (Ca(OH)2) and iron (III) oxide (Fe2O3) were employed as precursors of the catalysts and compared to the Raw-form (no catalyst added) to investigate the effects of potassium (K), calcium (Ca) and iron (Fe) on pyrolysis and combustion. Pyrolysis tests were carried out in 100% N2 and 100% CO2 ambient conditions which are the main diluting gases in air and oxy-fuel combustion. These experiments revealed that the major difference between pyrolysis in these two ambient conditions was observed above 720 °C and DTG (Derivative Thermogravimetric) profiles experienced sharp peaks at 785 °C in 100% CO2 which can be attributed to a char-CO2 gasification reaction. Furthermore, K2CO3 was found to be the most effective catalyst in the char gasification reaction during pyrolysis tests in 100% CO2. Combustion experiments were carried out in various oxygen concentrations from 21% to 35% O2 in N2 and CO2 ambient conditions. Combustion tests carried out in O2/CO2 ambient conditions revealed that for 30% and 35% O2, the relative active sequence of catalysts to the reaction rates of devolatilization can be described as Fe ≫ K > Ca > Raw-form, and Fe > Ca > Raw-form ≫ K respectively. Furthermore, potassium catalyst had the best char reactivity due to its much higher reaction rates for all oxygen concentrations. The burnout temperature (Tb) also experienced a significant drop in the case of the K-based catalyst. Finally, emission profiles of the evolved gases, CO2, CO, H2O, SOx and COS, were analyzed during pyrolysis tests in both N2 and CO2 ambient conditions using the FTIR method.