Combustion behaviour of Turkish lignite in O2/N2 and O2/CO2 mixtures by using TGA-FTIR

The pyrolysis and combustion behaviour of a low calorific value Turkish lignite with high sulphur and ash content in air and oxy-fuel conditions were investigated by using non-isothermal thermo-gravimetric method (TGA) coupled with Fourier-transform infrared (FTIR) spectrometer. Pyrolysis tests were carried out in nitrogen and carbon dioxide environments which are the main diluting gases of air and oxy-fuel environment, respectively. Pyrolysis results show that weight loss profiles are almost the same up to a temperature of 720 °C in these two environments, indicating that CO2 behaves as an inert gas in this temperature range. However, further weight loss takes place in CO2 atmosphere at higher temperatures due to CO2-char gasification reaction. Combustion experiments were carried out in four different atmospheres: air, oxygen-enriched air environment (30% O2-70% N2), oxy-fuel environment (21% O2-79% CO2) and oxygen-enriched oxy-fuel environment (30% O2-70% CO2). Combustion experiments reveal that replacing nitrogen in the gas mixture by the same concentration of CO2 does not affect the combustion process significantly but only leads to slight delay in combustion. Overall comparison of derivative thermogravimetry (DTG) profiles shows that oxygen content in the combustion environment is the most effective parameter irrespective of the diluting gas. As O2 concentration increases profiles shift through lower temperature zone, peak and burnout temperatures decrease, weight loss rate increases and complete combustion is achieved at lower temperatures and shorter times. During pyrolysis and combustion tests gaseous products CO2, CO, H2O, CH4, SO2 and COS in flue gas were identified and analyzed by using FTIR. Results indicate that higher CO and COS formation takes place during pyrolysis due to gasification reaction. Gaseous species evolution trends in combustion tests are found to be almost identical in oxygen enriched conditions independent of the diluting gas.