Pyrolysis characteristics of Turkish lignites in N2 and CO2 environments

Pyrolysis characteristics and kinetic parameters of two Turkish lignites having different ash contents (Orhaneli as low ash and Soma as high ash sample) were studied under N₂ and CO₂ atmospheres by means of thermogravimetric analysis. The isoconversional kinetic methods of Flynn?Wall??Ozawa, Kissinger??Akahira??Sunose, and Friedman were employed to estimate the activation energy and pre-exponential factors. The experiments were conducted at four different heating rates of 5, 10, 15, and 20°C/min within the temperature range of 50??950ºC. The obtained results indicated that changing the pyrolysis ambient had no significant effect on the devolatilization region up to 700°C. The char formation region in N₂ atmosphere was due to the CaCO₃ decomposition and was more significant for Soma lignite due to its high ash content. However, in CO₂ atmosphere, the gasification reaction took place at temperatures higher than 700°C. The decomposition process of CaCO₃ in CO₂ atmosphere was hampered up to temperatures higher than 900°C. The estimated activation energies were found to have approximately similar trends under different atmospheres. For Orhaneli lignite, the average activation energy values were higher in CO₂ environment. However, for Soma lignite due to decomposition of CaCO₃, the activation energy values were higher in N₂ atmosphere. The mean uncertainty values were assessed for the activation energy values obtained for all test cases.