Upon further heating the sample, evolution of gas is observed again at 563 ◦C. Moreover, the gaseous fraction for the thermal decomposition of pyrite is consisted chiefly of SO2. This will be further proved by the in situ FTIR spectroscopic evolved gas analysis. Therefore, it is concluded that the thermal decomposition of the coal-derived pyrite which started at about 400 ◦C was complete at 600 ◦C; the gas evolved during the thermal decomposition of pyrite can be established by combining the DTG peak, the Gram–Schmidt curve and in situ FTIR spectroscopic evolved gas analysis. As expected, the Gram–Schmidt plot show the presence of three regions of evolved gases, clearly related to the peak of the DTG curve. It is pointed out that the products of decomposition and reaction for pyrite vary depending upon the environment [3,30]. For instance, it is easily oxidized to different iron oxides such as Fe2O3 or Fe3O4 in the oxidizing environment [12], while pyrrhotite is one of the major products for the thermal decomposition of pyrite in nonoxidative environments [30]. However, as previous reported vary widely, no unanimous conclusion can be drawn. There are two main explanations accounting for the thermal decomposition process for the pyrite.