Fig. 4 shows the spectra of gas released out in the first pyrolysis stage, at 410 ◦C, 444 ◦C and 470 ◦C for the thermal decomposition of the coal-derived pyrite, respectively. According to the widely applied Lambert–Beer law, the absorbance at a specific wave number is linearly dependent on gas concentration [48]. Thus the variation of absorbance in the whole process can reflect the tendency of product yield of the gas species. The absorption bands of volatile for the sample appear to be at the same wave numbers, while some diversities of the absorbance exist. It can be seen from the spectra that the relative intensity of bands in 3964–3500