Study on the change of organic sulfur forms in coal during low-temperature oxidation process

Organic sulfur functional groups in coal not only have certain oxidation characteristics at low temperature and but also can break to produce free radicals, which is bound to have a considerable effect on coal spontaneous combustion. However, this area has not been reported yet. There are relatively few studies on variation laws of organic sulfur functional groups in coal in the coal spontaneous combustion process (especially low-temperature oxidation stage), and the release of SO2, the gaseous product of organic sulfur during the low-temperature oxidation, is also rarely reported. Therefore, it is necessary to systematically obtain the change laws of organic sulfur forms in coal during low-temperature oxidation. In this paper, the morphological changes of organic sulfur in coal during low-temperature oxidation were studied first, and then the changes of organic sulfur forms in coal before and after low-temperature oxidation and the release of the low-temperature oxidation product SO2 were studied by using the X-ray photon spectroscopy (XPS) technique and the XLZ-1090 infrared gas analyzer. The XPS analysis results show that the relative contents of mercaptan and sulfide are reduced; the content of thiophene sulfur decreases slightly or remains unchanged; and the contents of sulfone, sulfonate and sulfate all increase. This verifies that the low-temperature oxidation process of active organic sulfur functional groups in coal is: sulfide → sulfoxide → sulfone, mercaptan → disulphide → sulfoxide → sulfone → sulfonic acid. The study reveals that the release of gaseous SO2 is not detected during the low-temperature oxidation of the raw coal samples, whereas it is detected during the low-temperature oxidation of the acid-treated coal samples. This is because the acid treatment can remove alkaline minerals in coal, weakens the inhibitory effect of alkaline minerals on SO2 and enable SO2 to escape almost completely. In addition, the study also discovers that the time and amount of SO2 gas release during the low-temperature oxidation process are directly related to the metamorphic degree and organic sulfur content of coal.