Mechanism Insight into the Conversion between HS and Thiophene during Coal Pyrolysis: A Theoretical Study.

Clean coal technology is the important thrust to the achievement of "carbon neutralization"; clearing the transformation mechanism of thiophene, the dominant organic sulfur species in coal, is conducive to promoting the development of sulfur removal technology. DFT calculations were performed, and 28 reaction paths were proposed in this research, clarifying the decomposition mechanism of thiophene and the fixation mechanism of HS. Thiophene is pyrolyzed mainly through the hydrogen-transfer reaction, which occurs at above 2000 K rather than 800 K. The hydrogen transfer between the C-C bond rather than the C-S bond causes the ring opening. Hydrogen promotes the decomposition of thiophene, which happens at 800 K, with a molar ratio of hydrogen to thiophene of 5. Therefore, thiophene is decomposed at 800 K mainly through the hydrogenation reaction that occurs at para carbons and the C-S bond, the HS elimination reaction, and the generation of ethane. Furthermore, HS can be converted into thiophene through the addition reaction with unsaturated hydrocarbon, or the dehydration reaction with hydroxyl or carboxyl groups. The combination between HS and the aliphatic compound occurs at 800 K, which is mainly influenced by the species of the functional group rather than the composition and morphology of the carbon chain. Meanwhile, the conversion of aromatic compounds tends to the generation of aryl mercaptan rather than thiophene at around 800 K.