Progress in the migration and transformation mechanism of thiophene compounds during coal pyrolysis

Thiophene sulfide is an important organic sulfur form in coal. During coal pyrolysis，the thiophene sulfides will migrate into the pyrolytic products，affecting the quality of the products or causing environmental pollution. There⁃ fore，it is of great significance to clarify the migration and transformation mechanism of thiophene sulfide during coal pyrolysis，which is useful to the efficient and clean utilization of coal and the realization of carbon peaking and car⁃ bon neutrality goals. First，the presence of thiophene sulfide in coal and its formation characteristics during pyrolysis are summarized. The content of thiophene sulfide rises with the increase of coal rank. The thiophene sulfide in the py⁃ rolysis process mainly comes from the thermal cracking of macromolecular structure in coal，and partly originates from the transformation of inorganic sulfur and organic sulfur such as thioethers and thiols. Then，the relevant experi⁃ mental and computational results of the pyrolysis characteristics and mechanism of thiophene sulfide are reviewed. The decomposition of thiophene sulfide is mainly caused by C—S bond homolysis and hydrogen migration. Sulfur⁃containing gases of H2 S，SO2 ，COS，and CS2 are then produced from the sulfur⁃containing free radical intermedi⁃ ates. Other sulfur⁃containing groups will polymerize with each other or combine with the aromatic rings to generate sul⁃ fur⁃containing polycyclic aromatics which are retained in tar and coke. Afterward，the effects of various reaction condi⁃ tions on the pyrolysis of thiophene sulfides are summarized. The increase of pyrolysis temperature can generally pro⁃ mote the decomposition of thiophene compounds，and a slow heating rate is also beneficial for the thorough decomposi⁃ tion of thiophene compounds. H2 ，steam，and CO2 atmosphere can promote the decomposition of thiophene sulfides. H2 and steam promote the pyrolysis of thiophene sulfides by attacking the sulfur atom on the thiophene ring with hydrogen radicals，or reacting with C