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. Therefore,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 carbon 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 pyrolysis 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 experimental 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 H2S,SO2,COS,and CS2 are then produced from the sulfur-containing free radical intermediates. Other sulfur-containing groups will polymerize with each other or combine with the aromatic rings to generate sulfur-containing polycyclic aromatics which are retained in tar and coke. Afterward,the effects of various reaction conditions on the pyrolysis of thiophene sulfides are summarized. The increase of pyrolysis temperature can generally promote the decomposition of thiophene compounds,and a slow heating rate is also beneficial for the thorough decomposition 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 = C bonds through addition reactions. The oxidation of CO2 will reduce the difficulty of C—S bond breaking and promote the formation of more SCO2, CHO, and R—O free radicals in the thiophene sulfide,thus promoting the formation of sulfur-containing gases. The kaolin contained in coal have positive effect on the release of thiophene sulfides, and the calcareous mineral will react with thiophene sulfide to form CaS. Additives with different properties can affect the pyrolysis reactions of thiophene sulfides as reactants or catalysts. Based on the migration mechanism of thiophene sulfide, the control of sulfur release during coal pyrolysis can be handled through two ways, i. e., promoting the hydrogen migration reaction and C—S bond fracture to accelerate the decomposition of thiophene sulfide,and providing hydrogen radicals and oxygen-containing groups to inhibit the formation of polycyclic sulfur-containing aromatic hydrocarbons. In the future,it is also necessary to carry out in-depth research on the reaction mechanism of thiophene sulfide migration to liquid and solid phases,as well as the pyrolysis control mechanism of thiophene sulfide under multiple factors such as reactor types. © 2022 China Coal Society. All rights reserved.