Study On The Migration Mechanism Of Thiophene Compounds In Coal Pyrolysis

With the proposal of the national dual-carbon goal,the clean and efficient utilization of coal is of great importance.Pyrolysis is an effective means of coal utilization,but pyrolysis will release the inherent sulfur element in coal,resulting in environmental pollution and low product quality.Therefore,it is important to clarify the migration and transformation mechanism of thiophene compound,an important intermediate,for coal pyrolysis utilization.In this paper,density functional theory(DFT)and wave function analysis are used to study the possible pathways of the formation of sulfur-containing radicals(S,SH and CS radicals)and sulfur gases(H2S,SO2,COS and CS2)during the pyrolysis of benzothiophene,a key sulfur-containing intermediate in coal pyrolysis.Moreover,the effect of alkali metal elements(K and Na)on the pyrolysis mechanism of thiophene is also studied.The purpose is to clarify the migration and transformation mechanism of sulfur during coal pyrolysis and provide guidance for the development of desulfurization technology.For the pyrolysis of benzo thiophene,the reaction is mainly caused by the hydrogen transfer from the adjacent C atom of S atom on the thiophene ring to the intermediate C atom,and the energy barrier to be overcome is 269.9 kJ/mol.During the pyrolysis of benzothiophene,a large number of S and SH radicals will be generated via the intermediate 2-ethynylphenylmercaptan.The energy barrier of the generation of them is 408.0 and 498.7 kJ/mol,respectively.The strong competition of S and SH free radicals leads to the massive generation of H2S In addition,due to the high energy barrier(551.8 kJ/mol),there is a small amount of CS radicals generation.Notably,the increase of temperature in the pyrolysis process can promote the occurrence of the initial homolysis reaction,and promote CS radicals formation,and increase the amount of CS2.In the presence of alkali metal ions,K+ and Na+ ions will destroy the conjugation effect of thiophene ring,thus promoting the initial reaction of thiophene pyrolysis.The dominant initial reaction is still the hydrogen transfer from the intermediate C to the adjacent C,the energy barrier is 252.7 and 231.5 kJ/mol respectively in the presence of K+and Na+.Two alkali metal ions will inhibit the migration of thiophene sulfur to gas,inhibit the formation of SH and CS radicals,promote the formation of S radicals and H2S,and change the dominant product from CS radicals to H2S.Similar to benzothiophene,thiol compounds are also key intermediates in thiophene pyrolysis to generate SH radicals and H2S and inhibited by alkali metal ions.