Study on oxidation characteristics and conversion of sulfur-containing model compounds in coal

In order to reveal the evolution mechanism of organic sulfur in the process of coal spontaneous combustion, the reaction paths and thermodynamic characteristics of 2-methylthiophene, dibenzothiophene, phenyl mercaptan, diphenyl sulfide, diphenyl sulfoxide, and diphenyl disulfide have been investigated by quantum chemical calculations. The results indicate CH, -S-, -SS-, SH, -S-, and SO bonds to be the respective active sites for oxidation reactions of the six model compounds. Upon oxidation and heating, 2-methylthiophene and phenyl mercaptan undergo dehydrogenation reactions to form hydroxyl radicals. The oxidation of organic sulfur compounds involves multiple steps; mercaptans are converted into thioethers, and thioethers and thiophenes are converted into sulfoxides and sulfones. The activity order of the six organic sulfur compounds is diphenyl sulfoxide > phenyl mercaptan > diphenyl sulfide > dibenzothiophene > 2-methylthiophene > diphenyl disulfide, with corresponding activation energies of 21.00, 60.39, 76.14, 94.52, 102.39, and 202.16 kJ/mol, respectively. It can be concluded that diphenyl sulfoxide is the most reactive and is easily oxidized to the corresponding sulfone, but is only present at low levels in coal. Therefore, mercaptans play the dominant role in the oxidation of coal. The oxidation of 2-methylthiophene releases the most heat of 181.16 kJ/mol, which serves to accelerate coal spontaneous combustion.