Chemical Thermodynamics and Kinetics of Thiophenic Sulfur Removed from Coal by Microwave: A Density Functional Theory Study

The increasing use of high-sulfur coal contradicts with the stringent environmental pressure in metallurgical industry. Microwave has been confirmed to have efficient effect for coal desulfurization. However, the removal behavior of thiophenic sulfur, as the most difficult sulfur to remove from coal, is still unclear in microwave field. In this work, the feasibility of removing thiophenic sulfur from coal by microwave was evaluated by dielectric properties test, the chemical thermodynamics and kinetics of thiophenic sulfur migration with microwave irradiation were further analyzed by density functional theory (DFT) calculation. Dielectric properties test indicates that thiophenic sulfur is strong for converting electric field energy to heat energy than coal, which means the thiophenic sulfur will be selectively heated in the process of coal desulfurization by microwave. The magnetic loss powers of coal and thiophenic sulfur are very small and can be ignored. DFT calculation shows that under microwave irradiation, the energy barrier and activation energy of rate-limiting step for thiophenic sulfur removal decrease by 7.6 kJ/mol and 2.24 kcal/mol, and the logarithm of rate constant increases by 11.2 s−1. It suggests that the thiophenic sulfur removal is promoted by catalyzing the breaking of C–H and C–C bonds. In addition, the optimal removal path of thiophenic sulfur is also changed, accompanying the variation of sulfur-containing products from carbon monosulfide (CS) to thioketene (C2H2S). These findings reveal the catalytic mechanism of microwave on the thiophenic sulfur removal from coal, which are conducive to the sustainable utilization of high-sulfur coal in metallurgy process.