Theoretical study of the formation mechanism of sulfur-containing gases in the CO2 gasification of lignite

In this work, molecular dynamics based on a reactive force field (ReaxFF) and density functional theory (DFT) are used to investigate the formation mechanisms of H2S, COS and SO2 in the CO2 gasification process of lignite, which is very difficult to be obtained by recent experimental method. The molecular representation of lignite, provided by Wolfrum, is selected as the structural unit to construct the lignite model and the lignite-O-2 model. ReaxFF simulations of the two models are then performed for 1 ns at 3000 K. The evolution of H2S, COS and SO2 shows a good agreement with the reported formation tendency in the coal pyrolysis process. By analyzing ReaxFF simulation trajectories using C + + programs, it is inferred that the thermal decomposition of sulfur containing moieties is initiated mainly by cleavage of their carbon sulfur bonds, leading to the same alkylthio intermediates. Reaction energies from DFT calculation indicate that CO2 molecules, as well as alkoxy and aldehyde radicals produced by the reactions between CO2 and the carbon structures in lignite, can weaken the carbon-sulfur bonds by stabilizing the cleavage products. The formation reactions of HS, R-S-CO2 and R-S-O radicals are respectively key steps in the formation of H2S, COS and SO2, which can both be accelerated by CO2. The obtained mechanism can successfully explain the reported experimental phenomena and helps to understand the formation processes of sulfur-containing gas during the CO2 gasification of lignite.