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

• Published in: Fuel (Guildford) 2019-04, Vol.242, p.398-407
• Main Authors: Chen, Shi-Yun, Ding, Jun-Xia, Li, Guang-Yue, Wang, Jie-Ping, Tian, Yan, Liang, Ying-Hua
• Format: Article
• Language: English
• Subjects: Bonding strength; Carbon dioxide; Cleavage; Computer simulation; Density functional theory; Experimental methods; Gases; Gasification; Hydrogen sulfide; Intermediates; Lignite; Mechanism; Molecular dynamics; Pyrolysis; Radicals; ReaxFF; Sulfur; Sulfur dioxide; Thermal decomposition; Trajectory analysis
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2019.01.010

[Display omitted] •Formation mechanisms of sulfur-containing gases in CO2 gasification are simulated.•RS, SCO2, RSO and SO are important precursors for sulfur-containing gases.•CO2 can weaken CS bonds, and contributes to the formation of H2S, COS and SO2.•Simulation results have explained experimental phenomena in the molecular level. 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-CO2 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 carbonsulfur 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 carbonsulfur bonds by stabilizing the cleavage products. The formation reactions of HS, RSCO2 and RSO 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.