ReaxFF and DFT study on the sulfur transformation mechanism during the oxidation process of lignite

Two structural models for lignite and O-2-lignite systems were constructed to investigate the impact of O-2 atmosphere on the sulfur transformation in the lignite oxidation process. Reactive force field molecular dynamics was employed to simulate the reaction process of the two models at 1000-2000 K, and then the obtained reaction pathways were further confirmed by density functional theory. By analyzing simulation trajectories with C++ program, we have obtained the existing forms, distributions and elementary reactions of sulfur during the pyrolysis and oxidation processes. Different forms of sulfur transform into each other through sulfur radical intermediates in the pyrolysis process. The added O-2 molecules or the oxides produced by O-2 can react with sulfur radicals, forming sulfur oxide radical intermediates (such as R-S-O-center dot and R-S-C-O-center dot). The sulfur atoms in these intermediates transform into small sulfur-containing gas molecules. Thus, sulfur atoms in lignite are finally removed by O-2 molecules during the oxidation process. (C) 2016 Published by Elsevier Ltd.