A ReaxFF molecular dynamics study on the mechanism of organic sulfur transformation in the hydropyrolysis process of lignite

• Six categories of S-containing elementary reactions are found.
• There is a transforming cycle among different forms of S atoms.
• The lack of H2 leads to the formation of thiophene structure.
• H2 is helpful for the formation of hydrosulfuryl group and removal of S atoms.

Distributions and conversations of sulfur in hydropyrolysis of lignite greatly influence the quality of its products. In this work, two three-dimensional models of lignite for its pyrolysis and hydropyrolysis processes were built to study influences of H2 atmosphere on the organic sulfur transforming mechanism. The structural units of the two models are the molecular model provided by Wolfrum. Reactive force field (ReaxFF) molecular dynamics was then used in simulations of desulfurization process for the two models. C++ programs were developed to deal with the ReaxFF results, and analyze the effects of H2 molecules on the S-atom distributions and S-containing elementary reactions. We found that the temperature should be controlled in an appropriate range in desulfurization process to reach maximum products with minimum S content. The H2 molecules could help thiophene and thioether structures convert into thiophenol/thiol structures in lignite. Meanwhile, H2 molecules also help H2S release into the gas category, achieving the desulfurization from the liquid and solid products. This theoretical work will be helpful in understanding the hydropyrolysis process of lignite.

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