ReaxFF Study On The Sulfur Transfer Mechanism In The Pyrolysis Process Of Lignite

In the process of combustion and thermal transformings, lignite will produce SO₂ and H₂ S which are easy to form acid rain and haze, once these gases getting into the atmosphere. Furthermore, they will endanger human health seriously. Therefore, sulfur transfer of coal is an important issue in clean coal researeh. Reax FF can not only quickly capture or detect the sulfur radicals in pyrolysis process, but also handle chemical reaction process in complex molecular systems. Therfore, it can theoretically help study the sulfur transfer mechanism in the pyrolysis process of lignite.Based on Wolfrum model, three three-dimensional models were built for the pyrolysis process, the hydropyrolysis process and the O₂-atmosphere pyrolysis process of lignite, respectively. Then a series of pyrolysis reactions in the three systems of lignite were simulated by Reax FF. C++ programs were developed to search and cunt the Scontaining elementary reactions in the pyrolysis process. The effects of pyrolysis temperature and pyrolysis atmosphere on the sulfur transfer mechanism were then studied.The results show that, sulfur atoms can migrate from semi-coke component to tar and gas components in hydropyrolysis and the O₂-atmosphere pyrolysis for the H₂/O₂ molecules play an important role in the sulfur transfer in the pyrolysis process. In the hydropyrolysis process, H₂ molecules can provide H atoms at high temperatures, which accelerate the transformation of heterocyclic and thioether to hydrosulfuryl. Hydrosulfuryl is unstable and can be easily decomposed. SH groups in hydrosulfuryl structure then react with H radicals or H₂ molecules, forming H₂ S. Thus, the S atoms in lignite can be removed from solid component to the gas phase. In the pyrolysis process under the O₂ atmosphere, O₂ molecules can provide O atoms at high temperatures, which react with S radical and produce sulfur oxide radicals, decelerating all S transformation reactions in the semi-coke. Sulfur oxide radicals then produce S-containing gas. Thus, the S atoms in lignite can be removed from semi-coke products to tar and gas components.