Reaction Mechanisms Of Coal Pyrolysis And Coal Hydrodesulfurization By ReaxFF Molecular Dynamics

Coal plays a key role in the world’s energy resource, providing approximately 27% of the global energy needed. With the various requirements of new technology, economy and environment, clean and efficient utilization of coal is gradually replacing the traditional extensive use of coal. Coal pyrolysis is the initial process and accompanying reaction of coal combustion, gasification, liquefaction and other thermal processes. Therefore, the mechanisms of coal pyrolysis and coking have always been the main content of the research and is extremely important to the efficient and clean use of coal.As a high energy resources consumption and serious pollution industry, low consumption and clean use are the development direction of coking industry in recent years. Coking is the typical industrial application of coal pyrolysis. In this paper, as the background of coking process, the reaction mechanisms of coal pyrolysis and coal hydrodesulfurization were studied in details from the microscopic mechanism and macroscopic phenomena.Coking coal is the core material of coke production. Aiming at coking coals of different rank in North China, the proximate analysis and ultimate analysis of typical coking coals were obtained. The TGA and FTIR experiments were carried out to obtain the pyrolysis characteristics of coking coals, the IR parameters of coal structure and the change of functional groups. The results show that:the pyrolysis characteristic temperatures, the aromatic carbon ratio and the aromatic hydrogen ratio increase with the rising coal rank. Moreover, there is a good exponential relationship between the aromatic carbon ratio & the aromatic hydrogen ratio and H/C & VM, respectively. The changes of infrared absorption peaks of chars produced by different rank coking coals are substantially the same. In addition, as the temperature increases, the infrared absorption peak number and intensity of coal/cokes show a downward trend, the proportion of aromatic hydrogen is gradually increased.Considering the functional groups in the coal, a series of organic compounds containing oxygen, nitrogen and sulfur were selected as coal-related model compounds. The pyrolysis of those model compounds were studied by molecular dynamic simulations with reactive force field (ReaxFF). The temperature effect on production and the migration regularity of oxygen, nitrogen and sulfur were analyzed. Compared with the results of experiment and quantum chemistry, it shows that Reaxff-MD simulation can well estimate the pyrolysis process of coal-related model compounds. Therefore, the Reaxff-MD can be effectively applied to coal pyrolysis.Based on the Wiser coal Model, two practical coal models as well as some small molecule compounds in coals, the process of coal pyrolysis and coal hydropyrolysis were studied by ReaxFF-MD simulations. The product (gas, tar and char) evolution tendencies and the products distribution with time and temperature were obtained. In addition, the coking mechanism of coking coal was explained in a molecular level. The results show that:the initial pyrolysis position of coal model is unaffected by the temperature. Small molecule compounds have an important influence on the gas and light hydrocarbon product yield. Moreover, the coal macromolecules pyrolysis provides one source of metaplast, the suitable ratio of gas, liquid and solid phases in the metaplast is essential for the coking. The existence of hydrogen can intensify the broken of C-S bond, promote the formation of light hydrocarbons and increase the production and duration time of H2S. The maximum desulfurization rate can be obtained if a suitable temperature used and the best time for the adding of hydrogen can improve the economy of coal hydropyrolysis. In addition, the hydrogen can effectively be replaced by coke oven gas to reduce the content of sulfur in the coke.Finally, a transient coupled model of coal carbonization was developed to simulate the coking process including heat and mass transfer in the coking chamber. The simulated bed temperatures agree with the experimental data. The evolutions of bed temperature, gas temperature, gas component and flow path were illustrated. According to the results of microscopic molecular dynamics and macroscopic numerical simulations, an improved technology roadmap, which can sufficiently use the produced coke oven gas for desulfurization in the coking chamber, was proposed. Our work is expected to provide a theoretical basis and data for the development of the actual process.