Study of pyrolysis of brown coal and gasification of coal-water slurry using the ReaxFF reactive force field

Based on the Wender coal model, the processes of brown coal pyrolysis and coal-water slurry (CWS) gasification were studied by molecular dynamics simulations with the ReaxFF reactive force field. To examine the pyrolysis/gasification process and the initiation mechanism of brown coal and its CWS, some large-scale reactive systems containing different numbers of brown coal and water were built in this work. A relatively high simulation temperature, which was proven reasonable in other studies, was used to control the simulation within an acceptable period of time. The products and the change of potential energy of the systems were analyzed. The related initial reaction mechanisms and factors were discussed. It was found that the pyrolysis of brown coal began with the rupture of bridge bonds, closely followed by the separation of some functional groups such as carboxyl, methoxyl, and methyl. Then, gas products were generated from the reactions between small intermediate structures. Some mechanisms of CO and H-2 generation were discussed in the article. It was observed that temperature significantly enhanced the reactions in the brown coal pyrolysis process and the yields of gas products. For the gasification of CWS, it was observed that the reaction started from the pyrolysis of coal, and then water reacted with the fragments. The effects of temperature and mass fraction were taken into consideration. It was found that gas products were hardly generated and the consumption of water was relatively less at low temperatures and that rising temperature could significantly enhance gas yield and water consumption. There was an inflection point in the curve of water consumption when the temperature is greater than 3000 K. The mass fraction could not affect the gasification process as great as temperature. However, an appropriate amount of water could enhance the yield of gas products and the CWS with 70% mass fraction could provide enough water for H-2 and CO generation. Some of the important reactions and intermediate structures agreed with other experimental data from the literature.