The molecular dynamics simulation of lignite combustion process in O₂/CO₂ atmosphere with ReaxFF force field

Pressurized oxy-fuel combustion is a complex process consisting of free radical collisions and reactions. ReaxFF reactive Molecular Dynamics (MD) simulation method is a useful tool to investigate the chemical events in coal combustion at atomistic level. In this work, the impact of operating conditions on the distribution of main fragments and the reaction mechanisms were examined using a lignite coal model with ReaxFF force field to simulate the process of oxy-fuel combustion under pressures. The ReaxFF MD simulations found that the decomposition of coal molecular structure would be enhanced by increasing the pressure/density, oxygen concentration and temperature. The variety of initial reactions increased with temperature. Increasing pressure promoted the effective collision of molecules and the coal structure was more likely to be attacked by oxidants, so the chemical bonds in coal molecule broke earlier and the consumption of O₂ increased at higher pressure. The dehydrogenation reaction on the macromolecular structure of coal and small fragments was also promoted after increasing pressure/density. However, the number of fragments decreased as the pressure/density continued to grow, because large fragments further decomposed or some small molecular benzene ring structures with less than 6C atoms were re-polymerized. The dehydrogenation reaction of coal molecule and small fragments was promoted after increasing the pressure. O₂ attacked the benzene ring structure, the C atom on the outer side of the coal, and the carbon-containing fragments in the system, indicating that the increase of O₂ concentration promoted the ring opening and oxidation reactions, thereby promoting the decomposition and combustion of coal. The work in this paper has confirmed the findings obtained by other oxy-fuel experimental work from the microscopic point of view.