Reactive force field (ReaxFF) molecular dynamics investigation of bituminous coal combustion under oxygen-deficient conditions

The microscopic reactive behaviours of functional groups in the Wiser bituminous coal model under burning of coal in atmospheres with various O2 concentrations were simulated by integrating molecular dynamics simulations with the reactive force field. The findings indicated that the combustion of coal molecules occurred through oxidation and cracking. The hydroxyl groups (-OH) on coal were dehydrogenated, and additional free radicals then attacked the remaining oxygen atoms to form carbon oxides or water. The ether bond (-O-) was the most active functional group. Generally, the alkyl with the lowest molecular mass and on one side of the ether bond departs the functional group soon after the beginning of the simulation, and this was followed by the remaining oxygen atoms being attacked by free radicals to form carbon oxides or water. The carbonyl groups (-CO) were the most stable functional group, they were attacked by O-2 or center dot O, resulting in the formation of carbon oxides in the second half of the simulation. In most cases, the hydrogen atom dissociated from the carboxyl group (-COOH), generating CO2 from the remaining one carbon atom and two oxygen atoms. The concentration of O-2 had a strong effect on the production of free radicals, carbon oxides, and water. Temperature had great effect on the breaking of chemical bonds and progress of chemical reactions. Free radicals center dot OH, center dot HO2, and C2O2 were important in coal combustion. Their numbers changed with temperature, affecting the way coal burned.