Quantum Chemical Calculation of the Effect of NH3 on CO2 Adsorption and Storage in Goaf

The injection into a goaf of a flue gas issuing from a power plant and containing highly polluting CO2 as inert gas can allow, at the same time, disaster prevention and greenhouse gas emission reduction. However, it can happen that unreacted ammonia present in the power plant flue gas is injected underground, and ammonia is present in the coal seam itself. At present, it is unclear how NH3 can affect disaster prevention and storage. To explore the influence of NH3 on CO2 storage and coal spontaneous combustion, the micromechanism of adsorption between a coal model molecule and NH3/CO2 (taken separately or as a mixture) was studied using the density functional theory of quantum chemistry, including the dispersion correction method (DFT-D3). Results show that the lowest adsorption energy of CO2 adsorbed on the surface of the coal molecule is significantly lower than that of NH3, which indicates that the adsorption of CO2 on coal is preferred to that of NH3. In the mixed adsorption state, the adsorption stability of NH3 on the coal–CO2 system is lower than that of NH3 on the coal molecular surface alone. On the contrary, the adsorption stability of CO2 on the coal–NH3 system at the same location is higher than that of CO2 on the coal molecule alone; that is, the presence of NH3 promotes the adsorption of CO2 on coal, while the presence of CO2 promotes the desorption of NH3. Therefore, the presence of NH3 is positive in terms of CO2 storage in the goaf, reducing the environmental pollution caused by flue gas from a power plant and inhibiting the occurrence of coal spontaneous combustion.