Molecular simulation of coal-fired plant flue gas competitive adsorption and diffusion on coal

Better understanding the microcosmic mechanism of CO2, O-2, and N-2 competitive adsorption can benefit the effective CO2 storage and fire prevention by injecting coal-fired flue gas into the goaf. Toward this aim, macromolecular coal model was established. Grand Canonical Monte Carlo and Molecular Dynamics simulation were carried out at single, binary, and ternary component systems under the conditions of 298.15-318.15 K and up to 10000 kPa. Simulation results demonstrate that the absolute adsorption amount decreases with the increase of temperature and water content in coal, and increases with the pressure increasing and corresponding bulk mole fraction. The competitiveness is CO2 > O-2 > N-2 basing on the ternary adsorption selectivity of CO2/O-2 (5.6-24.2), CO2/N-2 (8.4-29.3), and O-2/N-2 (1.2-1.5). The isosteric adsorption heat of CO2 (about 29.1-30.9 kJ/ mol) is nearly double of O-2 or N-2 (about 16.6-17.3 kJ/mol), and CO2 occupies stronger adsorption sites without being affected by O-2 or N-2. The interaction energy between CO2 and coal is greater than O-2 or N-2 due to the electrostatic energy and much larger van der Waals energy. The adsorbed molecules swell the coal and the trend of self-diffusion coefficients with pressure is consistent with loading and volumes. So the flue gas injecting into the goaf is better than pure N-2, and can store large amounts of CO2 (0.406 mmol/g), meanwhile inhibit the coal spontaneous combustion, visually displayed by density distributions. The findings provide essential evidence for injection parameters such as temperature, pressure, gas concentration, injectivity, moisture, and so on.