Adsorption and purification of biogas inside graphitic nanopores: molecular dynamics simulation approach

Biogas is one of the most common sources of biomass energy. Due to the associated environmental pollution and costs, desulfurization, and purification are the most important challenges of biogas power generation. Using all-atom molecular dynamics (MD), we systematically simulated the isothermal adsorption behavior of biogas (comprising CH₄, CO₂, H₂O, H₂S, and H₂) in graphite (Gr) slit nanopores. The impact of slit width, system temperature, and moisture content on the adsorption energy, adsorption ratio, and diffusion coefficient of biogas molecules was investigated. Simulation results revealed that due to strong interactions between graphite and H₂S, graphite slits of width d = 48 ~ 80 Å displayed significant selective adsorption of H₂S molecules. At temperatures between 300 and 500 K, Gr slits can effectively separate H₂S in biogas. Moreover, as the moisture content of biogas (vol%) increases from 0 to 20%, the formation and interactions of hydrogen bonds between water molecules create H₂O films accumulating on the Gr surface and taking up the adsorption sites, which reduces the amount of hydrogen sulfide that can be adsorbed. Our findings provide important insights into the material design for biogas purification. A schematic representation of molecular interactions between adsorbates and the wall for biogas mixtures (comprising CH₄, CO₂, H₂O, H₂S, and H₂) inside graphitic nanopores.