Impact of Impure Gas on CO₂ Capture from Flue Gas Using Carbon Nanotubes: A Molecular Simulation Study

We used a grand canonical Monte Carlo simulation to study the influence of impurities including water vapor, SO₂, and O₂ in the flue gas on the adsorption of CO₂/N₂ mixture in carbon nanotubes (CNTs) and carboxyl doped CNT arrays. In the presence of single impure gas, SO₂ yielded the most inhibitions on CO₂ adsorption, while the influence of water only occurred at low pressure limit (0.1 bar), where a one-dimensional chain of hydrogen-bonded molecules was formed. Further, O₂ was found to hardly affect the adsorption and separation of CO₂. With three impurities in flue gas, SO₂ still played a major role to suppress the adsorption of CO₂ by reducing the adsorption amount significantly. This was mainly because SO₂ had a stronger interaction with carbon walls in comparison with CO₂. The presence of three impurities in flue gas enhanced the adsorption complexity due to the interactions between different species. Modified by hydrophilic carboxyl groups, a large amount of H₂O occupied the adsorption space outside the tube in the carbon nanotube arrays, and SO₂ produced competitive adsorption for CO₂ in the tube. Both of the two effects inhibited the adsorption of CO₂, but improved the selectivity of CO₂/N₂, and the competition between the two determined the adsorption distribution of CO₂ inside and outside the tube. In addition, it was found that (7, 7) CNT always maintained the best CO₂/N₂ adsorption and separation performance in the presence of impurity gas, for both the cases of single CNT and CNT array.