Biogas upgrading using single-walled carbon nanotubes by molecular simulation

Abstract

Biogas from anaerobic digestion of biological wastes is a renewable energy resource that mainly contains CH4, CO2, trace amounts of H2S and a fraction of H2O vapour. In order to transfer biogas into biomethane to meet the standards for use as vehicle fuel or for injection in the natural gas grid, removing H2S from biogas in advance is necessary. In addition, biogas is usually saturated with water vapour. It is significant to study the effect of the presence of H2O on the biogas separation performance. Adsorption of H2S/CO2/CH4 and H2O/CO2/CH4 ternary mixtures using single-walled carbon nanotubes (SWCNT) were investigated via the Grand Canonical Monte Carlo (GCMC) method. We studied the effects of carbon nanotube diameter, –COOH modification, temperature and pressure on H2S adsorption. The results indicate that the presence of hydrophilic –COOH groups does affect the separation of H2S/CO2/CH4 mixtures. Temperature swing adsorption is more suitable than pressure swing adsorption for the separation of H2S/CO2/CH4 mixtures. The effect of water vapour on the separation of CO2/CH4 was also investigated. The result shows that the presence of H2O has little effect on the selectivity of CO2/CH4 in pristine CNT, but the selectivity of CO2/CH4 with the presence of H2O is markedly enhanced after modification in –COOH modified SWCNT with specific modification degree. It is expected that this work could provide some useful information for biogas upgrading.