Biogas main components besides methane are carbon dioxide and water that often have to be removed to increase the calorific value of the gas. In this study the effect that the presence of water vapor has on CO2 separation from a biogas stream by means of adsorption on a lignocellulosic-based activated carbon is addressed. Binary CO2/CH4 breakthrough experiments at 30 °C to account for competitive adsorption under dry conditions were conducted at several feed compositions and adsorption pressures in a fixed-bed lab set-up. An adsorption selectivity factor was selected for analysis of the different cases. The results indicated that CO2/CH4 separation by adsorption on this material could be most promising at lower pressures.

Humid gas mixtures were then evaluated in the fixed-bed set-up purposely adapted. The effect of water vapor co-adsorption on the separation of CO2 and CH4 was studied for different CO2/CH4 ratios, at 30 °C and atmospheric pressure. Under humid conditions the CO2 uptake was reduced but the CH4 uptake showed negligible influence of water vapor co-adsorption. Results also indicated that the presence of pre-adsorbed water vapor (saturated bed) reduced the capacity of the bed to adsorb CO2 and CH4 but it prompted CO2 adsorption over CH4 and hence facilitated the selectivity towards CO2 with respect to the dry case.

Peer reviewed