Performance Evaluation of the Adsorption of a Binary Mixture (CH4/CO2) on Commercial Activated Carbons for Biogas Upgrading

This study investigates the adsorption behavior and separation performance of CH4/CO2 binary mixtures on commercial activated carbons under conditions relevant to biogas upgrading. Adsorption isotherms of a binary CH4/CO2 mixture (60% CH4/40% CO2, v/v), selected as a model mixture relevant to biogas upgrading, were measured at 303.15 K using a high-pressure manometric system coupled with gas chromatography. All investigated materials exhibited preferential adsorption of CO2 over CH4 (SCO2/CH4 > 1), adsorption performance strongly influenced by both textural properties and surface chemistry. While ROx 0.8 showed the highest selectivity (SCO2/CH4 = 6.57), the adsorption capacity was found to be primarily governed by micropore volume and BET surface area, while CNR-115 showed a more balanced adsorption behavior with lower CH4 co-adsorption, leading to improved overall performance. To provide a more application-oriented evaluation, a methane-penalized performance indicator (API*) was introduced, combining CO2 uptake, selectivity, and methane co-adsorption into a single metric. This approach revealed that CNR-115 exhibits the best overall performance, despite not having the highest selectivity, due to its lower CH4 uptake and balanced physicochemical properties. The results highlight the importance of considering both adsorption capacity and methane preservation when selecting adsorbents for biogas upgrading. This work provides insights into structure–performance relationships and proposes a criterion for the evaluation and optimization of carbon-based adsorbents for CO2/CH4 separation.