Towards bio-upgrading of biogas: Biomass waste-based adsorbents

The choice of an appropriate adsorbent for CO₂ separation by pressure swing adsorption remains a field of intense research and it would be of great help to rely on a parameter that could be used to quickly evaluate the performance of those materials. In this work, two activated carbons prepared in our laboratory from biomass wastes (olive and cherry stones) are studied for the separation of CO₂ from mixtures of CO₂ and CH₄. CO₂ and CH₄ adsorption isotherms at room temperature and up to 10 bar were determined in a high pressure magnetic suspension balance. Breakthrough adsorption experiments with simulated biogas binary mixtures were conducted in a fixed-bed lab-pilot adsorption unit. The performance of the tested carbons was assessed in terms of a dimensionless sorbent selection parameter (S), which is based on the selectivity but incorporates the nature of the adsorption isotherm under PSA operating conditions. A commercial activated carbon Calgon BPL was also tested and used as reference material for the separation process. Experimental results show that the performance of the different adsorbents, based on the calculated S parameter, may be very different depending on whether data is derived from pure component adsorption isotherms or from breakthrough adsorption experiments. Under dynamic flow conditions, activated carbons produced in this study show a great potential to be applied to a CO₂/CH₄ separation for biogas upgrading purposes. Further research is envisioned in order to define a PSA process that maximizes the purity and recovery of CH₄.