Analysis of Dual, Onboard Storage and Separation of Biogas in Carbon-Based Adsorbed Gas Systems

Direct utilization of “low-grade” biogas, as an important sustainable energy resource, provides a viable approach to avoid the energy-intensive upgrading step that is often required to separate CO2 from CH4 in order to produce a pipeline-grade fuel gas. This study investigated dual, onboard storage and separation of biogas in an adsorbed gas system (AGS) over two highly porous carbon sorbents, Br-318 and Nuchar, in the pressure range of 0–55 bar. The AGS was pressurized with a 50/50 vol % CH4/CO2 feed, and working capacities as a function of charge–discharge pressures and temperatures were determined and compared with those obtained from the pure CH4-pressurized AGS. Also, the outlet composition of the discharged gas during depressurization was determined experimentally as a function of vessel void fraction. Both sorbents exhibited an acceptable storage and separation performance; but Br-318 outperformed Nuchar by displaying a 20% higher storage capacity at a 40% void fraction. This, however, only improved the purity of delivered CH4 by 10% at elevated temperatures. As the void fraction of the Br-318-filled AGS decreased from 40 to 10%, the weight of biogas stored increased by 35.3% and the purity of CH4 at the outlet increased to 89%. Analysis of the temperature profiles indicated large temperature fluctuations of 52.0 and 40.3 °C during charge and discharge steps, respectively, necessitating a proper thermal management to maintain the vessel’s temperature. Additionally, cyclic test results showed capacity losses of 7.4 and 5.7% for Br-318 and Nuchar, respectively, after the fourth cycle, highlighting the importance of consistent regeneration to maintain the storage/separation performance of the biogas-filled AGS. Lastly, the biogas-filled vessel demonstrated an increased gas storage performance compared to the pure CH4-filled vessel by exhibiting 61.8% higher storage capacity under the same conditions.