Analysis of Dual,\nOnboard Storage and Separation of\nBiogas in Carbon-Based Adsorbed Gas Systems

Direct utilization of “low-grade” biogas,\nas an important\nsustainable energy resource, provides a viable approach to avoid the\nenergy-intensive upgrading step that is often required to separate\nCO<sub>2</sub> from CH<sub>4</sub> in order to produce a pipeline-grade\nfuel gas. This study investigated dual, onboard storage and separation\nof biogas in an adsorbed gas system (AGS) over two highly porous carbon\nsorbents, Br-318 and Nuchar, in the pressure range of 0–55\nbar. The AGS was pressurized with a 50/50 vol % CH<sub>4</sub>/CO<sub>2</sub> feed, and working capacities as a function of charge–discharge\npressures and temperatures were determined and compared with those\nobtained from the pure CH<sub>4</sub>-pressurized AGS. Also, the outlet\ncomposition of the discharged gas during depressurization was determined\nexperimentally as a function of vessel void fraction. Both sorbents\nexhibited an acceptable storage and separation performance; but Br-318\noutperformed Nuchar by displaying a 20% higher storage capacity at\na 40% void fraction. This, however, only improved the purity of delivered\nCH<sub>4</sub> by 10% at elevated temperatures. As the void fraction\nof the Br-318-filled AGS decreased from 40 to 10%, the weight of biogas\nstored increased by 35.3% and the purity of CH<sub>4</sub> at the\noutlet increased to 89%. Analysis of the temperature profiles indicated\nlarge temperature fluctuations of 52.0 and 40.3 °C during charge\nand discharge steps, respectively, necessitating a proper thermal\nmanagement to maintain the vessel’s temperature. Additionally,\ncyclic test results showed capacity losses of 7.4 and 5.7% for Br-318\nand Nuchar, respectively, after the fourth cycle, highlighting the\nimportance of consistent regeneration to maintain the storage/separation\nperformance of the biogas-filled AGS. Lastly, the biogas-filled vessel\ndemonstrated an increased gas storage performance compared to the\npure CH<sub>4</sub>-filled vessel by exhibiting 61.8% higher storage\ncapacity under the same conditions.