Novel Adsorption-Reaction Process for Biomethane Purification/Production and Renewable Energy Storage

This work proposes an innovative method for the simultaneous upgrading of biogas streams and valorization of the separated CO₂, through its conversion to renewable methane. To this end, two sorptive reactors were filled with a layered bed containing a CO₂ sorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru/Al₂O₃). The continuous cyclic operation of the parallel sorptive reactors was carried out by alternately feeding a biogas stream (CO₂/CH₄ mixture) or H₂. The CO₂/CH₄ mixture is fed to the sorptive reactor during the sorption stage, with CO₂ being captured by the sorbent and CH₄ exiting as a purified stream (i.e., as biomethane). During the reactive regeneration stage, the inlet stream is switched to pure H₂, which reacts with the previously captured CO₂ at the methanation catalyst active sites thus producing additional methane. For continuous operation, the two sorptive reactors were operated 180° out of phase and cyclic steady-state could be reached after ca. five cycles. The performance of the cyclic sorptive-reactive unit was assessed through a parametric study to evaluate the influence of different operating conditions, namely, the inlet flow rate and CO₂ content during the sorption stage, the hydrogen inlet flow rate during the reactive regeneration stage, the stage duration, and temperature. The inclusion of an inert purge after the reactive regeneration stage was also tested. The performance of the unit was compared to the case of direct hydrogenation of biogas, and conclusions were drawn regarding future optimization, with special attention being given to CH₄ productivity and purity. During the parametric study, a compromise between these process indicators, i.e., a productivity of 1.63 mol_CH4 kg_cat ^-1 h^-1 with 70.3% of CH₄ purity, was obtained at 350 °C. However, biomethane purities above 80% were easily achieved, though at the expense of methane productivities.