A cyclic sorption-reaction process for continuous synthetic methane production from flue gas and green hydrogen

Abstract

In the scope of Power-to-Methane processes, green H2 and CO2 (captured from flue gas, for instance) can be catalytically converted to CH4 (or synthetic natural gas), an easily stored and distributed energy vector. From the perspective of process intensification, the CO2 capture (by sorption) and conversion can be performed in the same multifunctional reactor, thus requiring fewer operational units and potentiating the synergy between the exothermic methanation and the endothermic CO2 desorption. In this regard, two reactors were individually packed with alternating layers of two commercial materials, a CO2 sorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru-based), and tested to carry out a cyclic sorption/reaction process. The feed during the sorption stages consists of a synthetic flue gas stream containing 15 %vol. of CO2 balanced in N2. In the reactive regeneration stage, the inlet is pure H2, which reacts with the previously sorbed CO2, producing CH4. The two sorptive reactors were operated in parallel and out of phase until a cyclic steady-state was reached. The employed materials showed no signs of deactivation during over 150 h of time on stream. The performance of the cyclic unit was tested under different operating conditions, in a parametric study that assessed the influence of each variable on several performance indicators like CH4 productivity and CH4 purity. The study was carried out at atmospheric pressure and centered on a reference experiment in which a CH4 productivity of 1.12 molCH4·kgcat–1·h−1 and a purity of 60.8 % were obtained, using 200 mLN·min−1 of synthetic flue gas (15 %vol. CO2) and 100 mLN·min−1 of H2, at 350 °C and a switching time of 30 min. Nevertheless, all performance indicators could be improved in other operating conditions (CH4 purity up to 71.6 % and CH4 productivity up to 1.32 molCH4·kgcat–1·h−1).