Process intensification concepts for CO2 methanation- A review

The combination of Power-to-Methane (PtM) and Carbon Capture and Utilization (CCU) concepts allows the simultaneous decrease of the greenhouse gas emissions (GHG), and the use of renewable electricity to produce synthetic natural gas (SNG) that can be stored and/or distributed by current gas infrastructure. In this way, CO2 and green H2 can be catalytically converted into synthetic methane via CO2 methanation reaction. This review focuses on studying process intensification strategies in CO2 methanation by using multifunctional reactors, more specifically units that simultaneously perform chemical reaction and separation processes. Instead of being separated and concentrated in a preliminary stage, the CO2 present at low concentration in flue gas (<= 15 vol%) or in the biogas streams (ca. 25-50 %) can be captured by adsorption and converted to CH4 in the same unit. Thus, in this type of device, a mixture of CO2 adsorbent and methanation catalyst (typically based on Ni or Ru) is placed inside the reactor to combine the two process units into a single device. In the last years, for such type of multifunctional reactor, dual function materials (DFMs) were also prepared and tested. DFM materials simultaneously contain a CO2 adsorbent and a methanation catalyst; this allows easier reactor packing and having the captured CO2 near the catalyst active sites during the reactive regeneration step (processing intensification is thus also reached at the particle level, including heat integration). Apart from flue gas sources of CO2, some projects related to Direct Air Capture (DAC) of CO2 are also addressed. Besides that, and for biogas streams, the high CH4 content (ca. 50 - 75 %) is unfavorable for the CO2 methanation reaction due to its reversible nature. Hence, the in situ capture/removal of water, the other reaction product, by using an H2O-selective adsorbent or a permselective membrane is also considered and allows to improve the performance of the CO2 methanation for biogas upgrading application, i.e., by shifting the reaction towards the production of more CH4. This concept (sorption-enhanced reactor) also presents benefits when considering other CO2 streams.