Ex-situ biomethanation for CO₂ valorization: State of the art, recent advances, challenges, and future prospective

Ex-situ biomethanation is an emerging technology that facilitates the use of surplus renewable electricity and valorizes carbon dioxide (CO₂) for biomethane production by hydrogenotrophic methanogens. This review offers an up-to-date overview of the current state of ex-situ biomethanation and thoroughly analyzes key operational parameters affecting hydrogen (H₂) gas-liquid mass transfer and biomethanation performance, along with an in-depth discussion of the technical challenges. To the best of our knowledge, this is the first review article to discuss microbial community structure in liquid and biofilm phases and their responses after exposure to H₂ starvation during ex-situ biomethanation. In addition, future research in areas such as reactor configuration and optimization of operational parameters for improving the H₂ mass transfer rate, inhibiting opportunistic homoacetogens, integration of membrane technology, and use of conductive packing material is recommended to overcome challenges and improve the efficiency of ex-situ biomethanation. Furthermore, this review presents a techno-economic analysis for the future development and facilitation of industrial implementation. The insights presented in this review will offer useful information to identify state-of-the-art research trends and realize the full potential of this emerging technology for CO₂ utilization and biomethane production.