A review of biogas upgrading technologies: key emphasis on electrochemical systems.

Biogas, consisting mainly of CO and CH, offers a sustainable source of energy. However, this gaseous stream has been undervalued in wastewater treatment plants owing to its high CO content. Biogas upgrading by capturing CO broadens its utilisation as a substitute for natural gas. Although biogas upgrading is a widely studied topic, only up to 35% of produced raw biogas is upgraded in the world. To open avenues for development research on biogas upgrading, this paper reviews biogas as a component in global renewable energy production and upgrading technologies focusing on electrochemically driven CO capture systems. Recent progress in electrochemical CO separation including its energy requirement, CO recovery rate, and challenges for upscaling are critically explored. Electrochemical CO separation systems stand out for achieving the most affordable technology among the upgrading systems with a low net energy requirement of 0.25 kWh/kg CO. However, its lower CO recovery rate compared to conventional technologies, which leads to high capital expenditure limits the commercialisation of this technology. In the last part of this review, the future perspectives to overcome the challenges associated with electrochemical CO capture are discussed.