Emerging green strategies for biogas upgrading through CO2 capture: From unconventional organic solvents to clathrate and semi-clathrate hydrates

Biogas produced from anaerobic digestion of organic materials is emerging as a renewable energy source. Unfortunately, the large quantities of CO2 contained in the raw biogas greatly decreases its calorific value, thus efficient CO2 separation is needed to upgrade biogas to biomethane. Several technologies, based on physical or chemical sorption of CO2, are already available but they suffer of some drawbacks, primarily related to their negative impact on the environment and/or high energy requirements. Development of alternative green biogas upgrading technologies is therefore needed, and research is growing exponentially in this field. As a consequence, a frequent assessment of the state of the art of these rapidly evolving technologies is required. This review summarizes the latest advances on emerging strategies for biogas upgrading based on unconventional organic solvents such as ionic liquids and deep eutectic solvents and clathrate/semi clathrate hydrates, for which low cost, ease of operation and environmental sustainability represent attractive advantages over conventional technologies. In particular, the present work is focused on the performance of these CO2 sorbent systems in terms of CO2/CH4 selectivity rather than CO2 absorption capacity. Indeed, CO2/CH4 selectivity is the most important parameter for evaluating the suitability of a sorbent for practical applications, as high selectivity means negligible CH4 losses. The scientific results published in the last few years have been collected and discussed to serve as a guide for researchers in choosing the optimal technology and operating conditions in the perspective of efficient and environmentally friendly separation of CO2 from biogas. Ionic liquids and deep eutectic solvents are excellent candidates for biogas upgrading, especially in the form of hybrid systems, which show the best performance in terms of selectivity. Similarly, biogas upgrading via clathrate hydrates can be considered efficient, environmentally friendly and cost-effective. However, some drawbacks still need to be addressed.