A research group from the Tsuji Laboratory at the Graduate School of Engineering, the University of Tokyo, has proposed a threshold CO₂ concentration for economically viable geological storage in Direct Air Capture (DAC) systems. This finding provides practical guidance for the design of DAC systems and for planning Carbon Capture and Storage (CCS) projects that involve impure CO₂.
DAC is a key technology in the path toward net-zero CO₂ emissions, as it enables removal of CO₂ directly from the atmosphere. It plays an essential role in offsetting emissions from sectors where CO₂ reduction is difficult. However, the large-scale implementation of DAC is currently limited by the high energy cost associated with purifying captured CO₂ to a high concentration.
To overcome this challenge, the use of impure CO₂, containing air components like nitrogen and oxygen, has been proposed to reduce purification energy and overall system costs. However, these impurities decrease CO₂ storage efficiency. This study investigated how storage efficiency declines with increasing impurity content across a wide range of pressures and temperatures. The team found that this reduction in efficiency correlates with molecular-level interactions, particularly van der Waals forces.
To quantitatively assess the impact of impurities, the researchers introduced a new index: the Normalized Storage Efficiency caused by Impurities (NSEI). Applying this index to six representative CCS projects worldwide, including CCS sites in Iceland, the U.S., Norway, China, and Japan, they found significant variation in the effectiveness of impure CO₂ storage depending on local geological conditions. Among them, the Cranfield site in the U.S. demonstrated relatively high storage potential even at lower CO₂ purities.
Finally, a trade-off analysis between CO₂ capture cost and storage efficiency revealed that a CO₂ concentration of 70 mol% serves as the minimum threshold for economically viable geological storage. This concentration represents the point where reductions in purification costs are balanced by acceptable losses in storage efficiency. This work contributes to optimizing the balance between CO₂ purity and system cost, providing a solid foundation for the future development of DAC technologies and practical implementation of CCS projects using impure CO₂.
Moreover, since DAC can remove CO₂ directly from the atmosphere regardless of location, it enables the deployment of standalone CO₂ reduction systems even in remote areas such as deserts or offshore platforms, which were previously considered unsuitable for carbon capture. This new concept of combining DAC with geological storage of low-purity CO₂ could represent a scalable approach for reducing atmospheric CO₂ on a global scale.
Schematic of the Direct Air Capture (DAC) process and its integration with geological storage, illustrating how the Normalized Storage Efficiency caused by Impurities (NSEI) varies with CO₂ concentration.
Papers
Journal: Communications Engineering
Title: Economically Viable Geological CO2 Storage from Direct Air Capture has Critical Threshold of 70% CO2 Concentration
Authors: Le Zhang, Yunfeng Liang*, Arata Kioka, and Takeshi Tsuji*