Geological storage of CO₂–N₂–O₂ mixtures produced by membrane-based direct air capture (DAC)

Carbon capture and storage has been considered as a realistic approach to reducing atmospheric CO₂ concentrations. However, the cost of capturing high-purity CO₂ typically used for geological storage (e.g., 98%) is high. Direct air capture (DAC), a technology that extracts CO₂ of relatively low-purity from the ambient atmosphere, has been recently proposed as a means to achieve negative CO₂ emissions when the product is sequestered underground. Although the CO₂ produced by DAC is of low-purity, the other gaseous components (mainly nitrogen and oxygen) are not hazardous materials like NO_x and SO_x that must be typically dealt with by conventional projects in coal-burning plants. Here, we evaluate geological storage of the low-purity CO₂ captured via advanced membrane-based DAC technology. The ubiquity of ambient air is important in reducing transport costs and ensuring social acceptance as the CO₂ product can be both produced and stored at sites in remote areas, such as deserts and offshore platforms. We calculated the density of CO₂–N₂–O₂ mixtures via molecular dynamics simulation and evaluated the cost of the low-purity CO₂ storage. Our evaluation suggests that the storage of low-purity CO₂ in geological formations is environmentally acceptable and economically viable.