(672d) New Negative Emissions Technology: Indirect Ocean Capture Separating CO2 from Air

Negative emissions technologies (NETs), which result in the removal of CO₂ from the atmosphere, will likely prove crucial to limiting global warming to 2 °C. Few viable NET options exist, and detailed, transparent cost assessments are scarce, hindering policy development and strategic investment.

We propose a new platform of membrane-based negative emissions technologies, which we term Indirect Ocean Capture, and provide detailed experimental results demonstrating real-world feasibility. Our research presents the design, pilot-scale construction, testing, and fully integrated techno-economic analysis of a new platform of negative emissions technologies for removing CO₂ from the atmosphere. The technological platform has two unique implementations but both rely on bi-polar membrane electrodialysis in series with hydrophobic membranes used for water-gas separations. The fully automated system extracts CO₂ in the following manner: A custom build bi-polar membrane electrodialysis system is used to efficiently produce HCl and NaOH in low concentrations. In the first implementation the continuous production of HCl at a stable concentration is pumped into degassed seawater. The seawater is acidifed to pH 4.5 and pumped across hydrophobic membrane contactors under vacuum pressures. High purity CO₂ is evolved from the seawater and extracted across the membrane. The continuous production of NaOH at a stable concenrtaion is pumped into the acidified seawater to return the seawater to its original alkalinity, and then returned to the oceans. In the second implementation, NaOH is added to seawater such that the seawater achieves a 9.3 < pH < 9.6. CaCO₃ is precipitated from solution while basic seawater is removed. HCl is pumped into the basic seawater to return the seawater to its original pH, and then returned to the oceans.

We built a pilot-scale platform of this technology and quantified the performance of this previously untested negative emissions platform. Four different membranes from two different companies were analyzed to optimize acid and base production during electrodialysis. Further membranes for CO₂ separation from water were evaulated for efficiency. Finally, we used our experimental measurements as input into a fully integrated techno-economic quantitative model that calculates the cost of indirect ocean capture, including the effects of co-location with other industrial processes that use seawater, and the resultant costs of equivalent emissions in dollars per ton of captured CO₂. Our analysis takes the important step of determining the optimized design in pilot-scale, as well as quantifying the economics for a real-world implementation verified through industrial partner. This represents the first and only detailed process design and cost assessment for indirect ocean capture.