Direct Air Capture of CO2 with Chemicals

Executive Summary

This report explores direct air capture (DAC) of carbon dioxide (CO₂) from the atmosphere with chemicals. DAC involves a system in which ambient air flows over a chemical sorbent that selectively removes the CO₂. The CO₂ is then released as a concentrated stream for disposal or reuse, while the sorbent is regenerated and the CO₂-depleted air is returned to the atmosphere.

To guide the reader to an understanding of the factors affecting costs, a benchmark system is introduced that could be built today. With optimistic assumptions about some important technical parameters, the cost of this system is estimated to be of the order of $600 or more per metric ton of CO₂. Significant uncertainties in the process parameters result in a wide, asymmetric range associated with this estimate, with higher values being more likely than lower ones. Thus, DAC is not currently an economically viable approach to mitigating climate change. Any commercially interesting DAC system would require significantly lower avoided CO₂ costs, and thus would likely have a design very different from the benchmark system investigated in this report. This report identifies some of the key issues that need to be addressed in alternative designs.

The physical scale of the air contactor in any DAC system is a formidable challenge. A typical contactor will capture about 20 tons of CO₂ per year for each square meter of area through which the air flows. Since a 1000-megawatt coal power plant emits about six million metric tons of CO₂ per year, a DAC system consisting of structures 10-meters high that removes CO₂ from the atmosphere as fast as this coal plant emits CO₂ would require structures whose total length would be about 30 kilometers. Large quantities of construction materials and chemicals would be required. It is likely that the full cost of the benchmark DAC system scaled to capture six million metric tons of CO₂ per year would be much higher than alternative strategies providing equivalent decarbonized electricity. As a result, even if costs fall significantly, coherent CO₂ mitigation would result in the deployment of DAC only after nearly all significant point sources of fossil CO₂ emissions are eliminated, either by substitution of non-fossil alternatives or by capture of nearly all of their CO₂ emissions.

Nonetheless, DAC is one of a small number of strategies that might allow the world someday to lower the atmospheric concentration of CO₂. The wide-open science and engineering issues that will determine ultimate feasibility and competitiveness involve alternative strategies for moving the air and alternative chemical routes to sorption and regeneration.

Ultimate judgments about the future role for DAC and its future cost are necessarily constrained by the scarcity of experimental results for DAC systems. No demonstration or pilot-scale DAC system has yet been deployed anywhere on earth, and it is entirely possible that no DAC concept under discussion today or yet to be invented will actually succeed in practice. Nonetheless, DAC has entered policy discussions and deserves close analysis. This report provides insights into how DAC relates to greenhouse gas emissions. This report was prepared for the APS Panel on Public Affairs (POPA). POPA routinely produces reports on timely topics so as to inform the debate with the perspectives of physicists and other scientists working in the relevant issue areas, including energy and the environment. Most reports prepared for POPA are policy studies, often making policy recommendations and suggesting priorities for research support. This report, by contrast, is a technology assessment and contains no policy or funding recommendations. The analysis is the outcome of a two-year study conducted by a 13-member committee whose members work in industry, academia, and national and government laboratories.