In systems achieving direct air capture (DAC) of carbon dioxide (CO2), ambient air flows over a chemical sorbent, either liquid or solid, 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.
DAC is now included in discussions of climate change policy because it is among the few strategies that might lower the atmospheric concentration of CO2 to reduce the negative impacts of climate change. However, the intent of this assessment is not to make specific policy recommendations.
The assessment 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. It concludes that DAC would play a very limited role in a coherent CO2 mitigation strategy for many decades. Deployment of DAC would not be pursued aggressively until the world has largely eliminated centralized sources of CO2 emissions, especially at coal and natural gas power plants, either by substitution of non-fossil alternatives or by capture of nearly all of their CO2 emissions. For example, it makes little sense to ignore the emissions of CO2 in the flue gas from a coal power plant while removing CO2 from ambient air where it is 300 times more dilute.
The assessment estimates that removing CO2 from the flue gas of a coal power plant would be seven or more times less expensive, relative to a benchmark DAC system that, in the assessment committee’s judgment, is well enough described in the published literature that its costs can be estimated today. The benchmark system removes 1 MtCO2/yr from the atmosphere. Applying a simplified costing methodology used in industry for early-stage projects, its avoided cost is estimated to be at least $600/tCO2. Using the same methodology, the estimated avoided cost for “post-combustion capture” of CO2 from the flue gas of a reference coal power plant is about $80/tCO2.
A variety of science and engineering issues will determine the ultimate feasibility and competitiveness of DAC. If DAC were to ever have a substantial role in removing CO2 from the atmosphere, it would need to be much less costly than the benchmark system considered in the report. Today, few relevant experimental results have been published, and no demonstration or pilot-scale DAC system has yet been deployed. Improved designs would involve alternative strategies for bringing air into contact with chemicals, new chemistries for sorption and regeneration, materials that can operate effectively and efficiently over thousands of consecutive cycles, and low-carbon energy sources for power and heat in order to avoid emitting more than one CO2 molecule into the atmosphere for each CO2 molecule captured. From what is now known, it would not be wise to delay dealing with climate change on the grounds that at some future time DAC could be available as a significant compensating strategy.
Robert Socolow (Princeton University) served as a co-chair of the DAC study.
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