Projected energy and dollar costs of air capture processes that have appeared in recently published technical analyses. The projected dollar costs are in the range of $100–$200/tCO2—although the energy requirements vary widely, with most of those for NaOH scrubbing/lime causticization systems clustering around 500–800 kJ primary energy/mol CO2. By contrast, a larger body of work has focused on systems to capture and purify CO2 from coal-fired power plant flue gases, where the CO2 concentration is approximately 12% by volume (“flue-gas capture”), approximately 300-fold higher than air. Estimates of avoided cost for flue-gas capture using current-generation capture and compression technologies are in the range of $50–$100/tCO2 (15). The most developed flue-gas-capture solvents currently used for absorbing CO2 from industrial gas streams are aqueous solutions of amines (16), particularly monoethanolamine. The primary energy required to strip CO2 from the rich amine stream (115–140 kJ/mol CO2; ref. 13) dominates the energy requirements of the process. The driving question of our study is how the energetics and costs will scale with input CO2 concentration ranging from those found in air capture systems to those found in flue-gas-capture systems.

...In all of these analyses, it is clear that the cost to separate a given substance from a mixture scales inversely with the initial concentration of that substance...

Our empirical analysis of energetic and capital costs of existing, mature, gas separation systems indicates that air capture processes will be significantly more expensive than mitigation technologies aimed at decarbonizing the electricity sector. Unless a technological breakthrough that departs from humankind’s accumulated experience with dilute gas separation can be shown to “break” the Sherwood plot and the second-law efficiency plot—and the burden of proof for such a process will lie with the inventor—direct air capture is unlikely to be cost competitive with CO2 capture at power plants and other large point sources.

Our assessment indicates that air capture will cost on the order of $1,000/t of CO2. Through 2050, it is likely that CO2 emissions can be mitigated for costs not exceeding about $300/t of CO2 (33). However, at some point in time, air capture conceivably could be a useful tool to mitigate emissions from distributed sources, and may even be deployed to reduce atmospheric concentrations of CO2 below current concentrations. Air capture for negative net CO2 emissions would follow the decarbonization of our electricity system and other large anthropogenic point sources and assumes abundant and inexpensive non-carbon energy sources.