Air Capture

Guest Commentary by Frank Zeman

[This is one of an occasional series on the science of mitigation/adaptation/geo-engineering that we hope to continue. Since this isn’t our core expertise, we’d especially appreciate balanced contributions from other scientists.]

One of the central challenges of controlling anthropogenic climate change is developing technologies that deal with emissions from small, dispersed sources such as automobiles and residential houses. Capturing these emissions is more difficult as they are too small to support infrastructure, such as pipelines, and may be mobile, as with cars. For these reasons, proposed solutions, such as switching to using hydrogen or electricity as a fuel, rely on the carbon-free generation of electricity or hydrogen. That implies that the fuel must be made either by renewable generation (wind, solar, geothermal etc.), nuclear or by facilities that capture the carbon dioxide and store it (CCS).

There is however an alternative that gets some occasional attention: Air Capture (for instance, here or here). The idea would be to let people emit the carbon dioxide at the source but then capture it directly from the atmosphere at a separate facility.

The removal of carbon dioxide directly from the atmosphere is a natural phenomenon that occurs in the surface ocean or during photosynthesis. Ocean absorption is a result of both the higher concentration of CO2 in the atmosphere and the alkaline nature of seawater (Note that this absorption that is leading to the “other” CO2 problem, ocean acidification – which may prove detrimental to coral reefs and other organisms that use carbonate). Land-based air capture is an effort to enhance this mechanism at an industrial scale so that CO2 can be removed from the atmosphere under controlled conditions. Given that it is performed under controlled conditions, we can use more alkaline solutions to improve the rate of capture without adversely affecting the biosphere.

Industrial air capture is based on the absorption of CO2 using alkali earth metals such as sodium or potassium. The process is a variant of the Kraft Process used in most pulp and paper mills and as such, benefits from a long industrial history. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which an electrical voltage is applied across the carbonate solution to release the CO2. While simpler, the electrical process consumes more energy as it splits water at the same time. It also depends on electricity and so unless the electricity is renewable or nuclear, will result in the storage of more CO2 than the chemical process.

If the technology is well established and, aside from the oxygen combustion of lime, dates back over 50 years, what stops it from being used and what might change in the future?

The main barrier is the efficiency of the energy requirements during the reducing process. Air capture requires energy to move the air, manufacture the absorbing solutions and solids as well as to produce the oxygen, fuel and make up chemicals. All of these items will result in additional CO2 emissions, which reduce the efficiency and therefore the benefits. The second important consideration, and maybe the dominant one, is cost. Air capture has to be more economical than the proposed alternatives (hydrogen, electricity, renewables, greater efficiency etc.). It should be stated clearly that air capture is not a viable alternative to capture at large, point source emitters such as power plants since it will always be more efficient to capture and store carbon dioxide from more concentrated streams. So while there are any non-CCS fossil fuel plants, Air Capture is a non-starter.

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