While conventional CCS is an attractive option for centralized power stations, there may be opportunities for other CCS technologies that may be more economic or environmentally preferable in certain situations (e.g., Rau et al., 2007) or could be used to remove CO2 released by many small sources (e.g., Lackner et al., 1999). There have been many initial forays into the possibility of capturing GHGs directly from the atmosphere via technological means, but research in this area is generally only in preliminary stages. The only strategy for direct air capture that has emerged thus far involves physical or chemical absorption from airflow passing over some recyclable sorbent such as sodium hydroxide. A few research groups are developing and evaluating prototypes of such systems (Rau, 2009; Stolaroff et al., 2006). Major challenges remain in making such systems viable in terms of cost, energy requirements, and scalability. Direct capture approaches must also deal with the same challenges of long-term storage of the captured CO2 as conventional CCS.
Other proposed approaches to direct capture from air involve fertilizing the ocean or modifying agricultural or ecosystem management practices (see Chapters 9 and 10). Further details and discussion about direct air capture approaches can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c). As noted in Chapter 15, sometimes direct air capture approaches are grouped together with solar radiation management approaches under the rubric of geoengineering (e.g., The Royal Society, 2009).
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