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airplanes is currently impractical. While the electrification of passenger vehicles is (very slowly) shifting some energy use for transportation to large electricity-generating point sources, some mobile CO2 sources, such as planes, will likely never be electrified. Thus, alternative technologies for addressing CO2 emissions from mobile sources are needed.

DIRECT CAPTURE OF CO2 FROM AIR (“AIR CAPTURE”)

In 1999, Klaus Lackner first proposed the widespread development and deployment of devices that extract CO2 directly from the atmosphere as a way to address global CO2 emissions and climate change (Lackner et al. 1999). Although initially considered an alternative to capture from large point sources, the direct capture of CO2 from the air, or “air capture,” is generally considered a complementary technology to point source capture. Implementation of the two technologies together could allow long-term use of fossil energy while slowing or mitigating the impacts of anthropogenic CO2 emissions on climate change. Furthermore, unlike other climate mitigation options—often described as geo-engineering, whereby humans tinker with the planet to influence climate—CO2 capture from air may be a safer option, a form of traditional pollution control.

Why, then, have PCC and air capture not been widely implemented? Because, in the absence of a price on emitted carbon, there is no incentive for the private sector to adopt such technologies. A recent study published by the US Department of Energy suggests that 90% of the coal-fired power plants in the United States could implement PCC at a cost of approximately $60 per ton of CO2 captured (Nichols 2011). However, as an emerging technology, there are far fewer detailed technoeconomic descriptions of air capture processes, and the limited reports offer a wide array of estimated costs. One study of air capture processes based on CO2 absorption using basic alkaline hydroxide solutions suggested costs of $500–1,000/ton CO2 (House et al. 2011), whereas an evaluation of a second-generation technology based on use of supported amine adsorbents estimated costs closer to $100/ton CO2 (Kulkarni and Sholl 2012).

TECHNICAL CHALLENGES OF AIR CAPTURE

Most gas separation processes considered for air capture are based on CO2-absorbing liquids or CO2-adsorbing solids, with the overall process passing through cyclical stages of adsorption and desorption, as shown in Figure 1. This approach is common and used in a variety of scalable gas separation technologies. However, compared to most large-scale gas separation processes, air capture has a unique challenge associated with the ultradilute nature of CO2 (~400 ppm) in the atmosphere. Yet it also has some key advantages over PCC; for example, it can be located anywhere in the world, because ambient air is largely uniform in composition; in addition, impurities in fossil fuel exhaust (e.g., nitrogen and sulfur



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