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FIGURE 1 Gas separation processes based on adsorption onto solid are cyclical in nature. An adsorption cycle (top) selectively removes some gas species (small solid sphere) by adsorption on a solid (large shaded sphere), yielding a purified exhaust, followed by a desorption cycle (bottom) that liberates a concentrated product, most often induced by a pressure and/or temperature change.

oxides) are present in ambient air at a much lower level, precluding the need for gas pretreatment to remove these species. This allows for siting processes at locations appropriate for CO2 use or sequestration, negating the need for transport of concentrated CO2 in pipelines over long distances.

There are five important criteria for an economically scalable air capture process.

1.   Because of the low concentration of CO2 in air, very large volumes of air must be moved through the process—about 125 times and 375 times more than for CO2 capture from a natural gas- or coal-fired power plant, respectively, assuming an equivalent capture fraction. Thus, to prevent excessive energy requirements for gas movement, the process must have very low pressure drops associated with the air flow.

2.   Also associated with the low ambient CO2 concentration, the process must use materials and/or fluids with high CO2 capture capacities, such as those with a very high density of adsorption sites and/or very strong CO2-adsorbent interactions.

3.   Favorable adsorption kinetics are important to enable short cycle times (long cycle times lead to impractical plant sizes associated with large inventories of adsorption media).

4.   Because absorption and adsorption are exothermic processes, the removal of CO2 from the capture media for concentration is endothermic and can require significant energy input. This regeneration energy must be provided at low cost, ideally in the form of low-grade waste heat.

5.   Finally, the process equipment and adsorption media must have a suitably long lifetime because the above factors will make air capture a capital-

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