Flawed analysis of the possibility of air capture.

In the article entitled " Economic and energetic analysis of capturing CO 2 from ambient air, " House et al. (1) drew an analogy between air capture and other gas separation processes. It concludes that (i) " unless air capture significantly outperforms these systems, it is likely to require more than 400 kJ of work per mole of CO 2 " and (ii) " costs of air capture systems will be on the order of $1,000 per tonne of CO 2 " (1). The underlying logic in this conclusion is clearly circular because the key phrase is " unless " and is flawed in making a connection between energy used and cost. Furthermore, the article does not claim or prove that any fundamental law of thermody-namics or physics prevents the air capture system from out-performing the specific processes used for comparison, or that it cannot take considerably less than 400 kJ/mol of work. In fact, the notion of minimum work does not apply to the capture of CO 2 , because the capture process is exothermic. This is a basic flaw in using a second law of thermodynamics analysis to compare CO 2 capture (air or flue gas) with analogous types of physical separation processes. From this perspective, we point out that the only fundamental difference between air capture at 400 ppm and flue gas capture at 10% is the well-known entropy difference of about 10 kJ/mol associated with concentrating the CO 2 (2). Furthermore, this implies that beyond that difference, there is no fundamental reason why CO 2 capture from relatively clean air at ambient temperatures need be more costly (or less costly) than flue gas capture of CO 2 from contaminated and hot flue gas. There is one area of work and cost that could be an energy and cost problem for CO 2 from ambient air. That is the work required to move the large amount of air, about 2,500-fold or more than CO 2 captured, over a contactor containing a sorbent that will exothermally capture the CO 2. A straightforward analysis will show that for this exothermic process, the work is related to the pressure drop in the contactor. In the well-studied parallel channel monolith contactors used in automobile catalytic converters to remove mono-nitrogen oxides chemically, pressure drops of 100 Pa have adequate surface area to capture a specific component effectively from the input gas stream. …