What is it?
Direct air capture (DAC) is nothing new. It’s been happening in nature for over three billion years in the form of photosynthesis. While plants make taking carbon (CO2) out of the atmosphere look easy, removing CO2 at an industrial scale is much more challenging.
The concentration of CO2 in the atmosphere is 400 ppm (parts per million1), about 0.04%. Putting that into perspective, the exhaust from a jet engine contains about 4.0% CO2, 100 times more concentrated. Since chemical separations are inherently concentration dependent, it would make sense that removing CO2 from combustion exhaust would be easier than from the atmosphere.
The concentration of CO2 makes it so capturing 1 tonne of it (assuming 100% capture) would require processing the entire volume of air in a modern NFL stadium2. Moving this volume of air requires significant power. In this example, moving the volume of air in the referenced stadium over a 24-hour period at a differential pressure of 3.4 kPa (0.5 psi) requires about 140 kW (180 hp).
Why do we do it?
Given the challenges of removing low-concentration CO2 from the atmosphere via direct air capture and the power requirements, why would we choose this method of carbon capture? It’s relatively simple. While emission control carbon capture is good for capturing CO2 before it enters the atmosphere, direct air capture removes CO2 that’s been emitted over the past several hundred years. In other words, direct air capture gets the emissions we didn’t catch before they entered the atmosphere.
How does it work?
There are two prevailing technologies for direct air capture: solvents and sorbents. DAC solvent technology works by drawing air through large solvent contactors where CO2 dissolves into the aqueous phase, forming a carbonate ion. The carbonate ions react with cations in the solvent (Ca2+, Na+, K+), forming a solid precipitate. The solid precipitate is separated, and cations are thermally regenerated, producing a pure stream of CO2. The product CO2 is compressed and stored underground.
Sorbent technologies use highly structured micro-porous materials to capture CO2 in low concentrations. One of the most promising classes of microporous materials are Metal Organic Frameworks, (MOFs). The pores in these materials have structures giving them high affinities for CO2. Air is drawn through large contactors containing MOF (or another sorbent) pulling CO2 from the atmosphere. The MOF is regenerated by either heating it, putting it under vacuum, or a combination of both. Like solvent technology, the product CO2 is then compressed and stored underground.
DAC marks a significant step in greenhouse gas (GHS) reduction, and conversations surrounding this technology will certainly continue as CO2 removal becomes even more critical.
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1It’s common practice that gas concentrations are reported on a volume basis. For liquid solutions, ppm typically refers to a weight basis.
21.3 million NM3
Article originally published on the POWER Engineers website.
