294 — A Process for Capturing CO2 from the Atmosphere

Pulling CO₂ from the air is one of the easiest ways to scrub carbon from the environment.

Aside from…like… planting trees.

But I digress!

If one must use machines to scrub CO₂, Keith et al propose in this paper a powerful and cost-effective direct-air-capture (DAC) technique that outcompetes all previous methods of carbon capture and storage (CCS).

The DAC process works like this: A chamber is opened to the surrounding air; at normal atmospheric conditions, the CO₂ binds to some substrate (a sorbent) — often a solid material. The chamber is then closed, and some property is “cycled”: Perhaps the pressure, humidity, or temperature is varied drastically from atmospheric conditions. This induces the sorbent to release the captured CO₂ back into the chamber, where it can be vacuumed off into storage.

But this method has been hugely inefficient in the past for a whole variety of reasons, including impure ambient air and the requirement that the chamber be alternatingly opened and closed. Liquid (aqueous) sorbents resolve some of this problem because the contactor can run without needing to swap a chamber open and closed, and impurities in the air generally stay in the liquid with only minimal impact to its sorbency.

But dry environments — one of the best places to scrub CO₂ to begin with — are not great for aqueous systems, since the dry air wicks away the water from the liquid sorbent. And due to the volume and complexity of the system, liquid systems are much more expensive to run.

The process pitched in this paper uses a two-cycle process, the first of which is akin to a normal aqueous system, and captures CO₂ in an ionic solution. The second loop reacts CO₃^2- with Ca²⁺ to form CaCO₃, where dissolved Ca(OH)₂ maintains a steady supply of calcium ion.

This technique means that it costs somewhere between 100 and 200 USD to pull a ton of CO₂ from the air; existing methods generally cost dramatically more. A prototype system currently pulls one Mton of CO₂ per year; for comparison, this is equal to the CO₂ absorption of around 50,000 large trees — nearly 1000 acres of forest.