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Solid amine sorbents are limited by amine lifetime

Updated: Jun 28, 2023

Amines make excellent sorbents for carbon dioxide because they are rich in electrons, and therefore are ready to react with the electron-poor carbon at the heart of a molecule of CO2. However, this useful reactivity comes with a dark side: Amines can also react irreversibly with oxygen, permanently deactivating them.

Side-reactions are common in traditional liquid amine sorbent systems that are used for point-source capture, and in this case they are solved by continually adding a 'make-up" flow of amine to replace the molecules that are lost. This makes traditional amine systems sensitive to the rate of amine lost, and the cost of amines, but not absurdly so - the make-up stream is estimated to add 6-8% of the cost of point source capture, depending on the assumptions used. The problem is real, but not devastating.

Solid sorbents use different amines, which will have slightly different reactivities than liquid ones, but the problem persists. A new paper, co-authored by researchers at Georgia Tech, Laurence Livermore National Lab, and the startup Global Thermostat digs into the chemistry of solid sorbents, and shows that the problems arise from oxidation of the activated amine-CO2 complex, which form a functional group called a carbamate. These carbamates are heated later in the carbon capture process to re-release CO2, but if they react with an oxygen molecule first there is a chance they will irreversibly change. The mechanism of failure seen by the researchers requires both a dilute CO2 stream and the presence of oxygen. In the system they studied, degradation is measurable in only 30 sorption-desorption cycles. And notably, the failure mechanism is not something that is likely to be seen in conventional amine sorbent systems, which usually operate in mostly oxygen-free environments. The amine chemistry is old, but the application creates new failure mechanisms, and therefore new economics.

For direct air capture (DAC) applications, it is critical for the amines to last tens of thousands of cycles at least. This is because DAC sorbents are both more expensive than monoethanolamine and similar liquid systems, but also retain less CO2 per ton of sorbent. Even the future, scaled cost of solid sorbents will be 15X or more or expensive than monoethanolamine when compared in terms of the cost of material needed to capture a ton of CO2. (Monoethanolamine is a smaller molecules, as well as a cheaper one!) So under conditions where the rate of deactivation of solid amine sorbents is equivalent to the rate of deactivation of monoethanolamine, the cost of materials would be 15X higher, rising from 6-8% of system costs to the dominant part of the system. And as discussed, there is reason to expect the rate of deactivation in oxygen-rich environments to be higher. The lifetime of the amines turns out to be the single most important parameter in the system.

Amines are excellent for CO2 capture, providing a high quality output stream even when using the 0.04% CO2 input stream from air. But the lifetime issues of amines must be resolved for Global Thermostat, Climeworks, and others to be successful.

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