Carbon capture: what is it, and what crucial role does humidity play?
Direct Air Capture (DAC), the direct removal of CO₂ from the atmosphere, is one of the most promising climate technologies of the moment. But humidity is anything but a side issue in this process: it helps determine the efficiency, the lifetime of the sorbent and the purity of the end product.
⏱ approx. 5 minutes reading time
In Direct Air Capture (DAC), ambient air is passed through a system that selectively captures CO₂. The CO₂ concentration in the atmosphere is only ~420 ppm, a small fraction compared with factory exhaust gases. This makes DAC technically more challenging than post-combustion capture, but also more unique: it can be deployed at any location, independent of an emission source.
The captured CO₂ can be stored permanently (geological storage, mineralisation) or reused as a raw material for synthetic fuels, plastics or building materials, an approach also known as Carbon Capture & Utilisation (CCU).
Step 01: Adsorption
Ambient air flows past a solid sorbent (e.g. amine-treated porous material). CO₂ molecules bind to the surface. Humidity in the air plays a direct role here.
Step 02: Regeneration
The sorbent is regenerated via temperature, pressure or moisture swing. The concentrated CO₂ is released for further processing.
Step 03: CO₂ processing
The released CO₂ gas still contains moisture and impurities. For storage or use, it must be cleaned and conditioned; this is where gas conditioning comes in.
The complex role of humidity in DAC
Humidity plays an ambivalent role in a DAC system: it is both a useful process ingredient and a potential problem. Understanding this distinction is essential for designing a properly functioning system.
| Process step | Effect of humidity | Status |
|---|---|---|
| Adsorption (sorbent) | Moderate humidity (40–70% RH) increases CO₂ uptake capacity with amine-based sorbents | Positive |
| Moisture swing regeneration | High humidity (> 70% RH) triggers CO₂ desorption; this is the driving force | Functional |
| CO₂ stream quality | Released CO₂ is saturated with water vapour, which disrupts downstream sensors and compressors | Problematic |
| Sorbent lifetime | Excessive moisture or condensation degrades the sorbent and reduces CO₂ capacity per cycle | Harmful |
| CO₂ analysis & measurement | Moisture interferes with NDIR sensors and MFCs; accurate drying is required for reliable data | Manage critically |
Wet CO₂ is a measurement problem
The CO₂ gas released after regeneration of the sorbent is almost always saturated with water vapour. If this gas goes directly to sensors, MFCs or analysers without drying, it leads to measurement errors, condensation on optical elements and a shorter service life; the same problems as with any other wet gas stream.
Gas conditioning in DAC systems: the FC-Series
For DAC applications in which large gas volumes need to be conditioned, both for the incoming air stream and for the released CO₂, the Perma Pure FC-Series Humidity Exchanger offers a scalable solution.
The FC-Series also works with Nafion membrane technology and is specifically designed for higher flows. This model is normally used mainly for humidifying gas, but it can also be used for drying. Always ask for appropriate advice on this. Depending on the model, gas flows of up to 7,000 alpm are possible, suitable for pilot plants up to full-scale DAC systems.
From pilot to full scale: scalability is key
DAC installations are growing rapidly, from lab to pilot to industrial scale. The FC-Series can be deployed modularly: start with an FC125 for a pilot set-up and scale up to an FC400 or FC600 for production environments. The Nafion technology remains identical across all sizes.
In summary: managing humidity at every stage
In a DAC system, humidity is not a side issue but a process parameter that must be actively managed. During adsorption, the right humidity is a friend; in the CO₂ product and in downstream instrumentation, it is an enemy. A well-designed gas conditioning strategy, with the right dryer in the right position, is the difference between a reliable system and one that constantly requires maintenance.
