A previous post discussed temperature swing adsorption, wherein carbon dioxide is captured when the sorbent is at low temperature and released when raised to sufficiently high temperature. Desorption temperatures of five to seven hundred degrees Celsius are typical with known sorbents, imposing a substantial energy cost to heat and cool the material.
There are other sorbent materials where the capture and release cycle is controlled not be temperature but by other factors. The two most common are:
- pressure-swing, where adsorption is controlled by the pressure of the gases in the process. In one study, activated carbon was used as the sorbent to capture carbon dioxide.
- moisture-swing, where the presence of water or water vapor controls the adsorption cycle. A great deal of recent work on moisture swing sorbents for carbon dioxide has been done at the Arizona State University, apparently focused on a Metal Oxide Framework material containing zirconium.
The goal with both of these technologies is for a carbon dioxide removal process requiring less energy than for temperature swing adsorption. The Temperature Swing Adsorption processes are much further along in development, with several commercial carbon capture systems (detailed in the earlier post). Pressure Swing Adsorption is used to scrub CO2 in high-oxygen feeds like for hospitals, but is not currently used at scale for carbon capture from the atmosphere. So far as I can tell, Moisture Swing Adsorption has thus far only been used in the lab and small scale trials.
Companies and organizations in this technology space
- The Center for Negative Carbon Emissions at Arizona State University originated the moisture swing adsorption technology for CO2, and continues to evangelize its potential.
- The Healthy Climate Alliance and Engineer Jim Aldridge demonstrated a direct air capture prototype using a moisture-swing sorbent at an affiliated event of the Global Climate Action Summit in September 2018.
- Infinitree appears to be working in this space and using moisture swing adsorbents for carbon removal, though there are few details. Infinitree also uses the Carbon Sink brand, as mentioned in a paper by Ishimoto et al.
- Skytree in the EU is a spinout from the European Space Agency. Its technology is a moisture-swing sorbent originally developed for air recycling in space.
- SRI International has developed a pressure-swing adsorption technology using ammonia and potassium salts. It is claimed to require very low energy inputs to cycle the sorbents and be ready for another round of adsorption.