Saturday, June 23, 2018

Carbon Capture: Temperature Swing Adsorption

Adsorption: the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.

Temperature Swing Adsorption (TSA) for carbon capture relies on a set of materials, called carbon dioxide sorbents, which attract carbon dioxide molecules at low temperature and release them at a higher temperature. Unlike the Calcium Loop described previously, there is no chemical reaction between the sorbent and the CO2. Adsorption is purely a physical process, where the CO2 sticks to the sorbent due to the slight negative charges of the oxygen atoms and positive charge of the carbon.

There are a relatively large number of materials with this sorbent property for carbon dioxide, enough to have a dedicated Wikipedia page. These materials contain porous gaps. The gaps in the most interesting materials for our purpose are the right size to hold a CO2 molecule, with a slight charge at the right spot to attract the charges of different points on the CO2. To be useful for carbon capture, the sorbent has to attract CO2 molecules but readily release them with a change in temperature. They can be cycled from cold to hot to repeatedly grab and release carbon dioxide.

Unfortunately most of the known materials have drawbacks which make them unsuitable for real-world use, such as being damaged by water vapor.

The most recent class of sorbents developed are Metal-Organic Frameworks (MOFs), which are chains of organic molecules bound up into structures with metals. Metal-Oxide Frameworks are interesting because they are much more robust than the previously known sorbents, not being easily damaged by compounds found in the air and capable of being cycled in temperature without quickly wearing out.


Companies in this technology space

  • Inventys in Canada builds a carbon capture system using Temperature Swing Adsorption materials. Their system uses circular plates of a sorbent material, stacked vertically, and rotates the plates within a cylindrical housing. At different parts of the revolution the plates spend 30 seconds adsorping CO2, 15 seconds being heated to 110 degrees Celsius to release the concentrated CO2, and 15 seconds cooling back down to 40 degrees to do it again.

    Inventys goes to some length to explain that their technology is in the whole system, not tied to any particular sorbent material. I suspect this is emphasized because Metal Oxide Frameworks are innovating rapidly, and indeed the entire class of MOF materials was developed after Inventys was founded, so they ensure that the system can take advantage of new sorbent materials as they appear.

  • Skytree in the EU is a patent licensing firm which is fairly coy about the technologies it licenses but says they were developed as part of the Advanced Closed Loop System for the International Space Station. One of the main innovations in the ACLS is the development of a solid resin adsorbent Astrine, which means the technology is adsorption-based.

  • Climeworks in Switzerland describes their process as a filter which is then heated to release the carbon dioxide. This is clearly an adsorption process, and almost certainly using Metal-Organic Frameworks as it is described as being reusable for a large number of cycles.

  • Global Thermostat in New York describes their process as an amine-based sorbent bonded to a porous honeycomb ceramic structure.