Summary
DAC refers to CDR technologies (engineered processes) which draw CO2 from the air. Fans move air into contact with material (liquid or solid) that attracts CO2 molecules. After the CO2 is captured by the material, the CO2 is extracted in concentrated form and the material is reused. Of the operational multi-ton per year facilities, one employs alkaline absorption and the other employs a proprietary adsorption. Both of these facilities use heat to extract the CO2 and regenerate adsorbent. Numerous other process paths are in development around the world - evaluating various materials for capturing CO2 and various methods for regenerating those materials.
There are two fundamental routes of direct air capture:
- dissolution - carbon dioxide dissolves, into a solvent - water or other liquid phase (and then often undergoes chemical reactions). The carbon dioxide molecules and the solvent molecules are mixed in a single phase and individual molecules move freely within the phase.
- adsorption - carbon dioxide molecules adhere to an attractive portion of an adsorbent substance and are retained there, or are absorbed into the sorbent phase - the two-step process of adsorption followed by absorption often being shortened to “sorption”. The carbon dioxide molecules are not free to move independently of the adsorbent. Adsorbents of carbon dioxide are commonly liquid or solid, but gas-phase adsorption is also possible. Adsorbents may cover, flow over, or be impregnated into a solid substrate or be incorporated into a membrane. The initial process of absorbance necessarily occurs at a phase interface, so adsorption always precedes absorption
Indeed, at molecular scale dissolution and adsorption represent the ONLY routes for carbon dioxide capture currently thought to be feasible - natural or technological. Dissolution and many variations of adsorption rely on the chemical property of carbon dioxide that there is a dipole moment between carbon and each oxygen - the central carbon is electrophilic. DAC technologies are variations primarily in what adsorbent is used and how the captured carbon dioxide is subsequently managed. DAC is used to denote engineered processes, but natural processes of CO2 removal also begin with sorption or dissolution.
Physical properties of CO2 are not known to be helpful for bulk air separation - although CO2 will deposit as dry ice at about -100C (cryogenic separation), the entire air stream must be cooled to this temperature - requiring complex multi-stage heat-exchange. Thus cryogenic separation is only employed when the gas stream is more than about 50% carbon dioxide.
Membrane separation of CO2 and some other gases from natural gas streams is practiced. OpenAir does not know of demonstrations of CO2 separation from atmosphere.
It may be possible to destroy carbon dioxide rather than capture it, but no technologies for achieving this have yet been devised. Imparting enough energy to CO2 molecules for them to ionize cannot be done electrostatically because oxygen molecules have lower ionization energy - before CO2 ionizes, all the oxygen must be ionized first. Perhaps CO2 can be ionized selectively with suitable wavelengths radiation, but this has not been demonstrated conclusively.
History
Preliminary draft - February 2023
Forms and Variations
alkaline stripping (CO2 + 2OH- = CO3(2-) + H2O) – dissolution and chemical adsorption
adsorption in metal-organic framework (MOF)
adsorption on activated carbon
adsorption on PEI or other amine-rich polymers
adsorption on zeolite (microporous aluminosilicate)
Notable Projects
Climeworks - located in Iceland, capacity 4000 tons/year
Carbon Engineering - located in Texas, projected capacity 500,000 tons/year (2025)
Global Thermostat early 2023 - see 6 Things to Know About Direct Air Capture}
OpenAir research and development missions:
AirSynth - a DAC device with electrochemical synthesis
CYAN - an inexpensive DAC device with alkali adsorption
PriSMa - a computer effort to identify suitable metal-organic framework (MOF) sorbents
Thursday - a DAC system with PEI sorbent on silica gel, and temperature-swing desorption
Violet - a DAC system with Arduino controls
Viridian - experiments with DAC via microalgae
{policy issues and initiatives related to DAC}
Further Learning
This Is CDR E05 - Introduction to DAC
This Is CDR E09 - Climeworks DAC
This Is CDR E15 - NOYA DAC in cooling towers
This Is CDR E18 - DAC and Concrete
This Is CDR E23 - Heriloom DAC/Mineralization Project
This Is CDR E27 - Sustaera DAC Process
This Is CDR E30 - Verdox Electrochemical DAC
This Is CDR E34 - Mission Zero: a modular, heat-free, low-cost DAC
This Is CDR E51 - Electrochemical DAC
CDR Horizons E01 - Moisture-swing Adsorption
CDR Horizons E06 - PriSMa identifying MOF adsorbent candidates
CDR Horizons E08 - Biocatalytic textile CDR
{universities and companies engaged in DAC projects}
Aspiring Materials, New Zealand is recovering magnesium hydroxide for direct air capture from olivine - this is a DAC process that may be related to mineralization such as EWR
{literature and conferences related to DAC}
June 6, 2023 - Direct Air Capture Summit, hosted by Climeworks
September 27, 2023 - https://www.carboncapture-expo.com/
{foundations, trusts, and agencies funding DAC-related projects}