Direct Air Capture

DAC refers to CDR technologies (engineered processes) which draw CO2 from the air. There are two fundamental routes of direct air capture:

  • dissolution - carbon dioxide dissolves into a solvent such as 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.

{list of DAC processes: 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)

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.

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.