Most carbon capture targets concentrated flue gas at a smokestack. Direct air capture (DAC) is harder and more ambitious: it extracts CO₂ from the open atmosphere, where the concentration is only ~420 ppm. Because it addresses diffuse and even past emissions, DAC is one of the few routes to genuine net-negative carbon removal.
Working principle
Large fans draw air across a sorbent that selectively binds CO₂. Two chemistries dominate. Liquid solvent systems use a hydroxide solution (e.g. KOH) that captures CO₂; the carbonate is then processed at high temperature to release pure CO₂ and regenerate the solvent. Solid amine sorbents adsorb CO₂ at ambient conditions and release it when gently heated (a temperature/vacuum-swing cycle). The captured CO₂ is then stored underground or used.
| Aspect | Point-source capture | Direct air capture |
|---|---|---|
| CO₂ concentration | ~5–15% | ~0.04% (420 ppm) |
| Energy per tonne | Lower | Higher (dilute feed) |
| Addresses | New emissions at source | Diffuse + legacy CO₂ |
| Siting | At the plant | Anywhere (near storage/clean energy) |
Key challengeDAC's challenge is energy and cost: capturing a dilute gas is thermodynamically demanding, so it must run on cheap, low-carbon energy to be net-negative. Sorbent durability and regeneration heat are key levers.
Applications
- Permanent carbon removal and carbon-credit generation
- Synthetic e-fuels (CO₂ + green H₂ → hydrocarbons)
- Supplying CO₂ for industrial and food-grade use
References & further reading
- Keith et al., “A Process for Capturing CO₂ from the Atmosphere,” Joule, 2018.
- Sanz-Pérez et al., “Direct Capture of CO₂ from Ambient Air,” Chemical Reviews, 2016.
- IEA, “Direct Air Capture: A key technology for net zero,” 2022.