Often, the question is posed why one would capture CO₂ out of atmospheric air instead of a power plant's flue gas, where the CO₂ concentration is about 300 times higher than in the air.

A first part of the answer to that question is that, compared to flue gas capture, air capture is not as hard as it seems to be. Thermodynamics reveal that, while the CO₂ concentration differs by a factor of about 300 between flue gas and air, the minimum energy required to extract pure CO₂ differs only by a factor of about 3. Also, flue gas capture systems have to deal with complications like high inlet temperatures and contaminants like SO_x and NO_x, which is not the case for air capture.

Air Capture as Complementary Technology

Nonetheless, air capture will always be somewhat harder than flue gas capture in the near future, especially due to the high volume flows that need to be treated when capturing CO₂ from air. If two systems are under direct competition (same location, energy, labor and investment costs, etc.), capturing the same amount of CO₂ out of flue gas will most likely be somewhat cheaper than capturing the same amount out of ambient air. The actual cost difference will only be evident with satisfying accuracy after first large-scale air capture systems have been set into operation.

Yet CO₂ capture from ambient air and flue gas capture need not be considered as mutually exclusive alternatives. Rather, air capture should be regarded as a complementary technological instrument for location-independent, carbon neutral supply of CO₂ in the short- to mid-term and for capturing the emissions of distributed CO₂ sources like planes and cars in the long-term.

Air Capture Today

There are several reasons why CO₂ capture from air is meaningful in many situations today:

• There is a huge market for CO₂ as a feedstock. Many consumers need small to medium amounts of CO₂ at remote locations. Through on-site capturing systems logistics efforts, costs and energy consumption can be avoided.

• By air capture, the processes of CO₂ emission and capturing are decoupled in time and location. Hence, CO₂ capture does not need to take place next to the site of emmision. An air capture plant can instead be constructed at a more favorable location in terms of CO₂ transportation, space, costs, and other surrounding conditions.

• For instance, the emissions of an old coal power plant could be compensated through an air capture system located on a different continent, where the CO₂ is further processed (for fuel production, sequestration, etc.)

Unique Opportunities through Air Capture in the Future

The described situations open up large markets, which render the commercial development of air capture systems feasible today. Further, in the mid- to long-term, CO₂ capture from atmospheric air opens many more, unique opportunities:

• CO₂ capture from air is the only option of creating a closed carbon cycle for the production of synthetic fuels without relying on biomass taking up large amounts of valuable agricultural area.

• Air capture is the only feasible way to capture CO₂ emitted by distributed sources, which make up nearly half of the global CO₂ emissions.

• Air capture is the only option to capture emissions that occurred in the past and to potentially “go carbon negative,” i.e. reduce the atmosphere’s CO₂ content.

• Even though the dynamics of the influence of the CO₂ content of the atmosphere on the climate are relatively slow and not yet fully understood, CO₂ capture from ambient air might at some point provide a life boat to counteract climate change.