Don't Write Off DAC

4 November 2025
The energy penalty of capturing CO₂ from a diffuse source is smaller than you might expect.

Many people believe that direct air capture (DAC) is fundamentally misguided. Capturing CO₂ from the atmosphere requires separating it from the much more abundant nitrogen and oxygen. The concentration of CO₂ in the atmosphere, while rising, is only 0.425%. By contrast, flue gas from a power plant contains about 10% CO₂ by volume. If we want to remove carbon, surely it’s wiser to extract it from flue gases rather than from atmospheric CO₂, right?

Thermodynamics can tell us the minimum energy required to separate a mixture of two gases. I’ve plotted the results below for 1) the atmosphere and 2) a flue gas with 10% CO₂. What is striking is that, even though the flue gas contains 225 times more CO₂, it only requires 2.5 times more energy to “unmix” it and obtain a mole of CO₂ than from the atmosphere.

I was really surprised by this because I expected the difference to be closer to ~100 times, since the difference in concentration is ~225.

So the energy penalty of capturing CO₂ from a diffuse source is smaller than you might think.

If we plot more values, we can see that the higher the CO₂ concentration in a gas, the easier it is to unmix and capture a mole of CO₂. At the far right of the graph, it takes zero energy to extract CO₂ from a stream with 100% CO₂—which seems obvious. But it’s worth noting that the energy difference between 425 ppm (atmosphere) and a 10% is only about 40%. Granted, flue gases drastically differ in their composition of CO2, but 5-30% is a pretty solid range

DAC can still be cost competitive

Let's compare the lifetime costs of a DAC plant compared with a CCS plant designed to capture CO₂ from a flue gas with 10% concentration. We will further assume that 50% of the lifetime cost of the DAC plant goes directly towards capturing the CO₂. The rest of the plant needs to be 37.5% cheaper than a CCS plant in order for the two to have the same lifetime cost, as shown in the following diagram:

Don't write off DAC!