michael_nielsen

Ah: thermodynamics is easier to overcome: you “just” need free power/heat, for example from an Allam cycle plant or SMR. Kinetics = at 450ppm you need large air contractors that are hard to make cheap enough (including structural support) to get too far below $100/tCO2.

I'm afraid I don't understand either argument. (For calibration in explanation: my background is mainly theoretical physics, with a smattering of other stuff. ) I'd be very interested if you wouldn't mind expanding over a few tweets!

Most common argument against DAC is that CO2 is so diffuse in atmosphere (~420ppm) that it takes way more energy to capture than from flue gas. True enough, but only relevant as long as your DAC cost is dominated by energy requirements.

With a source of waste heat, you can cook the CO2 off a compound that then passively reabsorbs CO2 from the atmosphere, and at that point you just need a source of cheap zero-carbon electricity to run the fans to blow air over it (much like a big air conditioner does).

The kinetics side is: because the CO2 is so diffuse, and you have to have a contractor that passively reabsorbs CO2, you have to blow a lot of air over the contractor to capture meaningful amounts of CO2.

The contractors can’t be very light, which means you need a big strong structure to hold them in place against gravity, and against the air you’re blowing through/over them. That puts a lower bound on the capital cost of your DAC equipment.

These sound very design-specific. Are they fundamental arguments about physics, or more heuristic, about particular approaches? (That's what they sound like - much like arguments I used to hear that so-and-so was the limit to transistor density. But maybe I"m misunderstanding?)

They’re heuristic, but I think they’re general enough to apply to all the best DAC approaches under development, but a quantitative analysis from first principles is beyond my current skills.