Subject: Re: OT: Direct Air Capture (DAC) of CO2 and XOM
OK, thanks, that helps.

2. If it is sequestering the carbon underground, wouldn't it make more sense to just use the smokestack emissions of a big natural gas power plant, where CO2 is a high percentage of (say 50%) as opposed to ambient air that has a concentration of about 0.04% CO2?
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Smokestack emissions are a mixture of CO2 emissions with nitrogen gases and other combustion products. The concentrations are much lower than 50%, and sequestering them is a far different animal than just CO2. CO2 dissolves, and can be compressed to a liquid. The other components don't. So the CO2 must be separated.

You're right about concentration mattering in cost. The highest concentration sources now are from ethanol production and ammonia production. So these will be immediate targets followed by emissions from cement plants, power plants, and industrial plants such as refineries and petrochemical plants...


Smokestack emissions can be quite high, and there are some attempts to sequester CO2 this way already. Coal generating plants typically have about 13% CO2, which is already 300 times more than what is found in atmospheric air, so it seems a priori like a more sensible place to start. And if you burn methane (natural gas), you can get 25% CO2 if you burn it cleanly enough (combined cycle generators), and even higher (for some reason that I don't understand) if you use ethanol or LNG (https://thundersaidenergy.com/...).

Of course, there are problems with using a dirty mix of hot gases, and for practical purposes, you have to capture the CO2 near where you can bury it, which is not always the optimal place to put a power plant.

Is CO2 really liquefied before it is sequestered? If so, I didn't know that, but this seems to confirm your statement: https://www.1pointfive.com/geo...

"Once captured, the CO2 is purified and compressed into a supercritical phase—a state with combined properties of gas and liquid.

Next, this CO2 is injected deep below the surface via a high-integrity well and intoan underground reservoir—often more than 5,000 feet down. Once the CO2 reaches the reservoir, it is securely sequestered through four trapping mechanisms..."

dtb