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When it comes to Chapter 6, Direct Air Capture, by a MIT author, the news is not as rosy as IPCC estimates.
It is unlikely that DAC will reach a cost of $100-300 per t CO2 captured. In fact, the current cost is $1,200 per t CO2 captured. The author believes the challenges will likely keep the costs higher as scaleup happens.
The author gives an example for the masses that describe the differences of CO2 capture from a coal plant to air. In a coal fired plant, we have 400 red marbles and 3600 blue marbles and we need to remove all 400 marbles. In the case of air, 400 ppm, now ~420 ppm, we have 400 red marbles and 999,600 blue marbles. It is much more difficult to "find" (extract) the red marbles. Real DAC processes remove 0.6 to 0.7 fraction for Climeworks adsorption process, and 0.745 fraction for Carbon Engineering absorbent process.
The thermal energy requirement for Climeworks is about 2.6 times that of CCS amine processes at 770 kWh per t CO2. Replacement of sorbent after 24 months is a significant maintenance cost. This is usually caused by the combination of oxidation and SO2 and NOx pollution. Climeworks is researching better sorbents, and a 20% reduction in electricity and thermal requirements.
Carbon Engineering absorbers operate continuously, while Climeworks is cyclic. The absorption process uses potassium hydroxide which increases the second law efficiency, work min. reversible/Work actual, to 0.66. Climeworks has a lower second law efficiency.
Climeworks regenerate CO2 at 80-120 deg C as compared to Carbon Engineering Circulating Fluidized Bed (CFB) Reactor at 900 deg C. There is great discrepancy of the costs to operate the plants.
Currently, Carbon Engineering uses gas to heat up the CFB and provided electricity to the plant, the CO2 from NG is 35% of the total CO2 processed. For the base case, 8.81 GJ energy per t CO2 captured. Also, much water is used in the absorber due to evaporation, and humid conditions are actually better, whereas adsorption produces ~ 1 tonne H2O per tonne CO2 captured.
I must admit that an MIT PhD researcher knows more than I. I thought DAC was likely to become more advantageous. I thought circulating fluidized beds would make the adsorption process more economically friendly, I know the DOE is looking at this path as well as RTI corporation, but the MIT author didn't even mention it as a possibility. He was a very practical author. I often talk about Metal Organic Frameworks, but they have challenges. Still, much research is being done.
I suggest the reader read Chapter 6 for all the great details about Climeworks and Carbon Engineering, down to pressure drop across the beds and linear velocity requirements.
#CarbonDioxideRemoval #DirectAirCapture #ChemicalEngineer #RetiredLife #FriendOfMIT
References:
[1] Greenhouse Gas Removal Technologies. (2022). United Kingdom: Royal Society of Chemistry.