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Per Carbon Engineering's Website: Direct Air Capture (DAC).

They claim to be able to remove 1 million tons of CO$_2$ per year with a single plant. My question is whether they needed to account for the fact that by running a DAC plant, they would decrease local CO$_2$ concentrations. Is this a factor? Or does the air in the atmosphere circulate quickly enough that they didn't need to account for changing levels of CO$_2$ in the area?

Qmechanic
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Nikhil Murali
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1 Answers1

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Direct capture of CO2 from gas streams is known technology. It is one of standard industrial methods to produce CO2 of very high purity (i.e. 99.995% pure).

However those processes use exhaust gas streams which are rich in CO2 - i.e. concentrations of the order of 10% to 50%, not 400ppm as in the general atmosphere.

Carbon Engineering's claim is just that, a claim. If you browse the site, they have "plans" to build a first operating plant starting in 2021, and "plan" that it will be operational by 2023.

Ignoring the issue of processing the same air repeatedly, and assuming the process is 100% efficient at capturing the CO2, it is easy enough to work out how much air needs to pass through their extraction plant.

Working to one significant figure, one year is 30 million seconds. So to extract 1 million tons of CO2 per year, they need to extract about 30 kg per second.

The mass of CO2 in air is about 0.07 kg/liter, so they need to process about 400 liters of air per second.

To get some idea of what that means "in real life," the air flow through a large jet engine at full power is of the order of 1000 liters per second.

Large gas turbine engines (based on aircraft engines) used in natural gas pumping run continuously for months on end without any issues depleting the local atmospheric oxygen content, so an isolated plant of this size probably won't have problems with processing recycled air either.

So on the face of it the claim is not obviously impossible to achieve, though time will tell.

However, one such plant would extract about 1 millionth of the total CO2 in the atmosphere in about 3 years of operation. To restore a "pre-industrial" CO2 concentration, requires removal of about 30% of the global CO2, or about 1 million "plant-years" of this type of device.

Draw your own conclusions whether this is a realistic technical solution to the problem, or simply a way to make money from concerned investors.

alephzero
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  • Interesting analysis, so 10,000 plants would take 100 years to do the same as 1 plant in a million years. What is the form of the CO2 and where could we put it? No answer required, just some thoughts. – PhysicsDave Jul 23 '19 at 15:19
  • I'd be curious to see the energy costs per kg of CO2. I get the feeling that one million plant years would have a pretty hefty energy bill associated with it. – JMac Jul 23 '19 at 15:24
  • Assuming that these will be government-funded, the cost of the energy bill would be cheaper than the cost of medical bills. – LostCause Jul 23 '19 at 15:31
  • @PhysicsDave based on the funding they have published for the first plant, the cost of building 10,000 plants would be around $110 billion. There was an energy use figure somewhere on a picture on their website, but I don't feel like trying to find it again. – alephzero Jul 23 '19 at 15:31
  • @alephzero I think (s)he was asking about the cost of operation and maintenance. Whatever it claims to be, it is just an estimation and could be off by even +/- 75% – LostCause Jul 23 '19 at 15:33
  • @LostCause sure, but the operation and maintenance cost is zero if you can't afford to build the plants. – alephzero Jul 23 '19 at 15:34
  • Apparently, humans put on avg of 40B tonnes of extra CO2 every year. 10k plants would potentially reduce it by 25%. A single country is not going to do it. A pool of 10 countries can easily afford $110B but its all based on the priorities. – LostCause Jul 23 '19 at 15:54
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    That's true, 10 countries could afford it but we still need to know the operating energy cost .. which maybe could be solar ... and if the CO2 is stably preserved. And then back to the the originator's question ... does the CO2 get exhausted in the locality of the plant ... probably not because it is working so slowly. – PhysicsDave Jul 23 '19 at 16:01
  • @PhysicsDave We would also probably need to check into the carbon associated with the material that goes into the plant, and how much more they need to offset for that. This is the type of thing that might actually be a really bad idea if the supply chain that builds it isn't also ecologically friendly. – JMac Jul 23 '19 at 16:17
  • The scientists who developed the machine would have considered this .... the machine's sole purpose is to reduce CO2. But yes it would be a good sanity check. I guess it's even possible the CO2 ends up in some material that could be expensive ... but I think the scientists are on top of this too. I should reread the article, its been a while. – PhysicsDave Jul 23 '19 at 18:05
  • @PhysicsDave I would like to think they considered that... but sometimes people's ideas and the funding for them can get quite a bit ahead of reality. – JMac Jul 23 '19 at 18:19