Using enrichment technology to fight climate change: how ridiculous?
March 26, 2015 6:43 AM   Subscribe

If enriching uranium with a gas centrifuge costs 50 to 60 kW·h (180–220 MJ) of electricity per SWU, is it possible to adapt the formulas given there to calculate the energy needed to extract pure CO2 directly from the atmosphere? (assuming e.g. 0.04% concentration). Asking as a complete science n00b.
posted by ianso to Science & Nature (10 answers total)
A centrifuge is a very inefficient way to separate chemically distinct gases, even though it's the only way to separate compounds that are chemically identical but formed from different isotopes. Carbon capture and storage is the keyword you're looking for.
posted by ambrosen at 7:11 AM on March 26, 2015 [2 favorites]

No, because as it says in the article you linked,
Separative work is not energy. The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology.
Carbon dioxide is also much, much easier to separate from the rest of the atmosphere than U235 is from U238, because it's quite chemically distinct. Relying on a mass-discriminating separation method such as centrifuging would almost certainly be way more wasteful than taking advantage of the chemistry.
posted by flabdablet at 7:11 AM on March 26, 2015 [1 favorite]

The answer is going to be yes (unless literal 100% purity is a requirement), as the underlying physics of a centrifuge applies to CO2 molecules too. The basic idea is that as you spin things around, heavier things experiance a greater force, and we can use that differential to seperate off components of different mass.

However, there are going to be way easier ways to extract CO2 from the atmosphere as it's chemically distinct from all of the other components of the atmosphere. Centrifuges are used for uranium as, to all intents and purposes, U238 is chemically identical to U235, so there's only the difference in mass we can use. So we have to use centrifuges or something similar, even though it's really hard. Chemical production generally isn't a difficult problem for big companies / nation states to solve, wheras getting enough material for an atom bomb generally is. So unless there's a really compelling reason to use a centrifuge, this doesn't sound like a good way of going about it.
posted by Ned G at 7:12 AM on March 26, 2015

CO2 could be enriched by a centrifuge, yes. The density of gases vary with height in a gravitational field according to a known law (see the second equation under the "In the context of the Earth's atmosphere" section here), and furthermore, that variation is greater for gases with higher molecular weights than with lower molecular weights. In a 300m tall building at 20°C, the concentration of oxygen at the top of the building is 96.2% of what it is at the bottom, but the concentration of heavier carbon dioxide at the top is 94.8% of what it is at the bottom (vastly oversimplifying with no air circulation and other unlikely assumptions).

With a centrifuge you're essentially creating a much stronger gravitational field (not really, but for the purposes of the calculations it can be treated as such), so that separation can be more effective and take place over a shorter column. And you can calculate, given a specification of the initial mixture of gases and what you want the final CO2 concentration to be (which will never be absolutely pure, but can be significantly enriched), what parameters for the centrifuge you would need to achieve that — the radius of the centrifuge rotor, length of the tube, and what RPMs you would need.

However, translating that into energy required is going to be more difficult. It's going to depend on the specifications of the centrifuge itself (most of the spinning weight is the rotor itself, not the gases in the tubes), and its efficiency in energy usage. In a sort of idealized, frictionless, physics-101 way, doubling the rotation rate of a centrifuge would require four times as much energy, but things such as friction and other real-world considerations will throw a monkey wrench into those calculations. You'd need to get your hands on the actual centrifuge you were using and empirically measure its energy consumption to have a good idea of that. Or at least have the manufacturer's documentation on the centrifuge's energy usage.

But as others have already pointed out, it's going to be far easier to separate carbon dioxide from other gases by chemical means, some of which are already known. This is not an option for uranium enrichment, which is why the centrifuge method is used.
posted by DevilsAdvocate at 8:01 AM on March 26, 2015 [1 favorite]

The easy way is to bubble air through water, because some (not all) of the CO2 will go into solution. (If you have calcium oxide in solution, then you'll precipitate out calcium carbonate.)

But the numbers are terrifying: since CO2 is (as you point out) only 0.04% of the atmosphere, you have to process A LOT OF AIR to extract out any significant amount of CO2. Like hundreds of cubic kilometers of air, or even more.
posted by Chocolate Pickle at 8:55 AM on March 26, 2015 [2 favorites]

There have been some proposals for facilities (citation needed) where a substance is used to absorb CO2. The substance is then purged, using added energy, and the CO2 is then sequestered in a deep well or in the deep ocean. Again, energy is used for that purpose. The advantage of this process, compared to your centrifuge suggestion, is that there would be a natural affinity of this substance to the CO2.

The current goal of these proposed facilities is that they use less than 10% of the power generated by the fossil fuel powered generation plant that they are offsetting. That is a lot of power. Powering with wind or solar would be better. There would be a substantial investment in materials and facilities, and the energy to produce them.
posted by Midnight Skulker at 10:09 AM on March 26, 2015

The least energy-intensive way yet devised to process hundreds of cubic kilometres of air is to employ hundreds of square kilometres of land or, more efficiently, ocean area.

CO2 dissolves readily in water, and the ocean needs no particular encouragement to pull it out of the air at a vastly greater rate than anything we could hope to achieve by industrial, agricultural or silvicultural means. It even sequesters it long-term as shell-forming microorganisms build calcium carbonate out of it and sink to the ocean floor, eventually to be subducted back under the crust or heaped up into mountains. Every single molecule of chalk, limestone and marble that exists in the Earth's crust today was once part of a shell around some squidgy living thing.

The absolute fastest method we could possibly use for reducing atmospheric carbon dioxide concentration has to involve not dumping long-sequestered fossil carbon into the atmosphere faster than the ocean can suck it down. We can do that either by not digging it up in the first place, or grabbing it before it leaves the smokestacks.

Designing industrial fantasies to harvest diffuse carbon from the atmosphere at large is like sitting outside an open stable door trying to work out what color wool to use for knitting a scarf long enough to lasso a thundering herd of escaped horses bolting over the horizon. Any industrial solution is going to be so punily undersized, compared to the problem, as to be laughable.
posted by flabdablet at 10:20 AM on March 26, 2015 [1 favorite]

There are two different answers to your question. The literal answer is that yes, the power requirements for operating a gas centrifuge for uranium separation are probably approximately the same as operating a gas centrifuge for separating other types of gases, since the principles of operation would be the same. I've never seen a gas centrifuge for CO2 separation though.

Using a gas centrifuge wouldn't be a preferred method of separating CO2 from air. Extracting CO2 from air is also not the most economical way of producing CO2 in general, either.

If you want CO2, the most efficient way of producing it absent anything else—cost efficiency, anyway—is by oxidizing hydrocarbon fuels, catalyzing the exhaust to convert CO to CO2, and then 'scrubbing' the CO2 from the flue gas with monoethanolamine. You can buy a monolithic machine that takes fossil fuels and emits pure CO2, if you want. Obviously that is sort of environmentally questionable, since you are getting the CO2 from fossil carbon and atmospheric oxygen, but it is cheap. (It's something of a less offensive waste of fossil fuels if you are burning the fossil fuels for some other purpose to begin with, and just taking advantage of the waste CO2.) And if your goal is to capture CO2, it's very much easier to capture it where the concentration is high (exhaust gases from fossil-fuel combustion) than it is to take it out of the air where it's a low concentration. Hence most carbon sequestration systems take this or a similar route.

You can also get CO2 by cryogenic distillation of air. Since CO2's boiling (and melting!) point is much higher than other components of air, it would be one of the first products. However, I think most commercial-scale air separation plants, which are really designed to produce N2 and O2 and other stuff like Argon economically, don't actually retain and concentrate the CO2; they basically pre-filter it out, because the machinery isn't designed to deal with solid crystals. But on a small scale you can absolutely do this in a lab, if you have the equipment to compress air up to ~850 PSI.

So... coming back to your original question: if you were looking to extract CO2 from the air, I think that you would probably see much higher efficiencies by taking advantage of the relatively high boiling point of CO2 relative to other components of air, vs. trying to separate it using the difference in mass using a centrifuge, but best still would be to find a CO2-rich source and use chemical scrubbers.
posted by Kadin2048 at 10:27 AM on March 26, 2015 [1 favorite]

Any approach to try to capture CO2 out of the planet's atmosphere must necessarily involve processing a significant percentage of the total atmosphere of the planet.

The surface of the earth is 510 million square kilometers, and the troposphere (the lowest level of the atmosphere) is generally considered to be 12 kilometers. So just the troposphere is on the order of 6 billion cubic kilometers. To have any meaningful effect you're going to have to process at least a sixth of that, 1 billion cubic kilometers.

How big is your centrifuge?
posted by Chocolate Pickle at 11:13 AM on March 26, 2015

The least energy-intensive way to extract CO2 from the atmosphere is to grow stuff. You can gasify biomass, use the gas for fuel, and leave [a proportion of] the carbon in a handy solid form - biochar.

There are industries that currently extract atmospheric CO2 - brewing alcohol produces [originally atmospheric] CO2 as a byproduct - capturing that would be very efficient as the separation stage is already done for you, cost-free.
posted by HiroProtagonist at 6:22 PM on March 26, 2015

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