ELI5: the entropy of carbon capture
February 15, 2025 7:09 PM Subscribe
According to my understanding of thermodynamics, if you get X joules of useful energy from burning a ton of coal, then the amount of energy you'd need to capture the resulting CO2 is more than X, making the whole exercise worse than useless. So how does carbon capture work?
I know this is a naive question that must have been easily answered long ago, but internet searching for "the entropy of carbon capture" did not explain in terms that I could understand. Can someone more physics-savvy than I explain this apparent paradox in simple terms?
I know this is a naive question that must have been easily answered long ago, but internet searching for "the entropy of carbon capture" did not explain in terms that I could understand. Can someone more physics-savvy than I explain this apparent paradox in simple terms?
if you get X joules of useful energy from burning a ton of coal, then the amount of energy you'd need to capture the resulting CO2 is more than X
This is only true if you assume that the way the CO2 is captured is by reversing the combustion process.
Carbon capture does not work that way. They're not turning the CO2 back into oil. They're separating it out and locking it away somewhere ("capturing" it) so it doesn't go into the atmosphere.
There's no thermodymic reason that needs to take more energy than was released in burning the fuel.
posted by ManInSuit at 7:25 PM on February 15 [10 favorites]
This is only true if you assume that the way the CO2 is captured is by reversing the combustion process.
Carbon capture does not work that way. They're not turning the CO2 back into oil. They're separating it out and locking it away somewhere ("capturing" it) so it doesn't go into the atmosphere.
There's no thermodymic reason that needs to take more energy than was released in burning the fuel.
posted by ManInSuit at 7:25 PM on February 15 [10 favorites]
You can "capture carbon" by burying trees in the ground. It is not a reverse combustion process.
posted by so fucking future at 8:09 PM on February 15 [1 favorite]
posted by so fucking future at 8:09 PM on February 15 [1 favorite]
The amount of energy that you need to turn the captured carbon dioxide back into coal would be greater than the energy you'd get from burning the coal, but the energy to simply capture the carbon dioxide is unrelated to the energy you get from burning coal.
Think of it this way: if you capture carbon dioxide from the air, it could be from burning coal, from burning wood, from burning natural gas, from a human oxidizing sugars and then breathing out carbon dioxide, or from any other number of sources, all with different amounts of energy released per molecule of carbon dioxide created. The carbon capture process is just trapping and concentrating that carbon dioxide from the air and it doesn't matter which source the carbon dioxide came from (the process doesn't "know" where the carbon dioxide came from, because it is all identical). So there can't be a relation between the energy released in burning a source of carbon dioxide and the energy required to capture that carbon dioxide from the air.
posted by ssg at 8:32 PM on February 15 [3 favorites]
Think of it this way: if you capture carbon dioxide from the air, it could be from burning coal, from burning wood, from burning natural gas, from a human oxidizing sugars and then breathing out carbon dioxide, or from any other number of sources, all with different amounts of energy released per molecule of carbon dioxide created. The carbon capture process is just trapping and concentrating that carbon dioxide from the air and it doesn't matter which source the carbon dioxide came from (the process doesn't "know" where the carbon dioxide came from, because it is all identical). So there can't be a relation between the energy released in burning a source of carbon dioxide and the energy required to capture that carbon dioxide from the air.
posted by ssg at 8:32 PM on February 15 [3 favorites]
Yeah, the thermodynamics are fine, since you just need to convert the gas into something liquid or solid that's easier to contain / control, not reverse the entire combustion process. The problem with CCS is in execution: how do you know that the sequestered material isn't just leaking back out someplace? The CCS provider will have NO incentive to make sure that it's really staying down. (Legislation? Enforcement? Ha ha.)
This is similar to the large-scale switch to natural gas / methane generation over recent decades. The methane folks have claimed for forever that a) they don't leak, and b) if they do it's a trivial amount. It took us until the 2020s to finally get a satellite up (over the opposition of you know who) to detect methane plumes and WHOOPS there are tons (literally) of them. Oh and methane is a 20-to-80 times worse GHG than CO2 so geez sorry about that ...
But there's money to be minted in this geoengineering foolishness, so I have no doubt that large scale CCS will happen, especially with our new Grifter In Chief and his easy manipulability. Maybe in the 2040s we'll find out that all the CO2 was actually leaking out from the other side of the rug, but by then the investors will have their Cote d'Azur condos and liability isolation. Carry on.
posted by intermod at 8:44 PM on February 15 [4 favorites]
This is similar to the large-scale switch to natural gas / methane generation over recent decades. The methane folks have claimed for forever that a) they don't leak, and b) if they do it's a trivial amount. It took us until the 2020s to finally get a satellite up (over the opposition of you know who) to detect methane plumes and WHOOPS there are tons (literally) of them. Oh and methane is a 20-to-80 times worse GHG than CO2 so geez sorry about that ...
But there's money to be minted in this geoengineering foolishness, so I have no doubt that large scale CCS will happen, especially with our new Grifter In Chief and his easy manipulability. Maybe in the 2040s we'll find out that all the CO2 was actually leaking out from the other side of the rug, but by then the investors will have their Cote d'Azur condos and liability isolation. Carry on.
posted by intermod at 8:44 PM on February 15 [4 favorites]
Quite a bit of recovered CO₂ is used to repressurize oil wells to enable more oil to be recovered and burned. The energy usage of that is completely bananapants
posted by scruss at 8:44 PM on February 15 [2 favorites]
posted by scruss at 8:44 PM on February 15 [2 favorites]
Best answer: One strategy for removing CO2 is "carbon capture and sequestration" (CCS). This involves capturing carbon dioxide at the places where it's produced. It's analogous to putting a filter on a smokestack to get rid of particles and then throwing the filter away: You aren't converting the CO2 to anything else so there's no thermodynamic violation. (For CO2, the "filter" is frequently a solvent.) This doesn't mean it's energy efficient; that depends on the exact process. And economically, to make the storage of the captured CO2 "work" people do things like using it to pump more oil with the CO2. Which is a net benefit only if you imagine we were going to pump the oil no matter what. You can see why fossil fuel companies love this strategy.
Beyond this, there are two natural processes that remove CO2, the biological one and the geological one. This NASA page has a decent description of them both (though it calls them "fast" and "slow", respectively.). Human carbon capture attempts use both.
The biological one is basically growing stuff. This one validates your intuition: You end up with something (like wood) that you could burn, so it takes energy, and technically more than you got from burning it. But the energy is sunlight we aren't using for "human" stuff already. (Note that for a mature forest, plant matter is growing and decomposing at the same rate, so using that to do carbon removal requires short circuiting the natural cycle*, such as harvesting and treating the wood, then using it heavily in construction.)
The geological carbon cycle is different. It converts CO2 to carbonate (CO32-), then the carbonate is sequestered (via combining with calcium to form calcium carbonate, for example.) Biological organisms like shellfish are often involved in this, but the thermodynamics is such that it happens in a completely lifeless world, too. Not only does it work, it's so effective that one theory of the Cryogenian period ice age is it was caused by an increase in seismic/volcanic activity producing a lot of new rock, which absorbed so much CO2 the planet became a giant snowball in a reverse greenhouse effect.
It's an extremely slow process. Glacially slow, if you'll excuse the pun. Human attempts to speed this up include something as simple as grinding up existing rock that hasn't absorbed CO2 yet (search online for "olivine"), or developing artificial materials that do the same thing, only faster. The H2O + CO2 -> CO32- + H2 reaction is thermodynamically favorable under the right conditions, so there's no theoretical problem with the idea. But obviously digging up and grinding rock would use lots of power, so whether we could ever do this practically is an open question.
* You could also just try to plant new forests and not cut them down, so the carbon gets "stored" by living trees. I feel this is a very appealing approach for certain kinds of act-locally tinged environmentalism. And I love forests, who doesn't? But this isn't practical; you'd rapidly fill up all existing arable land with new forests if you tried.
posted by mark k at 10:17 PM on February 15 [9 favorites]
Beyond this, there are two natural processes that remove CO2, the biological one and the geological one. This NASA page has a decent description of them both (though it calls them "fast" and "slow", respectively.). Human carbon capture attempts use both.
The biological one is basically growing stuff. This one validates your intuition: You end up with something (like wood) that you could burn, so it takes energy, and technically more than you got from burning it. But the energy is sunlight we aren't using for "human" stuff already. (Note that for a mature forest, plant matter is growing and decomposing at the same rate, so using that to do carbon removal requires short circuiting the natural cycle*, such as harvesting and treating the wood, then using it heavily in construction.)
The geological carbon cycle is different. It converts CO2 to carbonate (CO32-), then the carbonate is sequestered (via combining with calcium to form calcium carbonate, for example.) Biological organisms like shellfish are often involved in this, but the thermodynamics is such that it happens in a completely lifeless world, too. Not only does it work, it's so effective that one theory of the Cryogenian period ice age is it was caused by an increase in seismic/volcanic activity producing a lot of new rock, which absorbed so much CO2 the planet became a giant snowball in a reverse greenhouse effect.
It's an extremely slow process. Glacially slow, if you'll excuse the pun. Human attempts to speed this up include something as simple as grinding up existing rock that hasn't absorbed CO2 yet (search online for "olivine"), or developing artificial materials that do the same thing, only faster. The H2O + CO2 -> CO32- + H2 reaction is thermodynamically favorable under the right conditions, so there's no theoretical problem with the idea. But obviously digging up and grinding rock would use lots of power, so whether we could ever do this practically is an open question.
* You could also just try to plant new forests and not cut them down, so the carbon gets "stored" by living trees. I feel this is a very appealing approach for certain kinds of act-locally tinged environmentalism. And I love forests, who doesn't? But this isn't practical; you'd rapidly fill up all existing arable land with new forests if you tried.
posted by mark k at 10:17 PM on February 15 [9 favorites]
Other folks upthread have pointed out that CCS doesn't propose to completely undo fossil fuel combustion, and so the energy required is less than you'd expect from the First Law of Thermodynamics.
However, there is a limit placed on the process by the Second Law of Thermodynamics, which says that the entropy of a closed system cannot increase. In practical terms, this means that if we want to take the gases in the atmosphere from a less ordered state (mixed) to a more ordered state (some CO2 sequestered), this decreases their entropy. So we have to "create" a certain amount of entropy elsewhere to compensate, which itself requires some expenditure of energy. (∆S = Q/T for the boffins in the audience.)
I did the back-of-the-envelope calculation on this once. I don't remember the results precisely, but I seem to recall that the energy required to bring the atmosphere back to pre-industrial levels of CO2 was something like half of annual world energy production — a number where it was not inconceivable that you could do it over a period of decades with significant capital investment, but it was not something that could be done in the next five years either.
posted by Johnny Assay at 4:30 AM on February 16 [2 favorites]
However, there is a limit placed on the process by the Second Law of Thermodynamics, which says that the entropy of a closed system cannot increase. In practical terms, this means that if we want to take the gases in the atmosphere from a less ordered state (mixed) to a more ordered state (some CO2 sequestered), this decreases their entropy. So we have to "create" a certain amount of entropy elsewhere to compensate, which itself requires some expenditure of energy. (∆S = Q/T for the boffins in the audience.)
I did the back-of-the-envelope calculation on this once. I don't remember the results precisely, but I seem to recall that the energy required to bring the atmosphere back to pre-industrial levels of CO2 was something like half of annual world energy production — a number where it was not inconceivable that you could do it over a period of decades with significant capital investment, but it was not something that could be done in the next five years either.
posted by Johnny Assay at 4:30 AM on February 16 [2 favorites]
So how does carbon capture work?
Short answer to this part of your question is: badly.
There isn't a CCS pilot program on the planet that has come close to meeting its stated capture quantity goals, and even those goals are absurdly low. Fossll fuel extraction outfits like to spruik their own carbon capture programs but the only carbon that those are even putatively designed to capture are the emissions from their extraction operations rather than the much, much larger emissions that will inevitably happen when what they're extracting is burnt.
CCS is not a serious emissions mitigation proposal, just one hell of a fossil fuel industry talking point.
posted by flabdablet at 9:31 AM on February 16 [4 favorites]
Short answer to this part of your question is: badly.
There isn't a CCS pilot program on the planet that has come close to meeting its stated capture quantity goals, and even those goals are absurdly low. Fossll fuel extraction outfits like to spruik their own carbon capture programs but the only carbon that those are even putatively designed to capture are the emissions from their extraction operations rather than the much, much larger emissions that will inevitably happen when what they're extracting is burnt.
CCS is not a serious emissions mitigation proposal, just one hell of a fossil fuel industry talking point.
posted by flabdablet at 9:31 AM on February 16 [4 favorites]
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posted by Ausamor at 7:21 PM on February 15 [1 favorite]