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The impact of direct air carbon capture on climate change

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Re: The impact of direct air carbon capture on climate change

#21

Post by [deleted] »

zozbot234 wrote: post #19
gibybo wrote: post #17

Earlier quoted context omitted.

> extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon. This sounds true at first glance, but is it always? It seems like it's obviously true for simple reactions like burning methane and converting it back to methane since the energy you get from it is equal to or less than the energy required to turn it back into methane, but might there be some othe…

Yes, there are rocks that will "extract" carbon from the air on their own if crushed into sand. They are even reasonably abundant, so it seems like they would be a feasible tool for carbon capture. It would basically amount to speeding up the natural carbon cycle, which normally keeps carbon dioxide concentrations relatively stable in the very long run.

[deleted]

Re: The impact of direct air carbon capture on climate change

#22

Post by iso1337 »

jarvist wrote: post #6

I'm not sure if this article hammers home the key point: extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon. This is inevitable due to the entropy cost of de-mixing CO2 from the air, and from the limited efficiencies of thermal power generation (~33-40% for a coal power plant, perhaps 20% in an internal combustion engine), and limited efficiency of t…

There is a well known relationship summarized in the Sherwood plot.

https://www.pnas.org/content/pnas/108/51/20428/F1.large.jpg

related paper: https://www.pnas.org/content/108/51/20428

Your basic intuition is correct in that the more dilute something is, the more costly it is to extract/purify it.

Re: The impact of direct air carbon capture on climate change

#23

Post by Robotbeat »

jarvist wrote: post #6

I'm not sure if this article hammers home the key point: extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon. This is inevitable due to the entropy cost of de-mixing CO2 from the air, and from the limited efficiencies of thermal power generation (~33-40% for a coal power plant, perhaps 20% in an internal combustion engine), and limited efficiency of t…

Not objectively true, as plants and even rocks grab CO2 out of the air pretty efficiently. From a thermodynamics standpoint, you can burn fossil fuels at 50-70% efficient (50% is not uncommon), and the actual theoretical lower bound of CO2 direct air capture is ~20 kJ/mol,[0] or about 1.7MJ/kg of elemental carbon, compared to 33MJ/kg for burning elemental carbon (same as high grade coal).

(Burning methane is even better as it releases much more energy per unit CO2 i.e. ~860kJ-per-mole-of-CO2 vs anthracite's ~400kJ.)

And as others have said, rocks actually are able to exothermally absorb CO2 permanently. It's possible to speed this up by grinding rocks to a powder, placing in water with a catalyst.

...none of this is to say I support relying on direct air capture and just go ahead and burn fossil fuels. We need to keep that in the ground and transfer the whole civilization to non-fossil fuels, otherwise if some populist jerks wer elected, they could just order the expensive direct air capture program to stop (can't do that if you're already transitioned to clean energy and all your wellheads and drills and coal mining equipment are rusted or melted down for scrap).

Source: [0] https://www.researchgate.net/publication/257177020_The_therm...

Re: The impact of direct air carbon capture on climate change

#24

Post by AnthonyMouse »

aidenn0 wrote: post #16
ForHackernews wrote: post #7

Earlier quoted context omitted.

> Maybe it has a future role (>2050? >2100?) It absolutely must, or else it's basically already too late: https://www.vox.com/2016/10/4/13118594/2-degrees-no-more-fos... > This image should terrify you. It should be on billboards. > As you can see, in either scenario, global emissions must peak and begin declining immediately. For a medium chance to avoid 1.5 degrees, the world has to zero out net carbon emissions by…

My dad sees articles like this and he'll just say that maybe the damage needed to prevent warming is worse than the damage that will be done by the warming.

The cost to ordinary people would actually be relatively minor. You buy an electric car instead of a gasoline one. The average new car price in the US is ~$36,000. A new Nissan Leaf starts below $30,000, Model 3 only a little more than that. Replace coal with alternatives. We can do at least as well as nuclear, which is barely any more expensive than coal has been. Best case is that storage gets a lot cheaper than it is and then the price of electricity goes down from the historical norm.

Capture could cost as little as zero. Renewables inherently have to build over-capacity to compensate for intermittency, but that means there are days when they generate more power than there is demand. Use the rest for carbon capture and it's free.

The real "cost" to fixing it, and the big lobby against it, is that the oil and coal companies would take a nosedive. Literally trillions of dollars in lost revenue. It moves to alternatives. It isn't economic damage, it's displacement. But the specific people who own the oil and coal reserves stand to lose their shirts, and these are not people lacking in political influence. Everything you hear about the high cost -- it's only really a cost to them. To you it's just an upgrade to electric.

But we still have to actually do it. A carbon tax so that people do all buy electric cars and stop burning coal. It's a thing we can do but not a thing we have done yet.

Re: The impact of direct air carbon capture on climate change

#25

Post by theothermkn »

jarvist wrote: post #6

I'm not sure if this article hammers home the key point: extracting carbon from the air always takes more energy that was gained by burning the fossil fuel to make that carbon. This is inevitable due to the entropy cost of de-mixing CO2 from the air, and from the limited efficiencies of thermal power generation (~33-40% for a coal power plant, perhaps 20% in an internal combustion engine), and limited efficiency of t…

I think that you're thinking of the cost of turning the CO2 (and water) back into fuel, which can be argued along thermodynamic and irreversibility lines to show that unscrambling that egg always requires more energy than obtained from combusting. However, I don't think there is a similar thermodynamic argument on the energy costs of mere capture. It's not a priori impossible that a fractional distillation column with a clever configuration of heat exchangers, for example, could capture all the carbon released by the fossil-fueled plant that powered the column in the first place (and more).

Arguing that mere capture would require more energy than is released would be akin to arguing that cleaning the fireplace would require more energy than was released by the fire, on the mistaken grounds that ash removal took as much energy as regrowing the wood that was burned. Both the ash and the CO2 are in a lower energy state, so there's thermodynamic room to maneuver.

Re: The impact of direct air carbon capture on climate change

#26

Post by mmanfrin »

tathougies wrote: post #9

Good analysis, but fails to take into account the storage costs of whatever CO2 is sequestered (or the potential to offset those costs by selling the CO2 to whatever industry uses them).

> storage costs of whatever CO2 is sequestered

CO2 is not spent uranium.

Re: The impact of direct air carbon capture on climate change

#27

Post by scarygliders »

Here goes for further downvotes...

The entire edifice of this "human caused climate change" hypothesis rests on the premise that carbon dioxide is /the/ control knob for this planet's temperature.

There is zero proof that this is true. Zero. So if I were you, I'd be very, very cautious before going ahead with capturing CO2, by any method.

It's way too early for this sort of thing. Vastly more research is required before taking a potentially disastrous decision to try to reduce the amount of CO2 - a gas which Earthly vegetation /needs/ in order to survive!

Re: The impact of direct air carbon capture on climate change

#28

Post by whytaka »

Michael Nielsen, the author, also wrote a multi-part intro to neural networks (http://neuralnetworksanddeeplearning.com) which really cleared up for me the inner-workings and mathematics of machine learning quickly but without skimping on details.

I'm very excited to read what he has to say here.

Re: The impact of direct air carbon capture on climate change

#29

Post by pipermerriam »

scarygliders wrote: post #27

Here goes for further downvotes... The entire edifice of this "human caused climate change" hypothesis rests on the premise that carbon dioxide is /the/ control knob for this planet's temperature. There is zero proof that this is true. Zero. So if I were you, I'd be very, very cautious before going ahead with capturing CO2, by any method. It's way too early for this sort of thing. Vastly more research is required bef…

Can you provide anything beyond conjecture that there is any merit to what you've stated? My understanding is that the effects of CO2 as a greenhouse gas are pretty well understood at this point.

Re: The impact of direct air carbon capture on climate change

#30

Post by dv_dt »

If you're going to continue to burn fossil fuels, it seems better to burn it in a plant that inherently captures the carbon then trying to wrangle it back after it's release. The interesting part of the Allam cycle for power generation is that supercritical co2 is the working fluid for the turbine, and when you burn the fuel it increases the mass in the closed circuit. Both the size and the efficiency of the plant also purportedly improve with the Allam cycle too.

https://en.wikipedia.org/wiki/Allam_power_cycle

https://www.netpower.com/technology/

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