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A comment on The impact of direct air carbon capture on climate change. Parent comment

Re:The impact of direct air carbon capture on climate change by gibybo on Thursday November 21, 2019 @07:21PM (#21601248)
> 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 other chemical reactions where you can turn it into something less energetic than the molecule it came from?

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Re:The impact of direct air carbon capture on climate change by zozbot234 on Thursday November 21, 2019 @07:31PM (#21601312)
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.
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Re:The impact of direct air carbon capture on climate change by rapnie on Friday November 22, 2019 @03:20AM (#21603820)
The rocks are called olivine, and are very common. They indeed look very promising for CO2 absorption.

https://www.betterworldsolutions.eu/is-olivine-the-solution-...

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Re:The impact of direct air carbon capture on climate change by pfdietz on Thursday November 21, 2019 @10:38PM (#21602211)
It's not true at all. Turning the CO2 back into fuel would take as much energy as burning it released, but simply capturing the CO2 from the air need not.
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