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Using such a process on plain air is not likely to ever be economically feasible as air only contains a tiny amount of CO₂, around 400 parts per million. If you

Using such a process on plain air is not likely to ever be economically feasible as air only contains a tiny amount of CO₂, around 400 parts per million. If you want to run such a process run it on the exhaust stack of a gas-fired power plant since that contains a good amount of CO₂. Of course this only makes sense if the capture takes less energy than the power plant produces which can not be the case since it would violate the second law of thermodynamics [1]. [1] https://www.britannica.com/science/second-law-of-thermodynamics https://www.britannica.com/science/second-law-of-thermodynam...

brutusborn 4y ago
Can you please explain why it violates the second law of thermodynamics? I thought the 2nd law would only apply if you are reversing the reaction, i.e. CO2 -> C + O2 , whereas we are simply moving the CO2 underground. To me the critical measure would be how much energy the CO2 needs to be compressed to get it underground.
andrewmutz 4y ago
Well the good news is that every year that 400ppm goes up
sbierwagen 4y ago
>violate the second law of thermodynamics This is directionally correct, in that coal emissions sequestration is unlikely to be economical, but with two caveats: 1) Most coals are not actually pure carbon, they do contain a small percentage of hydrogen. If you don't capture the resulting water vapor† then that gives you some combustion energy for free. 2) If you had to spend energy to split the carbon off CO2 and sequester the carbon, then yes, it would be a pointless round trip. Which is why most "clean coal" schemes just compress the CO2 and sequester it underground. --- †: Water vapor is itself a powerful greenhouse gas, but it varies between 4,000 and 25,000 ppm, so clean coal, which is intended to be a short term stopgap measure operated only for a few decades until the global PV solar infrastructure is built out, shouldn't have time to affect global gas balances.
- pfdietz 4y ago
  CO2 can also be sequestered by reaction with silicates to make carbonates. This process is mildly exothermic, if slow, and is how nature will draw down the CO2 we've been releasing, eventually.

brutusborn 4y ago

Can you please explain why it violates the second law of thermodynamics? I thought the 2nd law would only apply if you are reversing the reaction, i.e. CO2 -> C + O2 , whereas we are simply moving the CO2 underground. To me the critical measure would be how much energy the CO2 needs to be compressed to get it underground.

andrewmutz 4y ago

Well the good news is that every year that 400ppm goes up

sbierwagen 4y ago

>violate the second law of thermodynamics This is directionally correct, in that coal emissions sequestration is unlikely to be economical, but with two caveats: 1) Most coals are not actually pure carbon, they do contain a small percentage of hydrogen. If you don't capture the resulting water vapor† then that gives you some combustion energy for free. 2) If you had to spend energy to split the carbon off CO2 and sequester the carbon, then yes, it would be a pointless round trip. Which is why most "clean coal" schemes just compress the CO2 and sequester it underground. --- †: Water vapor is itself a powerful greenhouse gas, but it varies between 4,000 and 25,000 ppm, so clean coal, which is intended to be a short term stopgap measure operated only for a few decades until the global PV solar infrastructure is built out, shouldn't have time to affect global gas balances.

pfdietz 4y ago
CO2 can also be sequestered by reaction with silicates to make carbonates. This process is mildly exothermic, if slow, and is how nature will draw down the CO2 we've been releasing, eventually.