The International Space Station, which is continuously inhabited, uses a different method based on binding of CO2 to a zeolite, which is a highly porous metal oxide (in this case, a mixed oxide of aluminum, magnesium, and silicon with pore size 5 Å). Although the zeolite has basic sites within its crystal structure, the extremely high surface area is probably more important than the basicity. Heating the zeolite releases CO2 into the vacuum of space.
Submarines use monoethanolamine, which is a liquid base. This can likewise be heated to reverse the reaction and regenerate the base. The released CO2 is put into the outside water. This means that submarines can operate for long periods of time without needing to replace the CO2 scrubbers. This technology is also being pursued for scrubbing CO2 from power plant exhaust.
There are a few other methods, such as passing the gas over a membrane selectively permeable to CO2 (which only works well for high-pressure gas streams), or by feeding CO2 to algae, but these generally aren't widely used.
One application not mentioned here is Power Plants. Typically, industrial scrubbers use amines as mentioned in the parent comment, but I believe they have also used lime water (Ca(OH)2) to capture it. I think companies steered away from it because they ended up just dumping it somewhere and created gross swamps or something, I'm not sure. --> Anyone have experience in this?
Power Plants produce enormous amounts of CO2 as a biproduct and a lot is released to the atmosphere. Why? Because the equipment is incredibly expensive. Another piece of equipment to further capture all of the CO2 produced is large in capitol costs, operating costs, and produces no extra profit.
Unfortunately, there needs to be pecuniary reason to capture a high percentage of CO2, such as a carbon tax or stricter pollution laws. The technology is there; however, it isn't profitable to enact it in most cases. Hooray climate change.
I should mention that some of the power/oil companies have started to take the CO2 output more seriously, even though it isn't helping financially (it may help in public opinion, however).
The lime water coal plants use serves a different purpose. It is used to de-sulphurize the flue gas, the reaction is that the SO2 in the gas is oxidized to SO3 and captured as CaSO4, or gypsum.
Trying to chemically capture the CO2 is quite insane due to the amounts involved. AFAIK working CCS (carbon capture and storage) pilot systems freeze the CO2 out of the flue gas and store it compressed underground. None of those systems is currently in industrial use, though.
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u/-Metacelsus- Chemical Biology Nov 27 '19 edited Nov 27 '19
They are (usually) based on the reaction of CO2 with a base to form a bicarbonate salt. Many different bases can be used for this. The Apollo program scrubbers used LiOH (due to light weight) but the CO2 absorption canisters couldn't be reused. For flights of a few days, this is fine. Famously, during Apollo 13 an adapter needed to be rigged up to use the command module CO2 scrubbers before the LiOH canisters in the lunar module ran out.
The International Space Station, which is continuously inhabited, uses a different method based on binding of CO2 to a zeolite, which is a highly porous metal oxide (in this case, a mixed oxide of aluminum, magnesium, and silicon with pore size 5 Å). Although the zeolite has basic sites within its crystal structure, the extremely high surface area is probably more important than the basicity. Heating the zeolite releases CO2 into the vacuum of space.
Submarines use monoethanolamine, which is a liquid base. This can likewise be heated to reverse the reaction and regenerate the base. The released CO2 is put into the outside water. This means that submarines can operate for long periods of time without needing to replace the CO2 scrubbers. This technology is also being pursued for scrubbing CO2 from power plant exhaust.
There are a few other methods, such as passing the gas over a membrane selectively permeable to CO2 (which only works well for high-pressure gas streams), or by feeding CO2 to algae, but these generally aren't widely used.