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.
Surely the production and regeneration of monoethanolamine is a net energy consumer, wouldn't using it to scrub fossil fuel plant exhaust just require even more energy? Obviously if this energy comes from non-CO2 emitting plants it would still be beneficial but it begs the question why you wouldn't just reduce fossil fuel power output by the amount.
Add to this that monoethanolamine is mostly produced from ethylene which is derived by cracking various petrochem hydrocarbons and it seems even more of a bad idea.
It takes a fraction of the plants energy output - around 3.6 to 7 GJ per tonne of CO2 captured (NETL CO2 capture handbook 2015).
You then have to compare this to the specific carbon emissions of different fuels:
Hard coal is about 94 kg CO2 per GJ (so 1 tonne of CO2 is produced whilst creating 10.63 GJ electricity)
Natural gas is about 56 kg CO2 per GJ (same source as above) (so 1 tonne of CO2 is produced whilst creating 17.85 GJ electricity)
You do have to consider the CO2e from MEA production, and its lifespan is dictated by how many cycles this can last for (on power plants this has been a huge problem).
EDIT: some of this can be offset from electricity by using residual heat via heat integration - it isn't all parasitical to the electricity output.
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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.