Much of the currently used techniques of scrubbing the carbon dioxide from power plant exhaust uses sophisticated amine and other organic compounds. These compounds are then forced to release the CO2 with electric current or heat so that the concentrated CO2 can be pumped underground or otherwise sequestered. Why not just spray the power plant exhaust with a finely atomized Sodium Hydroxide mist to directly precipitate out the CO2 as Sodium Carbonate or Sodium BiCarbonate? The carbonates can be sold as carbon credits and/or be useful products. Sodium Hydroxide can be obtained in quantity cheaply from electrolysis of sea water and other methods.
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asked Apr 26, 2017 at 13:40
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2 Answers
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TL;DR we use stupendous amounts of fossile fuels, capturing the carbon would require even more stupendous amounts of sodium hydroxide.
The advantage of amines is that removing CO2 from amines only requires heat input and the amines can be recycled (though some degradation does occur).
You describe a caustic scrubber, chemistry as outlined here. you would need about 2 mol NaOH / 1 mol CO2 or about 6 kg NaOH per kg Carbon in the fuel. So imagine a coal plant, and for every train car full of coal six train cars of NaOH prills arrive.
This paper (that I only skimmed) estimates the carbon footprint of NaOh tp be 1.9 kg CO2 equivalents (per kg). Caustic scrubbing will not provide a net carbon benefit.
It is possible that someone, somewhere built a bicarbonate factory using exhaust gas as a CO2 source. The driving consideration re. sizing this plant will be: How much bicarbonate can I sell, not how much electricit do I need => how much coal do i burn etc.
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$\begingroup$ thank you mart. I actually had natural gas based co-generation plants in mind where the carbon to hydrogen ratio of the fuel is much lower than coal. I don't think NaOH would have a significant carbon footprint if your energy source is solar, geothermal, tidal, hydro, or wind. $\endgroup$ Commented Apr 26, 2017 at 15:26
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$\begingroup$ A scenario would be a solar-electric, tidal, or wind powered electrolysis plant located adjacent to a desalination plant powered from the same sources. The concentrated salinated seawater from the desalination plant would be fed to the electrolysis plant instead of dumped back into the sea. The electrolysis would yield the NaOH, chlorine, hydrogen, and other useful and marketable products. I certainly agree that transportation of the NaOH would increase its carbon footprint. The natural gas energy plant would have to be close proximity; near a CNG delivery import/export. $\endgroup$ Commented Apr 26, 2017 at 15:39
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$\begingroup$ Feel free to do a mass and energy balance for your idea and see where you land. I wouldnt be surpised if somehwere, Bicarbonate plant is usung exhast gas as a carbon source. I would be surprised if your idea is feasible outside a few small niches. I suggest you hunt down the paper refrenced in the first link, this should be an in-depth treatment of (air-)carbon sequestion using NaOH. $\endgroup$– martCommented Apr 26, 2017 at 19:45
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$\begingroup$ @lunarorlunacyexplorer If you are going to build a renewable-energy plant big enough to provide the NaOH to clean a gas-fired plant, why not just forget the NaOH and gas-fired generator and use the renewable electricity directly? $\endgroup$ Commented Apr 27, 2017 at 14:05
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$\begingroup$ @alephzero Storing the energy from renewable energy sources with current technology is expensive and impractical. You need a conventional plant to deliver energy to counter the troughs of energy output from renewables. $\endgroup$ Commented Apr 27, 2017 at 15:07
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I would guess it's because the amines are able to be regenerated, so they don't need to buy more that often. Where as with the sodium hydroxide, it would not be regenerated because the CO2 took it's oxygen. Basically, the amines do not react with the CO2, but rather they absorb it and then are sent to a desorber where the amines and CO2 are separated and the pure amines get reused.
