From some research into the feasibility of an indoor CO2 scrubber that targets pre-industrial concentrations (100~200 ppm), using a sodium hydroxide solution in a simple counter- or cross-flow packed-bed wet scrubber does a good job at scavenging CO2 from the air down to <100 ppm on the exhaust, while the hygroscopic sodium hydroxide has an equilibrium with ambient humidity at all relevant indoor living room temperatures and humidities.
Regeneration is easy in another counter-flow packed bed reactor, this time reacting with a CaOH bed to exchange the carbonate ion. The output is mostly CaCO3, with some NaOH contamination. This can probably be washed for home-scale disposal (and recuperation of the NaOH), while the industrial scale process follows up with thermally decomposing the CaC03 into CaO and CO2. This can be very pure CO2 suitable for direct sequestration, if the thermal energy is provided electrically or by combusting a hydrocarbon with purified oxygen.
So the lower cost would seem to be that of calcinating the limestone (at 900~1050°C), and a trade-off between cap-ex and op-ex for the scrubbers. The lower the flow rate, the less energy is needed to force the solution and air through the packed bed.
But afaik freezing the CO2 out of the exhaust from fossil fuel power plants and industrial processes requires less energy than the calcination, and is therefore economically favored until all easy opportunities have been converted.
The calcination seems to require about 800 Wh/kg of CO2. At typical electricity rates in favorable locations of 10 ct/kWh, this makes 1 kg DAC-CO2 cost >~8 ct. If you want the carbon out of this, you're looking at 1.25 $/kg of DAC carbon. Assuming perfect electrolyzation of the CO2.
Regeneration is easy in another counter-flow packed bed reactor, this time reacting with a CaOH bed to exchange the carbonate ion. The output is mostly CaCO3, with some NaOH contamination. This can probably be washed for home-scale disposal (and recuperation of the NaOH), while the industrial scale process follows up with thermally decomposing the CaC03 into CaO and CO2. This can be very pure CO2 suitable for direct sequestration, if the thermal energy is provided electrically or by combusting a hydrocarbon with purified oxygen.
So the lower cost would seem to be that of calcinating the limestone (at 900~1050°C), and a trade-off between cap-ex and op-ex for the scrubbers. The lower the flow rate, the less energy is needed to force the solution and air through the packed bed.
But afaik freezing the CO2 out of the exhaust from fossil fuel power plants and industrial processes requires less energy than the calcination, and is therefore economically favored until all easy opportunities have been converted.
The calcination seems to require about 800 Wh/kg of CO2. At typical electricity rates in favorable locations of 10 ct/kWh, this makes 1 kg DAC-CO2 cost >~8 ct. If you want the carbon out of this, you're looking at 1.25 $/kg of DAC carbon. Assuming perfect electrolyzation of the CO2.