The Ergosphere
Saturday, November 09, 2019
 

Direct atmospheric remediation

Word out of MIT is that researchers have produced a reversible electrochemical CO2 capture system which consumes 40-90 kJ/mol depending on conditions (h/t David B. Benson at BNC forum).  This is a range from about 0.91 GJ/ton to slightly over 2 GJ/ton.  The lowest concentration mentioned is 6000 ppm, which is certainly the highest energy consumption due to reduction in entropy during the uptake and consequent heat dissipation.  No word on whether the material functions as low as 400 ppm.

If we can grab CO2 out of the atmosphere for 2 GJ/ton, 1 TW(e) would capture 15.8 gigatons/year; at 2.5 GJ/ton you'd get almost 13 GT/yr.  The world only emits about 35 GT/yr; 3 TW(e) would likely get it all, and then some.  Of course the best solution is to use carbon-free (e.g. nuclear) energy to avoid generating CO2 in the first place, but if we need to reduce CO2 levels rapidly we now have something in the toolbox.

3 TW(e) or even 1 TW(e) is a lot, but since the atmosphere is global you can do CO2 capture anywhere, any time; you can e.g. overbuild nuclear and use surplus generation to scour CO2, or put floating wind farms in the wind belts like the "roaring forties" and have them grab CO2 and put it on the sea floor in bags.  Excess atmospheric CO2 is rapidly becoming a problem with a real engineering solution.

I wondered what the characteristics of such a capture system might be.  If this polyanthraquinone worked down to a concentration of 300 ppm given sufficient driving voltage, and consumed 110 kJ/mol (2.5 GJ/ton) in the process, what would it look like?

Assuming a Roaring Forties wind speed of no less than 8 m/s, blowing through an ocean-borne capture system with internal air speed of 5 m/s, each square meter of frontal area processes 5 m3 of air per second.  At 400 ppmv CO2 concentration at the inlet and 300 ppmv at the outlet, CO2 would be removed at the rate of 0.5 l/sec.  Given CO2 gas density of 1.907 g/liter at 10°C, this comes to 0.954 gCO2/m2/sec; at 2.5 GJ/ton (2.5 kJ/g) that's about 2.38 kW/m2 of collector area.  Assuming the full power input is dissipated as heat, the air temperature rise through the collector would be less than 0.5°C.

12 m/s wind speed is about where most commercial wind turbines reach their rated output.  The output curves, where given, do not follow a straight cubic function as one would expect from the raw physics; the rise is more rapid at low speeds and reaches maximum at an asymptote, not a corner.  This makes it difficult to estimate the output at lower speeds.  However, extrapolating from the cubic curve at 8 m/s, a minimum of 29.6% of rated output can be assumed.  Full rated output is generated at 12 m/s which is at the maximum of typical wind speeds.  Splitting the difference, an average generation of 65% seems reasonable.

Soaking up the full rated output of a 6 MW wind turbine at 2.38 kW/m2 of collector area would require 2521 m2 of collector.  This is large, but hardly impossible; it's a square slightly more than 50 m on a side, compared to a machine with a rotor diameter over 200 m.  Higher wind speeds might increase the flow through the collector, thus requiring less area.  At full power, a 6 MW(e) wind turbine could power a collector extracting 2.4 kg/sec of CO2 from the atmosphere.  That's 8640 kg/hr, 207 tons/day, 75.7 thousand tons/year; this gets to serious quantities very quickly.

So yes, it does appear likely that we can remediate the earth's atmosphere to any CO2 concentration that we deem desirable and appropriate.  If we don't have the technology yet, we are well on our way to having it in time; we don't have the energy yet, but we have every reason to get it for other reasons.  At this point, all we really need is the will to get the job done.
 
Comments:
The MIT capture tech is interesting; nice post. I suspect storing the captured CO2 via "bags" dropped in the ocean, at at rate of up to 35 GT/yr, is not yet feasible. Liquification of all annual CO2 emissions (35 GT CO2/yr) at 163 kJ/lb CO2, and it has to be sunk to 3000 M to become heavier than seawater (the CO2 deep 'lake' scheme), though 2000M is about the limit of deep water petroleum operations now. The volume of total global CO2 annual emissions at 2kg/M^3 liquified is 70,000 km^3/yr.

Caldeira has written on the topic.
https://sequestration.mit.edu/pdf/direct_injection.pdf
 
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