DAC in the clouds - freezing CO2 at high altitude

[Robert Höglund]

I don't think this company "High-hopes" has been discussed here. Would be interesting to hear what people think about the proposed method. 

"instead of pulling in the air near the ground, they could use machines at high altitudes, where temperatures can dip to minus 70 degrees Celsius (minus 94 Fahrenheit). At minus 80 degrees Celsius, carbon dioxide freezes into what are essentially snowflakes. With relatively little energy, the air can be cooled enough to freeze the carbon out of the air and capture it in an adsorbent material. Then it’s brought back to earth, to be buried underground or used in manufacturing products"

https://www.fastcompany.com/90627344/ten-miles-above-the-earth-these-machines-can-capture-co2-from-the-air

Best regards

Robert Höglund
Independent consultant 
www.twitter.com/roberthoglund 
www.linkedin.com/in/roberthoglund 

+46 8 559 25 515 

[Renaud de RICHTER]

Older proposals were made in Antarctica.

• Honjou, Takako, and Hiroshi Sano. "Huge CO2 storage in Antarctic ice sheet." Energy conversion and management 36.6-9 (1995): 501-504.
• Agee, Ernest, Andrea Orton, and John Rogers. "CO2 snow deposition in Antarctica to curtail anthropogenic global warming." Journal of applied meteorology and climatology 52.2 (2013): 281-288.

Can really 420 ppm of CO2 generate snowflakes at -80°C?

The lowest natural temperature ever directly recorded at ground level on Earth is −89.2 °C (−128.6 °F; 184.0 K) at the Soviet Vostok Station in Antarctica on 21 July 1983 by ground measurements. Did CO2 snow deposition occured? 

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[jamesmat...@gmail.com]

I believe the authors have neglected partial pressure. CO2 may desublimate at -80 degC at 1 atm of CO2. At 420 ppm of CO2 at 1 atm total pressure, that's a partial pressure of CO2 of only 42 mbar. If you go to 30,000 feet the total pressure is about 1/3 of ground level and the CO2 is still 420ppm, so the CO2 partial pressure is < 20 mbar. Desublimating CO2 at that partial pressure will require a much lower temperature than -80 degC.

This challenge is known. Post combustion carbon capture is incrementally less efficient at higher altitude sites (think coal CCS in Wyoming) than at sea level. While the volumetric composition of the flue gas may be the same as a facility at sea level, the overall lower atmospheric pressure lowers the partial pressure of CO2. Cryogenic carbon capture startups (like Sustainable Energy Solutions) use temperatures down to -140 degC for a similar reason. As they capture more CO2, the partial pressure of CO2 in the flue gas drops so they need a lower temperature to desublimate additional CO2.

[Andrew Lockley]

Bearing in mind the difficulty of launching a single large balloon for SRM (Mt/y), the idea of scaling 3 orders to remove Gt/y seems... ambitious. 

Andrew 

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