John,
How would the "H+ on a membrane" process work if the seawater were saturated with CO2 at 10 to 50 atm (10,000 to 50,000 ppm)?
One possible biomethane purification component of the Ocean Afforestation ecosystem is differential dissolution inside the anaerobic digestion. It should be possible to obtain biogas directly from the digester at about 90% biomethane: 10% bioCO2 (at 1 atm we get 60:40). Our current vision is to recover the bioCO2 by reducing pressure (pump liquid to the ocean surface). We then recompress the 90% bioCO2:10% biomethane to liquefy the bioCO2 and recover the biomethane.
Recompression of the 90% bioCO2 uses about 10% of the system-produced energy.
Do you suspect the membrane process would allow us to recover most of the bioCO2 independent of pressure? If so, we might operate the digesters at conditions where the CO2 is produced as either a hydrate or a liquid (without the need for the compression step). Or we operate the digesters at whatever and pump the liquid to the desired depth and temperature for membrane CO2 removal. (Still need all the liquid back at the surface with its dissolved plant nutrients and dissolved biomethane recovery.)
Lots of issues to work around here. I would rather operate the digester above 20C. The membrane might not work with thick biofilms. We may need more energy to chop seaweed so that it will flow through our combination fixed film digester and CO2 extractor. ...
---------- Original Message --------
Subject: Re: [geo] (Recovering CO2 from seawater for) A Zero Emissions
Vehicle Fuel? | The Energy Collective
From: John Morgan <john.d.p.mor...@gmail.com>
Date: Thu, January 17, 2013 2:51 am
To: geoengineering@googlegroups.com
Cc: john.d.p.mor...@gmail.com
Hi,I'm the author of the seawater article. Thanks for your comments.On whether or not its costly, I did try to construct as defensible a costing as possible, given that the carbon capture technology has not left the lab. The cost assumptions are described through the article, and at the end in more detail in the Appendix on costings. At the end of the introduction is a link to a spreadsheet containing the detailed cost calculations, so you can see what I've done.There are many caveats, but given the assumptions, which are largely those of the US Navy researchers, it turns out to be surprisingly cheap. For nuclear electricity generated from cheap Chinese nuclear build, the cost for carbon capture comes out at ~$40/tonne, and the cost of the synfuel derived therefrom is about 80 cents a litre. If you think this is unrealistic, what would be valuable would be if you can point to errors or omissions in the assumptions.Note too that the CO2 capture process is not a molecular separation from seawater, at least not in the sense that I think Greg meant. CO2 is not separated by a membrane process, and the seawater is not passed through a membrane. Instead, H+ is generated electrolytically on the membrane surface, and seawater is pumped past the membrane. The pH drops, and the CO2 bubbles out of solution to be captured. The acid stream is recombined with a base stream that was generated as a complement, and returned to the ocean. This is a very energy efficient, and clean process. The difference in power between this process and a true membrane separation process is huge. And without costing it, I imagine its cheaper than solar calcining of limestone.kind regards,john--
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