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Comment by smurph | Hacker News Reader

It unclear where all of the carbon dioxide goes after they capture it. Does the process actually use the carbon dioxide for a second reaction? Or do they just put it in a tank somewhere? If it's the later, this would be difficult to do at scale.

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There's very little you can do with the captured CO2. It's a pretty inert gas that can only be broken up and reused for a second reaction if you add energy which is contrary to the goal of creating electricity. So the only option is storage, which is a bit of a problem at scale. There are some test power plants in germany that try this, but the basic idea is to collect the CO2 and push it into old, used natural gas fields. However, obviously nobody really feels safe sitting on a giant bubble of CO2...

I generally store captured CO2 in homebrew. Clean. Cheap. Delicious.

Great idea. Does it scale?

Uh, temporary?

Well, some of these tripels only seem to get better with age, so extrapolating from that, we can assume that storing it forever will produce an infinitely tasty beverage. Drinking it will, of course, truncate the 'forever' timeline, thus making it less than 'infinitely tasty', so the only way to preserve the extreme flavor is to never open it, thus: sequestered.

waves hands

It's meant to be injected underground.

But given coal has perhaps 500 kg/m^3 of carbon while pure CO2 at ambient pressure has about 0.5 kg/m^3. A thousand fold difference.

How are you going to dig something up from the ground, expand it thousand fold and put it back in to the same hole?

I don't understand this carbon storage business. Clearly coal is the best carbon storage method by far, and it doesn't cost anything as it's already stored.

You can also store it in the holes that you took natural gas out of. I'm not aware of it being done on any large scale, but it's been talked about a lot.

It would be pressurized before storage. However, to reach 500kg/m^3 it looks like it will be close to becoming a liquid: http://www.wolframalpha.com/share/clip?f=d41d8cd98f00b204e98...

The critical point is at 7.4 MPa (74 bars) and 304 K (30 C) - there the density is 470 kg/m^3.

Here's the phase diagram. http://www.wolframalpha.com/input/?i=co2+critical+point&...

According to this http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/45_4... the pressures in depleted natural gas wells are 20-40 bars so it's not as bad as I expected.

Never seen any numbers so had to go look.

I think once you consider the water vapor that is also generated from burning natural gas, storing the resultant CO2 in the same space might actually be possible. I suppose I could do an actual calculation based on the molecular weight of H20 and reading up on the combustion stoichiometry of methane et al., but I'm not a chemist by any stretch of the word, and I've got other things to do :).

1 CH_4 + 2 O_2 -> 1 CO_2 + 2H_2O

So the amount of CH4 and CO2 is the same in number of molecules, which also means they are the same volume of gas (at low pressures). Of course the mass of CO2 is about three fold but that doesn't mean much.

Sell it to PepsiCo!

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