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Thread: Govt may stash carbon dioxide underground

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    Site Caretaker Dave A's Avatar
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    Govt may stash carbon dioxide underground

    The government is looking at underground storage of carbon dioxide from coal-fired power stations as a way of reducing the millions of tonnes of greenhouse gases the plants belch into the country's atmosphere each year.

    Speaking at the end of a media briefing on climate change at Cape Point, south of Cape Town, on Wednesday, Minister of Environmental Affairs and Tourism Marthinus van Schalkwyk said it is still too early to make a final decision on the matter.

    "We are looking at it ... but we have a particular problem with the geological structures in [South Africa], which from a carbon storage point of view may make it a risk. But we don't have enough evidence on the table at the moment to make a decision," he said.

    Power utility Eskom's 10 coal-fired power stations are responsible for much of the carbon dioxide -- a major contributor to climate change and global warming -- emitted in South Africa.

    According to its annual report, the stations released a total of 203,7-million tonnes of carbon dioxide during the year ending March 2006. The utility is planning to bring three more coal-fired plants on line over the next few years, which will push this figure up considerably.
    full story from M&G here

    I must say I'm quite curious about the technology behind this. Compressed CO2 is "dry ice", isn't it. Is this the plan?
    Also, given that it takes a fair amount of heat to release gaseous CO2 from solid CO2, is a lot of heat released in the comression process? Could that energy also be realised as electricity?
    The trouble with opportunity is it normally comes dressed up as work.

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    just me duncan drennan's Avatar
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    I don't really understand this approach, it seems a bit backwards. It may be slightly more difficult, and take a bit longer, but surely it makes more sense and is more sustainable to focus on energy savings, and reduction? There are plenty of ways that we in SA could be more energy concious, and reduce emissions.

    Quote Originally Posted by Dave A View Post
    I must say I'm quite curious about the technology behind this. Compressed CO2 is "dry ice", isn't it. Is this the plan?
    Also, given that it takes a fair amount of heat to release gaseous CO2 from solid CO2, is a lot of heat released in the comression process? Could that energy also be realised as electricity?
    I'm also curious. Not too sure about the harnessing the heat, but I am sure that it will take energy to change the CO2 from gas to solid - how much CO2 is being created as a side product to the energy that is needed to convert the CO2 to a solid?
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    Site Caretaker Dave A's Avatar
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    This is a bit of a shot in the dark, but here's my logic.

    If we take solid CO2 and wish to convert it to gaseous CO2, all we have to do is put it in a tub with some warm frosties. The solid CO2 rather usefully chills the frosties as it sucks heat energy from them so that it can go gaily wandering off into the atmosphere.

    Of course, this method of turning solid CO2 into gaseous CO2 is fairly energy efficient, and has some rather useful spin-offs.

    Now by my rather primitive understanding of science this means that when converting gaseous CO2 into solid CO2, exactly the same amount of energy is going to have to be sucked out of the CO2 as we put in from our frosties. However, rather than simply refrigerate gaseous CO2 to achieve this effect (which is rather a pain actually because it has to get really cold before gaseous CO2 is going to transform into solid CO2 all by itself), we can take advantage of a particularly interesting feature of CO2 - it doesn't have a liquid phase.

    So now we can take gaseous CO2, put it in a big space within something like a piston chamber, and slam it into a really small space where it conveniently turns solid.

    *Just a word of caution here to anyone wanting to try to turn steam into ice this way: It's not going to work and you'll probably break your piston because steam (gaseous H2O) first turns into a liquid (which is kinda OK), but then has this irritating quirk of expanding a little to become a solid. Fortunately, this trait proves useful elsewhere. Anyway, I digress and happily CO2 does not even become a liquid so I have no idea why that even came up*

    I'm betting that piston chamber gets really hot in the process of slamming the gaseous CO2 into solid CO2. Now we could use that heat to generate steam to drive a turbine which is connected to an electrical generator (by now you have hopefully caught my drift) and in this way recover some of that energy and convert it into electricity. Just like we do with burning coal which is our source of CO2 in the first place.

    Even if we can't get this efficient enough to produce a nett positive in terms of energy invested vs energy returned (power to drive piston vs electrical power generated) it still has to be way better than just dumping that energy through evaporator coolers into the atmosphere, thereby adding to the heat of the atmosphere which somehow I recall was vaguely connected to why we were trying to capture the CO2 in the first place.

    But given how well solid CO2 chills frosties, I've got a sneaking suspicion there might just be a nett surplus. Which means we get a double bite at the energy potential of coal.

    Of course, now we move onto the complexities of storing this solid CO2 underground, but I think I'll save that for another restless night.
    The trouble with opportunity is it normally comes dressed up as work.

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    Site Caretaker Dave A's Avatar
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    And on a more serious note - we're talking about the latent energy associated with a transition from gas to solid. It should be a fair bundle.
    The trouble with opportunity is it normally comes dressed up as work.

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