Exhaust sequestration vs operating temperature

Discussion in 'General Science & Technology' started by Aqueous Id, Nov 24, 2015.

  1. Aqueous Id flat Earth skeptic Valued Senior Member

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    Suppose a fossil fuel is used in some kind of engine in which the temperature of the exhaust is kept at, say, a few degrees above the critical point for liquid CO2. Using a little more energy, we might reason that the CO2 could be captured in the vehicle as a liquid (or solid) and cheaply recovered for sequestration while refueling. Without digging into this, it seems intuitively to me that there should be some energy left to power the vehicle, since even liquid exhaust products are lower in energy content than the fuel. But since (as far as I know) no one has invented such an engine, I'm left to assume that it's impossible.

    What do you think?
     
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  3. Russ_Watters Not a Trump supporter... Valued Senior Member

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    I'm not sure why you would conclude it is impossible just based on the fact that nobody has done it (it might be, but there are better reasons for why it isn't done). You could fairly easily calculate the energy balance to see if it is possible, but the obvious reasons why it isn't done are cost and performance. Oh, and the lack of a "what now?" for after you've recovered the co2.
     
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  5. Aqueous Id flat Earth skeptic Valued Senior Member

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    I mean it must not be a new idea. Someone must have looked at it and found it to be infeasible, even before you get to the sequestration piece. And for that, I suppose we could ask how long it would take to fill up all of the proposed sequestration sites if every vehicle on Earth could recover its CO2 in the liquid or solid phase.

    I stopped short of trying to calculate the net energy, once I realized how weird it would be to try to supercool, say, an internal combustion engine. So I would have to come up with some kind of hypothetical engine to even get started.

    Gasoline freezes at around -70C and CO2 at about -79C. Suppose you loaded solid gasoline to start with, and let it sublimate without too much loss of temperature. I guess, ideally, you could load liquid oxygen, too. (At least in a test vehicle.) Another idea I wondered about is to try to react frozen crude with liquid oxygen, as if it might be possible to defray the cost of refining (esp if the reaction in this type of engine for some reason made this possible). Some of this seems to have enormous mechanical advantages, especially where crossing from solid to vapor phase. But I'm not sure how such engines (or should I say reactors) might practically be feasible. In particular, this "reactor" would need to be very efficient, so that there is as little waste heat as possible.
     
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  7. Russ_Watters Not a Trump supporter... Valued Senior Member

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    Well, you don't need to design the cycle completely, but what you are talking about is a CO2 liquefaction plant attached to the car. It's basically compressors, nozzles and heat exchangers. You didn't specify pressure or temperature, so I'll just pull some easy ones:

    Gasoline has an energy content of about 44 MJ/kg and a good car engine is about 30% efficient, so that leaves 13 MJ/kg. And a kg of gas produces 2.6 kg of CO2.

    CO2 condenses at atmospheric pressure at -57C. Assuming you can get it down to room temperature with heat exchangers, cooling it further to -57C requires 0.16 MJ for the 2.6 kg. Not too. Condensing it requires a further 0.9 MJ, for a total of about 1.1 MJ, or about 8% of the power of the car. Let's assume you can do the liquefaction for 30% efficiency. Now you're at 25% of the power of the car. That's not a small penalty. Add to that the storage 130 kg of CO2 and the cost and bulk of the liquefaction plant (basically the entire trunk) and that's quite a burden to add to a car.
    From a chemistry standpoint, I think you are barking up the wrong tree there. Liquid or frozen substances don't burn: burning both produces and requires heat. You could use the liquid/solid gas to help you liquefy the CO2, though that's just transferring some of the extra cost and energy to the production of the fuel.
     
  8. exchemist Valued Senior Member

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    Yes, isn't the problem with this the fact that the waste heat from any heat engine needs to get rejected to the ambient environment? I cannot see how you can run a heat engine cycle at temperatures below that of the environment and then rely on a heat pump (another heat engine running backwards) to lift the waste heat temperature up to a level above that of the environment in order to get rid of it.
     
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