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Comment by cogman10 | Hacker News Reader

Yup.

The problem we have is CO2 is mixed into the atmosphere at ~420 ppm. Isolating just the CO2 and concentrating it enough for these processes to work is one of the major energy sinks.

This is compounded by the fact that the energy it takes to turn CO2 back into fuel can never be less than the energy released by the fuel.

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> This is compounded by the fact that the energy it takes to turn CO2 back into fuel can never be less than the energy released by the fuel.

That doesn’t seem right. Most fossil fuels have multiple CH3 complexes. Even without pulling up the enthalpy of combustion of the molecules, I’d still wager that the 3-H’s combing with O2’s will produce more than the balance of re-capturing the C’s.

That would violate the conservation of energy. Put simply: The energy that is released from those CH3 complexes has to be put back into them to make them be CH3 complexes again. Otherwise you could, for example, heat your house by burning the same bit of fuel over and over again (using some of the energy released by the combustion to recapture the carbon and rebind it to the hydrogen molecules and leaving the rest as heat to enjoy all through the winter). Alternately, you could generate a tremendous amount of electricity by the same method.

Sadly, thermodynamics says, "No."

Ah my mistake. I misread and was thinking in regards to just doing the CO2 capture portion of generating fuel. Yes making new fuel from captured CO2 will take as much or more energy than released from burning it.

Oh, yes that makes more sense (I don't know whether it's correct or not, but it is far more plausible!)

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