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I don't see how we're going to use CO2 as energy again. It's not like the original hydrocarbons we dug up.

Right, which is why mostly the idea is to just bury the CO2.

However, if you do want to make fuel from it, say for airliners, then it's totally possible, you just have to add hydrogen, and more energy than you'll get from burning the fuel. E.g. the Sabatier process to make methane.

But if you already have loads of hydrogen to play with you might as well just burn that, right?

Spend much time working with hydrogen as a fuel and you'll mostly be learning just how amazingly convenient sticking your hydrogens on to carbon chains is compared to the alternative.

Carbon is the ideal hydrogen storage system. It raises the critical point by 160K and increase the density achievable at normal temperatures and pressures drastically. Don't discount the energy that needs to go in to pressurizing and cooling hydrogen, it results in significant CO2 production. I'd be very interested in seeing a comparison of the energy requirements for methane production vs. hydrogen storage, I have a feeling that it would favour methane if you accounted for all of the variables.

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