When anyone comes up with a "plan" that turns CO2 into fuel, remember that CO2 is ash in gaseous form. Only a massive injection of energy will turn ash back into fuel.
It has already been done - biofuels.
Yeah -- what I'm excited to see develop, in this domain, is
efficient electrochemistry, or photochemistry, converting CO2 + H2O into useful feedstocks plus oxygen.
We already have such an end-to-end process -- however, it's embarrassingly inefficient, a percent or two, no better than plants are.
There are some processes for reducing carbonates into formate, a key step. (On heating, formate decomposes to CO + CO2, the CO of which can be used to reduce other things, and ultimately make any other feedstock. Though that's kind of heroic to go through, and hence inefficient.)
A catalyst that reduces carbonate (or CO2) down to, say, methanol, would have a product that is directly usable as fuel, and easily integrated into existing synthesis chains.
Methane would be a close second. It absolutely sucks to transport, but is about as usable, and can be oxidized one tick back up, to methanol, which is easy to transport.
The purpose, by the way, is again not to "make" energy -- that is manifestly impossible here -- it is to transport energy, in the most economical way possible. Batteries
SUCK. No ifs, ands or buts about it. There is very good reason why hydrocarbon fuels will never go away. We can't possibly try to compete with them, we must harness them.
Note: currently, anything producing methane, and by extension methanol, needs to compete with natural gas, which is one hell of a challenge. Not only that, but methane is currently only economical from high pressure wells -- it's distributed at 100s of bar, where its gaseous energy density is
comparable to its heat value! At those pressures, it pumps itself through purification and distribution systems, no external power required.
The best possible dream, would be a carbon fuel cell, that is reversible, so can be charged, capturing CO2 and H2O from the air and producing fuel; and which does the opposite in reverse to produce electrical energy. This would perform about an order of magnitude better than any battery we can possibly come up with, and be compatible with existing hydrocarbon infrastructure with none, or minor changes.
I'm not so optimistic to expect that to happen even in the next half a century, but the first one can, and I think will, happen. If I were to guess, I'd guess some of the catalysts will be figured out in the next decade, with mass roll-out in the next decade or two after that, driven by the rising costs of petroleum. You will know it has arrived, when we have indirectly-solar-powered jet transportation dominating the skies! (Also assuming solar power -- or anything else reasonably carbon neutral, like nuclear -- expands to fill the then-massive electrical demand. There are also direct thermal routes known, like the iodine process for producing hydrogen, which has been suggested as an application of mid-level waste heat from high-level power stations, e.g., high temp coal or natural gas, pebble bed nuclear, etc.)
Tim
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