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The other day, I was wondering, if cheap and easy nuclear fusion were developed tomorrow and fossil fuels disappeared, would there be a way to fuel all the gas-powered cars out there? After all, we have the Haber-Bosch process for producing ammonia. But then gasoline is not a chemical but a mixture of different organic compounds. So, let me structure a question to get at the heart of my curiosity:

How might we produce gasoline from electricity on an industrial scale without the use of fossil fuels as input?

Assume the electrical power came from an unlimited source, such as nuclear fusion, solar arrays in space, or very specific magic.

You may also assume that the gasoline doesn't need to be replicated perfectly -- that which can fuel anything that uses gasoline, or which satisfies an industry definition of gasoline, is sufficient. Ethanol does not count, since most cars cannot use it solely.

Feel free to comment on the environmental friendliness of the solution, excepting the electricity generation itself.

This basic question was originally asked here, and where it was suggested to migrate to chemistry SE.

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Historically, this sort of process has been done with the Fischer–Tropsch process.

The core of this process is to use "syngas" (a mixture of $\ce{CO}$ and $\ce{H2}$) in the presence of a catalyst to make higher molecular weight hydrocarbons:

$$\ce{(2n + 1) H2 + n CO → C_{n}H_{2+2n} + n H2O}$$

Under normal conditions, this will actually make mostly straight-chained hydrocarbons, which are great for diesel fuel, but not so great for gasoline/petrol engines. However, there are standard catalytic cracking techniques in the petrochemical industry which can adjust the branchedness of hydrocarbons, making them more "gasoline like".

Now, the only question is where to get the $\ce{CO}$ and $\ce{H2}$ from? The $\ce{H2}$ is easy, as it is readily obtained from the hydrolysis of water.

The $\ce{CO}$ can be obtained from $\ce{H2}$ and $\ce{CO2}$ by the water-gas shift reaction (here technically run in reverse):

$$\ce{CO2 + H2 \rightleftharpoons CO + H2O }$$

As I mentioned, the Fischer–Tropsch process was actually used during WWII by the Germans to produce motor fuel, as conventional sources were blocked off by the Allies. Their syngas didn't come from $\ce{CO2}$ and water, though, but from reformed coal, which they had a ready supply of.

This process is not done much because it's hugely inefficient, compared to just digging oil out of the ground. But if you have a "too cheap to meter" supply of electricity, the efficiency concerns go away. The main issue is getting a ready supply of $\ce{CO2}$, and there are a number of techniques being proposed at the moment for scrubbing $\ce{CO2}$ from the atmosphere - either by capturing it from point-source generators like power plants and cement manufacturers, or from "artificial trees" which capture $\ce{CO2}$ directly from standard atmospheric concentrations.

These "artificial trees" tend not to use normal photosynthesis for carbon capture, but instead often use something like a (strong) base in aqeous solution or $\ce{CaO}$. In these systems the base or the $\ce{CaO}$ reacts with dissolved $\ce{CO2}$ to form a carbonate salt, which either stays in solution or precipitates out. This carbonate salt formation is favorable, and happens spontaneously. The energy intensive portion of the process is the regeneration of the basic solution/$\ce{CaO}$. This normally happens by heating the carbonate salt, which causes $\ce{CO2}$ release. If done correctly, this can produce a stream of relatively pure carbon dioxide. But again, the regeneration step is quite energy intensive, and so would not be feasible without a cheap source of power.

Other environmental considerations would be minimal, aside from the land needed for the $\ce{CO2}$ devices (you would need a fair number to get a suitable steady volume of $\ce{CO2}$), and any disposal concerns about the process catalysts (which shouldn't be any more onerous than dealing with the catalysts now used in the petrochemical industry).

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  • $\begingroup$ Thank you for the answer, R.M. I like especially how you go through the whole process. Might you be able to put some rough numbers on the efficiency? I'm wondering what order of magnitude we looking at -- for every 100J of electrical energy put in, are we getting something like 10J or 1J or 0.1J of chemical energy back? Or worse? $\endgroup$ – einnocent Nov 2 '15 at 6:06
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    $\begingroup$ @einnocent I'm not a chemical engineer, so such efficiency estimates are not my area of expertise. A quick Google search estimates thermal efficiency of Fischer–Tropsch is ~60%, electrolysis is ~70%, and $CO2$ capture is ~$100/ton, which I think back-of-the-envelopes to the ~60% range. Combined, you're probably running in the low double digits: so 10J of electricity to 1J of fuel is probably the right ballpark. All that depends massively on how you run your plants, though - there's opportunities for heat scavenging, etc. from a combined operation. $\endgroup$ – R.M. Nov 2 '15 at 15:00
  • $\begingroup$ Thank you taking the time to look in to those numbers. This addresses another aspect of my question, which is the practicality of industrialized fusion to gasoline as replacement for current world consumption. I recognize fusion-to-gasoline is not an efficient process, but I wonder does that mean that it's not worth bothering until electricity is "too cheap to meter", or does it mean covering the surface area of the planet ten times over with fusion reactors? It sounds like the former. $\endgroup$ – einnocent Nov 2 '15 at 18:57
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How might we produce gasoline from electricity on an industrial scale without the use of fossil fuels as input? Assume the electrical power came from an unlimited source, such as nuclear fusion, solar arrays in space, or very specific magic.

I love this sort of question.

If we had magic and wanted gasoline, why would we make electricity?


To get serious...

You could make gasoline from the CO2 in the air and electricity, but it is very energy inefficient. So it is something like using 100 units of power in the form of electricity to make 1 unit of power in the form of gasoline. So to say that electricity can't be used to make gasoline, really means that in this case that it isn't practical to do so.

The deeper rub is that even solar panels are not "free energy". It costs energy to make the solar panels, to ship them, to install them, and to maintain them.

But certainly converting energy types is a valid way to mull over the energy problem.

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  • $\begingroup$ Thanks for the response. Remember, we have as much energy as we want, in the form of electricity, so efficiency isn't important, unless the waste heat of production causes environmental issues. What chemical reactions, as part of a larger industrial process, would produce the gasoline? $\endgroup$ – einnocent Oct 30 '15 at 22:19
  • $\begingroup$ Fundamentally CO2 + H2O yields gasoline and O2. $\endgroup$ – MaxW Oct 30 '15 at 22:27
  • $\begingroup$ You can think of gasoline as mixture of various $C_{x}H_{y}$ compounds. $\endgroup$ – MaxW Oct 30 '15 at 22:45

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