[cmurf]

What's an example of a synthetic fuel suitable for this purpose? I wouldn't call methane synthetic, for example.

[ncmncm]

Anhydrous ammonia stores liquid at room temperature under light pressure, stores energy per unit volume close to hydrocarbons, and can be burned where they can.

But in the near future, synthetic methane will be cheaper than mined natural gas. More expensive than ammonia, necessarily, because it needs carbon.