If you can produce synthetic ammonia without fossil fuel (natural gas) inputs, you can create synthetic hydrocarbon analogues of petroleum-based petrol (gasoline), kerosene (jet fuel), diesel, and possibly heavier grades of oil (marine bunker fuel). Though it's worth noting that the latter are used not so much for their suitability in marine use as their <i>unsuitability</i> elsewhere, and lighter grades (kerosene or diesel) would be likely substitutes.<p>Ammonia is harmful or deadly when inhaled, produces a large quantity of NOx in normal combustion (TFA specifically addresses <i>this one point</i>), and is highly corrosive. It's one of the five most harmful chemicals in regular use already, given limited industrial and agricultural applications. As a general-application fuel, particularly in scenarios in which fuel-based power systems cannot readibly be substituted (marine and aviation uses) would greatly increase hazards of such operations.<p>Synthetic fuels are lossy as all heck in round-trip energy return (on the order of 15--25% recovery as motive or generation output), but offer a very-well-understood, well-behaved, largely safe, and high-energy-density (by both weight and volume) option for very-long-term (100+ million years proved) energy storage. There's very little else that fits that bill.<p>Fossil fuels are principally nonviable going forward due to net CO2 emissions. Synthetic fuels based on ocean or atmospheric CO2 extraction would be net-neutral. Costs are higher than present fossil fuels, but present fossil fuels are grossly mis-priced based on both pollution and cost-of-formation externalities excluded from present market prices. (The pricing error could be as high as millions of times --- markets can in fact be remarkably inefficient and inaccurate.)<p>Other externalities including localised polluton (NOx, CO, VOCs, particulates) would remain an issue, but can be reasonably well controlled with extant technologies. If net fuel usage is decreased through electrified transportation and changes in land-use, lifestyle, architectural, and industrial behaviours, use in shipping, aviation, mobile, and remote applications should be within reason.<p>Ammonia not so much.