You'll be hearing a lot more about ammonia as a clean fuel option as the race to zero carbon by 2050 progresses. In particular, it looks like a strong option for long-haul shipping and trucking. So what is it, how is it made, and how does it shape up as a green fuel?

Chemically, ammonia is a molecule comprising three hydrogen atoms, all linked to one central nitrogen atom. Both very common elements; the Earth's atmosphere is mostly nitrogen, and hydrogen is of course the most abundant element in the universe. That doesn't mean it's simple to produce, but we'll get to that.

At atmospheric pressure, ammonia is a very stinky gas with a boiling point of −33.3 °C (−28.0 °F). Kept cold or under a modest amount of pressure, it's relatively easy to liquefy, making it a much easier green fuel to transport and store than hydrogen. You can truck it about or keep it in tanks, cheap as chips. Hydrogen is nearly 30 times more expensive to store.

Indeed, in many ways, ammonia does a better job of storing hydrogen than hydrogen gas itself; H2 is notorious for leaking away through the metal walls of containers, for embrittling steel it comes into contact with, and for taking a lot of energy to liquefy at cryogenic temperatures. And then there's density: it may sound weird, but there's one and a half times more hydrogen in a gallon of ammonia than there is in a gallon of hydrogen, all else being equal.

Ammonia is dangerous to humans. It's caustic in high concentrations, and classified as an "extremely hazardous substance" in the United States, with strict reporting requirements for any facility that uses a significant quantity.

Today, it's most frequently used in agriculture, where as a salt or in solution it's a powerful fertilizer leading to improved yields of some cereal crops. This accounts for nearly 90 percent of commercial ammonia use in the United States. The rest includes industrial use as a precursor to virtually all synthetic nitrogen compounds, use as a general purpose household cleaning agent, use as a nitrogen source in the fermentation process, use as an antimicrobial agent, notably to kill E. coli bacteria in super-fine beef mince, and other uses.

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