In the last seven years I've written extensively about a wide variety of alternative fuels, including ethanol, methanol, and higher alcohols like butanol, along with compressed and liquefied natural gas (CNG and LNG), hydrogen, and electricity, but I find I haven't said anything about anhydrous ammonia. It turns out that there is a small but enthusiastic group of people promoting its use as an alternative fuel, going back to at least the 1940s. Much of the recent interest in this stems from the fact that ammonia releases little or no greenhouse gas when burned, and that it's possible to produce it by means that involve minimal GHG emissions throughout its lifecycle. However, when you dig into this a little deeper, you discover that almost all ammonia today is produced by the Haber process, using hydrogen sourced from natural gas. And if that weren't enough of a deterrent, the physical properties of ammonia render it an unattractive candidate for a mass-market fuel.
So-called "green ammonia" would avoid natural gas by substituting hydrogen from electrolysis using wind, solar or other renewable electricity. As long as natural gas remains abundant, it's hard to envision this growing beyond a small niche, because the price of ammonia will ultimately be set by the price of natural gas, which remains a cheaper source of hydrogen than electricity from any source, let alone from expensive renewable power sources. Moreover, electricity is fungible, and the best use of renewable or other low-emission power (e.g., nuclear) is probably in backing out power from higher-emitting sources, rather than diverting it into inefficient production of chemicals. As a result green ammonia, like green power, would require subsidies for at least the near-to-medium term if it is to compete with conventional ammonia, which seems like a crucial prerequisite for competing with conventional fuels. And without green ammonia, the whole rationale for an ammonia fuel-and-vehicle network looks questionable--why not just use the gas as CNG or LNG instead, with a fraction of the headaches?
Even if that weren't the case, ammonia faces serious obstacles as a consumer fuel, compared to either conventional fuels or to many other alternatives. Start with energy density, which is less than half that of gasoline by weight, and about 40% by volume. So a gallon of ammonia would only take you about 40% as far as a gallon of gas, even if you could burn pure ammonia in your engine--and from what I've read it still requires help from another fuel to sustain combustion. (That means two fuel tanks, which constitutes another major hurdle with consumers.)
Then there are the economics. Ammonia itself isn't exactly cheap, if you adjust for its energy content. The price of bulk ammonia for agricultural use appears to be around $550-$600/ton, which equates to $1.55-1.70/gal. But when you factor in its lower energy density, that raises it to at least $3.85/gal. of gasoline equivalent, without any fuel taxes. And while a distribution system exists to supply farms with ammonia, this is a long way from what would be required to fuel anything beyond farm vehicles. Because ammonia boils well below ambient temperature, it must either be refrigerated or stored under pressure, and dispensed through special equipment. And if all that weren't daunting enough for any service station owner considering adding an ammonia pump on the forecourt, the safety aspects of ammonia handling look even worse.
A glance at a typical material safety data sheet (MSDS) for anhydrous ammonia reveals that the recommended exposure limits are very low, under 50 parts per million in air, and the consequences of exposure include caustic burns and much more serious outcomes. Gasoline has its own issues, but spilling some on your hand won't send you to the hospital, and a larger spill or leak doesn't require first responders in hazmat suits. I simply can't imagine any fuel retailer wanting to take on the liabilities that would go along with this, even if there were an attractive margin in it, which there doesn't appear to be.
I concluded long ago that we're heading into a period of much greater fuel diversity, and that certainly seems to be true, with LNG catching on for big-rig trucks and CNG for a few cars but more fleet vehicles and buses, and even hydrogen appearing in a few places for fuel cell vehicles. However, it's very hard to imagine a substance with as many drawbacks as ammonia coming into wide use for consumers or even fleets. Our range of alternative fuel options seems sufficiently broad already, without having to consider a fuel that turns into a poison gas at atmospheric pressure and temperature.