Ammonia has been commonly used as a fertilizer since the development of the
Haber Process in 1913. It has also been used as a fuel for small scale applications since World War II, when some diesel buses in Belgium were converted to run on it. While hydrogen has received significant attention as a potential alternative fuel, ammonia has a key advantage over it due to having a much higher boiling point than hydrogen, -33.34 °C/-28.01 °F, versus -252.879 °C/-423.182 °F for hydrogen. This means that it can potentially be stored in a liquid state without using tanks with extensive insulation or pressurization. According to
Wikipedia, at -30 °C it could remain liquid at 27.5 psi, and at 30 °C it would require a 170 psi tank. It is somewhat self-stabilizing, in that if pressure is reduced some ammonia will turn to gas and increase pressure to the required level.
There are some other interesting points in favor of ammonia. Diesel engines can be easily converted to operate on it, provided about 5% of the fuel has a high
cetane level (cetane being the inverse of the octane number used for gasoline engines). Gasoline and jet engines can run on ammonia too. Gasoline engines can run directly on ammonia with minimal modifications, although they are inefficient unless specifically designed to run on the fuel. This is probably due to the slow flame speed of ammonia combustion. There is also the disadvantage of ammonia having about half as much energy as gasoline on a gallon per gallon basis (
here), as well as for gasoline (
here).
There are some environmental and operational advantages for ammonia though. Just like hydrogen, it doesn't produce carbon dioxide when burnt. Also, because ammonia contains no carbon, there is no possibility of carbon buildup on engine components. Interestingly for rocket use, it is apparently relatively similar in density to liquid oxygen, which helped in developing the X-15 rocket. Ammonia has also been used as a working fluid in the
Kalina cycle, helping steam cycle electric power plants achieve
combined cycle level efficiency with less complexity and cost. Unlike hydrogen, ammonia cannot explode, and according to
this source it's safer than gasoline when it comes to fire and explosion risk. While its vapors are potentially fatal, it poses no cancer risk, unlike gasoline. Also, ammonia can use existing infrastructure developed for the fertilizer industry to help with distribution, and standard fuel distribution and other infrastructure can be converted to handle ammonia as well according to
this source.
Ammonia can be fairly easily produced from a variety of sources, ranging from
electrolysis of water to
steam reforming (which requires hydrocarbon feedstock). There was research on using portable nuclear reactors to produce it as part of the United States Army Energy Depot Program in order to simplify logistics, but the program was abandoned for a variety of reasons.
That's a somewhat lengthy overview of the general aspects of ammonia as a fuel and working fluid, and more than I expected when I sat down to write this. While ammonia is more expensive than hydrogen, and historically more conventional fuels such as gasoline (although according to this source, at over $3 per gallon it becomes competitive), it has some interesting properties that seem to give it some potential as an alternative fuel and working fluid. I'm wondering how it would fare as a fuel source for vehicles in terms of environmental, health, safety, efficiency, and economic factors compared to conventional technologies, as well as some of the proposed alternatives such as hydrogen, electricity, biodiesel, synthetic fuels, etc. I'm also wondering how it might fare as a working fluid for coal, nuclear, and renewable energy facilities compared to the steam cycle and other alternatives.