By McClintock, Joe; Holbrook, John
National Defense , Vol. 93, No. 657
The search for alternative transportation fuels has become a major national challenge. One substitute fuel that could help reduce the nation's dependence on petroleum is anhydrous ammonia, which is widely used as a fertilizer.
Due to its hydrogen content, anhydrous ammonia can be used in internal combustion engines with minor modifications, can be used in direct ammonia fuel cells, and also provides a hydrogen feed stock for standard hydrogen fuel cells.
The use of ammonia as transportation fuel became cost effective once gasoline broke the $3-barrier. The United States consumes approximately 25 million barrels of petroleum daily.
For electricity generation there are alternatives including coal, natural gas, hydropower, nuclear energy, and increasingly, renewable sources such as wind and solar cells. In the case of transportation fuels, the options are more restricted.
Plant-derived liquid fuels such as ethanol and bio-diesel can be domestically produced and have the further benefit of being carbon neutral when consumed. What is less clear is the total cost and carbon balance when fossil fuels are used in farming, transportation, and processing bio-fuels. The diversion of food crops such as corn and soy beans to fuel production diminishes the world food supply and may lead to political unrest. Liquid fuels can be produced from coal although the investment cost is high and this approach has a heavy burden of carbon dioxide emissions.
Gaseous fuels, mainly methane and hydrogen, are candidates for transportation fuels. Methane and hydrogen may be stored on a vehicle as cryogenic liquids or as high pressure gasses. The complexity and energy cost of cryogenic liquids has made them unpopular in transportation applications.
Ammonia fuel is a variation of hydrogen fuel. It is a molecule composed of one atom of nitrogen and three atoms of hydrogen. It has similar physical characteristics to propane; it is a gas at normal temperatures and atmospheric pressure but becomes liquid at higher pressure, about 150 pounds per square inch at 75 degrees Fahrenheit. The ability to become a liquid at moderate pressure allows ammonia to store more hydrogen per unit volume than compressed hydrogen or even cryogenic liquid hydrogen. In addition to providing a practical means to store and transport hydrogen, ammonia can he burned directly in internal combustion engines and direct-ammonia fuel cells
In 1935 Ammonia Casale Ltd. received a patent for a system to burn a mixture of ammonia and hydrogen in internal combustion engines. The hydrogen was derived from the stored ammonia and was included to improve combustion characteristics. Ammonia has a high octane rating of approximately 120, but a slow flame speed. So a combustion promoter is advantageous for some engine conditions. During World War II, because of a severe shortage of diesel fuel in Belgium, municipal buses were operated using a mixture of coal gas and ammonia, which was readily available. The coal gas contained hydrogen and served as the combustion promoter. A picture of one of those buses can be found at www.ammoniafuelnetwork.org.
The Defense Department also studied ammonia as a potential fuel in the 1960s on the Energy Depot Program, since it could be manufactured from water, air and electricity. The concept was that a portable nuclear reactor could drive a generator to produce electricity and ammonia could be manufactured to fuel vehicles. The relatively low energy density of ammonia made this approach impractical.
The lower energy density, about half of that of gasoline on a gallon-per-gallon basis, makes ammonia suitable for short-range transportation but not for long-haul aviation, for example, because it would cut the range of the aircraft roughly in half, compared to conventional jet fuel.
Safety and inhalation hazards--although ammonia is not strictly toxic--are major concerns. …