Alternative Fuels: Taking A Second Look at Ammonia
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 be 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. When compared to gasoline and ethanol, ammonia has a higher ignition energy, higher flash point, and a narrower explosive range when mixed with air. Explosion and fire would be less likely with a ruptured ammonia tank than with gasoline or ethanol.
Ammonia vapors cause irritation at low concentration and are life-threatening at high concentration. This is because of ammonia’s extraordinary affinity for water, including water in human flesh and organs. Ammonia, however, is lighter than air and rapidly dissipates in open spaces, or can be controlled with water. Both gasoline and diesel fuel can contain carcinogenic components in their vapors, but ammonia is not carcinogenic.
Gasoline, and to a greater extent diesel fuel, can produce fine carbon soot particles if incompletely burned, which are extremely hazardous when breathed. Ammonia cannot produce soot since it contains no carbon.
The Department of Transportation keeps records of deaths and injuries due to transportation accidents. Ammonia has a lower death and injury rate than gasoline per ton-mile transported. Ammonia is also widely used by farmers as fertilizer and has a good safety record in this application. The safety message on ammonia is that the vapor is more hazardous but the strong odor provides early warning of its presence.
For ammonia to be widely used as a transportation fuel, design standards for on-board ammonia fuel tanks must be established as well as procedures for ammonia transfer from storage to vehicle tanks.
The best features of ammonia are those it shares with hydrogen. It can be used both in internal combustion engines and in fuel cells, produces no greenhouse gasses on combustion, and can be produced from a wide variety of fossil and renewable resources.
The world produces more than 100 million tons of ammonia every year. The United States consumes in excess of 15 million tons annually. The 100 million-ton annual world output is equivalent to about 325 million barrels of gasoline, a relatively small number compared to the 27 billion barrels of petroleum produced. By comparison, the United States produces about 100 million barrels of ethanol per year.
Ammonia became widely available after 1913 when Fritz Haber developed a process for directly combining hydrogen and nitrogen at elevated temperature and pressure to produce ammonia. Karl Bosch engineered a large scale plant based on the reaction and the Haber-Bosch process is still the basis for industrial ammonia production. Haber received the Nobel Prize in Chemistry in 1918 for this development. It was not long before ammonia was used as a vehicle fuel.
The Haber-Bosch process is still the fundamental basis for manufacturing ammonia. It requires a source of hydrogen, nitrogen is taken from the air, and the two are combined at elevated temperature and pressure to make ammonia.
The most common source of hydrogen for ammonia production is natural gas, but hydrogen can be produced by using coal, petroleum coke, or heavy petroleum fractions to reduce water and produce hydrogen. Hydrogen too can be produced by the electolytic splitting of water using any source of electricity, including hydropower, wind and solar cell power, or nuclear power.
An example is the wind-to-ammonia demonstration project at the University of Minnesota. The project uses “stranded” wind resources that are not near electric transmission lines to produce ammonia for use as fertilizer.
Since ammonia can be domestically produced from a variety of energy sources it provides a path to reduce dependence on imported petroleum. In the longer term, it provides an avenue to produce liquid transportation fuel from renewable solar or wind power to reduce carbon dioxide emissions.