Yes, we are talking about anhydrous ammonia (NH3) as a fuel and it's the same stuff that farmers inject into the ground as fertilizer. Because it does not occur naturally in its pure form on our planet and must be manufactured, we can consider it an energy carrier rather than an energy source. Be that as it may, we will consider anhydrous ammonia as a fuel.

Some might think that ethanol and biodiesel are the ultimate green fuels. However, there is no way to grow enough biofuel feedstock (typically from corn in the USA) for this fuel to displace petroleum to any great extent. As crops are grown for fuel rather than food, this diversion of resources places upward pressure on the price of food. Please refer to an interesting OECD article about rising food prices as well as the OECD report entitled "BIOFUELS: IS THE CURE WORSE THAN THE DISEASE?" found the Food and Agriculture Organization of the United Nations web site:

The OECD has said biofuels may "offer a cure that is worse than the disease they are seeking to heal".

"The current push to expand the use of biofuels is creating unsustainable tension that will disrupt world markets without generating significant environmental benefits."

"When such impacts as soil acidification, fertilizer use, biodiversity loss and toxicity of agricultural pesticides are taken into account, the overall environmental impacts of ethanol and biodiesel can very easily exceed those of petrol and mineral diesel."

The holy grail of "green" fuels is hydrogen, an element that is also very scarce in its pure form on earth. Green in the sense that it is produced from renewable sources, the most common being the electrolytic cracking of water. Hydrogen may also be produced from "brown" sources such as the refining of petroleum. Brown in the sense that byproducts of this production are greenhouse gases and other forms of pollution. Almost all of the world's H2 is produced by steam reforming of natural gas, or as by-products of petroleum refining. Very little is currently produced by electrolysis although there is no technical reason that it can't be produced in large-scale wind farms in the US Midwest or even Patagonia.

Why would anyone consider using anhydrous ammonia rather than hydrogen? Hydrogen, after all, contains more LHV (lower heating value) energy than ammonia (51,500 BTU/lb vs 7,987 BTU/lb or 119.93 kJ/g vs 18.577 kJ/g) on a weight basis. However, on a volume basis ammonia is a much better hydrogen carrier than even liquefied hydrogen. The energy density of liquefied hydrogen is 8,491 kJ/litre compared to ammonia's 11,308 kJ/litre. Although ammonia contains 17.65% of hydrogen by weight, the fact that there are 3 hydrogen atoms attached to a single nitrogen atom allows ammonia to contain about 48% more hydrogen by volume than even liquefied hydrogen. That is to say, a cubic meter of liquid hydrogen contains 71 kg of hydrogen compared with 105 kg for liquid anhydrous ammonia.

Hydrogen's physical properties make it very difficult to handle. Because it is such a low density gas, very high pressures must be used to transport compressed hydrogen gas and this results in very low energy densities:

  • 48,900 Btu/ft3 gas @ 3,000 psig & 60 ºF
  • 121,000 Btu/ft3 gas @ 10,000 psig & 60 ºF

in metric, this is:

  • 1,825 kJ/litre gas @ 200 barg & 15 ºC
  • 4,500 kJ/litre gas @ 690 barg & 15 ºC

The low energy density of compressed hydrogen gas makes storage and transport very expensive. Transporting compressed hydrogen gas any significant distance by truck can consume more energy in diesel fuel than what is contained in hydrogen. Liquefied hydrogen is obviously more energy dense than compressed hydrogen gas but a significant amount of energy must be expended to liquefy hydrogen and keep it refrigerated because its boiling point is –423 ºF (–253 ºC). Liquefaction requires about 30% of the energy content of liquid hydrogen while compression to 800 bar requires about 10-15% of energy carried by the hydrogen.

Hydrogen's molecules are very small and difficult to contain. Hydrogen will slowly leak out from hoses and its rate of leakage is much higher than larger molecule gases like ammonia and propane. Hydrogen also causes embrittlement in metals which requires periodic replacement of metallic tubing, valves, and tanks.

Hydrogen is typically transported as a compressed gas and a 40 ton truck that can carry 26 tons of gasoline can only carry about 400 kg (0.4 tonnes) of compressed hydrogen due to the weight of the high pressure hydrogen tanks.

Ammonia, in comparison, stores and handles very much like LPG. Its boiling point is -33.35 °C (-28.03 °F). Propane, the main constituent of LPG, has a boiling point of -42.07 °C (-43.73 °F). On a hot day, a tank of NH3 at 50°C (122°F) will have a pressure of 2032 kPa (295 psi) compared with propane at 1729 kPa (251 psi) so it is important to keep these fuels out of the sun.

The design pressure of both anhydrous ammonia and propane tanks (with a corrosion allowance) is 250 psi which corresponds to a temperature of 44°C (111°F) for ammonia and 47°C (116°F) for propane. If these tanks were designed to the 312 psi (propane tank without a corrosion allowance), that corresponds to a temperature of 57°C (135°F) for ammonia and 60°C (140°F) for propane.

As for fuel properties, let's compare some relevant fuels:

Property Ammonia Hydrogen Propane CNG / Methane Ethanol Gasoline Diesel
Energy Density LHV (BTU/gal) 40,571 30,459 @ -423°F 84,500 19,800 @ 2400 psi   116,090 ~129,050
Energy Density LHV (MJ/litre) 11.2 8.4   23.3 21.1 29.8 35.8
Minimum Ignition Energy (mJ) 680 0.011 - 0.017   0.28 - 0.3 n/a 0.8 n/a
Octane Number 130+ 130+ 104 105 - 122   87 - 93 N/A
Auto Ignition Temperature (°C) 630 500   580 363 246 - 280 210
Flash Point (°C) 11 -253   -188 13 - 17 <-40 >62
Latent heat of vaporization (BTU/gal) 3356 N/A 775 N/A   ~900 ~710
Boiling Point (°C) -33 -253   -162 78 126 287
Critical Temparature (°C) 132 -240   -83 -- -- --

Using anhydrous ammonia as an engine fuel is not a blue-sky concept. There are already three companies in the business of building NH3-fueled engines or NH3 engine conversions: Hydrogen Engine Center, Hydrofuel Inc., and NH3CAR.

For more information, please visit the following links:

Ammonia Fuel Association

Iowa Energy Center's Ammonia Site

Raso Enterprises' Ammonia Fuel Forum

Potential Roles of Ammonia in a Hydrogen Economy

The Ammonia Economy

Ammonia: Key to US Energy Independence

AIR LIQUIDE's Gas Encyclopaedia - Ammonia

R.M. Technologies Technical Information