Ammonia as a Transportation Fuel
AgMRC Renewable Energy Newsletter
Retired Ag Economist Emeritus
Agricultural Marketing Resource Center
(second in a series of two)
Anhydrous ammonia (ammonia without water) can be a substitute for petroleum as a transportation fuel. It has the potential to make the hydrogen economy a reality in the near-term, at an affordable cost. It is an energy form that is manufactured. It can be made from all primary energy sources so production sources can be diversified or production can focus on the cheapest, cleanest and greenest source. Ammonia can be used in internal combustion engines with minor modifications. It can be used in gas turbines and ammonia fuel cells are being developed. Substantial ammonia distribution infrastructure already exists in the Midwest. Other existing infrastructure can be converted. The existing retail fuel dispensing infrastructure can be converted to ammonia. Although there are safety issues with ammonia, the issues are no more severe than those with gasoline and diesel fuel.
An ammonia molecule is composed of one atom of nitrogen and three atoms of hydrogen (NH3) and is commonly found in nature. The great majority of ammonia in the environment comes from the natural breakdown of manure and dead plants and animals. When ammonia combusts it produces nitrogen and water vapor (4NH3 + 3O2 yields 2N2 + 6H2O)
Similar to propane or liquid petroleum (LP) gas, ammonia is a gas at normal temperature and atmospheric pressure but a liquid at higher pressures (about 150 pounds per square inch at 75 degrees Fahrenheit). So it can be stored and transported as a liquid but used as a gas.
Anhydrous ammonia is used extensively in the nitrogen fertilizer industry where it can be applied directly as anhydrous ammonia or used as a feedstock in liquid nitrogen solutions, urea and others. The U.S. annually stores, transports and uses 15 to 20 million tons of ammonia and ammonia based fertilizers in the nation’s heartland.
Ammonia powered internal combustion engines
Gasoline and diesel fuel internal combustion engines can be converted to run on ammonia. The first utilization of liquid anhydrous ammonia as a fuel for motor-buses took place in Belgium during the year 1943. The motor-bus fleet logged thousands of miles during WWII with no difficulties.
Source: Ammonia Fuel Network
Ammonia has a high octane rating (about 120 versus gasoline at 86-93). So it does not need an octane enhancer and can be used in high compression engines. However, it has a relatively low energy density per gallon – about half of gasoline. The fuel mileage of ammonia is about half of gasoline’s mileage.
Along with hydrogen, ammonia is the only fuel that has no carbon emission when combusted because it doesn’t contain carbon. It may contribute a small amount of nitrous oxide emission, which can be controlled.
Ammonia can also be used in diesel engines. However, ammonia will not compression ignite except at very high pressures. So a small amount of high-cetane (the combustion quality during compression ignition) fuel is added. Research is showing that a 5 percent biodiesel and 95 percent ammonia blend works well in farm machinery.
Ammonia and fuel cells
We often hear that hydrogen is our ultimate renewable and green fuel source. Hydrogen fuel cells take in hydrogen (H2) and oxygen (O2), produce electricity to power the motor vehicle and emit water (H2O). Hydrogen is the most abundant element in the universe but it is relatively rare in its elemental (H2) form on earth. Although hydrogen has high energy density by weight, it is the lightest of all elements and requires large volumes to power a motor vehicle. So, elemental hydrogen is difficult to store and transport. Hydrogen volumes can be reduced by compressing it as either compressed hydrogen or liquid hydrogen. However, the pressures required to do either of these are substantial and create a potential safety hazard.
Ammonia is sometimes called the “other hydrogen” due to its structure of three hydrogen molecules and one nitrogen molecule. The ability of ammonia gas to become a liquid at low pressures means that it is a good “carrier” of hydrogen. Liquid ammonia contains more hydrogen by volume than compressed hydrogen or liquid hydrogen. For example, ammonia is over 50% more energy dense per gallon than liquid hydrogen. So ammonia can be stored and distributed easier than elemental hydrogen. Fueling stations are much easier to convert to dispensing ammonia than elemental hydrogen. Ammonia could be stored onboard a motor vehicle where the elemental hydrogen and nitrogen are separated just before the hydrogen is fed into the fuel cell.
In addition to hydrogen fuel cells there are several fuel cells designed to use ammonia directly. This would eliminate the need to separate the ammonia into its hydrogen and nitrogen elements before it is used in the fuel cell. These cells enable high efficiency conversion of ammonia to electric power.
Although ammonia has a good safety track record, it has safety concerns if used as a transportation fuel for motor vehicles. Ammonia is safer than propane and comparable to the safety of gasoline when used as a transportation fuel. Ammonia vapors cause irritation to humans at low concentrations and is life threatening at high concentrations. This is because ammonia can pull the water out of human tissue. However, ammonia can easily be detected by its strong odor. It is lighter than air and rapidly dissipates in the atmosphere. Regardless, safety precautions need to be implemented for fueling stations where the ammonia is transferred from trucks to storage tanks and from pumps to motor vehicles. In addition, there are also safety issues for the storage of ammonia onboard motor vehicles. Research is underway to develop new and improved ammonia tanks, fittings and tubing to ensure safe operation.
There are also positive safety aspects to ammonia when compared to gasoline. Gasoline may produce carcinogenic vapors but ammonia is not carcinogenic. Ammonia does not burn readily or sustain combustion except under narrow fuel-to-air mixtures of 15-25% air. Explosions and fire are less likely with a ruptured ammonia tank than with gasoline. Also, gasoline can produce carbon soot particles that are hazardous when breathing. Because ammonia contains no carbon, it cannot produce soot.
Currently ammonia is produced primarily from natural gas and coal. China is the number one producer. Currently U.S. ammonia is made primarily from natural gas. Hydrogen is taken from the natural gas and nitrogen comes from the atmosphere. Although U.S. ammonia has traditionally been produced domestically, imports of ammonia have increased substantially in recent years due to the high price of U.S. natural gas versus foreign natural gas.
Natural gas produces greenhouse gases. So, although ammonia does not produce greenhouse gases during combustion, the production of ammonia from natural gas does emit greenhouse gases. Carbon sequestration at the point of production could greatly reduce these emissions. However, carbon capture and sequestration technology is still in the developmental stage and would add additional cost to the fuel. If this technology is developed, it will provide a carbon free transportation fuel from the use of natural gas and coal.
Ammonia can be produced from a variety of renewable energy sources. For example, wind energy (February 2009 article) can be used to create hydrogen through the electrolysis cracking of water into hydrogen and oxygen. Ammonia can also be produced from solar energy, municipal sewage systems, nuclear power, geothermal energy, ocean and tidal energy and others.
Sweet sorghum growing at a research station in China.
Source: Food and Agriculture Organization of the United Nations
Cellulosic ammonia may be more viable than cellulosic ethanol. Biomass ammonia from renewable crops such as sweet sorghum have been shown in Iowa to supply enough hydrogen through anaerobic digestion or thermal gasification to produce a quantity of ammonia that contains more energy per acre than corn ethanol or cellulosic ethanol. Crops like sweet sorghum will thrive under drier and warmer conditions than many other crops. So, marginal land can be used for energy production.
Regardless of the energy source for the hydrogen, nitrogen is taken from the atmosphere (78% of earth’s atmosphere is nitrogen) and hydrogen and nitrogen are converted to ammonia through the Haber-Bosch process. As fossil fuel reserves become more costly to recover and the technology for producing renewable energy improves, the comparative cost of production for renewable energy hydrogen sources will improve.
Ammonia is easy to store and deliver in large quantities. Ammonia is one of the most transported chemicals worldwide. A storage and delivery infrastructure of pipelines, barges, rail and truck already exists for ammonia in the Midwest. Existing ammonia pipelines and storage terminals are shown below. Storage is in refrigerated, liquid, above-ground steel tanks of 10–60,000 tons each. The pipeline is 8-10 inches in diameter and constructed of plain carbon steel with a total length of approximately 3,000 miles with regularly spaced pump stations. Retail ammonia outlets exist in practically every state. Iowa has 800 outlets. However, these outlets tend to be concentrated primarily in the rural Midwest. Additional infrastructure would be need for densely-populated urban areas on the east and west coasts.
Source: Ammonia Fuel Network
As an alternative for accessing these areas, existing natural gas and petroleum pipelines can be cost-effectively converted to carry ammonia. Almost two million miles of natural gas pipelines could be converted to transporting ammonia, making it available to nearly every community in the U.S. By contrast, storing and transporting elemental hydrogen (H2) requires large volumes and intense pressures as discussed above. So, ammonia has a clear advantage in storage and distribution.
The ability to use one fuel source, ammonia, in a variety of engines, fuel cells, turbines, etc., is a distinct advantage in storage and distribution. Fewer fuel formulations are needed, only one product is storage and transported, and fueling stations can be standardized for one fuel source.
Moving to an ammonia/hydrogen economy has policy implications. Although the ultimate vision is to make large amounts of ammonia from a variety of renewable energy sources, ammonia in the interim will need to be made from natural gas. Care must be used to not adversely impact the use of ammonia as a nitrogen fertilizer. Increasing the demand for ammonia could adversely impact the price and availability of nitrogen fertilizer for farmers unless supplies are expanded along with demand. This could potentially impact cropping patterns in the U.S. and elsewhere. The impact on electric power generation which is also a larger user of natural gas must be taking into account.
Related Web sites
Ammonia – Iowa Energy Center - Proceedings of the annual Ammonia Fuel Network conferences.
NH3 Car - an ammonia-gasoline fueled S10 pickup that drove from Detroit to San Francisco in summer 2007.
Iowa Energy Center - Conducts research on ammonia, energy efficiency, and renewable energy.
Hydrogen Engine Center - a company that makes ammonia-fueled internal combustion engines and generator sets.
Raso Enterprises - ammonia fuel information page
Potential Roles of Ammonia in a Hydrogen Economy – A study of issues related to the use of Ammonia for on-board vehicular hydrogen storage. US Department of Energy white paper
Comments on Potential Roles of Ammonia in a Hydrogen Economy - Sandia National Laboratories
Wind to Ammonia - University of Minnesota update
Freedom Fertilizer - Wind to Ammonia project
NH3 – “The Other Hydrogen”; N. Olson P.E., and J. Holbrook PhD.
Ammonia Fuel Network
Iowa Energy Center
Alternative Fuels: Taking a Second Look at Ammonia; National Defense Magazine, August 2008.
New Alternative Fuel Vehicle; Green Building Elements; August 2007