Ammonia is an important industrial chemical. It is widely used as a fertilizer, a refrigerant, for pollution control, and as a building block for many other chemicals, including explosives. Hence, ammonia is vital to the viability of modern economies.
The manufacture of ammonia uses large amounts of carbon-based fuels, usually in the form of methane. Consequently the industry’s emissions of CO2 are correspondingly large; it is estimated that the manufacturing of ammonia contributes around 1.8% of world’s total CO2 emissions. Therefore, some method for making “green” ammonia is needed in order to achieve a “Net Zero” economy.
Most ammonia is manufactured using the Haber-Bosch process. It uses methane (CH4) as a feedstock — the carbon from the methane becomes CO2 and is emitted to the atmosphere. The process also uses large amounts of carbon-based fuels. Overall, the ammonia industry contributes about 1.8% of the world’s total CO2 emissions. Therefore, it is important to develop new ways of manufacturing ammonia that do not require the use of methane or carbon-based fuels. Doing so will create a “green ammonia” industry.
Once it is manufactured, ammonia may be help reduce carbon emissions by being used as a fuel to replace traditional carbon-based fuels such as gasoline, diesel or propane. When burned, ammonia produces nitrogen and water vapor, neither of which affect the climate. However, ammonia’s low energy of combustion and poor flammability characteristics limits its appeal as a fuel.
The use of ammonia as a fuel is at Phase III of the simple Phase-Gate diagram being used to evaluate the various technological options. It has passed the commercialization stage because ammonia is already so widely used and is manufactured in bulk. However, ammonia is not yet used as a fuel commercially, either directly or in fuel cells.
Ammonia as a Fuel
When discussing ammonia as a fuel it is important to distinguish between anhydrous ammonia and ammonia water solution (ammonium hydroxide). Anhydrous ammonia is a vapor under normal conditions. It is used by farmers as a fertilizer (the ammonia is injected directly into the soil from nurse tanks that are towed behind a tractor). Anhydrous ammonia is also used as the chemical building block for a wide range of products, including fertilizers, refrigerants, plastics, textiles, pharmaceuticals and explosives. It is also used as a means of NOx control in the exhaust of vehicles.
Ammonium hydroxide solution does not have such a wide range of industrial applications. However, it is used as a general purpose cleaner in many households.
The following conclusions can be drawn from Table 7.1.
Gasoline and diesel are liquids under normal conditions. This property makes them simple to transport and handle. Ammonia is a gas under the same conditions. It can be liquefied without needing refrigeration at around ten times atmospheric pressure, so it is reasonably portable.
Ammonia is typically stored at a pressure of around 10 bar (10 atmospheres or 145 psig) so it flows from the storage tank to the user under its own pressure; a pump is not required.
There is not a large difference between the densities for the three fuels.
The energy of combustion of ammonia is only about 45% of the other fuels.
Its energy density is similar to that of wood. This means that ammonia fuel tanks will have to be about two and half times bigger than gasoline or diesel tanks to provide the same range of travel.
Like ammonia, propane is a gas under normal conditions. However, it provides more energy per kilogram than either gasoline or diesel.
Ammonia Health and Safety
The discussion in this section is to do with health and safety issues of anhydrous ammonia, which has the formula NH3, i.e., it is made up of one nitrogen atom and three hydrogen atoms. Ammonium hydroxide solution is considerably less hazardous.
Safety and health issues to do with any particular chemical can be summarized in Safety Diamonds (National Fire Protection Association, 2017). Each of the four sections of the diamond contains a number in the range 0 to 4. A value of ‘0’ indicates that there is no concern, a ‘4’ indicates a high risk. Table 7.2 shows the legend that is associated with the Safety Diamond. (These signs are used by emergency personnel who may not be familiar with the properties of the chemical, but who need to make quick decisions.)
A diamond has four sections, each with its designated color.
This is for the fire hazard. It will provide the chemical’s flash point. (The temperature to which the liquid must be heated such that it can be ignited by an ignition sources such as a spark.)
This covers chemical instability, including the potential for detonation.
Hazards specific to that chemical, such as a high degree of corrosivity.
This covers health issues resulting from exposure to the chemical.
Table 7.2 shows three diamonds — for anhydrous ammonia, gasoline (petrol), and diesel.
Safety Diamond Legend
Starting with the red section, we see that anhydrous ammonia has a rating of ‘1’. This means that it can be ignited, but not easily. The flash point of is above 200 °F (93 °C). In practice, the ammonia would have to be preheated before it caught fire in normal circumstances. Gasoline, on the other hand, rates a ‘3’, indicating that it is a much more serious fire hazard than ammonia. Its flash point is below 73 °F (22.8 °C); it can be ignited under almost all ambient conditions if an ignition source is present.
The yellow section for both chemicals is ‘0’, indicating that the material is stable, and it will not react with water. The white section is blank — there are no special issues to be concerned about in either case.
The blue section for gasoline is ‘1’ meaning that exposure to it can cause irritation, but there should be no residual or long-term injury. Ammonia, however, rates a ‘3’. This means that short exposure could cause serious temporary injury or moderate residual injury.
It should also be noted that ammonia is widely used in agriculture. Small tanks for ammonia known as nurse tanks are towed across fields, with the ammonia being directly injected into the soil. Widespread use of the chemical in this manner shows that there is a track record of handling ammonia safely. This video shows a nurse tank in use.
Overall, the conclusion is that,
Ammonia is considerably less flammable than the fuels it replaces.
Ammonia creates a significant health hazard. However, there is much industry experience to do with the safe handling of the chemical.
In a marine environment, sea water can be used to knock down a leak.
The following articles, blogs and videos provide more information on the topics discussed in this chapter.