Ammonia safety

Safety is one of the most important issues to be tackled for widespread adoption of ammonia. This page reviews some of the ammonia case studies and points towards gaps in the literature.

Unlike natural gas, ammonia is toxic, and therefore not suitable as a replacement fuel for domestic applications.

The main applications for ammonia are storage, shipping, rail and aviation - each of which have specific safety considerations.

Some properties and handling procedures for ammonia are well understood as a result of its widespread use as feedstock for fertilisers. These procedures seek to contain the risks to the public through safeguards and restrictions in use. For a more widespread application of ammonia, new means of storage and transportation may need to be considered, which could increase the inherent safety of ammonia. Among such solutions are liquefied and solid ammonia, which can prevent harmful dispersion. Further research in this area is urgently required.

Storage

Graph of vapour pressure of ammonia
Figure 1: Vapour pressure of ammonia. At ambient temperatures ammonia can be liquefied at around 8 bar. Below 33.3°C ammonia is liquid at ambient pressure.

Ammonia has been handled and stored for 100 years, even in the midst of urban areas. The risks are generally perceived to be low.

Ammonia can be liquefied at modest pressures. 8 bar are sufficient to store and transport ammonia as a liquid (Figure 1). The ease of compression has made this the dominant form of liquefaction. From a safety perspective ammonia at lower temperatures could yield a range of benefits, including reduced evaporation or plume formation. Figure 2 shows the drop in evaporation rate for lower temperatures.

Odor

The odor threshold for ammonia is between 5 - 50 parts per million (ppm) of air (Laval 2020). This is well below the Irreversible Effect Threshold of 343ppm (Tissot, 2003). Unlike other gases, like carbon monoxide, ammonia can be detected by smell before harmful thresholds are reached.

Toxicity

Ammonia vapour is lethally toxic to humans at concentrations greater than 4800ppm.

At low concentrations below 50ppm, repeated exposure to ammonia produces no chronic effects to human body. However, even in small concentration in the air it can be extremely irritating to the eyes, throat and breathing ways. (Laval 2020)

Citric acid neutralises ammonia effectively, and ammonium citrate is a safe food preservative. A filter mask containing 75g of citric acid crystals could theoretically allow up to 15 minutes of safe breathing for concentrations up to 55,000ppm.

Combustion of ammonia can create polluting nitrous oxides. These must be removed from exhaust fumes. Conversion in fuel cells does pose no such pollutants.

Ammonia toxicity exposure levels (from Laval 2020)
Concentration (ppm)Exposure (hours)Effect
10000 Promptly lethal
5000 – 10000 Rapidly fatal
700 – 1700 Incapacitation from tearing of the eyes and coughing
500 0.5 Upper respiratory tract irritation, tearing of the eyes
140 2 Severe irritation, need to leave the exposure area
100 2 Nuisance eye and throat irritation
50 – 80 2 Perceptible eye and throat
20 – 50 Mild discomfort, depending on whether and individual is accustomed to smelling ammonia

For animals symptoms are observed for concentrations of 100ppm. Lethal concentrations for pigs are at around 5000ppm exposure for one hour (EFMA-IFA, 1990).

For aquatic life ammonia is toxic. However, ammonia dissolved in water forms salts, which are not toxic.

Flammability

“Although ammonia is designated as a non-flammable gas for shipping purposes by the United Nations and the U.S. Department of Transportation, it is flammable in air within a certain range of concentrations. Because these concentrations are quite high, it would be extremely difficult to reach those conditions in an outdoor shipping situation. The fact that ammonia gas is lighter than air and that it diffuses readily in air makes it difficult to create a flammable situation outdoors.” IIAR Handbook of Ammonia

The self-ignition temperature of ammonia is 650°C (Chaineaux, 1991), compared to 610°C form methane, 535°C for hydrogen and 210°C for diesel. The fire risk from ammonia is greatly reduced compared to natural gas, oil or kerosene. (Jansohn, 2013)

To extinguish an ammonia flame it is advisable to only use CO2, especially if liquid ammonia could be present. Due to its hydrophilic properties, contact with water can transmit heat and accelerate vaporisation. (Ineris, 2005)

For the avoidance of doubt, the devastating explosion that killed over 200 people in Beirut on 4 August 2020, was caused by inappropriate storage of ammonium nitrate (NH4NO3), not ammonia (NH3). Ammonium nitrate is used as an explosive in mining and as a fertilizer. Ammonia is a feedstock to make ammonium nitrate.

Evaporation

Graph of evaporation rates at different temperatures
Figure 2: Ammonia evaporation rates at different temperatures

Large scale studies have explored the concentration levels surrounding an ammonia gas leak. Tests with up to 3.5t of ammonia released with flow rates up to 4.5kg/s show that dispersion is sensitive to the liquefaction behaviour of the plume and interactions with solid surfaces. In many cases concentrations drop from over 50,000 ppm to below 100ppm over a radius of 500m from the point of release. Ammonia clouds behave like a heavy gas and do not rise. When de-pressurised, the temperature of the ammonia release can drop as low as -70°C, resulting in liquefaction of more than half of the released ammonia. (Ineris 2005).

The evaporation rates of liquid ammonia are low and, by comparison, poorly understood (See Figure 2).

Applications

Shipping

Safety considerations for ammonia in shipping concern both cargo and propulsion fuel.

Ammonia as a cargo is well understood. $55bn worth of ammonia are manufactured and traded annually. International rules and regulations govern the safe handling and transportation, including the safeguarding of personnel. (Laval 2020)

At present ammonia cannot be used as a fuel for ships. The IMO International Gas Carrier Code (IGC) prohibits toxic products as fuel. The International Code of Safety for Ship Using Gases or Other Low-flashpoint Fuels (IGF Code) does not cover the case of ammonia. (Laval 2020)

The dispersion of ammonia in a marine environment is not well understood. Anhydrous ammonia gas is lighter than air and would disperse quickly. However, ammonia is hydrophilic and immediately reacts with the humidity in the air, such that it could remain near the surface for longer.

Aviation

The overall safety of aviation is constantly improving. In the event of a catastrophic crash the fuel could form an ammonia mist or vapour. The behaviour of ammonia in such circumstances needs to be studies with a view to understanding both toxicity and explosivity.

Three mitigation routes could be considered:

  1. Foam filled tanks: tanks with multiple compartments can reduce the mist volume. Such approaches are widely used in military aircraft to prevent fire and explosion. Droplets and misting can be prevented by entrapment and coalescence. A fibreglass layer surrounding the tanks could have this effect.
  2. Frozen ammonia: in its frozen state ammonia does not form a mist and the evaporation is self limiting, due to the high latent heat of vaporisation. For fuelling the ammonia can be handled as a slush.
  3. Rheology modifiers: Increase the viscosity of ammonia to create a honey-like consistency

References

Alfa Laval. 2020. Ammonfuel - an industrial view of ammonia as a marine fuel. Technical report, Alfa Laval, Hafnia, Haldor Topsoe, Vestas, Siemens Gamesa

Chaineaux Abiven. 1991. Caractéristiques d'inflammabilité et d'explosibilité de l'ammoniac, Rapport INERIS, EXP-FAB/DG F42 e/349 91.78.1120

P. Jansohn, ed. 2013 Modern Gas Turbine Systems. High Efficiency, Low Emission, Fuel Flexible Power Generation. Woodhead Publishing Series in Energy.

Ineris. 2005. Ammonia large-scale atmospheric dispersion tests. Work study n◦ 10072, INERIS – Accident Risks Division

Pichard Tissot. 2003. Seuils de toxicité aiguë, Ammoniac.