The chemical formula for ammonium nitrate is NH₄NO₃. It is manufactured in the form of small porous granules or prills and is one of the most widely used fertilizers in the world.
It is also the main component of many types of explosives for mining, where it is mixed with fuel oil and detonated by an explosive charge.
For an industrial loss due to ammonium nitrate to happen, many things have to go wrong. Unfortunately, that seems to have been the case with what happened in Beirut.
What could have caused the explosion?
Ammonium Nitrate does not burn on its own. Instead, it acts as a source of oxygen that can accelerate the combustion (burning) of other materials.
For combustion to occur, there must be oxygen. Ammonium nitrate granules provide a much more concentrated supply of oxygen than the air around us. Therefore, it is effective in mining explosives, where it is mixed with oil and other fuels.
However, at high enough temperatures, ammonium nitrate can decompose violently on its own. This process generates gases, among which are nitrogen oxides and water vapor. This rapid release of gases is what causes the explosion.
The decomposition of ammonium nitrate can be triggered by an explosion at the storage location or by an intense fire nearby. The latter is what happened in the 2015 Tianjin explosion, which killed 173 people after jointly storing flammable chemicals and ammonium nitrate at a chemical plant in eastern China.
Although it is not known with certainty what caused the explosion in Beirut, the images of the incident indicate that it could have been caused by a fire, visible in a part of the port area of the city before the explosion.
It is relatively difficult for a fire to cause an ammonium nitrate explosion, as the fire should be prolonged and kept in the same area as the ammonium nitrate granules.
Also, the granules themselves are not combustible for fire, so they would have to be contaminated or packed with some other combustible material.
Risk to the health of the inhabitants
It is reported that 2,700 tonnes of ammonium nitrate had been stored in a warehouse in Beirut for six years without proper security controls.
This likely contributed to the tragic circumstances that led to a common industrial fire that caused such a devastating explosion.
An explosion of ammonium nitrate produces enormous amounts of nitrogen oxides. Nitrogen dioxide (NO₂) is a red, smelly gas. Images from Beirut reveal a marked reddish color in the plume of gases from the explosion.
Nitrogen oxides are commonly found in air pollution in cities and can cause irritation to the respiratory system. Elevated levels of these pollutants are of particular concern to people with respiratory illnesses.
In Beirut, the gases will pose a risk to the health of its inhabitants until they dissipate naturally, which could take several days, depending on the local climate.
An important reminder
Large quantities of ammonium nitrate are produced and imported in some countries, mainly for use in mining. It is made by combining ammonia gas with liquid nitric acid, which in turn is made from ammonia.
Ammonium nitrate is classified as a dangerous good , and all matters relating to its use are strictly regulated. The Beirut explosion shows how important these regulations are.
Gabriel da Silva, Senior Lecturer in Chemical Engineering, University of Melbourne
This article was originally published on The Conversation. Read the original.
