- Ammonium nitrate is an odourless crystalline substance commonly used as a fertilizer
- It has been the cause of numerous industrial explosions over the decades
- These include a Texas fertilizer plant in 2013 that killed 15 and a chemical plant in Toulouse, France, in 2001 that killed 31
- In agriculture, ammonium nitrate fertilizer is applied in granule form and quickly dissolves under moisture
- This allows nitrogen which is key to plant growth to be released into the soil
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Lebanese authorities said the Beirut blast on Tuesday, August 4 was fueled by a stockpile of over 2,700 tons of ammonium nitrate, confiscated and stored for approximately six years at the port.
This is an odourless crystalline substance commonly used as a fertilizer that has been the cause of numerous industrial explosions over the decades.
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These include notably at a Texas fertilizer plant in 2013 that killed 15 and was ruled deliberate, and another at a chemical plant in Toulouse, France, in 2001 that killed 31 people but was accidental.
The explosive potential of ammonium nitrate is well understood and has also been used in numerous terrorist attacks including the Oklahoma City bombing of 1995.
So what is ammonium nitrate, why is it explosive and why do we use it?
Under normal storage conditions and without very high heat, it is difficult to ignite ammonium nitrate, said Jimmie Oxley, a chemistry professor at the University of Rhode Island.
“If you look at the video (of the Beirut explosion), you saw the black smoke, you saw the red smoke, that was an incomplete reaction,” she was quoted by The Telegraph.
“I am assuming that there was a small explosion that instigated the reaction of the ammonium nitrate — whether that small explosion was an accident or something on purpose I have not heard yet," she added.
When combined with fuel oils, ammonium nitrate creates a potent explosive widely used by the construction industry, but also by insurgent groups like the Taliban for improvised explosives.
That is because ammonium nitrate is an oxidizer — it intensifies combustion and allows other substances to ignite more readily, but is not itself very combustible.
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In agriculture, ammonium nitrate fertilizer is applied in granule form and quickly dissolves under moisture, allowing nitrogen — which is key to plant growth — to be released into the soil.
Plants rely on photosynthesis, converting sunlight to energy using chlorophyll.
Nitrogen is an essential element for this process and ammonium nitrate is particularly good, compared with other fertilizers, at delivering plants nutrients.
The chemical is also an explosive compound and is used around the world in mining and construction operations requiring detonations.
Under normal conditions, the chemical is highly stable. It can explode after exposure to contaminants or fuel oil and then heated, which sets off a cascade of reactions.
When heated to above 170 degrees Fahrenheit, ammonium nitrate begins to undergo decomposition.
But with rapid heating or detonation, a chemical reaction can occur that converts ammonium nitrate to nitrogen and oxygen gas and water vapour.
The products of the reaction are harmless - they are found in our atmosphere - but the process releases huge amounts of energy.
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Ian Rae, a professor at the University of Melbourne also said in the explosion, not all of the ammonium nitrate is used up and exploded.
According to him, some of it decomposes slowly creating toxic gases like nitrogen oxides.
It is these gases that are responsible for the red-brown plume of smoke seen in the aftermath of the Beirut explosion, Rae said.
For these reasons, there are generally very strict rules about where it can be stored: For example, it must be kept away from fuels and sources of heat.
In fact, many countries in the European Union require that calcium carbonate to be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.
In the United States, regulations were tightened significantly after the Oklahoma City attack.
Under the Chemical Facility Anti-Terrorism Standards, for example, facilities that store more than 2,000 pounds (900 kilograms) of ammonium nitrate are subject to inspections.
Despite its dangers, Oxley said legitimate uses of ammonium nitrate in agriculture and construction have made it indispensable.
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“We would not have this modern world without explosives, and we would not feed the population we have today without ammonium nitrate fertilizer,” she said.
“We need ammonium nitrate, we just need to pay good attention to what we’re doing with it," she added.
In 2015, explosions partly caused by the detonation of around 800 tons of ammonium nitrate rocked the Beijing port of Tianjin, killing 173 people.
An explosion of ammonium nitrate was responsible for the worst industrial accident in US history in 1947.
A ship in the harbour of Texas City, carrying approximately 2,300 tons of ammonium nitrate caught fire, supposedly via a discarded cigarette, causing a series of explosions that killed 581 people.