What is ammonium nitrate and how could it go so wrong in Beirut?

What good is it? What is it used for? And how can it lead to such an explosion?

The Lebanese capital Beirut was startled yesterday afternoon by a huge explosion in the port area, near the center. The event currently has 78 deaths and thousands injured. The havoc is also immense. The massive blast wave shattered windows, knocked down doors, and shook buildings to their foundations. According to Lebanese Prime Minister Hassan Diab, the devastating explosion was caused by dusting an estimated 2,750 tons of ammonium nitrate, which had been in a warehouse for six years. But what good is this? And how could things go so wrong in Beirut?


Ammonium nitrate
Ammonium Nitrate (NH4NO3) is a highly water-soluble salt of nitric acid and ammonia and is often used in fertilizers. The pure substance itself has a low sensitivity and is therefore difficult to detonate (although this has sometimes gone wrong in the past). But in combination with, for example, gasoline or kerosene, ammonium nitrate can become explosive, which means that the stuff is frequently used in mining. “Ammonium nitrate is classified as a hazardous substance,” said Flinders University associate professor Stewart Walker. “Although it is known as a ‘non-flammable’ substance, it can still explode under certain conditions.”

Ammonium nitrate is therefore not flammable on its own, but it can serve as an oxidizing agent to facilitate combustion of other substances. When ammonium nitrate is heated, the solid passes through a number of phase transitions to eventually decompose into gaseous products. “When it gets hot enough, gases can develop, including possibly nitrogen dioxide,” explains Professor Emily Hilder of the University of South Australia. “This increases the risk of explosion, especially in a closed environment. The risk is also increased if pollutants are present. Only a very small amount can make ammonium nitrate unpredictable. ” If only a small amount of fuel is added – such as oil or organic material – ammonium nitrate can therefore become very dangerous. In combination with heat, such a mixture can easily have catastrophic consequences. The scale in Beirut also suggests that this involved large amounts of dust.

In short, an explosion therefore occurs when a large amount of an energetic substance detonates, creating a large volume of trapped hot gases that expand and cause a shock wave. “The video footage of the Beirut incident initially shows white-gray smoke, followed by an explosion that produced red-brown smoke and a large white ‘mushroom cloud’,” Walker said. “This means that the released gases consisted of white ammonium nitrate vapors, toxic nitrous oxide and water.”

Bomb attacks
Due to the above mentioned properties, ammonium nitrate is sometimes used in improvised explosives. “The Oklahama bombing in 1995 was carried out using three to four tons of ammonium nitrate,” researcher David Caldicott of Australian National University lists. In addition, the bomb attack by Anders Behring Breivik, in the government district of the Norwegian capital Oslo on July 22, 2011, was also caused by explosives based on ammonium nitrate extracted from fertilizers. So, if ammonium nitrate falls into the wrong hands, it can have serious consequences. Exactly how the explosion started in Beirut remains to be uncovered. “The cause of such an explosion has yet to be investigated, such as whether it happened accidentally or intentionally,” Caldicott continues. “This is still possible, given the geo-political history of the region.”

Although the explosion has already claimed many casualties at the moment, it is expected to increase even further. “This is clearly a very large explosion and the reported death toll is likely to be much higher than currently identified,” said Caldicott. The dismayed Lebanese Prime Minister has therefore vowed to punish those responsible.

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