Beirut explosion: What is ammonium nitrate and why is it so explosive?

On the morning of Wednesday April 19th, 1995, far-right terrorist Timothy McVeigh parked a pick up truck outside the Murrah Federal Building in Oklahoma City.

The vehicle was packed with nearly 2,300kg of ammonium nitrate fertiliser mixed with fuel oil and attached to blasting caps.

McVeigh left the truck after setting a five-minute fuse to detonate the mixture.

The resulting explosion destroyed the entire front facade of the 117 foot structure, killing 168 people.

It was the worst terrorist attack in US history until the September 11th attacks.

In comparison, Tuesday’s explosion at Beirut’s port reportedly involved 2,700 metric tonnes of ammonium nitrate - potentially having yielded more than a thousand times the explosive force of the Oklahoma City bombing.

The Beirut explosion was powerful enough to create a mushroom cloud, usually associated with nuclear explosions, and was seen and heard from the island of Cyprus, around 130 miles away. Arab media outlets, citing the Jordanian Seismological Observatory, said the explosion caused a tremor that registered 4.5 on the Richter scale - the equivalent of a small earthquake.

Residents of Beirut, living miles away from the epicentre of the explosion, told TRT World that the windows on their shops and apartments were blown out. Videos shared on social media showed cars far away being brought to a halt by the shockwave and the shower of debris it carried.

So what is Ammonium Nitrate?

As the name suggests, the compound is rich in nitrogen, which combined with its easy solubility and low production cost makes it ideal for use as a fertiliser.

At between $65 to $100 a metric tonne to produce, the compound is also a cost effective option for farmers, explaining its popularity and its storage and transportation in large quantities.

But nitrate compounds also come with a downside, while stable when not exposed to flames and other potential triggers, they can easily be converted into explosives.

Both terrorists, such as in Oklahoma City, and miners have exploited that destructive potential. Turning the fertiliser from agricultural additive to high power explosive requires basic knowledge of chemistry and readily available household and industrial accelerants.

It took just a carelessly disposed cigarette to spark a very similar disaster to the Beirut explosion in Texas City, Texas, in 1947. There, the detonation of a 2,300 tonne stockpile of ammonium nitrate created one of the largest non-nuclear man-made explosions in history.

Nitrogen in itself is very unreactive, demonstrated in the composition of our atmosphere, which is 78 percent made up of the gas.

Nitrogen-based compounds, such as nitrates, are less stable than the element from which they are composed. With the assertion of enough energy, such as from a detonator or a flame, bonds holding the molecular structure of the nitrate breakdown into gases, such as Nitrogen, Oxygen, and water vapour.

The outward expansion of these gases is responsible for the blast energy that causes physical harm to buildings and living organisms.

Put simply, the more nitrates that are involved in the reaction, the more energy there will be being released. 

Regulation

Having recognised the dangers, governments have long sought to regulate the sale and storage of ammonium nitrate.

In the US, producers and purchasers must register with the federal authorities.

After a spate of Daesh and PKK terrorist attacks in 2015 and 2016, Turkey moved to ban the sale of some ammonium nitrate based fertilisers and seized more than 64,000 tonnes. 

In both Turkey and the EU, manufacturers are required to keep nitrate levels in fertilisers below an accepted threshold, and also required to include additives that inhibit the function of explosive accelerants, such as diesel.