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Video: Why Does Fertilizer Sometimes Explode?

On August 12, 2015, chemicals exploded at a container storage station at the Port of Tianjin killing 173 people and caused so much damage that it could be seen from space.

PortandTerminal.com, November 2, 2020

HALIFAX, NOVA SCOTIA – Last August an explosion at the port in Beirut killed at least 220, injured more than 5,000 and left an estimated 300,000 people homeless. The blast was caused by 2,750 tonnes of ammonium nitrate, a chemical compound commonly used as an agricultural fertilizer, which had been stored for 6 years at a port warehouse.

30 disasters and terrorist attacks

Tianjin Explosion video (August, 2015)

The fact is, what happened in Beirut is not an isolated incident. Over the last century, the compound ammonium nitrate has been involved in at least 30 disasters and terrorist attacks. 

In April 1995, a truck bomb ripped apart a building in downtown Oklahoma City killing at least 168 people and injuring 680 more.

More recently, in August 2015, a series of explosions shook the port of Tianjin and northern China. Investigators found that the blast was triggered when stocks of nitrocellulose, a flammable compound used as a binding agent with medical applications and as an ingredient of lacquer, became too dry and caught fire in the August heat.

The flames then spread to illegal stores of the combustible fertilizer ammonium nitrate, triggering a series of blasts that flattened the warehouse, destroyed an adjacent auto lot and caused extensive damage to neighbouring apartment blocks.

What makes ammonium nitrate turn deadly?

The American Chemical Society produced a video (we’ve included it below) that explains the hows and whys of what causes ammonium nitrate to turn so deadly in certain circumstances. Here’s a quick summary of what they have to say:

So what makes this normally harmless compound turn deadly?

First, you need a lot of it. That bag of it in your garage isn’t going to cause any problems. There’s this thing called the critical diameter, which is essentially the amount of a compound you would need for it to detonate. Ammonium nitrate’s critical diameter is huge, about 100 times bigger than TNT’s.

By itself, ammonium nitrate is quite inert. But sometimes if you have enough of it, add fire to the mix and it starts to burn, that’s when you have a problem. Once ammonium nitrate reaches 410 degrees Fahrenheit, it starts to melt, quickly producing a bunch of gases, mainly nitrous oxide, water vapor and nitrogen gas.

If those gases can’t escape, temperatures go up and the reaction gets faster and faster.

Now this on its own could lead to an explosion. In the case of Beirut, 6 million pounds of ammonium nitrate had been improperly stored in a warehouse, sitting around and soaking up moisture for nearly seven years. When the nitrate has been a bit wet, it becomes very hard like cement, it’s not a fluffy powder anymore. So add fire, and that hardened ammonium nitrate starts to heat up and melt.

And then the pressure starts to increase where the gases can’t escape. The burn rate or the speed at which ammonium nitrate is burning starts to pick up. When the burn rate is slower than the speed of sound, that’s deflagration, but when it becomes faster than the speed of sound, that causes a shockwave through the ammonium nitrate which detonates it.

But ammonium nitrate doesn’t need to be in a compressed form to create a disaster. At the port of Tianjin, 1.7 million pounds of ammonium nitrate was being stored in a warehouse near a place where nitrocellulose was also being stored.

Unlike ammonium nitrate, nitrocellulose is highly flammable. The nitrocellulose caught fire, spreading to the stored ammonium nitrate, causing it to detonate.

Tianjin and Beirut were some of the deadliest accidental ammonium nitrate disasters over the last century.

In April 1995, a truck bomb ripped apart a building in downtown Oklahoma City killing at least 168 people and injuring 680 more.
In April 1995, a truck bomb ripped apart a building in downtown Oklahoma City killing at least 168 people and injuring 680 more.

But terrorists have also weaponized the chemical. Like in the 1995 Oklahoma City bombing, where terrorists detonated a truck loaded with 5,000 pounds of ammonium nitrate fertilizer, killing 168 people.

When I see a large explosion on the news, my first instinct is to ask, “Was that an accident? Or was it planned?”

If it’s a terrorist attack, you wouldn’t have a fire, it will happen very quickly. You wouldn’t have time for people to video it, it’ll be done within minutes. Because what they’re doing, they have the explosive, they will detonate it, cause a blast do the utmost damage they can and that’s it.

Because of its prevalence and terrorist attacks, Some countries have outlawed ammonium nitrate. In one instance in 2011, the US Army detonated about 2,000 pounds of ammonium nitrate fertilizer that it collected in Afghanistan.

It was done very far away from homes or people so the resulting shockwave wouldn’t cause damage or injury.

Sometimes the shockwave is more deadly than the explosion itself.

Shockwave from blast seen over city center
PHOTO: The shockwave from the explosion at the port in Beirut last August is shown here.

A shockwave is compressing the air going through the atmosphere, and at the tip of the shockwave, the temperatures are three to 4,000 degrees centigrade and the speed of the shockwave is supersonic.

So as a shock wave going through the atmosphere as it hits windows of course it shatters them because it pushes them out.

As it hits people, it’ll break our bones because they’re brittle, but the rest of it will go through but it’ll burn us because it’s very hot. It’ll flip over cars, it will flip over ships. It’ll break anything brittle. It will shatter anything brittle as it goes through the atmosphere.

So if you find yourself in a place where you see a massive fire burning, get as far away from it as possible and away from any windows because you don’t know what’s burning. If it is a large quantity of ammonium nitrate burning, you’ve got a problem.

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