Why Does Fertilizer (Sometimes) Explode? [Video]

Over the last century, the compound ammonium nitrate has been involved in at least 30 disasters and terrorist attacks. Under normal circumstances, it’s totally harmless and used in things like fertilizer, so what makes ammonium nitrate turn deadly?:

Video Transcript:

In April 1995, a truck bomb ripped apart a building in downtown Oklahoma City. In August 2015, a series of explosions shook the port of Tianjin and northern China. And in August 2020, a warehouse fire set off a blast in Beirut so powerful that neighboring countries could feel it.

These were all caused by the compound ammonium nitrate, which over the last century has been involved in at least 30 disasters and terrorist attacks.

But what is ammonium nitrate?

Well, it’s used in all kinds of things from making antibiotics to instant cold packs. And it’s a key ingredient in fertilizer.

You might be wondering, Why are we using something that has the potential to be so deadly to grow food?

I like to think of ammonium nitrate as a Jekyll and Hyde chemical.

When it’s stored properly, it’s the mild mannered Dr. Jekyll. Plants love it because it supplies them with nitrogen which they need to grow. And although our atmosphere is mostly nitrogen, plants can’t actually use it in that form, but they can use it in the form of nitrate.

Under the wrong conditions, ammonium nitrate can turn into Dr. Jekyll’s evil alter ego, Mr. Hyde.

– [MAN] Window
– [BOY] Are you serious?
– [MAN] Yeah. (explosion)

So what makes this normally harmless compound turn deadly?

First, you need a lot of it. There’s this thing called critical diameter, which is essentially the amount of a compound you would need for it to detonate.

Ammonium nitrate is critical diameter is huge, about 100 times bigger than TNTs.

It’s quite inert by itself. But sometimes if there’s a fire quite near the ammonium nitrate, and ammonium nitrate is in a huge pile, ammonium nitrate can catch on fire.

Once ammonium nitrate reaches 210 degrees Celsius, 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.

And particularly though nitrate has been a bit wet, it becomes very hard like cement, it’s not fluffy powder anymore.

So add a 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.

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.

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.

So if it detonates, the shockwave that follows could be devastating.

Yes, all of this is scary. But Dr. Akhavan made it clear that we should not be freaked out about ammonium nitrate.

Because people worry now, so they shouldn’t be worried about ammonium nitrate. It is safe to use. It’s nothing more. It’s not classed as an explosive. So very rarely does it go up.

It’s only when it’s old, and there’s a fire next door and also stored in huge quantities. Otherwise, it’s fine.

So having a bag of ammonium nitrate fertilizer in your garage is not cause for concern. So keep on using it, your tomatoes will probably thank you.

Happy gardening.

3 Comments on "Why Does Fertilizer (Sometimes) Explode? [Video]"

  1. Thanks. I could not understand how Ammonium Nitrate could explode.

  2. Ammonium nitrate melts on engulfment by fire, the hot molten ammonium nitrate runs to the lowest point and pools, it can also become contaminated by carbon, in a fire situation, making it more sensitive to impact and propagation of an explosion.

    However, the hot molten ammonium nitrate by itself is more sensitive to impact than nitroglycerin. This is the key fact not mentioned in the article. Falling girders etc are then enough to start the detonation, which propagates through the remaining pile.

  3. Clyde Spencer | March 24, 2021 at 2:27 pm | Reply

    “Like in the 1995 Oklahoma City bombing, where terrorists …

    That should be singular — Terrorist!

    What the article neglects to mention is that ANFO (Ammonium Nitrate/Fuel Oil) is probably the most common explosive used in the mining industry. The pelletized AN is mixed with diesel fuel and then detonated with dynamite or other high-explosives. So, the Mr. Hyde persona is in evidence frequently, albeit for constructive purposes.

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