The science behind Ammonium Nitrate Explosion

I will be honest. I didn’t read up much about the tragic accident that took place in Beirut, when I first heard of it. But last night, I read briefly in The Economist that Ammonium Nitrate (AN) has been causing major such explosive accidents regularly since the beginning of the 20th century! And then today morning I read in the papers that ~700 tonnes of seized Ammonium Nitrate is stored in a Container Freight Station in Manali – not the hill station but an industrial region north of Chennai . And this bit of news is definitely scary. Could that explode too? What is the science behind the AN explosion anyway?

As a wedding photographer / film-maker, this clip hits me harder

As I am writing this blog, I’ve just discovered a new information:

18,000 tonnes? Wow! But the NewsMinute article above doesn’t mention if this AN is explosive grade or not. I hope not! Anyway, I just got interested in reading up a bit on the science behind this explosion – sharing it here.

Ammonium Nitrate (AN) is a fertilizer. Why does it explode?

In general, AN (or NH4NO3) is a stable compound and doesn’t explode. You may have read / heard that it also used as an explosive material in mines and landslide clearance – but that is not just AN. That explosive is a mixture of AN and FO (fuel oil, usually diesel) – in short ANFO. The typical ratio of AN and FO is 96:04

But there have been multiple instances of AN explosion too – starting from the beginning of the 20th century.

Wikipedia has a tabulated list of each major AN related disaster.

Anyway, so what causes AN explosion even without the FO? Typically fire. Overall, there are only so many ways to trigger any ‘explosive’ material:

  • heat (or sometimes just spark)
  • impact
  • friction
  • electromagnetic radiations

The quantum of the above parameters needed to make a chemical / compound explode tells us how ‘sensitive’ the material is (and different materials could be differently sensitive to different parameters).

One one of the most sensitive materials that exists is NI3 – Nitrogen Tri-iodide. See for yourself in the video below.

As you can see, NI3 is pretty extreme. But there are several other more usable highly sensitive chemicals like Mercury Fulminate or Lead Azide – they are called primary explosives. They explode by decomposition and it is easy to trigger them to explode.

Then we have the secondary explosives that require relatively more amount of energy to get triggered. Guess what, AN is not even that!

AN (and even ANFO) is a tertiary explosive.You simply cannot make it explode easily. In fact for ANFO, explosion occurs from ‘combination’ instead of ‘decomposition’.

But yes, beyond a certain temperature (or massive force of impact) AN does decompose and explosion can happen – the way it did in Beirut (and in all the other instances listed in The Economist’s chart shared above).

Once a temperature of 170 C is reached, ammonium nitrate starts breaking down, emitting nitrous oxide, better known as laughing gas. Any sudden ignition causes ammonium nitrate to decompose directly into water, nitrogen and oxygen, which explains the enormous explosive power of the salt.

DW, 05 Aug 2020

What’s up with the terrifying white cloudy ‘shock-wave’ that that we see in the videos from Beirut?

To understand the answer to this question, you need to know the two phases of decomposition of an explosive material.

In the first phase, a “chemical reaction wave” travels through the material, slower than the speed of sound. The term for it is deflagration. In many explosive materials, the chemical reaction wave speed will never cross this limit (they are called low explosives – propane, gasoline, gunpowder etc.).

But AN is a high explosive (note: not highly). Here as the chemical reaction wave continues to travel, eventually its speed crosses that of sound (detonation), transforming the chemical reaction wave into what we call a “shock wave”.

Shock-waves are high pressure waves that travel through air (or water). The white misty wave thing that you see in the big explosion in the Beirut videos is basically water vapor condensing out of the air, because of really low pressure right behind the high-pressure shock wave. And then you see it disappear right away as the condensed mist evaporates back, once the pressure equalizes (to atmospheric pressure).

Anyway, so that’s the science behind the explosion. What happened really sucks. Other than the 130+ dead, a single explosion has rendered 3 lakh folks homeless (houses damaged or outright destroyed). 3 lakh! In just few seconds!


Research sources:

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