Yesterday there was news of a huge fertiliser explosion in the town of West, near Waco, Texas and as I write the search for survivors is ongoing.  It’s a dreadful tragedy:  the blast all but destroyed a school and a nursing home a few hundred metres away, and dozens of homes were also levelled.  More than 160 people have been injured and so far twelve have been found dead.

At the moment the full details are still unknown.  Fertilisers have long been associated with explosives, and terrorists have been known to use fertiliser bombs (something I shall not be discussing in more detail for fear the men in dark suits might come knocking), although it seems that there’s no indication of malicious intent in this case.  Obviously factories make fertiliser all over the world, and they don’t all blow up on a regular basis, so clearly something went very wrong at 8pm local time on the 17th of April.

So why is fertiliser such potentially dangerous stuff?  Can we make it safer?

First of all, we should probably clarify what we mean by ‘fertiliser‘ (or fertilizer, for our American cousins).  Actually the clue is in the name; it’s something which makes the soil more fertile.  In essence, anything that’s added to the soil to supply one or more of the nutrients that plants need.  In particular, most fertilisers supply nitrogen.  If you were paying attention at school, you’ll remember that most of the solid stuff in plants actually comes from the air in the form of carbon dioxide (see that wooden table over there? A plant made most of that out of air. Air. How cool is that?)

However, just like us, plants also need to make protein for growth, and to do that they need nitrogen.  Unlike us, they can’t (with a few notable exceptions) get that protein from eating animals or other plants, on account of not having teeth, the ability to move and so on.  Except for triffids and that plant in Little Shop of Horrors obviously.  But good old air is about 80% nitrogen, so surely if they can get the carbon from carbon dioxide from air they can get nitrogen too?

Well, there are a few plants that can do that, but most can’t.  The problem is that the nitrogen in air, N₂, has one of the strongest bonds between its atoms.  It’s very difficult to break, which means it doesn’t get involved in chemical reactions very easily.  And since growing is basically one big complicated mix of chemical reactions, plants can’t easily use the nitrogen in the air.  Before we started chucking fertiliser on the soil plants managed of course, because useable forms of nitrogen do get into the soil from natural processes.  But if you want to grow large quantities of crops year after year, you need to provide a bit of a helping hand, and that’s what fertiliser does, whether it comes from a factory or, ahem, the back of a cow.

But, and here’s the thing, it’s that strong, triple, bond in N₂ that makes fertilisers potentially explosive.  Because if it takes a lot of energy to break those bonds, then exactly the same amount of energy is released when they’re formed.  There is no way around this: energy cannot be created or destroyed, or made to disappear.  (Not in real life, anyway – Harry Potter and co follow different rules.  But they’re not real.  Sorry.)

Why do things explode?  Essentially an explosion occurs when a chemical reaction produces lots of hot gases, very quickly.  If these gases have nowhere to go, because they’re in an enclosed space, they put immense pressure on their immediate surroundings as they rapidly expand.  Ultimately those surrounding are apt to give way, with a bang. (High explosives, like dynamite and TNT, are a little different – but fertilisers aren’t high explosives, so we’ll save that topic for another day.)

Compounds that contain nitrogen have the potential to produce nitrogen gas.  Gases take up a lot more space than solids because their particles are further apart and, as I’ve already mentioned, when that hugely strong nitrogen triple bond forms lots of energy is released.  So there you are, hot (that’s the energy bit) gas.  Lots of it.  Surround it with walls – say in a container in a factory – and you have the potential for an explosion.

The fertiliser in this case appears to have been ammonium nitrate.  This is made by reacting ammonia (if you remember, Fritz Haber figured out how to produce that) with nitric acid.  Ammonium nitrate’s chemical formula is NH₄NO₃ – so plenty of nitrogen there.  In fact when ammonium nitrate decomposes it forms water vapour, nitrogen gas and oxygen gas (via some nitrous oxide, aka laughing gas, along the way).  Lots of gases.  Lots of heat.

The factory also contained lots of anhydrous ammonia.  Not especially surprising this, since you need ammonia to make ammonium nitrate – this was a fertiliser factory.  Anhydrous just means ‘no water’, in other words pure ammonia, NH₃.  The boiling point of pure ammonia is -33 ^oC, so you have a bit of a problem right there if your cooling systems fail; it will quickly turn into vapour at room temperature.  This vapour is pretty nasty.  You know that smell when you use hair dye or perming solution (if you’re still in the 80s)?  That.  Times a hundred.  It’s toxic and corrosive (it poisons you while damaging your lungs), and environmentally damaging.  Oh yes, and flammable.  Not as flammable as say, petrol, but flammable enough.

Reports are that there was a fire at the plant before the explosion, so it looks as though the ammonia might have caught fire.  Ammonium nitrate isn’t easy to ignite, but if the fire is contained and it’s exposed to sustained heat it’ll start reacting.  It decomposes at about 210 ^oC and once it’s started it’s very difficult to stop, because the reaction gives out a lot of heat which causes the surrounding material to react, and so on in a catastrophic spiral – something chemists call a runaway reaction – ultimately leading to detonation.

So fertilisers are potentially dangerous because they contain nitrogen in a more reactive form, which plants can use.  There’s nothing you can do to make fertilisers explosion-proof.  You can’t say, put additives in to make them less explosive.  It’s in their nature.  Take away their explosiveness and you take away their ability to act as fertilisers.

Factories, though, should be following detailed safety procedures and have numerous protective backup systems to prevent disasters like this.  We don’t yet know what went wrong here, but let’s hope some serious lessons are learned.