Why is fertilizer explosive?
The answer is not obvious, and for years after the invention of ammonium nitrate, people didn’t actually know it was explosive. It was only after the Texas City Disaster that the penny dropped.
Ammonium nitrate is of course a completely synthetic product, which was only created after Fritz Haber in 1913 worked out how to convert nitrogen from air into ammonia. This process opened the way for the manufacture of nitrogen containing compounds as fertilizers.
So why is it explosive? Well it’s not too hard to understand if we look at the anatomy of an explosion. An explosion is simply a rapid combustion (oxidation) process. Coal burns, wearers coal dust explodes. The only difference is the rate.
When something burns, two components are required – the fuel and the oxidant. Usually, the oxidant is simply the oxygen in air. If we were to consider, for example, the combustion of the simplest hydrocarbon, methane, the reaction would be:
CH4 + 2O2 = CO2 + 2H2O
In other words, the flammable methane requires air to burn. This is why your car needs air to run. Without air, the fuel cannot burn. This is also the reason why people put turbochargers on engines – the extra air gives you extra oxygen and therefore a more rapid combustion rate.
Well, this is the chemical structure of ammonium nitrate
you will notice something interesting about it – it contains three oxygens already. What this means is that for it to burn, (or explode) it doesn’t need any air, as it supplies its own oxygen. This means that once the combustion process begins, there is nothing to stop the rate escalating rapidly, and hence the explosion.
Incidentally, this is the same principle behind rocket fuels, but that’s another story.