The explosion at a Texas, US, fertiliser plant in mid-April 2013 was stunning in its ferocity. ‘Some media outlets are speculating about exploding anhydrous ammonia tanks, but that is a very rare event,’ notes fire expert Guillermo Rein at Imperial College London, UK. A more likely culprit is ammonium nitrate (AN) present in the fertiliser, a known fire and explosion hazard.
On average, one fertiliser plant explosion occurs worldwide about every three years. An explosion at a factory in 2001 in Toulouse, France, for example, killed 30, injured over 2000 people and shattered windows up to 3km away.
The most likely sequence of events in Texas, according to Rein, is that an accidental heat source started a decomposition reaction that was self-sustaining and led to the fire. ‘Self-sustaining decomposition of fertilisers is promoted by accidental contamination with organic materials and can start at around 100°C. This is significantly lower temperature than pure AN decomposition,’ Rein notes.
The ensuing chemical reactions lead to oxygen production, which would have fed the fire and rapidly heated very large quantities of AN, leading to an explosion. To make matters worse, the burning AN would have become airborne and ignited fires further away.
Andrea Sella at University College London notes that vast quantities of AN are manufactured because of its effectiveness as a fertiliser. ‘This is one of the simple compounds that underpins civilisation,’ he says, adding that problems occur when towns start to grow around fertiliser plants as planners are apparently unaware of the risks.
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