What is ammonium nitrate, the chemical that exploded in Lebanon's Beirut?

For an industrial ammonium nitrate disaster to occur, a lot needs to go wrong. Tragically, this seems to have been the case in Beirut.

The Lebanese capital Beirut was rocked on Tuesday evening local time by an explosion that has killed at least 78 people and injured thousands more.

The country’s prime minister Hassan Diab said the blast was caused by around 2,700 tonnes of ammonium nitrate stored near the city’s cargo port. Video footage appears to show a fire burning nearby before the blast.
Ammonium nitrate has the chemical formula NH4NO3. Produced as small porous pellets, or “prills”, it’s one of the world’s most widely used fertilisers.

It is also the main component in many types of mining explosives, where it’s mixed with fuel oil and detonated by an explosive charge.

For an industrial ammonium nitrate disaster to occur, a lot needs to go wrong. Tragically, this seems to have been the case in Beirut.

What could have caused the explosion?

Ammonium nitrate does not burn on its own.

Instead, it acts as a source of oxygen that can accelerate the combustion (burning) of other materials.

For combustion to occur, oxygen must be present. Ammonium nitrate prills provide a much more concentrated supply of oxygen than the air around us. This is why it is effective in mining explosives, where it’s mixed with oil and other fuels.

At high enough temperatures, however, ammonium nitrate can violently decompose on its own. This process creates gases including nitrogen oxides and water vapour. It is this rapid release of gases that causes an explosion.

Ammonium nitrate decomposition can be set off if an explosion occurs where it’s stored, if there is an intense fire nearby. The latter is what happened in the 2015 Tianjin explosion, which killed 173 people after flammable chemicals and ammonium nitrate were stored together at a chemicals factory in eastern China.