What is ammonium nitrate which is being blamed for blast in Beirut?

The Beirut explosion which shook the entire capital of Lebanon and killed 100 people on Tuesday, is said to be caused by thousands of tonnes of ammonium nitrate stored in a warehouse.

Following the massive blast, Lebanon’s Prime Minister, Hassan Diab said the blast occurred after an estimated 2,750 tons of ammonium nitrate, lying unsecured in a warehouse for six years, was ignited.

However, it remains a mystery what caused the common industrial chemical to ignite. Ammonium nitrate, with the chemical formula NH4NO3, is a naturally occurring, highly soluble white crystalline solid found in the Atacama Desert in Chile.

Videos: Huge explosion in port area shakes Beirut

It is used mainly for fertilizer as it is a good source of nitrogen for plants. The chemical is also the main component in mining explosives. It accounts for 80% of all the industrial explosives used in the US.

Although it is not regarded as particularly dangerous, most countries have regulations controlling its storage to make sure it is safe.

Gabriel da Silva, a senior lecturer in chemical engineering at the University of Melbourne, said that ammonium nitrate is not explosive on its own, rather it ignites only under the right circumstances.

"While ammonium nitrate can in fact put out a fire, if the chemical itself is contaminated, for example with oil, it becomes highly explosive," the Guardian quoted da Silva.

If there is a sufficient quantity of ammonium nitrate, it can generate enough heat to catch fire and keep the fire going. As it burns, it goes through chemical changes that leads to the production of oxygen, which causes the fire to get bigger.

As it heats up, the chemical can fuse together, creating a plug which on heating, forms gas. The hot gas then expands behind the plug, and breaks through the plug and the force of that will trigger an explosion.

If yesterday's blast was in fact caused by ammonium nitrate, then this will make it even bigger than the explosion that occurred in Texas City in 1947, when a consignment of 2,300 tonnes of ammonium nitrate exploded.

Nearly 500 people were killed while the blast created a 4.5-metre tidal wave.