What Is Ammonium Nitrate And Why Did It Devastate Beirut?

Ammonium nitrate, the material which appears to have caused the massive explosion which devastated Beirut yesterday, has been behind some of the largest accidental explosions and terrorist bombings on record. The Beirut explosion has marked similarities with the Texas City disaster of 1947, the deadliest industrial accident in U.S. history.

The main use of ammonium nitrate is as an additive to fertilizer, for helping nitrogen-poor soil. Global production runs to over twenty millions tons a year, so there is a lot of it around. Smaller quantities are employed industrial blasting products. It is a highly efficient oxidizer, a cousin of saltpeter which is the oxidizing ingredient in gunpowder. For this application, ammonium nitrate is typically mixed with fuel oil to create ANFO (Ammonium Nitrate Fuel Oil) , a low-explosive blasting mix which is much less powerful weight-for-weight than modern explosives like C4, but extremely cheap to produce.  Low explosives burn rapidly but at less than the speed of sound, unlike high explosives, making them distinctly less destructive. ANFO is used in mining, construction, and quarrying.

Ammonium nitrate as fertilizer is a bringer of life, but ANFO can be a bringer of death, being a favorite with terrorists as it is so easy to produce in large quantities: just mix diesel with bags of ammonium nitrate fertilizer. The disadvantage is that it is relatively difficult to detonate. When Timothy McVeigh carried out the Oklahoma City Bombing in 1995, he was unable to get hydrazine, his explosive of choice, or a sufficient quantity of TNT. However, he easily obtained over 5,000 pounds of ammonium nitrate which formed the heart of a device which killed 168 people.

While not used as an industrial explosive on its own, ammonium nitrate is unstable and at high temperatures it decomposes into nitrogen oxide gas and water vapor. Again, it is a relatively low-energy explosion, but dangerous when large amounts are involved. It is reckoned to be about 40% as powerful as TNT. According to the Lebanese Prime Minister, some 2,700 tons of ammonium nitrate were stored in dockside warehouse in Beirut (having been confiscated), so the resulting blast might have been almost equivalent to a one-kiloton weapon. The seismic waves recorded were equal to a magnitude 3.3 earthquake.

In April 1947, the SS Grandcamp was docked in Texas City, Texas. Grandcamp, built in WWII as a Liberty Ship, was loaded with 2,300 tons of ammonium nitrate en route from Nebraska. Allegedly someone discarded a still-smoldering cigarette butt in the hold. Smoke was seen rising from the cargo hold and several attempts were made to put it out, but the fire kept returning. Eventually gas pressure inside the ship blew the hatches open, releasing clouds of yellow-orange smoke. The color is indicative of nitrous oxide, and was also seen in the cloud that appeared seconds before the main blast in Beirut.

When the Grandcamp exploded it killed over four hundred people, including many spectators who had gathered to watch the dockside firefighting operation, and injured more than five thousand others. At least a thousand buildings were leveled and windows were shattered in Galveston ten miles away. The Grandcamp’s two-ton anchor was thrown more than a mile; it was later retrieved and set up in a memorial park to the disaster.

A similar event had occurred more than twenty years previously at Oppau in Germany in 1921, when a store containing over 4,000 tons of ammonium nitrate exploded.  In this case the problem was that a mix of ammonium nitrate and ammonium sulphate had set hard in a storage silo, and workers were breaking it up with dynamite.  The blast killed more than five hundred people and destroyed the factory along with most of Oppau, and was heard in Munich almost two hundred miles away.

The explosive properties of ammonium nitrate have been well proven; the official statement, as well as photographs from the port of Beirut, indicate that ammonium nitrate was being storied there. Video of the event suggests that the initial fire was in a warehouse containing fireworks. Ammonium nitrate is not a powerful explosive, and is not easy to set off, but a large enough quantity will produce a massive blast.

Beirut has seen more than its share of warfare, though when I stayed there in 2018 – a short distance from the port, with a  view of loading operations – I was struck by how few signs there were of the past conflict. However, conspiracy theories aside, the latest disaster seems to have nothing to do with politics and, like the Oppau and Texas City incident, everything to do with failed safety procedures around deadly ammonium nitrate.