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Industrial Safety

The chemistry behind the Beirut explosion

Devastating blast at port in Lebanon blamed on poorly stored ammonium nitrate

by Laura Howes
August 5, 2020


Credit: Elizabeth Fitt/SIPA/2008051043
The red-orange smoke plume was one of the first clues that ammonium nitrate was involved in the explosion in Beirut.

On Aug. 4, a devastating explosion struck the area around a port in Beirut. Lebanese officials are blaming the blast, which has killed over 100 people and left 300,000 homeless, on the common fertilizer compound ammonium nitrate (NH4NO3). The chemistry behind ammonium nitrate’s explosive risk is well known, leading some officials to blame the accident on negligence.

According to Lebanese officials, 2,750 metric tons of ammonium nitrate had been stored in a hangar at the city’s port. The stores had been there since September 2013 after the ship carrying the material was forced to make an unplanned stop in Beirut where it was then abandoned by its owners and crew.

Before the explosion, a fire had started in the area, creating a plume of white smoke and small explosions. When the ammonium nitrate stores exploded, a white condensation cloud spread out in a sphere from the site, followed by a huge plume of red-orange smoke rising from the hanger. Many chemists on Twitter identified that color as being a signature of NO2 gas, possibly produced from the incomplete decomposition of ammonium nitrate. Others also used the video footage to estimate the detonation velocity of the explosion as being around 3,000 m/s, which is also consistent with an explosion involving ammonium nitrate.

University College London chemist Andrea Sella was one of those who quickly identified ammonium nitrate as the likely culprit based on the plume’s color. Sella told C&EN that under normal conditions the fertilizer is inert. It will slowly decompose over time but can explode if it is set off by a fire, especially if poorly stored.

When energy is applied to ammonium nitrate, like from a fire, the molecule is no longer stable. Because ammonium nitrate contains nitrogen in two different oxidation states, an exothermic reaction occurs between the two nitrogen species: the nitrate acts as an oxidizer, while the ammonium acts as a reducing agent. If the reaction is completely clean, the only products are dinitrogen, water, and a little oxygen, but side products like NO2 are common. Because all the products are gaseous, there is a sudden, large increase in pressure that will then travel outwards at supersonic speeds, which is referred to as the detonation.

The condensation cloud caused by the blast led some people on social media to speculate that the explosion was caused by a nuclear device. But these types of clouds can also occur with a large enough conventional explosion in humid air, Sella says. What happens, he says, is that immediately behind the shock wave is an area of lower pressure that causes water to condense into microscopic droplets. The differences in pressures on either side of the shockwave can also create optical effects as light bends as it passes through the different air masses.

Sella also points out that ammonium nitrate detonations have caused previous industrial accidents, which is why most nations have strict regulations on the proper storage of the chemical. For example, the US Occupational Safety and Health Administration has multiple rules regarding ammonium nitrate, including that warehouses for bulk storage should have adequate ventilation and fire protection. It is also not common for such a large amount of ammonium nitrate to be stored so close to urban areas, according to Stella.

On Twitter, Lebanese President Michel Aoun called the failure to deal with the ammonium nitrate stores for so many years “unacceptable” and vowed the “harshest punishment” for those responsible. An investigation into the incident is ongoing.



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Blair Duff (August 5, 2020 7:33 PM)
Good source and explanation from the NFPA and the people who write the Standards
Germie l.jala (August 6, 2020 3:35 AM)
Explosive components such as ammunition nitrates should be stored away from urban centers under the safety and health standards provision,,so very sad of what happened my deepest sympathy to the victims..
Antonio Frau (August 6, 2020 6:48 AM)
"When energy is applied to ammonium nitrate, like from a fire, the molecule is no longer stable." Ammonium nitrate is an ionic compound, there are no molecules involved. Please fix this.


Antonio Frau
Firmin (August 12, 2020 4:15 PM)
Antonio is correct
Joe Atkinson (August 6, 2020 8:55 PM)
I have carried out the controlled decomposition of 15N ammonium nitrate as follows:

15NH4NO3 → 15NNO + 2H2O

It is a clean way to make terminally labelled laughing gas. The Wikipedia article includes discussion of the decomposition of ammonium nitrate:
Joseph Maughan (August 7, 2020 9:17 PM)
Well you guys got it backwards, I have watched the videos many times over and the rust colored cloud of smoke rose WELL before the big white mushroom cloud. a fire and small white explosions can be seen all the way from the beginning.
One thing no one is talking about is that the grain elevator was completely opened up on the explosion side. When that fell apart and opened the grain dust was then a definite factor and contributor to the story, it must have added to the explosive force in some way.
Also noted that when it came time for the big explosion smaller eruptions can be seen all around the complex well outside the warehouse area as if something underneath was a factor.
RT (August 12, 2020 11:38 AM)
Thank you!
David Fisher (August 12, 2020 7:46 PM)
There was a long [+4hr] video on Al-Jazeera filmed the day before the explosion, showing two fires and black smoke on each side of the silos, both about the same magnitude. The video started on sunlight and moved into nighttime, meaning this fire was going for at least 12 hours.
Philip Jewess (August 12, 2020 10:31 AM)
I believe the small white explosions that could be seem were fireworks, which were also stored in or near the warehouse.
Paul Krebaum (August 12, 2020 2:43 PM)
I wonder why a country which can't feed itself would let a bunch of great fertilizer go unused. They could have at least distributed this abandoned property to their farmers.
James R. Madden (August 16, 2020 9:51 AM)
If the officials involved could have decided upon a way they could have personally profited from such a plan, it would have been immediately undertaken. Distributing without selling does not involve the transfer of money.
Brad Phillips (August 14, 2020 1:54 AM)
Let us not forget West, TX. A much smaller quantity of ammonium nitrate destroyed a school, apartment building, houses, and other structures a few years ago as well as killing a good part of the fire department personnel who were trying to extinguish the original fire.
Arian van Mourik (August 18, 2020 5:37 AM)
Very very said this could still happen in 2020. The people, who decided to store this amount of material must have had little notion of history with ammonium nitrate and similar compounds. One of the earliest big incindents with the substance happened already nearly 100 years ago at one of the (now) BASF sites near Ludwigshafen Germany. Explosion of about 400 tons resulted in over 550 casualties. So the consequences in Beirut could have been even more disastrous compared to what they are, given the 2750 t stored there.

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