The two explosions that hit Beirut on August 4, 2020, killing at least 100 people dead and thousands wounded, are the latest in a long list of disasters involving ammonium nitrate. Widely used to manufacture agricultural fertilizers, it also enters the composition of some explosives, and made part of this title, of the substances monitored by the service’s counter-terrorism around the world. Ammonium nitrate is a chemical compound composed of nitrogen, hydrogen and oxygen, which is in the form of a salt, the white powder granular.
Vanessa La Grange, chemical engineer at the prevention unit of the CNRS chemical risk. © Laurent Leclercq / CNRS” It is a product very used in fertilizers for agriculture, as it is a source of nitrogen, but it is also used for the manufacture of explosives, sealants, or adhesives, ” explains Vanessa of The Barn, an engineer at the prevention unit of the CNRS chemical risk. “This is not an explosive nor a fuel : it is classified as an oxidizing solid. This means that the substance provides oxygen and promotes the combustion of other material “, she adds. “In general, three conditions must be met for combustion to take place, it is what is called the fire triangle : fuel or flammable material, such as paper, wood, or a chemical product ; an oxidizer, such as oxygen of the air or ammonium nitrate ; and an activation energy, like a flame, heat, spark or static electricity “, says the chemist.
“It would appear that there has been a detonation”
Before the second explosion in Beirut, the most serious, a fire seems to destroy the warehouses in the port. “The ammonium nitrate, in the presence of a source of heat, may undergo a decomposition in several steps, and there may be explosion quickly if the ammonium nitrate is in a confined space,” says Vanessa of The Barn. “The decomposition is difficult to obtain with a pure product, but the presence of incompatible products, such as acids or reducing agents such as greases, oils, heating oil, sugar, sulphur, phosphorus, chlorides, or metals in powder, increases the risk,” she says. It is for this reason that the rules of storage, very strict requirements are imposed to this substance. The explosion of the AZF factory in Toulouse in 2001, had probably been caused by non-compliance of these rules, which have been strengthened since.
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“What defines an explosion, that is, in a confined space, the reaction to sudden combustion or decomposition with an increase in either the pressure or the temperature, or both,” she continued. “During an explosion, the shock wave propagates at a certain speed : if this speed is supersonic (greater than the speed of sound, editor’s NOTE), then there will be a detonation ; if it is subsonic (below the speed of sound, editor’s NOTE), then an explosion will take place “, she says. “Here, it would seem that there has been a detonation, the mushroom cloud, characteristic of detonations, though often associated with nuclear explosions,” details Vanessa of The Barn.
” ammonium nitrate, when it is used as a source of nitrogen in fertilizer, could be replaced by the ammonium sulfate, or urea, which are coupled to other molecules, according to the experiments of a team of the american university of Missouri dating back to 2006 “, specifies the scientist.
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