Explosive Ammonium Nitrate: What, How, Why?

By now you have all seen the horrific footage of the explosion in Beirut which occurred on the 4th August, injuring more than 5,000 and killing at least 135 people (as of Thurs 6th Aug), whilst the search for survivors continues. Here we will discuss what is likely to have caused the explosion, and how and why it happened. There are links to donation pages at the bottom. (Feature image courtesy of The Wall Street Journal).

It is believed that the Beirut explosion was caused by a huge quantity of ammonium nitrate (2,750 tonnes) being stored at the port. So what is ammonium nitrate and what is it used for? Ammonium Nitrate or NH4NO3 is a synthetic product (meaning it is created rather than produced naturally), and is used in a variety of industries; mainly in fertilisers for nitrogen release (nitrogen is a key component in chlorophyll which plants use to photosynthesise). It is also used in explosives for mining, or as a nutrient in the production of antibiotics and yeast. Generally, it is a white crystalline solid (looks like shiney crystals) and is made by reacting ammonia (NH3) with nitric acid (HNO3).

ammoniumnitrate

In its pure form, ammonium nitrate is not flammable, relatively safe to handle and can dissolve readily in water. However, if a large amount is left to decay over time and stored incorrectly, ammonium nitrate becomes a disaster waiting to happen.

How does ammonium nitrate become explosive?
Ammonium nitrate is an oxidiser. This means that, at an atomic level, it removes electrons from other molecules in a chemical reaction. We all learnt at school that fire needs 3 components to stay lit: fuel, heat and oxygen. Therefore, an oxidiser increases the amount of oxygen available to a fire. For this reaction to occur however, ammonium nitrate must come into contact with an open flame or other ignition source. Experts suggest that fireworks were involved in the Beirut disaster and you can read more about their reasoning here: LINK. So if ammonium nitrate only ignites under the right circumstances, which are difficult to achieve, how did this explosion happen? If ammonium nitrate is contaminated with a combustible substance like oil for example, the burning process is accelerated and it becomes highly explosive. Andrea Sella, professor of chemistry at University College London says, “the real problem is that over time it will absorb little bits of moisture and it eventually [becomes] an enormous rock.” Therefore, if a fire reaches the ammonium nitrate rock, the chemical reaction will be much more intense.

oxidising

During the footage, you see a large red cloud of smoke. This is the result of toxic chemicals (nitrogen dioxide – an air pollutant), which are produced during the combustion of ammonium nitrate. If there is not much wind following the explosion, this could become a danger to those nearby. Everyday we breathe in nitrogen dioxide as it is produced by cars, but only in trace (tiny) amounts. If you see a huge red cloud however, it is time to run. Not long into the footage, after the red cloud and smoke from the fire, a mushroom cloud follows the blast. A supersonic shockwave travels through the air, and can be seen as the white spherical cloud which travels out from the centre, expanding upwards, flattening the port and surrounding area. Prof. Sella says, “the air expands rapidly and cools suddenly, and the water condenses…causing the cloud.” The following video has upsetting scenes from the explosion, video courtesy of The Guardian Newspaper.

 

Why did this happen and has it happened before?
The ammonium nitrate exploded after being stored unsecured in a warehouse at the port for six years. Although fireworks have been reported by one source, the official cause of ignition is still unconfirmed. Unfortunately this is not the first time ammonium nitrate has caused disasters either. In 1921, ~4,500 tonnes caused an explosion at a plant in Oppau, Germany; more than 500 people died. In 1947 at least 581 people died in the deadliest industrial accident in US history at Galveston Bay, Texas, when more than 2,000 tonnes of ammonium nitrate detonated on-board a ship which had docked. It has also been used in several terrorist acts, including the Oklahoma City bombing in 1995, killing 168 people, in the 2002 nightclub bombing in Bali which killed 204 people, and in the 2011 Oslo bombing that killed eight people. Even as recently as 2015, an explosion involving ammonium nitrate (amongst other chemicals) killed 173 people in the port of Tianjin, northern China.

After these tragedies, there are generally very strict rules about where ammonium nitrate can be stored (i.e. must be kept away from fuels and sources of heat). Despite the disasters, ammonium nitrate remains indispensable in agriculture and construction. Jimmie Oxley, a chemistry professor at the University of Rhode Island said, we wouldn’t have this modern world without explosives, and we wouldn’t feed the population we have today without ammonium nitrate fertiliser. We need ammonium nitrate, we just need to pay good attention to what we’re doing with it.”

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