Ammonium nitrate and the Beirut explosion, 4 August 2020

\(\require{mhchem}\)

On 4 August 2020 a massive explosion devastated the city of Beirut, Lebanon. It is thought a fire amongst a shipment of fireworks ignited a warehouse containing 2,750 tons of ammonium nitrate that had been stored at the docks. More than 130 people died and 4,000 were injured.

Ammonium nitrate is used as a fertilizer and in explosives for mining and construction. In its pure form it is not highly explosive, but when mixed with something to act as a fuel, explosive. The mixture of 94% ammonium nitrate and 6% number 2 fuel oil is commonly used as an explosive and this is given the name ANFO. (Number 2 fuel oil comes from fractional distillation of crude oil; this fraction is used as heating oil and its carbon chains are 14 to 20 long.)

In Beirut, the ammonium nitrate fertilizer had been impounded from a shipment in 2014 and stored since then.

Ammonium nitrate is made industrially by reacting concentrated nitric acid with ammonia (an \(\ce{acid-base}\) reaction): $$\ce{nitric~acid + ammonia -> ammonium~nitrate}$$ $$\begin{equation} \ce{HNO3(aq) + NH3(g) -> NH4NO3(aq)} \end{equation}$$

At temperatures below 300\(^\circ\)C, ammonium nitrate decomposes into nitrous oxide and water: $$\ce{ammonium~nitrate -> nitrous~oxide + water}$$ $$\begin{equation} \ce{NH4NO3(s) -> N2O(g) + 2H2O(g)} \end{equation}$$ At higher temperatures, a more dangerous reaction happens: $$\ce{ammonium~nitrate -> nitrogen + oxygen + water}$$ $$\begin{equation} \ce{2NH4NO3(s) -> 2N2(g) + O2(g) + 4H2O(g)} \end{equation}$$ In the high temperatures of this exothermic reaction, nitrogen and oxygen can combine to make the red-brown nitrogen dioxide, and we could certainly see clouds of that in the Beirut explosion. $$\ce{nitrogen + oxygen -> nitrogen~dioxide}$$ $$\begin{equation} \ce{N2(g) + 2O2(g) -> 2NO2(g)} \end{equation}$$

The energy of the explosion has been estimated as 1.2 kilotons, i.e. equivalent to the explosive power of 1200 tons of TNT; for comparison, when the United States dropped the atomic bomb on Hiroshima in 1945, the explosive power of that blast was about 16 kilotons. So the Beirut blast was about 1/13 of Hiroshima. 1.2 kilotons converts to \(5.8 \times 10^9\) kJ. (Examination of the blast crater led to revised estimates of 200 to 500 tons.)

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