The Beirut explosion may be the worst ammonium nitrate disaster in history, but it is not the first such calamity. Let’s go back to the early morning of April 16, 1947 when spectators flooded to the docks in Texas City, Texas, drawn by the bright orange flames and the massive plume of black smoke that enveloped the S.S. Grandcamp, a French ship that had caught fire in the harbour. Then, as people marveled at the inferno, and quick-thinking vendors circulated with peanuts and other refreshments, there was a reverberating explosion. Hot pieces of metal from the disintegrated ship rained down, a devastating shock wave rolled across the land and sea, and within minutes much of Texas City was in flames. Almost six hundred people perished, many of them the onlookers who had come to gawk at the spectacle.
What cargo was responsible for the disaster? Nitroglycerine? TNT? Dynamite? None of the above. It was fertilizer! Not any old fertilizer, mind you. The Grandcamp had been loaded with two million kilos of ammonium nitrate destined for Europe. Ammonium nitrate is rich in nitrogen and can yield bumper crops or green up a lawn. But it can also explode and cause terrible bloodshed. An explosion can best be described as a “sudden going away of things from the place where they have been.” The cause of such swift departures is a shock wave formed by the very rapidly expanding gases that characterize an explosion. In the case of ammonium nitrate, the gases are water vapour, oxygen, nitrogen and oxides of nitrogen. Don't get the impression though that ammonium nitrate explodes easily. It doesn’t. Various conditions have to be met for an ammonium nitrate explosion to occur.
Let's get back to the S.S. Grandcamp. A fire broke out in the hold, most likely due to an improperly discarded cigarette. Fearing damage to his cargo, the captain decided not to try to extinguish the flames with water. Instead, he ordered the hatches to be battened down, hoping to cut off the fire’s oxygen supply. It didn’t work and the cargo of ammonium nitrate began to heat up. At first, it just decomposed into steam and nitrous oxide, better known as “laughing gas.” But it was no laughing matter when the high temperatures triggered the breakdown of the laughing gas into nitrogen and oxygen. The fire, now well supplied with oxygen, intensified. Still, there probably would have been no explosion had it not been for two other factors. The ammonium nitrate was packed in paper bags which began to burn with great intensity, and more significantly, the ship had been filled with 1500 tons of fuel oil. When the oil caught fire and its hot vapors mixed with the ammonium nitrate, which by now was venting massive amounts of oxygen, the conditions were right for “a very sudden going away of things from the place where they had been!”
The Texas City disaster was an accident. But the bombing of the Federal Building in Oklahoma City in 1995 certainly was not. The chemistry, however, was the same. The perpetrators of this horrific crime were aware of the explosive nature of mixtures of ammonium nitrate and fuel oil (ANFO). Indeed, most commercial explosives in North America used in mining and construction fall into this category. ANFO mixtures are actually remarkably safe to use, they have to be detonated by an explosive charge. Of course, Timothy McVeigh and his cronies had no access to commercial explosives, so they decided to make their own. Either they researched their subject remarkably well, or they were very lucky. Homemade brews of fertilizer grade ammonium nitrate and fuel oil are very difficult to detonate without the use of TNT, dynamite or blasting caps. Unfortunately, as history has shown, terrorists can be remarkably resourceful.
The Beirut explosion in all probability was an accident, but exactly how the ammonium nitrate ignited may never be determined. Certainly, there was negligence involved. Such massive amounts of the chemical should never have been stored in such a fashion. However, as far as the ammonium nitrate fertilizer you may have purchased from a garden supply store goes, the chance that it will spontaneously blow up is roughly zero.