The perfect mixture of ammonia vapors and heat in narrow ranges would have been required to cause yesterday’s deadly explosion at a Texas fertilizer plant.
Anhydrous ammonia, a nitrogen-hydrogen combination used as a crop nutrient, has a tight explosive window that needs a precise ratio of liquid to vapor, said John Goodpaster, an assistant professor at Indiana University-Purdue University Indianapolis who researches explosives. The “sweet spot” is 16 percent to 25 percent vapor by volume, he said.
“If the mixture of ammonia gas and air is in the right range, then it can be very flammable,” Goodpaster said by phone. “If you have a fire going on outside a large container that has this mixture in it, and the fire is hot enough, then the mixture, if it’s right, would spontaneously ignite.”
There really are no procedures to regulate the amount of vapor and no ability to vent the tanks because of regulations preventing ammonia from being released into the air, Goodpaster said. Anhydrous ammonia is typically safer than other volatile chemicals and gases, such as hydrogen, which tend to have broader ranges of gas and air mixtures, Goodpaster said.
If anhydrous ammonia is stored in a 100 gallon tank and some of it is left empty, ammonia vapors would need to make up 16 gallons to 25 gallons of the tank to create the hazard.
Yesterday’s explosion at Adair Grain Inc.’s plant in West, Texas, killed as many as 15 people and injured at least 160 in what may be the worst U.S. industrial disaster since at least 2010, when 29 coal miners perished in Massey Energy Co.’s Upper Big Branch mine.
Anhydrous ammonia and ammonium nitrate, which is a solid in the form of small white pearls, can both be used to make fertilizer that farmers use to add nitrogen to the soil and boost crop yields. Ammonium nitrate can also be mixed with any type of fuel to create an explosive, such as the truck bomb detonated by Timothy McVeigh in 1995 at a federal office building in Oklahoma City, killing 168 people.
“Ammonium nitrate makes a pretty good explosive and it’s available because it’s a fertilizer,” Goodpaster said. “You can buy it in pretty large quantities.”
Ammonium nitrate was responsible for another Texas disaster in 1947, when the ship SS Grandcamp exploded at the docks in Texas City, according to the Texas State Historical Association. The ship was carrying the chemical and caught fire, destroying the dock and the nearby businesses and grain warehouses.
Ammonium nitrate and anhydrous ammonia also are used in one of the most popular methods of making methamphetamine, Goodpaster said.
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