Finally, Some Accurate Information

NYT:

The plant had large amounts of two commercial fertilizers, anhydrous ammonia and ammonium nitrate. Both chemicals have been linked to explosions in the past.

Anhydrous ammonia is a colorless, corrosive gas that is stored as a liquid under pressure; farmers inject it into the soil. “People mostly think of it as a toxic chemical that can cause breathing problems,” said Sam Mannan, a professor of chemical engineering at Texas A&M University. “But it’s also a flammable and explosive material.”

The gas must mix with air in relatively high proportions to ignite, so it is less dangerous than natural gas or gasoline under ordinary conditions. But Dr. Mannan suggested one way an anhydrous ammonia explosion might occur: If during a fire an ammonia tank were to be breached, the gas would mix with the air until it reached the proper concentration, at which point it would be ignited by the fire. The catastrophe at the West plant began with a fire.

Ammonium nitrate, which is usually sold in granular form, can be mixed with fuel oil to become a powerful explosive that is used often in industry and occasionally by terrorists. But Dr. Mannan said that even by itself the chemical can explode under the right conditions — if it is heated in a confined space during a fire, for example.

It is amazing how frequently people have interchanged anhydrous ammonia and ammonium nitrate this week. The takeaway is, fire at the fertilizer plant is bad news.  It is probably worse if there is ammonium nitrate in the building that was on fire, but it isn't good to have a fire burning by a giant pressure tank of anhydrous, either.