A little about the explosion at West, Texas. A Chemical Safety Board go-team is at the site, along with investigators from the Texas State Chemist . This news report says experts have identified the epicenter of the explosion, but aren't releasing that info yet. Later, we'll get the official word about exactly what exploded and why.
But the center is pretty clear from this photo in the Fort-Worth Star-Telegram, given streaks and the location of debris (Tony Gutierrez, Associated Press):
Note, for example, where the company's big ammonia tank sat (the round yellow foundation at upper right) and where the walls of that tank ended up. A line connecting those two locations leads to the center point of the explosion. While I haven't seen an official diagram of the plant, the epicenter looks like the north end of the dry barn mentioned in some news reports.
Here's a labeled photogram of the blast site, along with an aerial posted on Bing.com from last year. (Kudos to Bing and Pictometry for having recent aerial photographs on line):
We're told that West stored a large quantity of dry ammonium nitrate fertilizer in the dry barn, perhaps 270 tons at times. From a good piece in Chemistry World about ammonium nitrate, here's a snip:
"It is an intriguing feature of ammonium nitrate is that it should not, according to thermodynamics, even exist. It is a compound that contains nitrogen in two forms - surrounded by oxygen in one and by hydrogen in the other. In chemical parlance the nitrogen is, side-by-side, in oxidised and reduced forms. Normally one would expect them to react together, combining to form the more stable dinitrogen, dioxygen and water in a complicated partner swapping process called comproportionation. But in ammonium nitrate, this process does not happen at room temperature, making it a metastable compound, like a lake of water trapped behind a dam. Even if you are careful, ammonium nitrate is an accident waiting to happen."
When heated in a fire, ammonium nitrate doesn't always blow up but it might. If the smoke changes color to some shade of orange, that may mean the material is in a runaway reaction generating oxides of nitrogen. I interviewed a worker at a dynamite plant who once saw fumes from an overheating vat of metriol trinitrate change color in such a fashion, and he ran fast enough and far enough to survive the blast.
And one witness in West mentioned that the smoke from the fire at West Fertilizer changed color shortly before the blast.