That was Pepcon, and they made a component of rocket fuel, ammonium perchlorate, used for the shuttles, not jet fuel. They had been stock piling the stuff because the Challenger disaster shut down shuttle missions for a while. In addition, there was a 16 inch diameter high pressure natural gas line running through the middle of a plant making an explosive chemical. The large explosion was due to the gas line. The initial fire was due to a welding accident. Wiki link:
http://en.wikipedia.org/wiki/PEPCON_disaster
In the video made by Dennis Todd, who was working on an antenna on a mountain overlooking the side, you can see the approaching shock waves from the two large explosions, then hear the sound. Destroyed in seconds split up that video with interviews. There are numerous versions at youtube.