More about possible causes

The Guardian 18 April 2013 at 15:31

Guardian community coordinator Ruth Spencer reached out to commenter bigbobcolorado who is an industrial chemist. He has 35 years experience in fine chemicals manufacture and has worked as a plant manager at a facility that handled anhydrous ammonia. The West Fertilization fire that preceded the explosion is thought to have started in an anhydrous ammonia tank.
He shared some of his knowledge with Ruth in an email.

It seems clear that there was an initial fire before the explosion occurred. So the CSB investigation will probably identify how the fire was able to get near the bulk storage tanks holding anhydrous ammonia. Bulk storage tanks are normally protected by bunds/dykes or drainage systems that deflect or prevent burning materials accumulating underneath the tank.
The damage sustained indicates that a high energy explosion took place. In such events, a very high pressure wave is generated by the explosion that travels extremely rapidly through the surrounding environment resulting in catastrophic consequences over a large area. Such events usually involve large quantities of flammable vapor clouds which are constrained i.e. enclosed or confined. A BLEVE scenario as mentioned earlier would cause similar catastrophic consequences.

Another possible scenario is that flammable dust was ignited, but I have heard no mention of dust in any reporting so far.
Whatever the cause I fear the casualty numbers will increase. The only good news is that the event took place at night when the facility was closed, people were in bed and the adjacent school was closed.