Beirut blast: How does ammonium nitrate create such devastating explosions?

Looking at recent sat pics, and looking at google maps(always old), I see only one small cargo ship near by. It's still there and I doubt it exploded. From comparing the images, it looks like the center of explosion was the grain elevators. At least the greatest of the explosions.

Makes one wonder, could they have been storing the fertilizer in the grain bins(silos)?