Contrary to what's been suggested earlier in this thread, heavy fuel oil (bunker) tanks can certainly explode. (Over the course of my career, I've surveyed many ships on which this has happened). The flash point of HFO is usually specified at 60C minimum but typically around 70-100C. If the fuel is heated in excess of this by adjacent fire, flammable vapours will be produced, and the mixture in the tank ullage space will tend towards the flammable range. If an ignition source is then introduced, there will be an explosion.