In a modern utility/Power boiler- The tubes carry the high-pressure water/steam, but the boiler itself has fans blowing into it and sucking out of it (at mine, the air moves around 60mph through the boiler).
Ideally, you 'balance' the fans so that the 'sucking' fans run a big harder than the 'blowing' fans - this puts the boiler at a slight negative pressure (think in inches of water, tenths of a psi) - this keeps the 'fire' from leaking out.
In a coal fired boiler, the explosion works like this:
Coal is pulverized (made into powder) before being blow into the running boiler (the boiler is started on natural gas or fuel oil). If, for some reason, the fire goes out- and you dont stop blowing in coal dust- you create a large build-up of ultra-flammable coal dust. If you stop the fans (loss of electric power?) and subsequently restart a burner (with oil, nat gas or just a spark) it could ignite the coal dust and cause a large explosion.
Another way to blow up a boiler
