Ash Clumps

I'm having trouble with the ash in my outdoor boiler, I'm burning coal but the ashes form a solid clump, then I can not shake them down, can anyone tell me what is causing this.

The clumps are called clinkers. They form at high temperature. Some coal makes more clinkers than other coal. I don't know how to stop it from happening other than running the fire at a slower pace.

Are you burning anthracite or bituminous?

I'm burning Anthracite coal - Pea size.

Out door boilers are typically inefficient (not sure what you have so this statement might be way off). The amount of heat needed to produce a decent water temp is way over what you really need, most hand fed indoor boilers shut down the air over and under the fire to reduce temps, the clinkers are caused by high heat as Freddy already said. If you have a way of reducing the air under and over the fire and at the same time maintain water temps that will help the situation.

Rev the fire up first and then poke through it from the top with a steel bar in a bunch of places to break it up. Pea is kind of small for a big unit, I would try some nut or even stove size coal and fill it up a lot more if you can. The hotter it burns, the more trouble you will have with clinkers, more coal won't have to be pushed as hard to make the same heat.