I burned some red ash coal in my KAA-2 boiler last winter. (4 tons of it) I will not use red ash again. I probably didn't have clinkers, but it caked up. The cakes would not break and fall easily. The coal bed got taller, unburned coal fell over the side rails and I could hear the pusher motor/gearbox straining.
So I reduced air as recommended by some people on this forum. That helped until the cold weather came. Could not keep the house warm. Kitchen pipes froze. I had to mix good white ash coal with the red. And... there was a lot more ash, probably 30% more. So I was hauling more bags of coal down into my basement, and more pans of ash out. I am still putting bags of that red ash out for trash pickup.
No red ash for this Keystoker again! Maybe a larger unit could handle it better.