Try closing the silver colored air flap on the bottom all the way closed. That looks to be a very big and hot fire you have going and the high heat causes the fusion. As Rob stated you may be asking too much of your boiler for these real cold days.
My KA 6 is oversized for my house so I run it at a slightly reduced feed rate, buy my coal from Superior, and have never, ever produced a clinker bigger than about 2” and they usually fall apart if I try to pick them up.
Some coal does not form clinkers as readily as others but most of the suppliers are selling and processing coal from various sources and those sources and the blending may never be the same from day to day or week to week so someone’s good coal this month may be the worst coal next month. I would be hesitant to recommend you waste more time and money trying to track down better coal when better coal may not exist.