"I also noticed that when burning Bituminous coal , if I set the natural draft ( over the fire draft ) to work with my combustion blower ( under the grates for coal) I don't see the dreaded bridging problem as bad as when I have the natural draft all the way closed. "
this is because with bituminous, especially those coals prone to swelling, heat must be added above the coal bed to burn away the tars and bitumin that causes midphase "bridging". cracks form in the solidified coal bed from the intense heat burning away the tars, if the heat is only from the bottom, the tars will only burn away and cracks will only began to form in the bottom of the coal, the top will stay solidified, leaving a "crust" that prevents air from going all the way through the coal bed and creats low heat output, and in some cases can put the fire out or be a general pain in the ass. the results are even better and no poking is needed if either A. preheated air is entering above the coal bed, and directed somewhat downwards toward the bed of coal, thus limiting the timespan of "bridging" to only a few minuites, or B. there are insulating, reflective refractory panels or brick in an arch, which effectively direct heat back towards the burning coal bed creating the high temps neccecery to prevent "bridging". both of these methods either alone or in combination reduce soot and prevent fuel waste. burning a high volitile bituminous, the smoke should turn quickly from yellowish white to a lite black or light grey for the remainder of the volitile flaring cycle, not remain thick white/yellowish which is what will happen if burning high vol coal w/out one of these methods in place.