Hey Rich, Don't give up on that furnace for coal yet!! I have a Clayton with a very simalar design. I discovered that there are small openings in the front and back of the firebox around the front and back liner. These openings were allowing combustion air coming in from my ash pan air feed valve to actually bypass the coal bed. In othere words the air coming in thru the ash pan could "go around" the grates instead of going up thru the grates to feed the fire from the bottom. THis isn't a problem UNTIL you get some ash cutting off air supply up thru the grates, then the draft would pull up thru between the liner and firebox, bypassing the coal bed and slowly killing the fire.
SO, I took some fiberglass insulation and plugged MOST of these bypasses. Problem solved, I'm getting more heat in the house, less up the chimney and right about now I'm gonna go crack a window cuz I'm sweating lol.
SO take a good look inside the firebox, see if you can spot these bypassing ports. Just be careful - I'm sure these ports are designed for allowing air to come up over the fire for burning wood or BIT coal, but they seem to be a hinderance for burning ANT coal.
Check out this thread Fly ash and/or residue insulating firebox?