making coke and not heat

I have the same furnace and I installed my forced draft where my plug was. When I am burning coal, I use the damper on the fan to close off my blower, which stops all air from above. I also close my door damper. I modified my ash door damper for more air from underneath. I think its a possibility of getting too much air from below. It was designed to have the blower in back for reasons. I have never gotten clinkers from soft coal like that, and what I have burned has burned up completely. US Stove says you can use your forced draft for coal, but I shut mine down and use my ash door damper only. Never had good results with the forced draft and coal. I do know by having that plug out in back, I get some serious heat and no smoke from wood. I get good secondary combustion in the back of the firebox all night.

I have the same furnace and it does have a design flaw for Coal burning. The grate area is not large enough for a good complete coal burn.

Also the baffle plates above the fire (you know the ones you knock down when you put too much wood in) are a problem as well. I removed mine to burn coal (do not do this with wood as it is a good flue igniter) I eventually left them out and built a kidney for mine to keep the flue fires down (I burn almost all wood with the price of coal rising) and recover the heat. These baffles are to keep the heat in the box longer but created alot of smoke for me.