The biggest challenge you will have is shaking the coal ash. With wood, I'm told, much the ash provides a support for heat. But with coal you've got to get rid of the ash and keep air flowing up through the coal. Years ago I bought a wood/coal furnace from a wisconsin company called BK Industries. The furnace burned wood fine but the shaker grated were terrible! I had to let the fire burn out, clean out the ash and then build a new fire every 3 days. That furnace went out the door never to return!
I saw an out door unit advertised on line built in the mid west. They demonstrated their grates. I would not buy one for love nor money. The grates re not held down, they jump up and down. They'll be jammed after the first or second rake! Trouble in the making.
Good luck if you don't have good shaking grates.