A neighbor once gave me some coal so I tried a piece in with the wood in my pot belly stove. I had a heat exchanger on the exhaust to blow warm air and keep the house warm. The coal burned so hot the stove glowed red and it cooked the paint. I thought the house was going to burn down. We had to open the French doors to let the air blow through to cool the house down. This was a small home on Sullivans Island in SC. No more coal for me. It did contain lots of energy , and burned the wood much hotter. No need to clean the flue that year.
Ours a was a really heavy cast iron potbelly.