I burn bit mixed with wood in my OWB, which has a fan blowing air from underneath up through the grate. When I have a bed of coals, I throw on a bucket or two of coal, then fill with wood. The coal I'm using now is stoker size, not a lot of ash and little bridging. I get a 12 hour burn and the coal really helps in these 0 temps. Helps stretch the wood supply too.