We had a corn burner that would also burn coal, if you broke it down to a size the feed auger could handle. A neighbor burnt coal in his, and said the process for coal and corn was the same.
The key is that you need to burn it on a grate, so you have air flow up through the burning coal. Look at how coal forges are built. Draft controls burn rate and temperature. Unlike wood, coal forms clinkers that will plug up the grate and cut off air flow.