Shaver makes 4 coal furnaces and 5 wood furnaces.
The wood furnaces can only burn wood. The Coal furnaces are labeled as "dual fuel" wood or coal.
We'd need to know what model you have.
But according to Shaver's website, and I'm paraphrasing because their site isn't the easiest to navigate for me (I'm color blind and shades of red and green are hard to see so reading their website is incredibly difficult for me as I have trouble reading a lot of it.)
Most of their coal furnaces are "set-up" for stove coal. But they are adjustable grates so you can set them to run anything from stove to pea or rice.
But a model number will be helpful in determining if you can even burn coal with your particular furnace.http://www.outdoor-wood-furnace-boiler.com/