Welcome to the site 430...
I assume you have some kind of grate for the floor of your firebox. I did a search for your furnace and looked at the instalation/owners manual. It described the install of firebrick but either I missed it, or it didn't say anything about the grate system.
Coal needs air, lots of air. And you have to pretty much force air throught the gaps and passageways around each piece of coal for it to burn. A natural draft will work but you have to follow the guidelines like I describe below.
If you have a small fire going in the front of the firebox, you must block the grate on the rear of the firebox. You might try just laying firebrick over the grate. You have to have ALL available air that can come in and go up the chimney go through the bed of coal. Otherwise the air will take the easy route and bypass the coal, and it will starve for air and go out.
If you have a chimney good enough to support a wood fire, you should have enough draft to pull air through a coal fire.
Next, a coal fire needs to be deep, coal won't burn very good if you just add a few pieces on top of a wood fire. A big chunk of Bituminous coal will burn this way, but not completely or effeciently. You need to heap coal on at least 4-6" deep as a minimum and keep it that deep, or passageways will burn through the bed of coal, the air will bypass most of the coal and it won't burn.
I hope this helps you, Greg L