Hi Crs7300, my 'Big Bertha' is a home designed and built hand feed boiler, it is modeled after the OWB-styled firebox and grate system.. I struggled with it with both wood and coal.. It is a great wood burner, but to make the amount of BTU's I need, it needed feeding every 4-5 hours, even with a huge firebox. I reduced the firebox and burnt both Bituminous and Anthracite coal in it for a season. Then I found an antique Bituminous stoker [an Iron Fireman] I used this antique stoker to convert my 'Big Bertha' to a stoker fed boiler, this was better, but the limitations of the heat exchanger design [great for wood and creosote removal, but not great for coal] caused me to keep looking for a better boiler.
I found an Axeman Anderson 260 on ebay, bought it, friend on this forum removed it for me [it was in Penn.] and I retrieved it a month later, refurbished /rebuilt it and have been using it for heating the farm and DHW since.. BTW, in my avitar, if it is painted white, I heat it. maybe not really warm, but i heat if to protect it from freezing. So I consume a lot of BTU's.
Personally I don't recommend OWB's for burning coal,, they burn wood much better. The only way an OWB can burn coal well is if it has a good chimney, most OWB's are set up with only a few feet of chimney,, a wood fire will smolder and re-ignite with little or no draft or air, but a coa fire will quickly go out without a constant airflow through the coalbed from underneath, Without a chimney, the only way to provide this air is a two-speed blower, or a sophiticated blower with variable vent setup.. the coal must have a small amount of air all the time,, then have a lot to ramp up the heat to respond to BTU demands.. then go back to idle.. Made-for-coal boilers are set up to burn this way,, but most OWBs cannot, without some retrofitting of new controls.
The other problem you will find, is you mentioned buying 3-400# of coal to try,, well in most OWB's that is enough for one or two fillups of the firechamber.. and you MUST FILL the firechamber wall to wall , front to back, and at least 6-8" deep, or the coal will NOT burn.. So 3-400# of anthracite coal will not show you much.. It is worth a try, but read everything you can about starting a hand feed coal fire before adding coal to your OWB,, or you will just be wasting your coal and $$. Coal is a 'social or community' fuel,, it must have a lot of it to burn well, if at all.