Unfortunately we don't have a great selection of coal here anymore since the mines closed. I bought what I'm burning two years ago. I think the coal dug from seams near the surface. It's not screened and is everything from fist sized lumps to dust, rocks and mud. Right after I bought it the grates broke and I just found a set this fall. The previous owners obviously didn't keep the ash pit cleaned out, the original grates were warped and cracked. I had already bought wood and planned on installing a steel plate to burn wood in the boiler.
I covered the front grate with fire brick today and the wood is burning better and I seem to be getting very little wood smoke out the door now.
I found some tables that list the max btu that can be connected using different stove pipes and chimney liners and sizes.http://inspectapedia.com/chimneys/Chimney_Flue_Size.htm#C9F
I found a formula in another thread that estimates the btu input based on firebox dimensions.http://nepacrossroads.com/about32189.html
My boiler's estimated btu capacity of 141,750 btu exceeds what the 6" pipe can handle, something less than 124,000 btu, according to the table, since the pipe has no rise, it goes straight into the chimney. The chimney liner is 6" square and can handle a total of something less than 195,000 btu because a square liner develops less draft than a round liner. My oil boiler is 105,250 btu so I could never run both boilers at the same time, not that I would want to but the chimney couldn't handle it.
If I reduce the firebox length dimension by the width of a fire brick the estimated btu becomes 111,375 btu, pretty close to my oil boiler and within the capability of the stove pipe. I wonder if with coal it would be enough to just cover the grate to reduce the open area or if it would have to be filled in to the top of the fire box?