I have a Harman SF360 boiler, burning sub-bituminous coal. After some trial and two winters I now use and prefer stoker size coal. Much easier to handle, burns evenly, each load is about the same. Tried lump(large lumps), tried 3" size, stoker best. I buy from two different mines, the stoker size is slightly different. One is from 1"to3" with few fines, the other has much more fines. One is 8500BTU per pound the other 7000BTU per pound. I know one has more heat, but they burn about the same. I load one end full, wait at least 1 hour, if first is burning evenly (mostly) I then fill other half. No booms that way. As long as there is flame the volitiles are burn off. Sounds easy, but took me awhile to get it. You will have to do some trial and error on your stove. Good Luck
PS: If your getting the coal from Montana, it is most likely the same as I use. I am straight North.