Each type of coal has different burning characteristics. All coal seems to have more heat energy in the first 30% of it's burn cycle, that 's from catching fire till it is burnt out to ash.
IWhen I was hand stoking my self designed and home [shop] fabricated 'big Bertha' boiler, I noticed that the fresh loads of coal produced the most heat, and a few hours later the coal was all red, making heat but at a much lower level than when first added to the fire.
I think this is why stokers appeal to me, whenever the water temp drops, the stoker fires up to reheat the water, and fresh coal is added, and this fresh coal burns much hotter than coal that is at it's 'half life'..
Since your coal has much lower BTU content than either Bit or Anthracite, I think you are doing as good as you can.
Can you get a ton or so of high quality Bit coal, and give it a try for a comparrison? maybe you could mix your free sub-bit with good bit and still
be on the 'less expensive' side compared to everyone else here on the forum.