Much has recently been written about tuning our coal stokers.
As I see it we have three fundamental issues here, in no particular order.
a) Fuel / Air ratio.
b) Coal variables.
c) Heat exchanger effectivness.
Fuel / Air ratio
I set mine with adiquate air to avoid black smoke, and create a hot fire. The first monitored by looking at the vapor coming from the stack and the second measuerd by reaching my ash fusion temperature, i.e. making some clinkers. I would like to set it with a Bacharach Kit, http://www.bacharach-inc.com/fyrite-gas-analyzers.htm a few years ago the PDF instructions included coal in the charts, I imagine that information could be found. My friend that tuned diesel guns said the electronic meters were nice but he always brought the old mechanical units for back up. He recomends cleaning your combustion blowers, boiler rooms can be dirty, especially coal boiler rooms. Draft also effects this, a blower blowing into a vacume blows differently then when blowing against resistance. On natural gas burners you set for no yellow flame, indicating excess oxygen, don't tune coal this way, at least not my coal burner. Stack temperature is effected by fuel air ratio, and heat exchanger cleanliness.
This probalby includes everything from types and sizes of coal and moisture content. I know that my coal somtimes has snow in it and that must effect overall combustion efficiency.
Heat exchanger effectivness
For me this is the biggest issue, the more I clean my fire tubes the less coal I use, I'm sure there is a limit but, I need to clean the heat exchanger more. I'll bet this is where most efficiency can be gained. I have seen used oil burners with draft guages conected permanatly to indicate when to clean the heat exchanger. This probalby has more effect on stack temperatures than fuel air ratio.
I think each of these topics could be expanded on and probalby should be. I'll bet some fine tuning and clenaing efforts are worthwhile, but this is probalby not an exact science like a more consistant fuel that dose not foul it's heat exchanger. Lets get happy, and control the urge to over analyze this.