Hi JKP, if you think you are not burning all the coal, and getting unburnt coal in the ash, then reduce the temperature setting on the anthrastat. If your's is marked in degrees, it should be set at 140* from the factory, set it down to 135* and see if you notice a difference in the ash contents. If your anthrastat is marked in 1-8 or something similar, just turn it back a fraction [counter clockwise, to a smaller number] And see how or if that helps.
How much coal are you consuming right now?? In mild weather, you can reduce the boiler's temperature setting and save some coal. What is the Aquastat low setting at?? I keep mine at 155-160 when the weather is above 20*, I'll set it 10* higher for nights in the teens. This works in my home, with my baseboards, a lower water temp reduces the heat output of each baseboard. But if you don't need 180* water, then don't heat it that hot untill you need it.
Give us some consumption numbers. And how big your house is , how well insulated, and how good the windows are. In this mild weather, I'm burning only a little more than a five gallon bucket every 12 hours, and my home is 3500sqft, poor insulation and windows.