When a unit is designed a “Performance Coal” is selected to base the design of the boiler on. There may be ranges of some specifications included, but the unit guarantee performance is usually based on the performance fuel. If the performance coal is higher calorific value coal, the mills and coal pipe design may be set for this type of coal.
Two things happen if you bring in the lower cost, lower Btu/lb, calorific value coal. The first is more coal flow to make up for the lost CV. If the mill is tonnage limited you may, and some recommend that you can, open the classifiers and grind the coal less to allow for more tonnage flow. The lower CV low rank coals like to burn so there may not be much carbon in the ash when larger coal sizing is used. The second is that the velocity of the coal pipes goes up due to the higher coal flow and the use of more primary air in the mill to help dry the coal.
Wall slag forms when large molten ash particles fly through the flame and onto the wall due to high velocity. This is why the higher moisture lower CV coals may increase slag using the “Rod Hatt Slat Factor” concept by increasing the coal pipe velocity and increasing the size of rock and ash.