Preventing performance drops of coal mills due to high moisture content

Coal mills pulverize and dry the coal dust before it is blown into the furnace in coal-fired power plants. The coal mills can only deliver the requested coal flow if certain conditions are fulfilled. These are normally considered as constraints on individual variables. However, combinations of more than one variable might cause problems even though these individually variables are in an acceptable region. This paper deals with such a problem. The combination of a high load of the power plant, a large load change and high moisture content in the coal, can force the coal mill into a state where coal is accumulated instead of being blown into the furnace. This paper suggests a simple method for preventing the accumulation of the coal in the mill, by limiting the requested coal flow considering the coal moisture content and the temperature outside the mill.