Enhanced control system for non-standard pulverized fuel mills using individual mill capability indices

Concerns coal-burning boilers in power stations. They handle pulverised fuel, i.e. coal ground into a fine powder in ball or tube mills. Primary air carries the fuel in suspension to the furnace for combustion, this primary air making up some 30% of the total combustion air. The most typical primary air control arrangement is a dedicated fan for each mill. The primary air is preheated in a gas/air heat exchanger to provide drying of the coal. Mills have been traditionally controlled to a "load-line" relationship between the differential pressure across the mill and the primary air flow. This load-line calibration is identified during commissioning, but is affected by the exact geometry of each mill and the wear which has taken place on the components within it. In addition to the mill loading control loops described in this paper, a modulating loop provides control for the dryness of the coal sent to the furnace.