Capacity Mapping for Optimum Utilization of Pulverizers for Coal Fired Boilers

Abstract

Pulverizers play a pivotal role in coal-based thermal power generation. Improper coal fineness or drying reflects a qualitywise deterioration. This results in flame instability, unburnt combustible loss, and a propensity to slagging or clinker formation. Simultaneously, an improper air-coal ratio may result in either coal pipe choking or flame impingement, an unbalanced heat release, an excessive furnace exit gas temperature, overheating of the tube metal, etc., resulting in reduced output and excessive mill rejects. In general, the base capacity of a pulverizer is a function of coal and air quality, conditions of grinding elements, classifier, and other internals. Capacity mapping is a process of comparison of standard inputs with actual fired inputs to assess the available standard output capacity of a pulverizer. In fact, this will provide a standard guideline over the operational adjustment and maintenance requirement of the pulverizer. The base capacity is a function of grindability; fineness requirement may vary depending on the volatile matter (VM) content of the coal and the input coal size. The quantity and the inlet temperature of primary air (PA) limit the drying capacity. The base airflow requirement will change depending on the quality of raw coal and output requirement. It should be sufficient to dry pulverized coal (PC). Drying capacity is also limited by utmost PA fan power to supply air. The PA temperature is limited by air preheater (APH) inlet flue gas temperature; an increase in this will result in efficiency loss of the boiler. Besides, the higher PA inlet temperature can be attained through the economizer gas bypass, the steam coiled APH, and the partial flue gas recirculation. The PA/coal ratio, a variable quantity within the mill operating range, increases with a decrease in grindability or pulverizer output and decreases with a decrease in VM. Again, the flammability of mixture has to be monitored on explosion limit. Through calibration, the PA flow and efficiency of conveyance can be verified. The velocities of coal/air mixture to prevent fallout or to avoid erosion in the coal carrier pipe are dependent on the PC particle size distribution. Metal loss of grinding elements inversely depends on the YGP index of coal. Besides that, variations of dynamic loading and wearing of grinding elements affect the available milling capacity and percentage rejects. Therefore, capacity mapping is necessary to ensure the available pulverizer capacity to avoid overcapacity or undercapacity running of the pulverizing system, optimizing auxiliary power consumption. This will provide a guideline on the distribution of raw coal feeding in different pulverizers of a boiler to maximize system efficiency and control, resulting in a more cost effective heat rate.