Capacity Mapping for Optimum Utilization of Pulverizers for Coal Fired Boilers

Abstract

The pulverizer plays a pivotal role in coal based thermal power generation. The improper coal fineness or drying reflects a quality-wise 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 the coal pipe choke or the flame impingement, an unbalanced heat release, an excessive FEGT, overheating of the tube metal, etc, resulting on the reduced output and excessive pulverizer rejects. In general, the base capacity of the pulverizer is a function of coal and air quality, conditions of grinding elements, classifier and other internals. The capacity mapping is a process of comparison of standard inputs with actual fired inputs to assess the available standard output capacity of the pulverizer. In fact, this will provide a standard guideline over operational adjustment and maintenance requirement of the pulverizer. The base capacity is a function of grindability; fineness requirement may vary depending upon the volatile matter content of the coal and the input coal size. The quantity and inlet temperature of primary air limits the drying capacity. The base airflow requirement will change depending upon the quality of raw coal and output requirement. It should be sufficient to dry pulverized coal. Drying capacity is also limited by utmost P.A. fan power to supply air. The P.A. temperature is limited by APH inlet flue gas temperature — an increase of this will result in efficiency loss of the boiler. Besides, the higher P.A. inlet temperature can be attained through economizer gas by-pass, the SCAPH, partial flue gas recirculation. The primary air/coal ratio, a variable quantity within the pulverizer operating range, increases with decrease in grindability or pulverizer output and decreases with decrease in volatile matter. Again, the flammability of mixture has to be monitored on explosion limit. Through calibration, the P.A. 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 pulverized coal particle size distribution. Metal loss of grinding elements inversely depends on the YGP index of coal. Besides, variations of dynamic load on grinding elements, wearing of pulverizer internal components affect the available pulverizing capacity and percentage rejects. Therefore, the capacity mapping is necessary to ensure the available pulverizer capacity to avoid overcapacity or under capacity running of 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 operating system efficiency and control resulting a more cost effective heat rate.