Research On The Optimization Of Pulverizer Rejects Discharge Of Medium Speed Mill

Medium speed mill is widely used in thermal power plant because of its characteristic of rapid start, flexible regulation and high security. The pulverizer rejects is an important parameter about safe and economical of the mill. The more pulverizer rejects discharged, the more excessive entrained combustible components, which will lead to low pulverizer output, worse economy, high differential pressure of mill and more labor to discharge the rejects. What’s more, if the pulverizer rejects accumulate in the primary chamber of mill with high temperature for a long time, the rejects may spontaneously combust and damage the equipment. In the standard DL/T467-2004stipulated the pulverizer rejects discharge ratio should be less than0.05%and the heat value less than6.27MJ/kg.One of HP863medium speed mill which has pulverizer rejects problem was tested in this paper. The influence of mill operation parameters like primary air quantity, feed-coal quantity, grinding roll loaded-force and outlet temperature on pulverizer rejects discharge was analyzed and the most important influencing factors is primary air quantity or nozzle ring velocity. But increasing the nozzle ring velocity by increase primary air quantity may affect the stable operation of the boiler. So the nozzle ring velocity can be enhanced by mill internal structure optimization.The influence of different nozzle structure on pulverizer rejects discharge was studied by numerical simulation in this paper and a nozzle ring structure improvement scheme was presented. This scheme can increase the nozzle ring velocity from47.55m/s to73.69m/s, reduce the pulverizer rejects ratio by81.49%but the differential pressure rose0.79kPa.The nozzle ring of power plant mill was transformed based on the scheme and the mill performance was tested and optimized. The average pulverizer rejects ratio fall85%, from0.5%to0.05%, the mill operate normally but the differential pressure rose about0.6kPa.