The Classification Mechanism Of Pulverized Fuel Within The Static Air Classifier Of Vertical Spindle Mill

In 2016,the proportion of coal in China's energy consumption was 62%,of which 65%was used for power generation.In coal-fired power plants,the implementation of energy saving and emission reduction is facing great challenges as problems of high energy consumption and low classification efficiency exist in pulverizing system.In this paper,the static classifier of vertical spindle mill is selected as the research object.By comparing the sensitivity of overflow yield and classification efficiency of particles with different density,effects of particle density on the classification are investigated.Meanwhile,the necessity of treating coal as a multi-component material is clarified.And the classification efficiency curve model including particle density is modified.Through the analysis of on-line sampling data of an industrial vertical spindle mill,it is revealed that the classification efficiency of static classifier needs to be improved to reduce the circulating ratio.The distribution and classification mechanism of coal samples with different property in static classifier are analyzed applying CPFD numerical simulation.The main conclusions of the paper are as follows:Based on results of the response surface methodology,particle density has a significant influence on the yield of overflow.The overflow yield and fineness R₉₀ of low-density material are much higher than those of high-density material.The actual classification efficiency curves of different material present a same reverse distorted S-shape,while the classification efficiency is negatively linearly related to the density.The contour lines which is shown as irregularly shaped peaks,represents the combination effect of particle size and density on the classification efficiency.Actual classification efficiency curve and comprehensive classification efficiency were applied as indicators to evaluate the classification result for coal sample from power plant.Compared with actual classification efficiency curve,the comprehensive classification efficiency can reasonably evaluate the overall classification effect of the classifier.A notable thing is that the ash in Pulverized Fuel is much lower than that in the feed for the same size fraction.And variation becomes more obvious with the increase of size fraction and at a lower air amount.It is fully proved the accumulation of minerals with high density in the classifier reject.There is a large deviation in each classification indicator between treating coal sample as a homogeneous or heterogeneous material.Synergistic effect of particle size and density on classification efficiency can be expressed by an improved Whiten's model by analyzing and validating the results of the classification test for the pure mineral and coal from power plant in the laboratory.Based on the results of online sampling of industrial mill under different operation conditions,the circulating ratio of classifier is up to 7-10,which mainly attributes to the large amount of high-density mineral in power coal.In addition,the fineness of the classifier feed is too coarse(R₉₀ is over 58%-75%).And the classification efficiency of fine particles in the size fraction of-90?m is low.It results in a too small cut size?the particle size is only 35-45?m when classification efficiency is around 50%?.The air amount has a direct influence on the circulating ratio of the classifier.If the air amount was controlled in a suitable range,a relatively low circulating ratio can be obtained.Coal type,air amount and hydraulic loading force are important influence factors for the classification efficiency of classifier,especially to the fine particles.The air amount affects the flow rate and size distribution of classifier feed,while the coal type and hydraulic loading force has impact on the flow rate of classifier feed by affecting the grinding rate.The results of CPFD numerical simulation show that flow rate and fineness R₉₀of overflow are increase with the increase of density.In the first classification zone of the classifier,the airflow velocity of the inner wall is larger than that of the outer wall.Therefore,particles near the inner side wall have a short residence time,and particles near the outer wall tend to slide down and have a longer residence time as well as larger volume fraction.Because of the excessive gravity of the dense coarse particles,it is difficult for them to enter the second classification zone with the air,finally form a deposit at the bottom of the first classification zone.At the same height of the second classification zone,the velocity of air decreases gradually as the radius increases.Fine particles with low density follow well with the air and enter the second classification zone at a relatively faster velocity,and exit from the overflow.And particles at the lower portion of the second classification zone mainly move downwardly.With the increase of material density,the size distribution of particles which can enter the second classification zone becomes fine,and mass flow rate also decreases.In the outlet area of pulverized fuel,the axial velocity of air gradually decreases with the increase of the radius,and the particle size,axial velocity and residence time are obviously stratified along the radius.Coarse particles are farther away from the center feed pipe with a smaller axial velocity and longer residence time.There is a part of particles,neither exits from the overflow with the air in time,or falls to be re-grinding,just remains in the classifier in the whole simulation time.That part of particles will have a certain impact on the efficiency of the classifier as well as the mill.