Accumulation Of Minerals In Circulating Materials Of Power Plant Mill And Its Impact On The Performance Of The Mill

In China,the coal consumed by power plants accounts for 52% of the total coal consumption.More than 90% of the coal power plants are pulverized,of which the pulverizing system is an essential part.In the milling process,circulating materials will reduce the mill processing capacity,increase the energy consumption of the mill and the abrasion of equipment.Besides,the combustion of enriched minerals in the pulverized coal will increase the air pollutants.To investigate the reasons for high energy consumption and serious abrasion of the mill,circulating grinding and mixed grinding experiments are performed with high sulfur coal,lignite and coking coal.The self-made roller mill and Hardgrove machine are used in the experiments.Through the experiments,the accumulation and migration law of minerals in the miller are revealed gradually.Meanwhile,on the basis of the pulverizing energy consumption model,the effect of mineral enrichment in coal on the energy consumption of the pulverizing system are studied.The research results also provide theoretical basis for the removal of minerals in the pulverizing system and energy conservation and emission reduction.To simulate selective enrichment of minerals in coal-pulverizing by industrial mill with intermediate speed,the three kinds of coal samples are milled,which the yield of –0.09 mm powder is equivalent to the R90 of industrial conical body's feed particle size composition based on the particle size composition of the conical body in industrial sampling.Mineralogical analysis is carried out to figure out the mineral composition of coal samples of sulphur coal,lignite and coking coal.The coal samples mainly contain minerals of kaolinite,quartz and pyrite and others.Coalpulverizing experiments show that: pulverized coal with size fraction of +0.09 mm mainly contains minerals with large hardness and grindability poorer than coal.The larger the particle size of coal sample,the higher the ash content.Affected by the enrichment differences of clay minerals and calcium minerals in coal,the variation trends of ash with different particle size in-0.09 mm coal powder of coal samples are significantly different.The ash content of-0.09 mm sulphur coal powder decreases with the decrease of particle size,while the ash content of-0.09 mm lignite powder and coking coal powder increase as the particle size decrease.Based on the operational state of industrial mill,the accumulation and migration law of main minerals of the three coal samples are analyzed through cycling experiments of grinding and separation.The variation trends of ash content in cycling products of the three coal samples are markedly different.The ash contents of circulating materials and qualified coal in high sulfur coal increase with the increase of the number of cycles.The ash contents of circulating materials and qualified coal powder in lignite derease as the number of cycles increases.The variation of ash content in cycling products of coking coal is not obvious with the change of the number of cycles.The ash content can be used to evaluate the content of minerals with different particle size in circulating materials.The ash content of circulating materials of the high-sulfur coal is aggregated in the fine-grained coal.The ash content of the middle particle size of the lignite is lower than that of the coarsegrained and fine-grained pulverized coal.The ash content of the coking coal have a little difference,in which the distribution is more uniform.Embedded in the coal quality of grain size indicator to reflect the coal mineral content of circulating materials,the content of minerals in the three kinds of coal materials show a decrease tendency of coarse-grained minerals and a increase tendency of fine-grained mineral content with the increase of the number of cycles,and-0.5+0.125 mm coal powder has the largest mineral content.The semi-quantitative analysis of the main minerals in the product are carried out by standard analysis.Quartz and iron minerals accumulated in the circulating materials,and the hard minerals content in the circulating materials are higher than that in the pulverized coal.Kaolinite accumulate in the pulverized coal,and kaolinite's content in pulverized coal is higher than that in circulating materials.The contents of calcium-containing minerals in high-sulfur coal and lignite are different.The content of calcite minerals are similar in each product while the anhydrite minerals are increased in the products with the circulation experiment.The mineral enrichment coefficient Em is used to evaluate the enrichment characteristics of the minerals in the coal,compare the degree of enrichment and loss of different minerals,which has a guiding effect on the evaluation o of accumulation and migration of minierals.Based on the circulation rates and the property of circulating materials in industrial mill,the simulation method is used to study the effect of circulating materials on the pulverization process.The classical crushing model can be used to show the relationship between the energy of the coarse pulverized coal and the particle size of that coal.When the mineral content of circulating materials is close to that of feed,powder fineness t10 of feed reaches to a maximum value,which indicates that it is benefical to the breakage of feed.When the breakage energy reaches to 10 kw.h.t^-1,the power fineness t10 of circulating materials with density of +1.8g/cm3 is about 19% lower than that of circulating materials with density of 1.5-1.6g/cm3.The high density level of circulating coal can reduce the mechanical efficiency of the mill running and increase the unit breakage enengy of coarse pulverized coal.The results show that the accumulation of minerals of return material have a adverse affect to breakage energy consumption and mechanical efficiency.