| China’s energy structure shows the characterization of rich in coal and short of oil. In 2015, coal accounted for 64.4% of the energy consumption for China. High consumed energy and environmental consequences during the coal utilization have attracted more attention. In China, 65% of coal is used for generating electricity. While inhalable particles and polluted gas are produced in the combustion of pulverized fuel(PF). Also, the huge energy consumption for particle breakage and relatively higher house-service consumption rate lead to a big challenge for energy conservation and emission reduction of thermal plants. In this case, this thesis focuses on the vertical spindle pulverizer(VSP) which is widely used in power plants. Simulated research in laboratory and industrial sampling are comprehensively used to investigate the breakage and energy consumed characteristics of coal in VSP. Operational efficiencies of different types of VSPs are compared and grinding behaviors of components in the multi-component breakage are studied. Energy-size reduction model,which consists of material properties, is established and calculation method of energy split factors of components is proposed. Response mechanism of consumed energy to raw coal property and removal of minerals from circulating loads is analyzed. Main conclusions of this thesis are shown as follows.Simulated study method is used to avoid the limitation of high temperature and pressure in the closed VSP on the research of particle breakage. The Hardgrove machine and a lab-scale roller mill, modified with the addition of a power meter, have been employed to investigate the grinding behavior of particles in E and MPS type VSPs. Repeat experiments indicate the relatively low experimental error, due to the little difference in mineralogy property among coal in narrow size. These insure of the veracity of further studies. Generated particles would accumulate on the grinding table in the batch and closed grinding tests, and finally the grinding kinetics of the original particles change from linearity to non-linearity. The classical energy-size reduction model can describe the grinding process of coal in narrow size in the fixed parameters. The breakage model, embedded with particle size, can realize the precious representation of grinding process in multiple sizes and parameters.According to the multiple properties of materials on the grinding table of VSP, several mixture grinding experiments are designed to analyze the energy consumed characteristic and breakage behavior of particles. Breakage of each component in the mixture grinding of different minerals of the same size follows the first-order breakage kinetics. Compared with the single component breakage, effects of components in different hardness finally lead to the change of breakage rate. Breakage model embedded with the mass weighted hardness index of mixture is established, and influence among different phases in the mixture grinding is reflected in the grinding energy. Based on the energy balance, energy split factors of super clean coal, pyrite and calcite are calculated, respectively. Grinding experiments of multi-size coal demonstrate that the modified breakage model, which bases on the breakage of coal in the narrow size, can reflect effects of size and ash in the multi-size mixture breakage of super clean coal and coking coal middling. Mixture grinding of coarse and fine coal shows that the added fine coal would occupy spaces among coarse particles and grinding media, which leads to the decrease of frictional efficient and energy input, and finally makes the breakage rate of coarse particles and generating rate of fine coal in-0.074 mm see a decreasing trend.Differences in particle size and ash content result in the scatter distribution of specific energy and product fineness t10 for the breakage of coal in narrow size. Based on the response degree of particle breakage to size and ash, breakage model incorporated with size and ash parameters is established. For coal samples in different ash content, comparison of difference in product fineness at the same energy input and difference in energy input for yielding the same product fineness is conducted by the modified breakage model. Considering the large amount of tests for this study, the simplified testing program, consisting of all the size, ash content and energy input level, is designed based on the optimized combination method. Experimental results of simplified program show a good fitting with the original model. For the study of effects of circulating load control on the size distribution of progeny, product fineness and energy efficiency, simulated circulating materials with decreased ash and sulfur content, and lower circulating ratio, are utilized. Experimental results indicate that the reduction of circulating ratio could not only increase the breakage rate of coarse particles and yield of fine coal, but also decrease the consumed energy.In order to validate the energy efficiency comparison of roller mills in the lab-scale and the response of consumed energy to particle property, industrial sampling experiments are conducted to E and MPS type VSPs. Results indicate that the primary hot air should overcome a high ventilating resistance to realize the transportation and classification of PF. In this case, operational efficiency of E type VSP is lower than that of MPS type VSP. Breakage model and fitting parameters, which base on experimental data of lab-scale roller mill, can describe the grinding process of two pulverizers. These depict the same grinding energy efficiency of two simulated machines. But the difference in structure makes the breakage events in the Hardgrove machine 1.3 times higher than that in the lab-scale roller mill. Based on the simulated tests and industrial sampling results, the mathematical model in relation to specific grinding energy and fineness index tn is established to compare the energy efficiency of E and MPS type VSP under the same energy input or fineness of coal fines. For the continuously operated VSP, fluctuation in ash content of raw coal results in the change of circulating ratio, and finally changes the specific breakage energy. The industrial sampling tests indicate that if the ash content of raw coal decreases from 51% to 35%, energy consumption will drop about 14%. |
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