| The shortage of coal supply in power plants became serious in recent years. Due to the usage of inferior coal, the complexity of coal source and the frequent fluctuation of coal type, there are several problems often occurred in power plants, such as the insufficient output of double-inlet & double-outlet tube mill based direct fired system, poor running economy of milling system under low-load condition, excessively coarse of pulverized coal under high-load condition etc. In order to resolve those problems, the researcher chose a double-in & double-out tube mill based direct fired system for a PC-fired W-flame boiler of 315MW unit as the research object. Several capacity-increasing retrofit scheme which were based on the output of power plant's milling system and boiler operation status were proposed. Eventually, a scheme recycling the previously pulverized coal stored in a bunker back into the present mills was confirmed as the best scheme after a preliminary technical and economic comparison. The scheme introduces that we can store the surplus pulverized coal in the added bunker under low-load condition, while the boiler under high-load condition, we can inject the stored pulverized coal into the classifier from the top or circumferential position through primary air, which to meet the requirement for pulverized coal. The key technology of retrofit scheme included: equipments and piping layout, bunker capacity, pulverized coal injection system, classifier with pulverized coal injection, pressure and capacity of primary fan, mill pulverizing surplus.Depending on the analytical research of the pulverized coal recycle-direct fired system, the calculation result of mill pulverizing surplus shows that:(1) The retrofit scheme of pulverized coal recycle-direct fired system is totally feasible. The two added bunkers'effective volume is 1750 m3, storage capacity is 1225~1400 tons.(2) The boiler operating 24 hours under BMCR(boiler maximum continuous rating) working condition and low caloric, the supplement of pulverized coal is 1176.0~1742.4 tons ; In medium caloric condition, the supplement of pulverized coal is 600.0~1166.4 tons. When R90=10% and the boiler operating 24 hours under BMCR working condition, the minimum surplus output when the coal were burned by a boiler under the designed condition is far more than 1176.0~1742.4 tons, it can meet the injection requirements of variation coal under BMCR condition. Between low caloric coal and designed coal condition, the storage capacity of pulverized coal in the bunker is capable to run 6~9.8 hours under BMCR working condition, when the four mills were stopped working.During the design and experiment of pulverized coal injection classifier, the researcher made an experiment on the original and lengthened structure of classifiers, and it embedded three air guiders on the flow fields and flow resistances in the classifiers by the numerical and physical simulation method. The result shows that:(1) The structure and dimensions of the gas guider has greater effect on the original classifier's flow resistance. The radius of windward arc surface too small or too large will lead to higher flow residence when air through the classifier.(2) An appropriate heightening length of classifier's cylinder section can reduce flow residence in it and it will become less sensible to the structure. Therefore the dimension of the gas guider can greatly reduced the technical requirement of the gas guider's design.(3) Under varying operating conditions, the classifier which under a wide load and the flow pressure range promises the upper and lower pulverized coal will be inject fluently, the mixed and distributed of pulverized coal will be homogeneously and without secondary classification or sedimentation. |
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