| It is well known that the sensible heat contained in blast furnace slag is a high-grade heat source.However,such valuable thermal energy cannot be efficiently recovered by using water quenching technique that are currently commonly employed in steel industry worldwide,which also wastes water resources and generates pollutants like sulfur dioxide and hydrogen sulfide.In contrast,the technique of centrifugal granulation of molten slag based on spinning cup and disc can offer the advantages of recovering slag heat,protecting environment,saving water,and ease of control,thus making it a potentially viable slag treatment technology to replace water quenching method in future.However,to date researches on molten slag centrifugal granulation technology have focused mostly on the process of molten slag breaking into droplets in cold and hot granulation experiments and numerical simulations on such process;there have been limited researches having been reported in literature on the cooling and heat transfer of the in-flight slag particles during the granulation process.Therefore,in order to better understand the process of the cooling heat transfer of groups of in-flight slag particles in the process of centrifugal granulation,CFD numerical simulations were performed in the present research work.According to the proper operating parameters obtained from the numerical simulations,high-temperature centrifugal granulation experiments were carried out,whose results were used to verify the developed CFD model.Then,a tortuous shaped counter-current heat exchanger was designed and added to the granulation chamber,on which CFD model simulations were conducted to further optimize the structures of the heat exchanger and the granulation chamber.The following are the major contents and findings of the present study:(1)A CFD model of particle group in-flight heat transfer in the granulation chamber was established.The effects of particle initial temperature and particle size on heat transfer were examined using the developed CFD model.It was found that the heat transfer between the particles and the cooling air is mainly influenced by the particle size distribution,the average temperature of the particle system decreases by731 K in 0.5 s,which has reached the rapid cooling.It was expected that the glass phase content of the obtained slag particles would reach more than 90%.(2)The design and operation parameters,such as the appropriate cold air flowrate necessary for heat transfer,were obtained by means of numerical modeling using the slag particle size distributions reported in literature.On this basis,high-temperature experiments on spinning cup centrifugal granulation of molten blast furnace slag were performed.The result shows that the temperature of the particle system only decreases by 12K when the air velocity of the fan changes from 0m~3/h to 900m~3/h.It can be seen that the change of air velocity has little effect on the heat transfer effect of the particle system.It is found that the average diameter of the particles obtained by increasing the spinning cup speed from 2250 RPM to 2500 RPM only increases from 0.599 mm to 0.609 mm,and the value is not much different.The experimental results indicate that,when the spinning cup speed exceeds a certain level(≥2250 RPM),its influence on the particle size becomes limited.(3)A tortuous counter-current heat exchanger was designed and installed below the granulation chamber,and the structures of the heat exchanger and the granulation chamber were numerically modeled to evaluate the suitable structural size and velocity of the fluidization cooling air.This research results demonstrate that the installation of the tortuous counter-current heat exchanger to the granulation chamber can effectively enhance the heat transfer between slag particles and cooling air to achieve hot air with high grade heat.The average temperature of particles obtained by the improved device decreases from 848 K to 700 K,and the average temperature of air increases from 306 K to 320 K. |
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