| Iron and steel industry is not only a high energy consumption and high emission industry,but also a key industry of energy conservation and emission reduction in China.In each process of iron and steel process,the energy consumption of ironmaking system accounts for 70%of the energy consumption per ton of steel,and the energy consumption of blast furnace also accounts for about 70%of the energy consumption of ironmaking system.The energy saving and consumption reduction of blast furnace process is of great significance to the reduction of energy consumption in iron and steel industry.At present,more than 90%of iron and steel enterprises in China adopt wet slag treatment process,and large amounts of waste heat resources in blast furnace slag are transferred to slag flushing water.Due to the low temperature of slag flushing water and vulnerable to seasonal factors,the utilization of this waste heat resources is inefficient.In order to promote the recovery of waste heat resources of blast furnace slag based on the existed treatment of it,a wet recovery process of waste heat of blast furnace slag based on depressurization flash evaporation technology is proposed in this paper.In order to improve the feasibility of the process,two key nodes,the heat transfer of high-temperature slag mass in flowing liquid and the depressurization flash of slag flushing water in flash chamber,were selected to study the heat and mass transfer mechanism of the corresponding process.Firstly,the impact heat transfer mechanism of high-temperature slag mass in water quenching process is clarified,which provides theoretical support for the determination of flash initial parameters,such as initial temperature and back pressure;Secondly,the influence of the development of the two phase mixing zone,the size of the leakage port in the flash chamber and the convective intensity of the flash working medium on the flash efficiency are studied.On this basis,the intermittent flash mode is proposed,and its feasibility is analyzed and verified in detail.The main conclusions of this paper are as follows:(1)Aiming at the characteristics of low thermal efficiency,low waste heat grade and narrow application range of the existing wet regression process of blast furnace slag,a recovery process of blast furnace slag waste heat based on step-down flash evaporation technology is proposed.Detailed theoretical analysis and experimental research are carried out for the two core thermal processes of hightemperature slag impact heat transfer process and slag flushing water flash process,which provides a solution for the efficient recovery of blast furnace slag waste heat resources.(2)The effect of gas film rupture on solid-liquid heat transfer during boiling heat transfer of high-temperature slag in slag pool was studied.In order to obtain a stable boiling gas film,a small metal ball is used to represent the blast furnace slag.The experimental results show that the heat flux on the surface of the ball is only 1/10 to 1/6 of the maximum heat flux in the presence of boiling gas film;The movement of the quenching front formed by the gas-liquid-solid three-phase interface in the process of gas film rupture is affected by the surface temperature of the sphere,the liquid flow rate and the subcooling degree of the liquid.The influence of the surface temperature of the sphere and the subcooling degree of the liquid on the movement of the quenching front is more obvious than that of the liquid flow rate;After the quenching front moves,local micro-bubble emission boiling will occur on the surface of the sphere,and the heat flux on the surface of the sphere will reach the maximum.The above conclusions are helpful to predict the intensity distribution and duration of slag exothermic process under different working conditions,and are of great significance to determine the appropriate slag fishing time and provide accurate initial water temperature for flash process.(3)Three different static flashing experiments were carried out to study the generation and development of foam columns in static depressurization flash evaporation in cylindrical cavity,the leakage flashing process and the vibration flash processes in the vertical/horizontal direction in the cuboid cavity,in order to understand the influence of the shape of the flash chamber,the gas flow caused by the leakage point and the enhancement of convection induced by vibration on the efficiency of depressurization flash.The main conclusions are as follows:1)The experimental results of static flash in cylindrical cavity show that the foam column hinders the heat transfer process of the bottom liquid in the regular expansion stage,and the foam column develops more fully in the axisymmetric flash chamber.Therefore,the flash chamber with non axisymmetric design is easier to achieve higher flash efficiency.In view of this,the non axisymmetric rectangular cavity structure is adopted in the gas leakage and vibration flash experiments;2)The experimental results show that the air entering the flash chamber through the leakage point interferes with the flash process by affecting the pressure in the chamber and the pressure gradient from the liquid level to the exhaust port.At the same initial pressure or liquid temperature,the steam quality produced in the flash process is only related to the size of the air leakage point.Based on the quantitative study of the influence mechanism of gas leakage point size on flash process,an empirical formula describing the relationship between the pressure rise rate of flash system caused by gas leakage and the final amount of flash steam is proposed:me=10-2×p0vp+7.4×10-5Ψ;3)The experimental results of vibration flash show that when the vibration frequency is from 0 to 20Hz and the amplitude is from 1 to 4mm,the promotion effect of transverse vibration on the flash process is stronger than that of vertical vibration.The vertical vibration with the frequency in the range of 14 to 16hz can promote the flash process.The lower the amplitude and the higher the frequency,the more conducive the transverse vibration is to the flash process.The effect of vibration enhanced heat transfer on flash efficiency is lower than that of pressure change caused by gas flow in flash chamber.(4)Through experimental verification,a new flash mode,intermittent flash,is proposed in this paper.Compared with the results of continuous static flash experiment,the analysis shows that under the appropriate ratio of inlet and exhaust time,the flash process in a single cycle of intermittent flash will be more sufficient than that of continuous static flash.After a large amount of steam is generated in the initial stage of flash,intermittent flash can produce 93%of the steam quality of the latter in less than 33.3%of the extraction time of continuous static flash.The wet recovery process of blast furnace slag waste heat based on depressurization flash evaporation technology proposed in this paper has been successfully applied in the seawater desalination project of a domestic iron and steel company,which provides a technically feasible and economically reasonable process for the recovery and utilization of blast furnace slag waste heat resources. |
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