Numerical Study On Evaporation And Flow Characteristics Of Desulfurization Wastewater In High Temperature Cyclone Evaporator

The installed capacity of thermal power accounts for more than 60.2%of the installed capacity of power generation in China,and the main thermal power units are coal-fired units.In order to control the production of SO₂,in the process of coal combustion,limestone-gypsum method is often used for flue gas desulfurization in thermal power plants in China,and a certain amount of desulfurization Wastewater will be produced in the treatment process.The desulfurization Wastewater is treated by a high temperature bypass cyclone evaporator.The evaporation heat source is the high temperature flue gas between the SCR and the air preheater,which can make the desulfurization Wastewater evaporate rapidly and The droplets that not evaporate can enter the secondary evaporation of the tail flue,prevent the droplets from corroding the electrostatic precipitator,realize the zero discharge of desulfurization Wastewater and solve the problems of Cl^-enrichment and removal.In this paper,the coal-fired power plant of a 600MW unit is taken as the engineering application background,according to the treatment capacity of desulfurization Wastewater,a set of cyclone spray evaporation system for desulfurization Wastewater was designed.The distribution of flow field in cyclone evaporator with different structures,the evaporation track of droplet group and the wall collision rate of droplets in desulfurization Wastewater were studied by means of numerical simulation.On this basis,a suitable evaporator structure is selected for the improved cyclone evaporator,The effects of spray angle,droplet initial particle size,flue gas temperature,flue gas velocity and steam content in flue gas on the complete evaporation time,droplet diameter and evaporation rate of droplet group in desulfurization Wastewater were studied.At the same time,the movement of droplets with different diameters in cyclone evaporator,the influence of particle relative flow field and the collision between atomized droplets and fly ash are simulated.The following conclusions are obtained:（1）The inlet of cyclone evaporator is located in the outlet flue of SCR reactor.In order to ensure the flue gas temperature to be higher than 413K,the amount of flue gas needed to treat 4t/h desulfurization Wastewater is 40000 Nm³/h.Different evaporator structures have great influence on the smoke flow field and droplet impingement on the wall.The flue gas flow field in the evaporator with gradual expansion structure at the top is more stable and the wall collision rate is lower.the smaller the atomization particle size is,the shorter the complete evaporation time of the droplet is,the maximum evaporation times corresponding to the droplet diameters of 30μm,60μm,90μm,120μm,150μm and 180μm are 0.11s,0.33s,0.41s,0.57s,0.71s and 0.95s,respectively.The higher the temperature,the faster the evaporation.The time ratios of droplet group evaporation in hot flue gas at 593K,608K and 623K are 0.67s,0.65s and0.57s,respectively.When the axial velocity of flue gas increases with the tangential velocity,the complete evaporation time of droplets becomes longer,and the angles between the axial velocity and tangential velocity of flue gas are 0°,30°,45°and 60°,respectively.The complete evaporation time of droplet group is 0.37s,0.51s,0.57s and0.69s,respectively.With the increase of angle,the temperature distribution of evaporator is more uniform.The effect of flue gas water content on evaporation of desulfurization Wastewater is relatively not obvious.（2）The higher the salt content of desulfurization Wastewater is,the longer the evaporation time of droplet group is.The complete evaporation time of desulfurization Wastewater droplets with salt mass fractionωof 1.5%,2.0%and 2.5%is 0.7s,0.8s and0.9s,respectively.The salt content mainly affects the end of desulphurization and evaporation.The evaporation trend of droplets with different salt content is basically the same at the initial stage of evaporation.With the increase of flue gas temperature,the more droplets is evaporated at the same time,the complete evaporation time of droplets is shorter,which is the same as the evaporation trend of salt-free droplets.With the increase of the droplet diameter,the complete evaporation time of the droplet becomes longer,the complete evaporation time of droplets with diameters of 90μm,120μm and150μm was 0.49s,0.57s and 0.71s,the diameters of residual solid crystalline particles after evaporation are 11.1μm,21.9μm and 32.9μm,respectively.With the increase of tangential velocity of flue gas,the complete evaporation time of droplets becomes longer,which is similar to that of desulfurization Wastewater without salt.When the angles between axial velocity and tangential velocity of flue gas are 0°,30°,45°and 60°,respectively,the complete evaporation time of droplet group was 0.37s,0.45s,0.57s and0.81s,respectively.（3）The atomized air enters the cyclone evaporator at the speed of 210m/s,and the compressed air has a great disturbance to the flow field of the evaporator in the initial stage,and finally reaches the same velocity as the flue gas.When the fly ash in the flue gas is not considered,the average velocity of the droplet group decreases gradually along the direction of the evaporator.when the atomization angle is 30°,the average velocity of the droplet group decreases with a faster speed at x=0⁰.25m,and then the droplet velocity decays slowly,the flue gas velocity is basically stable at x<1.35m,but slightly higher than that of flue gas.When t=0.0025⁰.05s,the number of droplet collisions increases with the increase of droplet group diameter.When the atomization angle is 60°,the rapid decay zone of droplet group velocity is advanced,about 0⁰.15m,and the flue gas velocity is reached at x=1.85m and follows the flow of flue gas.When t=0.0025⁰.05s,the number of droplet collisions is disordered,and when t>0.3s,the number of droplet collisions becomes more stable.When considering the fly ash in the flue gas,due to the momentum exchange between the droplet and the fly ash particle,the average velocity of the droplet group decreases gradually along the direction of the evaporator and the attenuation rate is higher than that without considering the fly ash in the flue gas.When t=0.025⁰.1s,with the spray process,the particle group collision frequency increases gradually.When t=0.125⁰.25s,the particle collision frequency increases unsteadily,and when t>0.25s,the flow field in the cyclone evaporator is relatively stable,and the collision frequency of the particle group is basically stable.