Numerical Simulation Of Spray Distribution And Evaporation In Rotary Spray-drying Desulfurization Tower

Spray-dry flue gas desulfurization technology has the advantages of no waste liquid,simple process,and low water consumption.It is widely used in flue gas deacidification treatment of municipal solid waste(MSW)incineration power generation.However,the poor mixing of flue gas and spray and the rapid evaporation of spray from flue gas will lead to the reduction of flue gas desulphurization efficiency.Therefore,the studies of mixing and heat transfer process of spray and flue gas are significant for improving the gas-liquid two-phase flow in the desulfurization tower,improving the utilization rate of droplets,the reliability of the desulfurization system,and the desulfurization efficiency.For the problems of gas-liquid local distribution and two-phase heat and mass transfer in desulphurization tower,the numerical simulation method is adopted to study the distribution characteristics of droplets in the desulfurization tower.The two parameters of mixing variance and droplet mass-weighted life are put forward to quantify the evaporation time of droplet group and gas-liquid mixing degree to evaluate the utilization rate of spray droplets.Then,the effects of several factors on the flue gas temperature distribution,spray evaporation,and droplet motion in the desulfurization tower are explored.The heat and mass transfer and spray distribution characteristics between gas and liquid are obtained.Finally,according to the statistical results,the effects of various parameters on the droplet mass-weighted life and mixing variance are obtained,and the methods and ways to improve the evaporation time and gas-liquid mixing of droplets in the desulfurization tower are obtained.The main conclusions are as follows:(1)An appropriate swirl number of inlet flue gas can effectively improve the droplet utilization rate.When the inlet flue gas is a direct current jet,the 7.6% droplet is not fully evaporated escaping from the flue gas outlet,accompanied by a large mixing variance.When the inlet flue gas is swirling jet,it can effectively solve the problem of droplet incomplete evaporation.However,the excessive swirl number makes the droplets move along the radial direction,and droplets even collide with the wall of the desulfurization tower.In addition,it is not conducive to gas-liquid mixing.When the swirl number is 0.5,16% of the droplets collide with the wall.Due to the difference in local gas-liquid distribution,the mixing variance increases.(2)The effects of gas-liquid ratio on mixing variance and droplet mass-weight life are studied by adjusting the flue gas flow rate and the spray mass flow rate.The results show that the droplet mass-weighted life decreases linearly with the increase of the gas-liquid ratio.The decrease of gas-liquid ratio reduces the flue gas temperature in the desulfurization tower and decreases the temperature gradient between the flue gas and the spray.It decreases the droplet evaporation rate and prolongs the evaporation time.The decrease of gas-liquid ratio promotes the migration of gas-liquid two-phase along the radial direction,changes the mixing variance.Because the flue gas flow changes the flue gas field,the effects of flue gas flow on the mixing variance are greater than droplet mass flow.Under different inlet flue gas flow,the range of mixed variance is 0.314.The range of mixing variance is 0.079 when the mass flow rate of spray increases from1.2kg/s to 2.0kg/s.(3)The spray incident velocity has important effects on the mixing variance and the droplet mass-weighted life.When the inlet flue gas is direct current,the droplet mass-weighted life decreases linearly with the increase of spray incident velocity,and the mixing variance increases gradually.When the inlet swirl number of flue gas is 0.4,the droplet mass-weighted life increases linearly with the increase of spray incident velocity,and the mixing variance decreases first and then increases.The droplet mass-weighted life has the opposite trend with the increase of spray incident velocity.Because the droplets are entrained by the flue gas in a short time,the influence of the droplet initial velocity is small.Therefore,the flue gas velocity and temperature around the droplets are the main factors affecting spray evaporation.(4)The decrease of spray incidence angle is not conducive to the radial diffusion of the spray and increases the mixing variance.The simulation results show that the mixing variance decreases linearly with the increase of the hollow cone spray angle.In addition,when the nozzle is a solid cone,the spray is concentrated in the central part of the desulfurization tower,which weakens the heat exchange between the flue gas and droplets.The average temperature between the cross-section(Z=1)and the cross-section(Z=3)is only 1～2K,and the mixing variance of the solid cone nozzle increases by about10 times compare with the hollow cone condition.(5)The droplet mass-weighted life increases linearly with the increase of average droplet diameter,increases of the droplet diameter reduces the heat transfer area between gas and liquid,decreases the evaporation rate of the spray,and increases the evaporation time of the droplet group.The increase of spray mean diameter increased the radial displacement of the spray.When the gas-liquid distribution was adapted,the mixing variance decreased,and the mixed variance is the minimum when the average droplet diameter is 170μm.