Numefieal Simulation On Desulfurization Wastewater Evaporation Characteristics In Flue Gas Flow And Process Design In Thermal Power Plant

Currently,limestone-gypsum wet flue gas desulfurization process is commonly used in the flue gas desulfurization systems of the majority of domestic and foreign coal-fired power plants,which periodically discharges a certain amount of flue gas desulfurization wastewater with special water quality and high water pollution.With the gradually increasing emission standards,zero discharge of desulfurization wastewater has attracted widely attention of the world.Combining desulfurization wastewater treatment and boiler flue gas waste heat utilization,spraying desulfurization wastewater in flue gas duct evaporation treatment technology and other emerging technologies have been proposed to meet the new demands for the current energy saving and emission reduction in thermal power industry.Spraying flue gas desulfurization wastewater in flue duct evaporation treatment technology is one of the economically feasible technical ways to achieve zero discharge of desulfurization wastewater from power plants.To deal with the scale and corrosion on flue walls and subsequent equipment that exist in the practical application of this technology,the physical and mathematical models were established based on a 300 MW class coal-fired power plant.The flue gas in the flue duct between the air preheater and precipitator was taken as the research object.The Eulerian-Lagrangian approach was used to describe the thermo-fluid behavior of desulfurization wastewater in flue gas,by which the turbulent flow heat transfer of flue gas was described by the Eulerian framework and the evaporation of water droplets was described using the Lagrangian framework.Through the numerical simulation method,the flow evaporation characteristics and influencing factors of desulfurization wastewater droplets sprayed in flue gas were studied.The optimal installation distance between two adjacent nozzles and between the nozzle and the flue duct wall as well as the diffusion range and trajectories of atomized droplets under different operating conditions were obtained.The following results were gained that the smaller the installation distance between two adjacent nozzles with the same spray flow rate is,the longer time and distance are required for the complete evaporation of droplets.Higher flue gas temperature and smaller diameter of the atomized droplets contribute to shorter time and distance for complete evaporation.Besides,an optimal arrangement of multiple nozzles having a small flow rate is proposed to improve the evaporation rate of droplets.It is concluded that spaying droplets in the co-flow direction of flue gas can achieve the maximum evaporation rate of droplets.However,the initial velocity of droplets,the spray full cone angle of nozzles and the velocity of flue gas have little influence on the evaporation rate of droplets.Due to the collision,break up and coalescence of droplets,the Sauter mean diameter of droplets decreases initially and then increases and finally decreases in the direction of the flue gas flow.Based on the numerical simulation results of the flow evaporation characteristics of desulfurization wastewater droplets sprayed in flue gas,a least-square support vector machine prediction model with n input variables and m output variables was established.The results showed that the proposed model can be used to expeditiously predict the evaporation rate of droplets along the flue duct with high accuracy.Finally,in view of the technical defects of the desulfurization wastewater treatment process currently used in a 300 MW class coal-fired power plant,this paper developed an efficient and low-cost spraying flue gas desulfurization wastewater in flue duct evaporation treatment technology operating scheme,and proposed and numerically verified an improved nozzle optimization arrangement.The optimal schemes are as follows.The designed flow rate and full cone angle of a spray nozzle is 60 L/h and 30°～90°,respectively.Desulfurization wastewater droplets should be sprayed in the co-flow direction of flue gas.Besides,the droplet velocity is 10 m/s and the average diameter of droplets is 45.08 μm.Moreover,the spray nozzles are arranged along the rectangular cross section of the flue duct.The installation distance between two adjacent nozzles is 0.1 m,while the installation distance between the nozzle and the flue duct wall are 0.9 m horizontally and 0.7 m vertically,respectively.The total number of spray nozzles required is 176,of which 46 nozzles are arranged in the horizontal direction(z-axis direction)and 42 nozzles are arranged in the vertical direction(y-axis direction).The results indicated that the proposed improved treatment process and nozzle arrangement can meet the actual engineering requirements for a 300 MW class coal-fired power plant,which periodically discharges 8 m3/h amount of flue gas desulfurization wastewater.The findings in this paper can provide theoretical basis for the design and the optimization of spraying desulfurization wastewater treatment technology in thermal power plants.