Numerical Simulation On Transfer Characteristics Of Spray Evaporation In The Flue For Desulphurization Wastewater

The limestone-gypsum wet desulphurization process has been widely used for flue gas desulphurization in coal-fired power plants.Although it has the advantages of mature stability,strong adaptability and high desulphurization efficiency,a large number of desulphurization wastewater with complex composition and strong corrosion is produced in this process.At present,the flue gas spray evaporation technology has attracted a large number of researchers’ attention because it can achieve zero discharge of wastewater.In this thesis,aiming at the flue in front of the electrostatic precipitator of a 330 MW coal-fired unit,the difference of evaporation characteristics between pure water and salt-crust droplets was analyzed by CFD simulation method.The influence of operating parameters on evaporation characteristics of salt-crust droplets was investigated,and an optimization scheme for the internal components that can shorten the evaporation time of pure water droplets was proposed.The research content and main conclusions of this thesis are as follows:Firstly,in view of the flue spray evaporation process of desulfurization wastewater,by considering the thermal conductivity difference is caused by the salt crust formed on the surface of desulfurization wastewater in the evaporation process,Euler-Lagrange method was used to establish the two-phase momentum,heat and mass transfer model between flue gas and liquid droplets.The model was also applied for a 330 MW coal-fired unit in order to numerically simulate the flue gas spray evaporation characteristics on pure water and salt-crust droplets,and the results were compared and analyzed.The results show that the differences of heat and mass transfer lead to the differences of evaporation characteristics between pure water and salt-crust droplets.Secondly,since temperature differences and convective conditions are the two main influencing factors that control droplet evaporation process,the effects of flue gas temperature and flow velocity on the evaporation characteristics of the droplet population of salt-crust wastewater were investigated.The results show that in the flue gas temperature range of 413.15～473.15 K,the complete evaporation time of droplets becomes shorter with the increase of flue gas temperature,and the resistance caused by the salt-crust effect can be effectively weakened with the increase of flue gas temperature.In addition,as the gas velocity becomes larger,the complete evaporation time of both droplet populations becomes shorter,and this influencing factor has more influence on the droplets containing salt crust.At the above range of flue gas temperature and velocity,the complete evaporation time of salt-crust droplets is longer than that of pure water droplets,with an increase range of 6.3%-52.3%.Finally,in order to realize the complete evaporation of droplets in the finite flue,according to the idea of increasing the disturbance of the flue flow field to prolong the residence time of droplets in the flue,a flow field optimization scheme was proposed to add baffles to control the flow pattern and strengthen the heat and mass transfer.The influences of baffle type,layout and key geometric parameters were investigated,and the pressure drop energy consumption caused by increasing the internals was comprehensively considered.The layout mode of internals is at the section of Z=3 m,effective length L=0.3 m,and windward-crest angle α=30° is determined as an optimization scheme to improve the flow of flue gas and liquid droplets in the flue and enhance the evaporation effect.Compared with the flue without optimized component installed,the complete evaporation time is shortened by 57.22%,which has a great application prospect.