Simulation And Experiment Study On Air-Based Evaporation And Concentration Of Desulphurization Wastewater

At present,most domestic thermal power plants use the limestone-gypsum wet process for flue gas desulphurisation,which produces a complex composition of desulphurisation wastewater and is the most difficult high-salt wastewater to treat at the end of the power plant system.With the introduction of China’s environmental protection policies,conventional FGD wastewater treatment methods are no longer able to meet the increasing emission standards.In this context,zero discharge technology for desulphurisation wastewater has emerged.However,most of the existing zero-discharge technologies suffer from low system energy efficiency,high operation and maintenance costs and poor operational stability.To address this issue,this paper proposes an evaporation and concentration technology for FGD wastewater using air as the water vapour carrier.Through theoretical analysis,numerical simulation and pilot experiments,the influence of the state parameters of air and FGD wastewater on the evaporation and concentration effect is investigated,and the energy efficiency of the evaporation and concentration system is comprehensively evaluated.Firstly,based on the theory of air humidification and moisture reduction,a detailed analysis of the evaporation and concentration realization path of desulphurization wastewater using air as a water vapour carrier is presented.By analyzing the physical parameters of airliquid,the temperature and humidity diagrams of wet air and the equivalent moisture content of solution are established,and the migration law of moisture from solution to air is studied accordingly,and the feasibility of air-carried moisture evaporation and concentration technology is demonstrated.On this basis,a gas-liquid counter-current contact heat and mass transfer model was established to investigate the heat and mass transfer mechanism of the aircarrying moisture evaporation process,and to derive the volumetric mass transfer coefficient and the equation of evaporated water volume.Afterwards,the distribution of the gas-liquid two-phase flow field in the spray tower was simulated and the effect of air and FGD wastewater state parameters on the temperature field and water vapour mass fraction distribution in the tower was analysed.The results show that during the spraying process,the gas phase flow line in the tower is uniformly distributed,and the pressure in its longitudinal central section gradually decreases along the tower height direction.When the inlet air temperature,wastewater temperature and liquid to gas ratio increase,the outlet air temperature and moisture content increase significantly;when the relative air humidity and wastewater chloride ion concentration increase,the outlet air temperature and moisture content show a decreasing trend.Based on the simulation parameters,an air-based FGD wastewater evaporation and concentration experimental rig was built to investigate the effect of air and FGD wastewater state parameters on the evaporated water volume and volumetric mass transfer coefficient,and to verify the accuracy of the simulation results.The results showed that the effects of wastewater temperature,liquid to gas ratio and air relative humidity on the evaporated water volume and volumetric mass transfer coefficient were more significant than those of other factors.The evaporated water volume increased from 1.67 kg/h to 1158.4 kg/h and the volumetric mass transfer coefficient increased from 2.5 g/(m3·s)-1 to 294.6 g/(m3·s)-1 as the temperature of the wastewater increased from 10℃ to 90℃.The evaporated water volume increased from 225.5 kg/h to 501.7 kg/h and the volumetric mass transfer coefficient increased from 87.8 g/(m3·s)-1 to 187.4 g/(m3·s)-1 as the liquid to gas ratio increased from 3 L/m3 to 12 L/m3.The evaporated water volume decreases from 543.3 kg/h to 260 kg/h and the volumetric mass transfer coefficient decreases from 175 g/(m3·s)-1 to 51 g/(m3·s)-1.In addition,the concentration of salt ions during the wastewater concentration process was studied.The change pattern.The results show that as the cycle progresses,the rate of increase in the concentration multiplier of the wastewater ions slows down until the crystallisation point of the salt ions is reached.At the same time,a very small amount of chloride ions migrate with the air in the form of liquid droplets during the process,with a migration rate of less than 0.1%,which ensures good water quality of the wet air condensation recovery water.Finally,the energy efficiency of the air-based desulphurisation wastewater evaporation and concentration system was evaluated using the grey correlation degree energy efficiency evaluation method.The results of the study show that the overall energy efficiency of the system is good.Under the working conditions of high wastewater temperature,moderate air temperature and liquid to gas ratio,the system can achieve the effective unification and balance of high evaporation level,low electricity consumption and operating economy,so that the system can operate at a high energy efficiency level.