Research On Mechanism Of Flash Treatment Of Desulphurization Wastewater From Coal-fired Power Plant

Under the call of national policy of energy conservation,emission reduction and environmental protection,efficient and economic treatment of desulfurization wastewater from coal-fired power plants is one of the important problems that need to be solved,which has important research significance.Multistage flash is a commonly used technology for wastewater treatment and traditional multistage flash uses free liquid level flash which leads to large equipment area and high investment cost.In this paper,the traditional structure is replaced by vertical structure and the orifice plate is used as the pressure isolation device.The flash solution enters the next stage from the upper stage by gravity,and collides with the pyramid at the outlet of the small orifice to atomize,replacing the atomizing nozzle,so as to avoid the problem of easy scaling and blockage and achieve the purpose of efficient and economic treatment of desulfurization wastewater.Based on flash solution from upper stage rely on gravity flow into the next stage of the whole process for research,explore the solution through the orifice of the flashing flow resistance and the rectangular pyramid after spray droplets flash mechanism,multistage flash equipment structure design for the desulfurization wastewater provide theoretical guidance,a coal-fired power plant in Qingdao city as an example,a demonstration project is carried out to verify the theoretical research results of this paper.(1)Build a single orifice flash flow resistance experiment table,take water as the working medium,carry out a comprehensive flash experiment,and use the control variable method to study the flow through the small orifice,the relationship between the flow coefficient and the superheat pressure difference,and the relationship between the flow through the small orifice and the liquid level.The results show that the flow coefficient decreases with the increase of superheated differential pressure,and the decrease is slower and slower.The flow through the small orifice increases with the increase of superheated differential pressure and with the increase of liquid level.Based on the experiment,the experimental correlation between orifice flow coefficient and superheated differential pressure in the flash temperature range of 20～ 80℃ is fitted.(2)The depressurization flash phase change model is established and coupled with the multiphase flow model.The internal flash phase change process of water flow through small orifice is simulated by FLUENT and the model is verified by experiments.Through simulation research,the effects of inlet temperature and flash pressure on the flash phase change characteristics of water in the small orifice are explored.The results show that the greater the pressure difference between the inlet and outlet of the small orifice,the greater the degree of superheat of water.At the same position,the greater vapor volume fraction,the more intense the flash;vapor volume fraction near the wall is larger than that near the center,and the velocity and temperature of two-phase flow are lower than that near the center.When the pressure difference between the inlet and outlet of the small orifice is greater than 8.6k Pa,the difference of vapor volume fraction in the small orifice is small;the pressure difference between the inlet and outlet of the small orifice has little effect on the temperature which is basically about 1K.(3)The flash mechanism of the liquid column at the outlet of the small orifice is analyzed,and the Sauter Mean Diameter of the droplet after the impact of the liquid column and the pyramid is derived.Taking the atomized single droplet as the research object,the flash mathematical model is established,and the droplet heat and mass transfer and motion equations are obtained.It is solved by MATLAB to study the effects of different initial conditions on the droplet flash characteristics.The results show that the flash evaporation is basically completed within 0.2s,the droplet diameter affects the flash time and the flash pressure affects the equilibrium temperature and vaporization rate of droplets.(4)Taking a coal-fired thermal power plant in Qingdao as an example,the theoretical research results of this paper are used to guide the construction of desulfurization wastewater multistage flash equipment in practical engineering,and the flow at all levels of the multi-stage flash equipment is measured to verify the proposed theoretical research results of this paper.The results show that the relative deviation between the predicted flow and the actual flow of the fourth and sixth stages is large,and the rest are within 10%.The calculated vaporization rate and the actual vaporization rate of each stage is basically about 15%.