Experimental And Mechanism Research On Desulfurization Wastewater Evaporation Using Bypass Flue Gas

At the beginning of 2013 in China,the FGD units capacity of thermal power plants was more than 90%were wet limestone-gypsum flue gas desulfurization.WFGD needs to discharge a certain amount of desulfurization wastewater to maintain the Cl-concentration below 20000mg/L.On April 16,2015,the State Council issued the "Water Pollution Action Plan" to control water pollution strictly,desulfurization wastewater zero discharge has become another hot environmental issue after the flue gas ultra-low emission.In recent years,various zero discharge technologies had emerged,among which the bypass evaporation technology using high temperature flue gas before air preheater has many advantages and get industry attentions.Therefore,exploring the mechanism of high temperature evaporation process to solve the key scientific problems has important theoretical significance and application value.In this paper,the self-designed high temperature flue and evaporation tower was used to conduct evaporation experiments.At the same time,CFD was used to simulate the evaporation characteristics under different working conditions.When the inlet flue gas temperature is 300℃,the atomizer rotation speed is 18000r/min,and the deflector angle is 20°,maximum evaporation capacity of the evaporation tower reaches 4.38kg/h;When inlet flue gas temperature is 300℃,the inlet liquid flow rate is 150mL/min,the compressed air flow rate is 15L/min,and the inlet flue gas flow rate is 300m3/h,the bypass flue gets optimal.The CFD simulation results show that the parameters for the best evaporation effect are that the evaporator diameter is 2.4m,the atomization particle size is 100-150μm,the flue gas flow rate is 3-4m/s,and the flue gas temperature is 250-260℃;The proportion extraction design before and after the air preheater can achieve the economical,efficient and energy saving treatment effect of desulfurization wastewater.Furthermore,the liquid and solid phase volatilization mechanisms of different salt systems were explored in detail,established the gas-solid phase distribution model of Cl-,and proposed economical and effective methods for inhibiting HCl volatilization.Results show that the classic gas-liquid equilibrium model is adapted for gas-solid phase distribution in mixed salt systems;Rising temperature and decreasing pH can promote the HCl volatilization,special attention should be paid to temperature and pH during the evaporation of desulfurization wastewater;Gaseous HCl mainly comes from the combination of free Cl-and H+dissociated from H2O and the hydrolysis of NaCl,CaCl2,and MgCl2;Gaseous HCl in solid products originates from M9Cl2·6H2O hydrolysis;Ca(OH)2 can be used to adjust the pH of the desulfurization wastewater to 9-10,which will effectively inhibit the HCl volatilization.After that,based on the "Three-stage evaporation" process,a dynamic model of droplets groups in high temperature flue gas evaporation process was established.Calculation results show that the higher temperature of the flue gas and the smaller the droplet size,the higher mass transfer diffusion rate per unit area and the shorter evaporation time;Relative velocity of droplets mainly affects the constant speed drying stage,increasing the relative velocity of droplets is beneficial to shorten the evaporation time;There is also a critical salt content in the evaporation process.Before the critical point,constant speed evaporation is dominant,after the critical point,falling speed drying is dominant.Controlling the salt content below the critical point is conducive to the complete evaporation of droplets.Finally,crystallization experiment was conducted to collect evaporation products,the solid products were characterized and.The effects of bypass flue gas evaporation on existing equipment in coal-fired power plants were analyzed combine with laboratory calculations,characterization results and field test data.Flue gas evaporation of desulfurization wastewater can improve the dust removal efficiency and reduce the boiler thermal efficiency.When 5%bypass flue gas is extracted,the boiler exhaust heat loss increases by 0.19829%and the boiler coal consumption increases to 0.59g/kW·h;It will not cause heavy metals in dust exceed the standard except Cl-,the proportion of HCl volatilization should be controlled in 6%;Desulfurization wastewater evaporation can synergistically remove SO3,and the removal efficiency can reach 58.49%.In this thesis,stage results have been achieved in evaporation characteristics of bypass flue gas evaporation,the chloride salt distribution mechanism,evaporation kinetics mechanism and impact evaluation.The obtained experimental data and the proposed control ideas provide theoretical support for industrial applications of the technology.