Research On Key Issues Of Desulfurization Wastewater Concentration Using Low Temperature Flue Gas

With the increasingly strict national water pollution prevention and control policies and industry standards,zero discharge of desulfurization wastewater from coal-fired power plants is the general trend.Concentrating and reducing desulfurization wastewater reduces the cost and workload of subsequent zero-discharge treatment,and is a key step in zero-discharge of desulfurization wastewater.Using the low-temperature waste heat carried by the flue gas after the dust collector in the power plant,combined with the bypass concentration tower to realize the concentration and reduction of desulfurization wastewater has become a new idea of zero discharge of desulfurization wastewater.However,due to the high chlorine content and strong acid environment of desulfurization wastewater,the volatilization and migration of chlorine during the evaporation and concentration process will affect the chlorine balance of the subsequent absorption tower,causing concern.At the same time,most of the actual projects of the bypass concentration tower use the density index of the concentrated slurry to control the discharge of the slurry based on experience,lacking relevant theoretical basis.To this end,this paper conducts research on the key issues of low temperature flue gas concentration desulfurization wastewater,and provides a theoretical basis for the industrial application of this technology.For the volatilization and migration of chlorine in the process of evaporation and concentration of wastewater,single-factor influence experiments and orthogonal experiments were carried out on the volatilization and migration of chlorine in the process of evaporation and concentration of high-chlorine brine with high chlorine content by using a self-made simulation test bench.The study found that the migration of chlorine in the process of evaporation and concentration of high brine has two modes:vapor-phase chlorine volatilization and liquid-phase chlorine carry-over,among which liquid-phase chlorine is mainly carried,accounting for more than 70%of the total volatile chlorine.The temperature of flue gas changes the gas-liquid mass transfer coefficient,which is the main factor affecting the volatilization of hydrogen chloride gas;the change of the flow rate of the flue gas by the amount of air intake is the main factor affecting the chlorine carried by the droplets.In addition,p H and salt concentration also affect the volatilization and migration of chlorine.certain influence.The volatilization model of hydrogen chloride in high brine environment was established according to the multi-component hydrogen chloride system,and the chlorine balance model of the bypass concentration tower was established based on the principle of material balance,and the volatilization of hydrogen chloride gas during the evaporation and concentration process was predicted.Combined with the experimental results and the prediction model,suggestions are put forward for the regulation of chlorine volatilization and migration during the evaporation and concentration of desulfurization wastewater.For the problem of slurry discharge control indicators in the process of wastewater evaporation and concentration,first,the solution characteristics of calcium sulfate,which is easy to scale,in single-salt and mixed-salt systems were explored.The results show that Mg Cl₂ and Na Cl can promote the solubility of calcium sulfate,and Mg Cl₂ has the most obvious promoting effect.Ca Cl₂ has an inhibitory effect on the dissolution of calcium sulfate,and Na₂SO₄ inhibits the dissolution of calcium sulfate at lower temperature,and the solubility of calcium sulfate increases slightly with the increase of temperature.During the evaporative concentration of desulfurization wastewater,there is a good linear relationship between Cl^-concentration and TDS content,so the slurry density and TDS content can be used in combination to judge the slurry discharge.Secondly,the physicochemical properties of the liquid phase and the composition analysis of the solid phase during the evaporation and concentration of the salt solution were explored.The results showed that the concentration of SO₄^2-appeared a small peak when the concentration ratio of the slurry was about 7 times.Ca SO₄·0.5H₂O and Ca SO₄ in the concentrated slurry are in a saturated state,and Na Cl,Na₂SO₄,Mg SO₄,Mg Cl₂,Ca Cl₂,Mg SO₄·6H₂O,6Na₂SO₄·7Mg SO₄·15H₂O,Ca Cl₂·4H₂O are precipitated in sequence.When the concentration ratio reaches 7.1429,there is no precipitation of Ca SO₄,and the solubility of Ca SO₄ is the largest at this time.As the concentration continues,Ca SO₄ is precipitated again.According to the research on the physicochemical properties and crystal precipitation sequence of the concentrated slurry,it is recommended to control the concentration ratio of the concentrated slurry of wastewater to be about 7 times.At this time,the corresponding slurry density is 1.117g/cm³ and TDS is 200000ppm.The low-temperature flue gas evaporation concentration desulfurization wastewater process has high operational reliability and low investment and operating costs.It is an economical,efficient and practical desulfurization wastewater concentration reduction technology.The research results provide theoretical support and data reference for the application and development of this technology.