Study On Migration And Transformation Characteristics Of Chloride And Mercury During Evaporation Of Desulfurization Wastewater

The evaporation characteristics of desulfurization wastewater and the migration and transformation of volatile components during the evaporation process were the basis of hot flue gas evaporation to achieve zero discharge of desulfurization wastewater.However,it was limited to the investigation of macro phenomena,and relevant quantitative researches were lack.Under the background that the direct hot flue gas evaporation and concentration process of desulfurization wastewater without pretreatment had gradually become the mainstream route of zero emission in China,combined with previous studies,this work studied the droplet evaporation process of desulfurization wastewater and the evaporation process conditions,the influence mechanism of wastewater physical properties,as well as the migration and transformation of volatile components and their distribution in gas and solid phase,so as to reveal the law of hot flue gas evaporation of desulfurization wastewater.It was expected to provide theoretical and data support for the popularization and application of desulfurization wastewater hot flue gas evaporation technology and the suppression of secondary pollution.The release characteristics of chloride during evaporation process of desulfurization wastewater droplets were studied by single droplet drying experiments.The mass,size and temperature variation of droplets during the evaporation process were measured and release mechanism of chloride was revealed.Besides,the influence of temperature,p H value and metal cation species on the release characteristics of chloride were explored.Combined with the evaporation process,the chloride release could be divided into three stage:stage of free water evaporation,shell instability and crystallization chloride salt hydrolysis.In the initial stage,moisture removal was the dominant phenomenon and almost no chloride release was observed;During the stage of shell instability,cycles of droplet inflation and rupture took place,and the release amount of chloride accounted for 51.2%of the total release amount in the typical working conditions;Hydrolysis of crystallization chloride salt played a key role in chloride release and about 46.0%chloride release could be traced during the last stage.High temperature would boost the chloride release;With p H value increasing from 3 to 9,the chloride release decreased from 39.7%to 28.8%;The higher magnesium concentration would enhance the chloride release due to the greater hydrolysis strength.The chloride in the wastewater droplet would not transfer into the gas phase until the evaporation proceed to the stage of shell instability.During the rotary spray evaporation process,the increase of inlet temperature and gas-liquid ratio would increase the temperature along the drying tower,thus promoting the release of chloride;The increase of p H value would inhibit the chloride release.The evaporation rate,degree of droplet shrinkage and expansion and release rate of chloride of concentrated high-salt wastewater decreased during the evaporation process.The chloride release characteristics investigated in this work are expected to control the HCl emission in practical engineering of hot flue gas evaporation.The mercury release process could be divided into three stages.In the initial evaporation process,gaseous mercury was the main source of mercury release.Subsequently,due to the hydrolysis of chloride,chloride oxidation atmosphere was generated,resulting in the uniform oxidation of Hg⁰.At the same time,Hg Cl₂ would also be decomposed by heating.At the end of evaporation,the pyrolysis of Hg S would release a small amount of mercury.In addition,the effects of evaporation temperature,Cl^-and SO₃^2-content on mercury release were also studied.Higher evaporation temperature was not conducive to the stability of mercury.The ratio of gaseous Hg⁰ and Hg²⁺was highly sensitive to the content of SO₃^2-and Cl^-.Mercury in waste water during hot flue gas evaporation process would undergo release oxidation adsorption process.Due to the effect of SO₃^2-and Cl^-,about 93.6%of mercury in wastewater would be released first.Then,the release of Mercury would be further affected by oxidation and adsorption mechanisms.The generated HCl gas atmosphere and the active substances in the solid components（chloride free radicals and Fe₂O₃）would oxidize most of the released Hg⁰and increase the proportion of Hg²⁺in the flue gas.In addition,the adsorption effect promoted the conversion of gaseous mercury to mercuric chloride,resulting in the proportion of mercury in the evaporation product as high as 89.4%.The risk assessment results showed that the mercury released by the evaporation of wastewater would not affect the total mercury content in the main flue gas or the comprehensive utilization of solid phase.Hot flue gas evaporation technology would not disturb the distribution of mercury,nor would it cause additional mercury emissions to the whole power plant.However,the water/acid soluble portion of mercury in the evaporated product should be considered during storage and disposal.