A Study On The Conversion Of Mercury Speciation And Its Emission Control Mechanism In The Wet Flue Gas Desulfurization System

The wet flue gas desulfurization(WFGD)system is considered as a key pollutant control device in a coal fired power plant,and is capable of co-benefit with Hg removal in the flue gas.It is of commercial value to enhance Hg emission control for present coal fired power plants by a deep insight of the migration and conversion of Hg in the WFGD systems as well as the contribution that the systems make in Hg emission control.In order to provide the direction and the theoretical reference in enhancing Hg emission control in coal fired power plants,the present study investigated the mechanism of Hg emission control by the WFGD system,and studied the re-emission behavior of Hg in the system.In addition,the migration behavior and conversion mechanism of Hg in a typical WFGD system were conducted.The potential risks of the by-products of the WFGD system for leaching to the environment were also evaluated.First of all,the present study investigated the effect of halogen elements on WFGD divalent mercury control including Cl^-.The results showed that the halogen elements had a remarkable ability in Hg emission inhibition in the WFGD system..The halogen elements exhibited an increasing trend in Hg emission control by the order Cl^-<Br^-<I^-.Increasing reaction temperature and p H value are positive for Hg emission control.Further increase in halogen concentration from 5.0 m M to 50.0 m M for Cl^-,Br^-,and I^-,the amount of Hg emission decreased 23.5%,53.8%,and 62.8%,respectively.Considering the kinetic simulation results,a serious of outer-orbital coordination compounds were rapidly produced by the halogen elements and Hg,which could inhibit Hg emission.The study regarding S₂O₈^2-performance in elemental Hg control and its removal mechanism was conducted.The results showed that S₂O₈^2-exhibited Hg removal ability to a certain extend.The efficiency of Hg removal increased when the temperature and S₂O₈^2-concentration,and O₂concentration increased.Lowering p H value led to a pronounced improvement in Hg emission control.When the p H value decreased from 5 to 3,the efficiency of Hg removal reached 54.7%.The introduction of Fe²⁺resulted in a further improvement in Hg removal,the efficiency reached52.8%at the end of experiment.The results of mass transfer-kinetic reaction combined simulation indicated that the efficiency of Hg removal was conditioned by the process of mas transfer,and Fe²⁺introduction not only accelerated the reaction,but also deepened the reaction limitation of Hg oxidation.In order to reveal the Hg re-emission behavior in WFGD system,the present study investigated various parameters under which the Hg re-emission took place,including reaction temperature,p H value,O₂concentration,SO₃^2-concentration,and SO₄^2-concentration in WFGD system,then the possible mechanism involving Hg reduction and re-emission was concluded.The results showed that the SO₃^2-was the key factor that was responsible for Hg reduction and re-emission.Increasing the concentration of SO₃^2-resulted in a decrease in Hg re-emission,and it showed an opposite trend when the temperature increased where the amount of Hg re-emission enlarged over twice times when the temperature increased from 45?to 55?after90 min reaction.Large amount of Hg was re-emitted as a result of the protonation effect of Hg SO₃when the p H value decreased.The reaction mechanism was changed when the O₂presented,and O₂showed an inhibition effect on Hg re-emission overall.On one hand,SO₄^2-could suppress Hg re-emission directly;on the other hand,a new product was produced by SO₄^2-and an intermediate species in Hg reduction and re-emission,which was positive for Hg re-emission control.In the meanwhile,no obvious difference was found in increasing SO₄^2-concentration or change the ratio of SO₄^2-/SO₃^2-.Additionally,the migration and conversion of Hg in the solid fraction and the liquid fraction of desulfurization slurry were systematically studied based on the onsite sampling in WFGD system,and Hg partitioning and evolution in WFGD system were also included.The results showed that,Hg could transport from the liquid fraction to the solid fraction of the desulfurization slurry in the gypsum production process.There was a relatively high concentration of Hg in the finer fraction of desulfurization slurry,which was separated to the effluent treatment process by the first stage hydrocyclone.In the meanwhile,Hg S was predominated stepwise in various Hg species in this process,ending with the percentage of Hg S in the gypsum 77.8%.In the effluent treatment process,the conversion of Hg from the liquid fraction to the solid fraction of the desulfurization slurry could happen.The three-linked tank in this process could nearly all the Hg in the solid fraction of the slurry.The Hg concentration in the effluent met the corresponding emission standard.Moreover,the proportion of Hg O in various Hg species increased stepwise in this process.Based on the comprehensive investigation on Hg migration and evolution in WFGD system,it was concluded that a small amount of Hg output was carried by the gypsum,on the contrary,the desulfurization sludge was the major Hg output as a WFGD by-product compared with the gypsum.According to the Hg migration and evolution results in the whole WFGD system,Hg partitioning and species in the desulfurization sludge was conducted in three selected desulfurization sludges,especially for the finer fraction of the sludge which generally occurred with Hg enrichment.The results showed that the concentration of Hg in the desulfurization sludge was much higher than that of the gypsum.The study of Hg-TPD method and improved BCR method regarding Hg species in the sludge was performed in this work.It was indicated that Hg Cl₂,Hg S,Hg O,and Hg SO₄were identified in the desulfurization sludge with Hg O dominant.Among these for Hg species,Hg Cl₂,Hg O,and Hg SO₄tended to occur in the leachable fraction,and Hg S was mainly found in oxidizable and residual fraction.The proportion of Hg S increased with the particle diameter of desulfurization sludge decreased.The gypsum was divided into no-risk or low-risk group substance while the desulfurization sludge was assigned to moderate-risk or high-risk level.In comparison with gypsum,the desulfurization sludge should receive more attention and be carefully disposed of in case of leaching risk.