Study On Selenium Migration Behaviors And Enhanced Removal Methods In Wet Flue Gas Desulfurization Systems

Unique resource structure determines that the thermal power is the cornerstone of power supply of China.Emissions of conventional pollutants from coal-fired power plants have been strictly controlled,and trace heavy metals have increasingly attracted global attention.Coal-fired power plant is the main man-made source of selenium in the environment,and selenium pollutants are extremely harmful to the ecological environment.Attention must be paid to selenium emissions during coal combustion.The utilization of existing air pollutant control devices to achieve synergetic removal of multiple pollutants is currently the main strategy for heavy metal control.Due to the similarity between selenium and sulfur,selenium pollutants mostly exist in gaseous form in the flue gas,and the wet flue gas desulfurization system has become the core equipment for selenium control.However,available researches reported that the wet flue gas desulfurization system has an extremely unstable collaborative control effect on selenium.The selenium emission of most units exceeds the standard,and the enrichment of selenium in the desulfurization wastewater is difficult to treat.It is urgent to develop an enhanced removal and stabilization technology of selenium in wet flue gas desulfurization system.Based on this situation,this thesis combined theoretical modeling and experimental testing,to study the migration mechanism and removal behavior of selenium pollutants in wet flue gas desulfurization devices,and proposed enhanced removal techniques based on mass transfer optimization and flue gas temperature control.An in-situ solidification method for selenium in desulfurization slurry was developed.This study can provide theoretical guidance and technical reserves for improving the capability of the wet flue gas desulfurization system to synergistically remove selenium.Firstly,a model of selenium migration behaviors in the wet flue gas desulfurization spray tower was constructed,revealing the removal paths of gaseous selenium in the desulfurization tower.One is to be directly absorbed by droplets,and the other is to condense on the surface of particulate matter,then the particles are trapped by the droplets.The migration process of selenium can be divided into three stages:preparation,condensation and removal.In the condensation stage,the conversion of gaseous selenium to particulated selenium occurred.In the removal stage,gaseous selenium and particulated selenium were both removed by droplets,but the gaseous selenium exhibitied a poor mass transfer and absorption effect.The inlet particle size and outlet flue gas temperature were the key factors that affect the efficiency of selenium removal.Combined with the unique removal mechanism of selenium in the tower,it can explain the current instable performance of WFGD on selenium control.Secondly,a lab-scaled simulated wet flue gas desulfurization device and method for heavy metal pollutant removal test was established.The removal capacity and mechanism of the three internal components were explored,including tray,demister,and synergistic ring.It was found that the tray can significantly improve the gas-liquid contact in the absorption area,and the Se concentration at the outlet was reduced by 49.2～60.1%.The main mechanisms included,（1）improving the uniformity of flue gas,（2）reducing the flow area of flue gas and liquid droplets,（3）droplet spurted and formed massive secondary droplets,（4）the liquid film in the tray channel repeatedly underwent a random"formation-breaking"process,and when broken,small mists were generated,which can increase the gas-liquid contact area.The mist eliminator intercepted the fine droplets and formed a water film on the surface,which can perform secondary absorption of the gaseous selenium in the clean flue gas,and the Se concentration at the outlet was reduced by 52.2-61.7%.The synergistic ring could improve the"short circuit"phenomenon of the gas/droplets and promoted the movement of the gas/droplets droplets to the central area of the tower,and the Se concentration at the outlet was reduced by 47.8-56.5%.It was found in the internal component coupling experiments that the mechanism of the tray and the synergistic ring is overlapping.Considering removal performance and operating cost,"tray+synergistic ring+demister"is the preferred technical solution,which can reduce the selenium concentration to5.35μg/m³ under typical conditions,reaching the selenium emission limit regulated by US EPA for coal-burning flue gas.Subsequently,the influence of the flue gas temperature on the selenium removal behavior was explored.It was found that reducing the circulating slurry temperature could significantly reduce the outlet flue temperature,and at the same time,the Se concentration at the outlet sharply decreased,which was consistent with the model prediction results.Based on this phenomenon,a secondary condensing tube was designed to achieve deep cooling of the clean flue gas.The convective heat exchange between the flue gas and the tube wall was the main control step in the heat exchange process.Increasing the number of condensing tube layers（ie,heat convection area）can enhance the overall heat transfer capacity.The secondary condensation tube can significantly enhance the removal effect of gaseous selenium.When the flue gas temperature was reduced by 6.7℃,the Se concentration at the outlet was reduced by 79.0～87.4%.The enhancement mechanisms included,（1）reducing the flue temperature to trigger the secondary condensation of gaseous selenium,（2）providing additional condensation surfaces,（3）forming a water film on the wall of the tube to facilitate gaseous selenium transfer and absorption.Coupling the mass transfer optimization mtehod with the flue gas temperature control method,the outlet selenium concentration can be reduced to7.77μg/m³ at a liquid-to-gas ratio of 5,which has great application potential.Finally,the transformation mechanism of selenium in the desulfurization byproduct was explored.It was found that Fe³⁺in the desulfurization slurry could significantly promote the migration of selenium to the solid phase.S₂O₈^2-can oxidize selenite to highly soluble selenate due to its strong oxidizing property,and it would inhibit the effect of Fe³⁺on selenium.A selenium in-situ solidification method was developed,using or Fe³⁺/Fe²⁺Fe³⁺/Mn²⁺as the composites of in-situ solidification agent,which can counteract the adverse effects of S₂O₈^2-.The selenium in-situ solidification method performed well in both simulated slurry and field slurry.The mechanism of selenium removal was to combine Fe³⁺with selenite to form insoluble Fe₂（Se O₃）₃,which would enter gypsum after precipitation from the solution,and this reaction was superior to the hydrolysis reaction of Fe³⁺.The stability of selenium in gypsum after adding the in-situ solidification agent was also investigated.It was found that the thermal decomposition temperature of selenium compound in gypsum was about 425.4℃.Hence there was no risk of secondary release during the reprocessing of gypsum.The leaching stability of selenium containing gypsum after adding the in-situ solidification agent was significantly better than that of original gypsum,but its stability was poor under strong acid（p H=1.8）or strong alkaline（p H=12.7）solution,so it must be avoided during disposal.