Experimental Research And Mechanism Analysis Of Improving The Elemental Mercury Removal Efficiency In Flue Gas Using Non-thermal Plasma

The combustion of coal brings a lot of environmental problems,and mercury emission is one of the major concerns.It is very important to reduce mercury emissions because mercury does great harm to human body health.Usually mercury presents in coal combustion flue gas in three forms:elemental mercury(Hg0),particle-bound mercury(HgP),oxidized mercury(Hg2+).Particle-bound mercury can be effectively captured with fly ash in the particulate matter electrostatic control device.Oxidized mercury which reacts readily with water vapor can be removed by wet flue gas desulfurization equipment.Among them,elemental mercury is the most difficult to be removed from flue gas due to its high volatility and poor water-solubility.As a result,elemental mercury removal becomes the primary task of coal-fired power plants.There are basically two kinds of mercury removal technologies;one is oxidation based technology which utilizes oxidant or catalyst to convert elemental mercury to oxidized mercury or particle-bound mercury,the other is adsorption based technology which utilizes adsorbents to transform elemental mercury into particle-bound mercury.Non-thermal plasma contains some highly reactive free radicals which readily react with some refractory pollutants,thereby it is used for volatile organic compounds(VOCs)disposal and automobile exhaust disposal.In addition,it is widely used to modify adsorbent or catalyst to achieve better performance because of its high reactivity.Non-thermal plasma shows great potential in pollutant disposal,however,a few researchers has ever applied it in mercury removal from flue gas.Consequently,this study introduced non-thermal plasma into coal-fired power plant to enhance the mercury removal efficiency.Firstly,this study combined non-thermal plasma with oxidation based technology to improve the mercury removal efficiency.A non-thermal plasma based homogenous oxidation apparatus was designed and established to investigate the effect of non-thermal plasma on mercury oxidation.The results showed that non-thermal plasma treatment effectively promoted the mercury oxidation by O2,HCI and Cl2.That is because O2,HCl and Cl2 were transformed to highly reactive free radicals by non-thermal plasma treatment,and the highly reactive free radicals can react with the mercury easily and rapidly.However,increasing the reaction temperature inhibited the mercury oxidation by O2,HCl and Cl2 due to that the increasing temperature shortened the life of the free radicals Aside from O2,HCl and Cl2,other composition in flue gas also have some influence on mercury oxidation efficiency.It was found that CO2 can slightly promote the mercury oxidation,and NO can largely inhibit the mercury oxidation,while SO2 has no obvious effect on the mercury oxidation.Secondly,this study combined non-thermal plasma with adsorption based technology to improve the mercury removal efficiency.An activated carbon(AC)treatment apparatus and a mercury adsorption apparatus were built to study the effect of non-thermal plasma treatment on mercury adsorption by ACs.The results showed that AC treated with non-thermal plasma had higher mercury removal efficiency than others,and longer treatment time resulted in higher efficiency.The main reason was that non-thermal plasma treatment increased active sites of AC,especially increased carbonyl groups and ester groups of AC,which played an important role in mercury absorption.By further analysis,it was found that SO2 and H2O inhibited mercury adsorption efficiency of AC,while NO and HC1 promoted mercury adsorption efficiency of AC,and their promotion were related with oxygen functional groups of AC.Moreover,a temperature programmed desorption setup was built to investigate the mercury adsorption mechanism.The results indicated that the chemisorption by carbonyl groups and ester groups occurs in the temperature range of 30-170 ? and 30-210 ?,respectively,and physisorption occurs in the temperature range of 30-130 ?.Chemisorption is prior to physisorption,and chemisorption by ester groups is prior to that by carbonyl groups.Thirdly,in order to further enhance the mercury removal efficiency of ACs,this study used non-thermal plasma treatment to add Cl/Br active sites on ACs.The results showed that ACs treated with non-thermal plasma with a small amount of Cl2/Br2 presented very high mercury adsorption efficiency,and the capacity of mercury adsorption was positive correlation with the concentration of Cl2/Br2.Similarly,SO2 and H2O in flue gas had an inhibition effect on mercury adsoption by AC,while HCl had a promotion effect.XPS analysis indicated the chemisorption of mercury was attributed to the C-Cl groups generating on the AC surfaces during CI2 non-thermal plasma treatment.The C-Cl groups had strong adsorption energy for mercury,whereas converting element mercury to HgCl2.