Study On The Mercury Removal Performance And Kinetics Of Biomass Ash Modified Calcium-based Adsorbent

Sorbent injection is a very promising mercury emission control technology in coal-fired power plants.Development of a cheap and efficient sorbent is the key for industrial application of this technology.The calcium-based sorbent is a common sorbent for flue gas desulfurization in coal-fired power plants.In addition to the efficient removal of SO2,the calcium-based sorbent also has a good removal effect on the oxidized mercury(Hg2+)in the flue gas,but the removal efficiency of elemental mercury(Hg0)is pretty low.Therefore,it is of great theoretical significance and engineering application to develop a modified calcium-based sorbent for simulantous control of multi-pollutant in coal-fired flue gas.Biomass ash(BA)has abundant physical and chemical structures,which has potential catalytic oxidation and adsorption ability for Hg0 in the flue gas.In this paper,the calcium-based sorbent was modified with biomass ash and modifier,and mercury removal performance over modified calcium-based sorbents was evaluated by adsorption experiment in a fixed-bed reactor,and mercury removal mechanism was discussed based on the characterization of physicochemical properties and adsorption kinetics of calcium-based sorbents.The main work and the conclusions are listed as follows:Modified calcium-based sorbent was prepared by the method of impregnation and calcination with CaCO3 as the base object.The biomass ash and the modified sorbents were characterized by surface area analyzer,XRD analyzer,SEM-EDS analyzer,FTIR analyzer and XPS analyzer.The effect of MnCl2 addition,the ratio of CaCO3 and BA and calcination temperature(Te)on physical and chemical properties of modified calcium-based sorbents were studied.The results show that:(1)The surface of biomass ash is mainly mesoporous structure,there are also much microporous structure;the pore structure of modified sorbents are improved,and when calcination temperature is 800℃ the sorbent pore structure achieves best.(2)The biomass ash is composed mainly of SiO2,also contains a small amount of metal oxides such as Al2O3 and Fe2O3;the modified sorbents contain more Mn,MnO2,Mn2O3 and Mn3O4 are produced after calcining,and the valence state of Mn is most abundant when calcination temperature is 700℃.(3)Biomass ash contains oxygenic groups such as carboxyl and lactone;the type of oxygenic groups in sorbents increases after modifying.The experimental study on mercury adsorption was carried out in a fixed-bed reactor,the effects of different preparation conditions,inlet Hg0 concentration(Ci),adsorption temperature(Ta),flue gas composition and moisture content of flue gas(ω)on mercury removal efficiency were investigated.The results show that:(1)In the range of Tc=500～800℃,mercury removal efficiency and absorption capacity over unit sorbent both decrease first and then increase then decrease again with increasing of calcination temperature.When Tc =700℃,the sorbent shows the best adsorption characteristic.(2)In the range of 0.5%-2%,the more addition of MnCl2,the better the mercury removal performance.(3)The greater the mass ratio of biomass ash in the sorbent,the higher the mercury removal efficiency and the greater the mercury absorption capacity over unit sorbent.(4)The mercury absorption capacity over unit sorbent increases first and then decreases when adsorption temperature rises.The sorbent shows the best adsorption performance when Ta=70℃.(5)In the range of Ci=14.9～34.5μg/m3,an appropriate increase of mercury concentration is beneficial to mercury removal,but when the concentration is too high,the mercury removal efficiency decreases significantly.(6)In the range of CO2=0～12%,the higher the oxygen content,the higher the mercury removal efficiency and the greater the mercury absorption capacity over unit sorbent.(7)Appropriate SO2 concentration slightly promotes mercury removal;NO significantly promotes mercury adsorption.In the range of CNO=0～200μL/L,the higher the NO concentration,the better the mercury removal performance.(8)In the range of co=0～10%,suitable moisture content of flue gas can improve mercury removal of the sorbent.Four kinds of kinetic models were used to fit the mercury adsorption process,the relevant kinetic parameters were obtained and the kinetic mechanism of mercury removal reaction of the modified calcium-based sorbent was discussed.The results indicate that:(1)The fitting degree between Weber-Morris model and experiment is low,and the correlation coefficient R2 obtained from Pseudo first-order,Pseudo second-order and Elovich kinetic model is all above 0.999.(2)The mercury adsorption process is affected by external mass transfer and particle diffusion.The chemisorption of vapor-phase mercury molecules at the active sites is the control step during mercury adsorption.(3)In the initial stage,main form of mercury adsorption is surface adsorption,the internal diffusion rate is small,and the mercury adsorption rate is faster;as reaction proceeds,mercury molecules diffuse into the internal micropores,then the internal diffusion rate increases,and the mercury adsorption rate continuously decreases.(4)As the inlet Hg0 concentration increases,the mercury adsorption rate increases,and the effect of external mass transfer on mercury adsorption process is smaller;the higher the adsorption temperature,the faster the movement rate of vapor-phase mercury molecules,and the greater the diffusion coefficient,the faster the diffusion rate.