Study On Mercury Adsorption Form And Desorption Characteristics In Simulated Flue Gas

Coal combustion is one of the largest anthropogenic sources of atmospheric mercury emission,and coal-fired power plants occupy the main part in China,which is the largest fixed source of mercury emissions.Therefore,mercury control technology in coal fired power plant has been a hot topic.Activated carbon injection(ACI)is deemed to be one of the most commercial promising mercury removal technologies for coal-fired power plants but costly.ESP fly ash has attracted wide attention of scholars due to its inexpensive and mercury removal capability potentially characteristics.However,the experimental study of the factors on mercury adsorption in flue gases and the mercury speciation on the adsorbents surface are seldom.The purpose of this study was to investigate a kinds of cheap and efficient mercury removal adsorbent.The effects of bromine modified activated carbon(AC)and fly ash(FA)on the adsorption of mercury in flue gas were discussed.And then the temperature programmed desorption(TPD)was applied to study the adsorption mechanism and desorption characteristics of mercury species on the surface of the adsorbed bromine modified activated carbon and fly ash.Firstly,four kinds of fly alises(FA)from coal-fired power plants ESP were selected as the mercury removal adsorbents and impregnated with 1%NH4Br solution(NH4Br-FA,NBFA).The samples were characterized by N2 adsorption/desorption,X-ray fluorescence spectrometry(XRF)and scanning electron microscopy(SEM).Experimental study on the Hg0 adsorption by original and modified fly ash under simulated flue gas conditions were conducted on a fixed bed reactor.The results showed that the active sites were enhanced when bromine was impregnated on the surface of UBC in fly ashes,which promoted the adsorption and oxidation of Hg0 in N2 and simulated flue gas.Acid gases in simulated flue gas oxidized Hg0 obviously so that 75%of Hg0 was oxidized and released from the surface of fly ash samples and others adsorbed.The content of UBC in NBFA has a great influence on mercury removal.The higher amount of UBC,the stronger adsorption is,and the lower content is,the stronger catalytic oxidation is.Secondly,the effect of single component such as O2,SO2 and NO on mercury capture and adsorption under simulated flue gas were investigated in a fixed-bed reactor.The modification method on several metallic oxidizations contained in fly ash were the same with fly ash.Then TPD was applied to identify the mercury species of adsorption products.The results showed that the effect of bromine on Fe2O3 and TiO2 in fly ash was mainly to increase the lattice oxygen content and promote the oxidization of Hg0,forming HgO,and O2 in the flue gas replenishes the consumption of lattice oxygen continuously.NO could react with adsorbed Hg0 to form Hg2(NO3)2 on the surface of fly ash,then some would decompose to Hg(NO3)2 and HgO.The amount of UBC in the fly ash affected the amount of active sites,which had a great impact on mercury removal.Lower content of UBC in the flyash,higher percent of Hg2+ at the outlet of fixed-bed reactor was detected.The adsorption and oxidation of Hg0 by UBC followed the Langmuir-Hinshelwood mechanism,while metallic oxidization containing bromine catalic oxidized Hg0 mainly following the Mars-Masessen mechanism.Therefore,the process of adsorption and oxidation of Hg0 by NBFA is accompanied with two kinds of synergies mechanisms.At last,the Hg0 adsorption properties of NH4Br-AC in the presence of N2,O2,SO2,NO and simulated flue gas were investigated.The FAC and NBAC were characterized by N2 adsorption/desorption,Fourier transform infrared spectroscopy(FTIR),and scanning electron microscopy/X-ray energy dispersive spectrometer(SEM-EDX),which provided the scientific basis for the experimental results analysis.Then TPD were carried out on pure mercury compounds,the mercury adsorption products of FAC and NH4Br-AC,respectively,to study the mercury species on the surface of the samples.The experimental results showed that the Hg-OM(Hg Organic Matter)was formed by the reaction between Hg0 and organic matter which existed on the surface of AC.Though some Hg0 could be oxidized by the functional group.to form HgO,the efficiency is very low and the penetration rate was up to 94%.The embedding of bromine atom increased the activity of its neighboring site which in turn increases the adsorption capacity of Hg0,which explained why the modification promoted the mercury removal perfectly.TPD results indicated that large amount of HgBr2 was formed on the surface of NBAC.The single flue gas component O2 and NO promoted the adsorption of mercury because of Hg2(NO3)2 formed on the surface of NBAC.SO2 competed with Hg0 on the NBAC ester group,which inhibited Hg0 adsorption.Additionally,the mechanism of Hg0 removal by NBAC was similar to the UBC contained in NBFA,which was mainly affected by NO in the flue gas and followed Langmuir-Hinshelwood mechanism.