SO2/Hg removal from flue gas by dry FGD

Intemational Jourmal of Mining Science and Technology 22(2012)107-110Contents lists available at SciVerse Science DirectInternational Journal of Mining Science and TechnologyELSEVIERjournalhomepageswww.eisevier.com/locate/iimstSO2/Hg removal from flue gas by dry FGDWang Fan", Wang Hongmei, Zhang Fan, Zhu Jinwei, Tian Gang, Liu Yu, Mao JixianChinese Research Academy of Environmental Sciences, Beijing 100012. ChinaARTICLE INFOABSTRACTAmide hTo study the mechanism of SO, and Hg removal from flue gas, an experimental packed bed reactor wasReceived 1 April 2011designed to simulate the dry FCD, where a mixture of lime and fly ash in ratio 1 3 w/w was used as thAvailable online 20 March 2012he activation time set to 20 min. The experimental factors including the SO2/Hg sorbent characteristics,50% breakthrough time for SO2/Hg removal sorbent packed bed depth and reaction temperature wereinvestigated. The experimental results show that after steam activation, the Bet specific surface arand specific pore volume increased from 37. 8 to 45.5 m2/g and from 0.42 to 0.51 cm, respectivelyWith activation of the sorbent by steam, the 50% breakthrough times of Soz and Hg removal increasedfrom 34 to 42 min and from 23 to 45 min, respectively. when the packed bed depth was increased fromSorbent activation5 to 25 mm, the 50% breakthrough times for Hg and So2 removal increased from 12 to 52 min and from 6to 47 min, respectively with the increase of the reaction temperature, the 50% breakthrough of So2/Hgremoval decreased accordingly. Steam activation can efficiently improve $O2/Hg removal simultaneously.o 2012 Published by Elsevier B V. on behalf of China University of Mining Technologyas soz sorbent. fly ash and spent So adsorbents are recovered asaddictive sorbents and in the process lime is converted into Ca(oHhCoal utilization is recognized as the major source of So, and Hg by steam to achieve higher SO2 and Hg removal efficiencies [18, 19).mission, contributing over 1/3 of total anthropogenic merocury To simulate the dry FGD process, experimental studies wereemission in China(1 Mercury is one of the most toxic heavy metal ducted to investigate the mechanism of soz and Hg removalelements and is severely dangerous to human health and environ- factors including 50% breakthrough time sorbent packed bedment. Mercury has aroused a global concen due to its toxicity. and reaction temperature were examined. The experimental resultsand various mercury emission control methods have widely at- might be effectively applied to dry FCD in incinerators or powertracted interests from many researchers [2-6 ] The current research plants for So2 and Hg removal.results show that the commonly used flue gas purification equiments for coal-fired boilers such as electrostatic precipitator(ESP) 2. Experimentaland wet Alue gas desulfurization system( WFGD) are beneficial tomercury emission control [7-9The experimental packed bed reactor was designed with aIt is accepted that dry FGD technology is capable of reducing Hg 40 mm diameter and 800 mm length inner cylindrical quartz tubeemission, and conventional dry FGD process is typically employed held in a vertical position. the simulation flue gas consisted of Oz,in power plants that bums low to medium sulfur coal as fuel, CO,, SO,, N, and mercury vapor the schematic diagram is shownwhere calcium-based desuifurization agent (lime slurry or hy- in Fig. 1. The packed bed reactor was designed capable of beingdrated lime)is sprayed or injected into the desulfurization reactor. heated to the temperatures between 75 and 100C. The velocityTo improve the SO2 removal efficiency. spent SO2 adsorbent and fly of the flue gas was set 1 m/min, and the retention time of approx-ash are recirculated as addictive sOz adsorbent 10-15 Related re- imately 0.8s.search achievements show loi (Loss on Ignition) contained in flyMercury vapor was generated by adjusting the temperature of aash and used as cheap Hg adsorbent can improve Hg adsorption U-shaped mercury permeation tube(vIc Metronics ), and pure16-19gases of O2, COz, Soz and Nz were released and mixed as a simula-A new dry FGD technology has been developed in this study, tion flue gas before entering into the packed bed reactor. The outletwhich is a co-benefit approach for Hg reduction. where lime is used flue gas was analyzed with flue gas analyzer and Hg CEMS, and exhaust gas was discharged after being cleaned by activated carbon.凵中国煤化工-ME 9106 and detectingailaddress:fanwangsd@yahoo.com(Fwang)the so,zCNMHGfront matter o 2012 Published by Elsevier B.V. on behalf of China University of Mining technology0. 1016/j-jmst201106.011F Wang er al/mtemarional Joumal of Mining Sciemce and Technolog Z2 (2012)107-110Fig. 3 shows the SEM images of SO2/Hg sorbent before and aftersteam activation, respectively. Comparison analyses show thatmore minute pores appeared on the surfaces of So/Hg sorbentafter steam activation, the Bet specific surface area significantly in-creased from 37.8 to 45.5 mig, while the specific pore volumecreased from 0.42 to 0.51 cm/g. this show that steam activationcan improve SO2/Hg removal efficiency simultaneously, sinceute pores can enhance the adsorption of SO2 and HgThe experimental studies for examining the son and Hg concetration were conducted according to Fig. 1, where the inlet concentrations of Soz and Hg were 1500 mg/ Nmand 20 Hg/NmHR CEMSrespectively. The temperature in the packed bed reactor was con-trolled at 120oC with a precision of #o5C, and the SO2/Hg sorbentpacked bed depth was set to 20 mm. Fig. 4a depicts the breakthrough curve of SO2, the breakthrough curve of So2 removal post-poned with activation of the sorbent and 50% breakthrough timeappears at the 34 and 42 min for the sorbent without steam activa-tion and with steam activation were obtained respectively. whileFlE 1 Schematic diagram of the So,/ Hg removal system.Fig. 4b depicts breakthrough curves for Hg removal, which showsthat 50% breakthrough time for Hg removal appears at the 23and 45 min for the sorbent without steam activation and withsteam activation, respectively.Electric heaterFixed bedThe experimental tests were conducted accorbed. thethe packed bed reactor was controlled at 120C. Thetrations of so, and Hg were 1500 mg/Nm and=The relationship between depth of packed bed and $o2/HgFig 2 Steam sorbent activation systemoval efficiency was studied. Fig. 5 shows that the SO2 and Hg removal efficiency increased with increasing of the sorbent packebed depth. When packed bed depth was increased from 5LUMEX RA-915 by using atomic absorption spectromet25 mm. the 50% breakthrough time for Hg removal increased fromused for continuous measurement of mercury concentratio12 to 52 min while that of So2 removal increased from 6 tothe detection limit was as low as below O 1 ug/Nm. Since th47 min, therefore, it can obviously be deduced that packed bedcentration of SOz is below 10 x 10- after adsorption by=depth can influence efficient removal of both SOz and Hg from fluebed, any interference from SO2 was considered negligiblThe mixture of lime and fly ash in a ratio of 1: 3 w/w was employed as So 2 and Hg sorbent. the lime was sampled from a limekiln,while fy ash was collected from the baghouse filter of a50 MW coal-fired boiler. Table 1 shows chemical compositions of 3.3. Impact of reaction temperature on SO/Hg reductionthe so/Hg sorbent.The experimental sorbent preparation steam activation temperatures were set at 75, 87.5, 100, 115.5 and 130 C and the packedbed depth was 20 mm. The inlet concentrations of Soz and Hg were1500 mg/Nm and 20 ug/Nm, respectively3. 1. Influence of sorbent on SO, Hg removalIt should be noted that increasing the reaction temperature fordry Fgd decreases the SO2 removal efficiency accordingly. Fig. 6Fig. 2 shows the SO, /Hg sorbent activation system, where a provides a relationship between the reaction temperature andmixture of lime and fly ash in a ratio of 1: 3 w/w wasin 50% breakthrough time for both SOz and Hg removal. Howeverquartz tube and activated by steam at 100oC. During the activation the curve of Hg removal drops faster than that of So removal. Thisocess, the quartz tube was heated to 100.C for preventing forlearly shows that the reaction temperature obviously had moremation of water droplets. The SO2/Hg sorbent activation time influence on Hg removal. Hence for dry FGD, to attain high Hg re-was set to 20 minmoval efficiency the reaction temperature should be decreased.Table 1Chemical analysis results of fly ash and lime (X中国煤化工wSiO)w(Fe,0)wTIO2lIncCNMHGFly ash63321.046.3251,46F. Wang et aL/Intermational Journal of Mining Science and Technology 22(2012)107-110孩钙(a) Beforeb)AfterFig 3. Sorbent before and after steam activation.160Without steumalvarionReacting time(minReacting time(min(a)So2(b)HFig 4. Relationship between reaction time and concentration of O2 and Hg outlet.4. Conclusion(1)The experimental results show that after sorbent activationby steam, minute pores appeared on the surface of the sor-Evet, the Bet specific surface area increased from 37. 8 to. 5m/g. while the specific pore volume increased from0. 42 to 0.51 cm'ig(2)The breakthrough curves for both So2 and Hg removal pponed after sorbent activation by steam, i.e., the 50% breakthrough time of So2 removal increased from 34 to 42 min,while that of Hg removal increased from 23 to 45 min.(3)When the packed bed depth was increased from 5 to 25 mm.Depth of packed bed (mm)the 50% breakthrough time for Hg removal increased from12 to 52 min, while that of Soz removal increased from 6Fig. 5. Influence of packed bed depth on $O2/Hg removaL.to 47 min(4)The reaction temperature influences the performance of So2and Hg removal respectively, and more obvious influenceappeared for Hg remov55e SO breakthroughThis work was supported by the National High-Tech R&D Pro-gram of China(No. 2008AA06Z318)and thery of Environmental Protection of China(Nos. 20100904800909025)Referen中国煤化工[11 JiarCNMHGReacting temperature(C)[21 Wangso Da tsr in cet a. Atmos chemo PhysFig. 6. Influence of reaction temperature on SO2/Hg removal.2010:10:1183-92F Wang et aL/intemational Joumal of Mining Sciemce and Technology 22(2012)107-110l3 Tang TM.‰ J. Lu RI. wo IⅪuⅪH. Enhanced Hg" removal and Hg[12] Shih SM. Charaemission control from wet fuel gas desulfurization liquors with additi[4] Wu CL Cao Y, He CC, Dong ZB, Pan WP.1131Zhao Y Liu ST Ma XY, Yao]. Experimentaltion on mercury adsorptiocharacteristics by modified Ca-based sorbent. Proc CSEE 2009: 29(8): 50-4 [ in[5 Zhao YC. Zhang Y, Liu Diaznano[14] Wang LG, Peng SP Chen CH. The experimental study to Hg adsorption of fly161 Lopez-Anton MA, Abad-valle P. Diaz-Somoano115] Gao HL Zhou JS, Luo ZY, wuEffect of sulfur dioxideeciation of mercury in flue gases. A161 Hwang Y Sun X. Liz Unburned carbon from fy ash for mercurycontrol202060et FGD [171 Antonia LA, Mercedes DS, Patriciaxidation with20108911)3167-77[9] Wang Y]. Duan YF, Yang LG, Jiang YM, Wu C). Wang Q et al. Comparison of [181 Lin HY. Chen wC. Yuan CS, Hung CH Surface functional characmercury removal characteristblack before and after steam acti(10) Garea A Vigur JR.Manage Assoc 2008; 58(1): 78-8A Kinetics of flue gas des[191 Ffy ash/calcium (3/1) sorbent behaviour.N, Antonio T Gian LV. Edward JA, Piero S. Steam hydrationreactivation of FBc ashes for enhanced in situ desul97:52(5}:715-32009:88(6}1092-8[111 Sanders JE, Keener TC. Wang ]. Heated ny ash/hydrated lime slurries for SOremoval in spray dryer absorbers. Ind Eng Chem Res 1995: 34: 302-7中国煤化工CNMHG