Multi-pollutants simultaneous removal from flue gas

Multi-pollutants simultaneous removal from flue gasGao Xiang, Wu Zuliang, Luo Zhongyang, Ni Mingjiang, Cen KefaState Key Laboratory of Clean Energy Utilization of Zhejiang University, Hangzhou 310027, ChinaAbstract: The multi-gtages humidifier semi-dry flue gas cleaning technology, the CRS plasma flue gas cleaning technolo-gas cleaning technology using multi-stages humidifier and additive can improve oxidation and absorption, and oeflue gas cleaning technology were investigated for multi-pollutants removal. The semi-dryhieve high multi-pollutants removal efficiency. The CrS discharge can produce many OH radicals that promote NO oxidation. Combining NaOH absorption can achieve high deso, and deNO, efficiencies. It is fit for the reconstruction ofprimary wet flue gas desulfurization(WFGD ). In addition, using NaC102 as additive in the absorbent of WFGD can ob-Key words: simultaneous removal; semi-dry plasma; additiveIntroductionprocesses,e.g, dust collection by electrostatics pre-cipitation(ESP), $, reduction by wet flue gas desulCoal combustion produces various atmosphere pol- furization( WFGD ) and NO, reduction by selectivelutants such as dust, SO2, NO,, heavy metal, etc. catalytic reduction(SCR). Fig. I shows the traditionalThrough long-term investigation and engineering prac- pollution control system from flue gas. Subsequentlytice, the pollution problem about dust, So2 and No, high investment cost, large installation space and com-has been solved to a certain extent in China. For their plex craft system will appear certainly. This is a spinyremovals, the commonly adopted pathway is that these problem for pollution controlpollutants are treated respectively using differentESPer removal devices(de-dust)(Hg. PCDDs, PCDFs.)He PCDDsFig1 TraditionTo overcome the shortcoming of traditional combi- electron beam 15). But most technologies are in thetion pollution control method, simultaneous removal demonstration stage due to unripe craft, high energytechnology for two or more pollutants from flue gas was consumption or high investment. So developing a highbrought forward. Currently, developing these technolo- efficiency, stable-operation and inexpensive multi-pol-gies has become a research hotspot at home and lutants simultaneous removal technology is very press-abroad. The simultaneous removal technologies focus ing and necessary. The fundamental theory researchon SO, and No, mainly. The concrete technologies an中国煤化工 ys go on theas follows: active carbon SNOX. SNRB. NOXSOX. seCNMHGts simultaneousVol 8 No. 1. Mar 2010 27moval using additives or plasma. In the paper, someSchematics ostage humidifiercurrent research results will be discussed2 Semi-dry multi-pollutants flue gas2cleaning technologyIn the semi-dry flue gas cleaning technology formulti-pollutants simultaneous removal, acidity pollMoisture contentCa(OH)2-based absorbent from flue gas and conversedinto saline materials. In addition due to the activesurface of absorbent, NO,, heavy metal and other polon: 2--Three-stage humidified.- stage humidification, 4--Single-stage humidi-lutants are absorbed through some physic and chemicalactions. Composite additives with multi-componentswo-stage humidification; 9--lonic reaction stage for theand high activity can oxidize NO to NO,, and prolonge-stage humidificationthe time of liquid phase ionic reaction. At the meanFig 2 Multi-stages humidifierime, due to the developed stoma configuration andhuge inner surface area of absorbent and assistant ac-and accelerates pollutants absorption. In addition, oxi-tive carbon, heavy metal and organic compounds are dative additive oxidizes No into No, which can be neu-and removeed. Eventually, most pollutants de-tralized by alkali matter. Furthermore, the modifiedposit in the outcome ash collected by dust precipitator. absorbent can improve removal efficiency of mercuryhemical reactions in absorber can be divided intotwo stages: constant reactions and deceleration reac-effectively. The specific surface area and porosity ofabsorbent increases from 22 mtions. In the constant reaction stage, absorption rate ofSO2 is high and falls slowly with time. Therefore, when53 % to 63 respectively as hygroscopic additive ratiois from 0 to 1 % The microcosmic characteristic ofother parameters(inlet temperature, inlet SO, concentration, Ca/s and circulation rate)are the samemproving absorbent is good for its capability of purifi-multi-stages humidifier Fik 2)are used to distributecation. Composite absorbent with multi-component andwater reasonably to avoid over-humid absorbent locallhigh activity and multi-stage humidifier were usedand prolong constant reaction stage and promote desocontrol multi-pollutants simultaneously. SO2, NOperformance. The effect of humidifier staand Hg can be reduced effectively. From Fig 3, theremoval efficiencies of So,, No. and Hg can reachperformance is shown in Table 1. The results indicat95.7 %, 41. 11 and 73. 1.% respectivelyat aiency. The multi-stages humidifier also can avoid the 3 Plasma multi-pollutants flue gas cleaningale formation inside the absorber. Considering thetechnologyfactors of drop collision and system complexity, two orPlasma has an important role in treating the com-three-stages humidifier is recommended in the actualplex flue gas pollutants. It expresses a special ability ofTable 1 The deso, efficiency under differentnon-selectivity, high reduction efficiencies and rapidhumidifier stageschemical reaction, so using plasma to treat various gas-eous pollutants at one time have beening gaining moreand more attention recently. In order to improve energyefficiency, Chang, et al. brought forward a corona rad-89.2ical shower(CRS)system 69. A nozzle electrode wasused as a discharge electrode instead of corona wire491.5Under strong electric field, the stable and intensive co-rona can be formed near the nozzle. What's betterThe microcosmic characteristics can be promoted the high-energetic electrons mainly collided with addiby increasing its specific surface area and porositytional gas from nozzle, which makes the energy loss ob-through hygroscopic and oxidative additive. Hygroscop- o中国煤化工 nology was chosenic additive prolongs the time of liquid drop evaporation for fI in our researchCNMHGProportion of A additive°Efficiency 95.7Ficiency 41.11%Fig 3 Multi-pollutants removal using semi-dry flue gas cleaning technologyThe diagnosis of the produced radicals is important to know the reactive mechanism clearly. The excitn, dischargeair dischargeed OH radicals have strong UVemission arou-Simulationam 10, ] The interference from excited N, emission ataround 309 nm can be neglected).Therefore,theOHAE一Xgeneration and distribution of OH(A) radicals were in-vestigated based on the OH(A2Σ’→X2∏,00)emission peaked at 309 nm. Fig 4 illustrates recordedemission spectra from 300 nm to 320 nm with humid315320Wavelength/nmair and humid N, as background gas. The additionalgas was N2. The observed zone was at the exit of theFg4 Emission spectra of OH(A2Σ*→x2∏,040nozzle. The largest intensity was normalized to 1. Theapplied voltage was 20 kV. An OH emission spectra with LIFBASE. The corona discharge became easiersimulated with IFBASE was also showed in Fig. 4. when Ar was added and the streamer became filament-The peaks around 309 nm are from OH(A2'-+X, ry. From the figure, it can be seen that OH emission0-0)and the peaks from 314 nm to 318 nm are from intensity( around 309 nm) is stronger when Ar is add-the second positive band of N2C∏。→B'∏n). From ed into the discharge region as additional gas compa-the figure, it can be seen that the observed OH emis- ring with n, This result was in agreement with thesion spectra generated with humid N 2 as background vestigation of Kocik, et all i4. The effect of Ar on OHgas can be well reproduced with LIFBASE. The OH geneon was obvIousThe peaks from 314 nm to 318(A) generated in humid N, was substantially more nm are the second positive band of N2(C'l.-than that with air. The result was accordance with B'I,). However, when oxygen was sprayed as addi-Ono, et al. and can be explained by the electro-nega tional gas, the OH emission intensity decreased compa-tivity characteristic of oxygen which makes OH decay ring to the one with Ar/ H, 0 as additional gasveryquickly)OH emission spectra under different additional ga- and ain our research, the simultaneous removals of SO2e achlegses are shown in Fig. 5. The background gas was Nshe中国煤化工entration after theThe additional gas was Ar/02/H20 and N2/H,0 CICN MH Grption(thecurve(RH =80 %) The applied voltage was 15 kv. The signed with AB-NO, ) The No, concentration dropsdash lines represent OH emission spectra simulated after the flue gas passes through the absorption bottleVol 8 No. 1. Mar 2010 29e gas cleaningWFGD is a most commonly applied method for(c∏BTsO2 removal from flue gas. But it hasn t obvious func-tion to No, removal. So the method of adding some ox-ward. The oxidative additives can converse No into305NO,, which can make NO. absorbed easier in the rou.wavelength/nmAr09Itine absorbent. This is very promising for the reforma-L/mintion of WFGD equipment. The spraying tower is thetype that is used widely in WFGD systeSimulations applicationFig5 OH(AY-X, 0-0)emission spectrasimultaneous removal based on WFGD system, weunder different additional gasesstudy multi-pollutants simultaneous removal in a spraunder different reaction parameters were studiedSsesying tower. Simultaneous deso, and deNo, procee absorbed by the naOHtion. The transition curve of N(. concentration is analLiquid-gas ratio is one of the most important fac-ogous with that of NO concentration, this proves that tors in the WFGD. We investigate the effect of liquidsolution absorbs NO, completely and dis-gas ratio under the additives of 0. 03 mol/L KMnOsolves NO in small quantities. Due to being absorbeand 0. 03 mol/L NaC1O2. From the Fig.7,withby the NaoH solution, the nO, decreases to some ex-growing of liquid-gas ratio, deNO efficiency increasestent. Apparently, the overall NO, reduction rate in-a little while desO, efficiency is almost invariant. Increases as the discharge power increases. With a coro-the experiment of KMnO. as additive, the competitivena power of W. 75 of the No, is reduced corre- reaction of So, and NO, could occur. But in the expesponding to a relative humidity of 68iment of NaClO, as additive soal evidently. The reason could be that the addition ofsO2 increases concentration of sulfite ion in liquidwhich is helpful to improvement of No, removal 7+ SO, (B, Br,deso, and deNo)NO(B, deso, and de)s=0.03M亠NO(B,deNoso, "1000 ppm +NO(B, deSo, and deNO)a)No reduction under 68%RHPH-5.5,125C,4 3s B,: KMnO. B Nacio,Fig. 7 The effect of LG on deso, and deNoFrom Fig. 8, deNO, efficiency increases slightlywith theof Iwhile deno, efficiency decreases a little under the Na-(b)deNo efficiencyC102 as additive. The result is consistent with that inFig 6 NO, removal in ththe bubbling reactor. DeNO, efficiency decreases fromCRS and NaoH absorption99中国煤化工 ges from4 4mo7.hC N MH Operation value inthe actual application is within the experiment scopethe change of pH during the operation of WFGD systemwill not influence deNO, process. It is an important ad-NO, control technology from flue gas [J]. Guangxi Electric Power,vantage of using WFGD system of spraying tower for2003,26(4):6468simultaneous deso, and deNO[2] DOE/FTSNo TM-flue gas cleaningmonstration project, DOE/FE 0395[R]. KNO, ville: ABB En-ronmental System, 1999[3] DOE/NETL. SO, NO, -RO, BOx TM flue gas cleanup demonstra-on, DOE/NETL-2001/1135[R]. Pittsburgh: National Ener-gy Technology Laboratory, 2000[4 Zhong Qneering Instance from Coal-fired Flue Gas[ M]. Beijing: Chemicaldustry Press, 2002[5] Radoiu M T, Calinescu D IM L. Emission control of So, and NO,by iradiation mehods[ J]. Joumal of Hazardous Materials, 200397(13):1454158.[6Kanazawa S, Nomoto Y, et al. NO, removal by awith mole- plate electrode corona discharge system[J]. IEEErans. on Indus. Appli., 1994, 30, 856-861Fig8 The effect of pH on deNo[7]Ohkubo T, Kanazawa S, Nomoto Y, et al. Time dependence ofNO, removal rate by a corona radical shower system[J]. IEEETrans. on Indus, Appli., 1996, 32, 1058-10625 Conclusion[8]Kanazawa S, Chang JS, Round G F, et al. Removal of NO, fromflue gas by corona discharge activated methane radical shower[ J]he flue gas multi-pollutants simultaneous removaJournal of Electrostatics, 1997, 40&41: 651-656is an inevitable trend in the field of flue gas purifica- [9] Urashima k, Chang JS, Park JY, et al. Reduction of NO, fromtion. The multi-stage humidifier semi-dry technologynatural gas combustion flue gas byCRS plasma technology and additive oxidization technol-on techniques[ J ] IEEE Trans. on Indus. Appli., 1998, 34ogy show a good foreground for flue gas multi-pollutants [10] Enhov A, Borysow J. Dynamics of OH(X, l, v=0)in highsimultaneous removal according to our previous re-energy atmospheric pressure electrical pulsed discharge[J]. Jsearch. The semi-dry flue gas cleaning technology using (1 Falkenstein Z. The influence of ultraviole illumination on OHcirculating suspension and multi-stage humidificationformation in dielectric barrier discharges of Ar/O,/H,0: thehas found large-scale industry application. It can a-Joshi effect[ J]. J. Appl. Phys., 1997, 81: 7158-7162.removal efficiency[12] Wang W C, Wang S, Liu F, et al. Optical atudy of OH radical inmulti-stage humidification and improving additive. Thewire-plate pulsed corona discharge[J]. Spectrochim. Acta.ACRS of oxygen as additional gas combining alkali solu006,63:477482[13] Ono R, Oda T. Measurement of hydroxyl radicals in pulsedtion can achieve high deso, and deno efficienciesna discharge[J]. J. Electrostat., 2002, 55: 333-342.Furthermore, it is fit for the reconstruction of primary [14] Kocik M, Mizeraczk I, Kanazawa S, et al.Observation ofWFGD. In addition, the additive oxidation technologyound-gtate OH by LIF technique in DC nozzle-to plate positiusing NaCIO, can obtain very high removal efficiency ofstreamer coronas J).[AS2004, 2004, 1: 244-249SO, and NO, because the sulfite ion formed in the de. [15] Takeuchi H, Ando M, Kizawa N. Absorption of nitrogen oxidesin aqueous sodium sulfite and bisulfite solutions[ J]. Ind EngSO2 process can improve NO, removalChem Process Des., 1977, 16: 303-308References[1] Peng H H, Hu H Y, ZhaoG C, et al. The review of so, andAuthorGao Xiang, male, was born in 1968. He obtained Ph. D. degree from Zhejiang Universitya professor in thermal energy engineering. Mr. Gao has published over 60 papers and obtained 10 invent patents innear 5 years. His current research is multi-pollutants removal from flue gas, etc. He can be reached by E-mailxga@zju.edu.cnFoundation item The work is supported by NSF of Zhejiang(Y507079), EOP of Zhejiang(Y200702725)andPSF of China(20080431325)中国煤化工CNMHGvol.8No.1,Mar.201031