Experimental and mechanism studies on simultaneous desulfurization and denitrification from flue gas

Science in China series b: Chem2007in China presser verlaExperimental and mechanism studies on simultaneousdesulfurization and denitrification from flue gas using aflue gas circulating fluidized bedZHAO Yi, XU PeiYao, SUN XiaoJun& WANG LiDongSchool of Environmental Science Engineering, North China Electric Power University, Baoding 071003, ChinaThe oxidizing highly reactive absorbent was prepared from fly ash, industry lime, and an oxidizingadditive M. Experiments of simultaneous desulfurization and denitrification were carried out in a fluegas circulating fluidized bed (CFB). the effects of influencing factors and calcium availability were alsoinvestigated on the removal efficiencies of desulfurization and denitrification Removal efficiencies of95.5% for So2 and 64.8%for No were obtained respectively under the optimal experimental conditionsThe component of the spent absorbent was analyzed with chemical analysis methods. The results indicated that more nitrogen species appeared in the spent absorbent except sulfur species. A scanningelectron microscope(SEM)and an accessory X-ray energy spectrometer were used to obmicro-properties of the samples, including fly ash, oxidizing highly reactive absorbent and spentabsorbent. the simultaneous removal mechanism of so, and No based on this absorbent was proposed according to the experimental resultsflue gas circulating fluidized bed, simultaneous desulfurization and denitrification, oxidizing highly reactive absorbent, mechanismIn recent years, the interests in combined desulfurization The technology of flue gas desulfurization with flueand denitrification from flue gas increased rapidly. gas CFB was firstly proposed by Lurgi Lentjes Bisch-Many technologies have been proposed, among which off(LLB) Company in Germany and developed rapidlythe stage treatment technology is considered to be a ma- by the comprehensive investigation for the process andture one. In this traditional technology, a separate NO theation of engineering practice in the past 20control system, e.g., the selective catalytic reduction years. It has achieved increasing commercial application(SCR)or selective non-catalytic reduction (SNCR), all over the world as it reduced the cost of investmentshould be installed to the back of the desulfurization and running to 50%-70% of that for wet process. Theequipment. Although it succeeded in combined removal large-scale installation of flue gas CFB has been appliedof the SO2 and NO, it is not easy to achieve wide indus- in the world and China, however, this technology lackstrial application because of the large occupying area and the ability of simultaneous denitrification.high running cost. To reduce the cost of flue gas purifiThe research results show that the key point for si-cation,development of new technologies and equip- multaneous desulfurization and denitrification techniquements of simultaneous flue gas desulfurization and deni- in CFB is to oxide NO into NO2 rapidly in the flue gas,trification has become the leading research direction in the latter is easily soluble in water. There is less literathe air pollution control field. There are many investigaReceived April 29, 2005; accepted May 22. 2006tions in the world but most of them have technical and doi: 10. 1007/s11426-007-0013-0economic defects. and cannot develop to practicable Corresponding author (email: zhaoyi 9515@163. corSupported by the Significant Pre-research Foundation of North China Electric PowertechnologiesUniversity(D03-035中国煤化工www.scichina.comwww.springerlink.comSci China SerHCNMHG11135144An investigation of simultaneous desulfurization anddenitrification with oxidant/Naoh solutions was carriedout by Chu and Chien et al. - A technique to oxidizeNO rapidly by adding hydroxyl radicals into the fluewas proposed by Hori. Based on the researches fomany years, the oxidizing highly reactive absorbenthas been exploited, of which the capability of simultaneous denitrification and denitrification was verifiedfirstly by fixing bed and duct injection., and the satis-factory results were obtained. The experiments of si-multaneous desulfurization and denitrification withsimulated flue gas were done in a self-designed flue gas Figure 1 The experimental apparatus of flue gas CFB system.1,AirCFB, and the higher removal efficiencies for SO2 and inlet; 2, steel bottle of Soz: 3, steel bottle of No: 4, glass-rotor flow meter,5. buffer bottle: 6. electric heater: 7. fluidized bed reactor :8. water tank: 9NO were achieved. The removal products of SO2 and high-pressure pump; 10, spray nozzle; Il, screw material-fed machine; 12,NOx contained in spent absorbent were analyzed with vortex dust remover; 13, material-circling leg: 14, gas analysis instrumentchemical analysis methods. The micro-properties of fly15, induced-draft fan; 16, ash hopper.ash, oxidizing highly reactive absorbent and the spentabsorbent were characterized by a SEM and an acces- sulting from a high-pressure pump were sprayed into thesory X-ray energy spectrometer. The mechanism of reactor from the bottom of the reactor to adjust the adedesulfurization and denitrification based on the oxidiz. quate flue gas humiditying highly reactive absorbent was studied. The innoThe concentrations of So2 and NO, at inlet and outletvative research results have significant academic sense were determined by a flue gas analyzer(MrU 95/3CDand application valueflue gas analyzer, Germany) hand-running. The flue gashumidity was measured by the flue gas analyzer. The1 Experimentalflue gas temperature was measured with thermocouple1. 2 Preparation of the oxidizing highly reactive ab-1.1 Experimental system and the experiment ofsorbentmultaneous desulfurization and denitrificationThe oxidizing highly reactive absorbentExperiments of simultaneous denitrification and desul- from fly ash, industrial lime and oxidizing additive Mfurization were carried out in a self designed experiwhich was described in refs. [5, 6]. The preparing proce-mental system, as shown in Figure 1. The key part of thedure is as follows: The mixture of oxidizing additive mfluidized bed reactor is a vertical cylinder with theand water, including fly ash and industrial lime at thelength 4500 mm and inner diameter 250 mm, on whichseveral temperature station points are locatedratio of 3: I in weight, was stirred and digested at 363 KIn this experiment, the gas mixture containing SOand dried after six hours. Therefore, the oxidizing highlyreactive absorbent was obtainedNO, H2O and air was used to simulate actual flue gasand heated by an electric heater, in which SO, and NO 1.3 Definition of removal efficiencies, characterizacame from each cylinder. The experiment system oper- tion and analysis of removal productsated at the negative pressure by using an induced draft The removal efficiencies are calculated from the meas-fan. The pressure drop was 800 Pa in the reactor.ured concentrations of So, and no at inlet and outlet. aThe oxidizing highly reactive absorbent was fed to SEM(KYKY-2800B type, Scientific Instrument Corpothe reactor by a screw feeder. The feed-in amounts were ration of Academy of Science in China)was used to obcontrolled by adjusting the divergence of the feeder. serve the micro-structures of fly ash, oxygen-enrichedMost of the solid materials discharged from the reactor highly reactive absorbent and the spent absorbent, thewere collected by a cyclone cleaner and re-circulated composition of oxygen-enriched highly reactive abinto the reactorsorbent and the spent absorbent was determined by anDuring the operation, the atomized drops of water re- accessory X-ray energy spectrometer( Vantage DIS type,136ZHAO Yi et al. Sci China Ser B-Chem I February 2007 I volTH中国煤化工CNMHGThermo nOran Company in USA)2. 1.3 Calculation of the feed-in amount of absorbentThe contents of sulfate. sulfite. nitrate and nitrite in The feed-in amount of the absorbent is calculated fromthe spent absorbent were analyzed using the methods ofchemical analysis. Sulfate and sulfite were determinedby barium chromate photometry Nitrite was determinedby N-(1-naphthyl)-ethylenediamine photometry, andnitrate was determined by the process of the reduction of6C mg 10g l mol 1 Ih°m3mg30g260minzinc powder.74g4100+x1.4 Circulating flow rate, circulating multiple andnol0.9100100+xthe concentration of solid materials in the reactor=n(8.560Cso+9.13QCN0)The availability of the absorbent in the reactor is char- where n= Ca/(S+N), x is the mass percent of the addiacterized by circulating flow rate, circulating multiple tive, g is the gas flow rate in m/h, Cso, is the concen-and the concentration of solid materials in the reactor. tration of SO, in mg/m, and CNo is the concentration ofCirculating flow rate(kg/(m s)is obtained by measurement and calculation. The concentration of solid ma-terials in the reactor (kg/m), equal to the ratio of the2. 1. 4 Calculation of availability of calcium. Thesolid materials mass to volume of the reactor is calcicalculating formula for availability of calcium in fluelated. Circulating multiple is obtained by expression of gas simultaneous desulfurization and denitrificationGs Almds, where Gs is circulating flow rate, A is the sec- obtained referring to that of flue gas desulfurizationtional area of the reactor. and mde is the amount of ab- Availability of Casorbent. All of the three parameters can be adjusted by a Reacted amounts of Ca in desulfurization and denitrificationscrew feederAdded amounts of ca in absorbentOsO2 Results and discussion2.1 Calculation methodsCNO.n2. 1.1 Calculation of Ca/(S+N. In flue gas desulfurization technology, an important index Ca/s is usually5CSO, . YSO. +I6CNNoNO(4)used to evaluate the utilization of absorbents and serves15C0,+16CN0as a critical technological parameter to determine theddition of absorbent. As flue gas simultaneous desul2.2 Effects of the running parameters onrization and denitrication process in this paper, Ca/(S+Ndesulfurization and denitrification in flue gas cFBis used instead of Ca/s and expressed asEffect of content of m additive on removal effi(1) ciencies for SO2 and NO. In order to get the optimalwhere nI and n2 stand for the molar weights of calciumdditive quantity of M, the comparing experiment ofoxide in absorbents and the mixtures (SO2+ 1/2NO)in desulfurization and denitrification was done. The ex-flue gas, respectively. The definition of stoichiomperimental results are shown in Figure 2relationship is based on the expected removal productsThe experimental results show that the content of Mmainly including CaSO /CaSO4 and Ca(NO3)2/Ca(NO,) has no obvious effect on desulfurization, but has a sig-in the spent absorbentnificant effect on the removal of no. the efficiencies ofdenitrification increased rapidly as the content M in2. 1.2 Removal efficiencies for SO, and NO. Thecreased at lower content of m. and reached maximum atremoval efficiencies for SO2 and NO are calculated from 1.6%(wt) M, and then the efficiencies increased slowly100%(2) with the content of M, because M is highly dispersed onthe surface of the prepared absorbent during the coursewhere"”' can be SO2orNO,"in”and"our” stand for oflation which made it contain abundant oxidiz-inlet and outlet, respectivelying-point. The removal reaction was primarily on theZHAO Yi et al. Sci China Ser B-Chem February 2007 volcontrol of the conversion from No to nO2 at lower content of M. with the content of M, the "oxidizing-pointand the conversion from no to No, also increasedgreatly, therefore the efficiencies of denitrification increased significantly. However, the"oxidizing-point"onthe surface of the absorbent tended to saturation whenthe content of M reached 1.6%(wt). Correspondingly,20the efficiencies of denitrification increased very slowlywith the content of m. The optimal content of M was1.6%(Wt)Residence time (sIFigure 3 Effect of the residence time on the efficiencies of simultaneoudenitrification annd desulfurization. The inlet temperature of flue gas influe gas CFB is 130C; the flue gas humidity is 6%(v). Ca/(S+N)is 1.2the content of additive M is 1. 6%(wt); the concentrations of SO2 and NOare 2965 and 963 mg/m, respectivelyflue gas was determined to be 2. 4 s in the experiment2.2.3 Effect of Ca/S+N) on removal efficiencies forSO2 and NO. Figure 4 shows the effect of Ca/(S+Non efficiencies of desulfurization and denitrificationcan be seen that the efficiencies of desulfurization andAdditive content(%)denitrification increase with the value of Ca/(S+N), andFigure 2 Effect of the content of additive M on the efficiencies of si- the removal efficiencies appeared to be manifest trend ofmultaneous denitrification and desulfurization. The inlet temperature of increase when the Ca/(S+N) lay between 0 and 1.2flue gas in flue gas CFB is 130C, the flue gas humidity is 6%(v); the However, the turning point of removal efficiencies occirculation multiple is 85: the concentrations of So, and NO are 3087 and curred at 1.2 Ca/(S+N), thereafter, the removal efficien1032 mg/m, respectivelycies stayed on nearly constant at the higher value whenthe Ca/(S+N lay between 1. 2 and 2.0. The optimal2. 2.2 Effect of residence time of flue gas on removal Ca/(S+N)is set to be 1. 2 according to Figure 4efficiencies for SO, and No. The removal efficienciesare affected by gas-solid contact and NO oxidization,100which is controlled directly by the residence time of fluegas in a flue gas CFB. The optimal residence tirflue gas was achieved by the experiment. The results areshown in Figure 3It can be seen from figure 3 that efficiencies ofdesulfurization and denitrification increase with theresidence time of flue gas in flue gas CFB, especially fordenitrification. This could be that the contact of flue gas10152.0Ca(N+S)with absorbents is more sufficient as an increase of theresidence time of flue gas, which leads to the rise of re- Figure 4 Effect of Ca/(S+N)on the efficiencies of simultaneous denitri-moval efficiencies. with removal of no. due to the lowfication and desulfurization. The inlet temperature of flue gas in flue gaswater solubility of NO, it can be absorbed hardly by the the flow rate is 400 m /h: the content of additive M is 1.6%wt): the con-bsorbent, therefore the oxidation of no to no? is nec- centrations of SO2 and NO are 3110 and 978 mg/m, respectivelyessary, which can be carried out by an increase of theresidence time of flue gas. However, excessive residence 2. 2. 4 Effect of circulating multiple on availability oftime of flue gas would increase the volume capacity of calcium. Compared with that of circulating flow rate,reactor and running cost. The optimal residence time of circulating multiple and the concentration of solid mate138ZHAO Yi et al. Sci China Ser B-Chem I February 2007 I volrials, the effect of circulating multiple on availability of drop but also the burden of solid-gas separator. Basedcalcium is more visual. The relationship between avail- on expression of the ratio of Ca to(S+N) and previousbility of calcium and circulating multiple is shown in work the feed-in velocity of the absorbent was calcu-Figure 5, with the flue gas CFB inlet at the concentra- lated as 2. 19 g/s, while the concentrations of SO2 andtions of SO2 3087 mg/mand No 1032 mg /m, respec- NO at the flue gas CFB inlet were 3087 and 1032 mg/mtively, Ca/S+N of 1.2mthe circulating flow rate was 3.8 kg/(m2s), and the circulation multiple was 852.2.5 Effect of inlet flue gas temperature on removalmoval efficiencies for So, and No removal as a func-tion of the inlet temperature of flue gas in flue gas CFBare shown in Figure 6. Satisfactory efficiencies wereobtained in the inlet flue gas temperature range from 120to 130C. When the temperature was higher than 130Cthe removal efficiencies decreased with the temperatureCirculating multipleThe optimal inlet flue gas temperature was selected atFigure 5 Effect of circulating multiple on Ca availability. The inlet 130C accordingly. In fact, removal for SO2 and NO is atemperature of flue gas in flue gas CFB is 130C; the flue gas humidity is semi-dry reaction process between porous solids and6%() the content of additive M is 1. 6%(wt); the residence time is 2.4 s: gases. The adsorption, absorption and diffusion of SO2and 1032 mg/m, respectively.and No on the surface of absorbent are affected greatlyby the flue gas temperature. When the temperature wasFrom the expression of calcium availability in Selower than a critical value, adsorption and absorption2.1.4. the efficiencies of desulfurization and denitrifica- were predominant and the efficiencies increased with thetion varied with the change of calcium availability in a temperature.. However, when the temperature of fluegiven concentration of SO2, NO and Ca/S+N in flue gas gas is higher than the critical value, the desorption of gasCFB. When circulation ratio was 1, the entering solid molecules on the surface of the absorbent enhanced andmateriel into flue gas CFB will entirely be discharged the equilibrium adsorptive capacity of gas decreasedfrom the reactor. Therefore the flue gas CFB experiment Therefore, the removal efficiencies decreased evidentlywas regarded as the same in the duct injection systemwith the temperature. There is a turning point for aband the flue gas CFB reactor became the duct in which sorption, adsorption and desorption when temperature ofreaction occurred among the absorbent SO2 and NOSince SO2, NO and the absorbent contacted only oncethe residual calcium in absorbent was not reused todesulfurization and denitrification and the absorbentshowed a low calcium availabilityThe concentration of solid materials in the reactor in-reased with the circulating multiple. Accordingly, thequantity of materiel taken out by flue gas would alsoincrease in unit time. Therefore. the absorbent can bediffused better in the reactor by air flow, which leads to130the increase of gas-solid contact area. The contact probFlue gas temperature℃ability between absorbent and reacting gas also in- Figure 6 Effect of the inlet flue gas temperature on efficiencies of deni-reased and the availability of calcium was promoted.trification and desulfurization. The flue gas humidity is 6%(v); theCa/(S+N)is 1.2; the residence time is 2.4 s; the flow rate is 400 m/h;theHowever the excessive concentration of solid materi-content of additive M is 1. 6%(wt); the circulation multiple is 85; concenals in the reactor would increase not only the pressure trations of So2 and NO are 2912 and 939 mg/m, respectivelyZHAO Yi et al. Sci China Ser B-Chem February 2007 volH中魏temperature in coal-fired power plant after dust removal. 6% due to humidity of practical flue gas exceeding o oflue gas reaches 130C, which closes to the flue gasThe flue gas humidity should be controlled at aboIt is convenient for flue gas simultaneous desulfurization according to Figure 7. The higher removal efficiencyand denitrificationcan be obtained2.2.6 Effect of flue gas humidity on removal efficien- 2.2.7 Effect of concentrations of sOz and No on re-cies for SO2 and NO. Flue gas humidity is one of the moval efficiencies for SOz and NO. The emissionimportant factors affecting removal efficiencies. The concentrations of SO2 and NO vary with the content ofexperimental results of denitrification and desulfuriza- the sulfur in burned coal and the operating conditions oftion are shown in Figure 7 in the humidity range from the boiler. Experiments of simultaneous desulfurizationto 8%. The results indicated that removal efficiencies and denitrification were carried out in order to examineincreased with the humidity, and markedly increased in the adaptability of flue gas CFB process for coal speciesremoval efficiencies as flue gas humidity increased at and conditions of the boiler(Figure 8). It can be seenthe lower humidity. However, increasing degree of the that the removal efficiencies varied very little in theremoval efficiencies is lessened little by little as humi- range of 1000-3500 mg/m, indicating that the flue gasdity increased. It is considered that the removal reactions CFB technology is good in adapting to different coalsare mainly physical adsorption and slow gas-solid reac- and combustion conditionstion at low humidity the water membrane would beformed gradually on the surface of absorbent as flue gahumidity increased, especially for the formation of themonomolecular layer before, hence, the SO2 and NO.80could be easily dissolved on the surface of absorbent andthe rapid ionizing reactions between Ca(OH)2 and SO2and nO occurred, so the removal efficiencies were enhanced obviously. Figure 7 also shows that the removalefficiencies appeared constantly when the humidity ex100015002000250030003500ceeded 6%. at which adsorbed water on the surface ofConcentration(mg/m)diffusion of SO2 and NO, to the surface of the absorbent Figure 8 Effect of SOz and NO concentrations on the efficiencies ofsimultaneous denitrification and desulfurization. The inlet temperaturewould be delayed. Therefore, the velocity of ionizingflue gas CFB is 130C; the flue gas humidity is 6.0%(v); the residencereactions slowed down, which is consistent with that of time is 2.4 s: the flow rate is 400 m/h; Ca/(S+N) is 1.2; the content ofYoon and Stouffe 9-llIadditive M is 1. 6%(wt); the circulation multiple is 851002.3 Parallel experiments of simultaneous desulfuri-zation and denitrification in flue gas CFB80The optimal conditions of flue gas CFB were achievedthrough the above experiments, with the inlet tempera-ture of flue gas of 130C, the flue gas humidity of 6%(v)the residence time of 2, 4 s. the flow rate of 400 m/hCa/(S+N) of 1. 2, the content of additive M of 1.6%(wt),and the circulation multiple of 85. Five parallel experiments of simultaneous desulfurization and denitrificaFlue gas humidity (etion in flue gas CFB were carried out using oxidizingFigure 7 Effect of flue gas humidity on the efficiencies of simultaneous highly reactive absorbent when the inlet concentrationsdenitrification and desulfurization. The inlet temperature of flue gas in of SO2 and NO were 3087 and 1032 mg/m'respecflue gas CFB is 130C; the residence time is 2.4 s; the flow rate is 400m/h; Ca/(S+N)is 1.2; the content of additive M is 1.6%(wt); the circula- tively. The results are showed in Table 1. It can be seention multiple is 85; the concentrations of SO, and NO are 2487 and 980 that higher efficiencies of simultaneous denitrificationand desulfurization were achieved. which was valuableZHAO Yi et al. Sci China Ser B-Chem I February 2007 I vol中国煤化工CNMHGTable 1 resultsmultaneous flue gas desulfurization and denitrification with a flue gas CEEfficiAverageSample variance SSO2(%)9595.7NO(%)for industrial applicationgrid of Si-o, al-O, and finally makes activated Sio22.4 Chemical analysis of removal productsand Al2O3 react with calcium hydroxide. Hence, theThe removal products of SO2 and NO in the spent ab-achieved highly active absorbent showed a highly spesorbent were analyzed with chemical analysis methodscific surface area and adsorption activation. From thedescribed in sec. 13[12] The theoretical contents of sulSEM images shown in Figures 9, 10 and ll, the surfafur and nitrogen species in the spent absorbent werecial micro-structure for material particles changed sig-calculated according to the inlet concentrations of sonificantly before and after digestingand No and removal efficiencies. The contents of sulfurFigure 9 shows that distinct spherical bodies withglossy and tight surface can be seen in the un-preparaand nitrogen species in the spent absorbent resulting tive fly ash, indicating that the main crystal should befrom theoretical and measured values are listed in Table Sio, and Al,0, 3. Figure 10 reveals that the surface of2. It is indicated that the complicated reactions of soNOx and calcium species contained in the absorbentoxidizing highly reactive absorbent appears coarse andhave been carried out. The molar ratio of sulfate anddestroyed seriously. Some gelling amorphous materialssulfite in the spent absorbent was 1. 85, and that of nitritend plentiful loopholes are formed on the surface, dem-and nitrate was 2.98. which shows that sulfate was theonstrating that oxidizing highly reactive absorbent has amain desulfurization product and nitrite was the mainhighly specific surface area. Figure 11 shows thedenitrification one. The suggestion can be provided by mage for the spent oxidizing highly reactive absorbent.the above results for the assessment and utilization of Compared with Figure 10, the micro-structures for thethe removal productsspent particles are different, demonstrating that after thereaction there are plentiful sediments and the surface2.5 Desulfurization and denitrification mechanism turns to be glossy, because the surface of absorbent isfor oxidizing highly reactive absorbentCalcium absorbent is widely used in flue gas desulfurization. However, the availability of the absorbent is low-dry or dry pithis disadvtage and realize simultaneous desulfurization and denitrification especially, the oxidizing highly active absorbent was prepared( 4, 5)by fly ash, lime and additive. Asthe key material for preparation of the oxidizing highlyactive absorbent, fly ash is chiefly composed of Sio2Al2O3, Fe2O3 and Cao, which is regarded as volcarash because it contains a great amount of non-crystalSio and AlO3. Plentiful silicon and aluminum oxidesare gradually activated by the effect of alkali excitantCalcium hydroxide diffuses to the spherical glass body25kVKYKY-2800B SEM SN: 1101surface on fly ash to cause chemical adsorption and ero-sion, which dissolves the glass body and destroys theFigure 9 Surface of fly ash particleTable 2 Theoretical and measured values of sulfur and nitrogen species in spent absorbent(mmol/g)Measured valuesRelative errorContent of s Content of NNS species0.9970.555a) The calculated values resulting from the contents of S and N mthe removal efficiencies; b) the measured values of S and N in speZHAO Yi et al. Sci China Ser B-Chem February 2007 volH中魏141Figure 13 shows the energy spectrum on the surfaceof one single particle of the spent absorbent. Comparedith Figure 12, it is obvious that sulfur speciesin the image. Unfortunately, the Vantage DIS X-ray energy spectrometer can only analyze the elements between Na and U, as the nitrogen element could not beanalyzed. The absorption of nitrogen species was veri-fied by chemthe oxidizing highly reactive absorbent is rich in calciumfrom Table 2, it is clear that the chemical absorptions ofSO2 and NO2 and calcium species contained in the ab-25kV700100 Hm KYKY-2800B SEM SN: 1108Masanori sakai,'s research on simultaneous denitrifi-cation and desulfurization by hydrated lime!4hasFigure 10 Surface of oxidizing highly reactive absorbentdemonstrated that the coexistence of so, and No couldpromote the removal with each other. The infrared bandfor the spent sample showed that without No in flue gas,lin product of So2 absorbed bywas SO3. While with the coexistence of No, the mainproduct was SO4, showing that the existent NO erhances the oxidation of So3 to SO4. The analysis ofdesulfurization product in the experiment showed thatthe molar ratio of sulfate radical to sulfite radical in thespent absorbent was 1.85, which was similar to ours.Izukainvestigated the reactionmeans of infrared band and pre-organization temperature100 Hm KYKY-2800B SEM SN: 1109desorption technology. The experimental results furtherFigure 11 Surface of the spent absorbentdemonstrated that surface species formed from NO adsorption was relevant to the conversion from SO2 tocovered by removal products including calcium sulfate, SO4. However, NO/O2 was not effective to such con-calcium sulfite. calcium nitrate, and calcium nitrite and version, namely that the adsorbed SO2 on the surfaceso on, after the resorption and adsorptionwhich had already pre-adsorbed NO2 could not beFigure 12 shows the energy spectrum on the surface verted to SO4. The coexistence and pre-adsorbed oxy-of fly ash, providing the main elementary compositions gen were of no use in this conversion, whereas useful inand the relative contentsthe presence of No O2 became useful人MMke0.000Figure 12 Energy spectrum at 1# point of Figurel0ZHAO Yi et al. Sci China Ser B-ChemI February 2007 vol中国煤化工CNMHG180015.360Figure 13 Average energy spectrum on the surface of one single particle for the spent absorbentThe predominant form of NO in flue gas is presentedCaSO3+O2+NO-reactive complexas no. whose solubility in water is much lower thanNO, HNO2 and HNO3 and so on[! which is the maincompounds—>CaSO4+NOreason why NO, cannot be removed together with SO, NO-+M(oxidant)->NO2+M(reductive products)(8)from flue gas in the conventional desulfurization tech3NO2+H2O—>2HNO3+NO(9)niques. Therefore, the key for simultaneously removingNO2+NO+H,O2HNONO and SO2 from flue gas is to oxidize no to NOCa(OH)2+2HNO3- Ca(NO3)2+ 2H20 (I1)rapidly. It was found in our experiments that the absorbent containing no additive M showed scarce removalCa(oh)2+2HNO2- Ca(NO2)2+2H20(12)for NOr. However, with the absorbent containing addiThe previous work of Chironna et al. showed thattive M, many oxygen-rich points were formed on the the ratio of NO/NO2 is extremely important in the corsurface of it where NO can be oxidized to NO2 within ventional scrubbing technology, and the highest removalthe residence time 2. 4 s of flue gas in the reactor. Firstly, efficiency occurs when the molar ratio of no to NO2 isNO2 could be adsorbed physically together with SO2 on 1. In this work, the molar ratio of nitrite to nitratethe surface of absorbent, when atomizing water was denitrification products was measured to be 2.98sprayed into the reactor, water film formed on the surResearch resultsshow that the heavy metals, suchface of absorbent SO2 and NO2 were dissolved into the as Hg, Cd, Ni, Cr, V, Se, Fe, Mn, Cu, Zn, Pb and As, arewater film and reacted rapidly with Ca(OH)2 in the ab- usually attended in environment. Quite a few heavsorbent. According to the results of Masanori Sakai 4 et metals are essential trace elements for organisms, whilal. and our results in this paper, the final products of the some are environmental pollutions, such as Cd, Pb, As,desulfurization and denitrification process were calcium Cr and Hg, excluding additive M. Moreover, additive Msulfate, calcium sulfite, calcium nitrate and calcium ni- applied in this experiment is in trace and will cause littletriteenvironmental influenceThere are two functions of additive M for removingNOr. As a strong oxidant, it can oxidize no to NO2 3 Conclusionrapidly. On the other hand, itshave catalytic activity. Furthermore the effect of the(1)Oxidizing highly reactive absorbent baased on limeoxygen content in flue gas should not be neglected, flyly ash and additive M was prepared. The analysis re-which should be investigated in the futuresults of SEM and X-ray energy spectrometer showedCombined with the results of the X-ray energythat volcanic ash reaction occurred between fly ash andometer and those of the chemical analysis, the relime during the digesting. Rough and plentiful poreswere observed on the surface of the oxidhigaction mechanism in flue gas CFB can be deduced asfollowsreactive absorbent, which indicated that oxidizing highlyreactive absorbent had a highly specific surface areaSO2+H2O—H2SO3Additive m dispersed uniformly on the absorbent surCa(OH)2+H2SO3- CaSO3+2H20facZHAO Yi et al. Sci China Ser B-Chem February 2007 vol中国煤化工CNMHG(2) The experiments of simultaneous desulfurization mass percent of additive M significantly affects the deand denitrification in flue gas CFB were carried out by nitrification, while weakly affects the desulfurizationusing oxidizing highly reactive absorbent. The results (3)The experimental results in flue gas CFB alsoshowed that flue gas humidity, flue gas temperature, flue showed that the technology of simultaneous denitrifica-gas residence time, content of additive M, Ca/(S+N)and tion and desulfurization appears stable and simplecirculating multiple are principal influencing factors for which is valuable in industry applicationremoval of SO2 and NOr from flue gas. The achieved4)According to the analysis of SEM and X-ray en-optimal parameters of simultaneous desulfurization and ergy spectrometer and the chemical analysis of the redenitrification using flue gas CFB are as follows: the moval products, the mechanism of desulfurization andinlet temperature of flue gas in flue gas CFB is 130C; denitrification using oxidizing highly reactive absorbentthe flue gas humidity is 6%(v); the residence time is 2.4 can be described as follows: No is oxidized to no2s: the flow rate is 400 m /h; Ca/(S+N) is 1.2; the content firstly, then reacted with Ca(OH)2 contained in the ab-of additive M is 1.6%(wt)and the circulation multiple is sorbent together with SO2. Therefore, calcium sulfate,85 Removal efficiencies for SO2 and NO under the op- calcium sulfite, calcium nitrate and calcium nitrite aretimal conditions are 95.5% and 64.8% respectively. The formedI Zhong Q. Techniques of Flue Gas Desulfuriztion and Denitrification 9 Yoon H, Stouffer MR, Rosenhoover W A, et al. Pilot process variablefor Coal-fired and Examples(in Chinese). Beijing: Chemical Industrystudy of coolside desulfurizatin prog,1988,7(2):l0l-1lPress,2002.182-20310 Stouffer M R, Yoon H F, Burker P. An investigation of the mecha2 Chu H, Chien T W, Li SY. Simultaneous absorption of SO and Nonisms of flue gas desulfurization by in-duct dry sorbent injection. Indom flue gas with KMnO4/NaOH solutions. Sci Total Environ, 2001275(3):127-13511 Cook J L, Khang S J, Lee S K, et al. Attrition and changes in particle3 Hori m. Matsunga N. malte P C. et al. The effect of low concentraize of lime sorbents in a circulating fluidized bed absorber. Powertion-fuels on the conversion of nitric oxide to nitrogen dioxide. In:Twenty-Four Symposium(International) on Combustion. The Com- 12 Christopher H N, Gary TR Simultaneous sulfur dioxide and nitrogenbustion Institute, Sydney, Australia, 1992. 909-916dioxide removal by calcium hydroxide and calcium silicate solids. J4 Zhao Y, Ma S C, Li Y Z, et al. Experimental investigation of desulAir Waste Manage, 1998, 48(9): 819-8furization and denitrification from flue gas by absorbents based on fly 13 Zhao P G. Synthetic Utilization of Fly Ash(in Chinese). Shenyangash. Zhongguo Dianji Gongcheng Xuebao/ Proceedings of the ChineseLiaoning Science and Technology Publishing House, 1993. 68-99Society of Electrical Engineering(in Chinese), 2002, 22(3): 108-112 14 Sakai M, Su C L, Sasaoka E J Simultaneous removal of SO and5 Zhao Y, Ma SC, Huang J J, et al. Experimental study on SO2 and NOusing slaked lime at low temperature. Ind Eng Chem Res, 2removal and mechanism by highly reactive sorbent. Proc Chin Soc41(20):5029-5033Electr Eng(in Chinese), 2003, 23(10): 236-24015 Tomohiro I, Hajime K, Tsutomu Y, et al. Initial step of flue gas6 Fan B G, Qi H Y, YuCk, et al. Mass balance and chemical change ofdesulfurizatior-An IR study of the reaction of SO2 with NO, onbed materials in circulating fluidized bed during desulfuriztionCao. Environ Sci Technol, 2000, 34(13): 2799--2803Therm Energy Power Eng(in Chinese), 2001, 23(10): 236-24016 Ervin B M J, Thomas JO. Hydrogen peroxide scrubber for the control7 Gao X, Luo Z Y, Chen Y F, et al. Study on effect of moisture onof nitrogen oxides. Environ Eng Sci, 2002, 19(5): 321-32desulfurization characteristic of calcium-based sorbent Combust Sci 17 Chironna R J, Altshuler B Chemical aspects of NO, scrubbing PollutTechnol (in Chinese), 1999, 5(1): 39-45Eng,1998,31:32-38 Gao X, Luo Z Y, Liu N. Desulfurization characteristic of calcium- 18 Gu J G, Zhou, QX, Wang X Reused path of heavy metal pollution inbased sorbent during activation process. J Chem Eng JPN, 2001, 34(9)soils and its research advance, J Basic Sci Eng(in Chinese), 20031114-111912(2):143-151ZHAO Yi et al. Sci China Ser B-ChemI February 2007 vol中国煤化工CNMHG