Mechanism of flue gas simultaneous desulfurization and denitrification using the highly reactive abs

Science in China ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--705Mechanism of flue gas simultaneous desulfuri-zation and denitrification using the highlyreactive absorbentXU Peiyao, MA Shuangchen, WANG Lidongliu FengSchool of Environmental Science Engineering, North China Electric Power University, Baoding 071003ChinaCorrespondenceshouldbeaddressedtoZhaoYi(email:zhaoy9515@163.com)Apri29,2005Abstract Fly ash, industry-grade lime and a few oxidizing manganese compoundadditive were used to prepare the Oxygen-riched highly reactive absorbent forsimultaneous desulfurization and denitrification. Experiments of simultaneousdesulfurization and denitrification were carried out using the highly reactive absorbentthe flue gas circulating fluidized bed(CFB)system. Removal efficiencies of 94.5% for SO2and 64.2% for No were obtained respectively. The scanning electron microscope (SEM)and accessory X-ray energy spectrometer were used to observe micro-properties of thesamples, including fly ash, common highly reactive absorbent, Oxygen-riched" highlyeactive absorbent and spent absorbent. The white flake layers were observed in theSEM images about surfaces of the common highly reactive absorbent and"Oxygen-riched one, and the particle surfaces of the spent absorbent were porous. The content ofcalcium on surface was higher than that of the average in the highly reactive absorbentThe manganese compound additive dispersed uniformly on the surfaces of the"Oxygenriched" highly reactive absorbent. There was a sulfur peak in the energy spectra picturesof the spent absorbent. The component of the spent absorbent was analyzed withchemical analysis methods, and the results indicated that more nitrogen speciesappeared in the absorbent except sulfur species, and so 2 and No were removed bychemical absorption according to the experimental results of X-ray energy spectrometerand the chemical analysis Sulfate being the main desulfurization products nitrite was themain denitrification ones during the process, in which no was oxidized rapidly to no 2 andsorbed by the chemical reactionKeywords: highly reactive absorbent, fly ash, simultaneous desulfurization and denitrification, micro-property, scanning electron microscope X-ray energy spectraDOI:10.1360/03ve0041Using slurry mixed by fly ash and alkaline m中国煤化工 industrialwaste gas had been reported in the 1970s, whichHCNMHGSCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrification693then, there have been many reports about flue gas desulfurization using the reactive ab-sorbent prepared with flying ash. There are four kinds as follows:(1) Using slurry of flyash as absorbent/2-, which mainly utilize the property of fly ash for desulphurization,but the removal efficiency is not high. (2)Using fly ash and lime/slaked lime/asabsorbent. There have been a lot of reports about the preparation conditions of this kindof absorbent, such as digesting temperature, time, pressure ratio by weight of fly ash andcalcium hydroxide, and ratio of water and solid, etc. 6-9, 1. 15.(3)Using highly reactivesorbent. The additives, such as CaSO4 8-24, CaCl2 and other compounds9,areadded to the fly ash/ime to improve the desulfurization activity. Unfortunately, the men-tioned absorbents are only used for desulphurization, while simultaneous denitrificationcannot be realized. The Oxygen-riched"absorbent for simultaneous desulfurization anddenitrification was prepared by adding oxidative reagent to fly ash and lime. With theduct injection system and CFB as the working platform, the experiments about simultaneous removal of so2 and No in flue gas have been done by using the proposed absor-bent,which are reported in our papers 25-7. The"Oxygen-riched"absorbent is namedthe fourth kindThe present investigations of the former three kinds of absorbents showed that thesurface characteristic of the absorbents has a great effect on desulphurization. However,the surface characteristics of the fourth kind of absorbent and its reactions with so, andNO in the CFB system have not been reported. SEM and the accessory X-ray energyspectrometer were used to analyze micro-properties and contents of significant elements on the surface of the fly ash, the fly ash/lime absorbent, " Oxygen-riched"absorbent and reaction products in this paper. The chemical analysis methods weredopted to determine the compositions of the products of desulfurization and denitrification. Thus the detailed mechanism of desulfurization and denitrification in the CFB sys-tem was investigated1 Preparation of the"Oxygen-riched"highly reactive absorbentAccording to our previous experimental results/25, 26] and other research conclu-sions 3.12, the"Oxygen-riched"highly reactive absorbent was prepared from fly ashindustrial lime and oxidizing manganese compound additive. The preparation processwas as follows. The mixture including fly ash and industrial lime with the ratio of 3: 1 inweight, manganese compound and water, was stirred and digested in 363 K and driedafter six hours. Thus, the "Oxygen-riched" highly reactive absorbent was achieved.The fly ash used in the experiments came from Baoding Thermoelectricity Plant,whose compositions are shown in Table 1. The content of the effective calcia in the in-dustrial lime was 90.77%0, measured by the method of cane sugarTable 1 Contents of components in fly ashSioCao中国煤化工 OthersContent(%)46.65,2ILI3.3YHCNMHG 1.4Science in China Ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--7052 Desulfurization and denitrification experiments in the flue gas CFB systemUsing the simulative flue gas containing SO2 and NO, five parallel experiments of thesimultaneous desulfurization and denitrification were carried out by using"Oxygen-riched"highly reactive absorbent in the flue gas FCB experimental system(shown in Fig1). The optimum experimental conditions are listed in Table 2, and the results are in Table 31412○9210Fig. 1. The experimental apparatus of the flue gas CFB system. 1, Air inlet; 2, steel bottle of SO2; 3, steel bottle ofNO: 4, glass-rotor flow meter; 5, buffer bottle; 6, electric heater; 7, fluidized bed reactor; 8, water tank; 9.high-pressure pump; 10, spray nozzle; Il, screw material-fed machine; 12, vortex dust remover; 13,mate-rial-circling leg: 14, gas analysis instrument: 15, induced-draft faTable 2 Optimum experimental conditions of CFBInlet gasWet of flue(mg/m)time(s)Ca/(S+Ntemperature℃)30871032130Table3Parallel experimental results of desulfurization and denitrificationEfficienciesA94.95,494.893.594.794.5NO(%)63.163.80.44Table 3 shows that the removal efficiencies of 94. 5%and 64.2%, respectively for SO2and No could be obtained by using the"Oxygen-riched"highly reactive absorbent un-der the optimized experimental conditions. Variance of the S in five parallel experiments was relatively small, which showed that the data was highly reproducible andprecise. The capacity of the"Oxygen-riched "highly reactive absorbent was stable in theimultaneous denitrification and desulphurization3 Analytical studies on the micro-properties on surface of fly ash, industrial limeabsorbents and the spent absorbent中国煤化工The profile about micro-properties on surface ofHCNMHGecommonCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrification695highly reactive absorbent, Oxygen-riched highly reactive absorbent and the productswere observed and analyzed by using SEM(KYKY-2800B type). The preparation ofsamples was as follows. The conductive two-sided gum paper was agglutinated on sample flat. Then the sample powder was uniformly sprinkled on the paper and the residualpowder was blown out. After the conductive film layer was plated on the sample powderwith the ion sputter method, it was observed with the electron microscope. The scannedmages of samples are shown in Fig. 2-Fig. 725 kV 3.00KX I0 um KYKY 2800B SEM SN: 110725kV300Kx 0 um KYKY-2S00B SEM SN: 109Fig. 2. Surface of fly ash parIndustrial lime particles(3000x)25 kv 700x 100 um KYK Y-2S0OB SEM SN: 110225 kV 700X 100 um KYKY-2800B SEM SN: 1108Fig 4. Surface of common highly reactive absorbent Fig. 5. Surface of"Oxygen-riched"highly reactiveparticle(700×)absorbent(700×)25 kV 980x 100 um KYKY-2800B SEM SN: II1023o×T0 m KYKY-2 SEM SN: ILIOSurface of an absorbent中国煤化工nice(980x)CNMHGin China Ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--705It was shown that particles of fly ash were relatively smooth, although there weresome abnormal protuberances on the surface. The surface of industry lime particlesmostly in 2 i m diameter was obviously coarse than that of fly ash, which was usually intens of microns diameter. The common highly reactive absorbent partucles were vecoarse. There were obviously erosive traces on the surface, which was attached to thewhite flake layer. The scanned surface image of the"Oxygen-riched"highly reactiveabsorbent particles was similar to that of the common highly reactive ones. Seeing themicro-surface, the spent absorbent particles were evidently different from fresh onesThe surface of the spent absorbent no longer had the white flake layer, while the porouscharacteristic was observed, which would be caused by complicated physics and chemical processes on the absorbent surfaceDuring the process of simultaneous desulfurization and denitrification in the flueCFB system, the substances on the absorbent surface may transform as followssolved reacted. dried. and circulated in materialIn the above process, the surface of absorbent particles was covered with productssuch as calcium sulfate, sulfite, nitrate and nitrite, etc. These products were porous underthe condition of the flue gas CFB. Accompanying by collision and abrasion between theparticles, the products flaked off continuously, which led to exposure of fresh absorbentIt was one of the key reasons for high removal efficiencies of So2 and NO4 Energy spectra analyzing of fly ash, industrial lime absorbent and productsAroscope(sEM, KYKY-2800B type)equipped with the X-rayenergy spectrometer(EDS, Vantage Dis type, Thermo NORaN Company in USA)wasused to analyze the surface compositions of the samples. The ordinate of energy spectrum figure was photon counts and the abscissa was energy. Elements from Na toUcould be analyzed by the X-ray energy spectrometer4. 1 Average energy spectra pictures on surface of fly ashAverage energy spectra pictures on surface of fly ash are shown in Fig 8. The relative content of the main elements on the surface of fly ash is shown in table 4eH中国煤化工Fig. 8. Average energy spectra picture of the surface of fly ash-m the au filmused as conductive medium in the preparation of samples and theCNMHGCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrificationTable 4 Relative contents of the main elements on the surface of fly ash18.2326.220.621.073.87From Fig. and Table 4, the compounds of Si, Al, Fe and Ca were the main composition in fly ash, in which there was also a small quantity of Ti and K elements4.2 Energy spectra pictures of the common highly reactive absorbent(1) Energy spectra pictures of point a of the common highly reactive absorbent in Fig4. From Fig. 9, relative content of Ca at point a in Fig. 4 was high, while that of Si, Aland k was low with others in trace. Hence, the main substance in point a might be cal-cium hydroxide, which was hydrated from the industrial lime in the preparation of ab-sorbent. Abundant heat was discharged with dissolution the industrial limes, whichtransformed calcium oxide particles into micro particles of calcium hydroxide. A littlecalcium hydroxide dissolved into water. As micro-particles of calcium hydroxide coexisted with fly ash, part of them and relative in solution would cover the surface of fly ashin the process of drying, which appeared as the white flake layer(ii)Energy spectra picture of common highly reactive absorbent at point b in Fig. 4Compared Fig. 10 with Fig. 8, the relative content of Ca at point a was higher than that15360Fig. 9, Scanned energy spectra pictures at point a in Fig. 4KFeAl中国煤化工15360Fig.10. Scanned energy spectra picturCNMHGScience in China ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--705in Fig. 8. It was shown that external Ca entered the surface of fly ash after digestion. Itwas also shown in Fig 8 that the relative content of Si was close to that of Al in fly ashwhich was different from that of Ca. However in Fig 10 the relative content of si wmuch larger than that of Al. At the same time, there was less difference between Si andCa. It was assumed that the silicon compound in fly ash reacted with Ca(Oh) intoCasio3 on the surface during the digesting process, which was consistent with refs. [9,13, 18, 20]. During the reaction process, the surface of fly ash was corroded by Ca(oh)2which led to the rough surface of prepared absorbent(ii) Energy spectra pictures of the common highly reactive absorbent at point c. Figl I shows the energy spectra pictures of the beading surface at point c in the highly reac-tive absorbent, which is similar to that in Fig. 10. Because sio was the main substancein the beading, there was more Si in Fig. ll, which was more than that in fly ash. Therelative contents of the main elements at point c of the samples are shown in Table 5IAu K/. feFig. 11. Scanned energy spectra pictures at point c in Fig. 4Table 5 Relative contents of the main elements on sof beadiy reactive absorbent31.070.00(iv) Average energy spectra pictures on surface of the common highly reactive abent. The average energy spectra pictures on surface of the samples are shown in Fig12. The relative content of the main elements are shown in table 6Ti中国煤化工-15360Fig. 12. Average energy spectra on surface of coCNMHGCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrification699Table 6 Relative contents of the main elements on surface of common highly reactive absorbentTi13.6118.730.511360.621.88Fig. 12 and Table 6 indicate that the relative content of Ca was higher than that of Sand Al, because part of Ca(OH)2 reacted with the substance on surface of the fly ash andsome other Ca(OH)2 absorbed into the surface of fly ash. However, from the amounts ofthe adding industrial lime and fly ash with the weight ratio of 3: I during preparation ofcommon highly reactive absorbent, the relative content of calcium should be about Ipercent, which was lower than that in Fig. 12. It might be that Ca(Oh)2 could not com-pletely spread into the inside of fly ash during digestion. Thus the relative content of Caon surface of absorbent was more than the average content, which enhanced the availability of Ca in the common highly reactive absorbent. Because SO2 and NO were easilyreacting with Ca on the surface 0-12, the preparation method of absorbent was beneficial to promote the efficiency of desulfurization and denitrification4.3 Energy spectra picture of"Oxygen-riched"highly reactive absorbent()Energy spectra pictures of"Oxygen-riched"highly reactive absorbent at point A inFig. 5. The energy spectra pictures of point A on surface of samples in Fig. 5 areshown in fig. 13. The relative contents of the main elements are shown in table 7Fig. 13. Energy spectra pictures at point A in FigTable 7 Relative contents of the main elements(wt %)of point A on surface of samples in Fig. 5KTiMnAccording to the relative contents of elements in Fig 13 and Table 7, it was assumedthat the main substance at point A in Fig. 5 was Ca(OH)2, which came from hydration ofindustry-grade lime in preparation of the highly reactive absorbent and adhered to thesurface of fly ash in the drying process. From the rather high content of silicon and aluminum, it was also assumed that there were compnino silicon and aluminum, such as hydration calcium silicate, calcium al中国煤化工;inosilicaetc. Different from the above energy spectra pictiCNMHgmanganeseScience in China ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--705element, which was that the"Oxygen-richedhighly reactive absorbent made by addingoxidizing manganese additive to the common highly reactive absorbent(ii) Energy spectra pictures of"Oxygen-riched" highly reactive absorbent at point Bin Fig. 5. The energy spectra pictures of point B on surface of samples in Fig. 5 areshown in fig. 14. The relative contents of the main elements are shown in table 8ViCar Mn E000Fig. 14. Energy spectra pictures at point B in Fig. 5Table8 Relative contents of the main elements at point B on surface of samples in Fig. 5KTiMn29.7052.171.770.50Seen from Fig. 14 and Table 8, the relative contents of silicon and aluminum werevery high and that of calcium was rather high. It was assumed that the main substancespoint B in Fig. 5 were calcium silicate, calcium aluminwhich contained silicon and aluminum, such as silicon dioxide, aluminate sesquioxide,(iii) Energy spectra pictures of point C in"Oxygen-riched highly reactive absorbentin Fig. 5. The energy spectra pictures of point C on surface of samples in Fig. 5 areshown in Fig. 15. The relative contents of the main elements are shown in Table 9. Theenergy spectra pictures and the relative contents of the main elements at point B were000中国煤化工15360Fig. 15. Energy spectra pictures atCNMHGCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrificationTable 9 Relative contents of the main elements at point C on surface of samples (wt %)in Fig. 5KMn314.361.99.45.310.58similar to that of point C in Fig. 5. It was assumed from the very high relative contentsof silicon and aluminum and rather high of calcium that the main substances at point Cwere calcium silicate, calcium aluminate and other compounds, which contained siliconand aluminum, such as silicon dioxide, aluminate sesquioxide, etc.(iv) Average energy spectra pictures on surface of"Oxygen-riched"highly reactiveabsorbent. The average energy spectra pictures are shown in Fig. 16. The relative con-tents of main element(wt % )are shown in Table 10Ti Mn FeFig. 16. Average energy spectra pictures on surface of"Oxygen-riched"highly reactive absorbentTable 10 Relative contents of the main elements on the surface of Oxygen-richedhighly reactive absorbent(wt. %AKTMn18.714.44By comparing Fig. 16 with Fig. 12, and Table 6 with Table 10, it is found that Fig. 16is very similar with Fig 12. However, there is a peak of manganese element in Fig. 16and there is the relative content of manganese element in Table 10. either from the av-erage value or the points a, B and c on the surface of the particle, the statistical tablesof the main elements show the existence of manganese element in the"Oxygen-richedhighly reactive absorbent. It is proved that the oxidizing additive spread uniformly dur-ing the preparation of the "Oxygen- riched "highly reactive absorbent4.4 Average energy spectra pictures on surface of"Oxygen-riched"highly reactiveabsorbent productThe average energy spectra pictures on the surface of the"Oxygen-rich"highly reactive absorbent after the desulfurization and denitrification in the cfb are shown in fig17. The relative contents of main elements(wt %)中国煤化工CNMHG702Science in China Ser. E Engineering and Materials Science 2005 Vol 48 No 6 692--705AlFig. 17. Average energy spectra pictures on the surface of the absorbent productsTable 11 Relative contents of main elements on the surface of spent absorbent( wt. o)Mn42.031.35It is shown in Table 10 there was no sulfur element on surface of the unreacted ab-sorbent, while the relative content of sulfur in the spent absorbent is given in Table ll. Itindicated that sulfur species were absorbed in the remove reaction. Unfortunately, theVantage Dis X-ray energy spectrometer can only analyze the elements between Na andU. As the nitrogen element could not be analyzed, the absorption of nitrogen speciescould not be reflected directly by the energy spectra pictures. The absorption of nitrogenspecies was verified by the chemical analytical methods5 Results of chemical analysisThe contents of sulfate, sulfite, nitrate and nitrite in the unreacted absorbent and theproducts were analyzed with chemical analysis. The contents of sulfate and sulfite weredetermined by barium chromate photometry. The content of nitrite was determined byN-(l-naphthyl)-ethylenediamine photometry, and that of nitrate by reduction of zincu The contents of sulfur and nitrogen species are listed in Table 12 for the unreactedsorbent and the products. It was seen that the molar ratio of sulfate and sulfite in thespent absorbent was 1.85, and that of nitrite and nitrate was 2.98. With regard to the un-reacted absorbent, a small quantity of sulfate was detected out, which might come fromfly ash. Moreover, sulfite, nitrate and nitrite were not detected out in unreacted absorbent,which indicated that sulfate was the main desulfurization product, and nitrite was theTable 12 Contents of sulfur and nitrogen species in unreacted absorbent and products(mmol/g)The contents of sulfur and nitrogen speciesThe contents of sulfur and nitrogen speciesacted absorbentin absorbent productsNO I0.002YHa中国煤化工0.186CNMHGCopyright by Science in China Press 2005Mechanism of flue gas simultaneous desulfurization and denitrificationmain denitrification one6 Mechanism of desulfurization and denitrification in CFB svstemIntegrated with the results obtained from the X-ray energy spectrometer and thechemical analysis, chemical absorption of SO2 and No was confirmed in the CFB system. Being the primary composition of NO in flue gas, dissolved NO in water wasmuch lower than that of other species such as NO2, HNO2 and HNO3. Although NOcould be oxidized to NO2 under natural conditions, it was not realized in practice because of the short settling time of about one to three seconds. The oxidizing additivecontained in the Oxygen-riched"highly reactive absorbent, NO might be rapidly oxi-dized to no, and absorbed by the absorbent The possible reaction process was inferredas follows.SO2+H2O→H2SO3Ca(oh)2+H2SO3+CaSO3+2H2O(2)CasO3+O2+No-(reactive complex compounds)-CaSO4+NO2NO+M(oxidant)NO+M(reductive results)4)3NO2+H2O→2HNO3+NO(5)NO2+NO+H2O→2HNOCa(Oh)+2HNO3-*Ca(NO3)2+ 2H,O(7)Ca(OH)2+2HNO2Ca(NO2)2+2H,Owhere(2)and (3)are the key steps of desulphurization, and (4),(6)and( 8)are the mainprocess of denitrification7 ConclusionsDuring the preparation of highly reactive absorbent, particles of Ca(OH)2 coexistedwith fly ash, which reacted in part with surface substances of fly ash. Thus the surface offly ash was corroded, and that of absorbent appeared very coarse. Some other Ca(oh)covered the surface of fly ash in the form of" white flake layer"during the hydration anddrying. The fly ash being the carrier of Ca(OH)2 in the process, dispersion of Ca(oh)2was enhanced and the effective reaction area was enlargedThe oxidizing additive was dispersed uniformly in the"Oxygen-riched"highly reactive absorbent. Thus NO could be oxidized and removed by the alkaline material in theabsorbeUsing the highly reactive absorbent, removal efficiencies of 94.5% for SO2 and 64.2%for noespectively in the CFB systemyH中国煤化工The sulfur element peak in the energy spectraCNMHOrbent conScience in China ser. e engineering and materials Science 2005 Vol 48 No 6 692--705firmed that the removal reaction was chemical absorption between SO2 in flue gas andthe absorbent, which was further verified by the results of chemical analysis. In the re-moval process of NO in the flue gas, it was oxidized firstly and then absorbed chemi-cally, which was confirmed by the chemical analysis. Sulfate and nitrite were the mainproducts of desulfurization and denitrification respectivelyAcknowledgements This work was supported by the Significant Pre-research Foundation of the North ChinaReferences1. Norman, K, Michael, A. M, Removal of So2 from industrial waste gases, Chemical Engineering, 1977.2. Gregory, D. R, Wayne, T. D, Randal, E. P. et aL., Analysis of coal fly ash properties of importance to sulfurdioxide reactivity potential, Environmental Science and Technology, 1984, 18(7): 548--5523. Tom, P, Hans, T. K, Significance of fly ash in wet-dry scrubbing of SO2, Chemical Engineering Technology,1988.11(5):298-3054. Izquierdo, J. F, Cunill, F, Martinez, J. C. et alL, Fly ash reactivation for the desulfurization of coal-fired utilitystations flue gas, Separation Science and Technology, 1992, 27(1): 61-725. Joseph, R. 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