Possibility of Reducing Formations of NOx and SO2 Simultaneously during Coal Combustion

Journal of University of Science and Technology Beijingo.7f2000),No.3,p165MetallurgyPossibility of Reducing Formations of NO and SO2Simultaneously during Coal combustionChunbao Xu Shengli Wu, Dagang Cang)Metallurgy School, University of Science and Technology Beijing, Beijing 100083, China2)Institute for Chemical Reaction Science, Tohoku University, JapanReceived 1999-10-15)Abstract: Comparing with other NO, and SO control technologies, in-bed reducing NO,, and SO2 simultaneously during coal combustionay lower the investment and operation cost. However, there are several possible contradictions between the reduction of NO, and thecapture of SO: during combustion: 1)CO rich atmosphere is favorable for the reduction of NO, whereas O rich favorable for the captureof so,; 2)higher preheating temperature of coal is favorable for reducing NO, but unfavorable for reducing SO: 3)sulphation of someminerals may deactivate their catalytic effect on the reduction of NO. The attempts to eliminate such contradictions by coating coalgranules with thin layer of monometallic oxides and mixed oxides were proposed. Ni O, and Fe, O showed high activity on NO, reductionand Cao and Cr O, showed good effect on sulfur capture The mixed metallic oxides, eg, Fe O, NiO, etc, showed effective for bothNO, reduction and SO, retention. It is possible to in-bed reduce NO, and SO; simultaneously if the adhering materials are properly chosento be difunctional materiais of both active catalysts for NO, reduction reactions and better sorbents for SO, retentionKey words: coal combustion; purification of NO, and SO2; catalyst; sorbent1 IntroductionNO /SO2 control have been proposed in recent years[7 One scheme of combined NO /SO2 control is toIn order to comply with the stringent regulations for mechanically combine the concept of in-bed NOx re-NO and SO: emissions from coal combustion, various duction and in-bed sulfur capture Some technologiestechnologies including pre-combustion coal cleaning, such as the combination of gas-reburning and sorbentcombustion modification and post-combustion flue gas injection have been developed [8]. As an advanced coaltreatment have been developed [1, 2 ]. Comparing with combustion technology, fluidized bed combustionflue gas denitrification and desulfurization, combustion (FBC) provides a good perspective for low emissionsmodification technologies for in-bed NO, reduction or of So: and NO. [9, 101, whereas it gives high emissionsulfur capture, possessing advantages of relatively lowof nitrous oxide (N O), which is regarded as a majorer cost and higher efficiency, hence they have been de- source of green house effect [11]. Although the techveloped rapidly. Some technologies, such as lower No nologies for combined NO /SO, control can achieveburner, air staging(overfire air), and fuel staging(re- joint reduction of NO, and SO, emissions, most of themburning),for in-bed NO, reduction are relatively inex- are of very high capital cost and solid or liquid waste70%NO, reduction [3]. disposal must be required. It is evident that ifwe canre-Sulfur pollutants can be efficiently removed during the duce NO, and SO2 within the fuel bed during coal com-combustion process by injecting sorbents(metal oxides bustion by modifying the combustion conditions,weof calcium, magnesium, zinc, iron and titanium, etc may lower, to a large extent, the capital and operationinto post-combustion gases, or by adding sorbents di- cost. However, according to the following analysesrectly into the fuel bed in the case of fluidized bed com- some contradictions may exist between the reductionbustion For pulverized coal combustion, successful ef- of NO, and the capture of SO. The following two reacforts have been made to mitigate SO2 emission in- bed tions中国煤化工 ucing NO. formationby pre-treating coal cores with precipitation of calcium duriCNMHGcarbonate, calcium acetate or magnesium acetate pno NO+C-2N*CO, AG=-2015412+8437Tlar to or even larger than that from untreated coal(7molTo meet the more stringent regulations for both NO, NO+CO+2N3-CO, AG=-3746610-1083Tand SO, emissions, numerous processes for combined (J/mol)of Univ. of Sci and Tech. Beijing, 7(2000 ), No. 3and the following heterogeneous reaction responsible cal reagents. The mixed metallic oxides: Fe O, NiO,or sulfur capture by the minerals in coal ash or added Fe: O, Cr O, Nio Cuo, Nio. MnO and Nio Cr O,sorbents [13](taking CaO as an example)were prepared by calcining the equimolar mixture ofCao-SO 20: CaSOa, AG--495919236996T crushing them into fine powder(<74 um). x-ray dif-(J mol)fraction measurement for all the mixed metallic oxideBy comparing equations(1), (2)and(3), it is obvicpowders. as given in figure 1, showed that complexmetallic oxides such as NiFe, O, were formedusly seen that for the reduction of NO, and the captureof So there is contradiction in atmosphere. CO-richatmosphere is favorable for the reduction reactions oft: NiONO, whereas O-rich atmosphere is favorable for SOe FeO,capture reaction. In addition, it has been found [14]thatG丶iFeOsome minerals showed catalytic effect on NO, reduction during coal combustion, but they could be deactiv-ated by sulphation of the minerals. Therefore, to realizesimultaneously reducing NO, and SO2 within the fuelbed, these contradictions should be properly eliminated. In the present paper, the attempts are proposed toeliminate such contradictions by coating coal granuleswith thin layer of monometallic oxide including CaO,40,060.00Fe-O Ni,O CuO. Cr-O and MnO, and mixed metallieoxide including Fe: O,NiO, Fe:, Cr:O,, NiO. Cuo,NiO.MnO and NiO.-O3. These metallic oxides haveFigure 1 XRD profile for mixed metallic oxide of Fe O3 NiObeen observed by the authors to show high catalytic activity on NO reduction by CO [15].2.2 Experimental apparatus and proceduresThe combustion of coal granules was carried out in a2 Experimentalpacked-bed combustion system which is schematically2.1 Coal sampleshown in figure 2. There is an alumina grid with air hoThe coal used in the experiments is an anthracite coalfrom Yang Quan in Shanxi province of China. CoalE Dust capturesamples were sieved to granules with particle size ofElectric1.0-3.0 mm. The proximate analysis (in d b. of thecoal is: ash, 12.6%; volatile, 9.9%: fixed carbon,77.5%. The ultimate analysis (in daf) is: C, 79. 5%; H,36%;N,1,2%;S,0.5%:O,15.2%(dif:). Two types ofcoal granules prepared in the experiments were namedas S' and S. S' type was coarse coal particle without ad-controllehering material, and S type was coal particle coatedwith adhering layer of fine powder of metallic oxide(the content of adhering material in S type coal granuleis 1.5%(0.1%, mass fraction). The preparation pro-N2O N2cedures of s type sample are given as follows: coal par-ticles(1.0-3.0 mm)after drying at 110 C in air for I hFigure 2 Schematic diagram of the packed-bed combustionwere surface wetted by spray ing a certain amount of di. systemstilled water in a small petri dish. The weighed metallic les中国煤化工 alumina-made reactoroxide powder(<74 um) was added into the dish andCNMHGmm in length).A thinshook repeatedly to coat the coal particle with a thin layer of alumina granules s mm in diameter )was plac-layer of metallic oxide powder. The obtained granules ed on the grid for well distributing the airflow. Belowwere air-dried at room temperature for 24h and then the grid, alumina granules were packed tightly for presubjected to combustion experiments. All the metallic heating the inlet gas. The reactor was pre-heated inoxides used in the present study are high purity chemi- pure N2 stream(1x 10 cm/min). As the specified tem.C Xu, SWu, D Cangperature was reached, the N, stream was switched to thegas mixture of 21%O2-N2 with the same flow rate as800(a)1073N:, and opened the valve of the charging funnel to(1073K)charge(1.50-0.05)g of coal sample into the reactor at⊙600o S(Cao)( 273 K)the same time. The contents of NO, and SO, in the tailgas were measured continuously with a chemilumin-escent NO, analyzer(Thermo Electron Co, USA)anda SO: analyzer(Kuri Physicochemical Co., Japan)re-200spectively.哪息!已R口e3 Results and Discussion1502002503.1 NO and So2 emissions from coal granules with250adhering layer of monometallic oxideS(Cao)(1 073 K)Table 1 gives the comparison of overall conversiono S(Cao)(I 273 K)ratio into NO, and SO,( FSo and Fso )in combustion ofS and S type coal with adhering layer of differentmonometallic oxides. As an example, figure 3 showsthe emission profiles of NO, and SO2 during combus-tion of S and S(Cao)at 1073 K and 1 273 K. The values of AF\, and A Fso in the table give an informationof the influence of different monometallic oxides on0200250300No, and SO. emissions. Ni O, decreased No, forma-tion during combustion at both preheating temperatFigure 3 Emission profiles of NO, and SO, during combus-ures, especially it decreased 63% of NO, in 1273K tion of s'and S(Cao) at 1073K and 1273Kcombustion. CaO and Fe O decreased No, in 1 273Kcombustion and an decrease of 48% in NO. emission whereas SO2 emissions were less in 1073 K combuswas observed for S(Fe. O, ), whereas they promoted NO, tion, with only one exception of S(Ni-O,)in 1 K combustion. It suggests that Ni_O, and Fe O3.2 NO, and SO2 emissions from coal granules withcan be effective for reducing NO, formation in higher adhering layer of mixed metallic oxideCr: O, promote NO, during combustion at both preheatTable 2 gives the comparison of FNo and Fso in com-ing temperatures On the other hand, all metallic oxidesbustion of s and s type coal with various mixed metal-decrease SO formation both in the 1073K and 1273K lic oxide adhering layer. As an example, figure 4 givcombustion (only with an exception of S(iO,)inthe profiles of NO, and SO: emissions during combus-1073K combustion). In terms of FN, and Fs. in the tion of s and S(Fe2O, NiO)at 1073 K and 1273 K Intable, NO, emissions were less in 1 273 K combustion,terms of the table and the figure, it is shown that all themixed oxides decreased both NO and SO, formationTable 1 Comparison of Fo and Fso in combustion of S' and Stype coal with adhering layer of different monometallic ori-during combustion. Especially, s(Fe O, NiO)reduceddes at 1073 K and 1273K30%40% of NO, and SO2 emissions simultaneously incomparison with S type coal. Such observation sugCoal type△Fb:%△F”/%S0.3060.23)0.86(0.98)Table 2 Comparison of F\o and Fso, in combustion of s and SS(CaO)0.400.20)0.63(0.66)+33(-13)-26(-33)type coal with adhering layer of different mixed metallic oxi-S(FeO)0.35(012)0.60.89)-18(-48)-30(-1)aeat1073Kand1273KSCuO)0.36(0.26)0.820.87)+2】(+14)-4(-11)Coal type△F!%△F”:%s(MnO)0.35(0.25)0.67(096)-18(+10)-22(-2)中国煤化工SCr:O)0.33.2710.62(067)-11(-17)-27(-32)S(Fe29(-44)-42(-30)SCI: O)029(0.08)090.78)-5(-63)+5(-20)S(Fe:OCNMHSote: a) FNn and Fs: overall conversion of coal containing ni-SCIO·CuO)0.24(0.17)0.56(0.74)-19(-28)-34(-24trogen and sulfur into NO, and So,; b) AFo and AFS. ratio of S(NiO. MnO)0. 21 (0. 15)0.65(0.78)-31(34)-24(-21)difference from Fso and Fsu for S type coals to those for S'type S(NiO- CrO,)0.25(0. 18)0.59(0.74)-15(-20)-32(-24)coal; c)The values in the parentheses are results from combustion at 1 273 KNote: a), b)and c)see table 1of univ of Sci. and Tech. Beijing, 7(2000), No, 3rS073(1073K)c S(Fe2O, NiO)(1073 K)O3NiO)(1073K)S(1273Ks"(1273K)o S(FeO.NO(1 273K)°SFe:ONjo)(1273K)xO100150200250300Figure 4 Emission profiles of NO, and SO, during combustion of S and S(Fe: O, -NiO)at 1073 K and 1273K.gests that the mixed oxides used in the present study areeffective in reducing both NO, and So formationsSimilar to the observation from table l and figure 3higher preheating temperature is favorable for nO, re-duction, but unfavorable for SO: reduction In terms ofAG of equations(1)(3), higher temperature seems beboth thermodynamically un favorable for the two, but ina real combustion process, both NO reduction and sul-COCO NOfur capture are controlled by chemical kinetics. HigherSO, H,O, Nytemperature generally enhances the reaction rates forboth reduction of NO, and capture of Soz. The influence of temperature on SO, formation is complicatedespecially as the presence of metallic oxides acting assorbents for sulfur capture reactions. Sulphation rate is Figure 5 A conceptual illustration of the combustion state fornot necessarily enhanced by increasing temperature,a granule of s type coal.observed in the present study that sO2 emissions werelarger in combustion at 1 273K than at 1073 K. It hass proposed by Levendis et al. [16], the effects of ad-been found [8]that significant sulphation only occurred hering layer of metallic oxide may include two partsin the 1 140-1 500 K temperature window, Below the physical and chemical. The physical effects may betemperature range, sulphation reaction rates are negdli- caused by the certain resistance to heat and mass transgible, whereas above the range all the reactions may bifer due to the adhering layer and the fast releasing ofinhibited by the agglomeration of the sorbent particlethe volatile matters in the early stage of combustion,or by the thermal decomposition of sulfates. It should which may lead to a local higher temperature regionbe noted that the preheating temperatures(1073 K and rich in volatile matters and CO inside the adhering lay1 K)referred in the present study is different fromer(denoted as region I in figure 5)and a lower tempera-the actual temperatures of coal particles. According to ture region rich in O2 outside of the layer as well(deno-the authors'mathematical simulation for packed bed ted as region II in figure 5). The atmosphere of regioncombustion, the highest fuel-bed temperature(Tb)cou-I is favorable for the reduction of NO, and the atmosId be about 400 K higher than the preheating tempera- phere of region II is favorable for the capture of SO,ture. Consequently, T, could reach about 1 473K andThe chemical effects are attributed to the catalytic ef-1 673 K respectively in the present study. 1 473 K is in fects of metallic oxide for NO formation or the sulfurthe temperature range of 1 140-1500K which is favo- capture effects as sorbents for SO2 reduction Metallicrable for sulphation and hence led to a lower SO2sion, whereas 1 673 K is much higher beyond the range tions [1中国煤化工 ysts for NO, foand led to a higher SO2 emission.mationopposing effects mi-CNMHch made the influ-3.3 Possible Role of metallic oxide layer in NO, andence of metallic oxide on NO, formation more compliSO, formationcated than on SO, formation In combustion of S( Cuo),The combustion state S type coal with metallic oxide S(MnO:) and S(Cr2O) at both temperatures and Slayer is tentatively conceptually illustrated in figure 5. (CaO) and S(Fe:O, )at 1073 K, larger amount of NOwas observed. This observations might be caused bythe following facts: (i)their catalytic effect on NO, for- [2] R.K. Lyon: Environ. Sci. Technol, 21(1987),p231mation reactions surpassing on the NO, reduction reac[3]S C Mitchell: NO, in Pulverized Coal Combustion, CCC/05ch,l998,tions;(ii) deactivation by SO2. On the other hand, the14]P. K. Sharama, G. R. Gavalas, R C. Flagan: Fuel, 66(1987),mixed metallic oxides studied in present study decreas-ed both NO and SO emissions simultaneously. It can [5] KK Chang, R C Flagan, G.R. Gavalas, P K.Sharamabe interpreted as follows: (i)difunctional characteris-tics with component as active catalyst for No reduc-[6] A. Atal, J. Steciak, Y, A. Levendis: Fuel, 74(1995), P. 495tion and component as good sorbents for SO, reduction; [7] A H. N. Soud K Fukasawa: /EACR/89, International Energ Agency Coal Research, London, U. K, August, 1996, P.i)the component for sulfur capture may prevent thecomponent as catalyst for NO, reduction from deacti-[8] US Department of Energy: Clean coal Technology: Reduevation by sulphationtion of NO, and SO: using gas Reburning. Sorbent injectionand Integrated Technologies, Washington, DC, U.S.A., Sep4 Conclusionstember. 1993. p 32Although it is difficult to in-bed reduce the forions of NO, and SO: simultaneously because some[10]Y. Lu, L. Hippinen, A Jahkola: Fuel, 74(1995),P. 317[11] L-E Amand, B. Leckner: [In: 24th Symposium (Int )oncontradictions may exist between them during coalCombustion, The Combustion Institute, Pittsburgh, 1992, pcombustion, pre-treating coal granules by coating athin layer of mixed oxide can be a possible measure, if [12]S. wu, C. Xu, D Cang: J of Univ of Sci. Tech Beying, Sthe adhering materials are properly chosen to be difunctional materials as both active catalysts of NO, re- [13JD. Cang, S. Wu, C. Xu: [Ln: I Proc. of the International Symeduction reactions and better SO, retention sorbentspossum on(SES98j, Beijing, China, 1998, p. 331Acknowledgements[14]R. F. w. Kopsel, S Halang: Fuel, 76(1997), p. 345[15]C. Xu, S. Wu, D Cang, Q Shou, M. Li, Y Deng: ChineseJournal of Environmental Science, 20(1999), No. l, p. 30The authors gratefully acknowledge the financialpewenberg, R C. Flagan, Gsupport of China National Educational Committee中国煤化工99p28Sincere thanks should also be given to the officers- in- [17CNMHGng: J of the Societ of Matercharge of Institute for Coal Chemistry of Beijing forproviding the coal sample[18]F. Verhoeff: [In: ]Proceedings of the ith International ConReferences[19]C. P. Fenimore, G. W Jones: J. Phys. Chemistry, 69(1965),[1]L.D. Smoot: Fundamental of Coal Combustion, Elsevier,