Nox reduction in the sintering process

International Journal of Minerals, Metallurgy and MaterialsVolume 16, Number 2, April 2009, Page 143MetallurgyELSEVIERNOx reduction in the sintering processYan-guang Chen'2), Zhan-cheng Guo'3), Zhi Wang", and Gen-sheng Feng)1) State Key Laboratory of Multiphase Complex System, Institute of Proccss Engincering, Chinese Academy Sciences, Beijing 100190, China2)Gnduate Universty of Chine Academy of Sciencs, Beijig 1000 China3) Key Laboratory of the Ministry of Education of China for Recycling & Ecologie Metallurgy, University of Science and Technology Beijing, Beijing 10083,Chin:(Received 2008-05-18)coke breeze and recycled fue gas on NO, reduction were investigated by sinter pot tests. The results show that the NOx reductionrate is over 10w% in the sintering of modified coke breeze, the efeetes of the additives on NO, reduction are: CeO2>CaO>K2CO3.The NO, reduction rate increases with the amount of rcycled flue gas, and is 2.35wt% in the sintering with reyecling 30vo1% of theflue gas. When 30vol% of the flue gas is recycled into the sintering of CeO2, CaO, and KzCO3 modified coke breeze, the NO; reduc-tion rates are 36. l0wt%, 32.56wt%, and 32.17wt%, respectively.Key words: nitrogen oxide; flue gas; coke breeze; cerium oxide; sintering[This study was financially suppored by the National Natural Science Foundation of China (No.50574085) and the Knowledge In-novation Program of the Chinese Academy of Sciences (No.082809).]1. Introductionmethod is the NOx reduction technology, through con-trolling operational conditions or mixing sinter mate-Annual total emissions of NOx are over ten millionrials with some additives. For example, NOx emissionmetric tons in China, which will increase yearly [1].can be reduced by treating the top sinter materialsThe minimization of NOx has been one of the mainwith microwaves before ignition [2], and it is also re-concermns because it can cause acid rain and photo-duced by mixing sinter materials with fine limestonechemical fog. The amount of NOx produced in sinter-(<2.5 mm) [3] or some hydrocarbons (rice husk, sugaring, representing the major NOx emissions in the ironcane slag, sawdust, sugar, and flour) [4].and steel industry, accounts for about 6wt% of the to-In sintering, over 90vol% of NO, comes from thetal nationwide NOx emissions. The majority of sinterfuel, the rest is from iron ores and fluxes [4], so theplants, in such developed countries as Japan and Ger-selective use of fuel with low nitrogen content is amany, have adopted denitrification equipments to treatpractical method for reducing NO, emission [5], al-the flue gas; however, the flue gas without any postthough low-nitrogen fuel is limited. Some reports in-process is directly discharged into the air in China.dicate that NO, emission in coke combustion isThere are two methods for reducing the amount ofgreatly affected by minerals in coke; active compo-NOx emitted from sinter plants. One method is thenents (such as Na and Fe) make the conversion oftreatment of flue gas. The flue gas volume is largefuel-N to NOx decrease, whereas, inert components(about 100 m'/(m'-min)) and the absolute level of NOr(such as Ca) cause an increase in the NOx emissionis reltively low (about 200x10*-300x 10), makinglevel中国煤化imatter sppesesit difficult to apply this method. Moreover, the hugethe cing coke combus-capital investment and high operational cost have re-tion [MYHC N M H Gion, by recyclingduced the competitiveness of this method. The otherlue gas and modifying coke breeze, is proposed [9]. ItCorresponding author: Zhan-cheng Guo, E mail: guozc@home.ipe.ac.cnAlso available online at ww.scencedirec.om。2009 University of Science and Technology Bejig. All rights reserved..144International Journal of Minerals, Mtallurgy and Materials, VoL.16, No.2, Apr 2009is based on the principle of in-situ catalytic denitrifi-fects of modified coke breeze and recycled flue gas oncation by modified coke breeze and NOx reduction byNO, reduction.recycling flue gas. In this process, the NOx reductionincludes: (i) in-situ catalytic denitrification of modi-2. Experimentalfied coke breeze; (i) the reduction of NOx in recycled2.1. Experimental materialsflue gas by burming coke breeze and CO produced incoke breeze combustion; (ii) NO, reduction by sinterSinter materials include iron ores (A, B, and C),ore. A comprehensive understanding of modified cokefluxes (dolomite, limestone, and lime), and cokebreeze and recycled flue gas on the formation and re-breeze. The chemical compositions of raw materialsduction of NO, during sintering would be of great useare shown in Table 1, and the particle size distribu-for contolling NOx emission. In this study, the sintertions of raw materials are shown in Table 2.pot tests have been performed to investigate the ef-Table 1. Chemical compositions of raw materialswt%MaterialFerouSiO2CaOMgOAl2O3PLOI (loss on ignition)_A59.194.170.0800.241.250.0060.0459.78B64.372.850.130.262.350.0360.0944.2968.120.450.160.230.170.0090.012Dolomite0.4933.8418.630.0870.00246.65Limestone-0.9950.593.690.270.0180.00743.681.4183.179.380.440.1200.0144.16Coke breeze0.475.950.520.123.570.0440.0118.43Table 2. Particle size distributions of raw materialsParticle size / mm85-8_3-1-30.5-18.259.3614.4625.3821.3321.222.936.2013.9020.0216.6040.351.400.631.953.2118.1374.684.2214.1110.2670.960.109.4525.4216.4848.55Lime4.2817.68 .16.8561.031.8318.0823.9117.3938.79The proximate and utimate analysis of coke breezetributed on the surface or in the pore of the cokeis shown in Table 3. The coke breeze was modified bybreeze by the adsorption capacity of the coke breezeCeO2, CaO, and K2CO3, respectively. To introduceitself. Water was eliminated by evaporation, and theCeO2, the CeO2 powder was first sieved to the sizeimpregnated samples were fully dried at 1109C for 6hbetween 47 and 74 μm, and then it was added intobefore use. The coke breeze samples loaded with CaOwater to make a certain concentration of CeO2 emul-or KzCO3 can be obtained in the same way. The load-sion at a stirring rate of 20 r/min. The coke breezeing amounts of CeO2，CaO, and K2CO3 are allsample was added into this CeO2 emulsion, and sub-2.0wt%.sequently the CeO2 particles were adsorbed and dis-Table 3. Proximate and utimate analysis of coke breeze (air-dry basis)Proximate analysisUtimate analysisFSHO*0.391.2986.750.6485.310.400.810.88Note: * By dfferenece.中国煤化工2.2. Experimental apparatussinterMYHCNMHGalways 40.0 kg.As ilustrated in Fig. 1, the apparatus consists ofaAboutgas, simulated asflue gas recirculation section, a pressure test section, athe recycled flue gas, was introduced to the top sur-sinter pot (200 mm in diameter, 750 mm in height),face of the sinter ore via a gas scatter, therefore, theand an online gas analysis section. The amount ofNOx gas and air flowed through the sintering bed un-Y.G Chen et ah, NO, reduction in the sintering process145der the suction pressure.20.9vol% to 12vol% during ignition. Afer ignition,the NOx concentration increases rapidly to 110x10 ，,2.3. Experimental methodand keeps a slight increasing trend till the NOx con-Iron ore fines, fluxes, and coke breeze were mixedcentration reaches the maximum value of 152x10。atwith water (7.0wt%) in a mixing drum to form gran-18.5 min. The O2 concentration fluctuates aroundules for 3 min, and then the granules were evenly laid16vol% during the process, and the variation trend ofinto the sinter pot. The addition of coke (4.2wt%),O2 is opposite to that of NOx, which lasts till 3 minSiO2 content (4.4wt%), and basicity (CaO/SiO2=1.9)before the bum-through point. Then the NOx concen-were kept constant for each run. The ignition tem-tration drops down to zero and the O2 concentrationperature was 1100+50°C, ignition time 1.5 min, andrapidly rises up to the O2 level in air, and the tem-ignition suction 6 kPa. The sintering suction was 10perature of the flue gas increases to the maximumkPa and the sintered ore was cooled for 15 min aftervalue at the same time, indicating the end of sintering.the sintering ended. The composition of the flue gasAfter ignition, the coke breeze in sinter materialswas monitored by an online gas analyzer, KM9106flue gas analyzer (Kane Ltd. Co., England), for theburned and the combustion zone gradually movedconcentrations of CO, O2, NO, NO2, and NOxdown to the bottom of the sintering bed under the suC-(NO,=NO+NO2), whereas, the concentration of CO2tion pressure. Therefore, the NOx produced in thewas calculated by the O2 concentration. The precisioncombustion zone could only be reduced by cokefor CO, NO, and NO2 was 1x10%, whereas, the preci-breeze or CO formed during coke breeze combustion,sion for O2 was 0.1vol%. The temperature of the fluewhich took place in or below the combustion zone.Coke had an important effect on NOx reduction [10].gas was measured by a Pt-Rh type thermocouple.The NOx concentration was low due to the thickerlayer of coke below the combustion zone at the earlystage of sintering. However, the NOx concentrationgradually increases with sintering time, which is re-lated to the steady decrease of the coke layer. More-over, the increase of thickness in the combustion zoneis another factor. The concentration of CO in the fluegas is about 1.0vol%-2.0vol%, which indicates thatthe chemical energy of coke breeze is not fully takenadvantage of. Therefore, the energy efficiency can beFig. 1. Schematic diagram of the sinter pot tests.improved by reducing CO emission in flue gas.The NO, in sintering is dominantly NO; the NO2 is) 700NO.24180。lower than 10x10*. The NO; reduction in flue gas is600expressed by the NO: reduction rate R (wt%):500 I168120400100R=cadt+co(t.-to)- [cdr .x100%(1)30080[Ccdur+co(1-1o)200，co CO2501。40where Cr is the NO, concentration in the base sintering0481216202428at a certain time, 10%; c' the NOx concentration inSintering time / minother sinter pot test at a certain time, 10 6; Co the NOxFig. 2. Emissions of NOx and O2 during sintering,concentration in the recycled flue gas, 10%; to is the .initial time of the flue gas recycled into sinter, min; 1e3.2. Effect of modified coke breeze on NOx reduc-the terminal time of the flue gas recycled into sinter,tionmin; t the total time of sintering, min.In the sintering of modified coke breeze, the NOx3. Results and discussionemissions are shqwn in Fie. 3. Apparently, the NOxemiss中国煤化工e base run. In the3.1. NOx emission in the sinteringrun 0MHCN M H G2e NO. concenta-The NO,, Co, CO2, and O2 concentrations and thetion is JUxIu, LOWCI Luiall ulal UL the base run, sug-flue gas temperature in the sintering are shown in Fig.gesting that CaO added in coke breeze, fiunctions to2. The NOx concentration increases from 0 to 37x10~ ,inhibit the conversion of fuel-N to NO元When thewhereas, the O2 concentration decreases fromcoke breeze modified by CeO2 is used for fuel, the146International Journal of Minerals, Mrtallurgy and Materials, VoL16, No.2, Apr 2009NOx concentration is 40x10° lower than that of the3.3. Effect of recycled flue gas on NOx reductionbase run, which is related with the redox property ofThe recycled flue gas, simulated by a certain con-CeO2 [11] and the catalytic effect of CeO2 on NOx re-centration of NOx gas, is introduced into the sintering,duction. The NO2 concentration is low at the earlyand the NOx emissions are shown in Fig. 5. Whenstage and is high at a later stage, with K2CO3 modifed10vol% of the flue gas is recycled into the sintering,coke breeze being used as fuel, it may be affected bythat is, an extra 15x10。NOx is added into the sinter-the catalytic combustion support and gasification ef-ing, the NO, emission is almost the same as that of thefects of K2CO3 [12]. .base run. The concentration of NOx emission is160slightly higher than that of the base run when 30vol%of the flue gas is recycled into the sintering. The NO,20 tconcentration becomes apparently higher in the run of00 trecycling 50vol% of the flue gas, indicating partial+-K,CO,80NO. in the flue gas is not reduced. In practical sinter-60ing, the initial concentration of O2 decreases with in-40creasing the amount of the recycled flue gas, resulting20in a slower sintering speed and lower sinter productiv-ity, therefore, the amount of the recycled flue gas510152025 30should be not more than 30vol% of the total amount ofSintcring time 1 minthe flue gas.Fig. 3. Effect of the modifned coke breeze on NO, emissionin sintering.160|The NOx reduction rates can be obtained by the in---S0vol% luegastegration of Fig. 3 and the calculation according to Eq.120(1). The results are shown in Fig. 4. The NOx reduc-tion rates are 18.8wt%, 13.5wt%, and 16.5wt%, re-spectively, when the coke breeze loaded with 2.0wt%CeO2, 2.0w1% CaO, and 2.0w% K2CO3 are used as .fuel. In the run of the modified coke breeze, the NOx048121620242832elimination reaction also occurs in or below the com-Sintering time 1 minbustion zone. In the sintering of CeO2 modified cokebreeze, NO2, produced by the oxidation of NO, reactsFig. 5. Effect of the amount of recycled fue gas on NO,with CeO2 and forms cerium peroxide or super oxide,emission.which subsequently undergoes secondary reactions toWhen the flue gas is recycled into the sintering, theyield N2 [13]. Although in the sintering of CaO modi-NOx reduction rates are shown in Fig. 6. The NO: re-fied coke breeze, CaO has a catalytic effect on theduction rate increases with the amount of the recycledNOx elimination. The K2CO3 loaded in coke breezeflue gas. The NOx reduction rates are 10.9wt%,promotes the gasification reaction in coke combustion,22. 4wt%, and 29.0wt%, respectively, in the sinteringso the local reducing atmosphere enhances with thewith recycling 10vol%, 30vol%, and 50vol% of theincrease in CO concentration, which is an importantflue gas, respectively. The NOx in the recycled fluedrive for NO, reduction.gas passes through the sinter bed, the NO, elimination2:reactions not only occur in or below the combustionzone, but also in the sintered ore zone. In or below the星20combustion zone, the NOx reduction is the same as15that in the base run. In the sintered ore zone, NO, willreact with FeO and be reduced to N2, it has been10demonstrated that the sintered ore has a positive effect中国煤化工3.4.MHCN M H Gd modifed cokeCaOCcO2K2CO,breerCoke loaded with different additivesIn the sintering of modified coke breeze with recy-Fig. 4. Effect of modified coke breeze on the NO, reduc-cling 30vol% of the flue gas, the NOx emissions aretion rate (additives addition: 2.0wt%).shown in Fig. 7. The concentrations of the NOx emis-y.G. Chen et aL, NO redution in the sintering process147sion are apparently lower than those of the base run.ergy efficiency will be improved.The existence of CeO2 and CaO not only has somedepressing effects on the formation of NO, in coke30、01% flue gas . 2wt% K.CO,breeze combustion, but also has some promoting ef-fect on the reduction of NOx in the recycled flue gas,30voul% tlue gas +2wt% CeO:whereas, the existence of K2CO3 has some positiveeffects on the gasification reaction, and the NO, re-30v01% flue gas↓2w%CaOduction is mainly due to the increase of CO concentra-tion.5301o1。flue ua401020 305030 INO, reduction rate 1 wt%25 tFig. 8. Effects of the modifed coke breeze and recyeled20flue gas on the NO, reduction rate.4. Conclusions(1) The NOx emission is reduced in the sintering ofmodified coke breeze, the order of NOx reduction is:35(CeO2>CaO>K2CO3.Circulation of flue gas 1 vol%(2) The NOx reduction rate increases with theFig. 6. Effect of the amount of recyeled fue gas on theamount of recycled flue gas. When 10vol%, 30vol%,NOx reduction rate.and 50vol% of the flue gas are recycled into the sinter,the NOx reduction rates are l0.89wt%, 2.35wt%, and马16030vo|% flue gas29.02wt%, respectively.20 t30vol% flue gas20wt% CaO(3) NOx reduction is over 30wt% in the sintering of十30vo1% flue gastmodified coke breeze with recycling 30vol% of the8020w1%K,cO.ASflue gas. .40 ,References04812162024283236[1] Y.X. Su, Y.R. Mao, and Z. Xu, Technologies of Control-Sintering time 1 minling NO, Emission during Coal Combustion (in Chinese),Chemical Industry Press, Beijing, 2005, p.11.Fig. 7. Effects of modified coke breeze and recyeled fuegas on NO, emission.2] H. Yozo, U. Akihito, N. Hirotaka, and K. 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