Fuel Processing Technology 90 (2009) 933–938

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Fuel Processing Technology

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Influence of CeO2 on NOx emission during iron ore sintering

Yanguang Chen a,b, Zhancheng Guo a,c,⁎, Zhi Wang a

a State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, PR Chinac Key Laboratory of Ecological and Recycle Metallurgy, Ministry of Education instead of School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing,Beijing 100083, PR China

⁎ Corresponding author. State Key Laboratory ofInstitute of Process Engineering, Chinese Academy ofChina. Tel./fax: +86 10 6255 8489.

E-mail address: guozc@home.ipe.ac.cn (Z. Guo).

0378-3820/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.fuproc.2009.03.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 August 2008Received in revised form 25 March 2009Accepted 30 March 2009

Keywords:NOx emissionCoke combustionCeO2

Iron ore sintering

The evolution of NOx during coke combustion in the presence and absence of CeO2 was studied in a quartzfixed bed reactor. The distribution of CeO2 in the coke was examined by SEM, and the effects of CeO2 loadingand CeO2 particle size on NOx emission were discussed. NOx emission was also investigated by sintering pottests with CeO2 modified coke as sintering fuel. The results showed that CeO2 was catalytically active inpromoting not only coke combustion but also NOx reduction. SEM examination indicated that the CeO2

particles were well distributed on the surface and in pore canals of coke. In coke combustion experiments,NOx and CO emission decreased with increasing CeO2 loading up to 2.0 wt.% and decreasing CeO2 particle size(28–150 µm), while sintering pot tests showed that NOx emission decreased by 18.8% with 2.0 wt.% CeO2

modified coke as sintering fuel.© 2009 Elsevier B.V. All rights reserved.

Table 1

1. Introduction

In China, the current NOx emission is about ten million tons ayear and may increase further [1]. The minimization of NOx has beenone of the main concerns for its main cause of environment pro-blems such as acid rain and photochemical fog. The NOx produced iniron ore sintering amounts to the major NOx emissions in the inte-grated iron and steel works, accounting for around 6% of nationaltotal NOx release. Since the absolute concentration of NOx isrelatively low (200–300 ppm) and the flow rate of flue gas is large(100 m3/m2·min) in sintering process, it is uneconomical to applythe treatment of flue gas to iron ore sintering due to the installationof ammonia NOx removal equipment.

In iron ore sintering, over 90% of NOx originated from thecombustion of fuel [2], so selective use of the fuel with low-nitrogencontent is a practical measure to decrease NOx emission [3],unfortunately, low-nitrogen fuel is limited. Some reports indicatedthat NOx emission during coke combustionwas greatly affected by theminerals in coke; active components (such as Na, Fe) in the mineralscould make the conversion of fuel-N to NOx decrease, while the inertcomponent (such as Ca) could cause rise of NOx emission concentra-tion [4–6]. In general, the mineral components suppressed theconversion of fuel-N to NOx during coke combustion [7]. Based on

Multiphase Complex System,Sciences, Beijing 100190, PR

ll rights reserved.

this principle, a new method, NOx reduction by using the modifiedcoke as the sintering fuel, was proposed [8]. Due to its properties ofredox [9] and NOx storage [10], Ceria, impregnated in coke, wasproposed to decrease NOx emission. Understanding the process of NOx

formation and clarifying the factors that affect the NOx emission inthe combustion of coke loaded with CeO2, should be important forcontrolling NOx emission. In this study, the NOx emission in the com-bustion of coke loaded with CeO2 was performed in a quartz fixed bedreactor, and the NOx reduction was also investigated by sintering pottests with CeO2 modified coke as sintering fuel.

2. Experimental

2.1. Experimental materials

2.1.1. Preparation of coke samplesCoke samples were derived from a Shanxi bituminous coal heated

in a crucible with a cover in a muffle furnace at 950 °C for 30 min,and then ground to a particle size of 0.5–0.8 mm. Table 1 shows theresults of proximate and ultimate analyses of coal sample and cokesample.

Proximate and ultimate analyses of coal and coke.

Sample Proximate analysis /wt.% Ultimate analysis /wt.%

Mad Aad Vad FCad St,ad Cad Had Nad Oada

Coal 1.02 9.69 24.03 65.26 0.98 76.83 4.12 1.18 6.18Coke 0.39 11.57 1.29 86.75 0.64 85.31 0.40 0.81 0.88

aBy difference

Table 2Composition of CeO2 reagent.

Composition CeO2 La2O3 Fe2O3 CaO Cl− SO42− L.O.I

Content /wt.% N99.5 ≤0.026 ≤0.008 ≤0.014 ≤0.019 ≤0.065 ≤0.5

Table 3BET surface area of CeO2 with different particle size.

Particle size /µm 28–38 38–47 47–74 74–150

BET surface area /(m2/g) 8.5 7.5 6.4 3.5

934 Y. Chen et al. / Fuel Processing Technology 90 (2009) 933–938

2.1.2. Introduction of CeO2

Through the screening, the CeO2 reagent (CeO2N99.5 wt.%, AR,Sinopharm Chemical Reagent Beijing Co., Ltd, China) was divided intofour particle size fraction: 28–38 µm, 38–47 µm, 47–74 µm and 74–150 µm. Composition of CeO2 reagent and BET surface area of CeO2wereshown in Tables 2 and 3, respectively. The CeO2 powder was addedinto water to make CeO2 emulsion with a certain concentration at a

Fig. 1. EM and EDS of coke with and without CeO2 addition.

stirring rate of 20 r/min, and then the coke sample was impregnated inthis CeO2 emulsion for 30min. The impregnated samplewas fully driedat 110 °C for 6 h before use.

2.1.3. SEM and EDS analysesScanning electron microscopy was performed with a microscope

(Leica-440, England) in order to study distribution of CeO2 (38–47 µm) in coke samples, as shown in Fig.1. The surface and pore canalsof raw coke are smooth and clear (Fig. 1a), while the surface andthe pore canals of CeO2 modified coke are filled with the fine CeO2

particles (Fig. 1b), which could be testified by the EDS analysis withinthe dotted line frame in the SEM photographs. Scattered with water,the CeO2 particle was adsorbed and distributed on the surface and inpore canals of coke, as shown in Fig. 1c.

2.2. Experiment and analyses

2.2.1. Coke combustionFor each run,1.0 g coke sample, mixedwith 10.0 g quartz sandwith

the same particle size, was packed into a quartz fixed bed reactor, andheated to 900 °C at a heating rate of 5 °C/min, maintaining at this

a: Raw coke, b and c: Coke loaded with 2.0 wt.% CeO2.

Table 4Analysis of residual carbon in ash.

Sample Rawcoke

CeO2 additiona /wt.% CeO2 particle sizeb /µm

0.5 1.0 2.0 28–38 38–47 47–74 74–150

Residual carbon /wt.% 0.019 0.014 0.011 0.009 0.009 0.009 0.010 0.011

aCeO2 particle size: 38–47 µmb CeO2 addition: 2.0 wt.%

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temperature for 30 minwith flowing Ar to prevent coke from burning,then Ar was switched to a mixture of Ar and O2 for coke combustion.The initial O2 concentration was always 21.0 vol.%, and the total inletgas flow rate was kept at 4.0 L/min by mass flow controllers (D07-12A/ZM, Seven Stars Limited Co, China).

2.2.2. Analysis of residual carbon and flue gasResidual carbon in ash was performed with a carbon/sulfur

determinator (LECO CS-344, U.S), as shown in Table 4.The flue gas compositions were monitored continuously by two

analyzers: AO2020 gas analyzer (ABB Ltd.) for O2, CO, CO2, NO, N2O,SO2 and a portable flue gas analyzer KM9106 (Kane InternationalLimited Co.) for NO2. It has been well established that NO2 formedduring coke combustion are mainly from the oxidation of NO, thusNOx is designated as the sum of NO and NO2. The data were recorded

Fig. 2. Results of duplicated runs for coke combustion.

every 30 s from the start to the end of coke combustion. The resultswere expressed in terms of NOx reduction ratio R:

R =R t0 ctdt −

R t0 cVtdtR t

0 ctdt× 100k ð1Þ

Where, ct is the NOx concentration in raw coke combustion at sam-pling time, c′t is the NOx concentration in the combustion of cokeloaded with CeO2 at sampling time; t is the total time of cokecombustion.

To guarantee the reproducibility, all experiments were repeatedseveral times. Fig. 2(a) presents some typical experimental resultsfor three duplicate runs of coke combustion, showing the relativeerrors of NOx concentrations less than 5% of the averaged values.During coke combustion, the N2O concentrations remained practi-cally unaltered after CeO2 addition, indicating that the NOx reductionwould be attributed to the conversion of NOx to N2, not to N2O, asshown in Fig. 2(b).

3. Results and discussion

3.1. Effect of CeO2 loading on NOx emission

The combustion of coke with and without CeO2 (38–47 µm)loading were performed under identical conditions, and the NOx

emission behavior was shown in Fig. 3. Generally, the NOx emis-sion starts once coke reacts with O2 and reaches its maximum valuequickly. In the combustion of coke loadedwith CeO2, the NOx emissiondecreases with increasing CeO2 loading. The maximum NOx concen-tration decreases by 24% (from 450 ppm to 341 ppm) as CeO2 loadingis 2.0 wt.%.

By integration of Fig. 3 and calculation according to Eq. (1), theNOx reduction ratio can be obtained, as shown in Table 5. NOx

reduction ratio increases with increasing CeO2 loading, suggestingCeO2 particles loaded in coke inhibit the emission of NOx. It has beenwell established that the NOx comes from the oxidation of fuel-N inthe combustion process, and part of NOx undergoes in-situ homo-genous and heterogeneous (catalytic and non-catalytic) reductionby reducing agents, i.e. coke or CO released during the combustion[11]. Undoubtedly, the CeO2 in coke catalyzes both the oxidation offuel-N and the reduction of NOx at the same time [12]. The higherreduction ratio of NOx in the presence of CeO2 indicates that CeO2

Fig. 3. Effect of CeO2 loading on NOx emission in coke combustion.

Table 5Effect of CeO2 loading on NOx reduction ratio in coke combustion.

CeO2 addition /wt.% Peak value /ppm R /%

0 450 00.5 401 5.411.0 376 10.662.0 341 14.84

Fig. 5. Effect of CeO2 particle size on NOx emission in coke combustion.

936 Y. Chen et al. / Fuel Processing Technology 90 (2009) 933–938

exerts more positive effects on the reduction of NOx than on theoxidation of fuel-N.

The consumption rate of O2 during coke combustion can reflect theextent of combustion process. Fig. 4(a) shows the variation of O2

concentration with time during coke combustion. The O2 concentra-tion decreases with increasing CeO2 loading at the later stage of thecombustion, suggesting that the combustion was postponed withincreasing CeO2 loading.

Fig. 4(b) shows the CO evolution profile during the combustion ofcoke with different CeO2 loading. It is evident that CeO2 can suppressthe formation of CO in coke combustion; simultaneously the contentof residual carbon in ash decreases slightly, suggesting more CO2 beproduced, so the coke combustion efficiency is improved. The reasonis that carbon adsorbs metal bound oxygen atoms faster and easierthan molecular oxygen [13].

Fig. 4. Effect of CeO2 loading on O2 and CO concentrations in coke combustion.

3.2. Effect of CeO2 particle size on NOx emission

The particle size of CeO2 is one of important factors in theimpregnation method. The CeO2 loading is always 2.0 wt.% for eachrun, and the NOx emission curves in the combustion of coke loadedwith different particle size CeO2 are shown in Fig. 5. The maximumNOx emission concentration diminishes with CeO2 particle size,which may be closely related to the extent of even distribution ofCeO2 on coke surface and the extent of contact between coke andCeO2. However, the NOx concentration increases with decreasing theCeO2 particle size at the later stage of combustion when the CeO2

particle size is below 47 µm, suggesting that the fine CeO2 particlesadsorbed on coke surface cause the slight postponement of NOx

emission.The NOx reduction ratios in coke combustion are listed in Table 6.

It can be seen that the NOx reduction ratio increases with thedecrease of particle size, indicating that the finer CeO2 particles favorthe reduction of NOx. When the particle size of CeO2 loaded in coke isbetween 28 µm and 38 µm, the NOx reduction ratio is 17.23%.

Fig. 6(a) shows the variation of O2 concentration with time incoke combustion. The O2 concentration increases with decreasingthe CeO2 particle size at the early stage of the combustion, whileincreases with increasing the CeO2 particle size at the later stage ofthe combustion, which illustrate that the coke combustion is slightlyaffected by the particle size of CeO2 loaded in coke. The reason maybe that the fine CeO2 particles distributed on coke surface restrainsthe reaction between coke and oxygen. Effect of the particle size ofCeO2 on CO emission during coke combustion is shown in Fig. 6(b).The fine CeO2 particles are prone to increasing the conversion ofcarbon to CO2, and have catalytic effect on the oxidation of CO toCO2. So the reduction of CO emission is obvious as CeO2 particles sizedecreasing.

Table 6Effect of CeO2 particle size on NOx reduction ratio in coke combustion.

CeO2 particlesize /µm

Peak value /ppm R /%

450 0

74–150 416 8.4947–74 374 11.9638–47 341 14.8428–38 312 17.23

CeO2 addition: 2.0%wt.%

Fig. 6. Effect of CeO2 particle size on O2 and CO concentrations in coke combustion.

Fig. 7. Physical model of NOx reduction in the combustion of coke loaded with CeO2.

937Y. Chen et al. / Fuel Processing Technology 90 (2009) 933–938

3.3. Proposed mechanism of NOx reduction by CeO2

The NOx storage is due to the interaction of NO2 with CeO2, whichhas been demonstrated by means of X-ray absorption near-edgespectroscopy [14]. An expression of the NOx reduction in the pre-sence of CeO2 during coke combustion is proposed based on theexperimental results presented in the previous sections. The NOx

reduction by CeO2 contains the following processes:

(a) NO was formed by the oxidation of Fuel-N in coke combustion

Fuel� N þ O2→NO ð2Þ

(b) NO was oxidized to NO2, and then interacted with CeO2 tocreate cerium peroxide or super-oxide

CeO2 þ NO2→CeðNO2ÞO2 ð3Þ

(c) The peroxide or super-oxide was decomposed and contributedto increase the conversion of NOx to N2 and the oxidation rateof carbon

2CeðNO2ÞO2 þ 2C→2CeO2 þ N2 þ 2CO2 ð4Þ

2CeðNO2ÞO2 þ 4CO→2CeO2 þ N2 þ 4CO2 ð5Þ

(d) NO and NO2 are reduced by coke or CO released in cokecombustion

2NO þ C→N2 þ CO2 ð6Þ

2NO2 þ 2C→N2 þ 2CO2 ð7Þ

2NO þ 2CO→N2 þ 2CO2 ð8Þ

2NO2 þ 4CO→N2 þ 4CO2 ð9Þ

The physical model of NOx reduction in the combustion of cokeloaded with CeO2 was shown in Fig. 7. NOx reduction was mainly dueto the reactions that NOx was reduced by coke or CO released in cokecombustion. In NOx reduction reactions, CO acted as one of theimportant reducing agents for suppressing NO emission, whichwould be one main cause for the decrease of the CO concentrationduring the combustion of coke loaded with CeO2. Certainly, the CeO2

also has catalytic effect on the oxidation of CO. It was reasonableto believe that CeO2 served as a catalyst for NO reduction by cokeand CO.

4. Application in iron ore sintering process

The sintering pot tests were carried out in a stainless steel pot(H. 750 mm, I.D. 200 mm). Iron ores and fluxes were sieved andmixed with water in the same manners, the addition of coke insinter materials and sintering conditions (such as suction pressure)were also kept constant in each test. The amount of sinteringmaterials for each case was always 40.0 kg. Fig. 8 shows theconcentration of NOx emission in every run. The NOx concentrationin the base case is around 150 ppm. Apparently, the NOx con-centration is 40 ppm lower than that of the base case when the cokebreeze loaded with 2.0 wt.% CeO2 is used as sintering fuel. The NOx

emission decreases by 18.8%.After the ignition, the coke breeze in sinter materials burned and

the combustion zone moved down to the bottom of sintering bedunder suction pressure. Therefore, NOx produced in the combustionzone only can be reduced by coke breeze or CO generated by theburning coke breeze, which took place in or below the combustionzone. The thickness of coke layer plays an important role in NOx

reduction [15]: at the early stage of sintering, the NOx concentration

Fig. 8. Effect of coke breeze loaded with CeO2 on NOx emission during sintering.

938 Y. Chen et al. / Fuel Processing Technology 90 (2009) 933–938

is low due to the thicker coke layer below the combustion zone;at the latter stage of sintering, the NOx concentration graduallyincreases for the steady decrease of coke layer. Certainly, theincrease in the thickness of the combustion zone is another factor.When the coke breeze loaded with 2.0 wt.% CeO2 was used assintering fuel, the NOx reduction was enhanced by the in-situcatalytic reduction by CeO2 added in coke breeze.

5. Conclusions

NOx reduction by CeO2 modified coke was performed both in cokecombustion and by sintering pot tests, the following conclusions canbe drawn:

(1) CeO2 particles are homogeneously distributed on the surfaceand in pore canals of coke by impregnation method.

(2) In coke combustion, the NOx reduction ratio increases withincreasing CeO2 loading up to 2.0 wt.% and decreasing CeO2

particle size (28–150 µm). The NOx reduction ratio reaches avalue of 17% as CeO2 (28–38 µm) loading is 2.0 wt.%.

(3) CeO2 suppress the formation of CO in coke combustion. The COemission decreases with increasing the CeO2 loading anddecreasing the CeO2 particle size.

(4) In sintering pot tests, the NOx reduction ratio is 18.8% with2.0 wt.% CeO2 modified coke as fuel.

Acknowledgements

This work was financially supported by the National Natural ScienceFoundation of China (No. 50574085) and the knowledge InnovationProgram of the Chinese Academy of Sciences (No. O82809).

References

[1] Y. Su, Y. Mao, Z. Xu, The technology of controlling NOx emission during coalcombustion, Chemical Industry Press, Beijing, 2005, p. 11.

[2] M. Koichi, I. Shinichi, S. Masakata, A. Koji, S. Takeshi, Primary application of the “in-bed-deNOx” process using Ca–Fe oxides in iron ore sintering machines, ISIJ Int. 40(2000) 280–285.

[3] C. Jin, H. Su, L. Nam, SOx and NOx reducing method of sintering discharging gas.Korean Patent, No.20020040506, 2002-05-30.

[4] Ralf F.W. Kopsel, S. Halang, Catalytic influence of ash elements on NOx formation inchar combustion under fluidized bed conditions, Fuel 76 (1997) 345–351.

[5] Z. Wu, Y. Sugimotob, H. Kawashima, Effect of demineralization and catalystaddition on N2 formation during coal pyrolysis and on char gasification, Fuel 82(2003) 2057–2064.

[6] Z. Zhao, W. Li, J. Qiu, H. Chen, B. Li, Influence of Na and Ca on the emission of NOx

during coal combustion, Fuel 85 (2006) 601–606.[7] Z. Zhao, W. Li, J. Qiu, B. Li, Effect of Na, Ca and Fe on the evolution of nitrogen

species during pyrolysis and combustion of model chars, Fuel 82 (2003)1839–1844.

[8] Y. Chen, Z. Guo, Z. Wang, A method of NOx reduction by coke modified withadditives in sintering process. Chinese Patent, Appl. 200710177237.7, 2007-11-13.

[9] A. Paulenova, S.E. Creager, J.D. Navratil, Y. Wei, Redox potentials and kinetics of theCe3+/Ce4+ redox reaction and solubility of cerium sulfates in sulfuric acidsolutions, J. Power Sources 109 (2002) 431–438.

[10] M. Piacentini, M. Maciejewski, A. Baiker, NOx storage-reduction behavior of Pt–Ba/MO2 (MO2=SiO2, CeO2, ZrO2) catalysts, J. Appl. Catal. B 72 (2007) 105–117.

[11] J. He, W. Song, S. Gao, L. Dong, M. Barz, J. Li, et al., Experimental study of thereduction mechanisms of NO emission in decoupling combustion of coal, FuelProcess. Technol. 87 (2006) 803–807.

[12] N. Takahashi, K. Yamazaki, H. Sobukawa, H. Shinjoh, The low-temperatureperformance of NOx storage and reduction catalyst, Appl. Catal. B Environ. 70(2007) 198–204.

[13] Q. Wang, J. Qiu, Y. Liu, C. Zheng, Effect of atmosphere and temperature on thespeciation of mineral in coal combustion, Fuel Process. Technol. 85 (2004)1431–1441.

[14] K. Krishna, M. Makkee, Coke formation over zeolites and CeO2-zeolites and itsinfluence on selective catalytic reduction of NOx, J. Appl. Catal. B 59 (2005) 35–44.

[15] Z. Zhao, W. Li, J. Qiu, H. Chen, B. Li, Influence of mineral matter in coal ondecomposition of NO over coal chars and emission of NO during char combustion,Fuel 82 (2003) 949–957.