Magnetic field effects on selective catalytic reduction of NO by NH3 over Fe2O3 catalyst in a magnetically fluidized bed

Energy 35 (2010) 2295e2300

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Magnetic field effects on selective catalytic reduction of NO by NH3 over Fe2O3 catalyst in a magnetically fluidized bed Gui-huan Yao, Fang Wang, Xiao-bo Wang, Ke-ting Gui* College of Energy and the Environment, Southeast University, Sipai Lou #2, Nanjing 210096, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2009 Received in revised form 7 February 2010 Accepted 10 February 2010 Available online 15 March 2010

Selective catalytic reduction (SCR) of NO from simulated flue gas by ammonia with Fe2O3 particles as the catalyst was performed using a magnetically fluidized bed (MFB). X-ray diffraction (XRD) spectroscopy and BrunauereEmmetteTeller (BET) method were used to analyze Fe2O3 catalyst. Important effects of magnetic fields were observed in the SCR of NO by ammonia over Fe2O3 catalyst. The apparent activation energies of SCR were reduced by external magnetic fields, and the SCR activity of Fe2O3 catalyst was improved with the magnetic fields at low temperatures. Thus the scope of temperature with high efficiency of NO removal was extended from 493e523 K to 453e523 K by magnetic fields. Magnetic fields of 0.01e0.015 T were suggested for NO removal on Fe2O3 catalyst with MFB. The results suggested that the magnetoadsorption of NO onto Fe2O3 surface together with NH2 and NO free radicals effects induced by the external magnetic fields both acted to improve the rate of SCR of NO on Fe2O3 catalyst. On the other hand, magnetic field effects were also attributed to improved gasesolid contact in MFB. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

Keywords: Magnetic field effects SCR Fe2O3 Magnetically fluidized bed Ammonia

1. Introduction Fossil fuels approximately account for 80% of worldwide energy supplies in human societies, but the combustion of fossil fuels results in many environmental problems. Therefore, energy-related environmental problems are considered in energy efficiency measures, and energy-related environmental studies are becoming important and hot issues [1,2]. Nitrogen oxides (mainly NO) are one of the major air pollutants emitted from fossil fuel combustion, and the ecological problem of air pollution caused by nitrogen oxides has become prevalent in recent years. The selective catalytic reduction (SCR) of NO by NH3 is an effective technique for the control of nitric oxide emissions in the flue gas of large combustion units [3,4]. The main SCR reaction is as follows: 4NH3 D 4NO D O2 / 4N2 D 6H2 O:

(1)

Many catalysts have been reported to be active for the above reaction, such as V-based catalysts, Mn-based catalysts, Fe-based catalysts, and other transition metal catalysts [5e13]. Iron oxides' catalysts, especially Fe2O3 [4,14e17] and Fe containing mixed oxides [17e22], have been investigated repeatedly for the removal

* Corresponding author. Tel.: þ86 25 83792506; fax: þ86 25 83613851. E-mail addresses: [email protected] (G.-h. Yao), [email protected] (K.-t. Gui).

of nitrogen oxides with ammonia due to its SCR activity and low cost. Most of these catalysts were in high SCR activity at medium and high temperatures, but the thermal stability of Fe2O3 was not good at high temperatures, and the oxidation of ammonia started above 573 K on iron oxide catalysts. On the other hand, the tail-gas arrangement is a favorable configuration of abatement technologies for existing power plants. However, the incoming flue gas in this arrangement has to be reheated by additional burners that mainly use natural gas to reach the operating temperature of the commercial catalysts, which is from 533 K to 593 K. If the operating temperature of the SCR catalysts can be reduced to about 473 K or below it, the heat exchange from the raw gas after the economizer to the cleaner gas will be usually sufficient, and then more energy and operating costs will be saved. Therefore, it is important to improve SCR activity of iron oxide catalysts at low and medium temperatures. Applying magnetic fields on magnetic particles in a fluidized bed, a magnetically fluidized bed (MFB) is formed. This special fluidized bed has good hydrodynamics behavior and mass transfer performance, and it has been designed as a chemical reactor for some scientific research in both laboratory and pilot scales [23]. Furthermore, as the magnetic field is a kind of energy field, it might influence the process and results of chemical reactions [24e32], such as rate of reaction and production, orientation, property and structure, and balance of reaction kinetics. In the SCR of NO, the reaction rate and reaction temperatures were related to the activation energy of the SCR reaction, thus magnetic fields could be

0360-5442/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.02.017

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applied to the SCR reaction to influence the activation energy of the reaction and improve the efficiency of SCR of NO at low and medium temperatures. The research was completed in a magnetically fluidized reactor using Fe2O3 catalyst as fluidized particles in this paper. The experimental results showed that the external magnetic fields reduced the activation energies of SCR and improved the SCR activity of the Fe2O3 catalyst at low temperatures. These results give the foundation for the application of magnetic fields on the heterogeneous SCR reaction over Fe2O3 catalyst. 2. Experimental 2.1. Experimental materials and setup The particulate Fe2O3 catalyst was prepared by the Research Institute of Nanjing Chemistry Industry Group, and collected in the diameters of 450e600 mm by sieving. The average diameter and the bulk density of Fe2O3 catalyst particles were 520 mm and 1.0 g cm3, respectively. The setup of the experiment was illustrated in Fig. 1. This setup mainly consisted of eight units: a simulated flue gas system, an MFB reactor, a magnetic field system, a Lortz vacuum pump, a flue gas analysis unit, an electric heating and heat exchange system, measurement and control unit for hydrodynamics, and dust removal. The simulated flue gas used as the fluidized gas for the chemical reaction was driven to the fluidized bed reactor with the assistance of the Lortz vacuum pump. The fluidized bed reactor was made up of stainless steel (316 L, China) with a height of 1.0 m and an inner diameter of 0.1 m. Two perforated stainless steel plates with 8.2% open area were used as a distributor. The stainless steel wire mesh of 120 meshes was clamped between these two plates. Helmholtz coils were wound from copper wire, and the coil had an average diameter of 0.4 m, a height of 0.14 m, and 2673 turns. The Helmholtz coils were coaxial with the fluidized bed, and the space between these two coils was 0.2 m. In addition, the center plane of the lower coil was at the same level as the distributor or slightly lower. A uniform axial magnetic field was generated by Helmholtz coils, and the magnetic field intensity could be controlled by adjusting the current of the D.C. stabilized voltage supply. The maximum power of the supply was 300 V  7 A. The pressure at different locations in the fluidized bed was measured by watercooled pressure transmitters in the range of 50 to e50 kPa.

2.2. Experimental procedure The fluidized bed reactor was filled with particulate Fe2O3 catalyst as a solid medium to a height of 0.1 m. The particles were fluidized by air in cold-model experiments, and the minimum fluidization velocity of 0.1 m s1 was ascertained by the pressuredrop method. The magneto fluidization was operated with external magnetic fields from 0 T to 0.025 T in magnetization FIRST mode, where the magnetic field was applied to a static powder bed before fluidization [33]. The SCR of NO on Fe2O3 catalyst was operated in an MFB reactor. The simulated gas was adjusted and measured by mass flow rate controllers (MFCs) (D07-19B, Sevenstar, China) and thermal mass flow meters (Series MPNH, EPI, USA). With the assistance of the pump, the flue gas was mixed with NH3 through a mixing chamber and distributors, and then was driven to the MFB. Ammonia reacted with NO under the conditions of external magnetic fields and catalysis of Fe2O3 catalyst in the fluidized bed. The inlet and outlet gas of the fluidized bed were analyzed by an online flue gas analyzer (rbr ecom-J2KN, Germany) with an accuracy of 5 ppm for NO and NO2. The NO conversion was calculated as follows:

h¼

Ci  Co  100%; Ci

(2)

where, h was NO conversion (%), Ci represented the NO concentration in the inlet of the fluidized bed (ppm), and Co represented the NO concentration in the outlet of the fluidized bed (ppm). After each experimental run in which ferrimagnetic Fe2O3 particles were subjected to an external magnetic field, the Fe2O3 particles had to be demagnetized by allowing the bed to bubble with cold air in the absence of magnetic field for 30 min. Therefore, the residual magnetization was negligible. The simulated flue gas consisted of 500 ppm NO, 5 vol.% O2 and the balance N2. The NH3/NO mole ratio used in the SCR of NO was 1:1. The reactions were performed at 423e618 K. Two superficial velocities of the simulated flue gas, 0.1 m s1 and 0.15 m s1, were applied with assisted magnetic induction from 0 T to 0.025 T. To make sure the reaction was in a steady situation, each case of SCR reaction in MFB lasted for 30 min with online measurement.

Fig. 1. Schematic diagram of testing apparatus for NO removal from flue gas in a magnetically fluidized bed. 1. NO; 2. NH3; 3. Purified air; 4. N2; 5. Mass flow rate controller (MFC); 6. N2 collector tank; 7. N2 thermal mass flow meter; 8. Air thermal mass flow meter; 9. Target flow meter; 10. Fluidized bed reactor; 11. Helmholtz coils; 12. D.C. stabilized voltage supply; 13. Flue gas analyzer; 14. Computer; 15. Agilent 34970A data acquisition instrument; 16. Electric heater; 17. Temperature control cabinet; 18. Mixing chamber composed of perforated plates; 19. Spiral heat exchanger; 20. Vortex fan; 21. Bag collector; 22. Lortz vacuum pump; 23. Valve; 24. tee; T, temperature; P, pressure; V, volume flow rate.

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2.3. Material characterization

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Table 1 BET surface area, pore volume and size of the particle Fe2O3 catalyst.

The specific surface area, pore volume and pore size distribution of the catalyst were measured by nitrogen adsorption isotherms at 77 K on a Micromeritics ASAP 2020M analyzer and determined by the BrunauereEmmetteTeller (BET) and BarretteJoinereHalenda (BJH) methods. The X-ray diffraction (XRD) measurement was carried out on an X-ray diffractometer (XD-3A, Analysis and Testing Center of Southeast University) with Cu Ka (l ¼ 0.1543 nm) radiation, 40 kV  30 mA. The sample was scanned in the 2q range from 10 to 90 at ambient temperature, and the crystalline phases of the catalyst were identified by PCPDFWIN with JCPDSeICDD PDF-2004. 3. Results and discussion 3.1. Characterization of the Fe2O3 catalyst The specific surface area, pore volume, and pore size of the Fe2O3 catalyst were summarized in Table 1. The BET surface area of the Fe2O3 catalyst was 90.8 m2 g1 as shown in Table 1, and this surface area was larger than that of commercial V-based catalysts. The XRD patterns for the Fe2O3 catalyst before and after the SCR reaction were shown in Fig. 2. The main phases found in the Fe2O3 catalyst were g-Fe2O3 (maghemite) and a-Fe2O3 (hematite); g-Fe2O3 was stable and didn't transform to a-Fe2O3 by thermal conversion in SCR at 423e618 K [34].

BET (m2 g1) 90.8 External surface area (m2 g1) 87.0

Pore volume (cm3 g1)

Average pore diameter (nm)

0.258

11.3

Micropore area (m2 g1) 3.80

Nitric oxide was observed in the oxidation of ammonia when the temperature exceeded 523 K, and 138 ppm NO was formed with 21 ppm NO2 at 623 K. Furthermore, ammonia was oxidized to more NO and NO2 above 623 K in Fig. 4. Combined with Fig. 3, the temperature below 523 K was suggested in the SCR of NO by ammonia over Fe2O3 catalyst to avoid the ammonia oxidation. With the aid of the magnetic fields, the range of temperature with high NO removal efficiency could be changed from 493e523 K to 463e493 K, which could make the temperature range of the SCR reaction apart from the oxidation of ammonia. 3.4. Influence of magnetic fields over the activation energy of SCR reaction A semi-empirical equation was deduced by Marangozis [35] to calculate the pseudo-first order apparent rate constant for SCR of

3.2. Influence of magnetic fields on the efficiency of NO removal Fig. 3 compared the results of the abatement of NO by SCR on Fe2O3 catalyst with and without the effects of a magnetic field in the fluidized bed. Fig. 3(a) showed a good SCR performance under the effects of a magnetic field at the fluidized flue gas velocity of 0.1 m s1. Without external magnetic fields, nitric oxide conversions of 78% and 85% were obtained at 453 K and 473 K, respectively. The conversion efficiency increased with increasing temperatures, reached a maximum at 523 K, and then decreased. On the other hand, the efficiency of NO removal reached 89% and 95% in the magnetic field of 0.015 T at 453 K and 473 K, respectively. It demonstrated that the conversion efficiency was raised by about 10% with the aid of the magnetic field. Nitric oxide conversion only got to 62% at a velocity of 0.15 m s1 at 453 K without an external magnetic field in Fig. 3(b). The conversion also increased to its maximum at 523 K, and then decreased with increasing temperatures. With the assisted magnetic fields, nitric oxide conversion efficiency was increased from 80% to 89% and from 85% to 94% at 473 K and 493 K, respectively. It also showed that the conversion efficiency increased by approximately 10% with the external magnetic fields. In order to take advantage of magnetic fields over the SCR of NO by NH3, the assisted magnetic fields of 0.01e0.015 T were recommended on ferrimagnetic Fe2O3 particles in the MFB. 3.3. Ammonia oxidation on the Fe2O3 catalyst The ammonia oxidation on Fe2O3 catalyst was studied in the same bed reactor in the presence of 5 vol.% oxygen to comprehend the phenomena that the NO conversion decreased above 523 K in Fig. 3, and the results were shown in Fig. 4. The initial concentration of NH3 in the flue gas was 500 ppm, and the superficial velocity of the flue gas was 0.1 m s1. The reaction of ammonia oxidation to NO could be illustrated by the following equation: Fe2 O3

4NH3 þ 5O2 ƒƒ! 4NO þ 6H2 O

(3)

Fig. 2. XRD patterns for the Fe2O3 catalyst: (a) before reaction; (b) after reaction at 618 K.

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Fig. 4. Characteristics of ammonia oxidation on the Fe2O3 catalyst: 500 ppm of NH3, 5 vol. % of O2, N2 balance, total feed flow velocity of 0.1 m s1.

Ea ¼ Rg

d ln ka : dð1=TÞ

(6)

According to the regression of experimental data, the relationship curves between ln ka and 1=T in different magnetic fields were drawn in Fig. 5. These curves were also called as Arrhenius plots. Thus the apparent activation energies for SCR of NO by NH3 over Fe2O3 catalyst were calculated according to Eq. (6) from the slope of the tangent at different positions of the curves, and tabulated in Table 2. It was reported that the activation energy of iron oxide catalysts for SCR of NO by NH3 was about 36.54 kJ mol1 [35,36]. This value was close to the apparent activation energies listed in Table 2 without a magnetic field. Comparing the apparent activation energies in the magnetic fields of 0 T and 0.005 T in Table 2, we concluded that the applied magnetic field brought down the apparent activation energies of the SCR reaction at relevant temperatures. Fig. 3. NO conversion as a function of reaction temperatures in the simulated flue gas on Fe2O3 catalyst with MFB; 500 ppm of NO, 500 ppm of NH3, 5 vol. % of O2, N2 balance, total feed flow velocity: (a) 0.1 m s1, (b) 0.15 m s1.

NO by NH3 and O2 with respect to NO, at constant oxygen concentration:

 ka ¼

 F ln½1=ð1  hÞ ; VR 1  3b

(4)

3.5. Transport effects by magnetic fields on SCR over Fe2O3 catalyst in MFB The good gasesolid contact was important and useful for a catalytic reaction when transport phenomena controlled catalytic performance. Dumesic et al. [37] reviewed regimes of catalytic rate control in heterogeneous catalytic reactions with respect to the

where, ka was the apparent rate constant (s1), F was the total volumetric feed flow rate (m3 s1), VR was the reactor-bed volume (m3), and 3b was the bed void fraction (%). VR ð1  3b Þ was the real volume of catalysts taking part in reactions in the MFB reactor, and it was a certain value in the same bed of same materials. This value could be ascertained in the static bed. The void fraction of the static bed was measured and calculated as 0.416 by a water filled method with respect to the micropore volume of the Fe2O3 catalyst in Table 1. The relationship between an apparent rate constant and reaction temperatures could be described by the Arrhenius equation:

d ln ka Ea ; ¼ dT Rg T 2

(5)

where, Rg was the universal gas constant (J mol1 K1), Ea was the apparent activation energy (kJ mol1), T was the reaction temperature (K). Rearranging Eq. (5), Eq. (6) would be obtained to calculate the apparent activation energy

Fig. 5. Comparison of Arrhenius plots: the relationship curves between ln ka and 1=T in the magnetic fields of 0 T and 0.005 T with the flue gas velocity of 0.15 m s1.

G.-h. Yao et al. / Energy 35 (2010) 2295e2300 Table 2 Apparent activation energies of SCR on Fe2O3 catalyst. External magnetic field (T)

Temperature (K)

Ea (kJ mol1)

0

423 453 473 493

40.81 35.85 28.32 14.48

0.005

453 473 493

33.15 22.44 9.34

effect of temperature, and Sun [38] summarized a quantitative criterion for reaction rate limiting regimes with apparent activation energy. When the apparent activation energy was 20e40 kJ mol1, the reaction rate was determined by both chemical reaction and ‘mass transfer and diffusion’ [38]. Therefore, the reaction rate in the SCR of NO on the Fe2O3 catalyst was controlled by chemical reaction and ‘mass transfer and diffusion’ together above 453 K, based on the calculated activation energies in Table 2. The Fe2O3 catalyst consisted of a-Fe2O3 and ferrimagnetic g-Fe2O3, so magneto fluidization could be carried out with a coaxial and uniform magnetic field in the fluidized bed reactor. The minimum fluidization gas velocity of Fe2O3 particles was not affected by the applied magnetic fields. In an MFB, the bed behavior was identical to that in a conventional bubbling bed up to the point of incipient fluidization. Rather than bubbling, the bed expanded in a piston-like manner and turned into a magnetically stabilized bed at superficial gas velocities above the minimum fluidization velocity. However, with high magnetic fields, the bed became a ‘frozen’ bed, and the particles were locked together in relative positions without their flowability. We have done previous work on the fluidization characteristics of magnetic particles and the determination of stable fluidization zone in MFB [39,40]. The magnetic field could not only check the bubbles, stabilize hydrodynamics and widen the range of the velocity of the particulate fluidization, but also eliminate the bubbles and change the aggregative fluidization into the particulate one in MFB. The magnetically stable fluidization zone extended further as the applied field intensity increased. Thus the gasesolid contact was strengthened by the magnetic field in MFB, especially in the stable fluidization. In addition, catalyst particles arranged forming chains following magnetic field lines and the bed of the particles expanded in magnetically stable fluidization, so the residence time of flue gas in the catalyst bed increased, and the probability of the heterogeneous catalytic reaction was improved. In combination with the results of NO removal with MFB in Fig. 3, the NO conversion on the Fe2O3 catalyst was promoted by good heat and mass transfer characteristics in modest magnetic fields of 0.01e0.015 T. 3.6. Magnetic field effects on SCR over Fe2O3 catalyst in MFB The adsorption of NO on Fe2O3 catalyst surface and NH2 production from H-abstraction of coordination NH3 were important in SCR on Fe2O3 catalysts with ammonia at low temperatures. Hence the effects of magnetic fields on SCR of NO by NH3 over Fe2O3 catalyst in MFB still could be expressed as follows. Firstly, the magnetoadsorption of NO onto Fe2O3 was enhanced by magnetic fields. Nitric oxide not only adsorbed readily on iron [41], but also on iron oxides [14,42,43]. Delbecq and Sautet [44] pointed out that chemisorption of NO on a magnetic surface was accelerated by the interaction between the singly occupied 2p spin-orbital on NO molecule and the vacant d spin-orbital on magnetic sites of the magnetic surface. According to the ferromagnetic theory of Néel, the net magnetic moment of g-Fe2O3 was

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closely approximated by the difference in the spin only moments between the tetrahedral Fe3þ and octahedral Fe3þ magnetic sublattices. The spin only moment of high spin Fe3þ was proportional to the number of unpaired d-electrons of each (5 unpaired spins in the case of Fe3þ). Nitric oxide had the ability to bond with Fe3þ  surface cations as NOþ, NO or NO, and the nature of the adsorbed NO was sensitive to the coordination of the Fe3þ cation. Unpaired delectrons of Fe3þ surface cations were reactive, and easier to bond with NO molecule. The chemical potential of substances could be changed with additional magnetic chemical potential by magnetic fields. The magnetic field on the ferrimagnetic g-Fe2O3 surface involved contributions by the external magnetic field and the magnetization of g-Fe2O3. There was a gradient magnetic field around the surface of g-Fe2O3 particles. Nitric oxide was transferred from the ambient space of g-Fe2O3 particles to the surface until the chemical potential around the surface was equal, since nitric oxide was paramagnetic. Therefore, the concentration of NO on the surface increased, and the adsorption of NO on g-Fe2O3 was strengthened. Ozeki [45,46] reported that magnetoadsorption occurred in the low magnetic field of less than 0.1 T on g-Fe2O3. A better NO conversion was found on the Fe2O3 catalyst after the Fe2O3 catalyst was subjected to the magnetic field in our experiments. This phenomenon might be attributed to the effects of residual magnetization on g-Fe2O3. Secondly, the production of NH2 active free radical and free radicals reaction between NH2 and NO were accelerated by the external magnetic field. NH3 adsorbed molecularly on Fe3þ site, and then generated NH2 intermediate under the oxidizing property of Fe3þ on the surface of the Fe2O3 catalyst. Grzybek [47] presented the stabilization of formed NH2 radical from adsorbed with NH3 on the surface of Fe2O3 in vacuum at 473e533 K. Zhang et al. [48] reported that the applied external magnetic field increased the formation rate of surface hydroxyl radical produced over Pt/TiO2 film for benzene degradation by photocatalysis. Moreover, the external magnetic field would impact on the electron-spin of an unpaired electron in a free radical, and alter the order of reaction systems, thereby change the overall rate of SCR of NO on the Fe2O3 catalyst. 4. Conclusions Magnetic field effects were observed in the SCR of NO by NH3 over Fe2O3 catalyst in an MFB. The external magnetic field reduced the apparent activation energies in SCR of NO on the Fe2O3 catalyst at 453e493 K, and improved low-temperature SCR activity of the Fe2O3 catalyst. With external magnetic fields, the efficiency of NO removal was improved to over 90% at 453e493 K, and increased by 10% compared with that without magnetic fields. Modest magnetic fields of 0.01e0.015 T were recommended for the NO removal on magnetic Fe2O3 catalyst with MFB. Magnetic field effects on SCR of NO by NH3 over Fe2O3 catalyst in MFB were concluded into two components. Firstly, the NO conversion was improved by magnetoadsorption of NO onto Fe2O3, increased formation of NH2 free radicals and the reaction between NH2 and NO free radicals with an external magnetic field. Secondly, additional magnetic field effects on SCR occurred, primarily due to the improved gasesolid contact in an MFB. These results show that an MFB is a promising reactor for the SCR of NO by NH3 over Fe2O3 particle catalyst at low and medium temperatures. Acknowledgements This research is financially supported by the National Natural Science Foundations of China (Grants No. 50576013). We appreciate the assistance of the Research Institute of Nanjing Chemistry Industry Group for the preparation of the iron oxide catalysts.

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