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Microwave Catalytic Desulfurization and Denitrification Simultaneously on Fe/Ca-5A Zeolite Catalyst Zaishan Wei,* Guihua Zeng, and Zhirong Xie School of EnVironmental Science and Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, People’s Republic of China ReceiVed NoVember 24, 2008. ReVised Manuscript ReceiVed March 12, 2009
Simultaneous desulfurization and denitrification has been investigated using a microwave reactor packed with ammonium bicarbonate (NH4HCO3) and Fe/Ca-5A zeolite or Fe/Ca-5A zeolite only. The experimental results showed that 95.6% of sulfur dioxide (SO2) and 97.3% of nitrogen oxides (NOx) could be removed in the microwave reactor filled with NH4HCO3 and Fe/Ca-5A zeolite; a microwave reactor with Fe/Ca-5A zeolite only could also be used to microwave catalytic oxidative 69.5% SO2 to sulfate and 79.3% NOx to nitrates. The desulfurization and denitrification effect of the experiment using a microwave reactor with Fe/Ca-5A zeolite only is close to that of catalytic reduction of SO2 and NOx using ammonium bicarbonate and Fe/Ca-5A zeolite together. The optimal microwave power and empty bed residence time (EBRT) on microwave catalytic reduction desulfurization and denitrification simultaneously are 280 W and 0.358 s, respectively. The mechanism for microwave catalytic reduction of SO2 and NOx simultaneously can be described as the microwave-induced reduction catalyst reaction between SO2, NOx, and ammonium bicarbonate, with Fe/Ca-5A zeolite being the catalyst and microwave absorbent. Microwave catalytic reduction reaction of SO2 and NOx with the Fe/Ca-5A zeolite catalyst follows Langmuir-Hinshelwood kinetics.
1. Introduction Sulfur dioxide (SO2) and nitrogen oxides (NOx), mainly emitted from power plants, incinerators, and boilers during coal combustion, are major air pollutants.1 Sulfur dioxide is generally accepted to be the most important precursor to acid rain.2 Nitrogen oxides contribute a lot to photochemical smog, acid rain, ozone depletion, and the greenhouse effect.3,4 Many technologies have been developed to remove SO2 and NOx from flue gas, among them, wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) are most effective for SO2 and NOx removal, respectively. Compared to the individual control techniques, simultaneous desulfurization and denitrification is advantageous due to less equipment demanded and low cost. SO2 and NOx from flue gas were simultaneous removed using copper on alumina catalyst sorbents (CuO/ Al2O3)5 or by a dual bed of potassium-containing coal-pellets and calcium-containing pellets.6 Microwave has been widely used in environment protection.7 Microwave was applied to a pyrolytic carbon such as activated carbon and char, enhancing the reaction of sulfur dioxide (SO2) and nitrogen oxide (NO) with carbon.8 The simultaneous treatment with the accelerated electronic beams and the micro* To whom correspondence should be addressed. Phone: +86-2084037096; fax: +86-20-39332690; e-mail:
[email protected]. (1) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q.; Wen, Z. C.; Liu, J. Z.; Cen, K. F. Fuel. Pro. Tech. 2007, 88, 817. (2) Zhang, X. L.; David, O. H.; Colleen, L. D.; Michael, P. M. Appl. Catal., B 2001, 33, 137. (3) Carja, G.; Kameshima, Y.; Okada, K.; Madhusoodana, C. D. Appl. Catal., B 2007, 73, 60. (4) Vicente, S. E.; Tania, M.; Guido, B. Appl. Catal., B 2005, 58, 19. (5) Xie, G. Y.; Liu, Z. Y.; Zhu, Z. P. J. Catal. 2004, 224, 42. (6) Bueno, L. A.; Garcı´a, G. A. Fuel. Pro. Tech. 2005, 86, 1745. (7) Jones, D. A.; Lelyveld, T. P.; Mavrofidis, S. D. Resour. ConserV. Recycl. 2002, 34, 75. (8) Cha, C. Y.; Kim, D. S. Carbon 2001, 39, 1159.
waves could increase the removal efficiency of NOx and SO2; about 80% of NOx and more than 95% of SO2 were removed by precipitation with ammonia.9 It was reported that the reaction efficiency of microwave reduction of NOx could be up to 98% when microwave energy was applied continuously.10 Some research has been conducted in simultaneous desulfurization and denitrification by plasma technologies such as calcium magnesium acetate,11 electron beam and electrical discharge induced nonthermal plasmas,12 pulsed corona discharge plasma and additives,13,14 and nonthermal plasma.15 In this paper, experimental investigations were conducted to simultaneous desulfurization and denitrification from stimulated flue gas by the novel microwave catalytic reactor with bicarbonate ammonium (NH4HCO3) and Fe/Ca-5A zeolite or Fe/Ca-5A zeolite only. The study evaluates the influence of microwave power, empty bed residence time (EBRT), microwave and catalyst on the performance of the microwave reaction system, and the mechanistic and kinetic analysis of microwave-induced catalytic SO2 and NOx reduction were elicited. An understanding of the role of microwave and catalyst on desulfurization and denitrification simultaneously can help evaluate the potential (9) Radoiu, M. T.; Martin, D. I.; Calinescu, I. J. Hazard. Mater. 2003, B97, 145. (10) Zhang, D. X.; Yu, A. M.; Jin, Q. H. Chem. J. Chinese. UniV. 1997, 18, 1271. (11) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E. Fuel 2004, 83, 149. (12) Ighigeanu, D.; Martin, D.; Zissulescu, E.; Macarie, R.; Oproiu, C.; Cirstea, E.; Iovu, H.; Calinescu, I.; Iacob, N. Vacuum 2005, 77, 493. (13) Shang, K. F.; Wu, Y.; Li, J.; Li, G. F.; Li, D.; Wang, N. H.; Zhu, J. Recent DeV. Appl. Electron. 2004, 61. (14) Onda, K.; Kasuga, Y.; Kato, K.; Fujiwara, M.; Tanimoto, M. Energy ConserV. Manage. 1997, 38, 1377. (15) Yu, Q.; Yang, H. M.; Zeng, K. S.; Zhang, Z. W.; Yu, G. J. EnViron. Sci. 2007, 19, 1393.
10.1021/ef801013w CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
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Figure 1. Experimental flow loop of reduction of SO2 and NOx with microwave ammonium bicarbonate over catalyst zeolite. (1) SO2 gas cylinder, (2) NOx gas cylinder, (3) air compressor, (4) the bottle of gas mixture, (5) flow meter, (6) quartz tube, (7) microwave reactor, (8) outlet port, (G) sampling port.
of applying this novel method as an effective SO2 and NOx emission control strategy for the flue gas cleaning. 2. Experimental Section 2.1. Microwave Reaction System. The experimental flow loop used in the study is shown schematically in Figure 1. A constant input microwave power of 164 ∼ 331 W was used, and the microwave frequency was 2450 MHz. The microwave reactor consisted of a quartz tube (i.d. 10 mm and 250 mm long) with ammonium bicarbonate (NH4HCO3) and Fe/Ca-5A zeolite or Fe/ Ca-5A zeolite only was set up to study removal of SO2 and NOx (NO, NO2) from stimulated waste gas. The SO2 and NOx supplied from the gas cylinders, were first diluted with compressed air, passed through an air mixture bottle, and flowed upward through the microwave reactor. The flow meter and the valve were used to monitor the gas flow through the reactor. SO2 and NOx (NO, NO2) concentrations were monitored by the analysis device of S2000 flue gas, and gas flow rate was monitored by the rotameter and the mass flow controllers. In the process of the experiments, the simulated SO2- and NOx-containing flue gas were supplied to the microwave reactor at a flow rate of 150 ∼ 350 L · h-1 (EBRD, 0.153 ∼ 0.358 s). 2.2. Measurement Methods. The periodic measurements of the gas concentration from sampling ports and the gas flow of the quartz tube in the microwave reaction system were carried out by using the following devices. A S2000 flue gas device was used for the analysis of sulfur dioxide (SO2) and nitrogen oxides NOx (NO, NO2). The rate of the gas flow was measured by Model LZB-1 flow meters with the units of 0.01 m3 · h-1. An infrared radiation thermometer was used to measure the surface temperature of catalyst.
Figure 2. Influence of concentration of NOx in inlet on denitrification (reaction conditions: gas flow ) 0.15 m3 · h-1; EBRT ) 0.358s; inlet concentration of SO2 ) 1000 mg · m-3; microwave power ) 280 W).
3. Results and Discussion 3.1. Simultaneous Microwave Catalytic Desulfurization and Denitrification. The microwave catalytic desulfurization and denitrification by microwave reactor with Fe/Ca-5A zeolite only is shown in Figures 2 and 3. The MWFe/Ca-5A zeolite and ABFe/Ca-5A zeolite profile are represented by Fe/Ca-5A zeolite under microwave and ammonium bicarbonate over Fe/ Ca-5A zeolite separately. SO2 removal efficiency decreases from 69.5 to 54.4% when the concentration of SO2 in inlet is increased; whereas NOx removal efficiency changes from 79.3 to 67.5% with increasing concentration of NOx. Figures 2 and 3 indicate that Fe/Ca-5A zeolite catalyst has good performance of microwave catalytic desulfurization and denitrification under no reducing agent; the desulfurization and denitrification effect using microwave and Fe/Ca-5A zeolite only is close to that of catalytic reduction of SO2 and NOx using ammonium bicarbonate as reducing agent and Fe/Ca-5A zeolite as catalyst. With no reducing agent such as ammonium bicarbonate, about 68.3% of sulfur dioxide and 76% of nitric oxide are converted in the microwave reactor with Fe/Ca-5A zeolite only under the conditions of gas flow of 0.15 m3 · h-1, microwave power (280 W), inlet concentration of 1000 mg · m-3 of SO2, and inlet concentration of NOx of 250 mg · m-3.
Figure 3. Influence of concentration of SO2 in inlet on desulfurization (reaction conditions: gas flow ) 0.15 m3 · h-1; EBRT ) 0.358s; inlet concentration of NOx ) 250 mg · m-3; microwave power ) 280 W).
During nonthermal plasma operation, ambient gas temperatures increase due to inelastic electron-molecule collisions.9 Consequently, microwaves, nonionizing radiation incapable of breaking bonds, are a form of energy that manifest as heat through their interaction with the medium or materials.7 Thermal effects produced by microwaves on mineral-supported catalysts seem to result from the formation of rapid superheating of the catalyst surface, which cause the high temperature;2 microwave heating has been used to promote desulfurization of poisoned NOx storage-reduction catalysts.16 However, the Fe/Ca-5A zeolite surface reaction temperature range of microwave catalytic (16) Ibe, M.; Gomez, S.; Malinger, K. A.; Fanson, P.; Suib, S. L. Appl. Catal., B 2007, 69, 235.
MicrowaVe Catalytic Desulfurization and Denitrification
desulfurization and denitrification was 90 ∼ 110 °C, obviously lower than the temperature of selective noncatalytic reduction (SNCR), 900-1100 °C. Microwave irradiation further enhanced the number of the catalyst active sites.17 This means that the observed reactions are not due to the microwave heating but to microwave- catalytic-generated hydroxyl radicals. The possible reason for this could be that a fast inducement over the surface of Fe/Ca-5A zeolite catalyst by microwave includes the production of hydroxyl radicals, which not only induces SO2 oxidation catalytic reaction to sulfate, but also induces NOx oxidation to nitrates. High performance liquid chromatography (HPLC) was used to identify hydroxyl radicals, the results showed that the active OH free radicals existed in the process and were involved in the microwave catalytic system (see Supporting Information). Basically, microwave-generated catalytic hydroxyl radicals play an important role in the oxidation of SO2 to SO3 and NO to NO2, simultaneously. 3.2. Simultaneous Microwave Catalytic Reduction of SO2 and NOx. The influence of concentration of SO2 and NOx in inlet on desulfurization and denitrification are shown in Figures 2 and 3, separately, under the conditions of gas flow of 0.15 m3 · h-1 and microwave power (280 W). The MWABFe/ Ca-5A zeolite profile is represented by ammonium bicarbonate and Fe/Ca-5A zeolite under microwave. As are shown in Figures 2 and 3, the additional use of microwave to ammonium bicarbonate over Fe/Ca-5A zeolite leads to the enhancement of SO2 removal efficiency from 17.9 to 23.3%, and NOx removal efficiency from 13.8 to 19%. For comparison, in the presence of microwave irradiation and Fe/Ca-5A zeolite, with/without ammonium bicarbonate (NH4HCO3) in flue gas treatment process, SO2 removal efficiency increases from 24 to 29.1% and the denitrification efficiency increases from 18 to 21.7%. The SO2 and NOx removal efficiency achieved 95.6 and 97.3%, respectively, in the microwave reactor with NH4HCO3 and Fe/ Ca-5A zeolite. The ammonium bicarbonate does not absorb microwave energy, and hence microwave would not induce SO2 and NOx reduction without a microwave absorbent such as Fe/Ca-5A zeolite. Because some zeolite catalyst has magnetic properties,18 the Fe/Ca-5A zeolite catalyst does absorb microwave energy but requires the reducing agent such as ammonium bicarbonate. The use of both Fe/Ca-5A zeolite and ammonium bicarbonate combined with microwave energy would induce SO2 and NOx catalytic reduction reaction significantly.19 Significant synergetic effects of microwave and catalyst treatment were observed. We could speculate that microwave accentuates catalyst reduction of SO2 and NOx and increases the desulfurization and denitrification efficiency.9 Thus, a major mechanism for simultaneous microwave catalytic reduction of SO2 and NOx can be described as the microwave-induced reduction catalyst reaction between SO2, NOx, and ammonium bicarbonate with Fe/Ca-5A zeolite being the catalyst and the microwave absorbent. Microwave catalytic reduction of SO2 and NOx effect simultaneously are much higher than that of catalytic reduction or microwave catalytic desulfurization and denitrification, which are generally in the order: (MWABFe/Ca-5A zeolite) > (ABFe/Ca-5A zeolite) > (MWFe/Ca-5A zeolite) (17) Reddy, P. S. S.; Babu, N. S.; Pasha, N.; Lingaiah, N.; Prasad, P. S. S. Catal. Commun. 2008, 9, 2303. (18) Pierella, L. B.; Saux, C.; Caglieri, S. C.; Bertorello, H. R.; Bercoff, P. G. Appl. Catal., A 2008, 347, 55. (19) Wei, Z. S.; Du, Z. Y.; Lin, Z. H.; He, H. M.; Qiu, R. L. Energy 2007, 32, 1455.
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Figure 4. Influence of desulfurization and denitrification with different microwave powers using ammonium bicarbonate and Fe/Ca-5A zeolite (reaction conditions: gas flow ) 0.15 m3 · h-1; inlet concentration of SO2 ) 1000 mg · m-3; inlet concentration of NOx ) 250 mg · m-3).
3.3. The Influence of Microwave Power on Microwave Catalytic Reduction of SO2 and NOx Simultaneously. Figure 4 shows the influence of microwave power on simultaneous desulfurization and denitrification using ammonium bicarbonate as reducing agent and Fe/Ca-5A zeolite as catalyst. Purifying efficiency of SO2 is gradually increased from 72.2 to 92.3%, and NOx removal efficiency increases from 83.6 to 94.8% when the microwave power is increased from 164 to 280 W. However, the removal of SO2 and NOx decrease with more than 280 W, showing excellent desulfurization and denitrification effect by microwave reactor with ammonium bicarbonate (NH4HCO3) and Fe/Ca-5A zeolite. The experimental results show that the optimum microwave power for simultaneous desulfurization and denitrification is supposed to be 280 W. On the basis of the experiments as above, we assume that SO2 and NOx from flue gas react with ammonium bicarbonate to produce sulfur and nitrogen when ammonium bicarbonate and Fe/Ca-5A zeolite are used together under microwave, which is critical to simultaneous microwave catalytic reduction desulfurization and denitrification. Fe/Ca-5A zeolite could adsorb SO2 and NOx, while ammonium bicarbonate could reduce SO2 and NOx to sulfur and nitrogen. Microwave could induce the SO2 and NOx reduction catalyst reaction using ammonium bicarbonate over Fe/Ca-5A zeolite with the zeolite being the catalyst and the microwave absorbent. 3.4. The Influence of EBRT on the Simultaneous Microwave Catalytic Reduction of SO2 and NOx. The influence of EBRT on simultaneous microwave catalytic reduction desulfurization and denitrification is presented in Figure 5, under the conditions of microwave power of 280 W, inlet concentration of 1000 mg · m-3 SO2, and inlet concentration of NOx of 250 mg · m-3 with ammonium bicarbonate and Fe/Ca-5A zeolite catalyst. With increasing EBRT, the purifying efficiency of SO2 increases from 77.9 to 92.3%, and NOx removal efficiency changes from 80.3 to 94.8%. This indicates the longer EBRT is a benefit on the removal of SO2 and NOx, in the case where the EBRT is too short to reduce SO2 and NOx to sulfur and nitrogen before release. The type of Fe/Ca-5A zeolite catalyst and the length of the quartz tube with catalyst and reducing agent are the key elements. Microwave irradiation dramatically increased reaction rates.20,21 Figure 5 also demonstrates that sulfur dioxide (SO2) and nitrogen (20) Hayes, B. L. CEM Publishing: Matthews, NC, 2002, 29. (21) Lai, T. L.; Wang, W. F.; Shu, Y.-Y.; Liu, Y. T.; Wang, C. B. J. Mol. Catal. A: Chem. 2007, 273, 303.
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Figure 5. Influence of EBRT on desulfurization and denitrification (reaction conditions: microwave power ) 280 W; inlet concentration of SO2 ) 1000 mg · m-3; inlet concentration of NOx ) 250 mg · m-3).
Figure 6. Plot of ln(Cin/Cout)/(Cin - Cout) vs 1/(Cin - Cout) on different SO2 concentration according to the L-H rate expression. (reaction conditions: microwave power ) 280 W; inlet concentration of NOx ) 250 mg · m-3; EBRT ) 0.358 s; gas flow rate of 150 L · h-1).
oxides (NOx) are rapidly microwave catalytic reduced by ammonium bicarbonate over Fe/Ca-5A zeolite under microwave irradiation. From Figure 5, in our experimental conditions, we can assume that the optimum EBRT is 0.358 s in the simultaneous microwave catalytic reduction desulfurization and denitrification system, and about 92% sulfur dioxide and 94% nitric oxide in the gas stream is converted. 3.5. Kinetic Analysis of the Simultaneous Microwave Catalytic SO2 and NOx Reduction. The LangmuirHinshelwood (L-H) rate expression has been widely used to describe the gas-solid phase reaction for heterogeneous catalysis.22 Assuming that the SO2 and NOx mass transfer is not the limiting step and that the effect of the intermediate product is negligible, the substrates SO2 and NOx are adsorbed onto the surface of the Fe/Ca-5A zeolite catalyst, respectively, and the reaction rate in a plug-flow reactor can be expressed as r ) -u
kK dC ) dL 1 + KC
(1)
where k and K are the L-H reaction rate constant and the L-H adsorption equilibrium constant, respectively; L is the length of the microwave reactor filled with ammonium bicarbonate (NH4HCO3) over Fe/Ca-5A zeolite catalyst; and u is the gas velocity through the microwave reactor. After rearrangement and integration of eq 1, the following linear expression can be obtained:
( )
Cin Cout kK(V/Q) ) -K (Cin - Cout) (Cin - Cout) ln
(2)
where Cin and Cout are the inlet and outlet concentrations of SO2 and NOx, respectively; V is the volume of the microwave reactor (14.92 mL); and Q is the flow rate through the reactor (150 L · h-1). If the L-H model is valid, a plot of ln(Cin/Cout)/(Cin - Cout) versus 1/(Cin - Cout) should be linear. As shown in Figures 6 and 7, the experimental data in removal of SO2 and NOx are in good agreement with the integral rate law analysis, and a linear relationship is observed (R2 (desulfurization) ) 0.993, R2 (denitrification) ) 0.999). The obtained values of the L-H reaction rate constant k of the microwave catalytic SO2 and NOx reduction are 13559.29 (22) Kim, S. B.; Hong, S. C. Appl. Catal., B 2002, 35, 305.
Figure 7. Plot of ln(Cin/Cout)/(Cin - Cout) vs 1/(Cin - Cout) on different NOx concentration oaccording to the L-H rate expression. (reaction conditions: microwave power ) 280 W; inlet concentration of SO2 ) 1000 mg · m-3; EBRT ) 0.358 s; gas flow rate of 150 L · h-1).
mg · m-3 · min-1, 1682.26 mg · m-3 · min-1 separately; the possible reason for this could be that concentrations of SO2 in inlet are more than concentrations of NOx in inlet under our experimental conditions. This demonstrates that the microwave reactor with ammonium bicarbonate and Fe/Ca-5A zeolite has good desulfurization and denitrification effect, showing that desulfurization capacity is higher than denitrification capacity. The obtained values of L-H adsorption equilibrium constant K of the microwave catalytic reduction desulfurization and denitrification are 0.00071 (mg · m-3)-1 and 0.008 (mg · m-3)-1, respectively. This finding suggests that reaction occurs on the catalyst surface through L-H mechanism and not in the gas phase. The microwave catalytic SO2 and NOx reduction rate matches Langmuir-Hinshelwood model in the microwave reactor with ammonium bicarbonate and Fe/Ca-5A zeolite. 4. Conclusion The paper revealed that the microwave reactor with ammonium bicarbonate and Fe/Ca-5A zeolite or the microwave reactor with Fe/Ca-5A zeolite only can be used for simultaneous desulfurization and denitrification, and the L-H kinetic model was successfully applied to describe this process on microwave
MicrowaVe Catalytic Desulfurization and Denitrification
catalytic reduction of SO2 and NOx. Microwave catalytic desulfurization and denitrification effect of the experiment using a microwave reactor with Fe/Ca-5A zeolite only is close to that of catalytic reduction of SO2 and NOx using ammonium bicarbonate and Fe/Ca-5A zeolite together. Microwave catalytic reduction of SO2 and NOx simultaneously can be described as the microwave-induced reduction catalyst reaction between SO2, NOx, and ammonium bicarbonate with Fe/Ca-5A zeolite being the catalyst and microwave as the absorbent. Acknowledgment. The authors gratefully acknowledge the financial support from the Research Fund Program of Guangdong
Energy & Fuels, Vol. 23, 2009 2951 Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology 2006K0013. Note Added after ASAP Publication. There were errors in Figures 2 and 3 in the version of this paper published ASAP April 14, 2009; the corrected version published ASAP April 17, 2009. Supporting Information Available: Catalysts preparation, the intermediate OH free radicals products detected by high performance liquid chromatography (HPLC) methods. This material is available free of charge via the Internet at http://pubs.acs.org. EF801013W
