Nonthermal Plasma-Enhanced Catalytic Removal of Nitrogen Oxides

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Nonthermal Plasma-Enhanced Catalytic Removal of Nitrogen Oxides over V2O5/TiO2 and Cr2O3/TiO2 Young Sun Mok,* Dong Jun Koh,† Kyong Tae Kim,† and In-Sik Nam‡ Department of Chemical Engineering, Cheju National University, Ara, Cheju 690-756, South Korea

A nonthermal plasma process (dielectric-packed bed reactor) was combined with catalyst to remove nitrogen oxides (NOx). Two different honeycomb catalysts such as V2O5/TiO2 and Cr2O3/ TiO2 were compared with respect to the removal characteristic of NOx. The effect of oxygen content, water vapor, feed gas flow rate, reaction temperature, and initial concentration on the removal of NOx was examined. The plasma discharge was found to largely enhance the removal of NOx on the catalyst. Without plasma discharge, V2O5/TiO2 was superior to Cr2O3/TiO2 in terms of NOx removal activity. However, the degree of enhancement in NOx removal as a result of plasma discharge was similar for each system. Cr2O3/TiO2 catalyst reduced NO2 to both NO and N2 while the reduction of NO2 back to NO was not significant on V2O5/TiO2 catalyst. The combined system of the nonthermal plasma with V2O5/TiO2 catalyst removed nearly 90% of NOx at 150 °C that is a relatively low temperature, compared to the typical temperature window of NOx reduction catalyst. 1. Introduction A nonthermal plasma technique such as pulsed corona discharge and dielectric barrier discharge has been considered as a prospective candidate for the removal of nitrogen oxides (NOx) emitted from a variety of industrial processes.1-3 Generally, the content of nitric oxide (NO) in NOx is more than 95% in practical exhaust, and the rest is nitrogen dioxide (NO2). Most of the nonthermal plasma processes that have been studied so far convert NO into NO2 (or HNO3) first by using the active species generated during the discharge, and then form ammonium nitrate through the reaction with ammonia.4-7 The same approach is not applicable to the other exhaust gases, namely, engine exhausts and small-scale industrial flue gases. The desired approach in such cases may be to reduce NOx to molecular nitrogen (N2). One important problem regarding this kind of approach is that nonthermal plasma alone cannot reduce NOx to N2 when oxygen exists in exhaust gas.3,4 As proved in many laboratories, the principle action of the nonthermal plasma in the presence of oxygen is the oxidation of NO to NO2.3,4,8,9 Accordingly, to attain the purpose of NOx reduction to N2, nonthermal plasma should be combined with another process. Yamamoto et al.10 made use of the wet scrubbing method using Na2SO3 solution to form N2 from NO2 previously produced by the nonthermal plasma. Bro¨er and Hammer,11 Yoon et al.,12 and Penetrante et al.13 combined nonthermal plasma with catalyst to achieve the same purpose. From the practical point of view, the combination of nonthermal plasma with catalyst rather than that with wet scrubbing may be advisable since both of them are dry processes. * To whom correspondence should be addressed. Tel.: 8264-754-3682.Fax: 82-64-755-3670.E-mail: [email protected]. † Air Protection Research Team, Research Institute of Industrial Science and Technology, Hyoja, Pohang, Kyungbuk 790-330, South Korea. ‡ Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja, Pohang, Kyungbuk 790-781, South Korea.

One major issue with the catalytic removal of NOx may be the high activation temperature.14,15 According to a published paper in the area of catalytic reduction of NOx, it is reported that the performance of catalyst is elevated by the increase in the ratio of NO2 to NO.16 This result implies that the temperature window for NOx removal on catalyst can be lowered when a part of NO is converted to NO2. As mentioned above, one easy method to increase the portion of NO2 in NOx may be nonthermal plasma discharge. Several researchers experimentally verified that installing a nonthermal plasma process in the front of the catalytic reactor lowers the activation temperature of NOx removal catalyst and enhances the removal efficiency.11-13,17 It can thus be said that nonthermal plasma discharge complements the demerit of the catalytic process and vice versa. In the present work, nonthermal plasma combined with catalyst for the removal of NOx has been studied. A dielectric-packed bed reactor was utilized as the nonthermal plasma reactor, and two different honeycomb catalysts such as V2O5/TiO5 and Cr2O3/TiO2 were put downstream from the nonthermal plasma reactor. The main objectives of this study are to examine the effects of several crucial variables including discharge power, reaction temperature, water vapor, oxygen content, and space velocity in the catalytic reactor on the removal of NOx. As well, a comparison between the two catalysts in terms of NOx reduction characteristics is also an important objective. 2. Experimental Details Experimental Apparatus. The schematic of the experimental setup composed of a nonthermal plasma reactor and a catalytic reactor is presented in Figure 1. The coaxial plasma reactor makes use of dielectric barrier discharge operated with ac high voltage (60 Hz). A glass tube (inner diameter, 25.8 mm; outer diameter, 30.2 mm) was utilized as the dielectric material and a 3/ in. stainless steel rod was used as the discharging 8 electrode to which ac high voltage was applied. The

10.1021/ie0208873 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003

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Figure 1. Schematic of the experimental setup.

space between the glass tube and the discharging electrode was filled with glass beads of 5 mm in diameter (Sigmund Lindner, Germany). The effective length of the plasma reactor whose outer surface of the glass tube was wrapped with aluminum foil is 31 cm. The apparent reactor volume excluding the electrode volume is calculated to be 140 cm3. The void fraction of the plasma reactor was estimated to be 0.3725, and accordingly, the actual reactor volume occupied by gas is 52 cm3. The honeycomb catalysts used were commercially available V2O5/TiO2 (20 channels per square inch) and Cr2O3/TiO2 (100 channels per square inch). The supplier of the commercial catalysts cannot be identified due to a secrecy agreement. The content of vanadium in V2O5/TiO2 catalyst was 5.0 wt %, and the content of chromium in Cr2O3/TiO2 catalyst was 10 wt %. The apparent volume of the honeycomb catalyst was 31 cm3 (1.8 × 1.8 × 9.7 cm3), which was used for the calculation of space velocity (feed gas flow rate/catalyst volume). Methods. The experiments were carried out at gas temperatures up to 200 °C. The reactor was kept in an oven to maintain the desired gas temperature. The main components of the feed gas stream were nitrogen and oxygen whose flow rates were adjusted by mass flow controllers (MFC) (Model 1179A, MKS Instruments, Inc.). The flow rates of NO (5.0% (v/v) balanced with N2), NH3 (5.0% (v/v) balanced with N2), and ethylene (pure) were also controlled by mass flow controllers, and they were mixed with N2 and O2. The water vapor was added to the feed gas stream by using its vapor pressure. The concentrations of NOx (NO + NO2) and NH3 at the reactor inlet were typically 300 ppm (parts per million, volumetric), and they were varied from 200 to 400 ppm. The concentration of ethylene added to the feed gas was kept 750 ppm. The voltage applied to the plasma reactor was varied from 5 to 11 kV (peak value) to change the discharge power. The feed gas flow rate was typically 5 L/min (based on room temperature), and it was changed in the range 2.5-7.5 L/min. The residence time of the feed gas in the plasma reactor calculated by considering the void fraction was 1.2, 0.6, and 0.4 s when the flow rate was 2.5, 5.0, and 7.5 L/min, respectively. Note that the flow rate of the feed gas depends on the feed gas temperature, and the residence time calculated above should be corrected using the real gas temperature when the feed gas is not at room temperature. The concentrations of NO and NO2 were analyzed by a chemiluminescence NO-NO2-NOx analyzer (model 42C, Thermo Environmental Instruments, Inc.). This kind of NO-NO2-NOx analyzer has a problem in measuring NO2 concentration when ammonia is present,

Figure 2. Relation of the discharge power to the input power in the absence of water vapor (a), and in the presence of 3% (v/v) water vapor (b).

and thus, a portable flue gas analyzer (Eurotron) was utilized to analyze NO2 concentration when ammonia was used. The voltage applied to the discharging electrode was measured by a 1000:1 high voltage probe (PVM-4, North Star Research, Corporation) and a digital oscilloscope (TDS 3032, Tektronix). For the measurement of the voltage between both ends of the 1.0 µF capacitor connected to the plasma reactor in series, a 10:1 voltage probe (P6139A, Tektronix) was used. The measurement of input power was carried out using a digital power meter (model WT 200, Yokogawa). Discharge Power Measurement. A 1.0 µF capacitor connected to the plasma reactor in series is for measuring the discharge power. The method adopted to measure the discharge power is described in detail in the literature.18-20 Figure 2 shows the relation of the input power to the discharge power. As can be seen, the discharge power showed an increasing trend as the temperature increased, and the presence of water vapor lowered the discharge power significantly. This decrease in the discharge power in the presence of water vapor may be attributed to the loss of electrons by dissociative attachment (H2O + electron f H- + OH). When highenergy electrons are lost by this way, it leads to the decrease in successive ionization of gas molecules, and eventually the decrease in the discharge power. The

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In the absence of oxygen, the decrease in the concentration of NO was very small, and NO2 was not produced. When oxygen is not present in the feed gas, the main removal pathway of NO is the reduction to N2 as follows:3,8

NO + N f N2 + O

(1)

The rate of this reaction depends on the amount of N radical produced by plasma discharge. Therefore, although the amount of NO reduced was small at this experimental condition, further increase in the energy density will remove more NO since the production of N radical is proportional to the energy delivered to the plasma reactor.3,8 Ethylene plays an important role in enhancing the oxidation of NO to NO2.9,22,23 Under plasma discharge, ethylene is decomposed into useful intermediates such as CH3 and CH3O for the oxidation of NO. In the presence of oxygen, the intermediates, CH3 and CH3O, can exhibit their abilities to oxidize NO as follows:9,23

Figure 3. Effect of the oxygen content on the NO concentration (a), and on the NO2 concentration (b) (flow rate, 5 L/min; initial NOx, 300 ppm; C2H4, 750 ppm; temperature, 150 °C).

increase in the discharge power with the temperature can be explained as follows. When pressure is constant, the gas density decreases with the increase in temperature, which causes the increase in the reduced electric field (electric field divided by gas density).21 The increase in the reduced electric field indicates that the gas can be more easily ionized. Therefore, higher gas temperature at constant pressure results in larger discharge power. The discharge power data in Figure 2 were used for the calculation of energy density in the plasma reactor. The energy density is defined as the ratio of discharge power to feed gas flow rate, and it has been a widely used parameter in the field of nonthermal plasma.1,4,13 Actually, the flow rate of gas is a function of temperature. Throughout this study, however, the energy density was calculated with the flow rate on the basis of room temperature for the sake of uniformity. 3. Results and Discussion Characteristics of Plasma Reactor. Figure 3 shows the variations of NO and NO2 concentrations as a function of energy density at different oxygen contents when 750 ppm of ethylene (C2H4) was added to the feed gas.

CH3 + O2 f CH3O2

(2)

NO + CH3O2 f NO2 + CH3O

(3)

CH3O + O2 f HCHO + HO2

(4)

NO + HO2 f NO2 + OH

(5)

As can be seen in Figure 3, the concentration of NO largely decreased with the increase in the energy density when ethylene is present. Regardless of the oxygen content (5-20%), most of the initial NO was depleted at an energy density around 70 J/L. Since the oxygen content of 5-20% is much higher than the initial concentration of NOx, the change in the oxygen content in this range did not significantly affect the oxidation of NO. Besides reactions 2-5, the oxidation of NO to NO2 in the presence of oxygen may additionally take place as follows:9,23,24

NO + O f NO2

(6)

As will be shown below, reaction 6 is not so fast especially at high temperature, and thus, the large decrease in the concentration of NO with the energy density as in Figure 3 obviously resulted from reactions 2-5. According to the previously reported results, ozone was proved to play an important role in the oxidation of NO at room temperature.3,25 As can be understood from the following reactions, however, generation of ozone decreases much with the increase in the temperature and, furthermore, the decomposition of ozone into molecular oxygen becomes significant as the temperature increases.23,24

O + O 2 f O3 k ) 5.6 × 10-34(300/T)2.23[M] (cm3/molecules/s) (7) O + O3 f 2O2 k ) 8.0 × 10-12 exp(-2060/T) (cm3/molecules/s) (8) As a result, the reaction of NO with O3 does not largely contribute to the oxidation of NO at high temperatures. Figure 4 shows the amount of NO oxidized to NO2 as a function of temperature when ethylene was not added.

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Figure 4. Effect of the reaction temperature on the oxidation of NO in the absence of ethylene and water vapor (flow rate, 5 L/min; initial NOx, 300 ppm; oxygen content, 10% (v/v)).

Figure 5. Effect of water vapor and ethylene on the oxidation of NO (flow rate, 5 L/min; initial NOx, 300 ppm; temperature, 200 °C; oxygen content, 10% (v/v)).

As the temperature increased from room temperature to 200 °C, the amount of NO oxidized greatly decreased. When the plasma reactor was operated at 200 °C, the amount of NO oxidized was only 10, 20, and 30 ppm at energy densities of 10, 30, and 50 J/L. As mentioned above, the dominant pathway for the oxidation of NO at room temperature is the reaction with ozone. On the other hand, the key reaction for the oxidation of NO at high temperature is reaction 6 since ozone is hardly formed, yet the rate constant of reaction 6 is inversely proportional to the temperature.23,24 In this context, the oxidation of NO decreases with the temperature. In addition, the formation of NO and the reduction of NO2 back to NO may result in the decrease in the amount of NO oxidized.3,8 Several reactions regarding the formation of NO are given below24

gas, the oxidation of NO was very small, but the oxidation of NO was enhanced considerably when water vapor was present. This enhancement in the oxidation of NO is obviously due to the formation of OH radical from water vapor, which induces the following reactions:

NO2 + O f NO + O2 k ) 5.21 × 10-12 exp(202/T) (cm3/molecules/s) (9)

NO + OH f HNO2

(12)

HNO2 + OH f NO2 + H2O

(13)

In the presence of ethylene, the oxidation of NO was greatly improved by reactions 2-5 given above, and the effect of ethylene on the oxidation of NO was more pronounced when water vapor coexisted. The OH radical formed from water vapor first combines with ethylene fast, and then, the intermediate (C2H4OH) oxidizes NO to NO2, regenerating OH radical as9

C2H4 + OH f C2H4OH

(14)

N + O f NO k ) 1.8 × 10-31[M]/T1/2 (cm3/molecules/s) (10)

C2H4OH + 2O2 + 2NO f 2HCHO + 2NO2 + 2OH (15)

N + O2 f NO + O k ) 4.4 × 10-12 exp(-3220/T) (cm3/molecules/s) (11)

As shown in Figure 5, the use of additive was able to largely increase the oxidation of NO. However, it should be emphasized that the NO was simply converted to NO2, and almost no reduction in NOx level (NO + NO2) was obtained when only the plasma reactor was used. Characteristics of Plasma-Catalytic Reactor. Figure 6 shows the NOx (NO + NO2) removal efficiencies obtained by the hybridization of the nonthermal plasma reactor and V2O5/TiO2 catalyst at different oxygen contents. When the energy density was zero, i.e., when high voltage was not applied, the catalyst separately exerted action on the removal of NOx. In the absence of oxygen, the catalytic activity was very low, and only a small amount of NOx was removed. The increase in the energy density up to 50 J/L did not increase the removal efficiency significantly. The small increase in the removal efficiency with the energy density is understood to have mainly resulted from the reaction 1 rather than catalytic activity. As the oxygen content increased, the catalyst itself was able to remove more NOx, but the removal efficiency was still low at this reaction temperature of 150 °C. However, the NOx

where M is the three-body reaction partner. The rate constants of reactions 9 and 10 decrease with the increase in temperature, and thus, the formation of NO by these reactions becomes slow as the temperature increases. On the other hand, the rate constant of reaction 11 increases with temperature, and its temperature dependence is very great due to the high activation energy. Furthermore, since oxygen that is one of major gas components in the feed gas that participates in this reaction, the rate of reaction 11 is very fast. Consequently, at high temperatures, reaction 11 has a very significant effect on the formation of NO, which can lead to the decrease in the amount of NO oxidized. The decrease in the oxidation of NO at high temperatures may justify the use of chemical additives such as ethylene and propylene. The effect of water vapor on the oxidation of NO at a temperature of 200 °C is presented in Figure 5. When neither water vapor nor ethylene was added to the feed

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Figure 6. NOx removal efficiency as a function of energy density at different oxygen contents (flow rate, 5 L/min; initial NOx, 300 ppm; C2H4, 750 ppm; temperature, 150 °C; oxygen content, 10% (v/v); catalyst, V2O5/TiO2).

Figure 7. NOx removal efficiency as a function of energy density at different oxygen contents (flow rate, 5 L/min; initial NOx, 300 ppm; C2H4, 750 ppm; temperature, 150 °C; oxygen content, 10% (v/v); catalyst, Cr2O3/TiO2).

removal efficiency was largely enhanced when high voltage was applied. For instance, the NOx removal efficiency was about 90% at an energy density of 50 J/L when oxygen content was 20%. As can be seen in Figure 3, the role of nonthermal plasma was nothing but the oxidation of NO to NO2. According to the literature,16,17 however, the gas mixture should contain NO2 for the efficient catalytic reduction of NOx. Since the adsorption capability of NO2 on the catalyst is larger than that of NO, the increase in the NO2 fraction can considerably enhance the catalytic activity. This is why the removal efficiency increased with the energy density. When ammonia is added as a reducing agent, NO2 not only reacts with the gas phase NO fast but also is easily decomposed into nitrogen as follows:11,26

NO + NO2 + 2NH3 f 2N2 + 3H2O

(16)

6NO2 + 8NH3 f 7N2 + 12H2O

(17)

Reaction 16 means that the concentrations of NO and NO2 should be equal, and reaction 17 signifies the importance of NO2, which leads to a conclusion that at least the concentration of NO2 should be the same with that of NO. Besides reactions 16 and 17, formation of ammonium nitrate via the reaction between NO2 and NH3 is possible, but it is known as a slow reaction.27 Figure 7 shows the NOx removal efficiencies when Cr2O3/TiO2 catalyst was employed for the plasmacatalytic rector. Without applying high voltage, the catalyst removed less than 10% of the initial NOx even in the presence of 5-20% oxygen, implying that the catalytic activity is lower than that of V2O5/TiO2. When oxygen was not present, the catalyst removed a negligible amount of NOx. As the energy density increased, however, the NOx removal efficiencies largely increased as in the case of V2O5/TiO2. Although the removal efficiency itself with Cr2O3/TiO2 catalyst was much lower than that with V2O5/TiO2, the degree of enhancement as a result of plasma discharge was similar to V2O5/TiO2 catalyst. Such enhancement emphasizes that Cr2O3/TiO2 catalyst needs NO2 for efficient treatment of NOx as V2O5/TiO2 catalyst does. However, this result does not mean that the two catalysts conform to the

Figure 8. Comparison of V2O5/TiO2 with Cr2O3/TiO2 catalyst (flow rate, 5 L/min; initial NOx, 300 ppm; C2H4, 750 ppm; oxygen content, 10% (v/v); temperature, 150 °C).

same reaction mechanism. As compared in Figure 8, V2O5/TiO2 and Cr2O3/TiO2 catalysts function in different ways. In case of Cr2O3/TiO2 catalyst, NO2 concentration was low while NO concentration was kept high. It can be clearly seen in Figure 3 that the plasma reactor converted most of NO into NO2. However, the concentration of NO at the outlet of the plasma-catalytic reactor was always higher than that at the outlet of the plasma reactor. This result can be evidence that NO2 is not only converted back into NO but also reduced to N2 on this catalyst. On the contrary, both NO and NO2 were found to be lessened when V2O5/TiO2 catalyst was used, indicating that reactions 16 and 17 played an important role and the reduction of NO2 back to NO was not significant. Figure 9 shows the effect of feed gas flow rate on the removal of NOx in the plasma-catalytic reactor. The flow rates of 2.5, 5.0, and 7.5 L/min on the basis of room temperature correspond to the respective space velocities of 6800, 13 600, and 20 500 h-1 in the catalytic reactor at 150 °C. As can be seen in Figure 9, a higher flow rate resulted in lower NOx removal efficiency,

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Figure 9. Effect of the space velocity on the NOx removal efficiency (initial NOx, 300 ppm; C2H4, 750 ppm; oxygen content, 20% (v/v); temperature, 150 °C; catalyst, V2O5/TiO2).

which is obviously due to the decrease in the oxidation of NO to NO2 in the plasma reactor and the decrease in the residence time in the catalytic reactor. At an identical discharge power, the oxidation of NO to NO2 in the plasma reactor decreases with the increase in the flow rate. In addition, the increase in the flow rate decreases the residence time in the catalytic reactor. That is why the NOx removal efficiency decreased with the increase in the flow rate. Meanwhile, the maximum removal efficiency obtained at each flow rate was similar although higher flow rate required more discharge power. This result means that the catalytic activity was largely improved by the plasma discharge and higher gas flow rates can be treated if sufficient power to convert NO into NO2 is delivered to the plasma reactor. In general, catalytic removal of nitrogen oxides is carried out around 300-350 °C.14,15 On the other hand, the plasma-catalyst combination system of this study gave nearly 90% of NOx removal efficiency at the relatively low temperature of 150 °C. Figure 10a,b shows the effect of the initial concentration on the removal of NOx. For this experiment, the concentration of NH3 was kept equal to that of the initial concentration of NOx; i.e., the injection ratio of NH3 to NOx was maintained at 1.0. As can be seen in Figure 10a, NO was almost completely converted into NO2 in the plasma reactor although the higher initial concentration required more discharge power to oxidize NO to NO2. As mentioned above, at least the concentration of NO2 should be equal to that of NO for effective treatment. When the concentration of the initial NOx was 200, 300, and 400 ppm, the plasma reactor gave an equimolar composition of NO and NO2 at 1.6, 2.0, and 2.1 W, respectively. At these small discharge powers, the respective sum of NO and NO2 (NOx level) measured at the outlet of the plasma-catalytic reactor was 55, 85, and 150 ppm, corresponding to 73%, 72%, and 63% of removal efficiency (see Figure 10b). Further increase in the discharge power resulted in more reduction in NOx level because the plasma reactor produced more NO2 capable of undergoing reaction 17. The removal efficiencies 73%, 72%, and 63% at the initial NOx concentrations of 200, 300, and 400 ppm are tantamount to energy yields of 33.6, 28.4, and 25.6 eV/ NOx-molecule, respectively.

Figure 10. Effect of the initial NOx concentration on the oxidation of NO in the plasma reactor (a), and on the removal of NO and NO2 in the plasma-catalytic reactor (b) (flow rate, 5 L/min; initial NOx/C2H4, 0.4; initial NOx/NH3, 1.0; oxygen content, 10% (v/v); temperature, 150 °C; catalyst, V2O5/TiO2).

Formation of Byproducts. Ethylene used as an additive generates formaldehyde via reactions 4 and 15, and it can be converted into carbon monoxide and carbon dioxide as follows:

HCHO + OH f HCO + H2O

(18)

HCO + O2 f CO + HO2

(19)

HCO + O f CO + OH

(20)

HCO + O f H + CO2

(21)

CO + O f CO2

(22)

The byproducts such as formaldehyde and carbon monoxide were measured at the outlet of the plasma reactor and at the outlet of the plasma-catalytic reactor. As shown in Figure 11, significant amounts of formaldehyde and carbon monoxide as a result of ethylene decomposition were emitted from the plasma reactor. The formation of carbon monoxide is believed to have resulted from reactions 18-20. The concentration of carbon dioxide was not directly analyzed, but estimated using a simple material balance. One molecule of

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pated because real exhaust gases contain some amount of unburned hydrocarbons, and further studies are required to bring down the CO level to an allowable limit for practical application of this system. 4. Conclusions The conclusions drawn from the present study are as follows. Although the presence of water vapor somewhat improves the oxidation of NO at high temperature, the use of an additive such as ethylene is necessary for the effective oxidation of NO in the plasma reactor. In the plasma-catalytic system, the main role of the plasma reactor is the oxidation of NO to NO2 with the sum of NO and NO2 almost kept constant, but the presence of NO2 leads to an enhancement in the NOx removal efficiency. Without plasma discharge, the NOx removal efficiency obtained with V2O5/TiO2 catalyst was around 50% at 150 °C, and that with Cr2O3/TiO2 was around 10%. However, more than 80% of NOx with V2O5/TiO2 and 40% of NOx with Cr2O3/TiO2 were removed when plasma was generated. Changes in the oxygen content from 5% to 20% (v/v) did not significantly affect the results either from the plasma or the catalytic reactor. High concentration of NOx up to 400 ppm was successfully treated in the present plasma-catalytic system. At an identical discharge power, a higher flow rate resulted in lower NOx removal efficiency because the oxidation of NO to NO2 in the plasma reactor decreased and the residence time in the catalytic reactor decreased. However, although a higher flow rate required more discharge power, the maximum removal efficiency was almost similar regardless of the flow rate because the catalytic activity was largely improved by the plasma discharge. The energy for the removal of NOx ranged from 25.6 to 33.6 eV/NOx-molecule, depending on the initial concentration of NOx and the removal efficiency. Figure 11. Concentrations of CO, CO2, and HCHO at the outlet of the plasma reactor (a), and at the outlet of the plasma-catalytic reactor (flow rate, 5 L/min; initial NOx, 300 ppm; C2H4, 750 ppm; oxygen content, 10% (v/v); temperature, 150 °C; catalyst, V2O5/ TiO2).

ethylene can generate two molecules of HCHO, CO, or CO2. Therefore, two times the amount of ethylene removed is equal to the sum of HCHO, CO, and CO2 unless any other byproduct is formed. According to the gas chromatogram obtained by using the flame ionization detector, no noticeable peaks other than ethylene were observed, implying that the formation of any other organic species from ethylene is negligible. The estimated concentration of carbon dioxide is also shown in Figure 11. As observed, the concentration of carbon dioxide at the outlet of the plasma reactor was lower than that of carbon monoxide. This result may be explained by the slow rate of reaction 22.24 While the emission of formaldehyde from the plasma reactor was significant, it was completely removed on the catalyst surface; i.e., the concentration of formaldehyde at the outlet of the plasma-catalytic reactor was always zero at this experimental condition. However, the concentration of CO at the outlet of the plasma-catalytic reactor was higher than that at the outlet of the plasma reactor. Such an increase in the concentration of CO at the outlet of the plasma-catalytic reactor is understood to have arisen from the decomposition of formaldehyde on the catalyst. In real situations, a similar problem is antici-

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Received for review November 4, 2002 Revised manuscript received April 14, 2003 Accepted April 23, 2003 IE0208873