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Temperature effect on hydrocarbon-enhanced nitric oxide conversion using a dielectric barrier discharge reactor
Fuel Processing Technology 81 (2003) 187 – 199 www.elsevier.com/locate/fuproc
Temperature effect on hydrocarbon-enhanced nitric oxide conversion using a dielectric barrier discharge reactor V. Ravi a, Young Sun Mok b,*, B.S. Rajanikanth a, Ho-Chul Kang b a
b
Department of High Voltage Engineering, Indian Institute of Science, Bangalore-560 012, India Department of Chemical Engineering, Cheju National University, Ara, Cheju 690-756, South Korea Accepted 1 November 2002
Abstract Experimental investigations into the effect of temperature on conversion of NO in the presence of hydrocarbons (ethylene, acetylene and n-hexane) are presented. An AC energized dielectric barrier discharge reactor was used as the plasma reactor. The experiments were carried out at different temperatures up to 200 jC. The discharge powers were measured at all the temperatures. The discharge power was found to increase with temperature. NO conversion in the presence of ethylene and n-hexane was better than that of acetylene at all temperatures. The addition of acetylene at room temperature showed no better conversion of NO compared to no additive case. While at higher temperatures, it could enhance the conversion of NO. A slight enhancement in NO and NOx removal was observed in the presence of water vapor. D 2003 Elsevier Science B.V. All rights reserved. Keywords: NO conversion; Dielectric barrier discharge; Temperature; Hydrocarbons
1. Introduction Non-thermal plasma associated with electrical discharges is fast gaining prominence because of its potential air cleaning applications. Among all air cleaning applications, electric discharge plasma is extensively studied for its removal of oxides of nitrogen. Oxides of nitrogen are the main pollutants emitted from diesel engines and thermal power
* Corresponding author. Tel.: +82-64-754-3682; fax: +82-64-755-3670. E-mail address: [email protected] (Y.S. Mok). 0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3820(03)00013-4
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plants. The application of electric discharge plasma in the removal of NOx from thermal power plants has been studied widely [1– 3] and at present the efforts are on to minimize the NOx emissions from diesel engine exhaust. The main role of electric discharge plasma in NOx removal has been understood to be the oxidation of NO to NO2. It was later found that the presence of hydrocarbons in the gas promotes the oxidation reactions [4– 8]. The presence of hydrocarbons not only reduces the energy required for the oxidation process such as NO to NO2 but also greatly reduces the possibility of HNO3 formation [9], which is not desirable from the vehicular exhaust treatment point of view. However, it has been established that the electric discharge plasmas alone cannot remove NOx from diesel exhaust. This demands the combination of the discharge plasma technique with heterogeneous catalysis. At present in the field of heterogeneous catalysis, selective catalytic reduction (SCR) is gaining more importance. Generally, SCR catalysts show maximum NOx removal at around 300 – 350 jC. However, as the portion of NO2 in NOx (NO+NO2) increases, the temperature window for NOx removal becomes wide. Broer and Hammer [10] showed that about 70% removal of NOx at 100 jC and more than 95% removal at 200 jC in SCR process was possible by oxidizing a part of NO to NO2 using electric discharge plasma. This removal efficiency obtained in the plasma/catalytic hybrid system is much higher than the summation of removal efficiencies obtained individually in the plasma reactor and in the catalytic reactor. Referring to the literature [11,12], this kind of synergistic effect seems to result from the increase in NO2 concentration by plasma discharges and it can be said that the crucial role of electric discharge plasma is to oxidize NO to NO2. In this context, the study on the oxidation of NO, especially at high temperature, is important because the chemistry related to the oxidation of NO and the role of hydrocarbons at high temperature are different from that at room temperature. For example, ozone can dominate the oxidation of NO and the decomposition of hydrocarbons at room temperature, but at high temperature, the generation of ozone will be suppressed and the other active species play an important role in the oxidation. As well, the rate of oxidation and the power consumption will depend on the reaction temperature. Up to now, there are few data available in the literature about the temperature effect on NO removal. This paper presents some experimental studies on temperature effect on NO oxidation in the presence of hydrocarbon additives using ac energized dielectric barrier discharge reactor. The experiments were carried out at different temperatures up to 200 jC. The hydrocarbon additives used were ethylene, acetylene and n-hexane.
2. Experimental setup Fig. 1 shows the reactor used in the present study (dielectric barrier discharge reactor) that is referred to as plasma reactor. A cylindrical glass tube (inner diameter: 25.8 mm; outer diameter: 30.2 mm) was used as the dielectric material and a copper rod (diameter: 9 mm) was used as the discharging electrode. An aluminium foil wrapped over the glass tube acted as the ground electrode. The space between the glass tube and the discharging electrode was filled with glass beads of diameter 5 mm. When the dielectric material such
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Fig. 1. Schematic of the dielectric barrier discharge reactor.
as glass beads are filled inside the plasma reactor, intense electric field is formed around the contact points of each pellet, causing partial discharges between the pellets. This partial discharge together with the discharge from the wire helps in increasing the radical production. The effective length of the plasma reactor, along which the discharge occurs, was 31 cm (reactor volume: 162 cm3). A 1.0 AF capacitor was connected to the plasma reactor in series to measure the discharge power. The experiments were carried out at different temperatures of the gas up to 200 jC since the typical temperature of the diesel exhaust is around 200 jC. For this, the plasma reactor was kept in a dry oven to maintain the desired gas temperature. To ensure proper heating-up of the feed gas to a given temperature, the stainless steel tube connected to the reactor inlet was wound several times within the dry oven, which acted as a heat exchanger. The experiments were carried out for two different gas mixtures such as (a) NO in N2 (300 ppm), O2 (10%) and N2 (balance), (b) NO in N2 (300 ppm), O2 (10%), H2O (3%) and N2 (balance). The hydrocarbon additives chosen in the present set of experiments were acetylene, ethylene and n-hexane. These hydrocarbons were injected into the gas stream in equal concentrations of 750 ppm. In a real diesel exhaust, the typical concentration of hydrocarbons is around 700 –750 ppm (under no load conditions). The flow rates of all the gas components used in the experiment were adjusted by mass flow controllers (MFC) (Model 1179, MKS Instruments) to get the desired concentrations. In the case of gas mixture (b), the content of water vapor was adjusted by using its vapor pressure. The flow rate of both the gas mixtures was kept at 2 l/min (based on room temperature) throughout the experiments. Since the reactor volume is 162 cm3, the residence time (apparent value) is calculated to be 4.86 s. However, actual residence time should be calculated by considering the void fraction of the reactor and the volume of the electrode (19.7 cm3). In this reactor packed with glass beads, the void fraction was found to be 0.3725. Therefore, the real reactor volume where the reactions take place corresponds to 53 cm3, and the real
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residence time is 1.6 s. There is one more thing that should be made clear in the calculation of residence time. The residence time (1.6 s) is based on the flow rate at room temperature, however, it should be corrected when the gas is not at room temperature, because the gas flow rate is a function of temperature. The concentrations of NO and NO2 were analyzed by a chemiluminescence NO – NO2 – NOx analyzer (Model 42C, Thermo Environmental Instruments). The reactor was energized by AC voltage at 60 Hz. The voltage was varied from 8 to 16 kV to change the discharge power. The voltage applied to the discharging electrode was measured by a 1000:1 high voltage probe (PVM-4, North Star Research) and a digital oscilloscope (TDS 3032, Tektronix). For the measurement of the voltage between both ends of the 1.0 AF
Fig. 2. Voltage waveforms measured at the discharging electrode and at the 1.0 AF capacitor (a); charge – voltage plot (b).
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capacitor, a 10:1 voltage probe (Tektronix P6139A) was used. The measurement of input power was carried out using a digital power meter (Model WT 200, Yokogawa). 2.1. Measurement of discharge power The method adopted to measure the discharge power in the present experiments is as follows. A 1.0 AF capacitor was connected in series with the reactor as shown in Fig. 1. Since the dielectric barrier discharge reactor can be considered as a capacitor, the charge stored in the capacitor (1.0 AF) is equal to that in the reactor. The charge stored in the capacitor ( Q) is the product of capacitance (C) and voltage (V) ( Q=CV), which can be directly read by the voltage between both ends of the capacitor. In other words, the charge stored in the capacitor is equal to 106 times the voltage. Fig. 2(a) shows the voltage waveforms measured at the discharging electrode and at the 1.0 AF capacitor, and Fig. 2(b) shows the charge–voltage plot at the corresponding voltage. The area of the parallelogram in Fig. 2(b) conforms to the discharge energy per one cycle, and the average discharge power can be obtained by multiplying the discharge energy per one cycle by AC frequency (i.e. 60 Hz). For example, in Fig. 2(b), the discharge energy per cycle at 12 kV was found to be 7.33 mJ/cycle. Hence, the average discharge power is 7.33103 J/cycle60 Hz=0.44 W. In principle, this method is similar to the one adopted in Ref. [13], in which a mathematical formula was employed for calculating the area of the parallelogram. For convenience, in the present case the area of parallelogram was calculated by comparing the total mass of the graph with that of the parallelogram. It was assumed that the thickness of the paper was uniform throughout. We compared both methods of calculating the area of the parallelogram and the results were found to agree well within 5%.
3. Results and discussion Fig. 3 shows the variation of discharge power for the mixture (a) at different temperatures as a function of input power. The discharge power tends to increase with temperature, which can be explained as follows. The gas at high temperature is more excited than that at low temperature and it can be more easily ionized. As the extent of ionization increases, the discharge current increases because of decreased gas resistance. Fig. 4 shows the discharge power variation at different temperatures for the gas mixture (b). The discharge powers obtained with the mixture (b) showed the similar trend to that of the gas mixture (a). Both Figs. 3 and 4 were used as basis for measuring the energy densities in the following results. Fig. 5 shows the concentrations of NO, NO2 and NOx as a function of energy density using plasma reactor at room temperature. As the energy density increased, the NO started oxidizing into NO2 while keeping the NOx concentration constant. The oxidation proceeded faster up to an energy density of around 50 J/l, at which the NO removal efficiency is around 60%, and becomes almost constant up to an energy density of 150 J/l. This is because at higher energy densities, the NO2 reduces back to NO (see (A3) in Appendix A). At higher energy densities there was very small production of NOx above its
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Fig. 3. Discharge power with mixture (a) at different temperatures.
initial value. This is because of the production of NO through reaction (A4). The NO produced will counterbalance the reduction in NO and some part of it will be oxidized to NO2 through the reactions (A1) and (A2) thereby increasing NOx level. The effect of temperature on NO removal for gas mixtures (a) and (b) at 150 J/l is shown in Fig. 6. The NO removal efficiency for the mixture (b) containing water was higher than that of the mixture (a) at all temperatures. This is because in the presence of water molecules, the plasma reactor releases OH radicals which in turn oxidize NO to NO2
Fig. 4. Discharge power with mixture (b) at different temperatures.
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Fig. 5. NO, NO2 and NOx concentrations as a function of energy density at room temperature.
through the reactions (A7) and (A8) in Appendix A. As the temperature of the gas increased, the NO removal efficiency decreased for both the gas mixtures. At room temperature, plasma reactor produces high concentration of ozone, which is mainly responsible for the oxidation of NO to NO2 (see (A2) in Appendix A). The decrease in NO removal efficiency with temperature for the gas mixture (a) is mainly because of two reasons: (i) the rate of the reaction (A1) decreases with increase in the temperature; (ii) the production of ozone is suppressed at high temperatures (see (A12) in Appendix A). In case
Fig. 6. Effect of temperature on NO removal efficiency (energy density: 150 J/l).
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of gas mixture (b), besides the above two reasons, the reaction rate in (A7) decreasing with increase in temperature accounts for the decrease in NO removal efficiency. The effect of hydrocarbon additives such as acetylene, ethylene and n-hexane on the NO removal was studied using plasma reactor at different temperatures. Initially, the gas mixture (a) was considered. Fig. 7 shows the NO removal efficiencies in the presence of hydrocarbon additives at room temperature. The presence of both ethylene and n-hexane enhanced the oxidation of NO to NO2 significantly, whereas the addition of acetylene was found to be no better than that without any additive. At an energy density of 50 J/l, the NO removal efficiency was as high as 80% with ethylene and 75% with n-hexane, while acetylene and no additive case showed only 60% removal. The NO removal in presence of hydrocarbons can be explained as follows. The radicals important for the oxidation of NO are alkyl (R), alkoxy (RO) and acyl (RCO) radicals, which are generated when hydrocarbons react with O, OH, O3, etc. Initially, alkoxy (CH3O, C2H5O, etc.) and alkyl radicals (such as CH3, C2H5, etc.) react with oxygen to liberate active species [5]: RO þ O2 ! HCHO þ HO2
ð1Þ
R þ O2 ! RO2
ð2Þ
The HO2 produced increases the NO oxidation (A10) and RO2 oxidizes NO by RO2 þ NO ! CH3 O þ NO2
ð3Þ
These are the important reactions enhancing the NO oxidation though there are varieties of other radicals, which also help in oxidizing NO to NO2.
Fig. 7. Effect of hydrocarbons on NO removal efficiency at room temperature.
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Fig. 8. Effect of temperature on NO removal efficiency in the presence of hydrocarbons (energy density: 150 J/l).
Fig. 8 shows the comparison between the NO removal efficiencies at 150 J/l obtained in the presence of additives as a function of gas temperature. Both ethylene and n-hexane with high reactivities with O3, O and OH (see Table 1) retained their abilities to oxidize NO at all temperatures. On the other hand, the effect of acetylene at room temperature was not significant, but at higher temperatures acetylene showed relatively better performance than that of no additive. This is because at room temperature the oxidation of NO is dominated by O3. As can be seen in Table 1, the reactivity of acetylene with O3, O and OH radicals is low and as a result there is less formation of fragments such as R, RO and RCO capable of oxidizing NO. But at higher temperatures, the effect of acetylene becomes significant because the production of ozone is greatly reduced. The variation of NOx concentration with energy density for different additives at room temperature is shown in Fig. 9. There was hardly any NOx removal without any additive, whereas it was negligible in the presence of acetylene. In the presence of ethylene and nhexane, the NOx removal efficiencies obtained in this experimental condition were 32%
Table 1 Reaction rate constants of hydrocarbons with OH, O and O3 [NIST Chemical Reactions Database-1997] Reaction partner
OH
O
O3
C2H2
4.98 10 12 exp( 553/T) 2.15 10 12 exp(411/T) 9.21 10 13 (T/298)2.2 exp(540/T)
3.05 10 13 (T/298)2.8 exp( 250/T) 1.01 10 12 (T/298)1.88 exp( 92/T) 2.48 10 12 (T/298)2.9 exp( 1000/T)
1.00 10 14 exp( 4100/T) 1.2 10 14 exp( 2630/T) No data
C2H4 C6H14
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Fig. 9. Effect of hydrocarbons on NOx concentration at room temperature.
and 35%, respectively. Although the data are not presented at temperatures higher than room temperature, the NOx removal was insignificant in all these cases. Further experiments were conducted by adding ethylene to the gas mixture (b). Ethylene was chosen because it showed better performance compared to other additives. Fig. 10 shows the NO removal efficiencies at an energy density of 150 J/l at room temperature and at 200 jC. NO removal efficiency at room temperature was 99% for the gas mixture (a) in the presence of ethylene and ethylene plus water, while at 200 jC, ethylene plus water showed slightly better performance than ethylene alone. This is
Fig. 10. NO removal efficiencies with water and water plus ethylene (energy density: 150 J/l).
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Fig. 11. NOx removal efficiencies with water and water plus ethylene (energy density: 150 J/l).
because OH radicals generated from water vapor can also be involved in the oxidation of NO as following reactions [7]: C2 H4 þ OH ! HOC2 H4
ð4Þ
HOC2 H4 þ 2O2 þ 2NO ! 2CH2 O þ 2NO2 þ OH
ð5Þ
Fig. 11 shows the NOx removal efficiencies at room temperature and at 200 jC at an energy density of 150 J/l. In the absence of ethylene or water vapor, no reduction in NOx level was observed. The gas mixed with only water exhibited 22% NOx removal efficiency. Ethylene and ethylene plus water exhibited NOx removal efficiencies of 32% and 37%, respectively. The measurement of byproducts was not carried out in the present study, but it can be easily understood that N2O, HNO2 and HNO3 may be produced according to the reactions (A6), (A7) and (A9). At higher temperatures, the NOx removal was not significant in any of the cases. It is in this context that the need of heterogeneous catalysis exists to ensure the NOx removal by converting it into molecular nitrogen.
4. Summary This paper was focused at studying the temperature effect on the NO conversion in the presence of hydrocarbons. The additives such as ethylene and n-hexane helped NO conversion to a greater extent at all temperatures. The effect of acetylene at room temperature was similar to that of no additive case, but at high temperatures it showed relatively better performance. In the present studies, the reduction in NOx level was negligible at high temperatures. The results obtained in this study will make the basis for our future studies, which aim at reducing NOx by combining electrical discharge plasma with heterogeneous catalysis.
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Acknowledgements This work was supported by the Basic Research Program of the Korea Science and Engineering Foundation (KOSEF) under Grant number R05-2001-000-01247-0.
Appendix A The following chemical reactions are considered during the discussion of the results [14,15]. k1 ¼ 5:0 1033 expð900=T Þ½M
NO þ O ! NO2
k2 ¼ 2:3 1012 expð1450=T Þ
NO þ O3 ! NO2 þ O2
k3 ¼ 1:7 1011 expð300=T Þ
NO2 þ O ! NO þ O2
ðA1Þ ðA2Þ ðA3Þ
N þ O2 ! NO þ O
k4 ¼ 4:4 1012 expð3220=T Þ
ðA4Þ
N þ NO ! N2 þ O
k5 ¼ 3:25 1011
ðA5Þ
k6 ¼ 3:0 1012
NO2 þ N ! N2 O þ O
k7 ¼ 7:4 1031 ðT =300Þ2:4 ½M
NO þ OH ! HNO2
HNO2 þ OH ! NO2 þ H2 O
k9 ¼ 2:6 1030 ðT =300Þ2:7 ½M
NO2 þ OH ! HNO3 NO þ HO2 ! NO2 þ OH OH þ O3 ! HO2 þ O2 O þ O2 ! O3
ðA7Þ ðA8Þ ðA9Þ
k10 ¼ 3:7 1012 expð240=T Þ
ðA10Þ
k11 ¼ 1:3 1012 expð956=T Þ
ðA11Þ
k12 ¼ 5:6 1034 ðT =300Þ2:23 ½M
H þ O3 ! OH þ O2 O þ O3 ! 2O2
k8 ¼ 1:8 1011 expð390=T Þ
ðA6Þ
k13 ¼ 1:4 1010 expð480=T Þ
k14 ¼ 8:0 1012 expð2060=T Þ
ðA12Þ ðA13Þ ðA14Þ
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References [1] L. Civitano, in: B.M. Penetrante, S.E. Schultheis (Eds.), Non-Thermal Plasma Techniques for Pollution Control: Part B, Springer-Verlag, Berlin, 1993, p. 103. [2] S. Masuda, in: B.M. Penetrante, S.E. Schultheis (Eds.), Non-Thermal Plasma Techniques for Pollution Control: Part B, Springer-Verlag, Berlin, 1993, p. 131. [3] G.E. Vogtlin, B.M. Pentrante, in: B.M. Penetrante, S.E. Schultheis (Eds.), Non-Thermal Plasma Techniques for Pollution Control: Part B, Springer-Verlag, Berlin, 1993, p. 187. [4] A. Mizuno, K. Shimizu, A. Chakrabarti, L. Dascalescu, S. Furutu, IEEE Trans. Ind. Appl. 31 (1995) 957. [5] Y.S. Mok, I.-S. Nam, J. Chem. Eng. Jpn. 31 (1998) 391. [6] Y.S. Mok, I.-S. Nam, Chem. Eng. Technol. 22 (1999) 527. [7] G. Sathiamoorthy, S. Kalyana, W.C. Finney, R.J. Clark, B.R. Locke, Ind. Eng. Chem. Res. 38 (1999) 1844. [8] R. Dorai, M.J. Kushner, SAE Paper 99FL-472, 1999. [9] J. Hoard, SAE Paper 01FL-63, 2001. [10] S. Broer, T. Hammer, Appl. Catal., B Environ. 28 (2000) 101. [11] S. Yoon, A.G. Panov, R.G. Tonkyn, A.C. Ebeling, S.E. Barlow, M.L. Balmer, Catal. Today 72 (2002) 243. [12] H. Miessner, K.P. Francke, R. Rudolph, T. Hammer, Catal. Today 75 (2002) 325. [13] L.A. Rosocha, G.K. Anderson, L.A. Bechtold, J.J. Coogan, H.G. Heck, K. Kang, W.H. McCulla, R.A. Tennant, P.J. Wantuck, in: B.M. Penetrante, S.E. Schultheis (Eds.), Non-Thermal Plasma Techniques for Pollution Control: Part B, Springer-Verlag, Berlin, 1993, p. 281. [14] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson Jr., J.A. Kerr, J. Troe, J. Phys. Chem. Ref. Data 21 (1992) 1125. [15] H. Ma¨zing, in: I. Prigogine, S.A. Rice (Eds.), Advances in Chemical Physics, vol. LXXXWiley, New York, 1991, p. 315.
Temperature effect on hydrocarbon-enhanced nitric oxide conversion using a dielectric barrier discharge reactor V. Ravi a, Young Sun Mok b,*, B.S. Rajanikanth a, Ho-Chul Kang b a
b
Department of High Voltage Engineering, Indian Institute of Science, Bangalore-560 012, India Department of Chemical Engineering, Cheju National University, Ara, Cheju 690-756, South Korea Accepted 1 November 2002
Abstract Experimental investigations into the effect of temperature on conversion of NO in the presence of hydrocarbons (ethylene, acetylene and n-hexane) are presented. An AC energized dielectric barrier discharge reactor was used as the plasma reactor. The experiments were carried out at different temperatures up to 200 jC. The discharge powers were measured at all the temperatures. The discharge power was found to increase with temperature. NO conversion in the presence of ethylene and n-hexane was better than that of acetylene at all temperatures. The addition of acetylene at room temperature showed no better conversion of NO compared to no additive case. While at higher temperatures, it could enhance the conversion of NO. A slight enhancement in NO and NOx removal was observed in the presence of water vapor. D 2003 Elsevier Science B.V. All rights reserved. Keywords: NO conversion; Dielectric barrier discharge; Temperature; Hydrocarbons
1. Introduction Non-thermal plasma associated with electrical discharges is fast gaining prominence because of its potential air cleaning applications. Among all air cleaning applications, electric discharge plasma is extensively studied for its removal of oxides of nitrogen. Oxides of nitrogen are the main pollutants emitted from diesel engines and thermal power
* Corresponding author. Tel.: +82-64-754-3682; fax: +82-64-755-3670. E-mail address: [email protected] (Y.S. Mok). 0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3820(03)00013-4
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plants. The application of electric discharge plasma in the removal of NOx from thermal power plants has been studied widely [1– 3] and at present the efforts are on to minimize the NOx emissions from diesel engine exhaust. The main role of electric discharge plasma in NOx removal has been understood to be the oxidation of NO to NO2. It was later found that the presence of hydrocarbons in the gas promotes the oxidation reactions [4– 8]. The presence of hydrocarbons not only reduces the energy required for the oxidation process such as NO to NO2 but also greatly reduces the possibility of HNO3 formation [9], which is not desirable from the vehicular exhaust treatment point of view. However, it has been established that the electric discharge plasmas alone cannot remove NOx from diesel exhaust. This demands the combination of the discharge plasma technique with heterogeneous catalysis. At present in the field of heterogeneous catalysis, selective catalytic reduction (SCR) is gaining more importance. Generally, SCR catalysts show maximum NOx removal at around 300 – 350 jC. However, as the portion of NO2 in NOx (NO+NO2) increases, the temperature window for NOx removal becomes wide. Broer and Hammer [10] showed that about 70% removal of NOx at 100 jC and more than 95% removal at 200 jC in SCR process was possible by oxidizing a part of NO to NO2 using electric discharge plasma. This removal efficiency obtained in the plasma/catalytic hybrid system is much higher than the summation of removal efficiencies obtained individually in the plasma reactor and in the catalytic reactor. Referring to the literature [11,12], this kind of synergistic effect seems to result from the increase in NO2 concentration by plasma discharges and it can be said that the crucial role of electric discharge plasma is to oxidize NO to NO2. In this context, the study on the oxidation of NO, especially at high temperature, is important because the chemistry related to the oxidation of NO and the role of hydrocarbons at high temperature are different from that at room temperature. For example, ozone can dominate the oxidation of NO and the decomposition of hydrocarbons at room temperature, but at high temperature, the generation of ozone will be suppressed and the other active species play an important role in the oxidation. As well, the rate of oxidation and the power consumption will depend on the reaction temperature. Up to now, there are few data available in the literature about the temperature effect on NO removal. This paper presents some experimental studies on temperature effect on NO oxidation in the presence of hydrocarbon additives using ac energized dielectric barrier discharge reactor. The experiments were carried out at different temperatures up to 200 jC. The hydrocarbon additives used were ethylene, acetylene and n-hexane.
2. Experimental setup Fig. 1 shows the reactor used in the present study (dielectric barrier discharge reactor) that is referred to as plasma reactor. A cylindrical glass tube (inner diameter: 25.8 mm; outer diameter: 30.2 mm) was used as the dielectric material and a copper rod (diameter: 9 mm) was used as the discharging electrode. An aluminium foil wrapped over the glass tube acted as the ground electrode. The space between the glass tube and the discharging electrode was filled with glass beads of diameter 5 mm. When the dielectric material such
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189
Fig. 1. Schematic of the dielectric barrier discharge reactor.
as glass beads are filled inside the plasma reactor, intense electric field is formed around the contact points of each pellet, causing partial discharges between the pellets. This partial discharge together with the discharge from the wire helps in increasing the radical production. The effective length of the plasma reactor, along which the discharge occurs, was 31 cm (reactor volume: 162 cm3). A 1.0 AF capacitor was connected to the plasma reactor in series to measure the discharge power. The experiments were carried out at different temperatures of the gas up to 200 jC since the typical temperature of the diesel exhaust is around 200 jC. For this, the plasma reactor was kept in a dry oven to maintain the desired gas temperature. To ensure proper heating-up of the feed gas to a given temperature, the stainless steel tube connected to the reactor inlet was wound several times within the dry oven, which acted as a heat exchanger. The experiments were carried out for two different gas mixtures such as (a) NO in N2 (300 ppm), O2 (10%) and N2 (balance), (b) NO in N2 (300 ppm), O2 (10%), H2O (3%) and N2 (balance). The hydrocarbon additives chosen in the present set of experiments were acetylene, ethylene and n-hexane. These hydrocarbons were injected into the gas stream in equal concentrations of 750 ppm. In a real diesel exhaust, the typical concentration of hydrocarbons is around 700 –750 ppm (under no load conditions). The flow rates of all the gas components used in the experiment were adjusted by mass flow controllers (MFC) (Model 1179, MKS Instruments) to get the desired concentrations. In the case of gas mixture (b), the content of water vapor was adjusted by using its vapor pressure. The flow rate of both the gas mixtures was kept at 2 l/min (based on room temperature) throughout the experiments. Since the reactor volume is 162 cm3, the residence time (apparent value) is calculated to be 4.86 s. However, actual residence time should be calculated by considering the void fraction of the reactor and the volume of the electrode (19.7 cm3). In this reactor packed with glass beads, the void fraction was found to be 0.3725. Therefore, the real reactor volume where the reactions take place corresponds to 53 cm3, and the real
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residence time is 1.6 s. There is one more thing that should be made clear in the calculation of residence time. The residence time (1.6 s) is based on the flow rate at room temperature, however, it should be corrected when the gas is not at room temperature, because the gas flow rate is a function of temperature. The concentrations of NO and NO2 were analyzed by a chemiluminescence NO – NO2 – NOx analyzer (Model 42C, Thermo Environmental Instruments). The reactor was energized by AC voltage at 60 Hz. The voltage was varied from 8 to 16 kV to change the discharge power. The voltage applied to the discharging electrode was measured by a 1000:1 high voltage probe (PVM-4, North Star Research) and a digital oscilloscope (TDS 3032, Tektronix). For the measurement of the voltage between both ends of the 1.0 AF
Fig. 2. Voltage waveforms measured at the discharging electrode and at the 1.0 AF capacitor (a); charge – voltage plot (b).
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capacitor, a 10:1 voltage probe (Tektronix P6139A) was used. The measurement of input power was carried out using a digital power meter (Model WT 200, Yokogawa). 2.1. Measurement of discharge power The method adopted to measure the discharge power in the present experiments is as follows. A 1.0 AF capacitor was connected in series with the reactor as shown in Fig. 1. Since the dielectric barrier discharge reactor can be considered as a capacitor, the charge stored in the capacitor (1.0 AF) is equal to that in the reactor. The charge stored in the capacitor ( Q) is the product of capacitance (C) and voltage (V) ( Q=CV), which can be directly read by the voltage between both ends of the capacitor. In other words, the charge stored in the capacitor is equal to 106 times the voltage. Fig. 2(a) shows the voltage waveforms measured at the discharging electrode and at the 1.0 AF capacitor, and Fig. 2(b) shows the charge–voltage plot at the corresponding voltage. The area of the parallelogram in Fig. 2(b) conforms to the discharge energy per one cycle, and the average discharge power can be obtained by multiplying the discharge energy per one cycle by AC frequency (i.e. 60 Hz). For example, in Fig. 2(b), the discharge energy per cycle at 12 kV was found to be 7.33 mJ/cycle. Hence, the average discharge power is 7.33103 J/cycle60 Hz=0.44 W. In principle, this method is similar to the one adopted in Ref. [13], in which a mathematical formula was employed for calculating the area of the parallelogram. For convenience, in the present case the area of parallelogram was calculated by comparing the total mass of the graph with that of the parallelogram. It was assumed that the thickness of the paper was uniform throughout. We compared both methods of calculating the area of the parallelogram and the results were found to agree well within 5%.
3. Results and discussion Fig. 3 shows the variation of discharge power for the mixture (a) at different temperatures as a function of input power. The discharge power tends to increase with temperature, which can be explained as follows. The gas at high temperature is more excited than that at low temperature and it can be more easily ionized. As the extent of ionization increases, the discharge current increases because of decreased gas resistance. Fig. 4 shows the discharge power variation at different temperatures for the gas mixture (b). The discharge powers obtained with the mixture (b) showed the similar trend to that of the gas mixture (a). Both Figs. 3 and 4 were used as basis for measuring the energy densities in the following results. Fig. 5 shows the concentrations of NO, NO2 and NOx as a function of energy density using plasma reactor at room temperature. As the energy density increased, the NO started oxidizing into NO2 while keeping the NOx concentration constant. The oxidation proceeded faster up to an energy density of around 50 J/l, at which the NO removal efficiency is around 60%, and becomes almost constant up to an energy density of 150 J/l. This is because at higher energy densities, the NO2 reduces back to NO (see (A3) in Appendix A). At higher energy densities there was very small production of NOx above its
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Fig. 3. Discharge power with mixture (a) at different temperatures.
initial value. This is because of the production of NO through reaction (A4). The NO produced will counterbalance the reduction in NO and some part of it will be oxidized to NO2 through the reactions (A1) and (A2) thereby increasing NOx level. The effect of temperature on NO removal for gas mixtures (a) and (b) at 150 J/l is shown in Fig. 6. The NO removal efficiency for the mixture (b) containing water was higher than that of the mixture (a) at all temperatures. This is because in the presence of water molecules, the plasma reactor releases OH radicals which in turn oxidize NO to NO2
Fig. 4. Discharge power with mixture (b) at different temperatures.
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Fig. 5. NO, NO2 and NOx concentrations as a function of energy density at room temperature.
through the reactions (A7) and (A8) in Appendix A. As the temperature of the gas increased, the NO removal efficiency decreased for both the gas mixtures. At room temperature, plasma reactor produces high concentration of ozone, which is mainly responsible for the oxidation of NO to NO2 (see (A2) in Appendix A). The decrease in NO removal efficiency with temperature for the gas mixture (a) is mainly because of two reasons: (i) the rate of the reaction (A1) decreases with increase in the temperature; (ii) the production of ozone is suppressed at high temperatures (see (A12) in Appendix A). In case
Fig. 6. Effect of temperature on NO removal efficiency (energy density: 150 J/l).
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of gas mixture (b), besides the above two reasons, the reaction rate in (A7) decreasing with increase in temperature accounts for the decrease in NO removal efficiency. The effect of hydrocarbon additives such as acetylene, ethylene and n-hexane on the NO removal was studied using plasma reactor at different temperatures. Initially, the gas mixture (a) was considered. Fig. 7 shows the NO removal efficiencies in the presence of hydrocarbon additives at room temperature. The presence of both ethylene and n-hexane enhanced the oxidation of NO to NO2 significantly, whereas the addition of acetylene was found to be no better than that without any additive. At an energy density of 50 J/l, the NO removal efficiency was as high as 80% with ethylene and 75% with n-hexane, while acetylene and no additive case showed only 60% removal. The NO removal in presence of hydrocarbons can be explained as follows. The radicals important for the oxidation of NO are alkyl (R), alkoxy (RO) and acyl (RCO) radicals, which are generated when hydrocarbons react with O, OH, O3, etc. Initially, alkoxy (CH3O, C2H5O, etc.) and alkyl radicals (such as CH3, C2H5, etc.) react with oxygen to liberate active species [5]: RO þ O2 ! HCHO þ HO2
ð1Þ
R þ O2 ! RO2
ð2Þ
The HO2 produced increases the NO oxidation (A10) and RO2 oxidizes NO by RO2 þ NO ! CH3 O þ NO2
ð3Þ
These are the important reactions enhancing the NO oxidation though there are varieties of other radicals, which also help in oxidizing NO to NO2.
Fig. 7. Effect of hydrocarbons on NO removal efficiency at room temperature.
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Fig. 8. Effect of temperature on NO removal efficiency in the presence of hydrocarbons (energy density: 150 J/l).
Fig. 8 shows the comparison between the NO removal efficiencies at 150 J/l obtained in the presence of additives as a function of gas temperature. Both ethylene and n-hexane with high reactivities with O3, O and OH (see Table 1) retained their abilities to oxidize NO at all temperatures. On the other hand, the effect of acetylene at room temperature was not significant, but at higher temperatures acetylene showed relatively better performance than that of no additive. This is because at room temperature the oxidation of NO is dominated by O3. As can be seen in Table 1, the reactivity of acetylene with O3, O and OH radicals is low and as a result there is less formation of fragments such as R, RO and RCO capable of oxidizing NO. But at higher temperatures, the effect of acetylene becomes significant because the production of ozone is greatly reduced. The variation of NOx concentration with energy density for different additives at room temperature is shown in Fig. 9. There was hardly any NOx removal without any additive, whereas it was negligible in the presence of acetylene. In the presence of ethylene and nhexane, the NOx removal efficiencies obtained in this experimental condition were 32%
Table 1 Reaction rate constants of hydrocarbons with OH, O and O3 [NIST Chemical Reactions Database-1997] Reaction partner
OH
O
O3
C2H2
4.98 10 12 exp( 553/T) 2.15 10 12 exp(411/T) 9.21 10 13 (T/298)2.2 exp(540/T)
3.05 10 13 (T/298)2.8 exp( 250/T) 1.01 10 12 (T/298)1.88 exp( 92/T) 2.48 10 12 (T/298)2.9 exp( 1000/T)
1.00 10 14 exp( 4100/T) 1.2 10 14 exp( 2630/T) No data
C2H4 C6H14
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Fig. 9. Effect of hydrocarbons on NOx concentration at room temperature.
and 35%, respectively. Although the data are not presented at temperatures higher than room temperature, the NOx removal was insignificant in all these cases. Further experiments were conducted by adding ethylene to the gas mixture (b). Ethylene was chosen because it showed better performance compared to other additives. Fig. 10 shows the NO removal efficiencies at an energy density of 150 J/l at room temperature and at 200 jC. NO removal efficiency at room temperature was 99% for the gas mixture (a) in the presence of ethylene and ethylene plus water, while at 200 jC, ethylene plus water showed slightly better performance than ethylene alone. This is
Fig. 10. NO removal efficiencies with water and water plus ethylene (energy density: 150 J/l).
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Fig. 11. NOx removal efficiencies with water and water plus ethylene (energy density: 150 J/l).
because OH radicals generated from water vapor can also be involved in the oxidation of NO as following reactions [7]: C2 H4 þ OH ! HOC2 H4
ð4Þ
HOC2 H4 þ 2O2 þ 2NO ! 2CH2 O þ 2NO2 þ OH
ð5Þ
Fig. 11 shows the NOx removal efficiencies at room temperature and at 200 jC at an energy density of 150 J/l. In the absence of ethylene or water vapor, no reduction in NOx level was observed. The gas mixed with only water exhibited 22% NOx removal efficiency. Ethylene and ethylene plus water exhibited NOx removal efficiencies of 32% and 37%, respectively. The measurement of byproducts was not carried out in the present study, but it can be easily understood that N2O, HNO2 and HNO3 may be produced according to the reactions (A6), (A7) and (A9). At higher temperatures, the NOx removal was not significant in any of the cases. It is in this context that the need of heterogeneous catalysis exists to ensure the NOx removal by converting it into molecular nitrogen.
4. Summary This paper was focused at studying the temperature effect on the NO conversion in the presence of hydrocarbons. The additives such as ethylene and n-hexane helped NO conversion to a greater extent at all temperatures. The effect of acetylene at room temperature was similar to that of no additive case, but at high temperatures it showed relatively better performance. In the present studies, the reduction in NOx level was negligible at high temperatures. The results obtained in this study will make the basis for our future studies, which aim at reducing NOx by combining electrical discharge plasma with heterogeneous catalysis.
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Acknowledgements This work was supported by the Basic Research Program of the Korea Science and Engineering Foundation (KOSEF) under Grant number R05-2001-000-01247-0.
Appendix A The following chemical reactions are considered during the discussion of the results [14,15]. k1 ¼ 5:0 1033 expð900=T Þ½M
NO þ O ! NO2
k2 ¼ 2:3 1012 expð1450=T Þ
NO þ O3 ! NO2 þ O2
k3 ¼ 1:7 1011 expð300=T Þ
NO2 þ O ! NO þ O2
ðA1Þ ðA2Þ ðA3Þ
N þ O2 ! NO þ O
k4 ¼ 4:4 1012 expð3220=T Þ
ðA4Þ
N þ NO ! N2 þ O
k5 ¼ 3:25 1011
ðA5Þ
k6 ¼ 3:0 1012
NO2 þ N ! N2 O þ O
k7 ¼ 7:4 1031 ðT =300Þ2:4 ½M
NO þ OH ! HNO2
HNO2 þ OH ! NO2 þ H2 O
k9 ¼ 2:6 1030 ðT =300Þ2:7 ½M
NO2 þ OH ! HNO3 NO þ HO2 ! NO2 þ OH OH þ O3 ! HO2 þ O2 O þ O2 ! O3
ðA7Þ ðA8Þ ðA9Þ
k10 ¼ 3:7 1012 expð240=T Þ
ðA10Þ
k11 ¼ 1:3 1012 expð956=T Þ
ðA11Þ
k12 ¼ 5:6 1034 ðT =300Þ2:23 ½M
H þ O3 ! OH þ O2 O þ O3 ! 2O2
k8 ¼ 1:8 1011 expð390=T Þ
ðA6Þ
k13 ¼ 1:4 1010 expð480=T Þ
k14 ¼ 8:0 1012 expð2060=T Þ
ðA12Þ ðA13Þ ðA14Þ
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