- Email: [email protected]
NOx treatment by DC streamer corona discharge with series gap
Vacuum 65 (2002) 469–474
NOx treatment by DC streamer corona discharge with series gap Penghui Guan*, Nobuya Hayashi, Saburoh Satoh, Chobei Yamabe Department of Electrical and Electronic Engineering, School of Science and Engineering, Saga University, 1, Honjo-machi, Saga City, Saga 840-8502, Japan
Abstract Among many methods used for removal of NO from flue gas, a positive DC streamer corona discharge method is presented in this paper. To generate a uniform streamer corona discharge, which is effective for the NO treatment, a series gap was connected to the main discharge reactor. The electrical and optical measurements showed that the series gap was useful for getting the uniform streamer discharge. Using the series gap, a voltage pulse whose dV =dt was from 0.7 to 2.72 kV/ns (depending on the series gap spacing) was produced in the main reactor. The energy consumption of the series gap was from about 6% to 20% of the total input power for different series gap spacings from 0.3 to 1.05 mm.The maximum NO removal rate of 90% was obtained at the main gap spacing of 30 mm and series gap spacing of 0.9 mm. Maximum removal efficiency of 6.0 g NO/kW h also occurred at a series gap spacing of 0.3 mm. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: NOx treatment; Streamer corona discharge; Series gap; Atmospheric discharge
1. Introduction Nowadays, the influence of the NO on the environment has become a serious problem, while humans improve their level of technology. Various kinds of non-thermal and thermal plasmas have demonstrated their technical and economical feasibility for removal of NO from flue gas, such as pulsed streamer corona discharge, microwave discharge, gliding arc, glow discharge, dielectric barrier, surface discharge, etc. For the electrode arrangements with large gap spacing (>5 cm), the positive DC streamer corona seems to be most suitable for inducing non-thermal plasma under *Corresponding author. Fax: +81-952-28-8651. E-mail address: [email protected] (P. Guan).
high pressure [1]. It has been well known that for the gap spacing of centimeters order in air, the morphology of corona discharge may change from onset streamer to Hermstein glow, pre-breakdown streamer and then spark breakdown by increasing the positive DC applied voltage [2]. While producing a large volume of streamer corona with a DC power supply, both streamer corona and glow discharge may be generated simultaneously. For this reason, many experiments have been carried out using other power supplies than positive DC, such as nano-seconds pulsed power supply, or, in a way to improve DC power supply, simplified DC/AC power supply [3]. Very few techniques have been reported using simple positive DC streamer corona reactors for inducing chemical reactions. However, due to its simplicity,
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 5 8 - 4
470
P. Guan et al. / Vacuum 65 (2002) 469–474
the positive DC streamer corona discharge has been chosen for our research of NO removal. In previous work by same group [4], DC streamer discharge was obtained by using wireto-plate geometry (diameter of wire was 0.1 mm). However, such streamer discharge was unstable, often changing into arc, and also the lifetime of the wire was very short under such discharge condition. The attempts to increase the diameter of wire to 0.5 or 1.0 mm failed as no stable discharge except arcing occurred as well. To produce uniform streamer discharge, a series gap was connected directly behind the main discharge reactor. The series gap was a sphereto-plane gap with gap spacing of the order of millimeters. with increasing voltage, the breakdown appeared firstly in the series gap, because of the shorter gap spacing. At the same time, the voltage across the main gap increased rapidly, resulting in the generation and stabilization of the streamer in the main gap. If we define the total applied voltage V1 ; the voltage drop across the series gap as V2 and the voltage of main reactor Vm ; we always have Vm ¼ V1 V2 : Using the series gap in the circuit, the V2 dropped rapidly almost to 0 V, while simultaneously the Vm rapidly increased as shown in Fig. 1. After the streamer crosses the gap, both the Vm and V1 decreased, and the streamer corona discharge disappeared until the next series gap breakdown occurred. The frequency of this process ranged from 2.5 to 7 kHz,
depending on both applied voltage and the series gap spacing. We suppose the dV =dt of the main reactor voltage characteristic is an important parameter for production of the streamer and following chemical processes. Studies on discharge characteristics and energy consumption of the series gap are reported.
Fig. 1. Typical voltage waveform of device with series gap.
Fig. 2. Electrical circuit.
2. Experimental setup In this experiment, a DC power supply was used, as shown in Fig. 2. The main discharge reactor used in the experiment was of rod-to-double slope geometry, shown in Fig. 2. In our previous experiments, a wire with 0.1 mm diameter was used as an anode, because of its advantage of easy streamer corona discharge generation. However, industrial application required electrodes with a long lifetime. Thus, the rod-to-double slope reactor was designed and used in this study instead. The diameter of the rod was 1 mm. To be able to measure the electrical characteristics for different discharge gap spacings, a movable double slop plate electrode as a cathode was designed and used. Thanks to such design, the main discharge spacing could be changed from 5 to 35 mm continuously simply by sliding the double-slope electrodes. A sphere-to-plane series gap was connected directly behind the main reactor (Fig. 3). The gap spacing of series gap could be changed in the order of millimeter.
P. Guan et al. / Vacuum 65 (2002) 469–474
471
Fig. 3. Experimental setup.
Fig. 4. Discharge photos with different discharge gap spacing.
In the experiment, the NO (500 ppm)/N2 gas mixture with a flow rate of 2 l/min was used as a sample gas. The total applied voltage (V1 ), total discharge current (I) and the voltage that on series gap (V2 ) were measured, as shown in Fig. 1, and the total energy and the energy costs of series gap were calculated. In a way to change the value of dV =dt; various gases were admitted into series gap (air, O2, CO2, NO (500 ppm)/N2 and N2). For each gas, the typical voltage waveforms were measured and recorded to consider the change of the dV =dt: The energy costs of series gap for each additional gas, and respective NO removal rate were calculated.
To observe the effect of series gap itself, we kept the series gap spacing constant (0.8 mm) at first, and moved the double-slope electrodes thus changing the main reactor discharge gap from 5 to 35 mm.The uniform streamer discharge was observed during whole process. The photos of the streamer discharge are shown in Fig. 4. In the second step, in a way to obtain the energy consumption of series gap, the main discharge gap spacing was kept at 30 and 30 mm has been turned out to be the best because of better NO removal result [5], while the series gap spacing was changed from 0.3 to 1.05 mm.The voltages and currents for different series gap spacing were measured and the
472
P. Guan et al. / Vacuum 65 (2002) 469–474
energy consumptions for each series gap spacing were calculated.
3. Experimental results Using series gap, we observed that positive streamer corona discharge in the main reactor was significantly uniform (see Fig. 4). In fact, without series gap, we did not get a stable streamer corona at all. According to Fig. 5, the dV =dt of series gap was the highest in the case of air, and the lowest for N2. However, in the case of N2, the operation of series gap was the most stable allowing to increase both the series gap spacing and the applied voltage even higher compared with other two gases. The energy consumption for different series gap spacing was calculated based on the experimental results. Fig. 6 presents the results of power consumption for series gap spacing changing from 0.3 to 0.75 mm in the case of additional gas air as an example, according to figure, with the increase of the series gap spacing, the energy consumption increased. The energy cost of the series gap was dependent mainly on the spacing of the series gap, more than the applied voltage. The energy cost of the series gap was a considerable part of total energy cost (maximum 20.8%). For another set of experiments to compare the energy consumption of series gap with different
Fig. 5. dV =dt as a function of series gap spacing for different gas in the series gap.
Fig. 6. Energy consumption of series gap with different series gap spacing in air.
Fig. 7. Power consumption of series gap with different additional gases.
additional gas, Fig. 7 shows the power consumption as a function of series gap spacing. The series gap with additional gas of N2 shows the lowest power consumption, which was due to its stable mode of operation style (see Fig. 8). The NO removal rates for different series gap additional gas were measured, while the main gap spacing was kept at 30 mm. According to Fig. 9, up to 14 W, the NO removal rates using NO/N2 as series gap additional gas were higher than others. By using a water bubbler behind the main reactor, the NO removal rate was improved up to 90%. Fig. 10 shows the treatment efficiency of NO as a function of total power for series gaps with different additional gas. Maximal treatment efficiency reached was 6.0 g NO/kW h, at a NO
P. Guan et al. / Vacuum 65 (2002) 469–474
473
Fig. 8. Discharge photographs for in series gap with different additional gas.
Fig. 9. NO removal rate as a function of total power for different series gap spacing.
removal rate of about 30%, with 0.15 mm series gap spacing and the NO/N2 additional gas. By increasing the input power, the NO removal rate could be increased up to 90%; however, the efficiency decreased to 4.0 g NO/kW h. It was also found that increasing the series gap spacing, the treatment efficiency decreased, as the increase of the series gap led to higher power consumption by the series gap. In this experiment, although the NO removal rate was rather high, the treatment efficiency was
lower than expected. It turn out that further work is required to decrease the power losses, concerning also the design of the reactor.
4. Conclusions In this research, the effect of series gap on the positive DC streamer discharge for NO treatment has been studied. The results have shown that the series gap can stabilize the discharge in the
474
P. Guan et al. / Vacuum 65 (2002) 469–474
Fig. 10. NO removal efficiency as a function of total power for different series gap spacing
streamer mode effectively. The series gap provides a steady pulsed voltage to obtain the intense streamer corona discharge in the main reactor. It was shown that the parameters of series gap (series gap spacing and different additional gas) influenced the generation of uniform streamer. Additional gas of air, led to the highest value of dV =dt (2–2.5 kV/ns), while the N2 gas caused the lowest power consumption by series gap (15–27%) (see Fig. 7). Changing the series gap spacing influenced both the dV =dt and the power consumption of the series gap. As for NO treatment, the maximum NO removal rate of 90% was obtained. The maximum treatment efficiency of 6.0 g NO/kW h at NO removal rate of 30% was obtained with the series gap spacing 0.15 mm. It should be pointed out that, increasing the series gap spacing, we always got lower NOremoval efficiency because of higher power con-
sumption of series gap. On the other hand, with bigger series gap spacing we could improve the maximum NO removal rate because of higher applied voltage (input power). It shows that a balance of high removal rate and high treatment efficiency should be studied further.
References [1] Yan K, Kanazawa S, Ohkudo T, Nomoto Y. Trans IEE Japan 1998;118-A:948–53. [2] Sigmond RS. In: Meek JM, Craggs JD, editors. Electrical Breakdown of Gases. New York: Wiley, 1978. [3] Yan K, van Veldhuizen EM, Rea M, Li R, J Adv Oxid Technol 1998. [4] Gasparik R, Mine N, Ihara S, Satoh S, Yamabe C. Vacuum 2000;59:220–7. [5] Guan P, Ihara S, Gasparik R, Yamabe C, Proceedings of the 1999 Joint Conference of Electrical and Electronics Engineers in Kyushu, Kitakyushu, p. 67.
NOx treatment by DC streamer corona discharge with series gap Penghui Guan*, Nobuya Hayashi, Saburoh Satoh, Chobei Yamabe Department of Electrical and Electronic Engineering, School of Science and Engineering, Saga University, 1, Honjo-machi, Saga City, Saga 840-8502, Japan
Abstract Among many methods used for removal of NO from flue gas, a positive DC streamer corona discharge method is presented in this paper. To generate a uniform streamer corona discharge, which is effective for the NO treatment, a series gap was connected to the main discharge reactor. The electrical and optical measurements showed that the series gap was useful for getting the uniform streamer discharge. Using the series gap, a voltage pulse whose dV =dt was from 0.7 to 2.72 kV/ns (depending on the series gap spacing) was produced in the main reactor. The energy consumption of the series gap was from about 6% to 20% of the total input power for different series gap spacings from 0.3 to 1.05 mm.The maximum NO removal rate of 90% was obtained at the main gap spacing of 30 mm and series gap spacing of 0.9 mm. Maximum removal efficiency of 6.0 g NO/kW h also occurred at a series gap spacing of 0.3 mm. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: NOx treatment; Streamer corona discharge; Series gap; Atmospheric discharge
1. Introduction Nowadays, the influence of the NO on the environment has become a serious problem, while humans improve their level of technology. Various kinds of non-thermal and thermal plasmas have demonstrated their technical and economical feasibility for removal of NO from flue gas, such as pulsed streamer corona discharge, microwave discharge, gliding arc, glow discharge, dielectric barrier, surface discharge, etc. For the electrode arrangements with large gap spacing (>5 cm), the positive DC streamer corona seems to be most suitable for inducing non-thermal plasma under *Corresponding author. Fax: +81-952-28-8651. E-mail address: [email protected] (P. Guan).
high pressure [1]. It has been well known that for the gap spacing of centimeters order in air, the morphology of corona discharge may change from onset streamer to Hermstein glow, pre-breakdown streamer and then spark breakdown by increasing the positive DC applied voltage [2]. While producing a large volume of streamer corona with a DC power supply, both streamer corona and glow discharge may be generated simultaneously. For this reason, many experiments have been carried out using other power supplies than positive DC, such as nano-seconds pulsed power supply, or, in a way to improve DC power supply, simplified DC/AC power supply [3]. Very few techniques have been reported using simple positive DC streamer corona reactors for inducing chemical reactions. However, due to its simplicity,
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 5 8 - 4
470
P. Guan et al. / Vacuum 65 (2002) 469–474
the positive DC streamer corona discharge has been chosen for our research of NO removal. In previous work by same group [4], DC streamer discharge was obtained by using wireto-plate geometry (diameter of wire was 0.1 mm). However, such streamer discharge was unstable, often changing into arc, and also the lifetime of the wire was very short under such discharge condition. The attempts to increase the diameter of wire to 0.5 or 1.0 mm failed as no stable discharge except arcing occurred as well. To produce uniform streamer discharge, a series gap was connected directly behind the main discharge reactor. The series gap was a sphereto-plane gap with gap spacing of the order of millimeters. with increasing voltage, the breakdown appeared firstly in the series gap, because of the shorter gap spacing. At the same time, the voltage across the main gap increased rapidly, resulting in the generation and stabilization of the streamer in the main gap. If we define the total applied voltage V1 ; the voltage drop across the series gap as V2 and the voltage of main reactor Vm ; we always have Vm ¼ V1 V2 : Using the series gap in the circuit, the V2 dropped rapidly almost to 0 V, while simultaneously the Vm rapidly increased as shown in Fig. 1. After the streamer crosses the gap, both the Vm and V1 decreased, and the streamer corona discharge disappeared until the next series gap breakdown occurred. The frequency of this process ranged from 2.5 to 7 kHz,
depending on both applied voltage and the series gap spacing. We suppose the dV =dt of the main reactor voltage characteristic is an important parameter for production of the streamer and following chemical processes. Studies on discharge characteristics and energy consumption of the series gap are reported.
Fig. 1. Typical voltage waveform of device with series gap.
Fig. 2. Electrical circuit.
2. Experimental setup In this experiment, a DC power supply was used, as shown in Fig. 2. The main discharge reactor used in the experiment was of rod-to-double slope geometry, shown in Fig. 2. In our previous experiments, a wire with 0.1 mm diameter was used as an anode, because of its advantage of easy streamer corona discharge generation. However, industrial application required electrodes with a long lifetime. Thus, the rod-to-double slope reactor was designed and used in this study instead. The diameter of the rod was 1 mm. To be able to measure the electrical characteristics for different discharge gap spacings, a movable double slop plate electrode as a cathode was designed and used. Thanks to such design, the main discharge spacing could be changed from 5 to 35 mm continuously simply by sliding the double-slope electrodes. A sphere-to-plane series gap was connected directly behind the main reactor (Fig. 3). The gap spacing of series gap could be changed in the order of millimeter.
P. Guan et al. / Vacuum 65 (2002) 469–474
471
Fig. 3. Experimental setup.
Fig. 4. Discharge photos with different discharge gap spacing.
In the experiment, the NO (500 ppm)/N2 gas mixture with a flow rate of 2 l/min was used as a sample gas. The total applied voltage (V1 ), total discharge current (I) and the voltage that on series gap (V2 ) were measured, as shown in Fig. 1, and the total energy and the energy costs of series gap were calculated. In a way to change the value of dV =dt; various gases were admitted into series gap (air, O2, CO2, NO (500 ppm)/N2 and N2). For each gas, the typical voltage waveforms were measured and recorded to consider the change of the dV =dt: The energy costs of series gap for each additional gas, and respective NO removal rate were calculated.
To observe the effect of series gap itself, we kept the series gap spacing constant (0.8 mm) at first, and moved the double-slope electrodes thus changing the main reactor discharge gap from 5 to 35 mm.The uniform streamer discharge was observed during whole process. The photos of the streamer discharge are shown in Fig. 4. In the second step, in a way to obtain the energy consumption of series gap, the main discharge gap spacing was kept at 30 and 30 mm has been turned out to be the best because of better NO removal result [5], while the series gap spacing was changed from 0.3 to 1.05 mm.The voltages and currents for different series gap spacing were measured and the
472
P. Guan et al. / Vacuum 65 (2002) 469–474
energy consumptions for each series gap spacing were calculated.
3. Experimental results Using series gap, we observed that positive streamer corona discharge in the main reactor was significantly uniform (see Fig. 4). In fact, without series gap, we did not get a stable streamer corona at all. According to Fig. 5, the dV =dt of series gap was the highest in the case of air, and the lowest for N2. However, in the case of N2, the operation of series gap was the most stable allowing to increase both the series gap spacing and the applied voltage even higher compared with other two gases. The energy consumption for different series gap spacing was calculated based on the experimental results. Fig. 6 presents the results of power consumption for series gap spacing changing from 0.3 to 0.75 mm in the case of additional gas air as an example, according to figure, with the increase of the series gap spacing, the energy consumption increased. The energy cost of the series gap was dependent mainly on the spacing of the series gap, more than the applied voltage. The energy cost of the series gap was a considerable part of total energy cost (maximum 20.8%). For another set of experiments to compare the energy consumption of series gap with different
Fig. 5. dV =dt as a function of series gap spacing for different gas in the series gap.
Fig. 6. Energy consumption of series gap with different series gap spacing in air.
Fig. 7. Power consumption of series gap with different additional gases.
additional gas, Fig. 7 shows the power consumption as a function of series gap spacing. The series gap with additional gas of N2 shows the lowest power consumption, which was due to its stable mode of operation style (see Fig. 8). The NO removal rates for different series gap additional gas were measured, while the main gap spacing was kept at 30 mm. According to Fig. 9, up to 14 W, the NO removal rates using NO/N2 as series gap additional gas were higher than others. By using a water bubbler behind the main reactor, the NO removal rate was improved up to 90%. Fig. 10 shows the treatment efficiency of NO as a function of total power for series gaps with different additional gas. Maximal treatment efficiency reached was 6.0 g NO/kW h, at a NO
P. Guan et al. / Vacuum 65 (2002) 469–474
473
Fig. 8. Discharge photographs for in series gap with different additional gas.
Fig. 9. NO removal rate as a function of total power for different series gap spacing.
removal rate of about 30%, with 0.15 mm series gap spacing and the NO/N2 additional gas. By increasing the input power, the NO removal rate could be increased up to 90%; however, the efficiency decreased to 4.0 g NO/kW h. It was also found that increasing the series gap spacing, the treatment efficiency decreased, as the increase of the series gap led to higher power consumption by the series gap. In this experiment, although the NO removal rate was rather high, the treatment efficiency was
lower than expected. It turn out that further work is required to decrease the power losses, concerning also the design of the reactor.
4. Conclusions In this research, the effect of series gap on the positive DC streamer discharge for NO treatment has been studied. The results have shown that the series gap can stabilize the discharge in the
474
P. Guan et al. / Vacuum 65 (2002) 469–474
Fig. 10. NO removal efficiency as a function of total power for different series gap spacing
streamer mode effectively. The series gap provides a steady pulsed voltage to obtain the intense streamer corona discharge in the main reactor. It was shown that the parameters of series gap (series gap spacing and different additional gas) influenced the generation of uniform streamer. Additional gas of air, led to the highest value of dV =dt (2–2.5 kV/ns), while the N2 gas caused the lowest power consumption by series gap (15–27%) (see Fig. 7). Changing the series gap spacing influenced both the dV =dt and the power consumption of the series gap. As for NO treatment, the maximum NO removal rate of 90% was obtained. The maximum treatment efficiency of 6.0 g NO/kW h at NO removal rate of 30% was obtained with the series gap spacing 0.15 mm. It should be pointed out that, increasing the series gap spacing, we always got lower NOremoval efficiency because of higher power con-
sumption of series gap. On the other hand, with bigger series gap spacing we could improve the maximum NO removal rate because of higher applied voltage (input power). It shows that a balance of high removal rate and high treatment efficiency should be studied further.
References [1] Yan K, Kanazawa S, Ohkudo T, Nomoto Y. Trans IEE Japan 1998;118-A:948–53. [2] Sigmond RS. In: Meek JM, Craggs JD, editors. Electrical Breakdown of Gases. New York: Wiley, 1978. [3] Yan K, van Veldhuizen EM, Rea M, Li R, J Adv Oxid Technol 1998. [4] Gasparik R, Mine N, Ihara S, Satoh S, Yamabe C. Vacuum 2000;59:220–7. [5] Guan P, Ihara S, Gasparik R, Yamabe C, Proceedings of the 1999 Joint Conference of Electrical and Electronics Engineers in Kyushu, Kitakyushu, p. 67.