Article pubs.acs.org/IECR
Comparative Study between Single- and Double-Dielectric Barrier Discharge Reactor for Nitric Oxide Removal Xiao-Long Tang, Feng-Yu Gao, Jian-Gen Wang, Hong-Hong Yi,* Shun-Zheng Zhao, Bo-Wen Zhang, Yan-Ran Zuo, and Zhi-Xiang Wang Department of Environmental Engineering, Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing, 100083, China ABSTRACT: The effect of several parameters, such as peak voltage, frequency, and geometry, on the NO removal efficiency and energy utilization efficiency of a dielectric barrier discharge (DBD) reactor has been investigated in this work. NO removal efficiency and specific input energy (SIE) was augmented with the increase of voltage from 2.5 kV to 6.5 kV. For a given voltage, the maximum of NO removal efficiency and SIE were attained at resonance frequency. Single-dielectric DBD reactor with a discharge gap of 3.5 mm obtained the highest discharge power and NO removal efficiency. The energy utilization efficiencies of different DBD reactors were also compared.
1. INTRODUCTION The combustion of fossil fuels inevitably leads to the formation of nitrogen oxides (NOx, i.e., NO and NO2), which are one of the major air pollutants and toxic gases. It can result in the acid rain, the photochemical smog, the ozone depletion, and the climate change, and are also harmful to the human body. In China, above 90% of total NOx emissions are in the form of NO emitting from stationary sources.1 Selective catalytic reduction (SCR)2,3 and selective noncatalytic reduction (SNCR)4,5 are well-known postcombustion methods used for the NO reduction. Comparing with the conventional methods of DeNOx, nonthermal plasma (NTP) technology operates at room temperature and has no secondary pollution. 6,7 NTP technology, featured by the nonequilibrium characteristic between high energy electrons and highly reactive species (including ions, radicals, and neutrals),8−10 has aroused great attention for DeNOx in the recent 20 years. The most distinctive characteristic of NTP is its ability to induce various chemical reactions at atmospheric and room temperature. The common method of NTP generation is electric discharge, such as dielectric barrier discharge (DBD),9−13 pulsed corona discharges,14,15 DC corona discharges,14,16 and surface discharges.17 DBD is obtained between two electrodes, at least one of which should be covered with dielectric material, when AC high voltage is applied on the electrodes. High discharge power levels can be reached by using a DBD reactor without using complex pulsed power supplies.6,7 The effects of several parameters, such as voltage, frequency, reactor geometry, and discharge power, on NO removal efficiency are discussed in this work. The current investigated results showed that NO removal efficiency is enhanced with the increase of peak voltage and discharge power,8,9,17 but an equilibrium concentration of NOx would be produced by plasma at sufficiently high power depositions. The effects of frequency on NO removal efficiency between different works were not consistent.8,9,17 For reactor geometry, more attention was focused on the length of the discharge gap and the © 2014 American Chemical Society
structure of the high voltage electrode for both cylinder and plate DBD reactors. The main idea of this work is to enhance the NO removal efficiency and energy utilization efficiency by changing the geometry of the DBD reactor.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. The experimental system (Figure 1) mainly consisted of a gas distribution system,
Figure 1. Schematic diagram of experimental system.
DBD reactor, plasma generator (peak voltage: 0−30 kV, frequency: 5−25 kHz), electrical measurement system, and flue gas analyzer. The flow rates of desired gas were controlled by mass flow controllers. Simulated gas, consisting of NO (500 ppm), O2 (3.5 vol %) (99.9% pure) in a balance of N2 (99.9% pure), with a flow rate of 0.2 L/min, was entered into the DBD reactor for all experimental runs. The main detected species in Received: Revised: Accepted: Published: 6197
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3. RESULTS AND DISCUSSION 3.1. Influence of Voltage and Frequency on Specific Input Energy. Peak voltage (Vp) and frequency applied on DBD reactor have a prominent influence on electric discharges. Specific input energy, which is the discharge power of the DBD reactor divided by the gas flow rate, is varied by changing Vp and frequency.8,9,17 Figure 3 gives a schematic diagram of DBD system. The system mainly consists of power supply (AC = 220 V), plasma generator,and DBD reactor.
this work were NO, NO2 and O2, and online analysis of the simulative gas was detected by flue gas analyzer (Kane, KM9106). The NO removal efficiency (η) was defined as follows: η=
outlet Cinlrt NO − C NO inlet C NO
Cinlet NO
× 100% (1)
Coutlet NO
where and are the NO concentration of the gas stream in the inlet and outlet of the reactor at steady state, respectively. A digital oscilloscope (Rigol, DS1052E, 50 MHz|GSa/s) was used for electrical measurements. The output voltage of plasma generator was measured by a high voltage probe (Pintek, HVP28HF, 1000:1). A low voltage probe (LV probe) was used to gather current waveforms from the voltage drop across a sampling resistor (R0 = 50 Ω) connected in series with the ground electrode of the DBD reactor. A noninductive capacitor (Cm = 0.47 μF) was inserted between the reactor and the ground electrode instead of the sampling resistor, when the discharge power (P) of the DBD reactor was accumulated. The V−Q lissajous method918 was used to determine the discharge power (P) dissipated in the DBD reactor. Discharge power was calculated using the relation: P(W ) = f × A × Cm
(2)
Figure 3. Comparison diagram of two kinds of DBD reactor (a: singledielectric DBD reactor, b: double-dielectric DBD reactor).
where f indicates the frequency of AC; A denotes the area of V−Q lissajous curve. Specific input energy (SIE) is used to compare NO removal efficiency at different discharge volumes, gas flow rate, and discharge powers. SIE is calculated using the relation: SIE(J/L) =
P(W) (1J/L = 2.78 × 10−4 Wh/L) Q (L/S)
The results presented in Figure 4 show that Vp of the plasma generator was maximized at 8.6 kHz for each input voltage (10,
(3)
where Q indicates the flow rate of gas entered into the DBD reactor. 2.2. DBD reactor. As shown in Figure 2, NTP was obtained in a coaxial cylinder-type DBD reactor. A glass tube with an
Figure 4. Influence of frequency on peak voltage.
20, 30 V; output voltage of voltage regulator set in the function generator). The possible reason is that the total impedance of the plasma generator reached the minimum when the operating frequency approached the resonance point (8.6 kHz), simultaneously as the output power of the plasma generator reached the maximum. Effects of frequency on SIE are shown in Figure 5. HV electrode with a diameter of 3 mm was used in this section. The NTP does not exist when Vp is less than the breakdown voltage (Vb) of gas. The electrons obtain enough energy to break the chemical bond of gas since Vp exceeds the Vb, meanwhile, discharge power of DBD reactors increases like na avalanche. At the frequencies of 8.5, 8.9, 9.5, and 10.0 kHz, the breakdown voltage demanded for electric discharges was about 4.5 kV. The DBD reactor would be broken down when the Vp is beyond 7 kV; therefore, the operating Vp should be settled between 4.5
Figure 2. Schematic configuration of DBD reactor.
inner diameter of 10 mm functioned as dielectric material. A stainless steel cylinder centered in the glass tube was used as a high-voltage electrode (HV electrode). The aluminum foil wrapped on the outer surface of the glass tube acts as a ground electrode. The area between the HV electrode and the ground electrode is the discharge area. The length of discharge area was 100 mm in this experiment. The HV electrodes had diameters of 0.5, 1.5, 3, 4, and 6 mm; correspondingly, the discharge gap lengths (Lg) of the single-dielectric DBD reactors were fixed at 4.7 mm, 4.2, 3.5, 3.0, and 2.0 mm. The form of doubledielectric DBD reactor is the following: the HV electrode was wrapped by a glass tube with the thickness of 1 mm, the Lg of this form was 2.5, 2, and 1 mm. 6198
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field changing alternatively. Therefore, under the given conditions of another reactor’s geometry, there is an optimal value for the discharge gap length (critical gap), which could regenerate the highest electron energy and obtain the best NO removal efficiency.19 It is generally known that the electrical characteristics of the discharge will change with the gap length.20 With increasing the discharge gap, the residence time of the gas in the plasma region increases, which was found to be beneficial for removing pollutants.21 Nevertheless, Lg is not “the longer the better”, because with the increase of Lg, the density of the micro-discharge channel and the average electron energy will decrease; as a result, the inhomogeneity and instability of electric discharge is increased. At constant applied voltage, the total charge transferred by the micro-discharges is higher for short gap lengths.20 In addition, when the discharge gap increases over the critical gap, the discharge will move away from the glow discharge due to the inhomogeneity and instability of electric discharge. In this section, when the Lg is over 3.5 mm, the electric discharge type was changed from glow discharge to corona discharge, which was accompanied by a “hiss” noise and sometimes with a faint glow. It will reduce the discharge power of the DBD the reactor, because the corona discharge is a partial discharge phenomena. The results presented in Figure 6 also show that single-dielectric DBD reactors obtain higher SIE than double-dielectric DBD reactors, when the diameter of the HV electrode is the same. Figure 7 shows the typical V−Q curves for single- and double-dielectric DBD reactors. It is obvious that the quantity of electric charge produced in a single-dielectric DBD reactor is larger than that produced by a double-dielectric DBD reactor. The areas of V−Q curves are augmented with the peak voltage increasing from 4.5 kV to 5.5 kV respectively for both singleand double-dielectric DBD reactors. The discharge power of single-dielectric reactor with Lg of 3.5 mm is 0.27 and 0.50 W for 4.5 and 5.5 kV, respectively. The discharge power of a double-dielectric reactor with Lg of 2.5 mm is 0.18 and 0.38 W for 4.5 and 5.5 kV, respectively. Figure 8 gives a direct comparison of the current waveform of two kinds of (the single- and double-dielectric) DBD reactors with different Lg at peak voltage of 5.5 kV. The current waveform consists of several current pulsed with amplitudes of tens to hundreds mA. The amplitudes of current pulses in Figure 8a and c are larger than those of the current pulses in b and d of Figure 8, respectively. It indicates that the electric discharge of the single-dielectric DBD reactor is more intense and greater than that of the double-dielectric DBD reactor.
Figure 5. Influence of frequency on the SIE.
kV and 7 kV. For a given voltage, the SIE was augmented with the increasing of frequency from 5.0 kHz to 8.9 kHz, but it would be reduced with the further frequency increasing. This phenomenon may be interpreted as follows: total impedance of DBD reactor reaches the minimum when the operating frequency approaches the resonance point,10 simultaneously, the maximum of discharge power is obtained. 3.2. Influence of geometry of DBD reactors on specific input energy. In order to optimize the geometry of the DBD reactor, the effects of discharge gap length (Lg) on NO removal and SIE were investigated. The frequency of this section was set at 8.6−8.9 kHz to make the discharge power maximum. Figure 6 presents the SIE as a function of the peak voltage for different Lg. It is obvious that the SIEs of DBD reactors augment with increasing applied voltage from 2.5 to 6.5 kV for both singleand double-dielectric DBD reactors. The rate of the SIE increase is higher when the peak voltage is greater than the breakdown voltage (Vb = 4.5 kV). When the peak voltage is over 4.5 kV, a single-dielectric DBD reactor with an Lg of 3.5 mm attains higher SIE than other reactors. The high-energy electrons play an important role on the formation of NTP, which also directly indicates the intensity of the electric discharge occurring in the DBD reactor and the activation degree of the gas. For a given applied voltage and frequency, the acceleration distance of electrons will be longer for the DBD reactor with larger Lg. However, electrons would not be infinitely accelerated, because of the direction of the electric
Figure 6. Influence of length of discharge gap on the SIE (a: single-dielectric DBD reactor, b: double-dielectric DBD reactor). 6199
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Figure 7. V−Q curves of DBD reactors at different peak voltage (a: single-dielectric DBD reactor with Lg of 3.5 mm, b: double-dielectric DBD reactor with Lg of 2.5 mm).
Figure 8. Characteristics of the current waveform of DBD reactors at peak voltage of 5.5 kV (single-dielectric DBD reactor a: Lg = 3.5 mm, c: Lg = 2.0 mm; double-dielectric DBD reactor b: Lg = 2.5 mm, d: Lg = 1.0 mm.).
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Figure 9. Influence of length of discharge gap on NO removal (a: single-dielectric DBD reactor, b: double-dielectric DBD reactor).
single-dielectric DBD reactors with the Lg of 3.5 mm at voltage of 6.5 kV, which followed the same trend with SIE (Figure 6a), and 29 ppm NO2 were produced simultaneously. For doubledielectric DBD reactors, the optimum NO removal efficiency is 84% at 6.5 kV. The production of high-energy electron, N, O, and O3 responsible for removal and oxidation of NO is proportional to the input energy. The radicals and oxygen atoms were produced largely when the applied voltage was greater than the Vb (4.5 kV); meanwhile, the reaction in the DBD reactor would be more intense. As eq 6 shows, NTP could generate not only energetic electrons but also chemically active radicals such as O by the ionization of oxygen. In addition, ppm-level ozone is formed in the NTP process (see eq 7); therefore, eq 8 demonstrates that the conversion of NO to NO2 by NTP is technologically feasible.17,22 Ozone is a strong oxidant, and iodine could be generated when the O3 was reacted with KI solution. The standard solution of Na2S2O3 (0.01 mol/L) was used to quantify the I2 with starch as indicator. The O3 concentration was calculated according to the eqs 4 and 5, and Figure 10 gives the O3 concentration as the function of peak voltage.
Each current pulse means a microdischarge channel in DBD reactor. It is obvious that the discharge occuring in a doubledielectric DBD reactor is more stable and homogeneous than that occuring in the single-dielectric DBD reactor. The amplitudes of current pulses in Figure 8a are larger than those of the current pulse in Figure 8c, which indicates the discharge power in a single-dielectric DBD reactor with the Lg of 3.5 mm is larger. The similar trend is found for doubledielectric DBD reactors. The effects of Lg on NO removal are presented in Figure 9, and the concentration of NO2 formation as the function of Vp is shown in Figure 10. The NO removal reached 88% over the
O3 + 2KI + H 2O → O2 + I 2 + 2KOH
(4)
I2 + 2Na 2S2 O3 → 2NaI + Na 2S4 O6
(5)
The dissociation energy of O2 is smaller than that of N2. Less useful species are formed by rotational and vibrational excitation reactions which are favorable for NO oxidation,
Figure 10. O3 concentration as function of peak voltage.
Figure 11. Influence of the length of the discharge gap on NO2 concentration (a: single-dielectric DBD reactor, b: double-dielectric DBD reactor). 6201
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when the voltage is below Vb (Vb = 4.5 kV). Therefore, eqs 6, 7, and 8 mainly occurred17 when the voltage was below 4.5 kV. e + O2 → e + O(3P) + O(3P)
(6)
O(3P) + O2 → O3
(7)
O3 + NO → NO2 + O2
(8)
e + N2 → e + N( 4S) + N( 4S)
(9)
N( 4S) + NO → N2 + O
(10)
NO2 + N → N2O + O
(11)
As shown in Figure 11, the concentration of NO2 formation is augmented with increasing Vp; however, it is reduced with the further increasing Vp. The energy required of O2 ionization is lower than that of N2 ionization. Therefore, eqs 8, 9 and 1017 mainly occur when the voltage is below 4.5 kV. Electrons will obtain higher energy to form active species by dissociation and ionization reactions, when the voltage applied on the DBD reactor is greater than Vb (Vb = 4.5 kV). NO could be reduced to N2 by atomic nitrogen species produced by electron impact of N2. As shown in eq 9, N(4S) is formed by dissociation and ionization reactions which is beneficial for NO reduction. It is well-known that N2O will be the major product by DBD plasma oxidation with the existence of oxygen. As presented in eq 11, N2O is mainly generated from the reaction of NO2 and N radical.23 The outlet concentration of N2O was detected by mass spectrometer (MAX300-LG). The results are presented in Figure 12. As we can see from Figure 12, the N2O
Figure 13. NO removal efficiency as a function of SIE.
indicates energy consumption for removal of NO per mol. As an important parameter, the SEC was calculated according to the following formula: SEC(KJ/mol) =
SIE/1000 × 22.4 outlet − C NO
inlet C NO
(12)
where NOinlet and NOoutlet indicate the concentrations of NO without and with DBD (ppm) respectively; SIE denotes the corresponding specific input energy (J/L). The SEC of different reactors as a function of the peak voltage is shown in Figure 14. When the diameter of the HV
Figure 14. Specific energy cost of different DBD reactors.
Figure 12. N2O concentration as a function of peak voltage.
electrode is the same, it is found that double-dielectric DBD reactors gain higher energy utilization efficiency than singledielectric DBD reactors. As shown in Figure 8, current waves occurring in double-dielectric DBD reactors were more stable and homogeneous than those occurring in single-dielectric DBD reactors. The homogeneous current waves indicate that discharge occurring in the DBD reactor is more stable and uniform, which may be favorable for energy utilization. The results presented in Figure 14 show that the SEC is augmented with increasing applied voltage from 4.5 to 6.5 kV for each reactor, which is similar to the trend of NO removal and SIE. As showed in Figure 6, the discharge power of the DBD reactor is augmented with applied voltage increasing. More discharge power is turned to heat when the DBD reactor is working at high voltage, which is detrimental for energy utilization. That the higher voltage is better for NO removal but
concentration increased with increasing the peak voltage. In addition, it also increased with the decrease of the discharge gap. As we mentioned above, under the given peak voltage, the discharge intensity increased with the discharge gap decreasing. The results presented in Figure 13 indicate that the NO removal is proportional to SIE for certain DBD reactors. It was found that the effects of Lg and Vp on SIE (Figure 6) and NO removal efficiency (Figure 9) followed the similar trend. The SIE indicates the discharge power per volume dispersed in gas. The reaction in NTP would be more intense with increasing the discharge power, which is favorable for NO removal. 3.3. Influence of Geometry of DBD Reactors on Specific Energy Cost. Specific energy cost (SEC) is used to compare the energy cost of different DBD reactors. SEC 6202
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detrimental for energy utilization efficiency is the same trend found for the double-dielectric DBD reactor.
4. CONCLUSIONS By comparing the NO removal performance in different DBD reactors, the effect of peak voltage (Vp), frequency of alternating current (f), and discharge gap length (Lg) on NO removal efficiency have been demonstrated clearly. In particular, the energy utilization efficiency of different DBD reactors was compared. The experiments provide the following conclusion. (1) For the DBD system, there is a resonance point of frequency at which the total impedance of DBD system is smallest. Simultaneously, the output power of the plasma generator and the discharge power of the DBD reactor reach the maximum at the same voltage. (2) Under the given conditions of the other reactor’s geometry, there is an optimal value of the discharge gap length (critical gap), which could regenerate the highest electron energy and obtain the best NO removal efficiency. When the discharge gap increases above the critical gap, the discharge will move away from glow discharge and may change to corona discharge for coaxial DBD reactors due to the inhomogeneity and instability of the electric discharge. (3) For a certain voltage, the electric discharge occurring in the double-dielectric DBD reactor is more stable and uniform which is favorable for energy utilization; a discharge occurring in the single-dielectric DBD reactor is more intense which is beneficial for NO removal efficiency. Therefore, the NO removal and energy utilization efficiency of DBD reactor could be improved by optimizing the discharge uniformity and intensity synergistically.
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AUTHOR INFORMATION
Corresponding Author
*Tel./fax:+86 010 62332747. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (20907018, 21177051).
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REFERENCES
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