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Ind. Eng. Chem. Res. 2008, 47, 2122-2130
REVIEWS Review of Packed-Bed Plasma Reactor for Ozone Generation and Air Pollution Control Hsin Liang Chen,† How Ming Lee,‡ Shiaw Huei Chen,‡ and Moo Been Chang*,†
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Graduate Institute of EnVironmental Engineering, National Central UniVersity, Chung-Li, Taoyuan County 32001, Taiwan, Republic of China, and Physics DiVision, EnVironmental and Energy Technology Center, Institute of Nuclear Energy Research, Longtan, Taoyuan County 32546, Taiwan, Republic of China
Packed-bed plasma reactors, constructed by packing noncatalytic dielectric pellets inside nonthermal plasma reactors, have been demonstrated to effectively alleviate the major bottleneck encountered by nonthermal plasma, i.e., the energy efficiency needs to be further improved. As far as the environmental issues are concerned, packed-bed plasma reactors are mainly applied to ozone generation and gaseous pollutant removal. According to the available experimental data, for a given specific energy density, the energy efficiency for ozone generation and gaseous pollutant abatement obtained with packed-bed reactors, if compared to that of nonpacked reactors, could be 1.1-4.3 and 1.1-12 times higher, depending on the type of pollutant, the reactor geometry, and the packing pellets used. Nevertheless, it is worth noticing that the packing pellets suitable for ozone generation and pollutant removal are quite different. The influences of material, dielectric constant, size, and shape of the packing pellets on the performance for ozone generation and gaseous pollutant removal are comprehensively reviewed in this paper and guidelines for pellet selection are provided as well. For the single-stage plasma catalysis system, in which catalyst pellets are directly packed inside the plasma reactor, the physical parameters of catalyst would also have significant influence on the plasma characteristics and the performance. Therefore, the content of this review paper could provide useful information for singlestage plasma catalysis system from the viewpoint of plasma characteristics. 1. Introduction Nonthermal plasma (NTP) technologies were first developed and applied to ozone generation1 and later have been successfully applied to remove varieties of gaseous pollutants, such as NOx,2 SOx,3 mercury,4 volatile organic carbons (VOCs),5,6 dioxins,7,8 and perfluoro compounds (PFCs).9-11 In addition, nonthermal plasma has the potential to simultaneously remove multiple pollutants from gas streams at low temperature,12,13 which is quite different from conventional air pollution control technologies, such as combustion and catalytic oxidation in which high-temperature operation is required. In NTP, the electrons, due to the tiny mass, acquire much higher energy than other charged species. The mean electron temperature in an NTP system, typically ranging from 10 000 to 100 000 K (corresponding to 1-10 eV), is 2-3 orders higher than the gas temperature, which is generally close to room temperature.14 Energetic electrons play an important role in the initiation of plasma chemistry reactions. Compared with combustion and catalytic oxidation, NTP has one advantage, i.e., fast ignition/ response, because of no requirement of high temperature. However, reduction of power consumption is still needed for industrial applications. Packed-bed reactors have a great potential to resolve the bottleneck. Packed-bed reactors have been * To whom correspondence should be addressed. Tel.: +886-34227151 ext. 34663. Fax: +886-34221602. E-mail: mbchang@ncuen. ncu.edu.tw. † National Central University. ‡ Institute of Nuclear Energy Research.
proved to achieve higher energy efficiency than conventional NTP reactors either for ozone generation15-17 or for pollutant removal.18-20 A packed-bed reactor consists of dielectric pellets within two electrodes, whereas the pellets could be either noncatalytic or catalytic. The latter, also termed as a plasma catalysis system, is not in the scope of the present work. Ozone generation and gaseous pollutant removals are two major applications of packed-bed plasma reactors regarding environmental issues. Relevant studies indicate that the role of packing pellets behaves quite differently between these two applications. Additionally, the physical properties of packing pellets would significantly influence the plasma characteristics as well as the performance. To date, a systematic review on this topic is not available. This paper aims to examine important parameters of noncatalytic packing pellets affecting the performance of a packed-bed reactor and to provide guidelines on pellet selection for different applications. 2. Characteristics of Packed-Bed Reactors Packed-bed reactors were initially developed to remove particulate matters.21,22 The so-called packed-bed reactor is constructed by placing dielectric pellets within the discharge region inside NTP reactor. Typically, the dielectric materials used include glass, quartz, aluminum oxides, ceramic, ferroelectrics, etc. One of the major differences among these dielectric materials is their dielectric constant. In general, the dielectric constants for glass, quartz, aluminum oxides, ceramic, and ferroelectrics range from 4-6, 4-7, ∼10, tens-10ks, and
10.1021/ie071411s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008
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hundreds-10ks, respectively. Owing to the high dielectric constant of ferroelectrics, some researchers specifically named “ferroelectric plasma reactor” for a packed-bed reactor filled with perovskite oxide. Barium titanate (BaTiO3) is the most commonly adopted perovskite oxide. The geometry of a packed-bed reactor, as shown in Figure 1, could be either parallel-plate or cylindrical (or so-called “coaxial”). Packed-bed reactors can also be constructed without a dielectric layer between packing pellets and electrode. High dielectric constant pellets bring about one important function, i.e., reducing the breakdown voltage,23,24 which is favorable for industrial application because operating at lower voltage is safer. In addition, a lower breakdown voltage generally leads to, at the same applied voltage, a higher discharge power. This fact is experimentally observed in numerous references,25-27 except for the study by Chang and Lin.18 The dielectric material used in refs 25-27 is BaTiO3 with dielectric constant ) 3 000-10 000, while glass beads ( ) 4-6) are used by ref 18. It seems that the discharge powers of packed-bed reactors strongly depend on the pellet material used. The major characteristic of packed-bed reactors is the presence of contact points between pellets and pellets/electrodes. Because of the short distance near these contact points, the electric field strength is significantly higher than the mean value in the reactor. The fact that packed-bed reactors could achieve higher electric fields has been confirmed in various simulation works.9,28-32 Among those studies, the dielectric constant tested ranges from 4 to 10 000, and the shapes investigated include sphere and cylinder. Takaki et al.33 developed a one-dimensional parallel-plate model to evaluate the important plasma parameters for the reactor packed with spherical pellets in N2 plasma. The time and cross-sectional averaged electric field (Ex) inside an approximately spherical void and corresponding electron density (ne) can be expressed by eqs 1 and 2,
Ex ≈
V 3p d 2p + g
(1)
P ≈ ne ≈ VRAe(µ0Eω0 )E1-ω x P V 3p VRAe(µ0Eω0 ) d 2p + g
(
)
1-ω
(2)
where V is the applied voltage, d is the separation distance between the electrodes, g and p are the dielectric constants (or relative permittivities) of background gas and packing pellets, respectively, R and A are the void fraction and cross-sectional area of a packed-bed reactor, respectively, e is the electric charge of electrons ()1.6 × 10-19 C), µ0 is the electron mobility at reference electric field E0, and ω is an empirical coefficient. According to eq 1, Ex is a function of g and p. For a given applied voltage and a fixed discharge gap, the electric field has a minimum value (Ex ≈ V/d) when p ≈ g ≈ 1. Then, the electric field increases with increasing dielectric constant and reaches a maximum when p . g. In other words, the augmentation factor of electric field is between 1.0 and 1.5, determined by p, as shown in Figure 2. It needs to be emphasized that the results obtained with eqs 1 and 2 represent the cross-sectional averaged values including the space occupied by pellets. On the other hand, eq 1 is derived for a spherical
Figure 1. Illustration of packed-bed reactors: (a) parallel-plate and (b) cylindrical types. In addition to the packing pellets, there might be one dielectric layer inserted between the electrodes, but two or none in some cases. The materials of dielectric layer and packing pellets could be different. In a cylindrical reactor, the dielectric layer could be adhered to either outer or inner electrode and the inner electrode could be in the shape of a wire, rod, screw, or tube.
Figure 2. Enhancement of electric field as a function of dielectric constant of pellets. The subscript i in Ex,i stands for the dielectric constant of packing pellets (p). Ex,i and Ex,1 stand for the electric field Ex with p ) i and p ) 1, respectively. The data are calculated based on eq 1.
void inside a uniform dielectric material. However, the void between spherical packing pellets is usually not spherical and the electric field inside is more complicated. In fact, the maximum electric field near the contact point between pellets can be 10-104 times higher than that in a spherical void, depending on the contact angle, curvature, and dielectric constant of the packing pellets.28,32,34,35 The enhancement of electric field in packed-bed reactors would result in higher electron energy. However, according to eq 2, ne is inversely proportional to Ex; therefore, for a given discharge power, increasing Ex results in a lower ne. Chang et al.9 indicate that, under the same discharge power, packed-bed reactors possess lower electron density compared to that of a nonpacked one due to higher electric fields. As a result, packedbed reactors can be categorized as high electron energy but low plasma density devices. In spite of the lower electron density, a packed-bed reactor still achieves a better energy efficiency either for ozone generation or pollutant removal, which has been confirmed in numerous studies.15-20 Among these studies, at a given specific energy density (SED), the ozone concentration and pollutant removal efficiency are 1.1-4.3 and 1.1-12 times higher when the pellets are packed inside nonthermal plasma reactors, depending on the type of pollutant, reactor geometry, and packing pellets. Here, the SED represents the ratio of discharge power to gas flow rate. As can be expected, electrons
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Table 1. Literatures Regarding Packed-Bed Reactors for Ozone Generationa packing material silica
background gas
glass
O2 O2/N2 mixture O2 O2 dry air
quartz Al2O3 BaTiO3
O2 O2 O2 dry and humid air
a
dielectric constant
pellets size (mm)
discharge gap (mm)
discharge length (mm)
7 4.5 1.5 1.5 4 7 10 12 1.5, 3.3 12 3-5
100 -
15 16 41 42 24
33, 150, 660, 1,500, 10 000
0.5-1.25, 1.25-3.2 0.5-1.25 0.16-0.315, 0.315-0.5, 0.5-0.8 0.5-0.8 3 5 2, 5 2, 3, 5 0.16-0.315, 0.315-0.5, 0.5-0.8 2, 5, 10 1-3
150 150 3-5
17 41 17 43
-
ref
Note: - represents that the data is not provided.
with higher electron energy tend to form active species by dissociation and ionization reactions, rather than to form less useful species by rotational and vibrational excitation reactions. That is, energy is favorable to be consumed by the electronimpact reactions that are mainly responsible for plasma chemistry as the electric field is increased. Hence, packed-bed reactors could serve as an alterative approach to improve the energy efficiency of nonthermal plasmas. In addition, for difficultly treated pollutant, which has insignificant activity with radicals, electron-impact reactions play an important role for its decomposition. Because of the characteristic of higher electron energy, packed-bed reactors could be a more suitable technique. 3. Applications of Packed-Bed Plasma Reactors on Environmental Issues Packed-bed reactors can be applied to ozone generation or gaseous pollutant removals. Relevant studies are summarized in Tables 1 and 2. From these tables, one can easily realize what dielectrics are mostly used. For example, glass and silica are commonly used as packing materials for ozone generation, while BaTiO3 is often applied to pollutant abatements. Namely, the dielectric constant of packing pellets for ozone generation and gaseous pollutant removal are usually 2 000, respectively. It is the most significant difference between the packed-bed reactors used to produce ozone and abate pollutants. Packed-bed reactors can be operated with alternating current (ac) or pulsed power, but the former is more commonly adopted. Only refs 26 and 43 and part of the experiments in refs 25 and 27 were conducted with pulsed power. It is believed that, for nonpacked plasma reactors, adopting pulsed power leads to two advantages over ac power, i.e., preventing the transition from streamer to spark discharge36-38 and achieving better energy efficiency.39 As for packed-bed reactors, comparison between ac and pulsed powers for diesel NOx removal had been experimentally tested by Yamamoto et al.26 BaTiO3 pellets with dielectric constant of 10 000 and diameter 1.7-2.0 mm were packed and tested. Better removal efficiency and energy efficiency were achieved with ac power than with pulsed power for the case of initial NO concentration of 100 ppm. Therefore, packed-bed reactors with ac power can achieve the same merits compared to that with pulsed power. Additionally, an ac power generator is relatively simple and cheap compared with the pulsed one.40 Hence, ac power might be a good and practical choice for packed-bed reactors. On the other hand, since SED90 would be significantly influenced by the initial concentration of pollutant, the SED90 listed in Table 2 is not used to compare the performance of plasma reactors in this paper. Unless the initial pollutant
concentration and gas compositions are similar, SED90 would not be a good indicator to evaluate the performance. The purpose for the SED90 listed in Table 2 is used to estimate the significance of power loss caused by pressure drop (see Section 5.4). 4. Important Parameters of Packing Pellets The important physical parameters of the packing pellets affecting the discharge performance include material, dielectric constant, size, and shape. The material of the packing pellets mainly influences the reactions between gas and solid phases, while other parameters influence the discharge characteristics. In this section, if not particularly mentioned, the shape of the packing pellet is spherical. The effects caused by each parameter are described in the following. 4.1. Material. Schmidt-Szalowski41 compared the energy efficiency for ozone generation in reactors packed with silica or quartz grains of the same size. The performance obtained with silica grains is better than that with quartz grains. Excluding the influence of dielectric constant, the most possible reason leading to the result is the different behavior of gaseous/solid reactions, i.e., the adsorption of O2 on silica grains. The mechanism of ozone generation has been well-studied by Eliasson et al.59 Important reactions are listed as the following:
e + O2 f e + O(3P) + O(3P)
(R1)
e + O2 f e + O (3P) + O(1D)
(R2)
O + O 2 + M f O 3* + M f O 3 + M
(R3)
O3* represents an excited species of ozone molecule, which is the initial reaction product. Schmidt-Szalowski41 proposed the following mechanism to support the better performance of silica grains,
O + O2(ad) f O3(ad) f O3
(R4)
where the surface of silica grains plays the role as third body to quench O3*. Nevertheless, some materials, like γ-Al2O3, have been demonstrated to effectively decompose O3.60 Although such material is not suitable for ozone generation, it might enhance the performance for pollutant removal because oxygen atom (O) is generally more active than O3 toward reactions with gaseous pollutants. Therefore, one should be conscientious in selecting appropriate material(s) as packing pellets based on the application.
Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2125 Table 2. Literatures Regarding Packed-Bed Reactors for Pollutant Abatementa
pollutant type volatile organic compounds (VOCs)
pollutant toluene
benzene
NOx
packing material
ozone-depletion substance
others
SED90b (J/L)
pellet sizec (mm)
dielectric constant
discharge gap (mm)
discharge length (mm)
ref
PbZrO3-PbTiO3
450
∼600
3 000
1-1.6
8
430
25, 27
BaTiO3 glass
50-1000 450 300 500 200 200
∼1 000 640 820 1 260
5 000 5 5 000 10 000 4 000
1, 3, 5 5 5 1 2 2
1 830* 1 440*
2
200
870 1 600, 2 600, 4 000, 10 000, 15 000 15 000
25 430 416 127 20 42 135 50
44 25, 27 45 46 20 47
200
25 8 8.4 15.4 20 10 6.4 10
20 100 330 1 100, 2 600 3 000
BaTiO3
48
Mg2TiO4 CaTiO3 SrTiO3
200 200 200
BaTiO3
200
1 440* 2 310* 3 000 1 440 -
PbZrO3-PbTiO3
160/75
-
2 000
5
14
320
27
Al2O3 BaTiO3
140 500, 1 000
108 -
5 000
2.3-3.2 3
4 25
30 25
19 44
BaTiO3 BaTiO3 BaTiO3
185 1 000 500
∼450 1 700* -
∼3 000 5 000 -
1.0-1.6 1 3.5
4.9 15.4 25
110 127 25
25 46 49
glass
300/0 70/5 80-500/∼40 125/40 10 000
72 -
10 000 660, 10 000
5 6 1.7-2.0 4 -
8.4 9.2 9.3 9.2 20
416 270 260 270 20
45 50 26 50 51
NF3 C2F6
294-5 200 3 000
-
127 15
9 11
-
20 10 9.9
300 50 290
52 53
glass
1 500, 3 000 960 50-500
1, 3 3.3 [a] [b] [b] 1 3
15.4 20
CH4 CCl4
10 000 660, 5 000, 10 000 10 000 10 000 10 000 15 000 -
CFC-12
ZrO2 BaTiO3
210-506 500 500 500 1 000, 10 000 1 000, 2 000 250-4000 1 000 1 000
785 1 300 893 144 -
methyl t-butyl ether (MTBE) acetone/toluene mixture HCHO methylene chloride (CH2Cl2) phenol trichloroethene (TCE) dichloromethane (DCM) NO/NO2 mixture
BaTiO3 greenhouse gases (GHGs)
pollutant conc. (ppm)
CO2
BaTiO3
1 3 2 2 2
10
50
10 10 10
50 50 50
1.0-1.6
4.9
110
25, 27
54
CFC-13 CFC-22 CFC-113 CNCl
BaTiO3 BaTiO3 BaTiO3 Al2O3 BaTiO3 BaTiO3
CCl2CHCl CH3CN
15 000 15 000 15 000 10 000 5 000 5 000
3.5 2 2 2 1 3-6 3-4 3-4
25 10 10 10 15.4 12.5
25 50 50 50 127 10-20
54 55 55 55 56 58
a - represents that the data is not provided. b SED stands for the specific energy density (ratio of discharge power to gas flow rate) required to obtain 90 90% removal efficiency. The value of SED90 with an asterisk as superscript represents it is calculated based on plug-in power (or primary power) instead of discharge power. In general, the ratio of discharge power to plug-in power ranges from 0.1-0.7. 1 J/L ) 0.278 Wh/m3. If not specified, the shape of packing pellets is spherical. c [a] stands for pellets that are cylindrical in shape, with the diameter and length of 3.2 and 4 mm, respectively. [b] means the pellets are hollow cylinders, whose inner diameter, outer diameter, and length are 3.2, 1.4, and 4 mm, respectively.
4.2. Dielectric Constant. According to the requirement of constant current flux at the boundary, the electric field in the discharge gap (Eg) can be expressed as
the packing pellets. Moreover, the electric field and mean electron energy are higher as the applied voltage or dielectric constant is raised.
p E g ) Ep g
Ferroelectric pellets with dielectric constant ranging from 20 to 15 000 have been employed to investigate the influence of dielectric constant for benzene removal.48 As shown in Figure 3a, at a fixed SED, benzene removal efficiency initially increases with increasing dielectric constant until dielectric constant is >1 100. Takaki et al.11 utilized the reactors packed with BaTiO3 pellets (dielectric constant ) 660, 5 000, and 10 000) to treat a gas stream containing C2F6 (Figure 3b). It seems that the amount of C2F6 removed is not a function of dielectric constant under the same SED.
(3)
where Ep is the electric field in the packing pellet and p and g are the dielectric constants of packing pellet and discharge gas, respectively. Apparently, dielectric constant affects the electric field in the void between packing pellets. As a result, the mean electron energy is also a function of dielectric constant. Chang et al.9 indicate that the electron density increases with discharge current and voltage but decreases with dielectric constant of
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Figure 3. Influence of dielectric constant of ferroelectric pellets on (a) benzene48 and (b) C2F611 removals. The figures are replotted based on the experimental results of refs 48 and 11. More details about the experimental parameters should be referred to the papers.
The above-mentioned experimental results suggest that increasing the dielectric constant could enhance the electric field but it would approach a certain value, just like the result for a spherical void (see Figure 2). Hence, for a given packed-bed reactor, raising dielectric constant of packing pellets could not unlimitedly improve the energy efficiency for pollutant removal. However, using higher dielectric constant pellets is still favorable from the engineering viewpoints. In most cases, the discharge power is limited by the occurrence of arcing discharge. The voltage to initiate arc is independent of dielectric constant;11 nevertheless, the discharge power could be enhanced by increasing dielectric constant at the same applied voltage.27,33 This means that utilizing higher dielectric constant pellets is a good approach to ensure appropriate discharge power. It is quite different between the influence of dielectric constant on ozone generation and pollutant control. Moon and Geum43 and Ogata et al.48 both indicate that there is an optimum dielectric constant for ozone generation. When the dielectric constant of BaTiO3 pellets is >660, ozone almost vanishes as observed in both studies. The local heating in the vicinity of pellet contact points proposed by Yamamoto et al.61 is adopted to interpret the results of Ogata et al.48 Nevertheless, it needs to be pointed out that the reactor used by Moon and Geum43 is different from the commonly used packed-bed reactors, i.e., there is a gap between one of the electrodes and the packing pellets. They claimed they did not observe corona discharge in the case with dielectric constant of 1 500 or 10 000, which implies the lower ozone generation might be stemmed from the occurrence of spark discharge. The fact that dielectric pellets with too high dielectric constant would deteriorate the ozonegeneration performance has been confirmed in various studies. For example, Holzer et al.27 compared the ozone concentration in DBD (i.e., nonpacked) reactor and packed-bed reactor with BaTiO3 pellets (dielectric constant ) 3 000). The DBD reactor achieved higher ozone concentration than the packed-bed reactor at the same specific energy density. A similar trend is also observed by Futamura et al.,46 in which BaTiO3 pellets with dielectric constant of 5 000 were tested. The tendency of ozone decomposition on the surface of BaTiO3 has been experimentally excluded by Holzer et al.27 They ascribed the lower ozone concentration in the packed-bed reactor to the occurrence of hot spots. To date, there is no direct evidence to verify the contribution to ozone decomposition caused by local heating or a hot spot. Measuring the variation in the amount of charge transferred by individual microdischarge and the frequency of pulse current with dielectric constant could provide useful information, because the local heating or hot spot is closely related to these two parameters. However, such data are still unavailable so far.
In addition to local heating, the high electric field resulted from the packing pellets with high dielectric constant might not be suitable for ozone generation. This phenomenon can be elucidated with G-values of O2 and O3 dissociations in O2/O3 mixture plasma (Figure 4). The G-values were calculated by ELENDIF code62 with the input of electron-impact cross sections adopted from refs 63 and 64. Here, G-value represents the number of specific electron-impact reactions per 100 eV energy input. The electrons not only dissociate oxygen molecules to form oxygen atoms and then synthesize ozone but also decompose parts of ozone formed simultaneously. Apparently in Figure 4, as the reduced field is higher than 50 Td, the dissociation of O3 can be ignored for a wide range of ozone concentration from 0.1% to 10%. Moreover, there is an optimum reduced field in terms of oxygen dissociation, i.e., ozone yield could be reduced at high reduced field. Hence, when it comes to ozone generation, the electric field must be controlled in a moderate range. The dielectric constant should neither be too low to weaken the electric field nor too high to deteriorate ozone generation. One should carefully choose a dielectric with proper dielectric constant for ozone generation. Overall speaking, the appropriate dielectric constant depends on the application. As for pollutant abatement, raising dielectric constant could always result in positive effects; however, there exists an optimum value for ozone generation. According to the available experimental results, authors would suggest the dielectric constants should be >1 000 for decontamination and 600 J/L (167 Wh/m3), the benzene removal efficiencies for 1 and 2 mm pellets were quite close and better than that for 3-mm pellet. As regard to ozone generation, Schmidt-Szalowski41 tested three different sizes of quartz beads and silica pellets, i.e., 0.160.315, 0.315-0.5, and 0.5-0.8 mm. In terms of the results with quartz beads, the greatest increase of ozone concentration and energy efficiency is observed in the reactor packed with the largest size range (0.5-0.8 mm). Nevertheless, compared with the case without packing, a reduction in ozone concentration and energy efficiency is noticed in the case with the smallest pellets tested. As with silica pellets, the medium size range shows the best performance instead. Chen et al.17 tested glass
ref 23 2.3 3.3 4 small 1 0.2 2.5
ref 11 4 3.2 4 large 3 1 3
3.2 1.4 4
beads (diameter ) 2, 3, and 5 mm) and Al2O3 pellets (diameter ) 2, 5, and 10 mm) for ozone generation. They found that, consistent with Schmidt-Szalowski’s result, the optimum pellet size is not the same for different materials. According to the available experimental results, it can be concluded that there exists an optimum pellet size for a specific packing material. But it is worth noticing that the discharge gap would affect the total contact points between pellets for a constant pellet size. It implies that the optimum pellet size might change in different reactors as well. Therefore, it would be helpful to develop a universal parameter for optimization of packed-bed reactors. The ratio of discharge gap to pellet size or void fraction might be a good choice, because both of these two parameters could reflect the number of contact points between packing pellets. Unfortunately, a systematic study is still not available. 4.4. Pellet Shape. Shaper edge leads to higher local electric field and thus highly energetic electrons. Most of the literatures regarding packed-bed reactors, as listed in Tables 1 and 2, adopted spherical pellets. Chang et al.23 tested BaTiO3 pellets in three different shapes, including sphere, cylinder and hollow cylinder. All the pellets tested are with dielectric constant of 10 000 and the dimensions are listed in Table 3. On the basis of the discharge powers measured at the same applied voltage, the electron density is in the following order: small hollow cylinder > large hollow cylinder > cylinder > sphere, which suggests packed-bed reactor with hollow cylinder inside could be operated at lower applied voltage. Takaki et al.11 investigated the discharge characteristics of BaTiO3 pellets in various shapes for C2F6 abatement. At the same applied voltage, the discharge current, was in the following sequence: hollow cylinder > cylinder > sphere; and the discharge power was in the order as cylinder > sphere > hollow cylinder. Moreover, the reactor packed with hollow cylinder showed the lowest breakdown voltage. As for the energy efficiencies for C2F6 removal, the results with hollow cylinder is 1.5 times higher than that with sphere, which are 3.7 and 2.5 g/kWh, respectively. In addition, their simulation showed that the electron density for hollow cylinder pellets was slightly higher than that for sphere pellets at the same discharge power. Among the different shaped pellets tested, the hollow cylindrical ones show the best performance. It can be concluded that increasing the sharp edge of pellet is beneficial for improving performance. Moreover, the pressure drop across pellets can be alleviated with hollow pellets,23 which is another benefit for industrial application. 5. Practical Considerations for Operation of Packed-Bed Reactors 5.1. Dielectric Layer. Similar to conventional nonthermal plasma reactors, packed-bed reactors can also be constructed with or without a dielectric layer between pellets and electrodes (see Figure 1). Ogata et al.47 adopted two reactors with BaTiO3 pellets inside to treat gas stream containing benzene, while one of them had a glass layer. Different behaviors were observed
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Figure 5. (a) Fluid power in a packed-bed reactor as a function of gas flow rate for pellet sizes D ) 1, 2, 3, and 5 mm and (b) the ratio of the fluid power to the discharge power. These data are valid for a wire-tube packed-bed plasma reactor, in which the inner diameter of reactor, diameter of inner electrode, and discharge length are 30, 2, and 400 mm, respectively. The background gas is dry air at temperature ) 300 K. A typical specific energy density of 100 J/L (27.8 Wh/m3) is assumed for calculating the discharge power.
between these two reactors even at the same discharge power and gas residence time. The reactor with glass layer achieved a higher removal efficiency and ozone yield and less byproduct like N2O and NOx; however, ∼30% of carbon removed deposited inside the reactor. On the contrary, ozone generation and carbon deposits were completely suppressed in the reactor without dielectric layer. It seems that the existence of dielectric layer plays some important role for ozone generation. The experimental results of Moon and Geum43 show consistency. They indicate that adding a mica layer between the BaTiO3 pellets and one of the electrodes could increase the ozone concentration. Besides, from the practical perspective, it would be a better choice to utilize packed-bed reactor employing an extra dielectric layer because the occurrence of arcing discharge can be avoided. As a result, the insertion of an extra dielectric layer is deemed necessary for ozone generation. Regarding the carbon deposits claimed by Ogata et al.,47 it can be overcome by using plasma catalysis with appropriate catalyst.67 5.2. Temperature Rising. Regarding the elevation of temperature, the heating of ferroelectric pellets like BaTiO3 resulted from dielectric loss would lead to a significant reduction in dielectric constant. Once it happens, the discharge power and corresponding pollutant removal efficiency decrease as well.23,52 The dielectric constant of BaTiO3 is affected by the operational conditions such as temperature, applied field strength, and applied frequency. In general, compared with the first factor, the influence of the other two is relatively insignificant under the common operating conditions of nonthermal plasmas. The dielectric constant of BaTiO3 almost stays the same as the temperature is below Curie temperature (TS). However, it would greatly increases as the temperature approaches TS and dramatically decreases once the temperature is above TS, leading to
the loss of dielectric property. Therefore, to ensure the appropriate working of packed-bed reactors, it is important to keep the temperature of packing material below its Curie temperature. Moreover, for industry application, in which high discharge power is necessary, the temperature would result in the formation of arcing discharge or even the melting of the reactor barrier. Therefore, an appropriate cooling system is required to ensure the proper operation, especially for ozone generation. 5.3. NOx Formation. The typical byproduct generated from packed-bed reactors is NOx with the concentration ranging from tens to hundreds ppm, depending on the oxygen content and the mean electron energy. It is noted that, in the field of environmental protection, NOx refers to nitric oxide (NO) and nitrogen dioxide (NO2) gases. Higher oxygen content and mean electron energy would result in higher NOx concentration. In air pollution control applications, the gas stream to be treated is usually the air. For the combustion off-gases, the oxygen content is typically 5-15%. Hence, the existence of oxygen in gas streams as well as the formation of NOx are inevitable. This drawback should be carefully considered in the commercial applications. To minimize NOx formation, a packed-bed reactor had better to be designed and/or operated at moderate electron energy. This idea provides a good direction, however, very limited research has been devoted to this topic. If NOx formation cannot be avoided, an after-treatment device would be needed. The speciation of NOx formed in plasmas is quite different compared with traditional combustion processes. For instance, NO2 dominates in plasma systems but more than 90% of NOx is NO for traditional combustion processes. It is also noted that NO2 is more water-soluble than NO. Wet-scrubbing is capable of NO2 removal without expensive chemicals. Accordingly, recent studies50,68,69 pay attention to the combination of plasma with wet scrubbing as a total solution for NOx treatment. 5.4. Pressure Drop. Pressure drop is another concern for packed-bed reactors. The pressure drop for a packed-bed reactor can be estimated through the following equation,70,71
150(1 - )µg + 1.75 dpUgFg D × ∆P ) 3 dp g c F U 21- g
(4)
g
where ∆P ) pressure drop (N/m2), ) void fraction, µg ) gas viscosity (kg/m/s), dp ) diameter of packing pellets (m), Ug ) gas superficial velocity (m/s), Fg ) gas density (kg/m3), D ) bed depth (m), and gc ) gravitational constant (1 kg‚m/s2). Once the pressure drop is known, the corresponding fluid power required can be calculated through the following equation,71
Wf ) Q∆P
(5)
where Wf ) fluid power (W), i.e., work input rate into the fluid, Q ) volumetric flow rate (m3/s), and ∆P ) pressure drop (N/ m2). The overall running cost should take into account the electrical power consumed by a plasma system and the fluid power. As shown in Table 2, one could see the pellet size, discharge gap and discharge length of packed-bed reactors are generally in the ranges of 1-5 mm, 4-15 mm and 40-400 mm, respectively. The fluid power in typical packed-bed plasma reactors are shown in Figure 5a. Here the inner diameter of reactor, diameter of inner electrode and discharge length are assumed to be 30, 2, and 400 mm, respectively. One can easily find that the fluid power increases exponentially with the gas flow rate, whereas the influence of pellet size is relatively smaller.
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As listed in Table 2, the specific energy density required to achieve 90% of removal efficiency is generally higher than 100 J/L (27.8 Wh/m3). In most cases, for a given plasma reactor and gas composition, the pollutant removal efficiency at the same specific energy density is independent of the gas flow rate.12,72,73 One can calculate the corresponding discharge power by simply multiplying the specific energy density by the gas flow rate. Accordingly, the ratio of the fluid power to the discharge power can be estimated, as shown in Figure 5b. With the gas flow rate of 10 L/min, the ratios for pellet sizes of 1, 2, 3, and 5 mm are 7.7%, 2.7%, 1.5%, and 0.8%, respectively. Gas flow rate of 50 L/min and D ) 1 mm or Q > 80 L/min and D ) 2 mm. Therefore, multiple reactors in parallel are required to treat high gas flow rate. 6. Conclusions In terms of ozone generation, the material and dielectric constant of packing pellets could impose significant effects. Inappropriate material and too high dielectric constant result in local heating and high electric field would accelerate ozone decomposition. As for pollutant abatement, it seems that the material has no significant influence. Moreover, increasing dielectric constant could guarantee better performance with appropriate discharge power. According to the available results, the suitable dielectric constants might be 1 000 for pollutant removal, respectively. The size of packing pellets is also important for both applications. The available data suggest that the optimum pellet size is determined by reactor geometry. Therefore, a universal parameter is needed. The ratio of discharge gap to pellet size or void fraction might be a good indicator because the number of contact points between packing pellets is highly related to these two parameters. Regarding the pellet shape, increasing the number of sharp edge is favorable for pollutant removal but its influence on ozone generation is not well understood. In short, packed-bed plasma reactor is an effective technique to enhance the energy efficiency for ozone generation and air pollution control. However, it needs to be emphasized that the principles for choosing appropriate packing pellets are quite different in these two applications. A comprehensive overview on the effects caused by several important physical properties of packing pellets is given, and suggestions for pellet selection are provided based on the available experimental data. More recently, a novel technology termed as plasma catalysis, in which catalyst could be placed inside plasma region (single-stage type) or at the downstream of plasma reactor (two-stage type), has been developed. For the single-stage plasma catalysis system, the physical parameters of catalyst such as dielectric constant, size, and shape also have a significant influence on the plasma characteristics as well as the performance. Therefore, the content of this review paper could provide useful information for the single-stage plasma catalysis system from the viewpoint of plasma characteristics. Acknowledgment The authors gratefully acknowledge the financial support provided by the Institute of Nuclear Energy Research (INER A1540R), Industrial Technology Research Institute (ITRI), and the National Science Council of Taiwan (NSC 95-EPA-Z-008002).
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ReceiVed for reView October 17, 2007 ReVised manuscript receiVed January 13, 2008 Accepted January 30, 2008 IE071411S
