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Effects of geometric parameters and electric indexes on performance of a vertical wet electrostatic precipitator
Journal of Electrostatics 72 (2014) 402e411
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Effects of geometric parameters and electric indexes on performance of a vertical wet electrostatic precipitator Masoud Molaei Najafabadi, Hassan Basirat Tabrizi*, Amin Aramesh, Mohammad Ali Ehteram Mechanical Engineering Department, Amirkabir University of Technology, No. 424, Hafez Ave, PO Box 15875-4413, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2013 Received in revised form 15 March 2014 Accepted 27 June 2014 Available online 18 July 2014
A single-stage, single-wire vertical wet electrostatic precipitator was designed and operated in airewater droplets flow to investigate its performance. The efficiency was compared with a glass micro fiber filter and proposed semi-empirical efficiency model, which was in good accuracy while considering the vapor content. Effects of geometric parameters on efficiency under different charge conditions were discussed. Due to evaporation mechanism, the corona current decreases for high flow rates at the same applied voltage. Findings indicated while developing flow is created inside the ESP, there exists an optimum wire-to-flow inlet spacing that provides maximum droplet collection efficiency. © 2014 Elsevier B.V. All rights reserved.
Keywords: Wet electrostatic precipitator Upward developing flow Water droplet Collection efficiency Geometric parameter Electric index
1. Introduction Electrostatic precipitators (ESPs) are one of the most commonly employed particulate control devices to collect fly ash emissions and particulate matters from exhaust gases originating from various production installations such as boilers, incinerators, coalfired power plants, mining installations, metallurgical and chemical industries and many other industrial processes. They can operate in a wide range of gas temperatures, pressures and flow rates as well as very fine suspended particles, achieving high particle collection efficiency compared with mechanical devices such as cyclones, scrubbers and bag filters [1]. Although the process of electrostatic precipitation involves several complicated and interrelated physical mechanisms between the fluid and particle flow such as turbulent and Brownian diffusion, particles collisions etc., it is definite the geometric parameters and electric field characteristics have significant effects on the efficiency of ESPs [2e4]. Several experimental studies on the collection efficiency of ESPs at different operation indexes have been carried out. In this regard, Navarrete et al. [5] investigated influence of plate spacing and ash resistivity on the efficiency of electrostatic precipitators through
* Corresponding author. Tel.: þ98 21 64543455; fax: þ98 21 66419736. E-mail addresses: [email protected], [email protected] (H. Basirat Tabrizi). http://dx.doi.org/10.1016/j.elstat.2014.06.005 0304-3886/© 2014 Elsevier B.V. All rights reserved.
real tests carried out on different types of coal with real flue gases in a pilot precipitator installed at a pulverized-coal power plant. Miller et al. [6] presented results of an experimental investigation conducted with a laboratory scale electrostatic precipitator for optimizing the geometric parameters. They showed the optimum ratio of discharge electrode distance to gap width depends on the duct width. Chang and Bai [7] presented experimental results for the influence of back corona on the performance of a laboratory scale single-discharge-wire ESP system. They indicated as back corona occurs, the output current and the power consumption increase under constant voltage operation. Kim and Lee [8] examined a laboratory-scale single-stage ESP to seek the operating conditions for increasing the particle collection efficiency. They found that as the diameter of the discharging wires and the wire-to-plate spacing are set smaller and further the air velocity increases, the higher collection efficiency can be obtained. Zhuang et al. [9] performed experimental and theoretical studies of ultra-fine particle behavior in cylindrical electrostatic precipitator. They observed the corona current decreases at the higher gas velocities because fewer ions reached the collector surface. Je˛ drusik et al. [10] presented the results of experimental research on the movement of fly ash particles in the model of an electrostatic precipitator with different corona electrodes. They employed photographic method for measuring the particle velocity using laser light and determined the migration velocity of particles with different size. Kim et al. [11] used a pilot
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scale electrostatic precipitator to get a modified Deutsch equation that can be applied to real conditions. They evaluated the effects of operational variables as resistivity and concentration of particles as well as temperature, moisture contents and velocity of gas on the ESP collection efficiency. Falaguasta et al. [12] investigated effects of some geometric, hydrodynamic and electric parameters on the collection efficiency of a laboratory scale plate-wire electrostatic precipitator, operating in the removal of airborne PM2.5. Haque et al. [13] evaluated influence of the inlet velocity profiles on the prediction of velocity distribution inside an electrostatic precipitator experimentally and theoretically. Khaled and Eldein [14] presented and experimentally validated a laboratory-scale model for prediction of the voltageecurrent characteristics of wire-plate electrostatic precipitators under clean air conditions. Je˛ drusik and Swierczok [15] tested different constructions of discharge electrode in the aspect of discharge current uniform distribution on collecting electrode surfaces to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Nevertheless, there are few experimental studies concerning the effects of geometric parameters on the efficiency of ESPs with such altered electric indexes as the electric field strength, the average current density and the corona power ratio based on nominal and consumed powers, especially covering collection of fine water drops size inside developing flow condition. Since in the core region of developing flow droplets have accelerated motion across the ESP, the wire-to-flow inlet spacing as a geometric parameter can affect the efficiency of ESPs. Wet ESPs were developed to control a wider variety of particulate pollutants and exhaust gas conditions compared to dry ESPs, especially for particles that are sticky, corrosive, or have high resistivity [16,17]. For example, wet ESP is good option for effective control of sulfuric acid aerosol emissions [18]. In addition, the periodic or continuous scrubbing water flow, used to wash the collection electrode surfaces, was found to prevent particle reentrainment caused by rapping, which occurs in dry ESPs [19]. However, various problems caused by materials and non-uniform distribution of water film have limited high performance of wet ESPs. Recently researchers on wet ESP technology have tried to find some appropriate methods to solve these imperfections and increase wet ESP performance. In this regard, Bologa et al. [20] designed a wet electrostatic precipitator for effective control of fine aerosol from humid gases which operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge and gives mass collection efficiency of 90e97% for one-filed and up to 99% for two-field electrostatic precipitator. Lin et al. [21] designed and tested an efficient parallel-plate single-stage wet electrostatic precipitator to control fine and nanosized particles. They compared the corona current between the dry and wet ESPs, at various supplied voltages and indicated at the same voltage, the corona current was decreased after supplying the scrubbing water on the collection plate surfaces, due to the resistivity of the water film. However, they did not consider the effect of different vapor content in the flow of wet ESPs on corona current variations. Several researchers focused on the collection electrode to solve the shortcomings of wet ESPs (see e.g. Refs. [22e25]). Chang et al. [26] employed single terylene or polypropylene collection electrodes for excellent removal of sulfuric acid aerosol in a wet electrostatic precipitator. Nevertheless, experimental research on effect of some geometric parameters and electric indexes such as wire-toflow inlet spacing and corona power ratio to enhance the performance of vertical wet ESP is scarce. Most of the researchers focused their research on the material of collection electrode to increase the efficiency of wet ESPs, while this method is crucially expensive.
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Added to the experimental studies, some researchers investigated effects of geometric parameters theoretically and presented helpful conclusions. In this regard, Hall [2] summarized several beneficial effects when employing wider duct ESPs. He reported that wider duct ESPs result in reduced gas flow shear stress, more uniform current and electric field distributions as well as more stable electrical operation due to the increased range between corona start and sparkover. Abdel-Sattar [3] and further Pontius and Sparks [27] investigated the effect of the corona wire diameter on the efficiency of ESPs, but presented different conclusions. Pontius and Sparks [27] pointed out that improved efficiency can be obtained with large corona wire by evaluating the efficiency of a pilot-scale ESP, whereas Abdel-Sattar [3] indicated that the precipitation efficiency increases by decreasing corona wire diameter. In addition, Abdel-Sattar [3] found that the most crucial geometric parameter, which affects the performance of ESPs, is the wire-toplate spacing. Chang and Bai [4] theoretically indicated that influences of wire-to-wire spacing and plate (wire)-to-plate spacing on the ESP performance do not have regular trends if the electric field strength is used as the comparison basis. The mentioned studies suggested that optimal design values exist but these values vary with changes in the electric field strength. However, there is no doubt that changing wire-to-plate spacing would alter the spatial arrangement of the wire and plate and consequently influences the collection efficiency. Yang et al. [28] mentioned that additional work is required to examine the effect of changing the spatial configuration of ESPs on their efficiency. The purpose of this study is to investigate experimentally the effects of geometric parameters on the efficiency of vertical wet electrostatic precipitator in a laboratory-scale size at different electric indexes to collect fine water droplets. The weighing technique is employed to measure the ESP efficiency. Vertical flow precipitators are mainly used for self-draining liquid and mist type particulates, where efficiencies in excess of 99 percent can be readily achieved from a single field. A high negative voltage is applied to the corona discharge wire. The upward airewater droplets flow is strongly under developing condition in the entire collection duct of ESP. This kind of flow can be created in small length, wide duct ESPs that serve in high flow rates such as some types of fly ash precipitators and those related to small power generators and boiler plants. In these experiments, an ultrasonic atomizer produces fine water droplets in a size range of 1e10 mm measured using laser light scattering technique. The efficiency was compared with a glass micro fiber filter and proposed semiempirical efficiency model. 2. Experimental apparatus and procedure 2.1. Test setup for measuring multiphase flow properties Fig. 1 shows the experimental setup of mechanism of generating droplets, conveying into the ESP and measuring instruments. Ultrasonic atomizer generates droplets with constant rate in a size range of 1e10 mm. A commercial type, Malvern Mastersizer 2000, laser particle sizer based on laser scattering technique is used to measure the droplets size. Fig. 2 shows the droplet size distribution produced by the ultrasonic method. In the experiment, air mixes with the produced droplets and enters into a primary separator. The primary separator used for gravitational sedimentation of large drops and consequently generates an outcome of more uniform size droplet especially at high flow rates. Then flow is conducted to the ESP. A piston type compressor is used which has the same power over the testing time and produces the suction. In order to control flow rate of the path, a bypass valve is positioned between the flow meter and the compressor units. It should be mentioned
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Fig. 1. Schematic diagram of the experimental setup for the ESP efficiency test.
that the frequency of ultrasonic droplet generator, the droplet mass fraction and air flow rate is identical for all measurements. This leads to the same droplets size and concentration in the suction method. It is assumed that the downstream condition does not significantly influence on the droplets size in the ESP. The sealing of the flow path is controlled by using a vacuum pump and a negative pressure indicator. The flow rate and temperature are measured with a hot plate mass flow meter. The flow meter accuracy for
Fig. 2. Droplet size distribution produced by the ultrasonic method.
flow rate and temperature measurement was ±3% and ±2 K, respectively.
2.2. Wet electrostatic precipitator Since most of industrial electrostatic precipitators are of the wire-plate type, the laboratory-scale of duct-type single-stage, single-wire precipitator was prepared to examine. Fig. 3(a) shows the side view of the mentioned wet ESP used in the experiments. Further, a schematic diagram of the ESP with the up view of the configuration is shown in Fig. 3(b). In this figure, L and Swp denote length of the collection plates and wire-to-plate spacing, respectively. As is seen in Fig. 3, the upward flow moves perpendicular to the corona wire. The ESP walls for attaching the plates are made of Plexiglas material with thickness of 15 mm. Further two Plexiglas transparent walls perpendicular to the plates are provided to allow optical observation of the spatial distribution of the corona. The ESP has internal dimensions of 160 mm height and 105 mm length. Two Aluminum plates with an area of 90 120 mm2 and 2 mm thickness were attached parallel to the Plexiglas walls, one on each side of the precipitator. One corona discharge wire made of copper was installed symmetrically midway between the Aluminum plates along the length of the precipitator. DC high voltage is applied between the corona wire and plates (negative and positive poles, respectively). The width of Aluminum plates has been chosen less than the width of Plexiglas plates. A gap between the wire and side plates (see Fig. 3(b)) has been provided both to prevent the electric
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Fig. 3. The used single-stage wire-plate ESP, (a) the side view, (b) schematic diagram of the up view of the configuration.
field breakdown and the applied voltage drop due to the water drops aggregation. In addition, in the experiment the water film formed on the plates is removed and dried. Since, thick water film leads to decrease the electric field strength due to water resistivity.
Uncertainty for the ESP efficiency in the weighing test can be described as:
2.3. Weighing method for measuring collection efficiency
where, dDmESP and dDmTc are the precision of weighing test for DmESP and DmTc. In addition, an estimate for total mass difference, DmTc, in Eq. (2) follows:
Similar weighing method of Ehteram et al. [29] is used to measure ESP collection efficiency. Accordingly, the ESP efficiency for droplets collection is defined as:
h¼
DmESP 100; DmT
DmT ¼ DmSource DmPsep
(1)
where,DmESP ; DmSource and DmPsep are the mass difference of the ESP, drops source (water vessel in Fig. 1) and primary separator, respectively. In the experiment, these differences were measured by weighing of the components before and after the test precisely with ±0.1 gr precision. The duration of each test was about 600 s. For each flow rate, the tests were repeated at least three times to ensure consistency of the data. It was found that the highest deviation in the data was about 3e5% for the ESP efficiency. However, averages of measurements are reported in this study. A typical example of measurement accuracy analysis is presented in Section 5.
Eu h ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vh vh dDmESP þ dDmTc DmESP DmTc
DmTc zm_ Droplet Time
(2)
(3)
In which m_ Droplet is droplets mass flow rate. Substituting Eq. (1) into Eq. (2) gives:
Eu h ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðdDmESP Þ2 þ ðh dDmTc Þ2 DmTc
(4)
Considering the ESP efficiency approximately 50% (i.e. h ¼ 0.5) and droplets mass flow rate 1 g per minute (i.e. m_ Droplet ¼ 1 gr/min), and taking ±0.1 gr precision for weighing the ESP, water vessel and primary separator (i.e. dDmESP and dDmTc ¼ ±0.1 gr), using Eqs. (3) and (4) the uncertainty of collection efficiency as a function of time is shown in Fig. 4. It is seen from Fig. 4, in 600 s test duration, this leads approximately to below 0.5% uncertainty in the measurement of ESP efficiency that is sufficient.
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velocity, VTE, is a key factor of collection efficiency, and it can be estimated as [31]:
VTE ¼
neECc 3pmd
(7)
In which n is total number of charges acquired by a particle with diameter d by different charging mechanisms, e is the charge on an electron, e ¼ 1.6 1019 C, E is the electric field strength, m is the dynamic viscosity of the gas and Cc is the Stokes-Cunningham slip correction factor which is calculated by[31]:
Cc ¼ 1 þ Knp 1:257 þ 0:4 exp 1:1 Knp
Fig. 4. Uncertainty of the measurements of ESP efficiency for different times.
3. Flow condition Table 1 indicates the dimensions, parameters and basic operating conditions of the ESP. The hydrodynamic entrance length inside ducts can be obtained from the Langhaar equation as follows [30]:
Lhd ¼ 0:058 Dh Reh
(5)
where Dh, is the hydraulic diameter of duct cross-section and Reh ¼ UDh/n is the Reynolds number based on average velocity, U, and Dh. It can be obtained; Lhd has the value of approximately 348 mm for 19 lt/min flow rate that is about three times larger than collection duct height (see Table 1). For higher flow rates the hydrodynamic entrance length increases. Therefore, flow is strongly under developing condition across the ESP collection duct for the all examined flow rates.
4. Semi-empirical model of droplet collection efficiency The collection efficiency of an ESP can be estimated from the Deutsch model as follows [31]:
hDe
VTE Ac ¼ 1 exp Q
(6)
where Q is the volume flow rate of carrier gas, Ac is the surface area of collecting electrodes and VTE is the mean migration velocity of the particle across the precipitator. This model is the most used model for predicting the ESP efficiency. The mean migration
Table 1 The dimensions and operating conditions for the present vertical ESP. Dimensions and operating conditions
Value
Diameter of discharge wire, Dw (mm) Wire-to-plate spacing, Swp (mm) Plate-to-flow inlet spacing, Spi (mm) Wire-to-flow inlet spacing, Swi (mm) Height of collection plate, H (mm) Length of collection plate, L (mm) Corona wire effective length (mm) Collection plate thickness (mm) Air flow rate, Q (lt/min) Nozzle diameter, dnozzle (mm) Inlet Reynolds number, Re Applied voltage on wire, V (kV)
0.5 22.5e27.5 30 60e120 120 90 90 2 19e33 10 865e1503 3.64e9.64
(8)
where p is the particle Knudsen number defined by Knp ¼ 2l=d, pKn ffiffiffiffiffiffiffiffiffiffiffiffiffiffi l ¼ m p=2RT =r is the molecular mean free path of the gas and R and T are universal constant and absolute temperature of gas. There are two mechanisms for aerosol charging in ESPs due to corona discharge. One is the diffusion charging results from the Brownian motion of the ions and particles and the other is the field charging results from presence of a strong electric field. These two mechanisms require the production of unipolar ions by corona discharge. Field charging is the dominant mechanism for particles larger than 1.0 mm [31]. Here, it is assumed the total number of charges acquired by a particle in ESP is the summation of number of charges due to diffusion charging and field charging. Concerning diffusion charging, an approximate expression for the number of charges nd(t) acquired by a particle with diameter d during charging time t is [31]:
nd t ¼
d pKE d ci e2 Ni t kT ln 1 þ 2 2kT 2KE e
(9)
where ci is the mean thermal speed of the ions (equal to 240 m/s at standard conditions), k ¼ 1:38 1023 ðJ=KÞ is the Boltzmann constant, KE ¼ 9.0 109 N m2/C2 is a constant and Ni is the ion number concentration usually 1013 per unit volume (m3). In addition, for field charging the number of charges, nf(t), acquired by a particle with diameter d during charging time t in an electric field E can be estimated from the following equation [31]:
nf t ¼
3ε εþ2
Ed2 pKE eZi Ni t 1 þ pKE eZi Ni t 4KE e
(10)
where ε is the relative permittivity of the particle or dielectric constant (¼ 80 for pure water droplet) and Zi is the mobility of the ions, approximately 0.00015 m2/V s. As is seen in Eq. (10), the greater the value of ε, the greater the number of charges. This is because greater number of field lines converges on the particle with greater dielectric constant. Charging time in Eqs. (9) and (10) has been estimated using the following equation which is a time constant for particle charging on the basis of Ohm's law [32]:
t¼
4ε0 Eav jav
(11)
where ε0 ¼ 8.854 1012 F/m is the vacuum permittivity and jav is the average electric current density at the collecting electrode. Finally n(t) in Eq. (7) is estimated as the summation of Eqs. (9) and (10). In case of water droplets as the particle, the inlet mass flow rate to the ESP is a summation of mass flow rate of droplets and water vapor. Nevertheless, the Deutsch efficiency Eq. (6) only considers the efficiency of ESP for droplets. In fact, the Deutsch model is the ratio of mass of separated droplets to mass of inlet droplets. Hence, this efficiency must be corrected to consider the effect of vapor
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Fig. 5. Collection efficiency characteristics of ESP as a function of specific collection area (SCA) at various applied voltages.
Fig. 7. Comparison of measured and estimated values of collection efficiency as a function of corona nominal power.
content in the flow and produce theoretical efficiencies comparable with our measurements. To consider this effect the ESP efficiency for combined droplets/vapor can be written as [29]:
by the parameters variation, throughout the results power trendline is used to fit the data accurately. Efficiency characteristics of the fabricated ESP as a function of specific collection area, SCA, at various applied voltages are illustrated in Fig. 5. The specific collection area defined as the ratio of the total collection area to the total gas volume flow rate. This important parameter characterizes the performance of ESP. In this study, the total collection area of the plates is fixed. Thus, the SCA value is changed by varying the flow velocity. Fig. 6 compares the measured collection efficiencies of ESP and Parker filter (P3K coalescing filter series) at different flow rates when applied voltage on ESP wire has the value of 9.64 kV. For this purpose Parker filter is positioned instead of the ESP in the setup and the same weighing technique is used for measuring its efficiency. As a typical example, the error bars for the collection efficiency is shown in Fig. 6. Parker filter is equipped to a 0.01 micron coalescer glass micro fiber element and has high filtration efficiency (99.97%) for liquid aerosols and sub micron (0.3e0.6 micron) particles. A good agreement between the efficiencies is noticed in Fig. 6. It is seen the collection efficiencies are less than 90% for both filters, so significant amount of vapor in air exists. Further, by increasing the flow rate the vapor amount and evaporation rate for drops becomes larger and consequently the collection efficiency of droplets for both filters decreases. Comparison of measured and estimated values of collection efficiency as a function of nominal power is illustrated in Fig. 7. The
:
hDrops=vapor ¼ ¼
mSeperated
:
min
:
Droplets
þ mVapor
hDe .: : 1 þ mVapor min
.: : mSeperated min Droplets .: ¼ : 1 þ mVapor min Droplets
Droplets
(12) :
:
:
In Eq. (12), mSeperated ,min Droplets and mVapor are, respectively, mass flow rates of separated droplets, inlet droplets and vapor content in the flow which are measured using weighing technique. In this study, it is assumed that the flow is fully saturated. Furthermore, the Sauter mean diameter of Fig. 2 (d32 ¼ 1.4 mm) is used as the droplet size in calculating theoretical efficiencies.
5. Results and discussion In this section, measurements are related to wire-to-plate spacing (Swp) of 27.5 mm and wire-to-flow inlet spacing (Swi) of 90 mm, except for when effects of Swp or Swi parameters are investigated. Further, to investigate the physical trend of efficiency
Fig. 6. Comparison of the measured collection efficiencies of ESP and Parker filter, repeatability of experiment for the ESP efficiency.
Fig. 8. Corona current as a function of applied voltage in wet ESP.
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Fig. 9. Effect of wire-to-plate spacing (Swp) on ESP efficiency as a function of (a) electric field strength, (b) average current density at the collecting plate, (c) average current density at the input area, (d) corona nominal power ratio, (e) corona consumed power ratio.
nominal power is defined as Pnom ¼ VI where I is the total corona current. In Fig. 7, hDrops=vapor estimated by the semi-empirical model in 19 lt/min are compared with those measured with weighing method. The minimum, maximum and average differences between the measured and estimated values are, respectively, 1.65%, 26.04% and 11.09%, so a good agreement between the results is found. It is seen form Fig. 7 that similar to the experimental findings, the semi-empirical prediction indicates the increasing of the corona power more than a critical value, in this case the critical value is about 30 W, can significantly affect the ESP separation. Furthermore, existing of the vapor content in the air can significantly drop the ESP operation. Fig. 8 shows comparison of corona current as a function of applied voltage for various flow rates operated in wet ESP. For the higher flow rates evaporation rate of drops becomes larger and
consequently higher moisture content in air is created. Therefore, the humidity near the collection plates increases that affects the operational indexes of ESP. It is seen from Fig. 8 that at the same voltage the corona current is decreased for higher flow rates. This result is compatible with experimental study of Lin et al. [21] for comparing the corona current between the dry and wet ESPs. However, they indicated at the same voltage the corona current is lower for wet ESPs due to the resistivity of the water film. As an outcome, the wet ESP collection efficiency is expected to be lower at higher flow rates at the same voltage due to lower corona current. In order to notice the effect of geometric parameters on the ESP efficiency, sets E-I are employed and the results are shown in Figs. 9 and 10. Here, the flow rate has value of 19 lit/min. The ESP efficiency is illustrated as a function of average electric field strength (Eav ¼ V/ Swp), average current density at the collecting wall area (jav),
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Fig. 10. Effect of wire-to-flow inlet spacing (Swi) on ESP collection efficiency as a function of (a) applied voltage, (b) average current density at the collecting plate, (c) corona nominal power ratio, (d) corona consumed power ratio.
average current density at the input area (Jav), corona nominal power ratio (Pnom/Q) and corona consumed power ratio (P/Q) in Fig. 9(a)e(e), respectively. It should be noted that the corona power ratio is related to the ESP operation cost [4]. Effect of wire-to-plate spacing (Swp) on ESP efficiency is illustrated in Fig. 9(a)e(e). It can be seen from Fig. 9(a) that the efficiency is increased as the wire-to-plate spacing increases under the same electric field strength. Therefore the wider wire-to-plate spacing seems to be more beneficial to enhance ESPs efficiency. This outcome is confirmed since wider ESP duct leads to lower gas shear stress and droplets velocity inside the ESP. However, it is seen higher electric field strength makes higher efficiency for ESP collection. It appears in Fig. 9(b) and (c) that when the average current density at the collecting plate and input have an intermediate value, the ESP with larger wire-to-plate spacing tends to perform better under the same average current density. This may be because the current and electric field distributions in narrow duct are more uneven than that in wide duct in this range of applied voltage. Further, a narrow duct leads to more unstable electrical operation. However, it is noticed with the increase of average current density at the collecting wall, the design of narrow duct has an inverse effect on the ESP efficiency and makes the ESP more beneficial. Therefore, there is certain value for jav for which the effect of wireto-plate spacing on ESP efficiency is reversed (alteration point). According to Fig. 9(c), it is predicted that such a point also exists for Jav, since the electric field strength in narrow duct is higher than that of ESP with wide duct. These results are compatible with theoretical study of Yang et al. [28] for intermediate values of average current density at the collecting wall. However, they
investigated very smaller average current densities than present study. Fig. 9(d) and (e) indicates the similar trend with Fig. 9(b) and (c), for corona power ratio instead of average current density and both of the results along with Fig. 9(a) show complex relationship of the ESP efficiency versus the wire-to-plate spacing. This may be due to change of wire-to-plate spacing influences average electric field strength, droplets inlet velocity to the collection duct and developing flow condition simultaneously. Smaller wire-to-plate spacing makes higher electric field strength along with higher droplets inlet velocity and more uneven electric field. These factors have adverse effects on ESP efficiency. Further, wire-to-plate spacing influences hydraulic diameter of duct and consequently hydrodynamic entrance length in developing flow. Therefore, the thickness of accelerated region inside the ESP collection duct is also affected by wire-to-plate spacing size. Effect of wire-to-flow inlet spacing (Swi) on ESP collection efficiency is indicated in Fig. 10(a)e(d), where the efficiency is shown as a function of wire-to-flow inlet spacing in various applied voltages, average current density at the collecting wall in various Swi, corona nominal power ratio in different Swi and corona consumed power ratio in various Swi, respectively. It should be mentioned, the corona power was positioned at ¼, ½ and ¾ of total height of collection duct (H ¼ 120 mm, see Table 1) for measurements (corresponding with Swi ¼ 60, 90, 120 mm, respectively). It is noticed from Fig. 10(a) that there exists an optimum wire-to-flow inlet spacing which provides maximum droplet collection efficiency. When the corona wire moves from the bottom toward top of collection duct, larger area of plates is interacted with corona wire and consequently stronger and wider electric field inside the duct is
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appointed. Such condition exists before the wire approximately reaches the middle of duct and ahead of this point an inverse condition occurs. Further, due to the developing flow condition, droplets have accelerated inside the core and their velocity increase across the collection duct. Therefore, when the wire is positioned upper to the initial half of the duct, higher collection efficiency is obtained. In the secondary half of the duct, droplets have larger velocity and further upper wire position leads to weaker electric field strength that cause the collection efficiency sharply decreases. Therefore existing of the peak point and the minimum efficiency for highest wire position in Fig. 10(a) are reasonable. For higher flow rates, the thickness of accelerated region becomes larger and consequently the maximum efficiency can occur in lower heights. Moreover, the slope of efficiency decreasing with the increase of wire height is larger in the secondary half of collection duct. The existence of optimum wire-to-flow inlet spacing also appears in Fig. 10(b)e(d). In addition, it is seen from these figures that there are the ranges for average current density at the collecting wall and corona power ratio for which the collection efficiency of wire positions of ¼ H and ¾ H are very close. This is may be due to that in parameters jav and P/Q both electric and hydrodynamic indexes are considered simultaneously.
Nomenclature :
Acknowledgments
mVapor mass flow rates of vapor content, kg/s Ci mean thermal speed of ions, m/s : mDroplets droplets mass flow rate, kg/s : min Droplets mass flow rates of inlet droplets, kg/s : mSeperated mass flow rates of separated droplets, kg/s Ac surface area of collecting electrodes, m2 Cc Stokes-Cunningham slip correction factor d particle diameter, m d32 Sauter mean diameter, m Dh hydraulic diameter, m dnozzle nozzle diameter, m Dw diameter of discharge wire, m dDmESP precision of weighing test for ESP, kg dDmESP precision of weighing test for total mass difference, kg e charge on an electron, ¼ 1.6 1019 C E electric field strength, V/m Eu uncertainty of measured efficiency H height of collection plate, m I corona current, A jav average electric current density at the collecting electrode, A/m2 Jav average electric current density at input area, A/m2 KE constant, ¼ 9.0 109 N m2/C2 Knp particle Knudsen number L length of collection plates, m Lhd hydrodynamic entrance length, m n total number of charges acquired by a particle nd number of charges acquired by diffusion charging nf number of charges acquired by field charging Ni ion number concentration, ¼ 1013/m3 P consumed corona power, W Pnom nominal corona power, W Q volumetric flow rate, m3/s R universal constant of gas, kJ/kg K Re inlet Reynolds number Reh Reynolds number based on hydraulic diameter SCA specific collection area, s/m Spi plate-to-flow inlet spacing, m Swi wire-to-flow inlet spacing, m Swp wire-to-plate spacing, m T absolute temperature of gas, K t time, s U average velocity, m/s V applied voltage on wire, kV VTE mean migration velocity of particle across precipitator, m/s Zi mobility of ions, m2/V s DmESP mass difference of ESP before and after test, kg DmPsep mass difference of primary separator before and after test, kg DmSource mass difference of drops source before and after test, kg DmT measured total mass difference, kg DmTc estimated total mass difference, kg ε relative permittivity of particle ε0 vacuum permittivity, ¼ 8.854 1012 F/m h ESP collection efficiency k Boltzmann constant, ¼ 1.38 1023 J/K l molecular mean free path of gas, m m dynamic viscosity, Pa s r gas density, kg/m3
The authors would like to appreciate many undergraduate students and specially Mr. Mojtaba Mohammadi who obtained data from the experiments.
Subscripts av average De Deutsch model
6. Conclusions A laboratory-scale single-stage, single-wire wet ESP was designed, built and operated in airewater droplets flow to measure collection efficiency using weighing technique. A short vertical collection duct was provided so that upward flow was under developing condition across the ESP. In the experiment, fine water droplets in a size range of 1e10 mm were produced using ultrasonic atomizer. The ESP collection efficiency was compared in similar conditions with a glass micro fiber filter, which has very high filtration efficiency (99.97%) for liquid sub micron aerosols, and a good agreement was noticed. In addition, comparison between the measured efficiencies and a semi-empirical model was provided and a good agreement was found. Effects of some geometric parameters on the efficiency of ESPs using four electric indexes were evaluated. It was observed while the effect of vapor content in the flow was considered via weighing technique; a good accuracy with an average error of 10% for semi-empirical Deutsch model was obtained. Further, at the same applied voltage, the corona current was decreased for high flow rates due to drops evaporation. Existing of the vapor content in the air and evaporation of droplets significantly affects the ESP operation via changing different factors such as corona current and droplets size. The experimental results indicated that the trends of the collection efficiency with the change of wire-to-plate spacing are considerably complex due to simultaneously developing flow condition and electrical operation. Furthermore, there is an alteration point for parameters of average current density and corona power ratio at which effect of wire-toplate spacing on ESP efficiency is reversed. Based on the findings, when developing flow is created inside the ESP, an optimum wireto-flow inlet spacing provides maximum droplet collection efficiency. In addition, lower wire position tends to obtain better collection efficiency than upper positions.
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References [1] K. Parker, Electrical Operation of Electrostatic Precipitators, first ed., The Institution of Engineering and Technology, London, United Kingdom, 2003. [2] H.J. Hall, Some electrode geometry, electric field and performance effects in electrostatic precipitation, in: Proceedings of 2nd International Conference on Electrostatic Precipitation, Kyoto, Japan, 1984, pp. 354e361. [3] S. Abdel-Sattar, Influences of geometrical parameters upon electrostatic precipitator efficiency, Int. J. Comput. Math. Electr. Eng. 10 (1991) 27e43. [4] C.L. Chang, H. Bai, Effects of some geometric parameters on the electrostatic precipitator efficiency at different operation indexes, Aerosol Sci. Technol. 33 (2000) 228e238. [5] B. Navarrete, L. Cafiadas, V. Cortrs, L. Salvador, J. Galindo, Influence of plate spacing and ash resistivity on the efficiency of electrostatic precipitators, J. Electrostat. 39 (1997) 65e81. [6] J. Miller, B. Hoferer, A.J. Schwab, The impact of corona electrode configuration on electrostatic precipitator performance, J. Electrostat. 44 (1998) 67e75. [7] C. Chang, H. Bai, An experimental study on the performance of a single discharge wire-plate electrostatic precipitator with back corona, J. Aerosol Sci. 30 (1999) 325e340. [8] S.H. Kim, K.W. Lee, Experimental study of electrostatic precipitator performance and comparison with existing theoretical prediction models, J. Electrostat. 48 (1999) 3e25. [9] Y. Zhuang, Y.J. Kim, T.G. Lee, P. Biswas, Experimental and theoretical studies of ultra-fine particle behavior in electrostatic precipitators, J. Electrostat. 48 (2000) 245e260. [10] M. Je˛ drusik, J.B. Gajewski, A.J. Swierczok, Effect of the particle diameter and corona electrode geometry on the particle migration velocity in electrostatic precipitators, J. Electrostat. 51e52 (2001) 245e251. [11] B. Kim, K. Ahn, Y. Jang, Electrostatic precipitability of the coal fly-ash by the pilot scale test, KSME Int. J. 15 (2001) 602e612. [12] M.C.R. Falaguasta, J. Steffens, E.E. Valdes, J.R. Coury, Overall collection efficiency of a plate-wire electrostatic precipitator operating on the removal of PM2.5, Lat. Am. Appl. Res. 38 (2008) 179e186. [13] Shah M.E. Haque, M.G. Rasul, M.M.K. Khan, A.V. Deev, N. Subaschandar, Influence of the inlet velocity profiles on the prediction of velocity distribution inside an electrostatic precipitator, Exp. Therm. Fluid Sci. 33 (2009) 322e328. [14] U. Khaled, A.Z. Eldein, Experimental study of V-I characteristics of wire-plate electrostatic precipitators under clean air conditions, J. Electrostat. (2012) 1e7. [15] M. Je˛ drusik, A. Swierczok, The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator, J. Electrostat. (2013) 1e5.
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[16] R. Altman, G. Offen, W. Buckley, I. Ray, Wet electrostatic precipitation: demonstrating promise for fine particulate controldpart I, Power Eng. 105 (2001a) 37e39. [17] R. Altman, W. Buckley, I. Ray, Wet electrostatic precipitation: demonstrating promise for fine particulate controldpart II, Power Eng. 105 (2001b) 42e44. [18] P. Saiyasitpanicha, T.C. Keener, S. Khang, M. Lu, Removal of diesel particulate matter (DPM) in a tubular wet electrostatic precipitator, J. Electrostat. 65 (2007) 618e624. [19] U.S. Environmental Protection Agency, Air pollution control technology fact sheet; EPA-452/F-03-030, EPA, Washington, DC, 2003. €scher, K. Woletz, Novel wet electrostatic [20] A. Bologa, H.-R. Paur, H. Seifert, Th. Wa precipitator for collection of fine aerosol, J. Electrostat. 67 (2009) 150e153. [21] G. Lin, C. Tsai, S. Chen, T. Chen, S. Li, An efficient single-Stage wet electrostatic precipitator for fine and nanosized particle control, Aerosol Sci. Technol. 44 (2010) 38e45. [22] M. Jedrusik, A. Swierczok, R. Teisseyre, Experimental study of fly ash precipitation in a model electrostatic precipitator with discharge electrodes of different design, Powder Technol. 135e136 (2003) 295e301. [23] M. Chai, M. Lu, T. Keener, S. Khang, Using an improved electrostatic precipitator for poultry dust removal, J. Electrostat. 67 (2009) 870e875. [24] A. Niewulis, J. Podlinski, J. Mizeraczyk, Electrohydrodynamic flow patterns in a narrow electrostatic precipitator with longitudinal or transverse wire electrode, J. Electrostat. 67 (2009) 123e127. [25] J. Kim, H. Lee, H. Kim, Electrospray with electrostatic precipitator enhances fine particles collection efficiency, J. Electrostat. 68 (2010) 305e310. [26] J. Chang, Y. Dong, Z. Wang, P. Wang, P. Chen, C. Ma, Removal of sulfuric acid aerosol in a wet electrostatic precipitator with single terylene or polypropylene collection electrodes, J. Aerosol Sci. 42 (2011) 544e554. [27] D.H. Pontius, L.E. Sparks, Effects of wire diameter on particle charging and collection efficiency in electrostatic precipitators, in: Proceedings of the 2nd International Conference on Electrostatic Precipitation, Kyoto, Japan, 1984, pp. 269e274. [28] X. Yang, Y. Kang, K. Zhong, Effects of geometric parameters and electric indexes on the performance of laboratory-scale electrostatic precipitators, J. Hazard. Mater. 169 (2009) 941e947. [29] M. Ehteram, H. Basirat Tabrizi, M. Mesbah, G. Ahmadi, M. Aghamirsalim, Experimental study on the effect of connecting ducts on demisting cyclone efficiency, Exp. Therm. Fluid Sci. 39 (2012) 26e36. [30] H.L. Langhaar, Steady flow in the transition length of a straight tube, J. Appl. Mech. 9 (1942) A-55eA-58. [31] W.C. Hinds, Aerosol Technology, Properties, Behavior and Measurement of Airborne Particles, second ed., John Wiley & Sons, Inc., New York, 1999. € ffler, Electrical similarity concerning particle transport in [32] C. Riehle, F. Lo electrostatic precipitators, J. Electrostat. 29 (1992) 147e165.
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Effects of geometric parameters and electric indexes on performance of a vertical wet electrostatic precipitator Masoud Molaei Najafabadi, Hassan Basirat Tabrizi*, Amin Aramesh, Mohammad Ali Ehteram Mechanical Engineering Department, Amirkabir University of Technology, No. 424, Hafez Ave, PO Box 15875-4413, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2013 Received in revised form 15 March 2014 Accepted 27 June 2014 Available online 18 July 2014
A single-stage, single-wire vertical wet electrostatic precipitator was designed and operated in airewater droplets flow to investigate its performance. The efficiency was compared with a glass micro fiber filter and proposed semi-empirical efficiency model, which was in good accuracy while considering the vapor content. Effects of geometric parameters on efficiency under different charge conditions were discussed. Due to evaporation mechanism, the corona current decreases for high flow rates at the same applied voltage. Findings indicated while developing flow is created inside the ESP, there exists an optimum wire-to-flow inlet spacing that provides maximum droplet collection efficiency. © 2014 Elsevier B.V. All rights reserved.
Keywords: Wet electrostatic precipitator Upward developing flow Water droplet Collection efficiency Geometric parameter Electric index
1. Introduction Electrostatic precipitators (ESPs) are one of the most commonly employed particulate control devices to collect fly ash emissions and particulate matters from exhaust gases originating from various production installations such as boilers, incinerators, coalfired power plants, mining installations, metallurgical and chemical industries and many other industrial processes. They can operate in a wide range of gas temperatures, pressures and flow rates as well as very fine suspended particles, achieving high particle collection efficiency compared with mechanical devices such as cyclones, scrubbers and bag filters [1]. Although the process of electrostatic precipitation involves several complicated and interrelated physical mechanisms between the fluid and particle flow such as turbulent and Brownian diffusion, particles collisions etc., it is definite the geometric parameters and electric field characteristics have significant effects on the efficiency of ESPs [2e4]. Several experimental studies on the collection efficiency of ESPs at different operation indexes have been carried out. In this regard, Navarrete et al. [5] investigated influence of plate spacing and ash resistivity on the efficiency of electrostatic precipitators through
* Corresponding author. Tel.: þ98 21 64543455; fax: þ98 21 66419736. E-mail addresses: [email protected], [email protected] (H. Basirat Tabrizi). http://dx.doi.org/10.1016/j.elstat.2014.06.005 0304-3886/© 2014 Elsevier B.V. All rights reserved.
real tests carried out on different types of coal with real flue gases in a pilot precipitator installed at a pulverized-coal power plant. Miller et al. [6] presented results of an experimental investigation conducted with a laboratory scale electrostatic precipitator for optimizing the geometric parameters. They showed the optimum ratio of discharge electrode distance to gap width depends on the duct width. Chang and Bai [7] presented experimental results for the influence of back corona on the performance of a laboratory scale single-discharge-wire ESP system. They indicated as back corona occurs, the output current and the power consumption increase under constant voltage operation. Kim and Lee [8] examined a laboratory-scale single-stage ESP to seek the operating conditions for increasing the particle collection efficiency. They found that as the diameter of the discharging wires and the wire-to-plate spacing are set smaller and further the air velocity increases, the higher collection efficiency can be obtained. Zhuang et al. [9] performed experimental and theoretical studies of ultra-fine particle behavior in cylindrical electrostatic precipitator. They observed the corona current decreases at the higher gas velocities because fewer ions reached the collector surface. Je˛ drusik et al. [10] presented the results of experimental research on the movement of fly ash particles in the model of an electrostatic precipitator with different corona electrodes. They employed photographic method for measuring the particle velocity using laser light and determined the migration velocity of particles with different size. Kim et al. [11] used a pilot
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scale electrostatic precipitator to get a modified Deutsch equation that can be applied to real conditions. They evaluated the effects of operational variables as resistivity and concentration of particles as well as temperature, moisture contents and velocity of gas on the ESP collection efficiency. Falaguasta et al. [12] investigated effects of some geometric, hydrodynamic and electric parameters on the collection efficiency of a laboratory scale plate-wire electrostatic precipitator, operating in the removal of airborne PM2.5. Haque et al. [13] evaluated influence of the inlet velocity profiles on the prediction of velocity distribution inside an electrostatic precipitator experimentally and theoretically. Khaled and Eldein [14] presented and experimentally validated a laboratory-scale model for prediction of the voltageecurrent characteristics of wire-plate electrostatic precipitators under clean air conditions. Je˛ drusik and Swierczok [15] tested different constructions of discharge electrode in the aspect of discharge current uniform distribution on collecting electrode surfaces to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Nevertheless, there are few experimental studies concerning the effects of geometric parameters on the efficiency of ESPs with such altered electric indexes as the electric field strength, the average current density and the corona power ratio based on nominal and consumed powers, especially covering collection of fine water drops size inside developing flow condition. Since in the core region of developing flow droplets have accelerated motion across the ESP, the wire-to-flow inlet spacing as a geometric parameter can affect the efficiency of ESPs. Wet ESPs were developed to control a wider variety of particulate pollutants and exhaust gas conditions compared to dry ESPs, especially for particles that are sticky, corrosive, or have high resistivity [16,17]. For example, wet ESP is good option for effective control of sulfuric acid aerosol emissions [18]. In addition, the periodic or continuous scrubbing water flow, used to wash the collection electrode surfaces, was found to prevent particle reentrainment caused by rapping, which occurs in dry ESPs [19]. However, various problems caused by materials and non-uniform distribution of water film have limited high performance of wet ESPs. Recently researchers on wet ESP technology have tried to find some appropriate methods to solve these imperfections and increase wet ESP performance. In this regard, Bologa et al. [20] designed a wet electrostatic precipitator for effective control of fine aerosol from humid gases which operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge and gives mass collection efficiency of 90e97% for one-filed and up to 99% for two-field electrostatic precipitator. Lin et al. [21] designed and tested an efficient parallel-plate single-stage wet electrostatic precipitator to control fine and nanosized particles. They compared the corona current between the dry and wet ESPs, at various supplied voltages and indicated at the same voltage, the corona current was decreased after supplying the scrubbing water on the collection plate surfaces, due to the resistivity of the water film. However, they did not consider the effect of different vapor content in the flow of wet ESPs on corona current variations. Several researchers focused on the collection electrode to solve the shortcomings of wet ESPs (see e.g. Refs. [22e25]). Chang et al. [26] employed single terylene or polypropylene collection electrodes for excellent removal of sulfuric acid aerosol in a wet electrostatic precipitator. Nevertheless, experimental research on effect of some geometric parameters and electric indexes such as wire-toflow inlet spacing and corona power ratio to enhance the performance of vertical wet ESP is scarce. Most of the researchers focused their research on the material of collection electrode to increase the efficiency of wet ESPs, while this method is crucially expensive.
403
Added to the experimental studies, some researchers investigated effects of geometric parameters theoretically and presented helpful conclusions. In this regard, Hall [2] summarized several beneficial effects when employing wider duct ESPs. He reported that wider duct ESPs result in reduced gas flow shear stress, more uniform current and electric field distributions as well as more stable electrical operation due to the increased range between corona start and sparkover. Abdel-Sattar [3] and further Pontius and Sparks [27] investigated the effect of the corona wire diameter on the efficiency of ESPs, but presented different conclusions. Pontius and Sparks [27] pointed out that improved efficiency can be obtained with large corona wire by evaluating the efficiency of a pilot-scale ESP, whereas Abdel-Sattar [3] indicated that the precipitation efficiency increases by decreasing corona wire diameter. In addition, Abdel-Sattar [3] found that the most crucial geometric parameter, which affects the performance of ESPs, is the wire-toplate spacing. Chang and Bai [4] theoretically indicated that influences of wire-to-wire spacing and plate (wire)-to-plate spacing on the ESP performance do not have regular trends if the electric field strength is used as the comparison basis. The mentioned studies suggested that optimal design values exist but these values vary with changes in the electric field strength. However, there is no doubt that changing wire-to-plate spacing would alter the spatial arrangement of the wire and plate and consequently influences the collection efficiency. Yang et al. [28] mentioned that additional work is required to examine the effect of changing the spatial configuration of ESPs on their efficiency. The purpose of this study is to investigate experimentally the effects of geometric parameters on the efficiency of vertical wet electrostatic precipitator in a laboratory-scale size at different electric indexes to collect fine water droplets. The weighing technique is employed to measure the ESP efficiency. Vertical flow precipitators are mainly used for self-draining liquid and mist type particulates, where efficiencies in excess of 99 percent can be readily achieved from a single field. A high negative voltage is applied to the corona discharge wire. The upward airewater droplets flow is strongly under developing condition in the entire collection duct of ESP. This kind of flow can be created in small length, wide duct ESPs that serve in high flow rates such as some types of fly ash precipitators and those related to small power generators and boiler plants. In these experiments, an ultrasonic atomizer produces fine water droplets in a size range of 1e10 mm measured using laser light scattering technique. The efficiency was compared with a glass micro fiber filter and proposed semiempirical efficiency model. 2. Experimental apparatus and procedure 2.1. Test setup for measuring multiphase flow properties Fig. 1 shows the experimental setup of mechanism of generating droplets, conveying into the ESP and measuring instruments. Ultrasonic atomizer generates droplets with constant rate in a size range of 1e10 mm. A commercial type, Malvern Mastersizer 2000, laser particle sizer based on laser scattering technique is used to measure the droplets size. Fig. 2 shows the droplet size distribution produced by the ultrasonic method. In the experiment, air mixes with the produced droplets and enters into a primary separator. The primary separator used for gravitational sedimentation of large drops and consequently generates an outcome of more uniform size droplet especially at high flow rates. Then flow is conducted to the ESP. A piston type compressor is used which has the same power over the testing time and produces the suction. In order to control flow rate of the path, a bypass valve is positioned between the flow meter and the compressor units. It should be mentioned
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Fig. 1. Schematic diagram of the experimental setup for the ESP efficiency test.
that the frequency of ultrasonic droplet generator, the droplet mass fraction and air flow rate is identical for all measurements. This leads to the same droplets size and concentration in the suction method. It is assumed that the downstream condition does not significantly influence on the droplets size in the ESP. The sealing of the flow path is controlled by using a vacuum pump and a negative pressure indicator. The flow rate and temperature are measured with a hot plate mass flow meter. The flow meter accuracy for
Fig. 2. Droplet size distribution produced by the ultrasonic method.
flow rate and temperature measurement was ±3% and ±2 K, respectively.
2.2. Wet electrostatic precipitator Since most of industrial electrostatic precipitators are of the wire-plate type, the laboratory-scale of duct-type single-stage, single-wire precipitator was prepared to examine. Fig. 3(a) shows the side view of the mentioned wet ESP used in the experiments. Further, a schematic diagram of the ESP with the up view of the configuration is shown in Fig. 3(b). In this figure, L and Swp denote length of the collection plates and wire-to-plate spacing, respectively. As is seen in Fig. 3, the upward flow moves perpendicular to the corona wire. The ESP walls for attaching the plates are made of Plexiglas material with thickness of 15 mm. Further two Plexiglas transparent walls perpendicular to the plates are provided to allow optical observation of the spatial distribution of the corona. The ESP has internal dimensions of 160 mm height and 105 mm length. Two Aluminum plates with an area of 90 120 mm2 and 2 mm thickness were attached parallel to the Plexiglas walls, one on each side of the precipitator. One corona discharge wire made of copper was installed symmetrically midway between the Aluminum plates along the length of the precipitator. DC high voltage is applied between the corona wire and plates (negative and positive poles, respectively). The width of Aluminum plates has been chosen less than the width of Plexiglas plates. A gap between the wire and side plates (see Fig. 3(b)) has been provided both to prevent the electric
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405
Fig. 3. The used single-stage wire-plate ESP, (a) the side view, (b) schematic diagram of the up view of the configuration.
field breakdown and the applied voltage drop due to the water drops aggregation. In addition, in the experiment the water film formed on the plates is removed and dried. Since, thick water film leads to decrease the electric field strength due to water resistivity.
Uncertainty for the ESP efficiency in the weighing test can be described as:
2.3. Weighing method for measuring collection efficiency
where, dDmESP and dDmTc are the precision of weighing test for DmESP and DmTc. In addition, an estimate for total mass difference, DmTc, in Eq. (2) follows:
Similar weighing method of Ehteram et al. [29] is used to measure ESP collection efficiency. Accordingly, the ESP efficiency for droplets collection is defined as:
h¼
DmESP 100; DmT
DmT ¼ DmSource DmPsep
(1)
where,DmESP ; DmSource and DmPsep are the mass difference of the ESP, drops source (water vessel in Fig. 1) and primary separator, respectively. In the experiment, these differences were measured by weighing of the components before and after the test precisely with ±0.1 gr precision. The duration of each test was about 600 s. For each flow rate, the tests were repeated at least three times to ensure consistency of the data. It was found that the highest deviation in the data was about 3e5% for the ESP efficiency. However, averages of measurements are reported in this study. A typical example of measurement accuracy analysis is presented in Section 5.
Eu h ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vh vh dDmESP þ dDmTc DmESP DmTc
DmTc zm_ Droplet Time
(2)
(3)
In which m_ Droplet is droplets mass flow rate. Substituting Eq. (1) into Eq. (2) gives:
Eu h ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðdDmESP Þ2 þ ðh dDmTc Þ2 DmTc
(4)
Considering the ESP efficiency approximately 50% (i.e. h ¼ 0.5) and droplets mass flow rate 1 g per minute (i.e. m_ Droplet ¼ 1 gr/min), and taking ±0.1 gr precision for weighing the ESP, water vessel and primary separator (i.e. dDmESP and dDmTc ¼ ±0.1 gr), using Eqs. (3) and (4) the uncertainty of collection efficiency as a function of time is shown in Fig. 4. It is seen from Fig. 4, in 600 s test duration, this leads approximately to below 0.5% uncertainty in the measurement of ESP efficiency that is sufficient.
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velocity, VTE, is a key factor of collection efficiency, and it can be estimated as [31]:
VTE ¼
neECc 3pmd
(7)
In which n is total number of charges acquired by a particle with diameter d by different charging mechanisms, e is the charge on an electron, e ¼ 1.6 1019 C, E is the electric field strength, m is the dynamic viscosity of the gas and Cc is the Stokes-Cunningham slip correction factor which is calculated by[31]:
Cc ¼ 1 þ Knp 1:257 þ 0:4 exp 1:1 Knp
Fig. 4. Uncertainty of the measurements of ESP efficiency for different times.
3. Flow condition Table 1 indicates the dimensions, parameters and basic operating conditions of the ESP. The hydrodynamic entrance length inside ducts can be obtained from the Langhaar equation as follows [30]:
Lhd ¼ 0:058 Dh Reh
(5)
where Dh, is the hydraulic diameter of duct cross-section and Reh ¼ UDh/n is the Reynolds number based on average velocity, U, and Dh. It can be obtained; Lhd has the value of approximately 348 mm for 19 lt/min flow rate that is about three times larger than collection duct height (see Table 1). For higher flow rates the hydrodynamic entrance length increases. Therefore, flow is strongly under developing condition across the ESP collection duct for the all examined flow rates.
4. Semi-empirical model of droplet collection efficiency The collection efficiency of an ESP can be estimated from the Deutsch model as follows [31]:
hDe
VTE Ac ¼ 1 exp Q
(6)
where Q is the volume flow rate of carrier gas, Ac is the surface area of collecting electrodes and VTE is the mean migration velocity of the particle across the precipitator. This model is the most used model for predicting the ESP efficiency. The mean migration
Table 1 The dimensions and operating conditions for the present vertical ESP. Dimensions and operating conditions
Value
Diameter of discharge wire, Dw (mm) Wire-to-plate spacing, Swp (mm) Plate-to-flow inlet spacing, Spi (mm) Wire-to-flow inlet spacing, Swi (mm) Height of collection plate, H (mm) Length of collection plate, L (mm) Corona wire effective length (mm) Collection plate thickness (mm) Air flow rate, Q (lt/min) Nozzle diameter, dnozzle (mm) Inlet Reynolds number, Re Applied voltage on wire, V (kV)
0.5 22.5e27.5 30 60e120 120 90 90 2 19e33 10 865e1503 3.64e9.64
(8)
where p is the particle Knudsen number defined by Knp ¼ 2l=d, pKn ffiffiffiffiffiffiffiffiffiffiffiffiffiffi l ¼ m p=2RT =r is the molecular mean free path of the gas and R and T are universal constant and absolute temperature of gas. There are two mechanisms for aerosol charging in ESPs due to corona discharge. One is the diffusion charging results from the Brownian motion of the ions and particles and the other is the field charging results from presence of a strong electric field. These two mechanisms require the production of unipolar ions by corona discharge. Field charging is the dominant mechanism for particles larger than 1.0 mm [31]. Here, it is assumed the total number of charges acquired by a particle in ESP is the summation of number of charges due to diffusion charging and field charging. Concerning diffusion charging, an approximate expression for the number of charges nd(t) acquired by a particle with diameter d during charging time t is [31]:
nd t ¼
d pKE d ci e2 Ni t kT ln 1 þ 2 2kT 2KE e
(9)
where ci is the mean thermal speed of the ions (equal to 240 m/s at standard conditions), k ¼ 1:38 1023 ðJ=KÞ is the Boltzmann constant, KE ¼ 9.0 109 N m2/C2 is a constant and Ni is the ion number concentration usually 1013 per unit volume (m3). In addition, for field charging the number of charges, nf(t), acquired by a particle with diameter d during charging time t in an electric field E can be estimated from the following equation [31]:
nf t ¼
3ε εþ2
Ed2 pKE eZi Ni t 1 þ pKE eZi Ni t 4KE e
(10)
where ε is the relative permittivity of the particle or dielectric constant (¼ 80 for pure water droplet) and Zi is the mobility of the ions, approximately 0.00015 m2/V s. As is seen in Eq. (10), the greater the value of ε, the greater the number of charges. This is because greater number of field lines converges on the particle with greater dielectric constant. Charging time in Eqs. (9) and (10) has been estimated using the following equation which is a time constant for particle charging on the basis of Ohm's law [32]:
t¼
4ε0 Eav jav
(11)
where ε0 ¼ 8.854 1012 F/m is the vacuum permittivity and jav is the average electric current density at the collecting electrode. Finally n(t) in Eq. (7) is estimated as the summation of Eqs. (9) and (10). In case of water droplets as the particle, the inlet mass flow rate to the ESP is a summation of mass flow rate of droplets and water vapor. Nevertheless, the Deutsch efficiency Eq. (6) only considers the efficiency of ESP for droplets. In fact, the Deutsch model is the ratio of mass of separated droplets to mass of inlet droplets. Hence, this efficiency must be corrected to consider the effect of vapor
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407
Fig. 5. Collection efficiency characteristics of ESP as a function of specific collection area (SCA) at various applied voltages.
Fig. 7. Comparison of measured and estimated values of collection efficiency as a function of corona nominal power.
content in the flow and produce theoretical efficiencies comparable with our measurements. To consider this effect the ESP efficiency for combined droplets/vapor can be written as [29]:
by the parameters variation, throughout the results power trendline is used to fit the data accurately. Efficiency characteristics of the fabricated ESP as a function of specific collection area, SCA, at various applied voltages are illustrated in Fig. 5. The specific collection area defined as the ratio of the total collection area to the total gas volume flow rate. This important parameter characterizes the performance of ESP. In this study, the total collection area of the plates is fixed. Thus, the SCA value is changed by varying the flow velocity. Fig. 6 compares the measured collection efficiencies of ESP and Parker filter (P3K coalescing filter series) at different flow rates when applied voltage on ESP wire has the value of 9.64 kV. For this purpose Parker filter is positioned instead of the ESP in the setup and the same weighing technique is used for measuring its efficiency. As a typical example, the error bars for the collection efficiency is shown in Fig. 6. Parker filter is equipped to a 0.01 micron coalescer glass micro fiber element and has high filtration efficiency (99.97%) for liquid aerosols and sub micron (0.3e0.6 micron) particles. A good agreement between the efficiencies is noticed in Fig. 6. It is seen the collection efficiencies are less than 90% for both filters, so significant amount of vapor in air exists. Further, by increasing the flow rate the vapor amount and evaporation rate for drops becomes larger and consequently the collection efficiency of droplets for both filters decreases. Comparison of measured and estimated values of collection efficiency as a function of nominal power is illustrated in Fig. 7. The
:
hDrops=vapor ¼ ¼
mSeperated
:
min
:
Droplets
þ mVapor
hDe .: : 1 þ mVapor min
.: : mSeperated min Droplets .: ¼ : 1 þ mVapor min Droplets
Droplets
(12) :
:
:
In Eq. (12), mSeperated ,min Droplets and mVapor are, respectively, mass flow rates of separated droplets, inlet droplets and vapor content in the flow which are measured using weighing technique. In this study, it is assumed that the flow is fully saturated. Furthermore, the Sauter mean diameter of Fig. 2 (d32 ¼ 1.4 mm) is used as the droplet size in calculating theoretical efficiencies.
5. Results and discussion In this section, measurements are related to wire-to-plate spacing (Swp) of 27.5 mm and wire-to-flow inlet spacing (Swi) of 90 mm, except for when effects of Swp or Swi parameters are investigated. Further, to investigate the physical trend of efficiency
Fig. 6. Comparison of the measured collection efficiencies of ESP and Parker filter, repeatability of experiment for the ESP efficiency.
Fig. 8. Corona current as a function of applied voltage in wet ESP.
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Fig. 9. Effect of wire-to-plate spacing (Swp) on ESP efficiency as a function of (a) electric field strength, (b) average current density at the collecting plate, (c) average current density at the input area, (d) corona nominal power ratio, (e) corona consumed power ratio.
nominal power is defined as Pnom ¼ VI where I is the total corona current. In Fig. 7, hDrops=vapor estimated by the semi-empirical model in 19 lt/min are compared with those measured with weighing method. The minimum, maximum and average differences between the measured and estimated values are, respectively, 1.65%, 26.04% and 11.09%, so a good agreement between the results is found. It is seen form Fig. 7 that similar to the experimental findings, the semi-empirical prediction indicates the increasing of the corona power more than a critical value, in this case the critical value is about 30 W, can significantly affect the ESP separation. Furthermore, existing of the vapor content in the air can significantly drop the ESP operation. Fig. 8 shows comparison of corona current as a function of applied voltage for various flow rates operated in wet ESP. For the higher flow rates evaporation rate of drops becomes larger and
consequently higher moisture content in air is created. Therefore, the humidity near the collection plates increases that affects the operational indexes of ESP. It is seen from Fig. 8 that at the same voltage the corona current is decreased for higher flow rates. This result is compatible with experimental study of Lin et al. [21] for comparing the corona current between the dry and wet ESPs. However, they indicated at the same voltage the corona current is lower for wet ESPs due to the resistivity of the water film. As an outcome, the wet ESP collection efficiency is expected to be lower at higher flow rates at the same voltage due to lower corona current. In order to notice the effect of geometric parameters on the ESP efficiency, sets E-I are employed and the results are shown in Figs. 9 and 10. Here, the flow rate has value of 19 lit/min. The ESP efficiency is illustrated as a function of average electric field strength (Eav ¼ V/ Swp), average current density at the collecting wall area (jav),
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409
Fig. 10. Effect of wire-to-flow inlet spacing (Swi) on ESP collection efficiency as a function of (a) applied voltage, (b) average current density at the collecting plate, (c) corona nominal power ratio, (d) corona consumed power ratio.
average current density at the input area (Jav), corona nominal power ratio (Pnom/Q) and corona consumed power ratio (P/Q) in Fig. 9(a)e(e), respectively. It should be noted that the corona power ratio is related to the ESP operation cost [4]. Effect of wire-to-plate spacing (Swp) on ESP efficiency is illustrated in Fig. 9(a)e(e). It can be seen from Fig. 9(a) that the efficiency is increased as the wire-to-plate spacing increases under the same electric field strength. Therefore the wider wire-to-plate spacing seems to be more beneficial to enhance ESPs efficiency. This outcome is confirmed since wider ESP duct leads to lower gas shear stress and droplets velocity inside the ESP. However, it is seen higher electric field strength makes higher efficiency for ESP collection. It appears in Fig. 9(b) and (c) that when the average current density at the collecting plate and input have an intermediate value, the ESP with larger wire-to-plate spacing tends to perform better under the same average current density. This may be because the current and electric field distributions in narrow duct are more uneven than that in wide duct in this range of applied voltage. Further, a narrow duct leads to more unstable electrical operation. However, it is noticed with the increase of average current density at the collecting wall, the design of narrow duct has an inverse effect on the ESP efficiency and makes the ESP more beneficial. Therefore, there is certain value for jav for which the effect of wireto-plate spacing on ESP efficiency is reversed (alteration point). According to Fig. 9(c), it is predicted that such a point also exists for Jav, since the electric field strength in narrow duct is higher than that of ESP with wide duct. These results are compatible with theoretical study of Yang et al. [28] for intermediate values of average current density at the collecting wall. However, they
investigated very smaller average current densities than present study. Fig. 9(d) and (e) indicates the similar trend with Fig. 9(b) and (c), for corona power ratio instead of average current density and both of the results along with Fig. 9(a) show complex relationship of the ESP efficiency versus the wire-to-plate spacing. This may be due to change of wire-to-plate spacing influences average electric field strength, droplets inlet velocity to the collection duct and developing flow condition simultaneously. Smaller wire-to-plate spacing makes higher electric field strength along with higher droplets inlet velocity and more uneven electric field. These factors have adverse effects on ESP efficiency. Further, wire-to-plate spacing influences hydraulic diameter of duct and consequently hydrodynamic entrance length in developing flow. Therefore, the thickness of accelerated region inside the ESP collection duct is also affected by wire-to-plate spacing size. Effect of wire-to-flow inlet spacing (Swi) on ESP collection efficiency is indicated in Fig. 10(a)e(d), where the efficiency is shown as a function of wire-to-flow inlet spacing in various applied voltages, average current density at the collecting wall in various Swi, corona nominal power ratio in different Swi and corona consumed power ratio in various Swi, respectively. It should be mentioned, the corona power was positioned at ¼, ½ and ¾ of total height of collection duct (H ¼ 120 mm, see Table 1) for measurements (corresponding with Swi ¼ 60, 90, 120 mm, respectively). It is noticed from Fig. 10(a) that there exists an optimum wire-to-flow inlet spacing which provides maximum droplet collection efficiency. When the corona wire moves from the bottom toward top of collection duct, larger area of plates is interacted with corona wire and consequently stronger and wider electric field inside the duct is
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appointed. Such condition exists before the wire approximately reaches the middle of duct and ahead of this point an inverse condition occurs. Further, due to the developing flow condition, droplets have accelerated inside the core and their velocity increase across the collection duct. Therefore, when the wire is positioned upper to the initial half of the duct, higher collection efficiency is obtained. In the secondary half of the duct, droplets have larger velocity and further upper wire position leads to weaker electric field strength that cause the collection efficiency sharply decreases. Therefore existing of the peak point and the minimum efficiency for highest wire position in Fig. 10(a) are reasonable. For higher flow rates, the thickness of accelerated region becomes larger and consequently the maximum efficiency can occur in lower heights. Moreover, the slope of efficiency decreasing with the increase of wire height is larger in the secondary half of collection duct. The existence of optimum wire-to-flow inlet spacing also appears in Fig. 10(b)e(d). In addition, it is seen from these figures that there are the ranges for average current density at the collecting wall and corona power ratio for which the collection efficiency of wire positions of ¼ H and ¾ H are very close. This is may be due to that in parameters jav and P/Q both electric and hydrodynamic indexes are considered simultaneously.
Nomenclature :
Acknowledgments
mVapor mass flow rates of vapor content, kg/s Ci mean thermal speed of ions, m/s : mDroplets droplets mass flow rate, kg/s : min Droplets mass flow rates of inlet droplets, kg/s : mSeperated mass flow rates of separated droplets, kg/s Ac surface area of collecting electrodes, m2 Cc Stokes-Cunningham slip correction factor d particle diameter, m d32 Sauter mean diameter, m Dh hydraulic diameter, m dnozzle nozzle diameter, m Dw diameter of discharge wire, m dDmESP precision of weighing test for ESP, kg dDmESP precision of weighing test for total mass difference, kg e charge on an electron, ¼ 1.6 1019 C E electric field strength, V/m Eu uncertainty of measured efficiency H height of collection plate, m I corona current, A jav average electric current density at the collecting electrode, A/m2 Jav average electric current density at input area, A/m2 KE constant, ¼ 9.0 109 N m2/C2 Knp particle Knudsen number L length of collection plates, m Lhd hydrodynamic entrance length, m n total number of charges acquired by a particle nd number of charges acquired by diffusion charging nf number of charges acquired by field charging Ni ion number concentration, ¼ 1013/m3 P consumed corona power, W Pnom nominal corona power, W Q volumetric flow rate, m3/s R universal constant of gas, kJ/kg K Re inlet Reynolds number Reh Reynolds number based on hydraulic diameter SCA specific collection area, s/m Spi plate-to-flow inlet spacing, m Swi wire-to-flow inlet spacing, m Swp wire-to-plate spacing, m T absolute temperature of gas, K t time, s U average velocity, m/s V applied voltage on wire, kV VTE mean migration velocity of particle across precipitator, m/s Zi mobility of ions, m2/V s DmESP mass difference of ESP before and after test, kg DmPsep mass difference of primary separator before and after test, kg DmSource mass difference of drops source before and after test, kg DmT measured total mass difference, kg DmTc estimated total mass difference, kg ε relative permittivity of particle ε0 vacuum permittivity, ¼ 8.854 1012 F/m h ESP collection efficiency k Boltzmann constant, ¼ 1.38 1023 J/K l molecular mean free path of gas, m m dynamic viscosity, Pa s r gas density, kg/m3
The authors would like to appreciate many undergraduate students and specially Mr. Mojtaba Mohammadi who obtained data from the experiments.
Subscripts av average De Deutsch model
6. Conclusions A laboratory-scale single-stage, single-wire wet ESP was designed, built and operated in airewater droplets flow to measure collection efficiency using weighing technique. A short vertical collection duct was provided so that upward flow was under developing condition across the ESP. In the experiment, fine water droplets in a size range of 1e10 mm were produced using ultrasonic atomizer. The ESP collection efficiency was compared in similar conditions with a glass micro fiber filter, which has very high filtration efficiency (99.97%) for liquid sub micron aerosols, and a good agreement was noticed. In addition, comparison between the measured efficiencies and a semi-empirical model was provided and a good agreement was found. Effects of some geometric parameters on the efficiency of ESPs using four electric indexes were evaluated. It was observed while the effect of vapor content in the flow was considered via weighing technique; a good accuracy with an average error of 10% for semi-empirical Deutsch model was obtained. Further, at the same applied voltage, the corona current was decreased for high flow rates due to drops evaporation. Existing of the vapor content in the air and evaporation of droplets significantly affects the ESP operation via changing different factors such as corona current and droplets size. The experimental results indicated that the trends of the collection efficiency with the change of wire-to-plate spacing are considerably complex due to simultaneously developing flow condition and electrical operation. Furthermore, there is an alteration point for parameters of average current density and corona power ratio at which effect of wire-toplate spacing on ESP efficiency is reversed. Based on the findings, when developing flow is created inside the ESP, an optimum wireto-flow inlet spacing provides maximum droplet collection efficiency. In addition, lower wire position tends to obtain better collection efficiency than upper positions.
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