CHAPTER 1 INTRODUCTION
1.3 Content of this thesis
In chapter 2, previous experimental studies on wet ESPs for particle control are reviewed.
The numerical models for predicting particle collection efficiency based on the Lagrangian and the Eulerian methods in ESPs are also discussed.
In chapter 3, the method to increase the hydrophilicity of the collection plates is introduced. Then the experimental methods for particle collection efficiency in the present wet ESP under initial clean and heavy loading conditions are presented. Next is the description of numerical methods for predicting the flow field, the electric field strength, and the ion concentration distribution. Charging models for predicting particle charges in the
transition and continuum regimes are described. Finally, the methods to solve charged particle concentration distribution and particle trajectory are presented.
In chapter 4, particle collection efficiency in the present wet ESP under initial clean and different operation conditions are discussed. Then the heavy particle loading effect on particle collection efficiency in the present wet ESP is compared with that of the dry ESP experimentally. Subsequently, the numerical results based on Lagrangian and Eulerian methods are compared with the experimental data published in the previous researches (Chang and Bai 1999; Huang and Chen 2002) and obtained for the present wet ESP. Finally, the numerical results for the parametric study of a pilot scale wet ESP are presented. In chapter 5, conclusions of this thesis are drawn and future work is recommended.
CHAPTER 2
LITERATURE REVIEW
2.1 Experimental studies on wet ESPs for particle control
Wet ESPs have been 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 (Altman et al. 2001a; Altman et al. 2001b). The periodic or continuous scrubbing water flow, used to wash the collection electrode surfaces, was found to prevent particle re-entrainment caused by rapping, which occurs in dry ESPs (USEPA 2003).
The traditional methods to distribute water uniformly on the collection plate surfaces include vapor condensation or spraying (Altman et al., 2001b; Bayless et al., 2004). In the vapor condensation method, moisture in the gas condenses to form water film on the collection plate surfaces due to the temperature differential between the cold surface and the hot gas. In the spraying method, water droplet is emitted by nozzles at the top or bottom of ESPs to wash the collection plates. However, aqueous mist used in these two methods will result in sparkover to reduce electric field strength and particle collection efficiency (Bayless et al. 2004).
Lungren et al. (1995) invented a wire-in-tube wet ESP in which the charging electrode consisted of a tapered section, a rod and four discs to enhance particles charging efficiency.
Scrubbing water was first introduced onto a diamond-shaped plate, which was used to equalize the water flow, and then flowed into the inner surface of the collection electrode to remove the collected particles. They found that the opacity of the flue gas was reduced from 35 to 0 % in the pilot unit when the applied voltage was increased from 30 to 75 kV. However, the collection efficiency for different particle diameters and particle loadings was not discussed. Similar types of wire-in-tube wet ESPs were used to control the emissions of incinerator flue gas (Kim et al. 2000) and diesel particulate matter (Saiyasitpanich et al. 2006;
10-1000 nm diesel particles was measured to be from 92 to 69 % when the gas velocity was from 1.38 to and 5.61 m/s (or the corresponding residence time from 0.4 to 0.1 sec) (Saiyasitpanich et al. 2007), respectively.
However, the collection efficiency under heavy particle loading conditions for long time periods was not studied. Moreover, the previous study used a high flow rate of scrubbing water (4.44-6.22 L/min/m2 of collection area), which resulted in a thick water film, thus decreasing the electric field strength due to water’s resistivity.
To improve the uniformity of the liquid film over the collection electrodes without having to supply the scrubbing water at high flow rate, Pasic et al. (2001) invented a membrane electrostatic precipitator which used hydrophilic membranes made of corrosion resistant woven fibers as the collection electrodes. The tensile apparatuses were needed to stretch the membranes during operation of the wet ESP to prevent channeling effect due to uneven surfaces. In the bench scale test, Bayless et al. (2004) compared the membrane-based and steel plate wet ESPs for the collection efficiency of fly ash, iron oxide and calcium carbonate having particle sizes of 20-25, 1-5 and 2-3 μm in diameter, respectively. The collection efficiency of the membrane-based wet ESP was found to be higher than the steel plate wet ESP. In the pilot scale test, the membrane wet ESP was used to remove iron-oxide particles with the peak gas velocity of 4 m/s or the corresponding residence time of 1.9 sec.
The collection efficiency of particle mass was approximately 96 % for particles from 1 to 5 μm in diameter. However, the collection efficiency of submicron particles under heavy particle loading conditions was not discussed. In addition, membranes may be expensive and not all have uniform wetting and corrosion-resistant properties, which must be carefully determined for optimal use in specific field applications (Bayless et al. 2004).
2.2 Numerical model for predicting collection efficiency of ESPs
In ESPs, particles are collected by electrostatic forces, and thus the electric field strength and particle charge are very important factors influencing the collection efficiency of ESPs.
The conventional theory used to predict particle collection efficiency is based on the Deutsch-Anderson equation, which is shown as follows:
)
where VTE is particle migration velocity (m/s), A is the total collection area (m2), Q is the air flow rate (m/s), q is the number of elementary units of charge, e is the elementary electrical charge (1.6×10-19 C), Eave is the average electric field strength (V/m), Cc is the Cunningham slip correction factor, μair is the viscosity (kg/m-s), and dp is the particle diameter (m). In order to predict collection efficiency more accurately, Matt and Ohnfeldt (1964) and White (1982) presented a modified Deutsch-Anderson equation as follows:
c c
where VTE,k is the average migration velocity of particles (m), kc is a constant (0.4~0.6). The above semi-empirical equations can not predict collection efficiency precisely because the following assumptions were made: (1) Particles are assumed to reach their saturation charge during the collection process. (2) Particles migration velocity is not affected by air velocity and nonuniform distribution of the electric field. (3) The effects of particle reentrainment,
uneven gas flow distribution, and back corona on collection efficiency are not taken into consideration.
Many researchers developed numerical models based on charging model for continuum regime (Kn=2λi/dp1, λi : mean free path of ions, dp: particle diameter) to predict collection efficiency of the wire-in-plate dry ESPs for particles ranging from 0.3-10 μm in diameter.
Good agreement between experimental data and numerical results was obtained (Goo and Lee 1997; Park and Kim 2000, 2003; Park and Chun 2002; Talaie 2001, 2005). For example, a numerical model based on Lagrangian method with turbulent electrohydrodynamic (EHD) flow field was developed to predict particle collection efficiency and further validated with experimental data of Kihm (1987) (Goo and Lee 1997). The particle charging rate was calculated by using combined charging model of Fuchs model (1947) for diffusion charging and Pauthenier and Moreau-Hanot’s model (1932) for field charging (The combined charging model shown in section 3.2.4 was also used in the present study to predict the charge for particles with dp≧100 nm). The calculated results were found to be lower than the experimental data at the applied voltage of 6.5~13 kV for 4 μm particles. The deviation between the calculated values and the experimental data is probably due to the difficulty in estimating exact charging properties of particles, the inlet conditions of flow and particles, and the experimental error.
The effects of the EHD flow and turbulent diffusion on the collection efficiency of particles in a wire-cavity plate dry ESP was further studied by using a numerical model based on Eulerian method (Park and Kim, 2000; 2003). Particle charges were obtained by solving White’s equation (1967), which is given by
ave p d d
whitee d E
q 0 2
1
3
(2.4)
where κd is the dielectric constant of particles, ε0 is the permittivity of free air (F/m). Good agreement between experimental and computational data of collection efficiency was obtained for 4.5 μm Al2O3 particles when the applied voltage and air velocity were 7.5~15 kV and 0.5~1 m/s, respectively.
Park and Chun (2002) developed a numerical model to investigate the effect of turbulent dispersion on particle collection efficiency without considering the nonuniform distribution of electric field and ion concentration in a wire-in-plate dry ESP. The eddy diffusivity of a particle is given as follows:
h
Deddy p y , 0≦y≦h (2.5)
p
Deddy , h≦y≦sy (2.6)
h
p u 4 *
.
0
(2.7)
sy
h0.1 (Oron et al., 1988) (2.8)
where ξp is the turbulent dispersion coefficient within the turbulent core of the flow (m2/s), h is the distance from the wall to the interface between the region of a linearly increasing dispersion coefficient and that of a constant dispersion coefficient (m), sy is the wire to plate spacing (m), u* is the velocity distribution of a fully developed turbulent flow (m/s), which is defined as
u* 8f u 2 (2.9)
where uave is the average air velocity (m/s), f r is the friction factor, which is given by
f 0.0791Re0.25 (2800<Re<105) (2.10)
where Re is the Reynolds number based on the hydraulic diameter as follows:
calculating turbulent dispersion coefficient of particles was adopted in the present parametric study (in turbulence flow case). To calculate particle charges, Cochet equation (1961) was used, which is given by
ave
The predicted collection efficiency was validated with published experimental data of Riehle and Löffler (1993) and good agreement was found for limestone particles in the size range of 0.3~10 μm at an applied voltage of 25 kV, a mean air velocity of 1 m/s, and a turbulent dispersion coefficient of 50 cm2/s. The collection efficiency was found to increase with increasing turbulent dispersion coefficient for particles below 1 μm. For particles larger than 1μm, the increase of turbulent dispersion coefficient led to a decrease of collection efficiency due to the effect of dispersing the particles back to the flow. The author also noted that a complete model should necessarily include a realistic particle charging model, an electrostatic
field strength distribution, and a more realistic flow field.
Except for the effect of EHD and the turbulent dispersion coefficient, the effect of polydisperse particle loading on the particle collection efficiency was also taken into consideration in Talaie et al. (2001), who developed a mathematical model based on Eulerian method to predict the collection efficiency of a double-stage wire-in-plate dry ESP. The particle charging rate was calculated by using White’s equation (1967). The numerical values were in good agreement with the experimental data of Leonard (1982) for 3.5 μm oleic acid drops.
As discussed above, the numerical models for predicting collection efficiency for particles in the size range of 0.3-10 μm in wire-in-plate ESPs have been well-developed. For particles with 100≦dp≦200 nm, the numerical model of Yoo et al. (1997) based on Fuchs diffusion charging model (1947) and the field charging model of Pauthenier and Moreau-Hanot (1932) was able to predict the particle collection efficiency accurately.
However, the combined charging model used by Yoo et al. (1997) was found to over-predict particle charge for particles with dp≦100 nm in Lawless (1996), who concluded that 100 nm was the limit of applicability of the combined charging model.
In the transition regime (Kn≈1), Fuchs model (1963), which was used by Zhuang et al.
(2000) and Li and Christofides (2006) to predict particle charge in ESPs, was shown to be accurate for particles with dp>30 nm (Adachi et al. 1985; Pui et al. 1988). However, the numerical model could not predict the experimental collection efficiency accurately for 30<dp<400 nm because the flow field and the non-uniform ion concentration distribution were not calculated in Zhuang et al. (2000). In the work of Li and Christofides (2006), non-uniform electric field and ion concentration were not considered either and simulated particle collection efficiencies were not compared with experimental data. Therefore, the applicability of the previous models (Yoo et al. 1997; Zhaung et al. 2000; Li and Christofides 2006) for
For particles with dp<30 nm, experimental data (Huang and Chen 2002) and numerical results (Yoo et al. 1997; Zhaung et al. 2000; Li and Christofides 2006) showed that a fraction of particles was uncharged and penetrated through ESPs, resulting in decreasing collection efficiency as dp was decreased from 30 nm to 5 nm. This is called the partial charging effect.
Marlow and Brock’s model (1975) was shown to provide accurate prediction of particle charge for dp<30 nm (Pui et al. 1975). However, Marlow and Brock’s model has not been applied to examine the partial charging effect on the collection efficiency of the ESP. The combined charging model used in Yoo et al. (1997) over-predicted particle charge in the transition regime, as compared to the experimental data of Fjeld and MacFarland (1986), leading to an overestimation of collection efficiency for particles below 30 nm. The Fuchs model (1963) used in Zhuang et al. (2000) and Li and Christofides (2006) also over-predicted particle charge for dp<30 nm, which also led to an overestimation of the collection efficiency.
In the traditional dry ESPs, particle collection efficiency, especially for nanoparticle, decreases with increasing operation time due to particle deposition on discharge electrodes and collection electrodes (Huang and Chen 2003), back corona (Chang and Bai 1999), and particle re-entrainment (USEPA 2003). In order to solve typical problems associated with dry ESPs, wet ESPs were developed to control fine and nanoparticle effectively (Saiyasitpanich et al. 2006; Lin et al. 2010). Saiyasitpanich et al. (2006) compared measured and calculated collection efficiency by using Deutsch equation in a wire-in-tube wet ESP for particles in the size range of 20~700nm. The particle charging was calculated by solving Cochet equation (1961) and Robinson equation, which is given by
Robinson ln 1 2
2
is the ion number concentration (m-3), kb is the Boltzmann’s constant (N·m/K), and T is the temperature (K). The comparison revealed that the predicted values based on Deutsch equation underestimated the measured values.
Huang and Chen (2002) investigated aerosol penetration for particles in the size range of 7-100 nm by using a single-stage and a two-stage wire-in-plate dry ESP. A significant increase aerosol penetration was found for particles below 20 and 50 nm in the single- and two-stage dry ESP, respectively. Their experimental data could serve as a good benchmark for validating the simulation models for the nanoparticle collection efficiency in ESPs.
The above literature reviews for previous experimental and numerical studies were arranged in Table 2.1 and 2.2, respectively. In summary, according to the above discussions, an efficient wet ESP should be further designed and developed for controlling fine and nanosized particles. The optimal operation conditions for collecting nanoparticle in wet ESPs, and the collection efficiency of submicron particles under heavy particle loading conditions are needed to be investigated.
For predicting nanoparticle collection efficiency in ESPs by using numerical methods, the existing models can’t predict collection efficiency very well because the electric field and ion concentration distribution were not simulated, or charging models were not adopted appropriately to calculate particle charges. Thus, a numerical model using appropriate charging model to calculate particle charges requires to be developed for predicting nanoparticle collection efficiency accurately in ESPs.
Table 2.1 Literature reviews in experimental studies of wet ESPs.
Investigator ESP type Particle residence
Table 2.2 Literature reviews in numerical studies of particle collection efficiencies in wire-in-plate ESPs.
Researcher Numerical method
The calculated results were found to be lower than the
experimental data.
Good agreement between experimental and computational data of collection efficiency was
obtained.
Park and Chun Eulerian method
Cochet
equation 0.3-10μm
The calculated collection efficiencies match with the
experimental data.
The numerical values were in good agreement with the
experimental data.
Talaie (2005) Lagrangian method
The secondary flow and the high particle loading result in a decrease of collection efficiency.
Chang and Bai
The simulated values match with the experimental data.
Yoo et al.
The combined charging model is only valid for particles with dp
≧100 nm Lawless (1996).
Zhuang et al.
Ion concentration distribution was not simulated. Simulated collection efficiency didn’t match with experimental data.
Li and
Electric field and ion concentration distribution were
not simulated. Simulated collection efficiency wasn’t compared with experimental
data.
The comparison revealed that the predicted values based on
Deutsch equation underestimated the measured
values.
CHAPTER 3
METHODS 3.1 Experimental method
3.1.1 The present parallel-plate wet ESP
The collection electrodes of the present parallel plate wet ESP were designed based the parallel-plate wet denuder in Tsai et al. (2008). As shown in Figure 3.1, the wet ESP consists of two plexiglass plates (M) on which a copper plate (G) (100 mm in length, 75 mm in width and 3.0 mm in thickness) was attached to the inner surface. A copper plate was used as collection electrode because of its high conductivity and ease with sandblasting. In order to make scrubbing water film flow uniformly on the collection electrodes (G) at low flow rates, a frosted glass plate (FG, 70 mm in length, 75 mm in width and 3.0 mm in thickness) coated with TiO2 nanopowder (Degussa AEROXITE TiO2 P25, Anatase, 20 nm) is attached to the inner surface (M) above the copper plate. Between the collection electrodes, a center piece (C) is sandwiched to form a 9 mm gap between the electrodes. On the center piece, three gold discharge wires (GW) (99% purity, 100 μm in diameter, Surepure Chemetals Inc.) spaced at 16 mm in the flow direction are fixed. These gold wires were used as discharge wires due to their long lifetime of more than 6 months (Asbach 2004). Two overflowing (OR) and collection reservoirs (CR) for continuous scrubbing water flow are installed at the top and bottom of the ESP, respectively. A pulse jet valve (Bag Filter Enterprise Co. Ltd., Taiwan) was used to generate pulse jet passing through 3 rows of small holes (diameter: 1.0 mm, 24 holes on each row) on the collection plates. The discharge wires were cleaned every 5 minutes with an instantaneous pressure of 2.95 atm (3.05 kg/cm2) and the instantaneous air flow rate passing through all 24 holes per wire was calculated to be 11 L/s. Pulse duration was about 0.5 sec. During pulse jet cleaning, the scrubbing water flow was stopped for two minutes to prevent sparking over due to the water mist generated by the pulse jet.
LI LI
Figure 3.1 Schematic diagram of the parallel-plate wet ESP. Plexiglass plates (M), enter piece (C), frosted glass plate (FG), sand-blasted copper plate (G), overflowing reservoir (OR), collecting reservoir (CR), golden wire (GW), liquid inlet (LI), liquid outlet (LO), aerosol inlet (AI), aerosol outlet (AO), pulse jet valve (PJ), air hole (AH).
In order to increase the hydrophilicity of the collection plates, which will enhance the uniformity of the water film and reduce the scrubbing water flow rate, the copper plate surfaces were first sand blasted to an average depth of 61.0 μm and then coated with TiO2
nanopowder (Degussa P25) based on the method described in Tsai et al. (2008). The surfaces were first sonicated with DI water (18.2 MΩ-cm) and then dried by purging with compressed air. 0.5 g of TiO2 nanopowder was well dispersed in 50 ml DI water by ultrasonication and then applied onto the sand blasted surfaces. 30 min later, the excess solution was removed, and the plates were calcinated at 300 °C for 90 min to facilitate thermal bonding of the TiO2
nanopowder onto the copper plate surfaces.
Tap water was used as the scrubbing liquid for the collection electrodes. Two peristaltic pumps (Model MP-1000, Eyela Co., Japan) were used to continuously pump water into and out of the overflow and collection reservoirs. The appropriate scrubbing water flow rate, important to water film thickness and uniformity (Tsai et al. 2008), was determined in this study. The goal was to minimize the flow rate in order to decrease the water waste and film thickness (which decreases the electric field due to water’s resistivity), while also ensuring the film is uniform.
To determine the hydrophilicity of the coated surfaces, the morphology of the TiO2
nanopowder coated on the frosted copper plate was first determined by SEM, and then the water contact angle was measured by using a Contact Angle System (FTA125, First Ten Angstroms, VA). As shown in Figure 3.2, there is a porous surface in our case which is not the same as the water-repellent leaves with epicuticular wax crystals in combination with papillose epidermal observed by Barthlott and Neinhuis (1997). When the water contact angle was measured, we found the frosted surface coated with TiO2 nanopowder sucked in the water droplet forcing the droplet to spread out quickly. The contact angle was measured to be only 6.0±4.2°, which is very small compared to 104.03±1.73° and 117.17±2.83° for the smooth copper plates and uncoated sand-blasted copper plates, respectively, as shown in Figure 3.3. It
is certain that hydrophilicity can be achieved by this porous morphology to enhance the uniformity of the scrubbing water film.
Particle collection efficiency experiments were conducted at different aerosol flow rates
Particle collection efficiency experiments were conducted at different aerosol flow rates