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原文

Experimental study of electrostatic precipitator

performance and comparison with existing

theoretical prediction models

S.H. Kim, K.W. Lee*

Kwangju Institute of Science and Technology, Department of Environmental Science and Engineering,

1 Oryong-dong, Puk-gu, Kwangju 500-712, South Korea

Received 1 February 1999; received in revised form 21 May 1999; accepted 2 June 1999

Abstract

A laboratory-scale single-stage electrostatic precipitator (ESP) was designed, built and

operated in a wind tunnel. As a "rst step, a series of experiments were conducted to seek the

operating conditions for increasing the particle collection e$ciency by varying basic operating

parameters including the wire-to-plate spacing, the wire radius, the air velocity, the turbulence

intensity and the applied voltage. As the diameter of the discharging wires and the wire-toplate

spacing are set smaller, the higher collection e$ciency has been obtained. In the

single-stage multiwire ESP, there exists an optimum wire-to-wire spacing which provides

maximum particle collection e$ciency. As the air velocity increases, the particle collection

e$ciency decreases. The turbulent #ow is found to play an important role in the relatively low

electric "eld region. In the high electric "eld region, however, particles can be deposited on the

collection plates readily regardless of the turbulence intensity. The experimental results were

compared with existing theories and Zhibin and Guoquan (Aerosol Sci. Technol. 20 (1994)

169}176) was identi"ed to be the best model for predicting the ESP performance. As the second

step, the in#uence of particle contamination at the discharging electrode and at the collection

plates were experimentally measured. The methods were sought for keeping the high collection

e$ciency of ESP over elapsed time by varying the magnitude of rapping acceleration, the time

interval between raps, the types of rapping system (hammer/vibrator) and the particle reentrainment.

The rapping e$ciency and the particle re-entrainment were increased with

increasing magnitude of rapping acceleration and time interval between raps. However, when

the thickness of deposited #y ash layer is su$ciently high, the concentration of re-entrained

particles starts decreasing abruptly due to the agglomeration force which can interact among

0304-3886/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 3 8 8 6 ( 9 9 ) 0 0 0 4 4 - 3

deposited particles. The combined rapping system is found more e!ective for removing

deposited particles than the hammer rapping system only. ( 1999 Elsevier Science B.V. All

rights reserved.

Keywords: Electrostatic precipitation; Turbulent #ow; Rapping; Particle re-entrainment; Collection e$-

ciency; Negative corona

1. Introduction

Electrostatic precipitators (ESPs) are one of the most commonly employed

particulate control devices for collecting #y ash emissions from boilers, incinerators

and from many other industrial processes. They can operate in a wide range of

gas temperatures achieving high particle collection e$ciency compared with mechanical

devices such as cyclones and bag "lters. The electrostatic precipitation process

involves several complicated and interrelated physical mechanisms: creation

of a non-uniform electric "eld and ionic current in a corona discharge, ionic

and electronic charging of particles moving in combined electro- and hydrodynamic

"elds, and turbulent transport of charged particles to a collection

surface.

Generally, the collection e$ciency of ESP decreases as the discharging electrode

and collection plates are contaminated with particulates. Thus, a rapping system is

needed for removing the collected particulates periodically. While there have been

numerous theoretical and experimental studies on particle collection characteristics of

electrostatic precipitators, a relatively small number of the studies addressed the

e!ects of particle accumulation both at the discharging electrodes and at the collection

plates. Both phenomena are known to in#uence adversely the performance of

electrostatic precipitators. Many researchers, such as Deutsch [1], Cooperman [2],

Leonard et al. [3], Khim et al. [4], Zhibin and Guoquan [5], and Kallio and Stock

[6], conducted particle collection measurements of ESP. However, they concentrated

mostly on the e!ects of both turbulent mixing and secondary wind in multiwire

single-stage electrostatic precipitators. Speci"cally, Cooperman [2] considered reentrainment

and longitudinal turbulent mixing e!ects, Leonard et al. [3] the "nite

di!usivity, and Zhibin and Guoquan [7] the non-uniform air velocity pro"le. Among

them, only Zhibin and Guoquan [7] measured the collection e$ciency of a singlestage

ESP covering a wide particle size range. Even though their experimental data

are considered to be practical and useful, their experimental conditions were not

identi"ed clearly.

In the present study, well-de"ned collection e$ciency data for an ESP are presented

covering the particle size range of 0.1}100 lm. The particles used in the present study

came from the Bo-Ryung power plant in Korea. In addition, the ESP performance

was evaluated in terms of optimum operating conditions. Finally, the optimum

rapping conditions were sought under which the rapping e$ciency increases and the

particle re-entrainment decreases.

4 S.H. Kim, K.W. Lee / Journal of Electrostatics 48 (1999) 3}25

Fig. 1. Schematic diagram of the wind tunnel for the eight wired single-stage ESP performance test.

2. Review of theoretical models

2.1. Particle charging

Fig. 1 shows the laboratory-scale electrostatic precipitator. The particle charging

system consists of discharge wires with diameter (D8) and two grounded parallel

plates of length (¸). A high negative voltage (<8) is applied to the corona discharge

wires, and suspended particles of diameter (d1) #ow with air between the plates at

a velocity (;) in the y-direction. In the whole range of particle sizes, both "eld

charging and di!usion charging mechanisms contribute to signi"cant charges [8,9].

In these theoretical analyses, it is nearly correct to sum the rates of charging from the

two mechanisms and then solve for the particle charging as follows:

dq1

dt

"q4

q A1!q

q4B2#d21

eN

0

4 S8k¹p

m

expA! 2qe

d1k¹B (1)

where q1 is the particle charge, q4 is the saturation charge,N

0 is the average number of

molecules per unit volume, e is the electronic charge ("1.6]10~19 C), b is the ion

mobility ("1.4]10~4 m2/V s), e0 is the permittivity of free space ("8.85]

10~12 F/m), d1 is the diameter of particle, k is the Boltzmann constant ("1.38]

10~23 J/K), ¹ is the absolute temperature ("293 K), m is the mass of a particle

("(p/6)d31

o1), and o1 is the particle density ("2.25]103 kg/m3).

2.2. Theoretical models of particle collection ezciency

Theoretical models of ESPs were provided by Deutsch [1], Cooperman [2],

Leonard et al. [3], Zhibin and Guoquan [7] and others. The Deutsch model for

S.H. Kim, K.W. Lee / Journal of Electrostatics 48 (1999) 3}25 5

calculating the particle collection in an ESP assumes complete mixing by turbulent

#ow and thereby uniform concentration pro"les. In order to improve the drastic

assumption of in"nite di!usivity in the Deutsch model, many researchers tried to

develop "nite di!usivity models by dealing with the convective-di!usion equation

with various boundary conditions.

Cooperman [2] developed a theory which modi"es the Deutsch model to account

for the e!ects of turbulence and particle turbulent di!usion. The major limitations of

the Cooperman model lie absence of a general method to estimate the re-entrainment

factor and the particle di!usivity. Leonard et al. [3] developed a more complicated

two-dimensional model using the method of the separation of variables from the

convective-di!usion equation. He assumed uniformity of velocity components of

charged particles and particle di!usivity. This assumption fails to adequately describe

the particle di!usivity near the collection plates, where it is governed mainly by the

molecular transport and, therefore, the di!usivity near the wall is signi"cantly lower

than the di!usivity in the turbulent core. Zhibin and Guoquan [7] suggested a new

model for the single-stage ESP which takes into account the e!ect of turbulence

mixing by electric wind. Predicted collection e$ciencies of the above theoretical

models are summarized as follows:

gDe"1!exp(!De), (2)

gCoo"1!expC;¸

2D

!SG A;¸

2DB2#(1!R)PeA¸

=B2HD, (3)

gLeo"1!P1

0

PA m!De

J2De/PeBdm, (4)

gZhi"1!S Pe

4pDeP1

0

expC!Pe

4De

(m!De)2Ddm, (5)

where <t is the migration velocity ("q1EC#/3pkd1), C# is the slip correction factor

("1#(2/Pd1)[6.32#2.01 exp(!0.1095Pd1)]), P is the absolute pressure

("76 cm Hg), E is the electric "eld intensity ("<8/=),= is the width of wire-toplate,

De is the Deutsch number ("<t¸/;=), Pe is the electric Peclet number

("<t=/D1), D1 is the particle di!usivity, and P(z) in Eq. (4) is the Gaussian probability

distribution function given by

P(z)" 1

J2pPz

~=

expA!B2

2 BdB. (6)

In order to evaluate the particle di!usivity for the calculation of De and Pe, the #ow

is assumed to be a fully developed turbulent channel #ow. The related physical

quantities are speci"ed like below [10]

1

f 1@2

"!1.8 log10A6.9

ReB, ;q

"Sf;2

8

,

D5"0.12;q=, DB"k¹C#

3pkd1

, D1"D5#DB (7)

6 S.H. Kim, K.W. Lee / Journal of Electrostatics 48 (1999) 3}25

Fig. 2. Comparison of measured fractional number of particles with existing theoretical predictions.

Experimental conditions: D8"1 mm, <8"50 kV, Sx"150 mm, Sy"37.5 mm, ;"1 m/s, ¹6"12%.

where f is the friction factor, Re is the Reynolds number ("2;=/v), ;q is the friction

velocity, D5 is the turbulent di!usivity, and D

B is the Brownian di!usivity.

With the measured data of fractional number of particles at the inlet of the

single-stage ESP, measured fractional number of particles at the outlet of the singlestage

ESP was compared with calculated results of each theoretical prediction model

as shown in Fig. 2. The grade e$ciency is computed over the particle size range

0.1}100 lm, and then integrated the grade e$ciency to obtain the overall mass

e$ciency, where the particle size distribution function is assumed to be lognormal.

The size distribution of most polydisperse aerosols is found very close to the lognormal

distribution. Thus, this assumption is quite reasonable. The lognormal particle

size distribution function is given by Herdan [11]:

f (d)" 1

d ln p'(2p)0.5

expC!(ln d!ln d')2

2 ln2 p' D (8)

where :=

0

f (d)dd"1, the geometric mean diameter d'"5.03 lm and the geometric

standard deviation p'"1.73 from the measured data. The fraction number of each

particle size at the outlet of ESP can be described by this particle size distribution

function. Finally, the theoretical overall collection e$ciency is calculated for comparison

with the experimental results.

S.H. Kim, K.W. Lee / Journal of Electrostatics 48 (1999) 3}25 7

Table 1

The dimensions and operating conditions for the present eight wire single-stage ESP

Dimensions and operating conditions Values

Diameter of discharge wire, D8 (mm) 1, 2, 3, 4

Wire-to-plate spacing, Sx (mm) 50}200

Wire-to-wire spacing, Sy (mm) 12.5}50

Length of collection plate, ¸ (m) 0.75

Height of collection plate, H (m) 0.3

Air #ow velocity, ; (m/s) 0.8}2.5

Applied voltage on wires, <8 (kV) 10}70

Turbulence intensity, ¹6 (%) 12, 15, 18

Air temperature, ¹ (K) 293

Air pressure, P (atm) 1

3. Experimental procedure

The experimental apparatus used in this study consisted of six components: an

aerosol generation system, a wind tunnel, a laboratory-scale ESP, a rapping system,

an aerosol sampling system, and a particle concentration measurement system. The

ESP was 30 mm (=)]500 mm (H)]750 mm (¸) in size and was equipped with eight

discharge wires. The schematic diagram of the ESP is shown in Fig. 1. The basic

operating conditions of the ESP and the parameters used are shown in Table 1. The

single-lane wind tunnel was made of plexiglas and operated at the ambient temperature.

It can provide air velocities ranging from 0.1 to 6 m/s. A thermo-anemometer

(Model 8525, Alnor Instrument Company) was used to measure the air velocity. The

air "ltered with a high e$ciency particulate "lter (HEPA) was supplied with a turbulence

intensity of about 12% and at a "xed mean velocity of 1 m/s. The #y ash

particles which came from the Bo-Ryung electric power plant in Korea were dispersed

using a microdust feeder (Model MF-2, Sibata Scienti"c Technology Ltd.). The #y ash

was analyzed using chemical, physical and electrical methods and the analysis results

are shown in Table 2. The microdust feeder utilizes a variable-speed turntable to

transport #y ash at a constant rate to the test section in the wind tunnel. The

laboratory-scale single-stage ESP described previously was installed in the test section

as shown in Fig. 1. For aerosol sampling, an isokinetic sampling tube was used to

measure the concentration and the size distribution of the #y ash particles. The

measuring points were positioned at the center of the cross-sectional area of the wind

tunnel. Measurements of the particle concentrations upstream and downstream were

made by Aerosizer (Model Mach II and LD, API) which is capable of measuring

individually the size of particles in the range of 0.2}200 lm regardless of the particle

shapes. Finally, the overall collection e$ciency, g%91, was evaluated with the mass

loading of the particles measured at inlet and outlet of the ESP:

g%91"[(m)*/-%5!(m)065-%5]

(m)*/-%5

, (9)

8 S.H. Kim, K.W. Lee / Journal of Electrostatics 48 (1999) 3}25

Table 2

Results of chemical, physical, and electrical analysis of #y ash

Classi"cation Values

Chemcial components of #y ash SiO2 (46.47 wt%)

Al

2

O

3

(24.48 wt%)

Fe2O3 (15.28 wt%)

CaO (4.06 wt%)

MgO (1.56 wt%)

Na2O (0.35 wt%)

K2O (1.17 wt%)

SO

3

(4.20 wt%)

TiO2 (1.18 wt%)

Measurement of particle size distribution GMD 5.03 m

GSD 1.73

d1)4.23 lm

d1'4.23 lm

Electrical resistivity 4.3]109 () m)

where (m)*/-%5 is the mass loading of particles at the ESP inlet. (m)065-%5 is the mass

loading of particles at the ESP outlet.

Presently, two philosophies are prevalent with regard to removal and transfer of the

particulate from the collection plates.

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