200 1 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

Modelling of the effect of electrode geometries on the corona discharge current-voltage characteristic for wire-plate electrostatic precipitators

D. Brocilo', J.S. Chang', R.D. Findlay', Y. Kawada' and T. Ito' 'McMaster University, Hamilton, Ontario, Canada

Musashi Institute of Technology, Tokyo, Japan

Abstract: An electrostatic precipitator (ESP) is used as a dust particle collection device in various power plants, steel manufactories, pulp and paper plants, cement plants, food processing, etc., as well as in commercial buildings and home ventilation systems. ESP operates on the principle of electrostatic separation of dust particles after charging, hence high ion density and electric fields are required. Therefore, the corona discharge current-voltage characteristic is very important and depends on several factors: discharge electrode shape, dimensions and surface condition, gas flow properties, dust particle loading and space charge, polarity of applied voltage, etc. In this work, a model for prediction of the current-voltage characteristic is presented and experimentally validated for round, threaded, rectangular, and rigid discharge electrodes in combination with plate type collecting electrodes. The experimental validation shows good agreement for the case of the round discharge electrode. The model predicts the effect of wire diameter, plate length and gas flow temperature. For the rigid discharge electrode, of the same electrode thickness as the diameter of the round electrode, much lower corona onset voltage was observed.

Introduction The collection efficiency of dust particles in ESPs depends on the charging and collection of the particles. Charging requires high current density (or ion density) and electric field, while the particle collection requires only high electric field.

From the point of view of particle charging, the current density increases the characteristic charging time, thus leading to a higher surface charge for the same gas residence time. The electric field also increases the surface charge. The magnitude of this effect depends on the dust particle size. For dust particle diameter (4) greater than 10 pm, the particles are mainly charged by field charging mechanism. Thus, the electric field has a greater impact on the surface charge then the ion density. For dust particle diameters smaller than 0.1 pm, the particles are mainly charged by the diffusion charging mechanism. Therefore, the ion density has a greater impact on the surface charge than the electric field. For dust particles in the transition region (O.1pm I dp IIOpm), the electric field and the

ion density contributed with same order of magnitude to the dust surface charge.

Related to the dust particle collection, the electric field increases the mobility of the charged dust particles, thus leading to higher collection efficiency. The magnitude of the current density may have a dual effect. On one hand, the current density may increase the packing density of the dust layer and the electrical forces generated by the dust particle contact-surface capacitance. This is considered as a positive effect, since it decreases the re-entrainment of dust particles. On the other hand, a high current density across the high resistivity layer may lead to the onset of back corona, thus leading to a lower collection efficiency.

The goal is to operate at the maximum allowable current, which is limited by sparking, and the breakdown of the dust layer, known as back-corona. The maximum allowed current can be smaller than the maximum theoretical current above spark voltages, since the nonuniform current density between discharge and collecting electrodes may lead to earlier breakdown.

In general, the current-voltage (I-V) characteristic of an electrostatic precipitator is governed by the electrode geometry, the presence of the dust layer on the collection electrode, dust particle parameters, and the gas flow conditions. Many other conditions, such as the surface condition of the discharge electrode, misalignment of the electrodes, non-uniform dust loading, etc. can affect the I-V Characteristic. It is very difficult to determine the discharge current based on a pure theoretical analysis [1,2,3]. In this work, the constitutive relationships for I-V characteristics were developed and experimental validations for various scale-up sues were conducted.

Modelling of Current-Voltage Characteristics The corona discharge I-V characteristic is governed by the charge transport and Poisson's equations as follows [41: ji = C ; V N , + p i NiB-DiVNi; V . J . - =-- aNi

1 at

- e (2)

V . E = - - N E

where Ji is the ion flux, Ni is the ion density, U, is the

0-7803-7053-8/1/$10.00 2001 IEEE 681

gas velocity, is the ion mobility, Di is the diffusion coefficient of ions, E is the electric field, e is the elementary charge, and E, is the electric permittivity of fiee space. For the conditions when g8

Table 2: Values of ri ,m and b according to DE-type. I I I I

DE ICE types

Round 0.23mm

Threaded 1.125mm 0.55 1.5 3mm 0.9

7 Rectangular # B O 1 1.5- I 0.9 I I Rectangular #45 I I I

1 I M I 1 0.9 I i:: Rigid &short CE Rigid &long CE Rectangular #BO.. . DE and CE are aligned. Rectangular k45.. . DE is rotated 45 related to the CE orientation.

Gas composition and temperature

Gas composition and temperature effects are incorporated in the mobility of the ions and the relative gas density. Mobility of the ions: The mobility of the ions is a h c t i o n of the mixture of gases, the gas temperature, the gas pressure, and the polarity of the applied voltage. A fairly substantial amount of data is available on the dependence of the ionic mobility in pure gases with gas temperature, and electric field versus pressure ratio [5]. However there is not enough experimental data for various gas mixtures. Therefore, the corona chemistry simulation of Chang and Kwan [6] is used to determine the dominant ion species, depending on the voltage

From Chang and Kwan [6], the dominant ion in air is N202- under negative corona andN,O+ for positive

polarity.

corona. The mobility of ions can be estimated fiom the reference mobility of 0,- ando ions and the molar mass ratio [6] as follows:

l(lN2%- -/?- and 2 -lk (6) p%- N2%- myo+

The effect of the gas temperature on the mobility of the ions can be approximated by [6]:

(7)

Current-Voltage (I-V) Characteristics Corona discharge currents were measured by an electrometer on the CE side. The experimental loop, shown in Figure 2, includes: (a) the gas conditioning section with electrical heater, humidifier, various gas cylinders, and a gas combustion chamber for simulation of combustion gasses; (b) the ESP chamber with wire- plate and wire-cylinder geometry; and (c) monitoring instruments: analogue voltage and current meters on the power supply side, current and voltage probes

connected to an oscilloscope for the waveform recording, and an electrometer for the time-averaged current recording on the collecting electrode side.

conditioniag U system I mpsection I-

Oscilloscope

Figure 2: Schematic diagram of the experimental apparatus.

The current-voltage characteristic, shown in (3), can be approximated by a straight line when displaying in square root of current per unit length versus applied voltage. Experimentally obtained results for various DE geometries are plotted in Figure 3. Figure 4 shows the comparison between experimental and predicted I-V curves for round and spike DE geometries. Results related to the CE length prediction are shown in Figure 5.

I 2 O 1

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100 E

s 5 f 60

40

E )

c

c

5 20 2

0 0 5 10 15 20 25 30

Negative Voltage [W

oround 0.15mm xround 0.23mm Around 1.5mm Oround 2mm Oround 3mm xthreaded m rectangular 90 deg 0 rectangular 45 deg fD rigid

Figure 3: Measured I-V curves for various DE geometries.

The corona current on the round DE starts in the form of corona tufts and then develops into an unsteady glow corona as the voltage level increases. The surface condition of the DE determines the form of the corona discharge. Negative glow corona along the whole length of the wire was observed only in the case of the thin wire with a clean surface. Larger diameter DES were

exhibited rapidly moving corona glow along short sections, interrupted by localised tufts caused by electrode surface imperfections. The rigid DE combined the advantages of the thin wire and the rod. Like a thin wire, the rigid DE has a low onset voltage and a high current. The rigid DE has also a high sparking voltage- and a rigid construction as a rode type DES.

~ m u n d 3mm (m)

0 - 5 10 15 20 25 30 Negative Voltage [kVl

Figure 4: Comparison between measured (m) and predicted @) I-V curves for round and spike DES. CE length was 10.15mm.

E 204 -. L .*, 10 .

L_( 0

0 ' ' ' ' : .:E , ; ' ' ' ; ' ' ' ' 1 ' ' ' ' i 0 5 10 15 20 25 30

Negative Voltage [kVl

Figure 5: Measured (m) and predicted @) I-V curves for short (CE- s) and long (CE-I) collecting electrodes.

0 5 10 15 20 25 30 Negative Voltage [W

The experimental results show different degrees of discharge current response to gas flow for rigid-plate ESP, as shown in Figure 6. Compared to the stationary conditions, discharge current and sparking voltage increase with the increasing gas flow velocity due to ion transport (1) and effective electrode cooling by the gas flow. The sparking voltage may increase, due to the loss of charge fiom the current channel by convective charge transport.

Conclusion From the experimental validation of current-voltage characteristics the following was concluded:

The surface condition factor for thin and thick wires is 0.9. For larger wire diameters with rough surface condition, the factor may reduce to 0.85. The threaded wire geometry can be modelled by the mean thread diameter and m~ of 0.55. The rectangular DE and its orientation were successllly modelled by side length. The geometry correction coefficients kc. of aligned and 45O-rotated DE are 7 and 9, respectively.

0 The surface condition of spikes and their spacing are very important. Namely, dual discharge points on the surface of the spike and initiation of corona glow between spikes were observed for the present geometry of the rigid DE.

Acknowledgements The authors wish to express their appreciation to S. Teii, S. Ono, and V. Morgan for valuable discussions and comments. This work is supported by NSERC of Canada;

References [l] 1.1. Thomson, Conduction of Electriciy through Gases,

[2] C.W. Rice, Phys. Rev. 7, pp. 228-229, 1947

[3] A.D. Moore, Electrostatics and its applications, Wiley New York, NY, pp. 194,1973

[4] J.S. Chang, F. Pontiga, P. Atten, and A. Castellanos , "Hysteresis effect of corona discharge in a narrow coaxial *pipe discharge tube with gas flow," IEEE Transaction on Industry Applications, vo1.32, 110.6, pp. 1250-1256, NovlDec. 1996.

Cambridge University Press, New York, pp. 267,1945.

[5] E.W. McDaniel and E.A. Mason, The mobility and d i f i ion of ions, John Wiley & Sons New York, NY, 1973

[6] J.S. Chang and A. Kwan, "Modeling of dry air chemistry in a coaxial wire-pipe negative corona discharge," Proceedings ESA- D 3 Joint Symposium on Electrostatics 1998, Morgan Hill, pp. 391-403, June 1998.

Figure 6: Effect of gas flow on I-V characteristic for rigid-plate ESP geometry

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