Experimental studies in the electrostatic precipitation of high-resistivity particulate

Environment International, Vol. 6, pp. 273-278, 1981

0160-4120/81/010273-06502.00/0 1982Pergamon Press Ltd.

Printed in the USA.

EXPERIMENTAL STUDIES IN THE ELECTROSTATIC PRECIPITATION OF HIGH-RESISTIVITY PARTICULATE John C. Modla, Robert H. Leiby, Thomas W. Lugar, and Kris E. Wolpert Buell EmissionControl Division,EnvirotechCorporation, Lebanon, Pennsylvania17042, USA

A pulsed charging system is describedthat eliminates reverseionization in electrostaticprecipitation. The motivating concept is a purely electricalapproach. The fundamental characteristicof this method is the relaxation time of the precipitated particulate. Laboratory studies indicate that this powering concept could increase precipitator performance on high-resistivity particulate by more than two orders of magnitude. Pilot results are also presented.

Introduction Electrostatic precipitation is the removal of suspended particulate from a gas stream by the application of an electric field. This method of pollution control appears simple on a microscopic level, but in reality the phenomenon o f electrostatic precipitation on a microscopic level is a very complex process. The fundamental processes of electrostatic precipitation consist of corona generation, particulate charging, and particulate collection. Corona generation is the formation of a self-sustained electrical discharge between a high-voltage electrode (usually at a negative potential) and a grounded or passive electrode. Negative ions are formed by the attachment of electrons to electronegative gases and are accelerated by means of an electric field. The ions become attached to the suspended particulate within the gas stream by a combination o f direct collisional and diffusional processes. The unipolar charged particulate is attracted to the passive electrode for subsequent removal. The role o f the corona generation is twofold. The first is to charge the particulate. Secondly, together with the static electric field, the unipolar space charge density also aids in the process o f particulate collection.

Precipitator Limitations

ionization occurs, thus reducing the effectiveness of the precipitator. Reverse ionization (Masuda, 1975) as associated with electrostatic precipitation, is categorized as an abnormal electrical discharge emanating from the dust-electrode interface. The light signal from the electrical discharge consists o f two light waves: a primary light wave that rises very rapidly in time and a secondary light wave that rises more slowly. The former proceeds into space while the latter proceeds along the precipitated dust layer surface. When a sufficiently high voltage pulse is applied at low pressure, reverse ionization is triggered by free electrons. As the pressure is increased, the triggering mechanism is due to negative ions.

Criteria for Reverse Ionization Reverse ionization is a phenomenon whereby the gas molecules among the porous precipitated dust layer are electrically overstressed to such an extent that the gas molecules become ionized. Positive ions are formed which, in some cases, produce a local surface disruption within the dust layer. The positive ions are accelerated toward the emitting electrode, thus neutralizing the negative space charge density. A simple analysis o f the occurrence o f reverse ionization is given by the following equation:

The surface and bulk electrical resistivity of the particulate is a crucial variable that limits the performance o f an electrostatic precipitator. It is a well-established fact that for materials with a resistivity in the range of 10'°-1013 ohm cm the phenomenon of reverse or back

E = JQ' where E: electric field strength, V / m J: current density, A / m 2 0': resistivity, ohm m 273

274

J.C. Modla, R. H Leiby, T. W. Lugar, and K. E. Wolpert

/ /;

I

jl

• O . O . O . O -

1. 2. 3. 4. 5. 6.

O.O.

II

O,O.

13. 14. 15. 16. 17.

7. 8. 9. 10. 11.

Fan Motor Dust delivery pipe Distribution Inlet sampling point 12. Orifice

Corona wires Tubular third electrode Grounded dust collecting electrode Compressed air inlet Fluidized bed dust feeder Cyclone dust collector

Outlet sampling point Enclosure Heater and fan Heater and fan Resistivity test cup

Fig. 1. Tri-Electrode Precipitator test model.

As can be seen from the above equation, a large electric field strength can be obtained for modest current densities through the precipitated dust layer, provided the resistivity of the particulate is large. The occurrence of reverse ionization generally causes a reduction in the breakdown voltage of the gas thus leading to a spark, a substantial increase in the precipitator current, and unsteadiness in the current-voltage characteristic curve (hysteresis). The immediate effects of reverse ionization as far as pollution control is concerned are a reduction in the charging and precipitating electric fields, a substantial increase in particulate reen-

TIME

trainment from the collecting plates, and an increase in emitting wire vibrations.

Method of Control A common method of minimizing the process of reverse ionization is to condition either the gas-entrained particulate, the flue gas, or both. The conditioning process is achieved by the addition of chemical additives to the main gas stream. There are two schools (Loeb, 1939) of thought which attribute the reduction in reverse ionization to the increase in surface and bulk conductivity of the particulate or to a modification in the electrical characteristics of the flue gas itself (i.e., the formation of ion clusters, which leads to lower ion mobilities). %

o

o

•

•

o

50 msec

F

oo o

o

•

o

o o o o

I

u~

4o

I

•

Tri-Electrode

O

Conve"tional

•

o o

o

ooo o o o a

t

oo

o

-20

I

~,--

o

o

I --~

eel •

•

o

o

6o

-

•

o o

-lO. -

•

•

2 msec

4 msec

Fig. 2. Typical static and dynamic potential for resistivity of 5 × 10' ' ohm-cm,

i0 I0

i0 II

a lO12

lo13

Fig. 3. Efficiency comparison of laboratory Tri-Electrode Precipitator with conventional precipitator.

Electrostatic precipitation of high-resistivity particulate I

100

I

•

?r~-Elactrod,

0

Conventional

I

I

I

275 I

ionization will occur when the relaxation time of the dust layer exceeds the relaxation time of the flue gas. These criteria imply that reverse ionization will occur because the conductivity of the flue gas is larger than the conductivity of the dust layer. This observation suggests a way to minimize reverse ionization by correlating the electric field due to the ionic space charge density to the relaxation time of the precipitated dust layer.

Relaxation Time Approach I

I

50

I

70

I

90

I

] OO

130

I

150

AIR SPEED, FT/MI•

Fig. 4. Precipitator performance as a function of air speed.

Another method (SMMC, 1978) of minimizing reverse ionization is to revert to the concept of "wide space" precipitators. The concept of wide spacing refers to the separation between emitting electrodes and collecting electrodes. The likelihood of the occurrence of a disruptive spark due to reverse ionization is reduced with larger spacing. Very recent studies (Penney and Gelfand, 1978) on the minimization of reverse ionization have concentrated on the control of the current density through the precipitated dust layer. This method of control can be accomplished by electrical means. The idea behind this control strategy is based on the electrical properties of the flue gas and particulate. The criteria (Cooperman, 1976) for the initiation of reverse ionization are based on steady state conditions and the continuity of the electric field and current density across a boundary. The crucial parameters that determine the occurrence of reverse ionization are the relaxation time of the flue gas compared to the relaxation time of the precipitated particulate dust layer. The relaxation time is defined as the product of the permittivity of the media in question and the resistivity of the same media. The initiation of

The patented Tri-Electrode Precipitator is a device which minimizes the occurrence of reverse ionization. The concept is based on the relaxation time of the precipitated dust layer. A derivation of this relaxation time approach is given below: Consider the continuity equation: OO ; 0: space charge density at But J =

Q'

Assume that the resistivity of the dust layer is uniform (no spatial variation) V. (E/ o' )= V(1/ O ' ), E+ o/ eO ' = ao/at

o(t)

=

Ooe-"'; r

=

o'e

where 0o is the space charge density at time t = o, and e is the permittivity of the particulate. The above describes how the space charge density decays with time. This equation is analogous to the decay of a typical capacitor-resistor circuit. To minimize the occurrence of reverse ionization, the space F -¸

,

r

80

~3

80

o 50

60 O

o

o o o

40

o

o

40 - -

o

•

TRI-E~CTRODE

o •

Tri-Ele~trod~

O

¢o~ve~t i o n a l

o

2O

CONVENTIONAL

0

0 0

io 11

1012

10]3

I

1011

1012

RESIS~IVZIY, OHMCM

Fig. 5. Laboratory full-scale Tri-Electrode conventional precipitator tests.

RESISTIVITY, OHM CM

Fig. 6. Efficiency comparison with tubes present.

J

1013

276

J.C. Modla, R. H. Leiby, T. W. Lugar, and K. E. Wolpert Table 1. Tri-Eiectrode Pilot Precipitator design parameters.

Number of fields Depth of field Plate height Total plate area Gas passages Gas volume Specific collection area Plate spacing Emitting electrode Total emitter length Tube electrode Dust removal Hopper

1 9 ft. (2.7 m) 6 ft. (1.8 m) 600 ft. 2 (55.7 m z) 6 7100 ACFM (3.35 m2/s) 16.6 m2/(m2/s) 9" (23 cm) Rigid frame rods 430 ft. (131 m) Standard pipe Vibrators Continuous discharge system, drage and screw conveyer

charge density must be allowed to relax to a level below the breakdown strength of the flue gas.

Architecture of the Tri-Electrode Precipitator The basic architecture of the Tri-Electrode Precipitator, as compared to a conventional precipitator, is the addition of a third non-ionizing electrode. The purpose of the non-ionizing electrode is to further enhance the performance of the precipitator by producing a strong long-range static electric field. The device employs two separate pulsating power supplies for the ionizing and non-ionizing electrodes. The two sets of electrodes are operated with variable DC reference voltages with superimposed high voltage pulses. The pulse forming network is a resonant charging scheme which is discussed by Glaso and Lebacqz (1948). By connecting many thyristors in series with the proper voltage grading circuits, the desired voltage rating can be achieved. This leads to a highly reliable solid-state switch (Shoup and Mason, 1981), as compared to using relays or vacuum tubes. T h e power supply has the capability of varying the pulse amplitude, pulse duration, pulse frequency, and rise and fall times of the pulses.

Mode of Operation The mode of operation is to pulse the corona emitting electrode at a rate which correlates with the relaxation

C) ,

°

•

O

time of the precipitated dust layer. The duration of the pulse (pulse width) and the amplitude of the pulsed voltage (more specifically, the peak electric field) determines the quantity and rate of negative ion production (i.e., the ionic space charge density). This, in turn, determines the degree of charge acquired by the particulate. The pulsed voltage that creates the corona current can have a pulse width of one to four milliseconds. Since the pulse duration is short, the applied voltage to the emitting electrodes can be larger than the charging voltage of a conventional precipitator. These short corona pulses have been found to be effective in charging the particulate. The frequency of the pulsed corona voltage depends on the resistivity of the particulate. As an example, for particulate with a resistivity of l0 ~1 ohm cm and a relative dielectric constant of 2, the relaxation time is 180 milliseconds. This means that the space charge density requires 180 milliseconds to decay to 37°70 or ~-~ of its initial value. This implies that if the electric field associated with the initial space charge density is near the threshold value for reverse ionization, the corona emitting electrode should be pulsed at a rate not exceeding 30 Hertz. Too long a period between pulses results in a low electric field through the dust layer (poor collection), while too short a period between pulses permits so little decay in the ionic space charge density that the dust layer cannot accept the full pulse current (reverse ionization). With the emitting electrode at a low potential (i.e., below the corona emitting potential), the unipolar charged particulate migrates towards the collecting electrode. This process of particulate collection is categorized as space charge precipitation. This technique of pollution control also has a characteristic relaxation time which is quite long. Thus, to aid in the particle collection, a non-corona emitting electrode is pulsed with a high voltage whenever the emitting electrode is at a voltage below corona generation. In essence, the Tri-Electrode Precipitator is basically a two-stage precipitator itself. The pulsed corona emitting electrode charges the particulate, while the noncorona emitting electrode aids in the collection process. The pulsing scheme controls the flow of current through the precipitated dust layer, so the occurrence of reverse ionization is minimized.

°

•

O

•

4

Tube Electrode Fig. 7. Pilot Tri-Electrode Precipitator gas passage.

Electrostatic precipitation of high-resistivity particulate

277

Table 2. Chemical analysis of fly ash. Parameter

Percent

Si 02 Fe2 03 AI2 03 CaO MgO K20 Na20 P205 TiO2 Li20

55.91 7.80 25.71 3.80 1.60 3.13 0.43 0.31 1.19 0.04

.q [~

O

0

t-4

x

Laboratory Test Laboratory tests were conducted under the supervision of Professor Gaylord Penney at Carnegie Mellon University for the Buell Division of Envirotech Corporation. A sketch of the test apparatus is shown in Fig. 1. The passive electrodes are 3 ft 2 (0.28m 2) aluminum sheets mounted 4 in. (10 cm) apart. The spacing from wire to ground is two in. with three in. between wires. The third electrodes (non-emitting) are tubes that are mounted midway between the wires. The system is operated in a closed-loop fashion. A calibrated orifice is used to meter the air flow. Compressed air is supplied to a fluidized-bed dust feeder which, in turn, supplies the test dust. Leaving the feeder, the entrained particulate passes through a small-diameter, high-efficiency cyclone to remove agglomerates. The temperature of the air is adjusted to give the desired resistivity as measured by a resistivity cup located in the outlet chamber of the precipitator. The system is seasoned before actual efficiency measurements are taken. A schematic of the pulsing scheme for resistivity in

I

l

I

o

O

•

80

~601

o

~NTIONAL

I

0

2 GI~ VELOCITY, FT/SEC Fig. 9. Calculated migration velocity from pilot tests.

the range of 10~ ohm cm is shown in Fig. 2. The pulse width of the corona emitting electrode is two milliseconds, with a millisecond on the rise and fall time. A comparison of the performance of the Tri-Electrode Precipitator and a typical Research Cottrell precipitator over the resistivity range of 101~ to 1013 ohm cm is shown in Figs. 3 and 4. To normalize comparison, the efficiencies for the two-electrode precipitator were taken with the non-emitting electrodes present and removed. The measurements indicate that the pulsed system of operation is far superior. Scale-up studies with the Tri-Electrode Precipitator, based on scale factors obtained from the Deutsch-Anderson equation, are shown in Figs. 5 and 6. The scale factor is 2 on the wire-to-plate spacing with additional plate area. Also shown in the same figures are results obtained for a conventional precipitator by the removal of the third electrode and the pulsed power supply. Operating under identical conditions, the Tri-Electrode Precipitator is roughly two orders of magnitude more efficient on the high resistivity materials.

u

.=

•

Tri-Electrode

O

Conventional

Pilot Studies

~, 40

20

I

I

I

1

2

3

VEDOCI~,

'FI'/SEC

Fig. 8. Pilot study of Tri-Electrode Precipitator.

A pilot unit has been constructed in a 45-ft. long, 13.5-ft. high, drop f r a m e - t a n d e m axle trailer. The primary elements of the pilot unit are listed in Table 1. The geometrical arrangement of the three-electrode system is shown in Fig. 7. An initial series of tests has been completed at a utility located on the east coast. A chemical analysis of the fly ash is given in Table 2. Laboratory measurements of the fly ash resistivity using planar electrodes varies from 2 × 10~ ohm cm to 5 × 10~z ohm cm within a temperature range of 232 °-268 °F (111-131 °C). Fig. 8

278 shows the experimental results of the efficiency as a function of the gas velocity for both the Tri-Electrode and conventional precipitator design. Note that the conventional design is powered with a DC voltage. The migration velocity, ¢o, can be obtained from the efficiency measurements via the Deutsch-Anderson equation. Fig. 9 displays the relationship between ~b and gas velocity. Initial conjecture as to the reason for this phenomenon may be related to the role of ionic wind, i.e. electrical relaxation time, in precipitation. Further tests are planned to quantify the effect of the rise time of the pulsed voltage. The effect of a positive pulsed scheme is also under investigation.

J.C. Modla, R. H. Leiby, T. W. Lugar, and K. E. Wolpert

References Cooperman, P. (1976) Back corona and relaxation time, IEEE Transactions on Industry and Applications, 1A-12, No. 1. Glaso, G. N. and Lebacqz, J. V. (1948) Pulse Generators, McGraw Hill, New York, N.Y. Loeb, L. B. (1939)Fundamental Processes o f Electrical Discharge in Gases, John Wiley & Sons, Inc., New York, N.Y. Masuda, S. (1975) Research on Electrostatic Precipitation, Dept. of Electrical Engineering, University of Toyko, Tokyo, Japan. Penney, G. W. and Gelfand, P. C. (1978) The Tri-Electrode Precipitator for collecting high resistivity dust, Air Poll. Cont. Assoc. J. 28, 53. Shoup, J. F. and Mason, C. A. (in Press) A high voltage thyristor valve for precipitator applications, Conf. Eng. Sumitomo Metal Mining Co., Ltd. (1978), personal communication.