DE3004474A1 - Operation of pulsed electrostatic precipitator - by combining pulsed and steady DC voltages to avoid adverse sparking conditions - Google Patents

Abstract

Multisection precipitator having at least three sections, each having 0.109-0.250 in. dia. wire electrodes spaced 3 1/2-5 1/2 ins. apart, and collecting electrodes spaced 9 ins. apart, the inter electrode spacing being 4 1/2 ins. and the collecting plate areas in each section >=8200 ft.2, is operated by (a) applying a DC base voltage of 30-50 kV between discharge and collection electrodes such that the value is near but just below the corona starting voltage and the resultant electric field is 3.0-7.5 kV/in.; (b) applying in parallel to the discharge electrodes of one or both of the first two sections, short pulses of 50-80kV, of the same polarity as the base DC voltage, at 80-200 pulses per second, such that the pulse plus base voltage is above normal DC breakdown value; (c) setting the pulse voltages to have a fast rise time is not >200 ns and a short decay time, and a pulse width of 250-1500 ns. to avoid excessive sparking; (d) adjusting the DC base voltage in coordination with adjustment of the applied voltage pulses to vary the charging and collecting fields independently and maintain an average current through the collected dust layer at or just below that value which would cause electrical breakdown of the dust layer. Problems of handling difficult dusts and their inherently deficient particle charging and collecting fields are overcome. The charging and collecting fields may be varied independently so that each may be adjusted to a level that results in optimum precipitator performance.

Description

The invention relates to electrostatic precipitators and, more particularly, to a pulsed electrostatic precipitator.

In known separators, a half-wave converter or full-wave converter creates a voltage to the discharge electrodes. This creates both corona loading fields and particle collecting fields. These fields are not independent and are limited to the spark breakdown voltage as seen from the separators. For collecting dust with high Resistivity, the breakdown in the dust layer on the collection electrodes can occur at very low voltage levels and the charging and collection of particles can be serious with special needs.'

The invention particularly relates to electrostatic Separators in which a pulse voltage is applied to the corona discharge electrodes via the basic direct current voltage.

The invention also relates to separators with multiple Stages at which a previous stage is pulsed

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can to cause the ionization and the particles to
load, and a subsequent step can be used to collect the
charged particles are used. ·. ·

The present invention is aimed at the optimum design and implementation of the operation on a separator
with multiple sections to overcome the difficulties of handling difficult dusts and the associated disadvantages of particle loading and. Sämmelfeldern
turn off.

According to the invention it has been found that a very high
Particle loading can get .that very short pulses
can be applied or superimposed on the basic direct voltage in very high voltage levels without causing an electrical breakdown within the separator.
Depending on the shortness of the pulsation or the oscillation or
Pulsing becomes very high instantaneous ion densities
and possible ' high electron densities emitted by the discharge electrodes, which expand to the collecting electrode as a result of the applied and underlying basic voltage. By appropriate settings of such parameters, such as. , B- the fundamental voltage, the pulse voltage, the pulse rise time, pulse width and pulse repetition frequency can be very effective particle charging
can be obtained at the correct particle collection fields.

13003370364 ORIGINAL BATHROOM

By adjusting the pulse properties in combination with the discharge electrode diameters and the distance between The electrodes give you a high degree of freedom with regard to the optimal adjustment of the charging and collecting fields.

An important feature of the invention is the ability to vary the loading and collecting fields independently of one another. By superimposing pulse voltages on top of a basic voltage With the same polarity, the pulse properties can be varied independently of one another, together with the basic voltage so that each can be adjusted to a level which leads to an optimal function or execution of the separator leads.

Further advantages, features and possible applications of the present invention emerge from the following description preferred embodiments in conjunction with the drawings. Show it:

Fig. 1 is a schematic view of a separator with several chambers, which is used in the implementation of laboratory try to be used

Fig. 2 is a schematic view of a pulse generator circuit for use in the experiments and

Fig. 3 is the schematic illustration of a folded or pleated line or transmission, which as a

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Section of the separator with multiple sections can be used.

Figure 1 describes a preferred embodiment of the invention. Dust or other fine-particle material in a.m exhaust gas flow is through a perforated plate 13 in front of the Inlet section of a separator brought up with a single channel and multiple sections. In Figure 1 are only three sections are shown, but preferably, at least four sections are provided.

In Figure 1, each section has a pair of opposing header plates 20 spaced about nine inches apart are arranged. The header plates can have different dimensions. Between a pair of high tension rods 31 and midway between each collector plate 20 in each section is a group of nine discharge wire electrodes 31, which are as long as the panels. These discharge electrodes are uniform in size and equally spaced in each Facility or system, in other facilities or systems, however, they can have different sizes and different distances from one another to have. In an embodiment according to FIG. 1, the The discharge wire electrode is 0.109 inches in diameter and the electrodes are 5 1/4 inches apart. In another embodiment, each has a discharge wire electrode 0.25 inches in diameter and the electrodes 3 1/2 inches apart. The distance from each electrode to a collection plate is 4.5 inches.

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■ - 8 -

After passing through the three or more sections of the separator shown in Figure 1, the gas passes through Outlet line 50 off. Funnels, magazines or bunkers 40 are in each section under the collecting plates 20 for collecting the particles collected on the plates 20 are provided.

In some modes of operation, the discharge electrodes 30 are pulsed in each section. In other modes only the discharge electrodes of the inlet section are pulsed. Where only the inlet section is pulsed is on the discharge electrodes A basic DC voltage is applied to the other sections, and these sections function as collecting sections.

For most systems, the configuration or design is of the discharge electrodes in such a way that direct current and pulse voltages are applied parallel to the individual electrodes of each section. In some cases, however, it can be beneficial be a folded transmission path as the discharge electrode to use. Such a folded discharge path or transmission line is shown in FIG is described below. The essential feature of the pulse generator circuit according to Figure 2 is the oscillating or pulsating switch 61 which is a physical gap between the high voltage electrode 62 and the earth electrode 6 3. For a pulse to be generated, the gap must break through briefly, with the energy from the capacitor 5 4 in a water glass resistor 70 is emptied, resulting in a voltage pulse or a pulse voltage. The rise time of the im-

GOPY ORIGINAL INSPECTED

pulses, and its decay time are given by R-C components 76-65 certainly. Der.Ladewiderstand 6 6 in the circuit determines the Frequency of the impulses. The separation of the gap determines the Voltage at which the gap breaks down, and consequently the pulse voltage. The pulse width is determined by the water load resistance 70, the storage capacity 64 and the capacity 'of the separator load are determined. The coupling capacitor 71 couples the basic voltage against the DC power supply, the pulse voltage being generated by the above devices.

In order to achieve optimal operating conditions for the separator of FIG. 1 To ensure the following were variable parameters examined:

variable

(1) pulse width

(2) pulse voltage

(3) pulse frequency

* i

Area ■

: 250-1500 nanoseconds: 30-80 kV peak 2

: 80-1000 pulses / sec.

(4) Discharge electrode diameter a. 0.109 inches at 5 1/4 with spacing between electrodes b. 0.156 inches at 5 1/4

c. 0.25 inch at 3 1/2

d. 0.50 inch at 1.75

(5) gas temperature. : 160-320

(6) Particle size and concentration

5, around bulk medium diameter, 3-4 grains scf d

(7) gas velocity

(8) basic tension

(9) Specific resistance of the particles

13003 * 3 / π ο c ♦

j COPY

^. .- - .. ORIGINAL · INSPECTED

4-6 feet / second 10 kV - 50 kV

10 ¹¹ - 10 ¹³ ohm / cm

^V;: -; ■;; ■ ■ r -; -: Γ 3ΟΟ44? Α

The observations made include the following:

(1) Pulse Width - Increasing the pulse width of 300 to 1000 nanoseconds was favorable for the implementation or the function of the separator. (Only the Cooldown has been exposed to changes. The rise time remained essentially 100 nanoseconds constant).

(2) Pulse Voltage - Optimal performance was obtained when the pulse generator was operating at the highest possible voltage for both the 0.25 inch and 0.109 inch discharge electrodes. Impulse voltages up to 80 kV peak were used. The pulse voltage and width were monitored as shown in FIG.

(3) Pulse Rate - Increasing the pulse rate from 100 pulses per second to 200 pulses per second did not have a beneficial effect on the function of the separator when both the pulse voltage and the pulse width was kept constant. The increase in pulse frequency had to be 100 pulses per second an adverse effect. The base voltage had to be reduced to low levels (around 100 kV), and the Current densities were difficult to control.

(4) Discharge Electrode Diameter and Distance Between Electrodes - The discharge electrodes had one 0.109 inches in diameter at 5 1/4 inches apart

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and a diameter of 0.25 inches with a spacing of 3 1/2 inches. The distance from plate to plate was 9 inches.

(5) Gas temperature - the dust was kept hot prior to being injected into the air stream to prevent agglomeration due to moisture. The .Versuche dust with high resistivity were carried out at gas temperatures between 280 and 320 ⁰ F.

(6) Basic Voltage - The basic voltage during pulse excitation need not be at their highest level prior to sparking. There is an optimal basic voltage depending on the specific resistance of fly ash, after which a further increase leads to a Impairment of the function of the separator leads. For an effective driving force, the. Basic voltages are below the normal DC breakdown. In cases of a very high specific Resistance, the basic voltage can preferably be below the direct current corona voltage.

(7) Current densities - For optimal operating conditions, the separator current densities varied, between 5 and 10 itiA per one thousand square feet collecting surface, "When in use high pulse voltages and setting the pulse frequency and the base voltage, separator currents can be

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that they are working under corona conditions (below back-corona situations).

The laboratory experiments

Figure 1 shows the arrangement of the separator system as it is at was used in the laboratory testing program. Dust was added to the system through a weight feeder, to precisely control the dust load. The dust was in the inlet air stream by means of an ejector manifold assembly in well dispersed in the line. With the help of electrical heaters, the inlet air was brought to its desired operating temperature warmed up before carrying the dust with it. Downstream from the dust feed, a mechanical collector with a smaller one became Effectively used to remove larger particles and therefore offer relatively fine particle sizes to the separator.

In all experiments, hydrated clay was used as the high resistivity dust. Its particle size distribution at the separator inlet was characterized by a mean diameter of 4.5 microns and a geometric standard deviation of 2.76. The loading of the separator inlet with finely divided material was carried out under 3 grains / scfd. Of the The specific resistance of the dust was controlled by changing the operating temperature. For those reported here Experiments have chosen the temperature levels so that one can operate at very high resistivity provided with under-corona delimitation; and with moderately high specific

.t spark limit:

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Fish resistance with spark limitation: At 300 F the

12 resistivity of dust 5 χ 10 ohm-cm; and at

Λ Ί

200 ° F the resistivity was 2.5. χ 10 ohm-cm.

The separator itself was a single pipe separator and consisted of three powered sections with a total header area of 54 square feet Each section was 4.5 feet long with an effective flow height of 2 feet. The collector plate spacing was 9 inches, and the discharge electrodes were at a distance of 5 1/4 inches apart and were sheathed or shielded 0.109 inch diameter wires. The air speed in the separator was 5 feet / sec for all runs. Thus the specific collection area (SCA) was 120 square feet / 1000 acfm. '',.

The separator was operated as a two-stage separator with the inlet section separate from the two downstream sections was supplied by energy. During the impulse excitation experiments, only the inlet section that was downstream was pulsed Sections were energized in a conventional manner and they served as. Collecting section. the Basic electrical excitation of both sections was achieved using 70 kV peak full-wave rectifier power supplies. Impulses were in the inlet section from the pulser system to the basic voltage hung up.

Prior to the experimental program, outlet concentrations of fines from the separator were measured over a range of separator efficiencies measured under the operating conditions described above. These stresses were related to the optical density

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correlated or connected, as they are at the separator outlet was measured using an RM4 optical diameter meter from Lear-Siegler. During the experimental program, the results of the flowmeter were taken, and the Correlation curve was used to calculate the outlet loading during each experiment for the calculation of the precipitation efficiency to determine.

Table I shows the individual test results obtained during the test program. The results are accordingly the level of resistivity and the mode of operation (pulsed or conventional), not chronological, grouped. Each experiment was carried out with its optimal excitation condition; i.e. for the conventional experiments that the tension has been set to a value that provides maximum effectiveness, and for the pulsed experiments the combination of basic voltage, pulse voltage, pulse width and frequency was set so that that one got the best function. Thus, the numbers given in Table I are optimal values for comparison under operating modes based on the best function. All attempts are shown to be an indication of the Provide reproducibility of the results.

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TABEL

Laboratory data for large pulses against conventional arousal

Operating mode

Temperature (degrees K)

W ₁

Specific precipitation resistance- ^ (m / sec.) (M / sec.) (Ohm. Cm) Effectiveness (%). ;

Conventional 357 pulsed

367

Conventional

422

Pulsed

422

.5 xlO ¹¹ 96 3 .140 .286 98 0 .166 .375 96 , 3 .140 .286 96 , 3 .140 .286 95. , 7 .133 .265 95. .3 .129 .253 97. , 5 .156 .342 97. .0 .148 .315 1.5 x 10 ¹¹ 98 .0 .166 .375 98 .0 .166 .375 98 .3 .172 .401 97 .3 .153 .330 99 .7 .246 .707 98 .3 .172 .401 98 .0 .166 .375 98 .0 .166 .375 5 X 10 « 82 .7 .0743 .104 82 .7 .0743 .104 80 .3 .0688 .0920 82 .3 .0733 .102 80 .0 .0681 .0906 82 .7 .0743 .104 5XlO ¹² 95 .3 .129 .253 96 .0 Il36 .275 95 .3 . 129 .253 94 .3 ■, 121 .228

All experiments were run with an inlet load = 3 grains / scf and a gas velocity = 5 feet / second '.

The electrodongeometricn dos.Arscheiders were conventional like

described.

Each experiment was carried out with optimal electrical excitation.

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:.: ': -: .- :: -: 30Q4474

: ■ ■ - 16 -

guided.

For each case, Table I shows a collection efficiency, German migration speed w and the modified migration speed w,. The modified speed of migration is preferably used for comparative evaluations of the pulsed with the non-pulsed function, for their size in a given case is independent of the level of effectiveness. The modified effectiveness equal is:

1 - η = exp I- ( _Wl A / V) ^ 7 (D

where applies

η = collect] effectiveness, partially

A = collective] area or area, m

.V = volumetric flow rate, m / sec. W ₁ = modified migration speed, m / sec. m = exponent, depending on the inlet particle

size distribution?

for laboratory dust m - 0.635.

Examination of Table I shows the expected trends in the data. The separator efficiencies for a specific

1 2

Resistance of 5 χ 10 ohms .cm are lower than for

1 1
2.5 χ 10 Ohm.cm, for both pulsed and conventional operation. For any size of spe-

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With a specific resistance, however, pulsed operation is more effective than the conventional one. For the quantitative determination of the improvement in the implementation or function or Efficacy attributable to pulsed excitation became a single value of w, for each mode of operation calculated based on the average penetration (1-n) for that mode. A very good quantitative one The measure of the improvement due to the pulsation is then the ratio of these w, values at each level of the specific Resistance. This is because w i is a direct indication of the level of the trap excitation. Since the w, ratio represents an increase or improvement in the excitation of the separator, it is called the "increase factor" H.

Table II shows the effective w, for each mode and the increase factors at each level of the specific Resistance. '·

Table II Laboratory average data for

every operating mode

Operating mode Temperature Specific separation- w. Gradient K Resistance fertilization ^: _{/ q} , r- (Ohm.cm) effective- = ^{/ ί3θ} "factor (%)

Conventional 367 1 1 2.5 χ 10 96 55 , 295 1. 33 Pulsed 98 20th .392 Conventional 422 5. 10 ¹² 81. 80 .0993 2. 53 Pulsed 95. 22 - .251

1300 33 / 0.3 6 4

Because of the large number of individual experiments in Table I and their good reproducibility in every operating mode it is assumed that the values or magnitudes of the increase factor and their difference at the two levels of the specific Resistance have a very remarkable meaning. The fact that the level or size of the increase of w, which increases with increasing specific resistance, supports the previously described view of the effect of the pulsed Excitement. The weaker the conventional excitation, the greater the degree of pulsing possible Improvement. It should be noted, however, that in spite of the increase in the increase factor with the resistivity the value of w, for both conventional and pulsed excitation, decreases. This shows that the pulsating excited deposition as well as conventional deposition are subject to resistivity-induced limits although the limitation on pulsed function or effectiveness is much less serious. This is justified based on the theory, because whatever the separator excitation is obtained, dielectric breakdown of the dust layer hinders further useful excitation. With pulsed excitation, however, larger and more uniform ion densities and higher effective field strengths exist when these Limit is reached.

It was mentioned above that only the inlet section of the laboratory separator was pulsed. This is in agreement with the expectation that pulsed excitation is first

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Line to increase the particle charge acts. Thus represents the laboratory set-up is a two-stage separator which ensures increased loading in the inlet section is, and the downstream sections act primarily as collecting sections. In practice, one has A series of laboratory tests found that the pulsation from more than just the inlet section didn't become too noticeable Effectiveness improvement compared to pulsing leads to the inlet section only. However, this was not in the Trial confirmed at life size. See below.

Results in natural size

Investigation of natural-size pulsed excitation has been made performed at a Research Cottrell separator, after a mechanical collector fired with coal powder Boiler is operationally assigned. Each of his two Felder was equipped with separate pulse generators. Each pulsed field contained a collector plate area or one Collecting plate area of 8200 square feet. The plate spacing was 9 inches and the discharge electrodes were 0.109 inches in wire diameter and 5 1/2 inches apart arranged from each other. The downstream, non-pulsed separator consists of two fields, each one with one Had a quilt area of 10800 square feet. The total collection area the separator was therefore 38,000 square feet.

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. 300447A

In order to characterize the pulsed and conventional operation of the separator, a full test program was set up on site carried out. During the test program it became bituminous Coal from the east burned with low sulfur content, and the boiler or boiler was constantly at full load operated. The coal burned during the test program had an average of about 1.1% sulfur with 18% ash content. The gas volume flow through the separator varied by 110,000 acfm. Data recorded during the test program indicated In situ measurements of the specific resistance, particle size distributions, speed crossings, layers or Stack opacity results (opacity = reciprocal of the transmittance) using a Lear-Siegler transmittance, Separator outlet loads, mechanical collector inlet loads, and all electrical and boiler operational data. Coal and ash samples were collected during each experiment. Because of unacceptable flow patterns between the outlet of the mechanical collector and the separator inlet it was not possible to directly determine the separator inlet loading and size distribution to eat. By giving up the effectiveness curves of the mechanical However, to the measured inlet data of the mechanical collector it was possible to check the loading and the particle size distribution to the separator for the purpose of isolating the separator effectiveness or separator function. The size distribution the ash to the inlet of the separator was found to have a mean diameter of 2.2 microns at one geometric standard deviation of 2.2.

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The separator was in operation during the test program typical of the moderately high limitation due to the specific resistance, which was characterized by heavy sparking very low current levels which resulted in poor excitation. The in situ measurements of the resistivity were compatible with this type of operation. They were in the range from 1 χ 10 to 9 χ 10 Ohm.cm and were in the mean

11
tel about 5 χ 10 ohms «cm. '

All tests carried out during the test program were at optimal operating levels for the operating mode being tested, i.e. both pulsed and unpulsed functions were adjusted to give maximum collector effectiveness. Pulse generator variables were previously set for optimally certain levels in all experiments. ;.

Because it was possible to pulse each of the two inlet fields independently, four modes of operation were tested:

1) All fields are conventionally excited,

2) The inlet field (A) is pulsed, the others are conventionally excited.

3) The second field (B) is pulsed, the others conventional.

4) Both inlet fields (A + B) are pulsed, the others conventional. . :

Table III presents data for experiments performed during the test program. The data is through

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the operating mode is grouped, as described above, not chronologically. As with the laboratory data, those are Separation efficiency w and w, recorded for each experiment. In addition, the average stack or layer opacity and log the specific collection area for each attempt; Changes to the SCA were announced Fluctuations in gas volume. Finally, the increase factor H is recorded for each pulsed experiment. It is based on the average value of w, of 0.085 for the non-pulsed attempts.

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TABEL

III For full-size separators, data for pulsed or conventional excitation

fields
pulsed SCA
(Square feet / 1000 acfm) Separator
effectiveness Stack opacity W.
(m / sec.) ^w k
(m / sec.) H I. O None 341 93.84 25.9 .0415 . .0768 - VJ
I ' 0447 None. 369 95.70 25.6 .0433 .0861 - None 351 94.58 26.7 .0422 .0802 - Λ ■ A ■ 340 95.29 23.7 .0457 .0893 1.11 OO33 / A.
A. 329
339 98.28
96.05 "15.6
21.5 .0627
.0484 .145
.0978 1.80
1.21 ο approx
CD
* «· B. 349 ■ 95.35 22.8 .0447 .0876 1.09 ■ B 371. 95.56 20.8 .0426 .0842 1.05 , A + 3 ·. 337 97.84 16.3 .0578 .129 1.60 .... A + B 355 ·. ', .97.99 17.0 .0.559; .127 1.58 A + B 371 98.24 16.3 ' .0553 .128 '"1.59 A + 3 371 97.17 16.7 .0488 .105 1. 30

For the particle size distribution determined at the inlet of the separator the exponent m in the modified Deutsch effectiveness equation is essentially the same as it was found for the laboratory dust, d. H. 0.625. The actual levels of w and w are for the tests at natural size are lower than for the corresponding laboratory tests. This is usually one expected difference.

Examination of the data in Table III shows a consistent improvement in function or effectiveness which the is attributable to pulsed excitation. The most favorable improvement was found in those experiments in which both fields A and B were pulsed; the mean increase factor for these four trials was 1.5. The pulsing either field A alone or field B alone also improved performance or effectiveness in each experiment, but not to the same extent as pulsing fields A and B together.

During the execution of the test program it was obvious by observing the stack or the layer alone that the pulsed modes of operation Improved the effectiveness of the separator. When applying the layer or stack opacity against the number of pulsed

themselves
Sections show / the benefits of advancing from zero to one or two pulsed fields in the separator. Indeed, the curve should show that there are other notable ones

. 130033/0384

Advantages can be realized by pulsing additional fields.

The improvement in progression from one to two pulsed Fields is shown in both the increase factors in Table III and in the decrease in opacity. this apparently contradicts the laboratory tests carried out in the essentially no additional improvement in pulsing showed more than one section. The explanation for this is likely to be found in the fact that the company is in natural size is restricted to a lower level of excitation than in the laboratory, both for the pulsed also the non-pulsed operation, it is therefore reasonable to expect that a longer pulsed separator length is necessary for the application in natural size, to ensure the benefits of increased arousal. It can actually be further expected that the increase factors, which are greater than those measured in the laboratory, are possible in this situation because of the very low excitation base of conventional operation. This is done through the data shown in Table III. ·

Laboratory tests and tests at natural size have confirmed that the pulsed excitation system remarkable the separator function and its effectiveness for the collection of High resistivity dust improved. The laboratory data showed increase factors in the range of 1.33

13003370364 7

up to 2.53 for moderately high to very high resistivity; Field data for ashes with a moderately high specific Resistance showed the ratio 1.5.

The improvement noted above can possibly be explained by the following hypothesis ": The pulse voltage is essentially responsible for the generation of corona. Ions and electrons become during the rising part of the pulse voltage generated. The greater the peak voltage, the greater the ion generation. Also those generated during the impulse Peak currents are orders of magnitude higher than the average current on the collector plates. It is believed that because of the Impulse extremely high ion densities can be realized. These Ions repel each other during the impulsive period and expand under the field as a result of the base voltage the collecting plates. It is assumed that particle charging takes place in the time between pulses, namely in the presence of very high ion concentration and the field due to the base voltage. Another hypothesis is that the ion gap charging field further improves the charging process. Also the nature or property of the corona or corona discharge during pulse excitation differs significantly from that of conventional methods. The pulsed corona or corona discharge is very even and well distributed over the discharge wires, while the corona is with conventional methods is point-like and irregularly changes locations along the wire. Also, the radius of the corona glow is during the pulsed excitation is much larger than the conventional one

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Corona glow radius. Thus, the pulsed corona is more likely to change with improved corona current distribution results in the separators.

Overall, the extremely high instantaneous ion densities, the high ionic space charge and the uniformity are effective the corona current distribution during the pulse excitation and cause a particle charge size that is superior to that those found with conventional pathogen methods.

Conclusions:

(a) The effect of the pulse voltage

One comes to the conclusion that increased pulse voltages too lead to higher field strengths and lead to better particle charging during each pulse. It is possible, that the ion density per pulse is also higher. Thus, pulse voltages for a separator assembly should be as much as possible to maximize the particle charge. The increase in the pulse voltage does not increase noticeably the mean current.

(b) Effect of the pulse frequency

For a given pulse voltage, the ion density can be per Second in an almost linear relationship by increasing the frequency of the impulses. This, leads to

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a higher average current. Because the average current on the plates for the placement of the back corona for dusts with high resistivity is / should be the pulse frequency carefully controlled to keep the average current always below the critical current density, which is required for dissolving the reverse corona.

(c) Effect of the pulse width

The pulse width is made up of the pulse rise time and pulse decay time together. It is believed that the pulse rise time is responsible for the generation of ions and electrons, while the pulse decay time for particle charging can provide high field strength conditions. The shorter the pulse rise time the higher the separator spark voltage and consequently the particulate charge may be higher. The increase in the pulse decay time can also reduce the particle charge increase in that the particles are charged for a longer time with the high field strength. There is a limit, however for increasing the pulse decay time. A longer decay time approaches DC operation, and therefore results then lower spark voltages. Increasing the rise time also leads to the same result and is for loading or charging not effective.

(d) Effect of the base voltage

The base voltage should be below the normal breakdown voltage for collecting dust with high resistivity

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300U74

be held (possibly below the corona start tension). It is known that the direct current corona or direct current corona discharge is a non-uniform corona discharge and, as a result, could be used in a high specific Resistance the back corona can be triggered by the combination of the direct current corona and the pulses. It is believed, that the basic or basic voltage is the particle collection function performs and a rapid expansion of the by the impulse generated ion cloud to the collecting plate, which leads to a uniform current distribution.

(e) Optimizing the separator efficiency

On the basis of the above, the separator be optimally designed and set up by using suitable discharge and collecting electrode designs, which increases the DC corona start voltage to work at higher collector fields. The separator should be at the basic voltage operated near or below the corona starting voltage. The pulse voltage should be as practical as possible Value can be increased. Pulse width and frequency should then can be set for a given separator so that it works with optimum function or at its optimum effectiveness.

For particles with moderate resistivity it is possible to use the concept of pulsed excitation by increasing the base voltage to values higher than its initial corona voltage are, because the back corona phenomenon is not a

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Problem. The pulse voltage could then be combined with the pulse frequency and the pulse width can be adjusted so that the separator efficiency is optimal.

It was mentioned above that · j η most systems in which the separator has several guides, lines or channels, the discharge electrodes are paj allelically connected, but that in some cases a folded transmission or conduction for the discharge electrode! can be used. Such a folded line is shown in FIG.

In Figure 3 there are seen four header plates 21-24 which form a path or string for gas flow between each pair of header plates. The four plates 21-24 according to FIG. 3 merely illustrate a larger number of plates, and an even larger number of strands or paths could also be used. In each strand between the collector plates ί1-24 discharge wire electrodes 32-35 are suspended. The gas flow goes in the direction shown by the arrow in FIG. The gas flow is therefore parallel to the side walls of the collector plates and transversely to the discharge electrodes 32-35. Jece of the corresponding discharge wire electrodes in each Pfcd are linked together either at the bottom or at the top to form a continuous, folded transmission line, v, ie shown in FIG. A pulse input when applied is transmitted to all discharge electrodes as in a folded transmission line. Of the

13Q033 / 0364 BAD ORIGINAL

Advantage of using a transmission line for the Transmission of the pulse voltage is that the pulse voltage is less affected by the separator capacity and, consequently, is less affected by the size of the separator. So it can be easier control the waveform of the impulses. . ·

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Claims (4)

Research-Cottrell, Incorporated, New Jersey, Somerville, New Jersey 08876 / USA Process for separating dust with high specific resistance 1. Procedure for operating a separator with several Sections for collecting dust with high resistivity, with the separator at least three sections and each section has discharge electrodes with wires that are 0.109 to 0.250 inches in diameter and spaced 3 1/2 to 5 1/2 inches apart, each section 9 inches apart arranged collecting plates, the distance from each electrode to a collecting plate is 4.5 inches and the header plates of each section have a total area of at least 8200 square feet, characterized in that that a basic equilibrium 1 30033/0364 in the range of 30 to 50 kilovolts between the discharge electrodes and applying the DC base voltage to a value close to the corona start voltage but is set immediately below this in such a way that the resulting electric field is in a range from 3.0 to 7.5 kilovolts per inch, parallel to the discharge electrodes of at least one but no more than the first two sections short pulse voltages in the area of 50 to 80 kilovolts can be applied, with the pulses having the same polarity as the basic DC voltage and with a pulse repetition rate in the range of 80 to Pulses per second are applied, the pulse voltages plus the basic DC voltage to reach a peak or a peak above the normal DC breakdown level can be set or applied, the pulse voltages such be adjusted so that a fast rise time of no greater than 200 nanoseconds achieves a short decay time this pulse being 250 to 1500 nanoseconds wide such that excessive sparking is prevented, and that the basic DC voltage coordinates with the setting of the applied voltage pulses in such a way is set up or adjusted that the loading and collecting fields can be varied independently of each other and an average flow through the layer of collected dust immediately is kept below or at the value which cause an electrical breakdown of the dust layer would. 130033/0384 f, · ;. ". Γ: -" :. Xf ": 3Q04474 - 3 - 2. The method according to claim 1, characterized in that the first two sections of the separator with several Sections are pulsed by separate pulse generators or pulse generators. 3. The method according to claim 1 or 2, characterized in that the pulse voltage rise time is on the order of 100 nanoseconds. · '. ^: 4. The method according to any one of claims 1 to 3 *, characterized in that that the mean separator current through the layer of collected dust is between 5 and 10 milliamps per one thousand square feet of the collecting surface. 130033/0364