JPS594184B2 - Electrostatic precipitation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Electrostatic precipitation is now widely accepted in commercial processes as the most practical method of separating solid or liquid particles from a moving gaseous body.

Basically, the precipitation process reduces the effects of polluting air effluents, maintains acceptable environmental standards, and on the other hand allows convenient and economical use of industrial fuels that would otherwise be contraindicated from an ecological point of view. With the goal.

The second is to take advantage of the economic value of the material itself recovered by precipitation.

However, even though the basics of electrostatic precipitation have been known for about 50 years and there has been considerable commercial development of the process to date, the efficiency of precipitation is still below desirable levels.

In order to reach such acceptable non-contamination standards, current precipitation equipment must be large and expensive.

Theoretical considerations that can achieve improved performance are presented.

The efficiency of the electrostatic precipitator is expressed by the following formula:

Here, A - Collection area (m') Q - Flow rate (rrVsec) Vd = Travel velocity (m/see) Since A and Q are given by the real device, increasing the drift velocity Vd It is possible to increase efficiency by

The moving speed is expressed by the following formula.

Here, q - charge per particle (coulombs) E = electric field (V
olt 7m) π2 kinematic viscosity (kg/771 Hsee) r - particle radius (r!t) Tuning the viscosity is difficult, especially in already existing systems, so to increase the moving speed the average electric field It is necessary to increase E and the charge q of the particle.

The particles are charged by negative ions formed from surrounding gas molecules by means of electrical adhesion.

The charge per particle is expressed by the following formula. That is, for particles with a diameter of at least 0.5 microns, where εr = relative dielectric constant of the particle t = charging time (see ) Coulombs ε〇 - dielectric constant of vacuum - s, 85x10'''() - m N - Ion concentration (per 1 m3 of ions) ε = main electron charge = 1.6X 1 g-19 (coulombs) K = ion mobility (m/V @ see
) Equation rm) shows that the charge on a particle is proportional to the electric field strength E.

From formula i), the moving speed changes according to the square of the electric field strength,
From #i), it can be seen that the efficiency is a function strongly influenced by the electric field strength.

A considerable increase in efficiency can be obtained by increasing the average electric field inside the settling duct.

Most precipitation devices today function at field levels of 5 to 10 KV/cIrL.

This is due to the breakdown of the uniform electric field gap in air under atmospheric conditions (
breakdown) is 10 to ↑ of the electric field.

This large discrepancy between the actual electric field strength and the ideally achievable strength is inherent and necessary in conventional precipitation equipment.

This is because the electric field must act to supply electrons via corona discharge.

Such corona charging requires a non-uniform electric field in the vicinity of the charging electrode.

Unfortunately, this need for non-uniformity is accompanied by a reduced average electric field, since even high potential levels cannot exceed the breakdown capacity of the system, thus reducing the particle charging ability and reducing the particle charging capacity. Reduces the ability to transfer to the collection electrode.

The essence of the invention lies in the separation of the means within the precipitation device for (a) charging the particles and transporting them to the collection electrode, and (b) supplying electrons by the corona.

The first mth order is performed with a relatively uniform DC electric field.

(Here, "direct current °" means a unidirectional current, which does not require constant amplitude, and in particular also includes full-wave rectified or partially rectified alternating current.

) Since the efficiency of charging and transport is preferably due to a high average electric field, the electrodes are designed to induce a uniform and high intensity electric field.

In many conventional precipitation devices, this will be accomplished by increasing the number and/or diameter of the wires in the duct.

The provision of the corona current necessary to charge the particles, the second function mentioned above, is accomplished by an independently variable potential superimposed on the DC voltage.

The superimposed potential is a short-time pulse.

The pulsed electric field induced in this way can be twice or three times as large as the underlying DC electric field.

This is because the dielectric breakdown strength of gases is high for short-term pulsed potentials.

(For this point, see Fe1senthal and
“Nano 5econd Pu” written by Proud
l se B reakdo wn 1nGase
s”, Phys, Rev, p 1796.5ep
t.

1965) Separating the charging and transport functions from the supply of electrons has a number of advantages, including: 1. The method of the present invention allows existing electrostatic precipitators to be adapted to the method of the present invention without the need for major equipment changes. can be applied to

The electric circuit of the present invention is extremely simple, and a specific example thereof will be described later with reference to FIGS. 1 and 2.

2. In conventional precipitation devices, the working voltage is chosen close to the breakdown voltage of the gas.

If this voltage is below a certain value, the corona stops producing electrons and the precipitator does not function.

Such conventional precipitation devices are very sensitive to fouling of the electrodes, which affects the working voltage.

In the system according to the invention, the pulsed electric field can be chosen high enough to ensure a corona current under virtually normal operating conditions, and the range of action of the DC electric field is greatly increased.

3. The action of the corona provides charge carriers to charge the particles to an equilibrium state.

Increasing the number of charge carriers well beyond the minimum level required to perform this function satisfactorily is not very advantageous, and merely serves to reduce the time required for the particles to reach an equilibrium charge level. Not too much.

This relationship is determined quantitatively by formula (iii), and is expressed by formula (Ill)
shows that the equilibrium charging depends only on the electric field, the particle radius, and to a lesser extent the permittivity of the particle.

The time required to charge the particles to an equilibrium state (approximately 2 m sec
) is the particle crossing time (typically about 5Or71sec>
, so the increase in efficiency from charging faster is minimal.

Furthermore, supplying too many charge carriers is 11
By creating a back corona +1, the efficiency of the system is adversely affected.

'Back corona' occurs when the resistivity of the collected dust is sufficiently high that the current through the dust layer increases the potential drop across this layer to a value that exceeds the electrical breakdown strength of this layer.

11 Back Corona'' development has a dust resistance value of approximately 2 x 1
It becomes particularly troublesome in conventional precipitation equipment when 0.3 Ω·α is exceeded.

In the present invention, the average value of the corona current can be precisely adjusted independently of the DC electric field.

This can be done by adjusting the superimposed pulsed voltage, pulse width or pulse repetition rate.

The most important of these is the adjustment of the pulse voltage.

This is because corona generation is exponentially related to peak voltage.

Referring to FIGS. 1 and 2, a typical electrostatic precipitator 2
Electrode arrangement 1 is shown.

Although the present invention is capable of using conventional electrode configurations, it may be preferable to modify conventional electrode configurations in certain cases, as described below.

As an essential component, the electrostatic precipitator is equipped with a grounded collection plate 4.
It consists of a single corona wire 3 surrounded by

The corona discharge that occurs between the wire 3 and the surrounding collection plate 4 produces ions.

These ions are then used to electrically charge the particles, and this charging of the particles is called Cottre.
ll ), or in a single-stage precipitation device, the charging of the particles occurs between the corona electrode 3 and the collecting electrode 4, and in a so-called two-stage arrangement (not shown) the charging of the particles occurs in separate areas.

The present invention can basically be used in a single stage precipitation device,
The following description is limited to this.

Simple pipe precipitators are used for small gas flows, for the collection of mist (mlst) or fumes (fog), and for applications requiring water-washed electrodes, with the precipitator placed between a pair of flat collection plates as a precipitator unit. Ducted precipitators with multiple corona wires arranged in parallel accommodate large gas flows and are used for dry collection and sometimes water wash applications.

Although the invention is not limited to any particular type of precipitation device, it will be described with particular reference to a duct type precipitation device.

In some indoor units where the generation of ozone is undesirable, a positive voltage is supplied to the corona wire, whereas in industrial units a negative voltage is generally applied to the corona wire 3 by a suitable voltage generator 5.

A voltage generator 5 delivers a negative voltage to the corona wire by means of a high voltage cable 6, and the collecting electrode or plate 4 is generally grounded.

In a duct sedimentation device, the collection plate is 6 feet high (183cIrL) and 2 to 3 feet wide (60,
Dimensions vary from 9CrfL to 91.5CrrL) to 20 to 25 feet high (61ocm to 762CrrL) and 6 to 8 feet wide (183cIrL to 244cfrL).

Atypical collection plates are so-called opzels, or stretched metal, as shown in FIG.
Including rod-shaped curtains and Vee-type curtains.

Many industrial precipitation apparatuses use steel or steel metal corona wire with a diameter of about 0.1 inch (2.5 mm).

Corona currents typically range from 0.01 mA to 1 rfLA per foot of discharge wire at voltages on the order of 30 KV to 100 mm for a Cottrell or single stage precipitation device.

Basically, the voltage supply means 5 supply a negative voltage to the corona electrode 3, usually a constant direct current is not used, but rather a ripple waveform (rip
It is used to send direct current with ple wave form).

With a pulse method somewhat similar to recent high-power radar equipment,
For example, methods have been proposed that operate at a pulse frequency of 100 pulses per second with a pulse time of 100 microIJ see.

Pulsed waveforms are still not commonly used; the conventional 60 cycles per second is generally used in today's precipitation equipment.

One electrical circuit embodying the invention is shown in FIG.

The section designated 10 contains the basic elements of a conventional electrostatic precipitation apparatus.

Here is the AC power source, transformer 1
4 and a rectifier 16 to generate a DC waveform that activates the electrodes 18 of the precipitation device.

Ground potential collection electrodes often include a plurality of substantially parallel plates.

A number of high potential wire electrodes parallel to each other and parallel to the plate electrodes are threaded between the plates.

The electrodes of the precipitator are positioned such that the gas stream containing the particles flows between the plate electrodes and passes through the high potential wire.

For example, the electrodes are positioned within a chamber through which the fuel exhaust gas passes as it travels toward the exhaust stack or chimney.

Near the electrode, particles contained in the gas stream become electrically charged and are transferred out of the stream to a collecting electrode at ground potential.

The particles collect as dust on the plates or on the floor of the chamber.

The collection electrode can have a variety of non-planar designs to improve particle collection, reduce particle re-entrainment, or otherwise improve the work of the precipitation device.

Such modifications, such as V-shaped plates, stretched metal plates, bar curtains, Opzel shield plates, etc., are also suitable for use with the present invention.

The precipitation apparatus of FIGS. 1 and 2 merely represents one possible application of the invention.

In combination with the conventional precipitator elements described above, the present invention generally includes pulse forming circuitry, such as shown at 20, which serves to provide a pulsed corona current. It may be a system of capacitors and inductance elements, or a cable, or any suitable pulse generation system readily apparent to those skilled in the art.

Powering the pulse forming network is often conveniently the same as for the underlying DC electric field.

However, the AC power source is provided as required by the parameters and design of the particular system.

When activated, the switch indicated at 22 superimposes a pulsed voltage onto the underlying DC waveform.

Inductance, capacitance and resistance circuit elements 24
, 26 and 28 respectively serve to isolate the pulse forming network from the DC circuit.

Another example is shown in FIG.

A conventional precipitation apparatus circuit, including an AC power source 12, a transformer 14, a rectifier 16, and a precipitation electrode 18, is combined in series with a pulse transformer 30 to create a superimposed pulsed electric field.

The effect on the electric field of reducing the spacing of the wires in the duct between the collecting electrodes is shown in FIGS. 5 and 6.

In FIG. 5, corona wires at a potential level of 50 KV are spaced 6 inches (15,2) apart in an 8 inch (20,3 CrrL) duct.

Solid lines represent equipotential curves. Figure 6 shows 8 inches (20,3
A similar configuration of corona wires spaced 3 in f (7,6c*) in a duct of CrIl) is shown.

The equipotential curves are more evenly spaced within the duct and more parallel to the plates of the duct than in the case of FIG.

FIG. 5 shows the duct and wire spacing of many of today's conventional precipitation devices.

FIG. 6 shows an advantageous arrangement adopted in the precipitation device according to the invention, which, as mentioned above, separates the particle charging and transport functions from the corona current supply functions, thereby achieving the former function. A more precise, uniform and therefore more efficient DC electric field can be employed.

Although the best wire spacing cannot be generalized for the present invention, it is preferred that the distance between adjacent wires be less than half the duct width in a typical duct-wire arrangement.

The diameter of the wire in the duct of a conventional precipitator is generally about 1
/10 inch to 1/8 inch (2.5 mm to 3.2
The thickness is mm).

In accordance with the purpose of the present invention to obtain a uniform DC electric field of relatively high potential, precipitation devices embodying the present invention are preferably constructed of wires of increased thickness.

For example, many tests have shown that wire thicknesses of 3/16 to 5/16 inch (4.8 mm to 7.9 mm) are suitable for the system of FIG.

In addition to the purpose of obtaining a more uniform DC electric field, increasing the wire thickness reduces the amplitude and improves the fatigue properties of the wire.

Furthermore, the wires are not necessarily of circular cross-section; other shapes may be adopted depending on the particular shape and parameters of the precipitation device (by which generation of sufficiently large corona currents is possible but desirable). A DC potential gradient is achieved.

In view of the fact that in a conventional precipitator, such as that combined with the fifth arrangement, the potential applied to the wire is about 50 KV, in the precipitator according to the invention the potential of the wire is much higher, and the potential of the wire is much higher. Limited only by breaking strength.

Experiments have shown that applying a voltage of 70 KV or more to the wire is suitable.

The corona current required to supply ions for charging the particles is determined by the pulse forming network shown at 20 in FIG.
It is obtained by a pulsed high potential superimposed by a pulse generating mechanism, such as a pulse transformer, shown at 30 in the figure.

The pulsed electric field thereby induced is considerably higher than the underlying DC electric field, without causing gas breakdown inside the duct, since the pulsed potential is short-lived.

More particularly, the superimposed voltage is at least 10% of the underlying DC wire voltage, and typically may be about the same magnitude as the DC wire voltage.

For example, tests have shown that with a device similar to Figure 6,
It has been shown that it is suitable to superimpose a pulsed voltage of 70 KV on the DC wire potential of .

The superimposed potential has a pulse width of 10-9 to 1O-
3 sec is preferable.

A typical pulse waveform is on the order of 100 nanoseconds.

Experiments with pulse waveforms with steeply rising fronts show that the latter part of the waveform has a relatively small contribution to the corona current.

Exponentially decaying waveforms or even damped oscillating waveforms are suitable, and a narrower region of the waveform is utilized.

The pulse repetition rate of the superimposed potential must be at least 60 pulses per second (60 pps), and preferably high (on the order of several hundred pps).

When the pulse repetition rate is low and close to the repetition rate of the underlying DC waveform, the superimposed pulse is placed at the maximum position of the DC waveform to help maintain the capacitance between the electrodes and direct the entire particle flow. need to be synchronized.

For higher pulse repetition rates this difficulty is avoided.

These are illustrative and non-limiting examples of the invention described above.

Departing from the specific examples described above would amount to applying the invention to special circumstances and parameters, and such application does not depart from the spirit of the invention as defined in the claims.
What is done will be obvious to those skilled in the art.

Embodiments of the present invention are as follows.

(1) A method of increasing the particle collection efficiency of an installed electrostatic precipitator by substantially increasing the number of wires in an insulated electrode array, thereby increasing the electrostatic field distribution between a grounded particle collection electrode and the array. on the other hand thereby reducing the corona-generating ability of the normally rectified DC working voltage applied to the electrode array to an insufficient level and continuously repeating short high voltage pulses to charge and insulate the electrode array. The method comprises superimposing the electrode array on the electrode array.

(2) Method 5 according to embodiment 1, characterized in that the repetitively pulsed potential is at least 10% of the potential associated with the underlying DC electric field.

(3) The method of embodiment 1, wherein the repetitive pulsed potential has a repetition rate of at least 50 pps.

(4) In the method of Embodiment 1, the voltage pulse is 100
A method characterized by having a corona initiation time of microseconds or less.

(5) A method according to embodiment 1, characterized in that the voltage pulses are generated by a power source independent of the normal DC source and separated by a DC blocking circuit element.

6. The method of embodiment 1, wherein the number of wires in the insulated array is increased by a factor of at least 1.5.

(7) A method according to embodiment 1, characterized in that both the number and size of the wires are increased by at least 1.5 times.

(8) An apparatus for charging and collecting particles contained in a gas flowing between a parallel, spaced-apart metal electrode of large surface area and a parallel, insulated, spaced-apart metal array of relatively small surface area, , the large surface area electrode is grounded, the insulated metal array is connected to a source of rectified direct current potential, and the insulated metal array is connected to a second potential source of a repetitively pulsed potential. The pulsed potential is superimposed on the DC potential, but the potential sources are electrically separated from each other by high DC impedance circuit elements.

A device characterized by:

(9) An apparatus according to embodiment 8, wherein the relatively small surface area insulating array is arranged to form a substantially uniform electric field between the large area grounded electrodes.

00) A device according to embodiment 8, characterized in that the relatively small surface area insulating array is characterized in that significant particle charge amounts are produced by the corona only during high duration periods of the repetitively pulsed potential.

0υ Apparatus according to embodiment 8, characterized in that the insulating array is assembled such that the spacing between the wires is less than half the spacing between adjacent ground electrodes.

(121) Apparatus according to embodiment 8, characterized in that the magnitude of the pulsed potential is at least 10% of the peak potential of the rectified potential.

σ3) In a device that collects electrostatic particles with increased efficiency and with a reduced spark rate, the electromechanical function of the physical movement of the charged particles toward the collection electrode is essentially a continuously applied approximately This is accomplished by the action of a uniform rectified DC electric field, and the function of charging particles is essentially achieved during short periods of repeated pulsed electric fields superimposed on the DC electric field, and these separate functions are based on the same physical A device characterized in that it is achieved in a physical space and with the same field-defining electrodes.

[Brief explanation of the drawing]

FIG. 1 is a three-dimensional illustration of a typical duct type for a settling device used to collect airborne ash and is suitable for use in the present invention. FIG. 2 is an enlarged view of the electrode configuration for the duct precipitation device of FIG. 1; FIG. 3 is a schematic diagram of a specific example of the circuit of the present invention. FIG. 4 is a schematic diagram of another embodiment of the circuit of the present invention. 5 and 6 are diagrams showing the electrolytic geometry for different wire spacing configurations between plate electrodes. 1...- Electrode arrangement, 2... Electrostatic precipitation device, 3
...Corona electrode, 4...Collection electrode, 5
... Voltage generator, 20 ... Pulse forming circuit diagram, 30 ... Pulse transformer.

Claims (1)

Claims: (1) at least one collection electrode plate; at least one corona electrode array; and means for establishing a direct voltage between the plate and the at least one array; , wherein the geometry of the at least one plate and the at least one row of plates generates a dense and uniform electrostatic field when the voltage is established therebetween. , thereby reducing the field at at least one array of corona electrodes to such a level that the generated corona current is insufficient to precipitate particles, and further reducing the uniform electrostatic field , comprising superposition means for superimposing superimposed electrostatic fields to produce a combined field of sufficient strength to generate a corona, said superposition means superimposing said superposition at a sufficiently high pulse repetition rate. means for generating a combined electrostatic field in a pulsed manner to generate the corona current at a level that precipitates the particles, the spacing between two adjacent corona electrodes in at least one row being such that the spacing between the corona electrodes and An electrostatic precipitation device for electrostatically precipitating particles by means of a corona current, characterized in that the spacing between the collecting electrode plate and the collecting electrode plate is smaller than that of the collecting electrode plate.