An electrostatic precipitator is provided capable of preventing a reduction in dust collection effect of ionic wind, and increasing dust collection efficiency. The electrostatic precipitator includes: a plurality of collecting electrodes (4) in the form of circular pipes arranged at predetermined intervals in a direction orthogonal to a longitudinal direction of the electrodes; and a plurality of protrusions (5a) protruding toward the collecting electrodes (4) and arranged offset in parallel with the orthogonal direction. An equivalent diameter of a cross section of the collecting electrode (4) is 30 mm to 80 mm. An opening ratio of the collecting electrodes (4) arranged at predetermined intervals is 10% to 70%.

TECHNICAL FIELD

The present disclosure relates to an electrostatic precipitator.

BACKGROUND ART

A known conventional electrostatic precipitator includes flat collecting electrodes arranged in parallel along a gas flow, and pointed discharge electrodes arranged between the collecting electrodes.

The electrostatic precipitator applies a high DC voltage between the collecting electrodes and the discharge electrodes, and performs stable corona discharge of the discharge electrodes to electrically charge dust in the gas flow. A conventional dust collection theory describes that electrically charged dust is collected by collecting electrodes by the action of a Coulomb force applied to the dust in an electric field between the discharge electrodes and the collecting electrodes.

Electrostatic precipitators in PTL 1 and PTL 2 include a collecting electrode having a plurality of through holes through which dust passes and a closed space for collecting the dust therein. In PTL 1 and PTL 2, the dust is passed through the through holes and trapped in the closed space to prevent the collected dust from scattering again.

An electrostatic precipitator in PTL 3 includes a collecting electrode that includes a ground electrode having an opening ratio of 65% to 85%, and a dust collecting filter layer that collects dust. In PTL 3, such a collecting electrode is provided to generate ionic wind in a section orthogonal to a gas flow, and to generate a spiral gas flow circulating between a discharge electrode and the collecting electrode, thereby efficiently collecting dust. In PTL 3, the ionic wind is positively used, but the dust is mainly collected by the dust collecting filter layer.

CITATION LIST

Patent Literature

[PTL 1] The Publication of Japanese Patent No. 5761461[PTL 2] The Publication of Japanese Patent No. 5705461[PTL 3] The Publication of Japanese Patent No. 4823691

SUMMARY OF INVENTION

Technical Problem

Dust collection efficiency of an electrostatic precipitator can be calculated by the well-known Deutsch's equation (Expression (1)) below:
η=1−exp(−w×f)  (1)
where w is an index of dust collection performance (particulate migration velocity) and f is a specific collecting area (collecting area per actual gas volume).

In Expression (1) above, the particulate migration velocity w of dust (particulate matter) is determined by a relationship between the action of a Coulomb force and viscosity resistance of gas. The Deutsch's equation (Expression (1) above) assumes that dust travels in an electric field from a discharge electrode, and does not directly consider an influence of ionic wind on performance. However, there is an assumption that a dust concentration as a basis of the performance design is always uniform in a dust collection space between the discharge electrode and the collecting electrode, and the ionic wind is considered to cause disturbance of gas to provide a uniform dust concentration.

When a negative voltage is applied between the electrodes, corona discharge of the discharge electrode generates negative ions, thereby generating the ionic wind. When a positive voltage is applied, positive ions generate the ionic wind. For considering an industrial electrostatic precipitator, an example of a negative voltage being applied is described below, but the same applies to a positive voltage.

The ionic wind generated from the discharge electrode flows toward the collecting electrode to cross the gas flow. The ionic wind having reached the collecting electrode is reversed at the collecting electrode and changes its flow direction. This causes spiral turbulence between the electrodes.

In the turbulence, a flow from the discharge electrode toward the collecting electrode carries dust close to the collecting electrode. The dust carried close to the collecting electrode is finally collected by a Coulomb force.

However, the ionic wind reversed at the collecting electrode moves the dust away from the collecting electrode as a collector, and may prevent dust collection.

PTL 3 describes the electrostatic precipitator considering the effect of ionic wind. However, this collector has a complex structure in which the ionic wind is fed to a filter layer behind the collecting electrode having an opening, and is intended to collect dust in a region without any influence of main gas. Also, for a dry type electrode, it is difficult to dislodge and collect dust adhering to the filter layer.

The present disclosure is achieved in view of such circumstances and has an object to provide an electrostatic precipitator capable of preventing a separation action of ionic wind that reduces a dust collection effect, and increasing dust collection efficiency.

Solution to Problem

An aspect of the present disclosure provides an electrostatic precipitator including: a plurality of collecting electrodes having a cylindrical shape and arranged at predetermined intervals in a direction orthogonal to a longitudinal direction of the electrodes; and a plurality of discharge portions protruding toward the collecting electrodes and arranged in parallel with the orthogonal direction, wherein an equivalent diameter of a cross section of the collecting electrodes is 30 mm to 80 mm.

The cylindrical collecting electrodes are arranged at predetermined intervals to allow part of ionic wind flowing from the discharge portions toward the collecting electrodes to escape behind the collecting electrodes. This can prevent the ionic wind from being reversed at and moving away from the collecting electrodes.

The equivalent diameter of the cross section of the collecting electrode is 30 mm or more. A smaller equivalent diameter increases electric field concentration to increase dust collection performance. However, too small an equivalent diameter increases a peak value of electric field strength with a current required for dust collection being ensured, thereby causing spark discharge. Thus, a lower limit of the equivalent diameter is 30 mm.

The equivalent diameter of the cross section of the collecting electrode is 80 mm or less. A larger equivalent diameter causes little rise in electric field strength near the collecting electrode, and only average electric field strength of a flat electrode is reached. A larger equivalent diameter also generates a swirl of a gas flow. Thus, an upper limit of the equivalent diameter is 80 mm.

The equivalent diameter refers to a diameter of a circle equivalent to a cross section of a predetermined shape. Thus, for a circular cross section, the equivalent diameter corresponds to a diameter thereof.

The collecting electrode may be, for example, a pipe-like member having a circular section. However, the cross section may have, other than the circular shape, an oval shape, an elliptical shape, a polygonal shape, or the like. The collecting electrode may be hollow or solid.

A direction of gas flowing in the electrostatic precipitator may be the orthogonal direction in which the collecting electrodes are arranged or the longitudinal direction of the collecting electrodes.

The collecting electrode may dislodge and collect dust by rapping, may be moved to scrape off dust with a brush, or may perform wet cleaning.

Further, in the electrostatic precipitator according to an aspect of the present disclosure, an opening ratio of the collecting electrodes arranged at predetermined intervals is 10% to 70%.

An opening ratio of less than 10% reduces an effect of preventing moving away of the ionic wind. An opening ratio higher than 70% reduces an effective dust collection area and reduces dust collection performance.

An opening ratio α is expresses as described below:
α={1−((d×3.14/2)/Pc)}×100
where d is an equivalent diameter and Pc is a center-to-center pitch between the collecting electrodes.

Further, in the electrostatic precipitator according to an aspect of the present disclosure, one and the other discharge portions are arranged on opposite sides of the collecting electrodes arranged in the orthogonal direction, the one and the other of the discharge portions being arranged such that ionic wind flowing from the one discharge portion toward the collecting electrodes does not oppose ionic wind flowing from the other discharge portion toward the collecting electrodes.

The one and the other of the discharge portions are arranged on opposite sides of the collecting electrodes arranged in the orthogonal direction such that ionic wind flowing from the one discharge portion toward the collecting electrodes do not oppose ionic wind flowing from the other discharge portion toward the collecting electrodes. This can prevent interference of ionic wind to hinder dust collection.

Advantageous Effects of Invention

The cylindrical collecting electrodes arranged at predetermined intervals are used to prevent ionic wind from moving away from the collecting electrodes and increase dust collection efficiency.

DESCRIPTION OF EMBODIMENTS

Now, an embodiment of an electrostatic precipitator according to the present disclosure will be described with reference to the drawings.

An electrostatic precipitator1is used, for example, in a thermal power generation plant using coal or the like as fuel, and collects dust (particulate matter) in a combustion exhaust gas guided from a boiler.

The electrostatic precipitator1includes a plurality of conductive collecting electrodes4made of, for example, metal. The collecting electrodes4are hollow cylindrical circular pipes having a circular cross section, and arranged at predetermined intervals in an orthogonal direction (direction of a gas flow G) orthogonal to a longitudinal direction. A plurality of rows of the collecting electrodes4arranged in the direction of the gas flow G are provided in parallel at predetermined intervals. Between the rows of the collecting electrodes4, discharge electrodes5are arranged. InFIG. 1, dashed lines show positions of the discharge electrodes5.

The collecting electrodes4are grounded. The discharge electrodes5are connected to a power supply (not shown) having a negative polarity. The power supply connected to the discharge electrodes5may have a positive polarity.

As shown inFIG. 2, each discharge electrode5has a plurality of pointed protrusions (discharge portions)5a. The protrusion5aprotrudes with a tip directed toward the collecting electrode4. Corona discharge occurs at the protrusion5a, and ionic wind is generated from the tip of the protrusion5atoward the collecting electrode4.

FIG. 3is a front view ofFIG. 1seen from the direction of the gas flow G. As shown, the protrusions5aare provided to be alternately directed in opposite directions (alternately directed to left and right inFIG. 3) in a height direction. Corresponding protrusions5aat the same height protrude in the same direction with the collecting electrode4therebetween. The protrusions5aare arranged in this manner so that the ionic wind flowing from the protrusions5atoward the collecting electrodes4is directed substantially in the same direction in the height direction. This can prevent interference of the ionic wind.

As shown inFIG. 4, all the protrusions5amay be directed in the same direction (right inFIG. 4) so that the ionic wind is directed in a uniform direction.

FIG. 5shows a positional relationship between the collecting electrodes4and the protrusions5a.FIG. 5is a sectional view taken at a position of the protrusions5aat a certain height inFIG. 2. Thus, the protrusions5ado not appear on opposite sides as inFIG. 2in plan view, but the protrusions5aare shown directed to only one side. As shown inFIG. 5, a center-to-center pitch Pc between the collecting electrodes4and a center-to-center pitch Pd between the protrusions5aare preferably equal. Also, the protrusions5aare preferably arranged to face spaces between adjacent collecting electrodes4in a staggered manner. With such an arrangement, as shown inFIG. 6, line of electric force are equally distributed to the collecting electrodes4, and can reach a deep side of the circular cross sections of the collecting electrodes4when seen from the protrusions5a. Reference character D inFIG. 5denotes a distance between the collecting electrode4and the protrusion5ain the orthogonal direction (vertical direction inFIG. 5), which is, for example, 125 to 250 mm.

Considering that the line of electric force reach the deep side of the collecting electrodes4in this manner, an opening ratio α of the collecting electrodes4in front view from the protrusions5ais expressed as below:
α={1−((d×3.14/2)/Pc)}×100

where d is an equivalent diameter of the collecting electrode4. The equivalent diameter refers to a diameter of a circle equivalent to (having the same area as) a cross section of a predetermined shape. Thus, when the collecting electrode4has a circular cross section as in this embodiment, the equivalent diameter corresponds to a diameter thereof.

The opening ratio α is 10% to 70%. The reason therefor will be described later with reference toFIG. 11.

The equivalent diameter d of the collecting electrode4is 30 mm to 80 mm.

The equivalent diameter d of the cross section of the collecting electrode4is 30 mm or more for the following reason. A smaller equivalent diameter d increases electric field concentration to increase dust collection performance. However, as shown inFIG. 7, too small an equivalent diameter d increases a peak value of electric field strength with a current density (for example, 0.3 mA/m2) required for dust collection being ensured, and the peak value exceeds spark electric field strength of 10 kV/cm, thereby causing spark discharge. Thus, a lower limit of the equivalent diameter d is 30 mm.

The equivalent diameter d of the cross section of the collecting electrode4is 80 mm or less for the following reason. A larger equivalent diameter causes little rise in electric field strength near the collecting electrode (described later with reference toFIG. 9), and only average electric field strength (2 kV/cm) of a flat electrode without any bore is reached. A larger equivalent diameter d also has an influence on gas flow and generates a swirl. Thus, the upper limit of the equivalent diameter d is 80 mm. For example, average electric field strength at the equivalent diameter d of 30 mm calculated under the same condition as the above is about 5.7 kV/cm.

The ordinate inFIG. 8represents average electric field strength, which is electric field strength averaged by a surface area of the collecting electrode4. The average electric field strength is different from peak electric field strength on the ordinate inFIG. 7. The peak electric field strength is electric field strength at a position with highest electric field strength on a surface of the collecting electrode4.

Next, with reference toFIG. 9, a rise in electric field strength near the collecting electrode4will be described. As shown, the abscissa represents a position, and the protrusion5ais assumed to be located at a position corresponding to the y-axis. The ordinate represents electric field strength. The electric field strength is highest at the position of the protrusion5a, reaches a minimum value between the protrusion5aand the collecting electrode4, and then increases again toward the collecting electrode4. Near the collecting electrode4, there is a region B with a high rate of increase in (gradient of) electric field strength. This is because the electric field strength is increased near the collecting electrode4due to space charge of dust or negative ions. The increase in electric field strength in this region B is referred to as “a rise in electric field strength”. In the region B, a Coulomb force is dominant, and dust P is effectively collected by the collecting electrode4.

In a region A closer to the protrusion5athan the region B, ionic wind is dominant. In the region A, the dust P in the gas is subjected to the Coulomb force, but mainly guided on the ionic wind to the collecting electrode4.

FIG. 10shows, as a reference example, electric field strength when a conventional flat electrode7without any bore is used as a collecting electrode. As seen fromFIG. 10, an absolute value of electric field strength near the flat electrode7is smaller than that of the collecting electrode4in the form of a circular pipe inFIG. 9, also with a smaller rise in electric field strength. Thus, it is found that dust collection performance is lower than that of the collecting electrode4in the form of the circular pipe.

FIG. 11shows a dust collection area ratio relative to an opening ratio α. The dust collection area ratio refers to a dust collection area providing the same dust collection performance with dust collection performance at an opening ratio of 0% (no gap) being1. Thus, a smaller dust collection area ratio refers to higher collection efficiency.

As shown inFIG. 11, the dust collection area ratio is 0.8 or less at the opening ratio α of 10% to 70%. Thus, the opening ratio α is preferably 10% to 70% (range of application).

Next, an operation of the electrostatic precipitator1of this embodiment will be described.

In the electrostatic precipitator1, a power supply applies a negative voltage to the discharge electrode5to cause corona discharge at the tip of the protrusion5a. Dust contained in the gas flow G is electrically charged by the corona discharge. By the collection principle of the conventional electrostatic precipitators, electrically charged dust is attracted to the grounded collecting electrode4by a Coulomb force, and collected on the collecting electrode4. However, ionic wind actually has a great influence.

When corona discharge occurs, negative ions are generated near the protrusion5a, and moved toward the collecting electrode4by an electric field to generate ionic wind. Thus, simultaneously with the Coulomb force acting on the dust, the ionic wind flowing toward the collecting electrode4moves the dust contained in the gas flow G close to the collecting electrode4. Then, due to a large rise in electric field strength in the region B (seeFIG. 9) near the collecting electrode4, the dust is effectively collected. Also, the collecting electrodes4in the form of circular pipes are arranged at predetermined intervals to allow part of the ionic wind flowing from the protrusions5atoward the collecting electrodes4to escape behind the collecting electrodes4. This can prevent the ionic wind from being reversed at and moving away from the collecting electrodes4, thereby increasing collection efficiency.

Part of the ionic wind containing dust flowing toward the collecting electrodes4passes between the collecting electrodes4. As shown inFIGS. 3 and 4, all the protrusions5aat the same height are directed in the same direction. Thus, the ionic wind is directed in a uniform direction and does not interfere.

The dust collected by the collecting electrode4is dislodged and collected by rapping. Alternatively the collecting electrode may be moved to scrape off the dust with a brush, or wet cleaning may be adopted.

This embodiment has the following effects.

The collecting electrodes4in the form of the circular pipes are arranged at predetermined intervals to allow part of the ionic wind flowing from the protrusions5atoward the collecting electrodes4to escape behind the collecting electrodes4. This can prevent the ionic wind from being reversed at and moving away from the collecting electrodes4.

The equivalent diameter d of the cross section of the collecting electrode4is 30 mm to 80 mm. This can increase dust collection performance of the collecting electrode4.

The opening ratio α is 10% to 70%. This can ensure an effective dust collection area to increase dust collection performance.

The ionic wind generated from the protrusions5aprovided at the same height are directed in a uniform direction so as not to interfere with the ionic wind generated from the protrusions5aprovided at different heights (seeFIG. 3). This can prevent the ionic wind from hindering dust collection.

The above embodiment may be varied as described below. InFIG. 1, the direction of the gas flow G is orthogonal to the longitudinal direction of the collecting electrode4. However, as shown inFIG. 12, the direction of the gas flow G may be the longitudinal direction of the collecting electrode4.

InFIG. 5, the pitch Pc between the collecting electrodes4and the pitch Pd between the protrusions5aare described to be equal. However, as shown inFIG. 13, the pitch Pc between the collecting electrodes4may be smaller than the pitch Pd between the protrusions5a. In this case, the collecting electrodes4and the protrusions5aare preferably aligned such that line of electric force are distributed as equally as possible to the collecting electrodes4.

In this embodiment, the collecting electrode4in the form of a circular pipe has been described. However, the cross section of the collecting electrode4may have, other than the circular shape, an oval shape, an elliptical shape, a polygonal shape, or the like. The collecting electrode4may be solid rather than hollow like the pipe.

REFERENCE SIGNS LIST