JP2006083751A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a form of a grounded electrode (a metallic mesh 5) capable of obtaining desired coagulation effect without worsening pressure loss. <P>SOLUTION: A coagulation device 1 has a housing 3 connected with an exhaust pipe 2 to form a part of an exhaust passage, a discharge electrode 4 attached to the housing 3, and the grounded electrode arranged on the downstream side in the direction of flow of exhaust gas more than the discharge electrode 4 to apply high voltage between the discharge electrode 4 and the grounded electrode to generate corona discharge and coagulate PM in exhaust gas by static electricity. The grounded electrode is formed by the circular metallic mesh 5 having many ventilation holes. Rate of hole area of the ventilation hole of the metallic mesh 5 is set to 30 to 90%, and individual ventilation hole has such size that does not prevent groups of coagulated and bulky PMs from passing through. To explain concretely, when diameter of inscribed circle inscribed in the ventilation hole is defined as scale division opening, this scale division opening is set to 0.5 to 2 mm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device including an aggregator that aggregates particulate matter contained in exhaust gas using corona discharge.

In recent years, regulations on exhaust gas from diesel engines have been strengthened year by year, and in particular, there is an urgent need to reduce particulate matter (hereinafter referred to as PM) mainly composed of carbon.
As a device for removing PM contained in exhaust gas, for example, a diesel particulate filter is known. In order to increase the PM collection rate by this filter, PM is electrostatically aggregated using corona discharge. A method is being considered. In this method, a corona discharge field is formed inside the exhaust pipe, and when the exhaust gas flows through the corona discharge field, the PM contained in the exhaust gas is charged, thereby aggregating the charged PM by electrostatic force. Is the method.
Patent Document 1 proposes an exhaust gas reducer that is configured such that a mesh body is disposed in a shearing direction in the exhaust passage and a high-voltage current flows through the mesh body.
Japanese Utility Model Publication No. 53-37813

In the above-described method of electrostatically aggregating PM, the inner peripheral wall of the exhaust pipe is used as a dust collecting electrode, and therefore the charged PM moves slowly toward the dust collecting electrode by electrostatic force, and efficiently aggregates. Is difficult. In particular, when the exhaust flow rate increases, PM may be pushed away by the exhaust flow and cannot move to the dust collecting electrode side, and may flow into the filter without being aggregated.
On the other hand, it is conceivable to use the mesh body described in Patent Document 1 as a dust collecting electrode. In this case, since the mesh body is arranged across the exhaust passage so as to be orthogonal to the exhaust flow, the PM can be efficiently aggregated and the PM in the metal mesh orthogonal to the exhaust flow can be obtained even when the exhaust flow rate is large. Can be electrostatically aggregated.

However, the above-mentioned Patent Document 1 only describes that the mesh body is disposed in the shearing direction in the exhaust passage, and the specifications of the mesh body (for example, the aperture ratio of the mesh, the shape of the mesh) , Mesh size, etc.) are not disclosed at all. In addition, Patent Document 1 is not a technique for electrostatically aggregating PM using corona discharge. Therefore, a desired aggregation effect can be obtained simply by using the mesh body described in Patent Document 1 as a dust collecting electrode. Can not expect. In particular, if the aperture ratio of the mesh is reduced in order to enhance the agglomeration effect, the pressure loss may increase, leading to a decrease in engine output.
The present invention has been made on the basis of the above circumstances, and an object of the present invention is to provide a conductive structure having a ground electrode disposed across the exhaust passage, the ground electrode having a number of openings. For example, in an exhaust gas purification apparatus constituted by a metal mesh, a porous body, and the like, it is an object of the present invention to provide a configuration of a ground electrode capable of obtaining a desired agglomeration effect within a range not deteriorating pressure loss.

(Invention of Claim 1)
In the present invention, when a high voltage is applied between the discharge electrode and the ground electrode disposed in the exhaust passage, a corona discharge field is formed in the exhaust passage, and the exhaust gas flows through the corona discharge field. In addition, the exhaust gas purification apparatus includes an aggregator that electrostatically aggregates particulate matter contained in the exhaust gas, and the ground electrode is disposed downstream of the discharge electrode in the exhaust flow direction and across the exhaust passage. And an electrically conductive structure having a large number of openings through which the aggregated particulate matter can pass. The ratio of the area of the openings to the total projected area in the exhaust flow direction of the structure is 30% to 90%. It is characterized by being set.

In the above configuration, since the ground electrode is disposed across the exhaust passage, that is, the ground electrode is disposed orthogonal to the exhaust flow, the particulate matter (PM) contained in the exhaust gas is effectively removed from the ground electrode. Capable of electrostatic aggregation.
The ground electrode is constituted by a structure having a large number of openings, and the hole area ratio of the structure (area ratio of the openings in the total projected area with respect to the exhaust flow direction of the structure) is 30% to 90%. Therefore, a desired agglomeration effect can be obtained within a range that does not deteriorate the pressure loss. That is, when the porosity of the structure is less than 30%, the surface area of the structure is increased, and the agglomeration effect is increased. However, on the other hand, the pressure loss is rapidly increased. In addition, when the opening ratio of the structure exceeds 90%, it is difficult to manufacture the structure itself. Therefore, by setting the opening ratio of the structure to 30% to 90%, an agglomerator having a high agglomeration effect can be realized without deteriorating pressure loss.

(Invention of Claim 2)
2. The exhaust gas purification apparatus according to claim 1, wherein the structure constituting the ground electrode has an opening of 0.5 mm to 2 mm when the diameter of an inscribed circle inscribed in the shape of the opening is defined as an opening. It is characterized by being set to.
If the opening is reduced, the agglomeration performance is improved, but clogging is likely to occur (pressure loss increases). On the other hand, if the opening is made larger than necessary, the agglomeration performance decreases. Therefore, when the minimum value of the opening is obtained from the relationship with the exhaust flow velocity and the maximum value of the opening is obtained from the relationship with the aggregation performance, the opening is set to 0.5 mm to 2 mm, thereby causing clogging. The desired agglomeration performance can be ensured without this.

(Invention of Claim 3)
3. The exhaust gas purifying apparatus according to claim 1, wherein the structure constituting the ground electrode is a metal in which the shape of the opening is a polygon including a square or a hexagon, or a circle including an ellipse or a circle. It is a mesh.
A metal mesh can be used as the structure constituting the ground electrode. The shape of the opening formed in the metal mesh is not necessarily a mesh shape (square), and a polygon including a quadrangle or a hexagon, a round including an ellipse, or the like can be adopted.

(Invention of Claim 4)
The exhaust emission control device according to claim 3 is characterized in that a plurality of metal meshes are arranged so as to overlap each other in the exhaust flow direction, or are separated by a predetermined distance.
The metal mesh does not need to be a single sheet, and by arranging a plurality of metal meshes in the exhaust flow direction or by separating them by a predetermined distance, the agglomeration effect can be enhanced without increasing pressure loss. Is possible.

(Invention of Claim 5)
The exhaust emission control device according to claim 1 or 2, wherein the structure constituting the ground electrode is a porous body having a depth in the exhaust flow direction.
By using a porous body having a depth as a structure constituting the ground electrode, the surface area of the ground electrode is increased, so that the aggregation efficiency is increased and the wall surface is passed when PM passes through the porous body. The agglomeration effect by the collision with can also be expected.

  The best mode for carrying out the present invention will be described in detail by the following examples.

FIG. 1 is a cross-sectional view showing the configuration of the exhaust emission control device.
As shown in FIG. 1, the exhaust emission control device shown in the first embodiment includes an aggregator 1 disposed in an exhaust passage of an internal combustion engine (for example, a diesel engine).
The aggregator 1 is disposed on the downstream side of the discharge electrode 4 with respect to the housing 3 connected to the exhaust pipe 2 and forming a part of the exhaust passage, the discharge electrode 4 attached to the housing 3, and the discharge electrode 4. And a ground electrode (metal mesh 5).

  The housing 3 is made of metal (for example, SUS), is provided in a cylindrical shape having an outer diameter larger than that of the exhaust pipe 2, and is grounded. The housing 3 has a cylindrical central axis facing the top-and-bottom direction, and an exhaust pipe 2 disposed upstream of the aggregator 1 is connected to the upper side surface of the housing 3 so that the interior of the housing 3 As shown in FIG. 1, the exhaust gas flows from the top to the bottom.

  The discharge electrode 4 is provided in a rod shape, is insulated and held by a cylindrical insulator 6, and is attached to the central portion of the upper end surface 3 a of the housing 3 via the insulator 6. One end side of the discharge electrode 4 protrudes into the housing 3, and its tip is exposed from the insulator 6. In addition, for example, a large number of projections 7 projecting radially in a star shape are provided at the tip portion. By providing a large number of protrusions 7 at the tip of the discharge electrode 4, the discharge rate can be increased, and corona discharge can be evenly generated inside the housing 3. The other end side of the discharge electrode 4 is taken out of the housing 3 and the rear end portion exposed from the insulator 6 is connected to a DC high voltage power source (not shown).

  As shown in FIG. 2, the ground electrode is formed of a circular metal mesh 5 having a large number of air holes 5a (openings of the present invention). The metal mesh 5 is arranged across the interior of the housing 3 so as to be orthogonal to the exhaust flow flowing through the interior of the housing 3, and its outer peripheral edge is fixed to the inner peripheral surface of the housing 3. Is connected to ground. In addition, the ventilation hole 5a of the metal mesh 5 is not limited to a specific shape. For example, as shown in FIG. 3, (a) a square, (b) a circle, (c) a hexagon, etc. are conceivable. .

In the metal mesh 5, the opening ratio of the vent holes 5a (the area ratio of the total vent holes 5a in the total projected area of the metal mesh 5 with respect to the exhaust flow direction) is set to 30% to 90%.
The individual vents 5a have a size that does not hinder the passage of the aggregated and coarsened PM group. Specifically, as shown in FIG. 3, when the diameter of an inscribed circle inscribed in the vent hole 5a (inner diameter when the vent hole 5a is circular) is defined as an aperture d, the aperture d Is set to 0.5 mm to 2 mm.

  The attachment position of the metal mesh 5 is such that the distance (axial distance) between the tip portion (projection 7) of the discharge electrode 4 and the metal mesh 5 is the tip portion (projection 7) of the discharge electrode 4 and the inner peripheral surface of the housing 3. It is desirable to arrange it to be longer than the distance (distance in the radial direction). As a result, when the exhaust gas flow rate is low, PM is aggregated mainly on the inner peripheral surface of the housing 3, and when the exhaust gas flow rate is high, PM that cannot move to the inner peripheral surface of the housing 3 due to the exhaust flow. The metal mesh 5 can be aggregated so as to be orthogonal to the exhaust flow.

Next, the operation of the exhaust purification system of this embodiment will be described.
a) At low flow rate with low exhaust gas flow rate (see Fig. 4)
When a negative DC high voltage (for example, −20 KV) is applied to the discharge electrode 4 from a DC high voltage power source, a corona discharge is generated in the vicinity of the tip (projection 7), and electrons are emitted (1 in the figure). Oxygen with high affinity is negatively ionized (2 in the figure). When this ionized oxygen adheres to nearby PM, the negatively charged PM (3 in the figure) moves due to the Coulomb force and the exhaust flow. However, since the exhaust flow is weak at a low flow rate, the charged PM is mainly attracted to the inner peripheral surface of the housing 3 by the Coulomb force (4 in the figure), and the PMs aggregate and become coarse (5 in the figure). ). The coarse PM group emits electrons to the grounded housing 3 (6 in the figure), and then leaves the inner peripheral surface of the housing 3 by its own weight or by an exhaust flow, and the ventilation holes of the metal mesh 5 It moves downstream through 5a (7 in the figure).

b) At high flow rate with large exhaust gas flow rate (see Fig. 5)
At this high flow rate, the exhaust flow becomes stronger than at the low flow rate, so that the PM attracted to the inner peripheral surface of the housing 3 by the Coulomb force decreases, and the PM that moves by the exhaust flow increases (4 in the figure). As a result, the PM that moves due to the exhaust flow is attracted to the metal mesh 5, and the PMs aggregate and coarsen in the metal mesh 5 (5 in the figure). The coarse PM group is released from the metal mesh 5 by its own weight or by exhaust flow after emitting electrons (6 in the figure) to the grounded metal mesh 5 as in the case of a low flow rate. Then, it moves downstream through the vent 5a of the metal mesh 5 (7 in the figure).

(Effect of Example 1)
The aggregator 1 has a metal mesh 5 that forms a ground electrode on the downstream side of the discharge electrode 4, and the interior of the housing 3 is arranged so that the metal mesh 5 is orthogonal to the exhaust flow flowing inside the housing 3. It is arranged across. According to this configuration, for example, even when PM is pushed away by the exhaust flow and cannot be agglomerated on the inner peripheral surface of the housing 3 at a high flow rate at which the exhaust flow becomes strong, the metal mesh is attracted to the metal mesh 5. 5 can be agglomerated. As a result, PM can be efficiently aggregated not only at a low flow rate but also at a high flow rate.

  Moreover, the metal mesh 5 described in Example 1 can obtain a desired aggregating effect within a range in which the pressure loss is not deteriorated because the opening ratio of the air holes 5a is set to 30% to 90%. That is, when the opening ratio of the air holes 5a is reduced, the surface area of the metal mesh 5 is increased, so that the agglomeration effect is increased, but on the other hand, the pressure loss is increased. Therefore, when the minimum value of the hole area ratio is determined from the relationship between the hole area ratio and the pressure loss, the minimum hole area ratio that satisfies the predetermined value of the pressure loss is 30% as shown in FIG. In addition, when the opening ratio of the air holes 5a exceeds 90%, the pressure loss is reduced, but the manufacture of the metal mesh 5 is difficult. Therefore, by setting the aperture ratio of the vent hole 5a to 30% to 90%, the agglomerator 1 having a high aggregating effect can be realized without deteriorating the pressure loss.

  Further, in the metal mesh 5, the opening d of the vent hole 5a is set to 0.5 mm to 2 mm. If the opening d is reduced, the agglomeration performance is improved, but the pressure loss is increased. On the other hand, when the opening d is increased more than necessary, the aggregation performance is lowered. Therefore, when the minimum value of the opening d is obtained from the relationship with the exhaust flow rate, as shown in FIG. 7, when the maximum exhaust flow rate is 20 m / sec, the minimum value of the opening d that satisfies the pressure loss predetermined value = 0. .5mm. That is, in the region where the exhaust flow rate is up to 20 m / sec, the pressure loss predetermined value can be satisfied by setting the aperture d to 0.5 mm or more.

On the other hand, when the maximum value of the opening d is obtained from the relationship with the aggregation performance, the maximum value of the opening d satisfying the predetermined aggregation value = 2 mm as shown in FIG. That is, when the mesh size d exceeds 2 mm, desired aggregation performance cannot be obtained.
As a result, by setting the aperture d of the vent hole 5a to 0.5 mm to 2 mm, desired aggregation performance can be ensured without causing clogging.

(Modification)
In Example 1, although the example which uses the metal mesh 5 as a ground electrode was described, the metal mesh 5 can also be used in piles. Alternatively, the plurality of metal meshes 5 can be arranged apart by a predetermined distance.
In addition to the metal mesh 5, a porous body (for example, a metal honeycomb) having a depth in the exhaust flow direction can be used as the ground electrode.
In the first embodiment, the aggregator 1 is disposed in the vertical direction, but may be disposed in the extending direction (horizontal direction) of the exhaust pipe 2.

It is sectional drawing which shows the structure of an exhaust gas purification apparatus. It is a top view of a metal mesh. It is a top view of the vent hole explaining the definition of the mesh opening. It is operation | movement explanatory drawing of an aggregator (at the time of low flow rate). It is operation | movement explanatory drawing of an aggregator (at the time of high flow rate). It is a figure which shows the relationship between the aperture ratio of a metal mesh, and pressure loss. It is a figure which shows the relationship between an exhaust gas flow velocity and opening. It is a figure which shows the relationship between an opening and an average particle diameter.

Explanation of symbols

1 Aggregator 4 Discharge electrode 5 Metal mesh (ground electrode, conductive structure)
5a Vent (opening)

Claims (5)

By applying a high voltage between the discharge electrode disposed in the exhaust passage and the ground electrode, a corona discharge field is formed in the exhaust passage, and when exhaust gas flows through the corona discharge field, An exhaust purification device comprising an agglomerator that electrostatically aggregates particulate matter contained in exhaust gas,
The ground electrode is disposed across the exhaust passage downstream of the discharge electrode in the exhaust flow direction, and is configured by a conductive structure having many openings through which aggregated particulate matter can pass. An exhaust emission control device, wherein an area ratio of the opening to a total projected area of the structure with respect to an exhaust flow direction is set to 30% to 90%. The exhaust emission control device according to claim 1,
The structure constituting the ground electrode is characterized in that when the diameter of an inscribed circle inscribed in the shape of the opening is defined as an opening, the opening is set to 0.5 mm to 2 mm. Exhaust purification device. The exhaust emission control device according to claim 1 or 2,
The structure constituting the ground electrode is a metal mesh formed with a polygonal mesh including a quadrangle or a hexagon, or a circular mesh including a circle or an ellipse. . The exhaust emission control device according to claim 3,
An exhaust emission control device, wherein a plurality of the metal meshes are arranged so as to overlap each other in the exhaust flow direction, or are separated by a predetermined distance. The exhaust emission control device according to claim 1 or 2,
The exhaust purification apparatus according to claim 1, wherein the structure constituting the ground electrode is a porous body having a depth in the exhaust flow direction.

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