Patent Application
20090049821
The present invention relates to an exhaust gas purifier. More particularly, the present invention relates to an exhaust gas purifier which can efficiently remove particulate matter via a reaction when introducing a treatment target fluid containing particulate matter into the exhaust gas purifier.
It is known that a silent discharge occurs when disposing a dielectric between two electrodes and applying a high alternating voltage or a periodic pulse voltage between the electrodes, and active species, radicals, and ions are produced in the resulting plasma field to promote a reaction and decomposition of gases, particulate matter, and the like. It is known that this phenomenon can be utilized to remove toxic components contained in engine exhaust gas or incinerator exhaust gas.
For example, a plasma reactor that allows engine exhaust gas or incinerator exhaust gas to pass trough a plasma field to treat NOx, carbon particles, HC, CO, and the like contained in the engine exhaust gas or incinerator exhaust gas has been disclosed (e.g., Patent Document 1).
When treating components contained in exhaust gas by utilizing plasma, a large amount of energy is required to treat particulate matter (PM) such as carbon particles as compared with other chemical compounds contained in exhaust gas.
Moreover, the PM removal rate is comparatively low although a large amount of energy is required. Specifically, the PM removal performance increases to only a small extent even if the amount of energy supplied to the exhaust gas purifier is increased.
When treating exhaust gas using an exhaust gas purifier that generates plasma between unit electrodes and treats PM via a reaction, PM is normally charged by plasma when exhaust gas is introduced between the unit electrodes. The charged PM moves and adheres to the surface of the electrode due to the force of an electric field generated between the unit electrodes. The PM is thus collected on the surface of the unit electrode. The collected PM is oxidized to be removed from the exhaust gas by oxidizing active species (e.g., ozone) produced by the effect of plasma.
As described above, the PM oxidation treatment process using the exhaust gas purifier that utilizes plasma includes the collection step and the oxidation step. Since the PM removal performance is not improved even if the amount of energy supplied is increased, the rate-limiting step of the PM removal process is considered to be the collection step. For example, when increasing the total area of the unit electrodes of an exhaust gas purifier having a PM removal rate of about 60% in order to improve the collection capability, it is estimated from the following Deutsch Equation that the PM removal rate increases by about 20% by doubling the total area of the unit electrodes, and increases by about 30% by increasing the total area of the unit electrodes by a factor of three.
Collection efficiency (PM removal efficiency)=100×(1−Exp(−G×A/Q))
Q: Gas flow rate
A: Effective (collection) electrode area
G: Separation rate (dust separation rate due to electrode)
Specifically, it is considered that the dust collection capability is improved by increasing the total area of the unit electrodes so that the PM removal rate tends to increase. However, the size of the plasma reactor of the exhaust gas purifier increases to a large extent when increasing the area of the unit electrodes by a factor of two or three. Moreover, it is necessary to use a large power supply.
The present invention was conceived in view of the above-described problems. An object of the present invention is to provide an exhaust gas purifier which can efficiently remove particulate matter via a reaction when introducing a treatment target fluid containing particulate matter into the exhaust gas purifier.
in order to achieve the above object, the present invention provides the following exhaust gas purifier;
[1] An exhaust gas purifier comprising a plasma reactor that includes a plasma generating electrode formed by hierarchically stacking a plurality of unit electrodes at specific intervals, the plasma reactor generating plasma in a space formed between the unit electrodes by applying a pulse voltage between the unit electrodes so that exhaust gas introduced into the space can be treated via a reaction and a power supply that generates the pulse voltage, each of the unit electrodes including a conductive film and a dielectric disposed on at least one side of the conductive film, the unit electrodes being stacked so that the dielectric is positioned on at least one of opposing sides of the unit electrodes, the pulse voltage having a pulse width of 1 μs to 1 ms and a field intensity of 5 kV/cm or more, and the amount of energy supplied when applying the pulse voltage being 10 J or less per microgram of a treatment target substance.
[2] The exhaust gas purifier according to [1], wherein the pulse voltage has a pulse width of 20 to 50 μs and a field intensity of 10 to 200 kV/cm and the amount of energy supplied is 0.05 to 10 J or less per microgram of the treatment target substance.
[3] The exhaust gas purifier according to [1] or [2], wherein the distance between the adjacent unit electrodes is 2 mm or loss.
[4] The exhaust gas purifier according to any one of [1] to [3], wherein a pulse cycle when applying the pulse voltage is 10 μs to 10 ms.
[5] The exhaust gas purifier according to any one of [1] to [4], wherein a rise time of the pulse voltage is 100 μs or less.
The exhaust gas purifier according to the present invention exhibits an improved PM collection capability since the pulse cycle of the pulse voltage applied to the unit electrodes is as long as 1 μs to 1 ms. Since the field density is 5 kV/cm or more, a stable discharge can be achieved. Moreover, charged PM can be effectively moved. Since the amount of energy supplied when applying the pulse voltage is 10 J or less per microgram of the treatment target substance (PM), PM can be oxidized while supplying a minimum amount of energy.
1: plasma generating electrode, 2: unit electrode, 3: dielectric, 4: conductive film, 10: plasma reactor, 11: power supply, 12; conductive line, 21: casing, 22: inlet port, 23: outlet port 31 and 32: waveform, P: one direction (gas flow direction), Q: another direction, V: space, W: distance between unit electrodes, 100: exhaust gas purifier
Embodiments of the present invention are described in detail below with reference to the drawings. Note that the present invention is not limited to the following embodiments. Various modifications, improvements, and the like may be made with regard to the design without departing from the scope of the present invention based on common knowledge of a person skilled in the art. In the drawings, identical elements are indicated by identical symbols.
As shown in
In the exhaust gas purifier 100 according to this embodiment, the pulse voltage applied between the unit electrodes 2 has a pulse width of 1 μs (microsecond) to 1 ms (millisecond) and a field intensity of 5 kV/cm or more, and the amount of energy supplied when applying the pulse voltage is 10 J or less per microgram of the treatment target substance.
The pulse width of the pulse voltage is 1 μs to 1 ms, and preferably 5 to 50 μs. The term “pulse width” of the pulse voltage used herein refers to a period (pulse width) of a pulse voltage shown in
If the pulse width is less than 1 μs, the PM collection capability cannot be improved. If the pulse width is greater than 1 ms, it is necessary to increase the amount of energy supplied for a PM oxidation reaction.
The field intensity when applying the pulse voltage is 5 kV/cm or more, preferably 10 to 200 kV/cm, and more preferably 20 to 150 kV/cm. A stable plasma discharge can be achieved by setting the field intensity at 5 kV/cm or more. Moreover, a strong force acts on the charged PM due to a high field intensity so that the PM can be effectively moved. If the field intensity is lower than 5 kV/cm, plasma cannot be stably generated. Moreover, a sufficiently strong force cannot be applied to the charged PM.
The amount of energy supplied when applying the pulse voltage is 10 J or less, preferably 0.05 to 10 J, and particularly preferably 0.1 to 2 J per microgram of PM (i.e., treatment target substance). Since the PM removal efficiency is not improved even if the amount of energy supplied is increased to a large extent, it is preferable to reduce the amount of energy supplied within such a range that PM can be oxidized from the viewpoint of a reduction in energy consumption and energy cost. If the amount of energy supplied is larger than 10 J per microgram of the treatment target substance, energy is unnecessarily consumed so that the energy cost increases. It is preferable that the amount of energy supplied be 0.05 J/microgram or more in order to reliably oxidize PM in a PM oxidation step of the PM oxidation treatment process. If the collected PM cannot be sufficiently oxidized, PM may be deposited on the surface of the unit electrode and may serve as a conductor, whereby plasma may tot be generated between the unit electrodes.
In the exhaust gas purifier according to is embodiment, it is preferable that the distance W between the adjacent unit electrodes 2 be 3 nm or less, and more preferably 0.1 to 2.0 mm. The field intensity can be easily increased by setting the distance between the unit electrodes 2 at 2 mm or less. Moreover since the distance from the unit electrode when PM moves due to the force of an electric field decreases, the PM collection capability can be improved. If the distance between the unit electrodes is longer than 2 mm, since PM moves over a long distance due to the force of an electric fields the PM collection capability may decrease. If the distance between the unit electrodes is long, it may be necessary to supply a large amount of power, because the voltage required to obtain a desired field intensity increases and it is necessary to maintain the PM oxidization capability. Note that the distance W between the adjacent unit electrodes 2 refers to the distance between the opposing surfaces of the adjacent unit electrodes 2 in other words the distance between the opposing surfaces of the dielectrics 3 of the it electrodes 2.
In the exhaust gas purifier according to this embodiment, the pulse cycle when applying the pulse voltage is set at 10 μs to 10 ms. The term “pulse cycle” used herein refers to a period between the rise timing (i.e., 0% of the peak value) of one pulse and the rise timing (i.e., 0% of the peak value) of the next pulse. The term “pulse cycle” used herein refers to a period between the rise timing (i.e. 0% of the peak value) of one pulse and the rise timing (i.e., 0% of the peak value) of the next pulse. Therefore, it is preferable that the pulse cycle be greater than the pulse width so that successive pulses do not overlap.
In the exhaust gas purifier according to is embodiment, it is preferable that the rise time of the pulse voltage be 100 μs or less, and more preferably 5 μs or less. It is preferable that the rise time of the pulse voltage is 100 μs or less, because an oxidizing excited species can be efficiently generated in the plasma generating space so that the PM oxidation performance is improved. If the rise time of the pulse voltage is longer than 100 μs, the collected PM oxidation performance decreases. Therefore, the PM collected on the surface of the electrode cannot be treated and is deposited on the surface of the electrode, whereby a discharge between the electrodes becomes unstable. As a result, the PM removal performance may deteriorate to a large extent. The term “rise time” of the pulse voltage used herein refers to a period of time of a pulse voltage shown in
In the plasma generating electrode 1 of the plasma reactor 10 shown in
As shown in
In this embodiment, the thickness of the conductive film 4 of the unit electrode 2 is preferably 0.001 to 0.1 mm, and more preferably 0.005 to 0.05 mm in order to reduce the size of the plasma generating electrode 1 and to reduce the resistance of the treatment target fluid which is caused to pass through the space between the unit electrodes 2 when treating an exhaust gas or the like.
The conductive film 4 used in this embodiment preferably includes a metal exhibiting excellent conductivity as the main component. As suitable examples of the main component of the conductive film 4, at least one metal selected from the group consisting of tungsten, molybdenum, manganese, chromium, titanium, zirconium, nickel, iron, silver, copper, platinum, and palladium can be given. In this embodiment, the term “main component” refers to a component that accounts for 60 mss % or more of components of the conductive film.
It is preferable that the conductive film 4 of the unit electrode 2 be applied to the dielectric 3. As suitable examples of the application method, printing, rolling, spraying, electrostatic painting, dipping, knife coating, and the like can be given. According to these methods, at conductive film 4 exhibiting excellent surface smoothness after application can be easily formed. In this case, it is preferable to use a sheet-shaped ceramic body as the dielectric 3.
When applying the conductive film 4 to the dielectric, a powder of a metal mentioned above as the main component of the conductive film 4, an organic binder, and a solvent such as terpineol may be mixed to prepare a conductive paste, and the conductive paste may be applied to the dielectric 3 using the above-mentioned method. When using a sheet-shaped ceramic body as the dielectric 3, an additive may optionally be added to the conductive paste in order to improve the adhesion between the conductive film 4 and the sheet-shaped ceramic body and improve sintering properties.
In this embodiment, it is preferable that the dielectric 3 that forms the unit electrode 2 be a sheet-shaped ceramic body formed using a high-dielectric-constant material as the main component. As the material for the sheet-shaped ceramic body, aluminum oxide, zirconium oxide, silicon oxide, mullite, cordierite, a titanium-barium type oxide, a magnesium-calcium-titanium type oxide, a barium-titanium-zinc type oxide, silicon nitride, aluminum nitride, or the like may be suitably used. It is preferable to form the dielectric 3 using a material appropriately selected from these materials and suitable for generating plasma appropriate for a PM reaction. The plasma generating electrode can be utilized at a high temperature using a material exhibiting excellent thermal impact resistance as the main component of the dielectric.
For example, copper metallization technology may be utilized for a rower-temperature cofired substrate material (lower-temperate cofired ceramic: LTCC) in which a glass component is added to aluminum oxide (Al2O3). Use of the copper metallization technology enables formation of an electrode exhibiting a low resistance and a high discharge efficiency, whereby the size of the electrode can be reduced. This enables a design which prevents thermal stress, whereby the strength of the electrode can be improved. When forming the electrode using a high-dielectric-constant material such as barium titrate, a magnesium-calcium-titanium type oxide, or a barium-titanium-zinc type oxide, the size of the electrode can be reduced due to high discharge efficiency. Therefore, structural design which can reduce occurrence of thermal stress due to high thermal expansion can be achieved.
The relative dielectric constant of the dielectric 3 may be appropriately determined corresponding to the plasma to be generated. The relative dielectric constant of the dielectric 3 is preferably 2 to 10.
The thickness of the dielectric 3 is preferably 0.1 to 3 mm, although the thickness of the dielectric 3 is not particularly limited. If the thickness of the dielectric 3 is less than 0.1 mm, a pair of adjacent unit electrodes 2 may not be electrically insulated. If the thickness of the dielectric 3 exceeds 3 mm, the size of the resulting exhaust gas purifier may not be reduced. Moreover, the applied voltage must be increased due to an increase in electrode-to-electrode distance, whereby efficiency may decrease.
When using a sheet-shaped ceramic body as the dielectric 3, it is preferable to fire a ceramic green sheet for a ceramic substrate to produce a sheet-shaped ceramic body. The ceramic green sheet may be obtained by forming a green sheet slurry or paste to a specific thickness using a known method such as a doctor blade method, a calender method, a printing method, or a reverse roll coating method. The resulting ceramic green sheet may be subjected to cutting, shaving, punching, or communication opening formation, or may be used as an integral laminate in which a plurality of green sheets are stacked and bonded by thermocompression bonding or the like.
As the green sheet slurry or paste, a mixture prepared by mixing an appropriate binder, sintering agent, plasticizer, dispersant, organic solvent, and the like into a specific ceramic powder may be suitably used. As suitable examples of the ceramic powder, alumina, mullite, cordierite, zirconia, silica, silicon nitride, aluminum nitride, ceramic glass, glass, and the like can be given. A suitable examples of the sintering agent, silicon oxide, magnesium oxide, calcium oxide; titanium oxide, zirconium oxide, and the like can be given. The sintering agent is preferably added in an amount of 3 to 10 parts by mass based on 100 parts by mass of the ceramic powder. As the plasticizer; dispersant and organic solvent, those used for a known method may be suitably used.
The porosity of the sheet-shaped ceramic body is preferably 0.1 to 35%, and more preferably 0.1 to 10%. This enables plasma to be efficiently generated between the unit electrodes 2 including the sheet-shaped ceramic body 3 so that energy consumption can be reduced.
The casing 21 of the plasma reactor 10 shown in
The material for the casing 21 used in this embodiment is not particularly limited. For example, it is preferable to use ferritic stainless steel having excellent conductivity, being lightweight and inexpensive, and showing only a small amount of deformation due to thermal expansion, as the material for the casing 21.
As shown in
The power supply 11 is not particularly limited insofar as the above-deseribed specific pulse voltage can be generated. As examples of a power supply that generates a pulse voltage by ON/OFF-controlling a high-voltage switch, “HVP-10K” (manufactured by Haiden Laboratory Co., Ltd.) and the like can be given.
The exhaust gas purifier 100 according to this embodiment configured as described above may be installed in an automotive exhaust system, for example. In this case, exhaust gas is caused to pass through a plasma generated in the space V formed between the unit electrodes, whereby PM contained in the exhaust gas can be efficiently oxidized.
A method of producing an exhaust gas purifier according to one embodiment of the present invention is described in detail below.
A plasma generating electrode that forms the exhaust gas purifier is produced. A ceramic green sheet used for the above-described sheet-shaped ceramic body is formed. For example a sintering agent, a binder such as a butyral resin or a cellulose resin, a plasticizer such as DOP or DBP, an organic solvent such as toluene or butadiene, and the like are added to at least one material selected from the group consisting of alumina, mullite, cordierite, mullite, silicon nitride, aluminum nitride, ceramic glass, and glass. The components are sufficiently mixed using an alumina pot and alumina cobblestone to prepare a slurry for forming a green sheet. The slurry for forming a green sheet may be prepared by mixing these materials by ball milling using a mono ball.
The resulting slur for forming a green sheet is stirred under reduced pressure to remove bubbles. The viscosity of the slur for forming a green sheet is then adjusted to a specific value. The slur for forming a green sheet thus prepared is formed in the shape of a tape using a tape forming method such as a doctor blade method to form a plurality of types of unfired ceramic bodies.
A conductive paste for forming a conductive film disposed on one side of the unfired ceramic body is prepared. The conductive paste may be prepared by adding a binder and a solvent such as terpineol to a silver powder, and sufficiently kneading the mixture using a triple roll mill, for example.
The resulting conductive paste is printed on the surface of one of the unfired ceramic bodies by screen printing or the like to form a conductive film with a specific shape to obtain a conductive film-containing unfired ceramic body. In order to supply electricity to the conductive film from the outside of the unit electrode after forming the unit electrode by holding the conductive film between the ceramic bodies, the conductive paste is preferably printed so that the conductive film reaches the outer edge of the unfired ceramic body.
The conductive film-containing unfired ceramic body and another unfired ceramic body are stacked so that the printed conductive film is covered. It is preferable to stack the unfired ceramic bodies at a temperature of 100° C. while applying a pressure of 10 MPa. Then, the unfired ceramic bodies that have been stacked with the conductive film held therebetween are fired to form a unit electrode including the sheet-shaped ceramic body and the conductive film.
The resulting unit electrodes are then stacked. In this case, in order to provide a specific interval between the unit electrodes, a ceramic rod in the shape of a quadrangular prism is formed using a raw material similar to that of the ceramic body, and is placed between the unit electrodes. The thickness of the ceramic rod corresponds to the distance between the unit electrodes. The ceramic rods are placed between the unit electrodes so that the ceramic rods are disposed in parallel to provide gas passages when treating exhaust gas or the like. The ceramic rod need not be in the shape of a quadrangular prism, but may be in the shape of a cylinder, a prism other than a quadrangular prism, or another prism shape. A plurality of protrusions may be formed on one side of the ceramic body, and a space may be formed by disposing the unit electrodes with the protrusions interposed therebetween. Projections and depressions may be formed on the ceramic bodies, and a space may be formed by combining the projections and the depressions. The plasma generating electrode of the exhaust gas purifier according to this embodiment can be obtained by hierarchically stacking the unit electrodes with the ceramic rods interposed therebetween. The casing 21 of the exhaust gas purifier according to this embodiment may be obtained by forming ferrite-type stainless steel or the like into a tube having the gas inlet port 22 and the gas outlet port 23 (see
The exhaust gas purifier 100 according to this embodiment may be obtained by providing the plasma generating electrode in the casing and connecting a specific power supply to the plasma generating electrode (see
The present invention is described in detail below by way of examples. Note that the present invention is not limited to the following examples.
A plasma reactor configured as shown in
A plasma generating electrode used in the exhaust gas purifier of this example was produced as follows. A conductive film (thickness; 10 μm) was screen-printed on the inner side of an unfired alumina tape substrate (thickness: 1.0 mm when stacking and firing two substrates) using a tungsten paste to produce a unit electrode. The outer dimensions of the unit electrode were 90 mm×50 mm. The dimensions of the printed conductive film were 84 mm×46 mm. Thirty-eight unit electrodes were produced and stacked to produce a plasma generating electrode. The distance between the unit electrodes was 0.5 mm. The outer circumference of the plasma generating electrode was held using a heat-resistant mat. The plasma generating electrode was placed in a casing made of 430SS to obtain a plasma reactor. A pulse power supply was connected with the plasma reactor to obtain an exhaust gas purifier. As the pulse power supply, a high-voltage pulse power supply of which the pulse width could be increased was used. The 38-stage unit electrodes were alternately connected to the pulse power supply and ground.
An exhaust gas treatment test was performed using the exhaust gas purifier utilizing exhaust gas (200° C., 1 Nm3/min) containing 21.6 mg of particulate matter (PM) to measure the PM removal rate. As the pulse voltage application conditions, the pulse width was set at 30 μs, the field intensity was set at 31.7 kV/cm, and the amount of energy supplied was set at 1.4 J per microgram of the treatment target substance. The term “PM removal rate (%)” refers to a value obtained by dividing the PM content (g/m3) in the gas discharged from the exhaust gas purifier by the PM content (g/m3) in the gas flowing into the exhaust gas purifier, and multiplying the resulting value by 100. The results are shown in Table 1.
An exhaust gas purifier was produced in the same manner as in Example 1. The exhaust gas treatment test was performed to measure the PM removal rate in the same manner as in Example 1, except that the pulse width was set at 4.7 μs, the field intensity was set at 35.2 kV/cm, and the amount of energy supplied was set at 1.78 J per microgram of the treatment target substance. The results are shown in Table 1.
A plasma reactor was produced in the same manner as in Example 1. An exhaust gas purifier was produced using a high-voltage pulse power supply that applied a voltage having a short pulse width as the pulse power supply. The exhaust gas treatment test was performed to measure the PM removal rate using the resulting exhaust gas purifier in the same manner as in Example 1, except that the pulse width was set at 0.9 μs, the field intensity was set at 30 kYV/cm, and the amount of energy supplied was set at 2.0 J per microgram of the treatment target substance. The PM removal rate was measured. The results are shown in Table 1.
As shown in Table 1, the exhaust gas purifiers of Examples 1 and 2 exhibited an excellent PM removal rate due to a due pulse width.
The exhaust gas purifier according to the present invention can be used to remove particulate matter contained in engine exhaust gas or incinerator exhaust gas. In particular, the exhaust gas purifier according to the present invention can be suitably utilized as an exhaust gas purifier which can efficiently remove particulate matter with a low energy loss and a low cost of equipment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-094339 | Mar 2006 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/056146 | Mar 2007 | US |
| Child | 12239385 | US |
