WO2005083241A1

WO2005083241A1 - Plasma reactor power source, plasma reactor, exhaust gas purification device and exhaust gas purifying method

Info

Publication number: WO2005083241A1
Application number: PCT/JP2005/003543
Authority: WO
Inventors: Masaru Kakinohana
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2004-02-27
Filing date: 2005-02-24
Publication date: 2005-09-09
Also published as: JP2005240738A

Abstract

There are provided a plasma reactor power source (38), a plasma reactor (30), an exhaust gas purification device and an exhaust gas purifying method, whereby both collection and combustion removal of PM are accomplished. The plasma reactor power source (38) of this invention is characterized by generating a superimposed voltage (c) having a direct current voltage component (a) and a pulse voltage component (b). The plasma reactor (30) is characterized by having an electrode pair sandwiching an exhaust gas flow channel, wherein a voltage is applied to the electrodes (34, 36) by a plasma reactor power source (38) according to the invention. The exhaust gas purification device is characterized by comprising a plasma reactor (30) of the invention and a PM collector downstream from the exhaust gas flow.

Description

DESCRIPTION

PLASMA REACTOR POWER SOURCE, PLASMA REACTOR, EXHAUST GAS PURIFICATION DEVICE AND EXHAUST GAS PURIFYING METHOD

Technical Field The present invention relates to purification of exhaust gas from an internal combustion engine and the like and, specifically, relates to the removal of particulate matter (hereinafter, "PM") emitted from a diesel engine.

Background Art Diesel engines are commonly installed in automobiles and, particularly, in trucks and, in recent years, it has been a strongly desired goal to reduce emissions of PM along with nitrogen oxides, carbon monoxide and hydrocarbons in the exhaust gas. A technology has therefore been developed for fundamentally reducing PM through engine modifications and optimization of combustion conditions, while techniques have also been established for efficiently removing PM in exhaust gas. Ceramic honeycomb filters, alloy filters and ceramic fiber filters are employed for removal of PM in exhaust gas. With prolonged use, however, such methods result in clogging of the filters by the collected PM, thereby increasing the air-flow resistance and increasing the load on the engine. In addition, it has not been possible to achieve adequate oxidation removal of PM by exhaust heat when the PM is collected using conventional filters . Discharge devices are known in the prior art as exhaust gas purification devices for diesel engines. For example, Japanese Unexamined Patent Publication No. 6- 146852 discloses a method of providing a pair of plasma treatment electrodes sandwiching the filter element of a ceramic filter, whereby a high-frequency voltage or pulse high voltage is applied between the electrodes to generate a discharge. This accomplishes continuous removal of PM accumulated on the ceramic filter as it is subjected to low temperature oxidation by plasma, whereby an increase in a pressure loss is prevented. It has therefore been disclosed that in the conventional exhaust gas purification devices which utilize discharge, PM collected on the ceramic filter is removed by low temperature oxidation caused by the discharge. However, as the collection of PM depends on the filter, the essential problem of pressure loss has not been solved. In order to overcome this problem, the inventers of the present application provides an exhaust gas purifying apparatus for trapping and burning PM which comprises electrodes and an insulative honeycomb structure having a number of cell passages in Japanese Unexamined Patent Publication No. 2004-239257. The apparatus is characterized in that the electrodes make an electric field in the honeycomb structure, the electric field being non-parallel to the direction of the cell passages of the honeycomb structure. For this purpose, the honeycomb structure particularly may have opposite outer surfaces, and the electrodes may comprise a pair of plate electrodes respectively placed on said opposite outer surfaces of the honeycomb structure as stated in Japanese Unexamined Patent Publication No. 2004-270467 of the inventors . According to these exhaust gas purifying apparatus, the PM in the exhaust gas passing through the cell passages of the honeycomb structure is deposited onto the sidewalls of the cell passages of the honeycomb structure by the Coulomb force between it and the electric field that is not parallel with the direction of the cell passages of the honeycomb structure. Further, the PM deposited in the honeycomb structure is burned with the use of thermal energy of an exhaust gas and also an electrical current that passes through the deposited PM rather than the insulative honeycomb structure.

Disclosure of Invention According to the present invention there is provided a plasma reactor power source, a plasma reactor, an exhaust gas purification device and an exhaust gas purifying method, whereby both collection and combustion removal of PM are preferably accomplished. The invention is particularly suited for purification of exhaust gas from diesel engines, but may also be used for purification of other exhaust gases, such as an exhaust gas from internal combustion engines such as gasoline engines. The plasma reactor power source of the invention is characterized by generating a superimposed voltage having a direct current voltage component and a pulse voltage component . According to the plasma reactor power source of the invention, the direct current voltage component promotes electrification and electrostatic collection of PM, while the pulse voltage component promotes activation of the exhaust gas component and combustion removal of the PM. The "direct current voltage component" according to the invention means that the voltage is applied in a constant direction during at least the residence time of the exhaust gas in the plasma reactor, and includes a change in the direction of the applied voltage. Thus, the direct current voltage component according to the invention may be a voltage applied in a constant direction during a predetermined period, for example 10 minutes. The direct current voltage component may, of course, be consistent in a direction. The "pulse voltage component" according to the invention means a voltage exhibiting a waveform which rapidly increases and then rapidly decreases, and includes not only direct current pulses but also alternating current pulses. For example, the pulse width of the pulse voltage component may be 10 nsec to 10 μsec. In one embodiment, the plasma reactor power source of the invention may alternately change the direction of the direct current voltage component, and especially the directions of both the direct current voltage component and the pulse voltage component. According to this embodiment, the direction of the electric field in the plasma reactor is changed to allow changing the direction to which the electrified PM is drawn by Coulomb force. Thus, accumulation of PM in the plasma reactor can occur in a relatively uniform manner. The plasma reactor of the invention is a plasma reactor having an electrode pair sandwiching an exhaust gas flow channel, and it is characterized in that a voltage is applied to the electrode pair by the plasma reactor power source of the invention. According to the plasma reactor of the invention, the direct current voltage component applied to the electrode pair promotes electrification and electrostatic collection of PM, while the pulse voltage component promotes activation of the exhaust gas component and combustion removal of the PM. In one embodiment of the plasma reactor of the invention, the electrode pair sandwiching the exhaust gas flow channel can produce an electric field which is non- parallel to the exhaust gas flow direction, particularly an electric field at an angle of larger than 45° or 60° with respect to the exhaust gas flow direction, and more particularly an electric field vertical to the exhaust gas flow direction. According to this embodiment, the exhaust gas flow direction is different from the direction in which the electrified PM is drawn by Coulomb force as the effect of the electric field, such that accumulation of PM is promoted in a relatively uniform manner in the plasma reactor . In another embodiment, the plasma reactor of the invention is characterized in that a PM collection structure is located in the exhaust gas flow channel between the electrode pair. According to this embodiment, electrification and electrostatic collection of PM is promoted by the direct current voltage component, while combustion removal of the collected PM is promoted by active radicals (for example, oxygen radicals, ozone and NO_x radicals) generated by the pulse voltage component. The PM collection structure may have any structure which is used for collection of PM in exhaust gas, and it may be a honeycomb structure or pellet filled layer, and particularly a honeycomb structure carrying either or both an exhaust gas purification catalyst or NO_x storage material. The exhaust gas purification catalyst may be a catalyst which promotes purification of exhaust gas, and particularly purification of PM, NO_x, CO and HC in exhaust gas; for example, it may be a PM oxidation catalyst, three-way catalyst, NO_x storage reduction catalyst or NO_x selective reduction catalyst. The NO_x storage material is an element selected from the group consisting of alkali metal elements, alkaline earth metal elements and rare earth elements. The exhaust gas purification device of the invention is characterized by comprising a plasma reactor according to the invention and a PM collector downstream thereof. According to the exhaust gas purification device of the invention, electrification of the PM by the direct current voltage component and activation of the exhaust gas component by the pulse voltage component are accomplished in the plasma reactor of the invention. The PM collector downstream thereof accomplishes collection of PM electrified by the plasma reactor, oxidation of PM by active components in the exhaust gas generated by the plasma reactor, and purification of NO_x, CO and HC. The PM collector may have any structure used for collection of PM in exhaust gas, and it may be a honeycomb structure or pellet filled layer, and particularly a honeycomb structure supporting an exhaust gas purification catalyst. The exhaust gas purification catalyst may comprises N0_X storage material. The PM collector used may be a plasma reactor with a honeycomb structure as shown in Fig. 3 and Fig. 4. The exhaust gas purification catalyst may be a catalyst which promotes purification of exhaust gas, and particularly purification of PM, NO_x, CO and HC in exhaust gas; for example, it may be a PM oxidation catalyst, three-way catalyst, N0_X storage reduction catalyst or NO_x selective reduction catalyst. The NO_x storage material is an element selected from the group consisting of alkali metal elements, alkaline earth metal elements and rare earth elements. The exhaust gas purifying method of the invention is characterized by applying a superimposed voltage having a direct current voltage component and a pulse voltage component to the electrodes of a plasma reactor having an electrode pair sandwiching an exhaust gas flow channel. According to the gas purifying method of the invention, the direct current voltage component promotes electrification and electrostatic collection of PM, while the pulse voltage component promotes activation of the exhaust gas components and combustion removal of the PM. The plasma reactor power source, plasma reactor, exhaust gas purification device and exhaust gas purifying method of the present invention can promote electrification and electrostatic collection of PM, as well as activation of the exhaust gas components and combustion removal of the PM, due to the pulse voltage component and direct current voltage component.

Brief Description of Drawings Fig. 1 is an illustration of an example of superimposed voltage applied by a plasma reactor power source according to the invention. Fig. 2 is an illustration of another example of superimposed voltage applied by a plasma reactor power source according to the invention. Fig. 3 shows perspective and cross-sectional views of a plasma reactor according to the invention. Fig. 4 shows perspective and cross-sectional views of another plasma reactor according to the invention. Fig. 5 shows an example using a plasma reactor according to the invention. Fig. 6 is a block diagram of the apparatuses used in the example and comparative examples . Fig. 7 is a partial perspective view of the plasma reactor used in the example and comparative examples. Fig. 8 is a graph showing PM collection ratios obtained in the example and comparative examples . Fig. 9 is a graph showing NO —» N0₂ conversion ratios obtained in the example and comparative examples. Fig. 10 is a graph showing exhaust gas purification ratios obtained in the example and comparative examples.

Best Mode for Carrying Out the Invention The present invention will now be explained in greater detail based on the embodiments shown in the accompanying drawings, with the understanding that these drawings serve only as a general description of the invention and are not intended to limit its scope in any way. <Plasma reactor power source> An example of a superimposed voltage applied by the plasma reactor power source of the invention will now be explained with reference to Fig. 1. From the plasma reactor power source of the invention, a superimposed voltage (c) including a direct current voltage component (a) and pulse voltage component (b) , as represented in Fig. 1, is applied to the plasma reactor. As explained above, application of a pulse voltage component (b) is unsuitable for electrification and electrostatic collection of the PM, because of the short period of the generated electric field. Application of a pulse voltage, however, means that a large discharge energy is instantaneously supplied, and it is therefore appropriate for activation of the exhaust gas components and combustion removal of the PM. As an electric field is continuously generated with application of the direct current voltage component (a) , this is optimally suited for electrification and electrostatic collection of PM, but unsuitable for activation of exhaust gas components. Thus, when a superimposed voltage (c) including a direct current voltage component (a) and pulse voltage component (b) is applied to the plasma reactor as with the plasma reactor power source of the invention, the effect of the pulse voltage component promotes activation of the exhaust gas components and combustion removal of the PM, while the direct current voltage component promotes electrification and electrostatic collection of PM. The plasma reactor power source of the invention may apply a superimposed voltage (c) , including a direct current voltage component (a) with changing positive/negative directions and a pulse voltage component (b) with similarly changing positive/negative directions, to the plasma reactor, as represented in Fig. 2. The direct current voltage component applied by the plasma reactor power source of the invention may be set as a voltage which will electrify and electrostatically collect PM. The direct current voltage may be, for example, of 1 to 50 kV, and particularly 20 to 40 kV.

The direct current voltage component is a voltage applied in a constant direction at least during the residence time of the exhaust gas in the plasma reactor; this may be a period of, for example, 1 second or longer, 10 seconds or longer, 1 minute or longer, 10 minutes or longer or 1 hour or longer. The direct current voltage component may, of course, be always in a constant direction. The pulse voltage component applied by the plasma reactor power source of the invention may be set with a pulse frequency, pulse width and pulse voltage which allows generation of corona discharge. The pulse frequency, etc. will sometimes be subject to certain restrictions such as the device design and economic considerations, but a high voltage and short pulse are preferred from the standpoint of generating a satisfactory corona discharge. The pulse frequency, pulse width and pulse voltage may be a pulse frequency of 0.5 to 50 kHz and especially 2 to 20 kHz; a pulse width of 10 nsec to 10 μsec and especially 1 to 5 μsec; and a pulse voltage of 100 V to 50 kV, and especially 5 to 30 kV. The plasma reactor power source of the invention may be operated with continuous application of a superimposed voltage including a direct current voltage component and pulse voltage component, but normally a direct current voltage may be applied for collection of PM, with superimposition of a pulse voltage component, as necessary, to promote activation of the exhaust gas components and oxidation of the PM. The plasma reactor power source of the invention may be used to apply either a positive voltage or a negative voltage, but application of a negative voltage is preferred. This is because application of a positive voltage draws away electrons from the PM and the gas, whereas application of a negative voltage supplies electrons and is therefore advantageous in terms of energy. <Plasma reactor> The plasma reactor of the invention may be shown in Fig. 3. Fig. 3(a) is perspective view of a first embodiment of a plasma reactor of the invention, and Fig. 3 (b) is a cross-sectional view of the same plasma reactor. In the plasma reactor 30, a ground or outer electrode 34 is provided around a cylindrical honeycomb structure 32, and a rod-like (discharge) electrode 36 is located on the central axis of the honeycomb structure 32 as a counter electrode of the perimeter (outer) electrode 34. The perimeter electrode 34 is grounded, while the rod-like electrode 36 is connected to the plasma reactor power source 38 of the invention. The plasma reactor 30 is used in such a manner that the exhaust gas passes through the honeycomb structure 32 in the direction indicated by the arrow 39. In use, in the plasma reactor 30 shown in Fig. 3, the plasma reactor power source 38 is activated to produce an electric field between the perimeter electrode 34 and the rod-like electrode 36. The electric field is in the direction transverse toward the flow direction of the exhaust gas flowing through the channel of the honeycomb structure 32. Thus, a coulomb force, due to the electric field, forces the electrified PM toward the honeycomb walls of the honeycomb structure 32, thus promoting collection of the PM. The plasma reactor in which a voltage is applied by the plasma reactor power source of the invention may be one such as shown in Fig. 4. Fig. 4(a) is a perspective view of a second embodiment of a plasma reactor of the invention, and Fig. 4(b) is a cross-sectional view of the same plasma reactor. In this plasma reactor 40, flat electrodes 44 and 46 are located on the upper and lower sides of a rectangular honeycomb structure 42. The upper electrode 44 is grounded, while the lower electrode 46 is connected to the plasma reactor power source 48 of the invention. This plasma reactor 40 is used with the exhaust gas flowing through the honeycomb structure 42 in the direction indicated by the arrow 49. For use of the plasma reactor 40 shown in Fig. 4, the plasma reactor power source 48 is activated to produce an electric field between the upper electrode 44 and the lower electrode 46. The electric field is in the direction transverse toward the flow direction of the exhaust gas flowing through the channel of the honeycomb structure 42. Thus, Coulomb force by the electric field forces the electrified PM toward the honeycomb walls of the honeycomb structure 42, thus promoting collection of the PM. Each of the constituent parts of the plasma reactors of the invention shown in Fig. 3 and Fig. 4 will now be explained in detail. The honeycomb structure 32(42) may be a ceramic honeycomb structure such as a cordierite honeycomb structure. The honeycomb structure may also be a straight-flow type or wall-flow type, but is preferably a straight-flow type from the standpoint of air resistance; satisfactory PM collection can be achieved even when using such a straight-flow type honeycomb structure. A PM oxidation catalyst may be supported on the honeycomb structure 32(42) for combustion of the PM. As catalysts, there may be mentioned Ce0₂, Fe/Ce0₂, Pt/Ce0₂, Pt/Al₂0₃, Mn/Ce0₂, or Ce0₂ carrying an alkali metal or alkaline earth metal. Ce0₂ may also be replaced with Ce0₂-Zr0₂ solid solution or Fe₂0₃. One or a combination of two or more of these metal oxides may be used. Any desired method may be used to carry the metal oxides on the honeycomb structure 32(42), such as wash coating followed by firing. According to these modes, the honeycomb structure 32(42) is located between the perimeter (outer) electrode 34 and rod-like (center) electrode 36 (or between the upper electrode 44 and lower electrode 46) , but alternatively, the electrodes may directly face each other with nothing between them, for the exhaust gas flowing through the space between the electrodes. In this case, an insulating material such as A1₂0₃ may be coated on the inner surface of the perimeter electrode 34 (or on the exhaust gas channel sides of the upper electrode 44 and lower electrode 46) so that stable barrier discharge occurs between the electrodes. The electrodes 34 and 36 (44 and 46) may be produced using a material which allows application of a voltage therebetween. As such materials, there may be used conductive materials and semiconductor materials, but metal materials are preferred. As specific metal materials, there may be mentioned copper, tungsten, stainless steel, iron, platinum, aluminum and the like, among which stainless steel is particularly preferred from the standpoint of cost and durability. The rod-like electrode 36 used will usually be a metallic wire, but a hollow rod-like electrode may also be used. The perimeter electrode 34 (upper electrode 44 and lower electrode 46) may be fabricated by forming the material into a metal mesh or metal foil, and winding or attaching it around or to the honeycomb structure 32(42), or applying a conductive paste to the honeycomb structure 32(42). For the embodiment shown in Fig. 4, the upper electrode 44 and lower electrode 46 may be made of metal meshes of different roughness to promote discharge. Here, the electrode 36(44) is connected to the plasma reactor power source 38(48) while the other electrode 34(48) is grounded, but the opposite arrangement may also be employed. Also, both of the electrodes may be connected to the plasma reactor power source without grounding either one, for application of an opposite voltage. Also, either electrode may be the cathode or the anode. <Mode of use for plasma reactor> The plasma reactor of the invention may used in the manner illustrated in Fig. 5(a). Specifically, in this case, it is possible to locate the present plasma reactor downstream from the engine primarily for electrification of PM and activation of the exhaust gas components, and to locate a PM collector, particularly a honeycomb structure carrying a catalyst for purification of PM, NO_x, HC and CO, for collection and oxidation of the PM. With the arrangement shown in Fig. 5(a), the electrodes of the plasma reactor of the invention are located such that the electrodes face each other either directly or via an insulating coat on the surface of either or both electrodes, the exhaust gas flowing between the electrodes . The plasma reactor of the invention may also be used in the manner illustrated in Fig. 5(b). Specifically, in this case, the plasma reactor of the invention may be used not only for electrification of PM and activation of the exhaust gas components, but also for purification of NO_x, HC and CO, and collection and oxidation of PM. With the arrangement shown in Fig. 5(b), most preferably, an insulating honeycomb structure such as a ceramic honeycomb is located between the electrodes of the plasma reactor of the invention, and an exhaust gas purification catalyst, for example, a NO_x purification catalyst such as a three-way catalyst, NO_x storage reduction catalyst or NO_x selective reduction catalyst, or a PM oxidation catalyst, is supported on the honeycomb structure . The present invention will now be explained in greater detail by examples, which in no way limit the invention. As shown in Fig. 6, the exhaust gas from a 2.2 L displacement diesel engine was passed through a plasma reactor for an exhaust gas purification test. A portion of the exhaust gas from the plasma reactor was supplied to an exhaust gas analyzer to measure the HC, CO and NO_x purification ratio. Another portion of the exhaust gas from the plasma reactor was passed through a full dilution tunnel, and a portion thereof was supplied to quartz collecting filter for measurement of the PM collection ratio by a gravimetric method. Fig. 7 shows a schematic view of a plasma reactor used for the experiment. In this plasma reactor 70, a converter vessel 74 holding a ceramic honeycomb structure 76 was located downstream from an exhaust pipe 72 and a cone 73 having an alumina cylinder on the inner perimeter. The exhaust pipe 72, cone 73 and converter vessel 74 were all made of stainless steel. The ceramic honeycomb structure 76 was cylindrical with a diameter of 103 mm and a length of 155 mm, and it was coated with 240 g/honeycomb structure-L of A1₂0₃ and 0.2 mol/honeycomb structure-L of K. An approximately 5 mm thick ceramic fiber buffer material was placed between the ceramic honeycomb structure 76 and the converter vessel 74. A rod-like electrode 75 was placed on the center axis of the exhaust pipe 72, cone 73, converter vessel 74 and ceramic honeycomb structure 76. The rod-like electrode 75 was connected to a power source 78, and the stainless steel exhaust pipe 72 and converter vessel 74 were grounded as a counter electrode. <Comparative Example 1> For Comparative Example 1, a direct current voltage such as shown in Fig. 1(a) was applied from the power source 78 to the rod-like electrode 75 at a magnitude of -30 kV. That is, V₀ in Fig. 1(a) was -30 kV. <Comparative Example 2> For Comparative Example 2, a pulse voltage such as shown in Fig. 1(b) was applied from the power source 78 to the rod-like electrode 75 at a magnitude of -30 kV. That is, Vi in Fig. 1(b) was -30 kV. The pulse width was 2 μsec, and the frequency was 10 kHz. <Example> For the Example, a superimposed voltage as shown in Fig. 1(c) was applied from the power source 78 to the rod-like electrode 75. Here, both the direct current voltage component and pulse voltage component were -30 kV. That is, V₀ in Fig. 1(c) was -30 kV, and V₀+Vi was -^■ 60 kV. The pulse width was 2 μsec, and the frequency was 10 kHz. In Comparative Examples 1 and 2 and the Example, discharge primarily was generated between the exhaust pipe 72 and rod-like electrode 75 having an alumina cylinder on the inner perimeter. <PM collection efficiency test> The engine rotation speed was 2000 rpm and the torque was 30 Nm. The input gas temperature was approximately 200°C. The PM in the exhaust gas was collected with a quartz collecting filter for 3 minutes without the plasma reactor 70, and the amount was measured. Next, the plasma reactor 70 was mounted and three different applied voltages were used for Comparative Examples 1 and 2 and the Example for collection of PM in the exhaust gas with the quartz collecting filter, and the amounts were measured. The measurement was carried out 12 times every 10 minutes. The PM collection efficiency was determined for Comparative Examples 1 and 2 and the Example, based on the PM collection amount without the plasma reactor 70. The PM collection ratios obtained in Comparative Examples 1 and 2 and the Example are shown in Fig. 8. As seen in Fig. 8, the PM collection ratio decreased with time in Comparative Example 1 which employed a direct current, reaching essentially 0% in 90 minutes. This was attributed to the accumulation of PM and concomitant exfoliation of the accumulated PM, and to the fact that the accumulation of PM allowed an electric current to flow through the PM, thereby lowering the electric field strength in the honeycomb structure 76. In Comparative Example 2 which employed a pulse voltage, the PM collection ratio was initially lower than in Comparative Example 1. This was attributed to the fact that only an instantaneous voltage was applied, thereby decreasing the effect of electrification and electrostatic collection of PM compared to Comparative Example 1. However, in Comparative Example 2, the PM collection efficiency was not reduced with time as occurred in Comparative Example 1. This was attributed to continuous oxidation of the accumulated PM by the exhaust gas components which were activated by the pulse voltage, thereby preventing exfoliation of the accumulated PM and an electric current flow through the accumulated PM. In the Example which employed a superimposed voltage, the PM collection efficiency was high from the start, and the collection efficiency was maintained throughout. This was attributed to electrification and collection of PM achieved by the direct current voltage component, with continuous oxidation of the accumulated PM by the pulse voltage component, thereby preventing exfoliation of the accumulated PM and current flow through the accumulated PM. <NO -» N0₂ conversion efficiency test> This test was carried out without coating the honeycomb structure with K/A1₂0₃. The NO -» N0₂ conversion efficiencies for Comparative Examples 1 and 2 and Example are shown in Fig. 9. As seen in Fig. 9, a high NO — > N0₂ conversion efficiency was confirmed in Comparative Example 2 which employed a pulse voltage and in the Example which employed a superimposed voltage with a pulse voltage component. The generated N0₂ is an important component for oxidation of PM. In the PM collection efficiency test for Comparative Example 2 and the Example, therefore, the presence of N0₂ in the exhaust gas also presumably promoted oxidation of the accumulated PM. Moreover, it is highly probable that 0₃ and 0₂ ^~ are produced in addition to N0₂ in the test, and it is thought that they are produced in amounts similar to that of N0₂. <Exhaust gas purification test> The engine rotation speed was 2000 rpm and the torque was 30 Nm. The input gas temperature was approximately 200°C. The volumes of NO_x, HC and CO in the exhaust gas were measured with an exhaust gas analyzer 66, without mounting the plasma reactor 70. The plasma reactor 70 was subsequently mounted and three different applied voltages were used for Comparative Examples 1 and 2 and Example, for measurement of the volumes of NO_x, HC and CO with the exhaust gas analyzer. The NO_x, HC and CO purification ratios were determined for Comparative Examples 1 and 2 and Example based on the volumes of NO_x, HC and CO in the exhaust gas without the plasma reactor 70. The NO_x, HC and CO purification ratios obtained in Comparative Examples 1 and 2 and Example are shown in Fig. 10. As seen from Fig. 10, a high exhaust gas purification efficiency was exhibited in Comparative Example 2 and the Example, compared to Comparative Example 1 which did not employ a pulse voltage. This was attributed to activation of the exhaust gas components by the pulse voltage. Also, a higher exhaust gas purification ratio was exhibited in the Example, compared to Comparative Example 2. This was attributed to maintenance of a higher electric field strength in the Example.

Claims

CLAIMS 1. A plasma reactor power source characterized by generating a superimposed voltage having a direct current voltage component and a pulse voltage component.

2. The plasma reactor power source according to claim 1, wherein the direction of said direct current voltage component is maintained in a constant direction, and the pulse width of the pulse voltage component is 10 nsec to 10 μsec.

3. The plasma reactor power source according to claim 1 or 2, wherein the direction of said direct current voltage component is alternately changed.

4. A plasma reactor having an electrode pair sandwiching an exhaust gas flow channel, characterized in that a voltage is applied to said electrode pair by a plasma reactor power source according to any one of claims 1 to 3.

5. The plasma reactor according to claim 4, characterized in that said electrode pair produces an electric field which is non-parallel to the exhaust gas flow direction.

6. The plasma reactor according to claim 4 or 5, characterized in that a PM trapping structure is located in said exhaust gas flow channel between said electrode pair.

7. The plasma reactor according claim 4 characterized in that said electrode pair produces an electric field which is non-parallel to the exhaust gas flow direction; that an ceramic honeycomb structure is located in the exhaust gas flow channel; and that said honeycomb structure carries a PM oxidation catalyst selected from the group consisting of Ce02, Fe/Ce02, Pt/Ce02 and Pt/A1203, and combination thereof.

8. An exhaust gas purification device characterized by comprising a plasma reactor according to any one of claims 4 to 7 and a PM trapping part downstream therefrom.

9. An exhaust gas purifying method characterized by applying a superimposed voltage having a direct current voltage component and a pulse voltage component to the electrodes of a plasma reactor having an electrode pair sandwiching an exhaust gas flow channel.