JP4591164B2 - Exhaust gas purification method and exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification method and apparatus, and more particularly to an exhaust gas purification method and apparatus for purifying hydrocarbon components in exhaust gas.

The exhaust gas from internal combustion engines such as automobile engines includes nitrogen oxides (NO _x ), carbon monoxide (CO), hydrocarbons (HC), etc., but these substances oxidize CO and HC. and also it can be removed by an exhaust gas purifying catalyst for reducing NO _x. Typical examples of the exhaust gas purification catalyst include a three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina. It has been known.

  Such an exhaust gas purification catalyst can sufficiently achieve warm-up of the catalyst. For example, when the catalyst is at a temperature exceeding 300 ° C., it can achieve a relatively good exhaust gas purification performance. However, when the engine is started, the exhaust temperature is low, such as during low-speed operation, and the catalyst is not sufficiently warmed up, the activity of the catalyst is not sufficient, such as nitrogen oxide, carbon monoxide, hydrocarbons, etc. It may be difficult to purify the components in the exhaust gas. In this regard, the treatment of hydrocarbon components that are generated relatively frequently when the engine is started at a low temperature, so-called cold HC, is a problem of many prior arts.

  In order to solve this problem, conventionally, the catalyst is heated quickly by electric energy and quickly warmed up, and a hydrocarbon adsorbent such as zeolite is disposed in the exhaust gas flow path, and the hydrocarbon component discharged at a low temperature. It has been proposed that the hydrocarbon component is adsorbed temporarily and the hydrocarbon component is desorbed after the downstream catalyst is warmed up.

  Further, for example, in Patent Document 1, a catalyst-adsorbent having an adsorbing element that adsorbs hydrocarbons in exhaust gas and a catalyst element is used, and hydrocarbons and the like in exhaust gas generated at a cold start of an internal combustion engine are used as catalyst-adsorbent. Discloses a method for purifying exhaust gas temporarily adsorbed on the surface. Here, in the process where hydrocarbons and the like are desorbed from the catalyst-adsorbent as the temperature rises due to the exhaust gas, the exhaust gas composition is made oxygen-excessive by adding oxidizing gas to the exhaust gas, etc. Hydrogen or the like is burned on the catalyst-adsorbent. In Patent Document 1, according to this exhaust gas purification method, the catalyst-adsorbent quickly adsorbs a large amount of HC and the like generated at the cold start, and before the desorption of HC and the like, the oxygen-excess exhaust gas composition It is said that the oxidation activity of the catalyst-adsorbent and / or the catalyst body can be improved and HC and the like can be purified efficiently.

  In addition to the hydrocarbon components in the exhaust gas, the presence of particulate matter in the exhaust gas, so-called PM (particulate matter) may be a problem. Usually, PM in exhaust gas is removed from exhaust gas by collection with a filter or the like. In order to prevent the filter from being clogged with the collected PM, the PM must be burned and removed on the filter. This combustion removal generally requires a relatively high temperature of over 600 ° C., and in order to achieve this, the filter is electrically heated, and the exhaust temperature is increased by controlling the engine to heat the filter. Etc. have been proposed.

  In this regard, Patent Document 2 discloses an exhaust gas purification catalyst in which a catalyst for burning PM is supported on the surface of a honeycomb filter and burned continuously simultaneously with the collection of PM. In Patent Document 2, alkali metal silicate, aluminate, zirconate, and the like are disclosed as catalysts for burning PM.

  Patent Document 3 discloses an exhaust gas purifying device including a PM filter and an active oxygen release agent. Here, this active oxygen release agent is shown as a substance that takes in oxygen when excess oxygen is present and retains oxygen, and releases active oxygen when the surrounding oxygen concentration decreases. Metals, alkaline earth metals, rare earths and transition metals are mentioned.

Further, Patent Document 4 discloses an exhaust gas purification device having a discharge reactor and a PM filter downstream thereof. In this exhaust gas purifying apparatus, a gas component having strong oxidizing power such as active oxygen and ozone is generated by a discharge reactor, and the PM oxidation catalyst coated on the PM filter is interacted with PM gas catalyst having a strong oxidizing power by the PM reactor. It is supposed to facilitate the collection and removal of combustion. Examples of the PM oxidation catalyst used here include metal oxides, specifically, for example, SiO ₂ , Al ₂ O ₃ , CeO ₂ , TiO ₂ , ZrO ₂ , and these metal oxides include Furthermore, transition metals and / or noble metals such as transition metals such as Fe, Mn, Ni, Co, and Cu, and noble metals such as Pt, Rh, Pd, and Ag, particularly Pt can be supported.

Japanese Patent No. 3311051 JP 10-118490 A JP 2001-271634 A JP 2004-11492 A

  As described above, the prior art has proposed various exhaust gas purification apparatuses for removing hydrocarbon components and PM in exhaust gas. However, the requirements for purification of these components are becoming stricter year by year, and there is still a need for an exhaust gas purification method and apparatus that can remove these components satisfactorily even under low exhaust gas temperature conditions such as when starting an engine or operating at low speed. It is said that.

  Accordingly, the present invention provides an improved exhaust gas purification method and apparatus for removing PM in exhaust gas.

  In the exhaust gas purifying method of the present invention, an exhaust gas purifying apparatus having a silver-carrying PM collecting part disposed in an exhaust gas flow path and an oxidizing gas supply device for supplying an oxidizing gas to the silver-carrying PM collecting part. When the temperature of the silver-carrying PM trapping part is below a predetermined temperature, particularly 300 ° C. or less, more particularly 200 ° C. or less, and even more particularly 150 ° C. or less, the oxidizing gas is supplied from the oxidizing gas supply device. including.

  According to the exhaust gas purification method of the present invention, the oxidizing gas supplied by the oxidizing gas supply device oxidizes silver in the PM trapping part, and the silver oxide is decomposed to produce atomic oxygen having strong oxidizing power. discharge. This atomic oxygen is very useful for the oxidation of PM collected in the PM collection part. Here, since this silver oxide starts decomposing at a relatively low temperature, for example, 100 ° C. to 160 ° C., the oxidation of PM can be promoted even at such a low temperature. Therefore, exhaust gas purification by silver oxide can be efficiently promoted by supplying the oxidizing gas from the oxidizing gas supply device when the temperature of the silver-carrying PM trapping portion is equal to or lower than the predetermined temperature.

  Further, a part of the oxidizing gas not consumed by the reaction with PM can be stored as silver oxide by oxidizing silver. This silver oxide promotes the oxidation of PM as described above.

  In one aspect of the exhaust gas purification method of the present invention, silver in a silver-carrying PM trapping part is carried on a ceria-zirconia composite oxide.

  In this embodiment, the PM oxidation reaction catalyzed by the ceria-zirconia composite oxide can be further accelerated.

  In one embodiment of the method of the present invention, the oxidizing gas supply device generates an oxidizing gas, particularly ozone and / or active oxygen, by plasma treating the entire exhaust gas stream.

  In one embodiment of the method of the present invention, the oxidizing gas supply device generates oxidizing gas, particularly ozone and / or active oxygen, by plasma treating a part of air or exhaust gas.

  According to this aspect, it is possible to reduce the space for generating plasma as compared with the case where plasma processing is performed on the entire exhaust gas flow, and therefore, the plasma density can be increased even when relatively small energy is used. Is possible.

  In one embodiment of the method of the present invention, the oxidizing gas supply device comprises a container holding an oxidizing gas, in particular ozone.

  The exhaust gas apparatus of the present invention includes a silver-carrying PM collecting unit disposed in the exhaust gas flow path, and an oxidizing gas supply device that supplies an oxidizing gas to the silver-carrying PM collecting unit, The silver of the PM collecting part is supported on the ceria-zirconia composite oxide.

  According to the exhaust gas purifying apparatus of the present invention, the oxidizing gas supplied by the oxidizing gas supply device oxidizes silver in the PM trapping part, and the silver oxide is dispersed to produce atomic oxygen having strong oxidizing power. discharge. This atomic oxygen is very useful for the oxidation of PM collected in the PM collection part.

  In the exhaust gas purifying apparatus of the present invention, part of the oxidizing gas not consumed by the reaction with PM is stored as silver oxide by oxidizing silver. This silver oxide promotes the oxidation of PM as described above.

  Furthermore, in the exhaust gas purification apparatus of the present invention, the PM oxidation reaction catalyzed by the ceria-zirconia composite oxide can be further promoted.

  In the following, the present invention will be specifically described based on the embodiments shown in the drawings. However, these drawings are merely for description, and the present invention is not limited to these embodiments.

  An exhaust gas purification apparatus that can be used in the exhaust gas purification method of the present invention will be described with reference to FIG. Here, FIG. 1 is a schematic view of the exhaust gas purification apparatus.

  An exhaust gas purification device 10 shown in FIG. 1 includes a silver-carrying PM collecting unit 14 disposed in an exhaust gas flow path, and an oxidizing gas supply device 12 disposed on the upstream side thereof.

  In the use of the exhaust gas purifying apparatus 10 shown in FIG. 1, when the temperature of the silver-carrying PM collecting unit is equal to or lower than a predetermined temperature, an oxidizing gas such as ozone and / or active oxygen is supplied from the oxidizing gas supply apparatus 12. Then, the silver trapped in the PM trapping section can be oxidized, and the PM trapped in the PM trapping section 14 can be oxidized and removed by the atomic oxygen having a strong oxidizing power released from the silver oxide.

  More specifically, the oxidizing gas is supplied by the oxidizing gas supply device 12 at a relatively low temperature immediately after starting the engine or at a low speed operation, and the PM collected in the silver-carrying PM collecting unit is oxidized. Can be removed.

  Optionally, immediately after starting the engine or during low-speed operation, an oxidizing gas is supplied by the oxidizing gas supply device 12 to oxidize silver in the silver-carrying PM collector, and silver oxide is added to the silver-carrying PM collector. Oxidizing components can also be stored. Further, after the engine is stopped, while the temperature of the silver-carrying PM trapping portion is decreasing, the oxidizing gas can be supplied from the oxidizing gas supply device to generate silver oxide. Furthermore, without stopping the engine immediately after stopping the engine, the engine is operated at a low speed for a certain period of time, and the oxidizing gas is supplied from the oxidizing gas supply device while the temperature of the silver-carrying PM collecting unit is decreasing. It can also be supplied to produce silver oxide. The silver oxide thus produced can be used to oxidize hydrocarbons generated at the start of the next operation.

  Below, each part which comprises the exhaust gas purification apparatus shown in FIG. 1 is demonstrated more concretely.

  The silver-carrying PM collecting part 14 can have a ceramic honeycomb filter made of a material such as cordierite or SiC as a base material, and can be obtained by carrying silver therein. Silver can be supported directly on the filter, but it is preferable to coat the support layer on the filter to increase the surface area and to support the silver here. As the carrier, it is preferable to use a carrier having PM oxidation catalyst performance such as ceria-zirconia composite oxide. A method for supporting silver is generally known. For example, it can be achieved by impregnating a carrier with a salt solution containing silver, and drying and baking it. Silver can also be supported by ion exchange.

  As the oxidizing gas supply device 12, any device that can supply oxidizing components such as ozone and active oxygen to the silver-carrying PM collecting unit 14 can be used. Therefore, as the oxidizing gas supply device 12, it is possible to use a device that has a tank holding an oxidizing component such as ozone, and that supplies ozone from this tank or a device that generates oxidizing components on the spot. it can.

  An oxidizing gas supply device that generates an oxidizing component in-situ can generate the oxidizing component in-situ by, for example, treating a part or all of exhaust gas or ambient air with plasma. This plasma may be, for example, a discharge plasma, a microwave plasma, or a coupled induction plasma.

  Oxidizing property that generates oxidizing components in-situ by treating part of the exhaust gas or surrounding air with discharge plasma, and supplies the resulting oxidizing gas to the exhaust pipe upstream of the silver-carrying PM trap The gas supply device may be as shown in FIG. The oxidizing gas supply device 12 shown in FIG. 2 has a cylindrical outer peripheral electrode 26 and a central electrode 24 disposed on the central axis of the outer peripheral electrode, and these are paired to ground. And a power source 22. Further, the outer peripheral electrode 26 and the center electrode 24 are insulated by an insulator 25. In the use of such an oxidizing gas supply device, the gas 28 to be treated by the discharge plasma is supplied from a gas supply path 29 provided in a part of the outer peripheral electrode 26, and the outer peripheral electrode 26 and the central electrode 24 are supplied by the power source 22. By applying a voltage between them and generating a discharge between these electrodes, an oxidizing gas can be generated in the plasma processing space 23 between the outer peripheral electrode 26 and the center electrode 24. The oxidizing gas 20 generated in this way is supplied from the oxidizing gas outlet 21 to the upstream side of the silver-carrying PM collecting unit.

  In the exhaust gas purification method of the present invention, as shown in FIG. 3, an oxidizing gas supply device 32 that generates an oxidizing gas by performing plasma treatment on the entire exhaust gas flow upstream of the silver-carrying PM trapping section 14. Can also be used.

  An oxidizing gas supply device that generates an oxidizing gas by performing plasma treatment on the entire exhaust gas flow can use any means such as discharge, microwave, and coupling induction as the plasma generating means. For example, when plasma is generated by discharge, a discharge reactor as shown in Document 4, that is, a discharge reactor having a reactor body that is an electrode for discharge and an insulator can be used. More specifically, this discharge reactor has electrodes, for example, mesh electrodes, at the upstream inlet and the downstream outlet of the exhaust gas flow, respectively, so that a voltage can be applied between these electrodes.

  When plasma is generated by discharge as described above, a portion used as an electrode can be manufactured using a material that can be used as a discharge electrode by applying a voltage between the electrodes. As such a material, a conductive material or a semiconductor material can be used, but a metal material such as copper, tungsten, stainless steel, iron, aluminum, or the like is preferable. However, particularly in arc discharge, the electrode becomes hot, so it is preferable to use a high melting point material such as tungsten. Moreover, in order to use barrier discharge, an insulating material can also be arrange | positioned on these electroconductive or semiconductive materials. It is preferable in terms of plasma stability, electrode durability, and the like to dispose an insulating material on the electrode to perform barrier discharge.

  Further, when plasma is generated using corona discharge, the power source used for the discharge may supply pulsed direct current or alternating current voltage. In general, a voltage of 1 kV to 100 kV, for example, 5 kV to 20 kV can be used as an applied voltage between the electrodes. The pulse width of the applied voltage can be 0.1 μs to 10 ms, particularly 0.1 to 10 μs.

  Hereinafter, the present invention will be described based on examples, but the present invention is not limited thereto.

  An experiment for clarifying the effect of the present invention was conducted using the experimental equipment shown in FIG. Here, in the experimental facility shown in FIG. 4, the temperature of the PM collection unit can be adjusted by an infrared image furnace, and the model gas can be supplied to the PM collection unit. Further, the gas flowing through the PM collecting section can be analyzed for its carbon monoxide and carbon dioxide concentrations by an analyzer.

[Preparation of PM collection part]
The PM collector for this experiment was as follows:
(Comparative example)
Cordierite diesel particulate filter (DPF) (12 mil, 300 cpsi (number of cells / square inch)) 30 mm in diameter and 50 mm in length was used to add ceria-zirconia composite oxide (Ce _0.6 Zr _0.4 ) using slurry. O ₂ ) was coated as a carrier. Thereafter, this DPF was baked at 500 ° C. for 2 hours to obtain a PM collecting part of a comparative example. The coating amount of the carrier was 100 g per liter of filter.

(Example)
The PM collection part of the comparative example was impregnated with a silver nitrate solution, and 5% by weight of silver was supported on the ceria-zirconia composite oxide and baked at 450 ° C. for 2 hours. It was a collection.

〔Test method〕
Using 2 L of diesel, PM was deposited in advance on the PM trapping parts of Examples and Comparative Examples for 1 hour at 2000 rpm × 30 Nm.

Thereafter, the infrared image furnace was set to 200 ° C. or 300 ° C., 5 volume% O ₂ , 50 volume ppm NO, 150 volume ppm NO ₂ , 80 volume ppm C ₃ H ₆ , 3 volume% H _2. A model gas (8 standard L / min) containing O and the balance N ₂ was supplied, and the oxidation rate of PM was determined from the integrated values of CO and CO ₂ analyzed by the analyzer with respect to time. As the injection gas, dry air or dry air containing 1000 ppm of ozone (O ₃ ) as an oxidizing gas (both 2 standard L / min) was used. The results are shown in FIGS.

  As understood from this result, by adding ozone, the oxidation rate of PM increases regardless of whether the temperature is 200 ° C. or 300 ° C. However, when the temperature of the PM trapping part is 300 ° C. (FIG. 6), when ozone is added, the PM oxidation rate in the PM trapping part of the example and the PM oxidation in the PM trapping part of the comparative example The difference from the speed is relatively small. On the other hand, when the temperature of the PM collection part is 200 ° C. (FIG. 5), when ozone is added, the PM oxidation rate in the PM collection part of the example and the PM collection part of the comparative example The difference from the PM oxidation rate is remarkable, which is close to three times here. This indicates that the silver supported on the PM collecting part of the present invention is particularly effective even at a relatively low temperature of 200 ° C.

It is a conceptual diagram showing one embodiment of this invention. It is a schematic sectional drawing of the oxidizing gas supply apparatus which can be used with the embodiment of FIG. 1 of this invention. It is a conceptual diagram showing other one embodiment of this invention. It is a schematic sectional drawing of the experimental equipment used in the Example. It is a figure which shows PM oxidation rate when using the PM collection part of an Example and a comparative example at 200 degreeC. It is a figure which shows PM oxidation rate when the PM collection part of an Example and a comparative example is used at 300 degreeC.

Explanation of symbols

10, 30 Exhaust gas purifying device 12, 32 Oxidizing gas supply device 14 Silver-carrying PM collecting unit 16 Arrow indicating exhaust gas flow 20 Oxidizing gas 22 Power source 24, 26 Electrode 28 Gas processed by discharge plasma

Claims (9)

  Using the exhaust gas purifying device having a silver-carrying PM collecting part disposed in the exhaust gas flow path and an oxidizing gas supply device for supplying an oxidizing gas to the silver-carrying PM collecting part, the silver-carrying PM collecting is performed. An exhaust gas purification method comprising supplying an oxidizing gas from the oxidizing gas supply device when the temperature of the section is equal to or lower than a predetermined temperature.   The exhaust gas purification method according to claim 1, wherein silver in the silver-carrying PM trapping part is carried by a ceria-zirconia composite oxide. The exhaust gas purification method according to claim 1, wherein the predetermined temperature is a temperature of 300 ° C. or less. Immediately after engine start-up or during low-speed operation, an oxidizing gas is supplied by the oxidizing gas supply device to oxidize silver in the silver-carrying PM collecting part, thereby oxidizing the silver-carrying PM collecting part as silver oxide. The exhaust gas purification method according to any one of claims 1 to 3, comprising storing components. In order to oxidize the silver oxide generated at the start of the next operation by producing silver oxide in the silver-carrying PM collector by at least one of the following operations (a) and (b): The exhaust gas purification method according to any one of claims 1 to 4, which is used for:
(A) After the engine is stopped, while the temperature of the silver-carrying PM trapping portion is decreasing, an oxidizing gas is supplied from the oxidizing gas supply device, Generating,
(B) The engine is operated at a low speed for a certain period of time without stopping the engine immediately after the engine is stopped, and the oxidizing gas supply device oxidizes while the temperature of the silver-carrying PM trapping portion is decreasing. Supplying gas to produce silver oxide in the silver-carrying PM collector. The method according to claim 1 , wherein the oxidizing gas supply device generates an oxidizing gas by plasma-treating the entire exhaust gas flow. The method according to any one of claims 1 to 5, wherein the oxidizing gas supply device generates oxidizing gas by plasma-treating a part of air or exhaust gas. The method in any one of Claims 1-5 with which the said oxidizing gas supply apparatus has the container holding oxidizing gas.   A silver-carrying PM collecting part disposed in the exhaust gas flow path, and an oxidizing gas supply device for supplying an oxidizing gas to the silver-carrying PM collecting part, and silver in the silver-carrying PM collecting part Is an exhaust gas purification device supported by a ceria-zirconia composite oxide.

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