JP2005282447A - Exhaust gas purification device - Google Patents

Abstract

A fuel is reformed without using a heater, and the reformed fuel can be added to exhaust gas.
A microreactor 5 for reforming fuel to lower the molecular weight and a heating means for heating the microreactor 5 are provided, and the reformed fuel reformed in the microreactor 5 is discharged into exhaust gas upstream of a catalytic converter 4. Added.
Since the fuel is reformed in the microreactor 5, the heat capacity is extremely small, the reaction field is very small, and the amount of heat supply can be reduced as compared with a conventional reforming catalyst. Therefore, power consumption can be reduced by heating using engine waste heat.
[Selection] Figure 1

Description

  The present invention relates to an exhaust gas purification device that purifies exhaust gas from a diesel engine or the like, and more particularly to an exhaust gas purification device used in a system in which fuel is added to exhaust gas.

  As for gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology that can cope with it. However, because diesel engines emit harmful components as particulates (particulate matter: carbon fine particles, sulfur fine particles such as sulfate, high molecular weight hydrocarbon fine particles, etc., hereinafter referred to as PM), regulations are also restricted. Technological progress is also slow compared to gasoline engines.

  As exhaust gas purification devices for diesel engines that have been developed so far, a trap type exhaust gas purification device (wall flow) and an open type exhaust gas purification device (straight flow) are known. Among these, as a trap type exhaust gas purification device, a ceramic plug-type honeycomb body (diesel PM filter (hereinafter referred to as DPF)) is known. This DPF is formed by alternately sealing both ends of the openings of the cells of the ceramic honeycomb structure, for example, in a checkered pattern, and is adjacent to the inflow side cells and the inflow side cells clogged on the exhaust gas downstream side. It consists of an outflow side cell clogged upstream of the exhaust gas and a cell partition partitioning the inflow side cell and the outflow side cell. The exhaust gas is filtered through the pores of the cell partition wall to collect PM, thereby suppressing emissions. To do.

  However, in DPF, exhaust pressure loss increases due to PM accumulation, so it is necessary to periodically remove and regenerate PM accumulated by some means. Therefore, conventionally, when exhaust pressure loss increases, high temperature exhaust gas is circulated, or PM accumulated by burning with a burner or an electric heater is burned to regenerate DPF. However, in this case, the higher the amount of PM deposited, the higher the temperature during combustion, and the DPF may be damaged by the thermal stress.

  Therefore, in recent years, as described in, for example, JP-A-09-173866, a coating layer is formed from alumina or the like on the surface of the DPF cell partition wall, and a noble metal such as platinum (Pt) is supported on the coating layer. Continuous regeneration type DPF (filter catalyst) has been developed. According to this filter catalyst, the collected PM is oxidized and burned by the catalytic reaction of the noble metal, so that the filter function can be regenerated by burning the PM simultaneously with the collection or continuously with the collection. Since the catalytic reaction occurs at a relatively low temperature and PM can be burned while the amount of trapped is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.

The coat layer, an alkali metal, are also known NO _x storage-and-reduction type filter catalyst further carrying _the NO _x storage material selected from alkaline earth metals and rare earth elements. According to the NO _x storage-and-reduction type filter catalyst occludes NO _x in _the NO _x storage material in a lean atmosphere, so reducing the NO _x released in a rich atmosphere, NO _x purifying performance is dramatically improved . Therefore, when purifying exhaust gas from a diesel engine using a NO _x storage reduction type filter catalyst, a system is used in which fuel is intermittently added to the exhaust gas to create a rich atmosphere.

For example, in Japanese Patent Laid-Open No. 2002-021544, an oxidation catalyst or NO _x storage reduction catalyst is disposed in front of a DPF or a filter catalyst, and post-injection for injecting fuel into a combustion chamber or adding fuel into exhaust gas A technique is disclosed in which HC is supplied to burn PM deposited on the DPF or filter catalyst by the heat of reaction in the oxidation catalyst or NO _x storage reduction catalyst, and NO _x is reduced and purified.

When a rich atmosphere thus by adding a fuel such as light oil into the exhaust gas, recovering _the NO _x storage capacity by adding fuel before _the NO _x storage amount of the NO _x storage reduction catalyst is saturated It is necessary to let Therefore, it is necessary to add fuel at relatively short intervals even during acceleration / deceleration at low speed. However, in such a case, the exhaust gas temperature is relatively low, and the exhaust gas temperature is further lowered by the addition of liquid fuel, so that the reaction between the fuel and NO _x hardly occurs. Further, the NO _x storage-reduction type filter catalyst has a problem that the oxidation activity of HC by the noble metal is low because the NO _x storage material is supported.

For this reason, when light oil is added to the exhaust gas using a NO _x storage reduction type filter catalyst, the added light oil adheres to the filter catalyst in an unreacted state, and the supported catalyst metal is poisoned and activated. Decreases. Further, there is a problem that PM adheres to the attached light oil and the front end face of the cell is blocked. Further fuel such as light oil for a polymer HC, characteristic of poor NO _{x storage-and-reduction} catalyst in the reaction activity of the NO _x is also a problem that not enough brought out.

Therefore in Japanese Patent Laid-Open No. 09-013956 includes a _the NO _x reduction catalyst, a reforming catalyst for reforming fuel, the exhaust gas purifying apparatus is disclosed having a fuel supply means for supplying liquid fuel to the reforming catalyst . According to this exhaust gas purifying apparatus, the liquid fuel is reformed by the reforming catalyst to be reduced in molecular weight to become reformed fuel, and the reformed fuel that is easily vaporized is supplied into the exhaust gas. _x The purification efficiency of the reduction catalyst can be sufficiently increased. Accordingly, if applied to _the NO _x storage reduction type filter catalyst, clogging of the end face can be suppressed.

However, since this exhaust gas purifying apparatus uses a reforming catalyst having a relatively large volume, there is a great space limitation for purifying automobile exhaust gas. Further, since it is necessary to heat the reforming catalyst in order to increase the reforming reaction activity, the above publication uses a heater to heat the reforming catalyst. However, since the reforming catalyst has a relatively large volume, there is a problem that the power consumption of the heater is large and the burden on the battery is heavy.
JP 2002-021544 JP 09-013956 A JP 2004-016870

  The present invention has been made in view of the above circumstances, and in an exhaust gas purifying apparatus used in a system in which fuel is added to exhaust gas, the fuel is reformed without using a heater or the like, and the reformed fuel is treated with exhaust gas. It is intended to be able to be added inside.

  The exhaust gas purifying apparatus of the present invention that solves the above problems is characterized in that an exhaust gas purifying catalyst disposed in an exhaust flow path from an engine, a microreactor that reforms fuel to reduce the molecular weight, and a heating means that heats the microreactor And an adding means for adding the reformed fuel reformed in the microreactor to the exhaust gas upstream of the exhaust gas purification catalyst.

  It is desirable that a tank for storing reformed fuel reformed in the microreactor is further provided between the microreactor and the adding means.

  The heating means preferably uses engine waste heat, and is preferably an oil circulation path for circulating engine oil through the microreactor.

  According to the exhaust gas purifying apparatus of the present invention, since a microreactor with a reaction field of several tens of μm to several hundreds of μm is used, it is possible to react efficiently with little influence of diffusion of reactants and to reduce the molecular weight of liquid fuel. it can. And since the temperature of a reaction field can be kept uniform, control of reaction is easy.

  Furthermore, since the fuel is reformed in a microreactor with a capacity of several tens of cc or less, the heat capacity of the microreactor itself is extremely small, the reaction field is very small, and the amount of heat supply is less than that of conventional reforming catalysts. . Therefore, heating can be performed using engine waste heat or the like, and a heater or the like is not necessary, so that power consumption can be significantly reduced.

In addition, although a filter catalyst for diesel exhaust gas generally uses a large amount of noble metal, the amount of noble metal used can be reduced because the NO _x purification activity is improved by adding reformed fuel.

  The exhaust gas purifying apparatus of the present invention includes an exhaust gas purifying catalyst, a microreactor that reforms fuel to reduce the molecular weight, a heating means that heats the microreactor, and a reformed fuel reformed in the microreactor. Adding means for adding to the exhaust gas on the upstream side.

As the exhaust gas purification catalyst, an oxidation catalyst, a three-way catalyst, a NO _x storage reduction catalyst, a filter catalyst, or the like can be used, but liquid fuel adheres to the end face, PM adheres to the liquid fuel, and the end face is blocked. It is preferable to use a filter catalyst that can solve the problem of occurrence. It is also preferable to use a NO _x storage-reduction catalyst that has the effect of improving the NO _x purification rate by the reformed fuel. Therefore, it is particularly preferable to use a NO _x storage reduction type filter catalyst.

The NO _x storage-and-reduction type filter catalyst, a clogged been inflow cells in the exhaust gas downstream side, outlet cells which are sealed on the adjacent inlet cells exhaust-gas upstream side, outlet cells and inflow cells And a porous cell partition wall having a large number of pores, and a catalyst layer formed on the surface of the cell partition wall and the inner surface of the pores and having a porous oxide supporting a noble metal and a NO _x storage material. It has a wall flow structure.

  The base material of the filter catalyst is a plug-type honeycomb body similar to the conventional DPF. This base material can be manufactured from heat-resistant ceramics such as cordierite and silicon carbide. For example, a clay-like slurry containing cordierite powder as a main component is prepared, and the slurry is formed by extrusion or the like and fired. Instead of cordierite powder, powders of alumina, magnesia and silica can be blended so as to have a cordierite composition. Thereafter, the cell opening on one end face is sealed in a checkered pattern with the same clay-like slurry, and the cell opening of the cell adjacent to the cell sealed on the one end face is sealed on the other end face. Thereafter, it can be produced by fixing the sealing material by firing or the like.

  In order to form pores in the cell partition walls of the base material, the combustible powder such as carbon powder, wood powder, starch, and resin powder is mixed in the above slurry, and the combustible powder disappears during firing. Thus, the pores can be formed, and the distribution and opening area of the diameters of the surface pores and the internal pores can be controlled by adjusting the particle size and addition amount of the combustible powder.

  The pore distribution in the cell partition walls of the substrate can be in the range of 40 to 80% porosity and 10 to 50 μm average pore diameter, similar to the conventional DPF. If the porosity or the average pore diameter is out of this range, the PM collection efficiency may decrease or the exhaust pressure loss may increase.

The catalyst layer is formed on the surface of the cell partition wall and the surface of the pores, and is formed by supporting a noble metal and a NO _x storage material on a porous oxide. As the porous oxide, oxides such as alumina, ceria, zirconia, and titania, or a composite oxide composed of a plurality of these oxides can be used.

  As the noble metal, it is preferable to use one or more kinds selected from platinum group noble metals such as Pt, Rh, Pd, Ir, and Ru. The amount of the noble metal supported is preferably 0.1 to 5 g per liter of the substrate. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.

The catalyst layer further supports a NO _x storage material selected from alkali metals, alkaline earth metals, and rare earth elements. By including the NO _x storage material in the catalyst layer, NO ₂ generated by oxidation with the noble metal can be stored in the NO _x storage material, and the NO _x purification activity is improved. The supported amount of the NO _x storage material is preferably in the range of 0.05 to 1.00 mol per liter of the substrate volume. If the supported amount is less than this, the activity is too low to be practical, and if the supported amount is more than this range, the catalyst metal is covered and the activity is lowered.

The formation amount of the catalyst layer is preferably 30 to 200 g per liter of the substrate volume. If the catalyst layer is less than these ranges, the durability of the noble metal or NO _x storage material is inevitably lowered, and if it exceeds these ranges, the exhaust pressure loss becomes too high, which is not practical.

The exhaust-gas upstream side of the filter catalyst, it is also preferable to place such _the NO _x storage reduction catalyst of the oxidation catalyst or a straight flow structure. In this way, the supplied reformed fuel is oxidized by the upstream catalyst, and the exhaust gas is heated by the reaction heat. Therefore, the oxidation reaction of PM collected on the filter catalyst is promoted, and PM collection The ability recovers easily.

  A microreactor used in the present invention forms a substrate by laminating a first chip and a second chip as described in, for example, Japanese Patent Application Laid-Open No. 2004-016870, and is elongated on at least one of both chips. By forming the concave groove, a reaction channel is formed in the substrate, and a fluid supply port for supplying fluid to the reaction channel is provided.

  In order to be able to reform the fuel in this microreactor, the cracking action of zeolite can be used. For example, by forming the substrate itself from zeolite or coating zeolite on at least the reaction channel of the substrate, the fuel flowing through the reaction channel can be cracked and reformed to low molecular HC. Zeolites such as mordenite, ZSM-5, and ultrastable Y-type zeolite (US-Y) exhibit particularly high cracking action. It is also preferable to coat the zeolite with a reforming catalyst supporting a noble metal such as Pt, Rh, Pd, or a copper-zeolite catalyst. By using such a catalyst, the reforming reaction can be further promoted.

  Further, instead of zeolite, a solid superacid having cracking ability like zeolite can also be used. As the solid super strong acid, a solid acid obtained by treating an oxide such as zirconia, alumina or titania with a strong acid such as sulfuric acid, tungstic acid or molybdic acid and attaching the strong acid to the oxide can be employed. In order to form a microreactor, a solid super strong acid may be supported on a reaction flow path of a substrate or a coating layer of a solid super strong acid may be formed as in the case of zeolite.

  Since the microreactor has a reaction capacity of several tens of cc or less, the fuel can be reformed at high speed. However, if the length of the reaction channel is short, the reaction may not proceed sufficiently. Therefore, it is preferable to make the reaction channel as long as possible. In addition, when the amount of reformed fuel produced is small, a plurality of microreactors can be used. Even when a plurality of microreactors are used, the volume is smaller than that of the conventional reforming catalyst, so that the space is not easily restricted. It is also preferable to increase the surface area by forming fine irregularities in the reaction channel.

  Since the reforming reaction by the microreactor is an endothermic reaction, it is desirable to promote the reaction by heating. Therefore, in the present invention, a heating means for heating the microreactor is used. As this heating means, a heater can be used, but it is desirable to use engine waste heat. By using engine waste heat, power consumption can be reduced or zero, and the burden on the battery is reduced.

  In order to use engine waste heat, there are a method of arranging the microreactor in contact with the engine block, a method of arranging the microreactor adjacent to the exhaust pipe, and the like. Since the reaction capacity of the microreactor is several tens of cc or less, it can be easily heated to the reaction temperature by the heat of the engine block or the exhaust pipe.

  It is also preferable to dispose the oil circulation path outside the microreactor and use engine oil as a heat medium. In order to do this, a part of the flow path of the oil pump may be disposed around the microreactor so that the reaction flow path of the microreactor can be heated by conduction heat or radiant heat of engine oil.

  The adding means is for adding the reformed fuel reformed in the microreactor to the exhaust gas upstream of the exhaust gas purifying catalyst, and a conventionally used injector or the like can be used.

  The exhaust gas purification apparatus of the present invention preferably further includes a tank for storing the reformed fuel reformed in the microreactor between the microreactor and the adding means. Since the microreactor has a reaction capacity of several tens of cc or less, the actual reforming rate may be less than the fuel addition rate. Moreover, it is normal that fuel addition is performed intermittently. Therefore, by storing the reformed fuel in the tank during the period when the fuel is not added, a sufficient amount of the reformed fuel can be added at the time of fuel addition.

  Hereinafter, the present invention will be described specifically by way of examples.

FIG. 1 shows an exhaust gas purification apparatus of this embodiment. The exhaust gas purifying apparatus includes an exhaust amount 2L diesel engine exhaust line 3 is housed in the catalytic converter 4 disposed in the oxidation catalyst 40 and _the NO _x storage reduction type filter catalyst 41 of 1, light oil from the fuel tank 2 A supplied microreactor 5, a tank 6 in which the reformed fuel reformed in the microreactor 5 is stored, and an injector 7 for spraying the reformed fuel supplied from the tank 6 into the exhaust gas upstream of the catalytic converter 4; , Is composed of.

  The oxidation catalyst 40 is formed by forming a coat layer made of a mixed powder of alumina, titania and ceria-zirconia solid solution on a honeycomb substrate having a straight flow structure, and carrying Pt on the coat layer. The filter catalyst 41 disposed on the downstream side of the oxidation catalyst 40 forms a coat layer made of alumina, titania, zirconia, and ceria powder on the cell partition wall surface of the honeycomb substrate having a wall flow structure and in the pores of the cell partition wall. The coating layer carries Pt, Li, Ba and K.

  The microreactor 5 is formed by forming a mordenite coat layer in a zigzag reaction channel and supporting Pt on the coat layer, and reforms light oil by cracking. The microreactor is installed in the diesel engine 1 and is configured to be heated to about 200 to 300 ° C. by conduction heat from the engine block.

  The tank 6 stores the reformed fuel reformed in the microreactor 5, and the stored reformed fuel is sprayed into the exhaust gas upstream of the catalytic converter 4 by the injector 7.

  In this exhaust gas purification device, when the diesel engine 1 is started, the microreactor 5 is constantly supplied with light oil from the fuel tank at a flow rate of 2 ml / min. In the microreactor 5, the reforming reaction by cracking of light oil proceeds smoothly by heating by conduction heat from the diesel engine 1, and the generated reformed fuel is stored in the tank 6.

  The diesel engine 1 is always operated in a lean atmosphere with excess oxygen. When PM accumulates on the filter catalyst 41 and the exhaust pressure loss rises above a predetermined value, the injector 7 is driven, and the reformed fuel stored in the tank 6 is sprayed into the exhaust gas at a flow rate of 1.5 ml / min. Becomes rich atmosphere. A part of the supplied reformed fuel is oxidized by the oxidation catalyst 40, and the rest flows into the filter catalyst 41. Since the exhaust gas including the reformed fuel flowing into the filter catalyst 41 is heated by the reaction heat in the oxidation catalyst 40, the accumulated PM burns and the filter catalyst 41 recovers the PM trapping ability.

Also the filter catalyst 41, together with the NO _x which had stored in the NO _x storage material in a lean atmosphere is released, the released NO _x is reduced and purified by reforming fuel. Since reforming fuel is low molecular weight modified with microreactor 5, while the reaction may readily vaporize in the exhaust gas NO _x and efficiency, itself is oxidized and purified. There is no problem that the liquid fuel adheres to the end face of the filter catalyst 41 and PM adheres to the end face of the filter catalyst 41 to close the end face.

  When the exhaust pressure loss decreases to a predetermined value, the injector 7 is stopped and the reformed fuel is stored in the tank 6. The tank 6 is provided with a float valve (not shown) so that the supply of light oil to the microreactor 5 is stopped when the liquid level of the reformed fuel reaches a predetermined level.

  In the present embodiment, the heat of the engine block is used as the heating means of the microreactor 5, but a part of the flow path of the oil pump is disposed around the microreactor 5 and is caused by conduction heat or radiant heat of the engine oil. The reaction channel of the microreactor 5 may be heated.

  The exhaust gas purification apparatus of the present invention can be used not only for exhaust gas purification of diesel engines but also for exhaust gas purification of gasoline engines.

It is a block circuit diagram showing an exhaust gas purification apparatus of one example of the present invention.

Explanation of symbols

1: diesel engine 2: fuel tank 3: exhaust pipe 4: catalytic converter 5: microreactor 6: tank 7: injector 40: oxidation catalyst 41: filter catalyst

Claims (4)

  An exhaust gas purification catalyst disposed in an exhaust passage from the engine, a microreactor that reforms the fuel to reduce the molecular weight, a heating means that heats the microreactor, and the reformed fuel reformed in the microreactor And an adding means for adding to the exhaust gas upstream of the exhaust gas purification catalyst.   The exhaust gas purifying apparatus according to claim 1, further comprising a tank for storing reformed fuel reformed in the microreactor between the microreactor and the adding means.   The exhaust gas purification apparatus according to claim 1 or 2, wherein the heating means uses engine waste heat. The exhaust gas purification device according to claim 3, wherein the heating means is an oil circulation path for circulating engine oil through the microreactor.

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