JP2006037811A - Exhaust emission control device for internal combustion engine and exhaust emission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify a replenishing device of NOx capturing component as compared with conventional technology solving out NOx capturing component of which NOx capturing capacity drops after a predetermined distance travel and newly supplying solution containing NOx capturing component and heating to dry and sinter to recover NOx capturing capacity of a lean NOx catalyst of which NOx capturing capacity drops due to SOx poisoning. <P>SOLUTION: A catalyst component replenishing device for replenishing catalyst component in an upstream of the lean NOx catalyst in an exhaust gas flow passage is provided to replenish catalyst component in a exhaust gas flow passage at a time of engine on, engine off or both. Since replenishing quantity of catalyst component per once can be a little, catalyst component can be sintered with using heat possessed by exhaust gas and replenished catalyst component can be carried on a lean NOx catalyst surface. This invention is suitable for replenishing NOx capturing component. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device and an exhaust gas purification method for purifying exhaust gas discharged from an internal combustion engine operated in a lean fuel lean atmosphere.

  From the viewpoint of resource saving, lean burn vehicles that move gasoline engine vehicles with lean burn have been developed. The lean burn vehicle cannot reduce the nitrogen oxide (NOx) contained in the exhaust gas by the three-way catalyst, unlike the conventional vehicle operating at the stoichiometric air-fuel ratio (stoichiometric). Therefore, a lean NOx catalyst has been developed that can remove NOx even in a lean atmosphere containing excess oxygen. The lean NOx catalyst contains NOx trapping components in the catalyst so that NOx in the lean exhaust gas can be trapped and removed by the catalyst. However, the lean NOx catalyst has a problem that the NOx trapping component is poisoned by sulfur oxide (SOx) contained in the exhaust gas, and the NOx trapping ability is lowered by this poisoning.

  As a countermeasure, when the NOx trapping ability is reduced is determined by the travel distance or the fuel consumption, if the NOx trapping ability is lowered, the NOx trapping component poisoned by SOx is eluted from the lean NOx catalyst and newly added. It has been proposed to replenish a solution containing a NOx trapping component (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2000-104521 (Abstract, Claims 5 and 6)

  The invention described in Patent Document 1 elutes poisoned NOx trapping components after stopping the engine, and then replenishes new NOx trapping components. Moreover, drying and baking are performed after replenishing the solution containing the NOx trapping component. As described above, in the invention described in Patent Document 1, it is necessary to remove the poisoned NOx trapping component, and it is necessary to perform heat treatment after newly supplying the NOx trapping component. Furthermore, the timing for newly supplying the NO trapping component is limited after the engine is stopped.

  An object of the present invention is to provide an exhaust gas purification apparatus and an exhaust gas purification method that can replenish NOx trapping components without performing heat treatment, paying attention to these problems.

  In the present invention, a catalyst component is replenished to the lean NOx catalyst when the engine is started, when the engine is stopped, or when both are operated. In the present invention, in order to replenish the catalyst component, a catalyst component replenishment mechanism is provided that replenishes the catalyst component to the lean NOx catalyst when the engine is switched on or off, or both. As this catalyst component supply mechanism, it is desirable to provide a device for supplying the catalyst component in a solution. Further, it is desirable that the catalyst component to be replenished is a NOx trapping component.

  In the present invention, since the replenished catalyst component is dried and sintered using the retained heat of the exhaust gas heated by the engine start or the exhaust gas after the engine is stopped, the heating device for heat-treating the replenished catalyst component Even if it is not necessary, the supplied catalyst component can be supported on the lean NOx catalyst.

  FIG. 1 shows an embodiment of the present invention. A lean NOx catalyst 4 is installed in a part of the exhaust gas flow path 2 directly connected to the engine 1, and a catalyst component replenishing device for replenishing the catalyst component to the exhaust gas flow path upstream from the position where the lean NOx catalyst is installed. Is installed. The catalyst component replenishing device includes a tank 7 for storing the catalyst solution, a supply pipe 8 for guiding the catalyst solution in the tank to the exhaust gas flow path 2, and an on-off valve 6 provided in the middle of the supply pipe pipe 8. is doing. The on-off valve 6 is set to open and close by an engine start signal or an engine stop signal sent from the engine control unit 3.

  In this embodiment, when the engine 1 is turned on or off, or turned on and off, the on-off valve 8 is opened by a signal from the engine control unit 3, and the solution containing the catalyst component is leaned from the tank 7 through the supply pipe 8. It is supplied to the exhaust gas flow path 2 upstream of the NOx catalyst 4. The solution containing the catalyst component supplied to the exhaust gas flow path 2 is sent to the lean NOx catalyst along with the flow of the exhaust gas, and is supported thereon.

  In the present invention, the replenishment amount of the catalyst component per one time may be very small, so that the drying and sintering can be sufficiently performed using the retained heat of the exhaust gas. Further, since the operation of removing the NOx trapping component whose NOx trapping ability has been reduced is not performed, the catalyst component replenishment operation is shortened and the replenishment system can be simplified as compared with the invention described in Patent Document 1. In addition, the catalyst component is replenished not only when the engine is stopped but also when the engine is started.

  As a method for supplying the catalyst component, a method of supplying the catalyst component as a solid powder and a method of supplying the catalyst component as a solution can be considered. Among these, the method of replenishing solid powder is difficult to carry the replenished catalyst component on the lean NOx catalyst, and therefore, the method of replenishing in the form of a solution is recommended in the present invention. In addition, when replenishing with a solution, it is necessary to evaporate the solution. Therefore, it is recommended that the solution be supplied not to the lean NOx catalyst directly but to the exhaust gas passage upstream of the lean NOx catalyst.

The components of the lean NOx catalyst, NOx trapping, and reduction mechanism will be briefly described. The lean NOx catalyst is a generic term for catalysts that can remove NOx contained in exhaust gas with excess oxygen and lean fuel, such as lean burn exhaust gas. The lean NOx catalyst includes a NOx trapping component capable of trapping NOx even in the presence of oxygen, and is characterized by this. The NOx trapping component is mainly selected from alkali metals such as sodium, potassium, lithium, cesium and the like or alkaline earth metals such as strontium, calcium, barium and magnesium, and one or more of these are used. . In a lean NOx catalyst, NOx in the lean exhaust gas is captured by the catalyst by either adsorption or absorption. When the alkali metal or alkaline earth metal in the lean NOx catalyst is contained in the form of a compound with a transition metal selected from titanium or zirconium, NOx may be trapped on the catalyst surface by chemisorption. Many. It is presumed that NOx in the exhaust gas becomes NO ₂ and NO ₂ is trapped on the catalyst surface by chemical adsorption.

The lean NOx catalyst, platinum, palladium, rhodium, the inclusion of precious metal such as iridium, can be oxidized to NOx contained mainly in the form of NO in the exhaust gas to NO _2, also the trapped NOx in the catalyst it can be reduced to N _2. However, in order to reduce the NOx trapped by the catalyst, it is necessary to switch the atmosphere of the exhaust gas to an atmosphere in which the stoichiometric air-fuel ratio (stoichiometric) or excess fuel (rich) oxygen does not coexist. The trapped NOx can be reduced by creating such an atmosphere for several seconds or tens of seconds. In order to give the lean NOx catalyst the function of CO purification or HC purification, it is desirable that the lean NOx catalyst contains the noble metal as described above. In addition, it is desirable to include rare earth metals such as lanthanum and cerium.

  In a lean NOx catalyst, catalyst components such as a NOx trapping component, a noble metal, and a rare earth metal are usually carried on a honeycomb substrate coated with alumina. Instead of alumina, titanium oxide, zirconium oxide, or the like can be used. As the honeycomb substrate, ceramics such as cordierite or metal such as stainless steel is used. In the case of a metal honeycomb, it can be made by forming or joining metal foils to form a honeycomb structure. By impregnating the alumina-coated honeycomb with the catalyst component-containing solution several times, drying and firing, a lean NOx catalyst carrying the catalyst component can be prepared. As a method other than impregnation, immersion, mixing, or the like may be used.

  The SOx poisoning of the lean NOx catalyst occurs when the NOx trapping component and SOx react to form a compound during the lean operation. This compound of NOx trapping component and SOx has a special effect such as heating for a long time at a high temperature by setting the air-fuel ratio of the exhaust gas to a stoichiometric air-fuel ratio in the vicinity of 14.7 or a fuel excess air-fuel ratio (rich) smaller than 14.7. It cannot be disassembled unless it is treated. This operation is very troublesome due to the timing of the operation, and causes a significant increase in fuel consumption. In this regard, in the present invention, when the engine is turned on or off, or both, the NOx trapping component is replenished and the heat of the exhaust gas is used for sintering, so there is no increase in fuel consumption.

  In this embodiment, the NOx trapping component is replenished, but other catalyst components such as noble metals or rare earth metals may be replenished. The noble metal or rare earth metal supported on the lean NOx catalyst grows during the operation of the internal combustion engine, gradually decreases in surface area, and decreases in activity. As a countermeasure against this, it is extremely effective to replenish noble metals or rare earth metals when the engine is on or off. Based on the above points, in the present invention, it is possible to replenish catalyst components in general widely without being limited only to replenishment of NOx trapping components. In the present invention, the catalyst component is not limited to a noble metal or a rare earth metal, but means a component supported on a carrier such as alumina.

  Since the rate at which the NOx trapping capacity decreases or the rate at which the activity of noble metals, rare earth metals, etc. decreases is empirically known, the amount of catalyst component replenishment per time based on those experiences or measured data is determined. It is possible to set. In the present invention, it is preferable to replenish a predetermined amount in this way.

  The catalyst component to be replenished is preferably stored and replenished in the form of a solution or slurry. In this way, since the replenishment amount per time is very small, the solution can be easily evaporated using the heat of the exhaust gas, and the catalyst component can be supported on the lean NOx catalyst. The catalyst component replenisher can be prepared by, for example, a method in which an aqueous solution having a sodium nitrate concentration of 30% is placed in a tank.

  FIG. 2 shows an embodiment in which the present invention is applied to a lean burn vehicle in which a lean NOx catalyst 4 is installed in an exhaust pipe immediately below an exhaust manifold, that is, an exhaust gas passage. The catalyst component replenishing device of this embodiment includes a tank 7 for storing a solution containing the catalyst component, a pump 5 for sending the solution in the tank 7 to the supply pipe 8, an on-off valve 6, and a solution containing the catalyst component in the exhaust gas flow path. It is comprised from the injector 9 for spraying in. The pump 5 and the on-off valve 6 are controlled by instructions from the engine control unit. In this embodiment, since the solution containing the catalyst component is supplied in a sprayed state by the injector 9, the solution is easily vaporized, and the catalyst component can be easily supported on the lean NOx catalyst.

  FIG. 3 shows an example in which a part of an exhaust pipe installed under the floor of a lean burn vehicle is processed to be lower in position than the other part, and a lean NOx catalyst is installed there. Shows the case. That is, the catalyst component is replenished upstream in the flow direction of the exhaust gas 10 from the position where the lean NOx catalyst 4 is provided in the exhaust gas flow path. In the present embodiment, since the catalyst component replenished by the catalyst component replenishing device is structured not to easily flow out downstream of the lean NOx catalyst, the replenished catalyst component is easily captured by the lean NOx catalyst.

  The present invention is an extremely effective method as an SOx poisoning countermeasure in a lean burn compatible internal combustion engine equipped with a lean NOx catalyst, and its practical applicability is extremely large.

1 is an exhaust system configuration diagram of a lean burn vehicle showing one embodiment of the present invention. The exhaust system block diagram of the lean burn vehicle which shows the other Example of this invention. The exhaust system block diagram of the lean burn vehicle which shows the further another Example of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Exhaust gas flow path, 3 ... Engine control unit, 4 ... Lean NOx catalyst, 5 ... Pump, 6 ... Open / close valve, 7 ... Tank, 8 ... Supply pipe, 9 ... Injector, 10 ... Exhaust gas.

Claims (5)

  An exhaust gas purification apparatus for purifying exhaust gas discharged from an internal combustion engine, comprising a lean NOx catalyst containing a NOx trapping component for trapping NOx in the presence of oxygen in an exhaust gas flow path of the internal combustion engine, wherein the engine of the internal combustion engine An exhaust gas purification apparatus for an internal combustion engine, comprising: a catalyst component supply mechanism configured to supply a catalyst component to the lean NOx catalyst when at least one of a start operation and an engine stop operation is performed.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the catalyst component replenishing mechanism supplies an aqueous solution in which the catalyst component is dissolved.   The catalyst component replenishment mechanism according to claim 1, wherein the catalyst component replenishment mechanism is a tank that stores an aqueous solution in which a catalyst component is dissolved, and an exhaust gas flow channel that is upstream of the position where the lean NOx catalyst is installed. An exhaust gas purifying device for an internal combustion engine, comprising a spraying device for spraying inside.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the catalyst component replenishment mechanism replenishes a NOx trapping component. NOx contained in lean burn exhaust gas is captured by a lean NOx catalyst containing NOx capture components installed in the exhaust gas flow path of the internal combustion engine, and the captured NOx is then reduced to N ₂ by switching the air-fuel ratio of the exhaust gas to stoichiometric or rich. In the exhaust gas purifying method, the exhaust gas of the internal combustion engine is characterized in that a catalyst component is replenished to the lean NOx catalyst at the time of at least one operation of the internal combustion engine when the engine is started and when the engine is stopped. Purification method.

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