JP3576504B2 - Exhaust gas purification device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification device used for an internal combustion engine such as a diesel engine.
[0002]
[Prior art]
Conventionally, in a diesel engine, a NOx reduction catalyst is provided in the middle of an exhaust pipe through which exhaust gas flows, and a required amount of a reducing agent is added to an upstream side of the NOx reducing catalyst so that the reducing agent is placed on the NOx reducing catalyst. There is an apparatus in which the reduction concentration of NOx can be reduced by causing a reduction reaction with NOx (nitrogen oxide) in exhaust gas.
[0003]
On the other hand, it is widely known that a lean burn can be performed at a higher air-fuel ratio than the stoichiometric air-fuel ratio to significantly improve fuel efficiency. However, it is assumed that such a lean-burn operation is performed. Reducing the NOx emission concentration has become an important issue for diesel engines as well.
[0004]
However, in general, in the presence of excess oxygen during the lean burn operation, the reducing agent reacts with oxygen on the NOx reduction catalyst before reacting with NOx. It was difficult to obtain a high NOx reduction effect.
[0005]
Therefore, as a catalyst capable of reducing NOx even during lean burn operation, NOx is oxidized during lean burn operation in which the oxygen concentration in the exhaust gas is high to temporarily store NOx in the state of nitrate and to reduce the NOx in the exhaust gas. Practical use of NOx storage reduction catalyst with the property of decomposing and releasing NOx to reduce and purify by rich spike (instantaneous rich combustion: rich air-fuel ratio combustion with low oxygen concentration) by direct addition or post injection during in-cylinder combustion Is currently being promoted.
[0006]
As this type of NOx storage reduction catalyst, a platinum-barium-alumina catalyst, an iridium-platinum-barium-alumina catalyst, etc., are already known as having the above-mentioned properties (for example, alkali metal such as barium or the like). , Alkaline earth metal is 1 g / l or more).
[0007]
[Problems to be solved by the invention]
However, in such an exhaust gas purifying apparatus using the NOx storage reduction catalyst, when occluding NOx in the NOx storage reduction catalyst, NO occupying most of the NOx in the exhaust gas is reduced to oxygen on the platinum with oxygen in the exhaust gas. Occlusion is achieved through a two-stage reaction in which NO ₂ is selectively reacted to become NO ₂ and then this NO ₂ reacts with barium and is temporarily occluded in the state of nitrate. Generally, an alkali metal such as barium or an alkaline earth metal, which contributes to substantial occlusion of NO ₂ , sinters a noble metal catalyst component which promotes an oxidation reaction such as platinum (the noble metal particles with time). (Agglomeration phenomenon) is easily promoted, and when a noble metal catalyst component such as platinum causes sintering due to long-term operation and the catalytic performance related to generation of NO ₂ is reduced, sufficient N There is a possibility that the Ox reduction effect cannot be obtained.
[0008]
The present invention has been made in view of the above-described circumstances, and provides a highly practical exhaust gas purifying apparatus that can maintain a high NOx reduction effect without incurring performance degradation even during long-time operation. It is an object.
[0009]
[Means for Solving the Problems]
The present invention provides a method for oxidizing NOx when the oxygen concentration in the exhaust gas is high and temporarily storing the NOx in the state of nitrate when the oxygen concentration in the exhaust gas is high, and reducing the NOx when the oxygen concentration in the exhaust gas is low. interposed NOx storage reduction catalyst for reducing and purifying by decomposing release NOx into the particulate filter together with the equipped by supporting integrally with the immediately before or immediately after the NO x storage-and-reduction catalyst, the catalyst monolith of the flow-through A supported auxiliary NO x storage reduction catalyst is separately provided, and NO ₂ in the exhaust gas is selectively reacted with NO ₂ in the exhaust gas upstream of the NO x storage reduction catalyst including the auxiliary NO x storage reduction catalyst to remove NO ₂ . The present invention relates to an exhaust gas purification device, which is provided with a NO ₂ generation catalyst having an enhanced function of generating, and configured so that a reducing agent can be appropriately added upstream of the NO ₂ generation catalyst.
[0010]
Thus, according to such an exhaust purification device, during lean burn operation in which the oxygen concentration in the exhaust gas is high, NOx in the exhaust gas is stored in the NOx storage reduction catalyst in the form of nitrate, so that NOx can be reduced. As will be understood, when the exhaust gas passes through the NO ₂ generating catalyst provided in front of the NOx storage reduction catalyst, NO, which accounts for the majority of NOx in the exhaust gas, selectively reacts with oxygen to produce NO. _2, and since the high NO ₂ of this reactivity is to be occluded by the flows in efficiently nitrate state downstream of the NOx storage reduction catalyst, occlusion reaction of NO ₂ in the NOx storage reduction catalyst is promoted drastically, NOx results in high NOx reduction effect than with storage reduction catalyst alone obtained, moreover, a catalyst with respect to the generation of NO ₂ in NOx storage-reduction catalyst by temporarily prolonged operation There even decreased, so that is borne generation process of NO ₂ is at its front stage of NO ₂ generation catalyst reduction of NOx occlusion capacity is avoided.
[0011]
That is, the first-stage NO ₂ generation catalyst is manufactured without containing the alkali metal or alkaline-earth metal component responsible for the storage of nitrate such as barium as in the case of the second-stage NOx storage reduction catalyst. Even if the operation for a long period of time, the performance of the catalyst component that promotes the oxidation reaction does not decrease, and even after the catalytic performance related to NO ₂ generation on the subsequent NOx storage reduction catalyst side decreases, the former NO ₂ generation catalyst remains The catalytic performance for the generation of NO ₂ will be kept high.
[0012]
On the other hand, the reducing agent is added by rich spike (instantaneous rich combustion: rich air-fuel ratio combustion with low oxygen concentration) by direct addition of the reducing agent to the exhaust gas or post injection during in-cylinder combustion, and the NOx storage reduction catalyst In order to regenerate the NOx storage-reduction catalyst by actively decomposing and releasing NOx from the NOx storage-reduction catalyst, and reacting the released NOx with the reducing agent on the NOx storage-reduction catalyst to reduce and purify the NOx storage-reduction catalyst, the former stage of the NOx storage-reduction catalyst is used. Since the reducing agent reacts with oxygen and thermally decomposes in the NO ₂ generation catalyst to generate CO and hydrogen, the increase in the amount of CO and hydrogen causes the decomposition and release of NOx from the NOx storage reduction catalyst and the reduction of the NOx. The purification reaction will be significantly accelerated.
[0013]
In the present invention, since the NOx storage reduction catalyst are integrally carried on the particulate filter, a high NO ₂ reactive generated in the previous stage of the NO ₂ generation catalyst with an oxidation reaction heat later stage As a result of flowing into the particulate filter and reacting with the particulates collected by the particulate filter, the combustion of the particulates is assisted, and the particulates are efficiently burned and removed at a relatively low ignition temperature. become.
[0014]
Further, in the present invention, NO x storage-immediately before the reduction catalyst or immediately after, the flow-through catalyst monolith has a supported auxiliary NOx storage-reduction catalyst separately equipped, but assisted catalytic monolith of the flow-through type NO x When the storage reduction catalyst is supported, it is possible to secure a large amount of catalyst in a compact manner without worrying about clogging.Especially, when targeting large vehicles, it is integrated with the particulate filter. It is possible to supplement the capacity of the NO x storage reduction catalyst, which tends to be insufficient simply by supporting the catalyst on the catalyst.
[0016]
Further, in the present invention, it is preferable that a reducing agent treatment catalyst having an enhanced function of oxidizing the unreacted reducing agent with oxygen in the exhaust gas is provided at the final stage. It is possible to prevent the possibility that the agent is released into the atmosphere together with the exhaust gas beforehand.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0018]
1 to 5 show an embodiment of the present invention. In the drawings, reference numeral 1 denotes an engine which is a diesel engine. In an engine 1 shown in FIG. 1, a turbocharger 2 is provided, and an air cleaner 3 is provided. The introduced air 4 is sent to a compressor 2a of the turbocharger 2 through an intake pipe 5, and the air 4 pressurized by the compressor 2a is further sent to an intercooler 6 for cooling. The air 4 is guided to the intake manifold 7 and introduced into each cylinder of the engine 1.
[0019]
In each cylinder of the engine 1, liquid fuel (light oil) from a fuel tank (not shown) is injected into each cylinder of the engine 1 and burned, and is discharged from each cylinder of the engine 1. Exhaust gas 8 is sent to a turbine 2b of the turbocharger 2 via an exhaust manifold 9, and the exhaust gas 8 that drives the turbine 2b is exhausted outside the vehicle via an exhaust pipe 10.
[0020]
In the middle of the exhaust pipe 10 through which the exhaust gas 8 flows, when the oxygen concentration in the exhaust gas 8 is high, NOx is oxidized and temporarily stored in the form of nitrate, and the oxygen concentration in the exhaust gas 8 is low. A NOx storage-reduction catalyst 12 that decomposes and releases NOx by the interposition of a reducing agent 11 (light oil), which will be described later, is provided integrally with the particulate filter 13.
[0021]
Here, as shown in FIG. 2, the particulate filter 13 has a porous honeycomb structure made of ceramic such as cordierite or the like, and the inlets of the channels 13a partitioned in a lattice shape are alternately plugged. The flow path 13a whose inlet is not plugged is configured such that its outlet is plugged, and only the exhaust gas 8 that has passed through the porous thin wall 13b that defines each flow path 13a flows downstream. And the particulates are trapped on the inner surface of the porous thin wall 13b. The NOx storage reduction catalyst 12 does not cause clogging of the entire particulate filter 13. To a certain extent.
[0022]
Further, the front stage of the NOx storage-reduction catalyst 12, is equipped with NO ₂ synthesizing catalyst 14 with improved functionality by selectively reacting NO in the exhaust gas 8 and oxygen to generate NO _2, this kind As the NO ₂ generation catalyst 14, for example, a carrier containing at least one kind of titania, silica, zirconia, and alumina is made of a noble metal such as platinum (1 to 10 g / l) and rhodium (0.1 to 5 g / l) in FIG. It is possible to support the catalyst by supporting it on a flow-through type catalyst monolith as shown in FIG.
[0023]
Further, immediately after the NOx storage reduction catalyst 12 is equipped with an auxiliary NOx storage reduction catalyst 15 carried on the same flow as NO ₂ generating catalyst 14 through type catalyst monoliths (see FIG. 3) described above separately, The capacity of the NOx storage reduction catalyst 12 is supplemented by the auxiliary NOx storage reduction catalyst 15.
[0024]
That is, when the auxiliary NOx storage reduction catalyst 15 is carried on the flow-through type catalyst monolith, a large amount of catalyst can be secured in a compact manner without worrying about clogging. Thus, the capacity of the NOx occlusion reduction catalyst, which tends to be insufficient if the catalyst is supported integrally with the particulate filter, can be efficiently supplemented.
[0025]
The last stage behind the auxiliary NOx storage reduction catalyst 15 is provided with a reducing agent treatment catalyst 16 having an enhanced function of oxidizing the unreacted reducing agent 11 with oxygen in the exhaust gas 8. The kind of the reducing agent treatment catalyst 16 can be constituted by, for example, a catalyst in which platinum (1 to 10 g / l) is supported on alumina as a carrier.
[0026]
At an appropriate position of the exhaust pipe 10 upstream of the NO ₂ generation catalyst 14, a reducing agent addition injector 17 is provided, which is capable of appropriately injecting the reducing agent 11 and adding it to the exhaust gas 8. When the NOx storage reduction catalyst 12 storing NOx is regenerated, the reducing agent 11 is injected to create a rich atmosphere.
[0027]
Thus, according to such an exhaust gas purification device, during the lean burn operation in which the oxygen concentration in the exhaust gas 8 is high, NOx in the exhaust gas 8 is stored in the NOx storage reduction catalyst 12 in the state of nitrate, and NOx is reduced. However, when the exhaust gas 8 passes through the NO ₂ generation catalyst 14 provided in front of the NOx storage reduction catalyst 12, NO, which accounts for the majority of NOx in the exhaust gas 8, becomes oxygen and NO. It selectively reacts to form NO ₂ , and this highly reactive NO ₂ flows into the subsequent particulate filter 13 and is efficiently stored in the NOx storage reduction catalyst 12 in the form of nitrate. The storage reaction of NO _{2 in} the catalyst 12 is remarkably accelerated, resulting in a higher NOx reduction effect than when the NOx storage reduction catalyst 12 is used alone.
[0028]
In fact, according to the average experimental results of a diesel engine 13 mode as shown in FIG. 4, those were NOx reduction ratio of the order of about 60% when there is no NO ₂ generating catalyst 14, NO ₂ synthesizing catalyst 14 It was possible to improve the NOx reduction rate to about 80% by using together.
[0029]
Moreover, even if the long-term operation catalyst performance relates to the generation of NO ₂ in NOx storage-reduction catalyst 12 decreases, occlusion of at preceding NO ₂ synthesizing catalyst 14 is borne by the generation process of the NO ₂ NOx A decline in performance will be avoided.
[0030]
That is, the first-stage NO ₂ generation catalyst 14 is manufactured without containing the alkali metal or alkaline-earth metal component responsible for storing nitrate such as barium as in the case of the second-stage NOx storage reduction catalyst 12. , without the catalyst component responsible for promoting the oxidation reaction by prolonged operation decreases the performance, even preceding NO ₂ generated after the catalyst performance is lowered on generating NO ₂ at a subsequent stage of the NOx occlusion reduction catalyst 12 side The catalyst performance of the catalyst 14 regarding the generation of NO ₂ is kept high.
[0031]
In addition, the NOx storage reduction catalyst 12 is actively decomposed and released to regenerate the NOx storage reduction catalyst 12, and the released NOx is reacted with the reducing agent on the NOx storage reduction catalyst 12 for reduction purification. At this time, if the reducing agent 11 is directly added to the exhaust gas 8 from the reducing agent addition injector 17, the reducing agent 11 reacts with oxygen in the NO ₂ generation catalyst 14 in the preceding stage and thermally decomposes to generate CO and hydrogen. Due to the increase in CO and hydrogen, the decomposition and release reaction of NOx from the NOx storage reduction catalyst 12 and the reduction and purification reaction of the NOx are remarkably promoted.
[0032]
On the other hand, in the present embodiment, since the NOx storage reduction catalyst 12 is integrally supported on the particulate filter 13, a high NO ₂ reactive generated in the previous stage of the NO ₂ generation catalyst 14 oxidation reaction heat As a result, it flows into the subsequent particulate filter 13 and reacts with the particulates collected by the particulate filter 13, so that the combustion of the particulates is assisted and the particulates are efficiently produced at a relatively low ignition temperature. Will be removed by combustion.
[0033]
For example, in a verification experiment at a rotation speed of about 60% in a general diesel engine, as shown in FIG. 5, the ignition temperature of the particulates was about 350 ° C. without the NO ₂ generation catalyst 14. However, the temperature could be lowered to about 250 ° C. by using the NO ₂ generation catalyst 14 in combination.
[0034]
The unreacted part of the reducing agent 11 added by the reducing agent adding injector 17 reacts with oxygen in the exhaust gas 8 by the final-stage reducing agent treatment catalyst 16 and is oxidized. The possibility that the unreacted reducing agent 11 is released into the atmosphere together with the exhaust gas 8 is prevented beforehand.
[0035]
Therefore, according to the above-described embodiment, the NO ₂ generation catalyst 14 provided before the NOx storage reduction catalyst 12 causes NO, which occupies most of NOx in the exhaust gas 8, to selectively react with oxygen to form NO _2. Since the highly reactive NO ₂ can be efficiently stored in the state of nitrate by flowing the highly reactive NO ₂ to the subsequent NOx storage reduction catalyst 12, the NO ₂ storage reaction in the NOx storage reduction catalyst 12 can be remarkably promoted. In addition, even when the catalytic performance relating to the generation of NO _{2 on} the side of the subsequent NOx storage reduction catalyst 12 is reduced, the catalytic performance relating to the generation of NO ₂ can be maintained high at the NO ₂ producing catalyst 14 of the preceding stage, so that NOx can be maintained. The storage capacity of the NOx storage-reduction catalyst 12 can also be avoided, and the NOx storage-reduction catalyst 12 can be regenerated by adding the reducing agent 11 to regenerate the NOx storage-reduction catalyst 12. The CO 2 and hydrogen are increased by reacting the reducing agent 11 with oxygen and thermally decomposing in the NO ₂ generating catalyst 14 at the preceding stage to decompose and release NOx from the NOx storing and reducing catalyst 12 and reduce and purify the NOx. Can be remarkably promoted, so that a high NOx reduction effect can be maintained without incurring performance degradation even by long-time operation.
[0036]
The exhaust gas purifying apparatus of the present invention, rather than be limited to the above embodiment e.g., N when the revitalization of Ox storage reduction catalyst is rich due post injection in-cylinder combustion in the engine side The spike (instantaneous rich combustion: rich air-fuel ratio combustion with low oxygen concentration) may be added to add the reducing agent (the thermal decomposition component of the unburned fuel), and the auxiliary NOx storage reduction catalyst may be particulated. Of course, it may be arranged immediately before the filter, and it is needless to say that various changes can be made without departing from the gist of the present invention.
[0037]
【The invention's effect】
According to the exhaust gas purification apparatus of the present invention described above, various excellent effects as described below can be obtained.
[0038]
According to the invention described in claim 1 of the present invention (I), it can significantly promote occlusion reaction of NO ₂ in the NOx storage reduction catalyst, the reduction purification reaction of the decomposition releasing reaction and the NOx of the NOx, moreover, reduction of the subsequent NOx storage reduction catalyst performance is able to maintain high catalytic performance for the generation of NO ₂ at even preceding NO ₂ generating catalyst was reduced NOx occlusion capacity for the generation of NO ₂ in the catalyst side Can be avoided, so that a high NOx reduction effect can be maintained without incurring performance degradation even during long-time operation.
[0039]
According to the invention described in claim 1, (II) the present invention, the pouring high NO ₂ reactive generated in the previous stage of the NO ₂ generation catalyst with oxidation reaction heat downstream of the particulate filter Since it is possible to cause an oxidation reaction with the particulates collected by the particulate filter, the particulates collected by the particulate filter can be efficiently burned and removed at a relatively low ignition temperature.
[0040]
(III) According to the first aspect of the present invention, when the auxiliary NOx storage reduction catalyst is carried on the flow-through type catalyst monolith, the amount of the catalyst can be reduced to a large amount without worrying about clogging. Therefore, especially for a large vehicle or the like, it is possible to efficiently supplement the capacity of the NOx storage-reduction catalyst, which tends to be insufficient simply by integrally supporting the particulate filter integrally.
[0041]
(IV) According to the invention described in claim 2 of the present invention, the unreacted reducing agent remaining in the exhaust gas can be detoxified by oxidation treatment with the final-stage reducing agent treatment catalyst. It is possible to prevent the risk that the fuel reducing agent is released into the atmosphere together with the exhaust gas.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of an embodiment for carrying out the present invention.
FIG. 2 is a sectional view showing details of the particulate filter of FIG. 1;
FIG. 3 is a perspective view showing details of the NO ₂ generation catalyst of FIG. 1 with a part thereof cut away.
FIG. 4 is a graph comparing the NOx reduction effect with a conventional one.
FIG. 5 is a graph comparing the ignition temperature of particulates with a conventional one.
[Explanation of symbols]
8 exhaust gas 10 exhaust pipe 11 reducing agent 12 NOx storage reduction catalyst 13 particulate filter 14 NO ₂ synthesizing catalyst 15 auxiliary NOx storage-reduction catalyst 16 reducing agent treatment catalyst 17 reducing agent addition injector

Claims (2)

In the middle of the exhaust pipe through which the exhaust gas flows, NOx is oxidized when the oxygen concentration in the exhaust gas is high and temporarily stored in the form of nitrate, and when the oxygen concentration in the exhaust gas is low, NOx is mediated by the reducing agent. with equipped by supporting integrally with the particulate filter and NOx storage reduction catalyst for reducing and purifying decomposes releasing, immediately before or immediately after the NO x storage-and-reduction catalysts, auxiliary carried on the catalyst monolith of the flow-through A NO x storage reduction catalyst is separately provided, and a function of selectively reacting NO and oxygen in the exhaust gas to generate NO ₂ is provided in front of the NO x storage reduction catalyst including the auxiliary NO x storage reduction catalyst. An exhaust gas purification apparatus comprising an increased NO ₂ generation catalyst and configured so that a reducing agent can be appropriately added upstream of the NO ₂ generation catalyst. The exhaust gas purifying apparatus according to claim 1 , wherein a reducing agent treatment catalyst having an enhanced function of oxidizing an unreacted reducing agent with oxygen in exhaust gas is provided in a final stage.

Images