JP2002097940A - Operating method of exhaust gas purification device - Google Patents

Abstract

(57) Abstract: In purifying exhaust gas of a diesel engine with a NOx storage reduction catalyst, it is possible to eliminate poisoning of the NOx storage reduction catalyst by sulfate without performing operation control on the diesel engine side. To SOLUTION: Exhaust gas 7 from a diesel engine 1
A NOx storage-reduction catalyst 11 is provided in the exhaust pipe 9 through which the NOx storage-reduction catalyst 11 is added. The reducing agent 10 is appropriately added to the NOx storage-reduction catalyst 11, and the exhaust gas 7 With respect to an operation method of an exhaust gas purifying apparatus configured to flow N 2, the reducing agent 10 is added by periodically adding the reducing agent 10 while flowing the exhaust gas 7 to the NOx storage reduction catalyst 11.
The temperature of the Ox storage reduction catalyst 11 is raised to a predetermined temperature or higher, and immediately thereafter, the NOx storage reduction catalyst 11 is bypassed and the reducing agent 1 is again supplied to the NOx storage reduction catalyst 11 while flowing the exhaust gas 7.
0 is added to perform desulfurization treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating an exhaust emission control device used for a diesel engine.

[0002]

2. Description of the Related Art Conventionally, in a diesel engine, a NOx reduction catalyst is provided in an exhaust pipe through which exhaust gas flows, and a required amount of a reducing agent is added upstream of the NOx reducing catalyst to reduce the amount of the reducing agent. Is reduced on the NOx reduction catalyst with NOx (nitrogen oxide) in the exhaust gas to produce NOx.
Some of them are capable of reducing the emission concentration of methane.

[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 greatly improve fuel efficiency. It is also an important issue to reduce the NOx emission concentration for a diesel engine that assumes the following.

[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 at a practical level.

Therefore, even during lean burn operation, NOx
As a catalyst that can reduce NOx, 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 the reducing agent is used during stoichiometric air-fuel ratio operation in which the oxygen concentration in the exhaust gas is low. At present, practical use of a NOx storage reduction catalyst having a property of decomposing and releasing NOx to reduce and purify is being studied.

For example, as this type of NOx storage reduction catalyst, a platinum-barium-alumina catalyst, an iridium-
Platinum / barium / alumina catalysts are already known as having the above-mentioned properties.

However, when the NOx stored in the NOx storage reduction catalyst is released to regenerate the NOx storage reduction catalyst, the operating state must be switched from lean combustion operation to stoichiometric air-fuel ratio operation. The advantage of improved fuel economy by lean burn operation is lost,
It is desired to be able to achieve good regeneration of the NOx storage reduction catalyst while maintaining the lean burn operation. However, in the presence of excess oxygen during the lean burn operation, the oxygen concentration in the exhaust gas is high. As in the case of the above-described NOx reduction catalyst, the reducing agent such as HC reacts with oxygen before being reacted with NOx and is consumed, whereby the reaction selectivity between the reducing agent and NOx is reduced. There was a problem that good regeneration of the NOx storage reduction catalyst could not be achieved.

Therefore, the present inventors have found that N
In addition to the provision of the Ox storage reduction catalyst, a bypass flow path is provided so as to bypass the NOx storage reduction catalyst, and the reducing agent is added while switching the exhaust gas to the bypass flow path side to add the reducing agent. By reducing the relative excess ratio of air to the amount, the reaction selectivity between the reducing agent and NOx is improved, whereby NOx is actively decomposed and released from the NOx storage-reduction catalyst to achieve good regeneration of the NOx storage-reduction catalyst. The inventors have come up with the idea of reacting the released NOx with the reducing agent on a NOx storage-reduction catalyst to purify the NOx, which has already been filed as Japanese Patent Application No. 11-328339.

[0009]

However, since SO ₂ derived from sulfur in the fuel is present in the exhaust gas of a diesel engine, this SO ₂ is oxidized on the NOx storage reduction catalyst in the same manner as NOx. The sulfate is more stable than the nitrate, and the NOx storage reduction catalyst is poisoned and deteriorated by the sulfate, which makes it impossible to store NOx. NOx
As for the problem that the purification rate decreases, there is no actual solution at the practical level.

However, the method itself for eliminating poisoning by sulfate is to perform rich combustion at an air-fuel ratio smaller than the stoichiometric air-fuel ratio and to maintain the atmosphere at a high temperature of about 600 ° C. or higher, so that the SO ₂ from the NOx storage reduction catalyst can be reduced. It is known that the NOx storage reduction catalyst is regenerated by releasing the NOx. However, realizing this by operation control on the diesel engine side requires the diesel engine to operate in an operation region where black smoke is inevitable. This means that it is practically difficult to adjust the conditions for the desulfurization treatment of the NOx storage reduction catalyst by the operation control on the diesel engine side while avoiding the generation of such black smoke.

The present invention has been made in view of the above-mentioned circumstances, and in purifying exhaust gas of a diesel engine with a NOx storage reduction catalyst, poisoning of the NOx storage reduction catalyst by sulfate on the diesel engine side is performed. It is an object of the present invention to be able to solve the problem without performing operation control.

[0012]

SUMMARY OF THE INVENTION The present invention oxidizes NOx in the exhaust pipe through which exhaust gas from a diesel engine flows when the oxygen concentration in the exhaust gas is high, and temporarily stores NOx in the state of nitrate. And a NOx storage reduction catalyst that decomposes and releases NOx through the use of a reducing agent when the oxygen concentration in the exhaust gas is low to reduce and purify the NOx. The reducing agent is appropriately added to the NOx storage reduction catalyst, and the NOx An operation method of an exhaust gas purification device configured to allow exhaust gas to flow by appropriately bypassing a storage reduction catalyst, and in which exhaust gas of a diesel engine is normally passed to a NOx storage reduction catalyst, whereby NOx in the exhaust gas is reduced. Is stored in the NOx storage reduction catalyst to reduce NOx.
A reducing agent is added to the NOx storage-reduction catalyst while flowing around the Ox storage-reduction catalyst, thereby actively decomposing and releasing NOx from the NOx storage-reduction catalyst to regenerate the NOx storage-reduction catalyst. A reducing agent is added while flowing exhaust gas to the NOx storage reduction catalyst periodically to raise the temperature of the NOx storage reduction catalyst to a predetermined temperature or higher, and immediately after that, the NOx storage reduction catalyst is bypassed and exhausted by flowing the exhaust gas.
It is characterized in that a reducing agent is added again to the storage reduction catalyst to perform a desulfurization treatment.

When exhaust gas having a high oxygen concentration of the diesel engine is passed through the NOx storage reduction catalyst, NOx in the exhaust gas is stored in the NOx storage reduction catalyst in the form of nitrate, thereby allowing the diesel engine to operate during operation. , A good reduction of NOx is achieved.

Further, when a reducing agent is added to the NOx storage reduction catalyst while exhaust gas is flowed around the NOx storage reduction catalyst periodically, the relative excess amount of air relative to the amount of the reducing agent is reduced and the reduction is performed. The selectivity of the reaction between the NOx storage and reduction catalyst and the NOx is improved, whereby the NOx storage and reduction catalyst is actively decomposed and released to achieve good regeneration of the NOx storage and reduction catalyst, and the released NOx is transferred to the NOx storage and reduction catalyst. By reacting with the reducing agent, reduction and purification can be performed well.

Further, when the reducing agent is added while flowing the exhaust gas to the NOx storing and reducing catalyst, the NOx storing and reducing catalyst generates heat due to an oxidation reaction of the reducing agent in a high-temperature atmosphere having a high oxygen concentration in which the exhaust gas flows. Rises above a predetermined temperature,
Immediately after that, when the reducing agent is added again to the NOx storage-reduction catalyst while flowing the exhaust gas while bypassing the NOx storage-reduction catalyst, the relative excess air ratio with respect to the added amount of the reducing agent decreases, and the relative air-fuel ratio becomes lower than the stoichiometric air-fuel ratio. Since a rich atmosphere with a small air-fuel ratio is realized, the conditions for releasing SO ₂ from the NOx storage reduction catalyst are set without performing operation control on the diesel engine side, and the NOx storage reduction catalyst is efficiently desulfurized. As a result, the poisoning by the sulfate is eliminated.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a diesel engine. In the diesel engine 1 shown here, a turbocharger 2 is provided, and an air cleaner 3 is provided. 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 to be cooled. The air 4 is guided to the intake manifold which is not used and introduced into each cylinder of the diesel engine 1.

In each cylinder of the diesel engine 1, liquid fuel (light oil) from a fuel tank (not shown) is injected into each cylinder of the diesel engine 1 and burned. Exhaust gas 7 discharged from each cylinder is sent to a turbine 2b of the turbocharger 2 via an exhaust manifold 8, and the exhaust gas 7 driving the turbine 2b
Is discharged outside the vehicle through the exhaust pipe 9.

In the middle of the exhaust pipe 9 through which the exhaust gas 7 flows, when the oxygen concentration in the exhaust gas 7 is high, NOx is oxidized to temporarily store NOx in the state of nitrate, and the exhaust gas 7
A NOx storage-reduction catalyst 11 that decomposes and releases NOx by the interposition of a reducing agent 10 (light oil) to be described below when the oxygen concentration is low is provided, and the exhaust gas is bypassed so as to bypass the NOx storage-reduction catalyst 11. The pipe 9 is provided with a bypass channel 12.

Here, the bypass passage 1 for the exhaust pipe 9 is provided.
At the two branch points, flow path switching valves 13 and 14 are provided so that the exhaust gas 7 flowing through the exhaust pipe 9 can be appropriately switched to the bypass flow path 12 and flow therethrough.

An injection nozzle 15 is provided on the exhaust pipe 9 at the inlet of the NOx storage reduction catalyst 11.
A reducing agent supply pipe 17 is provided between the injection nozzle 15 and a reducing agent tank 16 at a predetermined location (which can also be used as a fuel tank).
In addition, the injection nozzle 15 and an air tank 18 at a predetermined location are connected by an air introduction pipe 19, and a feed pump 20 in the middle of the reducing agent supply pipe 17 is driven to reduce the reducing agent. The reducing agent 10 in the tank 16 is guided to the injection nozzle 15 so that the reducing agent 10 can be sprayed from the injection nozzle 15 toward the inlet side of the NOx storage reduction catalyst 11. Also, if the amount of the reducing agent 10 is small, it is difficult to atomize, so as an assist for atomizing,
The opening / closing valve 21 in the middle of the air introduction pipe 19 is opened so that a small amount of pressurized air can be guided to the injection nozzle 15.

In the figure, reference numeral 22 denotes a muffler provided on the downstream side of the exhaust pipe 9.

When the exhaust gas 7 having a high oxygen concentration flows through the NOx storage reduction catalyst 11 by opening the flow path switching valve 13 and closing the flow path switching valve 14 during operation of the diesel engine 1, the exhaust gas 7 The stored NOx is stored in the form of nitrate, whereby good reduction of NOx during the lean burn operation of the diesel engine 1 is achieved.

Then, such NOx occlusion is reduced by about 30
After continuing for about 2 to 3 minutes, the flow path switching valve 14 is opened, the flow path switching valve 13 is closed, and the exhaust gas 7 is bypassed to the bypass flow path 12 to flow therethrough. The reducing agent 10 is guided from 16 and, if necessary, the on-off valve 2
1, the reducing agent 10 is sprayed from the injection nozzle 15 to the inlet side of the NOx storage reduction catalyst 11 by guiding a small amount of pressurized air from the air tank 18 as assist for atomization.

By doing so, the relative excess air ratio with respect to the amount of the reducing agent 10 added decreases, and the reducing agent 10 and NO
The reaction selectivity of x is improved, so that NOx is actively decomposed and released from the NOx storage-reduction catalyst 11, whereby good regeneration of the NOx storage-reduction catalyst 11 is achieved, and the released NOx
Reacts with the reducing agent 10 on the NOx storage reduction catalyst 11 to be reduced and purified.

Further, NOx storage reduction as described in detail below is performed periodically at a rate of about once every about 10 to 30 hours (about 10 to 15 hours for high load operation, about 20 to 30 hours for medium load operation). The desulfurization treatment of the catalyst 11 is performed.

That is, the flow path switching valve 13 is opened, the flow path switching valve 14 is closed, and the exhaust gas 7 flows to the NOx storage reduction catalyst 11 while the reducing agent 10 is supplied to the inlet side of the NOx storage reduction catalyst 11 in the same manner as described above. To the NOx storage reduction catalyst 11 by heat generated by the oxidation reaction of the reducing agent 10 in a high-temperature atmosphere with a high oxygen concentration in which the exhaust gas 7 is flowing.
The temperature is raised to about 650 to 700 ° C., and immediately after that, the flow switching valve 14 is opened and the flow switching valve 13 is closed, and the NOx storage reduction catalyst 11 Is added again with the reducing agent 10.

In this manner, the relative excess ratio of air to the amount of the reducing agent 10 is reduced, and a rich atmosphere having an air-fuel ratio smaller than the stoichiometric air-fuel ratio is realized. conditions for releasing SO ₂ becomes the ready without operation control of the diesel engine 1 side, so that the poisoning is eliminated by sulfate NOx occlusion reduction catalyst 11 is efficiently desulfurized from .

Here, as for the desulfurization treatment of the NOx storage reduction catalyst 11, as shown in FIG.
The desulfurization treatment by two-stage addition of 0 is about 15 cycles.
Preferably, a mode in which several cycles are repeated for about a minute is set in advance, and most of the sulfate in the NOx storage reduction catalyst 11 is released as SO ₂ to sufficiently recover poisoning deterioration.

Therefore, according to this embodiment, when purifying the exhaust gas of the diesel engine with the NOx storage reduction catalyst, the poisoning of the NOx storage reduction catalyst by the sulfate is eliminated without performing the operation control on the diesel engine side. Therefore, the application of the exhaust gas purification device using the NOx storage reduction catalyst to a diesel engine can be realized.

FIG. 3 shows another embodiment of the present invention. In this embodiment, the NOx storage reduction catalysts 11 shown in FIG.
The exhaust pipe 9 is branched into two branch passages 9A and 9B so that the exhaust gas 7 can be appropriately switched and flown to each of the storage reduction catalysts 11 and 11 so that the respective NOx storage reduction catalysts 1 are separated.
1 and 11 and are joined again on the downstream side to lead to the muffler 22.
Exhaust gas 7 flowing through the exhaust pipe 9
Are respectively provided so as to be able to alternately flow through the branch flow paths 9A and 9B.

As shown in FIG. 1, an injection nozzle 15 is provided on both sides of each of the NOx storage reduction catalysts 11, 11.
The reducing agent 10 in the reducing agent tank 16 is supplied to each of the injection nozzles 15 and 15 via the reducing agent supply pipes 17 and 17 by driving the supply pumps 20 and 20. Guided and pressurized air in the air tank 18 is guided through the air introduction pipes 19 and 19 as needed to open the on-off valves 21 and 21 to selectively supply the reducing agent 10 to the inlet side of each NOx storage reduction catalyst 11. So that it can be sprayed.

Thus, the NOx storage reduction catalyst 1
1 are provided in parallel, the flow path switching valve 13 is opened and the flow path switching valve 14 is closed to exhaust gas 7 into the branch flow path 9A.
While the NOx in the exhaust gas 7 is stored in the state of nitrate by the NOx storage reduction catalyst 11 in the branch flow path 9A, the flow rate of the exhaust gas 7 is restricted.
When the NOx storage reduction catalyst 11 on the B side is regenerated or subjected to a desulfurization treatment, one of the NOx storage reduction catalysts 11 and 11 is always in a usable state to continuously reduce NOx. However, it goes without saying that the operation method of the present invention can be similarly applied to the exhaust gas purification apparatus having such a configuration.

The operation method of the exhaust gas purifying apparatus of the present invention is as follows.
It is needless to say that the present invention is not limited to the above-described embodiment, and that various changes can be made without departing from the spirit of the present invention.

[0035]

According to the operation method of the exhaust gas purifying apparatus of the present invention, when the exhaust gas of the diesel engine is purified by the NOx storage reduction catalyst, the poisoning of the NOx storage reduction catalyst by the sulfate is performed on the diesel engine side. Therefore, it is possible to achieve an excellent effect that the application of the exhaust purification device using the NOx storage reduction catalyst to a diesel engine can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an embodiment for implementing the present invention.

FIG. 2 is a diagram showing a time schedule of a desulfurization processing mode.

FIG. 3 is a schematic diagram showing another example of the embodiment for carrying out the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine 7 Exhaust gas 9 Exhaust pipe 10 Reducing agent 11 NOx storage reduction catalyst

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/36 F01N 3/36 B

Claims (1)

[Claims] In the exhaust pipe through which exhaust gas from a diesel engine flows, when the oxygen concentration in the exhaust gas is high, NOx is oxidized and temporarily stored in the form of nitrate, and the oxygen concentration in the exhaust gas is reduced. When the pressure is low, N
A NOx storage reduction catalyst that decomposes and releases Ox to reduce and purify the exhaust gas is provided so that a reducing agent is appropriately added to the NOx storage reduction catalyst and the NOx storage reduction catalyst is appropriately bypassed to allow exhaust gas to flow. This is a method of operating the exhaust gas purification device thus configured.
While flowing NOx in the NOx storage reduction catalyst to thereby reduce NOx, NOx in the exhaust gas is reduced by the NOx storage reduction catalyst. Add a reducing agent to the reduction catalyst,
As a result, the NOx storage reduction catalyst is actively decomposed and released from the NOx storage reduction catalyst to regenerate the NOx storage reduction catalyst, and further, a reducing agent is added to the NOx storage reduction catalyst while periodically flowing exhaust gas thereto to store NOx. Exhaust purification characterized by raising the temperature of the reduction catalyst to a predetermined temperature or higher, and immediately thereafter adding a reducing agent to the NOx storage reduction catalyst again while flowing exhaust gas while bypassing the NOx storage reduction catalyst to perform desulfurization treatment. How to operate the device.