JP2002161732A - Exhaust gas cleaning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning device that can be adapted to obtain a high rate of NOX reduction from comparatively low temperature region even if urea solution is utilized for reduction agent. SOLUTION: With regard to the exhaust gas cleaning device which is constituted so that a NOX reduction catalyst 5 is provided intermediate an exhaust pipe 4, and reduction cleaning of NOX is carried out, on the upstream side of the foregoing NOX reduction catalyst 5, by adding urea solution 12 as a reduction agent; a urea decomposing catalyst 7 that decomposes the urea solution 12 into ammonia and carbon dioxide is provided between the position in the longitudinal direction of the exhaust pipe 4 where the urea solution is added and the NOX reduction catalyst 5, thereby decomposing the urea solution 12 into ammonia and carbon dioxide by the foregoing urea decomposing catalyst 7, and the highly reactive ammonia thus obtained enables efficient execution of reduction treatment of NOX.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust emission control device.

[0002]

2. Description of the Related Art Conventionally, in a diesel engine automobile, a NOx reduction catalyst (selective reduction type) having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen is provided in an exhaust pipe through which exhaust gas flows. Catalyst) and the N
A required amount of a reducing agent is added to the upstream side of the Ox reducing catalyst, and the reducing agent reacts with NOx (nitrogen oxide) in the exhaust gas on the NOx reducing catalyst, thereby reducing the NOx emission concentration. There is something like that.

For example, this type of NOx reduction catalyst includes noble metal catalysts such as platinum and palladium, vanadium,
Base metal catalysts such as oxides of copper and iron are already known to have the properties as described above.

[0004] In the field of industrial flue gas denitrification treatment in plants and the like, the effectiveness of a technique for reducing NOx using ammonia (NH ₃ ) as a reducing agent is already well known. In such a case, since regulations on traveling with a toxic substance such as ammonia mounted thereon are strict, in most cases, hydrocarbons (light oil) are used as a reducing agent.

[0005]

However, in recent years, non-toxic urea has been used as a reducing agent to reduce NO.
It has been studied that x can be reduced with high efficiency, and the present inventors have added urea, which is usually present in a solid powder state, as urea water dissolved in water to the inlet side of the NOx reduction catalyst. However, when such urea water is used as a reducing agent, N2 is only used in a relatively high temperature range exceeding about 300 ° C. in the exhaust gas temperature range.
There was a problem that Ox could not be efficiently reduced and purified, and the NOx reduction rate in a low temperature range was low.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an exhaust gas purifying apparatus capable of obtaining a high NOx reduction rate from a relatively low temperature range even if urea water is used as a reducing agent. It is an object.

[0007]

According to the present invention, a NOx reduction catalyst is provided in the middle of an exhaust pipe, and urea water is added as a reducing agent upstream of the NOx reduction catalyst to reduce and purify NOx. Exhaust purification device, characterized in that a urea decomposition catalyst for decomposing urea water into ammonia and carbon dioxide is provided between the urea water addition position in the longitudinal direction of the exhaust pipe and the NOx reduction catalyst. It is.

Therefore, in the present invention, the urea water added as a reducing agent is decomposed into ammonia and carbon dioxide by the urea decomposition catalyst in the former stage, so that NOx is efficiently reduced by the highly reactive ammonia in the latter NOx reduction catalyst. As a result, the urea water is reduced and purified from a relatively lower temperature range than when the urea water is reacted with NOx as it is.

Further, in the present invention, it is preferable to provide a first oxidation catalyst upstream of the urea water addition position in the longitudinal direction of the exhaust pipe, so that most of the NOx in the exhaust gas is provided. NO accounts for highly reactive NO ₂
Thus, the reactivity of the NOx reduction catalyst with ammonia is enhanced, and the effect of reducing and purifying NOx in a relatively low temperature range is further enhanced.

Further, in the present invention, it is preferable to provide a second oxidation catalyst downstream of the NOx reduction catalyst in the longitudinal direction of the exhaust pipe, so that the NOx reduction catalyst can be passed unreacted. and had traces of leak ammonia to be discharged is oxidized to lower NO and harmless N ₂ toxicity as compared to ammonia.

[0011]

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

FIGS. 1 to 3 show an example of an embodiment of the present invention. As shown in FIG. 1, in an exhaust gas purifying apparatus of this embodiment, exhaust gas is discharged from a diesel engine 1 through an exhaust manifold 2. In the middle of an exhaust pipe 4 through which exhaust gas 3 flows, a NOx reduction catalyst 5 having a flow-through type honeycomb structure (see FIG. 2) is mounted and held in a casing 6.
In front of the x reduction catalyst 5, an iron-based urea decomposition catalyst 7 such as iron oxide (Fe ₂ O ₃ ) having a weak oxidizing effect is provided.

An injection nozzle 8 is provided at the most upstream position of the casing 6 on the inlet side of the urea decomposition catalyst 7, and a space between the injection nozzle 8 and a urea water tank 9 provided at a required location is provided. The urea solution 12 (reducing agent) in the urea solution tank 9 is connected to the urea solution supply tube 10 through the injection nozzle 8 by driving a supply pump 11 provided in the middle of the urea solution supply tube 10. 7 can be added to the inlet side.

The NOx reduction catalyst 5 suitable for the case where urea water 12 is used as the reducing agent includes vanadium, titanium,
A catalyst combining tungsten oxide, a catalyst combining copper and zeolite, and a catalyst combining platinum and zeolite are preferable.

Further, a first oxidation catalyst 13 for improving the NOx reduction / purification action is held by a casing 14 on the upstream side of the injection nozzle 8 installation position (urea water addition position). On the other hand, a second oxidation catalyst 15 is provided downstream of the NOx reduction catalyst 5 in the casing 6 as a measure against leakage ammonia.

The diesel engine 1 includes:
A rotation sensor 16 for detecting the engine speed is provided, and a rotation speed signal from the rotation sensor 16 and a load signal from an accelerator sensor 17 (a sensor for detecting the depression angle of an accelerator pedal) are transmitted to a controller 18. To be entered.

A temperature sensor 19 for detecting the temperature of the exhaust gas 3 flowing through the exhaust pipe 4 is provided near the inlet of the casing 6, and near the outlet of the casing 6, A NOx sensor 20 for detecting the NOx concentration is provided, and detection signals from the temperature sensor 19 and the NOx sensor 20 are input to the control device 18.

On the other hand, in the control device 18, the rotation speed signal from the rotation sensor 16 and the accelerator sensor 1
The amount of generated NOx is estimated based on the current operating state determined from the load signal from the engine 7 and the amount of urea water 12 added to the estimated amount of generated NOx is further calculated. When it is confirmed from the detection signal from that that the exhaust gas temperature is in the activation temperature range of the NOx reduction catalyst 5, the control device 18 directs the supply pump 11 so that the required amount of urea water 12 is added. A drive command signal is output. On the other hand, the feedback control is applied to the calculation of the addition amount of the urea water 12 based on the final NOx concentration determined from the detection signal from the NOx sensor 20.

In FIG. 1, reference numeral 21 denotes a muffler (silencer) provided on the exhaust pipe 4 on the downstream side of the casing 6.

When the exhaust gas temperature is in the activation temperature range of the NOx reduction catalyst 5, the supply pump 11 is driven by the drive command signal from the control device 18, and the NOx generation amount estimated from the current operation state. Amount of urea water 12 suitable for
Is injected from the injection nozzle 8, the urea water 12 added as a reducing agent in the urea decomposition catalyst 7 in the preceding stage

## STR1 ## Since (NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ decomposes into ammonia and carbon dioxide gas, the highly reactive ammonia in the subsequent NOx reduction catalyst 5
Mainly, the following equation

## STR2 ## NOx in the exhaust gas 3 is efficiently reduced to nitrogen by 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O, and the temperature is relatively lower than when the urea water 12 is reacted with NOx as it is. NOx can be reduced and purified from the region.

The reason why such a reaction between NO ₂ and ammonia is predominant is that the first oxidation catalyst 13 is provided upstream of the position where the urea water 12 is added by the injection nozzle 8. More specifically, when the exhaust gas 3 passes through the first oxidation catalyst 13, the N
NO that the majority of the Ox is because the highly reactive NO _2.

If the NO in the exhaust gas 3 is oxidized to NO ₂ on the upstream side, the reactivity of the NOx reduction catalyst 5 on the downstream side with ammonia is greatly increased, and The effect of reducing and purifying NOx in a low temperature range is further enhanced.

However, even if the first oxidation catalyst 13 is not provided on the upstream side of the position where the urea water 12 is added by the injection nozzle 8, it does not mean that NO ₂ does not exist in the exhaust gas 3.

6NO + 4NH ₃ → 5N ₂ + 6H ₂ O or the following formula

## STR4 ## NOx in the exhaust gas 3 is also favorably reduced and purified by 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O.

Furthermore, in the present embodiment, in particular, the second oxidation catalyst 15 is provided after the NOx reduction catalyst 5 in the casing 6 as a countermeasure against leak ammonia.
x A small amount of leaked ammonia that has passed unreacted through the reduction catalyst 5 has NO toxicity that is lower than that of ammonia.
Or harmless N ₂ is oxidized and discharged.

Therefore, according to the embodiment described above, the urea water 12 added as a reducing agent is decomposed into ammonia and carbon dioxide by the urea decomposition catalyst 7 in the former stage, and the resulting highly reactive water is obtained. NOx can be efficiently reduced by ammonia, and NOx occupying most of NOx in the exhaust gas 3 by the first oxidation catalyst 13.
As highly reactive NO ₂ , it is possible to greatly increase the reactivity of the downstream NOx reduction catalyst 5 with ammonia. Therefore, even if urea water 12 is used as the reducing agent, a high NOx reduction from a relatively low temperature range is achieved. Rate can be obtained, and the practicability of the exhaust gas purification device using the urea water 12 as the reducing agent can be greatly improved.

In fact, according to the results of experiments conducted by the present inventors, as shown in the graph of FIG. 3, a case A in which urea water 12 was added in the above-described apparatus configuration of the present embodiment, a urea decomposition catalyst 7 and the case B in which only the NOx reduction catalyst 5 was provided without the first oxidation catalyst 13 and the case B in which the urea water 12 was added, the case A was higher than the case B in the lower temperature range. It was confirmed that the NOx reduction rate could be obtained. The vertical axis in the graph of FIG. 3 indicates the NOx reduction rate,
The horizontal axis shows the catalyst inlet temperature of the exhaust gas 3 respectively.

Further, in the present embodiment, since the second oxidation catalyst 15 is provided as a countermeasure against the leak ammonia in the subsequent stage of the NOx reduction catalyst 5 in the casing 6, the NOx reduction catalyst 5 is passed without being reacted. The small amount of leaked ammonia can be oxidized to NO or N ₂ by the second oxidation catalyst 15, and there is a possibility that ammonia may remain in the exhaust gas 3 finally discharged to the atmosphere. Can also be avoided.

It should be noted that the exhaust gas purifying apparatus of the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the spirit of the present invention.

[0029]

According to the exhaust gas purifying apparatus of the present invention described above, the following various excellent effects can be obtained.

(I) According to the first aspect of the present invention, urea water added as a reducing agent is decomposed into ammonia and carbon dioxide by the urea decomposition catalyst in the preceding stage, and the resulting reactive NOx can be efficiently reduced with ammonia having a high concentration, so even if urea water is used as the reducing agent, a high NOx reduction rate can be obtained from a relatively low temperature range, and urea water is used as the reducing agent. The utility of the exhaust gas purification device can be greatly improved.

(II) According to the second aspect of the present invention, NO, which accounts for the majority of NOx in the exhaust gas by the first oxidation catalyst, is converted into strong oxidizing NO ₂ by N ₂ on the downstream side.
Since it can be led to the Ox reduction catalyst, the reactivity of the NOx reduction catalyst with ammonia can be greatly increased, and the action of reducing and purifying NOx in a relatively low temperature range can be further improved.

(III) According to the third aspect of the present invention, a small amount of leaked ammonia that has passed through the NOx reduction catalyst without being reacted is converted into N by the second oxidation catalyst.
O or N ₂ can be oxidized to make it harmless, and the possibility that ammonia remains in exhaust gas finally discharged to the atmosphere can be avoided beforehand.

[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 partially cutaway perspective view showing details of the NOx reduction catalyst of FIG. 1;

FIG. 3 is a graph showing a relationship between a NOx reduction rate and an exhaust gas temperature by the apparatus of FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine 3 Exhaust gas 4 Exhaust pipe 5 NOx reduction catalyst 7 Urea decomposition catalyst 8 Injection nozzle 12 Urea water 13 First oxidation catalyst 15 Second oxidation catalyst

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference)

Claims (3)

[Claims] 1. An exhaust gas purifying apparatus comprising a NOx reducing catalyst provided in the middle of an exhaust pipe and configured to reduce and purify NOx by adding urea water as a reducing agent upstream of the NOx reducing catalyst. An exhaust gas purification apparatus comprising a urea decomposition catalyst that decomposes urea water into ammonia and carbon dioxide between a position where urea water is added in the longitudinal direction of the exhaust pipe and the NOx reduction catalyst. 2. The exhaust gas purifying apparatus according to claim 1, wherein a first oxidation catalyst is provided upstream of a urea water adding position in a longitudinal direction of the exhaust pipe. 3. The exhaust gas purification apparatus according to claim 1, wherein a second oxidation catalyst is provided downstream of the NOx reduction catalyst in a longitudinal direction of the exhaust pipe.