JPH05171921A - Exhaust gas purifying device - Google Patents

Abstract

PURPOSE:To effectively increase an NOx purifying ratio when an HC quantity in exhaust gas is small. CONSTITUTION:Catalyst 6 comprising metal-including silicate carrying transition metal is provided in an exhaust passage 3, a means 12 to supply HC to the catalyst 6 is provided, and HC is supplied to the catalyst 6 by actuating the HC supplying means when a ratio of HC/NOx in exhaust gas is a specified value or less and simultaneously when a temperature of exhaust gas or the catalyst 6 is a specified value or more.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art As a catalyst for purifying engine exhaust gas,
Oxidation of CO (carbon monoxide) and HC (hydrocarbons), N
A three-way catalyst that simultaneously performs reduction of Ox (nitrogen oxide) is generally known. This three-way catalyst is, for example, γ-alumina on which Pt (platinum) and Rh (rhodium) are supported, and when the air-fuel ratio (A / F) of the engine is controlled to be near the theoretical air-fuel ratio of 14.7. , High purification efficiency can be obtained.

On the other hand, there is a demand to increase the air-fuel ratio to reduce the fuel consumption of the engine.
A lean-burn engine has been developed. That is,
This engine is designed to improve the atomization of the air-fuel mixture and achieve stable combustion even with a lean air-fuel mixture.When the engine temperature is low, the air-fuel ratio is set near the stoichiometric air-fuel ratio. After the engine temperature rises and the combustion stability of the air-fuel mixture becomes high, the air-fuel ratio is usually set by switching to the lean side. In that case, since the exhaust gas contains excess oxygen, the three-way catalyst described above does not emit CO or HC.
Although it can be oxidized and purified, NOx cannot be reduced and purified.

Therefore, in recent years, zeolite (aluminosilicate) -based NOx in which a transition metal is supported by ion exchange is carried out.
Research on purification catalysts is ongoing. In the case of this catalyst, even in a lean atmosphere, N ₂ and O ₂ can be directly mixed with NOx or by a coexisting reducing agent (eg, CO, HC).
It can be catalytically decomposed with.

However, the above NOx purification catalyst has a problem that the NOx purification rate is low when the amount of HC in the exhaust gas is small. On the other hand, it may be considered to replenish the exhaust gas with HC (see Japanese Patent Laid-Open No. 63-283727).

[0006]

However, it is not preferable to use HC more than necessary for purifying NOx from the viewpoint of reducing the fuel consumption rate. That is, an object of the present invention is to efficiently increase the NOx purification rate when the amount of HC in the exhaust gas is small.

[0007]

[Means for Solving the Problem and Its Function] NOx
When the present inventor examined the problem of the reduction of the purification rate, first, the results shown in FIG. 9 were obtained regarding the relationship between the HC / NOx ratio and the NOx purification rate. The decrease in the NOx purification rate is not so great when the temperature of the exhaust gas or the catalyst is low, and becomes remarkable when the temperature is high. Therefore, it is sufficient to replenish the catalyst with HC when the temperature is high. Further, it has been found that, when the temperature is low, supplementing oxygen to the catalyst can improve the NOx purification rate.

That is, the first means for solving the above problems is that a transition metal is supported on a metal-containing silicate,
A catalyst for purifying exhaust gas containing NOx, an HC supply means for supplying HC to the catalyst, a component ratio detection means for detecting a component ratio of HC and NOx in the exhaust gas, and the exhaust gas Alternatively, the temperature detection means for detecting the temperature of the catalyst and the HC detected by the component ratio detection means
Based on the / NOx ratio and the temperature detected by the temperature detecting means, the control for operating the HC supply means is performed when the HC / NOx ratio is below a predetermined value and when the temperature is above a predetermined value. And an exhaust gas purification device.

As described above, when HC is supplied to the catalyst under a specific condition, the reduction of the NOx purification rate of the catalyst is reduced. The reason is considered as follows. That is, when the HC / NOx ratio is equal to or less than a predetermined value, for example, when the ratio is close to 1 or less than 0, the exhaust gas or the catalyst temperature is low. When it becomes higher (for example, at 400 ° C. or higher), the following HC combustion reaction is likely to occur.

CnHm + (n + m / 4) O ₂ → nCO ₂ + m / 2 · H ₂ O This is a complete combustion reaction of HC, which is why HC
Does not sufficiently contribute as a reducing agent in the decomposition reaction of NOx, and the purification rate of NOx decreases.

On the other hand, in the case of the above apparatus, since HC is supplied from the HC supply means, it is considered that the incomplete combustion reaction of HC proceeds as follows.

[0012] For _{CnHm + X1 O 2 → X2 CO} 2 + X3 H 2 O + X4 Cn'Hm ' That is, the intermediate Cn'Hm by incomplete combustion' is produced, which reacts with NOx in the exhaust gas, NOx
It is thought that the purification rate will increase. In this case, even if the HC is replenished when the exhaust gas temperature or the catalyst temperature is low, the combustion itself of the HC is not smoothly performed because of the low temperature, and the effect is small.

Here, a catalyst in which a transition metal is supported on a metal-containing silicate will be described. The metal-containing silicate is a crystalline porous body having micropores,
Zeolite (aluminosilicate) is suitable as the metal-containing silicate. Of course, instead of this zeolite, other silicates, for example, Al and Fe, Ce, Mn, Tb, C as the metal forming the crystal skeleton are used.
A silicate formed by combining other metal (semi-metal) such as u, B, or P, or a non-aluminosilicate not containing Al can also be adopted, and these are effective in obtaining heat resistance. Also, from the viewpoint of improving heat resistance, Na
The H type is preferable to the type, and the H type zeolite is particularly preferable. As the above zeolite, ZSM-5 is preferable, but A type, X type, Y type and the like may be used.

As the transition metal to be supported on the metal-containing silicate, Cu is preferable, but C other than that is preferable.
It may be o, Cr, Ni, Fe, Mn or the like. The transition metal is preferably supported on the metal-containing silicate by ion exchange, but may be supported by impregnation.

When the catalyst is used, it may be in a pellet type, but it may be supported on a carrier. In this case, cordierite is preferable as the carrier, and other inorganic porous materials may be used. A plastid can also be used.

As the HC to be supplied to the catalyst, for example, in the case of an automobile engine, the fuel (gasoline) of the engine may be used, and other hydrocarbons such as C ₃ H ₆ and C ₄ H ₁₀ may be used. Good.

As the component ratio detecting means for detecting the component ratio between HC and NOx in the exhaust gas, an air-fuel ratio sensor (O
₂ Concentration sensor) is used as a substitute, and HC is calculated based on the A / F value.
It is possible to determine whether the / NOx ratio is less than or equal to a predetermined value. In the case of an engine such as an automobile, the A / F can be detected based on the engine speed and the intake negative pressure. Of course, the respective concentrations of HC and NOx may be detected by a sensor to obtain the HC / NOx ratio.

A second means for solving the above-mentioned problems is as follows.
A transition metal is supported on a metal-containing silicate,
a catalyst for purifying exhaust gas containing x and O
O ₂ supply means for supplying ₂ and component ratio detection means for detecting the component ratio of HC and NOx in the exhaust gas,
Temperature detecting means for detecting the temperature of the exhaust gas or catalyst, and HC / N detected by the component ratio detecting means.
Control for operating the O ₂ supply means based on the Ox ratio and the temperature detected by the temperature detecting means when the HC / NOx ratio is below a predetermined value and when the temperature is below a predetermined value. And an exhaust gas purification device.

Also in the second means, the NOx purification rate is improved by supplementing O _{2 under} the specific conditions. This means that when the HC / NOx ratio is below a predetermined value and the temperature of the exhaust gas or catalyst is below a predetermined value, O
_{Since the} replenishment of ₂ promotes the combustion of HC by this replenishment, an intermediate product due to incomplete combustion similar to the case of the first means is generated, which contributes to purification (reduction) of NOx. Conceivable.

In this case, even if the O ₂ is replenished as described above when the temperature of the exhaust gas or the catalyst is high, the NOx purification rate cannot be improved. When the temperature is high,
This is because the combustion of HC is naturally easy to proceed. That is,
If O ₂ is supplied in such a state, the combustion of HC is further promoted, and the above-mentioned incomplete combustion of HC cannot be expected, and the promotion of the combustion of HC causes the catalyst temperature to rise. There is a concern that it will cause an excessive rise in

As O ₂ supplied to the catalyst, oxygen itself may be used, but air may be used, and in the case of an automobile engine, a method of introducing secondary air from the intake passage to the exhaust passage is preferable.

As is clear from the above description, the above HC
When the / NOx ratio is equal to or lower than a predetermined value and the temperature of the exhaust gas or the catalyst is equal to or higher than a predetermined value, the above H
When the C supply means is operated and the HC / NOx ratio is equal to or lower than a predetermined value and the temperature is equal to or lower than the predetermined value, operating the O ₂ supply means solves the above-described problem of the present invention. This is a preferable third means.

[0023]

Therefore, according to the first means, the HC
When the / NOx ratio is equal to or lower than a predetermined value and the temperature of the exhaust gas or the catalyst is equal to or higher than the predetermined value, HC is separately supplied to the catalyst. Therefore, effective supply of HC, that is, HC NO while keeping the usage to a minimum
x Purification rate can be increased.

According to the second means, HC / NOx
When the ratio is less than or equal to the predetermined value and the temperature of the exhaust gas or the catalyst is less than or equal to the predetermined value, O ₂ is supplied to the catalyst, so that the NOx purification does not occur without causing an excessive rise in the catalyst temperature. The rate can be increased.

According to the third means, HC / NOx
When the ratio is equal to or lower than a predetermined value and the temperature of the exhaust gas or the catalyst is equal to or higher than the predetermined value, HC is supplied to the catalyst, and when the HC / NOx ratio is equal to or lower than the predetermined value and the temperature is Since O ₂ is supplied to the catalyst when the temperature is equal to or lower than the predetermined value, the NOx purification rate can be increased in a wide temperature range from when the temperature is low to when the temperature is high.

Further, a catalyst in which Cu is supported on the zeolite by ion exchange is used, and an exhaust gas temperature of 400
According to the one in which the supply of HC or the supply of O ₂ is performed on the basis of ° C, it is possible to surely improve the NOx purification rate.

[0027]

EXAMPLES Examples of the present invention will be described below. -Mechanical structure of the exhaust gas purifying device-Fig. 1 shows the mechanical structure of the device. In the figure, 1 is a lean burn engine, 2 is an intake passage, 3 is an exhaust passage, 4 is a fuel (gasoline) tank,
5 is an air cleaner. The exhaust passage 3 is provided with a NOx purification catalyst 6 that mainly purifies NOx in the exhaust gas, and further, an oxidation catalyst 7 is provided downstream thereof. Further, a secondary air passage 9 extends from a portion of the intake passage 2 upstream of the throttle valve 8 to a portion of the exhaust passage 3 immediately upstream of the NOx purification catalyst 6.

A fuel supply passage extending from the fuel tank 4 branches downstream of the fuel pump 10, and one of them is connected to a first injector 11 for injecting fuel to a portion of the intake passage 2 downstream of the throttle valve 8. The other is connected to a second injector (HC supply means) 12 for injecting fuel into a surge tank 9a provided in the middle of the secondary air passage 9. A pressure regulator 13 is provided in the return passage for returning the fuel to the fuel tank 4.

A surge tank 9a is provided in the secondary air passage 9.
An on-off valve (O ₂ supply means) 14 is provided in the upstream portion of the. The exhaust passage 3 is provided with a first temperature sensor 15 and a first air-fuel ratio sensor 16 upstream of the opening of the secondary air passage 9, and a second temperature sensor downstream of the opening. 17 and a second air-fuel ratio sensor 18 are provided. The first sensors 15 and 16 are the same as the engine 1
Is for detecting the temperature and the air-fuel ratio of the exhaust gas (before the supply of HC and O ₂ ) discharged from the exhaust gas. The second sensors 17 and 18 supply HC and O ₂ to the exhaust gas. It is for detecting the temperature (catalyst inlet temperature) of the gas after being burnt, and the air-fuel ratio. Further, the NOx purification catalyst 6 is provided with an exhaust gas temperature sensor 19 for detecting the temperature of the exhaust gas.

The operation of the second injector 12 and the on-off valve 14 is controlled by the control means (CPU) 20 based on the outputs of the sensors 15 to 19 described above.

The NOx purifying catalyst 6 is a zeolite (ZSM-5) on which Cu is supported by ion exchange. Further, the above-mentioned oxidation catalyst is prepared by supporting noble metal (Pt, Pd) on γ-alumina.

-Control Example 1- An example of control of the second injector 12 and the on-off valve 14 by the control means 20 is shown in FIG. That is, in FIG.
(-450 ° C), the on-off valve 14 is opened to supply a constant amount of secondary air per unit time, and the area and (A / F = 1)
5 to 18, the catalyst inlet temperature is 400 ° C. or higher, the second injector 12 is turned on and a fixed amount of fuel (H
C) is injected.

In this case, basically, the start of the control of each area is determined based on the output from the first sensor 15, 16, and the control of the area is performed by the first sensor 15, 1.
6 and the second sensor 17, 18 ends when the output value deviates from the area.

The supply amount of the secondary air is set so that the oxygen concentration in the exhaust gas becomes 10% or more, preferably 13%. On the other hand, the amount of fuel supplied is equal to the amount of H in the exhaust gas.
C concentration is 2000 ppmC or more, preferably 3000
The amount is ppmC.

In the above device, the surge tank 9
"a" functions as a mixer of the secondary air and the fuel.

-Control Example 2- Another control example of the second injector 12 and the on-off valve 14 by the control means 20 is shown in FIG. That is, in the figure, the region (catalyst inlet temperature 350 to 45
(0 ° C.), the on-off valve 14 is opened to supply a constant amount of secondary air per unit time, and the second injector 12 is turned on in the region (A / F = 15 to 18, catalyst inlet temperature 350 ° C. or higher). At the same time, the on-off valve 14 is opened, and fuel (HC) and oxygen (secondary air) are supplied according to the addition amount characteristics shown in FIG. In this case, the secondary air supply amount in the region is the same as the secondary air supply amount in the control example 1. In addition, in FIG. 4, the oxygen amount in the addition amount 10 is the same as the secondary air supply amount in the above region, and the fuel amount in the addition amount 10 is the same as the fuel amount in the control example 1 region.

The determination of the start and end of the control of each area in the control example 2 is the same as that in the control example 1.

-Relationship between A / F and HC / NOx- Now, the relation between the A / F and HC / NOx will be described. That is, as shown in FIG. 5, the exhaust gas from the engine 1 has a component concentration that varies depending on A / F, and there is a substantially constant relationship between A / F and HC / NOx. Therefore, in the present embodiment, instead of directly detecting HC / NOx, the above A / F is detected and replaced with detection of HC / NOx. In this case, A / F = 15-18
In the above, HC / NOx is about 1.0 to 1.8.

-Purification test- FIG. 6 shows each purification test data according to the control example 1 and the control example 2 in comparison with the non-control test data. In this purification test, a simulated exhaust gas having the following composition was used. That is, the simulated exhaust gas is passed through the catalyst while changing the temperature, and the NOx purification rate is checked without separately supplying oxygen and fuel without control, and in the control example 1 and the control example 2, the oxygen or fuel is adjusted depending on the temperature. Supply NOx
This is an examination of the purification rate. HC is C ₃ H ₆ .

HC; 350 ppm NOx; 260 ppm CO; 650 ppm CO ₂ ; 10% O ₂ ; 6% As a result of the purification test, in both control examples 1 and 2, the NOx purification rate was higher than that of the uncontrolled one. There is.
In particular, in the case of the uncontrolled type, the NOx purification rate sharply decreases when the catalyst inlet temperature exceeds 400 ° C, whereas in the control examples 1 and 2, the reduction of the purification rate is small even after 400 ° C and the low temperature The rise of activity in is getting steeper. From this, in exhaust gas with a low HC / NOx ratio, the supply of O ₂ at low temperature and the supply of HC at high temperature are
It can be seen that it greatly contributes to the improvement of the Ox purification rate.

-Regarding Effect of Supplying HC- FIG. 7 investigates and compares the NOx purification rate with the above-mentioned simulated exhaust gas composition and the NOx purification rate with the simulated exhaust gas composition in which only HC in the same composition is 10 times 3500 ppm. It was done. From the figure, it is confirmed that the supply of HC is effective in improving the NOx purification rate.

-Effect of O ₂ Supply- FIG. 8 shows the O ₂ concentration of 6 based on the following simulated exhaust gas composition.
The results of investigating the NOx purification rate by changing to three types of%, 12% and 17% are shown.

HC; 350 ppm NOx; 1000 ppm CO; 650 ppm CO ₂ ; 10% From the results shown in FIG. 8, it is found that the supply of O ₂ is effective for improving the NOx purification rate, especially at 450 ° C. or lower, for improving the NOx purification rate. It is confirmed that it contributes well.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an exhaust gas purification device according to an embodiment.

FIG. 2 is a control area diagram according to a control example 1 of the embodiment.

FIG. 3 is a control region diagram according to a control example 2 of the embodiment.

FIG. 4 is a graph showing the addition characteristics of oxygen and fuel in Control Example 2.

FIG. 5 is a graph showing the relationship between air-fuel ratio and HC and NOx emissions.

FIG. 6 is a graph showing the purification test results in each of control example 1, control example 2 and non-control.

FIG. 7 is a graph showing the results of examining the relationship between the amount of HC and the NOx purification rate.

FIG. 8 is a graph showing the results of examining the relationship between the O ₂ amount and the NOx purification rate.

FIG. 9 is a graph showing the results of examining the relationship between HC / NOx and the NOx purification rate.

[Explanation of symbols]

1 Engine 2 Intake Passage 3 Exhaust Passage 4 Fuel Tank 6 NOx Purification Catalyst 9 Secondary Air Passage 12 Injector (HC Supply Means) 14 Open / Close Valve (O ₂ Supply Means) 15, 17 Temperature Sensor 16, 18 Air Fuel Ratio Sensor (HC / NOx ratio detection means) 20 control means

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI technical display location F01N 3/10 A 7910-3G (72) Inventor Takashi Takemoto 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture No. Mazda Co., Ltd.

Claims (5)

[Claims] 1. A catalyst in which a transition metal is supported on a metal-containing silicate and which purifies exhaust gas containing NOx, HC supply means for supplying HC to the catalyst, and HC in the exhaust gas. Component ratio detecting means for detecting the component ratio with NOx, temperature detecting means for detecting the temperature of the exhaust gas or catalyst, HC / NOx ratio detected by the component ratio detecting means and the temperature detecting means. And a control means for operating the HC supply means when the HC / NOx ratio is equal to or lower than a predetermined value and when the temperature is equal to or higher than the predetermined value. Gas purification device. 2. A catalyst comprising a metal-containing silicate carrying a transition metal and purifying NOx-containing exhaust gas, O ₂ supply means for supplying O ₂ to the catalyst, and exhaust gas in the exhaust gas. By the component ratio detecting means for detecting the component ratio of HC and NOx, the temperature detecting means for detecting the temperature of the exhaust gas or the catalyst, the HC / NOx ratio detected by the component ratio detecting means and the temperature detecting means. Control means for operating the O ₂ supply means when the HC / NOx ratio is below a predetermined value and when the temperature is below a predetermined value based on the detected temperature. Exhaust gas purification device. 3. An O ₂ supply means for supplying O ₂ to the catalyst, wherein the control means comprises an HC / NOx ratio detected by the component ratio detection means and a temperature detected by the temperature detection means. Based on the above, when the HC / NOx ratio is equal to or lower than the predetermined value and the temperature is equal to or higher than the predetermined value, the HC supply means is operated and the HC / NOx ratio is equal to or lower than the predetermined value. The exhaust gas purifying apparatus according to claim 1, wherein the O ₂ supply means is operated when the temperature is equal to or lower than a predetermined value. 4. The catalyst according to claim 1, wherein Cu is supported on the zeolite by ion exchange, and the temperature condition for the control means to operate the HC supply means is when the exhaust gas temperature is 400 ° C. or higher. Exhaust gas purification device. 5. The catalyst according to claim 2, wherein Cu is supported on the zeolite by ion exchange, and the temperature condition for the control means to operate the O ₂ supply means is when the exhaust gas temperature is 400 ° C. or lower. Exhaust gas purification device described.