JP3443223B2 - Diesel engine exhaust purification system - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an exhaust emission control device for a diesel engine.

[0002]

2. Description of the Related Art A diesel engine usually contains a relatively large amount of nitrogen oxides (hereinafter referred to as NOx) in its exhaust gas in order to burn fuel in an excess air state. As a device for purifying harmful NOx,
It is known to have a NOx catalyst arranged in the exhaust system and a means for supplying a reducing agent such as ammonia to the exhaust upstream side of the NOx catalyst.

In the NOx purifying device disclosed in Japanese Patent Laid-Open No. 5-222923, it has been proposed to use a light oil similar to a diesel engine fuel as a reducing agent in consideration of the vehicle mountability of the reducing agent. There is. Light oil does not vaporize well even if it is directly supplied to the NOx catalyst, because light oil has a high boiling point and contains hydrocarbons with a large number of carbon atoms.
It cannot sufficiently reduce NOx. Thereby,
In this purification device, light oil is supplied to the reforming catalyst heated by the heater before it is supplied to the NOx catalyst, and only the reformed hydrocarbon having a small carbon number is supplied to the exhaust gas upstream side of the NOx catalyst. It is like this.

[0004]

The above-mentioned prior art makes it possible to use light oil as a reducing agent.
In order to heat the reforming catalyst, it is necessary to constantly supply electric power to the heater, and such a large amount of new energy is consumed.

Therefore, an object of the present invention is to purify nitrogen oxides by reforming a hydrocarbon fuel such as light oil into a hydrocarbon reducing agent without consuming a large amount of new energy.
An object of the present invention is to provide an exhaust emission control device for a diesel engine, which can favorably reduce NOx in exhaust gas with an Ox catalyst.

[0006]

According to a first aspect of the present invention, there is provided an exhaust emission control device for a diesel engine, wherein an NO _x catalyst for purifying nitrogen oxides arranged in an engine exhaust system and heat of exhaust gas. And a reforming catalyst for reforming the hydrocarbon-based fuel into a hydrocarbon-based reducing agent, and a hydrocarbon-based fuel supply means for supplying the hydrocarbon-based fuel to the NO _X catalyst via the reforming catalyst. And an air supply means for supplying to the reforming catalyst air sufficient to burn a part of the hydrocarbon fuel supplied to the reforming catalyst by the hydrocarbon fuel supplying means to the reforming catalyst, And the above
The reforming catalyst uses the NO _x catalyst having a large heat capacity.
The heat exchange between the reforming catalyst and the exhaust gas is surrounded by
It is characterized in that it is carried out via a NO _X catalyst .

In this exhaust emission control system for a diesel engine, the reforming catalyst is heated by the heat of the exhaust gas,
The exhaust gas temperature is low and the reforming catalyst is a hydrocarbon because the air supply unit supplies air to the reforming catalyst to burn only a portion of the hydrocarbon-based fuel supplied to the reforming catalyst to the reforming catalyst. Even if the temperature of the system fuel does not reach the level at which it can be reformed to the hydrocarbon reducing agent, a part of the hydrocarbon fuel burns in the reforming catalyst, and the heat of combustion causes the reforming catalyst to reform the hydrocarbon fuel. Maintained at possible temperature. Also, the reforming catalyst has a large heat
Surrounded by a NO _x catalyst with capacity , reforming
The heat exchange between the catalyst and the exhaust gas is performed via the NO _X catalyst.
In order to change the temperature of the exhaust gas, the temperature of the reforming catalyst
The changes are kept small.

According to a second aspect of the present invention, there is provided an exhaust emission control device for a diesel engine, wherein an NO _x catalyst for purifying nitrogen oxides arranged in an engine exhaust system and an exhaust gas.
Reforming catalyst for reforming a hydrocarbon-based fuel into a hydrocarbon-based reducing agent, which is heated by the heat of fuel, and a hydrocarbon-based fuel for supplying the hydrocarbon-based fuel to the NO _x catalyst via the reforming catalyst. Fuel supply means and air supply for supplying to the reforming catalyst air sufficient to burn a part of the hydrocarbon fuel supplied to the reforming catalyst by the hydrocarbon fuel supplying means with the reforming catalyst. Means, and the reforming catalyst comprises
It is surrounded by a cover with a large heat capacity,
Heat exchange between the high quality catalyst and the exhaust gas is performed through the cover.
Characterized in that it is.

In this exhaust emission control system for a diesel engine, the reforming catalyst is heated by the heat of the exhaust gas,
Since the air supply means supplies the reforming catalyst with the air that burns part of the hydrocarbon fuel supplied to the reforming catalyst with the reforming catalyst, the temperature of the exhaust gas is low and the reforming catalyst is
The temperature must be high enough to reform the elementary fuel into a hydrocarbon-based reducing agent.
At least some of the hydrocarbon fuel burns in the reforming catalyst
The combustion heat causes the reforming catalyst to modify the hydrocarbon fuel.
Maintained at a temperature that allows quality. Also, the reforming catalyst has a large heat
Surrounded by a cover with capacity, reforming catalyst
And the heat exchange with the exhaust gas is done through the cover
In addition, the temperature change of the reforming catalyst with respect to the temperature change of the exhaust gas
Maintained small.

[0010]

1 is a schematic sectional view showing a first embodiment of an exhaust emission control device for a diesel engine according to the present invention. In the figure, 1 is an exhaust passage of a diesel engine. A NOx catalyst 2 for purifying NOx in the exhaust gas is arranged in the expanded portion 1a of the exhaust passage 1. The NOx catalyst 2 has a large heat capacity such as cordierite or a ceramic such as silicon carbide (SiC) as a base material and supports copper ion-exchanged zeolite (Cu-ZSM-5) and the like, and is excellent in reducing hydrocarbon as a reducing agent. To N
It reduces Ox. The NOx catalyst 2 has a through hole 2a at its center.

The exhaust emission control system of this embodiment uses light oil, which is a fuel for a diesel engine, as a hydrocarbon as a reducing agent. Since the main component of light oil is a hydrocarbon with a large number of carbon atoms, its boiling point is high and the NOx catalyst 2
It is difficult to vaporize even when supplied to and does not function well as a reducing agent. Therefore, before supplying to the NOx catalyst 2, it is necessary to reform with a small number of carbon atoms, and the reforming catalyst 3 for that needs to be reformed.
The NOx catalyst 2 is arranged on the exhaust downstream side of the through hole 2a.

The reforming catalyst 3 is an oxidation catalyst in which a noble metal is supported on the same base material as the NOx catalyst 2, and is activated at a temperature equal to or higher than a predetermined temperature to reform light oil. The plate member 4 is disposed on the exhaust downstream side end surface of the reforming catalyst 3. As a result, the through hole 2a of the NOx catalyst 2 is closed and the exhaust gas is prevented from passing through the through hole 2a.
Exhaust gas does not flow into the reforming catalyst 3. Reforming catalyst 3 and N
The reforming catalyst 3 is provided in a portion in contact with the Ox catalyst 2.
Is shielded from the NOx catalyst 2 so that the reducing agent does not pass therethrough. Alternatively, this portion is provided with a structure that makes it difficult for the reducing agent to pass from the reforming catalyst 3 to the NOx catalyst 2.

A supply pipe 5 for supplying light oil and air to the reforming catalyst 3 is connected to the plate member 4. Supply pipe 5
Are branched into two branches extending to the outside of the exhaust passage 1, one of which is connected to a fuel pump (not shown) through a first control valve 5a for controlling the amount of light oil supply, and the other is air. It is connected to an air pump (not shown) via a second control valve 5b for controlling the supply amount.

Reference numeral 20 denotes a control device for controlling the first and second control valves 5a and 5b, which includes a rotation sensor 21 for detecting engine rotation, an air flow meter 22 for detecting intake air amount, A temperature sensor 23 for detecting the temperature of the exhaust gas is connected to the exhaust passage 1 on the downstream side of the NOx catalyst 2.

In the present embodiment, the reforming catalyst 3 is NO
It is surrounded by the x catalyst 2 and the inflow of exhaust gas is prevented by the plate member 4. Therefore, the heat exchange between the reforming catalyst 3 and the exhaust gas is performed via the NOx catalyst 2 having a large heat capacity, and the exhaust gas temperature of the reforming catalyst changes greatly with respect to the exhaust gas temperature that greatly changes according to the engine operating state. Temperature changes are kept small. Since the reforming catalyst 3 is surrounded by the exhaust downstream portion, which is the least affected by the temperature change of the exhaust gas, among the respective portions of the NOx catalyst 2,
Furthermore, the temperature change is kept small.

Therefore, the temperature of the reforming catalyst 3 is always above a certain temperature. As a result, when the light oil and the air are supplied from the supply pipe 5 to the reforming catalyst 3, a part of the light oil starts to burn well. As a result, the temperature of the reforming catalyst 3 further rises due to this combustion heat, and the reforming catalyst 3 reaches a temperature suitable for reforming the remaining light oil, and surely reforms and vaporizes the remaining light oil. Is possible. The light oil thus reformed and vaporized is N
It is possible to move to the upstream side of the exhaust gas of the Ox catalyst 2 and sufficiently mix with the exhaust gas, and as a reducing agent, the NOx in the exhaust gas can be favorably reduced in the NOx catalyst 2.

As described above, the amount of light oil supplied to the NOx catalyst 2 through the reforming catalyst 3 depends on the NO contained in the exhaust gas.
It is desirable to correspond to the amount of x. FIG. 2 is a map showing the engine speed N and the generated amount of NOx with respect to the intake air amount Q / N per unit speed as the engine load.
As described above, since the NOx generation amount changes according to the engine operating state, the control device 20 controls the current engine speed N detected by the rotation sensor 21 and the air flow meter 22.
Also, based on the intake air amount Q, the present NOx generation amount is grasped so that the corresponding light oil amount is supplied to the NOx catalyst 2, and the light oil amount burned in the reforming catalyst 3 is also taken into consideration. The opening degree of the first control valve 5a is controlled.

The amount of air supplied to the reforming catalyst 3 is
A part of the light oil is burned in the reforming catalyst 3 and the temperature of the reforming catalyst 3 is determined so that the remaining light oil is surely reformed and vaporized in the reforming catalyst 3, and the control device 20 controls the air amount. The opening degree of the second control valve 5b is controlled so that For example, the light oil supplied to the reforming catalyst is 2000
It is about 1 cc / min in normal running with a cc engine, and about 10% of that, that is, 0.1 cc / min, is burned by the reforming catalyst. The air required for this combustion is about 1 liter / minute, and this amount of air is supplied to the reforming catalyst.

The NOx catalyst 2 cannot sufficiently reduce NOx outside the predetermined temperature range. Therefore,
The control device 20 detects NO detected by the temperature sensor 23.
The temperature of the NOx catalyst 2 is estimated from the exhaust gas temperature on the downstream side of the x catalyst 2, and the light oil and air are supplied to the reforming catalyst 3 as described above only when the temperature is within a predetermined range.

FIG. 3 is a schematic sectional view showing a second embodiment of the exhaust emission control system for a diesel engine according to the present invention. Only the differences from the first embodiment will be described below. In this embodiment, the NOx catalyst 2'is not provided with a through hole. The reforming catalyst 3'is the NOx of the tube expansion portion 1a '.
It is arranged on the exhaust upstream side of the catalyst 2 '.

The reforming catalyst 3'has a spherical shape and is supported by a supply pipe 5'for supplying light oil and air to the reforming catalyst 3'at substantially the center of the pipe expanding portion 1a '. A ceramic cover 6 is provided to cover the exhaust upstream half of the reforming catalyst 3 '. This cover 6 has a very large heat capacity.

In the present embodiment, the reforming catalyst 3'is surrounded by a cover 6 having a large heat capacity, and the cover 6 prevents the inflow of exhaust gas. Therefore, as in the first embodiment, heat exchange between the reforming catalyst 3 ′ and the exhaust gas is performed via the cover 6 having a large heat capacity, and the exhaust gas temperature that greatly changes according to the engine operating state. On the other hand, the temperature change of the reforming catalyst 3'is kept small. As a result, by supplying light oil and air to the reforming catalyst 3'as in the first embodiment, the light oil is satisfactorily reformed and vaporized, and the NOx catalyst located on the exhaust downstream side by the air flow and the exhaust gas flow. 2'is supplied.

In the above-mentioned two embodiments, the reforming catalyst is brought to a temperature at which the light oil can be burned by the heat of the exhaust gas. In order to reliably burn a part of the light oil, the reforming catalyst is surrounded by a member having a large heat capacity so that the temperature of the reforming catalyst does not drop so much. However, this is not a limitation of the invention, for example:
Even if the reforming catalyst is simply placed inside the exhaust passage or adjacent to the outside of the exhaust passage, the reforming catalyst is light oil due to the heat of the exhaust gas except in the specific engine operating state where the exhaust gas temperature becomes considerably low. When the temperature reaches a temperature at which combustion is possible and a portion of the light oil is burned, the temperature of the reforming catalyst can be raised by this heat of combustion to a temperature at which the light oil can be well reformed. By repeating this combustion, the reforming catalyst This temperature is maintained at. If kerosene or the like having a lower boiling point than light oil is used as the hydrocarbon-based reducing agent, it can be burned by supplying air even if the temperature of the reforming catalyst is lower.

FIG. 4 is a schematic sectional view showing a third embodiment of the exhaust purification system for a diesel engine according to the present invention. Only the differences from the second embodiment will be described below. In the present embodiment, the reforming catalyst 3 ″ is housed in the case 7 and arranged outside the exhaust passage 1 ″.

A supply pipe 5 "similar to that described above is connected to one end of the case 7. The connection pipe 7a connected to the other end of the case 7 is NOx in the expanded pipe portion 1a" of the intake passage.
It extends to the exhaust gas upstream side of the catalyst 2 ″. An electric heater 8 is arranged around the case 7.

The controller 20 "controls the opening of the first and second control valves 5a" and 5b "as described above, and
The energization of the electric heater 8 is controlled. In this control, the energization is started at the same time as the engine is started, and the energization is stopped after the reforming catalyst 3 ″ is brought to a temperature at which the light oil can be burned. When the combustible temperature is reached, as described above, a part of the light oil is burned to raise the temperature of the reforming catalyst 3 ″ to a temperature at which the light oil can be satisfactorily reformed, and this is repeated for reforming. The catalyst 3 "is maintained at this temperature. As a result, the modified light oil is connected to the connecting pipe 7
It is adapted to be supplied to the NOx catalyst 2 ″ via a.

In all the above-described embodiments, when the fuel is burned in the reforming catalyst, carbon or the like is generated, which may cause clogging of the reforming catalyst. In order to prevent this, it is possible to burn this carbon by periodically supplying only air from the supply pipe.

[0028]

As described above, according to the exhaust gas purification apparatus for a diesel engine of the present invention as set forth in claim 1, the reforming catalyst is heated by the heat of the exhaust gas, and the air supply means is supplied to the reforming catalyst. In order to supply the reforming catalyst with air sufficient to burn a part of the hydrocarbon-based fuel with the reforming catalyst,
Even if the temperature of the exhaust gas is low and the reforming catalyst does not reach a temperature at which the hydrocarbon-based fuel can be reformed into the hydrocarbon-based reducing agent, part of the hydrocarbon-based fuel burns in the reforming catalyst, and the combustion heat Thus, the reforming catalyst can be maintained at a temperature at which the hydrocarbon fuel can be reformed. Also, the reforming catalyst has a large heat capacity.
And a reforming catalyst surrounded by a NO _x catalyst having
The heat exchange between the exhaust gas and the exhaust gas is performed via the NO _X catalyst.
Change the temperature of the reforming catalyst against the temperature change of the exhaust gas.
Is kept small and the temperature of the reforming catalyst is
The temperature is above the temperature, and hydrocarbons are always present in the reforming catalyst.
It is possible to burn part of the system fuel. As a result, the supply means supplies the hydrocarbon-based fuel to the NO _X catalyst device via the reforming catalyst, and the heat of the exhaust gas is used for reforming without consuming a large amount of new energy. The hydrocarbon-based reducing agent thus generated can be supplied to the NO _X catalyst,
A good NO _X reduction on the O _X catalyst is achieved.

Further, according to the exhaust gas purifying apparatus for a diesel engine of the present invention as defined in claim 2, the reforming catalyst is exhausted.
Exhaust gas is supplied to the reforming catalyst in order to supply the reforming catalyst with air which is heated by the heat of the gas and is burned by the reforming catalyst to a part of the hydrocarbon fuel supplied to the reforming catalyst by the air supply means.
At low temperature, the reforming catalyst returns hydrocarbon fuel to hydrocarbon fuel.
Even if it does not reach the temperature at which it can be reformed into the base material,
Part of the hydrocarbon fuel burns, and this combustion heat
Maintaining the reforming catalyst at a temperature at which hydrocarbon fuels can be reformed
be able to. Also, the reforming catalyst has a large heat capacity.
It is surrounded by a cover, and the reforming catalyst and exhaust gas
The heat exchange of the exhaust gas is carried out through the cover.
The temperature change of the reforming catalyst is kept small with respect to the temperature change.
Therefore, the temperature of the reforming catalyst is always above a certain level.
The reforming catalyst always burns part of the hydrocarbon fuel.
It can be baked. As a result, the supply means supplies the hydrocarbon-based fuel to the NO _X catalyst device via the reforming catalyst, without consuming a large amount of new energy,
The using heat of exhaust gas reformed hydrocarbon reductant can be supplied to the NO _X catalyst, reduction of good NO _X in the NO _X catalyst is achieved.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a first embodiment of an exhaust emission control device for a diesel engine according to the present invention.

FIG. 2 is a map showing NOx generation amount with respect to engine speed and engine load.

FIG. 3 is a schematic sectional view showing a second embodiment of an exhaust emission control device for a diesel engine according to the present invention.

FIG. 4 is a schematic sectional view showing a third embodiment of the exhaust emission control system for a diesel engine according to the present invention.

[Explanation of symbols]

1,1 ', 1 "... Exhaust passage 2,2 ', 2 "... NOx catalyst 3,3 ', 3 "... Reforming catalyst 4 ... Plate member 5,5 ', 5 "... Supply pipe 6 ... Cover 8 ... Electric heater 20, 20 "... Control device 23 ... Temperature sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI F01N 3/24 B01D 53/34 ZAB ZAB 53/36 ZAB 3/36 ZAB 101A (72) Inventor Yasuo Takada Nagakute Town, Aichi District, Aichi Prefecture 1 in 41, Yokomichi Yokomichi, Chuo Research Institute, Toyota Central Research Institute Co., Ltd. (56) Reference JP-A-6-212952 (JP, A) JP-A-7-127447 (JP, A) JP-A-6-137138 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) F01N 3/08-3/36 B01D 53/34 B01D 53/86 B01D 53/94

Claims (2)

(57) [Claims] 1. A NO _X catalyst arranged in an engine exhaust system for purifying nitrogen oxides, and a reformer for reforming a hydrocarbon-based fuel into a hydrocarbon-based reducing agent which is heated by heat of exhaust gas. Quality catalyst, and hydrocarbon-based fuel supply means for supplying a hydrocarbon-based fuel to the NO _X catalyst via the reforming catalyst,
An air supply means for supplying to the reforming catalyst air sufficient to burn a part of the hydrocarbon fuel supplied to the reforming catalyst by the hydrocarbon fuel supplying means to the reforming catalyst. However , the reforming catalyst has a large heat capacity.
Surrounded by the NO _X catalyst,
An exhaust emission control device for a diesel engine, characterized in that heat exchange with air gas is performed through the NO _X catalyst . 2. A NO _X catalyst arranged in an engine exhaust system for purifying nitrogen oxides, and heated by heat of exhaust gas.
Is a reforming catalyst and a hydrocarbon fuel supply means for supplying hydrocarbon fuel through the reforming catalyst to the NO _X catalyst for reforming hydrocarbon fuel to hydrocarbon-based reducing agent,
The air supply means for supplying only air to burn a portion of the hydrocarbon-based fuel supplied to the reforming catalyst in the reforming catalyst by the hydrocarbon-based fuel supply unit to the reforming catalyst, comprising a city However, the reforming catalyst has a large heat capacity.
Is surrounded by a cover, the reforming catalyst and exhaust gas
Exhaust gas purification device for a diesel engine, characterized in that heat exchange is performed via the cover .