JPH09122447A - Purifying process for engine exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove NOX and particle substances in exhaust gas in all operation areas by providing an EGR system and a denitration reaction system on an engine and using a γ-alumina or a silica alumina carrier carrying a transition metal element as a denitration catalyst for the denitration system. SOLUTION: An EGR system and a denitration reaction system are provided on an engine, and an EGR regulating valve 82 is interposed in an EGR flow path 81 connecting an air suction line 101 with an air exhaust line 102 on the EGR system. The denitration reaction system is provided with a denitration catalyst and a reducing agent adding device, and the denitration catalyst is provided in a denitration catalyst reaction layer 6, and an engine fuel is used as a reducing agent. For the denitration catalyst, γ-alumina or silica alumina is used as a carrier carrying a transition metal element, preferably, titanium, silver or the like. Exhaust gas containing excessive oxygen is purified by operating the EGR system only at the time of low load of the engine.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas in an oxygen-excess atmosphere emitted from a diesel engine or a lean-burn gasoline engine.

[0002]

2. Description of the Related Art Carbon monoxide (CO) and hydrocarbon (HC) are respectively used as carbon dioxide (CO ₂ ) as a catalyst for reducing harmful substances in exhaust gas of a gasoline engine.
A three-way catalyst that simultaneously performs a reaction of oxidizing water and water and a reaction of reducing nitrogen oxide (NO _x ) to nitrogen gas has been put into practical use.

However, the efficiency of this catalyst depends on the air-fuel ratio of the engine. That is, in lean side combustion with a large air-fuel ratio, oxygen in the exhaust gas becomes excessive, so that the oxidation reactions of carbon monoxide and hydrocarbons become active, but the reduction reaction of nitrogen oxides is hindered. vice versa,
In combustion with a small air-fuel ratio, the oxidation reaction is inhibited and the reduction reaction becomes active. Therefore, the three-way catalyst has a theoretical air-fuel ratio (A
Must be used near /F=14.7) (Hamm
erle, Wu. “Three-Way Catalyst PerfomanceCharacter
rization ”SAE Paper 810275 (1981)), it cannot be applied to exhaust gas of diesel engine containing excess oxygen and exhaust gas of lean-burn gasoline engine.

Exhaust gas recirculation (EGR) systems have begun to be used to reduce NO _x in lean-burn gasoline engines and diesel engines, which are relatively small in size. However, when the EGR system is used for a large engine, NO _X can be reduced in the low load region, but CO, HC and particulate matter increase in the medium and high load regions. In addition, when using the EGR system for a large engine, problems such as deterioration of fuel consumption and wear of piston rings (Sato, Akai, Automotive Engineering, Vol. 44 (8) 67-73 (199)
0)) occurs. Therefore, the EGR system has not been put to practical use in a large engine.

In order to reduce NO _x , various denitration catalysts using zeolite or alumina as a carrier have been developed. However, since the operating temperature of the denitration reaction is as high as 350 ° C. or higher, when starting the engine or idling, The denitration reaction is difficult to proceed in a region where the exhaust gas temperature is low, such as when the load is low.

As described above, it has not been done so far to reduce the emission amounts of NO _X , CO, HC and particulate matter in the exhaust gas in an oxygen excess atmosphere, and its practical application is desired. ing.

[0007]

An object of the present invention is to provide a, in a diesel engine or a lean burn gasoline engine for discharging exhaust gas of excess oxygen atmosphere, NO _X, CO, HC and particulate matter in the exhaust gas in the entire operating range Is to be eliminated or reduced.

[0008]

[Means for Solving the Problems] The above-mentioned object is as follows (1)
This is achieved by any one of the configurations (1) to (9). (1) An EGR system and a denitration reaction system are installed in an engine, and the EGR system includes an EGR passage for returning exhaust gas to an intake passage and an EGR control valve for opening and closing the EGR passage. The denitration reaction system has a denitration catalyst and a reducing agent addition device, and the denitration catalyst is γ-alumina or silica.
Alumina is used as a carrier, and a transition metal element is supported on the carrier. The reducing agent addition device adds an organic compound as a reducing agent. By operating the EGR system only when the engine load is low. , A method of purifying engine exhaust gas for purifying exhaust gas in excess of oxygen. (2) The EGR system has an engine load factor of 20 to 6
The method for purifying engine exhaust gas according to the above (1), which operates on the low load side and stops the operation on the high load side with 0% as a boundary. (3) The EGR system has an exhaust gas temperature of 300 to 5
The method for purifying engine exhaust gas according to the above (1) or (2), which operates on the low load side and stops the operation on the high load side with 00 ° C as a boundary. (4) EGR when the EGR system is operating
The method for purifying engine exhaust gas according to any one of (1) to (3) above, wherein the rate is 50% or less. (5) When the EGR system is operating, the EGR rate is varied according to the engine load amount (1) to
The method for purifying engine exhaust gas according to any one of (4). (6) Operation control of the EGR system is performed by detecting at least one selected from oxygen concentration in the intake passage, pressure in the intake passage, engine speed, cooling water temperature, fuel injection amount, and exhaust gas temperature. The method for purifying engine exhaust gas according to any one of (1) to (5) above. (7) The method for purifying engine exhaust gas according to any one of (1) to (6) above, wherein the denitration catalyst uses γ-alumina as a carrier, and silver and cobalt are supported on the carrier. (8) The method for purifying engine exhaust gas according to any one of the above (1) to (6), wherein the denitration catalyst uses γ-alumina as a carrier and titanium, silver and cobalt are carried on the carrier. (9) The method for purifying engine exhaust gas according to any of (1) to (8) above, wherein the organic compound used as the reducing agent is engine fuel.

[0009]

As described above, when the EGR system is operated over the entire operating range of the engine, CO, HC and particulate matter emissions become a problem in the medium and high load range, and at the same time, deterioration of fuel consumption and wear of the piston ring occur. Will occur.

On the other hand, when a denitration catalyst, the exhaust gas temperature is low low load region, removal of the NO _X for the following operating temperature of the denitration reaction becomes impossible.

On the other hand, in the present invention, the EGR system is operated when the engine is in the low load range, and the EGR system is stopped in the medium and high load range.

In the low load region, the concentration of oxygen in the intake gas becomes 21% or less due to the recirculation of exhaust gas, and the combustion temperature drops, so that NO _x production is suppressed.

On the other hand, in the medium to high load range, the entire amount of exhaust gas is supplied to the denitration reaction system having a denitration catalyst and a reducing agent addition device. Since the exhaust gas is at a high temperature in the medium to high load range, the temperature of the denitration catalyst becomes higher than the working temperature of the denitration reaction, and the denitration reaction proceeds efficiently. That is, the reducing agent (organic compound such as engine fuel) supplied from the reducing agent addition device performs a selective reduction reaction of NO _X and converts it into nitrogen gas and oxygen gas. Further, CO and HC are oxidized by coming into contact with the denitration catalyst to become carbon dioxide gas and water, respectively. Since the EGR system is not operating in the medium and high load regions, the increase in the emission of particulate matter can be suppressed.

[0014]

[Specific Configuration] Hereinafter, a specific configuration of the present invention will be described in detail.

The present invention is applied to engines in which exhaust gas in excess of oxygen is generated, such as diesel engines and lean-burn gasoline engines. In the present invention, the engine is provided with the EGR system and the denitration reaction system. The EGR system has an EGR passage for returning exhaust gas to the intake passage, and an EGR control valve for opening and closing the EGR passage. The denitration reaction system has a denitration catalyst and a reducing agent addition device.

FIG. 1 shows an example of the configuration when the present invention is applied to a diesel engine. This diesel engine has an intake passage (intake pipe including an intake manifold) 101 and an exhaust passage (exhaust pipe including an exhaust manifold) 102 that communicate with the combustion chambers in the engine body 1.
A transmission 2 and an eddy current dynamometer 3 are connected to the engine body 1, and fuel is supplied from a fuel tank 5 to a combustion chamber via a fuel injection pump 4.

The EGR system in this engine is
EGR passage 8 connecting the intake passage 101 and the exhaust passage 102
1 and further provided in the EGR flow path 81,
An EGR control valve 82 having a function of adjusting the R rate is provided. The operation control of the EGR system, that is, the adjustment of the EGR rate is performed by at least one of parameters selected from oxygen concentration in the intake passage, pressure in the intake passage, engine speed, cooling water temperature, fuel injection amount, and exhaust gas temperature. Is detected and is performed based on these parameters.

In the illustrated example, the intake oxygen sensor output 1 indicating the oxygen concentration in the intake air before the recirculated exhaust gas is mixed
0, an intake / exhaust gas mixture oxygen sensor output 11 indicating the oxygen concentration in the intake air after mixing the recirculated exhaust gas, and an exhaust oxygen sensor output 12 indicating the oxygen concentration in the exhaust gas are sent to the control unit 9. The EGR control output 13 from the control unit 9 controls the opening of the EGR control valve 82 to change the EGR rate. Further, based on the engine load factor calculated from the engine speed output 18 and the fuel injection amount output 14 and the exhaust gas temperature sensor output 16 immediately before the denitration catalyst reaction layer 6, the EGR control valve 8
The opening / closing timing of 2 is determined, and the EGR control valve 13 opens / closes the EGR control valve 82. The load factor is
Also check from the output 15 of the eddy current dynamometer 3.

The denitration catalyst is provided in the denitration catalyst reaction layer 6. In this engine, engine fuel is used as a reducing agent for the denitration reaction system. The fuel used as the reducing agent is supplied from the fuel tank 5 to the reducing agent addition pump 20.
And it is added immediately before the denitration catalyst reaction layer 6 through the reducing agent supply passage 21. The reducing agent is used for the denitration catalyst reaction layer 6
It may be added immediately before the exhaust stroke by the piston or immediately before the exhaust manifold part.

In the present invention, the operation control of the EGR system is preferably performed as follows.

The EGR system operates when the engine load is low and the exhaust gas temperature is low. The timing of fully closing the EGR control valve 82 is when the engine load amount is preferably 20 to 60%, more preferably 30 to 50%, and the exhaust gas temperature is preferably 30.
It is 0 to 500 ° C, and more preferably 350 to 450 ° C. When the engine load decreases, E
The timing for opening the GR adjustment valve 82 is also the same.

The EGR operation timing is not always constant, and varies depending on the operation pattern even with the same engine. That is, even if the load factor is the same, the exhaust gas temperature at that time differs depending on the operation pattern, so it is preferable to control the operation timing by making a comprehensive judgment from the load factor, exhaust gas temperature, and the like. The operation timing also differs depending on the engine displacement and the like. For this reason, the preferred actuation timing has a range as described above.

The EGR rate when the EGR control valve 82 is open is preferably 50% or less, more preferably 1%.
0 to 40%. The EGR rate may be constant, but the EGR rate may be changed within the above range.
It is preferable to variably operate the R adjustment valve 82. In this case, it is preferable to variably operate so that the EGR rate decreases as the load increases. Nitrogen oxides can be reduced by increasing the EGR rate in the low load region such as idling, but as the load increases, CO, H
Since C increases and the emission of particulate matter also increases, it is preferable to lower the EGR rate in order to prevent these increases.

The engine load is determined by the engine speed, load and torque. Since the engine output required to maintain a constant rotation speed changes according to the load at that time, the load factor is calculated from the engine output required to maintain a constant rotation speed. Specifically, the load is changed while maintaining the rotation speed of 60% of the rated engine rotation speed (= maximum torque generation rotation speed), and the maximum value Pmax of the output required to maintain the rotation speed is obtained. The load factor at this time is 100%. The output Pma when the engine speed is maintained at 60% of the rated engine speed under a certain load
The ratio to x is the load factor at that time.

The EGR rate is 100 (V ₀ -V _S ) when the intake air amount when the EGR is not operating is V ₀ and the intake air amount when the EGR is operating is V _S. / V ₀ [%].

In the illustrated example, a bypass exhaust passage 103 is branched from the exhaust passage 102 in front of the denitration catalyst reaction layer 6, and an exhaust gas flow rate control valve 7 is provided at the branched portion. The bypass exhaust passage 103 and the exhaust gas flow rate control valve 7 are for controlling the amount of exhaust gas supplied to the denitration catalyst reaction layer 6. In the low load region of the engine, since the exhaust gas temperature is low and the denitration catalytic reaction does not substantially occur, the exhaust gas may or may not be supplied to the denitration catalytic reaction layer 6.
When exhaust gas is supplied to the denitration catalyst reaction layer 6 in the low load region,
Water vapor in the exhaust gas is likely to condense to produce water, resulting in an excessive amount of water on the surface of the denitration catalyst, which delays the start of the denitration reaction when shifting to the medium load range. Therefore, in order to prevent water from adhering to the catalyst surface, the exhaust gas supply to the denitration catalyst reaction layer may be stopped at low load, but on the other hand, if the exhaust gas supply is stopped, the temperature of the denitration catalyst becomes too low. After all, there is a delay in the start of the denitration reaction when shifting to the medium load range. Therefore, when the load is low, adhesion of water to the catalyst surface is minimized, and the exhaust gas flow rate control output 17 is based on the exhaust gas temperature sensor output 16 so that the temperature of the denitration catalyst can be maintained at a predetermined value or higher. It is preferable to control the exhaust gas flow rate control valve 7 to control the exhaust gas flow rate to the denitration catalyst reaction layer 6.

In the present invention, as the denitration catalyst, γ-alumina or silica-alumina is used as a carrier and a transition metal element is supported on this carrier. Preferably, γ-alumina is used as the carrier and silver and Cobalt-supported one is used, and more preferably, γ-alumina is used as a carrier, and titanium, silver and cobalt are supported on this carrier.

The γ-alumina and silica-alumina used as the carrier have excellent heat resistance and can be easily formed into a monolith, and they themselves have a catalytic function.

The γ-alumina carrier is composed of γ in the alumina component.
-Alumina is preferably contained in an amount of 50% by weight or more, particularly preferably 90% by weight or more. This content is
It can be determined from a fluorescent X-ray chart. Note that γ-
The alumina carrier preferably contains 90% by weight or more of Al ₂ O ₃ component and preferably 10% by weight or less of SiO ₂ .
In addition, the content of impurities such as Na ₂ O and Fe ₂ O ₃ is preferably 1% by weight or less.

The γ-alumina is obtained by thermal decomposition of crystalline alumina hydrate, a method of neutralizing sodium aluminate with aluminum sulfate and calcining (“Catalyst course” “Catalyst design” Kodansha 1985), aluminum metal alkoxide. It may be obtained by any method such as a method of preparing by hydrolysis of
A commercially available product may be used.

Among alumina, γ-alumina is used because it has a higher removal rate of nitrogen oxides, that is, a higher denitrification rate than α-alumina.

Silica and alumina are SiO ₂ and Al ₂ O.
₃ is included, and various ones can be prepared by changing the ratio of both. In the present invention, Al ₂ O ₃
The content is preferably 1% by weight or more, more preferably 10%.
% By weight or more. The total content of impurities such as Fe ₂ O ₃ and Na ₂ O is preferably about 3% by weight or less. The halo of silica / alumina can usually be confirmed by X-ray diffraction, and is either amorphous or a mixture of amorphous and polycrystalline.

The silica / alumina may be prepared by any of the coprecipitation method and the kneading method.

In the coprecipitation method, tetraethylsiloxane and aluminum nitrate, sodium silicate and sodium aluminate, and the like, a silicate salt and an aluminate salt are dissolved and mixed to form a precipitate, which is subjected to post-precipitation treatment, if necessary. ,
It is obtained by firing at about 500 to 600 ° C.
This coprecipitation method will be published separately in "Catalysis Course" (Kodansha 19
85) and the like. In addition to this, a commercially available product may be used.

Γ-alumina or silica-alumina may be used as a carrier as primary particles or as secondary particles, or may be used as a carrier by molding primary particles or secondary particles, on a suitable base material. What was coated by various methods may be used as a carrier. For example, using a honeycomb-shaped cordierite or the like as a base material, when coating this base material with γ-alumina or silica-alumina, a coating is formed by a sol-gel method, or an alumina sol is coated as an adhesive, Alumina sol may be wash-coated on the substrate to form a film, which is then dried and baked. Both γ-alumina and silica-alumina have a firing temperature of 4
The temperature is about 00 to 700 ° C., and since it can be fired in air, it is extremely easy to manufacture. Also, the adhesive strength to various base materials is extremely high.

The carrier may have a granular shape, a monolithic shape such as a honeycomb shape, or a cloth shape.

As the γ-alumina particles, it is preferable to use particles having a specific surface area of 100 m ² / g or more as measured by a nitrogen adsorption method (BET method). The upper limit of the specific surface area is not particularly limited, but is usually about 1000 m ² / g.

As the silica-alumina particles, it is preferable to use particles having a specific surface area of 100 m ² / g or more and usually about 1000 m ² / g or less.

Although γ-alumina and silica-alumina are usually used alone, they may be used as a mixture.

Although γ-alumina and silica-alumina can function as a catalyst by themselves, if one or more transition metal elements are loaded and used as a carrier, the catalytic activity is remarkably increased. It will be expensive.

As the transition metal element, various ones can be used, among which silver, copper, vanadium, manganese,
At least one metal element selected from cobalt, molybdenum, nickel, iron, lanthanum and chromium is suitable. Among these, at least one selected from silver, copper, vanadium, manganese, cobalt, molybdenum, nickel, lanthanum and chromium is preferable in terms of high denitration effect, and a combination of silver and cobalt is more preferable, It is more preferable to use titanium in addition to silver and cobalt.

By using silver and cobalt in combination, the ability to remove nitrogen oxides (NO _x ) in the engine exhaust gas does not decrease even in the presence of water and sulfur oxides (SO _x ), and it can be used for a long period of time. The catalytic activity can be maintained. On the other hand, with silver alone, the coexisting sulfur oxides show a decrease in catalytic activity, and with cobalt only, the coexisting water shows a decrease in catalytic activity.

However, with only silver and cobalt, 50
At a catalyst set temperature of 0 ° C. or higher, the catalyst activity is maintained well, but at 400 ° C. or lower, the catalyst activity is significantly reduced. It is considered that this is because, when the catalyst supporting silver and cobalt is used for a long period of time, sulfur oxides gradually accumulate in the catalyst in some form during that period. However, it is considered that the decomposition or elimination reaction of the sulfur oxide or the compound derived therefrom occurs at a high temperature, the accumulation of the sulfur oxide and the like is prevented, and the catalytic activity does not decrease. On the other hand, 400 ℃
Since decomposition or elimination reaction does not occur at the temperature below, it is considered that the active sites of the catalyst may be blocked due to sulfur oxides.

On the other hand, if titanium is further supported in addition to silver and cobalt, it is considered that the accumulation of sulfur oxides in the catalyst is prevented by some action of titanium even at a temperature of 400 ° C. or lower. Therefore, the catalytic activity does not decrease. The reason is that by supporting titanium on γ-alumina, the solid acidity increases, contributing to the improvement of the oxidation reaction of the organic compound reducing agent, and the sulfur oxide and titanium react to form titanium sulfate. For example, it is possible to protect the γ-alumina carrier from sulfur oxides because the main component is generated. Regarding the protection from sulfur oxides, in the absence of titanium, sulfur oxides and γ-
Alumina containing aluminum sulfate as a main component is produced, but the produced aluminum sulfate is 650 to 8
Decomposition does not occur unless the temperature is as high as 00 ° C, and as a result, γ
-While it is accumulated in the alumina carrier and deteriorates the carrier function, when titanium is present, a substance containing titanium sulfate as a main component is produced from sulfur oxide and titanium, and the produced titanium sulfate is 150 to 400 ° C. As it is decomposed in some degree,
It is presumed that accumulation is inhibited.

The amount of the transition metal element supported in the entire denitration catalyst (including the carrier) is generally 1 to 50% by weight. If the loading amount is too small, the activity will be low, and if the loading amount is too large, the effect will be rather reduced.

The form of supporting two or more metals such as silver and cobalt, or silver, cobalt and titanium is not particularly limited. That is, the components may be mixed and supported, or each may be supported separately. Among these, it is preferable that each component is supported on the carrier in layers. That is, it is preferable to form each layer of a single kind of metal. In such a case, after the layers are formed, for example, silver may be diffused and mixed in the cobalt layer or cobalt in the silver layer in the vicinity of the contact surface between the adjacent layers. In addition, a metal such as titanium may be dispersed in the carrier, and a component in the carrier may be diffused in a metal layer such as a titanium layer. Further, titanium, silver, and cobalt are usually present as oxides, but part or all of them may be in a metallic state.

When silver and cobalt are to be supported, it is preferable that the layers of each metal component are supported on the carrier in a layered manner, but it is particularly preferable to have a two-layer structure. in this case,
If the first layer and the second layer are arranged from the carrier side, the first layer and the second layer
The ratio of the amount of metal carried in the second layer to the total amount of metal carried in the layer is preferably 0.1 to 50% by weight, more preferably 0.5 to 40% by weight, still more preferably 1 to 30% by weight, most preferably It is preferably 1 to 20% by weight. By controlling the amount of metal carried in this way, a higher effect can be obtained. On the other hand, if the amounts of metal carried by the first layer and the second layer are opposite to the above, the effect is reduced. Specifically, when the second layer is silver, as the amount of silver increases, the amount of silver oxide produced increases, and organic compounds such as hydrocarbons that are reducing agents are consumed by the oxidation reaction. As a result, the reduction efficiency of nitrogen oxides decreases. On the other hand, when the second layer is made of cobalt, when the amount of cobalt is large, the affinity of cobalt with water is good, so that the catalytic activity is significantly reduced due to water.

Therefore, it is preferable that the amount of metal supported on the first layer and the second layer is within the above range. Among them, it is preferable to form a silver layer as the first layer and a cobalt layer as the second layer. preferable. With such a layer structure,
As the initial catalytic activity increases, this high catalytic activity will be maintained.

When the first layer is a cobalt layer and the second layer is a silver layer, the initial catalytic activity is particularly inferior to that of the above structure.

In the case of supporting titanium, silver and cobalt, it is preferable to support each metal component as a layer on the carrier in a layered manner, but in particular, a three-layer structure is adopted, and the first layer from the carrier side,
When the second layer and the third layer are used, the first layer is preferably a titanium layer. This helps maintain the catalytic activity, especially 4
The effect of maintaining the catalytic activity at a low temperature of 00 ° C. or lower is improved.
The ratio of titanium to the total amount of metal supported is preferably 10 to 60% by weight, more preferably 15 in terms of metal element.
˜55% by weight, more preferably 20 to 40% by weight. When the ratio of titanium to the total metal decreases, it becomes difficult to obtain the effect due to the addition of titanium, and when the ratio of titanium increases, the total supported amount of silver and cobalt decreases and the sufficient denitration effect cannot be obtained.

It is preferable that one of the second layer and the third layer is a silver layer and the other is a cobalt layer. In this case, the ratio of the metal of the second layer serving as the lower layer to the total metal supported amount is preferably 30 to 70% by weight, more preferably 35 to 65% by weight, and further preferably 40 to 60% by weight in terms of the metal element. The ratio of the metal of the upper third layer to the total amount of metal supported is preferably 5 to 25% by weight, more preferably 5 to 20% by weight, and further preferably 10 to 15% by weight in terms of metal element. . Further, the metal amount ratio of the second layer and the third layer is preferably 1 or more, more preferably 1 to 20 in terms of the weight ratio of the metal amount of the metal of the second layer / the metal of the third layer. The degree is about 1.2 to 15, and more preferably about 1.2 to 15. In this way, the denitration effect is improved by controlling the metal loading amounts of the second layer and the third layer. On the other hand, when the amount of metal carried in the second layer and the third layer is opposite to the above, the denitration effect is reduced. Specifically, when the third layer is silver, as the amount of silver increases, the amount of silver oxide produced increases, and organic compounds such as hydrocarbons that are reducing agents are consumed by the oxidation reaction. As a result, the reduction efficiency of nitrogen oxides (NO _x ) decreases. On the other hand, when the third layer is cobalt, when the amount of cobalt is large, the affinity of cobalt with water is good, and the catalytic activity is significantly reduced due to water. Therefore, it is preferable that the amount of metal supported on the second layer and the third layer be within the above range, but it is particularly preferable to form the silver layer as the second layer and the cobalt layer as the third layer. With such a layered structure, the initial catalyst activity is increased and this high catalyst activity is maintained. In addition, the second layer is a cobalt layer,
When the third layer is a silver layer, the initial catalytic activity is particularly inferior to that of the above structure.

When silver and cobalt are supported, a multi-layered structure of three or more layers may be used. For example, a silver layer and a cobalt layer may each be formed in a multi-layered structure to form a cobalt layer on the silver layer, or a silver layer. Alternatively, the cobalt layer and the cobalt layer may be alternately formed.

In the case of supporting titanium, silver and cobalt, it may have a multilayer structure of four or more layers. For example, a titanium layer may have a multilayer structure, or a silver layer and a cobalt layer may be formed in a multilayer structure to form a silver layer on the silver layer. It is also possible to form a cobalt layer on the above, or alternately form a silver layer and a cobalt layer. However, even if the titanium layer has a multi-layer structure, it is preferable that all the titanium layers are provided as layers under the silver layer and the cobalt layer.

When silver and cobalt are supported, the total supported amount of both metals is preferably 0.5 to 20% by weight, more preferably 0.5 to 20% by weight in terms of metal element, based on the whole catalyst.
10% by weight. The reason for setting such a loading amount is that the activity is low when the loading amount is small, and the loading effect is reduced when the loading amount is too large. When two layers are formed, the first layer is 0.
5 to 10% by weight, preferably 0.5 to 6% by weight, the second layer is preferably 0.05 to 8% by weight,
The supported amount is preferably 0.05 to 4% by weight.

When titanium, silver and cobalt are supported, the total supported amount of these metals is preferably 0.1 to 20% by weight in terms of metal element, based on the whole catalyst.
More preferably 0.1 to 15% by weight, even more preferably 0.5 to 12% by weight, most preferably 0.5 to 6% by weight.
It is. The reason for setting such a loading amount is that the activity is low when the loading amount is small, and the loading effect is reduced when the loading amount is too large. When three layers are formed, the amount of titanium supported in the first layer is a metal element conversion amount, and is preferably 0.
05 to 12% by weight, more preferably 0.05 to 6% by weight, still more preferably 0.1 to 4% by weight, and the second layer is preferably 0.05 to 12% by weight, more preferably 0. 1 to 8% by weight, more preferably 0.1 to 6% by weight
And the third layer is preferably 0.01 to 12% by weight,
It is more preferably 0.03 to 6% by weight, still more preferably 0.05 to 4% by weight.

As a method of supporting the metal on the carrier, a method of adding the carrier to an aqueous solution of a metal salt and carrying it with stirring is generally used. In this case, the carrier may be granular or monolithic. Then, it may be dried at a temperature of about 120 ° C. for about 10 to 20 hours and baked at about 400 to 700 ° C. for about 1 to 4 hours. As the metal compound used at this time, titanium compounds such as titanium trichloride,
Titanium tetrachloride, titanium sulfate, titanium alkoxide (for example, titanium butoxide) and the like can be mentioned. As the silver compound,
Examples thereof include silver nitrate and silver acetate, and examples of the cobalt compound include cobalt nitrate, cobalt acetate and cobalt sulfate. At this time, the concentration of the aqueous solution of the metal compound used is about 0.005 to 1 mol / l.

When supporting two or more kinds of metal elements, a method may be used in which the carrier is added to an aqueous solution containing salts of these metals, the mixture is stirred, dried and baked. Alternatively, the carrier may be added to an aqueous solution containing a salt of at least one of the metals to be supported, stirred and dried, and then added to an aqueous solution containing a salt of another metal, and the mixture may be stirred, dried, and baked. Good. Further, the carrier is put into an aqueous solution containing a salt of at least one of the metals to be carried, and after stirring, drying and firing, the carrier is put into an aqueous solution containing a salt of another metal, followed by stirring, drying,
You may bake.

First, the case of supporting silver and cobalt will be specifically described. When the supporting metal is formed in a two-layer structure, and the first layer is formed as a silver layer and the second layer is formed as a cobalt layer, the first layer of silver is, for example, a silver nitrate solution with a carrier having a solid-liquid ratio of 3 ( v / v) and added while stirring. After the supporting, it is dried at a temperature of about 120 ° C. for about 12 hours, and baked at a temperature of about 500 ° C. for about 2 hours in an air atmosphere. The second layer of cobalt is supported by, for example, adding a silver nitrate solution to a cobalt nitrate or cobalt acetate solution in the same proportion as in the case of the above silver, and carrying it with stirring. Drying and firing after supporting are performed in the same manner as the first layer.

Next, the case of supporting silver, cobalt and titanium will be specifically described. When the supporting metal is formed in a three-layer structure, the first layer is a titanium layer, the second layer is a silver layer, and the third layer is a cobalt layer, the first titanium layer is, for example, titanium trichloride. When either titanium tetrachloride or titanium sulfate is used, the carrier is added to these titanium salt solutions at a ratio such that the solid-liquid ratio is about 3 (v / v), and the carriers are supported with stirring. After loading, ammonia water is added to make it neutral, titanium hydroxide is produced on the surface of the carrier, dried at a temperature of about 120 ° C. for about 12 hours, and at a temperature of about 500 ° C. for about 2 to 3 hours. , Firing in an air atmosphere. As a result, the titanium layer contains anatase type titanium oxide as a main component. In the case of titanium alkoxide, the carrier is immersed in this solution, a hydrolysis reaction is performed using nitric acid as a catalyst, and solid-liquid separation is performed, followed by drying in the same manner as above.
Titanium oxide is prepared by performing air calcination. At this time, the titanium layer also contains such titanium oxide as a main component. The silver of the second layer is supported by, for example, adding titanium supported in a silver nitrate solution in the same ratio as in the case of titanium, and carrying it with stirring. After loading, drying and firing are performed as in the case of the first layer. The loading of cobalt in the third layer is
For example, a solution of titanium nitrate and silver supported in a cobalt nitrate or cobalt acetate solution is added at the same ratio as in the case of silver, and the solution is supported with stirring. Drying and firing after loading is the second
Do the same as the first layer.

The other layer constitution may be carried out in accordance with the above.

The shape of the denitration catalyst may be granular, monolithic or the like depending on the shape of the carrier.

The denitration catalyst used in the present invention has high heat resistance and can be effectively used up to about 900 ° C. Generally, use at 300 to 600 ° C. is effective. Also, in the case of supporting silver, cobalt and titanium,
The catalytic activity is maintained even at a temperature of 400 ° C. or lower.

Although the present invention is applied to the exhaust gas purification of diesel engines and lean-burn gasoline engines, the exhaust gas of diesel engines is generally NO _x : 700.
~1500ppm, O _2: 10~20 vol%, SO _{2: 2}
0 to 200 ppm, H ₂ O: 5 to 20% by volume of each component is included. Further, the exhaust gas of a gasoline engine in the lean combustion region generally has NO _x : 3000 to 500.
0 ppm, O ₂ : 0.5 to 10% by volume, H ₂ O: 10 to 2
It contains about 0% by volume of each component.

In the present invention, an organic compound is present as a reducing agent when the catalyst and the engine exhaust gas are brought into contact with each other.

As the organic compound used as the reducing agent, hydrocarbons, alcohols, ketones, ethers, etc., and fuels such as light oil, gasoline, heavy oil, etc. are preferable. The use of the reducing agent simplifies the reducing agent addition device. Examples of the fuel include light oil and propane gas, and light oil is particularly preferable. Diesel oil can stably obtain nitrogen oxide removing ability in a temperature range of 300 to 600 ° C., which is a catalyst set temperature (contact temperature between catalyst and exhaust gas, generally exhaust gas temperature) preferably used in the present invention. .

The reducing agent is nitrogen oxide (NO) in the exhaust gas.
It is preferable to add about 0.5 to 5 times (weight ratio) of _x ).

As described in Japanese Patent Application No. 3-293719, the fuel may be decomposed to obtain a low molecular weight component, and the low molecular weight component may be used as a reducing agent. Specifically, a pretreatment device is provided, and the fuel from the fuel tank is passed through the pretreatment device, where low molecular weight or low boiling point components are enriched by thermal decomposition or catalytic decomposition, and then passed through a cooler and a gas-liquid separator. The gas component is separated, and this is injected into the denitration catalytic reaction layer or the preceding stage. In this way, if an effective component as a reducing agent of the denitration reaction is enriched and separated and added,
The effect as a reducing agent can be enhanced.

However, it is considered that the same decomposition reaction as in the pretreatment occurs in the exhaust pipe, and the pretreatment device complicates the engine structure. Therefore, the pretreatment device may not be provided.

[0069]

EXAMPLES Hereinafter, the present invention will be described in more detail by showing specific examples of the present invention.

Comparative Example 1: No Exhaust Gas Treatment> Exhaust Volume 2
A 953cc diesel engine that complies with 1989
An internal combustion engine performance test device equipped with an eddy current dynamometer and measuring equipment was prepared.

[0071] exhaust gas composition of the diesel engine, the engine speed 2870Rpm, when more than 80% load _{factor, NO X: 700~800ppm, CO:} 700ppm
, CO ₂ : 11%, HC: 200ppm, SO ₂ 12ppm
Met.

Using this internal combustion engine performance test apparatus, the exhaust gas concentration in the diesel engine 13 mode (mode applied to diesel vehicles with a vehicle weight exceeding 2500 kg in the report of the Central Pollution Control Council) was measured. Table 1 shows the results. Table 1 also shows the engine load factor and the exhaust gas temperature in each mode. However, the exhaust gas temperature is a value when the temperature is stable after continuous operation in each mode for a long time.

[0073]

[Table 1]

<Comparative Example 2: Exhaust Gas Treatment by EGR System Only> An EGR system was installed in the engine of the internal combustion engine performance test apparatus used in Comparative Example 1. This EGR system has an EGR passage for returning a part of the exhaust gas from the exhaust pipe after the exhaust manifold to the intake pipe, and an EGR adjustment valve provided in the middle of this EGR passage. Is to adjust.

For this diesel engine, the EGR rate was 1 based on the diesel engine 13 mode test method.
The nitrogen oxide emission amount in the exhaust gas at 0% or 20% was measured for all modes. Table 2 shows the results.

The EGR rate is the oxygen concentration in the intake gas (air), the oxygen concentration in the exhaust gas, and the oxygen concentration in the gas mixture of the recirculated exhaust gas and the intake gas, with the intake amount kept constant. Was calculated and calculated.

[0077]

[Table 2]

The reduction rate of nitrogen oxides relative to Comparative Example 1 was as follows:
When the EGR rate was 10%, it was 35%, and when the EGR rate was 20%, it was 64%. However, in the operation modes 3 and 6 to 12 in which the load factor is 40% or more in the medium and high load region, the discharge of the particulate matter was very large. As for the reduction rate of nitrogen oxides, the total value of nitrogen oxide emissions in all modes was calculated for each of Comparative Example 1 and Comparative Example 2, and the total value of Comparative Example 1 and the total value of Comparative Example 2 were calculated. Calculated by comparing.

<Example 1: Exhaust gas treatment by EGR system + catalytic reaction system> Preparation of catalyst A A denitration catalyst A (Ti-Ag-Co) in which titanium, silver and cobalt were supported on γ-alumina was prepared.

Using γ-alumina (manufactured by Mizusawa Chemical Industry Co., Ltd.) as a carrier, this carrier was added to a 0.05 mol / l titanium trichloride solution at a solid / liquid = 1: 3 (volume ratio),
Immerse at 30 ° C for 1 hour with stirring, then perform solid-liquid separation,
Wash 3 times with 3 volumes of distilled water, add ammonia water while stirring in 3 volumes of distilled water to make it neutral (pH = 7), and leave it at 30 ° C for 1 hour while maintaining pH = 7. did. After solid-liquid separation, it was dried at 120 ° C. for 12 hours and then baked in air at 500 ° C. for 3 hours to form a titanium layer.

Next, this was immersed in a 0.1 mol / l silver nitrate solution at 25 ° C. for 30 minutes, dried at 120 ° C. for 12 hours, and baked in air at 500 ° C. for 2 hours to form a silver layer.

Further, this was immersed in a 0.02 mol / l cobalt acetate solution at 25 ° C. for 30 minutes, dried at 120 ° C. for 12 hours, and calcined in air at 500 ° C. for 2 hours to form a cobalt layer. , As a denitration catalyst.

The metal loading amount of this denitration catalyst was such that titanium was 0.25% by weight, silver was 0.4% by weight, and cobalt was 0.1%.
% By weight.

Preparation of catalyst B A denitration catalyst B (Ag-Co) in which γ-alumina was loaded with silver and cobalt was prepared in the same manner as the denitration catalyst A except that titanium was not loaded. The amounts of silver and cobalt supported were the same as those of catalyst A.

The amount of metal supported on each catalyst was measured by ICP emission spectrometry after the catalyst was dried at 120 ° C. for 3 hours, accurately weighed and wet-decomposed with nitric acid (15 mol). The obtained analytical value was converted into an element and determined as a supported amount per catalyst weight.

Catalyst Performance Test Using Simulated Exhaust Gas Next, a catalyst performance test was conducted using each of the above catalysts.

The catalyst performance test was carried out by using a mixed gas of nitrogen: 90% by volume and oxygen: 10% by volume as a diluent gas, NO concentration: 1000 ppm, water concentration: 7% by volume, SO ₂ : 15
Using a simulated exhaust gas with ppm added, a fixed flow system was used in which the space velocity (SV) was aerated at a flow rate of 20,000 hr ^-1 . Light oil was used as the reducing agent, and nitrogen oxide concentration (NO
3 times the amount (weight ratio) of _x ) was added immediately before the catalyst layer.

The analysis of nitrogen oxides in the gas was carried out by the atmospheric pressure chemiluminescence method, and the removal rate (denitration rate) of nitrogen oxides was obtained by dividing the nitrogen oxide concentration at the catalyst layer outlet by the inlet concentration by 100. The value was subtracted from%.

Regarding the catalyst performance, the dependency of the denitration rate on the catalyst set temperature was examined when the denitration reaction became stable after the exhaust gas was passed through the catalyst layer. First, set the catalyst temperature to 500 ° C.
Other than that, the same conditions such as the use of the above exhaust gas are used to stabilize the denitration reaction, and then the catalyst set temperature is set to 500.
℃ to 300 ℃, then 300 ℃ to 500 ℃
The denitrification rate at the time when the denitrification reaction at each catalyst set temperature was stabilized in each range of 50 ° C was examined, the denitrification rate at this time was taken as 0 hour, and the catalyst set temperature was 500 ° C and running for 100 hours, 25 hours, 50 hours, 7
The same operation as at 0 hour was performed at 5 hours and 100 hours, and the denitration rate at each catalyst set temperature was examined. In addition,
The time during which the test is performed is excluded from the running time. For example, the denitration rate at 50 hours is 500 ° C at 25 hours, and the operation for obtaining the denitration rate is completed for another 25 hours at 500 ° C. It is the denitrification rate obtained by lowering the temperature when running at.

The test results are shown in Table 3 for the catalyst A (Ti-Ag-Co) and in Table 4 for the catalyst B (Ag-Co).

[0091]

[Table 3]

[0092]

[Table 4]

From the results shown in each table, it can be seen that the initial characteristics of the catalyst A loaded with titanium and the catalyst B not loaded with titanium are slightly better in the catalyst A, but not so different. However, in the 100-hour continuous test, the catalyst B not supporting titanium was significantly deteriorated at the catalyst set temperature of 400 ° C. or lower, but it was found that the catalyst supporting titanium was excellent in maintaining the catalytic activity.

Next, in Catalyst A, catalysts having different titanium loadings were prepared in the same manner except that the loadings of titanium were changed to 0.12% by weight and 0.53% by weight.

The above catalyst performance test was conducted on these catalysts. Table 5 shows the initial characteristics among these. Table 5 also shows the results of the catalyst A (titanium supported amount = 0.25 wt%).

[0096]

[Table 5]

From Table 5, it can be seen that the initial characteristics deteriorated when the amount of titanium supported increased or decreased, and the initial characteristics deteriorated at any catalyst set temperature when the amount of titanium carried increased. From this, it can be said that it is preferable to use the amount of titanium supported near 0.25% by weight.

Thus, depending on the amount of titanium supported, the initial characteristics are worse than those without titanium,
In a 100-hour continuous test, the catalyst activity is maintained because the characteristic deterioration is small.
It has been found that, even after 0 hour, the catalyst supporting titanium has a better denitrification rate than the catalyst not supporting titanium, regardless of the amount of titanium supported.

Catalytic properties of diesel engine exhaust gas
Performance test Each of the catalysts A and B was filled in a stainless steel tube having an inner diameter of 36 mm to form a denitration catalyst reaction layer. Catalyst loading is 4
It was set to 0cc.

Each denitration catalyst reaction layer was installed behind the exhaust manifold of a direct injection diesel engine with an displacement of 2290 cc equipped with an AC generator. Further, a reducing agent (light oil) for the denitration reaction can be added immediately before the denitration catalyst reaction layer.

For each diesel engine, the dependency of exhaust gas denitration rate on the catalyst set temperature (exhaust gas temperature) was measured, and the nitrogen oxide removal performance was investigated. The exhaust gas was passed through the denitration catalyst reaction layer as it was without removing water and particulate matter. Incidentally composition of the exhaust _gas, NO X: 900
～1300Ppm, oxygen: 10-20% (by volume), water: 5 to 20%, SO _2: was 10 to 30 ppm. The measurement was performed by setting the space velocity (SV) to 20,000 hr ⁻¹ and changing the catalyst set temperature in the range of 350 to 500 ° C. at every 50 ° C. Reducing agent (gas oil) is 3 times the amount of NO _X (the weight ratio) was added just before the denitration catalyst reaction layer. NO _x after the reaction is atmospheric pressure chemiluminescence method (NO-NO
₂ ) and the denitration rate was calculated. The results are shown in Table 6 for catalyst A and Table 7 for catalyst B, respectively.

[0102]

[Table 6]

[0103]

[Table 7]

As is clear from the results of Tables 6 and 7, the same results as in the case of using the simulated exhaust gas in Tables 3 and 4 are obtained for the catalyst A supporting titanium and the catalyst B not supporting titanium. I can say. That is, with the catalyst A supporting titanium, the catalytic activity does not decrease even at the catalyst set temperature of 400 ° C. or lower.

In the catalysts A and B, the Co layer was the lower layer, the Ag layer was the upper layer, and the Co loading was the Ag of the catalysts A and B.
The supported amount is almost the same, and the supported amount of Ag is Co of catalysts A and B.
A catalyst was prepared with the same loading amount. A catalyst having a Ti layer is referred to as a catalyst C, and a catalyst having no Ti layer is referred to as a catalyst D. When the above catalyst performance test was conducted using catalysts C and D, T
It was found that the same tendency as the catalysts A and B was exhibited depending on the presence or absence of the i layer. However, since the catalyst C has inferior initial characteristics to the catalyst A, the catalyst A is superior.

By EGR system + catalytic reaction system
Exhaust gas treatment The internal combustion engine test apparatus with EGR system used in Comparative Example 2 was provided with the above-mentioned denitration catalyst reaction layer filled with catalyst A (Ti-Ag-Co) to produce a diesel engine having the configuration shown in FIG. did.

The reducing agent (light oil) used in the denitration catalytic reaction is
Fuel tank 5 → Reductant addition pump (micrometer pump) 2
It was added immediately before the denitration catalyst reaction layer 6 in the order of 0 → reducing agent supply passage (pipe having an inner diameter of 3 mm) 21. The amount of the reducing agent
The amount was 3 times (weight ratio) with respect to the amount of nitrogen oxides.

The composition analysis of the exhaust gas was performed by a gas analyzer. During the operation of the EGR system, the exhaust gas flow rate control valve 7 completely closed the exhaust pipe leading to the denitration catalyst reaction layer 6, and the composition of the exhaust gas discharged from the bypass exhaust pipe 103 was analyzed. Further, when the EGR system was stopped, the bypass exhaust pipe 103 was fully closed by the exhaust gas flow rate control valve 7, and the composition of the exhaust gas discharged from the exhaust pipe leading to the denitration catalyst reaction layer 6 was analyzed.

Using this engine, the emission of nitrogen oxides in the diesel engine 13 mode was measured. The EGR rate was set to 10% or 20% when the load rate was 40% or less, and the operation of the EGR system was stopped when the load rate exceeded 40%. The denitration catalyst was used at a space velocity (SV) of 20,000 hr ^-1 . Table 8 shows the results.

[0110]

[Table 8]

From Table 8, the effect of the present invention is clear.
The reduction rate of nitrogen oxide with respect to Comparative Example 1 was 1 when the EGR rate was 1.
When it was 0%, it was 39.1%, and when the EGR rate was 20%, it was 43.9%. At a high load factor of 60% or more, the nitrogen oxide concentration is slightly higher than in Comparative Example 2 in which the EGR is operated. However, in this example,
The particulate matter in the exhaust gas can hardly be confirmed by visual observation, which is superior to Comparative Example 2 in which the particulate matter is extremely discharged.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration example of an engine to which the present invention is applied.

[Explanation of symbols]

 1 Engine Body 101 Intake Path 102 Exhaust Path 103 Bypass Exhaust Path 2 Transmission 3 Eddy Current Dynamometer 4 Fuel Injection Pump 5 Fuel Tank 6 Denitration Catalyst Reaction Layer 7 Exhaust Gas Flow Rate Control Valve 81 EGR Flow Path 82 EGR Control Valve 9 Control Unit 10 Intake oxygen sensor output 11 Intake and exhaust gas mixture oxygen sensor output 12 Exhaust oxygen sensor output 13 EGR control output 14 Fuel injection amount output 15 Eddy current dynamometer output 16 Exhaust gas temperature sensor output 17 Exhaust gas flow control output 18 Engine rotation Number output 20 Reductant addition pump 21 Reductant supply channel

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F02M 25/07 550 B01D 53/36 102B 102H

Claims (9)

[Claims] 1. An EGR system and a denitration reaction system are installed in an engine, and the EGR system recirculates exhaust gas to an intake passage and an EGR control valve for opening and closing the EGR passage. Wherein the denitration reaction system has a denitration catalyst and a reducing agent addition device, and the denitration catalyst has γ-alumina or silica-alumina as a carrier, and a transition metal element is carried on the carrier. The reducing agent adding device adds an organic compound as a reducing agent, and operates the EGR system only when the engine has a low load to purify exhaust gas with excess oxygen, thereby purifying the engine exhaust gas. . 2. The method for purifying engine exhaust gas according to claim 1, wherein the EGR system operates on a low load side and stops operating on a high load side with an engine load factor of 20 to 60% as a boundary. 3. The exhaust gas temperature of the EGR system is 3
The method for purifying engine exhaust gas according to claim 1 or 2, wherein the operation is performed on the low load side and stopped on the high load side at a boundary of 00 to 500 ° C. 4. The method for purifying engine exhaust gas according to claim 1, wherein an EGR rate when the EGR system is operating is 50% or less. 5. The method for purifying engine exhaust gas according to claim 1, wherein the EGR rate is varied according to the engine load when the EGR system is operating. 6. The operation control of the EGR system detects at least one selected from oxygen concentration in the intake passage, pressure in the intake passage, engine speed, cooling water temperature, fuel injection amount and exhaust gas temperature. The method for purifying engine exhaust gas according to any one of claims 1 to 5, which is performed by the method. 7. The method for purifying engine exhaust gas according to claim 1, wherein the denitration catalyst uses γ-alumina as a carrier and silver and cobalt are carried on the carrier. 8. The method for purifying engine exhaust gas according to claim 1, wherein the denitration catalyst uses γ-alumina as a carrier, and titanium, silver and cobalt are carried on the carrier. 9. The method for purifying engine exhaust gas according to claim 1, wherein the organic compound used as the reducing agent is engine fuel.