JPH06235318A - Exhaust emission control method - Google Patents

Abstract

PURPOSE:To provide exhaust emission control method which can effectively remove oxygen nitrides contained in exhaust gas of a Diesel engine. CONSTITUTION:A porous exhaust gas purification material 3 is arranged on the way of an exhaust gas flow passage. Liquefied hydrocarbon is atomized on an upstream side of the exhaust gas purification material 3, and then oxygen nitrides are reduced and removed. In such exhaust emission control method, the hydrocarbon is atomized by means of a hydrocarbon atomization device 4 with grain sizes being 5 to 50mum in diameter.

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 discharged from an internal combustion engine such as an automobile engine, and more particularly to effectively reducing and removing nitrogen oxides contained in the exhaust gas such as a diesel engine. The present invention relates to an exhaust gas purification method capable of performing

[0002]

2. Description of the Related Art In recent years,
Global environmental pollution has become a problem, but air pollution is particularly serious, and problems such as photochemical smog due to nitrogen oxides in the atmosphere and acid rain remain unsolved. Air pollution caused by nitrogen oxides is particularly serious in metropolitan areas, and countermeasures are urgently needed. Examples of sources of nitrogen oxides include automobile engines, especially diesel engines, and large combustion devices (for example, internal combustion engines for cogeneration).

As a method of removing nitrogen oxides in exhaust gas, for example, for exhaust gas of a gasoline engine, there is a method of using a so-called three-way catalyst, and a large-scale fixed combustion device (large combustion in a factory or the like). For example, there is a selective catalytic reduction method in which ammonia is mixed into the exhaust gas to reduce nitrogen oxides in the exhaust gas.

However, for exhaust gas having a relatively high oxygen concentration such as exhaust gas emitted from a diesel engine, nitrogen oxides can be removed efficiently by the three-way catalyst system used for purifying exhaust gas of a gasoline engine. I can't do it. In addition, the method of reducing and removing nitrogen oxides by mixing ammonia in the exhaust gas, ammonia is expensive,
Further, ammonia has problems that it is toxic and that the device is generally large in size, so it cannot be applied to a moving exhaust gas source such as an automobile.

Therefore, it has been desired to establish a new method for reducing (removing) nitrogen oxides in exhaust gas having a relatively high oxygen concentration such as found in exhaust gas of diesel engines, and various attempts have been made so far. Has been done.

Particularly recently, liquid hydrocarbons such as light oil and kerosene and nitrogen oxide removing catalysts such as complex inorganic oxides have been used.
A method of purifying exhaust gas by reducing nitrogen oxides by adding gaseous hydrocarbons such as propane and acetylene to the exhaust gas (Jpn.
0th Internal Combustion Engine Joint Symposium Proceedings pp. 253-2
58). As a method for removing nitrogen oxides in exhaust gas of a diesel engine, a method for removing nitrogen oxides by using a hydrocarbon and a reducing catalyst has been known. However, when it is installed in an actual engine, the nitrogen oxide removal performance is not yet sufficient, and the nitrogen oxide removal by this method has not yet reached the practical range.

Therefore, an object of the present invention is to provide an exhaust gas purification method capable of effectively removing nitrogen oxides contained in exhaust gas of a diesel engine and the like.

[0008]

As a result of intensive studies in view of the above problems, the present inventor has found that a porous exhaust gas purifying material having heat resistance and carrying a nitrogen oxide reduction catalyst is installed in the middle of an exhaust gas conduit. However, it was discovered that if the liquid hydrocarbon was sprayed at a specific position on the upstream side of the exhaust gas purifying material so as to have a specific particle size, nitrogen oxide in the exhaust gas could be efficiently reduced and removed by the introduced hydrocarbon. The present invention has been completed.

That is, the exhaust gas purifying method of the present invention in which a porous exhaust gas purifying material is installed in the middle of the exhaust gas flow path, and liquid hydrocarbons are sprayed on the upstream side of the exhaust gas purifying material to reduce and remove nitrogen oxides. Is characterized in that a hydrocarbon spraying device sprays hydrocarbons to fine particles having a diameter of 5 to 50 μm.

[0010]

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing an example of an exhaust gas purifying apparatus used in the method of the present invention. The exhaust gas purification apparatus 1 includes a heat-resistant porous exhaust gas purification material 3 installed in the middle of the exhaust gas conduit 2, an apparatus 4 for spraying hydrocarbons into the exhaust gas on the upstream side of the exhaust gas purification material 3, and a spraying apparatus 4 And a hydrocarbon tank 6 connected to the supply control device 5, and a temperature sensor 7 for measuring the exhaust gas temperature at the purification material inlet.

In the present invention, the hydrocarbon spraying device 4 is used.
Is composed of a spray nozzle. The spraying device 4 is installed so that the distance from the spraying port of the spraying device 4 to the purification material is 100 to 1000 mm. If the distance from the spray port to the purification material exceeds 1000 mm, the hydrocarbons ejected are thermally decomposed before reaching the purification material, and the reduction rate of nitrogen oxides is reduced. If this distance is less than 100 mm, the sprayed hydrocarbons cannot spread sufficiently and are concentrated in the central portion of the purification material, so that the utilization efficiency of the purification material is poor. The distance from the spray port to the purification material is 200-60
It is preferably 0 mm, and particularly preferably 200 to 400 mm. Examples of the material forming the nozzle include heat resistant materials such as stainless steel and ceramics.

The hydrocarbon supply control device 5 is a device for supplying the hydrocarbons in the hydrocarbon tank 6 to the hydrocarbon spraying device 4. In the present invention, as a supply method, a two-fluid spraying method is used in which compressed air is supplied to the nozzle of the hydrocarbon spraying device 4 and the hydrocarbon is sprayed by the air flow at the nozzle.

The hydrocarbon added to the purifying material 3 is as fine particles as possible, and the diameter of the hydrocarbon particles atomized by the spraying device 4 is 5 to 50 μm. If the diameter of the hydrocarbon particles exceeds 50 μm, the hydrocarbons cannot completely react and the reduction rate of nitrogen oxides decreases. Further, it is technically difficult to reduce the diameter of the hydrocarbon particles to less than 5 μm. The preferable diameter of the hydrocarbon particles is 5 to 20 μm. It should be noted that the diameter of the hydrocarbon particles referred to here means the 50% representative particle diameter of the particle distribution.

The structure of the nozzle of the spraying device 4 is appropriately devised so as to achieve atomization of hydrocarbons. Although there are some changes depending on the hydrocarbon used, the orifice diameter of the nozzle is set to 0.
It is about 1 to 2.2 mm. Orifice diameter is 2.2 mm
If it exceeds, fine hydrocarbon particles cannot be obtained, and if the orifice diameter is less than 0.1 mm, the required hydrocarbon flow rate cannot be obtained. The orifice diameter of the nozzle is preferably 0.5 to 2 mm, particularly preferably 0.5 to 1.5 m.
m. Further, in the hydrocarbon supply device 5, it is preferable that the pressure of the compressed air is 98 to 980 KPa and the air is discharged. If the pressure of the compressed air is less than 98 KPa, the hydrocarbon particles will not become fine, and if it exceeds 980 KPa, the sprayed hydrocarbons cannot be uniformly dispersed in the purification material, and the reduction rate of nitrogen oxides will decrease. The preferred compressed air pressure is 1
It is 50 to 300 KPa.

Next, the exhaust gas purifying material 3 will be described.
The exhaust gas purifying material used in the present invention is obtained by coating a molded body with a porous inorganic oxide carrying a catalytically active species. As the molded body of the purification material 3 installed in the exhaust gas purification apparatus 1, a porous ceramic or metal porous molded body having excellent heat resistance and impact resistance is used. Further, it may be a purifying material in which a casing is filled with a porous pellet, a granular material, or a heat resistant fibrous material. It is preferable to use a ceramic molded body in consideration of durability and the like, and it is preferable to use a foam-type or honeycomb-type molded body in view of ease of manufacturing.

As the ceramics forming the molded body,
Alumina, silica, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia,
Mullite, cordierite or the like can be used. Preferred ceramic materials include cordierite, mullite, alumina and composites thereof.

When the exhaust gas passes through the purifying material, the nitrogen oxides in the exhaust gas react with the hydrocarbons finely added and gasified in the exhaust gas to reduce and remove the nitrogen oxides. In order to effectively proceed the reduction reaction, it is preferable to use a purifying material that has a pressure loss within an allowable range and a large contact area with the gas. Is preferably 20 to 90%. If the porosity is less than 20%, it becomes difficult for the exhaust gas to pass through the purification material, the reduction reaction of nitrogen oxides cannot be promoted, and the removal rate of nitrogen oxides decreases. On the other hand, if it exceeds 90%, the strength of the purifying material decreases, and the exhaust gas passes through too easily, so that the nitrogen oxide removal rate also decreases.

In particular, when the purification material is a honeycomb type molded body, the porosity is preferably 30 to 80%. The pore size at that time is preferably 0.5 to 3 μm. If the pore diameter exceeds 3 μm, the contact area with the exhaust gas becomes too small and the reduction efficiency decreases.

In the present invention, as the exhaust gas purifying material 3, on the surface of the ceramic molded body made of the above-mentioned material,
The one provided with a porous ceramic layer is used. Examples of the porous ceramic layer include silica, alumina, titania, zirconia, titania-alumina, silica-alumina, zirconia-alumina, silica-titania, silica-zirconia, titania-zirconia and the like.
In particular, it is preferable to use γ-alumina or its composite inorganic oxide, or an inorganic oxide ceramic material such as zirconia, titania, titania-zirconia.

By forming a porous ceramic layer having a large surface area on the surface of the molded body, the contact area of the purifying material with the exhaust gas is increased, so that the nitrogen oxides in the exhaust gas and the hydrocarbons added to the exhaust gas are increased. And will react effectively. Also, the reaction temperature range becomes wider,
Better purification of nitrogen oxides is possible.

There are several methods for forming the porous ceramic layer on the surface of the porous ceramic molded body, but the well-known wash coating method or sol-gel method is preferable.

Further, in the present invention, the above-mentioned exhaust gas purifying material carries the following metals or their compounds as catalytically active species. Examples of the catalytically active species include copper and silver as main components. Other than that, (a) alkali metal elements,
(b) Periodic Table IB, IIB, VA, VIA, VIIA
One or more elements selected from Group C, Group VIII transition metals, (c) rare earth elements and the like can be used. By using the catalytically active species having such a structure, the reaction between the hydrocarbon and the nitrogen oxide can be promoted, and the effective reduction of the nitrogen oxide can be performed in a wide temperature range.

As a method of supporting the above-mentioned catalytically active species, a method of directly supporting it on a porous molded body by a known impregnation method, a precipitation method or the like, or a catalyst on a porous inorganic oxide layer provided on the molded body There are a method of supporting an active species, a method of coating a molded body with a porous inorganic oxide carrying a catalytically active species, and the like.

Next, an exhaust gas purification method using the exhaust gas purification apparatus 1 shown in FIG. 1 will be described. In the present invention, the above-mentioned exhaust gas purifying material 3 is installed in the middle of the exhaust gas conduit 2 communicating from the engine to the exhaust port, and hydrocarbon is sprayed into the exhaust gas at the exhaust gas upstream side of the purifying material 3.

In the present invention, hydrocarbon is used as the reducing agent. In particular, it is a hydrocarbon in a liquid state in the standard state and has a boiling point of 9
Fractions from 0 to 350 ° C are preferred. Specific examples include kerosene, light oil, cetane, heptane and the like. If a hydrocarbon having a boiling point higher than 350 ° C. is used, the hydrocarbon is difficult to vaporize at the temperature of the exhaust gas under normal engine operating conditions, and the reduction reaction of nitrogen oxides does not proceed so much. Preferably, a hydrocarbon having a boiling point of 160 to 340 ° C is used. Considering practicality, it is particularly preferable to use light oil.
An oxygen-containing organic compound such as ethanol can also be used.

The amount of hydrocarbons added is preferably adjusted appropriately according to the amount of nitrogen oxides contained in the exhaust gas. For that purpose, a throttle valve is provided to control the flow rate to control the exhaust gas of hydrocarbons. Supply inside.

The temperature of the exhaust gas in the exhaust gas purifying device (specifically, the temperature of the exhaust gas passing through the exhaust gas purifying material 3, which is actually measured by measuring the temperature in the vicinity of the upstream end of the exhaust gas purifying material 3 Can be changed to some extent depending on (the boiling point of) the hydrocarbon used, but at least 200 to 50
It is better to keep it at 0 ° C. Below this temperature range,
No effective reduction of nitrogen oxides is obtained. Further, when the temperature is higher than this temperature range, the added hydrocarbon itself burns and the reaction of carbon dioxide and water is prioritized, so that the reduction rate of nitrogen oxides also decreases. The exhaust gas temperature in the exhaust gas purification device is more preferably 300 to
The temperature is 500 ° C.

By the way, the actual exhaust gas temperature of an automobile is
It changes every moment depending on the operating condition of the engine. Therefore, in order to ensure the purification of nitrogen oxides, it is preferable to control the exhaust gas temperature within the above temperature range. As an example of the control, there is the following method. That is, a valve for adjusting the flow rate of exhaust gas is provided on the upstream side of the exhaust gas purification device, the exhaust gas temperature in the exhaust gas purification device is monitored, and when the exhaust gas temperature falls below the above range, the valve is throttled to raise the exhaust gas temperature. When lowering the exhaust gas temperature, the opposite operation to the above operation may be performed.

The method of the present invention will be explained in more detail by the following specific examples, but the present invention is not limited thereto.

Example 1 An exhaust gas purifying apparatus shown in FIG. 1 was produced. Light oil (JIS No. 2) was used as the hydrocarbon, and a two-fluid spraying method was used in which compressed air and light oil were mixed and sprayed. The spray nozzle of the spray device 4 is made of stainless steel, and the spray spread angle is 15 °.
And the distance from the spray port to the purification material was 300 mm.

The hydrocarbon supply control device 5 comprises an air compressor and an air pressure adjusting device, and controls the flow rate of air in the spray nozzle to adjust the amount of light oil added. The pressure of compressed air is fixed at 196 kpa, and the air flow rate in the nozzle is 27 Nl / min. The amount of light oil sprayed is 17 ml / min
And

Next, a cordierite honeycomb type molded body (diameter: 7.5 inch, length: 7 inch, cell number: 400 / square inch, porosity: 70%) was coated with 50% by weight of alumina by a wash coat method (molded). (On the body) and carrying 8% by weight (on the molded body) of a mixed catalyst consisting of copper, lanthanum, and cesium by the slurry method was used as the purification material 3, and as shown in FIG. I installed it on the way. Further, a direct-injection diesel engine with a displacement of 4 liters, 6 cylinders was installed upstream of the exhaust gas conduit 2 as an exhaust gas generation source. The engine speed of diesel engine is 2000
The engine load was set so that the temperature at the inlet of the exhaust gas purification material (temperature sensor 9) was 400 ° C. at rpm.

A spraying device 4 of the exhaust gas purifying apparatus 1 shown in FIG.
At the orifice diameters of 0.7, 1.2, 1.8,
The reduction rate of each nitrogen oxide was measured using four types of 2.2 mm nozzles. The experimental results are shown in FIG. The particle size distribution of hydrocarbons in each nozzle was examined by an optical particle size analyzer, and the 50% representative particle size and nozzle orifice size are shown in Table 1.

Table 1 Nozzle orifice diameter and sprayed hydrocarbon particle diameter Orifice diameter (mm) 0.7 1.2 1.8 2.2 50% Representative particle diameter (μm) 5.1 10.4 14. 5 45.0

[0035]

As described above in detail, according to the exhaust gas purifying method of the present invention, nitrogen oxides in exhaust gas can be effectively reduced. The method of the present invention is particularly suitable for exhaust gas in an oxidizing atmosphere such as that found in diesel engine exhaust gas.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of an exhaust gas purifying apparatus used in the present invention.

FIG. 2 is a graph showing a relationship between a nozzle orifice diameter and a nitrogen oxide reduction rate in Example 1.

[Explanation of symbols]

 1 Exhaust Gas Purification Device 2 Exhaust Gas Conduit 3 Exhaust Gas Purification Material 4 Hydrocarbon Spraying Device 5 Hydrocarbon Supply Control Device 6 Hydrocarbon Tank 7 Temperature Sensor

Claims (4)

[Claims] 1. An exhaust gas purification method in which a porous exhaust gas purification material is installed in the middle of an exhaust gas flow path and liquid hydrocarbons are sprayed on the upstream side of the exhaust gas purification material to reduce and remove nitrogen oxides. A hydrocarbon atomizer with a diameter of 5
A method for purifying exhaust gas, which comprises spraying fine particles of ˜50 μm. 2. The exhaust gas purification method according to claim 1, wherein the hydrocarbon spraying device atomizes the hydrocarbons using compressed air having a pressure of 98 to 980 KPa. 3. The exhaust gas purifying method according to claim 1, wherein the hydrocarbon atomizer has an orifice diameter of 0.
A method for purifying exhaust gas, which comprises atomizing hydrocarbons using a spray nozzle having a diameter of 1 to 2.2 mm. 4. The exhaust gas purification method according to claim 1, wherein the distance from the spray port of the hydrocarbon spraying device to the purification material is 100 to 1000 mm.