KR20110035353A - Apparatus for reducing nitrogen oxide of exhaust from lean-burn engine - Google Patents

Abstract

PURPOSE: A device for reducing nitrogen oxide of exhaust gas from a lean-burn engine is provided to easily control the composition ratio of exhaust gas supplied to a nitrogen oxide reducing catalyst. CONSTITUTION: A device for reducing nitrogen oxide of exhaust gas from a lean-burn engine comprises an oxygen separation unit(20), a bypass exhaust passage(30), and a nitrogen oxide reducing catalyst(40). The oxygen separation unit is formed on an exhaust pipe foremd between an exhaust manifold of a lean-burn engine and a catalyst with a reduction function. The bypass exhaust passage is formed on the side surface of the exhaust pipe and bypasses oxygen passing through the oxygen separation unit.

Description

Apparatus for reducing nitrogen oxide of exhaust from lean-burn engine

The present invention relates to a device for reducing nitrogen oxides of lean-burn engine exhaust gas, and more particularly, to a porous support and a porous support, which reduces oxygen in the exhaust gas to oxygen ions and selectively permeates only the reduced oxygen ions. An apparatus for reducing nitrogen oxides in lean-burn engine exhaust gas comprising an oxygen separation unit composed of an oxygen ion conductor thin film.

Nitrogen oxides called NOx, among the compounds included in the exhaust stream of the combustion process, must be controlled to avoid meeting or exceeding government emission regulations because they degrade the air quality when discharged to the surrounding environment. Various combustion processes that produce NOx include coal or oil furnaces, reciprocating internal combustion engines (gasoline and diesel engines) and gas turbines.

In order to remove harmful substances (e.g., nitrogen oxides, carbon monoxide and hydrocarbons) from the engine exhaust stream, three-way catalytic converters are mounted on gasoline engine vehicles, in which case engine engines are mainly chemical It operates in stoichiometric mode (ie stoichiometric quantities of fuel and air are fed to the engine). Hazardous substances in the exhaust gas stream after combustion of gasoline engines are mainly hydrocarbons, carbon monoxide and nitrogen oxides.Three-way catalyst converts nitrogen oxides into nitrogen and oxygen by converting them into nitrogen and oxygen. By using oxygen as a reducing agent, more than about 95% of the harmful substances are removed by oxidizing carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water. However, in gasoline engines operating under lean burn conditions where fuel is lean, the amount of oxygen in the exhaust gas is too high to completely reduce nitrogen oxides to nitrogen.

On the other hand, lean-burn engines operate primarily in fuel-lean mode (ie, because more air than the stoichiometric amount of air is supplied into the engine cylinder with fuel), compared to stoichiometric engines. Combustion engines offer superior fuel savings. Typical examples of lean-burn engines are diesel engines. However, the exhaust gas from lean-burn engines operating under lean burn conditions with too much fuel has too much oxygen to reduce nitrogen oxides to nitrogen by a three-way catalytic converter. NOx emissions cannot be reduced to an appropriate level. Thus, reducing the nitrogen oxides in the exhaust gas emitted from lean-burn engines requires a different exhaust gas purification system than that of an engine operating in stoichiometric mode. Specifically, the exhaust gas purification system of an example lean-burn diesel engine is generally a diesel particulate filter (Diesel Particulate) disposed on the upstream side and collecting particulate matter (PM) on an exhaust pipe connected to the engine exhaust manifold. Filter, DPF); It is disposed downstream of the diesel particulate filter (DPF) and oxidizes soluble organic substances (SOF) in hydrocarbons, carbon monoxide, and particulate matters (PM) among the harmful substances contained in the exhaust gas. Diesel oxidation catalysts (DOC) which convert to water; It is disposed downstream of the Diesel Oxidation Catalyst (DOC) and consists of a Selective Catalyst Reduction (SCR) catalyst which reduces nitrogen oxides to nitrogen using ammonia as the reducing agent.

As described above, in the case of the lean-combustion engine or the engine operating in the lean-fuel mode, the amount of fuel is scarce and the amount of air is rich compared to the case of operating at the theoretical air-fuel ratio. In order to remove oxides of nitrogen and to remove nitrogen oxides, a reduction reaction is required. However, due to the large amount of oxygen present in the exhaust gas, gasoline engines do not easily remove nitrogen oxides, and diesel engines require a separate SCR catalyst. . In order to solve this problem, Japanese Laid-Open Patent Publication No. 2003-027926 discloses an oxygen enrichment system composed of an oxygen enrichment membrane, a porous support membrane, and a continuous foam. Specifically, a part of the exhaust gas passing through the exhaust passage passes through the oxygen enrichment membrane having higher oxygen permeability than the nitrogen, the porous support membrane supporting the oxygen load membrane, and the combustion foam in order to become an oxygen enrichment gas having a relatively high oxygen concentration. In addition, the remainder of the exhaust gas becomes an oxygen reducing gas having a relatively low oxygen concentration, and bypasses the oxygen enrichment gas to be discharged out of the vehicle body, and promotes the reduction of nitrogen oxide by supplying the oxygen reducing gas to the nitrogen oxide reduction catalyst. However, Japanese Laid-Open Patent Publication No. 2003-027926 uses a three-dimensional crosslinking compound of a silicone-based polymer that alternates a siloxane ring and an oligo oxyalkylene ring as an oxygen enrichment membrane, so that not only oxygen but also The permeation of nitrogen, carbon monoxide, hydrocarbons and nitrogen oxides cannot be excluded, and there is a problem that it is not easy to control the composition ratio of the oxygen reducing gas. In addition, the oxygen enriched gas contains oxygen as well as other harmful substances, it is difficult to reuse them, and even when discharged out of the vehicle body, additional hazardous substance verification may be required.

The present invention is to solve the conventional problems, an object of the present invention is to selectively control only the oxygen of the exhaust gas discharged from the lean-burn engine to accurately control the composition ratio of the exhaust gas supplied to the nitrogen oxide reduction catalyst, The present invention provides a device for reducing nitrogen oxides of lean-burn engine exhaust gas, which can supply oxygen reduction gas to the nitrogen oxide reduction catalyst to effectively reduce nitrogen oxides and reuse oxygen separated from exhaust gas.

An object of the present invention is formed on an exhaust pipe between an exhaust manifold of a lean-burn engine and a catalyst having a nitrogen oxide reduction function, and an oxygen separation unit for selectively separating only oxygen of exhaust gas discharged from the lean-burn engine. ; A bypass exhaust passage formed on the side of the exhaust pipe and in communication with the oxygen separator to bypass the oxygen passing through the oxygen separator from the exhaust gas; And a nitrogen oxide reduction catalyst, wherein the oxygen separation unit is composed of a porous support and an oxygen ion conductor thin film coated on the porous support and reducing oxygen of exhaust gas to oxygen ions and selectively transmitting only the reduced oxygen ions. It can be achieved by providing a nitrogen oxide reduction device for lean-burn engine exhaust gas.

The oxygen separation part may be formed by forming a part of the exhaust pipe or may be formed in an exhaust passage inside the exhaust pipe. When the oxygen separation part is formed by forming a part of the exhaust pipe, the oxygen is coated on the porous support with the porous support in the form of a sheet. When the ion conductor thin film becomes an inner surface of the exhaust pipe, and the oxygen separation portion is formed in the exhaust passage inside the exhaust pipe, the oxygen separation portion has a monolith type porous support having a plurality of exhaust gas flow paths in the exhaust gas flow direction, and It consists of a thin film of oxygen ion conductor coated on the surface.

In addition, the nitrogen oxide reduction device of the lean-burn engine exhaust gas of the present invention may further include a heating element for controlling the temperature of the oxygen ion conductor, the heating element is installed around the oxygen ion conductor thin film. In addition, the temperature of the oxygen ion conductor may be controlled by using an underfloor catalytic converter (UCC), which is a nitrogen oxide reduction catalyst, directly below the oxygen separation unit and using heat generated from the UCC.

In addition, the oxygen passing through the oxygen separation unit may be discharged out of the vehicle body together with the exhaust gas at the bottom of the UCC through the bypass exhaust passage, but may be preferably supplied to the interior of the vehicle body and used to purify the indoor air.

The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to the present invention comprises a porous support and a thin film of oxygen ion conductor coated on the porous support and reducing oxygen of the exhaust gas to oxygen ions and selectively transmitting only the reduced oxygen ions. By selectively separating only the oxygen of the exhaust gas by using the oxygen separation unit, the composition ratio of the exhaust gas supplied to the nitrogen oxide reduction catalyst can be easily controlled, and even when the engine is operated in the fuel-lean mode, the theoretical air-fuel ratio is operated. Nitrogen oxides can be reduced to nearly equivalent levels. In addition, since the exhaust gas passing through the oxygen separation unit is composed of pure oxygen only, it does not need a separate hazardous substance verification to be discharged out of the vehicle body, and in particular, it can be used to purify the indoor air when supplying it to the interior of the vehicle body.

The present invention relates to an apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas, wherein the apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas is provided between an exhaust manifold of a lean-burn engine and a catalyst having a nitrogen oxide reduction function. An oxygen separation unit formed on an exhaust pipe of the gas for selectively separating only oxygen of the exhaust gas discharged from the lean-burn engine; A bypass exhaust passage formed on the side of the exhaust pipe and in communication with the oxygen separator to bypass the oxygen passing through the oxygen separator from the exhaust gas; And a nitrogen oxide reduction catalyst.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 schematically shows a device for reducing nitrogen oxides of lean-burn engine exhaust gas according to an embodiment of the present invention, and FIG. 2 shows a device for reducing nitrogen oxides of lean-burn engine exhaust gas according to another embodiment of the present invention. It is shown schematically. 1 and 2 have the same reference numerals.

A lean-burn engine means that the engine operates primarily in a fuel-lean mode (Lean-burn condition where fuel is lean), but in the present invention, a lean-burn engine is a fuel-lean mode (fuel Operating under lean burn conditions. Engines that operate primarily in fuel-lean mode (lean-burn conditions) are typically diesel engines and are temporarily operated in fuel-lean mode (lean-burn conditions). There is a general gasoline engine, but the gasoline engine is also limited to the lean-combustion engine of the present invention, since the gasoline engine can be adjusted to operate mainly in a fuel-lean mode (lean-burn condition where fuel is lean). It doesn't happen.

Exhaust gases emitted from an engine operating in a fuel-lean mode (lean-burn condition in which fuel is lean) are passed through the exhaust passage of the exhaust pipe 10 to reduce the nitrogen oxides according to the present invention (100, 200). It contains an excess of oxygen compared to the exhaust gas from the engine operating at a theoretical air-fuel ratio, which reduces the reducing capacity of the nitrogen oxide reduction catalyst.

The exhaust gas then passes through oxygen separation sections 20 and 120 formed on the exhaust pipe between the exhaust manifold (not shown) of the lean-burn engine and the catalyst 40 having the nitrogen oxide reduction function. . The oxygen separation unit performs a function of selectively separating only oxygen of the exhaust gas discharged from the lean-burn engine, and is coated on the porous supports 21 and 121, and the oxygen of the exhaust gas is reduced to oxygen ions and reduced. It consists of the oxygen ion conductor thin films 22 and 122 which selectively permeate only oxygen ions.

The shape of the oxygen separation unit on the exhaust pipe may be various, and may be formed by forming a part of the exhaust pipe as shown in FIG. 1, and may be formed in the exhaust passage inside the exhaust pipe as shown in FIG. 2. have.

When the oxygen separation unit 20 is formed by forming a part of the exhaust pipe, the porous support has a sheet shape. Specifically, when the exhaust pipe has a circular cross section, the porous support has a circular sheet shape that is the same as the exhaust pipe shape. In addition, the surface of the porous support in the form of a sheet is coated with an oxygen ion conductor thin film, which becomes the inner surface of the exhaust pipe (or is formed to face the inner surface). In this case, the oxygen of the exhaust gas is reduced to oxygen ions, passes through the oxygen ion conductor thin film 22, is converted into oxygen, and then passes through the porous support 21 to reach the bypass exhaust passage 30. On the other hand, when the oxygen separation unit 120 is formed in the exhaust passage inside the exhaust pipe, the porous support has a monolithic shape having a single body and a plurality of exhaust gas flow paths in the exhaust gas flow direction. It is coated on the surface of the exhaust gas flow path formed inside the porous support. In this case, the exhaust gas is supplied to the front surface of the porous support body, and FIG. 3 (FIG. 3 shows a cross section when the oxygen of the exhaust gas passes through the monolithic porous support and the oxygen ion conductor thin film having a plurality of exhaust gas flow paths. As shown in FIG. 2, the plurality of exhaust gas passages h1 are formed in the monolithic porous support 122. Oxygen in the exhaust gas is coated on the surface of the exhaust gas flow path in a radial direction while the exhaust gas passes through the plurality of exhaust gas flow paths h1 formed in the porous support 122. After passing through the oxygen ion conductor thin film, the oxygen reaches the side of the porous support body through the pores of the porous support. Oxygen of the exhaust gas reaching the porous support side is discharged together with the exhaust gas which is recycled or purified to the room through the bypass exhaust passage 30. In this case, the amount of oxygen in the exhaust gas passing through the oxygen separation unit may be controlled by various factors such as the flow rate of the exhaust gas, the surface area of the oxygen ion conductor thin film, and the temperature of the oxygen ion conductor.

Oxygen ion conductor according to the present invention selectively separates only the oxygen of the exhaust gas, Figure 4 schematically shows a process of the oxygen ion conductor selectively separates only oxygen. As shown in FIG. 4, when a force is applied to both ends of an oxygen ion conductive material having oxygen holes or dielectric properties, oxygen molecules are reduced to oxygen ions and passed through the inside of the oxygen ion conductive material in the form of reduced oxygen ions. On the other surface, electrons are lost and released in the form of oxygen molecules. Such oxygen ion conductors include yttria-stabilized zirconia (YSZ), ceria (CeO ₂ ) -based oxides, yttria-stabilized bismuth oxides, or oxygen-deficient perovskite (Perovskite). Specific examples of the type perovskite (Perovskite) is a material represented by the following formula (1).

[Formula 1]

Ln _{1 - x} A _x Co _y B _{1 - y} O _{1 -δ} (where Ln is lanthanum or gadolindium, A is any one selected from strontium, barium, calcium, sodium, and cobalt, B is iron, Any one selected from copper and nickel, wherein 0 ≦ x <1, ≦ 1, 0 ≦ y <1, and 0 ≦ δ ≦ 1)

In general, since the oxygen ion conductor has a high oxygen ion conductivity (about 10 ⁻¹ to 10 ² S / m) at 700 to 900 ° C. or higher, the nitrogen oxide reduction device of the lean-burn engine exhaust gas of the present invention is an oxygen ion conductor. It is preferable to further include a heating element (not shown) for controlling the temperature of the. The heating element is preferably installed around the oxygen ion conductor thin film to maintain the oxygen ion conductor at a temperature of 700 to 1000 ° C. When the oxygen ion conductor temperature exceeds 1000 ° C., the increase in oxygen ion conductivity due to the temperature is not large. This is because the energy consumption by the heating element is large. In addition, the temperature of the oxygen ion conductor may be controlled by the position of the nitrogen oxide reduction catalyst described below. Specifically, an underfloor catalytic converter (UCC), which is a nitrogen oxide reduction catalyst, is installed directly under the oxygen separation unit and heat generated from the UCC is removed. Can be used to control the temperature of the oxygen ion conductor.

The oxygen of the exhaust gas passes through the oxygen separator to reach the bypass exhaust passage 30. The bypass exhaust passage is formed on the side of the exhaust pipe and communicates with the oxygen separator to bypass the oxygen passed through the oxygen separator from the exhaust gas. Do it. Bypass exhaust passages can be connected to the exhaust pipe at the bottom of the UCC and / or the interior of the vehicle. When connected to the exhaust pipe at the bottom of the UCC, the oxygen passing through the oxygen separator is discharged out of the body along with the exhaust gas at the bottom of the UCC and the interior of the body. When connected to the oxygen passing through the oxygen separation unit is supplied to the interior of the vehicle body can be used to purify the indoor air.

On the other hand, the exhaust gas discharged from the lean-burn engine may have an exhaust gas composition ratio equivalent to that emitted from the engine operating at a theoretical air-fuel ratio after some oxygen is removed by the oxygen separation unit. The reduced exhaust gas is supplied to the nitrogen oxide reduction catalyst 40 and purified and then discharged out of the vehicle body. Nitrogen oxide reduction catalysts are catalysts containing a material capable of reducing nitrogen oxides. The type and form thereof are not particularly limited, and are generally three-way catalysts when the lean-burn engine is a gasoline engine. If the lean-burn engine is a diesel engine, it is a Selective Catalyst Reduction (SCR) catalyst. Among them, since the SCR catalyst of the diesel engine has a separate reducing agent, the nitrogen oxide reduction device of the lean-burn engine exhaust gas according to the present invention will be more useful for the three-way catalyst of the gasoline engine. In addition, the nitrogen oxide reduction catalyst is preferably an underfloor catalytic converter (UCC) installed below the vehicle floor, because the general exhaust gas is discharged after the nitrogen oxide is finally reduced.

1 schematically shows a device for reducing nitrogen oxides of lean-burn engine exhaust gas according to an embodiment of the present invention, and FIG. 2 shows a device for reducing nitrogen oxides of lean-burn engine exhaust gas according to another embodiment of the present invention. It is shown schematically.

3 is a cross-sectional view when the oxygen of the exhaust gas passes through the monolithic porous support and the oxygen ion conductor thin film having a plurality of exhaust gas flow paths.

4 schematically illustrates a process in which the oxygen ion conductor selectively separates only oxygen.

Claims (11)

An oxygen separator formed on an exhaust pipe between an exhaust manifold of the lean-burn engine and a catalyst having a nitrogen oxide reduction function, and selectively separating only oxygen of exhaust gas discharged from the lean-burn engine; A bypass exhaust passage formed on the side of the exhaust pipe and in communication with the oxygen separator to bypass the oxygen passing through the oxygen separator from the exhaust gas; And It includes; nitrogen oxide reduction catalyst, The nitrogen separation unit of the lean-burn engine exhaust gas, characterized in that the oxygen separation unit is composed of a porous support and a thin film of oxygen ion conductor coated on the porous support to reduce oxygen of the exhaust gas to oxygen ions and selectively permeate only the reduced oxygen ions. Oxide reduction device. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to claim 1, wherein the oxygen separation part is formed by forming a part of the exhaust pipe or formed in an exhaust passage inside the exhaust pipe. 3. The lean-burn engine according to claim 2, wherein the oxygen separator formed by forming a part of the exhaust pipe has a sheet-like porous support and a thin film of oxygen ion conductor coated on the porous support is an inner surface of the exhaust pipe. Nitrogen oxide reduction device. The oxygen separation unit of claim 2, wherein the oxygen separation unit formed in the exhaust passage inside the exhaust pipe includes a monolith-type porous support having a plurality of exhaust gas flow paths in an exhaust gas flow direction, and a thin film of oxygen ion conductor coated on the surface of the exhaust gas flow path. An apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas, which is configured. 3. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to claim 2, further comprising a heating element for controlling the temperature of the oxygen ion conductor. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to claim 2, wherein the nitrogen oxide reduction catalyst is an underfloor catalytic converter (UCC) installed directly under the oxygen separation unit. The oxygen ion conductor according to any one of claims 1 to 6, wherein the oxygen ion conductor is yttria stabilized zirconia (YSZ), ceria (CeO ₂ ) -based oxide, yttria stabilized bismuth oxide, or oxygen deficient type. Apparatus for reducing nitrogen oxides of lean-burn engine exhaust gases, characterized in that it is at least one material selected from the group consisting of Perovskite. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to claim 7, wherein the oxygen deficient perovskite is represented by the following Chemical Formula 1. [Formula 1] Ln _{1 - x} A _x Co _y B _{1 - y} O _{1 -δ} (where Ln is lanthanum or gadolindium, A is any one selected from strontium, barium, calcium, sodium, and cobalt, B is iron, Any one selected from copper and nickel, wherein 0 ≦ x <1, ≦ 1, 0 ≦ y <1, and 0 ≦ δ ≦ 1) The nitrogen oxide reduction device for lean-burn engine exhaust gas according to any one of claims 1 to 6, wherein the oxygen ion conductor is maintained at a temperature of 700 to 1000 ° C. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to any one of claims 1 to 6, wherein the oxygen passing through the oxygen separation unit is supplied to the interior of the vehicle body through a bypass exhaust passage. The apparatus for reducing nitrogen oxides of lean-burn engine exhaust gas according to any one of claims 1 to 6, wherein the nitrogen oxide reduction catalyst is a three-way catalyst or an SCR catalyst.

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