JP2020090921A - Exhaust emission control device - Google Patents

Abstract

To suppress peeling of a coating material in a selective reduction type catalyst.SOLUTION: An exhaust emission control device of the present invention includes a selective reduction type catalyst and a reducing agent addition valve disposed in an exhaust passage of an internal combustion engine. The selective reduction type catalyst reduces nitrogen oxide in exhaust gas. The reducing agent addition valve is an addition valve for supplying a reducing agent to the selective reduction type catalyst. A passage through which exhaust gas passes in the selective reduction type catalyst comprises a base material. The surface of the base material includes a coated part in which a catalyst coating is formed and an uncoated part in which the catalyst coating is not formed. The uncoated part is formed at a position including a range where the reducing agent supplied from the reducing agent addition valve reaches in a liquid state. In contrast, the coated part is formed at a position outside the range where the reducing agent reaches in the liquid state and on the side opposite to the reducing agent addition valve relative to the uncoated part.SELECTED DRAWING: Figure 2

Description

The present invention relates to an exhaust emission control device having a selective reduction type catalyst that reduces nitrogen oxides in exhaust gas with a reducing agent supplied from a reducing agent supply device.

An exhaust gas purification device is known that includes a selective reduction catalyst and an addition valve that adds a reducing agent to the selective reduction catalyst. In this exhaust gas purification device, the selective reduction catalyst adsorbs the reducing agent added from the addition valve, and reduces and purifies nitrogen oxides (hereinafter also referred to as “NOx”) in the exhaust gas by the adsorbing reducing agent.

The gas passage of the selective reduction catalyst is generally formed by coating a base material forming the gas passage with a coating material containing a NOx adsorption catalyst or a NOx adsorption reduction catalyst. For example, Patent Document 1 describes an exhaust gas purification device having a selective reduction catalyst in which a catalyst carrier as a base material is coated with a NOx adsorption catalyst layer and an SCR catalyst layer. In the selective reduction catalyst of Patent Document 1, the coating amount of each catalyst layer increases from the upstream side to the downstream side in the SCR catalyst layer and conversely decreases in the NOx adsorption catalyst layer.

JP, 2014-152661, A Japanese Patent No. 5888312

When the reducing agent is supplied to the selective reduction catalyst, the reducing agent may come into contact with the coating material in a droplet state before being evaporated. In this case, the coating material may peel off from the base material due to the shearing force of the reducing agent. Further, when the liquid reducing agent adheres to the coating material, the water-soluble binder contained in the coating material melts, the adhesive strength of the coating material to the base material decreases, and the coating material is more easily peeled off. The peeling of the coating material deteriorates the purification performance of the exhaust gas purification device, and the metal oxide in the peeled coating material may be discharged to the downstream side together with the exhaust gas, which is not preferable.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a technique for suppressing peeling of a coating material in a selective reduction catalyst.

In one aspect of the present invention, an exhaust emission control device is provided. The exhaust gas purification device includes a selective reduction catalyst and a reducing agent addition valve arranged in an exhaust passage of an internal combustion engine. The selective reduction catalyst reduces nitrogen oxide in exhaust gas. The reducing agent addition valve is an addition valve for supplying the reducing agent to the selective reduction catalyst.

The passage through which the exhaust gas in the selective reduction catalyst passes is made of a base material. The surface of the base material includes a coated portion where the catalyst coat is formed and an uncoated portion where the catalyst coat is not formed. Here, the uncoated portion is formed at a position including a range where the reducing agent supplied from the reducing agent addition valve reaches in a liquid state. On the other hand, the coated portion is formed at a position outside the range where the reducing agent reaches in a liquid state, and at a position opposite to the reducing agent addition valve with respect to the uncoated portion.

According to the exhaust emission control device of the present invention, the base material is not coated with the catalyst coating within the range where the liquid reducing agent reaches the selective reduction catalyst. As a result, the peeling of the catalyst coat due to the direct impact of the liquid reducing agent can be suppressed.

It is a figure which shows typically the whole structure of the system of embodiment of this invention. It is a figure which shows typically the structure of the SCR catalyst of embodiment of this invention. It is the figure which expanded and represented a part of AA cross section in FIG. 2 of the SCR catalyst of embodiment of this invention. It is the figure which expanded and represented a part of BB cross section in FIG. 2 of the SCR catalyst of embodiment of this invention. It is a figure which shows an example of the time change of the evaporation rate of the urea water added from the urea addition valve in embodiment of this invention. It is a figure which shows an example of the time change of the droplet arrival distance of the urea water added from the urea injection valve in embodiment of this invention. It is a figure which shows an example of the specific relationship between the distance from the end surface of an SCR catalyst, and the evaporation rate of urea water. It is a figure explaining the injection state of urea water from a urea addition valve in the urea SCR system of an embodiment of the invention. In the urea SCR system of an embodiment of the present invention, it is a figure for explaining the state of the surface of the SCR catalyst when urea water is injected from a urea addition valve. In the conventional urea SCR system, it is a figure for demonstrating the surface state of the SCR catalyst when urea water is injected from a urea addition valve. It is a figure which shows typically the other structural example of the system of embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and the description thereof will be simplified or omitted.

FIG. 1 is a diagram schematically showing the overall configuration of the system according to the first embodiment of the present invention. As shown in FIG. 1, the system of this embodiment has an engine 2. The number of cylinders of the engine 2 and the arrangement thereof are not limited. The engine 2 is a diesel engine that includes an injector and self-ignites fuel injected from the injector in a cylinder.

In the exhaust passage 4 of the engine 2, a DOC (Diesel Oxidation Catalyst) 6 and a DPF (Diesel Particulate Filter) 8 are arranged adjacent to each other in order from the upstream side of the exhaust. The DOC 6 carries an oxidation catalyst that oxidizes HC, which is unburned gas in the exhaust gas. The DPF 8 is a member that collects PM (fine particle components) such as soot in exhaust gas, and is made of, for example, a porous ceramic.

A urea SCR system having an SCR (Selective Catalytic Reduction) catalyst, which is a selective reduction catalyst, is installed downstream of the DPF 8 in the exhaust passage 4. The urea SCR system includes an SCR catalyst 10 and a reducing agent supply device 20 that supplies a reducing agent to the SCR catalyst 10.

The reducing agent supply device 20 has a urea addition valve 22. The exhaust passage 4 on the downstream side of the DPF 8 and on the upstream side of the SCR catalyst 10 has a curved portion 4a in which the flow of the exhaust gas changes approximately 90 degrees, and the urea addition valve 22 has its tip inserted into the exhaust passage 4. In the bent state, it is installed on the bending portion 4a. The urea addition valve 22 is connected via a supply pipe 24 to a reducing agent tank 26 in which urea water is stored. A pressure pump 28 is installed in the supply pipe 24. The urea addition valve 22 and the pressure feed pump 28 are electrically connected to an ECU 30 that is a control device.

The urea addition valve 22 injects urea water toward the downstream side of the urea addition valve 22 in the exhaust passage 4, that is, toward the SCR catalyst 10 side. The addition of urea water from the urea addition valve 22 is controlled by the ECU 30. Urea water is an ammonia precursor, and when the urea water is hydrolyzed by exhaust heat, ammonia that is a reducing agent (hereinafter also referred to as “NH ₃ ”) is generated.

The SCR catalyst 10 adsorbs NH _3, and the adsorbed NH ₃ and the nitrogen oxides in the exhaust gas (hereinafter also referred to as “NOx”) are expressed by, for example, the following formula 1, formula 2, or formula 3 Exhaust gas is reduced and purified by causing the reduction reaction shown.

FIG. 2 is an enlarged view of the SCR catalyst 10. As shown in FIG. 2, the SCR catalyst 10 of the present embodiment is divided into an upstream portion 11 and a downstream portion 12. FIG. 3 shows a part of the AA cross section in FIG. 2 in an enlarged manner. FIG. 4 shows an enlarged part of the BB cross section in FIG. That is, FIG. 3 shows a part of the cross section of the upstream portion 11, and FIG. 4 shows a part of the cross section of the downstream portion.

As shown in FIGS. 3 and 4, the SCR catalyst 10 has a monolith substrate (hereinafter simply referred to as “substrate”) 14 in which a plurality of cells 13 are arranged in a lattice shape or a honeycomb shape. have. As shown in FIG. 4, the cell 13 of the downstream portion 12 is formed by adhering the coating material 15 to the surface of the base material 14. However, as shown in FIG. The surface of the base material 14 is not coated.

The base material 14 is made of, for example, one or more materials selected from cordierite, ceramics such as SiC, and metal. The coating material 15 is composed of a porous material such as zeolite, a catalyst component adhered to the surface of the porous material, and a binder. The catalyst component is a catalyst that promotes the reduction reaction of NH ₃ and NOx, and is made of, for example, a metal oxide such as copper, iron, vanadium, molybdenum, or tungsten. The binder is for adhering the catalyst component to the porous body and for adhering the porous body to the base material 14. The SRC catalyst 10 may have a function as a filter.

5: is a figure which shows an example of the time change of the evaporation rate of the urea water injected from the urea addition valve 22, and FIG. 6 shows the droplet which the droplet of the urea water injected from the urea addition valve 22 reaches. It is a figure which shows an example of a time change of an arrival distance. As shown in FIG. 5, the evaporation rate of the urea water increases with the passage of time, and eventually the evaporation rate becomes 1 to complete the evaporation. In the example shown in FIG. 5, evaporation is complete at time th.

In the example shown in FIG. 6, the droplet of urea water reaches Lth by the time th when the evaporation is completed. Here, the droplet reach distance Lth is a distance from the urea addition valve 22. When the distance from the urea addition valve 22 to the tip surface of the SCR catalyst 10 is L0, the droplet arrival distance in the SCR catalyst 10 is Lh.

上記の数４及び数５式において、ｍは尿素質量、ｔは時間、ｋは熱伝導率、Ｃｐは比熱、Ｈは蒸発線熱、Ｔｇは周囲のガス温度、Ｔｄは液滴温度、Ｒｅはレイノルズ数、Ｄ０は初期液滴径であり、Ｋｂは蒸発速度定数である。 The specific evaporation rate can be obtained, for example, by calculating the evaporation amount from the droplet diameter D per unit time using the following equations 4 and 5.

In the above formulas 4 and 5, m is urea mass, t is time, k is thermal conductivity, Cp is specific heat, H is evaporation line heat, Tg is ambient gas temperature, Td is droplet temperature, and Re is The Reynolds number, D0, is the initial droplet diameter, and Kb is the evaporation rate constant.

Similarly, the specific liquid droplet arrival distance Lh in the SCR catalyst 10 can be obtained, for example, by the following formulas 6 to 8. In the equations 6 to 8, Lth is a droplet arrival distance from the urea addition valve 22, L0 is a distance from the tip of the urea addition valve to the end surface of the SCR catalyst 10 on the urea addition valve 22 side, ρf is a liquid density, ρa is the atmosphere density, dn is the nozzle hole diameter of the urea addition valve, w0 is the spray initial velocity, θ is the spray angle, Pr is the injection pressure, and Pa is the atmospheric pressure.

In the present embodiment, the upstream portion 11 and the downstream portion are arranged so that the entire range within the droplet arrival distance Lh obtained by using the above equations 6 to 8 is included in the upstream portion 11 of the SCR catalyst 10. A boundary with the section 12 is set. That is, the length Lf of the upstream portion 11 in the gas flow direction is set to be at least larger than the droplet arrival distance Lh. As a result, the downstream portion 12 is located at least downstream of the position of the droplet arrival distance Lh.

FIG. 7 is a diagram showing an example of a specific relationship between the distance from the end surface of the SCR catalyst and the evaporation rate obtained according to the above equation. In the example of FIG. 7, the total length of the SCR catalyst 10 is 100 mm, the evaporation rate is 1, that is, the droplet reach distance Lh at which evaporation is completed is about 25 mm. Therefore, in this example, for example, the length Lf of the upstream portion 11 is set to the length of the frame 1/4 of the total catalyst length Lb (=100 mm).

FIG. 8 is a diagram illustrating the injection state of urea water from the urea addition valve. 9 and 10 are diagrams for explaining the surface state of the SCR catalyst when urea water is added, FIG. 9 is the surface state of the SCR catalyst 10, and FIG. 10 is a conventional example as a comparative example. It is a figure showing the state of the surface of the SCR catalyst of.

When the collision plate and the mixer are not arranged between the urea addition valve 22 and the SCR catalyst 10 as in the present embodiment, and the distance between the urea addition valve 22 and the SCR catalyst 10 is short. The urea water injected from the urea addition valve 22 does not evaporate and is likely to directly hit the end surface of the SCR catalyst 10 and the upstream portion near the end surface.

However, in the present embodiment, the portion of the SCR catalyst 10 having the length Lf from the end surface of the urea addition valve 22 on the side close to the injection hole is the uncoated portion where the coating material 15 is not applied (that is, the upstream portion 11). Is. The length Lf is set to be longer than the droplet arrival distance Lh in the SCR catalyst 10. Therefore, as shown in FIG. 9, the droplets are completely evaporated before reaching the coating portion (that is, the downstream portion 12) to which the coating material 15 is applied, and the droplets are not directly hit on the coating material 15. Avoided.

On the other hand, in the case of the conventional structure in which the coating material 15 is applied to the entire surface of the base material of the SCR catalyst, as shown in FIG. 10, a droplet of urea water directly hits the coating material 15. Therefore, the coating material 15 may be peeled off by the shearing force of the direct hit. Further, when the urea water remains in a liquid state on the surface of the SCR catalyst, the binder contained in the coating material 15 dissolves in the urea water, and the adhesion of the coating material decreases. If the urea water is further directly hit in the state where the adhesion of the coating material 15 is reduced, the coating material 15 is more likely to be peeled off.

As described above, according to the present embodiment, the upstream portion 11 of the SCR catalyst 10 is an uncoated portion to which the coating material 15 is not applied, whereby peeling or dissolution of the coating material 15 can be suppressed. .. As a result, the purification performance of the SCR catalyst 10 can be maintained at a constant level, and the outflow of the coating material 15 to the downstream side can be suppressed.

In the above embodiment, the case where the urea addition valve 22 is installed on the upstream side of the SCR catalyst 10 has been described. However, the urea addition valve 22 may be installed downstream of the SCR catalyst 10. FIG. 11 is a schematic diagram showing the configuration of a system in which the urea addition valve 22 is installed on the downstream side of the SCR catalyst 10.

In this configuration example, the exhaust passage 4 has a curved portion 4b on the downstream side of the exhaust flow from the SCR catalyst 10 in which the exhaust flow changes by approximately 90 degrees. The urea addition valve 22 is arranged with the injection hole at the tip inserted in the curved portion 4b. That is, the urea water is injected from the downstream side of the exhaust flow toward the SCR catalyst 10 on the upstream side.

In this configuration, the cell 13 of the upstream portion 16 of the SCR catalyst 10 has a configuration in which the coating material 15 is adhered to the base material 14, and the cell 13 of the downstream portion 17 has only the base material 14. Here, the length (Lb-Lf) of the downstream portion 17 in the gas flow direction is determined in the same manner as the length Lf of the upstream portion 11 when the above-described urea addition valve 22 is installed on the upstream side. That is, with the distance from the urea addition valve 22 to the downstream end surface of the SCR catalyst 10 being L0, the droplet arrival distance Lh with respect to the downstream end surface in the SCR catalyst 10 is calculated, and the droplet arrival distance Lh is calculated. Also, the distance of the downstream portion is set so that the length (Lb-Lf) of the downstream portion 17 becomes long. Thereby, peeling of the coating material can be avoided as in the case where the urea addition valve 22 is installed on the upstream side.

In the above embodiments, when the number of each element, the number, the amount, the range, or the like is referred to, the reference is made unless otherwise specified or in principle specified by the number. The invention is not limited in number. Further, the structure and the like described in this embodiment are not necessarily essential to the present invention, unless otherwise specified or clearly specified in principle.

2 engine 4 exhaust passage 4a, 4b curved portion 6 DOC
8 DPF
10 catalyst 11 upstream part 12 downstream part 13 cell 14 base material 15 coating material 16 upstream part 17 downstream part 20 reducing agent supply device 22 urea addition valve 24 supply pipe 26 reducing agent tank 28 pressure pump

Claims (1)

A selective reduction catalyst arranged in the exhaust passage for reducing nitrogen oxides in the exhaust;
A reducing agent addition valve for supplying a reducing agent to the selective reduction catalyst,
Equipped with
The surface of the base material forming the passage through which the exhaust gas in the selective reduction catalyst passes,
Including a coated portion where the catalyst coat is formed and an uncoated portion where the catalyst coat is not formed,
The uncoated portion is formed at a position including a range where the reducing agent supplied from the reducing agent addition valve reaches in a liquid state,
The coated portion is located outside the range, and is formed at a position opposite to the reducing agent addition valve with respect to the uncoated portion.
An exhaust purification device characterized by the above.

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