JP2013119773A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress occurrence of corrosion in an outer pipe of an exhaust emission control device having a double-pipe structure exhaust pipe.SOLUTION: An exhaust emission control device comprises: an SCR catalyst that is arranged in an exhaust pipe of an engine, receives supply of ammonia generated from a urea aqueous solution, and selectively reduces and purifies NOx in exhaust gas; and an injection nozzle that injects and supplies the urea aqueous solution to the exhaust upstream of the SCR catalyst. At least a part of an exhaust pipe 30 located between the injection nozzle and the SCR catalyst is formed into a double pipe structure composed of an inner pipe 36 and an outer pipe 38. Ends, located at least at the exhaust upstream end, of both ends of the inner pipe 36 and the outer pipe 38 are welded to each other over the whole circumference via a flange 34. It is configured to prevent exhaust gas, mixed with the urea aqueous solution, from entering between the inner pipe 36 and the outer pipe 38 in order to prevent urea-derived deposits from being deposited on the inner peripheral surface of the outer pipe 38, thereby suppressing the occurrence of corrosion in the outer pipe 38.

Description

  The present invention relates to an exhaust purification device that selectively reduces and purifies nitrogen oxides (NOx) in engine exhaust.

  As a catalyst purification system for removing NOx in engine exhaust, an exhaust purification device described in JP-A-2005-226528 (Patent Document 1) has been proposed. Such an exhaust purification device injects and supplies a liquid reducing agent or a precursor thereof according to the engine operating state to the exhaust upstream of an SCR (Selective Catalytic Reduction) catalyst disposed in an exhaust pipe. A reduction agent is selectively reduced to reduce NOx into harmless components. Further, in such an exhaust purification device, the exhaust pipe into which the liquid reducing agent or its precursor is injected and supplied includes an inner pipe and an outer pipe in order to suppress the liquid reducing agent or its precursor from being cooled and deposited. Double pipe structure is adopted.

JP 2005-226528 A

  By the way, in an exhaust emission control device mounted on a vehicle, a urea aqueous solution that is excellent in safety and handleability may be used as a precursor of a liquid reducing agent. In this case, the exhaust gas mixed with the urea aqueous solution passes not only between the inner tube and the outer tube but also between the inner tube and the outer tube. Since the outer peripheral surface of the outer tube is in contact with the outside air, the temperature is likely to be lower than that of the inner tube. When the temperature of the outer tube becomes less than about 200 ° C. according to the engine operating condition, etc., the urea aqueous solution droplets adhering to the inner peripheral surface of the outer tube generate biuret while generating ammonia. It will be deposited on the inner peripheral surface of. The biuret deposited on the inner peripheral surface of the outer tube generates cyanuric acid and ammonia when the exhaust gas temperature rises as the engine operating state changes. Ammonia is corrosive to metals, and therefore, when its concentration is locally high, there is a risk of corroding the outer tube, particularly the welded portion applied thereto. Incidentally, the corrosion of the outer tube is not limited to biuret, and may occur in other precipitates derived from urea.

  Therefore, an object of the present invention is to provide an exhaust emission control device that suppresses the occurrence of corrosion of the outer tube.

  The exhaust purification device is disposed in the exhaust pipe of the engine, receives the supply of ammonia generated from the urea aqueous solution, selectively reduces and purifies NOx in the exhaust, and injects the urea aqueous solution upstream of the SCR catalyst exhaust And an injection nozzle to be supplied. And at least a part of the exhaust pipe positioned between the injection nozzle and the SCR catalyst has a double pipe structure consisting of an inner pipe and an outer pipe, and at least of both ends of the inner pipe and the outer pipe. End portions located at the exhaust upstream end portion are welded to each other over the entire circumference.

  By preventing the exhaust gas mixed with urea aqueous solution from entering between the inner and outer pipes and making it difficult for urea-derived precipitates such as biuret to deposit on the inner peripheral surface of the outer pipe, corrosion of the outer pipe is prevented. Can be suppressed.

Top view of exhaust purification system Perspective view of exhaust purification device Explanatory drawing of the inside of the exhaust purification device The concrete structure of a communicating pipe is shown, (A) is a longitudinal cross-sectional view, (B) is the side view seen from the arrow X direction in (A).

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.
1 to 3 show an example of an exhaust emission control device.
The exhaust purification device communicates the first casing 10 having a cylindrical shape, the second casing 20 having the same cylindrical shape, and the far ends of the first casing 10 and the second casing 20 with each other. A communication pipe 30 to be provided. The first casing 10 and the second casing 20 are arranged in close proximity to each other so that the axis 10A and the axis 20A are substantially parallel (appears to be parallel in appearance. The same applies hereinafter). . The communication pipe 30 is also piped such that its axis 30A is substantially parallel to the axis 10A of the first casing 10 and the axis 20A of the second casing 20. Here, the 1st housing | casing 10, the 2nd housing | casing 20, and the communicating pipe 30 correspond to some exhaust pipes of an engine.

The first casing 10 has an inlet 12 at the exhaust upstream end and an outlet 14 at the exhaust downstream end. In the case of this embodiment, the inlet 12 is formed at the end face of the exhaust upstream end, and the outlet 14 is formed at the side of the exhaust downstream end. A DOC (Diesel Oxidation Catalyst) 16 that oxidizes at least NO (nitrogen monoxide) to NO ₂ (nitrogen dioxide) and PM (Particulate Matter) are collected between the inlet 12 and the outlet 14. A diesel particulate filter (Diesel Particulate Filter, hereinafter referred to as “DPF”) 18 to be removed is housed with the DOC 16 at the exhaust upstream side.

  The second casing 20 has an inlet 22 at the exhaust upstream end and an outlet 24 at the exhaust downstream end. In the case of this embodiment, both the inflow port 22 and the discharge port 24 are formed on the end side surface. An SCR catalyst 26 that selectively reduces and purifies NOx using ammonia as a reducing agent and an oxidation catalyst 28 that oxidizes ammonia that has passed through the SCR catalyst 26 are provided between the inlet 22 and the outlet 24. Is stored with the exhaust upstream side.

  The communication pipe 30 is the exhaust downstream downstream end of the first casing 10, which is the far ends of the first casing 10 and the second casing 20, in other words, the far ends positioned on opposite sides of each other. And the inflow port 22 at the exhaust upstream end of the second housing 20 are in communication with each other. Accordingly, engine exhaust flows into the first housing 10 from the inlet 12, passes through the DOC 16 and the DPF 18, and enters the communication pipe 30 from the outlet 14. Then, the exhaust gas flows into the second housing 20 from the inlet 22 through the communication pipe 30, passes through the SCR catalyst 26 and the oxidation catalyst 28, and is discharged from the outlet 24. That is, the exhaust passage extending from the first housing 10 to the second housing 20 via the communication pipe 30 is folded once by the communication pipe 30.

  The communication pipe 30 is a straight straight pipe having bent portions formed at both ends for connection between the outlet 14 of the first casing 10 and the inlet 22 of the second casing 20. An injection nozzle 32 is attached to a bent portion with respect to the outlet 14, and an aqueous urea solution is injected and supplied into a straight pipe. Thereby, the linear length required for the uniform diffusion of the urea aqueous solution into the exhaust gas is secured. In order to promote uniform diffusion of the urea aqueous solution, for example, a mesh-like diffusion member may be installed in the communication pipe 30.

  According to such an exhaust purification device, since the first casing 10 and the second casing 20 are arranged close to each other in parallel, the silhouette has a compact size comparable to that of a square muffler. Further, since the far ends of the first casing 10 and the second casing 20 are communicated with each other by the communication pipe 30, the first casing 10 and the first casing 10 and the second casing 20 are secured while ensuring the tube length necessary for uniform diffusion of the urea aqueous solution. The communication pipe 30 can be accommodated in the silhouette of the second casing 20, and the size in the length direction from the upstream side of the exhaust to the downstream side has been successfully reduced.

  As shown in FIG. 4, both end portions of the straight portion sandwiched between the bent portions of the communication pipe 30 are detachably fastened to the bent portions via flanges 34. A straight portion that can be attached and detached by the flange 34, in other words, at least a part of the exhaust pipe positioned between the injection nozzle 32 and the SCR catalyst 26 improves the heat retaining property of the communication pipe 30, and is supplied by injection from the injection nozzle 32, for example. In order to promote hydrolysis of the aqueous urea solution, a double tube structure comprising an inner tube 36 and an outer tube 38 arranged concentrically is formed. Specifically, the inner pipe 36 and the outer pipe 38 have a stepped shape in which the exhaust upstream end has a large diameter while the exhaust downstream end has a small diameter. Then, both ends of the outer pipe 38 are annularly formed by, for example, TIG (Tungsten Inert Gas) welding that provides a high-quality and beautiful weld bead (weld trace) so that the exhaust gas flowing through the communication pipe 30 does not leak to the outside. The entire surface of each of the shaped flanges 34 is welded. Further, both ends of the inner tube 36 are formed by, for example, TIG welding so that a sealed space is formed by the outer peripheral surface of the inner tube 36, the inner peripheral surface of the outer tube 38, and one surface of the flange 34. The entire surface is welded.

  Therefore, since the straight portion of the communication pipe 30 is a sealed space between the inner pipe 36 and the outer pipe 38, the exhaust gas mixed with the urea aqueous solution is improved in heat retention while the inner pipe 36 and the outer pipe 38 are mixed. Will not be introduced between. For this reason, precipitates derived from urea such as biuret are hardly deposited on the inner peripheral surface of the outer tube 38, and the absolute amount of ammonia generated from the precipitates is reduced even when the exhaust temperature rises. 38 corrosion can be suppressed. In addition, since both ends of the inner pipe 36 are welded to the inner peripheral surface of the flange 34, the welding operation is easily performed from the side, and the number of welding steps can be reduced.

  In order to suppress corrosion of the outer pipe 38, at least at the exhaust upstream end of the straight portion of the communication pipe 30, the end of the inner pipe 36 and the end of the outer pipe 38 are connected via the flange 34. What is necessary is just to weld each other over the perimeter. Further, the inner pipe 36 and the outer pipe 38 may be directly welded at the end portions without the flange 34 interposed therebetween. Further, when the straight portion of the communication path 30 is integrated with the bent portion (cannot be attached / detached) and the inner tube 36 is fixed to the outer tube 38 by a known means, for example, the inner tube What is necessary is just to expand the diameter of the edge part of 36 and to weld the front-end | tip part to the inner peripheral surface of the outer tube | pipe 38 all over. In addition, the inner tube 36 and the outer tube 38 may not have a stepped shape but may have a straight tube shape having the same diameter over the entire length.

  The exhaust purification device is not limited to the one shown in FIGS. 1 to 3, and may be one in which the DOC 16, the DPF 18, the SCR catalyst 26, and the oxidation catalyst 28 are linearly arranged. Further, the exhaust purification device only needs to have at least the SCR catalyst 26. Furthermore, the welding method is not limited to TIG welding, and arc welding such as MIG (Metal Inert Gas) welding and MAG (Metal Active Gas) welding may be used.

20 Second casing 26 SCR catalyst 30 Communication pipe 32 Injection nozzle 34 Flange 36 Inner pipe 38 Outer pipe

Claims (3)

An SCR catalyst that is disposed in the exhaust pipe of the engine, receives ammonia supplied from an aqueous urea solution, and selectively reduces and purifies nitrogen oxides in the exhaust;
An injection nozzle for supplying an aqueous urea solution to the exhaust upstream of the SCR catalyst;
Comprising
At least a part of the exhaust pipe located between the injection nozzle and the SCR catalyst has a double pipe structure composed of an inner pipe and an outer pipe, and of both ends of the inner pipe and the outer pipe. , At least the ends located at the exhaust upstream end are welded to each other over the entire circumference,
An exhaust purification device characterized by that. The inner pipe and the outer pipe can be attached to and detached from the exhaust pipe by a flange, and at least an end portion of the inner pipe and the outer pipe located at the exhaust upstream end is interposed via the flange. Welded around the entire circumference,
The exhaust emission control device according to claim 1. The welding is TIG welding.
The exhaust emission control device according to claim 1 or 2, characterized in that