JP2013124651A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of preventing ammonia not contributing to the purification of NOx from being released as it is into the atmosphere while the device being of space-saving as having flexibility of its disposition.SOLUTION: The exhaust emission control device is equipped with a reflux passage 8 whose one end 8a is opened toward a portion R of the outlet end face 6a of an SCR catalyst 6, while the other end 8b thereof is opened toward a portion R' of the inlet end face 6b of the SCR catalyst 6, with these one end 8a and the other end 8b of the reflux passage 8 being located at different positions as viewed from the direction of the central axis O that connects the outlet end face 6a of the SCR catalyst 6 and the inlet end face 6b, as the one end 8a of the reflux passage 8 is opened toward a portion P where the ammonia flow-out rate at the outlet end face 6a of the SCR catalyst 6 is greater than a prescribed value. The ammonia that has flowed out by a slip from the portion R of the outlet end face 6a of the SCR catalyst 6 is circulated through the reflux passage 8 to be flowed back to the portion R' of the inlet end face 6b of the SCR catalyst 6.

Description

  The present invention relates to an exhaust gas purification device, and more particularly, to an exhaust gas purification device that purifies nitrogen oxide (NOx) contained in exhaust gas of a diesel engine using an SCR (Selective Catalytic Reduction) catalyst.

  An exhaust gas purification device that purifies NOx contained in exhaust gas of a diesel engine using an SCR catalyst has been developed. In such an exhaust gas purification device, an SCR catalyst is provided in the middle of an exhaust path from the engine, and urea water is added from the upstream of the SCR catalyst. Ammonia generated by hydrolysis of urea water is adsorbed inside the SCR catalyst, and NOx is purified into nitrogen and water by a reduction reaction between ammonia and NOx.

  In the exhaust gas purification apparatus as described above, ammonia adsorbed on the SCR catalyst may desorb and flow downstream without causing a reduction reaction with NOx. Such a phenomenon is called “ammonia slip”. It is. Since ammonia itself is a harmful substance, it must be avoided as much as possible that ammonia that does not contribute to NOx purification flows downstream of the SCR catalyst and is released into the atmosphere.

Conventionally, an oxidation catalyst has been provided downstream of the SCR catalyst in order to render ammonia that has flowed downstream of the SCR catalyst due to slippage harmless.
In Patent Document 1, the exhaust path upstream of the SCR catalyst is bent, and a flow path changing plate is provided at the inlet of the SCR catalyst so that the exhaust gas is uniformly supplied to each part of the SCR catalyst. According to the above, matters for avoiding ammonia slip are described.

JP 2011-99415 A

  However, the conventional method of providing an oxidation catalyst for ammonia purification downstream of the SCR catalyst increases the size of the exhaust gas purification device. Further, in the method of bending a specific portion of the exhaust path as described in Patent Document 1, the exhaust path cannot be freely routed, and the degree of freedom of arrangement of the exhaust gas purification device is limited. . These restrictions can be a serious problem especially when the exhaust gas purifying device is mounted on a vehicle.

  The present invention has been made in order to solve such problems. It is considered that ammonia that does not contribute to the purification of NOx while being space-saving and has a degree of freedom in arrangement is directly released into the atmosphere. It is an object of the present invention to provide an exhaust gas purification device that can be prevented.

  In order to solve the above-described problem, an exhaust gas purification apparatus according to the present invention is an exhaust gas purification apparatus in which an SCR catalyst is provided in the middle of an exhaust path from an engine, and a reducing agent is added from upstream of the SCR catalyst. Is opened to a part of the outlet end face of the SCR catalyst, and the other end of the reflux path is provided to open to a part of the inlet end face of the SCR catalyst. It is provided at different positions when viewed from the direction of the central axis connecting the outlet end face and the inlet end face of the catalyst.

  Preferably, one end and the other end of the reflux path are provided at point-symmetric positions with respect to the center axis as viewed from the direction of the center axis of the SCR catalyst.

  One end of the reflux path may be provided so as to open toward a portion where the ammonia outflow amount at the outlet end face of the SCR catalyst is larger than a predetermined value.

  One end of the reflux path may be provided so as to open toward a portion where the nitrogen oxide outflow amount at the outlet end face of the SCR catalyst is larger than a predetermined value.

  The exhaust gas purifying apparatus according to the present invention can prevent the ammonia that does not contribute to the purification of NOx from being released into the atmosphere as it is while saving space and having a degree of freedom in arrangement.

It is a figure which shows the structure of the exhaust-gas purification apparatus which concerns on Embodiment 1 of this invention. (A) is a perspective view showing the peripheral structure of the SCR catalyst in the exhaust gas purification apparatus according to Embodiment 1 of the present invention, and (B) is a perspective view of the view of (A) seen from the direction of the arrow X. It is. (A) is a perspective view which shows the periphery structure of the SCR catalyst in the exhaust gas purification apparatus which concerns on Embodiment 2 of this invention, (B) is the perspective view which looked at the figure of (A) from the arrow X direction. It is. It is a figure which shows the modification of the exhaust-gas purification apparatus which concerns on Embodiment 1 of this invention.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 shows the configuration of an exhaust gas purification apparatus according to Embodiment 1 of the present invention.
The exhaust gas purification device 1 includes an oxidation catalyst 4, a DP filter 5, and an SCR catalyst 6 in the middle of an exhaust path 3 through which exhaust gas from the diesel engine 2 flows. Further, an addition nozzle 7 is provided between the DP filter 5 and the SCR catalyst 6 to communicate with a urea water tank (not shown) and add urea water as a reducing agent from the upstream of the SCR catalyst 6.

The oxidation catalyst 4 oxidizes a part of nitrogen monoxide (NO) contained in the exhaust gas to form nitrogen dioxide (NO ₂ ). The DP filter 5 removes particulate matter (PM) contained in the exhaust gas. Ammonia produced by hydrolysis of urea water added from the addition nozzle 7 is adsorbed on the SCR catalyst 6, and NOx is purified into nitrogen and water by a reduction reaction between ammonia and NOx.

  Next, the peripheral structure of the SCR catalyst 6 and its operation in the exhaust gas purifying apparatus according to Embodiment 1 will be described with reference to FIG. 2A is a perspective view showing the peripheral structure of the SCR catalyst 6 in the exhaust gas purification device, and FIG. 2B is a perspective view of the view of FIG. .

  The SCR catalyst 6 has a cylindrical shape formed by a honeycomb structure and is accommodated in the housing 7. One end 8 a of a reflux path 8 that opens toward a part of the outlet end surface 6 a of the SCR catalyst 6 is connected to the outlet side 7 a of the housing 7, and the inlet end surface of the SCR catalyst 6 is connected to the inlet side 7 b of the housing 7. The other end 8b of the reflux path 8 that opens toward a part of 6b is connected. Note that support means and a partition wall (not shown) are provided between the outer periphery of the SCR catalyst 6 and the inner periphery of the housing 7, and the SCR catalyst 6 is fixed to the housing 7 and the exhaust gas passes between the SCR catalyst 6 and the housing 7. I try not to pass.

  Ammonia slip in the SCR catalyst 6 does not occur uniformly within the SCR catalyst 6, but frequently occurs in a specific region determined by the shape of the exhaust passage 3, the direction in which gravity acts, and the like. In other words, the bent pipe is biased by centrifugal force in the exhaust flow, or is biased downward by gravity. For example, in the example shown in FIG. 2, a lot of ammonia slip occurs in the region V in the SCR catalyst 6, and a large amount of ammonia flows out from the portion R on the outlet end face 6 a of the SCR catalyst 6.

  In the exhaust gas purifying apparatus according to the first embodiment, a portion R in which the ammonia outflow amount at the outlet end surface 6a of the SCR catalyst 6 is larger than a predetermined value is specified in advance by experiment, and one end 8a of the reflux path 8 is It is provided so as to open toward the portion R. The ammonia flowing out from the portion R of the outlet end surface 6a of the SCR catalyst 6 due to the ammonia slip flows through the reflux path 8 and is returned to the inlet end surface 6b of the SCR catalyst 6. This prevents ammonia that does not contribute to NOx purification from flowing through the outlet of the housing 7 and being released into the atmosphere.

  Further, as shown in FIG. 2B, the one end 8a and the other end 8b of the reflux path 8 are in the direction of the central axis O connecting the outlet end face 6a and the inlet end face 6b of the SCR catalyst 6 (X of the arrow). Are provided at different positions as viewed from the direction. More specifically, the one end 8a and the other end 8b of the reflux path 8 are provided at point-symmetrical positions with the central axis O as a symmetric point when viewed from the direction of the central axis O of the SCR catalyst 6. . In FIG. 2A, when a large amount of ammonia slip occurs in the region V of the SCR catalyst 6, in the region V ′ that is symmetric with respect to the region V and the central axis O, ammonia is insufficient and NOx is reduced. Although there is a possibility that the purification cannot be performed sufficiently, in the exhaust gas purification apparatus according to the first embodiment, the ammonia flowing out from the portion R of the outlet end surface 6a of the SCR catalyst 6 due to the slip causes the inlet end surface 6b of the SCR catalyst 6 to flow. Reflux to part R ′. As a result, the shortage of ammonia in the region V ′ is resolved, and NOx can be sufficiently purified also in this region V ′.

  As described above, in the exhaust gas purifying apparatus according to Embodiment 1 of the present invention, one end 8a opens toward part R of the outlet end surface 6a of the SCR catalyst 6, and the other end 8b is the inlet of the SCR catalyst 6. A reflux path 8 is provided so as to open toward a part R ′ of the end face 6b. One end 8a and the other end 8b of the reflux path 8 are a center connecting the outlet end face 6a and the inlet end face 6b of the SCR catalyst 6. The one end 8a of the reflux path 8 opens toward the portion R where the ammonia outflow amount at the outlet end face 6a of the SCR catalyst 6 is larger than a predetermined value. The ammonia flowing out from the outlet end face 6a of the SCR catalyst 6 due to the slip flows through the reflux path 8 and is returned to the inlet end face 6b of the SCR catalyst 6. As a result, ammonia that does not contribute to NOx purification can be prevented from being released into the atmosphere as it is, and an oxidation catalyst for ammonia purification can be provided downstream of the SCR catalyst 6 as in the prior art. Since it is not necessary to bend a specific portion of the exhaust path as described in No. 1, it is possible to save space and to have a degree of freedom in arrangement as compared with the prior art.

  The one end 8a and the other end 8b of the reflux path 8 are provided at point-symmetrical positions with the central axis O as a symmetric point when viewed from the direction of the central axis O of the SCR catalyst 6. The ammonia flowing out from the portion R of the outlet end surface 6a of the No. 6 is refluxed to the portion R ′ in a point-symmetrical position with respect to the portion R and the central axis O on the input end surface 6b. As a result, the shortage of ammonia in the region V ′ where the ammonia slip is generated and the region V ′ located symmetrically with the central axis O as the symmetry point is resolved, and the NOx can be sufficiently purified also in this region V ′. become. As a result, the amount of urea water added is reduced in order to suppress the amount of ammonia released into the atmosphere without contributing to NOx purification, or the urea water is excessively added to increase the NOx purification rate. This eliminates the need for NOx purification.

Embodiment 2. FIG.
An exhaust gas purification apparatus according to Embodiment 2 of the present invention will be described with reference to FIG. Since the difference between the first and second embodiments is only the peripheral structure of the SCR catalyst 6 and its operation, only that part will be described. In the following description, the same reference numerals as those in FIG. 2 are the same or similar components.

  As described in the first embodiment, when a large amount of ammonia slip occurs in the region V of the SCR catalyst 6, ammonia is insufficient in the region V ′ that is point-symmetric with respect to the region V and the central axis O. NOx purification cannot be performed sufficiently. As a result, a large amount of NOx flows out from the portion P on the outlet end face 6a of the SCR catalyst 6.

  In the exhaust gas purifying apparatus according to the second embodiment, a portion P where the NOx outflow amount at the outlet end face 6a of the SCR catalyst 6 is larger than a predetermined value is specified in advance by experiment, and one end 208a of the reflux path 208 is It is provided so as to open toward the portion P. The NOx flowing out from the portion P of the outlet end surface 6a of the SCR catalyst 6 flows through the reflux path 208 and is returned to the portion P 'of the inlet end surface 6b of the SCR catalyst 6. As a result, a large amount of NOx is sent to the region V where a lot of ammonia slip occurs, and the reduction reaction between ammonia and NOx in the region V is sufficiently performed. This prevents Ammore that does not contribute to the purification of NOx from flowing through the outlet of the housing 7 as it is and being released into the atmosphere.

  As described above, in the exhaust gas purification apparatus according to Embodiment 2 of the present invention, the one end 208a of the recirculation path 208 is directed toward the portion P where the NOx outflow amount at the outlet end surface 6a of the SCR catalyst 6 is greater than a predetermined value. Open. Even if comprised in this way, it can prevent that ammonia which does not contribute to purification | cleaning of NOx is discharge | released in air | atmosphere as it is.

Other embodiments.
In the first and second embodiments, the reflux paths 8 and 208 are provided outside the housing 7, but the reflux paths may be provided inside the housing 7. In the example shown in FIG. 4, in the exhaust gas purifying apparatus according to the first embodiment, a reflux path 308 is provided inside the housing 7.
In the first and second embodiments, urea water is used as the reducing agent, but other reducing agents may be used.

  DESCRIPTION OF SYMBOLS 1 Exhaust gas purification apparatus, 2 Exhaust path | route, 6 SCR catalyst, 6a Outlet end surface of SCR catalyst, 6b Inlet end surface of SCR catalyst, 8, 208, 308 Return path, 8a, 208a, 308a End of return path, 8b, 208b, 308b The other end of the reflux path, the central axis that connects the outlet end face and the inlet end face of the OSCR catalyst, P a portion with a large nitrogen oxide outflow, and a portion with a large ammonia outflow.

Claims (4)

In an exhaust gas purification apparatus in which an SCR catalyst is provided in the middle of an exhaust path from an engine, and a reducing agent is added from upstream of the SCR catalyst,
A reflux path provided such that one end opens toward a part of the outlet end face of the SCR catalyst and the other end opens toward a part of the inlet end face of the SCR catalyst;
The exhaust gas purifying apparatus according to claim 1, wherein the one end and the other end of the reflux path are provided at different positions when viewed from a direction of a central axis connecting the outlet end face and the inlet end face of the SCR catalyst.   2. The exhaust gas purifying apparatus according to claim 1, wherein the one end and the other end of the recirculation path are provided at point-symmetric positions with respect to the central axis as viewed from the direction of the central axis of the SCR catalyst. .   The exhaust gas purification apparatus according to claim 1 or 2, wherein the one end of the reflux path is provided so as to open toward a portion where the ammonia outflow amount at the outlet end face of the SCR catalyst is larger than a predetermined value.   The exhaust gas purification device according to claim 1 or 2, wherein the one end of the reflux path is provided so as to open toward a portion where the nitrogen oxide outflow amount at the outlet end face of the SCR catalyst is larger than a predetermined value. .

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