JP2007218108A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device in which high concentration SO<SB>2</SB>gas is not generated in desulfurization of NOx occlusion/reduction catalyst. <P>SOLUTION: In the exhaust emission control device, the NOx occlusion/reduction catalyst 5 is disposed along an exhaust pipe 4, and fuel 8 can be added to an inlet side of the NOx occlusion/reduction catalyst 5. The exhaust emission control device comprises: a water spray means 18 disposed downstream of the NOx occlusion/reduction catalyst 5 and atomizing water 15 and spraying the water 15 into exhaust gas 3; and a mist catcher (a mist catch board 19, a rear wall part 20 of a desulfurization case 12, or a tail pipe 21) catching sulfuric acid mist downstream of the water spray means 18, and reacted with the sulfuric acid mist to form sulfide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust purification device using a NOx storage reduction catalyst.

  Conventionally, in diesel engines, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and when the oxygen concentration in the exhaust gas decreases, unburned HC, CO, etc. A NOx occlusion reduction catalyst having the property of decomposing and releasing NOx through the intervention of the catalyst to reduce and purify is provided in the middle of the exhaust pipe, and this NOx occlusion reduction catalyst is used to reduce the NOx emission concentration.

  However, in the NOx occlusion reduction catalyst, if the occlusion amount of NOx increases and reaches the saturation amount, no more NOx can be occluded, so the oxygen concentration of the exhaust gas that periodically flows into the NOx occlusion reduction catalyst Needs to be reduced by a reducing agent such as HC to decompose and release NOx.

  For example, when used in a gasoline engine, by reducing the operating air-fuel ratio of the engine (operating the engine at a rich air-fuel ratio), the oxygen concentration in the exhaust gas is reduced and unburned HC in the exhaust gas is reduced. Although it is possible to promote the decomposition and release of NOx by increasing the reducing components such as CO, it is difficult to operate the engine at a rich air-fuel ratio when the NOx occlusion reduction catalyst is used as an exhaust purification device of a diesel engine. .

Therefore, by adding fuel to the exhaust gas upstream of the NOx storage reduction catalyst, the added fuel is used as a reducing agent to react with oxygen on the NOx storage reduction catalyst, thereby reducing the oxygen concentration in the exhaust gas. There is a need (see, for example, Patent Document 1).
JP 2002-81111 A

However, in the exhaust gas of the diesel engine, SO ₂ gas (sulfurous acid gas) derived from sulfur contained in the light oil of the fuel is present, so this SO ₂ gas is combined with NOx on the NOx storage reduction catalyst. Similarly, it is unavoidable that it is oxidized and occluded as a sulfate, but since this sulfate is more stable than nitrate, even if fuel addition is performed to regenerate the NOx occlusion reduction catalyst, the NOx As a result of the fact that only a part of the sulfate stored in the storage reduction catalyst is released and remains, there is a problem that the recovery rate of the NOx storage site of the NOx storage reduction catalyst is reduced and the storage capacity is reduced. In reality, no practical solution has yet been established for the problem of residual sulfate.

That is, the method of desorbing sulfate from the NOx occlusion reduction catalyst itself involves increasing the catalyst bed temperature of the NOx occlusion reduction catalyst to a desulfurization temperature condition of about 600 to 700 ° C. It is already known that the NOx occlusion reduction catalyst can be desulfurized by releasing sulfate as SO ₂ gas from the SOx, but the SOx that has been stored in the NOx occlusion reduction catalyst over time during the desulfurization treatment There is a problem that a high concentration of SO ₂ gas is generated by releasing _two gases at once, and the generation of this high concentration of SO ₂ gas greatly hinders the practical use of the NOx storage reduction catalyst desulfurization treatment. It was a factor.

The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exhaust emission control device that prevents high-concentration SO ₂ gas from being generated during the desulfurization treatment of the NOx storage reduction catalyst.

  The present invention provides an exhaust gas purification apparatus provided with a NOx storage reduction catalyst in the middle of an exhaust pipe so that fuel can be added to the inlet side of the NOx storage reduction catalyst. A water spraying means for atomizing and spraying water, and a mist catcher that catches sulfuric acid mist on the downstream side of the water spraying means and reacts with the sulfuric acid mist to form a sulfide. .

Thus, when desulfurization treatment is performed on the NOx storage reduction catalyst, if fuel is added to the inlet side of the NOx storage reduction catalyst, the added fuel reacts with oxygen on the NOx storage reduction catalyst, so that the surrounding exhaust gas As a result, the catalyst bed temperature is raised to the desulfurization temperature condition by the reaction heat, and the sulfate is released as SO ₂ gas from the NOx storage reduction catalyst.

At this time, when water is atomized and sprayed by the water spraying means on the downstream side of the NOx occlusion reduction catalyst, the SO ₂ gas reacts with water in the exhaust gas to become sulfuric acid (H ₂ SO ₄ ), and is liquefied. The liquefied sulfuric acid mist is captured by the mist catcher and reacts with the mist catcher to form a sulfide, which is retained as a part of the mist catcher. Therefore, during the desulfurization treatment of the NOx storage reduction catalyst, ₍₂₎ There is no risk of gas being collected outside the vehicle.

  As the mist catcher becomes more difficult to take in the sulfuric acid mist as sulfide as the sulfidation by the sulfuric acid mist progresses, the mist catcher is new when the mist catcher is sufficiently sulfided to become a sulfide as a whole. Replace with a mist catcher.

Further, in the present invention, it is preferable to include a moisture retention filter that allows exhaust gas to pass while retaining moisture between the water spraying means and the mist catcher. Since moisture is retained by the moisture retention filter and the contact frequency between the SO ₂ gas and the water in the exhaust gas passing through the moisture retention filter is increased, the reaction efficiency between the SO ₂ gas and water is improved.

  Furthermore, in the present invention, it is preferable that the moisture retention filter is constituted by a metal filter that can react with the sulfuric acid mist to form a sulfide, and in this way, only the reaction of sulfuric acid generation occurs in the moisture retention filter. Instead, the generated sulfuric acid immediately reacts with the metal filter forming the moisture retaining filter and is retained as a sulfide.

  According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

(I) According to the first aspect of the present invention, the SO ₂ gas released from the NOx occlusion reduction catalyst during the desulfurization treatment is reacted with water to form sulfuric acid (H ₂ SO ₄ ), and the liquefied sulfuric acid Mist is captured by the mist catcher and reacted with the mist catcher to form a sulfide, which can be retained as a part of the mist catcher and processed reliably, so that a high concentration of SOx can be treated during the desulfurization treatment of the NOx storage reduction catalyst. _{2 The} problem of gas generation can be solved.

(II) According to the invention described in claim 2 of the present invention, the water sprayed from the water spraying means is held by the moisture holding filter, and the contact frequency between the SO ₂ gas in the exhaust gas and the water is significantly increased. Therefore, the reaction efficiency between the SO ₂ gas and water can be greatly improved, and the SO ₂ gas can be treated more reliably.

(III) According to the invention described in claim 3 of the present invention, a reaction for producing sulfuric acid occurs in the moisture retention filter, and the produced sulfuric acid is immediately reacted with the metal filter constituting the moisture retention filter and retained as sulfide. Therefore, the reaction from SO ₂ gas to sulfide can be efficiently advanced.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows an example of an embodiment for carrying out the present invention. In the exhaust purification apparatus of this embodiment, the exhaust pipe 4 through which the exhaust gas 3 discharged from the diesel engine 1 through the exhaust manifold 2 flows is shown. In addition, a NOx occlusion reduction catalyst 5 having a flow-through type honeycomb structure is mounted and mounted on a catalyst case 6, and fuel 8 (light oil) in a light oil tank 7 is placed at the inlet of the catalyst case 6. There is provided a fuel addition injector 10 which is guided through a light oil pump 9 and injected.

  In the example shown here, a catalyst-carrying particulate filter 11 carrying an oxidation catalyst is mounted on the downstream side of the NOx storage-reduction catalyst 5 in the catalyst case 6, so that the NOx in the front stage is provided. Particulates contained in the exhaust gas 3 that has passed through the occlusion reduction catalyst 5 are collected so that further exhaust purification can be achieved.

  Further, immediately after the catalyst case 6, a desulfurization case 12 is connected via a connecting pipe 13, and water 15 in the water tank 14 is guided to the inlet portion of the desulfurization case 12 via a water pump 16. The water tank 14, the water pump 16, and the spray nozzle 17 constitute a water spray means 18 that atomizes and sprays the water 15 into the exhaust gas 3. It has become.

  Further, a mist catch plate 19 made of an iron-based metal is disposed on the rear stage side in the desulfurization case 12 as a mist catcher that catches sulfuric acid mist, which will be described later, and reacts with the sulfuric acid mist to form a sulfide. The mist catch plate 19 has a ventilation structure made of a punching metal, a metal mesh or the like.

  Further, the rear wall portion 20 of the desulfurization case 12 and the tail pipe 21 connected to the lowermost position of the rear wall portion 20 are each made of an iron-based metal so that it can serve as a mist catcher. .

  In addition, the material of these mist catchers should just be a metal which reacts with a sulfuric acid mist and forms a sulfide, and is not necessarily limited to producing with a ferrous metal.

  On the other hand, on the upstream side in the desulfurization case 12, a dispersion plate 22 that diffuses the exhaust gas 3 and promotes the dispersion of moisture is provided. This dispersion plate 22 is also punched metal like the mist catch plate 19 described above. And a ventilation structure composed of a metal mesh or the like.

  In the space between the dispersion plate 22 and the mist catch plate 19 in the desulfurization case 12, a porous ceramic made of cordierite or the like is used as a moisture retaining filter that allows the exhaust gas 3 to pass while retaining moisture. A filter 23 is disposed. The ceramic filter 23 has a honeycomb structure substantially the same as that of the particulate filter 11, and the inlets of the respective flow paths partitioned in a lattice pattern are alternately sealed, and the inlets are closed. About the flow path which is not sealed, the exit is sealed.

Thus, with regard to the exhaust purification apparatus configured as described above, when performing a desulfurization treatment in which the sulfate stored in the NOx occlusion reduction catalyst 5 is released as SO ₂ gas, the NOx occlusion reduction catalyst 5 is placed on the inlet side. When the fuel 8 is added from the fuel addition injector 10, the added fuel 8 reacts with oxygen on the NOx occlusion reduction catalyst 5, thereby reducing the oxygen concentration in the surrounding exhaust gas 3 and the heat of reaction causes the catalyst bed temperature. Is raised to the desulfurization temperature condition, and the sulfate is released from the NOx occlusion reduction catalyst 5 as SO ₂ gas.

At this time, when the water 15 in the water tank 14 is guided through the water pump 16 and atomized into the exhaust gas 3 by the spray nozzle 17, the SO ₂ gas reacts with the water 15 in the exhaust gas 3 and sulfuric acid ( H ₂ SO ₄ ) is liquefied, and the liquefied sulfuric acid mist is captured by the mist catch plate 19 and reacts with the mist catch plate 19 to become iron sulfide (sulfide). Since it is held as a part, there is no possibility that SO ₂ gas is collected and released outside the vehicle during the desulfurization treatment of the NOx storage reduction catalyst 5.

  Here, the same reaction also occurs in the rear wall portion 20 and the tail pipe 21 of the desulfurization case 12, and sulfuric acid mist is also captured by the rear wall portion 20 and the tail pipe 21 of the desulfurization case 12 to cause iron sulfide (sulfide). And is held as a part of the rear wall portion 20 and the tail pipe 21 as it is.

  The mist catcher such as the mist catch plate 19, the rear wall portion 20 of the desulfurization case 12, and the tail pipe 21 becomes difficult to take in the sulfuric acid mist as iron sulfide as the sulfidation by the sulfuric acid mist progresses. What is necessary is just to replace | exchange for the new thing at the time when the sulfidation of the board 19 fully progressed and the whole became iron sulfide.

In particular, in the present embodiment, the ceramic filter 23 is provided between the water spraying means 18 and the mist catch plate 19, so that the water sprayed from the water spraying means 18 is held by the ceramic filter 23, and the ceramic As a result of increasing the contact frequency between the SO ₂ gas in the exhaust gas 3 passing through the filter 23 and the water 15, the reaction efficiency between the SO ₂ gas and the water 15 is improved.

Therefore, according to the above, the SO ₂ gas released from the NOx occlusion reduction catalyst 5 during the desulfurization treatment is reacted with the water 15 to form sulfuric acid (H ₂ SO ₄ ), and the liquefied sulfuric acid mist is converted into the mist catch plate 19 or the desulfurization. Caught by a mist catcher such as the rear wall portion 20 of the case 12 and the tail pipe 21 and taken in as iron sulfide, and held as part of the mist catch plate 19, the rear wall portion 20 of the desulfurization case 12, and the tail pipe 21 for reliable processing. Therefore, it is possible to solve the problem that a high concentration of SO ₂ gas is generated during the desulfurization treatment of the NOx storage reduction catalyst 5.

In particular, in the present embodiment, the water sprayed from the water spraying means 18 can be held by the ceramic filter 23 so that the contact frequency between the SO ₂ gas in the exhaust gas 3 and the water 15 can be remarkably increased. The reaction efficiency between the _two gases and the water 15 can be greatly improved, and the SO ₂ gas can be treated more reliably.

  FIG. 2 shows another embodiment of the present invention. In this embodiment, instead of the ceramic filter 23 of FIG. 1, a metal filter 24 that can form sulfide by reacting with sulfuric acid mist is used as a moisture retaining filter. This type of metal filter 24 has a micron-order metal fiber laminated and sintered, a metal powder sintered body, a metal mesh laminated and sintered, a metal mesh with metal What sintered the powder etc. can be used.

Thus, in this way, not only the reaction of sulfuric acid generation occurs in the metal filter 24, but the generated sulfuric acid immediately reacts with the metal filter 24 constituting the moisture retention filter and is held as sulfide. Therefore, the reaction from SO ₂ gas to sulfide can be efficiently advanced.

  The exhaust purification device of the present invention is not limited to the above-described embodiment. The mist catcher may be any mist catcher that captures sulfuric acid mist and forms a sulfide. Of course, the present invention is not limited to the rear wall portion of the case and the tail pipe, and various modifications can be made without departing from the scope of the present invention.

It is the schematic which shows an example of the form which implements this invention. It is the schematic which shows another form example of this invention.

Explanation of symbols

3 Exhaust gas 4 Exhaust pipe 5 NOx storage reduction catalyst 8 Fuel 15 Water 18 Water spray means 19 Mist catch plate (mist catcher)
20 Rear wall (mist catcher)
21 Tail pipe (mist catcher)
23 Ceramic filter (moisture retention filter)
24 Metal filter (moisture retention filter)

Claims (3)

  In an exhaust gas purification apparatus provided with a NOx occlusion reduction catalyst in the middle of an exhaust pipe so that fuel can be added to the inlet side of the NOx occlusion reduction catalyst, water is atomized into the exhaust gas downstream from the NOx occlusion reduction catalyst. An exhaust purification device comprising: a water spraying means for spraying and a mist catcher that catches sulfuric acid mist on the downstream side of the water spraying means and reacts with the sulfuric acid mist to form a sulfide.   The exhaust emission control device according to claim 1, further comprising a moisture retention filter that allows exhaust gas to pass while retaining moisture between the water spray means and the mist catcher.   The exhaust gas purifying apparatus according to claim 1, wherein the moisture retention filter is formed of a metal filter that can react with sulfuric acid mist to form sulfide.

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