JP2005351160A - Exhaust emission control method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To markedly improve NOx reduction efficiency more than conventional reduction efficiency when reducing and purifying NOx with urea water as a reducing agent. <P>SOLUTION: This exhaust emission control method reduces and purifies the NOx by adding the urea water 17 (reducing agent) on the upstream side of a selective reducing catalyst 10 by equipping the selective reducing catalyst 10 in the middle of an exhaust pipe 9. A first reaction area 19 for decomposing NH<SB>3</SB>generated from the urea water 17 up to NH<SB>2</SB>and a second reaction area 20 for reducing and purifying NO by the NH<SB>2</SB>are divided, and the exhaust temperature is lowered when the NH<SB>2</SB>is generated in the first reaction area 19, and reaction is advanced under a temperature condition lower than the first reaction area 19 in the second reaction area 20. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust purification method and apparatus for reducing and purifying NOx using urea water as a reducing agent.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust pipe through which exhaust gas circulates. A required amount of reducing agent is added upstream of the catalyst so that the reducing agent undergoes a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the selective catalytic reduction catalyst, thereby reducing the NOx emission concentration. There is what I did.

On the other hand, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method for reducing and purifying NOx using NH ₃ (ammonia) as a reducing agent is already widely known. Since it is difficult to ensure safety when traveling with NH ₃ itself, in recent years, the use of non-toxic urea water as a reducing agent has been studied (for example, Patent Document 1 and Patent Document 2).
JP 2002-161732 A JP 2002-166130 A

That is, if urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is decomposed into NH ₃ and CO under a temperature condition of about 170 ° C. or higher, and the exhaust gas is exhausted on the selective catalytic reduction catalyst. The inside NOx is reduced and purified well by NH ₃ .

In the NOx reduction and purification reaction using this type of urea water as a reducing agent, it is known that the main body of the reaction that directly reduces and purifies NOx is NH ₂ , and the urea water passes through NH ₃ to NH _2. Since it is also known that a high temperature condition of about 600 to 700 ° C. or more is necessary for the decomposition, the temperature of the exhaust gas reaching the selective catalytic reduction catalyst is positively increased when the exhaust temperature is low and the load is low. Many proposals have been made so far to improve the NOx reduction effect.

However, the reaction that reduces and purifies NO by NH _2, high temperature enough necessary to generate the NH ₂ is unnecessary, rather better to lower the temperature conditions of the reduction purification reaction of NO to 500 ° C. or less Although the fact that activity is finding that high is obtained, the one that has been proposed so far, only to produce a large amount of NH ₂ has been considered as a primary, and NH ₂ is generated No consideration was given to temperature control after that.

That is, as shown in FIG. 5, the temperature change of the reaction rate constant of NH ₂ and NO is represented by an Arrhenius plot in which the vertical axis represents the reaction rate constant logarithmically and the horizontal axis represents temperature in the form of temperature T / T. When graphed, as shown by curve X here, the reduction reaction of NO by NH ₂ has the lowest reaction rate constant in the high temperature region on the left side of the graph, and moves to the low temperature region on the right side of the graph. The reaction rate constant tends to increase gradually as it moves.

However, as indicated by the curve Y in this graph, the reaction rate constant of the reaction in which NH ₃ generated from urea water is decomposed into NH ₂ gradually increases as it moves to the higher temperature region on the left side of the graph. Since it shows a tendency, the reaction rate constant of the NO reduction reaction with NH ₂ falls in the temperature region on the left side where the reaction rate constant at which NH ₂ is generated is high.

  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 purification method and apparatus that can significantly improve NOx reduction efficiency as compared with conventional NOx reduction purification using urea water as a reducing agent. Yes.

The present invention is an exhaust purification method for reducing and purifying NOx by installing a selective catalytic reduction catalyst in the middle of an exhaust pipe and adding urea water as a reducing agent upstream of the selective catalytic reduction catalyst. A first reaction region that decomposes the generated NH ₃ to NH ₂ and a _second reaction region that reduces and purifies NO with NH ₂ are separated, and when NH ₂ is generated in the first reaction region, the exhaust temperature is reduced. The second reaction region is lowered and the reaction proceeds under a temperature condition lower than that of the first reaction region.

Thus, in this way, a high temperature condition suitable for decomposing NH ₃ generated from urea water to NH ₂ is given to the first reaction region, while a temperature condition lower than the first reaction region is provided. To the second reaction region, the reduction and purification reaction of NO by NH ₂ can be efficiently advanced.

  In carrying out the exhaust purification method of the present invention, for example, a selective reduction catalyst is provided in the middle of the exhaust pipe, and urea water is added as a reducing agent upstream of the selective reduction catalyst to reduce and purify NOx. It is preferable to use an exhaust purification device in which a selective catalytic reduction catalyst is formed so that the cross-sectional area of the flow path suddenly increases in the exhaust gas flow direction.

  If such an exhaust purification device is used, the exhaust gas flowing in the selective catalytic reduction catalyst adiabatically expands and decreases in temperature as it goes downstream, and this temperature decrease causes the temperature condition of the rear portion of the selective catalytic reduction catalyst. It becomes lower than the front part, and the front part of the selective catalytic reduction catalyst functions as the first reaction region, while the rear part of the selective catalytic reduction catalyst functions as the second reaction region.

  Also, instead of this exhaust purification device, the selective catalytic reduction catalyst is divided into two stages in the front and rear, and the downstream selective catalytic reduction catalyst is formed so as to have a larger cross-sectional area than the selective catalytic reduction catalyst in the previous stage. An exhaust emission control device that has been used may be used.

  If such an exhaust purification device is used, the exhaust gas flowing out from the upstream selective reduction catalyst diffuses and flows into the downstream selective reduction catalyst, so that it adiabatically expands and the temperature decreases. The temperature condition of the downstream selective reduction catalyst is lower than that of the upstream selective reduction catalyst, and the upstream selective reduction catalyst functions as the first reaction region, while the downstream selective reduction catalyst functions as the second reaction region. Will function as.

  Furthermore, it is also possible to use an exhaust purification device in which a cooling means is mounted on the rear part of the casing that holds the selective catalytic reduction catalyst. In this way, the rear part of the selective catalytic reduction catalyst is caused by the cooling means. Since the temperature lowers due to the cooling action, the temperature condition of the rear part of the selective catalytic reduction catalyst is lower than that of the front part, and the front part of the selective catalytic reduction catalyst functions as the first reaction region, while the selective catalytic reduction type The rear part of the catalyst will function as the second reaction zone.

  It is also possible to use an exhaust purification device in which the selective catalytic reduction catalyst is divided into two stages before and after and a cooling means is interposed between the two selective catalytic reduction catalysts. Since the exhaust gas flowing out of the selective catalytic reduction catalyst is cooled by the cooling means and the temperature is lowered, the temperature condition of the selective catalytic reduction catalyst at the latter stage becomes lower than that of the selective catalytic reduction catalyst at the former stage due to this temperature drop, The selective catalytic reduction catalyst functions as the first reaction region, while the selective catalytic reduction catalyst at the latter stage functions as the second reaction region.

According to the exhaust gas purification method and apparatus of the present invention described above, the first reaction region is provided with a high temperature condition suitable for decomposing NH ₃ generated from urea water to NH _2. The NO ₂ reduction and purification reaction by NH ₂ can be efficiently performed by applying a low temperature condition to the second reaction region, and therefore, it is possible to achieve an excellent effect that the NOx reduction efficiency can be significantly improved as compared with the conventional case. .

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

  FIG. 1 shows a first embodiment of the present invention. Reference numeral 1 in FIG. 1 denotes an engine which is a diesel engine. In the engine 1 shown here, a turbocharger 2 is provided, and an air cleaner is provided. 3 is sent to the compressor 2a of the turbocharger 2 through the intake pipe 5, and the air 4 pressurized by the compressor 2a is further sent to the intercooler 6 to be cooled. The air 4 is guided from 6 to an intake manifold (not shown) and introduced into each cylinder of the engine 1.

  Further, exhaust gas 7 discharged from each cylinder of the engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 8, and the exhaust gas 7 that has driven the turbine 2b goes out of the vehicle through the exhaust pipe 9. It is supposed to be discharged.

  In the middle of the exhaust pipe 9 through which the exhaust gas 7 circulates, the selective reduction catalyst 10 formed so that the cross-sectional area of the flow path suddenly increases in the flow direction of the exhaust gas 7 is held by the casing 11. The selective catalytic reduction catalyst 10 is a flow-through type honeycomb structure, and has a property capable of selectively reacting NOx with ammonia even in the presence of oxygen.

  Further, an electromagnetic addition valve 13 is arranged on the upstream side of the casing 11, and a urea water supply line 15 having a supply pump 16 is provided between the addition valve 13 and a urea water tank 14 provided at a required place. The urea water 17 (reducing agent) in the urea water tank 14 is driven to the upstream side of the selective catalytic reduction catalyst 10 through the addition valve 13 by driving a supply pump 16 that is connected and is provided in the middle of the urea water supply line 15. These addition valves 13, urea water tank 14, urea water supply line 15, and supply pump 16 constitute urea water addition means 18.

Thus, when such an exhaust purification device is used, the exhaust gas 7 flowing in the selective catalytic reduction catalyst 10 adiabatically expands and decreases in temperature as it goes downstream. The temperature condition of the rear part of the catalyst is lower than that of the front part, and the front part of the selective catalytic reduction catalyst 10 functions as the first reaction region 19 that decomposes NH ₃ generated from the urea water 17 to NH _2. The rear portion of the reduction catalyst 10 functions as a second reaction region 20 that reduces and purifies NO with NH ₂ .

That is, a high temperature condition of about 600 to 700 ° C. or more suitable for decomposing NH ₃ generated from the urea aqueous solution 17 to NH ₂ is given to the first reaction region 19, while lower than the first reaction region 19. Since the temperature reduction condition of 500 ° C. or less can be applied to the second reaction region 20 and the NO reduction / purification reaction with NH ₂ can be carried out efficiently, the NOx reduction efficiency can be greatly improved as compared with the prior art.

  FIG. 2 shows a second embodiment of the present invention. In this embodiment, the selective catalytic reduction catalyst 10 is divided into two front and rear stages, and the subsequent selective catalytic reduction catalyst 10B is replaced with the upstream selective reduction catalyst. The connecting pipe is formed so that the cross-sectional area of the flow path is larger than that of the catalyst 10A, and the cross-sectional area of the flow path rapidly increases in the flow direction of the exhaust gas 7 between the two selective reduction catalysts 10A and 10B. 21 is connected.

  In addition, a baffle plate or a guide member (not shown) may be appropriately provided in the communication pipe 21 so that the exhaust gas 7 flowing out from the selective catalytic reduction catalyst 10A in the previous stage is diffused well.

  Thus, if such an exhaust purification device is used, the exhaust gas 7 flowing out from the upstream selective reduction catalyst 10A diffuses and flows into the downstream selective reduction catalyst 10B, thereby adiabatically expanding and lowering the temperature. Therefore, due to this temperature decrease, the temperature condition of the downstream selective reduction catalyst 10B becomes lower than that of the upstream selective reduction catalyst 10A, and the upstream selective reduction catalyst 10A functions as the first reaction region 19, while The selective catalytic reduction catalyst 10B functions as the second reaction region 20.

Therefore, even in the second embodiment, the first reaction region 19 is given a high temperature condition of about 600 to 700 ° C. or more suitable for decomposing NH ₃ generated from the urea water 17 to NH _2. Since the temperature condition of 500 ° C. or lower which is lower than that of the first reaction region 19 is given to the second reaction region 20, the reduction and purification reaction of NO by NH ₂ can be advanced efficiently. As in the case of the example, the NOx reduction efficiency can be greatly improved as compared with the conventional case.

  FIG. 3 shows a third embodiment of the present invention. In this embodiment, fins 22 for promoting heat dissipation are provided as cooling means in the rear part of the casing 11 that holds the selective catalytic reduction catalyst 10. It has become an exterior.

  By doing so, the rear portion of the selective catalytic reduction catalyst 10 is cooled by the heat radiation promoting action by the fins 22 and the temperature is lowered, so that the temperature condition of the rear portion of the selective catalytic reduction catalyst 10 becomes lower than the front portion, The front part of the selective catalytic reduction catalyst 10 functions as the first reaction region 19, while the rear part of the selective catalytic reduction catalyst 10 functions as the second reaction region 20.

Therefore, even in the third embodiment, the first reaction region 19 is given a high temperature condition of about 600 to 700 ° C. or more suitable for decomposing NH ₃ generated from the urea water 17 to NH _2. Since the temperature condition of 500 ° C. or lower which is lower than that of the first reaction region 19 is given to the second reaction region 20, the reduction and purification reaction of NO by NH ₂ can be advanced efficiently. As in the case of the example and the second embodiment of FIG. 2, the NOx reduction efficiency can be significantly improved as compared with the prior art.

  FIG. 4 shows a fourth embodiment of the present invention. In this embodiment, the selective catalytic reduction catalyst 10 is divided into two stages in the front and rear, and between the selective catalytic reduction catalysts 10A and 10B, The connecting pipe 24 encapsulated by the water cooling jacket 23 is interposed as a cooling means, and the cooling water 12 is supplied to and discharged from the water cooling jacket 23.

  In this way, the temperature of the exhaust gas 7 that has flowed out of the selective catalytic reduction catalyst 10A in the preceding stage is lowered by the cooling action of the water cooling jacket 23 while passing through the connecting pipe 24. The temperature condition of the reduction catalyst 10B is lower than that of the selective reduction catalyst 10A in the preceding stage, and the selective reduction catalyst 10A in the preceding stage functions as the first reaction region 19, while the selective reduction catalyst 10B in the subsequent stage is in the second state. It will function as the reaction region 20.

Therefore, in the fourth embodiment as well, the first reaction region 19 is given a high temperature condition of about 600 to 700 ° C. or more suitable for decomposing NH ₃ generated from the urea aqueous solution 17 to NH _2. Since the temperature condition of 500 ° C. or lower which is lower than that of the first reaction region 19 is given to the second reaction region 20, the reduction and purification reaction of NO by NH ₂ can be advanced efficiently. As in the case of the example, the second embodiment shown in FIG. 2, and the third embodiment shown in FIG. 3, the NOx reduction efficiency can be significantly improved as compared with the prior art.

  The exhaust purification method and apparatus of the present invention are not limited to the above-described embodiments, and various cooling methods other than those shown in the drawings can be adopted as appropriate for the cooling means. Of course, various changes can be made without departing from the scope of the invention.

It is the schematic which shows the example of 1st form of this invention. It is the schematic which shows the 2nd form example of this invention. It is the schematic which shows the 3rd form example of this invention. It is the schematic which shows the example of a 4th form of this invention. Is a graph showing the temperature change of the reaction rate constants of the NH ₂ and NO.

Explanation of symbols

7 exhaust gas 9 exhaust pipe 10 selective reduction catalyst 10A selective reduction catalyst 10B selective reduction catalyst 11 casing 12 cooling water 17 urea water 18 urea water addition means 19 first reaction region 20 second reaction region 22 fin (cooling) means)
23 Water cooling jacket (cooling means)
24 Communication pipe (cooling means)

Claims (5)

An exhaust purification method for reducing and purifying NOx by installing a selective reduction catalyst in the middle of an exhaust pipe and adding urea water as a reducing agent to the upstream side of the selective reduction catalyst, the NH ₃ generated from urea water The first reaction region in which NH _{2 is} decomposed to NH ₂ and the _second reaction region in which NO ₂ is reduced and purified by NH ₂ are separated. When NH ₂ is generated in the first reaction region, the exhaust temperature is lowered, An exhaust purification method characterized in that the reaction proceeds in the second reaction region under a temperature condition lower than that in the first reaction region.   An exhaust emission control device that is equipped with a selective reduction catalyst in the middle of an exhaust pipe and that reduces and purifies NOx by adding urea water as a reducing agent upstream of the selective reduction catalyst, wherein the selective reduction catalyst is discharged as an exhaust gas. An exhaust emission control device formed so that a cross-sectional area of a flow path suddenly expands toward a flow direction of the exhaust gas.   An exhaust purification device that is equipped with a selective catalytic reduction catalyst in the middle of an exhaust pipe and that reduces and purifies NOx by adding urea water as a reducing agent upstream of the selective catalytic reduction catalyst. An exhaust emission control device characterized by being divided into stages and having a downstream selective catalytic reduction catalyst having a larger channel cross-sectional area than a selective catalytic reduction catalyst in the previous stage.   An exhaust emission control device that is equipped with a selective catalytic reduction catalyst in the middle of an exhaust pipe and adds urea water as a reducing agent upstream of the selective catalytic reduction catalyst to reduce and purify NOx, and holds the selective catalytic reduction catalyst An exhaust emission control device comprising a cooling means externally mounted on a rear portion of a casing.   An exhaust gas purification apparatus equipped with a selective catalytic reduction catalyst in the middle of an exhaust pipe and reducing and purifying NOx by adding urea water as a reducing agent upstream of the selective catalytic reduction catalyst. An exhaust emission control device characterized in that it is divided into stages and a cooling means is interposed between the selective reduction catalysts.

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