JP2006009608A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To miniaturize device constitution related to NO<SB>x</SB>reduction purification using urea water as a reducing agent, to improve mountability onto a vehicle. <P>SOLUTION: This exhaust emission control device is equipped with a selective reduction type catalyst 10 at an intermediate part of an exhaust pipe 9 and constituted to add urea water 17 as the reducing agent upstream of the selective reduction type catalyst 10 to carry out reduction purification of NO<SB>x</SB>. A mixer 20 equipped with a plurality of dispersion plates 19 for allowing exhaust gas 7 to meander to equally disperse the spray of urea water 17, is provided between the added position of the urea water 17 and the selective reduction type catalyst 10. Each dispersion plate 19 and an internal wall 20' of the mixer 20 carry hydrolytic catalysts 25 for decomposing the urea water 17 into ammonia and carbon dioxide. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device.

  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 ammonia (NH ₃ ) as a reducing agent is already widely known. Since it is difficult to ensure safety with respect to traveling with ammonia itself, in recent years, the use of non-toxic urea water as a reducing agent has been studied.

That is, if urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas, and NOx in the exhaust gas is converted to ammonia on the selective catalytic reduction catalyst. It is reduced and purified well by ammonia (see, for example, Patent Document 1).
JP 2002-161732 A

  FIG. 2 shows an example of an exhaust gas purification device that reduces and purifies NOx using urea water as a reducing agent. Reference numeral 1 in FIG. 1 denotes an engine that is a diesel engine. The turbocharger 2 is provided, and the air 4 guided from the air cleaner 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 supplied to the intercooler. The air 4 is led to an intake manifold (not shown) from the intercooler 6 and introduced into each cylinder of the engine 1.

  The 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, a selective reduction catalyst 10 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen is held and equipped by a casing 11. In addition, an iron-based hydrolysis catalyst 12 having a weak oxidizing action, such as iron oxide (Fe ₂ O ₃ ), is provided in the front stage of the selective reduction catalyst 10 in the casing 11.

  A urea water injection valve 13 with an injection nozzle 13 ′ is installed in the exhaust pipe 9 upstream of the casing 11, and a urea water supply line is provided between the urea water injection valve 13 and a urea water tank 14 provided at a required location. 15, the urea water 17 (reducing agent) in the urea water tank 14 is driven through the urea water injection valve 13 by the drive of the supply pump 16 provided in the middle of the urea water supply line 15. These urea water injection valves 13, the urea water tank 14, the urea water supply line 15, and the supply pump 16 constitute a urea water adding device 18.

Then, the spray of the urea water 17 can be evenly diffused by meandering the exhaust gas 7 between the addition position of the urea water 17 (opening position of the injection nozzle 13 ′) by the urea water addition device 18 and the casing 11. A mixer 20 equipped with a plurality of dispersion plates 19 is provided, and immediately after the selective reduction catalyst 10 in the casing 11, an NH ₃ slip catalyst that oxidizes surplus ammonia as a countermeasure against leaked ammonia. 21 is equipped.

  Further, a NOx sensor 22 that detects the NOx concentration in the exhaust gas 7 at an appropriate position upstream of the addition position of the urea water 17 (opening position of the injection nozzle 13 ′) and an appropriate position downstream of the casing 11. 23 are respectively provided, and the detection signals 22a and 23a from these NOx sensors 22 and 23 are input to a control device 24 constituting an engine control computer (ECU: Electronic Control Unit), and the added amount of urea water 17 is determined. Whether or not a suitable NOx reduction rate is obtained is monitored.

  On the other hand, the control device 24 outputs a valve opening command signal 13a and a drive command signal 16a to the urea water injection valve 13 and the supply pump 16, respectively. The addition amount of the urea water 17 is appropriately controlled by the operation, and an injection pressure required when the urea water 17 is added can be appropriately obtained by driving the supply pump 16.

Thus, according to the exhaust gas purification apparatus configured in this way, the spray of urea water 17 injected from the injection nozzle 13 ′ by the urea water adding apparatus 18 is introduced into the mixer 20 and a plurality of the dispersion plates 19. Meanderingly flows between the two components and is evenly diffused in the mixer 20 and enters the hydrolysis catalyst 12. In the hydrolysis catalyst 12, the following formula [Chemical Formula 1]
(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂
Thus, the urea water 17 is efficiently decomposed into ammonia and carbon dioxide gas from a relatively low temperature range.

Next, ammonia having high reactivity in the selective catalytic reduction catalyst 10 in the latter stage mainly uses the following formulas [Chemical Formula 2]
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O
[Chemical formula 3]
4NH ₃ + 2NO + O ₂ → 3N ₂ + 6H ₂ O
[Chemical formula 4]
2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O
Thus, NOx in the exhaust gas 7 is efficiently reduced to nitrogen, and it is possible to reduce and purify NOx from a relatively lower temperature range than when the urea water 17 is reacted with NOx as it is. Become.

However, in such an exhaust purification device, even if urea water 17 is used as the reducing agent, a high NOx reduction rate can be obtained from a relatively low temperature range, but on the other hand, the mixer is placed before and after the selective catalytic reduction catalyst 10. 20, the hydrolysis catalyst 12, and the NH ₃ slip catalyst 21 are required, so that the apparatus configuration related to NOx reduction purification is long in the longitudinal direction of the exhaust pipe 9 and can be mounted on a vehicle. There was a problem that it became extremely worse.

  Although not particularly shown in FIG. 2, in the case of a diesel engine, it is necessary to mount a particulate filter in the middle of the exhaust pipe 9 as a measure against particulates, which adversely affects the mountability of the particulate filter. Therefore, it is desired to make the apparatus configuration related to NOx reduction purification compact.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to reduce the size of the apparatus related to the reduction and purification of NOx using urea water as a reducing agent, and to improve the mountability in a vehicle.

  The present invention is an exhaust emission control device equipped with a selective 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 reduction catalyst, A mixer equipped with a plurality of dispersion plates is provided between the urea water addition position and the selective catalytic reduction catalyst so that the exhaust gas can meander and the spray of urea water can be evenly diffused, and each dispersion plate of the mixer is provided. And a hydrolysis catalyst for decomposing urea water into ammonia and carbon dioxide is supported on the inner wall.

  Thus, in this way, the spray of urea water added as a reducing agent is evenly diffused while flowing in a meandering manner in the mixer, and at the same time, hydrolysis carried on each dispersion plate and inner wall in the mixer. Since the decomposition reaction that decomposes urea water into ammonia and carbon dioxide in contact with the catalyst is promoted, as in the past, urea water is efficiently decomposed from a relatively low temperature range to select highly reactive ammonia. It is possible to lead to a reduction catalyst, and a high NOx reduction rate can be obtained from a relatively low temperature range.

  In this case, since the catalytic action of the hydrolysis catalyst that has been arranged independently in the subsequent stage of the mixer is handled by the mixer, it is not necessary to arrange the hydrolysis catalyst in the subsequent stage of the mixer, and selective reduction immediately after the mixer. By arranging the mold catalyst, the overall length of the apparatus is shortened, and the apparatus configuration can be made compact.

  Further, in the present invention, the first NOx sensor that detects the NOx concentration in the exhaust gas upstream from the urea water addition position and the NOx concentration in the exhaust gas that detects the NOx concentration in the exhaust gas downstream from the selective catalytic reduction catalyst. Two NOx sensors, a temperature sensor arranged near each of these NOx sensors to detect the temperature of exhaust gas, and the degree of deterioration of each NOx sensor due to heat based on a detection signal from each temperature sensor. It may be configured to be able to correct the detected value of each NOx sensor according to the estimation and the degree of deterioration.

  In this way, it is possible to estimate the degree of deterioration due to heat of the NOx sensor exposed to the high temperature exhaust gas, and to correct the detected value of each NOx sensor according to the degree of deterioration. It is possible to reduce the difference between the detected value by the sensor and the actual NOx concentration as much as possible and maintain high detection accuracy.

Further, in the present invention, it is preferable to provide an NH ₃ slip catalyst for oxidizing excess ammonia immediately after the selective reduction catalyst in the longitudinal direction of the exhaust pipe. Excess ammonia that has passed through the reaction can be rendered harmless by oxidizing it with NO or H ₂ O.

  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 invention described in claim 1 of the present invention, the spray of urea water is evenly diffused while flowing through the mixer while being in contact with the hydrolysis catalyst carried on each dispersion plate and the inner wall. Therefore, it can be efficiently decomposed into ammonia and carbon dioxide, so that it is possible to reduce the size of the apparatus by eliminating the need for a conventional hydrolysis catalyst that has been arranged independently after the mixer. Mountability can be greatly improved.

  (II) According to the invention described in claim 2 of the present invention, even if the NOx sensor exposed to the high temperature exhaust gas deteriorates due to heat, the detected value of each NOx sensor according to the degree of deterioration. Therefore, the deviation between the detected value of each NOx sensor and the actual NOx concentration can be reduced as much as possible to maintain high detection accuracy.

(III) According to the invention described in claim 3 of the present invention, surplus ammonia that has passed through the selective reduction catalyst in an unreacted state is oxidized to NO or H ₂ O to be rendered harmless. It is possible to avoid the possibility of ammonia remaining in the exhaust gas finally discharged into the atmosphere.

  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 emission control device of this embodiment shown in FIG. 1, in the conventional example of FIG. The disposed hydrolysis catalyst 12 is abolished, and instead, each dispersion plate 19 and inner wall 20 ′ of the mixer 20 is loaded with a hydrolysis catalyst 25 that decomposes the urea water 17 into ammonia and carbon dioxide. Immediately after that, the selective catalytic reduction catalyst 10 is arranged.

  In particular, in the present embodiment, temperature sensors 26 and 27 for detecting the temperature of the exhaust gas 7 are arranged in the vicinity of the NOx sensors 22 and 23, respectively, and detection signals from the temperature sensors 26 and 27 are provided. 26 a and 27 a are input to the control device 24.

  On the other hand, in the control device 24, the degree of deterioration due to heat of the NOx sensors 22 and 23 is estimated based on the detection signals 26a and 27a from the temperature sensors 26 and 27, and the degree of deterioration depends on the degree of deterioration. The detection values of the NOx sensors 22 and 23 can be corrected.

  That is, the NOx sensors 22 and 23 are gradually deteriorated by being exposed to the high-temperature exhaust gas 7, and the output is lowered. The degree of the deterioration depends on what temperature zone and how many hours it has been exposed. Since it is different, the temperature zone and time at which each NOx sensor 22, 23 is exposed are recorded in the control device 24, and based on this, the degree of deterioration of each NOx sensor 22, 23 is estimated to calculate a corrected deterioration coefficient, This detected deterioration coefficient is multiplied by the detected value of each NOx sensor 22, 23 to correct the detected value.

  Thus, in this way, the spray of urea water 17 added as a reducing agent is evenly diffused while meandering and flowing in the mixer 20, and at the same time, each dispersion plate 19 and inner wall 20 'in the mixer 20. Since the decomposition reaction of decomposing urea water 17 into ammonia and carbon dioxide is promoted in contact with the hydrolysis catalyst 25 supported on the catalyst, the urea water 17 is efficiently decomposed from a relatively low temperature range as in the prior art. Thus, highly reactive ammonia can be guided to the selective catalytic reduction catalyst 10, and a high NOx reduction rate can be obtained from a relatively low temperature range.

  At this time, since the catalytic action of the hydrolysis catalyst that is conventionally arranged independently in the subsequent stage of the mixer 20 is handled by the mixer 20, it is necessary to arrange the hydrolysis catalyst 12 (see FIG. 2) in the subsequent stage of the mixer 20. By arranging the selective catalytic reduction catalyst 10 immediately after the mixer 20, the overall length of the apparatus is shortened and the apparatus configuration can be made compact.

  Therefore, according to the above embodiment, the spray of urea water 17 is evenly diffused while meandering in the mixer 20 and is brought into contact with each of the dispersion plates 19 and the hydrolysis catalyst 25 carried on the inner wall 20 ′. Since it can be efficiently decomposed into ammonia and carbon dioxide, the conventional hydrolysis catalyst 12 (see FIG. 2), which is arranged independently after the mixer 20, is not required, and the apparatus configuration can be made compact. It is possible to greatly improve the mountability on the vehicle.

  Further, particularly in this embodiment, even if the NOx sensors 22 and 23 exposed to the high temperature exhaust gas 7 are deteriorated due to heat, the control device 24 controls the NOx sensors 22 and 23 according to the degree of deterioration. Since the detection value is corrected, the deviation between the detection value of each NOx sensor 22 and 23 and the actual NOx concentration can be reduced as much as possible to maintain high detection accuracy. The amount control of the urea water 17 by 24 can be executed accurately.

Also in the present embodiment, as in the conventional example of FIG. 2 described above, an NH ₃ slip catalyst 21 that oxidizes excess ammonia immediately after the selective reduction catalyst 10 in the longitudinal direction of the exhaust pipe 9 is provided. Therefore, surplus ammonia that has passed through the selective catalytic reduction catalyst 10 unreacted is represented by the following formula [Chemical Formula 5].
4NH ₃ + 5O ₂ → 4NO + 6H ₂ O
Thus, it can be rendered harmless by oxidizing it into NO or H ₂ O, and it is possible to avoid the possibility of ammonia remaining in the exhaust gas 7 finally discharged into the atmosphere.

  Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist 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 an example of the conventional exhaust gas purification apparatus which carries out reduction purification of NOx.

Explanation of symbols

7 Exhaust gas 9 Exhaust pipe 10 Selective reduction type catalyst 17 Urea water 18 Urea water addition device 19 Dispersion plate 20 Mixer 20 'inner wall 21 NH ₃ slip catalyst 22 NOx sensor (first NOx sensor)
22a Detection signal 23 NOx sensor (second NOx sensor)
23a Detection signal 24 Controller 25 Hydrolysis catalyst 26 Temperature sensor 26a Detection signal 27 Temperature sensor 27a Detection signal

Claims (3)

  An exhaust gas purification apparatus equipped with 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 to reduce and purify NOx. A mixer equipped with a plurality of dispersion plates is provided between the position and the selective catalytic reduction catalyst so that the exhaust gas can meander and the spray of urea water can be evenly diffused. An exhaust emission control device comprising a hydrolysis catalyst for decomposing urea water into ammonia and carbon dioxide.   A first NOx sensor that detects the NOx concentration in the exhaust gas upstream from the urea water addition position, a second NOx sensor that detects the NOx concentration in the exhaust gas downstream from the selective catalytic reduction catalyst, and these A temperature sensor that is disposed in the vicinity of each NOx sensor and detects the temperature of the exhaust gas, and the degree of deterioration of each NOx sensor due to heat is estimated based on the detection signal from each temperature sensor and the degree of deterioration The exhaust emission control device according to claim 1, wherein the exhaust purification device is configured to be able to correct the detected value of each NOx sensor according to the conditions. The exhaust emission control device according to claim 1 or 2, wherein an NH ₃ slip catalyst for oxidizing excess ammonia is provided immediately after the selective reduction catalyst in the longitudinal direction of the exhaust pipe.

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