JP2005334681A - Urea scr system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a urea SCR (selective catalytic reduction) system preventing ammonia slip, enlarging a range of application temperature and solving a conventional problem. <P>SOLUTION: In the urea SCR system, a front stage oxidation catalyst part 3 for enhancing oxidation activity of NO in an exhaust gas from an upstream side of an exhaust passage 2 of an internal combustion engine 1, or the like, an SCR catalyst part 4 for reducing NOx in the exhaust gas by ammonia, and a rear stage oxidation catalyst part 5 for removing ammonia are successively arranged, and a urea water liquid 6 is added between the front stage oxidation catalyst part 3 and the SCR catalyst part 4. The SCR catalyst part is arranged in series by combining a low temperature catalyst carrier A of an adsorbent having weak acid strength and a high temperature catalyst carrier B of an adsorbent having strong acid strength. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a urea SCR system for purifying NOx in exhaust gas by reducing with ammonia.

  As a general means for purifying NOx in exhaust gas of an internal combustion engine or the like, an exhaust gas purification system in which an oxidation catalyst, a hydrogen adsorption / desorption portion, and a NOx purification catalyst are sequentially arranged from the upstream side of the exhaust passage is provided.

  Incidentally, recently, a urea SCR system, which is a selective reduction catalyst method of SCR (abbreviation of Selective Catalytic Reduction) using urea for NOx reduction technology, has been developed. This is a system in which NOx in exhaust gas is selectively adsorbed by a catalyst, an aqueous urea solution is sprayed thereon, and NOx is decomposed into nitrogen and water by a reduction reaction and discharged.

  As shown in FIG. 4, the conventional urea SCR system uses a pre-stage oxidation catalyst unit 3 for increasing NO oxidation activity in the exhaust gas from the upstream side of the exhaust passage 2 of the internal combustion engine 1 and the like, and NOx in the exhaust gas. An SCR catalyst unit 4a for reducing with ammonia and a post-stage oxidation catalyst unit 5 for removing ammonia are sequentially arranged, and urea aqueous solution is added between the stage oxidation catalyst unit 3 and the SCR catalyst unit 4a. The SCR catalyst unit 4a uses an adsorbent 8 in which acid sites are mixed. Reference numeral 7 denotes ammonia.

In the conventional urea SCR system, urea SCR control is difficult due to the following factors.
1. There is a response delay until NOx can be purified after urea injection.
2. There is a temperature range where urea cannot be injected.
3. The amount of reducing agent that can be stored is insufficient.
4). It is difficult to estimate the storable amount of reducing agent in the system.

  Above 1. There is a possibility that a response method of the above can be solved in the process of elucidating where the rate is limited in the reaction mechanism for purifying NOx from urea. Urea SCR catalysts have been researched since the 1960s. Conventionally, research has been focused on increasing activation at high temperatures, and little consideration has been given to response delays at low temperatures. Therefore, there was room for research on improving response delay.

  2. The temperature range in which urea cannot be injected is mainly related to the temperature dependence of the reaction for converting isocyanate produced from urea into ammonia. Although it is conceivable to use a catalyst to lower the thermal decomposition temperature of isocyanate, it is difficult to selectively decompose isocyanate to ammonia at 170 ° C. or lower. In addition, it is considered that the decomposition of isocyanate is promoted at the nozzle portion by heating the nozzle that injects urea, but when the nozzle is heated, the water of the urea water evaporates, causing urea to clog the nozzle. A malfunction occurs. Moreover, if the NOx emission amount is small at low temperatures and the storable amount of the SCR system is increased, the shortage of reducing agent at low temperatures can be dealt with.

  3. above. Insufficient storable amount of the reducing agent cannot be dealt with simply by increasing the amount of SCR catalyst. The conceptual diagram is shown in FIG.5 and FIG.6. That is, even when the catalyst amount is increased, the temperature at which ammonia is desorbed does not change depending on the catalyst amount. When the catalyst amount is increased, when the catalyst temperature is shifted to a higher temperature side, the ammonia desorption amount also increases, which causes ammonia slip. Therefore, it is necessary to increase the ammonia adsorption capacity by shifting the curve of the SCR catalyst temperature and the ammonia adsorption capacity to the high temperature side. However, the catalyst whose ammonia adsorption capacity curve has shifted to the high temperature side shows that the adsorption power of ammonia and the catalyst is strong. Such a catalyst has low reaction activation with NOx at low temperatures.

Thus, the conventional urea SCR system has technical problems to be solved, such as the possibility that reactive ammonia may be discharged when the temperature of the exhaust gas is insufficient, and NOx not being reduced when urea is insufficient.
JP2001-2221036 JP 2003-286832 A JP 2003-343252 A

  An object of the present invention is to provide a urea SCR system that prevents ammonia slip, expands the application temperature range, and solves the conventional problems.

  In order to achieve the above object, according to a first aspect of the present invention, there is provided a pre-stage oxidation catalyst unit for increasing the oxidation activity of NO in exhaust gas from an upstream side of an exhaust passage of an internal combustion engine or the like, and an exhaust gas The SCR catalyst part for reducing NOx in the ammonia with ammonia and the rear oxidation catalyst part for removing ammonia are sequentially arranged, and urea aqueous solution is added between the preceding oxidation catalyst part and the SCR catalyst part. In the urea SCR system configured as described above, the SCR catalyst unit is characterized in that a plurality of catalyst carriers having different desorption temperatures of ammonia are combined and arranged in series.

  As described in claim 2, in the urea SCR system according to claim 1, the SCR catalyst unit is characterized in that a low-temperature catalyst carrier is arranged in front and a high-temperature catalyst carrier is arranged in series in the rear. It is.

  As described in claim 3, in the urea SCR system according to claim 2, the low-temperature catalyst support in the first stage is a weak acid strength adsorbent, and the high-temperature catalyst support in the second stage is a strong acid strength adsorbent. It is characterized by that.

  According to the present invention, ammonia desorbed by the upstream catalyst carrier of the SCR catalyst part is adsorbed by the downstream catalyst carrier to prevent ammonia slip. As a result, ammonia can be adsorbed to a high temperature, and the upper limit of the adsorbable amount can be increased.

  The best mode for carrying out the present invention will be described below with reference to the drawings. In FIG. 1, 1 is an internal combustion engine, and 2 is its exhaust passage. In this exhaust passage 2, from the upstream side, a pre-stage oxidation catalyst part 3 for increasing the oxidation activity of NO in the exhaust gas, an SCR catalyst part 4 for reducing NOx in the exhaust gas with ammonia, and ammonia A post-stage oxidation catalyst unit 5 for removal is sequentially arranged, and a urea aqueous solution is added 6 between the pre-stage oxidation catalyst unit 3 and the SCR catalyst unit 4. This configuration is the same as the conventional urea SCR system.

  The point of the present invention is that the structure of the SCR catalyst unit 4 is arranged in series by combining a plurality of catalyst carriers having different ammonia desorption temperatures.

  Specifically, a low temperature catalyst carrier A is arranged in the front stage and a high temperature catalyst carrier B is arranged in series in the front stage. The low temperature catalyst carrier A in the front stage is a weak acid strength adsorbent and is used for a high temperature in the rear stage. The catalyst carrier B is an adsorbent having a strong acid strength.

  As the adsorbent, for example, a zeolitic carrier is suitable. When the acid strength is changed by changing the structure of the zeolite, the adsorption power (desorption temperature) of ammonia also changes. One technique for measuring acid strength is ammonia TPD, in which ammonia is adsorbed at an acid point, and then desorption of ammonia is observed while the temperature is raised.

  In the present invention, as shown in FIG. 1, the ammonia 7 entering from the inlet of the SCR catalyst unit 4 is desorbed from the low-temperature catalyst carrier A upstream of the weak acid strength adsorbent, and then the strong acid strength adsorbent. Trapping is performed by the subsequent high-temperature catalyst carrier B. That is, by degrading the acid strength in ascending order, the ammonia desorbed when the catalyst temperature becomes higher at the low-temperature catalyst carrier A in the first stage can be held by the high-temperature catalyst carrier B at the second stage. . Thereby, as shown in FIG. 3, desorption at a high temperature is reduced and ammonia slip is prevented.

  Further, as shown in FIG. 2, in addition to the ammonia adsorption region of the solid curve by the low-temperature catalyst carrier A at the front stage of the low acid strength adsorbent, the high-temperature catalyst support B at the back stage of the strong acid strength adsorbent. The ammonia adsorption region of the dotted curve is expanded, ammonia can be adsorbed up to a high temperature, and the upper limit of the adsorbable amount can be expanded.

  Thus, in the present invention, the temperature at which ammonia is desorbed can be shifted to a high temperature by combining the low temperature catalyst carrier A and the high temperature catalyst carrier B in the SCR catalyst unit 4. Basically, the combination of these catalysts prevents ammonia from desorbing without reacting with NOx, but if ammonia still desorbs, it is combined with an ammonia removal catalyst. The ammonia removal catalyst has a problem that the operating temperature is high, but the ammonia removal catalyst works sufficiently in a high temperature region where ammonia desorption occurs from the high temperature catalyst.

System configuration diagram of the present invention The figure which shows the catalyst temperature and ammonia adsorption quantity with this system Conceptual diagram of the system of the present invention Conventional system configuration diagram Conceptual diagram of conventional system Conceptual diagram of conventional system

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 3 Pre-stage oxidation catalyst part 4 SCR catalyst part 5 Post-stage oxidation catalyst part 6 Add urea aqueous solution 7 Ammonia A Pre-stage low-temperature catalyst carrier B Rear-stage high-temperature catalyst carrier

Claims (3)

In order to remove the ammonia, a pre-stage oxidation catalyst part for increasing the oxidation activity of NO in the exhaust gas from the upstream side of the exhaust passage of an internal combustion engine, etc., an SCR catalyst part for reducing NOx in the exhaust gas with ammonia, In the urea SCR system in which the subsequent stage oxidation catalyst part is sequentially arranged and the urea aqueous solution is added between the preceding stage oxidation catalyst part and the SCR catalyst part,
The urea SCR system, wherein the SCR catalyst unit is arranged in series by combining a plurality of catalyst carriers having different desorption temperatures of ammonia.   2. The urea SCR system according to claim 1, wherein the SCR catalyst unit includes a low-temperature catalyst carrier disposed in a front stage and a high-temperature catalyst carrier disposed in a rear stage in series. 3.   3. The urea SCR system according to claim 2, wherein the low-temperature catalyst carrier in the former stage is a weak acid strength adsorbent and the high-temperature catalyst carrier in the latter stage is a strong acid strength adsorbent. system.

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