JP4455159B2 - Urea SCR system - Google Patents

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.

  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 includes a pre-stage oxidation catalyst unit 3 for increasing the oxidation activity of NO 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 the reducing agent in the system.

There is a possibility that the response delay of the above (1) 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 delay at low temperatures. Therefore, there was room for research on improving response delay.

The temperature range in which urea cannot be injected in the above (2) 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.

Insufficient storable amount of the reducing agent (3) cannot be dealt with by simply 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 is 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.

As described above, 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 exhaust gas is insufficient, and that NOx is not 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.

The urea SCR system according to the present invention for achieving the above object is characterized by the following problem solving means. That is, the present invention
The exhaust passage of an internal combustion organizations, from upstream to downstream of the exhaust passage, a pre-stage oxidation catalyst unit for increasing the oxidation activity of the NO in the exhaust gas, the NOx in the exhaust gas for reducing with ammonia SCR catalyst part and a rear-stage oxidation catalyst part for removing ammonia are sequentially arranged , and a urea SCR in which a urea aqueous solution is added between the front-stage oxidation catalyst part and the SCR catalyst part in the exhaust passage In the system,
The SCR catalyst portion is composed of a combination of a plurality of catalyst carriers having different ammonia desorption temperatures, and the plurality of catalyst carriers are arranged in series before and after each other , and in the SCR catalyst portion, In the relative relationship between the catalyst carrier located on the front stage side and the catalyst carrier located on the rear stage side, the catalyst carrier on the front stage made for low temperature is composed of a weak acid strength adsorbent and the latter stage made for high temperature. The catalyst support on the side is composed of a strong acid strength adsorbent .

  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 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 applying the slope in order of decreasing acid strength, the ammonia desorbed due to the catalyst temperature becoming higher in the low-temperature catalyst carrier A portion in the preceding stage can be held in the subsequent high-temperature catalyst carrier B. Thus, as shown in FIG. 3 , desorption at a high temperature is reduced, so that ammonia slip can be prevented .

Further, in the present invention, as shown in FIG. 2, in addition to the ammonia adsorption region of the solid curve by the low-temperature catalyst carrier A in the preceding stage of the adsorbent having a weak acid strength, the high-temperature catalyst in the subsequent stage of the adsorbent having a strong acid strength. since expanded ammonia adsorption region of the dotted line curve due to carrier B, it is possible to adsorb ammonia to a high temperature, it is possible to increase the upper limit of the adsorption capacity.

The present invention thus can transition from those which constitute the SCR catalyst unit 4 by combining the catalyst carrier A and the high temperature catalyst support B for the low-temperature, the temperature at which desorption of ammonia at a high temperature. 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 possible quantity with this invention system Conceptual diagram of the system of the present invention Conventional system configuration diagram Conceptual diagram of conventional system Conceptual diagram of conventional system

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 (1)

The exhaust passage of an internal combustion organizations, from upstream to downstream of the exhaust passage, a pre-stage oxidation catalyst unit for increasing the oxidation activity of the NO in the exhaust gas, the NOx in the exhaust gas for reducing with ammonia SCR catalyst part and a rear-stage oxidation catalyst part for removing ammonia are sequentially arranged , and a urea SCR in which a urea aqueous solution is added between the front-stage oxidation catalyst part and the SCR catalyst part in the exhaust passage In the system,
The SCR catalyst portion is composed of a combination of a plurality of catalyst carriers having different ammonia desorption temperatures, and the plurality of catalyst carriers are arranged in series before and after each other , and in the SCR catalyst portion, In the relative relationship between the catalyst carrier located on the front stage side and the catalyst carrier located on the rear stage side, the catalyst carrier on the front stage made for low temperature is composed of a weak acid strength adsorbent and the latter stage made for high temperature. A urea SCR system characterized in that the catalyst support on the side is composed of an adsorbent with strong acid strength .

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