JP2013099715A - Catalyst for removing nitrogen oxide contained in combustion exhaust gas, method for removing nitrogen oxide using the catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method that can efficiently remove a nitrogen oxide contained in an exhaust gas at a temperature around an ammonium sulfate precipitation temperature.SOLUTION: The method for removing a nitrogen oxide contained in an exhaust gas includes bringing exhaust gas into contact with a catalyst at a temperature ranging from 200 to 350°C, wherein the catalyst is produced by ion exchange of an MFI-type zeolite with at least one element selected from Fe, Co and Mn.

Description

  The present invention relates to a catalyst for removing nitrogen oxides (hereinafter referred to as NOx) in combustion exhaust gas and a method for removing NOx using the catalyst.

  A method for removing NOx contained in combustion exhaust gas is an ammonia selective catalytic reduction in which ammonia is added to exhaust gas as a reducing agent and NOx is removed by contacting the exhaust gas with a denitration catalyst mainly composed of vanadium or titania. The method (hereinafter referred to as the ammonia SCR method) is the mainstream.

Sulfur oxide (hereinafter referred to as SOx) is present together with NOx in the combustion exhaust gas of fuel containing sulfur such as heavy oil. The denitration temperature of such combustion exhaust gas must be equal to or higher than the precipitation temperature of ammonium sulfate from the viewpoint of preventing the precipitation of ammonium sulfate on the catalyst. The precipitation temperature of ammonium sulfate is related to the SO ₃ and ammonia concentrations in the exhaust gas. The relationship is shown in FIG. As can be seen from FIG. 1, as the SO ₃ and ammonia concentrations increase, the precipitation temperature of ammonium sulfate increases.

The flue gas of a coal-fired boiler, generally SOx concentration S0 ₃ concentration thereof of 10% 10ppm for a 100 ppm, ammonia concentration for a NOx concentration is 500ppm is maximum 500ppm. The precipitation temperature of ammonium sulfate under this condition is about 250 ° C. The exhaust gas temperature at this time is usually 300 to 400 ° C., and since the ammonium sulfate precipitation temperature is higher than 250 ° C., ammonium sulfate does not precipitate on the catalyst, and stable catalyst performance can be maintained.

  On the other hand, since the exhaust gas discharged from the marine engine is C heavy oil, the SOx concentration in the exhaust gas is 1000 ppm, the NOx concentration is as high as 1000 ppm, and the precipitation temperature of ammonium sulfate is as high as about 280 ° C. However, since the exhaust gas temperature is as low as 250 ° C., ammonium sulfate precipitates under these conditions, and stable catalyst performance cannot be maintained.

  Therefore, in order to treat marine engine exhaust gas by the selective catalytic reduction method, it is necessary to heat the exhaust gas temperature to a temperature higher than the ammonium sulfate precipitation temperature.

In order to use an ammonia SCR catalyst for the treatment of marine engine exhaust gas having a high concentration of SOx and NOx and a low exhaust gas temperature, it is necessary to set the precipitation temperature of ammonium sulfate below the exhaust gas temperature. Precipitation temperature of ammonium sulfate is determined by S0 ₃ concentration and ammonia concentration as shown in FIG. Since the ammonia concentration is determined by the NOx concentration in the exhaust gas and the target denitration rate, this value cannot be reduced. Therefore, the ammonia SCR method cannot be used in a marine engine having a low exhaust gas temperature.

  As a denitration method other than the ammonia SCR method, a direct decomposition method (such as Patent Document 1) in which NOx is simply brought into contact with a catalyst made of a metal composite oxide having ionic conductivity and decomposed into nitrogen and oxygen, or as a reducing agent. There is a denitration method in which NOx is brought into contact with a catalyst using hydrocarbons. As the latter catalyst, a catalyst in which a metal is supported on β zeolite (Patent Document 2), a catalyst mainly composed of alumina, aluminum sulfate, and silver (Patent Document 3) )It has been known. However, the reaction temperature is as high as 600 ° C. or more in the former method, and about 350 ° C. in the latter method.

  Patent Document 4 describes a direct cracking method that does not use a reducing agent. In Comparative Example 3, a catalyst in which a metal is supported on hydrogenated zeolite is used, and the NOx removal rate is 10% or less at a temperature of 400 ° C. It has been shown. However, in this catalyst, a part of the proton acid point, which is an acid point stronger than the metal acid point, remains even after the metal is supported, so that NO preferentially attacks the stronger acid point and the denitration reaction does not proceed easily.

Japanese Patent Laid-Open No. 11-151440 JP 2004-358454 A JP 2010-29783 A Japanese Patent Publication No. 07-106300

  As described above, conventionally, no catalyst having practical catalytic performance has been found for low-temperature exhaust gas at a temperature around 250 ° C. like a marine engine.

  The cause is that in the direct decomposition method and the reduction removal method, oxygen generated when NO decomposes on the catalyst is not removed from the catalyst surface. Therefore, if it is possible to remove oxygen on the catalyst surface, it is considered that a direct decomposition method and a reduction removal method are possible.

  As a result of repeated investigations, the inventors of the present invention conducted exhaust gas to a temperature of 200 to 350 ° C., preferably 220 to 320 ° C., more preferably, a catalyst in which MFI type zeolite was ion-exchanged with at least one of Fe, Co, and Mn. It has been found that NOx can be effectively reduced by contacting in the range of 230 to 300 ° C, more preferably 230 to 270 ° C.

  The MFI type zeolite may be a commercially available product.

  The ion exchange method may be, for example, a method of suspending MFI-type zeolite in an aqueous solution containing a precursor compound of Fe, Co and / or Mn, drying and then firing the zeolite after filtration and washing. The precursor compound may be an inorganic acid salt (eg, nitrate, sulfate) or an organic acid salt (eg, acetate) of Fe, Co and / or Mn.

The zeolite after ion exchange with at least one of Fe, Co, and Mn has an acid point in the sodium, the above ion exchange metal, and the proton, and the acid strength at each acid point is proton> ion exchange metal> Sodium order. The denitration reaction proceeds by decomposing nitric oxide at the acid sites of the ion exchange metal.

It is a graph which shows that the precipitation temperature of ammonium sulfate / ammonium hydrogen sulfate is related to sulfur trioxide concentration and ammonia concentration. It is a schematic flowchart which shows the test apparatus for performing the performance evaluation of a catalyst.

  Next, the present invention will be specifically described based on examples.

Example 1
After Fe / zeolite catalyst commercially available MFI zeolite 4g of 0.05M of _{Fe (NO 3) 3 · 9H} 2 0 aqueous 500m1, and the suspension 6 hours at 50 ° C., filtered, washed, 12 at 120 ° C. It was dried for 4 hours and then calcined at 400 ° C. for 6 hours. Thus, an Fe ion exchange zeolite catalyst was obtained. The Fe loading of this catalyst was 2.0 wt%.

Example 2
Co / zeolite catalyst 4 g of commercially available MFI type zeolite was suspended in 500 ml of 0.01 M Co (NO ₃ ) ₂ aqueous solution at 80 ° C. for 18 hours, then filtered, washed, and dried at 120 ° C. for 12 hours. Furthermore, it baked at 500 degreeC for 4 hours. In this way, a Co ion exchange zeolite catalyst was obtained. The amount of Co supported by this catalyst was 2.31 wt%.

Example 3
Mn / zeolite catalyst 4 g of commercially available MFI-type zeolite was suspended in 500 ml of 0.01M Mn (CH ₃ COO) ₂ aqueous solution at 80 ° C. for 18 hours, then filtered, washed and dried at 120 ° C. for 12 hours. Further, it was calcined at 500 ° C. for 4 hours. In this way, a Mn ion exchange zeolite catalyst was obtained. The amount of Mn supported by this catalyst was 4.0 wt%.

Comparative Example 1
A1 ₂ 0 ₃ 4g of 0.05M of _{Fe (NO 3) 3 · 9H} 2 0 was suspended 6 hours at 80 ° C. in an aqueous solution 500 ml, which was filtered, dried at 120 ° C. 12 hours, further 400 ° C. For 6 hours. Thus, an Fe-supported A1 ₂ 0 ₃ catalyst was obtained. The Fe loading of this catalyst was 2.0 wt%.

Comparative Example 2
After suspending 4 g of commercially available β-type zeolite in 500 ml of 0.01 M Co (NO ₃ ) ₂ aqueous solution at 80 ° C. for 18 hours, this was filtered, washed, dried at 120 ° C. for 12 hours, and further at 500 ° C. Baked for 4 hours. In this way, a Co ion exchange zeolite catalyst was obtained. The amount of Co supported by this catalyst was 3.8 wt%.

Evaluation of Catalyst Performance The catalyst obtained above was pulverized after press molding and sized to a mesh size of 28 to 14, and the obtained granular catalyst was subjected to an evaluation test using the test apparatus shown in FIG. In FIG. 2, (1) is a denitration reactor, (2) is an evaporator, (3) is a pump, and (4) is a water tank. Table 1 shows the test conditions.

Table 2 shows the test results. The reason why NO ₂ is generated at the outlet is due to the equilibrium relationship. NO is oxidized to NO ₂ by oxygen in the air in the supply gas. The denitration rate can be improved by increasing the amount of catalyst.

  In the examples, the denitration rate higher than that of the comparative example was obtained in any of the catalysts, indicating that the catalyst of the present invention has a high catalytic performance at a reaction temperature of 250 ° C.

Claims (3)

  A catalyst for removing nitrogen oxides in combustion exhaust gas, wherein MFI-type zeolite is ion-exchanged with at least one of Fe, Co, and Mn.   A method for removing nitrogen oxides in exhaust gas, wherein the exhaust gas is brought into contact with the catalyst according to claim 1 at a temperature of 200 to 350 ° C.   The method according to claim 2, wherein the exhaust gas is brought into contact with the catalyst at a temperature of 220 to 320 ° C.

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