JP3321423B2 - Exhaust gas purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification method,
The present invention relates to an exhaust gas purification method for removing nitrogen oxides (NOx) in exhaust gas and not discharging unreacted substances in ammonia (NH ₃ ) injected as a reducing agent for the nitrogen oxides to the outside of the exhaust gas purification system.

[0002]

2. Description of the Related Art NOx in flue gas emitted from power plants, various factories, automobiles and the like is a cause of photochemical smog and acid rain. As an effective method of removing NOx, ammonia (NH ₃ ) is reduced. The flue gas denitration method by selective catalytic reduction as an agent has been widely used mainly in thermal power plants. As a catalyst, titanium oxide (Ti) containing vanadium (V), molybdenum (Mo) or tungsten (W) as an active component is used.
O ₂ ) -based catalysts are used. Particularly, those containing vanadium as one of the active components have not only high activity but also little deterioration by impurities contained in exhaust gas.
Because it can be used at lower temperatures, it has become the mainstream of current denitration catalysts (Japanese Patent Laid-Open No. 50-128681, etc.).

Meanwhile, in order to cope with the increase in power demand in recent years, especially in summer, the construction of a gas turbine or the construction of a cogeneration system using a gas turbine or the like is increasing mainly in the city center. Since these facilities are often installed adjacent to artificially crowded areas and the regulations on the emission of nitrogen oxides (NOx) are total, the amount of NOx in the exhaust gas discharged from the facilities is extremely low. It is hoped that it will be suppressed. For this reason, a method of operating the denitration apparatus at a high denitration rate by increasing the amount of catalyst and increasing the amount of injected ammonia has been studied.

[0004] With the demand for such advanced denitration, it has become essential that unreacted ammonia (hereinafter referred to as "leakage ammonia") not used in the denitrification reaction be reduced to the NOx level or less, and the leakage ammonia is reduced. Therefore, installation of an ammonia decomposition catalyst in a downstream part of the denitration catalyst is being studied. Research on catalysts for decomposing this leaked ammonia has been conducted in the past, and catalysts using copper (Cu), iron (Fe), or the like as an active component having excellent ammonia oxidation activity have been known (Japanese Patent Application Laid-Open (JP-A) No. 2002-110572). Showa 52-43
767, etc.).

[0005]

Among the above prior arts, the method of simply increasing the denitration catalyst has a problem that unreacted ammonia increases. This is because the denitration device is NH ₃ / N
FIG. 5 shows the behavior of the NOx concentration and the leak ammonia concentration at the catalyst layer outlet when the operation was performed while changing the Ox ratio.
As shown by the solid line, when the NH ₃ / NOx ratio is operated near 1, a high denitration rate can be obtained, but the leak ammonia is also reduced to NH 3
₃ / NOx suddenly increases from around 1.

In order to reduce the leakage ammonia, the catalyst layer is divided into two layers, a conventional denitration catalyst is installed in the upstream part, and a conventional ammonia decomposition catalyst is installed in the downstream part as shown in FIG. Although the leak ammonia when the NH ₃ / NOx ratio is increased certainly decreases, the NO generation reaction as expressed by the following equation occurs simultaneously due to the oxidative decomposition of NH ₃ , and the high denitration rate is increased as shown in FIG. There was a problem that it could not be obtained. NH ₃ + 5 / 4O ₂ → NO + 3 / 2H ₂ O (1)

Accordingly, an object of the present invention is to provide a method for purifying exhaust gas which removes nitrogen oxides in exhaust gas and does not discharge unreacted substances in ammonia injected as a reducing agent for the nitrogen oxides to the outside of the exhaust gas purification system. To provide.

[0008]

The above object of the present invention is solved by the following constitution. And nitrogen oxides in the exhaust gas, and ammonia was injected as a reducing agent for nitrogen oxides, in contact with the catalyst, a method of removing nitrogen oxides in an exhaust gas upstream side portion, nitrogen oxides by catalytic reduction Treated using a denitration catalyst to remove titanium, titanium (Ti),
Ti-V of at least one element selected from vanadium (V), tungsten (W), and molybdenum (Mo);
Ti-Mo, Ti-W, Ti-VW or Ti-Mo
A composition comprising an oxide composed of any combination of -V or a zeolite supporting copper (Cu) or iron (Fe) as a first component, and previously supported on a porous material selected from zeolite, alumina, and silica Platinum (P
t) Oxidative decomposition of residual ammonia when treated with the denitration catalyst using a catalyst comprising a composition containing a precious metal-containing composition selected from palladium (Pd) and rhodium (Rh) as a second component An exhaust gas purifying method comprising removing nitrogen oxides generated after treatment and oxidative decomposition of the residual ammonia .

Further, the exhaust gas purifying method of the present invention provides a catalyst for decomposing unreacted ammonia among nitrogen oxides in the exhaust gas and ammonia injected into the exhaust gas as a reducing agent for the nitrogen oxide. This is an exhaust gas purification method using an ammonia decomposition catalyst having a denitration function.

As the first component and the second component of the ammonia decomposition catalyst having the above-mentioned denitration function, for example, the following components are used, and both are used so that the concentration of the noble metal element becomes more than 0 and less than 1000 ppm. After mixing and kneading by adding water, the mixture is formed into a plate, a honeycomb, or a granule by a known method, and then fired at a predetermined temperature using a catalyst.

(A) As the first component, Ti-V, Ti-
An oxide of any combination of Mo, Ti-W, Ti-V-W or Ti-Mo-V, or a zeolite such as mordenite supporting copper (Cu) or iron (Fe) is used.

(B) As the second component, a salt of a noble metal such as chloroplatinic acid, palladium nitrate or rhodium chloride or a composition in which the above noble metal element is previously supported on zeolite, porous silica or porous alumina by ion exchange impregnation or the like. Use things.

[0013]

FIG. 1 shows a model of pores of a catalyst used in an exhaust gas purifying method according to the present invention. It has a structure in which micropores such as zeolite are formed in places of macropores formed by the denitration catalyst component (first component), and the noble metal element-containing second component is supported in the micropores. . With such a structure, the ammonia that is easily adsorbed by the denitration catalyst component is selectively adsorbed by the denitration catalyst component at the entrance of the macropore, and is consumed by reacting with the diffused NOx as shown in FIG. For this reason, it does not reach the noble metal in the micropore having a large diffusion resistance, and shows a high NOx removal rate similar to that of a normal denitration catalyst.

On the other hand, when the NOx decreases and the adsorbed ammonia is no longer consumed, the ammonia diffuses into the micropores and reaches the noble metal, and (1) and (2)
The oxidation reaction by oxygen represented by the formula proceeds. NH ₃ + 5 / 4O ₂ → NO + 3 / 2H ₂ O (1) NH ₃ + 3 / 4O ₂ → 1 / 2N ₂ + 3 / 2H ₂ O (2)

The NO generated here collides with the ammonia adsorbed on the inner surface of the macropores in the process of diffusing from the micropores to the macropores as shown in FIG. It is reduced to nitrogen by the reaction. NO + NH ₃ + ／ O ₂ → N ₂ + 3 / 2H ₂ O (3) Therefore, NO is not generated, and the denitration rate is not reduced.

As described above, the catalyst used in the present invention operates in the same manner as a normal denitration catalyst when NO is present, and when NOx is consumed and ammonia becomes excessive,
This is a novel catalyst that can add ammonia to nitrogen by the synergistic action of the oxidation action of ammonia by the catalytic action of noble metal and the action of the denitration catalyst.

[0017] Therefore, in the case of using a catalyst used in the exhaust gas purifying method of the present invention alone, the leak ammonia amount shown by the solid line in FIG. 5 that occurs with increasing the NH 3 _/ NO ratio in question in the prior art As can be seen from the broken line in FIG.

Further, the catalyst used in the exhaust gas purifying method of the present invention is installed in the downstream part of the two-layer reactor shown in FIG. 6 (an ordinary denitration catalyst is installed in the upstream part) to oxidatively decompose leaked ammonia. In this case, NOx is not generated, so that a high denitration rate can be obtained as shown by a broken line in FIG. 7 without deterioration of the denitration rate which is a problem in the conventional ammonia decomposition catalyst as shown by the solid line in FIG. .

Further, in the prior art, a large amount of ammonia decomposing catalyst is required as much as the denitration catalyst. On the other hand, when the catalyst used in the present invention is used, when NOx is present, it functions as a denitration catalyst. Can be reduced,
As a result, the total amount of catalyst can be extremely reduced.

As described above, the catalyst used in the present invention may be used alone or may be installed downstream of a conventional denitration catalyst to constitute a system having a small amount of leaked ammonia while maintaining a high denitration rate. Is what makes it possible.

The catalyst used in the present invention is characterized by the constitution of the catalyst as described above, and it goes without saying that any preparation method can be adopted as long as it can realize such a structure. . However, a better catalyst can be obtained by using the following method.

Among the catalyst components, the first component may be any one of the various components described above. Particularly, as the catalyst component, Ti-V, Ti-V-Mo, Ti-W-V, etc. Good results are obtained when an elemental oxide catalyst is used. These are prepared by adding salts such as vanadium, molybdenum, and oxyacid salts of tungsten to a slurry of hydrous titanium oxide such as metatitanic acid, forming a paste while evaporating water using a heating kneader, drying, and then drying at 400 ° C. To 700 ° C., and if necessary, pulverized.

Further addition of the second component is pre-Me zeolite,
It is preferable to prepare a porous material such as silica or alumina supported by ion exchange or kneading in a micropore, and to add it to the first component. The zeolite used for the second component is mordenite, clinoptilolite, erionite, a hydrogen-substituted zeolite selected from among Y-type zeolites,
Sodium type and calcium type can be used. Further, silica and alumina having a surface area of 100 m ² / g to 500 m ² / g obtained by calcining a hydrated oxide at a low temperature are used. These particles have a particle size of about 1 to 10 μm, and can be used after being ground to such an extent that the structure of zeolite or the like is not destroyed. Add these precious metals to their chlorides, nitrates,
Alternatively, the powder is immersed in an aqueous solution dissolved in the form of an ammine complex for ion exchange, or is evaporated to dryness together with the aqueous solution to obtain a powder carrying 0.01 wt% to 0.1 wt% of a noble metal,
Used as the second component.

The obtained first and second components have a weight ratio of the second component / the first component (hereinafter, the ratio of the second component / the first component) of 20 /
80 to 0.5 / 99.5, preferably 10/90 to 1 /
99, and water, an inorganic binder, a molding aid, an inorganic fiber, and other well-known moldability improvers are added, and the mixture is kneaded by a kneader to form a paste-like catalyst mixture. The obtained paste catalyst is applied to a network substrate made of inorganic fibers, a metal substrate roughened by thermal spraying or the like, and formed into a plate catalyst, or formed into a column shape or a honeycomb shape by an extruder.

The second component / first component ratio is particularly important in the present invention, and when the second component / first component ratio is larger than the above-mentioned range, NOx is generated and the denitration rate is reduced,
When it is small, there is a problem that the decomposition rate of ammonia cannot be increased. Particularly, in the above-mentioned range, zeolite, silica, alumina or the like having a large noble metal loading is selected so that the second component / first component ratio is small, and the noble metal loading of the entire catalyst is desirably 1 to 1000 ppm. A range of 10 to 100 ppm gives good results. Which, as indicated in the model of Figure 1, micropores formed in the second component is sparsely be present in the macropores to form in the first component, NH ₃ to be selectively adsorbed denitration reaction in the first component This is to make it easy to use.

In addition to the economical effect that the unit price of the catalyst can be reduced by reducing the amount of the noble metal, there is also an effect that the denitration reaction and the oxidation reaction of ammonia are easily separated depending on the presence or absence of NO.

Further, when using a single catalyst, the ratio of the second component / first component is selected to be small, and when using a two-layer reactor as shown in FIG. It is easy to obtain a good result if the first component ratio is large and the noble metal content is large.

[0028]

The present invention will be described below in detail with reference to examples. Example 1 Metatitanate slurry (TiO ₂ content: 30 wt%, S
O ₄ content: 8 wt%) 67 kg of ammonium paratungstate _{((NH 4) 10 H 10} · W 12 O 46 ·
6H ₂ O) and 3.59 kg of ammonium metavanadate were added and kneaded while evaporating water using a heating kneader to obtain a paste having a water content of about 36%. This was extruded into a column of 3 mmφ, granulated, dried with a fluidized drier, and then fired at 550 ° C. in the atmosphere for 2 hours. The obtained granules were pulverized with a hammer mill to a particle size of 1 μm to 60% or more to obtain a denitration catalyst powder as a first component. The composition at this time is V / W / Ti = 4/5/91 (atomic ratio).

On the other hand, chloroplatinic acid (H ₂ [PtC ₁₆ ].
To a solution of 0.665 g of (6H ₂ O) in 1 liter of water was added 500 g of H-type mordenite having an Si / Al atomic ratio of about 21 and an average particle size of about 10 μm, and evaporated to dryness on a sand bath to remove Pt. Carried. This was dried at 180 ° C. for 2 hours, and calcined in air at 500 ° C. for 2 hours to obtain 0.05 wt% Pt-.
Mordenite was prepared and used as the second component.

Separately, a net obtained by plain weaving a twisted yarn consisting of 1400 E glass fibers having a fiber diameter of 9 μm with a roughness of 10 yarns / inch was used to obtain 40% of titania, 20% of silica sol,
Impregnated with 1% polyvinyl alcohol slurry, 15
It was dried at 0 ° C. to give rigidity, and a catalyst substrate was obtained.

To 20 kg of the first component and 408 g of the second component, 5.3 kg of silica-alumina-based inorganic fiber and 17 kg of water were added and kneaded with a kneader to obtain a catalyst paste. The prepared paste-form catalyst mixture was placed between the two catalyst substrates, and the catalyst was pressed between the stitches and the surface of the substrate by passing through a pressure roller to obtain a plate-like catalyst having a thickness of about 1 mm. . The obtained catalyst was dried at 180 ° C. for 2 hours, and then dried in air at 50 ° C.
It was baked at 0 ° C. for 2 hours. The second component / first component ratio of the first component and the second component in the present catalyst is 2/98, and the Pt content corresponds to 10 ppm excluding the catalyst base material and inorganic fibers.

Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that the second component was not added.

Comparative Example 2 A catalyst was obtained by using titania (manufactured by Ishihara Sangyo Co., Ltd., trade name: CR50) produced by a chlorine method in place of the first component of Example 1.

Test Example 1 The catalysts of Example 1 and Comparative Examples 1 and 2 were
Three reactors cut into 100 mm × length 100 mm were filled into the reactor at intervals of 3 mm, and the denitration rate and the unreacted ammonia at the reactor outlet were measured when the amount of ammonia was changed under the conditions shown in Table 1.

[0035]

[Table 1]

FIG. 4 shows the obtained results. FIG.
As shown in the above, when the catalyst of Example 1 was increased in the amount of injected ammonia and the NH ₃ / NO ratio was increased, although the denitration rate was equivalent to that of the denitration catalyst component of Comparative Example 1 alone, the reaction did not proceed. The ammonia concentration at the outlet of the vessel is several ppm
And low. On the other hand, in Comparative Example 1, as the NH ₃ / NO ratio increases, high-concentration ammonia is detected at the reactor outlet. On the other hand, in the case of using titania containing the second component of Comparative Example 2 but having no denitration activity, although the NH ₃ concentration at the reactor outlet was low, a large amount of NOx was generated, and the denitration rate became negative.

As can be seen from the results, the catalyst according to the present embodiment has a high denitration rate as a normal denitration catalyst when the NH ₃ / NO ratio is low due to the concerted action of the first component and the second component as described above. This shows that when NOx is consumed in the denitration reaction, excess ammonia can be reduced without generating NOx.

Examples 2 and 3 In place of H-type mordenite in Example 1, fine silica powder (Microcomputer F, manufactured by Tomita Pharmaceutical Co., Ltd.) (Example 2) and γ-alumina powder (Example 3) A second component was prepared in the same manner as described above, and a catalyst was prepared using the second component and the first component in Example 1 at a ratio of the second component / first component of 1/9.

Example 4 A catalyst was prepared in the same manner as in Example 1 except that 833 ml of an aqueous solution of chloroplatinic acid (Pt concentration: 1.2 mg / ml) was used instead of the second component in Example 1.

Example 5 In place of ammonium paratungstate in the first component preparation method of Example 1, ammonium paramolybdate ((N
Other using _{H 4) 6 · Mo 7 O} 24 · 4H 2 O) was similarly catalyst preparation as in Example 1.

Examples 6 and 7 The chloroplatinic acid in Example 1 was replaced with palladium nitrate (Pd
(NO ₃ ) ₃ ) (Example 6) and rhodium nitrate (Rh)
(NO ₃ ) ₃ ) (Example 7) was changed to the nitric acid solution, and mordenite having a palladium or rhodium loading of 0.05 wt% was prepared. This was added to the first component of Example 1 in the same manner as in the case of Pt-mordenite to prepare a catalyst.

Examples 8 to 10 The ratio of the second component / first component in Example 1 was from 2/98 to 0.5 / 99.5 (Example 8), 1/9 (Example 9),
2/8 (Embodiment 10), and the others were changed to Embodiment 1
A catalyst was prepared in the same manner as described above.

Examples 11 to 13 The second component was prepared in the same manner as in Examples 8 to 10 except that the amount of chloroplatinic acid added in Example 1 was changed to 2.66 g . Catalysts were prepared at respective component ratios.

Comparative Example 3 In Example 5, a catalyst was prepared using only the first component without adding the second component.

Comparative Examples 4 to 7 Catalysts were prepared using the same titania as used in Comparative Example 3 except that the first component in Examples 2, 3, 6 and 7 was replaced.

Test Example 2 For the catalysts of Examples 1 to 13 and Comparative Examples 1 to 7, the ammonia concentration was kept constant at 280 ppm under the conditions shown in Table 1.
The denitration rate and the decomposition rate of unreacted ammonia were measured. Table 2 summarizes the obtained results. Here, the decomposition rate of unreacted ammonia was determined by the following equation. Ammonia decomposition rate (%) = ｛NH ₃ concentration at reactor outlet /
(Reactor inlet NH ₃ concentration - NH consumed by denitration reaction
₃ concentration) x 100

[0047]

[Table 2]

As is clear from Table 2, the catalysts used in the examples of the present invention showed higher denitration rates and decomposition rates of unreacted ammonia than those of the comparative examples. It can be seen that the catalyst is an excellent catalyst that can prevent the leakage of water.

When the catalyst used in the present invention is used alone, the outflow of unreacted ammonia when the denitration apparatus is operated at a high NH ₃ / NOx ratio can be extremely reduced. In addition, if the catalyst used in the present invention is installed downstream of another high-activity denitration catalyst and is used to decompose unreacted ammonia (leakage ammonia), the outflow of unreacted ammonia due to imbalance in the amount of injected ammonia is eliminated. This makes it possible to operate a denitration apparatus desired in the suburbs of a city at a high denitration rate.

Further, the catalyst used in the present invention functions as a denitration catalyst when NOx is present, and functions as an ammonia decomposition catalyst when NOx is not present. In addition, NOx is hardly generated even by decomposition of ammonia, so that compared to a case where a denitration catalyst and a conventional ammonia decomposition catalyst are formed in two layers,
Another feature is that the amount of catalyst used can be significantly reduced.

According to the exhaust gas purifying method of the present invention, high NH
The outflow of unreacted ammonia when the denitration device is operated at a ₃ / NOx ratio can be extremely low, and unreacted ammonia generated due to imbalance in the amount of injected ammonia does not flow out of the exhaust gas purification system. It is possible to operate a rarely used denitration apparatus at a high denitration rate.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a catalyst for illustrating the characteristics of the catalyst used in the present invention.

FIG. 2 is a conceptual diagram showing the action of a catalyst used in the present invention.

FIG. 3 is a conceptual diagram showing the action of a catalyst used in the present invention.

FIG. 4 is a diagram showing a comparison between the denitration performance and the amount of unreacted ammonia of the catalysts of Example 1 and Comparative Examples 1 and 2.

FIG. 5 is a view showing the behavior of NOx and unreacted ammonia at the outlet of a denitration apparatus using a conventional catalyst and the catalyst used in the present invention.

FIG. 6 is a configuration diagram when a denitration catalyst and an ammonia decomposition catalyst are used in two layers.

FIG. 7 is a diagram showing a problem when a denitration catalyst and an ammonia decomposition catalyst are used in two layers.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94

Claims (1)

(57) [Claims] 1. A method for removing nitrogen oxides by contacting a nitrogen oxide in an exhaust gas and ammonia injected as a reducing agent for the nitrogen oxide with a catalyst, wherein a catalytic reduction Using a denitration catalyst that removes nitrogen oxides by means of a catalyst. In the downstream part of the exhaust gas, titanium (Ti), vanadium (V), tungsten (W), molybdenum (M
o) one or more elements selected from the group consisting of Ti-V, Ti
-Mo, Ti-W, Ti-V-W or Ti-Mo-V
Oxide or copper (C
u) or a composition comprising a zeolite supporting iron (Fe) as a first component, and platinum (P) previously supported on a porous material selected from zeolite, alumina and silica.
t) Oxidative decomposition of residual ammonia when treated with the denitration catalyst using a catalyst comprising a composition containing a precious metal-containing composition selected from palladium (Pd) and rhodium (Rh) as a second component An exhaust gas purification method comprising performing a treatment and removing nitrogen oxides generated after oxidative decomposition of the residual ammonia.