JPH08309152A - Purification method for exhaust gas - Google Patents

Abstract

PURPOSE: To perform denitration at high efficiency while minimizing the discharge of unreacted ammonia by placing a carbon monoxide removal catalyst layer on the upper step side of an ammonia addition apparatus and the first denitration catalyst layer, an ammonia decomposition catalyst layer, and the second denitration catalyst layer in sequence from above on the lower step side and adding an equivalent or more amount of ammonia to nitrogen oxides in exhaust gas. CONSTITUTION: A carbon monoxide removal catalyst layer 1 is placed on the upper step side of an ammonia addition apparatus 2, and carbon monoxide is removed by catalytic combustion. The first denitration catalyst layer 3 is placed downstream from the apparatus 2, an ammonia decomposition catalyst layer 4 is placed downstream from the layer 3, and the second denitration catalyst layer 5 is placed downstream from the layer 4. An equivalent or more amount of ammonia to NOX is added from an ammonia addition apparatus to the upstream side of the first denitration catalyst layer 3, and about 80% or more of the denitration is conducted in the first denitration catalyst layer 3. Unreacted ammonia flowing out is decomposed in a ammonia decomposition apparatus 4 and is led downstream into the second denitration catalyst layer 5. An ammonia decomposition catalyst is used in which Pd and/or Pt as active metals and metal oxide are supported on crystalline silicate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas capable of removing nitrogen oxides (NOx) and carbon monoxide (CO) in exhaust gas with high efficiency.

[0002]

2. Description of the Related Art As a method for removing NOx contained in combustion exhaust gas, a selective catalytic reduction method using NH ₃ as a reducing agent has been widely put to practical use mainly in thermal power plants. As the catalyst, a titanium oxide-based catalyst containing vanadium, tungsten, and molybdenum as active components is mainly used.

[0003]

In recent years, NOx emission regulations have been adopted.
It tends to become more and more severe, especially in large cities.
The amount of electricity is regulated, and at power plants adjacent to urban areas
More expansion of power generation equipment as power demand increases
Highly efficient denitration is required. In addition, the
The emission control of carbon monoxide emitted is also becoming strict. Obedience
The traditional denitration method is NH ₃Is a catalytic reduction method using
4NO + 4NH₃+ O₂→ 4N₂+ 6H₂O reaction formula
Therefore, NOx is N on the catalyst.₂Is decomposed into. This reaction formula
From a theoretical point of view, theoretically, NH is equimolar to NOx.₃With
If added, 100% of NOx can be removed. Only
However, in reality, NH in the exhaust gas₃And NOx are completely uniform
It is impossible to mix, and to perform highly efficient denitration
NH₃Needs to be added in excess of NOx. So
Unreacted due to NH₃Is discharged in a considerable proportion
there were.

To date, the present inventors have developed an ammonia decomposition catalyst to prevent the emission of unreacted NH ₃ (Japanese Patent Application No.
-127126, Japanese Patent Application No. 6-70486, etc.)
We proposed a process to install high-efficiency denitration by installing a finishing denitration catalyst in the rear (Japanese Patent Application No. 6-176494). But,
When carbon monoxide coexists in the exhaust gas, the NH ₃ decomposition rate and the nitrogen selectivity of the ammonia decomposition catalyst change.

In view of the above-mentioned state of the art, the present invention solves the drawbacks of the prior art and enables highly efficient denitration by suppressing the discharge of unreacted NH _{3 to} the atmosphere as much as possible, and further carbon monoxide It is intended to provide a method of purifying exhaust gas that can be removed.

[0006]

According to the present invention, (1) an exhaust gas containing nitrogen oxides and carbon monoxide is introduced into a reactor filled with a catalyst to catalytically remove nitrogen oxides using ammonia as a reducing agent. In the removal method, the carbon monoxide removal catalyst layer is on the upper side of the ammonia addition device, the first denitration catalyst layer is on the lower side of the ammonia addition device from the upstream side, and ammonia is oxidized to nitrogen and nitrogen oxides in the subsequent flow. An ammonia decomposing catalyst layer having a function of decomposing is further provided in the subsequent stream to the second
A denitration catalyst layer is installed, and ammonia added from an ammonia addition device is added to the exhaust gas at a reaction equivalent amount or more of nitrogen oxides in the exhaust gas at the inlet, a method for purifying exhaust gas,
(2) When the ammonia decomposition catalyst is dehydrated, (1.
0 ± 0.6) R ₂ O ・ [aM ₂ O ₃・ bAl ₂ O ₃ ] ・
cMeO.ySiO ₂ (R: alkali metal ion and /
Or hydrogen ion, M: at least one element selected from the group consisting of Group VIII elements of the Periodic Table, rare earth elements, titanium, vanadium, chromium, niobium, antimony and gallium,
Me: alkaline earth metal element, a + b = 1, a ≧ 0, b
≧ 0, c ≧ 0, y / c> 12, y> 12) and a crystalline silicate having an X-ray diffraction pattern shown in Table 1 below as a carrier, and platinum and / or an active metal. The method for purifying exhaust gas according to the above (1), which is a catalyst containing palladium, and (3) the carbon monoxide removing catalyst has Pt and / or Pd as an active metal of 0 in the porous heat-resistant carrier. The exhaust gas purifying method as described in (1) or (2) above, wherein the catalyst is supported by 0.001 to 10 wt%.

[0007]

[Table 1] VS: Very strong S: Strong M: Intermediate W: Weak (X-ray source: Cu)

[0008]

Hereinafter, one embodiment of the present invention will be described with reference to FIG.
The action is clarified. NH ₃On the upper side of the addition device 2
A carbon monoxide removal catalyst layer 1 is installed, and monoacid is generated by catalytic combustion
Combustion and removal of carbon dioxide. NH₃On the lower side of the adding device 2,
The first denitration catalyst layer 3 is fed with NH₃Decomposition catalyst layer 4
Further, the second denitration catalyst layer 5 is installed in the subsequent flow,
1 upstream of the denitration catalyst layer 3 the reaction equivalent amount or more for NOx
NH₃To NH₃The first denitration catalyst added by the addition device 2
80% or more of denitration is performed in layer 3 and the first denitration catalyst layer 3
Unreacted NH flowing out₃To NH₃By the decomposition catalyst layer 4
NOx, NH at the downstream second NOx removal catalyst layer inlet₃Dark
The NOx at the outlet of the second denitration catalyst layer 5 is adjusted to 0.
1ppm or less, NH₃To a level of 5 ppm or less. Up
V and V are applied to the first and second denitration catalyst layers 3 and 5 in the flow and the downstream.
TiO with W and Mo as active ingredients₂Conventional application of system
Any known catalyst can be used.

In the method of the present invention, since carbon monoxide is burned and removed by a catalyst, the performance of the NH ₃ decomposition catalyst is not changed, and furthermore, harmful carbon monoxide emission can be suppressed.

In the above embodiment of the present invention, the catalyst used in the NH ₃ decomposition catalyst layer preferably has a nitrogen selectivity% as defined below of 60% or more.

[0011]

## EQU1 ## Nitrogen selectivity (%) = [1- {ammonia decomposition catalyst outlet NOx (ppm) -ammonia decomposition catalyst inlet NOx (ppm)} / {ammonia decomposition catalyst inlet NH₃(Ppm) -Ammonia decomposition catalyst outlet NH₃(Ppm)} That is, the nitrogen selection of the ammonia decomposition catalyst defined above.
If the selection rate is small, NH ₃NH at the outlet of the decomposition catalyst layer₃＞ NO
The operating range of the plant that can be controlled to x is narrow
Control over a wide range of processing gas amounts and temperature conditions.
Therefore, the nitrogen selectivity should be at least 60% or less.
The above is preferable.

The NH ₃ decomposition catalyst having the above-mentioned nitrogen selectivity is (1.0 ± 0.6) R _{2 in the} dehydrated state.
O ・ [aM ₂ O ₃・ bAl ₂ O ₃ ] ・ cMeO ・ ySi
O ₂ (R: alkali metal ion and / or hydrogen ion,
M: one or more elements selected from the group consisting of Group VIII elements of the Periodic Table, rare earth elements, titanium, vanadium, chromium, niobium, antimony and gallium, Me: alkaline earth metal element, a + b = 1, a ≧ 0, b ≧ 0, c ≧ 0, y
/ C> 12, y> 12), and the above Table 1
A catalyst having a crystalline silicate having an X-ray diffraction pattern shown in (3) as a carrier and containing platinum and / or palladium as an active metal is preferable.

The carbon monoxide removal catalyst installed in the upper stage of the ammonia addition device is preferably a catalyst containing one kind of platinum or palladium as an active metal in a porous heat-resistant carrier such as alumina. Γ-Al ₂ O as a porous heat-resistant carrier
₃ , TiO ₂ , SiO ₂ , ZrO ₂ , Fe ₂ O ₃ , Si
O ₂ · Al ₂ O ₃ or the like can be used, and the amount of Pt and / or Pd supported is preferably in the range of 0.001 to 10 wt%.

Exhaust gas containing nitrogen oxides, in which carbon monoxide has been burned and removed in the carbon monoxide removal catalyst layer, contains excess NH ₃
It is supplied by the H ₃ addition device and is denitrated in the first denitration catalyst layer to NOx: 0 to 10 ppm and NH ₃ : 10 to ₃₀ ppm. This exhaust gas is reduced in NH _{3 in the} NH ₃ decomposition catalyst layer and NOx, NH ₃ The concentration is adjusted so that the NOx concentration in the exhaust gas is 0.1 ppm or less and the NH ₃ concentration is 5 ppm or less in the second denitration catalyst layer.

[0015]

EXAMPLES The method of the present invention will be described in more detail below with reference to examples.

Example 1 (Preparation of denitration catalyst): A powder catalyst in which 4 wt% of vanadium pentoxide (V ₂ O ₅ ) and 8 wt% of tungsten trioxide (WO ₃ ) were carried on a titania (TiO ₂ ) carrier. 3.
This catalyst was formed into a lattice honeycomb shape having a pitch of 3 mm and a wall thickness of 0.5 mm, and this catalyst was used as a denitration catalyst 1.

(NH₃Preparation of decomposition catalyst): Water glass No. 1
(SiO₂: 30%): 5616 g to water: 5429 g
It was dissolved and this solution was designated as solution A. On the other hand, water: 4175
Aluminum sulfate: 718.9 g, ferric chloride: 1
10 g, calcium acetate: 47.2 g, sodium chloride
Mu: 262g and concentrated hydrochloric acid: 2020g are mixed and dissolved
This solution was designated as solution B. Solution A and solution B are split
Are supplied together to form a precipitate, and the mixture is sufficiently stirred to pH = 8.
I got a zero slurry. 20 liter auto of this slurry
Charge into the clave, and then tetrapropylammonium
Add 500 g of bromide and water at 160 ° C for 72 hours
Thermal synthesis is performed, and after synthesis, it is washed with water and dried, and further 500
The crystalline silicate 1 was obtained by firing at ℃ for 3 hours. This result
Crystalline silicate 1 is the molar ratio of oxides (excluding water of crystallization)
0.5Na₂O ・ 0.5H₂O ・ [0.8Al₂O₃・
0.2Fe ₂O₃・ 0.25CaO] ・ 25SiO₂of
It is represented by the composition formula, and the crystal structure is shown in Table 1 by X-ray diffraction.
It was what was shown. 4N of the above crystalline silicate 1
NH_FourCl aqueous solution was stirred at 40 ° C. for 3 hours and NH_FourIo
Exchange was performed. Washing after ion exchange at 100 ℃, 2
After drying for 4 hours, the H-type
A crystalline silicate 1 was obtained.

The H-type crystalline silicate 1 was impregnated with an aqueous solution of chloroplatinic acid or an aqueous solution of palladium nitrate, evaporated to dryness, and then calcined at 500 ° C. for 3 hours to obtain a powder catalyst. Alumina sol as a binder: 3 g, silica sol: 55 g (SiO ₂ : 20 wt) with respect to the obtained powder: 100 g
%) And water: 200 g to make a slurry, and wash-coat on a monolith substrate for cordierite (lattice shape per 30 cells square inch) to obtain 200 g per substrate surface area.
The coating amount was / m ² . The obtained catalysts were designated as NH ₃ decomposition catalysts 1 to 4. The properties are shown in Table 2 below.

[0019]

[Table 2]

In the above method for preparing the NH ₃ decomposition catalyst, cobalt chloride: 112 g, titanium chloride: 105 g, vanadium chloride: 10 g, chromium chloride: instead of ferric chloride.
107 g, niobium chloride: 135 g, antimony chloride: 1
H-type crystalline silicate 2,3,4,5, by the same method as above except that 55 g and gallium chloride: 119 g are used.
6, 7, and 8 were prepared, and platinum was loaded on each H-type crystalline silicate by the same operation as the above-mentioned preparation method using an aqueous chloroplatinic acid solution for each of these H-type crystalline silicates. The monolith substrate for cordierite was wash-coated in the same manner as above to carry a coat amount of 200 g / m ² per substrate surface area. The obtained catalyst is used as an NH ₃ decomposition catalyst 5-1.
It was set to 1. The properties are shown in Table 3 below.

[0021]

[Table 3]

(Preparation of carbon monoxide removal catalyst) A γ-alumina (γ-Al ₂ O ₃ ) carrier was supported by a 3 wt% impregnation method using chloroplatinic acid aqueous solution as Pt, and dried at 100 ° C.
To 100 g of powder catalyst calcined at 500 ° C. for 5 hours, 55 g of silica sol (SiO ₂ : 2) as a binder
0 wt%) and water: 200 g to make a slurry, and wash coat it on a monolith substrate for cordierite (lattice shape per 30 cell square inch) to give 1 per substrate surface area.
A coated amount of 50 g / m ² was carried. This catalyst was designated as carbon monoxide removal catalyst 1.

(Denitration reaction test) 40 mm × 50 mm × 5
One 0 mmL of the carbon monoxide removing catalyst 1, 40 mm
X 50 mm x 400 mmL of the above denitration catalyst, 40 m
m × 50 mm × 150 mmL of the NH ₃ decomposition catalyst 1
And three denitration catalysts were placed in series and tested under the conditions shown in Table 4 below. The test results of Runs 1 to 11 are shown in Table 5 below.

[0024]

[Table 4]

Comparative Example 1 A system (= Ru) in which the carbon monoxide removing catalyst was omitted from the Run 1 system of Example 1.
The evaluation results of n12) are also shown in Table 5.

[0026] From the results shown in Table 5, in the second stage denitration catalyst outlet at Run1~11 which established the carbon monoxide removal catalyst, any NOx <0.1 ppm, whereas a NH ₃ <5 ppm, one Run1 without carbon oxide removal catalyst
At 2, the NH ₃ concentration at the outlet of the NH ₃ decomposition catalyst decreases and NH _{3
Since the 3} concentration was lower than the NOx concentration, NOx> 0.1 ppm at the outlet of the second stage denitration catalyst, and the target could not be satisfied.

[0027]

[Table 5]

As a carrier for the carbon monoxide removing catalyst,
Instead of γ-Al ₂ O ₃ , TiO ₂ , SiO ₂ , ZrO ₂ ,
The same effect was obtained by using Fe ₂ O ₃ and SiO ₂ .Al ₂ O ₃ .

[0029]

According to the method for purifying exhaust gas of the present invention, the emission of NH ₃ as a reducing agent can be maintained at a low level, and nitrogen oxides and nitric oxide can be removed with extremely high efficiency.

[Brief description of drawings]

FIG. 1 is an explanatory view of an exhaust gas purification method of the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 29/068 ZAB B01D 53/36 ZAB

Claims (3)

[Claims] 1. A method for catalytically removing nitrogen oxides by introducing an exhaust gas containing nitrogen oxides and carbon monoxide into a reactor filled with a catalyst and using ammonia as a reducing agent.
It has a function of decomposing a carbon monoxide removal catalyst layer on the upper side of the ammonia addition device, a first denitration catalyst layer on the lower side of the ammonia addition device from the upstream side, and oxidatively decomposing ammonia into nitrogen and nitrogen oxides in the subsequent flow. Purification of exhaust gas, characterized in that an ammonia decomposition catalyst layer and a second denitration catalyst layer are installed in the subsequent flow, and ammonia added from the ammonia addition device is added at a reaction equivalent amount or more of nitrogen oxides in the exhaust gas at the inlet. Method. 2. The (1.0 ± 0.6) R ₂ O. [aM ₂ O ₃ .bAl in the dehydrated state of the ammonia decomposition catalyst.
₂ O ₃ ] .cMeO.ySiO ₂ (R: alkali metal ion and / or hydrogen ion, M: selected from the group consisting of Group VIII elements of the Periodic Table, rare earth elements, titanium, vanadium, chromium, niobium, antimony and gallium 1
More than one element, Me: alkaline earth metal element, a + b =
1, a ≧ 0, b ≧ 0, c ≧ 0, y / c> 12, y> 1
A catalyst containing, as a carrier, a crystalline silicate having the chemical formula of 2) and having the X-ray diffraction pattern shown in Table 1 described in the detailed description of the invention and containing platinum and / or palladium as an active metal. Claim 1 characterized by the above.
Exhaust gas purification method described. 3. A carbon monoxide-removing catalyst containing Pt and / or Pd as an active metal in an amount of 0.001 to 0.001 on a porous heat-resistant carrier.
The exhaust gas purification method according to claim 1 or 2, wherein the catalyst is 10 wt% supported.