US20040168433A1

US20040168433A1 - Exhaust gas treatment system and exhaust gas treatment method

Info

Publication number: US20040168433A1
Application number: US10/704,784
Authority: US
Inventors: Shigeru Nojima; Kozo Iida; Masashi Kiyosawa
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2003-02-27
Filing date: 2003-11-10
Publication date: 2004-09-02
Also published as: JP2004255342A; EP1452222A1

Abstract

There is provided an exhaust gas treatment system for removing nitrogen oxide and sulfur trioxide in exhaust gas by using a catalyst characterized in that a SO₃reducing catalyst on the upstream side and a denitrifying catalyst on the downstream side are arranged in series with respect to the flow direction of exhaust gas. Also, there is provided an exhaust gas treatment method characterized by including a SO₃reducing process for reducing SO₃using a SO₃reducing catalyst on the upstream side and a denitrifying process for removing nitrogen oxides using a denitrifying catalyst on the downstream side with respect to the flow direction of exhaust gas.

Description

RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2003-051787 filed Feb. 27, 2003, the disclosure of which is incorporated by reference herein in its entirety.[0001]

FIELD OF THE INVENTION

The present invention relates to an exhaust gas treatment system and an exhaust gas treatment method and, more particularly, to an exhaust gas treatment method in which exhaust gas such as nitrogen oxides and sulfur trioxide (SO ₃) exhausted from a boiler or the like is removed by reduction using a catalyst.

BACKGROUND OF THE INVENTION

As a catalyst for removing sulfur trioxide (SO ₃) in exhaust gas by reaction, catalysts in which ruthenium (Ru) is carried on various carriers have been reported (see Japanese Patent Provisional Publication No. 10-24963 (No. 24963/1998).

A SO ₃reducing reaction using a ruthenium (Ru) carrying catalyst converts sulfur trioxide (SO₃) into sulfur dioxide (SO₂) using ammonia (NH₃) as a reducing agent according to the following reaction formula (1).

SO₃+2NH₃+O₂→SO₂+N₂+3H₂O (1)

However, in the case where the aforementioned ruthenium (Ru) carrying catalyst is used, in a system in which the concentration of ammonia (NH ₃) is low, the reaction expressed by the above formula (1) is extremely slow, so that there arises a problem in that the reducing reaction does not proceed sufficiently.

On the one hand, combustion exhaust gas exhausted from a coal-fired or oil-fired combustion boiler is a gas containing nitrogen oxides. In order to discharge this gas, it is necessary to remove nitrogen oxides in the exhaust gas. Therefore, a denitrifier is provided on the downstream side of the combustion boiler so that ammonia (NH ₃) is injected into the combustion exhaust gas through an injection nozzle to be caused to react with nitrogen oxides (NO, NO₂) by reduction, by which the exhaust gas is decomposed into nitrogen (N₂) and water (H₂O), which are harmless. At this time, in a method in which nitrogen oxides are removed from exhaust gas using a denitrifying catalyst, ammonia is usually added to cause sufficient denitrifying reaction.

In a system for treating exhaust gas exhausted from a combustion boiler, a technology in which sulfur trioxide (SO ₃) is not caused to flow to equipment on the downstream side as far as possible is demanded because SO₃contained in the exhaust gas is responsible for corrosion and deterioration of equipment.

On the other hand, the aforementioned ruthenium (Ru) carrying catalyst not only can remove sulfur trioxide (SO ₃) by reduction, but also can reduce nitrogen oxides (NOx) in the exhaust gas using ammonia (NH₃) according to the following reaction formula (2) to convert them into nitrogen gas (N₂).

4NH₃+4NO+O₂→4N₂+6H₂O (2)

SUMMARY OF THE INVENTION

In view of the above situation, the inventors carried out studies earnestly to develop a catalyst system and a treatment method capable of efficiently removing nitrogen oxides (NOx) and sulfur trioxide (SO ₃) by reaction at the same time in the treatment of exhaust gas containing NOx and SO₃.

As a result, the inventors found that the above problems can be solved by using a catalyst system in which a SO ₃reducing catalyst and a denitrifying catalyst are arranged in series and combined. The present invention was completed from the above-described viewpoint.

The present invention provides an exhaust gas treatment system for removing nitrogen oxide and sulfur trioxide in exhaust gas by using a catalyst, characterized in that a SO ₃reducing catalyst on the upstream side and a denitrifying catalyst on the downstream side are arranged in series with respect to the flow direction of exhaust gas.

In the present invention, the SO ₃reducing catalyst is preferably a catalyst in which Ru is carried as an active metal on a carrier. The carrier used for the SO₃reducing catalyst preferably contains at least one kind selected from a group consisting of titania, alumina, zirconia, silica, and zeolite. The SO₃reducing catalyst can contain at least one kind selected from a group consisting of Mo, W and V as a co-catalyst in the carrier. The denitrifying catalyst can use a catalyst that carries at least one kind selected from a group consisting of Mo, W and V as an active metal on a carrier containing titania. The shapes of the SO₃reducing catalyst and the denitrifying catalyst are not subject to any special restriction, and both of these catalysts can be, for example, a catalyst formed into a honeycomb shape.

Also, the present invention provides an exhaust gas treatment method for removing nitrogen oxide and sulfur trioxide in exhaust gas by using a catalyst, characterized in that the treatment method includes a SO ₃reducing process for reducing SO₃using a SO₃reducing catalyst on the upstream side and a denitrifying process for removing nitrogen oxides using a denitrifying catalyst on the downstream side with respect to the flow direction of exhaust gas. The treating method in accordance with the present invention preferably includes an NH₃adding process for adding ammonia NH₃, which is a reducing agent for nitrogen oxides and SO₃, in a mole ratio of NH₃/NOx favorably not lower than the equivalent at the preceding stage of the SO₃reducing process. Also, the concentration of NH₃at the downstream outlet of the SO₃reducing catalyst is preferably 3 ppm or higher in the SO₃reducing process.

According to the treatment method and treatment system in accordance with the present invention, in the treatment of exhaust gas containing nitrogen oxides (NOx) and sulfur trioxide (SO ₃), NOx and SO₃can be removed by reduction efficiently at the same time. Therefore, in the exhaust gas treatment using the system in accordance with the present invention, in particular, equipment on the downstream side of the denitrifier is less liable to be deteriorated or damaged, so that this treatment system has an advantage that the maintenance of the whole system is easy and the long-term use is ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are a block diagram showing one example of a system incorporating an exhaust gas treatment system in accordance with the present invention.[0015]
The reference numerals shown in these figures are defined as follows: [0016] 1, boiler; 2, SO₃reducing catalyst; 3, denitrifier; 4, air preheater; 5, dust collector; 6, gas-gas heater (heat exchanger); 7, desulfurizer; 8, stack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of an exhaust gas treatment system and an exhaust gas treatment method in accordance with the present invention will be described. The present invention is not subject to any restriction by the embodiment described below. [0017]
Exhaust Gas Treatment Method [0018]
In the exhaust gas treatment method in accordance with the present invention, a SO[0019] ₃reducing process is conducted on the upstream side and a denitrifying process is conducted on the downstream side with respect to the flow direction of exhaust gas containing nitrogen oxides. At the preceding stage of the SO₃reducing process, ammonia NH₃, which is a reducing agent for nitrogen oxides and SO₃in exhaust gas, is added by an NH₃adding process. In the NH₃adding process, ammonia should be added to the exhaust gas in a mole ratio NH₃/NOx of ammonia to nitrogen oxides NOx preferably not lower than the equivalent (not lower than 1), further preferably not lower than 1.2. Since the concentration of ammonia added to the exhaust gas is high at the preceding stage of the process in which reducing reaction takes place, the SO₃reducing process is first conducted in a state in which the concentration of ammonia is high.
In the SO[0020] ₃reducing process, SO₃is reduced by a SO₃reducing catalyst, described later. By using NH₃as a reducing agent, the SO₃reducing catalyst concurrently carries out three reactions: (1) SO₃reducing reaction, (2) denitrifying reaction, and (3) NH₃decomposing reaction.
SO₃+2NH₃+O₂→SO₂+N₂+3H₂O (1)
4NH₃+4NO+O₂→4N₂+6H₂O (2)
2NH₃+3/2O₂→N₂+3H₂O (3)
Thus, the SO[0021] ₃reducing catalyst not only removes sulfur trioxide (SO₃) by reduction, but also reduces nitrogen oxides (NOx) in the exhaust gas using ammonia (NH₃) to convert into nitrogen gas (N₂).
In the SO[0022] ₃reducing process, the concentration of NH₃at the downstream outlet of the SO₃reducing catalyst should preferably be 3 ppm or higher, further preferably be 10 ppm or higher. If the concentration of NH₃is too low, the reducing agent runs short in a downstream denitrifying process, so that sufficient denitrifying action cannot be brought about.
Next, on the downstream side of the SO[0023] ₃reducing process, the denitrifying process is conducted. In the denitrifying process, nitrogen oxides are removed using a denitrifying catalyst, described later, according to the following formula (2) or the like.
4NH₃+4NO+O₂→4N₂+6H₂O (2)
For an exhaust gas containing a large quantity of SOx, for example, in a system having no SO[0024] ₃reducing process, SO₂oxidizing reaction is produced by a denitrifying catalyst according to the following formula (4), so that the concentration of SO₃increases further in some cases. For this reason as well, a method in which SO₃is not yielded in exhaust gas as far as possible is needed. According to the treatment method in accordance with the present invention, SO₃can be removed effectively on the upstream side of the denitrifying process.
SO₂+1/2O₂→SO₃ (4)
Generally, in the treatment method for exhaust gas exhausted from various types of combustion equipment such as a boiler, the exhaust gas exhausted from the boiler is sent to a denitrifying catalyst, and a denitrifying process is conducted. In the denitrifying process, ammonia NH[0025] ₃added on the upstream side of the denitrifying catalyst is used as a reducing agent.
In the denitrifying process in accordance with the present invention, the SO[0026] ₃reducing process is usually provided on the upstream side of the denitrifying process, so that denitrification is carried out by using ammonia added at the preceding stage of the SO₃reducing process. However, a predetermined quantity of ammonia may be added as necessary at the following stage of the SO₃reducing process and at the preceding stage of the denitrifying process. In the exhaust gas after the denitrifying process, both NOx and SO₃are removed by reduction, and the added NH₃is also removed after acting as a reducing agent.
As described above, in the present invention, for the exhaust gas flowing from the denitrifying process, both NOx and SO[0027] ₃are removed by reduction, so that corrosion caused by SO₃is restrained even if the exhaust gas is caused to pass through the downstream equipment such as a dust collector and a wet type desulfurizer. Generally, between the denitrifying process and the desulfurizing process using a wet type desulfurizer, a heating process using a heater or steam, a dust collecting process using an electric dust collector, a heat recovery process using a gas heater, and the like process are included.
Exhaust Gas Treatment System [0028]
FIG. 1 shows one example of an exhaust gas treatment system incorporating an exhaust gas treatment system in accordance with the present invention. Hereunder, the whole of the exhaust gas treatment system in accordance with the embodiment shown in FIG. 1 will be described in detail with reference to the accompanying drawing. [0029]
In the present invention, a SO[0030] ₃reducing catalyst on the upstream side and a denitrifying catalyst on the downstream side are provided in series with respect to the flow direction of exhaust gas containing nitrogen oxides.
In the system shown in FIG. 1, on the downstream side of a [0031] boiler 1, a SO₃reducing catalyst 2, a denitrifier 3, an air heater (A/H) 4, which is heating means, a dust collector 5, a heat exchanger (GGH) 6 for recovering heat energy, a wet type desulfurizer (desulfurizing absorption tower) 7, a heat exchanger (GGH) 6 for reheating, and a stack 8 are arranged in that order. The SO₃reducing catalyst 2 is provided on the downstream side of the boiler 1 and on the upstream side of the denitrifier 3. On the upstream side of the SO₃reducing catalyst 2, NH₃adding means 9 for adding ammonia, which serves as a reducing agent, is provided.
In the system in accordance with the present invention exemplified in FIG. 1, as the NH[0032] ₃adding means 9 provided on the upstream side of the SO₃reducing catalyst 2, equipment formed by, for example, a reducing agent injection pipe or a plurality of spray nozzles is used. As a method for injecting a reducing agent containing ammonia, there is available a method in which the reducing agent is vaporized, and the vaporized reducing agent is diluted by adding air, inert gas, water vapor, etc. before the reducing agent is added to the exhaust gas. At this time, it is effective to arrange the nozzles so that the reducing agent flows uniformly to the SO₃reducing catalyst and the denitrifying catalyst. In some cases, a plurality of nozzles are arranged perpendicularly to the gas flow.
As the SO[0033] ₃reducing catalyst 2 used in this system, any catalyst can be used if it has a SO₃reducing function. For example, a catalyst in which Ru is carried as an active metal on a carrier can be cited. The kind of carrier used for the SO₃reducing catalyst is not subject to any special restriction, but the carrier is preferably at least one kind selected from a group consisting of titania, alumina, zirconia, silica, and zeolite. Also, the SO₃reducing catalyst can contain at least one kind selected from a group consisting of Mo, W and V as a co-catalyst in the carrier as necessary.
The shape of the SO[0034] ₃reducing catalyst is not subject to any special restriction. For example, a honeycomb-shaped catalyst, a catalyst formed by stacking the honeycomb-shaped catalysts, a catalyst formed by filling particulate catalysts, and the like can be used. In particular, a honeycomb-shaped catalyst is preferably used.
As a denitrifying catalyst used in the [0035] denitrifier 3 of this system, a catalyst formed by carrying at least one kind selected from a group consisting of Mo, W and V as an active metal on a carrier containing titania can be used preferably. In addition, a catalyst in which an active metal of at least one kind selected from a group consisting of Pt, Pd, Ir, Ru, Cu, Co, Fe, Ag, Mn, Ni, Zn and In is contained in the carrier of at least one kind selected from a group consisting of Al₂O₃, SiO₂, ZrO₂, TiO₂, metallosilicate, and zeolite can be used.
The shape of the nitrifying catalyst is not subject to any special restriction. For example, a honeycomb-shaped catalyst, a catalyst formed by stacking the honeycomb-shaped catalysts, a catalyst formed by filling particulate catalysts, and the like can be used. In particular, a honeycomb-shaped catalyst is preferably used. [0036]
In the treatment system in accordance with the present invention, as described above, the SO[0037] ₃reducing catalyst 2 on the upstream side and the denitrifying catalyst system 3 on the downstream side are arranged in series with respect to the flow of exhaust gas. On the downstream side of the catalyst system in which these two types of catalysts are combined with each other, various types of equipment as shown in FIG. 1 are provided according to the treatment of exhaust gas.
In the system shown in FIG. 1, the air heater (A/H) [0038] 4, which is heating means, and the dust collector 5 are provided on the downstream side of the denitrifier 3. The dust collector 5 is not subject to any special restriction if it can collect dust roughly before the exhaust gas is introduced into the desulfurizing absorption tower 7.
On the downstream side of the [0039] dust collector 5, the heat exchanger (GGH) 6 for recovering heat energy and the desulfurizer (desulfurizing absorption tower) 7 are provided. The desulfurizer 7 is not subject to any special restriction. For example, a wet type desulfurizer generally used for flue gas treatment or a desulfurizer in which a cooling tower is provided at the preceding stage of an absorption tower is used. As the desulfurizer 7, an ordinary wet type desulfurizer can be used. As an absorbent used for wet desulfurization, aqueous solution of absorbent (alkali absorbent) of calcium carbonate, calcium oxide, calcium hydroxide, sodium carbonate, sodium hydroxide, and the like can be cited.
In the system shown in FIG. 1, which uses a wet desulfurization method, the [0040] heat exchanger 6 for reheating is provided on the downstream side of the desulfurizing absorption tower 7, and the exhaust gas is discharged to the air through the stack 8 after passing through these types of equipment. In the heat exchanger 6 for reheating, the combustion exhaust gas whose temperature has been decreased is heated by heat energy recovered by the heat exchanger 6 for heat recovery at the preceding stage of the desulfurizer 7. Usually, the heat exchanger for heat recovery and the heat exchanger for reheating are formed by a gas heater of a system in which heat energy is exchanged by using a heat medium as a medium, and may be a separate system or may be a gas-gas heater for directly effecting heat exchange if they can cool and heat the exhaust gas, respectively.
In the above-described system shown in FIG. 1, on the downstream side of the catalyst system in which the SO[0041] ₃reducing catalyst 2 and the denitrifying catalyst 3 are combined with each other, since nitrogen oxides and SO₃component contained in the exhaust gas have been removed with high efficiency, corrosion and deterioration of equipment are restrained, and hence system control and trouble can be reduced.
The exhaust gas to be treated by the treatment system in accordance with the present invention is boiler exhaust gas generated in a thermal electric power station, a factory, etc. in which a fuel containing sulfur and mercury, such as coal and heavy oil, is burned or heating furnace exhaust gas generated in a metal factory, an oil refinery, a petrochemical plant, etc. The treatment system in accordance with the present invention is suitable for a large quantity of exhaust gas containing carbon dioxide, oxygen, SOx, soot, or water. [0042]
Hereunder, the present invention will be described in more detail with reference to examples. The present invention is not limited to these examples. [0043]

EXAMPLE 1 (PREPARATION OF CATALYSTS)

([0044] Honeycomb Catalysts 1 to 10)
Aqueous solution of ruthenium chloride (RuCl[0045] ₃) was impregnated into and carried on particles of compound 1: TiO₂(MC-90 manufactured by Ishihara Sangyo Kaisha, Ltd.), compound 2: γ-Al₂O₃(manufactured by Sumitomo Chemical Co., Ltd.), compound 3: SiO₂(manufactured by Fuji Silysia Chemical Ltd.), compound 4: zirconia (manufactured by Nikki Chemical Co., Ltd.), compound 5: silicalite (manufactured by Mobil Corp.), compound 6: TiO₂-WO₃(weight ratio 90:10), compound 7: TiO₂-WO₃(weight ratio 80:20), compound 8: TiO₂-WO₃-V₂O₅(weight ratio 90:9:1), compound 9: TiO₂-MoO₃-V₂O₅(weight ratio 90:9:1), and compound 10: TiO₂-SiO₂(weight ratio 90:10) so that the carrying amount of Ru per particle was 0.5%, and firing was accomplished in the air at 500° C. for about 5 hours after evaporation to dryness. As a result, powder catalysts 1-10 were obtained.
The powder catalysts 1-10 were put in water, and slurried by ball mill grinding. The slurries 1-10 were applied to and carried on a honeycomb (Ap: 445 m[0046] ²/m³) of TiO₂-WO₃(weight ratio 90:10) with a pitch of 7.0 mm and a wall thickness of 1.0 mm in an amount of 200 g/m²per honeycomb surface area, by which honeycomb catalysts 1-10 were obtained.
(Honeycomb Catalyst 11) [0047]
As the denitrifying catalyst, a honeycomb catalyst (pitch: 7.0 mm, wall thickness: 1.0 mm, Ap: 445 m[0048] ²/m³) consisting of TiO₂-WO₃-V₂O₅(weight ratio 91:8.8:0.2) was use

EXAMPLE 2 (EXHAUST GAS TREATMENT TEST A)

Catalyst systems were formed by combining honeycomb catalysts 1-10 (SO[0049] ₃reducing catalyst) prepared in example 1 and honeycomb catalyst 11 (denitrifying catalyst) with each other, and exhaust gas treatment tests were conducted under the conditions described below.
[Test Conditions][0050]
The gas composition was 150 ppm NOx, 2900 ppm SOx (including 80 ppm SO[0051] ₃), 180 ppm NH_{3, 1.3}% O₂, 10% H₂O, 10% CO₂, the balance being N₂.
The test conditions were as follows: temperature: 380° C., superficial velocity: 3.3 mN/s, gas quantity: 30 m[0052] ³N/s, catalyst formation: 50 mm square×800 mm (preceding stage: one SO₃reducing catalyst, following stage: one denitrifying catalyst), and GHSV: 15000 h⁻¹at outlet of SO₃reducing catalyst, 5000 h⁻¹at outlet of denitrifying catalyst (AV: 34 m³/m²·h at outlet of SO₃reducing catalyst, 11 m³/m²·h at outlet of denitrifying catalyst).
Table 1 gives exhaust gas treatment performance at a total GHSV of 5000 h[0053] ⁻¹in the case where honeycomb catalysts 1-10 serving as the SO₃reducing catalyst and honeycomb catalyst 11 serving as the denitrifying catalyst were arranged in series. The concentration of NOx was measured by the chemiluminescence method, the concentration of NH₃was measured by ion chromatography, and the concentration of SO₃was measured by arsenazo III method using a spiral tube. The NOx removal efficiency and SO₃reduction amount were calculated by the following formulae.
NOx removal efficiency (%)=(1−outlet NOx/inlet NOx)×100
SO₃reduction amount (ppm)=inlet SO₃−outlet SO₃
The target performance of this exhaust gas treatment test was set so as to be a target that satisfies all of the following performances. [0054]
NOx removal efficiency: 85% or higher [0055]
SO[0056] ₃reduction amount: 0 ppm or larger (SO₃is reduced)

Leak of NH ₃: 15 ppm or less

TABLE 1


Exhaust gas treatment performance of combined catalysts

			NOx	SO₃	Leak
	Preceding-stage catalyst		removal	reduction	of
	(coated carrier: SO₃	Following-stage cataly	efficiency	amount	NH₃
Test	reducing catalyst)	(denitrifying catalyst)	(%)	(ppm)	(ppm)

1	Catalyst 1	Catalyst 11	89	6	10
	(TiO₂carrier)	(TiO₂—WO₃—V₂O₅)
2	Catalyst 2	Catalyst 11	90	6	9
	(Al₂O₃carrier)
3	Catalyst 3	Catalyst 11	90	7	9
	(SiO₂carrier)
4	Catalyst 4	Catalyst 11	89	8	10
	(ZrO₂carrier)
5	Catalyst 5 (Silicalite carrie	Catalyst 11	90	7	9
6	Catalyst 6	Catalyst 11	95	7	7
	(TiO₂—WO₃carrier)
7	Catalyst 7	Catalyst 11	96	7	7
	(TiO₂—WO₃carrier)
8	Catalyst 8	Catalyst 11	96	6	5
	(TiO₂—WO₃—V₂O₃carrier)
9	Catalyst 9	Catalyst 11	96	7	8
	(TiO₂—WO₃—MoO₃carrier)
10	Catalyst 10	Catalyst 11	93	7	8
	(TiO₂—SiO₂
	carrier)

From the above results, it was confirmed that the catalyst systems in [0058] tests 1 to 10, in which the SO₃reducing catalyst is provided at the preceding stage and the denitrifying catalyst is provided at the following stage, are effective for denitrification, SO₃reduction, and decrease in leak of NH₃, and in particular, these catalyst systems are promising for achieving SO₃reduction.

EXAMPLE 3 (PREPARATION OF CATALYST)

Various tests were conducted based on [0059] test 6 having high performance in the above-described tests on catalyst system.
Honeycomb Catalysts 12-14) [0060]
Honeycomb catalysts 12 and 13 were prepared by the same preparation method as that for SO[0061] ₃reducing catalyst 6 (honeycomb catalyst 6) in example 1 except that slurries that carried 0.3% and 1% Ru, in place of 0.5% Ru, on TiO₂-WO₃carrier of compound 6 were prepared.
Also, honeycomb catalyst 14 was prepared by the same preparation method as that for SO[0062] ₃reducing catalyst 6 (honeycomb catalyst 6) in example 1 except that slurry 6 was applied to the honeycomb in an amount of 100 g/m²per honeycomb surface area in place of 200 g/m².

EXAMPLE 4 (EXHAUST GAS TREATMENT TEST B)

Exhaust gas treatment tests (tests 11, 12 and 13) were conducted in the same way as [0063] test 6 in example 2 on catalyst systems in which each one of the honeycomb catalysts 12, 13 and 14 (SO₃reducing catalysts) and one denitrifying catalyst 11 were combined with each other. Further, an exhaust gas treatment test (test 14) was conducted in the same way as described above on a catalyst system in which two honeycomb catalysts 6 (SO₃reducing catalysts) and one denitrifying catalyst 11 were combined with each other.

Table 2 gives the results of tests on the above-described catalyst systems.

TABLE 2


Exhaust gas treatment performance of
various combined catalysts

				SO₃	Leak
	Preceding-stage		NOx	reduction	of
	catalyst (SO₃	Following-stage cataly	removal	amount	NH₃
Test	reducing catalyst)	(denitrifying catalyst)	efficiency (%)	(ppm)	(ppm)

11	Catalyst 11 (Ru(0.3)/	Catalyst 11	95	6	8
	TiO₂—WO₃), one	(TiO₂—WO₃—V₂O₅)
12	Catalyst 12 (Ru(1.0)/	Catalyst 11	96	10	5
	TiO₂—WO₃), one
13	Catalyst 13 (Ru(0.5)/	Catalyst 11	95	7	7
	TiO₂—WO₃: 100 g/m²
	coated), one
14	Catalyst 6, two	Catalyst 11,	91	12	1
		one

From the above test results, it was confirmed that even when the Ru carrying amount or the coating amount of SO[0065] ₃reducing catalyst is changed, the catalyst systems in test 11 to 13 are effective for denitrification, SO₃reduction, and decrease in leak of NH₃, and in particular, these catalyst systems are promising for achieving SO₃reduction. Also, it was confirmed that even when the arrangement ratio of SO₃reducing catalyst to denitrifying catalyst is changed, the catalyst system in test 14 achieves the target performance in terms of SO₃reduction etc.

EXAMPLE 5 (EXHAUST GAS TREATMENT TEST C)

The activity evaluation was carried out under the following test conditions based on the catalyst combination and test conditions of [0066] test 6 that had exhibited high performance in the above-described tests on catalyst system.
Test 15 . . . An activity evaluation test was conducted by the same method as that of [0067] test 6 except that the concentration of NH₃in the gas at the inlet of catalyst was 153 ppm in place of the 180 ppm in test 6 in example 2.
Test 16 . . . An activity evaluation test was conducted by the same method as that of [0068] test 6 except that the concentration of SO₃in the gas at the inlet was 50 ppm in place of the 80 ppm in test 6 in example 2.
Test 17 . . . An activity evaluation test was conducted by the same method as that of [0069] test 6 except that the concentration of NOx in the gas at the inlet was 170 ppm in place of the 150 ppm in test 6 in example 2.
Test 18 . . . An activity evaluation test was conducted by the same method as that of [0070] test 6 except that the concentration of O₂in the gas at the inlet was 2% in place of the 1.3% in test 6 in example 2.
Test 19 . . . An activity evaluation test was conducted by the same method as that of [0071] test 6 except that the concentration of SOx in the gas at the inlet was 4000 ppm (including 80 ppm of concentration of SO₃) in place of the 2900 ppm (including 80 ppm of concentration of SO₃) in test 6 in example 2.

Table 3 gives the exhaust gas treatment test results in the above-described tests 15 to 19.

TABLE 3


Exhaust gas treatment performance of combined
catalysts under each test condition

		Preceding-stage	Following-stage	NOx		Leak of
	Changed item of	catalyst (SO₃	catalys	removal	SO₃reduction	NE
Test	test conditions	reducing catalyst)	(denitrifying catalyst)	efficiency (%)	amount (ppm)	(ppm)

15	NH₃	Catalyst 6	Catalyst 11	90	4	1
	Concentration
	153 ppm
16	SO₃	Catalyst 6	Catalyst 11	96	6	8
	concentration
	50 ppm
17	Nox	Catalyst	6	Catalyst 11	91	5	1
	concentration
	170 ppm
18	O₂	Catalyst 6	Catalyst 11	93	4	3
	concentration
	2%
19	SOx	Catalyst	6	Catalyst 11	95	6	8
	concentration
	4000 ppm

From the test results given in Table 3, it was confirmed that the catalyst systems in tests 15 to 19, in which the SO[0073] ₃reducing catalyst is provided at the preceding stage and the denitrifying catalyst is provided at the following stage, are effective for denitrification, SO₃reduction, and decrease in leak of NH₃under any of the test conditions.

Comparative Example 1 (Exhaust Gas Treatment Test)

Exhaust gas treatment tests were conducted under the following conditions using the aforementioned SO[0074] ₃reducing catalyst 6 singly.
[Test Conditions][0075]
The gas composition was 150 ppm NOx, 2900 ppm SOx (including 80 ppm SO[0076] ₃), 180 ppm NH₃, 1.3% O₂, 10% H₂O, 10% CO₂, the balance being N₂.
The test conditions were as follows: temperature: 380° C., superficial velocity: 3.3 mN/s, gas quantity: 30 m[0077] ³N/s, catalyst formation: 50 mm square×800 mm (three SO₃reducing catalysts), and GHSV: 15000 h ⁻1 at outlet of the first SO₃reducing catalyst, 7500 h⁻¹at outlet of the second one, and 5000 h⁻¹at outlet of the third one (AV: 34 m³/m²·h at outlet of the first SO₃reducing catalyst, 17 m³/m²h at outlet of the second one, and 11 m³/m²·h at outlet of the third one).
Table 4 gives the exhaust gas treatment performance results at the catalyst outlet at each GHSV ([0078] comparative tests 1, 2 and 3).

Further, an exhaust gas treatment test was conducted by arranging two denitrifying catalysts at the preceding stage and one SO ₃reducing catalyst at the following stage in series under the same test conditions as described above. Table 4 concurrently gives the exhaust gas treatment performance result at the catalyst outlet (total GHSV 5000 h⁻¹: comparative test 4).

TABLE 4


Exhaust gas treatment performance of comparative catalyst

			NOx removal
Comparative	Preceding-stage	Following-stage	efficiency	SO₃reduction	Leak of NH₃
test	catalyst	catalyst	(%)	amount (ppm)	(ppm)

1	Catalyst 6	—	52	10	70
	(one SO₃reducing catalyst,
	GHSV 15000 h⁻¹)
2	Catalyst 6	—	74	13	15
	(two SO₃reducing catalysts,
	GHSV 7500 h⁻¹)
3	Catalyst 6	—	80	5	0
	(three SO₃reducing catalysts,
	GHSV 5000 h⁻¹)
4	Catalyst 11	Catalyst 6	85	−5	3
	(two denitrifying catalysts)	(one SO₃reducing catalyst)		(Increase)

From the above-described test results, the following facts were found: In [0080] comparative test 1 in which one SO₃reducing catalyst is used, although a decrease in SO₃is recognized, NOx removal efficiency and leak of NH₃do not satisfy the target performance. Also, in comparative test 2 in which two SO₃reducing catalysts are used, although a decrease in SO₃is recognized as in comparative example 2 and leak of NH₃is little, NOx removal efficiency does not satisfy the target performance. Further, in comparative test 3 in which three SO₃reducing catalysts are used, SO₃reduction amount is small and NOx removal efficiency is still low.
Also, in [0081] comparative test 4 in which two denitrifying catalysts are arranged at the preceding stage and one SO₃reducing catalyst is arranged at the following stage, the SO₃reducing function is not recognized, and inversely SO₃tends to increase.
The above-described results revealed the following fact: In the case where a large quantity of NH[0082] ₃is contained, (1) SO₃reducing reaction, (2) denitrifying reaction, and (3) NH₃decomposing reaction take place concurrently, and the SO₃reducing catalyst can carry out denitrification and SO₃reduction at the same time.
On the other hand, in the case where a small quantity of NH[0083] ₃is contained, only (3) NH₃decomposing reaction proceeds, and (1) SO₃reducing reaction and (2) denitrifying reaction scarcely proceed.
For the above reason, it was clarified that in order to remove exhaust gas efficiently using the SO[0084] ₃reducing catalyst, it is preferable that the SO₃reducing catalyst be arranged at the preceding stage and the denitrifying catalyst be arranged at the following stage, by which NOx, SO₃and NH₃are reduced efficiently.

Claims

1. An exhaust gas treatment system for removing nitrogen oxide and sulfur trioxide in exhaust gas by using a catalyst, characterized in that a SO₃reducing catalyst on the upstream side and a denitrifying catalyst on the downstream side are arranged in series with respect to the flow direction of exhaust gas.

2. The exhaust gas treatment system according to claim 1, characterized in that said SO₃reducing catalyst is a catalyst in which Ru is carried as an active metal on a carrier.

3. The exhaust gas treatment system according to claim 1, characterized in that the carrier used for said SO₃reducing catalyst is at least one kind selected from a group consisting of titania, alumina, zirconia, silica, and zeolite.

4. The exhaust gas treatment system according to claim 2, characterized in that said SO₃reducing catalyst contains at least one kind selected from a group consisting of Mo, W and V as a co-catalyst in the carrier.

5. The exhaust gas treatment system according to claim 1, characterized in that said denitrifying catalyst carries at least one kind selected from a group consisting of Mo, W and V as an active metal on a carrier containing titania.

6. The exhaust gas treatment system according to claim 1, characterized in that both of said SO₃reducing catalyst and said denitrifying catalyst are a catalyst formed into a honeycomb shape.

7. An exhaust gas treatment method for removing nitrogen oxide and sulfur trioxide in exhaust gas by using a catalyst, characterized in that said treatment method includes a SO₃reducing process for reducing SO₃using a SO₃reducing catalyst on the upstream side and a denitrifying process for removing nitrogen oxides using a denitrifying catalyst on the downstream side with respect to the flow direction of exhaust gas.

8. The exhaust gas treatment method according to claim 7, characterized in that said treatment method includes an NH₃adding process for adding ammonia NH₃, which is a reducing agent for nitrogen oxides and SO₃, in a mole ratio of NH₃/NOx not lower than the equivalent at the preceding stage of said SO₃reducing process.

9. The exhaust gas treatment method according to claim 7, characterized in that the concentration of NH₃at the downstream outlet of the SO₃reducing catalyst is 3 ppm or higher in said SO₃reducing process.