Catalysts and methods for treating waste gas

Abstract

This invention provides a waste gas treatment catalyst that can improve its performance even when the composition of the waste gas changes. The catalyst is a waste gas treatment catalyst that promotes the reaction of nitrogen oxides and ammonia in a fluid containing nitrogen oxides and ammonia. It comprises titanium oxide as the main component and manganese as the active component, wherein the manganese content (converted to Mn2O3) is more than 0.2% by mass and less than 1.0% by mass.

Description

[Technical Field]

[0001] This invention relates to a catalyst for treating nitrogen oxides contained in waste gas and a method for treating waste gas. [Previous Technology]

[0002] Combustion plants that burn fossil fuels such as natural gas, oil, and coal, or biomass energy (such as gas turbines and boilers) are equipped with waste gas treatment devices to treat the exhaust gases produced after combustion. These waste gas treatment devices include denitrification equipment to reduce nitrogen oxides contained in the waste gases. The denitrification equipment supplies urea or ammonia to the waste gases, causing the ammonia to react with the nitrogen oxides, thereby reducing the nitrogen oxides. The denitrification device is equipped with a catalyst to promote the reaction between ammonia and nitrogen oxides.

[0003] As a denitrification catalyst (exhaust gas treatment catalyst), Patent Document 1 discloses a catalyst for removing nitrogen oxides with a NO2 / NO molar ratio of 1 or more from exhaust gas. This denitrification catalyst is composed of TiO2-V2O5-WO3, and at least one of Cr, Cu, Mn, and Co, expressed as oxides, is added to the catalyst at 0.1 to 10 wt%. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 04-18932 [Summary of the Invention]

[0005] [Problem to be Solved by the Invention] The catalyst described in Patent Document 1 can promote the reduction of nitrogen oxides. However, gas turbines, boilers, and other power generation equipment operate under variable loads, adjusting power generation based on the overall power output. The composition of the exhaust gas changes during variable load operation, low load operation, and operation at or above a set load. Therefore, even with variations in exhaust gas composition, it is still necessary to improve the performance of the catalyst.

[0006] The object of the present invention is to solve the above-mentioned problems and provide a waste gas treatment catalyst and a waste gas treatment method that can improve the performance of the catalyst even when the composition of the waste gas changes. [Means for solving the problems]

[0007] To achieve the above objective, the waste gas treatment catalyst of the present invention is a waste gas treatment catalyst that promotes the reaction of nitrogen oxides and ammonia in a fluid containing waste gas containing nitrogen oxides and ammonia. It contains titanium oxide as the main component and manganese as the active component, wherein the manganese is converted into Mn2O3 and is more than 0.2% by mass and less than 1.0% by mass of the total amount.

[0008] Furthermore, the waste gas treatment method of the present invention includes the following steps: introducing a fluid containing waste gas containing nitrogen oxides and ammonia into the system; and using a waste gas treatment catalyst containing titanium oxide as a main component and manganese as an active component, wherein the manganese content is less than 1.0% by mass of the total amount, to promote the reaction between nitrogen oxides and ammonia. [Effects of the Invention]

[0009] According to the present invention, the performance of the catalyst can still be improved even when the composition of the exhaust gas changes.

Implementation Method

[0011] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to these embodiments, and when multiple embodiments exist, it also includes configurations formed by combining the various embodiments. Furthermore, the constituent elements in the embodiments include: content readily conceived by those skilled in the art, substantially identical content, and content within a so-called equivalent scope.

[0012] Figure 1 is a schematic diagram showing the application of the exhaust gas treatment catalyst of this embodiment to a combustion plant. The denitrification equipment of this embodiment is applied to a combustion plant, which is a gas turbine that uses combustion gas generated from the combustion of gaseous fuel in a combustion chamber to drive a turbine. Although a gas turbine is used as the combustion device in this embodiment, the combustion device is not limited to this. It can also be used as a denitrification device for various combustion devices that burn fuel and discharge exhaust gas containing nitrogen oxides. For example, the combustion device can be a boiler that uses a burner to burn fuel and can recover the heat generated by the combustion. Furthermore, the denitrification equipment can also be used to treat exhaust gas discharged from a co-firing type combustion device that mixes and burns coal, liquid fuel, and gaseous fuel as fuel. Additionally, the denitrification equipment can also be used to treat exhaust gas discharged from combustion devices that burn biomass energy, etc.

[0013] The combustion plant 10 of this embodiment includes a fuel supply unit 12, an air supply unit 14, a combustion device 20, a flue 30, and a denitrification device 40. The combustion plant 10 may also be equipped with a heat exchanger or a waste gas treatment device other than the denitrification device 40, which exchanges heat with the exhaust gas discharged from the combustion device 20. The fuel supply unit 12 supplies fuel, such as LNG or shale gas, to the combustion device 20. The air supply unit 14 supplies air to the combustion device 20.

[0014] The combustion device 20 is a gas turbine, which includes a burner, a compressor, and a turbine. The compressor and the turbine are connected by a shaft. The combustion device 20 compresses air supplied by the air supply unit 14 through the compressor and mixes the compressed air with fuel supplied by the fuel supply unit 12, which is then burned in the burner to generate high-temperature, high-pressure combustion gas. The combustion device 20 uses the energy of the combustion gas to rotate the turbine, thereby discharging the exhaust gas after the turbine has rotated.

[0015] The flue 30 is connected to the combustion device 20, and the exhaust gas from the turbine of the combustion device 20 flows into it.

[0016] The denitrification equipment 40 supplies a reducing agent (such as ammonia, urea solution, etc.) that reduces nitrogen oxides into the flue 30, and causes the exhaust gas after the supply of the reducing agent to pass through a selective reduction catalyst that promotes the reaction between nitrogen oxides and the reducing agent, thereby removing or reducing nitrogen oxides in the exhaust gas. The denitrification equipment 40 has a reducing agent supply device 42, a first denitrification catalyst layer 50, and a second denitrification catalyst layer 52. The denitrification equipment 40 may also be further equipped with a control device for measuring ammonia concentration and controlling the supply amount of the reducing agent.

[0017] The reducing agent supply device 42 includes a reducing agent storage section 42a and an injection section 42b. The reducing agent storage section 42a is a tank for storing reducing agent, such as ammonia water, gaseous ammonia, or urea water. The injection section 42b is connected to the reducing agent storage section 42a, and its supply port is disposed inside the flue 30. The reducing agent supply device 42 supplies reducing agent from the reducing agent storage section 42a to the injection section 42b, and discharges (injects) the reducing agent through the supply port of the injection section 42b disposed inside the flue 30, thereby supplying reducing agent into the flue 30. The reducing agent supplied from the reducing agent supply device 42 moves with the exhaust gas and diffuses in the exhaust gas flow path.

[0018] The first denitrification catalyst layer 50 is disposed downstream of the reducing agent supply device 42 in the direction of exhaust gas flow. The first denitrification catalyst layer 50 has a catalyst and a support mechanism for fixing the catalyst to the flue 30. The shape of the catalyst is not particularly limited and may be plate-shaped or honeycomb-shaped. The composition of the catalyst will be described later. The support mechanism is only required to support the catalyst and its structure is not particularly limited.

[0019] The second denitrification catalyst layer 52 is disposed downstream of the first denitrification catalyst layer 50 in the direction of exhaust gas flow. The second denitrification catalyst layer 52 has a catalyst and a support mechanism for fixing the catalyst to the flue 30. Except for the different placement, the second denitrification catalyst layer 52 is the same as the first denitrification catalyst layer 50.

[0020] The denitrification equipment 40 supplies a reducing agent to the flue 30 via a reducing agent supply device 42. The exhaust gas mixed with the reducing agent passes through the first denitrification catalyst layer 50 and the second denitrification catalyst layer 52. In the first denitrification catalyst layer 50 and the second denitrification catalyst layer 52, the denitrification equipment 40 removes nitrogen oxides from the exhaust gas by promoting the reaction between nitrogen oxides and the reducing agent. Furthermore, although the denitrification equipment 40 of this embodiment has two denitrification catalyst layers, the total number is not particularly limited; it can have three or more layers or one layer. Additionally, a rectifying layer can also be provided upstream of the denitrification catalyst layers in the denitrification equipment 40.

[0021] Next, the catalysts of the first denitrification catalyst layer 50 and the second denitrification catalyst layer 52, i.e., the waste gas treatment catalyst of this embodiment, will be described. The waste gas treatment catalyst is mainly composed of titanium oxide and contains manganese as an active ingredient. In the waste gas treatment catalyst, the amount of manganese (Mn) added is less than 1.0% (wt.%) of the total amount of catalyst, converted from Mn2O3.

[0022] By controlling the manganese content within the above-mentioned range, the waste gas treatment catalyst can maintain its activity and improve its denitrification performance under NO2-rich conditions, and can also maintain its activity and improve its denitrification performance under high concentration HCHO (formaldehyde) coexistence conditions.

[0023] Furthermore, the waste gas treatment catalyst preferably contains vanadium, tungsten, etc., as active ingredients. That is, the waste gas treatment catalyst can be a binary TiO2-WO3 catalyst, a TiO2-V2O5 catalyst, or a ternary TiO2-V2O5-WO3 catalyst, and the amount of manganese (Mn) added is less than 1.0% (wt.%) of the total amount of catalyst, converted from Mn2O3. In addition, the waste gas treatment catalyst may also contain molybdenum (Mo) as an active ingredient. That is, it can be a TiO2-MoO3 catalyst or a TiO2-V2O5-MoO3 catalyst, etc.

[0024] The following explanation will use experimental and comparative examples. First, the composition of the catalysts used in the experimental and comparative examples will be explained.

[0025] (Experimental Example 1) In the exhaust gas treatment catalyst of Experimental Example 1, relative to the total mass percentage, TiO2 was 79.8% by mass, V2O5 was 2.6% by mass, WO3 was 6.2% by mass, Mn2O3 was 0% by mass, and SO3 was 0.45% by mass. In addition to the above components, it also contained binders and unavoidable impurities. Furthermore, as an active ingredient, it did not contain chromium, copper, or cobalt. Here, "not containing the target ingredient" means that its content is below the unavoidable contamination level.

[0026] (Experimental Example 2) In the waste gas treatment catalyst of Experimental Example 2, relative to the total mass percentage, TiO2 was 60% or more (not measured, but the main component), V2O5 was 4.8% by mass, WO3 was 5.3% by mass, Mn2O3 was 0.22% by mass, and SO3 was 0.27% by mass. In addition to the above components, it also contained binders and unavoidable impurities. Furthermore, as an active ingredient, it did not contain chromium, copper, or cobalt.

[0027] (Experimental Example 3) In the waste gas treatment catalyst of Experimental Example 3, relative to the total mass percentage, TiO2 was 60% or more (not measured, but the main component), V2O5 was 2.4% by mass, WO3 was 7.9% by mass, Mn2O3 was 0.23% by mass, and SO3 was 0.35% by mass. In addition to the above components, it also contained binders and unavoidable impurities. Furthermore, as an active ingredient, it did not contain chromium, copper, or cobalt.

[0028] (Experimental Example 4) In the waste gas treatment catalyst of Experimental Example 4, relative to the total mass percentage, TiO2 was 78.1% by mass, V2O5 was 2.5% by mass, WO3 was 5.3% by mass, Mn2O3 was 0.53% by mass, and SO3 was 1.1% by mass. In addition to the above components, it also contains binders and unavoidable impurities. Furthermore, as an active ingredient, it does not contain chromium, copper, or cobalt.

[0029] (Experimental Example 5) In the waste gas treatment catalyst of Experimental Example 5, relative to the total mass percentage, TiO2 was 80.0%, V2O5 was 2.4%, WO3 was 4.1%, Mn2O3 was 0.94%, and SO3 was 0.78%. In addition to the above components, it also contains binders and unavoidable impurities. Furthermore, as an active ingredient, it does not contain chromium, copper, or cobalt.

[0030] (Comparative Example 1) In the exhaust gas treatment catalyst of Comparative Example 1, relative to the total mass percentage, TiO2 was 60% or more (not measured, but the main component), V2O5 was 2.4% by mass, WO3 was 5.2% by mass, Mn2O3 was 2.9% by mass, and SO3 was 0.57% by mass. In addition to the above components, it also contained binders and unavoidable impurities. Furthermore, as an active ingredient, it did not contain chromium, copper, or cobalt.

[0031] (Comparative Example 2) In the exhaust gas treatment catalyst of Comparative Example 2, relative to the total mass percentage, TiO2 was 60% or more (not measured, but the main component), V2O5 was 2.3% by mass, WO3 was 5.0% by mass, Mn2O3 was 4.6% by mass, and SO3 was 0.67% by mass. In addition to the above components, it also contained binders and unavoidable impurities. Furthermore, as an active ingredient, it did not contain chromium, copper, or cobalt.

[0032] (Measurement Conditions) For the above-mentioned catalysts, as the first measurement, exhaust gas with the composition of NOx=47ppm, NO2 / NOx=0.85, NH3=54ppm, HCHO=179ppm, O2=15.1%, H2O=6.5%, CO2=4%, and the remainder being N2 was supplied to each catalyst at a temperature of 294°C and a volumetric flow rate AV=21Nm3 / m2 / hr. The amount of nitrogen oxides and HCHO after passing through the catalysts were measured, and the reduction ratio of nitrogen oxides before and after catalyst reaction (denitrification rate) and the reduction ratio of HCHO before and after catalyst reaction (HCHO removal rate) were calculated. In addition, Comparative Example 2 used exhaust gas rich in ammonia with NH3=142ppm and HCHO=200ppm for testing.

[0033] As the second measurement, a formaldehyde-free waste gas with the composition of NOx=47ppm, NO2 / NOx=0.85, NH3=54ppm, HCHO=0ppm, O2=15.1%, H2O=6.5%, CO2=4%, and the remainder being N2 was supplied to each catalyst at a temperature of 294℃ and a treatment gas flow rate AV=21Nm3 / m2 / hr. The amount of nitrogen oxides after passing through the catalyst was measured, and the reduction ratio of nitrogen oxides before and after catalyst reaction (denitrification rate) was calculated.

[0034] The measurement results are shown in Figures 2 to 5. Figure 2 is a graph showing the measurement results of denitrification performance. Figure 3 is a graph showing the measurement results of HCHO removal performance. Figure 4 is a graph showing the measurement results of denitrification performance. Figure 5 is a table showing the measurement results of denitrification performance.

[0035] Figure 2 is a graph showing the relationship between the denitrification rate (%) and the Mn2O3 ratio (mass%) of each experimental and comparative example measured in the first measurement (HCHO=179ppm). The horizontal axis of Figure 2 is the Mn2O3 ratio (mass%), and the vertical axis is the denitrification rate (%).

[0036] Figure 3 is a graph showing the relationship between the HCHO removal rate (%) and the Mn2O3 ratio (mass%) of each experimental and comparative example measured in the first measurement (HCHO=179ppm). The horizontal axis of Figure 3 is the Mn2O3 ratio (mass%), and the vertical axis is the HCHO removal rate.

[0037] Figure 4 is a graph showing the relationship between the denitrification rate (%) and the Mn2O3 ratio (mass%) of each experimental example and comparative example measured in the second measurement (HCHO=0ppm). The horizontal axis of Figure 4 is the Mn2O3 ratio (mass%), and the vertical axis is the denitrification rate (%).

[0038] Figure 5 is a table summarizing the catalyst composition, first measured denitrification rate, HCHO removal rate and second measured denitrification rate of the experimental and comparative examples.

[0039] The waste gas treatment catalyst is mainly composed of titanium dioxide and contains manganese as an active ingredient. The amount of manganese (Mn) added is less than 1.0% (wt.%) of the total catalyst volume when converted to Mn2O3. Therefore, as shown in Figures 2 and 4, it can remove nitrogen oxides from waste gases containing high levels of nitrogen dioxide at a high ratio, regardless of the presence of HCHO. Furthermore, the waste gas treatment catalyst is mainly composed of titanium dioxide and contains manganese as an active ingredient. The amount of manganese (Mn) added is less than 1.0% (wt.%) of the total catalyst volume when converted to Mn2O3. Therefore, as shown in Figure 3, it can remove HCHO at a high ratio.

[0040] In other words, when the manganese content in the waste gas treatment catalyst, converted to Mn2O3, exceeds 1.0% by mass of the total catalyst, the denitrification performance will decrease in an environment where nitrogen oxides and HCHO coexist in waste gas with high nitrogen dioxide content. However, if it is controlled within the range of this application, the denitrification performance can be maintained. In addition, by reducing the manganese content, the proportion of the main component titanium dioxide can be relatively increased, thereby further enhancing the function of the catalyst.

[0041] Here, if NH3 for denitrification is added to the exhaust gas containing a high concentration of HCHO, the NH3 on the catalyst will react with HCHO, producing the following reaction and generating byproducts, which may lead to insufficient NH3 required for the denitrification reaction:

[0042] The exhaust gas treatment catalyst of the present invention, by controlling the manganese content to below 1.0% by mass (Mn2O3 equivalent to the total catalyst amount), can suppress the promotion of the above-mentioned reaction, thereby suppressing the significant decrease in denitrification rate when HCHO coexists. In this way, the exhaust gas treatment catalyst of the present invention, while suppressing the reaction, can activate the reaction between nitrogen oxides and ammonia, the reaction between nitrogen oxides and formaldehyde, and the formaldehyde removal reaction through catalyst activation, thereby removing nitrogen oxides and formaldehyde as a VOC at a high ratio.

[0043] Furthermore, as shown in this embodiment, the exhaust gas treatment catalyst is suitable for treating exhaust gases with an NO2 / NOx ratio of 0.5 or higher and an HCHO concentration higher than that of NO2. For example, when using the natural gas turbine of this embodiment for power generation, it may operate at a low load during periods of high solar power generation due to output adjustments. Since the natural gas turbine operates at an unexpectedly low load during construction, combustion cannot be performed under optimal combustion conditions, potentially resulting in an NO2 / NOx ratio of 0.5 or higher and the generation of HCHO. The exhaust gas treatment method using the exhaust gas treatment catalyst of this embodiment can remove nitrogen oxides at a high ratio even under such conditions, and simultaneously remove both nitrogen oxides and HCHO at a high ratio.

[0044] Furthermore, the exhaust gas treatment catalyst of the present invention can still achieve the same level of nitrogen oxide removal rate as the comparative example when HCHO is absent, i.e., when the natural gas turbine is combusted under preferred conditions. In other words, the exhaust gas treatment method using the exhaust gas treatment catalyst of the present invention can maintain a high denitrification rate even when treating exhaust gas in an equipment where the conditions of NO2 / NOx ratio of the exhaust gas is 0.5 or higher and the HCHO concentration is higher than the NO2 concentration, to the conditions of no HCHO concentration, and can also effectively remove HCHO even when it is generated.

[0045] Here, in the waste gas treatment catalyst, it is preferable that the titanium dioxide content (based on TiO2) is 60% or more of the total content, and even more preferably 70% or more of the total content. Furthermore, it is preferable to use TiO2 as the titanium dioxide system. By controlling the titanium dioxide content of the main component within the above range, the denitrification rate can be improved.

[0046] In the catalyst for waste gas treatment, it is preferable that manganese, converted from Mn2O3, accounts for more than 0.2% by mass and less than 1.0% by mass of the total amount. By containing manganese, the denitrification rate of the catalyst for waste gas treatment can be improved.

[0047] In the catalyst for treating waste gas, at least one of tungsten oxide and vanadium oxide is preferably included as the active ingredient. In addition, molybdenum may also be included as the active ingredient.

[0048] In the catalyst for waste gas treatment, vanadium oxide is preferably present in an amount of 0.1% to 10% by mass (converted to V2O5) of the total amount, and even more preferably in an amount of 2% to 5% by mass. This can improve the denitrification rate.

[0049] In the catalyst for waste gas treatment, tungsten oxide (WO3 conversion) is preferably between 0.1% and 20% by mass, and even better if it is between 4% and 8% by mass. This can improve the denitrification rate.

[0050] The catalyst for waste gas treatment preferably further includes sulfur trioxide accounting for 0.1 wt% to 2% of the total amount, and more preferably includes sulfur trioxide accounting for 0.2 wt% to 2% of the total amount. This can improve the denitrification rate.

[0051] The catalyst for waste gas treatment, as the active ingredient, preferably does not contain chromium, copper, or cobalt. This simplifies the composition of the catalyst and helps to prevent a decrease in the proportion of the main component.

[0052] [Effects of this embodiment] The present invention has the following features. However, the present invention is not limited to the following: (1) A waste gas treatment catalyst, which is a waste gas treatment catalyst that promotes the reaction of nitrogen oxides and ammonia in a mixture of waste gas containing nitrogen oxides and ammonia, comprising titanium oxide as the main component and manganese as the active component, wherein the manganese, converted as Mn2O3, accounts for more than 0.2% by mass and less than 1.0% by mass of the total amount. Thereby, regardless of whether formaldehyde is contained, waste gas can be treated with a high denitrification rate, and formaldehyde can also be treated at the same time.

[0053] (2) The exhaust gas treatment catalyst as in (1) wherein the aforementioned titanium oxide, converted to TiO2, accounts for more than 60% by mass of the total amount. In this way, the denitrification rate can be improved.

[0054] (3) The waste gas treatment catalyst as in (1) or (2), wherein at least one of tungsten oxide and vanadium oxide is present as the active ingredient.

[0055] (4) As in the waste gas treatment catalyst of (3), vanadium oxide accounts for more than 0.1% and less than 10% of the total amount when converted as V2O5. In this way, the denitrification rate can be improved.

[0056] (5) The exhaust gas treatment catalyst as in (3) or (4) contains tungsten oxide, which, when converted to WO3, accounts for more than 0.1% by mass and less than 20% by mass of the total amount. This can improve the denitrification rate.

[0057] (6) The exhaust gas treatment catalyst of any one of (1) to (5) further comprises sulfur trioxide in an amount of more than 0.1 wt% and less than 2 wt%. This can improve the denitrification rate.

[0058] (7) The exhaust gas treatment catalyst of any one of (1) to (6) does not contain chromium, copper, or cobalt as active ingredients. This can improve the denitrification rate.

[0059] (8) The exhaust gas treatment catalyst of any one of (1) to (7) is suitable for treating exhaust gas with a NO2 / NOx ratio of 0.5 or higher and an HCHO concentration of NO2 or higher. This can improve the denitrification rate.

[0060] (9) A waste gas treatment method comprising the following steps: allowing a fluid containing nitrogen oxides and ammonia to flow in; and using a waste gas treatment catalyst comprising titanium oxide as a main component and manganese as an active component, wherein the manganese content is less than 1.0% by mass of the total amount, to promote the reaction between nitrogen oxides and ammonia.

[0061] (10) The waste gas treatment method of (9) is changed from the condition that the aforementioned waste gas has a NO2 / NOx ratio of 0.5 or higher and the HCHO concentration is higher than the NO2 concentration to the condition that it does not contain HCHO. [Simplified Explanation of the Diagram]

[0010] [Figure 1] Figure 1 is a schematic diagram showing the application of the exhaust gas treatment catalyst of this embodiment in a combustion plant. [Figure 2] Figure 2 is a graph showing the results of the measured denitrification performance. [Figure 3] Figure 3 is a graph showing the results of the measured HCHO removal performance. [Figure 4] Figure 4 is a graph showing the results of the measured denitrification performance. [Figure 5] Figure 5 is a table showing the results of the measured denitrification performance.

Claims

1. A catalyst for treating waste gas, which promotes the reaction of nitrogen oxides and ammonia in a fluid containing waste gas containing nitrogen oxides and ammonia, comprising: titanium oxide as a main component and manganese as an active component; the manganese, converted as Mn2O3, accounts for more than 0.2% by mass and less than 1.0% by mass of the total amount, and further comprises sulfur trioxide accounting for more than 0.1wt% and less than 2% by mass of the total amount.

2. As in the waste gas treatment catalyst of Request 1, wherein the aforementioned titanium oxide, converted to TiO2, accounts for more than 60% by mass of the total amount.

3. The exhaust gas treatment catalyst of claim 1, wherein at least one of tungsten oxide and vanadium oxide is included as an active ingredient.

4. As in the waste gas treatment catalyst of Request 3, vanadium oxide accounts for more than 0.1% and less than 10% of the total amount when converted as V2O5.

5. As in the waste gas treatment catalyst of request item 3, the tungsten oxide, converted to WO3, accounts for more than 0.1% by mass and less than 20% by mass of the total amount.

6. The exhaust gas treatment catalyst, as requested in item 1, does not contain chromium, copper, or cobalt as active ingredients.

7. For any of the exhaust gas treatment catalysts in requests 1 to 6, the NO2 / NOx ratio of the exhaust gas to be treated is 0.5 or higher, and the HCHO concentration is higher than the NO2 concentration.

8. A waste gas treatment method comprising the following steps: flowing in a fluid containing waste gas containing nitrogen oxides and ammonia; and using a waste gas treatment catalyst comprising titanium oxide as a main component and manganese as an active component, wherein the manganese content is less than 1.0% by mass of the total amount, to promote the reaction between nitrogen oxides and ammonia.

9. The waste gas treatment method of claim 8, wherein the aforementioned waste gas is changed from a condition where the NO2 / NOx ratio is 0.5 or higher and the HCHO concentration is 0.5 or higher than the NO2 concentration, to a condition where there is no HCHO concentration.

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