Methods for extending the service life of depleted SCR catalyst beds in NOx stationary source exhaust systems.

Abstract

NO x In the exhaust gas aftertreatment system of a stationary pollutant source, the exhaust gas emitted from the system to the atmosphere is NO x nitrogen oxides (NO) in the presence of nitrogen-containing reductants to continue to meet violation limits for both NO and ammonia emissions. x 1. A method for extending the useful life of a depleted selective catalytic reduction (SCR) catalyst bed that catalyzes the conversion of ammonia (NH 3 ) to dinitrogen (N 2 ), comprising retrofitting a honeycomb substrate monolith or plate-type substrate downstream of the depleted SCR catalyst bed, the honeycomb substrate monolith containing a catalyst (ASC) for converting ammonia (NH 3 ) in an exhaust gas that also contains oxygen to nitrogen and water, and x is greater than about 80 m / hr between about 300 and about 400°C, where kNO of a catalyst sample aged in 10% HO (as water vapor) in air at 450°C for 48 hours x is determined by SCR activity testing in a laboratory-scale reactor using a gas composition of 50 ppm CO, 30 ppm NO, 36 ppm NH, 15% O, 8% water, 3% CO, balance N.

Description

DISCLOSURE OF THEINVENTION

[0001] The present invention relates to a method for extending the useful life of depleted selective catalytic reduction catalyst beds in the exhaust systems of stationary sources of NOx.

[0002] Modern generators fueled by natural gas, light distillates or syngas (e.g., from coal) operate combined cycle gas turbines in which the gas is burned in a gas turbine to generate electricity in a first cycle and the heat from the combustion of the gas is recovered in a steam-operated heat engine in a second cycle. The combination of these two cycles improves the net overall efficiency of the system. The second cycle is usually performed in what is known as a heat recovery steam generator (HRSG) system, which usually also includes catalytic components for treating components including nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC) in the exhaust gas resulting from the combustion of the gas in the first cycle. The catalytic activity of these catalytic components is enhanced by the transfer of heat from the exhaust gas being treated to the catalytic components themselves. The location of the catalytic components in the HRSG system can be selected for temperature-optimized catalytic activity.

[0003] Emissions of NOx from stationary pollution sources, mainly from power plants, industrial heaters, cogeneration plants, e.g. wood-fired boilers, stationary diesel and gas engines, marine propulsion engines, diesel locomotives, industrial and municipal waste incinerators, chemical plants and glass, steel and cement manufacturing plants, are a major environmental problem. NOx causes ozone formation in the troposphere, acid rain formation and respiratory problems in humans. NOx is produced thermally in the combustion process by the combination of N2 and O2 present in the air. At temperatures above about 1500 °C, this reaction proceeds at a significant rate through a well-characterized mechanism called the Zeldovich mechanism.

[0004] In order to meet the NOx emission standards specified by various regulatory authorities, exhaust (flue) gas aftertreatment methods are required. Among such aftertreatment methods, the selective catalytic reduction (SCR) method is the best developed and the most widely used worldwide for the control of NOx emissions from stationary pollution sources due to its efficiency, selectivity (towards N2 products) and economics. The SCR reaction generally consists of the reduction of NOx by ammonia (NH3) to form water and nitrogen.

[0005] The main reactions involved in SCR NOx reduction are shown in reactions (1), (2), and (3): 4NO+4NH3+O2→ 4N2+6H2O (1) 6NO2+8NH3→ 7N2+12H2O (2) NO+NO2+2NH3→ 2N2+3H2O (3)

[0006] Three types of catalysts have been developed to promote reactions (1)-(3): precious metals, metal oxides, and metal-promoted zeolites. Precious metal SCR catalysts are primarily considered for low temperature and natural gas applications, since they may unnecessarily promote SO2 oxidation at temperatures above about 200 °C.

[0007] Among the various metal oxide SCR catalysts developed for 300-400°C applications, vanadia supported on anatase titania and promoted with tungsta or molybdena was found to resist sulfation and have low activity for the SO2 oxidation reaction.

[0008] Commercially available zeolite SCR catalysts for treating stationary source NOx emissions include mordenite (see RH Heck et al., "Catalytic Air Pollution Control - Commercial Technology", 3rd ed. (2009) John Wiley & Sons, Inc. Hoboken, New Jersey). See especially Chapter 12.

[0009] Fe promoted zeolite catalysts have been proposed for SCR primarily for use in gas-fired cogeneration plants at high temperatures, i.e., up to 600° C., where metal oxide catalysts can be thermally unstable.

[0010] Commercially available SCR catalysts are disposed as a coating on an extruded honeycomb monolith, a plate, or an inert honeycomb monolith.

[0011] For a more complete background on the application of SCR processes to stationary sources of NOx emissions, see P. Forzatti, App. Cat A: General 222 (2001) 221-236.

[0012] Reaction (3) is preferred as it is known to be a relatively fast reaction compared to either reaction (1) or especially reaction (2). A suitable oxidation catalyst, placed upstream of the SCR catalyst for this purpose, is also relevant for treating the CO and HC components in the exhaust gas. Reductant injection upstream of the SCR catalyst is generally performed downstream of the oxidation catalyst to avoid inefficient consumption of ammonia (by oxidation to NOx) with a consequent decrease in the overall system conversion performance. Ammonia injection can be performed via a suitable device such as an ammonia injection grid (AIG).

[0013] Any of the above SCR catalysts can be used to treat NOx in HRSG systems, but the gas flue gas temperatures generally require that the above metal oxide or metal promoted zeolites are suitable.

[0014] Gas turbine emission control systems typically include a CO oxidation catalyst (CO-Ox), followed by an ammonia injection grid (AIG), and finally a selective catalytic reduction (SCR) catalyst bed (SCR). As the catalyst system deteriorates, CO conversion decreases and SCR performance worsens. To maintain overall system NOx emissions below a prescribed minimum, plant operators increase the ammonia:NOx ratio to continue driving reactions (1), (2), and (3). However, this increases ammonia emissions ("slip") and increases operating costs through consumption of reductants. Ammonia slip refers to the emission of unreacted ammonia resulting from the incomplete reaction of NOx with reductants. Ammonia slip can cause the production of ammonium sulfate, which can clog or corrode downstream components and contribute undesirably to particulate matter (PM2.5) emissions into the atmosphere. In the United States, allowable ammonia slip levels are typically 2-10 ppm.

[0015] Eventually, the SCR catalyst activity will degrade to a point where it can no longer be increased without the ammonia:NOx ratio exceeding a predetermined maximum, at which point typical practice is to remove and dispose of the spent catalyst and replace it with a new catalyst so that the system can remain compliant with emissions standards.

[0016] The inventors have surprisingly found that NOx Regulated limits The ability of an end-of-life SCR catalyst bed in a gas turbine exhaust system to treat up to 100% of the exhaust gases exhausted from the exhaust pipes to be improved by retrofitting an ammonia slip treatment catalyst (referred to herein as an ammonia slip catalyst or "ASC") downstream of the end-of-life SCR catalyst bed more economically than by replacing the end-of-life SCR catalyst itself. Regulated NH 3 Slip Limit It has been found that the efficiency of the catalytic converter can be extended by up to 50% without exceeding the required operating temperature. This finding has the considerable advantage that the benefits can be obtained using a relatively small catalyst volume compared to the SCR catalyst bed volume, resulting in negligible additional backpressure added to the system, which does not significantly impact the overall power generation efficiency.

[0017] According to one aspect, the present invention provides a method for producing a cellular membrane comprising: x In the exhaust gas aftertreatment system of a stationary pollution source, the exhaust gas discharged from the system to the atmosphere is NO x Regarding emissions of both chlorine and ammonia Regulated limits In order to keep the nitrogen oxides (NO x A method for extending the service life of an aged selective catalytic reduction (SCR) catalyst bed that catalyzes the conversion of ammonia (NH) to dinitrogen (N2), comprising retrofitting a honeycomb substrate monolith or plate-type substrate downstream of the aged SCR catalyst bed, the honeycomb substrate monolith containing a catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water, and measuring the kNO of the honeycomb substrate monolith containing a catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water. x is about 80 m / hr or more between about 250°C and about 400°C, and kNO x is determined by SCR activity testing in a laboratory scale reactor using a gas composition of 50 ppm CO, 30 ppm NO, 36 ppm NH3, 15% O2, 8% water, 3% CO2, balance N2.

[0018] The present invention provides a number of important advantages: The useful life of the SCR catalyst can be extended by up to two times, which promotes sustainable use of resources in that relatively large SCR catalyst volumes do not need to be recycled or sent to landfills for significant periods of time.

[0019] In fact, the relatively small volume of retrofit honeycomb substrate monoliths or plate-type substrates containing catalyst for converting ammonia in exhaust gases that also contain oxygen to nitrogen and water is just one of the factors that makes it possible to manufacture retrofit catalysts at a cost lower than the replacement cost of SCR catalysts, which can be 20% or more lower than the replacement cost of SCR catalysts.

[0020] Ammonia slip catalysts are selectively designed to promote the following overall reaction: 4NH3+3O2→ 2N2+6H2O (4)

[0021] As described in more detail below, U.S. Patent Application Publication Nos. 2007 / 0110643 A1, 2016 / 0245139 A1 and 2016 / 0256823 A1 propose the use of an ammonia slip catalyst (also known as an ammonia oxidation ("AMOX") catalyst) downstream of or in combination with an SCR catalyst in the exhaust system of a gas turbine engine.

[0022] However, as described above, because SCR catalyst beds are capable of converting NOx with negligible ammonia slip, and because relatively low levels of ammonia are permitted to be emitted to the atmosphere anyway, power plant operators generally do not include an ammonia slip catalyst downstream of a newly installed SCR catalyst bed. Thus, adding an ammonia slip catalyst to the systems proposed by these publications is unnecessary and would undesirably increase the economic costs and burdens to the plant operators who install the systems.

[0023] U.S. Patent Application Publication No. 2007 / 0110643A1 discloses an exhaust gas treatment device for reducing the concentrations of NOx, HC and CO in an exhaust gas stream such as that produced by a gas turbine engine of a power plant. The treatment device includes a multi-functional catalyst element having an upstream reduction-only section and a downstream reduction-oxidation section disposed downstream of an ammonia injector. Injection of ammonia in excess of stoichiometric concentrations promotes selective catalytic reduction (SCR) of NOx in the upstream section of the catalyst element, and the resulting ammonia slip is oxidized in the downstream section of the catalyst element. Any additional NOx produced by the oxidation of ammonia is further reduced in the downstream section before being sent to the atmosphere.

[0024] US Patent Application Publication No. 2016 / 0245139A1 discloses an exhaust system for a power generation device including a heat source for burning fuel in air to generate electricity, the exhaust system configured to receive a flowing exhaust gas and including a catalyst system for treating the exhaust gas, the catalyst system including a first oxidation catalyst and a second catalyst, the first oxidation catalyst being disposed downstream of the heat source such that the flowing exhaust gas contacts the first oxidation catalyst before the second catalyst.

[0025] US Patent Application Publication No. 2016 / 0256823A1 discloses a system including a nitrogen oxide reduction catalyst fluidly connected to an exhaust conduit of an engine system. The nitrogen oxide reduction catalyst is configured to reduce nitrogen oxides in the engine exhaust. The system also includes an ammonia oxidation catalyst fluidly connected to the exhaust conduit downstream of the nitrogen oxide reduction catalyst and configured to reduce ammonia in the engine exhaust. The system further includes a reductant injection control system configured to control injection of a reductant into the exhaust conduit, determine a first nitrogen oxide conversion rate of the nitrogen oxide reduction catalyst, determine an ammonia storage value of the nitrogen oxide reduction catalyst, and determine a first temperature of the engine exhaust upstream of the ammonia oxidation catalyst. The reductant injection control system is also configured to increase or decrease injection of a reductant based on the first nitrogen oxide conversion rate, the ammonia storage value, and the first temperature.

[0026] US Patent No. 7,727,499 discloses a method for removing by oxidation excess ammonia (NH3) gas ("ammonia slip") resulting from flue gas and exhaust gas subjected to selective catalytic reduction (SCR) for the reduction of nitrogen oxides (NOx) with ammonia. More specifically, the method uses an ammonia oxidation catalyst consisting of a zeolite, one or more noble metals, and a base metal compound to catalyze the oxidation of both ammonia and carbon monoxide (CO) while minimizing the production of nitrogen oxides (NOx).

[0027] The rate constant kNOx is defined as: Rate constant (kNO x (m / hr))=-area velocity (Av (m / hr)×ln(1-NO x Conversion rate / 100) Here, the areal velocity A v (m / hr) = Q(Volumetric Flow Rate (m 3 / hr)) / A cat (Catalyst surface area (m 2 )).

[0028] From these equations, kNO of the catalyst x It can be seen that the constant is independent of the pitch (or cells / square inch) of the honeycomb substrate monolith or plate-type substrate, and therefore also of the volume of the honeycomb substrate monolith or plate-type substrate.

[0029] In practice, it is desirable for a catalyst for converting ammonia in exhaust gases that also contain oxygen to nitrogen and water, i.e., an ammonia slip catalyst (ASC), to have as low a pressure drop as economically possible. This is because, compared to a system containing an SCR catalyst alone, the addition of an ASC to a system introduces additional pressure drop and requires additional energy input to accommodate it. This additional energy input can be significant for gas turbine power generation, as the additional energy demands in the system can reduce the efficiency of the power cycle as a whole.

[0030] An important component of the pressure drop is the ASC bed depth, which can be affected by, for example, the operating temperature, the kNOx and the pitch (or honeycomb cell density) of the substrate monolith that makes up the ASC. So, for example, for a fixed ASC pitch, by lowering kNOx, more geometric ASC surface area - and therefore substrate monolith volume and, therefore, increased backpressure - is required to compensate.

[0031] Similarly, by lowering the pitch of an ASC substrate monolith, increased catalyst volume would be required to compensate and maintain a fixed kNOx.

[0032] Therefore, there should be a balance to be determined among at least the pitch of the substrate monolith, the volume of the substrate monolith, and the geometric surface area of the substrate monolith and the composition of the ASC according to the position of the ASC in the system and the temperature of the catalyst in use, and the selection of those parameters can be determined based on cost and operational considerations, comprehensively including kNOx.

[0033] The kNO of the honeycomb substrate monolith or plate-type substrate containing the catalyst x can be 300 m / hr or less.

[0034] Preferably, the kNOx of the honeycomb substrate monolith or plate-type substrate containing the catalyst is about 90 < kNOx < about 300 m / h between about 300 °C and about 400 °C.

[0035] The catalyst for converting ammonia in the exhaust gas containing oxygen into nitrogen and water preferably converts about 70% of NH3 at 250 °C or higher, and more preferably > about 80% of NH3 at about 300 °C or higher.

[0036] The catalyst for converting ammonia in the exhaust gas containing oxygen into nitrogen and water preferably has < about 20% of sNOx = NOx out divided by (NH3in - NH3out) at less than about 400 °C, and more preferably < about 10% of sNOx at less than about 350 °C, where sNOx is determined using the same conditions as those defined for determining the above kNOx.

[0037] The space velocity at which the exhaust gas contacts the post-attached catalyst for converting ammonia in the exhaust gas containing oxygen into nitrogen and water is 50000 - 500000 h -1 , for example, 100000 - 400000 h -1 or 150000 h -1 ~350000 h -1 can be.

[0038] A retrofit catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water preferably comprises a platinum group metal and a selective catalytic reduction catalyst supported on a refractory oxide support.

[0039] The platinum group metal in the platinum group metal supported on the refractory oxide support is about 1 to about 40 g / ft 3 , preferably about 1 to about 25 g / ft 3 Or about 1 to about 10 g / ft 3 Low loadings of platinum group metal are preferred because such loadings promote selectivity for reaction (4).

[0040] The SCR catalyst in a retrofit catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water may comprise: (i) vanadia supported on titania in the anatase form and promoted with tungsta or molybdena; (ii) a metal promoted molecular sieve; or (iii) a mixture of vanadia and metal promoted molecular sieve supported on titania in the anatase form and promoted with tungsta or molybdena.

[0041] When the SCR catalyst in a retrofit catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water contains a metal promoted molecular sieve, preferably the molecular sieve is an aluminosilicate zeolite.

[0042] Preferably, when the SCR catalyst in the retrofit catalyst for converting ammonia to nitrogen and water in an exhaust gas also containing oxygen comprises a metal-promoted molecular sieve, the molecular sieve has a maximum ring size of eight tetrahedral atoms, and more preferably is selected from the group consisting of framework type codes CHA, AEI and AFX. Aluminosilicate zeolites that are particularly resistant to hydrothermal degradation and hydrocarbon "coking" have a maximum ring size of eight tetrahedral atoms and are referred to in the art as "small pore" zeolites. Zeolites are classified by their framework type by the International Zeolite Association. However, in certain applications where long-term durability is not a primary requirement, for example in relatively low temperature applications, "medium pore" zeolites, such as those having framework type codes MFI or FER, or "large pore" zeolites, such as BEA or MOR, may be applicable as well.

[0043] The silica to alumina ratio (SAR) of the zeolite can be any suitable SAR for promoting reactions (1), (2) and (3). Generally, this is a balance between thermal stability, for which a relatively high silica content is preferred, on the one hand, and the promoting effect of the ion-exchanged transition metal, for which a relatively high alumina content is preferred. In practice, the SAR selected will depend on the framework type code of the zeolite, but is typically in the range of about 10 to about 256, with SARs of about 15 to about 40 being preferred for small pore zeolites such as CHA, AEI and AFX.

[0044] Preferably, the metal in the metal-promoted molecular sieve is copper and / or iron.

[0045] The refractory oxide support for platinum group metals in SCR catalysts in retrofit catalysts for converting ammonia in exhaust gases that also contain oxygen to nitrogen and water can be silica-doped alumina, titania or optionally doped zirconia.

[0046] A honeycomb substrate monolith or plate-type substrate in an SCR catalyst in a retrofit catalyst for converting ammonia in an exhaust gas that also contains oxygen to nitrogen and water has an axial length and can be coated with a first washcoat layer comprising a platinum group metal supported on a refractory oxide support and a second washcoat layer comprising an SCR catalyst.

[0047] A first washcoat layer can be disposed in a first zone coated from a first end of the honeycomb substrate monolith, and a second washcoat layer can be disposed in a second zone coated from a second end of the honeycomb substrate monolith or plate-type substrate, the second layer being disposed upstream from the first layer.

[0048] Alternatively, the first zone may be smaller than the axial length of the honeycomb substrate monolith or plate-type substrate and the second layer of the second zone may at least partially cover the first layer, in which case preferably the second layer may extend over the entire axial length of the honeycomb substrate monolith or plate-type substrate.

[0049] Alternatively, a first layer may extend the entire axial length of the honeycomb substrate monolith or plate-type substrate, and a second layer may cover the first layer and extend the entire axial length of the honeycomb substrate monolith or plate-type substrate.

[0050] Preferably, the NOx emissions in the method according to the present invention Regulated limits and / or ammonia emissions are below 2 ppm. Regulated limits is 10 ppm or less, for example less than 5 ppm or less than 2 pm.

[0051] The temperature at which the exhaust gas contacts the SCR catalyst bed can be from about 200°C to about 450°C.

[0052] The nitrogen-containing reductant is ammonia (NH3) itself, hydrazine, or any suitable ammonia precursor, such as urea ((NH2)2CO), ammonium carbonate, ammonium carbamate, ammonium bicarbonate, or ammonium formate. It is important that the mixing of the ammonia with the exhaust gas is as complete as possible before the exhaust gas / ammonia enters the SCR catalyst. This can be done using various baffles and static mixers, for example with herringbone and skew channel geometries, as needed, provided they do not contribute significantly to the increase in back pressure. Good mixing of the ammonia with the exhaust gas can be promoted by complete two-dimensional coverage of the reductant injection across the duct carrying the exhaust gas. Such an arrangement is known in the art as an ammonia injection grid (AIG).

[0053] To facilitate reaction (3) (and reactions (1) and (2)), the alpha ratio of ammonia molecules to NOx molecules used in the reaction is preferably from about 0.90 to about 2.00, preferably < about 1.8, for example from about 0.95 to about 1.5 or from 1.00 to about 1.20.

[0054] The SCR catalyst may be in the form of a washcoat coated onto a substrate, such as an inert ceramic honeycomb monolith formed from cordierite, or a metal monolith, or may be prepared as an extruded honeycomb body, where the catalyst is mixed with a paste of a binder (or matrix) component and then extruded into a desired shape having channels extending therethrough. The zeolite-containing washcoat compositions used in the present invention for coating onto monolith substrates or for producing extruded substrate monoliths may include a binder selected from the group consisting of alumina, silica, (non-zeolitic) silica-alumina, naturally occurring clays, TiO2, ZrO2, and SnO2.

[0055] In particular, the SCR catalyst may be any of those described for use in catalyzing the conversion of ammonia in an exhaust gas that also contains oxygen to nitrogen and water as described above, namely one of the following: (i) vanadia supported on titania in the anatase form and promoted with tungsta or molybdena; (ii) a metal-promoted molecular sieve; or (iii) Mixtures of vanadia and metal promoted molecular sieves supported on anatase form of titania and promoted with tungsta or molybdena.

[0056] The method according to the invention can be used to treat exhaust gases from any stationary source of NOx emissions. In particular, the exhaust gases can be the product of power plants, industrial heaters, cogeneration power plants, combined cycle power plants, wood-fired boilers, stationary diesel engines, stationary natural gas-fired engines, marine propulsion engines, diesel locomotives, industrial waste incinerators, municipal solid waste incinerators, chemical plants, glass manufacturing plants, iron and steel manufacturing plants or cement manufacturing plants.

[0057] In a preferred application, the exhaust gas is the product of a cogeneration plant, preferably a stationary natural gas combustion engine.

[0058] Preferably, the exhaust gas aftertreatment system comprises a heat recovery steam generator (HRSG).

[0059] The term "comprising" as used herein may be interchanged with the definitions "consisting essentially of" or "consisting of." The term "comprising" is intended to mean that the specified elements are essential, but other elements may be added and still form part of the claimed subject matter. The term "consisting essentially of" limits the scope of a claim to the specified materials or steps, and those that do not materially affect the basic and novel characteristics of the claimed invention. The term "consisting of" closes a claim to the inclusion of materials other than those recited, except for impurities ordinarily associated therewith.

[0060] For the avoidance of doubt, the entire contents of all documents cited herein are hereby incorporated by reference. [Brief description of the drawings]

[0061] A conventional HRSG exhaust system for a gas turbine including an SCR catalyst is shown in Figure 1, which has been adapted to the method of the present invention by inserting an ammonia slip catalyst downstream of the SCR catalyst in the location illustrated in Figure 2. Figures 1 and 2 are self-explanatory.

[0062] In order that this invention may be more fully understood, the following examples are given by way of illustration only. EXAMPLES

[0063] [Example 1] A dual layer ammonia slip catalyst (ASC) was prepared on a ceramic 230 cells per square inch (cpsi) honeycomb substrate monolith with a cell wall thickness of 7 mils (thousandths of an inch). The first (bottom) layer was coated directly onto the substrate with a continuous alumina washcoat layer along the entire axial length of the substrate. The resulting coated part was dried and calcined. Pt was then added at 5 g / ft from a platinum nitrate solution. 3 ASC was impregnated into the washcoat alumina layer until a Pt loading of 100% was reached. The resulting part was then dried and calcined. Finally, a second washcoat layer comprising Cu-impregnated CHA zeolite mixed with a binder was applied in a continuous layer covering 100% of the Pt-alumina layer along the entire axial length of the substrate, and the resulting part was then dried and calcined. A cylindrical core measuring 2 inches in diameter and 3.3 inches in length was cut from the finished honeycomb substrate monolith coated with dual layer ASC.

[0064] [Example 2 (SCR test)] The cylindrical ASC cores of Example 1 were degleined in 10% H2O in air (as water vapor) at 450°C for 48 hours and then activity tested in a laboratory scale reactor for SCR activity. The gas composition fed to the catalyst for activity testing was 50 ppm CO, 30 ppm NO, 36 ppm NH3, 15% O2, 8% water, 3% CO2, balance N2. CO, NOx and NH3 conversions were measured by holding the reactor at steady state temperature points ranging from 200 to 450°C. The gas hourly space velocity (GHSV) relative to the ASC volume was 180,000 hr -1 It was.

[0065] For various temperatures, the CO conversion, NOx conversion, and NH3 conversion are respectively shown in Table 1. kNOx, defined as kNOx=-(areal velocity)×ln(1-NOx conversion / 100), is also shown. TIFF0007624802000001.tif71167

[0066] [Example 3 (NH3 oxidation test)] The cylindrical ASC cores of Example 1 were degleined in 10% H2O in air at 450°C for 48 hours and then activity tested for NH3 oxidation activity in a laboratory scale reactor. The gas composition fed to the catalyst was 50 ppm CO, 30 ppm NH3, 10% O2, 4% water, 3% CO2, balance N2. CO, NOx and NH3 conversions were measured by holding the reactor at steady state temperature points ranging from 200 to 450°C. The gas hourly space velocity (GHSV) relative to the ASC volume was 120,000 hr -1 It was.

[0067] The results are shown in Table 2 for CO conversion and NH3 conversion. sNOx, defined as (NOxout divided by (NH3in-NH3out)), is also shown. TIFF0007624802000002.tif70159

[0068] Example 4 (SCR Test with High Ammonia to NOx Ratio (ANR)) The cylindrical ASC cores of Example 1 were degleined in 10% H2O in air at 450°C for 48 hours and then activity tested in a laboratory scale reactor for SCR activity. The gas composition fed to the catalyst for activity testing was 50 ppm CO, 30 ppm NO, 54 ppm NH3, 15% O2, 8% water, 3% CO2, balance N2. CO, NOx and NH3 conversions were measured by holding the reactor at steady state temperature points ranging from 200 to 450°C. The gas hourly space velocity (GHSV) relative to the ASC volume was 180,000 hr -1 It was.

[0069] For various temperatures, the CO conversion, NOx conversion, and NH3 conversion are shown in Table 1. The kNOx, defined as kNOx=-(areal velocity)×ln(1-NOx conversion / 100), is also shown. TIFF0007624802000003.tif70157

Claims

1. NO ｘ In an exhaust gas aftertreatment system for a fixed pollution source, the exhaust gas discharged from the system to the atmosphere is NO ｘ In order to maintain emissions of both nitrogen oxides (NO2) and ammonia below regulated limits, the ｘ ) to dinitrogen (N ２ 1. A method for extending the useful life of an deteriorated selective catalytic reduction (SCR) catalyst bed which catalyzes the conversion of CO to CO, comprising: downstream of the depleted SCR catalyst bed, retrofitting a honeycomb substrate monolith or plate-type substrate containing a retrofit catalyst for converting ammonia in an oxygen-containing exhaust gas to nitrogen and water; the retrofit catalyst for converting ammonia in an oxygen-containing exhaust gas to nitrogen and water comprises a platinum group metal and a selective catalytic reduction catalyst supported on a refractory oxide support; The platinum group metal supported on the refractory oxide support is 35 to 1413 g / m ３ (1-40g / ft ３ ) is filled, The process wherein the SCR catalyst in said retrofit catalyst is either vanadia supported on anatase form of titania and promoted with tungsta or molybdena, or a metal promoted molecular sieve.

2. 2. The method of claim 1, wherein the molecular sieve has a framework type code that is CHA, AEI, AFX, BEA, MOR, MFI or FER.

3. 3. The method of claim 1 or 2, wherein a honeycomb substrate monolith or a plate-shaped substrate has an axial length and is coated with a first washcoat layer comprising a platinum group metal supported on a refractory oxide support and a second washcoat layer comprising an SCR catalyst.

4. 4. The method of claim 3, wherein a honeycomb substrate monolith is retrofitted, a first washcoat layer is disposed in a first zone coated from a first end of the honeycomb substrate monolith, a second washcoat layer is disposed in a second zone coated from a second end of the honeycomb substrate monolith, and the second layer is disposed upstream from the first layer.

5. 5. The method of claim 4, wherein the first zone is smaller than an axial length of the honeycomb substrate monolith and the second layer of the second zone at least partially covers the first layer.

6. The method of claim 4 , wherein the second layer extends the entire axial length of the honeycomb substrate monolith.

7. 4. The method of claim 3, wherein the first layer extends the entire axial length of the honeycomb substrate monolith and the second layer covers the first layer and extends the entire axial length of the honeycomb substrate monolith.

8. The method of claim 1 or 2, wherein the retrofit catalyst comprises an SCR catalyst, and the SCR catalyst in the retrofit catalyst is a washcoat coated on a substrate or is a component of an extruded honeycomb body.

9. The SCR catalyst in the retrofit catalyst, (i) Vanadia supported on titania in the anatase form and promoted with tungsta or molybdena; (ii) a metal promoted molecular sieve; or (iii) Mixtures of vanadia and metal promoted molecular sieves supported on titania in the anatase form and promoted with tungsta or molybdena. The method of claim 8, wherein the

10. The rate constant kNO of the honeycomb substrate monolith containing the retrofit catalyst for converting ammonia in an oxygen-containing exhaust gas to nitrogen and water. ｘ is about 80 m / hr or more between about 250 and about 400° C., where 10% H ２ The kNO of the retrofit catalyst sample aged in air containing O (as water vapor) at 450° C. for 48 hours ｘ 50 ppm CO, 30 ppm NO, 36 ppm NH ３ , 15% O ２ , 8% water, 3% CO ２ , remainder N ２ was determined by SCR activity testing in a laboratory-scale reactor using a gas composition of ｘ teeth: Rate constant (kNO ｘ )=-area velocity (A ｖ (m / hr))×ln(1-NO ｘ Conversion rate / 100) [wherein the areal velocity A ｖ (m / hr) = Q (volume flow rate (m ３ / hr)) / A ｃａｔ (Catalyst surface area (m ２ )) 10. The method of any one of claims 1 to 9, wherein the honeycomb substrate monolith is coated with a first washcoat layer extending the entire axial length of the honeycomb substrate monolith and a second washcoat layer overlying the first layer and extending the entire axial length of the honeycomb substrate monolith, the first washcoat layer comprising alumina and the second washcoat layer comprising copper impregnated CHA zeolite mixed with a binder.

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