Patent Publication Number: US-2016245139-A1

Title: Exhaust system for power generating apparatus

Description:
FIELD OF THE INVENTION 
     The invention relates to an exhaust system for a power generating apparatus, such as a gas turbine, which exhaust system being adapted to receive a flowing exhaust gas and comprising a catalyst system for treating the exhaust gas, which catalyst system comprising a system of oxidation catalysts. In particular, the invention relates to methods of using such systems to reduce exhaust emissions from a power generating apparatus between start-up of the power generating apparatus and full-load. 
     BACKGROUND 
     In normal operation, gas turbine power plants generate significant amounts of carbon dioxide (CO 2 ), water, carbon monoxide (CO), volatile organic compounds (VOCs) and oxides of nitrogen (NOx) as part of the combustion process. Various regulatory agencies world-wide, such as the U.S. Environmental Protection Agency, are charged with reducing exhaust emissions. 
     Gas turbine engines typically operate by drawing air into a compressor to increase the gas pressure. A hydrocarbon fuel, typically natural gas, is combusted using the compressed air. The combustion normally takes place under relatively “lean” conditions, where more than the stoichiometric amount of oxygen necessary for complete combustion of the hydrocarbon fuel components is used. This helps to maintain a relatively low combustion temperature, which can improve the durability of materials used to make the turbines. 
     The high temperature, high pressure gas from a combustor is fed into a gas turbine engine where the gas expands and the temperature of the gas drops. In most applications, the gas turbine drives the compressor, as well as a generator that generates electric power. The gas leaving the turbine is at a relatively high temperature and can be used to generate steam in a heat recovery steam generator (“HRSG”) before being exhausted or treated in downstream operations to reduce unwanted emissions. Steam created by the heat recovery steam generator can be used as part of a combined cycle plant to drive a steam turbine. This increases the power generation efficiency of a power plant using the HRSG. 
     One of the problems with such a system is that the exhaust gas contains carbon monoxide (CO), volatile organic hydrocarbons (VOC) and oxides of nitrogen (NOx), all of which are being controlled by various regulatory agencies. 
     Two basic types of catalytic technologies are used in reducing emissions from power plants: the use of oxidation catalysts to convert CO and VOCs, and the use of selective catalytic reduction (“SCR” catalysts) to convert NO x . Emissions from mobile engines, e.g. automobile engines, use oxidation and SCR catalysts in conjunction with other technologies and changes in the operating parameters of the engine to reduce emissions. However, gas turbines differ from mobile engines in that once gas turbines reach their full load operation, the operating parameters of the system stay relatively constant, unlike mobile engines where dynamic driving conditions cause changes in the load demand on the engine, which results in changeable exhaust gas temperatures. Different phases of a mobile engine&#39;s duty cycle, e.g. on start-up from cold, can also affect the temperature of exhaust gas contacting the exhaust gas aftertreatment catalysts. These differences in operating parameters prevent some of the technologies used in mobile engines from being used in gas turbines. 
     Oxidation catalysts are used to treat CO and VOCs before NOx is treated. A number of oxidation catalysts are known. The selection of the oxidation catalyst is based on a number of factors, including, but not limited to, the temperature at which the catalyst will be used, the loading of the catalyst, the fuel being combusted, the flow rate of the exhaust gas, the percentage of reduction required, etc. The use of oxidation catalysts at high temperatures is known to affect the concentrations of different nitrogen oxides produced because there is less conversion of NO to NO 2  under these conditions. Having too much NO 2  in exhaust gas entering the SCR catalyst is problematic because the activity of the SCR catalyst is impacted, and it can also participate in the formation of a brown gas plume (possibly caused by the so-called Wisconsin Process). 
     The amount of NOx present in gas turbine exhaust streams is often controlled using selective catalytic reduction (SCR) or selective noncatalytic reduction. SCR relies on the selective reduction of NOx using ammonia. The basic reactions can be expressed as: 
       4NH 3 +4NO+O 2 →4N 2 +6H 2 O (fast)  (1);
 
       4NH 3 +2NO 2 +O 2 →3N 2 +6H 2 O (slow)  (2);
 
       and 
       NO+NO 2 +2NH 3 →2N 2 +3H 2 O (very fast)  (3).
 
     Selective non-catalytic reduction processes operate without any catalyst to convert the NOx through a reaction with ammonia to nitrogen and water as shown below: 
       4NH 3 +4NO+O 2 →4N 2 +6H 2 O  (4).
 
     Non-catalytic systems are often limited to a narrow reaction temperature range. This process only reduces NOx by about 60 to 80 percent while requiring a large molar volume of NH 3 . 
     Although the levels of CO created during combustion in a gas turbine can be reduced to currently acceptable regulatory levels, these levels do not include the start-up of the gas turbines. Thus there is a need to improve the reduction in the amounts of emissions during the start-up of gas turbines. 
     The industry standard method of reducing carbon monoxide and volatile organic compounds from a gas turbine is to pass the flue gas through one catalyst bed containing an oxidation catalyst. The single catalyst bed of catalyst must be sized appropriately to meet the emission requirements for different turbine loads. The bed of catalyst can be placed in different temperature regions between sets of heat transfer tubes inside of a heat recovery steam generator. The bed is currently placed in a temperature region that allows for the lowest pressure drop while still allowing for the gas turbine to meet the CO and VOC reduction requirement after the turbine has passed through a start-up period. As regulations change, gas turbines may be required to reduce CO and VOC emissions during start-up. Furthermore, higher overall reductions of CO and VOC during full load may be mandated. 
     SUMMARY OF THE INVENTION 
     It has been found that the use of a coupled oxidation catalyst system in the operation of a gas turbine can begin to reduce emission during start-up, as well as provide the desired reduction in CO and VOC when the turbine is running at its normal, full load. The coupled oxidation catalyst system according to the invention uses two beds of oxidation catalysts, where one bed of oxidation catalyst is located in a relatively high temperature region to allow the gas turbine to begin reducing emissions during start-up, and a second bed of oxidation catalyst is placed in a lower temperature region to provide additional catalyst surface and complete the desired CO and VOC reduction between start-up and full load. 
     The coupled catalyst system can provide superior overall performance in the reduction of CO and VOC, at reduced cost, and be less susceptible to poisoning than the current industry standard system that uses a single oxidation catalyst bed. The coupled oxidation catalyst system can provide for less formation of NO 2  from NO by having the first oxidation catalyst bed at the higher temperature. This is because an equilibrium exists between NO and NO 2  over an oxidation catalyst. At lower exhaust gas temperatures, NO oxidation is kinetically limited and so NO oxidation is promoted with increasing temperature. By appropriate location of the first oxidation catalyst in a relatively high temperature position, NO oxidation to NO 2  is promoted in a start-up phase so that the exhaust gas contains a mixture of NO and NO 2  in the NO x  component of the exhaust gas. The mixture of NO and NO 2  thus generated during the start-up phase beneficially promotes the more efficient fast reaction (3) of overall NO x  reduction on the SCR catalyst during start-up. Thus the location of the first oxidation catalyst promotes CO and VOC oxidation and NO x  reduction during the start-up phase. 
     However, as exhaust gas temperatures increases from start-up to full load operation, the equilibrium constant for the NO to NO 2  reaction is shifted so that the reaction is thermodynamically limited. This means that with increasing temperature the equilibrium favours NO instead of NO 2 . This is significant because NO oxidation over the first oxidation catalyst located in a relatively high temperature location is suppressed as temperatures increase and so excessive NO 2  formation—and thus a less efficient NO x  reduction reaction (2) on the SCR catalyst—is avoided. However, as the temperatures increase towards full load, the second oxidation catalyst, which is located in a lower temperature location, takes over the duty of oxidising NO to NO 2  (i.e. in the kinetically limited region of equilibrium reaction) as well as CO and VOC oxidation, thus maintaining the beneficial mixture of NO and NO 2  in the exhaust gas entering the downstream SCR catalyst. 
     The first and second oxidation catalysts oxidise CO and VOC between start-up and full load. However, immediately following start-up, the location of the first oxidation catalyst in a relatively high temperature location enables the catalyst to reach its light-off temperature more quickly and therefore to begin oxidising CO and VOC as soon as possible after start-up. 
     The coupled oxidation catalyst system also significantly reduces the likelihood of exhaust gas bypassing the oxidation catalyst system without any treatment. This is because each catalyst bed is independently sealed; if any gas is able to bypass the first catalyst bed, it is very likely to be treated in the second catalyst bed. This has the advantage of increasing the reliability of the catalyst system, and is a very important feature if high CO and VOC reduction is required. The use of two catalyst beds can also result in a lower pressure drop than the use of a single large bed. The first oxidation catalyst can be present in a first block and a second block, where the first block is positioned in the flow of an exhaust gas downstream from e.g. a gas turbine before the second oxidation catalyst and the second block is positioned in the flow of an exhaust gas downstream from the first block and before the second oxidation catalyst. It is preferred that the first and the second blocks be located a distance apart that allows for mixing of the exhaust gas after it passes the first block to allow for gas that was not treated in the first block to be exposed to the second catalyst to provide for higher conversion. For example, in a HRSG, a first first oxidation catalyst in the first block can be located between a duct burner and the Super Heater and the second first oxidation catalyst in the second block can be located between the Super Heater and the high pressure evaporator. This can be important when very high oxidation activity of CO and VOCs is needed to meet regulatory requirements. 
     According to a first aspect, the invention provides an exhaust system for a power generating apparatus, which exhaust system being adapted to receive a flowing exhaust gas and comprising a catalyst system for treating the exhaust gas, which catalyst system comprising a first oxidation catalyst and a second oxidation catalyst, wherein the first oxidation catalyst is positioned downstream from the heat source so that the flowing exhaust gas contacts the first oxidation catalyst before the second oxidation catalyst. 
     According to a second aspect, the invention provides the use of an exhaust system according to the first aspect of the invention for controlling CO and hydrocarbon emissions during both start-up and full-load operations. 
     According to a third aspect, the invention provides a power generating apparatus comprising: a heat source for combusting a fuel in air to produce power and a flowing exhaust gas comprising carbon monoxide (CO) and hydrocarbons (HC); and an exhaust system according to any preceding claim for receiving and treating the flowing exhaust gas prior to the exhaust gas being released to atmosphere. Preferably, the power generating apparatus according to the third aspect of the invention comprises a gas turbine. 
     According to a fourth aspect, the invention provides a method of treating an exhaust gas emitted from power generating apparatus comprising a heat source for combusting a fuel in air to produce power and a flowing exhaust gas, which exhaust gas comprising carbon monoxide and hydrocarbons, the method comprising: contacting the exhaust gas with a first oxidation catalyst in a relatively high temperature zone and subsequently contacting exhaust gas exiting the first oxidation catalyst with a second oxidation catalyst in a relatively low temperature zone, wherein the high temperature zone has a temperature of from 427° C. (800° F.) to 621° C. (1150° F.) when the power generating apparatus is at full load, and the relatively low temperature zone has a temperature of from 260° C. (500° F.) to 427° C. (800° F.), when the power generating apparatus is at full load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the percentage of oxidation vs. temperature at a constant flowrate. 
         FIG. 2  shows the percentage of CO oxidation vs. amount of active catalyst material at a constant flow and temperature in the mass transfer limited region. 
         FIG. 3  shows a diagram of a power plant using an HRSG system. In  FIG. 3  “LP” stands for “low pressure” and “HP” stands for “high pressure”. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In this invention, it has been realized that a coupled oxidation catalyst system, wherein a first bed of oxidation catalyst is placed in a higher temperature region to allow the gas turbine to begin reducing emissions during start-up, and a second bed of oxidation catalyst is placed in a lower temperature region to provide additional catalyst surface and complete the desired CO and VOC reduction once the power generating apparatus has started up, can provide superior overall performance, reduce cost, and be less susceptible to poisoning than the current approach of using a single oxidation catalyst bed. 
     The oxidation of CO and VOC in a catalytic system is dependent on the operating temperature and the reaction residence time. The temperature at which an oxidation catalyst begins to oxidize CO and VOCs is called the light-off temperature. The light-off temperature is related to the concentration of active catalyst material in the catalyst. The higher the concentration of the catalyst, the lower the temperature at which oxidation begins. Above the light-off temperature, increasing the reaction temperature at a constant gas flow rate will increase the percentage of oxidation that is achieved until the reaction transitions from a kinetically limited region to a mass transfer limited region (see  FIG. 1 ). Once the temperature is high enough that the reaction is mass transfer limited, increasing the temperature will no longer increase the percentage of oxidation. The primary means of increasing the percentage of oxidation is by adding catalyst surface area. In the mass transfer limited region, increasing the concentration of the active catalyst material does not significantly increase the percentage of oxidation ( FIG. 2 ). A minimum emission control load is set because the performance of the catalyst is not sufficient below that load. The use of the coupled oxidation catalyst system described herein can allow e.g. a gas turbine to be operated at a lower load because the system can reduce the emissions at a lower load. This provides a significant technology improvement over the current state-of-the-art systems. 
     In gas turbine applications, many different operating loads exist. However, emission reduction is typically only required once the gas turbine reaches a minimum load, called the minimum emission control load. At this turbine load, the temperature at the oxidation catalyst is typically above the light off region and a single oxidation catalyst bed approach is effective because excessive precious metal loadings are not required (see  FIG. 1 ), and the total catalyst surface area required is determined by only the required level of oxidation (CO and VOC and NO oxidation) at 100% load. 
     During gas turbine start-up, relatively large amounts of CO and VOCs are produced. In order for a system having only a single bed oxidation catalyst to be able to meet both the start-up and full load emission requirements, desirably it would have to (1) have sufficient catalyst surface area to meet the desired removal of CO and VOC at the maximum flow and (2) have sufficient active catalyst material at concentrations to remove CO and VOC at the relatively low temperatures (&lt;204° C. (&lt;400° F.)) encountered during start-up. This results in a large catalyst volume with a relatively high active catalyst material concentration. Because the active catalyst material is typically a precious metal, such as platinum or palladium, increasing the concentration and volume adds significantly to the cost of the catalyst. An alternate configuration would be to locate a single catalyst bed in a higher temperature zone so that the temperature of the catalyst during start-up would be sufficient to eliminate the need to increase the active catalyst material concentration and catalyst volume. However, the disadvantages of this approach are that the entire catalyst surface area required for full load CO oxidation would be located in the high temperature zone leading to higher pressure drop than a single bed in a lower temperature zone. This would require construction with a higher grade steel to support the larger volume of catalyst in the catalyst bed, and would significantly increase the undesired formation of SO 2 . 
     Because the gas turbine exhaust flow is lower during start-up, the oxidation catalyst surface area needed to meet the emission requirement is less than the oxidation catalyst surface area needed to meet the emission requirements at 100% load. Therefore, a CO and VOC reduction system comprising two catalyst beds located in two different temperature regions offers the most beneficial solution. One bed, located in an elevated temperature region, can be optimized to meet the start-up emission requirements and, and a second bed, located in a lower temperature region, can be optimized to meet the desired conversions for CO, VOC and NO x  after the start-up period when the temperature has risen past the catalyst light-off temperature. This optimization can include selection of the appropriate catalyst volume, cell density, and catalyst formulation for the oxidation catalyst bed in each of the two locations. Catalyst formulation options include adjusting the composition of the washcoat to add sulphur tolerance, provide additional thermal durability. The loading of the precious metal in the catalysts, the use of different precious metal compositions (such as Pd rich formulations) and other variations can be made to the catalysts in each of the two oxidation catalyst beds. In some cases, different catalysts, catalyst formulations and catalyst loading can be used for each of the first and second oxidation catalyst beds. In addition to the temperature where the catalyst becomes operational, i.e. light off temperature, another critical parameter is the time it takes for the first oxidation catalyst to heat-up to achieve its light-off temperature. In this regard, substrate monoliths e.g. honeycomb monoliths, having a relatively low heat capacity and relatively high thermal conductivity is preferred for the first oxidation catalyst. These types of potential changes in the catalysts can be made to increase the efficiency of the system. 
     In the current industry standard system using HRSG, the placement of the CO oxidation catalyst has been based on the needs of the selective catalytic reduction catalyst (SCR) system for converting oxides of nitrogen, including the NH 3  injection apparatus, mixing chamber and SCR catalyst. This has often led to oxidation catalysts being located at a location in the exhaust stream with an exhaust gas temperature that is advantageous to the operation of the SCR catalyst. However, this arrangement can be detrimental to the performance of the CO oxidation catalyst with respect to generation of NO 2 . NO 2  concentration in the gas turbine exhaust can be increased several fold by oxidation of NO over oxidation catalysts. The catalyst can oxidize NO to equilibrium concentrations of NO 2  at local exhaust temperatures at the location of oxidation catalyst in the exhaust gas stream. 
     Typical oxidation catalysts operate durably at temperatures up to about 760° C. (1400° F.), which may occur in HRSG exhaust gas stream. Placement of the oxidation catalyst within the exhaust stream at a location with a temperature range of about 399° C. (750° F.) to about 760° C. (1400° F.) can substantially reduce NO 2  production by the catalyst. Placement of the oxidation catalyst within the exhaust gas stream at a temperature range of between approximately 510° C. (950° F.) and approximately 760° C. (1400° F.) may provide for an extension of the life of the durable oxidation catalyst. It can also provide for NO 2  levels that allow smaller SCR catalyst volumes to be used or more efficient NH 3  usage (both because the fast reaction (3) is in use).  FIG. 3  shows possible locations of the high temperature and low temperature zones in an HRSG system. 
     Advantageous placement of the oxidation catalyst within the exhaust stream with respect to temperature therefore can have a significant positive impact on the amount of NH 3  needed for reduction of NOx and the size needed for the SCR catalyst. Smaller catalyst size and reduced NH 3  utilization for the same NO 2  ppm output from the exhaust stack can result in substantial cost savings, as well as reduced pressure drop in the exhaust gas flow allowing more power output from the combined cycle gas turbine or fuel savings for the same power output. 
     In the first aspect of the invention, the exhaust system preferably comprises a heat recovery steam generator (HRSG). In a preferred arrangement, the HRSG comprises HRSG tube bundles and the first oxidation catalyst (or a first block comprising a first oxidation catalyst) is positioned to receive the flowing exhaust gas upstream of any HRSG tube bundles. Alternatively, or in addition, the HRSG comprises HRSG super heater tube bundles and the first oxidation catalyst (or a second block comprising a first oxidation catalyst) is positioned to receive the flowing exhaust gas downstream of the HRSG super heater tube bundles. The second first oxidation catalyst can be disposed in a second block and positioned to receive the flowing exhaust gas downstream of the first first oxidation catalyst disposed in a first block and upstream of the second oxidation catalyst. 
     The second oxidation catalyst can be positioned to receive the flowing exhaust gas downstream of at least one HRSG tube bundle. For example, the second oxidation catalyst can be positioned to receive the flowing exhaust gas downstream of one or more high pressure evaporator tube bundles. 
     Most preferably the catalyst system comprises an ammonia injection apparatus (AIG) disposed downstream from the second oxidation catalyst and adapted for injecting ammonia into a flowing exhaust gas and a selective catalytic reduction (SCR) catalyst for reducing oxides of nitrogen with ammonia reductant. Preferably, an ammonia slip catalyst is positioned to receive flowing exhaust gas downstream from the SCR catalyst. Ammonia slip catalysts for treating exhaust gas, including at lower temperatures, from are known e.g. from WO 2014/120645 and whose duty is to catalyse the overall reaction 4NH 3 +3O 2 →2N 2 +6H 2 O. Preferably, the exhaust system comprises a mixing section disposed downstream of the ammonia injection apparatus and adapted for mixing injected ammonia with the flowing exhaust gas. The SCR and ammonia slip catalyst may be combined, for example, to save on space and additional structural supports within the exhaust system. 
     In existing exhaust systems e.g. for gas turbines having an oxidation catalyst in either the hot or cold temperature zones, the system of the invention can be made by adding another oxidation catalyst into the corresponding zone (cold or hot temperature zone as applicable) where an oxidation catalyst is not currently present. In existing exhaust systems e.g. for gas turbines not having an oxidation catalyst in either the hot or cold temperature zones, the system of the invention can be made by adding the two oxidation catalysts in both the hot and cold temperature zones. It is preferable to remove oxidation catalysts that are not located in these zones because of the increase in back pressure from having oxidation catalysts in addition to those required by the coupled at least two oxidation catalyst system described herein. 
     According to the fourth aspect, the method preferably comprises the steps of introducing ammonia into exhaust gas exiting the second oxidation catalyst and contacting the exhaust gas containing ammonia with a selective catalytic reduction catalyst, wherein the first oxidation catalyst and the second oxidation catalyst are each located in a position wherein oxidation of NO to NO 2  on the first oxidation catalyst and the second oxidation catalyst is limited relative to the respective location positions of maximal NO oxidation activity in the system. 
     In an alternative aspect, the catalyst system comprises a first bed of oxidation catalyst placed in a higher temperature region to allow the gas turbine to begin reducing emissions during start-up, as described and characterized throughout this application. In this aspect, however, the second bed catalyst bed comprises an SCR catalyst in combination with an ammonia slip catalyst (“ASC”—also known as an ammonia oxidation (“AMOX”) catalyst). In some embodiments, the SCR/ASC combination catalyst further includes an oxidation catalyst. Like the previously described second oxidation catalysts, the SCR/ASC combination catalyst is placed in a lower temperature region to provide additional catalyst surface and complete the desired CO and VOC reduction once the power generating apparatus has started up, and can provide superior overall performance, reduce cost, and save on space in the exhaust system. Most preferably the catalyst system comprises an ammonia injection apparatus (AIG) disposed downstream from the first oxidation catalyst and adapted for injecting ammonia into a flowing exhaust gas, prior to the SCR/ASC combination. This will allow the SCR catalyst to reduce oxides of nitrogen with the ammonia reductant, while the ASC is positioned to reduce excess ammonia from the SCR catalyst. Ammonia slip catalysts for treating exhaust gas, including at lower temperatures, are known e.g. from WO 2014/120645 and whose duty is to catalyse the overall reaction 4NH 3 +3O 2 →2N 2 +6H 2 O. Preferably, the exhaust system comprises a mixing section disposed downstream of the ammonia injection apparatus and adapted for mixing injected ammonia with the flowing exhaust gas. Use of the SCR/ASC may allow operation at higher ammonia NO x  ratios which can boost NO x  conversion with low NH 3  slip, improve HC conversion, provide CO conversion, and may eliminate the need for the second oxidation catalyst. 
     In one embodiment, the combined SCR/ASC catalyst comprises a combination of platinum on a support with low ammonia storage and a first SCR catalyst. The combination can be a blend of platinum on a support with low ammonia storage with a first SCR catalyst with a first catalyst. Alternatively, the combination can be a bi-layer arrangement with a top layer comprising a first SCR catalyst and a bottom layer comprising platinum on a support with low ammonia storage, where the bottom layer is positioned on a substrate monolith, such as an inert ceramic honeycomb, or on a second SCR catalyst located between the bottom layer and the substrate monolith. 
     In this embodiment, preferably the platinum is pre-fixed on the particles of the support material prior to combining the Pt/support with the SCR catalyst. By “pre-fixed” herein, we mean that by techniques known in the art such as impregnation, deposition (or precipitation), co-deposition (or co-precipitation), electrostatic adsorption, ion exchange, solid state incorporation etc. a precious metal or a base metal is incorporated in the particulate support material followed as appropriate by washing, drying and calcining the product. 
     Preferably the support with low ammonia storage is a particulate siliceous support comprising a silica or a zeolite with a crystalline molecular sieve having a silica-to-alumina ratio of ≧100, preferably ≧200, more preferably ≧250, even more preferably ≧300, especially ≧400, more especially ≧500, even more especially ≧750 and more preferably ≧1000. 
     In the embodiment, the weight ratio of the amount of the SCR catalyst to the amount of precious metal on a support with low ammonia storage can be in the range of 0.1 to 300:1 inclusive, preferably from 3:1 to 300:1 inclusive, more preferably 7:1 to 100:1 inclusive, and even more preferably in the range of 10:1 to 50:1 inclusive. 
     The platinum can be present in the catalyst in an active component loading from about 0.01 to about 0.3 wt. % inclusive preferably from about 0.03 to 0.2 wt. % inclusive, more preferably from about 0.05 to 0.017 wt. % inclusive and most preferably from about 0.07 to 0.15 wt. % inclusive. The term “active component loading” refers to the weight of the precious metal support+the weight of the precious metal+the weight of the SCR catalyst in the active interface layer of a population of porous cells embedded in the matrix. 
     The particulate siliceous support comprising a silica or a zeolite with a crystalline molecular sieve having a silica-to-alumina ratio of ≧100 has relatively low ammonia storage and it has been found that in combination with SCR catalysts, particularly with Cu- and/or Fe-crystalline molecular sieve SCR catalysts, is that the ASC (or AMOX) catalyst as a whole can provide an improvement in N 2  yield (i.e. increased selectivity) from ammonia at a temperature from about 250° C. to about 350° C. compared to a catalyst comprising a comparable formulation in which the SCR catalyst is present as an upper layer and the supported precious metal is in a layer that stores ammonia and is present in an underlayer, wherein the NH 3  passes through the SCR catalyst layer before contacting the supported precious metal layer. 
     In some embodiments, the ASC comprises a substrate monolith comprising a bottom layer of precious metal on a support and a top layer overlying the bottom layer of SCR catalyst. The top layer catalyses reactions (1) and (2) and also adsorbs ammonia. The bottom layer catalyses the oxidation of ammonia according to the reaction 4NH 3 +4O 2 →2NO+N 2 +6H 2 O. NO generated in the bottom layer contacts the SCR catalyst and adsorbed NH 3  as it exits the bottomlayer via the top layer. Thus the overall ammonia oxidation activity and NOx conversion in the ASC as a whole is improved. 
     The SCR catalyst for use in the present invention comprises preferably a crystalline molecular sieve promoted with a base metal or a vanadium compound supported a support comprising titanium. 
     Where the SCR catalyst is a vanadium compound supported on a support comprising titanium, different titanium-vanadium systems can be used. More particularly, oxidic systems comprising mixtures of titanium dioxide (TiO 2 ) and vanadium pentoxide (V 2 O 5 ) are used. Alternatively, the titanium-vanadium system comprises vanadium-iron compounds as the catalytically active component, including especially iron vanadate (FeVO 4 ) and/or iron aluminium vanadate (Fe 0.8 Al 0.2 VO 4 ). 
     In the case of the oxidic systems, these are especially titanium-vanadium-tungsten systems, titanium-vanadium-tungsten-silicon systems, titanium-vanadium-silicon systems. In the case of the second group comprising vanadium-iron compounds, these are titanium-vanadium-tungsten-iron systems, titanium-vanadium-tungsten-silicon-iron systems or titanium-vanadium-silicon-iron systems. 
     The titanium/vanadium weight ratio (Ti/V) is appropriately in the range between 35 and 90. In the case of oxidic titanium-vanadium systems, the weight ratio between titanium dioxide and vanadium pentoxide (TiO 2 /V 2 O 5 ) is typically in the range from 20 to 60. 
     The titanium-vanadium system typically has a proportion by weight of 70 to 90% by weight, based on the final catalytic converter. The remaining 10 to 30% by weight is divided between the porous inorganic filler component and matrix components, and possibly fibre components. 
     “Crystalline molecular sieve” is understood here to mean particularly zeolites in the narrower sense, namely crystalline aluminosilicates. Furthermore, crystalline molecular sieves are also understood to mean further molecular sieves which are not aluminosilicates but have a zeolitic framework structure, as apparent from the zeolite atlas of the Structure Commission of the International Zeolite Association, IZA-Sc. More particularly, this relates to silicoaluminophosphates (SAPO) or else aluminophosphates (ALPO), which are likewise included in the zeolite atlas mentioned. 
     Catalytically active components used in this context are especially molecular sieves having the CHA framework structure, especially aluminosilicate CHA such as SSZ-13, or SAPO-34, AEI, especially aluminosilicate AEI, SSZ-39 or ALPO 18, ERI, MFI, BEA, FAU especially Y zeolite, AFX or FER (the nomenclature used here refers back to the nomenclature used in the zeolite atlas). Most preferred are so-called small pore crystalline molecular sieves, such as CHA, AFX and AEI framework structures, having a maximum pore opening structure of 8 tetrahedral atoms. Preferably, aluminosilicates are used as molecular sieves, especially for extruded catalysts, since the network structures show no change in the lattice spacings on water uptake and water release. (The nomenclature used here draws on the nomenclature used in the Zeolite Atlas). 
     In the case of the crystalline molecular sieves having the framework structures according to the zeolite atlas, a distinction is generally made between small-pore, medium-pore and large-pore crystalline molecular sieves. Small-pore molecular sieves are those sieves having a maximum pore opening with a ring structure composed of eight tetrahedral atom structures. Medium-pore and large-pore molecular sieves, finally, are understood to mean those in which the maximum pore openings are formed by a ring opening having a ring of not more than 10 (medium-pore) or of not more than 12 (large-pore) atom structures in tetrahedral arrangement. The BEA framework structure mentioned is a large-pore framework structure, MFI is a medium-pore structure and CHA is a small-pore structure. The FAU framework structure mentioned is likewise a large-pore structure, preferably a Y zeolite. AEI is a small-pore framework structure, and preference is given here to using a zeolite with the SSZ-39 designation. FER is a medium-pore framework structure, and the material used is preferably ferrierite or ZSM-35. ERI is a small-pore structure, and the material used is preferably erionite. AFX is a small-pore framework structure, and the material used is preferably SSZ-16. The BEA, MFI and FAU framework structures (here especially zeolite Y) are preferably used as hydrocarbon traps. All the framework structures and materials mentioned can be used as SCR catalytic converters; they are suitably activated by a metal, especially ion-exchanged with copper and/or iron and/or cerium, preferably activated with copper or iron. 
     The molecular sieve generally comprises a metallic activator (promoter). This is especially copper, iron or cerium or a mixture thereof. More particularly, the molecular sieve is a molecular sieve, especially zeolite, exchanged with metal ions of this kind. As an alternative to the ion-exchanged molecular sieve in which the metal ions are thus incorporated into the framework structure, it is also possible that these metal activators are not incorporated in the framework structure and are thus present effectively as “free” metals or metal compounds (e.g. metal oxides) in the individual channels of the molecular sieves, for example as a result of impregnation of the molecular sieve with a solution containing the compound. Another possibility is a combination of ion-exchanged metals and free metal compounds in the molecular sieve. 
     Aside from the preferred base metal promoter/crystalline molecular sieve and vanadium compound/titanium compound SCR catalysts, SCR catalysts for use in the present invention can be based on a tungsten oxide-cerium oxide system or a stabilized tungsten oxide-cerium oxide system (WO 3 /CeO 2 ). 
     The stabilized tungsten/cerium system is especially a zirconium-stabilized system comprising Ce-zirconium mixed oxides. Preferably, a transition metal, especially iron, is distributed within such a support material. The transition metals used are especially selected from the group consisting of Cr, Ce, Mn, Fe, Co, Ni, W and Cu and especially selected from the group consisting of Fe, W, Ce and Cu. 
     The catalytic system is especially an Fe—W/CeO 2  or Fe—W/CeZrO 2  system, as described particularly in connection with FIG. 3 of WO 2009/001131, which is referenced here in full. The proportion of the transition metal in the catalytic converter is preferably in the range from 0.5 to 20% by weight based on the total weight of the catalytic converter. 
     The different catalytic systems described here are used either selectively or else in combination. More particularly, a mixture of the system based on titanium-vanadium with crystalline molecular sieves is used. A mixed catalytic converter of this kind comprises, as the first component, especially an aluminosilicate or iron silicate molecular sieve, which is either in the so-called H +  form or has been ion-exchanged with one or more transition metals, especially with iron. The second component is a vanadium oxide on a metal oxide support selected from aluminium, titanium, zirconium, cerium, silicon or combinations thereof. More particularly, the support material for the second component is titanium oxide. The first component is especially an iron-exchanged MFI, BEA or FER aluminosilicate molecular sieve (zeolite). The ratio of the first component to the second component in this mixture is in the range from 5:95 to about 40:60. 
     In some embodiments, the SCR/ASC catalyst may also include an oxidation catalyst, to provide additional CO oxidation performance. One approach to preparing an ASC-CO oxidation formulation is to coat a single inert substrate (for example, a cordierite honeycomb) with an oxidation catalyst layer (the bottom layer) continuously from the substrate inlet to the substrate outlet and then partially cover the bottom layer by a layer comprising an SCR catalyst (the top layer). This would leave a relatively short bottom layer zone in the rear of the substrate to act as a CO oxidation catalyst. By coating both the ASC and CO oxidation catalyst on a single, longer substrate, manufacturing and catalyst packaging issues would be avoided. 
     In some embodiments, the SCR-ASC-CO catalyst includes: (a) a substrate having an inlet end and an outlet end defining an axial length; (b) an oxidation layer comprising an oxidation catalyst comprising one or more noble metals, the oxidation layer being positioned on the substrate and covering the axial length of the substrate; and (c) an SCR layer comprising an SCR catalyst, the SCR layer being positioned on the oxidation zone and overlapping a portion of the oxidation layer, where the portion is less than 100%, and the article is configured for treating an exhaust gas stream containing at least one of NOx, hydrocarbons, CO, SOx and ammonia from a combustion turbine. 
     The SCR layer can be present in several configurations. The SCR layer can extend from the inlet end toward the outlet end. The SCR layer can extend from the outlet end toward the inlet end. The SCR layer can extend from a distance from the inlet end of the substrate towards the outlet end of the substrate and does not cover the inlet and outlet portions of the substrate. The SCR layer can extend from the inlet end toward the outlet end and from the outlet end towards the inlet end. 
     The catalytic article can provide higher CO/HC conversion than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. The catalytic article can provide reduced ammonia slip than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. The catalytic article can provide higher CO/HC conversion and reduced ammonia slip than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. 
     The SCR layer can comprise a first SCR catalyst and a second SCR catalyst where the first SCR catalyst is different than the second SCR catalyst and the first SCR catalyst is located on the inlet side of the article relative to the second SCR catalyst. The first SCR catalyst and the second SCR catalyst can differ based on the loading of the SCR catalyst. Preferably, the loading of the SCR catalyst is higher over the second oxidation catalyst than over the first oxidation catalyst. The first SCR catalyst and the second SCR catalyst can comprise different catalytic species. By different species is meant a chemically different catalyst. For example, the first SCR catalyst can be a base metal (such as vanadium) and the second SCR catalyst can be a metal containing molecular sieve (such as copper chabazite (Cu-CHA)), or the first SCR catalyst can be a metal containing molecular sieve (such as Cu-CHA) and the second SCR catalyst can be a different metal containing molecular sieve (such as iron-chabazite (Fe-CHA). 
     The oxidation layer can comprise a first oxidation catalyst and a second oxidation catalyst, where the first oxidation catalyst is different than the second oxidation catalyst and the first oxidation catalyst is located on the inlet side of the article relative to the second oxidation catalyst. The first oxidation catalyst and the second oxidation catalyst can differ based on the loading of the oxidation catalyst. Preferably the loading of the oxidation catalyst is higher in the second oxidation catalyst than in the first oxidation catalyst. The first oxidation catalyst and the second oxidation catalyst can comprise different catalytic species. 
     The catalytic article can further comprise a third catalyst coating, where the third catalyst coating extends from the outlet end toward the inlet end and the first catalyst coating contain an area that is not coated by the second catalyst coating or the third catalyst coating. The third catalyst coating can comprise an SCR catalyst that is different than the SCR catalyst in the second catalyst coating. 
     The SCR zone can extend over 95% or less, preferably over 90% or less, more preferably extends over 75% or less, even more preferably extends over 50% or less, of the axial length of the substrate. 
     In various configurations, the compositions can comprise a first SCR catalyst or a first SCR catalyst and a second SCR catalyst. The first SCR catalyst can be different from the second SCR catalyst by comprising a different active component, as described below, by having a different loading of the active component, or both. In some configurations, the loading of the second SCR catalyst is higher than the loading of the first SCR catalyst. The active component in the first and second SCR catalysts can be independently selected from the group consisting of a base metal, an oxide of a base metal, a mixed metal oxides, a molecular sieve, a metal exchanged molecular sieve or a mixture thereof. The base metal can be selected from the group consisting of cerium, chromium, cobalt, copper, iron, manganese, molybdenum, nickel, tungsten, vanadium and zirconium, and mixtures thereof. 
     The SCR catalyst can be present in the SCR layer in an amount from 0.5 g/in 3  to 3.0 g/in 3 , preferably from 1.0 g/in 3  to 2.5 g/in 3 , more preferably from 1.25 g/in 3  to 2.0 g/in 3 . The oxidation catalyst can be present in the oxidation layer from 0.2 g/in 3  to 1.6 g/in 3 , preferably from 0.35 g/in 3  to 1.25 g/in 3 , more preferably from 0.5 g/in 3  to 1.0 g/in 3 . 
     The oxidation layer can further comprise an SCR catalyst where the oxidation catalyst and the SCR catalyst in the oxidation layer are present as a blend. 
     A washcoat comprising the SCR catalyst or the oxidation catalyst is preferably a solution, suspension, or slurry that provides a surface coating. The top layer containing the SCR catalyst can contain the SCR catalyst in an amount of about 80% or greater based on the weight of the layer. The remainder comprises binders, etc. The noble metal coating preferably contains about 0.05-5 weight percent noble metal based on the weight of the refractory metal oxide support. 
     A washcoat can also include non-catalytic components, such as fillers, binders, stabilizers, rheology modifiers, and other additives, including one or more of alumina, silica, non-zeolite silica alumina, titania, zirconia, ceria. 
     A slurry can comprise a pore-forming agent, such as graphite, cellulose, starch, polyacrylate, and polyethylene, and the like. These additional components do not necessarily catalyze the desired reaction, but instead improve the catalytic material&#39;s effectiveness, for example by increasing its operating temperature range, increasing contact surface area of the catalyst, increasing adherence of the catalyst to a substrate, etc. 
     The bottom layer coating is preferably applied to the substrate in an amount sufficient to produce a washcoat loading of about 0.1-40 g/ft 3  of noble metal, more preferably about 0.5-20 g/ft 3 , and even more preferably about 1-10 g/ft 3 . 
     The top layer coating can be applied over the bottom layer to produce an SCR catalyst washcoat loading &gt;0.25 g/in 3 , such as &gt;0.50 g/in 3 , or &gt;0.80 g/in 3 , e.g. 0.80 to 3.00 g/in 3 . In preferred embodiments, the washcoat loading is &gt;1.00 g/in 3 , such as &gt;1.2 g/in 3 , &gt;1.5 g/in 3 , &gt;1.7 g/in 3  or &gt;2.00 g/in 3  or for example 1.5 to 2.5 g/in 3 . 
     The catalytic article having the SCR layer extending from the outlet end toward the inlet end can provide more sulfur tolerance and/or greater CO/HC conversion than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. 
     The catalytic article having the SCR layer extending from the outlet end toward the inlet end can provide higher CO/HC conversion than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. 
     The catalytic article having the SCR layer extending from the outlet end toward the inlet end can provide reduced ammonia slip higher CO/HC conversion than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. 
     The catalytic article having the SCR layer extending from a distance from the inlet end of the substrate towards the outlet end of the substrate, where the SCR layer does not cover the inlet and outlet portions of the substrate, can provide more sulfur tolerance than a comparable article where the second catalyst coating completely overlaps the first catalyst coating. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.