Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J15/00—Arrangements of devices for treating smoke or fumes
  • F23J15/003—Arrangements of devices for treating smoke or fumes for supplying chemicals to fumes, e.g. using injection devices
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/343—Heat recovery
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/86—Catalytic processes
  • B01D53/8643—Removing mixtures of carbon monoxide or hydrocarbons and nitrogen oxides
  • B01D53/8646—Simultaneous elimination of the components
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B1/00—Methods of steam generation characterised by form of heating method
  • F22B1/02—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers
  • F22B1/18—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers the heat carrier being a hot gas, e.g. waste gas such as exhaust gas of internal-combustion engines
  • F22B1/1807—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers the heat carrier being a hot gas, e.g. waste gas such as exhaust gas of internal-combustion engines using the exhaust gases of combustion engines
  • F22B1/1815—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers the heat carrier being a hot gas, e.g. waste gas such as exhaust gas of internal-combustion engines using the exhaust gases of combustion engines using the exhaust gases of gas-turbines
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B1/00—Methods of steam generation characterised by form of heating method
  • F22B1/02—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers
  • F22B1/18—Methods of steam generation characterised by form of heating method by exploitation of the heat content of hot heat carriers the heat carrier being a hot gas, e.g. waste gas such as exhaust gas of internal-combustion engines
  • F22B1/1861—Waste heat boilers with supplementary firing
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B37/00—Component parts or details of steam boilers
  • F22B37/008—Adaptations for flue-gas purification in steam generators
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J15/00—Arrangements of devices for treating smoke or fumes
  • F23J15/06—Arrangements of devices for treating smoke or fumes of coolers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2251/00—Reactants
  • B01D2251/20—Reductants
  • B01D2251/206—Ammonium compounds
  • B01D2251/2062—Ammonia
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J2215/00—Preventing emissions
  • F23J2215/40—Carbon monoxide
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J2219/00—Treatment devices
  • F23J2219/10—Catalytic reduction devices
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E20/00—Combustion technologies with mitigation potential
  • Y02E20/16—Combined cycle power plant [CCPP], or combined cycle gas turbine [CCGT]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E20/00—Combustion technologies with mitigation potential
  • Y02E20/30—Technologies for a more efficient combustion or heat usage

Definitions

• This invention relates to a system and method for maintaining a workable temperature range for a catalyst system in a heat recovery steam generator ("HRSG") system. These methods and systems are employed to reduce the NOx and CO emissions from the exhaust stream of a combined cycle gas turbine where in a waste heat boiler for recovering the steam (HRSG type) is used.
• the invention relates to means to provide the SCR catalyst the desired temperature window where it performs (preferably optimally), without sacrificing the HRSG efficiency.
• Gas turbines are used to generate electricity. Modern gas turbines are equipped with dual fuel capability and can run both on natural gas or other types of fuels such as diesel fuel, middle distillate, jet fuel etc.
• gas turbines may operate in two ways: simple cycle mode or a combined cycle mode.
• simple cycle operation the exhaust from the gas turbine is ducted out through a stack.
• a combined cycle operation the exhaust stream's heat is utilized typically to generate steam and/or hot water.
• the steam generated in a combined cycle mode can be used to run a steam turbine to improve the efficiency of the overall process or can be used as process steam.
• the process of removing waste heat from the exhaust stream of the gas turbine is typically carried out by HRSG systems. HRSG 's are also sometimes referred to as waste heat boilers.
• exhaust stream also referred to as "exhaust stream”
• exhaust stream coming out such combined cycle gas turbines is typically in the range of about 500 0 F to about 1100 0 F, typically about 825°F to about 1100 0 F.
• Stack temperatures of combined cycle gas turbines (measured at the exit of the stack) seldom exceed about 350 0 F.
• the HRSG HRSG
• 101633559 system is located after the gas turbine and before the stack. HRSG systems utilize the heat from the gas turbine exhaust stream, which would otherwise be wasted, thereby improving the overall energy efficiency of the turbine installation.
• r ⁇ HRSG's are often modularized and may have multiple heat exchangers. Some modular HRSG's consist of three major components: the Evaporator, Superheater, and Economizer, others include other types of heat exchangers. The different components are put together to meet the operating requirements of the unit. Often times the three components in the HRSG are also referred to as an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through super heaters to raise the temperature and pressure past the saturation point.
• LP low pressure
• reheat/IP intermediate pressure
• HP high pressure
• HRSGs are often packaged and designed to be shipped as a fully assembled unit from the factory.
• the packaged HRSG may have water- wall tubes (or membrane wall-tubes). Duct burners may then also be employed to allow for higher supplemental firing and better overall efficiency.
• HRSGs include supplemental, or duct, firing. These additional burners (referred to as “duct burners”) provide additional energy to the HRSG, which produces more steam. Generally, the use of duct firing produces more steam or hot water output.
• duct burners provide additional energy to the HRSG, which produces more steam. Generally, the use of duct firing produces more steam or hot water output.
• the HRSG is located between the turbine exit and the stack. The entrance of the HRSG is from the turbine side, and the temperature from the entrance to the exit of the HRSG can vary from 1100 0 F to 300 0 F, or more commonly from about 1000 0 F to about 500°F.
• HRSGs can also have diverter also called (by-pass) mechanisms to regulate in the inlet flow into the HRSG from the turbine.
• These by-pass mechanisms are typically designed to divert the entire flue gas around the HRSG, such that the gas turbine can continue to operate when there is no steam demand or if the HRSG needs to be taken offline for maintenance purposes.
• U.S. Patent Nos. 5,4618,53 and 7,191,598 disclose how a bypass can be installed in a combined cycle application.
• SCR selective catalytic reduction
• a reducing agent such as ammonia
• a catalyst is used to allow the reaction between the injected ammonia and NOx present in the exhaust stream, to form harmless nitrogen and water.
• Such catalysts are commonly referred to as ammonia SCR catalysts.
• US patent Nos. 4,833,113, and 4,961,917 (herein incorporated by reference) describe the use of such catalysts for removing NOx pollutants from exhaust streams.
• ammonia is preferably used as a reducing agent.
• Ammonia is injected and mixed with the exhaust streams from the turbine before the SCR catalyst for the purpose of NOx reduction.
• Ammonia is a toxic gas.
• Ammonia is either used in its anhydrous form (gaseous ammonia) or in liquid form.
• Ammonia is injected into the exhaust of the turbine before the SCR catalyst for the purpose of NOx removal, so that overall NOx levels are below the allowed limits for any particular commercial installation.
• liquid ammonia typical concentration of the liquid will have 19% by weight of ammonia in water or 29% by weight of ammonia in water as allowed by permit regulations worldwide.
• Liquid ammonia may be vaporized prior to injection to generate ammonia gas from the liquid and the vaporized gaseous ammonia is then injected before the catalyst for NOx removal.
• Ammonia generating compounds such as urea are also used as reducing agents in such ammonia SCR catalyst technology.
• the exhaust temperature with and without duct firing can vary about 300 to 500°F.
• the exhaust without duct firing is about 800F at the first economizer section, it can easily be greater than HOOF under duct firing conditions at the same location.
• the SCR catalyst has an optimal temperature range of about 700F, cooling from 800F (without duct firing) or HOOF (with duct firing), would be required for optimal performance.
• the first economizer for cooling from 800F to 700F cannot be the same as the economizer required to cool from 1100 to 700F.
• the present invention has the advantage of allowing narrow temperatures ranges (the optimal range) for the SCR catalyst even when the process conditions vary (e.g.
• a CO catalyst may also be located before or after the SCR catalyst, preferably in the same zone where the SCR catalyst is located for CO abatement when necessary or desired.
• a catalyst In SCR applications behind gas turbines, a catalyst is typically expected to work for more than 3 years or 24,000 hours without being replaced. This means that during operation of the turbine over this entire time period, the catalyst has to perform and meet emission requirements, which is difficult.
• These high temperature SCR catalysts are typically zeolite based catalysts. It is well known that the zeolite component which is the most active component in high temperature SCR catalysts, deactivate or degrade at a much faster rate in the presence of water vapor at temperatures around 1000 0 F. mperature SCR
• ammonia is a toxic gas used with SCR catalysts, in addition to the above NOx and CO emission requirements, gas turbine users are also required to meet certain ammonia emission requirements. While most of the ammonia is consumed for NOx reduction, there is still some un- reacted ammonia that escapes through the exhaust stream and reaches the atmosphere through the stack. This is commonly known as "ammonia slip". Typical regulations in the USA allow gas turbine users to emit no more than 10 ppm of NH 3 through their stack. More stringent regulations in states such as California and Massachusetts are forcing ammonia regulations not to exceed 2 ppm slip. Ammonia transportation, handling and permit issues also exist due to the toxic nature of ammonia.
• the turbine combined with HRSG's leads to several different operation modes in a combined cycle application. It is not necessary in a HRSG, that duct burners are fired continuously. Rather, it is dictated by the overall steam requirement and efficiency requirements of the plant.
• a typical ammonia SCR or HC-SCR catalyst is located inside a HRSG, the exhaust temperature that the catalyst is subject to significantly varies as operating conditions vary. For example, when the catalyst temperature is around 600 0 F without an operating duct burner, an operating duct burner can easily increase the catalyst temperature by several hundred degrees.
• the temperature window in the HRSG can vary significantly, well outside the optimal and maximum limits of the catalysts. This results in significant stress on the catalyst, damaging it faster than expected.
• HRSG's have been designed in order to meet the steam requirements of the customer.
• the requirements include, among others, the amount, pressure, and temperature of the steam.
• the HRSG engineer develops a design, which includes elements of the HRSG system such as the reactor size, tube bank(s) design, and duct fired burners.
• a space is created between tube banks, where the catalyst can be installed.
• the temperature of the flue gas in this zone is a result of the design of the HRSG, and can fall within a broad range of about 300 0 F to about 1100 0 F, typically about 500 0 F to about 900 0 F.
• the temperature of this zone changes in real time, as it is influenced by various factors such as the turbine load (the electrical load on the turbine), the amount of steam which is required (which can vary minute to minute), and the use of duct fired burners.
• the temperature of the traditional zone which is allocated to emission control catalysts tends to both broad and highly variable during turbine operation.
• some catalysts have narrower operating temperature ranges than others.
• the Hydrocarbon based SCR catalyst operates optimally in the range of 715°F to 815°F, a narrow temperature window.
• this catalyst In order for this catalyst to be used in a HRSG application, this catalyst must be placed in flue gas of this temperature, under all firing and HRSG conditions. No method of providing this narrow, permanent temperature zone exists in the prior art.
• This invention regulates the temperature in a specified area of the HRSG so that the catalysts in the catalyst section can be optimally active.
• a by-pass mechanism is located in the HRSG, which allows the exhaust stream, or a position thereof, to by-pass one or more heat exchangers and one or more superheaters. This by-pass amount is controlled so that the exhaust stream, as it enters the catalyst sector, is in the optimal temperature for the catalyst.
• a second alternative is positioning a heat exchanger, which can control the exhaust stream's temperature by lowering it, immediately before the SCR catalyst. After the exhaust stream has flowed through the catalyst section, it enters the remainder of the equipment in the HRSG.
• a suitable CO catalyst can be located downstream of the HC-SCR catalyst unlike the traditional ammonia SCR case, where the CO catalyst is located upstream of the ammonia injection grid.
• the hydrocarbon injection grid, HC-SCR catalyst and the CO catalyst can all be inside an HRSG.
• HRSG's can be modified to fit either vanadia titania type catalysts or more modern HC-SCR type catalysts.
• a "catalyst section” means a portion of the HRSG which comprises one or more reducing agent injection points, and one or more SCR catalysts; and optionally one or more CO catalysts.
• a "CO catalyst” means a catalyst that works to decrease the CO in the exhaust stream.
• a "SCR catalyst” means a catalyst that works to decrease NO x in the exhaust stream.
• An "HC-SCR” catalyst uses a reducing agent comprising one or more hydrocarbons.
• An "ethanol-SCR” catalyst uses a reducing agent comprising ethanol.
• An “ammonia-SCR” uses a reducing agent comprising ammonia and/or an ammonia generating compound (including but not limited to aqueous ammonia and/or urea).
• An "air preheater” means equipment that heats air to be mixed with the exhaust stream before exiting the stack.
• An “optimal temperature” or “optimal range” means the range of temperature at which the SCR catalyst is most active, resulting in greater than about 90% NO x conversion at all turbine operating conditions, and does not rapidly degrade, preferably about 715°F - about 815°F.
• Figure 1 is a plot of NOx conversion versus temperature for several prior art hydrocarbon-SCR catalysts disclosed in U.S. Patent Application No. 2006/0228283.
• Figure 2 is a plot of NOx conversion versus temperature for various prior art ammonia-SCR catalysts set forth in Byrne et al., Catalysis Today, Vol. 13, pgs 33-42 (1992) (hereinafter incorporated by reference).
• Figure 3A shows a diagram of a combined cycle plant which includes a HRSG as per a prior art embodiment.
• Figure 3B shows a diagram of a combined cycle plant with a HRSG as per an embodiment of this invention.
• Figure 4A shows a diagram of a combined cycle plant with a HRSG as per a prior art embodiment.
• Figure 4B shows a diagram of a combined cycle plant with a HRSG modified as per an embodiment of this invention.
• the present invention relates to a means of providing a narrow temperature range inside an HRSG, wherein this narrow temperature range is at the optimum performance window of the SCR catalyst for NOx emission reductions from combined cycle gas turbine applications. This arrangement will also result in lower SCR catalyst usage when compared to the prior art.
• Combustion turbine load demands shifts in steam requirements from HRSG, impact of duct firing on HRSG temperature profile (meaning the temperature all along the HRSG, not just at one point in the HRSG), and changes in performance of HRSG over time, result in vast deviations in the temperature at the SCR catalyst inside the HRSG, which results in reduced catalyst performance, premature degradation of the SCR catalyst and frequent catalyst replacement, excess usage of catalyst, excess usage of reducing agents, and frequent shutdowns to maintain catalysts amongst other things.
• Figure 1 shows a plot of NOx conversion versus temperature for several hydrocarbon SCR catalysts as disclosed in US Patent application (2006/0228283).
• gas turbine refers to a combined cycle turbine
• gas turbine typically contains about 25 ppm NOx, about 50 ppm CO, about 5%CO 2 , about 15% O 2 , about 10% H 2 O and balance nitrogen. Regulations require them to be below 2.5 ppm NOx, meaning 90% or higher NOx conversion is desired.
• ethanol was used as the hydrocarbon (reducing agent) for removing NOx using such hydrocarbon SCR catalysts.
• Tests were performed by recording NOx conversion data from a simulated feed gas representative of a exhaust stream, in a temperature range of about 350 0 C to about 450 0 C (about 662°F - about 842 0 F). As is evident from the plot, most catalysts result in greater than 90% NOx conversion when the temperature is in the range of about 700 to about 800 0 F. If one were to use this catalyst in a combined cycle application inside an HRSG, it is of obvious benefit to provide the desired range of temperature for such HC- SCR catalysts so that NOx emission reduction targets are met.
• FIG. 2 is a plot disclosed in prior art. This was taken from a paper published by Byrne et al., in Catalysis Today, Volume 13, pages 33-42, year 1992. On page 35, the authors disclose the various types of ammonia SCR catalysts.
• V/Ti is the most common type of SCR catalysts used in the industry today in HRSG's for combined cycle applications. As can be seen in this figure, the V/Ti catalysts result in greater than 90% NOx conversion in the temperature window of about 325°C to about 425 0 C. The chart shows that, if the operation is below about 325°C or above about 425 0 C, there is a significant drop in NOx conversion. Ammonia slip also increases significantly when temperature is below about 325 0 C.
• the temperature of exhaust stream entering the HRSG of a combined cycle combustion gas turbine can be about 600 0 F to about HOO 0 F, more typically about 825°F to about HOO 0 F.
• the amount of exhaust stream entering the HRSG varies considerably depending on the size of the combustion gas turbine used in the application and on varying operating conditions of a particular combustion gas turbine. Even though the amount of exhaust stream may vary, the temperature of this exhaust stream remains within the about 825°F to about HOO 0 F range. This exhaust stream temperature range, however, is too broad and not ideal for existing SCR catalyst technology, thus a much narrower temperature range is desired.
• Figure 3A shows an embodiment of the prior art.
• a hot exhaust stream from the gas turbine (10) passes through the HRSG (11) and then to the stack (12).
• Figure 3A also shows a duct burner (13) located at the entrance of the HRSG.
• the temperature of the exhaust stream going through the HRSG is different in duct firing mode versus when there is no duct firing.
• the temperature of the exhaust also varies as the turbine load varies.
• the first heat exchanger or the high pressure boiler (14) is upstream of the super heater section (15).
• An additional heat exchanger (16) known as the low pressure boiler is located upstream of the economizer (17).
• the CO catalyst (18), the reducing agent injection grid (19) and the SCR catalyst (9) are located inside the HRSG, typically in a location between the super heater section (15) and the low pressure boiler (16). In some installations where there is an intermediate pressure section (not shown in Figure 3A), the CO and SCR catalysts are located inside this intermediate pressure section.
• the CO and SCR catalysts are subjected to a broad range of temperature depending on operating conditions of the turbine and/or the duct heater inside the HRSG, and steam requirements of the plant from the HRSG. Typical exhaust temperature in the catalyst zone in such an arrangement is in the range from about 425°F to about 700 0 F, which is below the optimum temperature of SCR catalysts.
• FIG. 3B shows an embodiment of the present invention.
• Hot gas from point Z (22) is bypassed outside the HRSG using a duct (20).
• the difference from prior art to this embodiment of the invention is the use of the duct (20) to allow hot gas from the exhaust stream to flow directly to the catalyst section at (23), bypassing the first heat exchanger (high pressure boiler) (14) and the super heater (15).
• the flow through this duct (20) can be controlled by the use of a damper (21).
• the hot gases (exhaust stream) directly from the exhaust stream can be injected using injection nozzles (at 23) typically used to inject hot gases.
• the amount to be injected can also be controlled.
• the temperature of the mixed exhaust stream reaching the catalyst can be controlled to optimize the catalyst performance.
• the flow of the exhaust through this bypass and the temperature upstream of the catalyst can be measured and controlled using any suitable measuring and control devices and systems known in the art. Any known means of regulating the flow through this by-pass section can be used to arrive at a narrow temperature range at the catalyst section (which begins at 18).
• the duct (20) can be located on any side (top, left and/or right are preferred) to bypass the heat exchanger and superheater and allow hot exhaust streams into the catalyst section of the HRSG.
• One or more ducts can be used, to bring the exhaust stream to the desired catalyst operating temperature. When one or more ducts are used, they can be controlled together or independently. When more than one duct is used and are controlled either together or independently, they may have one or more exhaust stream injectors.
• the exhaust stream injector(s) can have one or more exhaust stream injection nozzles.
• the amount of exhaust injected and the rate injected through the injection nozzles are controlled so that during all turbine operations, the catalyst section (from (18) to (25)) is always at a constant narrow temperature range (the optimal temperature range) for the SCR catalyst.
• the temperature in the catalyst section is about 715°F to about 815°F, by controlling the exhaust bypass.
• an ammonia SCR catalyst is used, the CO catalyst (18) is located upstream, preferably immediately upstream, of the ammonia injection grid (19) but after the temperature sensor at (25).
• the basic HRSG components are arranged to consistently produce an exhaust stream temperature of around 765 0 F plus or minus 5O 0 F at a particular location in the HRSG.
• Exhaust stream temperature may be raised (very rarely required) via duct burners, or lower (mostly required) via a heat exchange section.
• An embodiment of the present invention produces a fixed exhaust downstream of any particular heat exchanger with the HRSG system.
• the use of a duct burner can generate excessive temperatures downstream of a particular heat exchanger (e.g. without limitation a tube bank), which can limit or reduce the effectiveness of pollution control catalyst systems.
• This invention addresses the problem of higher or lower than ideal temperatures that exist in the exhaust stream flowing through a HRSG with one or more duct burners.
• a heat exchanger e.g. without limitation a tube bank
• the invention embodied therein comprises a heat recovery steam generator (HRSG) generally designated (42) having an inlet plenum (41) spanned by a duct burner (46) and supplied with exhaust from a turbine.
• HRSG includes, for example, a high pressure drum (43), and associated heat exchanger surfaces including other heat exchangers (44 and 45), in a conventional manner.
• the exhaust gases are emitted to atmosphere via stack after the heat is removed and emissions are below permit limits.
• Heat exchanger bank (44) is in a dry state with no fluid circulating through this tube bank until the duct burners (46) are about ready to be started. Just before the duct burners (46) are fired, the circulation pump (38) and control valve(s) (39a, b and/or c and any combinations thereof) become active. They may be controlled by a control system, such as control system (40) which may be digital, to start flowing fluid through this tube bank (44) to absorb heat from the exhaust so that a pre-set temperature is constantly maintained downstream of this tube bank (44) and upstream of the catalyst.
• the control system (40) achieves the fixed temperature regulation by evaluating various process conditions and determining how much fluid to distribute across this tube bank (44). In some but not all embodiments, the fluid leaving this tube bank would remain a liquid because the heat picked up by this fluid would leave via steam that boils out through boiler (45).
• this bank (44) can be designed to become another steam generating bank, or could be designed to transfer heat via one of various other types of heat transfer media, such as Dowtherm. In all cases, the purpose is the same, which is generating a constant T 0 temperature in the catalyst section no matter how much duct firing heat is being generated.
• the duct burners (46) are off, the need to circulate and absorb heat from the tube bank (44) is no longer necessary and the bank may be purged of fluid and allowed to exist in a dry state that no longer absorbs and transfers heat from this particular location in the HRSG.
• the catalyst operates in an optimal temperature zone because the HRSG was designed to create an optimal temperature when duct firing was not present or was present.
• the tube bank (44) may be used and controlled to achieve a fixed temperature in the exhaust while the duct burners are on and/or off.
• the system is shown to have a HC-SCR catalyst (7) and a CO catalyst (9), wherein the CO catalyst (9) is downstream of HC-SCR catalyst (7).
• the system can be used with ammonia-SCR catalyst. If a CO catalyst is used, it is preferably located upstream of the reducing agent injection grid, but after a heat exchanger.
• control system (40) for example, without limitation flow, temperature and turning on and off.
• Thermocouples Tc 1 through Tc 7 or any combination thereof measure temperature at each respective location.
• the heat exchangers 43, 44 and 45 preferably tube banks
• the circulation pump (38) and valves (39a, 39b and 39c or any combination thereof) can be controlled using the control system (40).

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• 2008
  • 2008-06-19 US US12/214,541 patent/US8220274B2/en not_active Expired - Fee Related
• 2009
  • 2009-05-13 CN CN2009801274218A patent/CN102187078A/zh active Pending
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