Abstract:
A method for monitoring and/or controlling performance of a selective catalytic reduction (SCR) emission control system includes injecting a quantity of a pollution neutralizing gas into a combustion gas stream containing a pollutant gas. The method also includes passing the stream over a catalyst bed to facilitate a reaction of the pollution neutralizing gas with the pollutant gas to produce an effluent and measuring a ratio of the pollution neutralizing gas to the pollutant gas in the effluent.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to pollution reduction systems, and more particularly to methods and apparatus for monitoring and controlling selective catalytic reduction emission control systems. 
     Two issues faced by owners and operators of facilities equipped with Selective Catalytic Reduction (SCR) systems include proper control of the ammonia distribution across the face of the catalyst (or to different SCR modules) and deterioration of the SCR catalyst. 
     An SCR is capable of achieving high levels of NO x  destruction by injecting controlled quantities of ammonia into the NO x -laden gas stream and passing the mixture across a catalyst at a controlled temperature. The primary NO x  destruction reaction in an SCR can be described as:
 
4NO+4NH 3 +O 2 →4N 2 +6H 2 O
 
As indicated by this reaction, optimum NO x  destruction occurs when the molar flow of ammonia is essentially equal to the molar flow of NO. If there is a deficiency in ammonia, then the NO x  will not be completely destroyed. If there is excessive ammonia flow, then the ammonia (considered a hazardous air pollutant in several states) will pass through the system unreacted. This is referred to as ammonia slip. In general, SCR systems are equipped with hardware to continuously measure the molar flow rate of NO x  coming to the catalyst. That data are used to calculate and control the appropriate amount of ammonia to inject into the system at any point in time. A grid of spray nozzles (referred to as an Ammonia Injection Grid or AIG) is provided to distribute the ammonia across the flow field and (hopefully) to provide the proper mixture of NH 3  and NO x  at the face of the catalyst. One of the first challenges faced during start up of a new SCR system is to achieve proper balancing of the AIG. The optimal approach for meeting that start up challenge is to gather data on the NO x  and NH 3  concentration from multiple locations at either the inlet or exit of the SCR catalyst.
 
     Beyond the initial balancing, SCR systems can experience drift in the AIG performance. This can be caused by many system variables such as fouling of the injection nozzles, plugging of the ammonia transport lines, or shifts in the spatial distribution of the inlet NO x  at the inlet to the SCR (or between different SCR modules). To identify that the AIG balance has drifted and to provide guidance for adjusting the AIG requires gathering of continuous data similar to that suggested above for initial AIG tuning. 
     Also, the reactivity of an SCR catalyst degrades over time. Typically, catalyst performance will remain acceptably high for periods of 3 to 10 years but occasionally the reactivity can decline precipitously. A sharp drop in reactivity can occur due to many factors including catalyst poisoning or delamination of a wash coat type catalyst. Data are required to track the long-term performance of the catalyst and to discriminate the root cause of falling NO x  destruction efficiency between poor AIG performance, by passing of the catalyst, or loss of catalyst reactivity. 
     At least one known method for initial balancing of AIG systems is based on measurement of only NO x  concentration distribution at an outlet of the catalyst, measurement of NO x  destruction efficiency, or manual measurement of ammonia. This method has proven satisfactory for SCR systems that operate at sub-stoichiometric ammonia levels, but is less satisfactory for systems that operate at near 1:1 inlet NO x  to ammonia ratio or for facilities with stringent ammonia slip regulatory limits. The known method also does not provide continuous monitoring to detect deterioration in AIG balancing. 
     Also, at least one known method physically extracts catalyst samples on a regular basis and subjects the catalyst to reactivity testing at a remote laboratory, but does not monitor catalyst performance during yearlong periods between major plant outages. Some indication of catalyst performance is provided through continuous measurement of overall NO x  destruction efficiency but those data cannot distinguish the impacts of AIG tuning from catalyst reactivity. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Some configurations of the present invention therefore provide a method for monitoring and/or controlling performance of a selective catalytic reduction (SCR) emission control system. The method includes injecting a quantity of a pollution neutralizing gas into a combustion gas stream containing a pollutant gas, passing the stream over a catalyst bed to facilitate a reaction of the pollution neutralizing gas with the pollutant gas to produce an effluent, and measuring a ratio of the pollution neutralizing gas to the pollutant gas in the effluent. 
     Various configurations of the present invention also provide an apparatus for monitoring and controlling performance of a selective catalytic reduction (SCR) emission control system. The apparatus is configured to measure a ratio of pollution neutralizing gas to pollutant gas in a flue gas stream effluent downstream of a catalyst, and adjust an injected quantity pollution neutralizing gas in a flow of gas including the pollutant gas upstream of or at the catalyst in accordance with the measured ratio. 
     Some configurations of the present invention provide a gas turbine combustion plant having a selective catalytic reduction (SCR) emission control system that includes a catalyst bed. The plant also includes a supply of a pollution neutralizing gas for reacting with a pollutant gas in the SCR system and a measuring system configured to measure a ratio of the pollution neutralizing gas to pollutant gas in an effluent of the SCR system. 
     Still other configurations of the present invention provide a fossil fuel-fired boiler having a selective catalytic reduction (SCR) emission control system. The SCR include a catalyst bed, and the plant also has a supply of a pollution neutralizing gas for reacting with a pollutant gas in the SCR system. A measuring system that is configured to measure a ratio of the pollution neutralizing gas to pollutant gas in an effluent of the SCR system is also provided. 
     As a catalyst (or its modules) age and reactivity falls off, configurations of the present invention are able to keep an outlet pollution neutralizing gas to pollutant gas ratio in a proper range, although the concentration of both gases will increase. Through such monitoring, a drop off in catalyst reactivity can be segregated from injection gas maldistribution, thus allowing the catalyst life to be extended to its maximum limit. Moreover, using some configurations of the present invention, operators can identify whether all or only part of a catalyst bed or only one or a few modules need replacement. This identification permits catalyst replacement to be scheduled at an optimal time for facility operation while maintaining compliance with all environmental regulatory requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram representative of various configurations of an apparatus of the present invention for measuring and controlling a selective catalytic reduction (SCR) emission control system. 
         FIG. 2  is a graph showing actual NH 3 /NO x  ratio data gathered on a single day at an industrial facility from five modules out of ten using a configuration of the present invention. 
         FIG. 3  is a graph showing actual NH 3 /NO x  ratio data gathered on the same day as the data shown in  FIG. 2  at an industrial facility from the remaining five modules out of ten using a configuration of the present invention. 
         FIG. 4  and  FIG. 5  are graphs showing overall SCR efficiency data gathered for several hours on different days prior to an initial AIG balancing. 
         FIG. 6  and  FIG. 7  are graphs showing overall SCR efficiency data gathered for several hours on different days following an initial AIG balancing as provided in a configuration of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In some configurations of the present invention and referring to  FIG. 1 , a selective catalytic reduction (SCR) measuring and control system  10  is provided for reducing pollution from a combustion source  12 . A non-exhaustive list of such combustion sources includes gas turbine combustion sources, fossil fuel-fired combustion sources, and industrial processes (for example, quartz manufacturing). Combustion source  12  produces a combustion gas stream containing a pollutant gas, for example, NO x . A quantity of a pollution neutralizing gas  13  (e.g., NH 3 ) is injected into combustion gas stream  11  at or upstream from a catalyst  40  using an injection grid  90  such as an ammonia injection grid (AIG)  90 . 
     (It is recognized that ammonia is considered a hazardous air pollutant in many states. However, referring to ammonia as a “pollution neutralizing gas” should not result in any confusion to one of ordinary skill in the art. Ammonia is referred to herein as a “pollution neutralizing gas” and NO x  as a “pollutant gas” because the “pollutant gas” is produced as a consequence of combustion, whereas the “pollution neutralizing gas” is deliberately introduced in controlled quantities to react with the pollutant gas, thereby producing harmless reaction products. The use of the terms “pollution neutralizing gas” and “pollutant gas” herein helps make evident that the present invention is not limited solely to configurations in which NO x  is the pollutant gas and ammonia is the pollution neutralizing gas.) 
     Measurements are taken at a plurality of different locations. In some configurations, for example, samples are taken at ten different locations  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28 ,  30 , and  32 . Heated stainless steel sampling lines  34  are used for these measurements. Probes  36  at the ends of sampling lines  34  are positioned at different locations  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28 ,  30 , and  32  on an exit plane  38  of a catalyst bed  40  or downstream of separate catalyst modules  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 ,  56 , and  58  that comprise catalyst  40  in some configurations. Probes  36  are thus configured to sample a flue gas stream effluent downstream of catalyst  40 . 
     In some configurations, each probe  36  is filtered to capture small particles (e.g., all small particles&gt;2 μm). Samples from probes  36  are brought via sampling lines  34  into a weatherproof box  62  that is heated to a constant temperature greater than 125° C. and mounted on the side of the SCR. Each line  34  is routed to one of two rotary valves  64  or  66 , each of which is equipped with five inlets and one outlet. The outlets from rotary valves  64  and  66  are routed through a three-way valve  68  and from there to an outlet  70  of heated box  62 . By appropriately positioning rotary valves  64  and  66  and three-way valve  68 , each line  34  is isolated as sample gas is extracted from each individual probe  36  of the plurality of sampling probes. Also in some configurations, each sampling line  34  is flushed when not in use using instrument air (i.e., compressed air) and appropriate valves  72  and  74  to avoid diluting the active sample line with instrument air. Actuators (not shown) for rotary valves  64  and  66  and three-way valve  68  are located in a separate weather-proof enclosure (not shown) that is freeze protected. 
     It will be recognized that alternate plumbing configurations can be used in conjunction with various configurations of the present invention. By way of example and not by way of limitation, it may be advantageous in some configurations to utilize a 2-position, 4-port valve in place of three-way valve  68 . 
     After exiting heated box  62 , sample gas is transported from the side of the SCR to a remote instrument housing  76  using a heated sampling line  78  that is also operated at 180° C. In housing  78 , the heated sample passes through a heated head sampling pump  80  and appropriate valves (e.g.,  82  and  84 ) to control the sample flow and pressure delivered to an emissions monitor  86 . In some configurations, monitor  86  provides simultaneous measurement of NO x  NO 2  and NH 3 , and measurements are made on a hot wet basis to avoid requiring flow conditioning systems such as chillers that can cause NH 3  and/or NO 2  to condense and drop out of the sample stream prior to analysis. Also, monitor  86  in various configurations provides sufficient sensitivity to determine single digit concentrations, and span calibration for all three species using calibration cells rather than requiring special span gases, which can be expensive and introduce additional analysis error. In some configurations, monitor  86  is all digital and equipped with Ethernet capabilities for communication with both on-site and remote data storage and analysis systems. Also in some configurations, measurements of NO 2  and NH 3  are accomplished directly by monitor  86 , without the need to process samples with converters such as is required by chemiluminescent analyzers (an alternative measurement technique). 
     In other configurations, however, any suitable analysis method can be by monitor  86  that provides continuous, simultaneous determination of NO, NO 2 , and NH 3  concentration in a reliable manner at low concentration levels. In some configurations, monitor  86  can be replaced by manual monitoring and testing. As used herein, “low-concentration” refers to NO x  and NH 3  levels in a low single digit ppm range. 
     In some configurations, data from the analyzer are directed to a dedicated computer  88  that stores the appropriate data streams and controls positioning of the various valves in heated box  62  on the SCR. 
     In some configurations of the present invention, data are sequentially collected from each sampling probe  36  for a period of up to one hour. This sampling period is based on the absorption and desorption dynamics of SCR catalysts operating in the 300° C. range. A shorter sampling period can be used for SCR systems processing higher temperature flue gas. Dedicated computer  88  in some configurations determines, records, and reports a time-averaged ratio of NH 3 /NO x  as well as the actual value of all three measured species. 
     By comparing data from each sampling location  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28 ,  30 , and  32  (either manually or utilizing software or firmware to control computer  88 ), all necessary information is provided to guide manual balancing of ammonia injection grid (AIG)  90  or for control computer  88  to generate control signals for automatic adjustment of motorized drives on AIG valves. 
     More particularly, ratios of NH 3 /NO x  concentrations at catalyst  40  exits (in some configurations, time averaged ratios) as sampled by probes  36  provide direct information on the tuning of AIG  90  and allow a technician to determine which ammonia nozzle (or nozzles) of AIG  90  requires adjustment and in which direction. Some configurations of the present invention also provide continuous monitoring of the actual ammonia and NOx concentration. As catalyst  40  (or its modules) age and reactivity falls off, the outlet NH 3  to NO x  ratio will remain in the proper range as long as AIG  90  is balanced, although the concentration of both pollutants will increase. Through such monitoring, the drop off in reactivity can be segregated from AIG  90  maldistribution, thus allowing catalyst  40  life to be extended to its maximum limit. Moreover, using some configurations of the present invention, operators can identify whether all or only part of a catalyst bed  40  or only one or a few modules need replacement, renewal, or adjustment. This identification permits catalyst  40  replacement to be scheduled at an optimal time for facility operation while maintaining compliance with all environmental regulatory requirements. 
     For manual SCR systems, adjustment of AIG  90  balance may be required only infrequently. For other SCR systems, however, it may be appropriate to adjust AIG  90  any time there is a big swing in the facility  12  load. Thus, configurations of the present invention may be practiced in either a feed forward or a feedback control mode. In some configurations, AIG  90  injects a quantity of a pollution neutralizing gas (e.g., NH 3 ) in a flow of gas upstream or at catalyst  40 . The injected quantity of pollution neutralizing gas is adjusted in accordance with a determined ratio of the pollution neutralizing gas to the pollutant gas (e.g., NO x ). In some configurations, the determined ratio is a time-averaged ratio. Also, the injected quantity of pollution neutralizing gas is adjusted separately at different injection locations (e.g., at or upstream from different portions of catalyst  40  bed or at or upstream of different catalyst modules comprising catalyst  40 ). The separate adjustments are each in accordance with ratios determined from corresponding probe  36  locations. 
     A configuration of the subject invention has been developed and implemented in a non-automated fashion on an SCR system at a quartz manufacturing facility in Hebron, Ohio. The SCR system at this facility consists of ten separate modules, each processing 1/10 of the total flue gas generated at the facility. A metered, total quantity of ammonia reagent is delivered to the SCR system and then distributed to each module. Manual control valves are provided upstream to the ammonia spargers that distribute the reagent to each module. The ten modules are arranged in two banks of five SCR modules, which are referred to as the “East bank” and the “West bank.” Modules on the West bank are designated as modules  1 - 5  whereas modules on the East bank are designated as Modules  6 - 11 . Data in  FIG. 2  and  FIG. 3  show actual NH 3 /NO x  ratio data gathered from below each of the ten modules on the same day. Similar data had been gathered a week earlier and used to provide rough initial balancing of the ammonia delivery to each module. As shown by the data in  FIG. 2  and  FIG. 3 , initial balancing was not perfect. Modules  2  and  11  have excess ammonia whereas other modules are slightly ammonia starved. Data such as that shown in  FIG. 2  and  FIG. 3  have been used to further fine-tune the AIG balancing. 
     To determine the impact of AIG balancing, overall SCR efficiency data were gathered for several hours on two separate days prior to the initial balancing and for two days following the initial course balancing. Data presented in  FIG. 4  and  FIG. 5  were gathered prior to the balancing while the data in  FIG. 6  and  FIG. 7 ) followed the initial tuning. The line at the top of each figure indicates the SCR NO x  destruction efficiency. Each figure covers a period during which the facility inlet and outlet NO x  monitors were being calibrated, resulting in short periods of spurious calculated efficiency data. A comparison of the efficiency data in  FIG. 4  and  FIG. 5  with the data in  FIGS. 6 and 7  shows a dramatic improvement in efficiency. An approximately 6 to 8 percent improvement in average NO x  destruction efficiency was achieved through this initial tuning step. Subsequent tuning can improve the overall AIG balance with smaller, but still significant, improvement in NO x  control efficiency. 
     Configurations of the present invention can be applied to a wide spectrum of SCR installations, such as stationary gas turbines equipped with SCR and stationary diesel engines that require SCRs. Also, configurations of the present invention can be used to drive a service business for helping customers control and optimize performance of their SCR systems. In some configurations of the present invention, these adjustments can be performed manually but in others, the AIG adjustment is perfromed automatically. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.