Abstract:
A system includes a selective catalytic reactor and a bypass line. The selective catalytic reactor is located downstream of a furnace that generates flue gases. The selective catalytic reactor reduces nitrogen oxides to nitrogen. The bypass line is in fluid communication with the selective catalytic reactor. The bypass line contacts an input line to the selective catalytic reactor, where the bypass line is adapted to handle a volume of flue gases diverted from the selective catalytic reactor. A first control damper is disposed at an inlet to the selective catalytic reactor; and a second control damper is disposed at an inlet to the bypass line. The first control damper and the second control damper interact to divide the flue gas stream between the selective catalytic reactor and the bypass line in a ratio to reduce the amount of sulfur trioxide released from the system to a desirable value.

Description:
TECHNICAL FIELD 
     This disclosure relates to a flue gas stream bypass during selective catalyst reduction in power generation facilities. 
     BACKGROUND 
     During the combustion process, flue gases generated from furnaces contain nitrogen oxides (NO x ). It is desirable to reduce NO x  emissions into the atmosphere. One post-combustion process for the lowering of NO x  emissions is that of selective catalytic reduction (SCR). Selective catalytic reduction systems use a catalyst and a reactant such as ammonia gas, NH 3 , to dissociate NO x  to molecular nitrogen, N 2 , and water vapor. A utility steam generating power plant having, for example, a fossil fuel-fired furnace may utilize selective catalytic reduction (SCR) as a NO x  reduction technique. The furnace generally comprises a furnace volume in fluid communication with a backpass volume. Combustion of hydrocarbon fuels occurs within the furnace volume creating hot flue gases that rise within the furnace volume giving up a portion of their energy to the working fluid of a thermodynamic steam cycle. The flue gases are then directed to and through the backpass volume wherein they give up additional energy to the working fluid. Upon exiting the backpass volume the flue gases are directed via a gas duct through a selective catalytic reduction chamber and thence to an air preheater and flue gas cleaning systems thence to the atmosphere via a stack. 
     In a SCR system, at some point in the gas duct after the flue gas stream exits the back pass volume and upstream of the SCR chamber, a reactant, possibly ammonia, in a gaseous form, or a urea/water solution is introduced into, and encouraged to mix with, the flue gas stream. The reactant/flue gas mixture then enters the SCR chamber wherein the NO x  reduction takes place between the reactant and the flue gas mixture in the presence of the catalytic surfaces. The introduction of the ammonia or urea into the flue gas stream is generally achieved by the use of injector atomizing nozzles located at either the periphery of the gas duct, or immersed on injection lances within the flue gas stream. 
     While the SCR facilitates the reduction of NO x , sulfur trioxide (SO 3 ) emissions are increased because the catalyst used for the NO x  reduction, promotes oxidation of incoming sulfur dioxide (SO 2 ) to sulfur trioxide (SO 3 ). SO 3  emissions at the stack must be limited to very low levels (below 5 ppm) to avoid excess opacity and/or a visible blue plume. 
     Downstream of the SCR, SO 3  emissions are partially reduced by condensation in the combustion air preheater and captured in the particulate and SO 2  control equipment. If this reduction proves insufficient, specific SO 3  control measures are generally added to the process. One additional control measure is the addition of a spray dry absorber upstream of the flue gas desulfurization equipment (for removing both SO 2  and SO 3 ). Another is the addition of a condensing heat exchanger upstream of an electrostatic precipitator where the SO 3  is captured by the fly ash/condensate. Other methods include dry or wet sorbent injection using ammonia, lime, sodium bicarbonate, trona, and the like. All of these methods add both capital and operating cost. It is therefore desirable to reduce the SO 3  emissions to the atmosphere cost effectively with a minimum of capital expended on additional control equipment 
     SUMMARY 
     Disclosed herein is a system comprising a selective catalytic reactor; where the selective catalytic reactor is located downstream of a furnace that generates flue gases; the selective catalytic reactor being operative to reduce nitrogen oxides to nitrogen; a bypass line; the bypass line being in fluid communication with the selective catalytic reactor; the bypass line contacting an input line to the selective catalytic reactor, where the bypass line is adapted to handle a volume of flue gases that are diverted from the selective catalytic reactor; a first control damper disposed at an inlet to the selective catalytic reactor; and a second control damper that is disposed at an inlet to the bypass line; where the first control damper and the second control damper interact to divide the flue gas stream between the selective catalytic reactor and the bypass line in ratio that is effective to reduce the amount of sulfur trioxide released from the system to a desirable value. 
     Disclosed herein too is a method comprising discharging a flue gas stream from a furnace to a system comprising: a selective catalytic reactor; where the selective catalytic reactor is located downstream of a furnace that generates flue gases; the selective catalytic reactor being operative to reduce nitrogen oxides to nitrogen; and a bypass line; the bypass line being in fluid communication with the selective catalytic reactor r; the bypass line contacting an input line to the selective catalytic reactor, where the bypass line is adapted to handle a volume of flue gases that are diverted from the selective catalytic reactor; dividing the flue gas stream between the selective catalytic reactor and the bypass line in a ratio that is effective to reduce the amount of sulfur trioxide released from the system to a desirable value. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a depiction of a system having a bypass line around a selective catalytic reactor; and 
         FIG. 2  is a depiction of the system of the  FIG. 1 , with a reduced size selective catalytic reactor. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is a system for reducing SO 3  emissions comprising a selective catalytic reactor and a bypass line to the selective catalytic reactor that permits the flue gases to bypass the selective catalytic reactor. The selective catalytic reactor facilitates the reduction of NO x  present in the flue gas stream (emanating from the furnace) while the bypass line prevents or minimizes the conversion of SO 2  into SO 3  that generally occurs across the catalyst in the selective catalytic reactor with the consequent increase in the SO 3  that is released to the atmosphere. Permitting a portion of the flue gas stream to bypass the selective catalytic reactor facilitates a reduction in the SO 3  leaving the power plant. The use of a bypass line therefore facilitates reducing the amount of flue gas that passes through the selective catalytic reactor, thus permitting a reduction in the size of the selective catalytic reactor by an amount of up to 30%. 
     With reference now to the  FIG. 1 , the system  100  comprises a selective catalytic reactor  102  and a bypass line  104 . The selective catalytic reactor  102  (hereinafter SCR  102 ) lies downstream of a furnace (not shown) from which flue gases emanate. The SCR of the  FIG. 1  is scaled to catalytically reduce 100% of the flue gas flow. The flue gases may pass through particulate collection devices such as electrostatic precipitators or fabric filters (not shown) and flue gas desulfurization equipment (not shown) after contacting the system  100 . The flue gas enters the SCR  102  via input line  103  and exits the SCR  102  via output line  105 . The input line  103  contains a first isolation damper  106  and a first control damper  108 . The first control damper  108  lies downstream of the first isolation damper  106 , though in other embodiments, it may lie upstream of it. A second isolation damper  114  lies downstream of the SCR  102  on the output line  105 . 
     The bypass line  104  contacts the input line  103  at a point upstream of the first isolation damper  106  and contacts the output line  105  at a point downstream of the second isolation damper  114 . The bypass line  104  also comprises an optional third isolation damper  110  and a second control damper  112 , with the third isolation damper  110  lying upstream of the second control damper  112 . In an embodiment, the second control damper  112  lies upstream of the third isolation damper  110 . The isolation dampers  106 ,  110  and  114  each function to completely cut off the flow of flue gases in the line on which the respective isolation damper is disposed. The control dampers  108  and  112  can be variably adjusted to permit a selected percentage of the flue gas flow to proceed downstream of the respective control damper. The open position of the dampers can be set and automatically controlled according to unit load (i.e., MW rating). In addition, all of the isolation dampers and the control dampers can be manually adjusted or automatically activated and controlled. Both the isolation dampers and the control dampers may be electrically activated or pneumatically activated using actuators (not shown). 
     In an embodiment, the system  100  may comprise an optional SO 3  analyzer  202  disposed downstream of the SCR  102 . The SO 3  analyzer is in operative communication with an optional controller  204  that is in operative communication with one or more of the isolation dampers  106 ,  110  and  114 . The controller  204  is also in operative communication with one or more of the control dampers  108  and  112 . The SO 3  analyzer  202  measures the amount of SO 3  in the flue gas stream and transmits this information into the controller  204 . The controller  204  in turn facilitates adjusting the isolation dampers  106 ,  110  and  114  and the control dampers  112  and  114  to determine the percentage of the flue gas stream that is directed to the SCR and the percentage that is directed to the bypass (thereby bypassing the SCR). The controller  204  may be a computer, a microprocessor, or the like, and may contain software that is capable of compiling and retaining statistics on the type of fuel input and its relationship with the SO 3  content present in the flue gas stream. The controller  204  may also be capable of maintaining a correlation between the type of fuel input to the furnace and actuator (where the actuators are used on the respective isolation dampers and the respective control dampers) positions. 
     The controller  204  transmits messages to one or more actuators (not shown) that control the isolation dampers and/or to the control dampers. The communication between the controller  204  and the respective isolation dampers and the control dampers is shown by dotted lines in the  FIG. 1  (and in the  FIG. 2 , which will be detailed later). It is to be noted that the dampers can be controlled manually without any automation when the system does not contain analyzers and controllers. This is detailed later as well. 
     In one embodiment, in one method of operating the system  100  of the  FIG. 1 , the first isolation damper  106  may first be set to a closed position to permit the initial flue gas stream emanating from the furnace (that contains contaminants such as unburned carbon, oil mist carryover, and the like) to travel through the bypass line  104 . This prevents potential fires and/or degradation of the catalysts that are used in the SCR  102 . After the contaminated flue gas stream is discharged from the system  100 , the flue gas stream (without contaminants that can degrade catalyst reactivity within the SCR  102 ) is then discharged through the SCR  102 . 
     The first isolation damper  106  and the first control damper  108  permit the entire volume of flue gas from the furnace to travel along input line  103  to the SCR  102 . The first isolation damper  106 , the first control damper  108  and the second isolation damper  114  are completely opened during this portion of the process. The third isolation damper  110  is completely closed. The flue gas stream  102  travels through the SCR  102  and in the presence of the reactant, ammonia is catalyzed to dissociate NO x  to molecular nitrogen, N 2 , and water vapor. Some of the SO 2  present in the flue gas stream is converted to SO 3  during the catalytic reduction of the NO x . 
     The analyzer  202  measures the SO 3  content in the flue gas stream emanating from the SCR  102  along the output stream  105 . It is to be noted that the analyzer  202  may be a manually operated wet chemistry laboratory, where a sample of flue gas (after being treated by the SCR  102 ) is periodically collected in a device such as a bottle or bag and then analyzed via wet chemistry. In such a case, adjustments to the dampers (both the isolation and control dampers) are made manually. 
     In another embodiment, the analyzer  202  may be an automated chemical analyzer that is disposed in the flue gas stream and measures the sulfur trioxide content in-situ. If the SO 3  content is above an acceptable limit, then the controller  204  (which is automated) transmits a message to the controller  204  which activates the control dampers  108  and  112  respectively. In an embodiment, some of the dampers may be controlled automatically while others may be controlled manually. 
     In any event (when using either manual analysis or automated analysis), the first control damper  108  and the second control damper  112  are adjusted (i.e., partially opened) to prevent the entire amount of the flue gas stream from entering the SCR  102 . A portion of the flue gas stream flows through the bypass line  104 . In an embodiment, the first control damper  108  is partially opened to prevent an amount of up to 30% of the flue gas stream from entering the SCR  102  and instead letting it enter into the bypass line  104 . The second control damper  112  may be either completely opened at this stage or it may be partially opened to maintain the appropriate amount of back pressure in the lines  103  and  104 . During this configuration, the isolation dampers are all opened and the flow of the gases is controlled by the control dampers. The position of the control dampers can be adjusted (from the closed position) until the amount of SO 3  in the flue gas stream is reduced to the desirable value. In one embodiment, the amount of SO 3  in the flue gas stream after remediation in the system  100  is less than or equal to 5 ppm, preferably less than 3 ppm. 
     In one embodiment depicted in the  FIG. 2 , since the amount of flue gas passing through the SCR  102  is reduced, it may be desirable to reduce the size of the SCR reactor. The  FIG. 2  depicts a SCR  102  of a size reduced from that of the SCR  102  in the  FIG. 1 . The SCR may be scaled down in size to handle down to 70% of the volume flow, preferably down to 50 volume percent depending upon the outlet NO emission requirements. This design is compared to an SCR that does not have the bypass that is dedicated to handling 100% of the flue gas volume. The  FIG. 2  is essentially similar to that of the  FIG. 1  and functions in the same manner except that the SCR  102  is scaled down in size to depict the lower volume of flue gases that it will have to handle (because of using the bypass line) while releasing a desired volume of SO 3 . The smaller SCR  102  with its associated bypass line handles the entire flue gas stream while meeting stack NO x  and SO 3  requirements, but with a smaller cross-section, fewer catalyst modules, and less structural steel weight. 
     Bypassing some of the flue gas around the SCR reduces the amount of SO 3  produced and may eliminate the need for the addition of specific SO 3  control equipment. Reducing SO 3  content will also increase the effectiveness of mercury absorbents such as activated carbon. An additional benefit of partial flue gas bypass is easier installation and replacement of fewer catalyst modules over the life of the unit. The bypass design concept involves minimal added cost since a conventional SCR system includes a gas bypass with isolation dampers for other reasons (i.e. unit startup and shutdown). 
     It will be understood that, although the terms “first,” “second,” “third” etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, “a first element,” “component,” “region,” “layer” or “section” discussed below could be termed a second element, component, region, layer or section without departing from the teachings herein. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, singular forms like “a,” or “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to other elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower,” can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims. 
     The term and/or is used herein to mean both “and” as well as “or”. For example, “A and/or B” is construed to mean A, B or A and B. 
     The transition term “comprising” is inclusive of the transition terms “consisting essentially of” and “consisting of” and can be interchanged for “comprising”. 
     While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.