Abstract:
A heat recovery steam generator (HRSG) ( 10 ) including: an economizer ( 12 ) configured to heat a working fluid by extracting heat from a flow of flue gas ( 20 ). The HRSG includes a diluting fluid injector arrangement ( 60 ) configured to inject a diluting fluid ( 50 ) effective to dilute a concentration of a gaseous corrosive when compared to an undiluted concentration of the gaseous corrosive in the flow of flue gas. The HRSG also includes a preheater ( 18 ) configured to preheat the diluting fluid prior to injection.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to maximizing thermal efficiency of a combined cycle power generation plant via greater heat exchange in a heat recovery steam generator. In particular, the invention eliminates the need to preheat a working fluid entering an economizer, thereby enabling the economizer to extract more heat from flue gas. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a combined cycle power generation plant a heat recovery steam generator (HRSG) may be used to recover heat exhausted by a separate process such as the operation of a gas turbine engine. The HRSG receives the exhausted gas and uses various heat exchanging components to transfer the heat from the exhausted gas to a working fluid. In certain operations the exhaust gas may contain corrosive elements that may cause damage to the heat exchanging components if the flue gas is cooled below a threshold level. For example, gas turbine operations using high sulfur fuels generate flue gas having a relatively high concentration of sulfur oxides, including sulfur dioxide and sulfur trioxide. Sulfur trioxide forms when sulfur dioxide is oxidized. Gaseous sulfuric acid is then formed when sulfur trioxide combines with water vapor. If cooled below a sulfuric acid dew point, the sulfuric acid gas will form liquid sulfuric acid on HRSG interior surfaces, including heat exchanging element external surfaces and the liquid sulfuric acid will damage the interior surfaces, in particular the heat exchanging element external surfaces. When entering the HRSG the flue gas is at a temperature above the sulfuric acid dew point, and hence the formation of liquid sulfuric acid is not a problem at this location. As the flue gas traverses the HRSG and heat is drawn from the flue gas the temperature of the flue gas cools. In addition to corrosives, water vapor may condense and form liquid water on the heat exchanging elements if the flow of flue gas is cooled below the water vapor temperature. This liquid water may interfere with the heat exchanging process and accelerate the flow process in an undesired manner. 
         [0003]    Under conventional HRSG operations, care is taken to prevent the temperature of the flue gas from dropping below the sulfuric acid dew point and/or a water dew point at any location in the HRSG. This can be done by, for example heating the working fluid entering heat exchanging elements disposed within the flow of flue gas such that external surfaces of the heat exchanging elements remain sufficiently warm to prevent the unwanted condensation. However, under thermodynamically optimal operation of a HRSG, the working fluid entering at least one of the heat exchanging elements within the HRSG would be at a temperature below the sulfuric acid dew point and/or the water dew point of the flue gas. In this thermodynamically optimal scenario, the relatively cool working fluid would cause the external surface of the heat exchanging element to be below the dew point until heated. When the flue gas encounters the relatively cool surface, or a local volume within the flue duct that has been cooled by the relatively cool surface, the flue gas cools to below the sulfuric acid dew point. Liquid sulfuric acid then forms on the relatively cool surface of the heat exchanging element. The liquid sulfuric acid then acts as a thermal insulator which mitigates heat transfer from the flue gas to the working fluid. This results in the relatively cool working fluid staying cooler longer, which, in turn, expands the size of the relatively cool surface of the heat exchanging element upon which sulfuric acid will form. Over time this liquid sulfuric acid can damage and/or destroy the heat exchanging element. 
         [0004]    One conventional solution to this problem has been to preheat the working fluid entering the heat exchanging element to a temperature above the sulfuric acid dew point. In this case, since the working fluid is already above the sulfuric acid dew point when entering the heat exchanging element, liquid sulfuric acid will not form on the heat exchanging elements. However, heating the working fluid necessarily reduces the amount of heat that can be transferred from the flue gas to the working fluid. This reduction in heat transfer reduces a thermal efficiency of the heat recovery steam generator. Consequently, there is room for improvement in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in the following description in view of the drawings that show: 
           [0006]      FIG. 1  is a schematic representation of a heat recovery steam generator (HRSG) having an exemplary embodiment of the concentration dilution arrangement disclosed herein. 
           [0007]      FIG. 2  is a top view of an HRSG heat exchanging arrangement and an exemplary embodiment of the concentration dilution arrangement disclosed herein. 
           [0008]      FIG. 3  is a side view of the HRSG heat exchanging arrangement and the exemplary embodiment of the concentration dilution arrangement of  FIG. 2 . 
           [0009]      FIG. 4  is a view of a heat exchanging element of the HRSG heat exchanging arrangement of  FIG. 2  and an exemplary embodiment of an injection arrangement of the concentration dilution arrangement of  FIG. 2 . 
           [0010]      FIG. 5  shows plural heat exchanging elements arranged in accord with an exemplary embodiment of the concentration dilution arrangement of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The present inventors are aware that a thermal efficiency of a HRSG used in a combined cycle power plant, such as one using a gas turbine engine and a HRSG, is reduced by a need to preheat working fluid entering heat exchanging elements when there is a possibility that liquid sulfuric acid or liquid water may form on the portions of the heat exchanging elements and cause damage or interfere with the heat transfer process and flow of flue gas. They are further aware that without the preheating the liquid sulfuric acid or water formation would occur on these portions of the heat exchanging elements because these portions would be cooled by the working fluid to a temperature below a sulfuric acid dew point. The inventors have devised an innovative alternative solution to the corrosion problem that takes advantage of the fact that the sulfuric acid dew point varies with a concentration of the gaseous sulfuric acid in the flue gas. This solution can also be used to prevent the formation of liquid water and alleviate the problems associated there with. 
         [0012]    Instead of preheating the working fluid, or using sacrificial heat exchanging elements, the inventors propose to allow cooler working fluid to enter the heat exchanging unit. To reduce and/or prevent the formation of liquid sulfuric acid that would be anticipated in prior art HRSGs without preheating, the inventors locally dilute a concentration of gaseous sulfuric acid in a local volume within the flow of flue gas where liquid sulfuric acid might otherwise form. Since liquid sulfuric acid would be anticipated to form on the portion of the heat exchanging elements cooled by the working fluid to a temperature below the sulfuric acid dew point in the undiluted flue gas, the inventors propose to dilute the concentration of gaseous sulfuric acid in a local volume that separates the flue gas having undiluted gaseous sulfuric acid from the relatively cool portion of the heat exchanging exterior surface. Thus, the portion of the heat exchanging element may define a portion of that diluted local volume. Alternatively, the portion may be partially or entirely within the diluted local volume. Stated another way, the inventors propose to reduce/prevent the formation of liquid sulfuric acid by lowering the local sulfuric acid dew point within the local volume, where the local volume protects a surface where liquid sulfuric acid would otherwise be likely to form, by separating the protected surface from the flue gas having undiluted gaseous sulfuric acid. 
         [0013]      FIG. 1  is a schematic representation of an HRSG including a heat exchanging arrangement  12  and a dilution arrangement  14  shown generally. Not illustrated for purposes of clarity is the known evaporator portion of the HRSG. In the exemplary embodiment shown the heat exchanging arrangement  12  is shown to be a condensate preheater located generally at a colder end of the HRSG. However, the invention can be applied to any heat exchanging element where an undesirable liquid may form without dilution. For example, the heat exchanging element may be any economizer that extracts heat from flue gas and delivers the heat to a working fluid (i.e. a second fluid) that subsequently travels through an evaporator. Further, while the exemplary embodiment described herein addresses the formation of liquid sulfuric acid, the concepts and structure may be used to prevent the formation of any other liquid such as a liquid corrosive when a gaseous form of the corrosive is present in the flue gas. Major components of the dilution arrangement  14  that are visible include a dilution fluid source  16  and optionally a preheater  18 . The dilution fluid source  16  may be a fan or other means that cause the dilution fluid to flow. The preheater  18  may be any arrangement that preheats the dilution fluid to a desired temperature. In the exemplary embodiment shown the preheater  18  is shown as a flue gas air heat exchanger that transfers heat (i.e. permits thermal communication) from a flow of flue gas  20  (i.e. a first fluid) to the dilution fluid. In alternate exemplary embodiments the preheater may be, for example, an arrangement that receives heat from another working fluid in the HRSG  10 , or electrical, or gas powered etc. In such exemplary embodiments the preheater may be disposed external to the flow of flue gas  20 . In operation the working fluid flows through the preheater  18  where it is heated to the desired temperature. The working fluid then flows into an additional injection arrangement (not shown) adjacent a heat exchanging element (not shown) where it is injected into the flow of flue gas  20  to create a local volume (or volumes) of flue gas having a reduced concentration of gaseous sulfuric acid. Each volume may include a film over the surface to be protected. The dilution fluid source  16  may pressurize the dilution fluid as necessary, or this may be done separately, such as with a forced draft fan or the like. Alternately, when only local dilution is wanted, the preheater  18  may be dispensed with. 
         [0014]      FIG. 2  shows a top view and partial cutaway of the heat exchanging arrangement  12  and dilution arrangement  14  of  FIG. 1 . Within the heat exchanging arrangement  12 , or condensate preheater, there may be plural rows of heat exchanging elements  30 , including an upstream row  32  and a downstream row  34  with respect to the flue gas  20  flowing through a duct  36  in which the heat exchanging elements  30  are disposed. As the flow of flue gas  20  travels through the heat exchanging arrangement  12  it encounters an exterior/shell side  38  of these rows of heat exchanging elements  30 . Heat transfers from the flow of flue gas  20  to the working fluid on a second side  40  of (i.e. within) the heat exchanging elements  30 . Thus, as the flow of flue gas  20  travels from an upstream end  42  to a downstream end  44  of the heat exchanging arrangement  12  the flow of flue gas  20  cools. Working fluid enters the heat exchanging elements  30  starting from the downstream end  44  and working its way toward the upstream end  42 , during which time the heat from the flow of flue gas  20  heats the working fluid. 
         [0015]    From the foregoing flow description it can be seen that at the downstream end  44  the flow of flue gas  20  will be at its coolest temperature and the working fluid will also be at its coolest temperature. In this exemplary embodiment it is at the downstream end that the flow of flue gas  20  may encounter a heat exchanging element  30  having an exterior side  38  with a portion of the exterior side  38  that may be at a temperature below the dew point of the undiluted gaseous sulfuric acid in the flow of flue gas  20 . Thus, it is at the downstream end  44  that a volume  46  within the flow of flue gas  20  would be created having a diluted concentration of gaseous sulfuric acid. This volume  46  will be used to at least protect the portion of the exterior side  38  that may be at a temperature below the dew point of the undiluted gaseous sulfuric acid. 
         [0016]    The volume  46  may be formed by injecting the dilution fluid  50 , for example air, or other non corrosive fluids, proximate the portion of the exterior side  38  to be protected via the injection arrangement that may include injectors  48 . This can also be seen in  FIG. 3 , where the portion  52  of the downstream row  34  spans an entire length of the heat exchanging element  30  within the duct  36 . However, the portion  54  of the upstream row  32  spans less than the entire length of the heat exchanging element  30  within the duct  36 . This may occur in an exemplary embodiment when the working fluid heats while traveling through the downstream row  34  and continues to heat while traveling through the upstream row  32 . By the time the working fluid reaches an end  56  of the portion  54  of the upstream row  32  the working fluid has been heated sufficiently such that it is at a temperature above the sulfuric dew point of undiluted gaseous sulfuric acid in the flow of flue gas  20 . At or above this temperature dilution is no longer needed because the undiluted gaseous sulfuric acid will not condense on the exterior side  38  of the heat exchanging element  30 . Thus, as described above, the portions  52 ,  54  of the heat exchanging elements  30  that were likely to be at a temperature that would cause undiluted gaseous sulfuric acid to condensate on them were protected by the volume  46  having a diluted concentration of gaseous sulfuric acid. These portions  52 ,  54  then acted to define a part of the volume  46 . In an exemplary embodiment the dilution arrangement  14  may protect only those portions  52 ,  54  needing protection. These portions  52 ,  54  may account, for example, for about 20% of all heat exchanging surfaces exposed to the flow of flue gas  20  in the heat exchanging arrangement  12 . However, it is conceivable that other factors may influence design considerations and hence more or less of the exterior side  38  may be protected than would be likely to condense sulfuric acid. 
         [0017]    The preheater  18 , which is shown as external to the flow of flue gas  20  in this exemplary embodiment, may be configured to warm the dilution fluid to a temperature close to or the same as that of a temperature of the flow of flue gas  20  at the point of injection. For the injectors  48  on the upstream row  32 , this temperature could be a temperature of the flow of flue gas  20  immediately upstream of the upstream row  32 . Likewise, for the injectors  48  on the downstream row  34 , this temperature may be a temperature of the flow of flue gas  20  immediately upstream of the downstream row  34 . This may be desired to prevent an adverse thermal layer from forming between the flow of flue gas  20  and the exterior side  38  of the heat exchanging element  30  that might inhibit heat exchange from the flow of flue gas  20  to the working fluid. Some of this added heat may be recaptured via transfer to the working fluid during the heat exchanging process. 
         [0018]      FIG. 4  discloses a close up view of one of the heat exchanging elements  30  of  FIGS. 2 and 3  and an exemplary embodiment of the injection arrangement  60 . The injection arrangement may include injectors  48  having one or more injection manifolds  62 , for example a conduit channel etc., each having one or a plurality of outlets  64  (i.e. orifices). Dilution fluid  50  may travel longitudinally  68  through the injection manifold  62  and exit the outlets  64 . The outlets  64  may be formed so that streams  70  of dilution fluid  50  quickly merge to make the volume  46  uniform. In the exemplary embodiment shown the injection manifold  62  may be disposed on an upstream side  72  of the heat exchanging element  30  with respect to a direction of travel of the flow of flue gas  20 . The outlets  64  inject the dilution fluid  50  essentially tangential to the exterior side  38 . In this configuration the volume  46  includes a film  74  that blankets the exterior side  38 . Such an injection configuration does not interfere with heat transfer rates. Within this volume  46  the dilution fluid, which may be air, dilutes the concentration of gaseous sulfuric acid. This lowers the sulfuric acid dew point within the volume. With a lower dew point in the volume  46  adjacent the exterior side  38 , an inlet temperature of the working fluid can be reduced to a temperature at or slightly above the dew point of the diluted concentration of gaseous sulfuric acid in the volume  46 . Lowering the inlet temperature of the working fluid allows for more heat transfer from the flow of flue gas  20 . Hence, the HRSG operates more efficiently. 
         [0019]    The upstream row  32  and downstream row  34  can be seen in greater detail in  FIG. 5 . In this exemplary embodiment dilution fluid is injected at a plurality of outlets  80  around each heat exchanging element  30 . The outlets  80  form an upstream volume  82  that, in this exemplary embodiment, includes a first film  84  between the flow of flue gas  20  having the undiluted concentration of the corrosive, such as gaseous sulfuric acid, and the portion  54  of the upstream row  32  (i.e. the first heat exchanging element) that needs to be protected because it may be at the relatively cool temperature. The downstream row  34  (i.e. the second heat exchanging element) may have its own dilution fluid injected through its plurality of outlets  80  to form the a downstream volume  86  that, in this exemplary embodiment, includes a second film  88  between the flow of flue gas  20  having the undiluted concentration of the corrosive, such as gaseous sulfuric acid, and the portion  52  of the downstream row  34  that needs to be protected. 
         [0020]    In an exemplary embodiment the upstream row  32  and the downstream row  34  can be aligned within the flow of flue gas  20  such that diluting fluid injected into the upstream volume  82  (i.e. first volume) travels with the flow of flue gas  20  to contribute to the downstream volume  86  (i.e. second volume). The injectors  48  and its outlets  80  associated with the upstream row  32  (i.e. first injectors and first outlets) may inject the same amount of diluting fluid, more diluting fluid, or less diluting fluid than the injectors and outlets  80  associated with the downstream row  34  (i.e. second injectors and second outlets). Many factors may be considered when designing the desired arrangement. For example, as the flow of flue gas  20  travels from the upstream row  32  to the downstream row  34  the flow of flue gas  20  cools, as does the temperature of the working fluid. Thus, a greater amount of dilution may be needed in the downstream volume  86 . This can be accomplished by, for example, flowing more diluting fluid through the downstream row  34  to create a larger downstream volume  86  (and a thicker second film  88 ). Alternately, or in addition, the thicker second film  88  may result by having an arrangement where the dilution fluid used to form the upstream volume  82  is allowed to contribute to the downstream volume  86  as shown. This may permit the option of reducing the flow of dilution air injected at the downstream row  34 . Having injectors  48  that are the same from one heat exchanging element  30  to the next may simplify manufacturing. On the other hand, having injectors that vary may permit a degree of fine tuning that may be desired. In addition, there may be an injection air control system  90  to permit active control of the amount of diluting air being injected. 
         [0021]    Models incorporating the concepts disclosed herein have predicted as much as a 25% reduction in the concentration of gaseous sulfuric acids. This would enable the inlet temperature of the working fluid to the condensate preheater to be reduced by as much as 20 degrees Celsius. While exactly how much depends on many factors, this reduced inlet temperature may yields substantial savings that significantly outweighs the cost to implement and maintain the dilution arrangement. Consequently, the HRSG disclosed herein represents an improvement in the art. 
         [0022]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.