Patent Publication Number: US-2017356319-A1

Title: Exhaust Gas Heat Exchange for Ammonia Evaporation Using a Heat Pipe

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to turbine systems and, more specifically, to systems and methods for injecting cooling air into exhaust gas flow(s) produced by turbine systems. 
     Gas turbine systems typically include at least one gas turbine engine having a compressor, a combustor, and a turbine. The combustor is configured to combust a mixture of fuel and compressed air to generate hot combustion gases, which, in turn, drive blades of the turbine. Exhaust gas produced by the gas turbine engine may include certain byproducts, such as nitrogen oxides (NO x ), sulfur oxides (SO x ), carbon oxides (CO x ), and unburned hydrocarbons. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a gas turbine system includes an exhaust processing system fluidly coupled to an outlet of a turbine of a gas turbine engine, the exhaust processing system being configured to receive an exhaust gas having products of combustion generated by the gas turbine engine, and to process the exhaust gas before the exhaust gas exits the gas turbine system; an exhaust path of the exhaust processing system configured to flow the exhaust gas through the exhaust processing system. The system also includes an ammonia injection system having a source of ammonia and configured to introduce vaporized ammonia into the exhaust path; and a heat pipe having a first portion positioned within the exhaust path and a second portion positioned in a heat exchange relationship with a flow path of a heat exchange fluid used in the ammonia injection system. The heat pipe is configured to transfer thermal energy from exhaust gas in the exhaust path to the heat exchange fluid to enable the heat exchange fluid to vaporize the ammonia while cooling the exhaust gas to enable the exhaust processing system to more effectively process the exhaust gas. 
     In another embodiment, a heat pipe has a first portion positioned within an exhaust path of a gas turbine exhaust processing system and a second portion positioned in a heat exchange relationship with a flow path of a heat exchange fluid. The flow path of the heat exchange fluid includes an ammonia evaporator configured to evaporate ammonia received from an ammonia source. The heat pipe is configured to transfer thermal energy from exhaust gas in the exhaust path to the heat exchange fluid to enable the heat exchange fluid to vaporize the ammonia while cooling the exhaust gas to enable the gas turbine exhaust processing system to more effectively process the exhaust gas. 
     In a further embodiment, a gas turbine system includes a gas turbine engine configured to combust a mixture of fuel and an oxidant and to release exhaust gas resulting from the combustion; an exhaust processing system having an exhaust duct fluidly coupled to an outlet of a turbine of the gas turbine engine, the exhaust duct being configured to receive the exhaust gas released by the gas turbine engine. The exhaust processing system is configured to process the exhaust gas using a selective catalytic reduction (SCR) catalyst to reduce NO x  in the exhaust gas before the exhaust gas exits the gas turbine system. An exhaust path of the exhaust processing system is configured to flow the exhaust gas through the exhaust processing system. An ammonia injection system has an ammonia evaporator configured to receive aqueous ammonia from an ammonia source and vaporizes ammonia in the aqueous ammonia to enable the ammonia injection system to introduce vaporized ammonia into the exhaust path. A plurality of heat pipes is configured to receive thermal energy from exhaust gas in the exhaust duct to cool the exhaust gas before the exhaust gas reaches the SCR catalyst, transfers the thermal energy to a heat exchange fluid used in the ammonia evaporator to vaporize the ammonia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a diagrammatical overview of an embodiment of a gas turbine system having an exhaust processing system that uses heat pipes for exhaust gas cooling and ammonia evaporation; 
         FIG. 2  illustrates a side elevational view of an embodiment of the gas turbine system of  FIG. 1  in which the heat pipes have a first portion positioned in an exhaust duct and a second portion positioned in an ambient air heat exchanger; 
         FIG. 3  illustrates a schematic side elevational view of an embodiment of the exhaust processing system of  FIG. 1  in which an exhaust processing control system controls the flow of ambient air and the flow of aqueous ammonia to achieve levels of ammonia evaporation suitable for use in the exhaust processing system; 
         FIG. 4  illustrates a cross-sectional view of an embodiment of the heat exchange configuration of the heat pipes in accordance with various configurations of the present disclosure; and 
         FIG. 5  illustrates a schematic side elevational view of another embodiment of the exhaust processing system of  FIG. 1  in which the heat pipes are used to directly vaporize ammonia. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As set forth above, gas turbine engines may produce a number of products of combustion. These products may include nitrogen oxides (NO x ), sulfur oxides (SO x ), carbon oxides (CO x ), and unburned hydrocarbons. Generally, reducing the relative concentration of these products within an exhaust gas may include reacting such products with other reactants in the presence of a catalyst. The reaction between NO x  and a reductant such as ammonia (NH 3 ), for example, may occur within an exhaust duct in the presence of a metal oxide catalyst of a selective catalytic reduction (SCR) system. The catalyst lowers the activation energy of a reaction between the NO x  and ammonia to produce nitrogen gas (N 2 ) and water (H 2 O), thereby reducing the amount of NO x  in the exhaust gas before the exhaust gas is released from the gas turbine system. Such catalyst systems may be referred to as “DeNO x ” systems. 
     SCR systems may be used in a variety of different gas turbine systems, which range from relatively small scale systems to larger, heavy-duty gas turbine systems. Small scale systems produce exhaust gases having a relatively low temperature, while heavy-duty gas turbine systems produce exhaust gases with much higher temperatures. While exhaust gases from small scale systems (e.g., aero-derivative systems) have a temperature range that is generally amenable to the SCR process, the temperature of exhaust gases produced by heavy-duty systems is often much higher than acceptable operating ranges for the SCR process (e.g., temperatures suitable to maintain stability of the SCR catalyst). For example, in accordance with an embodiment of the present disclosure, the isotherm temperature of exhaust gases produced by a heavy-duty gas turbine engine may be greater than about 1000° F. (e.g., about 540° C.), such as between about 1100° F. and about 1300° F. (e.g., about 590° C. and about 705° C.), while an acceptable operating range of a “hot” SCR system (an SCR system having a relatively higher operating temperature range compared to other SCR systems) may be between about 800° F. and about 900° F. (e.g., about 425° C. and about 485° C.). 
     To reduce a temperature of these hot exhaust gases to the acceptable operating range for the SCR system, the exhaust gases may be mixed with tempering air to transfer heat from the exhaust gas to the tempering air and thereby cool the exhaust gas. Generally, the amount and temperature of tempering air therefore largely determines the amount of heat removed from the exhaust gas. 
     In an SCR system, as noted above, ammonia is reacted with NO x  in the exhaust gas to produce nitrogen and water. The SCR system may inject the ammonia into a stream of the exhaust gas, and the resulting mixture of ammonia and exhaust gas is directed to a catalyst of the SCR system. The source ammonia may include “wet” ammonia, which is an aqueous solution of ammonia, or “dry” ammonia, which is compressed or vapor ammonia that is substantially free of water. In embodiments where the source ammonia is wet ammonia, it may be desirable to separate the ammonia from the water in the aqueous solution. This may be accomplished by evaporating the ammonia away from the solution in an ammonia evaporator, which utilizes a feed of heated air to facilitate the evaporation process. 
     In accordance with aspects of the present disclosure, heat from the exhaust gas may be utilized to drive the ammonia evaporation process using one or more heat pipes. For example, it is now recognized that one or more heat pipes positioned along an exhaust path of the exhaust gas may conduct heat away from the exhaust gas and to one or more features used for ammonia evaporation. For instance, the one or more heat pipes may impart heat to an air flow to generate heated air for the ammonia evaporator. Additionally or alternatively, the one or more heat pipes may impart heat directly to an aqueous solution of ammonia to generate dry ammonia for injection by the SCR system. Accordingly, in general, the heat pipes of the present disclosure may be configured to transfer thermal energy from exhaust gas in an exhaust path to a heat exchange fluid (e.g., an air flow, or water within an aqueous ammonia solution) to enable the heat exchange fluid to vaporize the ammonia while cooling the exhaust gas. The cooling of the exhaust gas may enable the DeNOx catalyst to more effectively process the exhaust gas. 
     While the present disclosure may be applicable to a number of different gas turbine systems, such as combined cycle, the embodiments described herein may be particularly useful in simple cycle heavy-duty gas turbine systems that produce relatively high temperature exhaust gases (e.g., greater than 1000° F., about 540° C.). One example of a system having a configuration in accordance with certain aspects of the present disclosure is depicted in  FIG. 1 , which is a schematic view of an embodiment of a simple cycle gas turbine system  10 . However, it should be noted that the embodiments set forth herein may also be applied to combined cycle systems. 
     As illustrated, the simple cycle gas turbine system  10  includes a gas turbine engine  12 , which may include a heavy-duty gas turbine engine or an aero-derivative gas turbine engine. However, the present disclosure may be particularly applicable to embodiments where the gas turbine engine  12  is a heavy-duty gas turbine engine due to the much higher temperatures of exhaust gas  14  produced in such engines. Such aspects are discussed in further detail below. 
     The gas turbine system  10  may be part of a power plant, and may include a load  16  driven by the gas turbine engine  12  (e.g., a shaft  18  of the gas turbine engine  12  drivingly couples the gas turbine engine  12  to the load  16 ). By way of non-limiting example, the load  16  may include an electrical generator configured to output electrical power to an electric grid. The gas turbine engine  12  drives the load  16  by performing a combustion process, which produces the exhaust gas  14 . 
     The simple cycle gas turbine system  10  also includes an exhaust processing system  20  configured to receive the exhaust gas  14  from the gas turbine engine  12 , which may enable the exhaust gas  14  to be released from the simple cycle gas turbine system  10 . More specifically, the exhaust processing system  20  may include features configured to reduce a temperature, and/or concentration of certain products of combustion in the exhaust gas  14  before releasing the exhaust gas  14  via a stack and/or to another process  22 . Generally, the exhaust gas  14  flows along an exhaust path  24  from the gas turbine engine  12 , through the exhaust processing system  20 , and to the stack or other process  22 . 
     The exhaust processing system  20  includes a selective catalytic reduction (SCR) catalyst  26 , which may be a part of an SCR system configured to reduce a concentration of NO x  present within the exhaust gas  14 . More particularly, the SCR catalyst  26  lowers the activation energy for a reaction between the NO x  and ammonia (NH 3 ), which is a reducing agent, to produce nitrogen (N 2 ) and water (H 2 O). As noted above, while certain types of SCR catalysts are stable at relatively high temperatures, the exhaust gas  14  produced by the gas turbine engine  12  may still be much higher than is suitable for such catalysts. 
     To enable cooling of the exhaust gas  14  for more effective treatment of the exhaust gas  14  by the SCR catalyst  26 , a heat pipe  28  positioned in a heat exchange relationship with the exhaust path  24  transfers thermal energy from the exhaust gas  14  to ambient air  30 . This heat transfer may be facilitated by an ambient air heat exchanger  32  configured to place a flow of the ambient air  30  in a heat exchange relationship with the heat pipe  28 . More particular arrangements of the heat pipe  28 , the exhaust path  24 , and the ambient air heat exchanger  32  are described below. In addition, while the present disclosure refers to “ambient air,” such disclosures are intended to encompass treated (e.g., filtered) ambient air or untreated ambient air. Indeed, the use of untreated ambient air may provide the advantage of reduced capital and operating costs associated with the gas turbine system  10 . 
     The flow of the ambient air  30  is controlled using, by way of non-limiting example, an air flow control system  34 . The air flow control system  34  may include features configured to enable monitoring and control of a flow of the ambient air  30  into the ambient air heat exchanger  32 . Controlling the flow of the ambient air  30  into the ambient air heat exchanger  32  may also control the temperature and pressure of heated ambient air  36  produced by heat exchange between the heat pipe  28  and the ambient air  30 . 
     In accordance with an aspect of the present disclosure, the heated ambient air  36  facilitates ammonia vaporization in an ammonia injection system  38  to generate vaporized ammonia  40 . The vaporized ammonia  40 , in turn, reacts with the exhaust gas  14  in the exhaust processing system  20  as set forth above. The air flow control system  34  may control provision of the heated ambient air  36  to the ammonia injection system  38  within particular operating ranges. For example, the air flow control system  34  may adjust a flow rate, a temperature, a pressure, or any similar parameter of the heated ambient air  36  to within a particular operating range depending on characteristics of the heated ambient air  36  suitable to achieve a level of ammonia vaporization appropriate for the exhaust processing system  20 . The air flow control system  34  may be a part of a larger control system that is centrally located or distributed, as described in further detail below. 
     In situations where the ammonia injection system  38  does not necessarily need the total amount of the heated ambient air  36  exiting the ambient air heat exchanger  32 , the air flow control system  34  may direct at least a portion of the heated ambient air  36  to a vent or other process  42 . In this regard, the air flow control system  34  may control a split of the heated ambient air  36  between a first heated air flow path  44  leading to the ammonia injection system  38  and a second heated air flow path  46  leading to the vent or other process  42 . 
     A side elevational view of an embodiment of the simple cycle gas turbine system  10  is shown in  FIG. 2 . The gas turbine engine  12  may generally power the gas turbine system  10 , and includes one or more combustors  50  in which a fuel  52  and compressed oxidant  54  (e.g., compressed air) are mixed and undergo combustion. Other streams may also be present in the combustor to adjust combustion parameters as appropriate (e.g., exhaust gas diluent). Combustion products  56  generated in the one or more combustors  50  flow to a turbine  58 , which extracts work from the combustion products  56  to rotate the shaft  18  of the gas turbine engine  12 . The turbine  58  drives compression stages of an oxidant compressor  60  via the rotation of the shaft  18 . Staged compression within the oxidant compressor  60  creates a pressure gradient that draws in ambient air  30  to continue the compression and combustion cycle. 
     The combustion products  56  exit the turbine  58  as the exhaust gas  14 , which is directed into an exhaust duct assembly  62  fluidly coupled to an outlet  64  of the turbine  58 . The exhaust duct assembly  62  may include segments fluidly coupled to one another, or may include a single continuous duct. In certain embodiments, the exhaust duct assembly  62  may be segmented to allow for ready maintenance and replacement as appropriate. 
     The exhaust duct assembly  62  includes an exhaust inlet  66  configured to receive the exhaust gas  14  from the gas turbine engine  12 , and an exhaust gas outlet  68  in the form of a stack  70 . Generally, features of the exhaust processing system  20  are located within the exhaust duct assembly  62  along the exhaust path  24  and are configured to sequentially process the exhaust gas  14  as the exhaust gas flows from the exhaust inlet  66  to the exhaust outlet  68 . The processing may include encouraging turbulent flow of the exhaust gas  14  (which facilitates heat exchange), direct or indirect heat exchange, and catalytic byproduct removal, among others. 
     In the illustrated embodiment, such features include, but are not limited to, an ammonia injection grid  72  configured to inject the vaporized ammonia  40  into the exhaust path  24 , the SCR catalyst  26 , and a plurality of heat pipes  74  having respective first portions  76  (e.g., first ends) positioned along the exhaust path  24  upstream of the SCR catalyst  26 . The heat pipe  28  described above with respect to  FIG. 1  may be one heat pipe of the plurality of heat pipes  74  or, in other embodiments, the heat pipe  28  may be the only heat pipe positioned along the exhaust path  24 . 
     The respective first portions  76  of the plurality of heat pipes  74  are illustrated as being positioned between the ammonia injection grid  72  and the SCR catalyst  26 . This configuration may facilitate mixing of the exhaust gas  14  and the vaporized ammonia  40  by encouraging turbulent flow. Facilitating mixing in this manner may encourage homogeneity of the vaporized ammonia  40  and the exhaust gas  14  from both a compositional and thermal standpoint. However, the plurality of heat pipes  74  may have their respective first portions  76  positioned in any one or a combination of different locations along the exhaust flow path  24 , including upstream and/or downstream of the ammonia injection grid  72 . 
     During operation of the simple cycle gas turbine system  10 , the exhaust gas  14  flows along the exhaust path  24  in a bulk flow direction  78 . The first portions  76  of the plurality of heat pipes  74 , being oriented crosswise relative to the bulk flow direction  78 , contact the exhaust gas  14  (and vaporized ammonia  40 , in the illustrated embodiment) and receive thermal energy from (and cool) the exhaust gas  14 . Accordingly, the first portions  76  of the plurality of heat pipes  74  correspond to a “hot” side or end of the plurality of heat pipes  74 . 
     In one non-limiting example, a temperature of the exhaust gas  14  entering the exhaust duct assembly  62  from the gas turbine engine  12  is between about 1000° F. (about 540° C.) and about 1200° F. (about 650° C.). This temperature range may be higher than suitable for the SCR catalyst  26 . The plurality of heat pipes  74  reduces the temperature of the exhaust gas  14  to between about 800° F. (about 430° C.) and about 900° F. (about 480° C.) before the exhaust gas  14  reaches the SCR catalyst  26 , which may be more suitable for the SCR catalyst  26 . That is, the SCR catalyst  26  may be more efficient in catalyzing the reaction between the vaporized ammonia  40  and the NO x  in the exhaust gas  14  at such temperatures. 
     By way of non-limiting example, the plurality of heat pipes  74  may be arranged in rows of individual heat pipes  28  (e.g., substantially aligned along the bulk flow direction  78 ), columns of individual heat pipes  28  (e.g., substantially aligned crosswise relative to the bulk flow direction  78 ), staggered rows and columns of individual heat pipes  28 , or any combination thereof. Thus, any suitable arrangement of the plurality of heat pipes  74  may be utilized that enables the first portions  76  to contact the exhaust gas  14 . 
     Each heat pipe  28  of the plurality of heat pipes  74  is configured to rapidly conduct thermal energy from its respective first portion  76  (hot side or hot end) to a respective second portion  80  or end, which is a “cold” side or end of the heat pipe  28 . It is presently recognized that the second portions  80  of the plurality of heat pipes  74  may be placed in thermal communication (e.g., a heat exchange relationship) with one or more fluids (e.g., a heat exchange fluid) to integrate cooling and heating processes utilized in the exhaust processing system  20 . In the illustrated embodiment of  FIG. 2 , the cooling process involves cooling of the exhaust gas  14  and vaporized ammonia  40 , and the heating process involves heating a fluid to produce, either directly or indirectly, the vaporized ammonia  40 . Further, the embodiment depicted in  FIG. 2  is not limited to the specific heat exchange relationship shown. 
     For example, in one embodiment, a first set of the plurality of heat pipes  74  may have respective second portions  80  positioned in a heat exchange relationship with the ambient air  30  (e.g., a first flow path of a heat exchange fluid). Further, a second set of the plurality of heat pipes  74  may have respective second portions  80  in a separate heat exchange relationship with the ambient air  30  (e.g., a second flow path of the heat exchange fluid). The second flow path may be separate from and arranged in parallel with respect to the first flow path, and may lead to the same or different destinations (e.g., be used for the same or different purposes). 
     The one or more fluids may be capable of receiving thermal energy from the second portions  80  of the plurality of heat pipes  74  (e.g., rejecting heat from the second portions  80  of the plurality of heat pipes  74 ). In the illustrated embodiment, the heat exchange fluid is ambient air  30  taken into the ambient air heat exchanger  32 . However, other heat exchange fluids may be utilized. For example, the heat exchange fluid may be water in the aqueous ammonia subject to vaporization. 
     The air flow control  34  described with respect to  FIG. 1  may include, as illustrated, a heated air flow control device  82  configured to controllably close or open a heated air path  84  (e.g., a heated air conduit) coupling an outlet  86  of the ambient air heat exchanger  32  to a heated air motivator  88 . That is, the heated air flow control device  82  is configured to at least partially control a flow of the heated ambient air  36  to the heated air motivator  88 . By way of non-limiting example, the heated air flow control device  82  may include a damper  90  coupled to an actuation mechanism  92 . The actuation mechanism  92  may be communicatively coupled to an exhaust processing control system  94  configured to control operation of the damper  90  via the actuation mechanism  92 . In certain embodiments, the heated air flow control device  82  may include a plurality of flow control devices. 
     The exhaust processing control system  94  may also regulate other operational aspects of the exhaust processing system  20 . For example, the exhaust processing control system  94  is communicatively coupled to a variety of components that facilitate regulation of a flow rate, temperature, pressure, and so forth, of various fluids used to achieve suitable processing of the exhaust gas  14 . 
     The exhaust processing control system  94  may be implemented on any suitable programmable architecture, such as an architecture including one or more processors  96  and one or more memory  98 . Once programmed, the exhaust processing control system  94  may be considered to constitute a specially-configured device that is configured to control specific aspects relating to the exhaust processing system  20  based at least on algorithmic structure associated with its programming. In this way, the exhaust processing control system  94  be configured to perform certain functions, and these functions should be considered to denote a specific algorithmic structure of the exhaust processing control system  94 , for example a structure associated with the one or more processors  96  and one or more memory  98 . 
     By way of non-limiting example, the exhaust processing control system  94  may include one or more application specific integrated circuits (ASICs), one or more field programmable gate arrays (FPGAs), one or more general purpose processors, or any combination thereof. Additionally, the memory  98  storing instructions executed by processors  96  of the exhaust processing control system  94  may include, but are not limited to, volatile memory, such as random access memory (RAM), and/or non-volatile memory, such as read-only memory (ROM), optical drives, hard disc drives, or solid-state drives. Further, the exhaust processing control system  94  may be implemented as a part of a larger control system (e.g., a gas turbine control system), and/or as a variety of control devices and/or subsystems distributed throughout simple cycle gas turbine system  10  (e.g., a distributed control system). The control devices and/or subsystems, therefore, may include any one or a combination of the processing and memory circuitry configurations noted above. Additionally, the exhaust processing control system  94  will generally include various input devices, and may include a user interface in the form of a display, or in the form of a connector that is accessible through wired or wireless connection with a computing device of the user. 
     The exhaust processing control system  94  is also communicatively coupled to the heated air motivator  88 . The heated air motivator  88  is configured to motivate the heated ambient air  36  to the ammonia injection system  38 , and may include a blower, fan, pump, compressor, or similar device. The heated air motivator  88  may create a pressure gradient between the ambient air heat exchanger  32  and its outlet  96 , which functions to draw the ambient air  30  into the ambient air heat exchanger  32 . Accordingly, the operation of the heated air motivator  88  may be controlled to affect a residence time of ambient air  30  in the ambient air heat exchanger  32 , which in turn affects a temperature and pressure of the heated ambient air  36 . 
     Additional features may be present upstream of the heated air motivator  88  to process the ambient air  30  and/or the heated ambient air  36 . For example, one or more filters, silencers, and so forth, may be positioned upstream of an inlet  100  of the ambient air heat exchanger  32 , within the ambient air heat exchanger  32 , or along the heated air flow path  84 , or any combination. 
     Again, the heated air motivator  88  directs the heated ambient air  36  to the ammonia injection system  38  for ammonia vaporization. More particularly, in the illustrated embodiment, the heated ambient air  36  is directed into an ammonia evaporator  102  of the ammonia injection system  38  through a heated air inlet  104 . The ammonia evaporator  102  also includes an ammonia inlet  106  configured to receive ammonia (e.g., aqueous ammonia  107 ) from an ammonia source  108 , and a vaporized ammonia outlet  110  fluidly coupled to the ammonia injection grid  72  by a vaporized ammonia flow path  111  (e.g., a vaporized ammonia conduit). 
     In accordance with present embodiments, vaporization of the aqueous ammonia  107  (ammonium hydroxide) generates the vaporized ammonia  40 . The aqueous ammonia  107  may be held in a storage vessel  110  configured to store the aqueous ammonia  107  under controlled conditions (e.g., closed to the ambient environment). The storage vessel  110  may include a tank or similar vessel that allows the aqueous ammonia  107  to be controllably withdrawn. 
     To allow for such control, the ammonia injection system  38  may include various flow control and motivation features positioned along an aqueous ammonia flow path  112  coupling an outlet  114  of the storage vessel  110  to the aqueous ammonia inlet  106  of the ammonia evaporator  102 . In the illustrated embodiment, an ammonia motivator  116  positioned along the aqueous ammonia flow path  112  is configured to create a pressure gradient between the ammonia source  108  and the ammonia evaporator  102 . The pressure gradient causes the aqueous ammonia  107  to be withdrawn from the storage vessel  110  and motivated toward the ammonia evaporator  102 . The ammonia motivator  116  may include a pump or similar feature capable of motivating a fluid having properties of the aqueous ammonia  107  in a suitable manner. As an example, the aqueous ammonia  107  held in the storage vessel  110  may include between about 15% and about 20% by volume or by weight ammonia (NH 3 ), with the remainder being water. In one embodiment, the aqueous ammonia  107  is a 19% by weight solution of ammonia in water. 
     An ammonia flow control unit  118  positioned along the aqueous ammonia flow path  112  may further adjust the flow of the aqueous ammonia  107 , for example by controllably restricting the size of the flow path  112  (e.g., controllably closing or opening an orifice). The ammonia flow control unit  118  may be positioned downstream of the ammonia motivator  116  as shown, or may be positioned upstream of it (between the ammonia source  108  and the ammonia motivator  116 ). 
     The exhaust processing control system  94  is shown as being communicatively coupled to the ammonia motivator  116  and the ammonia flow control unit  118 . In accordance with the illustrated embodiment, the exhaust processing control system  94  may control one or more operating parameters of the ammonia motivator  116  and/or the ammonia flow control unit  118  to control the amount of aqueous ammonia  107  provided to the ammonia evaporator  102  over time. 
     The ammonia evaporator  102  is schematically depicted as having an injection nozzle  120  fluidly coupled to the aqueous ammonia flow path  112 . The injection nozzle  120  may be configured to inject a spray of the aqueous ammonia  107  into the ammonia evaporator  102  to encourage atomization. The aqueous ammonia  107  is also brought into heat exchange with the heated ambient air  36 , which further encourages evaporation of the aqueous ammonia  107  to produce the vaporized ammonia  40 . The heat exchange between the aqueous ammonia  107  and the heated ambient air  36  may be through direct contact of their associated flows, or indirect by way of heat exchange features within the ammonia evaporator  102 . The vaporized ammonia  40  may be discharged as an overhead vapor through the vaporized ammonia outlet  110 . 
     In the illustrated embodiment, the vaporized ammonia  40  is provided to the ammonia injection grid  72  via the vaporized ammonia flow path  111 . The ammonia injection grid  72  includes a plurality of spray injectors  122  configured to introduce the vaporized ammonia  40  into the exhaust path  24 . The plurality of spray injectors  122 , as shown, may have the same axial position along the flow direction  78  but different radial positions with respect to the exhaust duct assembly  62 . 
     The amount of vaporized ammonia  40  introduced into the exhaust path  24  may be controlled by the exhaust processing control system  94  to achieve a particular objective. For example, the amount of vaporized ammonia  40  introduced into the exhaust path  24  may be controlled over time to achieve a desired amount of NO x  reduction within the exhaust gas  14  (e.g., maximum NO x  reduction, reduction of NO x  to a mandated level). By way of non-limiting example, the amount of vaporized ammonia  40  introduced into the exhaust path  24  may be determined or otherwise controlled by the exhaust processing control system  94  a function of various parameters, such as the amount of exhaust gas  14  flowing through the exhaust path  24 , a composition of the exhaust gas  14  (e.g., the level of NO x  in the exhaust gas  14 ), activity of the SCR catalyst  26 , and so forth. 
     The amount of vaporized ammonia  40  used for NO x  reduction may, in turn, determine how the exhaust processing control system  94  controls intake and heating of the ambient air  30  in the ambient air heat exchanger  32 . By way of non-limiting example, the exhaust processing control system  94  may control a temperature of the heated ambient air  36  and a flow rate of the heated ambient air  36  to respective levels that are appropriate to produce suitable amounts of the vaporized ammonia  40  (e.g., as a function of time, as a function of exhaust gas composition, or a combination). 
     The manner in which the exhaust processing control system  94  may monitor and control elements of the system  10  may be further appreciated with respect to  FIG. 3 , which is a schematic side view of an embodiment of the gas turbine system  10 . More specifically, the embodiment of the gas turbine system  10  includes one or more exhaust sensors  130  positioned along the exhaust duct  62  at various positions in the exhaust flow direction  72 . The one or more exhaust sensors  130  may be communicatively coupled to the exhaust processing control system  94  to enable monitoring of one or more parameters of the exhaust gas  14  as it flows through the exhaust duct  62 . By way of non-limiting example, the exhaust sensors  130  may enable monitoring of temperature, pressure, oxygen levels, NO x  levels, CO levels, and/or similar parameters. 
     In the illustrated embodiment, for example, the exhaust sensors  130  may be configured to monitor one or more parameters of the exhaust gas  14  upstream of the ammonia injection grid  72 , between the ammonia injection grid  72  and the plurality of heat pipes  74 , between the plurality of heat pipes  74  and the SCR catalyst  26 , and/or downstream of the SCR catalyst  26 . As a more specific example, the exhaust gas  14  temperature may be monitored upstream of the SCR catalyst  26  to enable the exhaust processing control system  94  to determine appropriate flows and temperatures for the vaporized ammonia  40 , the ambient air  30 , the heated ambient air  36 , and so forth. A first of the exhaust sensors  130  positioned upstream of the ammonia injection grid  72  may monitor a temperature of the exhaust gas  14  before mixing with the vaporized ammonia  40 , while a second of the exhaust sensors  130  positioned between the ammonia injection grid  72  and the plurality of heat pipes  74  may monitor a temperature of a mixture of the exhaust gas  14  and the vaporized ammonia  40 . Feedback relating to cooling of this mixture by the plurality of heat pipes  74  may be obtained by a third of the exhaust sensors  130  positioned between the plurality of heat pipes  74  and the SCR catalyst  26 . Additionally or alternatively, the exhaust gas composition (e.g., NO x  levels) of treated exhaust gas  132  downstream of the SCR catalyst  26  may be monitored to determine appropriate flow rates and temperatures for the vaporized ammonia  40 , the ambient air  30 , the heated ambient air  36 , and so forth. 
     The exhaust processing control system  94  is also communicatively coupled to features that enable the exhaust processing control system  94  to monitor and control such flows and temperatures. For instance, an ambient air sensor  134  may be a temperature sensor configured to enable the exhaust processing control system  94  to monitor a temperature of the ambient air  30 . Based at least on this information, the exhaust processing control system  94  may determine the extent to which the ambient air  30  should be heated within the ambient air heat exchanger  32 . This may be at least partially accomplished by controlling a flow rate of the ambient air  30  using one or more ambient air flow control devices  136  (e.g., including a fan and/or baffle) positioned upstream of the ambient air heat exchanger  32  via associated actuators  138  and/or using the flow control devices  88 ,  90  downstream of the ambient air heat exchanger  32 . 
     The ambient air sensor  134  is positioned upstream of the ambient air heat exchanger  32  (e.g., upstream along a flow path of a heat exchange fluid), which may provide feed forward information to the exhaust processing control system  94 . Indeed, the exhaust processing control system  94  may include one or more air flow control modules  140  (e.g., code implemented in software) configured to provide air flow control using the feed forward information and, additionally or alternatively, feedback information from a heated ambient air sensor  142  positioned downstream of the ambient air heat exchanger  32 . 
     The exhaust processing control system  94  may also be communicatively coupled to features of the ammonia injection system  38 , and may include one or more ammonia injection control modules  144  (e.g., code implemented in software) configured to provide control over operational aspects of the ammonia injection system  38 . For example, the one or more ammonia injection control modules  144  may control the rate at which the ammonia injection system  38  produces the vaporized ammonia  40 , a temperature of the vaporized ammonia  40 , or similar parameters. As an example, the control may be performed based on a target NO x  level for the treated exhaust gas  132 , which may be a feed forward input, as well as feedback obtained from the one or more exhaust sensors  130 , such as a fourth of the exhaust sensors positioned downstream of the SCR catalyst  26 . The feedback information may include, as one example, a measured level of NO x  within the treated exhaust gas  132 . 
     The exhaust processing control system  94  may monitor parameters relating to the ammonia injection system  38 , such as a temperature of the aqueous ammonia  107 , flow rates of the aqueous ammonia  107  through the ammonia injection system  38 , and so forth, via communication with one or more ammonia sensors  146 . The exhaust processing control system  94  may use feedback generated by the one or more ammonia sensors  146  as a control input for the overall control of the injection of vaporized ammonia  40  into the exhaust duct  62 . 
     Again, embodiments of the present disclosure may utilize one or more heat pipes  28  (e.g., the plurality of heat pipes  74 ) to cool the exhaust gas  14  within the exhaust duct  62 . In this regard, while the illustrated embodiments of  FIGS. 2 and 3  depict the heat pipe  28  (or plurality thereof) as being positioned between the ammonia injection grid  72  and the SCR catalyst  26 , the present disclosure is not necessarily limited to this configuration. Indeed, embodiments of the present disclosure may use one or more heat pipes  28  positioned at any point along the exhaust flow direction  78  upstream of the SCR catalyst  26 . Thus, certain embodiments of the gas turbine system  10  may include one or more heat pipes  28  positioned upstream of the ammonia injection grid  72 , either in addition to or as an alternative to one or more heat pipes  28  positioned between the ammonia injection grid  72  and the SCR catalyst  26 . 
     A non-limiting example embodiment of the thermal configuration the heat pipe  28  or plurality of heat pipes  74  is depicted in  FIG. 4 . More specifically, a cross-sectional elevation view of the heat pipe  28  is shown in  FIG. 4 . The heat pipe  28  includes an exterior casing  160  defining an outer surface of the heat pipe  28 . An absorbent wick  162  is disposed inside of the exterior casing  160  and surrounds a vapor cavity  164 . A working fluid  166  such as a metal (e.g., sodium), a hydrocarbon, ammonia, or water, is disposed in the vapor cavity  164 . The first portion  76  of the heat pipe  28  (the hot side or hot end) is disposed such that the exhaust gas  14  flows across the first portion  76 , while the second portion  80  (the cool side or cool end) is positioned in a heat exchange relationship with a heat exchange fluid along a flow path of the heat exchange fluid. As illustrated, the heat exchange fluid may include the ambient air  30  as shown in  FIGS. 2 and 3 , or may include water present within the aqueous ammonia  107 , which his described in further detail below with respect to  FIG. 5 . 
     At the first portion  76 , thermal energy from the exhaust gas  14  transfers to the heat pipe  28 , causing the working fluid  166  in the wick  162  at the first portion  76  to evaporate and migrate into the vapor cavity  164 . This evaporation may also cause some evaporative cooling of the first portion  76  to thereby additionally cool the exhaust gas  14  and produce a cooled exhaust gas  168 . 
     The vapor migrates to the second portion  80  along the vapor cavity  164 . The vapor condenses at the second portion  80  and is absorbed by the wick  162 , releasing the thermal energy to the heat exchange fluid in a heat exchanger (e.g., the ambient air heat exchanger  32  or the ammonia evaporator  102 ). The working fluid  166  migrates via the wick  162  to the first portion  76 . 
     Additionally or alternatively, one or more of the heat pipes  28  may have other configurations. By way of non-limiting example, one or more of the heat pipes  28  may be a solid state heat pipe in which thermal energy of the exhaust gas  14  is absorbed by a highly thermally conductive solid medium disposed within the casing  160 . In such embodiments, the temperature difference between the first and second portions  76 ,  80  may cause thermal energy migration to enable the heat pipe  28  to heat the ambient air  30  or directly vaporize the ammonia. 
     As set forth above, in addition to or in lieu of heating air, the heat pipes  28  may be configured to directly heat and vaporize the aqueous ammonia  107 .  FIG. 5  is a schematic elevational view of an example embodiment having this configuration. In the illustrated embodiment, the heat pipe  28  or the plurality of heat pipes  74  have their respective second portions  80  positioned in the ammonia evaporator  102 . In this embodiment, the heat exchange fluid that is heated to effect ammonia vaporization may include water within the aqueous ammonia  107 . Similar to the embodiment in  FIG. 2 , the exhaust processing control system  94  may control the flow of the aqueous ammonia  107  to the ammonia evaporator  102  using one or more ammonia flow control devices such as the ammonia motivator  116  and/or the ammonia flow control unit  118  and associated actuators  180 . 
     While the ammonia motivator  116  and/or the ammonia flow control unit  118  may be positioned upstream of the ammonia evaporator  102 , one or more evaporated ammonia flow control devices  182  and associated actuators  184  may be positioned along the evaporated ammonia flow path  111  downstream of the ammonia evaporator  102 . The exhaust processing control system  94  may be in communication with the one or more evaporated ammonia flow control devices  182  and associated actuators  184  to enable additional control of evaporated ammonia injection via the ammonia injection grid  72 . The exhaust processing control system  94  of  FIG. 5  may have substantially the same configuration as set forth above with respect to  FIG. 3 , but may adjust ammonia flow as the primary and/or sole control parameter in response to feedback from the exhaust sensors  130 , the ammonia sensors  146 , and so forth. 
     Additional or alternative configurations for the system  10  of  FIG. 5  are also possible. For example, rather than causing direct and total vaporization of the aqueous ammonia  107 , the heat pipe  28  may be used to pre-heat the aqueous ammonia  107  to reduce reliance on other sources of heat. For example, the heat pipe  28  may be used to pre-heat the aqueous ammonia  107  to enable easier ammonia evaporation using heated ambient air generated by electric air heating of ambient air. This may reduce reliance on electrical energy to drive electric heaters while enabling tunable control of the final evaporated ammonia temperature using additional control mechanisms (e.g., electric heaters). 
     Indeed, any of the embodiments described herein may be used in lieu of or in addition to other independent heat exchange fluid flow paths, which may be independent and parallel to the flow paths described herein. As a more specific example, certain embodiments, such as the embodiment of  FIG. 3 , may also utilize an additional independent and parallel flow path for the ambient air  30  that flows the ambient air  30  over electric heaters to enable the use of an additional temperature control mechanism for ammonia evaporation. 
     Technical effects of the invention include the heat integration of exhaust gas release from a gas turbine engine with ammonia evaporation in an exhaust processing system that reduces NO x  in the exhaust gas. The heat integration may be accomplished using one or more heat pipes, where the one or more heat pipes are transfer thermal energy from the gas turbine flue gas (the exhaust gas) and to a heat exchange medium that is ultimately used for ammonia vaporization. This may reduce reliance on other forms of energy that would otherwise be required for ammonia evaporation, thereby enhancing efficiency of the exhaust gas treatment process. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.