Abstract:
A method for injecting a reductant into an exhaust gas stream of a combustion turbine engine for selective catalytic reduction. The method may include the steps of: directing the exhaust gas stream through an exhaust duct; receiving the directed exhaust gas for treatment by a catalyst positioned within the exhaust duct; providing a reductant in a liquid state; pressurizing and heating the reductant in a manner that maintains the reductant in the liquid state; and injecting the heated, pressurized reductant into the exhaust gas stream such that the reductant flash vaporizes upon injection due to a pressure differential.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present application relates generally to treatment of emissions in an exhaust path of a combustion system and, more specifically, to methods and systems for more efficiently reducing pollutant levels, such as nitrogen oxides (or “NO x ”), from combustion exhaust in power plants. 
         [0002]    Governmental agencies continue to tighten regulation of exhaust emissions from power plants. To satisfy these stricter standards, the after-treatment of combustion exhaust is being relied on to a greater extent. As will be appreciated, the reduction of NO x  from such emissions is of particular concern. Selective catalytic reduction (or “SCR”) of NO x  by nitrogen compounds has proven to be effective in industrial applications. More specifically, power plants, such as industrial gas turbines, steam turbines, and combined-cycle plants, may have exhaust systems that include a SCR system or system for removing NO x  from exhaust gases. In such cases, a reductant, such as ammonia or urea (or “NH 3 ”), is injected into the exhaust gas stream as it moves through the SCR system to remove NO x  from the exhaust gas. One problem associated with SCR systems is so-called “ammonia slip” in which ammonia passes through the SCR system without reacting and exits the exhaust system with the exhaust gases. As will be appreciated, ammonia that fails to react as expected can lead to higher levels of NO x  emissions. To avoid such issues, it is important that the reductant and exhaust gases are well-mixed such that the distribution of the reductant within the exhaust is substantially uniform through the exhaust flowpath. 
         [0003]    Conventional SCR systems include shortcomings that impair efficient performance in a number of ways, which, for example, may lead to issues such as ammonia slip. Conventional systems often are not cost-effective in that their design incurs unnecessary operational and equipment costs. In one type of conventional system, for example, aqueous or anhydrous NH 3  is vaporized prior to injection. In such cases, stored liquid NH 3  is vaporized external to the ducting of the HRSG and then injected into the flowpath as a gas. This is done because injection of the NH 3  as a gas promotes mixing so that the issue of ammonia slip is lessoned. However, as will be appreciated, this process requires overly-complex equipment and energy to operate, which may negatively impact operating costs and overall system efficiencies. Other conventional systems inject liquid NH 3  in the exhaust flowpath. In such cases, though, the adequate mixing of the ammonia in the exhaust gas becomes an issue. The time required for the liquid NH 3  to vaporize after being injected as a liquid so to fully disperse and attain a desired level of mixing inside an exhaust duct may result in increased duct length, which negatively impacts the size of the system and adds equipment and other costs as well. Mixing devices may be used to improve the mixing of the injected liquid ammonia with the exhaust gases so to reduce the required duct length, but this also leads to additional equipment that is typically expensive to install, operate, and maintain. 
         [0004]    In total, the manner in which reductants are injected by SCR systems remains a significant area for improvement in the technological field of after-treatment of combustion exhaust. Systems and methods that provide cost-effective injection while also promoting or improving mixing of the reductant and reduces such issues as ammonia slip would be commercially demanded. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The present application thus describes a method for injecting a reductant into an exhaust gas stream of a combustion turbine engine for selective catalytic reduction. The method may include the steps of: directing the exhaust gas stream through an exhaust duct; receiving the directed exhaust gas for treatment by a catalyst positioned within the exhaust duct; providing a reductant in a liquid state; pressurizing and heating the reductant in a manner that maintains the reductant in the liquid state; and injecting the heated, pressurized reductant into the exhaust gas stream such that the reductant flash vaporizes upon injection due to a pressure differential. 
         [0006]    The present application may further describe a treatment system for an exhaust gas stream from a combustion turbine system. The treatment system may include: an exhaust duct for directing the exhaust gas stream; a catalyst positioned within the exhaust duct for receiving the exhaust gas stream flowing therethrough; and an injection system for injecting reductant in the exhaust gas stream. The injection system may include: a reductant supply line for supplying the reductant; a nozzle disposed within the exhaust duct that connects to a downstream end of the reductant supply line; a pump coupled with the reductant supply line for pressurizing the reductant; a heater coupled with the reductant supply line for heating the reductant; and a flow controller for maintaining the reductant in the reductant supply line within predetermined temperature and pressure values such that: i) the reductant remains in a liquid state while moving through the reductant supply line; and ii) the reductant flash vaporizes upon injection into the exhaust gas stream due to a pressure differential between the reductant just prior to injection and the exhaust gas stream into which the reductant is injected. 
         [0007]    These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other features of this invention will be more completely understood and appreciated by careful study of the following more detailed description of exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a schematic illustration of an exemplary gas turbine system; 
           [0010]      FIG. 2  illustrates a simplified exemplary arrangement of a combined-cycle power plant with a heat recovery steam generator; 
           [0011]      FIG. 3  illustrates an exemplary internal arrangement of heat transfer equipment within a heat recovery steam generator within which embodiments of the present invention may be used; 
           [0012]      FIG. 4  schematically illustrates an injection system within a heat recovery steam generator according to an exemplary embodiment of the present invention; 
           [0013]      FIG. 5  schematically illustrates an injection system within a heat recovery steam generator according to an alternative embodiment of the present invention; and 
           [0014]      FIG. 6  schematically illustrates an injection system within a heat recovery steam generator according to an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
         [0016]    Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical designations to refer to features in the drawings. Like or similar designations in the drawings and description may be used to refer to like or similar parts of embodiments of the invention. As will be appreciated, each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. It is to be understood that the ranges and limits mentioned herein include sub-ranges located within the prescribed limits, inclusive of the limits themselves unless otherwise stated. 
         [0017]    Additionally, certain terms have been selected to describe the present invention and its component subsystems and parts. To the extent possible, these terms have been chosen based on the terminology common to the technology field. Still, it will be appreciate that such terms often are subject to differing interpretations. For example, what may be referred to herein as a single component, may be referenced elsewhere as consisting of multiple components, or, what may be referenced herein as including multiple components, may be referred to elsewhere as being a single component. In understanding the scope of the present invention, attention should not only be paid to the particular terminology used, but also to the accompanying description and context, as well as the structure, configuration, function, and/or usage of the component being referenced and described, including the manner in which the term relates to the several figures, as well as, of course, the precise usage of the terminology in the appended claims. Further, while the following examples are presented in relation to a certain type of gas turbine or combined-cycle system, the technology of the present invention also may be applicable to other types of turbine engines as would the understood by a person of ordinary skill in the relevant technological arts. 
         [0018]    By way of background, referring now to the figures,  FIG. 1  is a schematic illustration of an exemplary gas turbine system  110  including an intake section  112 , a compressor section  114  coupled downstream from the intake section  112 , a combustor section  116  coupled downstream from the compressor section  114 , a turbine section  118  coupled downstream from the combustor section  116 . At the aft end of the system  110 , an exhaust section  120  may be operably couple to the turbine section  18 . The turbine section  118  may be rotatably coupled to the compressor section  114  and to a load  122 , such as, but not limited to, an electrical generator or mechanical drive application. During exemplary operation, the intake section  112  channels air towards compressor section  114 . The compressor section  114  compresses the inlet air to higher pressures and temperatures. The compressed air is discharged towards to the combustor section  116  wherein it is mixed with fuel and ignited to generate combustion gases that flow to the turbine section  118  and induces rotation therein, which then drives compressor section  114  and load  122 . Exhaust gases exit the turbine section  118  and flow through the exhaust section  120 . The exhaust gases may be released to ambient atmosphere, or, as discussed below, may be fed to a heat recovery steam generator and the heat recovered therefrom used to produce steam that drives a steam turbine. 
         [0019]    During the combustion of natural gas and liquid fuels, pollutants such as, but not limited to, carbon monoxide (or “CO”), unburned hydrocarbons (or “UHC”), and nitrogen oxides (or “NO x ”) emissions may be formed and emitted into the atmosphere. As will be appreciated, CO and UHC are generally formed during combustion conditions with lower temperatures and/or conditions with an insufficient time to complete a reaction. In contrast, NO x  is generally formed under higher temperatures. At least some known pollutant emission sources include devices such as, but not limited to, industrial boilers and furnaces, larger utility boilers and furnaces, reciprocating engines, gas turbine engines, steam generators, and other combustion systems. 
         [0020]    Modern air quality regulations mandate continuingly reduced emission levels for power generating plants, while at the same time fuel efficiency requirements continue to increase. Due to stringent emission control standards, it is desirable to control NO x  emissions by suppressing the formation of it. As will be understood, nitrous oxides include NO and NO 2 , with NO 2  being known as a pollutant that produces a visible yellow plume from exhaust stacks and further creates “acid rain”. Combustion controls alone may prove inadequate to satisfy these often-conflicting performance objectives, and thus continued the improvement of post-combustion exhaust gas treatment systems is desired. 
         [0021]    As will be appreciated, one technology for the control NO x  that is widely used at large power generating stations is selective catalytic reduction (or “SCR”). The flue gases from such power generating stations have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. Because of thus, the nitrogen oxides that are present in the exhaust gases can be reduced to nitrogen and water only with difficulty. One way to address this problem is through selective catalytic reduction. With this process, the combustion exhaust of the power generating station are mixed with anhydrous or aqueous ammonia and then is passed over a suitable reduction catalyst at prescribed temperatures prior to release into the atmosphere. The ammonia is not a natural part of the combustion exhaust stream, but rather, it is injected into the exhaust stream upstream of the catalyst element for the specific purpose of supporting reduction reactions. 
         [0022]      FIG. 2  provides a simplified exemplary illustration of a combined-cycle power plant  200 . As will be appreciated, the combined-cycle power plant  200  may include a gas turbine  201 . Within the gas turbine  201 , air  210  is received in an air intake  215  of a compressor  220  to provide compressed air for mixing with fuel  225  in a combustors  230  to supply hot gases to a turbine  235  for driving a shaft connected to generator  240  for producing electricity output  245 . Exhaust gases  250  are discharged into an exhaust duct  255 , through a heat recovery steam generator  260  and out through stack  265  to atmosphere. The HRSG  260  includes heat exchangers  262  configured to extract heat from the exhaust gases  250  as well as emissions treatment equipment  264  for controlling emissions. The heat extracted from the exhaust gases is used to generate steam  280 . The steam  280  is supplied to a steam turbine  282  to drive shaft  290  of generator  292  for producing electricity. The steam  280  then passes to a condenser  284  where cooling water  286  passing through tube bundles condenses the steam to water  288 . The water  288  is then returned to the HRSG for completion of a closed cycle. An operating HRSG may include multiple heat exchangers and evaporators, steam systems and water systems for producing steam at different pressures and temperatures in many different configurations. Similarly, the emissions treatment equipment may include multiple treatment elements within the HRSG, which may be adapted to address different pollutants in different ways. 
         [0023]      FIG. 3  illustrates an internal arrangement of heat transfer equipment within an exemplary HRSG  300 . As illustrated, the HRSG  300  may include a conduit or duct  302  for receiving exhaust output or gases  304  from a combustion turbine engine. The HRSG, for example, may be adapted for generating steam at three pressures in a high pressure drum  305 , an intermediate pressure drum  310 , and a low pressure drum  315 , though other configurations are also possible. The steam generated by the HRSG  300  may then be supplied to a high pressure steam turbine, an intermediate pressure steam turbine and a low pressure steam turbine (turbines not shown). The HRSG  300  may include a plurality of superheater heat exchangers  320 , reheater heat exchangers  325 , and economizer heat exchangers  330 . In an exemplary case, the HRSG also may include a high pressure evaporator  340 , an intermediate pressure evaporator  345 , and a low pressure evaporator  350  adapted for producing steam for the associated high pressure drum  305 , intermediate pressure drum  310 , and low pressure drum  315 . The HRSG  300  also may include a duct burner  360  for supplying heat to exhaust gases  304  in order to enhance steam production output. Each of the above-described heat exchangers and evaporators removes heat and lowers temperature for the exhaust gases, while the duct burners add heat and increase temperature. As discussed in more detail below, emissions treatment equipment may be placed in the exhaust gas flow  304  among the heat exchangers, evaporators and burners to advantageously locate oxidation catalyst for reduced NOx production and discharge out exhaust stack  370 . As will be appreciated, HRSGs for use with gas turbine and other combustion systems may include other numbers and arrangements of evaporators and heat exchangers suited for the particular application. 
         [0024]      FIG. 4  illustrates an injection system  400  according to an exemplary embodiment of the present invention. As will be appreciated, the injection system  400  may be used to inject a reductant, such as ammonia, into the exhaust duct. The injection system  400  may serve an after-treatment system for treating exhaust gases produced by a combustion turbine engine, such as, for example, the exhaust from a gas turbine in a combined-cycle power plant having a HRSG, as described above in relation to  FIG. 2 . As will be appreciated, the present invention also may be used within an exhaust duct of a gas turbine system that does not include a HRSG. As will be provided herein, advantages of the injection system  400  include increased NO x  conversion and, therefore, decreased NO x  emissions from a power plant. As will be appreciated, due to the increased efficiency of the injection system described herein, advantages may further include a reduction in catalyst volume, which may lower operating costs. Additionally, the improved efficiency may result in a reduction in both the amount of injected ammonia and the surface area of the SCR catalyst. This reduction may decrease the pressure drop in the exhaust gas flow, which would result in greater output power from the gas turbine at a given level of fuel consumption. 
         [0025]    According to exemplary embodiments, the injection system  400  may be positioned in a conduit or exhaust duct  402 . For example, the exhaust duct  402  may serve as the exhaust conduit of a HRSG that is configured to receive the exhaust output of a gas turbine engine. In the direction of flow, the exhaust  404  may first pass through a pre-oxidation catalyst  406 . As will be understood, the pre-oxidation catalyst may treat unburned hydrocarbons and convert NO to NO 2 . This may be done due to the fact that NO 2  reacts more readily with NH 3 . Downstream from the pre-oxidation catalyst  406 , a SCR catalyst  408  may be located. The SCR catalyst may be, for example, platinum, vanadium, or zeolite, though other types are also possible. As will be appreciated, the SCR catalyst promotes the reaction of NH 3  with NO x  to form nitrogen and water, thereby reducing NO x  in the exhaust flow. According to an alternative embodiment, a hydrolysis catalyst  409  may be included. As illustrated, the hydrolysis catalyst  409  may be located upstream of the SCR catalyst  408 . The hydrolysis catalyst  409  may promote the reaction of urea with water to form ammonia and carbon dioxide (or “CO 2 ”), thereby helping to assure the availability of ammonia in the exhaust stream prior to entering the SCR catalyst  408 . According to another alternative, an oxidation catalyst  410  may be located downstream of the SCR catalyst  408 . As will be appreciated, the oxidation catalyst  410  promotes the breakdown of excess ammonia that does not react in the SCR catalyst. In other words, the oxidation catalyst  410  promotes the oxidation of excess ammonia, thereby limiting the release of ammonia from the SCR system. 
         [0026]    The injection system  400 , as illustrated, may further include a supply line  412  for delivering NH 3  for injection into the flow through exhaust duct  402 . The supply line  412  may connect to a local storage tank or other supply source. According to exemplary embodiments, the NH 3  in the supply line  412  is an aqueous NH 3  solution. The aqueous NH 3  solution, for example, may be between 25% and 40% NH 3  to water. According to certain preferred embodiments, the aqueous NH 3  solution is approximately 33% NH 3  to water, as this solution of aqueous NH 3  has the lowest freezing point and therefore is least likely to freeze during seasonal colder temperatures. The usage of anhydrous NH 3  is also possible. 
         [0027]    The supply line  412 , as illustrated, may be configured to direct the supply of liquid aqueous NH 3  solution to a nozzle  414  disposed within the duct  402  for injection into and mixing with the exhaust gases  404  moving therethrough. As indicated, the nozzle  414  may include multiple outlet ports. To promote an even application and mixing, the outlet ports of the nozzle  414  may be spaced about a cross-section of the duct  402  in an array. The nozzle  414  may be located upstream of the SCR catalyst  408 , the hydrolysis catalyst  409  (if present), and the oxidation catalyst  410  (if present). The nozzle  414  may be located downstream of the pre-oxidation catalyst  406  (if present). 
         [0028]    According to further aspects of the present invention, the injection system  400  may include a pressurizer  415 , a heater  418 , and/or a flow controller  420 . These components, as illustrated, may be connected in series along the supply line  412 . The pressurizer  415  may include a conventional high-pressure pump configured to pressurize the supply of aqueous NH 3  moving through the supply line  412 . Though other arrangements are possible, the heater  418  may be positioned on the supply line  412  downstream of the pressurizer  416 . The heater  418  may include any type of conventional heater or heat source, and may be configured to increase the temperature of the pressurized aqueous NH 3  moving through the supply line  412 . The temperature to which the pressurized aqueous NH 3  is heated may be one at which it remains liquid due to the pressurization. Downstream of the heater  418 , the flow controller  420  may control the flow of the aqueous NH 3  solution to the nozzles  414  and, in turn, the amount released therefrom. The flow controller  420 , for example, may include a valve, such as a solenoid valve, and may variably control the amount of liquid aqueous NH 3  solution flowing to the nozzles  414  for release within the exhaust duct  402 . According to an alternative embodiment, the function of the pressurizer  416  and flow controller  420  may be combined via the use of a variable speed pump. In such cases, the heater  418  may be positioned downstream of the variable speed pump, though other configurations are also possible. Pursuant to conventional systems and methods, a control unit  422  may be provided to control the operation of the pressurizer  416 , the heater  418 , and the flow controller  420 . The control unit  422  may receive engine operation data and information related to the combined-cycle power plant to aid in determining the timing and quantity of aqueous NH 3  released into the exhaust duct  402 . 
         [0029]    The release of pressurized, heated aqueous NH 3  into the exhaust  404  through the nozzle  414  may be controlled so to causes the aqueous NH 3  to rapidly turn to vapor or “flash vaporizes” upon release from the outlet ports. As will be appreciated, this rapid vaporization is due to the sharp drop in pressure between the pressure maintained in the supply line  412  just prior to injection and the pressure level within the exhaust duct  402 . More specifically, the aqueous NH 3  flashes to vapor due to the fact that it is pressurized and superheated within the supply line  412  prior to release. As will be understood, the boiling point of aqueous NH 3  rises with the increasing pressure. According to exemplary operation of the injection system  400 , the aqueous NH 3  in the supply line  412  may be pressurized to between approximately 50 to 100 psi. In contrast, the exhaust  404  with the duct  402  is typically near atmospheric pressure (approximately 14.6 psi). As will be understood, at such elevated pressures, the aqueous NH 3  may be heated to much higher temperatures without boiling. Therefore, after pressurizing the aqueous NH 3 , it may be heated by the heater  418  to an elevated temperature level that is selected as one being close to but below the boiling point of aqueous NH 3  at the elevated pressure. Plus, according to exemplary embodiments, the ammonia reductant is heated and pressurized so that it remains in a liquid state while moving through the reductant supply line and then flash vaporizes upon injection into the exhaust gas stream due to it being superheated and the pressure differential between the reductant just before injection and the exhaust gas stream into which the reductant is injected. Because of the pressurization, thus, the NH 3  may be heated without it vaporizing or resulting in a two-phase flow in the supply line  412 . As will be appreciated, two-phase flow in the supply line  412  may be undesirable because of the negative impact it has over controlling the amount injected. The flashing of the aqueous NH 3  may result in the NH 3  and water being quickly evaporated and, thus, effectively and evenly mixed with the exhaust gases over the cross-section of the duct  402 . 
         [0030]    As will be appreciated, the flash vaporization may atomize any portion of the reductant that remains in a liquid state, which results in the efficient mixing of substantially all of the reductant supply injected into the exhaust duct. That is, when the pressurized, heated aqueous NH 3  solution is released into the exhaust  404  through the nozzle  414 , the liquid NH 3  rapidly drops in pressure because of the typically low pressure maintained within the exhaust duct  402 . 
         [0031]    As stated, at the elevated temperature to which the reductant is heated, the sudden drop in pressure causes the aqueous NH 3  to almost instantaneously reach a pressure at which it boils. Any portion of the reductant that remains in a liquid state is broken up or atomized into sub-micron size droplets by this flash vaporization. The rapid expansion and resulting sub-micron size of the aqueous NH 3  droplets enables the highly effective mixing of the aqueous NH 3  with exhaust gases. Additionally, the small size of the ammonia droplets may result in their rapid evaporation, which also promotes uniform mixing with the exhaust gas stream. According to exemplary embodiments, the outlet ports of the nozzle  414  are sized and configured so that the sudden pressure drop that causes flash vaporization is achieved across the nozzle  414 . 
         [0032]    Additionally, as will be appreciated, the aqueous NH 3  loses little heat during the flash vaporization, which results in the atomized or vaporize NH 3  remaining at a relatively elevated temperature. Since the temperature of the aqueous NH 3  is higher than the saturation vapor pressure, the aqueous NH 3  resists condensation even when injected into an exhaust  404  that is at a much lower temperature than the aqueous NH 3 . Further, as the temperature of the aqueous NH 3  is increased, the hydrolysis of aqueous NH 3  becomes more efficient, which improves the formation of ammonia needed for the selective catalytic reduction of NO R . This may reduce the necessary volume for the hydrolysis catalyst. Also, given the functioning of the system, the NH 3  resulting from the hydrolysis of urea is at an elevated temperature and is well-mixed with the exhaust gases in the exhaust  404 , leading to more effective and complete reaction of the ammonia with NO R . As will be appreciated, this both increases NO x  conversion efficiency and reduces the amount of ammonia that is left unreacted after passing through the SCR catalyst, which may reduce ammonia slip and the volume size requirements for the oxidation catalyst. Further, since the ammonia is better utilized, less aqueous NH 3  may need to be consumed in order to achieve acceptable levels of NO x  conversion. The present invention, thus, improves NO x  conversion efficiency in ammonia-SCR after-treatment systems. The efficiencies achieved, as described herein, may make the operation of the system more cost-effective. 
         [0033]      FIGS. 5 and 6  provide alternative embodiments of injection systems  500 ,  600  in accordance with certain other aspects of the present invention. As illustrated in relation to the injection system  500  of  FIG. 5 , a heater  418  includes one that is configured to use the exhaust of the gas turbine as a heat source. This may be achieved via conventional heat exchangers and the like. According to the exemplary embodiment illustrated, the supply line  412  may be configured to extend through a section of the exhaust duct  402 . As will be appreciated, other heat exchange configurations between the reductant in the supply line  412  and the combustion exhaust are also possible. According to another arrangement, as illustrated in  FIG. 6  and with reference to injection system  600 , the heater  418  is positioned downstream of the pressurizer and flow controller, the function of which has been combined via the use of a variable speed pump  426 . In this example, the heater  418  may be configured in a section of the supply line  412  that is just upstream of the nozzle  414  that is configured to absorb heat from the exhaust  404  within the flow duct  402 . As will be appreciated, this arrangement may be used to further simplify the injection system. 
         [0034]    As will be appreciated, according to certain embodiments, the present invention may enable system efficiencies and cost savings, while also promoting effective NO x  conversion. As discussed, conventional systems for injecting ammonia as a liquid or a vapor have certain advantages as well as inherent shortcomings. For example, injecting ammonia as a liquid has the advantage over vapor due to the efficiencies related to handling and delivering a liquid. Specifically, because it is so much more dense, liquid ammonia may be delivered to the injectors much more cost-effectively, whereas the delivery of vapor at the same mass flow rate would requires a much greater volume and, therefore, greater equipment costs. However, when ammonia is injected into the exhaust flow stream as a liquid, it must first absorb sufficient heat to vaporize before it may be used efficiently in the SCR process. As will be appreciated, this vaporization of the ammonia liquid within the exhaust duct takes time, which necessarily adds to the length of the duct, which increases equipment costs significantly. Special mixing equipment may be used to hasten the vaporization, but such equipment also adds significant costs. As provided above, according to certain aspects present application, the inefficiencies associated with both liquid and vapor may be overcome by adding the vaporization heat to liquid ammonia external to the duct, but maintaining it as a liquid via pressurization, thereby creating superheated ammonia liquid for injecting. The superheated liquid ammonia may be heated to a sufficient temperature such that vaporization, partially or completely, occurs almost instantaneously upon injection into the duct. With this approach, as will be appreciated, the delivery efficiencies associated with liquid ammonia may be realized, while also taking advantage of the mixing efficiencies associated with vapor ammonia once inside the exhaust duct. 
         [0035]    As one of ordinary skill in the art will appreciate, the many varying features and configurations described above in relation to the several exemplary embodiments may be further selectively applied to form the other possible embodiments of the present invention. For the sake of brevity and taking into account the abilities of one of ordinary skill in the art, all of the possible iterations is not provided or discussed in detail, though all combinations and possible embodiments embraced by the several claims below or otherwise are intended to be part of the instant application. In addition, from the above description of several exemplary embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are also intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.