Abstract:
A method for reducing the amount of carbon monoxide and oxygen emissions in an oxyfuel hydrocarbon combustion system, comprising the steps of feeding defined amounts of hydrocarbon fuel and an oxidizer (e.g., air) to one or more combustors in the engine and igniting the mixture to form a first combustor exhaust stream; determining the amount of carbon monoxide present at the head end of a combustor in the initial combustor exhaust stream; identifying one or more target locations within the combustor at a point downstream from the first exhaust stream for injecting free hydrogen and a supplemental oxidizer; injecting hydrogen and the supplemental oxidizer into the combustor at specified downstream locations based on the amount of detected carbon monoxide; and injecting a diluent (e.g., CO 2 ) into the combustor at a point further downstream in the combustor exhaust to control the exhaust temperature.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to gas and liquid fuel turbine engines used in power generation plants and, in particular, to an improved combustor design for use in industrial gas turbine engines that provides reduced carbon dioxide and oxygen emissions using hydrogen fuel staging and a diluent feed in order to control the formation of OH radicals and co in the combustion exhaust. 
         [0002]    Gas turbine engines typically include a compressor section, a combustor section and at least one turbine that rotates to generate electrical power. The compressor discharges directly into the combustor section where hydrocarbon fuel is injected, mixed and burned to produce thermal and kinetic energy. The combustion gases are channeled into and through one or more stages of the turbine which extracts rotational energy from the exhaust gases, with the rotational energy then being used to generate electrical power. 
         [0003]    In order to achieve maximum operating efficiencies, gas turbine combustion systems, particularly those employing “multiple can” combustor designs, operate over a wide range of different fuel compositions, pressures, temperatures and fuel/air ratio conditions, preferably with the ability to use either liquid or gas fuels or a combination of fuels (known as “dual fire” systems). Many candidate hydrocarbon fuels used in gas turbine combustors contain unwanted contaminants and/or byproducts of other processes that tend to inhibit combustion and/or reduce the capacity and efficiency of the system. In addition, many existing gas turbine systems tend to operate in a slightly rich mode, i.e., with slightly less than the precise stoichiometric amount of oxygen in the feed necessary to burn the entire hydrocarbon fuel mixture, resulting in at least some incomplete combustion of the fuel and hence a certain amount of carbon monoxide and oxygen necessarily being present in the exhaust gases. Thus, in a conventional gas turbine system, the products of combustion invariably contain CO and small amounts of excess oxygen resulting from the equilibrium chemistry at the nominal temperatures of combustion with even some of the CO 2  products of combustion disassociating into CO and O 2 . 
         [0004]    In addition, perfect mixing of fuel and air cannot be achieved in most gas turbine engines, resulting in some of the oxygen present in the feed not reacting with the hydrocarbon fuel. As a result, the resulting exhaust stream from most gas turbine combustors, particularly multiple can combustors, comprise mostly N 2  and CO 2  but by design still contain residual amounts of CO and O 2  which eventually must be removed or reacted in downstream operations in order to maintain desired overall gas turbine efficiencies. 
         [0005]    One known solution to removing excess CO and O 2  from the exhaust streams of conventional gas turbine engines utilizes a catalyst system in which the products of combustion pass through the turbine section of the engine and then a heat recovery steam generator (“HRSG”) with a portion of the exhaust stream being treated by an oxidation catalyst which at the lower temperatures in the HRSG alters the equilibrium between CO and O 2  and CO 2  and forces trace amounts of CO and O 2  to convert to CO 2 , resulting in a more environmentally acceptable exhaust stream, i.e., with CO 2  and a lower oxygen content. In order for the catalyst to approach zero oxygen and CO content, however, a sufficient amount of CO must be present to consume all the O 2  in the exhaust. 
         [0006]    Given the practicality of most gas turbine operations (i.e., a slightly rich combustion environment in each combustor can), excess CO in the exhaust is known to have a negative effect on overall engine operation because the energy content of the CO will not be fully captured, eventually resulting in an efficiency penalty to the system. As a result, it has become important in the gas turbine industry to minimize and carefully control the amount of excess CO in the exhaust stream resulting from combustion in each “can” and to monitor and control the CO content leaving a bank of combustors. 
         [0007]    Two additional practicalities in the gas turbine art are known to drive the need to control the amount of CO formed in the turbine exhaust. First, the fuel to air ratio of the gas turbine engine is subject to finite control limits since only a limited ability exists to control the exact amount of fuel being fed to each combustor. Second, individual combustors may operate with a slightly different fuel to air ratio, resulting in slight variations in the emissions on a can-to-can basis. Those additional basic design features of combustors have contributed to the need to operate gas turbine engines using a slight amount of excess fuel and to create the right amount of CO such that all (or virtually all) of the oxygen still present in the exhaust eventually is consumed and/or eliminated using catalyst treatment. In the past, it has been difficult to control the exact amount of CO produced during the initial combustion, particularly on a can-to-can basis. 
         [0008]    Thus, a need exists for a combustion system that relies on a slightly rich, but sufficiently lean mixture of hydrocarbon fuel and air to yield low emissions of air pollutants, particularly CO, when operating an advanced, high efficiency gas turbine engine. A need also exists to carefully predict and control the amount of CO formed during the initial combustion in each can in order to maximize the operating efficiency of the entire engine, including any downstream catalyst treatment operations. 
         [0009]    The present invention addresses the above concerns through a new method for reducing and controlling the CO and oxygen emissions in an “oxyfuel” type gas turbine combustor using a hydrocarbon dominant fuel (such as natural gas or even “dual fire” fuel systems). As in conventional systems, hydrocarbon fuel is burned in each combustor using an oxidizer (e.g., air and/or an oxygen-rich feed). It has been found that the entire combustion dynamics can be improved by injecting additional, interim amounts of one or more auxiliary feed streams described below. The additional interim injections result in the formation of a controlled amount of an OH radical, which in turn helps to reduce the amount of CO and free O 2  formed during initial combustion. That is, the OH radical resulting from the injections combines with CO to produce additional CO 2 , thereby reducing the amount of unreacted CO and oxygen before the exhaust stream leaves the combustor. In the end, the removal of CO and O, in the exhaust improves the overall efficiency of the entire gas turbine engine. 
         [0010]    As noted above, the invention offers particular advantages in “rich” combustion systems, i.e., having a Phi value greater than 1.0 (and preferably between 1.0 and 1.1). A rich combustion typically leaves very little oxygen in the exhaust and creates an exhaust environment susceptible to the formation of the OH radical (a “chain breaking” type reaction). The resulting mixture created at that location near the combustor head tends to increase the OH radical density, which in turn reduces the amount of CO by virtue of a subsequent reaction to form CO 2 . Specifically, by injecting a small amount of free hydrogen, followed by an oxidizer element and then a diluent stream (e.g., nitrogen), the OH radical formed from the hydrogen injection reacts with CO to form carbon dioxide, which can then be more easily and efficiently removed by conventional means. 
         [0011]    The present invention also ensures that other unwanted emissions (particularly NO x ) do not exceed expected and/or mandated EPA exhaust limitations. That is, the invention provides a combustion system using a premixed combustion feed which would be characterized as slightly rich, but with a fuel/air mixture lean enough to ensure that thermal NOx formation in the zone remains negligible. The addition of hydrogen and oxidizer feeds followed by a diluent coolant are also sufficient to meet the inlet temperature demands of the gas turbine. The invention thus has particular advantages in applications where the inlet temperature demands of the turbines are so high as to preclude the possibility of achieving low thermal NOx emissions levels by lean premixed combustion alone. 
         [0012]    These and other advantages of the invention are achieved by providing a combustor for a gas turbine that includes the following basic features as described in greater detail in connection with  FIGS. 1-3 . At specified times and locations within a combustor, hydrogen gas is injected into the head of the combustor at the pre-defined target locations for a prescribed period of time and in a fixed amount, followed by injection of an oxidizer component, again in specified amounts for fixed durations. The exact amount of injected hydrogen and oxidizer will vary, depending on the amount of OH radical being formed, which in turn controls the eventual formation of CO 2 . The free hydrogen can be supplied directly or as a product of known reforming techniques away from the gas turbine engine. Once the equilibrium of the combustion products is accomplished using additional hydrogen and the oxidizer, a specific amount of diluent (including carbon dioxide) is injected into the combustor (or alternatively slightly downstream of the combustor) in order to control the temperature of the exhaust and preserve the formation of CO 2  before the exhaust stream reaches the turbine. 
         [0013]    In the exemplary embodiments described below in connection with  FIGS. 2 and 3 , free hydrogen is periodically injected into the system for a fixed duration and in an exact amount, followed by use of the oxidizer component, such as air or an oxygen-enriched feed steam. Balancing the amount of hydrogen and oxidizer injected into the combustor head serves to control the formation of the OH radical, which in turn controls the amount of any remaining CO. The oxidizer component tends to increase the formation of OH which then reacts with free CO in the systems to form carbon dioxide according to the empirical formula: 
         [0000]      CO+OH→CO 2  
 
         [0014]    Significantly, none of the known prior art processes rely on hydrogen and oxide injection to precisely control CO formation at that point in a combustor can, particularly in order to reduce the amount of CO down to the desired stoichiometric amount. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    By way of summary, the present invention covers a method for reducing the amount of carbon monoxide and oxygen emissions in an oxyfuel hydrocarbon combustion system, comprising the steps of feeding defined amounts of hydrocarbon fuel and an oxidizer component into one or more combustors, typically the type found in a gas turbine engine, igniting the mixture of hydrocarbon fuel and oxidizer to form a first combustor exhaust stream, monitoring the amount of carbon monoxide in the exhaust at the head end of each combustor in order to identify one or more target locations downstream of the initial exhaust stream for purposes of injecting free hydrogen and a supplemental oxidizer component into the combustor. Both injected components serve to reduce the amounts of carbon dioxide and oxygen remaining in the exhaust, and thus the amount of injected hydrogen and oxidizer component may vary, depending on the detected amount of carbon monoxide. 
         [0016]    The invention also contemplates injecting a fixed amount of a diluent component into one or more combustors at a point further downstream from the initial exhaust stream in order to control the temperature, particularly as related to the conversion of CO to CO 2 . It has been found that method has particular benefits in rich operating systems, namely those with Phi values greater than 1.0 in which a certain amount of carbon monoxide inherently results from the initial combustion reaction. 
         [0017]    The invention also includes a gas turbine engine comprising the following basic elements: A gas compressor, a plurality of combustors, a turbine driven by the expanded exhaust from the combustors, a primary fuel circuit for the combustors, means for feeding hydrocarbon fuel and air to each combustor, a supplemental fuel circuit for one or more of the combustors defined by a plurality of hydrogen and oxidizer injection nozzles for feeding additional hydrogen and an oxidizer component into the combustors, a carbon monoxide sensor disposed in the exhaust stream, and a feedback control for determining and controlling the amount of additional hydrogen necessary to reduce the level of carbon monoxide and oxygen present in the exhaust. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic of a conventional combustion system known in the art using known hydrocarbon fuels (such as natural gas), but without supplemental hydrogen, oxidizer or diluent injections; 
           [0019]      FIG. 2  is a cross sectional view of a premixed combustor forming part of a gas turbine as constructed in accordance with an exemplary embodiment of the invention; and 
           [0020]      FIG. 3  is a simplified mass flow profile illustrating the injection dynamics (including the injection time, duration and flow rate) for the hydrogen, oxidizer and diluent components used in an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    As noted above, a gas turbine engine typically includes a compressor section, a combustion section and a turbine section with the compressor section driven by the turbine through a common shaft connection. The combustion section nominally includes a circular array of a plurality of circumferentially spaced combustors with the fuel/air mixture being burned in each combustor to produce the hot energetic flow of gas which flows through a transition piece for directing the gas to the turbine blades of the turbine section. A conventional combustor is described in commonly-owned U.S. Pat. No. 5,259,184. For purposes of the present description, only one combustor is illustrated, it being understood that all of the other combustors arranged about the turbine will be substantially similar to the combustor illustrated in  FIG. 1 . 
         [0022]    The term “oxyfuel” as used in connection with  FIGS. 1-3  refers to combustion systems using natural gas (or other hydrocarbon fuels) and oxygen (e.g., an ambient air feed). A “diluent” as used herein refers to air, CO 2  and/or other feeds (including nitrogen) that tend to reduce the combustor exhaust temperature down to a point that inhibits additional CO from forming after the initial hydrogen injection described above. The term “Misc” as used herein refers to the time period in milliseconds for injection feeds to the combustor. The reference to “1.10 th ” is the ratio of hydrogen fed to the combustor as compared to the basic fuel feed amount (e.g., natural gas). The letter “M” refers to the predicted rate of formation of CO (based on modeling). 
         [0023]      FIG. 1-3  illustrate the primary aspects of the invention. As indicated above, by balancing the flow rate, position and timing of the hydrogen and oxidizer injections into the combustor, the amount of CO in the exhaust of each combustor can be reduced. Upon injection, the hydrogen and oxidizer react in a non-equilibrium reaction to form an OH radical, which in turn reacts with the CO formed during the initial combustion to form CO 2 . Carbon dioxide is not considered a pollutant and can be more easily removed from the system by downstream catalytic operations. Although a residual amount of oxygen may already be present in the combustion gasses, the additional oxidizer component helps to drive the CO to CO 2  reaction forward using the OH radical. 
         [0024]    Turning to the figures,  FIG. 1  depicts a general schematic of a conventional gas turbine engine known in the art, but without the advent of the additional injections to improve performance of the combustors as referred to herein. Gas turbine engine  10  typically comprises a gas inlet  12  for feeding an initial oxidizer, e.g., ambient air, into a compressor section  14 , a combustion section  16 , a turbine section  18 , a power turbine section  20  driven by the expanded combustion gases and an exhaust  22 . 
         [0025]    Nominally, a controlled amount of hydrocarbon fuel (such as natural gas) feeds directly into the gas turbine combustor where it ignites and produces thermal energy. The spent exhaust gases exiting from the combustor drive the gas turbine  18  and typically pass at a lower pressure into a heat recovery steam generator (not shown). As noted above, the combustion section often includes a circular array of a plurality of circumferentially spaced combustors. A prescribed fuel or fuel/air mixture is burned in each combustor to produce a hot energetic flow of gas which flows through a transition piece for transporting the gas to the turbine blades of the turbine section which serves as the main driving force to generate electricity via rotating shaft  26  and electrical generator  24  as shown. 
         [0026]    Referring now to  FIG. 2 , a combustor for a gas turbine engine, shown generally as  30 , having central axis  52  includes a primary combustion zone  36  with primary air and hydrocarbon fuel feeds,  42  and  40 , respectively, both of which are injected into the head end portion of the combustor  34  using mixing vane  46 . Typically, primary combustion chamber  30  includes at least one fuel feed that provides a first fuel source and at least one air, such as from compressor section  14  in  FIG. 1 , to form an inlet compressed air stream  42  entering the combustor can. Nominally, mixing vane  46  can be used to provide a mixed fuel/air stream having sufficient annular momentum to move through fuel/air passageway into primary combustion zone  36 . 
         [0027]    Immediately downstream of the mixing head, the fuel air mixture ignites using a pilot flame in the primary combustion zone  48 . Thereafter, an assembly such as that described in commonly-owned U.S. Pat. No. 6,192,688, directs the combined hot gases of combustion  50  into the turbine nozzles and turbine blades into turbine section  18 . An exemplary combustion system can also include a plurality of start-up fuel nozzles, premixing fuel nozzles and an ignition device (not shown) such as an electrically energized spark plug. The initial combustion in the primary combustion zone thus occurs as air under pressure mixes with fuel from the start-up fuel nozzles and/or the premixing fuel nozzles. 
         [0028]      FIG. 2  also illustrates exemplary means for accomplishing the stepped introduction of hydrogen, an oxidizer and the diluent as generally described above as first, second and third supplemental feeds to the combustor head. This secondary injection system includes a plurality of hydrogen injection nozzles  60 , shown by way of example in  FIG. 2 , circumferentially disposed at predefined locations near the top of the combustor head. Nominally, the hydrogen injection points into the combustor will be combined into a single manifold to allow for hydrogen to be injected at the designated peripheral points. However, it may desirable for only selected ones of the hydrogen injection ports  62  to be open and flowing, depending on the combustor operation in progress. 
         [0029]    In like manner, the periodic oxidizer injections downstream of the mixing head as described above can be made using individual and/or manifold type oxidizer injection ports  64 , with the number and size of the injection ports again dependent upon on actual combustor operation and the amount of OH radical being formed for reaction with CO. An exemplary feedback control system for determining the amount of hydrogen and oxidizer necessary to reduce the levels of carbon monoxide and oxygen in the final exhaust nominally will include a plurality of sensors for detecting the amount of carbon monoxide and OH radical in the exhaust (for example at or near the combined combustion gases  50  in  FIG. 3 ) together with a feed back control loop (not shown) to control the hydrogen and oxidizer feeds to the combustors. The control system can be part of a General Electric SPEEDTRONIC™ Gas Turbine Control System such as that described in Rowan, W. I., “SPEEDTRONIC™ Mark V gas Turbine Control System,” GE-3658D, published by GE Industrial &amp; Power Systems of Schenectady, N.Y. 
         [0030]      FIG. 3  of the drawings includes a simplified mass flow profile illustrating the injection dynamics (including the initiation time, duration and amount of flow during a supplemental injection period) for the hydrogen, oxidizer and diluent components as used in an exemplary embodiment of the invention. As  FIG. 3  illustrates, the injections are staged, with the hydrogen injection occurring for only about 2 milliseconds, followed by the oxidizer for a period of about 5 milliseconds and finally the diluent for about 6 milliseconds, making a total injection time of only about 20 milliseconds in this particular example. As noted above, the duration, amount and frequency of the injections has been found to control the OH radical formation immediately downstream of the combustor head. 
         [0031]      FIG. 3  also indicates that the combustor is operating in a slightly rich mode, with a Phi value of 1.01 (see Note  1  in  FIG. 3 ), which ensures that a certain amount of CO will always be present in the exhaust at the time of hydrogen injection following initial combustion.  FIG. 3  also indicates (see Note  2 ) that an effective amount of hydrogen during the injection step comprises approximately 1/10 the mass of the head end fuel itself, i.e., the level of fuel used during initial combustion. The changes in CO concentration resulting from the staged injection of hydrogen, oxidizer and diluent can be seen in the following table. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 With 
                 With H 2  injection 
               
               
                   
                 Base 
                 Dilution 
                 and dilution @ 
               
               
                 Component 
                 Case 
                 @2380° F. 
                 2380° F. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CO (ppm) 
                 9245 
                 3379.6 
                 2402.7 
               
               
                 O 2 (%) 
                 0.26366 
                 0.038749 
                 0.067324 
               
               
                   
               
             
          
         
       
     
         [0032]    The fourth column in particular confirms that the amount of CO in the exhaust drops significantly following the staged injections described above. 
         [0033]      FIG. 3  thus illustrates the effect of the diluent agent injected at a point further downstream from the initial combustion, namely to lower the combustion exhaust stream temperature slightly for the reasons outlined above, including inhibiting any further formation of unwanted CO in the main exhaust stream. 
         [0034]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.