Patent Publication Number: US-8117823-B2

Title: Method and system for increasing modified wobbe index control range

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 11/668,747, filed Jan. 30, 2007, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention may relate to methods and systems for controlling combustion dynamics in the combustor of a gas turbine and may particularly relate to methods for controlling combustion dynamics for variable fuel gas composition and temperature based on actual calculated fuel flow to the combustor and heat input to the gas turbine. 
     Industrial-based turbines are often gas-fired and are typically used at power plants to drive generators and produce electrical energy.  FIG. 1 , for example, schematically illustrates a simple cycle, single-shaft, heavy-duty gas turbine, generally designated  10 . The gas turbine comprises an axial flow compressor  12  having a rotor shaft  14 . Air enters the inlet of the compressor at  16 , is compressed by the axial flow compressor  12  and then is discharged to a combustor  18 , where fuel such as natural gas is burned to provide high-energy combustion gases which drive the turbine  20 . In the turbine  20 , the energy of the hot gases is converted into work, some of which is used to drive the compressor  12  through shaft  14 , with the remainder being available for useful work to drive a load such as a generator  22  by means of rotor shaft  24  for producing electricity. The heat exhaust from the turbine is illustrated at  26  and may be used for other purposes, for example, in a combined cycle system. Additionally, there is illustrated a heat exchanger  28  for heating the fuel inlet to the combustor  18  in accordance with the present invention. 
     A current method of heating the fuel gas is to take intermediate pressure (IP) feedwater from intermediate pressure economizer in the heat recovery steam generator (HRSG) and pipe it into the performance heat exchanger.  FIG. 2  schematically illustrates the current method for heating the fuel gas in a simple cycle, single-shaft, heavy-duty gas turbine. Generally, inputs to the turbine include fuel gas  202  and air  204 , and outputs include electrical energy  206 . Fuel gas  202  enters the system via conduit  210  and may be split at valve  212  into conduits  214  and  216 . Via conduit  214  fuel gas enters fuel gas saturator  218 , in which the fuel gas is moisturized. Other inputs to fuel gas saturator  218  include water, which enters via conduit  220 . Unused water exits the fuel gas saturator  218  via conduit  222 , and moisturized fuel gas exits the fuel gas saturator  218  via conduit  224 . Via conduits  216  and  224 , moisturized and unmoisturized fuel gas are mixed in mixer  226  and fed into heat exchanger  230 , which heats the fuel gas prior to introduction into turbine  240  via conduit  232 . 
     Air  204 , via conduit  234 , enters compressor  236 , where it is compressed then is discharged via conduit  238  to turbine  240 . Turbine  240  includes a combustor (not shown) where the fuel gas is burned in the presence of air to generate heat and generates electricity by driving a generator (not shown). Electrical energy  206  exits turbine  240  via carrier  250 . Exhaust exits the turbine  240  via conduit  242 , which connects to HRSG  244 . IP feedwater exits HRSG  244  via conduit  246 , which introduces the IP feedwater into heat exchanger  230 , where it heats the feed gas. After heating the feed gas, the IP feedwater exits heat exchanger  230  via conduit  248 . High pressure (HP) feedwater from the intermediate pressure economizer in HRSG  244  exits HRSG  244  via conduit  245 , which transports the HP feedwater so that it can heat a series of drums  247  (e.g., low pressure, intermediate pressure, and high pressure). 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, there is a method for controlling a temperature of a fuel gas, which may be introduced into a turbine for generating electricity. The method includes several steps, including (i) generating an intermediate pressure feedwater stream and a high pressure feedwater stream in a heat recovery steam generator using an exhaust from combustion involving the fuel gas; (ii) introducing the intermediate pressure feedwater stream into a mixer; (iii) introducing a high pressure feedwater stream into the mixer; (iv) mixing in the mixer the intermediate pressure feedwater stream and the high pressure feedwater stream; (v) outputting an output stream from the mixer; (vi) introducing the output stream into a heat exchanger; and (vii) heating the fuel gas in the heat exchanger. 
     In another embodiment, there is a system for generating electricity. The system includes a heat exchanger for heating fuel gas prior to introduction into a gas turbine; a gas turbine for receiving the heated fuel gas and air, wherein the gas turbine generates electrical energy; a heat recovery steam generator for generating intermediate pressure feedwater and high pressure feedwater; and a mixer for mixing intermediate pressure feedwater generated by the heat recovery steam generator and high pressure feedwater, wherein the output stream of the mixer is introduced into the heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a simple cycle, single-shaft, heavy-duty gas turbine 
         FIG. 2  schematically illustrates a prior art method for heating the fuel gas in a simple cycle, single-shaft, heavy-duty gas turbine. 
         FIG. 3  schematically illustrates an embodiment of the present invention in a simple cycle, single-shaft, heavy-duty gas turbine. 
         FIG. 4  is a chart illustrating the change in MWI as a function of nominal fuel temperature in accordance with an embodiment of the present invention. 
         FIG. 5  is a chart illustrating the change in MWI as a function of nominal fuel temperature using a prior art method. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A number of different types of fuel gases may be used for the combustors of turbines, including, for example, natural gas, LNGs such as propane and butane, refinery gases, and coal-derived gases. The energy content of each of these fuels may vary with its source, and of course, there may be variations in energy content among the various types of fuels. The temperature of the fuel gas supplied to the combustor may also be quite different from system to system. For example, many power plants generating electricity from the output of gas turbines may provide a fuel gas heater to provide a constant (or target) fuel gas temperature to the combustor. Other sites may have a number of boost compressors to elevate the temperature. Thus, different sites may provide fuel gas at different temperatures and pressures. Furthermore, several sites may source fuel gas from several different vendors, which implies that both the temperature and composition of the fuel gas may vary. 
     The standards for setting fuel gas composition and temperature may be characterized by a parameter called the Modified Wobbe Index (MWI). MWI allows comparison of the volumetric energy content of different fuel gases at different temperatures. Since the gas turbine reacts only to energy released in the combustors and the fuel flow control process is generally a volumetric flow control process, fuels of different composition with relatively close values for MWI can generally be provided in the same fuel control system. 
     MWI is defined as: 
     
       
         
           
             
               Modified 
               ⁢ 
               
                 
                     
                 
                 ⁢ 
                 
                     
                 
               
               ⁢ 
               Wobbe 
               ⁢ 
               
                   
               
               ⁢ 
               Index 
               ⁢ 
               
                   
               
               ⁢ 
               
                 ( 
                 MWI 
                 ) 
               
             
             = 
             
               
                 LHV 
                 
                   
                     
                       SG 
                       gas 
                     
                     × 
                     
                       T 
                       gas 
                     
                   
                 
               
               = 
               
                 LHV 
                 
                   
                     
                       
                         MW 
                         gas 
                       
                       28.96 
                     
                     × 
                     
                       T 
                       gas 
                     
                   
                 
               
             
           
         
       
     
     Where: 
     LHV=Lower Heating Value of the Gas Fuel (Btu/scf) 
     SG gas =Specific Gravity of the Gas Fuel relative to Air 
     MW gas =Molecular Weight of the Gas Fuel 
     T gas =Temperature of the Gas Fuel (° Rankine) 
     28.96=Molecular Weight of Dry Air 
     Allowable variations in MWI are generally less than +/− 5%. Variations in MWI from the specified value can lead to unacceptable levels of combustion dynamics. That is, combustion dynamics may be partially a function of MWI. Consequently, operation at high levels of variations in the MWI from a specified value may result in hardware distress or, possibly, a reduction in component life of the combustion system and/or a potential for power generation outage. 
     As defined above, MWI is a measure of the volume flow containing a certain amount of energy injected to a gas turbine combustor and, thus, may be a measure of the interchangeability of gas fuel in a given system design. Fuels from different sources or fuels composed of different mixes of gasses may have different energy contents. The fuel system in a gas turbine may be generally sized for a nominal volume flow rate having a certain energy content. Off-design volume flow rate may cause combustion dynamics or other issues. Thus, for a gas turbine to utilize a variety of blended fuels, a method of controlling the MWI may be needed in some circumstances. 
     Embodiments of the present methods and apparatuses control MWI by modifying the temperature of the fuel. The fuel may be heated above the design point by some percent. This may expand the acceptable range of MWI values for fuel that is to be combusted. This also may facilitate transient periods, such as start up, and decrease the time required for the system to achieve steady state operation. 
     In an embodiment, this control is proposed to be incorporated by mixing high pressure (HP) feedwater with the IP feedwater from the HRSG. This may increase the heat transfer in the performance heat exchanger allowing higher than nominal fuel temperatures. For example, the HP feedwater may be mixed with IP feedwater up to 1 hour or more after system start up. 
     At least certain embodiments of the present invention may be used in the fuel gas delivery system. At least certain embodiments of the present invention may increase the effective MWI control range allowing the turbine to operate close to the tuned point, even if the fuel supply is changed and the MWI increases outside the allowable 5% range. 
     In order to increase the effective MWI control range, the IP feedwater used to heat the gas in the performance heat exchanger must be brought to a higher temperature. In other words, the performance heating system must be designed with some margin that allows heating above the design point. This extra heat capacity may be used to bring the MWI back into spec should it drop below −5% of the nominal MWI set by combustion. 
     In order to increase the temperature of the IP feedwater, HP feedwater will be mixed with the IP feedwater. The pressure of the HP feedwater will be reduced such that a mixer can be used without the HP feedwater causing a flow reversal in the IP feedwater line. The system may also incorporate a cold water line that will be allowed to mix with the IP feedwater in the event that rapid gas cooling is required (MWI is below the acceptable range). The three streams may be blended; two at a time, to produce either increased temperature or decreased temperature IP feedwater. Of course, the IP feedwater may also be used by itself without any mixing at its natural temperature under normal operating conditions where the MWI is within the system&#39;s specification. 
     In order to support a flexible fuel supply (i.e., changes in the composition of fuel gas), Modified Wobbe Index (MWI) control possibilities have been explored. Certain embodiments of the present invention facilitate the ability to control the gas temperature, and therefore, MWI, using the performance heat exchanger. While MWI may be adjusted in current designs by reducing fuel temperature, the performance heat exchanger is already operating at the upper-limit of its gas heating ability, which permits the MWI to be controlled over a range of −15% to +1% of the nominal MWI as illustrated in  FIG. 5 . 
     Projected fuel Lower Heating Values (LHV) may range from nominal by +/−8%. Combustion limits dictate that the MWI should be controlled to +/−5% of the nominal value. Thus, under the current design, the error can only be reduced from +8% to 7%. In some embodiments of the present invention, the error may be reduced from +8% to +3%, which is within the acceptable range of MWI error. 
     While the current design will allow the MWI to be adjusted over a limited range, at least certain embodiments of the present invention will expand the range over which MWI may be controlled. The upper limit of MWI controllability may be increased further if the pressure of the water entering the performance heat exchanger is increased. This may be possible by using the HP feedwater to boost the mixed water pressure, thereby increasing the saturation point of the water and allowing its temperature to rise without forming steam. 
       FIG. 3  schematically illustrates an embodiment of the present invention in a simple cycle, single-shaft, heavy-duty gas turbine. Generally, inputs to the turbine include fuel gas  302  and air  304 , and outputs include electrical energy  306 . Fuel gas  302  enters the system via conduit  310  and may be split at valve  312  into conduits  314  and  316 . Via conduit  214  fuel gas enters fuel gas saturator  318 , in which the fuel gas is moisturized. Other inputs to fuel gas saturator  318  include water, which enters via conduit  320 . Unused water exits the fuel gas saturator  318  via conduit  322 , and moisturized fuel gas exits the fuel gas saturator  318  via conduit  324 . Via conduits  316  and  324 , moisturized and unmoisturized fuel gas are mixed in mixer  326  and fed via conduit  328  into heat exchanger  330 , which heats the fuel gas prior to introduction into turbine  340  via conduit  332 . By controlling the inputs into mixer  326 , the moisture content of the fuel gas introduced into turbine  340  can be controlled. 
     Air  304 , via conduit  334 , enters compressor  336 , where it is compressed then is discharged via conduit  338  to turbine  340 . Turbine  340  includes a combustor (not shown) where the fuel gas is burned in the presence of air to generate heat and generates electricity by driving a generator (not shown). Electrical energy  306  exits turbine  340  via carrier  350 . 
     Exhaust exits the turbine  340  via conduit  342 , which connects to HRSG  344 . IP feedwater from the intermediate pressure economizer in HRSG  344  exits HRSG  344  via conduit  346 , which introduces the IP feedwater into mixer  380 . HP feedwater from the intermediate pressure economizer in HRSG  244  exits HRSG  244  via conduit  245 , where it is split via valve  392  into conduits  249  and  251 . Valve  392  controls the relative flowrates of HP feedwater into conduits  249  and  251 . Conduit  249  carries the HP feedwater so that it can heat a series of drums  247  (e.g., low pressure, intermediate pressure, and high pressure). Via conduit  251  and valve  394 , some HP feedwater is fed into mixer  380  via conduit  390 . 
     In addition to IP feedwater via conduit  346  and HP feedwater via conduit  390 , another input to mixer  380  is cold water via conduit  382 . The output from mixer  380  is then introduced into heat exchanger  330 , where it heats the feed gas. After heating the feed gas, it exits heat exchanger  330  via conduit  348 . The operational parameters (e.g., temperature, pressure, etc.) of the output of mixer  380  are measured via measuring device(s)  396 . Measuring device sends a control signal to valves  392  and  384 , which control the flowrate of the HP feedwater and cold water. In addition, cold water entering the mixer  380  may be pressurized using pump  386 , which is fed by conduit  388  from the cold water supply. Similarly, the pressure of the HP feedwater may be controlled via pressure reducing valve  394 . 
     In an embodiment, the IP feedwater has a temperature of 380° F. and a mass flowrate of 50,000 pounds per hour (pph). The HP feedwater has a temperature of 600° F. and a mass flowrate of 4100 pph. In this embodiment, the output of mixer  380  has a temperature of 440° F. The fuel gas fed to heat exchanger  330  has a temperature of 55° F. Upon exiting heat exchanger  330  via conduit  332 , the fuel gas has a temperature of 335° F. In some embodiments, the fuel gas has a temperature of up to 425° F. The above temperatures and mass flowrates are approximate and may vary depending on specific operating conditions. 
       FIG. 4  provides a chart illustrating the change in MWI as a function of nominal fuel temperature using a method in accordance with an embodiment of the present invention.  FIG. 4  shows that the effective MWI control range can be above 5% using a method in accordance with an embodiment of the present invention. 
     In the prior art method using only IP feedwater (generally having a temperature of 380° F.), the effective MWI control range is 1%.  FIG. 5  provides a chart illustrating the change in MWI as a function of nominal fuel temperature using a prior art method. 
     Table 1 illustrates certain operating conditions of the heat exchanger (Horizontal with NTIW-Segmental Baffles) in accordance with an exemplary embodiment. Table 1 shows that the fuel gas may be heated to 410° F. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Operating conditions of the heat exchanger in 
               
               
                 accordance with an exemplary embodiment. 
               
            
           
           
               
               
               
            
               
                 Process Conditions 
                 Cold Shell 
                 Hot Tube 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Flow rate 
                 (1000-lb/hr) 
                   
                 75 
                   
                 55 
               
               
                 Inlet/Outlet Y 
                 (Wt. frac vap.) 
                 1 
                 1 
                 0 
                 0 
               
               
                 Inlet/Outlet T 
                 (Deg F.) 
                 55 
                 410 
                 440 
                 150 
               
               
                 Inlet P/Avg 
                 (psia) 
                 490 
                   
                 410 
               
               
                   
               
            
           
         
       
     
     It is noted that as described and claimed, all numbers and numerical ranges are approximate. 
     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.