Abstract:
Disclosed herein is a method for controlling a bypass air split for a gas turbine combustor, the method comprising determining a target exhaust temperature, wherein the target exhaust temperature is based on at least one parameter of a group of parameters consisting of low pressure turbine speed, high pressure turbine speed, inlet guide vane angle, and bypass valve air split. Using the target exhaust temperature to calculate a required percentage of bypass air split based on maintaining maximum CO levels or minimum NOx levels. And, applying the required percentage of bypass air split to control a position of the bypass air valve.

Description:
TECHNICAL FIELD  
       [0001]     This application relates generally to combustion system controllers for a two-shaft gas turbine. In particular, the invention relates to a combustor control algorithm for bypass air splits, carbon monoxide (CO) leveling, and nitrogen oxide/dioxide (NOx) leveling.  
       BACKGROUND OF THE INVENTION  
       [0002]     Industrial and power generation gas turbines have control systems (“controllers”) that monitor and control their operation. These controllers govern the combustion system of the gas turbine. Dry Low NOx (DLN) combustion systems are designed to minimize emissions of NOx from gas turbines. The controller executes an algorithm to ensure safe and efficient operation of the DLN combustion system. Conventional DLN algorithms receive as inputs measurements of the actual exhaust temperature of the turbine and the actual operating compressor pressure ratio. DLN combustion systems typically rely on the measured turbine exhaust temperature and compressor pressure ratio to set the gas turbine operating condition.  
         [0003]     Conventional scheduling algorithms for DLN combustion systems do not generally take into account variations in compressor inlet pressure loss, turbine backpressure, compressor inlet humidity, low pressure turbine speed, high pressure turbine speed, and bypass valve air split. Conventional scheduling algorithms generally assume that ambient conditions, e.g., compressor inlet humidity, compressor inlet pressure loss, and turbine back pressure remain at certain defined constant conditions or that variations in these conditions do not significantly affect the target combustor firing temperature.  
         [0004]     Compressor inlet pressure loss and turbine backpressure levels will vary from those used to define the DLN combustion settings. The NOx emissions and CO emissions from the gas turbine may increase beyond prescribed limits, if the conventional DLN combustion system is not adjusted as environmental conditions change. Seasonal variations in humidity or changes in turbine inlet humidity from various inlet conditioning devices, for example, evaporative cooler, fogging systems, can influence the operation of a DLN combustion system. As the ambient conditions change with the seasons, the settings of DLN combustion systems are often manually adjusted to account for ambient seasonal variations.  
         [0005]     The Dry Low NOx (DLN) combustion system was modified for application on a two-shaft, compressor drive, single can, combustion gas turbine. The program required that the combustion system meet both CO and NOx emissions requirements at 50% turndown operation. A combustion bypass valve was designed into the DLN system to change the fuel to air ratio at the head end and thus flame temperature to meet the CO requirements at low loads. In the prior art, there existed no way to schedule the bypass valve air split to meet the CO emissions requirements.  
         [0006]     A corrected parameter control approach was to be used to control the turbine operation. Exhaust temperature target adjustments were to be made based on specific humidity, compressor inlet pressure and compressor exhaust pressure. A two-shaft system added several more variables to the development of the exhaust temperature correction since its shaft speeds are not fixed. The addition of high pressure and low pressure turbine speeds, as well as second stage nozzle guide vanes and combustion bypass air increased the number of inputs into the algorithm and complicated the control. While turbine exhaust could be used to control high pressure turbine speed, low pressure turbine speed, and nozzle guide vanes, there existed no way to control the bypass air split.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Disclosed herein is a method for controlling a bypass air split for a gas turbine combustor, the method comprising determining a target exhaust temperature, wherein the target exhaust temperature is based on at least one parameter of a group of parameters consisting of low pressure turbine speed, high pressure turbine speed, inlet guide vane angle, and bypass valve air split. Using the target exhaust temperature to calculate a required percentage of bypass air split based on maintaining maximum CO levels or minimum NOx levels. And, applying the required percentage of bypass air split to control a position of the bypass air valve.  
         [0008]     Further disclosed herein is a method for determining a target exhaust temperature for a two-shaft gas turbine, the method comprising determining a target exhaust temperature based on a compressor pressure condition. Determining a temperature adjustment to the target exhaust temperature based on at least one parameter of a group of parameters consisting of low pressure turbine speed, high pressure turbine speed, inlet guide vane angle, and bypass valve air split. And, adjusting the target exhaust temperature by applying the temperature adjustment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
         [0010]      FIG. 1  is a schematic depiction of an exemplary two-shaft gas turbine having an air bypass valve for use in accordance with an embodiment of the invention.  
         [0011]      FIG. 2  is an exemplary block diagram of a system for selecting a desired turbine exhaust temperature and bypass air split schedule for use in accordance with an embodiment of the invention.  
         [0012]      FIG. 3  is a diagram of an exemplary algorithm for calculating a desired exhaust temperature for use in accordance with an embodiment of the invention.  
         [0013]      FIG. 4  is a diagram of an exemplary CO leveling algorithm for bypass air split scheduling for use in accordance with an embodiment of the invention.  
         [0014]      FIG. 5  is a graph of an exemplary bypass air split control for use in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  illustrates an exemplary configuration for a two-shaft gas turbine system  10  having a compressor  24 , a combustor  26 , a high pressure turbine  28  drivingly coupled, on a first shaft  44 , to the compressor  24 , a load compressor  48 , a low pressure turbine  32  drivingly coupled, on a second shaft  46 , to the load compressor  48 , a load controller  36 , and a gas turbine controller  38 . An inlet duct  20  to the compressor  24  feeds ambient air to the compressor  24  through a stage of inlet guide vanes (IGV)  22 . The flow path of air continues through the compressor  24  and into the combustor  26  towards fuel nozzles where it mixes with fuel  42  and is burned. Airflow can be redirected from going to the fuel nozzles through a bypass valve  40  where it is not mixed with the fuel and burned. The air redirected through the bypass valve  40  is discharged back into the main flow path towards the exit of the combustor  26 . The air continues through the high pressure turbine  28  to a stage of nozzle guide vanes (NGV)  30 . The nozzle guide vanes direct high pressure turbine  28  exhaust air to the low pressure turbine  32 . An exhaust duct  34  directs combustion gases from the outlet of the low pressure turbine  32  through ducts having, for example, emission control and sound absorbing devices. The exhaust duct  34  also applies a backpressure to the low pressure turbine  32 . Unlike single-shaft gas turbine systems that run at a constant speed, the two-shaft gas turbine system operates at variable speeds. In other words, the compressor  24  and high pressure turbine  28  coupled to shaft  44  rotate at a different speed than the load compressor  48  and low pressure turbine  32  coupled to shaft  46 .  
         [0016]     A load controller  36  sends a speed demand to a gas turbine controller  38 . The gas turbine controller  38  will then control low pressure turbine  32  speed using fuel flow and high pressure turbine  28  speed using nozzle guide vanes  30 . The load on the low pressure turbine  32  and the load compressor  48  is adjusted by regulating fuel flow, through the fuel system  42 , to the combustor  26 .  
         [0017]     The gas turbine controller  38  may be a computer system having a processor(s) that executes programs to control the operation of the gas turbine using system parameters, such as compressor  24  pressure ratio and exhaust back pressure, and instructions from human operators. The programs executed by the gas turbine controller  38  may include, for example, scheduling algorithms for regulating fuel and air flow to the combustor  26 . The commands generated by the controller  38  cause actuators on the gas turbine to, for example, adjust valves between the fuel supply and combustor  26  that regulate the fuel flow, adjust valves to regulate airflow, or stroke command, to the combustor  26 , adjust inlet guide vanes  22 , and activate other control settings on the gas turbine.  
         [0018]     The gas turbine controller  38  regulates the gas turbine based, in part, on algorithms stored in computer memory of the gas turbine controller  38 . These algorithms enable the gas turbine controller  38  to maintain the NOx and CO emissions in the turbine exhaust to within certain predefined emission limits and to maintain the combustor firing temperature to within predefined temperature limits. The algorithms have inputs for parameter variables including compressor  24  pressure ratio, ambient specific humidity, inlet pressure loss, turbine exhaust back pressure, compressor  24  exit temperature, low pressure turbine  32  speed, high pressure turbine  28  speed, inlet guide vanes  22 , and bypass valve  40  air split. Due to the parameters used as inputs by the algorithms, the gas turbine controller  38  accommodates seasonal variations in ambient temperature, ambient humidity, changes in the inlet pressure loss through the inlet duct  20  of the gas turbine and in the exhaust back pressure at the exhaust duct  34 . An advantage of including parameters for ambient conditions and for inlet pressure loss and exhaust back pressure is that the NOx, CO and turbine firing algorithms enable the gas turbine controller to automatically compensate for seasonal variations in gas turbine operation. Accordingly, the need is reduced for an operator to manually adjust a gas turbine to account for seasonal variations in ambient conditions and for changes in the inlet pressure loss or turbine exhaust back pressure.  
         [0019]     The combustor  26  may be, for example, a DLN combustion system. The gas turbine controller  38  may be programmed and modified to control the DLN combustion system. The DLN combustion control algorithms are set forth in  FIGS. 2-5 .  
         [0020]      FIG. 2  is a block diagram of an exemplary process  50  for selecting a desired output target exhaust temperature  66  and a bypass air split schedule  67 . The overall process  50  includes a selection algorithm  64  that selects a temperature target from a plurality of proposed exhaust temperatures by applying a certain logic, such as selection of the lowest temperature of the input temperature targets. These proposed exhaust temperature targets include: a desired exhaust temperature generated by a firing and combustor temperature leveling algorithm  60 , a maximum exhaust temperature  74 , and a desired exhaust temperature generated by a NOx leveling algorithm  62 . The output target exhaust temperature  66  is compared by the gas turbine controller  38  to the actual turbine exhaust temperature. The difference between the desired and actual exhaust temperatures is applied by the controller to regulate the fuel flow and air flow to the combustor or the angle of the inlet guide vanes  22  (when operating part load).  
         [0021]     The output target exhaust temperature  66  is also used as an input into the CO leveling algorithm  68  to calculate the required amount of bypass air to maintain NOx and CO within emission limits.  
         [0022]      FIG. 3  is a block diagram of an exemplary algorithm  51  that is representative of each of the algorithms  60 ,  62  and  68  that produce a target exhaust temperature reference  80 . The firing and combustor exit temperature leveling  60 , NOx leveling  62 , and CO leveling  68  algorithms each having their own unique schedules and correction factor exponent, but are otherwise similar and represented by algorithm  51 . The algorithms receive input data including the compressor  24  pressure ratio, the specific humidity of the ambient air entering the compressor  24 , the inlet duct pressure loss, exhaust back pressure, compressor  24  exit temperature, low pressure turbine  32  speed, high pressure turbine  28  speed, inlet guide vane  22  angle, and bypass valve  40  air split.  
         [0023]     The representative algorithm  51  includes a transfer function  52  for applying the compressor  24  pressure ratio, high pressure turbine  28  speed, and low pressure turbine  32  speed to derive corrected turbine exhaust temperature  53 . The transfer function  52  yields a corrected exhaust temperature that will result in a desired leveling objective for a defined reference load and ambient conditions.  
         [0024]     The desired turbine exhaust temperature is influenced by the load on the gas turbine, ambient humidity, inlet guide vane  22  angle, bypass valve  40  air split, ambient temperature, and so forth. However, a schedule that itself takes into account the previously mentioned parameters would be complex and difficult to apply in a controller. The corrected exhaust temperature transfer function  52  may be simplified by assuming that the load, ambient temperature, inlet guide vane  22  angle, bypass valve  40  air split, and ambient humidity are each at a defined condition. By defining certain conditions, the transfer function  52  is reduced to having three input variables, which are the compressor  24  pressure ratio, high pressure turbine  28  speed, and low pressure turbine  32  speed. A low pressure turbine  32  speed set point is defined by the compressor load controller  36 .  
         [0025]     To derive the desired actual exhaust temperature, the corrected exhaust temperature is adjusted to account for the load, inlet guide vane  22  angle, bypass valve  40  air split, ambient temperature, and ambient humidity. The corrected exhaust temperature  53  is first adjusted to account for changes in a gas turbine system  10  target. For example, if the NOx upper boundary were to increase, a new algorithm target would be updated as an input into a target schedule  81 . The new target would bias the corrected exhaust temperature reference from the transfer function  52  by a delta corrected exhaust temperature. The target schedule  81  may be provided as a tuning method for the algorithm  51  to better fit individual gas turbine systems.  
         [0026]     The corrected exhaust temperature  53  is then adjusted to account for the actual inlet guide vane  22  angle and the bypass valve  40  air split. Followed by adjustments to account for compressor  24  inlet pressure loss and exhaust back pressure. The next step in algorithm  51  is to “uncorrect” the adjusted corrected exhaust temperature to an absolute temperature level, such as degrees Rankine in step  73 . The absolute temperature is multiplied (step  73 ) by a correction factor  76 , which is a function (xy) of a correction factor exponent (y) and a compressor  24  temperature ratio (x). The correction factor exponent (y) may be empirically derived, and may be specific to each algorithm  60 ,  62 , and  68  and each class of gas turbine. The compressor  24  temperature ratio (x) is an indication of gas turbine load. The factor x is a function defined by the compressor  24  temperature ratio which is the current exhaust temperature minus the compressor  24  discharge temperature over a reference exhaust temperature minus the compressor  24  discharge temperature (delta Tref). By multiplying the function (xy) and the corrected target exhaust temperature, an uncorrected target exhaust temperature  77 , converted to a non-absolute temperature scale, is generated.  
         [0027]     The corrected turbine exhaust temperature  53  outputted from the transfer function  52  does not account for deviations in the compressor  24  inlet pressure loss, exhaust back pressure, changes in ambient humidity, high pressure turbine  28  speed, inlet guide vane  22  angle, or bypass valve  40  air split. Additional schedules  58 ,  59 ,  74 ,  78 ,  79 , and  81  are applied to adjust the target turbine exhaust temperature for changes in these conditions.  
         [0028]     The inlet guide vane  22  angle schedule  78  may be a function that correlates a delta exhaust temperature to the actual compressor  24  pressure ratio and the inlet guide vane  22  angle. For a first application the inlet guide vane  22  angle varies only during a small portion of the application while at a fixed high pressure turbine  28  speed. A delta exhaust temperature value  54  output from the inlet guide vane  22  angle schedule  54  is a corrected temperature value. Accordingly, the delta exhaust temperature value  54  is summed with the target corrected exhaust temperature  53  derived from the transfer function  52 . When the inlet guide vane  22  angle is at a predefined positioned, which may be open for example, the delta exhaust temperature  54  is zero.  
         [0029]     Similarly, the bypass valve  40  air split schedule  79  produces a corrected delta exhaust temperature adjustment  55  to be summed with the corrected exhaust temperature  53 . In the case that the bypass valve  40  is at a minimum setting, the delta exhaust temperature adjustment  55  is zero.  
         [0030]     The inlet pressure loss schedule  58  may be a function that correlates a delta exhaust temperature to the actual compressor  24  pressure ratio and the compressor  24  inlet pressure loss. The pressure loss is a function of corrected flow through the gas turbine and varies with the load on the gas turbine and therefore the inlet pressure loss schedule  58  is a function of compressor  24  pressure ratio. The delta exhaust temperature value  71  output from the inlet pressure loss schedule  58  is a corrected temperature value. Accordingly, the delta exhaust temperature value  71  is summed with the target corrected exhaust temperature  53  derived from the transfer function  52 .  
         [0031]     Similarly, the back pressure schedule  59  produces a delta value for the corrected exhaust temperature  53  based on a function of the compressor  24  pressure ratio and the actual back pressure. The turbine back pressure loss is a function of corrected flow through the gas turbine and varies with the load on the gas turbine and therefore the back pressure schedule  59  is a function of compressor  24  pressure ratio. The back pressure delta value  72  is summed with the target corrected exhaust temperature value  53 .  
         [0032]     The humidity schedule  74  applies a delta exhaust temperature versus a delta specific humidity. The delta specific humidity is the difference in the actual ambient humidity from a pre-defined level of humidity. The delta exhaust temperature is applied to the uncorrected target exhaust temperature. The schedule  74  is applied to determine the temperature difference to be used to adjust the corrected exhaust temperature. The temperature difference may be a positive or negative value.  
         [0033]      FIG. 4  is a block diagram of an exemplary CO leveling algorithm  68  for bypass air split scheduling. The CO Leveling algorithm  68  is nearly the same as the exemplary algorithm  51 , containing the same inputs as that of algorithms  60  and  62 , but uses the selected target exhaust temperature  66  as an input and calculates a new bypass air split control  67  to provide a bypass valve  40  position.  
         [0034]     In the CO Leveling Algorithm  68 , the CO upper limit is correlated to a minimum NOx boundary. This can be done since CO and NOx emissions have an inverse relationship. Large amounts of CO are generated at low temperatures and NOx at high temperatures. Thus when CO begins to encroach on its upper boundary, NOx is running very low. While the same functionality can be achieved, and in some cases must be achieved, using CO transfer functions, NOx transfer functions are more stable than CO transfer functions and thus are more desirable.  
         [0035]      FIG. 5  illustrates an exemplary bypass air split control  67  during gas turbine operation in response to the bypass valve  40  air split schedule  79  which prescribes the operation of the bypass valve  40 . Point  1  depicts the bypass split control  67  as fully closed due to CO being below its maximum allowable value (expressed as a NOx lower limit). In the absence of a CO limit violation, the preferred position of the bypass valve  40  is at a minimum position. Bypass air split control  67  from point  1  to point  2  represents a transient operation in the gas turbine where the exhaust temperature reference decreases. This moves the emissions above the maximum allowable CO emissions, and therefore this causes the bypass valve  40  to open in order to bring CO back down to maximum allowable limit. Bypass air split control  67  from point  2  to point  3  represents a second transient in the gas turbine which causes another decrease in the exhaust temperature reference, therefore the bypass valve  40  opens further to maintain CO at the maximum allowable limit. Thus the bypass valve  40  would continue to open (up to 100% split) as the exhaust temperature decreases to maintain the minimum NOx boundary. Inversely, the bypass valve  40  would continue to close as exhaust temperature increases to maintain the minimum NOx boundary.  
         [0036]     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.