Abstract:
A method for determining a target exhaust temperature for a gas turbine including: 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 specific humidity, compressor inlet pressure loss and turbine exhaust back pressure; and adjusting the target exhaust temperature by applying the temperature adjustment.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to controllers for a combustion system for a gas turbine. In particular, the invention relates to a combustor control algorithm for a Dry Low NOx (DLN) combustor.  
           [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. To minimize emissions of nitric-oxides (NOx), DLN combustion systems have been developed and are in use. Control scheduling algorithms are executed by the controller to operate DLN combustion systems. Conventional DLN algorithms receive as inputs measurements of the exhaust temperature of the turbine and of the actual operating compressor pressure ratio. DLN combustion systems typically rely solely on the turbine exhaust temperature and compressor pressure ratio to determine an operating condition, e.g., turbine exhaust temperature, of the gas turbine.  
           [0003]    Conventional scheduling algorithms for DLN combustion systems do not generally take into account variations in ambient operating conditions, such as seasonal variations in ambient air temperature and humidity. Similarly, conventional scheduling algorithms do not account for variations due to compressor inlet pressure loss and variations in the exhaust back pressure of the turbine. Rather, conventional scheduling algorithms generally assume that ambient conditions, e.g., humidity, compressor inlet pressure loss and turbine back pressure remain at certain defined conditions or that variations in these conditions do not affect the target exhaust turbine temperature.  
           [0004]    DLN combustion systems for gas turbines generally are sensitive to ambient conditions, such as outside ambient humidity. Seasonal variations in humidity can affect 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. In addition, compressor inlet pressure loss and turbine back pressure may drift from the loss and pressure levels used to define the DLN combustion settings. The NO x  and carbon monoxide (CO) emissions from the gas turbine may increase beyond prescribed limits, if the conventional DLN combustion system is not adjusted as the seasons change or to compensate for variations in the compressor inlet pressure or turbine back pressure.  
           [0005]    There is a long felt need for a combustion system controller, and especially a DLN controller, that accommodates seasonal variations in ambient conditions and changes in the inlet pressure and exhaust back pressure. Similarly, there is a long-felt need for a controller that reduces the need to manually adjust the DLN combustion controller of a gas turbine to account for seasonal fluctuations in ambient conditions, and for changes in the inlet pressure loss and turbine back pressure.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    In a first embodiment, the invention is a method for determining a target exhaust temperature for a gas turbine including: 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 specific humidity, compressor inlet pressure loss and turbine exhaust back pressure; and adjusting the target exhaust temperature by applying the temperature adjustment.  
           [0007]    In another embodiment, the invention is a method for determining a target exhaust temperature for a gas turbine comprising: determining a target turbine exhaust temperature based on a compressor schedule having as an input compressor pressure ratio and as an output target turbine exhaust temperature; adjusting the output target turbine exhaust temperature to compensate for a load condition of the gas turbine; determining a temperature change to be applied to the output target turbine exhaust temperature wherein the temperature change is derived from at least one parameter of a group of parameters consisting of specific humidity, compressor inlet pressure loss and turbine exhaust back pressure; changing the target exhaust temperature by the temperature change, and controlling the gas turbine based on the changed target exhaust temperature.  
           [0008]    In a further embodiment, the invention is a controller in a gas turbine having a compressor, combustor and turbine, where the controller comprises: a sensor input receiving data regarding an actual turbine exhaust temperature, a compressor pressure ratio level, a compressor pressure inlet loss, a turbine back pressure level and ambient humidity; a processor executing a program stored in the controller, wherein said program further comprises: a compressor versus turbine exhaust temperature schedule for generating a first target exhaust temperature based on the compressor pressure ratio level, and at least one additional schedule for generating a temperature change to be applied to the target turbine exhaust temperature wherein the temperature change is derived from at least one parameter of a group of parameters consisting of specific humidity, compressor inlet pressure loss, and the turbine back pressure level, and wherein said processor outputs a modified target exhaust temperature based on the first target exhaust temperature and the temperature change, and a combustion controller which outputs a combustor control signal based on the modified target exhaust temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The accompanying drawings in conjunction with the text of this specification describe an embodiment(s) of the invention.  
         [0010]    [0010]FIG. 1 is a schematic depiction of a gas turbine having a fuel control system.  
         [0011]    [0011]FIG. 2 is a block diagram of certain emission limiting and turbine firing algorithms applied to select a target turbine exhaust temperature.  
         [0012]    [0012]FIG. 3 is a block diagram of one exemplary algorithm for determining a target turbine exhaust temperature.  
         [0013]    [0013]FIG. 4 is a block diagram of a selection process for determining a target exhaust temperature. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1 depicts a gas turbine  10  having a compressor  12 , combustor  14 , turbine  16  drivingly coupled to the compressor and a control system  18 . An inlet  20  to the compressor feeds ambient air and possibly injected water to the compressor. The inlet may have ducts, filters, screens and sound absorbing devices that each may contribute to a pressure loss of ambient air flowing through the inlet  20  into the inlet guide vanes  21  of the compressor. An exhaust duct  22  for the turbine directs combustion gases from the outlet of the turbine through ducts having, for example, emission control and sound absorbing devices. The exhaust duct  22  applies a back pressure to the turbine. The amount of back pressure may vary over time due to the addition of components to the duct  22 , and to dust and dirt clogging the exhaust passages. The turbine may drive a generator  24  that produces electrical power. The inlet loss to the compressor and the turbine exhaust pressure losses tend to be a function of corrected flow through the gas turbine. Accordingly, the amount of inlet loss and turbine back pressure vary with flow through the gas turbine.  
         [0015]    The operation of the gas turbine may be monitored by several sensors  26  detecting various conditions of the turbine, generator and environment. For example, temperature sensors may monitor ambient temperature surrounding the gas turbine, compressor discharge temperature, turbine exhaust gas temperature, and other temperature measurements of the gas stream through the gas turbine. Pressure sensors may monitor ambient pressure, and static and dynamic pressure levels at the compressor inlet and outlet, and turbine exhaust, as well as at other locations in the gas stream. Further, humidity sensors, e.g., wet and dry bulb thermometers, measure ambient humidity in the inlet duct of the compressor. The sensors  26  may also comprise flow sensors, speed sensors, flame detector sensors, valve position sensors, guide vane angle sensors, or the like that sense various parameters pertinent to the operation of gas turbine  10 . As used herein, “parameters” and similar terms refer to items that can be used to define the operating conditions of turbine, such as temperatures, pressures, and flows at defined locations in the turbine that can be used to represent a given turbine operating condition.  
         [0016]    A fuel control system  28  regulates the fuel flowing from a fuel supply to the combustor  14 , a split between the fuel flowing into primary nozzles and the fuel mixed with air before flowing into a combustion chamber, and may select the type of fuel for the combustor. The fuel control system may be a separate unit or may be a component of a larger controller  18 .  
         [0017]    The controller may be a General Electric SPEEDTRONIC™ Gas Turbine Control System, such as is described in Rowen, W. I., “SPEEDTRONIC™ Mark V Gas Turbine Control System”, GE-3658D, published by GE Industrial &amp; Power Systems of Schenectady, N.Y. The controller  18  may be a computer system having a processor(s) that executes programs to control the operation of the gas turbine using sensor inputs and instructions from human operators. The programs executed by the controller  18  may include scheduling algorithms for regulating fuel flow to the combustor  14 . The commands generated by the controller cause actuators on the gas turbine to, for example, adjust valves between the fuel supply and combustors that regulate the flow and type of fuel, inlet guide vanes  21  on the compressor, and other control settings on the gas turbine.  
         [0018]    The controller  18  regulates the gas turbine based, in part, on algorithms stored in computer memory of the controller. These algorithms enable the controller  18  to maintain the NOx and CO emissions in the turbine exhaust to within certain predefined limits, and to maintain the combustor firing temperature to within predefined temperature limits. The algorithms include parameters for current compressor pressure ratio, ambient specific humidity, inlet pressure loss and turbine exhaust back pressure. Because of these parameters in the algorithms, the controller  18  accommodates seasonal variations in ambient temperature and humidity, and changes in the inlet pressure loss through the inlet  20  of the gas turbine and in the exhaust back pressure at the exhaust duct  22 . An advantage of the including parameters for ambient conditions and for inlet pressure loss and exhaust back pressure is that the NO x , CO and turbine firing algorithms enable the 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  14  may be a DLN combustion system. The control system  18  may be programmed and modified to control the DLN combustion system. The DLN combustion control algorithms are set forth in FIGS.  2  to  4 .  
         [0020]    [0020]FIG. 2 is a block diagram showing an exemplary process  34  for establishing a limiting turbine exhaust temperature based on a NOx (nitrous oxides) emission limiting algorithms  36 , a CO (carbon monoxide) emission limiting algorithm  38 , a target turbine firing temperature (Tfire) algorithm  40 , and a Tfire limiting algorithm  42 . These algorithms  36 ,  38 ,  40  and  42  each output a separate desired turbine exhaust temperature. The process  34  includes a selection logic  44  to select one of the input desired exhaust temperatures. The process  34  may be used to maintain turbine emissions and firing temperature at or below target levels, especially as ambient conditions, inlet pressure loss or exhaust back pressure vary. In addition, the process  34  allows for smooth transitions in the operation of the gas turbine as changes occur in ambient conditions and in inlet pressure loss and back pressure variations.  
         [0021]    [0021]FIG. 3 is a schematic diagram of an algorithm  45  that is representative of each of the algorithms  36 ,  38 ,  40  and  42  that produce a target turbine exhaust temperature  46 . The NOx, CO and Tf ire limiting algorithms and the Tfire Target algorithm each having their own unique schedules and correction factor exponent, but are otherwise similar and represented by algorithm  45 . The algorithms receive input data regarding such as, for example, the current compressor pressure ratio, the specific humidity of the ambient air entering the compressor  12 , the pressure lost of ambient air passing through the inlet duct  20 , and the back pressure on the turbine exhaust gas due to the exhaust duct  22 . Based on these inputs, the NOx, CO and Tfire limiting algorithms  36 ,  38  and  42 , and the Tfire Target algorithm each produced a desired target exhaust temperature  46 .  
         [0022]    The representative algorithm  45  includes a schedule  48  for applying the compressor pressure ratio to derive a corrected turbine exhaust temperature  50 . The compressor pressure ratio vs. exhaust temperature target schedule  48  may be a graph, look-up table or function that correlates the compressor pressure ratio to a corrected exhaust temperature target. The schedule  48  is generated for each gas turbine or gas turbine class in a conventional manner that is outside the scope of the present invention. The schedule  48  yields a corrected exhaust temperature, for a defined reference load and ambient conditions, e.g., humidity and temperature.  
         [0023]    The desired turbine exhaust temperature is influenced by the load on the gas turbine and the ambient humidity and temperature. However, a schedule that itself takes into account load, ambient humidity and temperature, and compressor ratio would be complex and could be difficult to apply in a controller. To simplify the compressor schedule  48 , the schedule was prepared assuming that the load and ambient temperature and humidity are each at a defined condition. By defining certain conditions, the compressor schedule  48  is reduced to having a single input variable, which is the compressor ratio. Because the effects of load and ambient temperature and humidity are assumed to be constant at the defined conditions, the output of the schedule is a “corrected exhaust temperature.” 
         [0024]    To derive the desired actual exhaust temperature, the corrected exhaust temperature is adjusted to account for the load and ambient temperature and humidity. The corrected exhaust temperature  50  (after being adjusted to account for compressor inlet pressure loss and exhaust back pressure) is converted to an absolute temperature level, such as degrees Rankine in step  52 . A temperature in Fahrenheit may be converted to Rankine by adding 459.67 degrees. The absolute temperature is multiplied (step  54 ) by a correction factor  56  which is a function (X y ) of a correction factor exponent (y) and a compressor temperature ratio (X). The correction factor exponent (y) may be empirically derived, and be specific to each algorithm  36 ,  38 ,  40  and  42  and to each class of gas turbine. The compressor temperature ratio (X) is an indication of gas turbine load. The compressor temperature ratio is the current compressor discharge temperature over a reference compressor temperature (Tref), such as the compressor temperature at full gas turbine load. The temperatures applied for the compressor temperature ratio are absolute temperatures. By multiplying the function (X y ) and the corrected target exhaust temperate, an uncorrected target exhaust temperature  58 , converted to a non-absolute temperature scale, is generated.  
         [0025]    The corrected turbine exhaust temperature  50  output from the compressor pressure ratio schedule  48  does not account for deviations in the compressor inlet pressure loss, exhaust back pressure loss or changes in ambient humidity. Additional schedules,  60 ,  62  and  64  are applied to adjust the target turbine exhaust temperature for changes in these conditions. The schedule  60  for the inlet pressure loss may be a function that correlates a delta exhaust temperature to the actual compressor pressure ratio and the compressor inlet pressure loss (or the change between actual inlet pressure loss and the defined inlet pressure loss applied in developing the compressor schedule  48 .). The inlet pressure loss schedule  60  is a function of compressor ratio because the pressure loss is a function of corrected flow through the gas turbine and does vary with the load on the gas turbine. The delta exhaust temperature value  66  output from the inlet pressure loss schedule  60  is a corrected temperature value. Accordingly, the delta exhaust temperature value  66  is summed with the target corrected exhaust temperature  50  derived from the compressor schedule  48 .  
         [0026]    Similarly, the back pressure schedule  62  produces a delta value for the corrected exhaust temperature  50  based on a function of the compressor pressure ratio and the actual back pressure (or the change between actual back pressure and the defined back pressure applied in developing the compressor schedule  48 .). The back pressure schedule  62  is a function of compressor ratio because the turbine back pressure loss is a function of corrected flow through the gas turbine and does vary with the load on the gas turbine.  
         [0027]    The humidity schedule  64  is of exhaust temperature delta versus delta specific humidity. The delta specific humidity is the difference in the actual ambient humidity from a pre-defined level of humidity. The exhaust temperature delta is applied to the uncorrected target exhaust temperature and is summed with that temperature value  58 . The schedule  64  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. The schedule  64  provides an exhaust temperature delta for a delta compressor inlet pressure loss, where the delta compressor inlet pressure loss is the difference between the inlet pressure loss and the defined inlet pressure loss. The schedule  50  provides an exhaust temperature delta for a delta exhaust back-pressure, where the delta exhaust back-pressure is the difference between the actual back-pressure and the defined back-pressure. The result is a target exhaust temperature  46  for the subject parameter (NOx, CO, or T-fire).  
         [0028]    [0028]FIG. 4 is an expanded schematic diagram that compresses the information from FIG. 3 into the blocks for each of the algorithms shown in FIG. 2. FIG. 4 shows that the representative algorithm  45  is tailored to and applied to each of the algorithms  36 ,  38 ,  40  and  48 . The selection logic  44  may include a maximum select logic unit  68  that identifies the hottest temperature between the target exhaust  46  from the CO limiting algorithm  38  and the Tfire Target Algorithm  40 . The hottest temperature identified by the maximum select  68  is applied to a minimum select logic unit  70  that identifies the coolest of the temperatures output from the maximum select logic unit  68 , the uncorrected target exhaust levels from the NOx limiting algorithm and the Tfire limiting algorithm, and a maximum exhaust temperature level  72 . The output of the minimum select unit  70  is applied as the uncorrected target turbine exhaust level  74 . The controller  18  adjusts the fuel control to achieve the target turbine exhaust level  74 .  
         [0029]    The selection logic  44  also provides smooth transition in target turbine exhaust during a transition from one selected limiting algorithm to the selection of another algorithm as operating conditions change. The selection of the exhaust target levels indirectly dictates the required combustor firing temperature and the level of the alternate emission when the schedule is in force.  
         [0030]    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.