Patent Publication Number: US-2016237916-A1

Title: Method of controlling the fuel distribution among different stages of a gas turbine combustion chamber

Description:
TECHNICAL FIELD 
     The present invention relates to a method of controlling the fuel distribution among different stages of a gas turbine combustion chamber. 
     BACKGROUND 
     Gas turbines are known to comprise a compressor for compressing an oxidizer such as air, a combustion chamber where a fuel is combusted with the compressed air generating hot gas, and a turbine where the hot gas is expanded, gathering mechanical work. 
     Combustion chambers can have staged fuel supply. A combustion chamber with staged fuel supply has a number of fuel nozzles that can be independently supplied with fuel, in order to adjust the fuel distribution within the combustion chamber by adjusting the fuel injected through the nozzles of different stages. 
     During operation fuel distribution can be adjusted according to the load or one or more parameters indicative of the load, in order to maintain the gas turbine operation in optimal or acceptable operating conditions over a large operating load window (for example the operating load window can vary from 40% to 100% of the nominal load). 
     During transient operation (regular transient operation for increasing/reducing the power of the gas turbine in response to a changed request of the grid) the load of the gas turbine or parameters indicative thereof are measured and the fuel distribution is determined accordingly. 
     The measuring of the load or parameters indicative of the load and the calculation or determination in other way (for example by look up tables) of the load requires some time, such that the fuel distribution corresponding to a measured load is implemented with a delay. 
     The delay is not troubling for the regular transient operation, because the regular transient operation is quite slow (in the order of tens of seconds or minutes), such that the load or parameters indicative of the load do not change substantially during the delay and the fuel distribution even implemented with a delay is substantially correct. 
     In some particular conditions, the transient operation can be abnormally fast. For example during protective load shedding (when the gas turbine is de-loaded because of a problem) or frequency response (when the gas turbine load has to be changed in response to a grid frequency change) the gas turbine is loaded or de-loaded in seconds, with a fast gradient that can be up to 10 times higher than during regular transient. For example during a regular transient operation the gas turbine can typically undergo a load change of 2 MW/min while during an abnormal transient operation the load change can be 4 MW/sec. 
     During these abnormal transient operations, the delay between the measurement of the load or parameters indicative of the load and the implementation of the fuel distribution can cause implementation of a fuel distribution not corresponding to the current load, with operation close to the lean blow off and risks of flame extension and/or pulsation. 
     SUMMARY 
     An aspect of the invention includes providing a method of controlling the fuel distribution among different stages of a gas turbine combustion chamber that is able to improve the gas turbine operation during abnormal transient operation. 
     These and further aspects are attained by providing a method in accordance with the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages will be more apparent from the description of a preferred but non-exclusive embodiment of the method, illustrated by way of non-limiting example in the accompanying drawings, in which: 
         FIGS. 1 and 2  show a gas turbine and a combustion chamber thereof; 
         FIGS. 3 and 4  show look up tables for implementing fuel distribution; 
         FIGS. 5 through 8  show different relationships between the Front Stage Ratio and parameters indicative of the load. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     With reference to the figures, these show a gas turbine  1  with a compressor  2  for compressing an oxidizer, such as air, a combustion chamber  3  for combusting a fuel with the compressed oxidizer generating hot gas, and a turbine  4  for expanding the hot gas and gather mechanical  15  power. 
     The combustion chamber can be of any type, in the following reference to a combustion chamber arranged for implementing a pre-mixed combustion is made. 
     The combustion chamber  3  has one or more burners connected to a combustor  7 . The burners have a substantially conical shape with slots  8  axially extending over the conical surface, for oxidizer  9  supply into the burners  6 . 
     The burners  6  are also provided with nozzles for fuel supply. 
     First nozzles  10  define a first fuel supply stage and are provided at the terminal part of the burner  6 , in order to supply fuel into the combustor  7  for combustion; fuel supplied via the first stage undergoes diffusion combustion. 
     Second nozzles  12  define a second fuel supply stage and are provided at the slots  8  (for example adjacent to the slots  8  or within the nozzles  8 ), in order to supply fuel into the burner for mixing the fuel with the oxidizer generating a fuel/oxidizer mixture that then passes into the combustor  7  for combustion; fuel supplied via the second stage undergoes pre-mixed combustion. 
     During operation the total amount of fuel defines the load and the fuel distribution between the first and second stages is adjusted in order to guarantee a correct operation. Typically the fuel distribution is defined according to the load or one or more parameters indicative of the load; for example parameters indicative of the load can be the VIGV position (i.e. the opening of the inlet guide vanes upstream of the compressor) and/or the TAT (i.e. the hot gas temperature upstream of the turbine). Other parameters are anyhow possible. 
     The fuel distribution between the stages is defined on the basis of the Front Stage Ratio, defined by 
       FSR= m   1 /( m   1   +m   2 ) 
     wherein. 
     FSR is the front stage ratio, m 1  is the fuel mass flow through the first stage, m 2  is the fuel mass flow through the second stage. 
       FIG. 3  shows an example of a look up table that can be used to implement the fuel distribution according to the load. 
     The look up table can contain the FSR corresponding to different loads, such that during regular operation the load is measured and on the basis of the measured load the FSR is adjusted according to the look up table. 
     According to the method, in reply to a fast load change being faster than a load change during a regular transient operation, the fuel distribution is determined according to an adjusted load or one or more parameters indicative of an adjusted load. 
     The method can be implemented both when the fast load change is a load, reduction or a load increase. 
     When the fast load change is a load reduction the adjusted load is a lower load than the measured load; when the fast load change is a load increase the adjusted load is a higher load, than the measured load. 
     For example  FIG. 4  shows a look up table that could be used to implement the method of the invention. 
     During normal operation (i.e. at steady state or with slow transient operation) the look up table provides the relationship between the load (or parameter indicative of the load) and the SFR (column “FSR (regular operation)”; during abnormal operation (i.e. with fast increase or reduction of the load) the table provides the SFR for load decrease or load increase. In particular:
         The column FSR (abnormal load decrease) provides for each measured load, FSR values corresponding to a lower load (real or theoretical value); for this reason the FSR values in this column are higher than in the column FSR (regular operation);   the column FSR (abnormal load increase) provides for each measured load, FSR values corresponding to a higher load (real or theoretical); for this reasons the FSR values in this column are lower than in the column FSR (regular operation).       

     In the following a specific example is described with reference to  FIGS. 5 through 7 . 
       FIG. 5  shows the relationship between the SFR and VIGV and TAT (parameters indicative of the load); in particular:
         curve A delimits the zone B wherein operation is not safe (for example because operation becomes close to lean blow off), therefore the gas turbine operation has to be outside of this zone B;   curve C indicates the relationship between the FSR and the parameters indicative of the load during regular operation (i.e. steady state operation and slow transient operation).       

       FIG. 6  shows curve D that indicates the actual FSR that is implemented in the gas turbine in case of fast load reduction, when FSR is adjusted on the basis of curve C, because of the delay between load measurement and adjustment implementation.  FIG. 6  shows that because of the delay the FSR can fall within the zone B. 
       FIG. 7  shows the curve B that indicates the relationship between the FSR and the parameters indicative of the load, to be used in case of abnormal transient, for example in case of fast load reduction; as it is shown the FSR is higher for given loads than in case of regular operation. 
       FIG. 8  shows the curve F that indicates the actual FSR that is implemented in the gas turbine, when adjusting the FSR on the basis of the curve F, in case of fast load reduction because of the delay between the load measurement and adjustment implementation.  FIG. 8  shows that notwithstanding the delay, the current FSR never falls within the zone B. 
     Naturally the features described may be independently provided from one another. 
     In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art. 
     REFERENCE NUMBERS 
     
         
         
           
               1  gas turbine 
               2  compressor 
               3  combustion chamber 
               4  turbine 
               6  burner 
               7  combustor 
               8  slot 
               9  oxidizer 
               10  first nozzle 
               12  second nozzle 
             A curve delimiting the not-safe operation zone 
             B not safe operation zone 
             C relationship between FSR and parameters indicative of the load during regular operation 
             D curve indicative of the actual FSR implemented for fast load reduction during abnormal transient 
             E relationship between FSR and parameters indicative of the load to be used during fast load reduction 
             F curve indicative of the actual FSR implemented for fast load reduction during abnormal transient using the curve E as the basis for the adjustment