Abstract:
In a method for reducing the NOx emissions from a burner arrangement comprising a plurality of burners (B 1 , . . . , Bn), in particular in a gas turbine, which burners (B 1 , . . . , Bn) are operated in parallel and each burner supplied fuel by means of combustion air to form a flame (F 1 , . . . , Fn), an effective drop is achieved in a simple way by virtue of the fact that at a predetermined time the flame temperatures of individual burners (B 1 , . . . , Bn) or burner groups or differences between the flame temperatures of individual burners (B 1 , . . . , Bn) or burner groups are measured directly or indirectly. The fuel supply to those burners or burner groups whose flame temperature exceeds a predetermined value for the flame temperature is selectively throttled in order to homogenize the flame temperatures of the burners (B 1 , . . . , Bn).

Description:
RELATED APPLICATION 
     The present application is a continuation application which claims the benefit of the filing date under 35 USC §120 of PCT/EP2004/051483, filed Jul. 14, 2004, and the priority of German Application No. 103 33 671.0, filed Jul. 24, 2003, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention deals with the field of combustion technology. It relates to a method for reducing the NOx emissions and to a burner arrangement for carrying out the method. 
     BACKGROUND OF THE INVENTION 
     Over the course of the last decade, the NOx emissions from gas turbine power plants have been lowered by a factor of 10 by means of lean-premix combustion. This is achieved in particular by virtue of the fact that the temperatures in the reaction zone of the combustion chamber (flame temperature) have been significantly lowered. A large number of burners which operate in parallel and generate the hot gas required for the turbine to operate are generally arranged in the combustion chambers of gas turbines (cf. for example EP-A1 1 273 776). A drawback in this context with regard to the NOx emissions is that on account of tolerances in burner and combustion chamber manufacture, not all the burners are operated at their optimum, low-pollutant operating point. Rather, there is a considerable variation in the flame temperature. As a result, the potential for lowering the emissions of NOx offered by the lean-premix burners can be only partially exploited. 
     JP-A2 10317991 has proposed that in a gas turbine having a plurality of premix burners the quantity of NOx be reduced, and at the same time the combustion be stabilized in the event of a load change in the turbine, by the temperatures of the flame stabilizers, of the combustion chamber lining and of the hot gases being measured, and the operating state of the burners being derived from this information. The fuel supply to the burners is then controlled on the basis of the measurement results in such a way that the combustion is stabilized with regard to partial misfires and deviations in the supplied fuel quantity. The objective in this context is to (dynamically) stabilize the combustion in the event of load changes in the gas turbine. At the same time, this prevents a rise in the quantity of NOx caused by the occurrence of burner instability. 
     The above cannot be used to permanently compensate for tolerances in the burner and combustion chamber configuration. 
     EP-A1 0 529 900 has disclosed a gas turbine and a method for controlling it in which the flow of fuel to a plurality of burners is controlled individually by means of corresponding control devices taking account of a predetermined combustion characteristic in accordance with specific temperature values recorded in the combustion chamber. Control of this type is highly complex and is susceptible to instabilities relating to control engineering. 
     EP-A1 0 969 192 has disclosed a method for equalizing the fuel distribution system in gas turbines having a plurality of burners, in which inhomogeneities in the fuel distribution system are equalized by the targeted alteration of the fuel mass flows by means of throttling members. To do this, certain pressure differences in the system are measured. This does not allow direct optimization of the flame temperatures. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method by which design-related and manufacturing-related inhomogeneities in the flame temperatures of a burner arrangement can be compensated for in a simple way, and as a result the additional NOx emissions associated with the inhomogeneities can be permanently lowered, and to provide a burner arrangement for carrying out the method. 
     The core concept of the invention consists in directly or indirectly measuring the flame temperatures of the individual burners or burner groups and then permanently throttling the fuel supply to those burners or burner groups whose flame temperatures are above a predetermined value, in order to compensate for the deviations in these burners caused by design engineering and manufacturing technology. This operation can be carried out one or more times and then leads to long-term homogenization of the flame temperatures and, as a corollary measure, to a reduction in the NOx emissions caused by the inhomogeneities. 
     A preferred configuration of the method according to the invention is distinguished by the fact that the individual burners or burner groups are each supplied with fuel via a fuel feedline, and that the throttling of the fuel supply is carried out by means of a throttling member arranged in the fuel feedline. This results in particularly simple adapting of the various burners with a view to homogenizing the flame temperature. 
     One possible way of measuring the flame temperatures consists in the measurement of the flame temperatures being carried out directly at the flames, with the measurement of the flame temperatures being carried out in particular by an optical route. 
     One possible way of measuring the flame temperatures consists in the measurement of the flame temperatures being carried out by an indirect route, in which case in particular the flame generated in a combustion chamber, and to measure the flame temperatures the temperatures of selected parts or regions of the combustion chamber are measured, or alternatively the hot gases generated in the flames by the burners are passed through a utilization device, in particular a gas turbine, and to measure the flame temperatures of burners or burner groups, the temperatures at the outlet of the utilization device are measured. Indirect measurement of the flame temperature is significantly simpler to realize and carry out in metrological terms. 
     To permanently throttle the fuel supply, it is preferable to use a settable throttling member, which may optionally be a settable valve, an adjustable throttling screw or an exchangeable diaphragm with a predetermined diaphragm opening. 
     One preferred configuration of the burner arrangement according to the invention is characterized in that the first means comprise a plurality of sensors which are connected to a measuring unit, with the sensors being designed either for direct measurement of the flame temperature, preferably by an optical route. 
     Alternatively, the sensors are designed to measure the temperature of components, the burners are accommodated in one or more combustion chambers and the sensors are arranged distributed in or on the combustion chamber(s). 
     As a further alternative to this, a utilization device for the hot gases, in particular in the form of a gas turbine, is arranged downstream of the burner arrangement, and the sensors are designed to measure the temperature of hot gases, and the sensors are arranged at the outlet of the utilization device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be explained in more detail below on the basis of exemplary embodiments in conjunction with the drawing, in which: 
         FIG. 1  shows a diagrammatic illustration of a burner arrangement in accordance with a preferred exemplary embodiment of the invention with direct measurement of the flame temperatures and throttling members in the fuel supply lines leading to the individual burners; 
         FIG. 2  shows an alternative exemplary embodiment of the invention to that shown in  FIG. 1 , in which the sensors for determining the flame temperatures are arranged on the combustion chamber and pick up material temperatures of combustion-chamber parts; 
         FIG. 3  shows an exemplary embodiment of the invention which represents an alternative to  FIGS. 1 and 2  and in which the sensors are arranged at the outlet of a device for utilizing the combustion chamber gases, in the form of a gas turbine, and determine the flame temperatures from the temperature distribution at the outlet of a utilization device; and 
         FIG. 4  shows a number of subfigures ( FIGS. 4   a, b  and  c ) to show various types of throttling members which can be used to implement the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The content of the present invention inter alia involves identifying the hottest burners of gas turbines having a plurality of burners or burner groups by direct or indirect measurement of the flame temperature or of the differences in the flame temperature. This measurement can be carried out in various ways (for example by measuring the temperature downstream of the turbine, by measuring the material temperature of parts of the combustion chamber, by direct optical measurement of the flame temperature). 
     After the flame temperature has been measured, the flame temperatures are homogenized by throttling the fuel supply to the burners with an excessively high flame temperature. This throttling can be effected by means of settable valves, settable throttling screws or fixedly installed throttling members (e.g. diaphragms). The operation of flame temperature measurement and throttling of the burners with an accessibly high flame temperature can be repeated until the desired homogeneity is achieved. 
       FIG. 1  diagrammatically depicts a burner arrangement in accordance with a preferred exemplary embodiment of the invention with direct measurement of the flame temperature. The burner arrangement  10  comprises a plurality (n; n=natural number) burners B 1 , . . . , Bn, which are arranged in a combustion chamber that is not shown ( 13  in  FIG. 2 ) and can be operated in parallel. The burners (B 1 , . . . , Bn) are designed, for example, as double-cone burners, as shown and described inter alia in EP-A2-0 807 787. The individual burners B 1 , . . . , Bn are each connected via fuel feedlines  19  to a common fuel supply  11 . The burners (B 1 , . . . , Bn) are usually arranged on one or more concentric circular rings. They may also be combined to form groups which are jointly supplied with fuel and operated. 
     In operation, each of the burners B 1 , . . . , Bn by combustion of the supplied liquid and/or gaseous fuel with the aid of compressed combustion air, generates a flame F 1 , . . . , Fn, the hot gases of which are then utilized for power engineering purposes in a downstream utilization device (turbine, steam generator etc.). On account of manufacturing and installation tolerances of burners and combustion chamber, the flames F 1 , . . . , Fn generated by the various burners B 1 , . . . , Bn in some cases have different flame temperatures, resulting in the presence of individual burners whose flame temperatures exceed a predetermined value. Although the mean value for the flame temperatures is within a tolerated range, the elevated temperatures of individual flames lead to high NOx emissions. In the exemplary embodiment shown in  FIG. 1 , a plurality of sensors S 1 , . . . , Sn are provided, which directly measure the temperatures of the individual flames F 1 , . . . , Fn by an optical route (e.g. spectral measurement). The sensors S 1 , . . . , Sn are connected to a measuring unit  12  in which the flame temperature measurements are evaluated and displayed. In particular, it is conceivable and advantageous to identify and indicate those burners whose flame temperature exceeds a predetermined value and is therefore too high. 
     If, following a flame temperature measurement of this type, selected burners are then indicated as operating at an excessively high flame temperature, these burners can be permanently corrected, with the result that the flame temperature of the corrected burner is reduced. This correction does not require any complex control devices, but rather can be carried out using relatively simple and operationally reliable means. In the exemplary embodiment shown in  FIG. 1 , throttling members D 1 , . . . , Dn which allow simple (partial) throttling of fuel supply to the respective burner are arranged in the fuel feedlines  19  leading to the burners B 1 , . . . , Bn. 
     An example of a selection of suitable throttling members Dn is illustrated in the partial figures (a) to (c) of  FIG. 4 . The throttling member Dn of  FIG. 4   a  is designed as a settable valve  15 . By partially closing the valve  15 , it is possible to throttle the fuel supply in the associated fuel feedline  19  to the desired extent.  FIG. 4   b  illustrates a throttling member Dn in the form of a throttling screw  16  which by being screwed in narrows the cross section of the fuel feedline  19  and thereby throttles the fuel supply. Finally,  FIG. 4   c  shows a throttling member Dn in the form of a diaphragm  17  which has a diaphragm opening  18  with a cross section that is smaller than the cross section of the unthrottled fuel supply line  19 . Different levels of throttling of the fuel supply can be achieved by installing different diaphragms  17  with different opening cross sections. 
     If, during the measurement of the flame temperatures, a burner with an excessively high flame temperature is identified, the supply of fuel to this burner is firstly throttled by a certain amount by means of the associated throttling member Dn. If the measurement is subsequently repeated and the excessively high flame temperature is still established, the throttling is boosted by a further step. This sequence can be repeated until the flame temperatures of all the burners B 1 , . . . , Bn are within a narrow tolerance range and have therefore been homogenized. The permanent throttling in steps ensures that it is impossible for any control oscillations to occur and that operation remains stable at any time. The use of simple throttling members keeps the costs low and leads to easy setting and a high operational reliability. 
       FIG. 2  shows a second exemplary embodiment of a burner arrangement according to the invention. The burners B 1 , . . . , Bn of the burner arrangement  10  are in this case illustrated together with the combustion chamber  13 . The flames F 1 , . . . , Fn from the burners B 1 , . . . , Bn lead, at different flame temperatures, to different heating of components (walls etc.) in the combustion chamber  13 . They can therefore be measured indirectly by measuring the temperature of certain components or regions of the combustion chamber  13  using appropriately fitted sensors S 1 ′, . . . , Sn′ (thermocouples, resistance thermometers or the like). These sensors S 1 ′, . . . , Sn′ are likewise connected to a measuring unit  12 , so that the burners or burner groups to be throttled can be displayed in an identifiable way there. The throttling members themselves are not shown in  FIG. 2 , for the sake of simplicity. 
       FIG. 3  illustrates a further exemplary embodiment of the invention. The combustion chamber  13  with the burners B 1 , . . . , Bn is in this case arranged on the inlet side of a utilization device, in this case a gas turbine  14 . The hot gas generated by the burners B 1 , . . . , Bn flows through the turbine  14 , performing work as it does so, and emerges at the outlet of the turbine  14 , where a temperature distribution which is characteristic of the flame temperatures of the burners B 1 , . . . , Bn is established in the hot-gas stream. If this temperature distribution is measured by means of sensors S 1 ″, . . . , Sn″, it is possible to ascertain the flame temperatures of the individual burners B 1 , . . . , Bn. Accordingly, it is possible to identify a burner with an excessively high flame temperature. The sensors S 1 ″, . . . , Sn″ are likewise connected to a measuring unit  12 . The throttling members for the fuel supply are not illustrated but are installed in the fuel feedlines in a similar way to that shown in  FIG. 1 . 
     Overall, the invention gives the following advantages:
         lowering of the maximum temperature in the combustion chamber, in particular of gas turbines;   reduction of the NOx emissions, in particular from gas turbines;   more uniform temperature distribution and therefore uniform thermal stressing of components;   simple setting and simple installation;   high operational reliability.       

     LIST OF DESIGNATIONS 
     
         
           10  burner arrangement 
           11  fuel supply 
           12  measuring unit 
           13  combustion chamber 
           14  turbine (gas turbine) 
           15  settable valve 
           16  throttling screw 
           17  diaphragm 
           18  diaphragm opening 
           19  fuel feedline 
         B 1 , . . . , Bn burner (e.g. double-cone burner) 
         D 1 , . . . , Dn throttling member 
         F 1 , . . . , Fn flame 
         S 1 , . . . , Sn sensors 
         S 1 ′, . . . , Sn′ sensors 
         S 1 ″, . . . , Sn″ sensors