Abstract:
A method for setting or regulating the steam temperature of the live steam and/or reheater steam, in particular under part load, in a combined-cycle power plant and a combined-cycle power plant so regulated. The power-plant has a water/steam cycle including a steam turbine, fired boiler and a means for superheating or reheating the steam generated in the boiler to form live steam or reheater steam, a gas turbine set, and a downstream heat recovery steam generator connected to the water/steam cycle in such a way that steam generated in the heat recovery steam generator is admixed with the live steam or reheater steam. Improved part-load behavior is achieved in that the steam temperature of the live steam and/or reheater steam is set or regulated by setting or regulating the steam temperature of the steam generated in the heat recovery steam generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/731,928 filed on Dec. 8, 2000, U.S. Pat. No. 6,497,101, which claimed priority under 35 U.S.C. §§119 and/or 365 to Application No. 100 01 955.01 filed in Germany on Jan. 19, 2000. The entire content of each of these applications is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of power plant technology. It relates to a method for setting or regulating the steam temperature of the live steam and/or reheater steam, particularly under part load, in a combined-cycle power plant and to a combined-cycle power plant for carrying out the method. 
     BACKGROUND 
     Power plant concepts, in which gas turbine sets having downstream heat recovery steam generators and water/steam cycles having fired boilers and corresponding steam turbines are combined with one another, have been known for some time from the prior art and are used increasingly, above all because of the increased overall efficiency, the additional power output (gas turbine set), along with low investment, and operating flexibility. 
     A variant of such combined power plant concepts is the combined-cycle power plant, in which the gas turbine set and the water/steam cycle are coupled to one another on the water/steam side. The flue gas discharged by the gas turbine set is conducted through a heat recovery steam generator for the generation of steam and is then discharged into the surroundings. The steam generated in the heat recovery steam generator is fed, for example as live steam and/or reheater steam, into the water/steam cycle at suitable points. Additionally or alternatively, the heat recovery steam generator may also be employed for condensate and/or feedwater preheating. Various types of combined-cycle power plants with different connections between the gas turbine system and the water/steam system are described, for example, in the article by G. Bauer et al., “Das Verbundkraftwerk—eine neue Variante des kombinierten Kraftwerkes”, [“The combined-cycle power plant—a new variant of the combined power plant”] VGB Kraftwerkstechnik 73 (1993), No. 2, page 120 ff. 
     Connection variants are in this case (1) the generation of live steam in the heat recovery steam generator, (2) the generation of reheater steam or medium-pressure steam in the heat recovery steam generator, (3) the generation of low-pressure steam in the heat recovery steam generator, (4) reheating in the heat recovery steam generator or (5) a combination of the first four connection variants with branch flows. 
     In combined-cycle power plants, essential importance is attached to keeping constant or regulating the live steam temperature, particularly with a view to operation under part load. 
     In the conventional steam generator, the flue gas temperature falls under part load. As a result of this, the live steam temperature cannot be maintained. However, a fall in the live steam temperature signifies a lowering of the efficiency of the cyclic process. If the steam temperature is to be capable of being regulated over a relatively wide power output range, water injection, together with a corresponding system design, may be provided in a known way. However, this signifies an undesirable loss of energy in the steam generator. 
     In the gas turbine set, the flue gas parameters depend on the load of the gas turbine and on the climatic conditions. This results in influence being exerted on the generation of steam in the heat recovery steam generator. 
     Overall, satisfactory part-load properties of a conventional steam generator cannot be achieved in a combined-cycle power plant by means of the above-mentioned known methods. A good part-load behavior is of great importance, particularly when the power plant is used in the medium-load range. 
     SUMMARY 
     An object of the invention is, therefore, to provide a method for setting or regulating the steam temperature of the live steam and/or reheater steam, particularly under part load, in a combined-cycle power plant, which leads to markedly improved part-load properties of the combined-cycle power plant, and a combined-cycle power plant for carrying out such a method. 
     The essence of the invention is to set or regulate the steam temperature of the live steam or reheater steam flowing to the steam turbine by admixing steam from the heat recovery steam generator with this steam, and by setting or regulating the steam temperature of this admixed steam. Improved part-load properties of the combined-cycle power plant can thereby be achieved in a simple way. 
     A first refinement of the method according to the invention is characterized in that, in order to set or regulate the steam temperature of the steam generated in the heat recovery steam generator, a corresponding change in the feedwater mass flow flowing through the heat recovery steam generator for steam generation is carried out. For this purpose, the feedwater mass flow may be controlled, for example, by the associated feedwater pump, as is known, for example from U.S. Pat. No. 5,237,816. 
     A second refinement of the method according to the invention is distinguished in that a heat recovery steam generator is used which comprises an evaporator and a downstream superheater and in which a separator is arranged between the evaporator and the superheater. The use of the separator limits the setting and regulating range of the steam temperature toward low temperatures. 
     The steam generated in the heat recovery steam generator may, within the scope of the invention, be fed into the water/steam cycle at different points: 
     One possibility is for the fired boiler to be followed by a superheater, for live steam to be generated in the heat recovery steam generator and for the live steam generated in the heat recovery steam generator to be fed into the water/steam cycle downstream of the superheater 
     Another possibility is for the fired boiler to be followed by a superheater with a plurality of successively arranged heat transfer surfaces, for live steam to be generated in the heat recovery steam generator and for the live steam generated in the heat recovery steam generator to be fed into the water/steam cycle between the heat transfer surfaces of the superheater. 
     Another possibility is for the steam turbine to comprise a high-pressure stage and a medium-pressure stage, for a reheater to be provided between the highpressure stage and the medium-pressure stage, for reheater steam to be generated in the heat recovery steam generator and for the reheater steam generated in the heat recovery steam generator to be fed into the water/steam cycle downstream of the reheater. 
     It is possible, furthermore, for the steam turbine to comprise a high-pressure stage and a medium-pressure stage, for a reheater with a plurality of successively arranged heat transfer surfaces to be provided between the high-pressure stage and the medium-pressure stage, for reheater steam to be generated in the heat recovery steam generator and for the reheater steam generated in the heat recovery steam generator to be fed into the water/steam cycle between the heat transfer surfaces of the reheater. 
     The combined-cycle power plant according to the invention for carrying out the method, which combined-cycle power plant comprises a water/steam cycle, in particular with a steam turbine, with a fired boiler and with means for superheating or reheating the steam generated in the boiler to form live steam or reheater steam, and also a gas turbine set with a downstream heat recovery steam generator, the heat recovery steam generator being connected to the water/steam cycle in such a way that steam generated in the heat recovery steam generator is admixed with the live steam or reheater steam, is characterized in that the means for superheating or reheating have a plurality of successively connected heat transfer surfaces, and in that the steam generated in the heat recovery steam generator is fed into the water/steam cycle between the heat transfer surfaces. 
     Further refinements of the invention may be gathered from the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below by means of exemplary embodiments in conjunction with the drawings in which: 
     FIG. 1 shows a diagrammatic illustration of a heat recovery steam generator with feedwater pump, such as may be used in the method according to the invention; 
     FIG. 2 shows the heat recovery steam generator from FIG. 1 with an additional separator between the evaporator and superheater; 
     FIG. 3 shows the live steam temperature t FD  as a function of the feedwater mass flow {dot over (m)} SPW  in a heat recovery steam generator according to FIG. 2; 
     FIG. 4 shows various possibilities for feeding the steam generated in a heat recovery steam generator according to FIG. 1 into the water/steam cycle of the combined-cycle power plant in the method according to the invention; 
     FIG. 5 shows various possibilities for feeding the steam generated in a heat recovery steam generator according to FIG. 2 into the water/steam cycle of the combined-cycle power plant in the method according to the invention; 
     FIG. 6 shows the plant diagram of a combined-cycle power plant according to the invention, in which steam in the high-pressure and medium-pressure range from the heat recovery steam generator is admixed with the live steam or reheater steam in the water/steam cycle; 
     FIG. 7 shows the plant diagram of a combined-cycle power plant comparable to that of FIG. 6, steam being recirculated from the reheater to the heat recovery steam generator; 
     FIG. 8 shows the plant diagram of a combined-cycle power plant according to the invention, in which steam in the medium-pressure and low-pressure range from the heat recovery steam generator is admixed with the reheater steam or low-pressure steam in the water/steam cycle; and 
     FIG. 9 shows the plant diagram of a combined-cycle power plant according to the invention, in which steam in the medium-pressure range from the heat recovery steam generator is admixed with the reheater steam in the water/steam cycle and the feedwater of the water/steam cycle is simultaneously preheated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The diagrammatic illustration of an exemplary heat recovery steam generator  10  with feedwater pump  14 , such as may be used in the method according to the invention, is illustrated in FIG.  1 . Feedwater conveyed by the feedwater pump  14  flows through the heat recovery steam generator  10  (from the bottom upward) and is successively first preheated in an economizer  11 , then evaporated in an evaporator  12  and thereafter superheated in a superheater  13 . The temperature of the steam emerging from the heat recovery steam generator  10  depends critically (assuming a uniform power output of the gas turbine set belonging to the heat recovery steam generator) on the feed, that is to say on the feedwater mass flow: when the feed, that is to say the feedwater mass flow, is increased, the associated steam mass flow rises and the steam temperature falls correspondingly. Conversely, when the feed is reduced, the associated steam mass flow decreases and the steam temperature rises correspondingly. The temperature of the superheated steam generated in the heat recovery steam generator  10  can thereby be set or regulated (on the once-through principle) by setting or regulating the feedwater mass flow. In this case, the feedwater mass flow may be set, for example, by setting the pumping capacity of the feedwater pump  14 . If, then, as described further below in connection with FIGS. 6 to  9 , steam generated in this way is admixed with the water/steam cycle of the conventional coal block within a combined-cycle power plant, the steam temperature of the live steam and/or reheater steam there can be suitably set and regulated for different load situations. 
     In the heat recovery steam generator according to FIG. 1, the entire feedwater mass flow is converted into a corresponding steam mass flow {dot over (m)} SPW . An increase or decrease in the feed, leads, in this case, to a fall or rise in the steam temperature. A behavior deviating from this (see the accompanying graph in FIG. 3 for the live steam temperature t FD  as a function of the feedwater mass flow) occurs when, according to FIG. 2, a separator  15  is arranged in the heat recovery steam generator  10 ′ between the evaporator  12  and superheater  13 : during normal operation (point P 1  in FIG. 3) slightly superheated steam enters the separator  15 , which therefore remains dry. There is no water offtake. As a result of the continuous increase in the overfeeding of the steam generator system, that is to say by a rise in the feedwater mass flow (on the right of point P 1  in FIG.  3 ), superheating initially decreases. The live steam temperature t FD  falls (the range between point P 1  and point P 2  in FIG.  3 ). When the wet-steam zone is reached, moisture is increasingly precipitated in the separator  15  and may be diverted, for example, into a feedwater tank, into a pressure stage located below it, into a preheater or into a condenser. The mass flow arriving at the superheater  13  decreases. As a result of the consequence of decreasing steam-side load on the superheater  13 , the live steam temperature t FD  begins to rise again (on the right of point P 2  in FIG.  3 ). The commencement of moisture precipitation is designated, here, by the point P 2 . When, starting from point P 1 , the feed is reduced (on the left of P 1  in FIG.  3 ), the live steam temperature t FD  rises within increasing under-feeding. However, the temperature rise is limited by the limited exhaust gas temperature of the gas. 
     The connection of the heat recovery steam generators  10  and  10 ′ according to FIGS. 1 and 2 to the water/steam cycle of the coal block can be made in various ways and is reproduced diagrammatically in FIGS. 4 and 5. Arranged in the water/steam cycle  20  is a conventional steam generator with a superheater or reheater  21  which conventionally comprises a plurality of heat transfer surfaces  18 ,  19 . The steam generated in the heat recovery steam generator  10  or  10 ′, then, is admixed via admixing lines  16  or  17 , selectively downstream of the heat transfer surfaces  18 ,  19  or between the heat transfer surfaces  18  and  19 , with the steam circulating in the water/steam cycle  20 . In this case, the steam temperature of the steam mixture is set and regulated by means of the steam temperature of the steam admixed from the heat recovery steam generator  10  or  10 ′. 
     The plant diagram of the entire combined/cycle power plant is reproduced in four different exemplary embodiments in FIGS. 6 to  9 , the same plant parts being designated by identical reference symbols: in the example of FIG. 6, the combined-cycle power plant  22  comprises, in the first place, the water/steam cycle  20 , in which a steam turbine  38  with a high-pressure stage  39 , a medium-pressure stage  40  and a low-pressure stage  41  is arranged and drives a generator  43 . A reheater  42  is provided between the high-pressure stage  39  and the medium pressure stage  40 . The low-pressure stage  41  is followed by a condenser  44  in which the exhaust steam condenses. The condensate is pumped into a feedwater tank  47  through a multi-stage condensate preheater  46  by means of a condensate pump  45 . The feedwater is conveyed from the feedwater tank  47  by means of a feedwater pump  48  through a multi-stage feedwater preheater  49  into a conventional (coal-fired) boiler  50 , and the steam arising is superheated in a downstream superheater  51  in order then to flow as live downstream superheater  51  in order then to flow as live steam to the high-pressure stage  39  of the steam turbine  38 . 
     In addition to the conventional coal block constructed in this way, the combined-cycle power plant  22  comprises a gas turbine set  23  with a downstream heat recovery steam generator  28  which is constructed in a similar way to the heat recovery steam generators  10  and  10 ′ of FIGS. 1 and 2 or  4  and  5 . The gas turbine set  23  itself comprises, in a way known per se, a compressor  25  and a turbine  27  which are arranged on a rotor and drive a generator  24  which may be identical to the generator  43 . In order to generate the hot flue gases, a combustion chamber  26  is connected between the compressor  25  and turbine  27 . The hot exhaust gases from the turbine  27  are conducted (in countercurrent) through the heat recovery steam generator  28  and there discharge heat to the water/steam cycle  20  at different temperature stages. 
     In the example of FIG. 6, a branch-flow condensate preheater  29  working in parallel with the condensate preheater  46  is provided at the lowest temperature stage. Arranged at the next higher temperature stage is a (split) economizer  30  which preheats feedwater extracted from the water/steam cycle  20  downstream of the feedwater tank  47  by means of two pumps  36 ,  37 . 
     One branch flow coming from the economizer  30  is led through an evaporator  32  and subsequently through a superheater  33  and is admixed as superheated steam with the live steam of the water/steam cycle  20  via the admixing line  52  or  53  in the superheater  51  or downstream of the superheater  51  (see also FIG. 4 or  5 ). The other branch flow coming from the economizer  30  is led through an evaporator  31  and subsequently through a superheater  33 ′ and is admixed as superheated steam with the reheater steam of the water/steam cycle  20  via the admixing line  54  or  55  in the reheater  42  or downstream of the reheater  42 . In this case, in the heat recovery steam generator  28 , the evaporators  31 ,  32  may be connected to the associated superheaters  33  and  33 ′ either directly or via separators  34 ,  35 , as was explained further above with reference to FIGS. 1 and 2. 
     In the exemplary embodiment of FIG. 7, in the combined-cycle power plant  56  illustrated, the connection of the heat recovery steam generator  28  to the water/steam cycle  20  at the high-pressure level is the same as in FIG.  6 . By contrast, at the medium-pressure level, instead of the superheater  33 ′, a two-stage superheater  57  in the heat recovery steam generator  28  is provided into which steam is fed via a recirculation line  58 , said steam having been branched off at the inlet of the reheater  42 . 
     In the exemplary embodiment of FIG. 8, in the combined-cycle power plant  60  illustrated, the connection between the heat recovery steam generator  28  and water/steam cycle  20  at the high-pressure level is dispensed with completely. Instead, preheated condensate is branched off upstream of the feedwater tank  47  by means of a pump  61  and is converted via an economizer  62 , an evaporator  63  and a superheater  64  into low-pressure steam which is then admixed via an admixing line  65  at the inlet of the low-pressure stage  41  of the steam turbine  38 . Here, too, a separator  59  may be provided between the evaporator  63  and the superheater  64 . 
     Finally, in the exemplary embodiment of FIG. 9, in addition to the steam mixing at medium-pressure level (heat exchangers  30 ,  31  and  33 ′), there is provided in the heat recovery steam generator  28 , in parallel with the feedwater preheater  49 , feedwater preheating in which feedwater is branched off at the outlet of the feedwater tank  47  by means of a pump  68 , is preheated in a feedwater preheater  67  and is subsequently recirculated into the water/steam cycle  20 . 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 LIST OF REFERENCE SYMBOLS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10, 10′ 
                 Heat recovery steam generator 
               
               
                 11 
                 Economizer 
               
               
                 12 
                 Evaporator 
               
               
                 13 
                 Superheater 
               
               
                 14 
                 Feedwater pump 
               
               
                 15 
                 Separator 
               
               
                 16, 17 
                 Admixing line 
               
               
                 18, 19 
                 Heat transfer surface 
               
               
                 20 
                 Water/steam cycle 
               
               
                 21 
                 Superheater or reheater 
               
               
                 22, 56, 60, 66 
                 Combined-cycle power plant 
               
               
                 23 
                 Gas turbine set 
               
               
                 24, 43 
                 Generator 
               
               
                 25 
                 Compressor 
               
               
                 26 
                 Combustion chamber 
               
               
                 27 
                 Turbine 
               
               
                 28 
                 Heat recovery steam generator 
               
               
                 29 
                 Branch-flow condensate preheater 
               
               
                 30, 62 
                 Economizer 
               
               
                 31, 32, 63 
                 Evaporator 
               
               
                 33, 33′, 57, 64 
                 Superheater 
               
               
                 34, 35, 59 
                 Separator 
               
               
                 36, 37, 61, 68 
                 Pump 
               
               
                 38 
                 Steam turbine 
               
               
                 39 
                 High-pressure stage 
               
               
                 40 
                 Medium-pressure stage 
               
               
                 41 
                 Low-pressure stage 
               
               
                 42 
                 Reheater 
               
               
                 44 
                 Condenser 
               
               
                 45 
                 Condensate pump 
               
               
                 46 
                 Condensate preheater (multi-stage) 
               
               
                 47 
                 Feedwater tank 
               
               
                 48 
                 Feedwater pump 
               
               
                 49 
                 Feedwater preheater 
               
               
                 50 
                 Boiler (fired) 
               
               
                 51 
                 Superheater 
               
               
                 52, . . . , 55, 65 
                 Admixing line 
               
               
                 58 
                 Recirculation line 
               
               
                 67 
                 Feedwater preheater 
               
               
                 {dot over (m)} SPW   
                 Feedwater mass flow 
               
               
                 P1, P2 
                 Point 
               
               
                 t FD   
                 Live steam temperature