Abstract:
The invention relates to a method for starting a gas and steam turbine system which comprises a gas turbine system which comprises at least one gas turbine, in addition to at least one steam turbine system which comprises at least one steam turbine and at least one steam system. Heat produced by the working fluid and which is released in the gas turbine is guided to the steam system in order to produce steam which drives the steam turbine. According to the invention, during starting, the gas turbine is started prior to the steam turbine and the steam turbine is started in the presence of the first steam in the system and is impinged upon by said steam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2006/061217, filed Mar. 31, 2006 and claims the benefit thereof. The International Application claims the benefits of European application No. 05007416.0 filed Apr. 5, 2005, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a method for starting up a gas and steam turbine system, and in particular a method for a fast startup of a system of said kind. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a gas turbine system a gaseous or liquid fuel, for example natural gas or crude oil, is mixed with compressed air and combusted. The pressurized combustion exhaust gases are supplied to the turbine of the gas turbine system as the working medium. The working medium sets the turbines under expansion into rotation, with thermal energy being converted into mechanical work, i.e. the rotation of the turbine shaft. When the expanded working medium is discharged from the gas turbine system said medium typically still has a temperature of 500-600° Celsius. 
         [0004]    In a gas and steam turbine system the expanded working medium, also called flue gas, from the gas turbine system is used to generate steam for driving a steam turbine. Toward that end the working medium is supplied to a heat recovery steam generator connected downstream of the gas turbine system on the exhaust gas side, in which steam generator heating surfaces are arranged in the form of pipes or pipe bundles. Said heating surfaces are in turn connected into a water-steam cycle of the steam turbine system which has at least one, but mostly a plurality of pressure stages. The pressure stages differ from one another in that the water supplied to the heating surface for the purpose of generating steam has different pressure levels. A gas and steam turbine system comprising a water-steam cycle having only one pressure stage is described in DE 197 36 888 A1, and such a system comprising three pressure stages, namely a high-pressure stage, a medium-pressure stage and a low-pressure stage, is described in DE 100 04 187 C1. 
         [0005]    Currently, in order to start a gas and steam turbine system, the gas turbine system is usually started up and the expanded working medium is supplied to the heat recovery steam generator of the steam turbine system. Initially, however, the steam generated in the heat recovery steam generator is not fed to the turbine part of the steam turbine system, but is directed past the turbine via diverter stations and supplied directly to a condenser which condenses the steam to water. The condensate is then supplied to the steam generator again as feedwater. In many embodiment variants of gas and steam turbine systems the diverted steam is also conveyed to the atmosphere. 
         [0006]    The steam turbine is only switched into the cycle when certain steam parameters in the steam lines of the water-steam cycle or in the steam lines leading to the turbine part of the gas turbine system, for example certain steam pressures and temperatures, are complied with. Complying with said steam parameters is designed to keep potential stresses in thick-walled components at a low level. 
         [0007]    After the startup of the gas turbine system there is a power increase which leads to an increase in pressure in the steam system. The load gradient at which the gas turbine system is started up, i.e. the power increase of the gas turbine system per time unit, is critically dependent on the implementation and mode of construction of the heat recovery steam generator as well as on the structural limitations within the steam turbine. As the gas turbine load and consequently the temperature or, as the case may be, the volume flow rate of the exhaust gas emitted from the gas turbine system increase, the steam temperature and the pressure in the steam system are also increased. 
         [0008]    Before the steam turbine starts up, the gas turbine is typically kept at a specific partial load until stationary states have come about in the gas turbine system and in the steam system. As soon as stable steam production has been reached, the steam contained in the steam system is channeled to the steam turbine, thereby accelerating the steam turbine. The turbine speed is then increased to nominal speed. Following synchronization of the generator coupled to the steam turbine with the power supply system, or in the case of single-shaft systems, following the engagement of the overrunning clutch, the steam turbine is subjected to further load as a result of an increase in the steam supply. At the same time the diverter stations close more and more in order to keep the steam pressure roughly constant and minimize level fluctuations in the heat recovery steam generator. 
         [0009]    As soon as the diverter stations are closed and the steam produced in the heat recovery steam generator is channeled in its entirety to the steam turbine, a further increase in the gas turbine power output takes place when there is a higher power requirement on the part of the system which is now operating in the gas and steam turbine mode. 
         [0010]    By definition, the startup operation of a gas and steam turbine system is terminated only when the gas turbine has reached the base load and all diverter stations are closed. 
       SUMMARY OF INVENTION 
       [0011]    The object of the present invention is to provide a method for starting up a gas and steam turbine system which enables a faster startup operation than the method described in the introduction. 
         [0012]    This object is achieved by means of a method for starting up a gas and steam turbine system as claimed in the claims. The dependent claims contain advantageous embodiments of the method. 
         [0013]    According to the invention, a method is provided for starting up a gas and steam turbine system, in particular for fast starting up of a gas and steam turbine system which has a gas turbine system comprising at least one gas turbine as well as a steam turbine system having at least one steam turbine and at least one steam system and in which the waste heat of a working medium expanding in the gas turbine is supplied to the steam system for the purpose of generating the steam driving the steam turbine. 
         [0014]    In the method according to the invention, at startup time the gas turbine is started first, before the steam turbine is started. The steam turbine is then already started up when the first steam is present in the steam system and is impinged upon by steam. 
         [0015]    In the method according to the invention, the steam turbine is started up at the earliest possible time and accelerated by means of the first steam from the heat recovery steam generator, without waiting for stationary states in the steam system. This measure enables the startup operation of the gas and steam turbine system to be shortened considerably. 
         [0016]    In contrast to the usual startup method, the steam temperature in the steam system at the time of starting the steam turbine can be less than the material temperature of the steam turbine or of its housing. The early channeling of the steam to the steam turbine can therefore lead to a cooling down of the components and to thermal stresses. However, a certain compensation can be achieved if the gradients are kept correspondingly low during the following increase in the steam temperatures. 
         [0017]    Advantageously, the tuning of the steam system during the startup operation is chosen in such a way that the steam pressure increases continuously. This can be achieved, for example, by opening a steam diverter station of the steam system only so wide that a minimum steam quantity required for accelerating and/or synchronizing the steam turbine is generated using a part of the waste heat of the working medium and a pressure increase in the steam system is produced by means of the remaining part of the waste heat of the working medium. 
         [0018]    In addition to a pressure increase in the steam system, the comparatively small opening of the steam diverter station leads to a reduction in the steam production in the heat recovery steam generator. As a result the thermal load to the condenser is reduced and the diverter station can close more quickly. 
         [0019]    In a special embodiment of the method according to the invention the diverter station is not opened at all. 
         [0020]    The method according to the invention can be embodied in particular in such a way that the gas turbine system experiences a load increase during the entire startup operation, in particular until the base load is reached. In other words, the method dispenses with keeping the gas turbine system at a certain partial load and waiting until the gas turbine system and the steam system of the steam turbine system have settled into stationary states. This measure also leads to a reduction in the startup time of the gas turbine system and thus enables a fast startup. 
         [0021]    In a special embodiment the gas turbine system&#39;s load is increased at maximum load ramp, which is to say that there is a maximum increase in the gas turbine power output per time unit. 
         [0022]    The gas and steam turbine system during the starting up of the gas turbine system to base load is preferably switched over into the gas and steam turbine operating mode, with the result that the startup operation is, by definition, terminated when the gas turbine base load is reached. The switchover into the gas and steam turbine operating mode can include in particular the synchronization of a generator coupled to the steam turbine with the power supply system or, in the case of single-shaft systems, the engagement of the automatic overrunning clutch. 
         [0023]    The described method according to the invention for starting up a gas and steam turbine system shortens the startup time of the system considerably. Compared with the method described in the introduction, a reduction in the starting time by approximately 50% is achievable. A gas and steam turbine operator can therefore respond very flexibly to short-term requirements, as a result of which the revenues from the purchase of power can be increased. As a result of the early steam takeover of the steam turbine and the reduced thermal load in the condenser, which leads to smaller power losses, there is also an increase in the averaged efficiency of the gas and steam turbine system, which is a significant factor in particular in the case of frequent starts and increases the cost-effectiveness of the system. 
         [0024]    Moreover, the lower steam production in the method according to the invention for starting up a gas and steam turbine system also enables smaller diverter stations to be installed, thereby reducing investment costs. 
         [0025]    The described startup method enabling a fast startup of a gas and steam turbine system can essentially be realized by means of software modifications. It is therefore also possible to convert existing gas and steam turbine systems to the startup method according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0026]    Further features, characteristics and advantages of the present invention will emerge from the following description of an exemplary embodiment with reference to the accompanying figure. 
           [0027]      FIG. 1  shows a schematic diagram for a gas and steam turbine system. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0028]    The gas and steam turbine system  1  represented schematically in  FIG. 1  comprises a gas turbine system  1   a  as well as a steam turbine system  1   b . The gas turbine system  1   a  is equipped with a gas turbine  2 , a compressor  4 , and at least one combustion chamber  6  connected between the compressor  4  and the gas turbine  2 . By means of the compressor  4 , fresh air L is drawn in, compressed and supplied via the fresh air line  8  to one or more burners of the combustion chamber  6 . The supplied air is mixed with liquid or gaseous fuel B fed via a fuel line  10  and the mixture ignited. The resulting combustion exhaust gases form the working medium AM of the gas turbine system  1   a , which working medium AM is supplied to the gas turbine  2 , where it produces work under expansion and drives a shaft  14  coupled to the gas turbine  2 . The shaft  14  is coupled not only to the gas turbine  2  but also to the air compressor  4  as well as to a generator  12  in order to drive the latter. The expanded working medium AM is conducted via an exhaust gas line  34  to a heat recovery steam generator  30  of the steam turbine system  1   b.    
         [0029]    In the heat recovery steam generator  30  the working medium output by the gas turbine  1   a  at a temperature of approx. 500-600° Celsius is used for generating and superheating steam. 
         [0030]    In addition to the heat recovery steam generator  30 , which can be embodied in particular as a once-through, forced-flow system, the steam turbine system  1   b  comprises a steam turbine  20  having turbine stages  20   a,    20   b,    20   c  and a condenser  26 . The heat recovery steam generator  30  and the condenser  26 , in combination with condensate lines and feedwater lines  35 ,  40  as well as steam lines  48 ,  53 ,  64 ,  70 ,  80 ,  100 , form a steam system which, together with the steam turbine  20 , forms a water-steam cycle. 
         [0031]    Water from a feedwater reservoir  38  is supplied by means of a feedwater pump  42  to a high-pressure preheater  44 , also known as an economizer, and from there is forwarded to an evaporator  46  which is designed for once-through operation and is connected to the economizer  44  on the output side. For its part, the evaporator  46  is in turn connected on the output side to a superheater  52  via a steam line  48  into which a water separator  50  is inserted. The superheater  52  is connected on the output side via a steam line  53  to the steam input  54  of the high-pressure stage  20   a  of the steam turbine  20 . 
         [0032]    In the high-pressure stage  20   a  of the steam turbine  20 , the superheated steam from the superheater  52  drives the turbine before it is passed on via the steam output  56  of the high-pressure stage  20   a  to an intermediate superheater  58 . 
         [0033]    After being superheated in the intermediate superheater  58 , the steam is forwarded via a further steam line  81  to the steam input  60  of the medium-pressure stage  20   b  of the steam turbine  20 , where it drives the turbine. 
         [0034]    The steam output  62  of the medium-pressure stage  20   b  is connected via an overflow line  64  to the steam inlet  66  of the low-pressure stage  20   c  of the steam turbine. After flowing through the low-pressure stage  20   c  and the driving of the turbine associated therewith, the cooled and expanded steam is output via the steam output  68  of the low-pressure stage  20   c  to the steam line  70 , which leads it to the condenser  26 . 
         [0035]    The condenser  26  converts the incoming steam into condensate and forwards the condensate by means of a condensate pump  36  to the feedwater reservoir  38  via the condensate line  35 . 
         [0036]    In addition to the already mentioned elements of the water-steam cycle, the latter also comprises a bypass line  100 , what is referred to as the high-pressure diverter line, which branches off from the steam line  53  before the latter reaches the steam inlet  54  of the high-pressure stage  20   a.  The high-pressure bypass line  100  bypasses the high-pressure stage  20   a  and flows into the feed line  80  to the intermediate superheater  58 . A further bypass line, referred to as the medium-pressure bypass line  200 , branches from the steam line  81  before the latter flows into the steam inlet  60  of the medium-pressure stage  20   b.  The medium-pressure bypass line  200  bypasses both the medium-pressure stage  20   b  and the low-pressure stage  20   c  and flows into the steam line  70  leading to the condenser  26 . 
         [0037]    Incorporated into the high-pressure bypass line  100  and the medium-pressure bypass line  200  are the shutoff valves  102 ,  202 , by means of which said lines can be shut off. Shutoff valves  104 ,  204  are also included in the steam line  53  and in the steam line  81 , in each case between the branching-off point of the bypass line  100  and  200 , respectively, and the steam inlet  54  of the high-pressure stage  20   a  and the steam inlet  60  of the medium-pressure stage  20   a,  respectively. 
         [0038]    Incorporated into the medium-pressure bypass line  200  is a shutoff valve  202  by means of which said line can be shut off. A shutoff valve  104  is also included in the steam line  53 , namely between the branching-off point of the bypass line  100  and the steam inlet  54  of the high-pressure stage  20   a  of the steam turbine  20 . 
         [0039]    The bypass line  100  and the shutoff valves  102 ,  104  are used during the starting up of the gas and steam turbine system  1  to divert a part of the steam for the purpose of bypassing the steam turbine  2 . 
         [0040]    An exemplary embodiment of the method according to the invention for starting up a gas and steam turbine system is described below based on the system  1  described with reference to  FIG. 1 . 
         [0041]    At the start of the method the gas turbine system  1   a  is started and the working medium AM being discharged from the system is supplied to the heat recovery steam generator  30  via an input  30   a.  The expanded working medium AM flows through the heat recovery steam generator  30  and exits the latter via an output  30   b  in the direction of a vent stack (not shown in  FIG. 1 ). As the working medium AM flows through the heat recovery steam generator  30 , heat is transferred from the working medium AM to the water or steam in the water-steam cycle. 
         [0042]    After the gas turbine system has been started up, the waste heat of the working medium in the heat recovery steam generator  30  leads to the start of steam production in the steam system. 
         [0043]    In this early phase of the startup operation the shutoff valves  102  and  104  or  202  and  204  are set in such a way that only a small part of the generated steam flows through the bypass lines  100 ,  200  and already in this phase of the startup operation the majority of the steam is supplied to the steam turbine  20 . The part of the steam supplied to the steam turbine  20  accelerates the steam turbine and preheats the latter insofar as the steam is hotter than the material of the turbine and the steam lines. 
         [0044]    Since only a small amount of steam flows directly to the condenser  26  via the medium-pressure bypass line  200 , the waste heat not used during the acceleration and preheating of the steam turbine  20  leads to a pressure increase in the steam system. In the further course of the startup operation the steam pressure therefore increases continuously in the steam system, as a result of which steam production in the heat recovery steam generator is reduced. This leads to a reduction in the heat input into the condenser  26  and as a result the shutoff valves  102  and  202 , which are not fully open anyway, can be closed quickly compared to prior art starting methods. 
         [0045]    Once the gas turbine system  1   a  has been started, the load of the gas turbine system is increased preferably at maximum load ramp until the base load is reached. 
         [0046]    If the steam temperature is less than the material temperature of the turbine  20  at the start of the introduction of steam into the steam turbine  20 , the steam temperature will steadily increase during the startup of the load of the gas turbine system and relatively soon exceed the material temperature of the steam turbine and the lines leading thereto. If the rapid rise from a relatively cool temperature of the turbine components to a high temperature would exceed a certain predefined limit of the thermal stresses in the material due to the starting up of the gas turbine system at maximum load ramp, the power output of the gas turbine system can also be increased at a lower ramp than the maximum load ramp, with the result that the steam temperatures rise more slowly. 
         [0047]    Since the bypass lines  100 ,  200  are closed at an early stage in the startup method according to the invention and the gas and steam turbine system  1  is switched over into the gas and steam turbine operating mode already during the starting up of the gas turbine system  1   a  to base load, the startup operation is terminated when the gas turbine base load is reached. 
         [0048]    Even if the steam turbine load were to reach only a magnitude of approximately 80-90% when the gas turbine base load is reached, the startup operation is deemed to be completed according to the definition whereby the startup operation is terminated when the base load of the gas turbine system is reached and the bypass lines are closed. Depending on the dynamic characteristics of the heat recovery steam generator, a further pressure increase will take place over several minutes and will be completed after approximately 10-20 further minutes. The amount of steam will increase accordingly, and steam turbine power output ratings in excess of 95% will be achieved as a function of steam temperature. 
         [0049]    The startup method according to the invention has been described with reference to a gas and steam turbine system comprising a water-steam cycle which has only one pressure stage. It should, however, be pointed out at this juncture that the method according to the invention can also be applied in the case of gas and steam turbine systems which have more than one pressure stage in the water-steam cycle. A gas and steam turbine system comprising three pressure stages, namely a high-pressure stage, a medium-pressure stage and a low-pressure stage in the water-steam cycle, for which the startup method according to the invention can also be used, is described for example in DE 100 04 187 C1, to which reference is made in relation to the embodiment of a gas and steam turbine system comprising a plurality of pressure stages.