Abstract:
The invention relates to a method for operating a steam power station and a power plant as well as a corresponding steam power station. According to the invention, essentially all of the water that is drained from at least one pressure stage of the steam power station is collected, stored, and recirculated into the water circuit of steam power station.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2005/056008, filed Nov. 16, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04028295.6 filed Nov. 30, 2004, both of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a method for operating a steam power plant and in particular a method for operating a power plant for generating at least electrical energy using a steam power plant, said steam power plant having a water circuit with at least one pressure stage and water being drainable if necessary from the water circuit or pressure stages. The power plant has at least one electrical generator which can be driven by the steam power plant. The invention additionally relates to a steam power plant for generating at least electrical energy on which the method according to the invention can be carried out. 
       BACKGROUND OF THE INVENTION 
       [0003]    Such a steam power plant usually contains one or more circulation-type steam generators having pressure drums with associated heating surfaces. The circulation-type steam generators are used to produce steam, particularly in different pressure stages, which can be fed to a steam turbine or rather the relevant pressure stage of the steam turbine. The steam power plant can also have one or more so-called once-through steam generators, also known as Benson boilers which, however, are mostly incorporated in the high-pressure stage. 
         [0004]    Conventionally, steam power plants are more or less heavily drained depending on the operating state of the steam power plant. Draining takes place e.g. during ongoing operation from long-closed pipework in which condensate has collected. For this purpose the relevant pipework is briefly opened, thereby draining it. This means that water is lost from the water circuit and must be replenished by supplying additional water known as deionate. Additional draining occurs during startup or shutdown of the steam power plant, as when the steam power plant is shut down, for example, the steam present in the water circuit gradually condenses and the resulting liquid water must not remain in the system sections, particularly the heating surfaces. During shutdown, more water is drained from the water circuit than is replenished, so that finally no more water is replenished. 
         [0005]    It is known to collect the drainings, i.e. to combine them. It is also known to store some of these drainings temporarily in a tank. As the drainings, i.e. the drained water, is conventionally discarded to the environment via a pump, the tank serves only to reduce the operating time and frequency of operation of the pump. It is also known to depressurize the drained water in a separator vessel and to separate the water and steam from one another. The separated steam is then discharged into the environment. 
         [0006]    The disadvantage with the prior art is in particular that the expensively produced deionate which is drained off is not returned to the water circuit but is discarded to the environment in the form of waste water. With conventional steam power plants, the deionate costs incurred are significantly increased, particularly in the event of frequent startups and shutdowns. Moreover, the environment is considerably impacted by the heavy waste water discharge. The re-supplied deionate has a high oxygen and carbon dioxide content requiring deaeration of the deionate, which means a longer startup time for the steam power plant. 
       SUMMARY OF INVENTION 
       [0007]    The object of the invention is to eliminate the disadvantages of the prior art. Specifically the object of the invention is therefore to reduce significantly the running costs of a steam power plant, and of a power plant for generating electrical energy using such a steam power plant, which result from deionate provision. A further object of the invention is to reduce significantly the environmental impact of waste water and the consumption of water. It is likewise the object of the invention to shorten the startup time of the steam power plant with minimal cost/complexity. 
         [0008]    This object is achieved according to the invention with a method having the features set forth in the claims. In respect of apparatus, the object is achieved by a steam power plant having the features set forth in the claims. 
         [0009]    The invention has the advantage compared to the prior art that the costs of providing deionate, particularly in the event of frequent startups and shutdowns, are markedly reduced. Using the invention it is additionally possible to operate steam power plants even in regions with a severe water shortage. In addition, the invention enables a large amount of water to be saved and the environment is less impacted by discharged waste water. The startup time of the steam power plant or of the power plant is shortened. In particular, this is achieved by recycling essentially all the drained water, which essentially means, for example, that about 99% of the drained water is fed back into the system. 
         [0010]    Advantageous further developments of the invention will emerge from the sub-claims. 
         [0011]    In an advantageous embodiment of the invention the drained water is collected, stored and completely fed back to the water circuit at least from the pressure stage with the highest pressure. Thus the largest part of the drained water can be fed back in a simple manner with little expense, as the amount of water flowing in the highest pressure stage constitutes the largest part of the water in the entire water circuit. 
         [0012]    In addition to the highest pressure stage, at least one other pressure stage whose pressure level is lower than that of the highest pressure stage can be advantageously included, all the pressure stages also being able to be included in a corresponding embodiment. In this way a larger part or all of the drained water is collected, stored and fed back to the water circuit, thus saving even more water. 
         [0013]    In a further advantageous embodiment of the invention, the drained water undergoes liquid water/steam separation, it being possible for the separated steam to be fed to the condenser of the steam power plant, thereby enabling the separated clean steam to be easily cooled and liquefied in the condenser. This largely eliminates the need for special cooling of the stored water. It also provides a simple means of feeding the collected water back into the water circuit. 
         [0014]    In another preferred embodiment of the invention, the drained water accumulating during a shutdown process is only ever returned to the water circuit to the extent that the drainable water, i.e. the maximum amount of water that can be drained off, is stored at the end of the shutdown process, i.e. at standstill. In addition, the amount of water thus drained off is then returned to the water circuit at the next startup. 
         [0015]    Advantageously, at least some of the drained water is fed back to the water circuit via a water treatment plant. At the same time at least some of the water leaving the condenser can likewise be fed via the water treatment plant, it likewise being possible to mix the two sub-flows before they enter the water treatment plant. Thus, for example, the quality, in particular the degree of contamination, of the water fed to the water treatment plant can be adjusted, thereby easily preventing overloading of the water treatment plant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    An exemplary embodiment of the invention will now be explained in greater detail with reference to the accompanying schematic drawing, in which; 
           [0017]      FIG. 1  shows an exemplary embodiment of an inventive steam power plant with three pressure stages. 
       
    
    
       [0018]    Throughout the following description, the same reference numerals will be used for elements that are identical and have the same effect. 
       DETAILED DESCRIPTION OF INVENTION 
       [0019]      FIG. 1  shows a first exemplary embodiment of a steam power plant  2  according to the invention. The steam power plant  2  is an integral part of a power plant  1 , which can also be implemented for instance as a combined gas and steam turbine power plant. The steam power plant  2  has a steam turbine  4  with, in this exemplary embodiment, three different pressure areas. In the exemplary embodiment, the steam power plant  2  also has a water circuit essentially comprising the steam turbine  4 , a condenser  6 , a condensate pump  7  and three pressure stages  8 ,  9 ,  10  each assigned to the respective pressure areas of the steam turbine  4 . The water circuit additionally comprises a feed water pump (not shown). The pressure stages  8 ,  9 ,  10  are connected to the pressure areas of the steam turbine  4  by steam pipes  11 . In the exemplary embodiment, the pressure stages  8 ,  9 ,  10  are made up of the first pressure stage  8  embodied as a high-pressure stage, the second pressure stage  9  embodied as a medium-pressure stage and the third pressure stage  10  embodied as a low-pressure stage. The first pressure stage  8  of the water circuit has a once-through steam generator  12  comprising a continuous-flow heating surface  16  and a separator vessel  15 . The second pressure stage  9  has a first circulation-type steam generator  13  comprising a first pressure drum  17  and a circulation-type heating surface  18  embodied as a circulation-type evaporator. The third pressure stage  10  constructed similarly to the second pressure stage  9  has a second circulation-type steam generator  14  with a second pressure drum  19  and a second circulation-type heating surface  20  embodied as a circulation-type evaporator. 
         [0020]    The heating surfaces  16 ,  18 ,  20  are disposed in a boiler  5  which can be embodied, e.g. as in the example, as a horizontal waste-heat boiler and is fed by the exhaust gases of a gas turbine (not shown). In the exemplary embodiment, a superheater  21  is disposed downstream of each of the steam generators  12 ,  13 ,  14 . The output of the respective superheater  21  is connected to the thereto assigned pressure area of the steam turbine  4  via the respective steam pipe  11 . Each steam pipe  11  is an integral part of the respective individual pressure stage  8 ,  9 ,  10 . 
         [0021]    During operation of the steam power plant  2  or of the power plant  1 , deionized water known as deionate is supplied by the feed water pump (not shown) to the steam generators  12 ,  13 ,  14  via piping which is not shown for simplicity&#39;s sake. As, in the example shown, different types of steam generators  12 ,  13  , 14  can be used which have different requirements in terms of the quality of the deionate supplied, in particular the ph value, the deionate is conditioned accordingly by a corresponding device (not shown) shortly before it enters the relevant steam generator  12 ,  13 ,  14 . The steam generator  12 ,  13  , 14  evaporates the water fed to it. In the once-through steam generator  12  further superheating mostly occurs. The evaporated water is superheated in the following superheater  21  and fed via the steam pipes  11  to the respective pressure area of the steam turbine  4 . 
         [0022]    The water leaving the high-pressure area of the steam turbine  4  in the form of steam is conventionally fed to the next-lower pressure stage via piping which is not shown for the sake of clarity. In the example, water leaving the high-pressure area of the steam turbine  4  in the form of steam is therefore fed to the second pressure stage  9 . Water leaving the medium-pressure area of the steam turbine  4  in the form of steam is fed to the third pressure stage  10 , and therefore finally also to the steam turbine&#39;s lowest pressure area  10 . 
         [0023]    The water leaving the low-pressure area of the steam turbine  4  is fed via an exhaust steam pipe  41  to the condenser  6  for cooling and liquefaction. The exhaust steam pipe  41  completes the water circuit of the steam power plant  2  between steam turbine  4  and condenser  6 . 
         [0024]    The water leaving the condensate pump  7  is mainly fed to the first pressure stage  8  via the feed water pump (not shown). In the exemplary embodiment, the amount of water flowing in the first pressure stage  8  during operation constitutes approx. 75% of the amount of water flowing in all the pressure stages  8 ,  9 ,  10 , as much more power is converted in it than with the other pressure stages  9 ,  10 . 
         [0025]    The energy supplied to the steam turbine  4  in the steam is converted to rotational energy in the steam turbine  4  and thus applied to the associated electrical generator  3 . 
         [0026]    During operation, particularly also during startup and shutdown, water is intermittently or in some cases continuously drained from the pressure stages  8 ,  9 ,  10 . For this purpose the drained water is first collected by a collecting apparatus  22  which in the example is embodied by a first pipe bundle  23  and a second pipe bundle  24 . For example, water is continuously drained from the pressure drums  17  and  19  during nominal operation of the steam power plant  2 . This process is also known as desludging, as circulating operation causes deposits to build up in the pressure drums  17 ,  18  which must be removed. For example, approx. 0.5 to 1% of the water throughput of the pressure drums  17 ,  18  must be continuously drained. As there is no such circulation in the once-through steam generator  12  during nominal operation, the separator vessel  15  in the exemplary embodiment does not need to be continuously drained, but mainly during startup and shutdown at the most. The superheaters  21  among other things are also drained, but again mainly during startup and shutdown only. In the exemplary embodiment, water is also drained from the steam pipes  11  and collected by the second pipe bundle  24 . Water can also be drained from other areas or sections of the pressure stages  8 ,  9 ,  10  that are not shown because of the simplified representation of the exemplary embodiment. 
         [0027]    In the exemplary embodiment, the water drained from the pressure stages  8 ,  9 ,  10  and collected is then stored. For this purpose a plurality of storage tanks  25 ,  26 ,  27  and  28  are provided which can be more or less filled depending on the operating state of the power plant  1 . Specifically in the exemplary embodiment the water drained from the pressure drums  17 ,  19 , the water drained from the separator vessel  15  and the water drained from the superheaters  21  is first fed to the first storage tank  25  where it is stored. The first storage tank  25  is made large enough to ensure that it can initially store for a time, and therefore buffer, the very high inflow of drained water during startup or shutdown of the steam power plant  2 . The first storage tank  25  also acts as first separating device  32 , as the hot drained water evaporates in the first storage tank  25 , liquid water being separated from steam and the per se contaminant-free steam being fed via a first feedback pipe  29  to the input of the condenser  6  and the liquid water being stored for the moment in the storage tank  25 . Liquid water stored in the first storage tank  25  is pumped if necessary into a third storage tank  27  by means of a first pump  34 . By means of a branch disposed downstream of the output of the first pump  34 , the pumped amount of water can be partially or completely pumped back into the first storage tank  25  via a first cooler  37  by an appropriate setting of a valve (not shown), thereby providing additional cooling of the water stored in the first storage tank  25 . In particular, by using the first cooler  37 , the amount of water evaporated can be reduced and the thermal loading of the condenser  6  can be lessened. 
         [0028]    In the exemplary embodiment, the water drained from the steam pipes  11  of the pressure stages  8 ,  9 ,  10  is drained by the second pipe bundle  24  and stored in the second storage tank  26 . Like the first storage tank  25 , the second storage tank  26  is also assigned a cooling circuit consisting of a second pump  35  and a second cooler  38 . The second storage tank  26  additionally has a second separating device  33  constituted as in the first storage tank  25 , the per se clean water vapor again being feedable to the input of the condenser  6  via a second feedback pipe  30 . The liquid water stored in the second storage tank  26  can once again be fed to the third storage tank  27  via the second pump  35  if necessary. 
         [0029]    In the exemplary embodiment, the liquid water stored in the third storage tank  27  is if necessary fed via a third cooler  39 , a third pump  36  and a water treatment plant  40  to the input of the condensate pump  7  via a third feedback pipe  31 . 
         [0030]    The water treatment plant  40  is connected and disposed in such a way that the entire liquid phase of the drained water is fed into it and conditioned before said liquid phase is fed back into the water circuit of the steam power plant  2 . All the water leaving the third storage tank  27  is fed via the water treatment plant  40  where it is conditioned. In the exemplary embodiment, the water treatment plant  40  is disposed in the secondary flow of the water circuit, a sub-flow of the water leaving a fourth storage tank  28  embodied as a condensate collecting tank being feedable to the water treatment plant  40  via the third pump  36 . In the exemplary embodiment, the sub-flow can be mixed with the liquid water coming from the third storage tank  27  before it reaches the water treatment plant  40 . Particularly during nominal operation of the steam power plant  2 , all the water leaving the condenser  6  can be fed via the water treatment plant  40 , the water treatment plant  40  then being in the main flow of the water leaving the condenser  6 . 
         [0031]    In the exemplary embodiment according to the invention, all the water drained over a particular period is collected, stored to a defined extent and then fed into the water circuit. In the exemplary embodiment, the water drained from all the pressure stages  8 ,  9 ,  10  is collected, stored and fed back. In other exemplary embodiments (not shown) the water drained from a single, preferably the highest, pressure stage  8  can be collected, stored and fed back in this manner. 
         [0032]    During shutdown, i.e. when the steam power plant  2  is being deactivated, drainings increasingly accumulate. This is also the case during startup, as the steam parameters required for nominal operation can only be attained gradually. The water circuit must also be maintained during shutdown, as heat must be removed from the pressure stages  8 ,  9 ,  10  by the circulating water. The accumulated amount of water to be drained is at its greatest at the end of the shutdown process. The drained water can also be fed back during the shutdown process, but this takes place in such a way that all the water is stored at the end of the shutdown process. The storage tanks are designed according to their size or capacity. The pumps  34 ,  35 ,  36  and  7  are controlled accordingly. Particularly during a restart, in this way only a small amount of new deionate needs to be added to the water circuit, thereby saving water and lessening the environmental impact through reduced waste water discharge. 
         [0033]    Particularly advantageous in the exemplary embodiment is the inventive disposition and use of the water treatment plant  40 , as a once-through steam generator  12  is used in the highest pressure stage  8 . Once-through steam generators  12  pose more stringent requirements in terms of water quality which can usually only be produced and ensured by the water treatment plant  40 . The different water quality requirements compared to the circulation-type steam generator  13 ,  14  relate in particular to the pH value and oxygen content. As the water treatment plant  40  is necessary anyway because of the once-through steam generator  12 , it is more advantageous to feed the comparatively small amounts of water drained from the circulation-type steam generator  13 ,  14  back to the water circuit likewise via the water treatment plant  40  than to discard them. This mainly applies also to the comparatively heavily contaminated quantities of water desludged from the pressure drums  17 ,  19 , or desludged from the separator vessel  15  during startup and shutdown. In order to relieve the water treatment plant  40 , however, it is conceivable not to feed the desludgings from the pressure drums  17 ,  18  of the circulation-type steam generator  13 ,  14  back into the water circuit. Steam/liquid water separation is nevertheless possible for these desludgings, the then per se clean steam accumulating being able to be fed back to the water circuit, in particular to the input of the condenser  6 . 
         [0034]    The water treatment plant  40  can have in particular a mechanical cleaner and a cation/anion exchanger. The water treatment plant  40  conditions the water fed to it, particularly in respect of its chemical properties. 
         [0035]    The entire water circuit, in particular the collecting apparatus  22 , the storage tanks  25 ,  26 ,  27 ,  28  and the feedback pipes  29 ,  30 ,  31 , are sealed to the atmosphere in order to prevent uncontrolled air input to the drained water. 
         [0036]    The features of the exemplary embodiment can be combined together.