Abstract:
The invention relates to a method for operating a steam power installation, whereby steam produced in a boiler is condensed in a condenser after passing through at least one turbine, and the condensate obtained is preheated and redirected back to the boiler as boiler feed-water. In order to preheat the condensate, said condensate is split into a first partial current and a second partial current. Only the first partial current is preheated and the second partial current is then mixed with the preheated first partial current. The power of the turbine can thus be increased as required, up to the boiler reserve of the steam power installation.

Description:
CROSS REFERENCE TO APPLICATIONS 
     This application is the continuation of International Application No. PCT1EP02/02023, filed Feb. 25, 2002 and claims the benefit thereof. The International Application claims the benefits of European application No. 01106600.8 EP, filed Mar. 15, 2001, both of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method for operating a steam power installation, whereby steam generated in a boiler is condensed in a condenser after flowing through at least one turbine, and the condensate obtained is preheated and fed back to the boiler as feed-water. The invention also relates to a steam power installation for implementing the method. 
     BACKGROUND OF THE INVENTION 
     A steam power installation is normally used for generating electrical power or for driving a machine. It involves a working medium, usually a water-water/steam mixture, which is fed through a steam-generating circuit of the steam power installation, being converted into steam in an evaporator or steam generator (boiler). The steam that is generated expands to produce work in a steam turbine and is then fed to a condenser. The working medium condensed in the condenser is then fed via a pump to the boiler again for generating steam. 
     In a generally known steam power plant of this type, the condensate used as feed-water is successively preheated to close to the boiling temperature by means of partial steam mass flows from the turbine steam volume, thereby increasing the thermodynamic efficiency of the whole process. By removing the steam from the turbine steam volume, however, the subsequent steam turbine stages can extract less power from the steam fluid. 
     A method for operating a steam power plant is known from EP-A2-1 055 801, in which the condensate used as feed-water is successively preheated to close to the boiling temperature by means of partial steam mass flows from the turbine steam volume. 
     In order to avoid the reduction in power extraction in the subsequent steam-turbine stages, it is provided that the heat emitted from fuel cells is used to preheat the condensate. Preheating the feed-water from the heat emitted from the fuel cells, and the associated increase in the amount involved in the expansion, achieves an increase in the steam process efficiency. The fuel-cell arrangement incorporated in the preheating line of EP-A2-1 055 801 is a relatively complex and costly way of achieving preheating by the external supply of heat via the fuel cells. 
     SUMMARY OF INVENTION 
     The object of the invention is to define a method of the type cited in the introduction, in which preheating of the boiler feed-water to be fed to the boiler is achievable while simultaneously increasing the power of the turbine. A further object of the invention is to specify a steam power installation that can be used to implement such an operating method. 
     This object is achieved according to the invention by a method for operating a steam power installation, in which steam generated in a boiler is condensed in a condenser after flowing through at least one turbine, and the condensate obtained is preheated and fed back to the boiler as feed-water, the condensate being divided for condensate preheating into a first partial flow and a second partial flow, only the first partial flow being preheated, and the second partial flow being re-mixed with the preheated first partial flow. 
     The invention is also based on the consideration that to increase the efficiency of a turbine connected in a steam power installation, not only does the steam mass flow through the turbine need to be taken into account, but also the preheat temperature of the boiler feed-water fed to the boiler. Both process parameters are coupled together by the bleeding of the turbine commonly performed in steam power installations, whereby a partial steam mass flow is removed from the steam turbine process to preheat the condensate obtained. This steam removal is at the expense of the turbine power, in particular of the overall efficiency of the steam power installation. In the known installations, the condensate obtained in the condenser is completely preheated using bleeder steam, being preheated to as high a temperature as possible close to the boiling temperature before being fed to the boiler as boiler feed-water. This rigid coupling between the condensate preheating and the steam removal means that the turbine power is fixed for constant live steam pressure. 
     A completely different way is now demonstrated by the invention, in which, if necessary, an increase in the turbine power of a steam power installation is achieved by flexible setting of the preheat temperature according to demand by means of mixing partial flows of condensate. To do this, the condensate is divided into a first partial flow and a second partial flow, only the first partial flow being preheated, and the second partial flow being re-mixed with the preheated first partial flow. The term partial flow is intended here as a true partial flow of the condensate condensed in the condenser. By mixing the first, preheated condensate flow with the second, non-preheated condensate flow, one is able to obtain, compared with preheating the whole condensate, a mixture temperature that is lower than the temperature of the preheated first partial flow of condensate prior to mixing with the second partial flow. Flexible setting of the mixture temperature is advantageously possible by adjusting the partial flows. 
     Of particular advantage is the fact that by preheating only a partial flow, a smaller amount of heat is required to preheat the first partial flow compared with preheating the complete condensate in the known installations. Thus process heat in the form of a higher steam mass flow through the turbine is available to increase the turbine power. The method provides, for the first time, the possibility of increasing the turbine power according to demand, frequently if necessary, up to the boiler reserve (not seconds reserves) of a steam power installation by partial and selective bypassing of the preheating by the second partial flow of condensate, without needing to raise the live steam pressure above the design value. 
     The division into the fist partial flow and the second partial flow can advantageously be flexibly set according to the power demand, so that correspondingly more or less process steam is available in the turbine to do work. 
     Also of advantage is the fact that, using the solution presented, it becomes possible for the first time to achieve an increase in power without limiting the lifetime of the components, in particular the preheating devices of the steam turbine installation, as a result of only part of the steam flowing through the preheating line. In particular, heat consumption is also clearly more efficient than when the preheating line is completely bypassed when, for at least some of the time, absolutely no condensate is preheated, i.e. the first partial flow equals 0. This is important, for example for high-pressure pre-heaters or the like. 
     In a particularly advantageous embodiment, the first partial flow is preheated using bleeder steam from the turbine. Using bleeder steam from the turbine to preheat only the first partial flow ensures that only a correspondingly smaller amount of bleeder steam is required for preheating compared with traditional bleeding. Thus more process steam in the steam turbine is directly available for increasing the turbine power. It is also advantageous if the condensate mass flow of the first partial flow is directly correlated to the bleeder-steam mass flow, so that the greater the first partial flow, the larger the amount of bleeder steam required to preheat the first partial flow to a desired temperature. By suitable coupling of the bleeder-steam flow to the first partial flow, the bleeder-steam requirement regulates itself automatically. This self-regulation effect makes the method a particularly cost-effective and flexible means of operating the steam power installation, in particular of increasing the turbine power. 
     In a preferred embodiment, the first partial flow is preheated in at least two stages. By preheating the first partial flow of condensate in more than one stage, it is possible to precisely set a desired temperature of the first partial flow after preheating. According to demand, all pre-heater stages or just a part of the pre-heater stages can be provided for preheating the first partial flow. In this way one advantageously has the option of utilizing to capacity individual preheating stages, and thus having additional process heat available for the turbine process. The precise setting of a desired temperature of the first partial flow after preheating and prior to mixing with the second partial flow, also enables the mixture temperature of the partial flows to be set precisely, so that the preheat temperature of the boiler feed-water can be set equally precisely. In an alternative embodiment, preheating of the first partial leg is also possible in just one stage, in particular precisely one stage. 
     Preferably, a preheat temperature of the boiler feed-water of 210° C. to 250° C., in particular 220° C. to 240° C., is set for the mixing of the partial flows. The pressure of the boiler feed-water then typically equals about 300 bar. By mixing with the second, non-preheated partial flow, the preheat temperature of the boiler feed-water is reduced by about 30° C. to 70° C. compared with the temperature of the preheated first partial flow. 
     In a preferred embodiment, the first partial flow and the second partial flow are divided in the ratio 0.4 to 0.8, in particular in the ratio 0.6 to 0.7. For example, in a typical operating mode of the steam power installation according to the method of the invention, the condensate obtained in the condenser is divided such that the first partial flow of condensate equals about 60% and the second partial flow of condensate about 40%. In this case, the first partial flow is preheated from a temperature of about 200° C. to a temperature of about 280° C., while the second partial flow is not preheated and continues at a temperature of 200° C. up to the mixing with the first partial flow. The pressure of the condensate flows remains substantially constant throughout at about 300 bar. 
     Advantageously, the preheat temperature of the feed-water to be fed to the boiler can be set according to demand by proportioning the amount of the second partial flow bypassing the preheating line, and mixing the two partial flows after preheating the first partial flow, where the division of the partial flows is preferably controlled or regulated. 
     In addition, after the mixing of the partial flows, the mixture is preferably fed as boiler feed-water to a fossil-fired steam generator. The method of the invention is intended in particular for use in steam power installations having a boiler that is fired with a fossil fuel, for instance coal or oil. 
     The object, directed at a steam power installation, is achieved according to the invention by a steam power installation for implementing the method described above, comprising a boiler for generating steam, at least one turbine, a condenser connected on the steam outlet side of the turbine, a condensate line for feeding the condensate back to the boiler, and a preheating device connected in the condensate line for preheating condensate, where a bypass line bypassing the preheating device is provided so that the preheating device only receives a first partial flow of the condensate. 
     Providing a bypass line that bypasses the preheating device ensures that the preheating device only receives the first partial flow of condensate, while a second partial flow flows through the bypass line without preheating. Bypass line is here understood to mean that it is taken parallel to the preheating device, the bypass line branching off from the condensate line upstream of the preheating device, and being reconnected to the condensate line downstream of the preheating device. A branching point is provided upstream of the preheating device for this purpose, while a mixing point is positioned downstream of the preheating device. The condensate from the condenser can be divided at the branching point into the first partial flow and a second partial flow complementary to this with respect to the whole condensate flow. After the branching point, viewed from the direction of flow of the condensate, the first condensate flow is fed along the condensate line in which the preheating device for preheating the condensate is connected. The second condensate flow and the preheated first condensate flow can be mixed at the mixing point, i.e. at the downstream-located connection point of bypass line to condensate line, whereby a mixture temperature can be set according to the mass flow of the first and the second partial flow of condensate, and according to the heat absorption of the first condensate flow in the preheating device. 
     In a particularly preferred embodiment, the preheating device is connected to the turbine via a bleeder line, achieving a direct coupling between bleeder steam as preheating medium in the heat exchange and the first partial flow of condensate in the preheating device of the steam power installation. The thermal energy needed for preheating can be set directly via the bleeder-steam mass flow, the bleeder-steam mass flow varying under self regulation according to the level of the mass flow of the first partial flow. The greater the first partial flow, the greater the heat requirement in the preheating device, and hence also the amount of bleeder steam removed from the turbine. 
     The bypass line preferably has a control valve for regulating a second partial flow of the condensate that bypasses the preheating device. The control valve is used to regulate or perhaps to preset the second partial flow, which does not flow through the preheating device and thus does not lead to a removal of bleeder steam. The second partial flow can be precisely set by the control valve in the bypass line, and hence also the amount of heat required for preheating in the preheating device the second partial flow complementary to the first partial flow. In addition, the mixture temperature that is established in the condensate line when the partial flows mix at the mixing point, can advantageously be regulated by the control valve. By this means, the amount of the second partial flow bypassing the preheating device in the bypass line can be set, in particular it can be regulated in a suitable control circuit, according to the level of demand for increased power from the steam turbine. 
     The bypass line preferably flows into the condensate line downstream of the preheating device, this inflow point also being the mixing point at which the first partial flow is mixed with the second partial flow, whereby after the mixing a desired preheat temperature of the boiler feed-water to be fed to the boiler establishes itself automatically. 
     The preheating device preferably has at least one heat exchanger, in particular a high-pressure preheater. A plurality of heat exchangers can also be connected one after the other, enabling multi-stage heating of the first partial flow of condensate. In the embodiment of the heat exchanger as a high-pressure preheater in a steam power installation, the preheater receives condensate at a pressure of about 300 bar, and is assigned to a high-pressure stage of the turbine. The turbine can, however, also have a high-pressure partial turbine and/or a medium-pressure partial turbine and/or a low-pressure partial turbine, as is usually provided in steam power installations. 
     The system design of the invention can consequently be applied very flexibly to different steam power installations comprising a combination of different turbine types (high-pressure, medium-pressure, low-pressure turbines) having corresponding preheating devices. 
     A diversion line that can be activated by a quick-shutoff fitting is preferably connected in parallel with the preheating device. This diversion line is provided to divert the condensate completely around the preheating device in the quick-shutoff situation, for instance in an emergency situation where there is the risk of flooding or overheating of the preheating device. In the quick-shutoff situation, the diversion line can be activated, i.e. switched open, by the quick-shutoff fitting, simultaneously interrupting the flow of condensate in the condensate line to the preheating device. For this purpose, the quick-shutoff fitting is designed, for example, as a three-way fitting, which directs at least the first partial flow of condensate via the diversion line after it is activated, so that preheating of condensate no longer takes place in the preheating device. In the normal situation, the diversion line is not activated, so that the first partial flow is fed to the preheating device via the condensate line. Advantageously, using the diversion line that can be activated via the quick-shutoff fitting achieves increased operational reliability of the steam power installation, in particular in combination with the bypass line according to the invention. 
     Further advantages of the steam power installation follow analogously to the advantages of the operating method of the steam power installation described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The single FIGURE is a schematic diagram showing an exemplary steam power installation. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The method according to the invention and a steam power installation for implementing the method are described with reference to an exemplary embodiment and a schematic diagram, in which the single FIGURE shows in simplified form a steam power installation. The steam power installation  1  shown in the FIGURE, which is part of a power plant, has a steam turbine  5  and a boiler  3  for generating steam D. A condenser  7  is connected to the steam outlet side of the turbine  5  via a bleeder line  51 . In order to feed condensate K back to the boiler  3 , the steam power installation  1  has a condensate line  13  that is connected to the outlet side of the condenser  7 . A first pump  41 , a feed-water container  45  and a second pump  43  are connected one after the other in the condensate line  13  in the direction of flow of the condensate. In addition, a preheating device  15  for preheating condensate K is connected in the condensate line  13 , positioned in front of the boiler  3  in the direction of flow of the condensate K. The preheating device comprises a first preheating stage  9 A and a second preheating stage  9 B connected to the outlet of the first preheating stage. The preheating stages  9 A,  9 B are here designed as heat exchangers  23 A,  23 B respectively. The boiler  3  has a fossil-fired steam generator  11 , which comprises a fuel supply  53  for supplying a fossil fuel  29 , for example coal or oil. A bleeder line  19 A leads from one stage of the steam turbine  5  to the heat exchanger  23 B. A bleeder line  19 B leads from a further stage of the turbine  5  to the heat exchanger  23 A. A respective amount of bleeder steam A 1 , A 2  can be fed via the bleeder lines  19 A,  19 B to the preheating device  15 , or more precisely to the heat exchangers  23 A,  23 B for preheating condensate K. 
     A bypass line  17  bypasses the preheating device  15 , the bypass line branching off from the condensate line  13  at a separation point  47 , bypassing the preheating device  15  and feeding back into the condensate line  13  at a mixing point  48  downstream of the preheating device  15 . A control valve  21  is provided in the bypass line  17  for regulating a partial flow K 2 , subsequently referred to as second partial flow K 2 , that bypasses the preheating device  15 . The control valve  21  has a motor actuator  33 , via which the desired valve setting of the control valve  21  and hence the first partial flow K 1  can be set. The condensate K delivered via the second pump  43  out of the feed-water container  45  can hereby be divided into a first partial flow K 1  and a second partial flow K 2  at the separation point  47 , the first partial flow K 1  being supplied to the preheating device  15  via the condensate line  13 , and the second partial flow K 2  bypassing the preheating device  15  via the bypass line  17 , so that the preheating device  15  only receives the first partial flow K 1  of condensate K. 
     A sliding valve  37 , which can be adjusted via a motor actuator  33 , and in normal operation is open, is connected in the direction of flow of the condensate K after the separation point  47  in the condensate line  13 . Connected in parallel with the sliding valve  37  is a branch line  55 , which is connected from the bypass line  17  to the condensate line  13  and has a low-load control valve  35  having an actuator element  35 A. The control valve  35  is closed in normal operation, so that no condensate K gets through via the branch line  55 . The low-load control valve  35  is only provided for the low-load situation, when the sliding valve  37  is closed, and, by means of the actuator element  35 A of the control valve  35 , a small amount of condensate K commensurate with the load demand reaches the preheating device  15  via the branch line  55 . 
     In addition, a diversion line  27 , which can be activated via a quick-shutoff fitting  25 , is connected in parallel with the preheating device  15 , a quick-shutoff fitting  25  being connected to the condensate line  13  upstream and downstream of the preheating device  15  respectively. The quick-shutoff fitting  25  can be switched quickly between two settings via an actuator  31 . The fitting  25  is designed as a three-way fitting for this purpose, the diversion line  27  being closed, i.e. not activated, in the normal operating state. Condensate K then flows in a first partial flow K 1  through the preheating device  15  and in a second partial flow K 2  via the bypass line  17 . In a quick-shutoff situation, the quick-shutoff fitting  25  is activated via the actuator  31 , thereby switching open the diversion line  27  and cutting off the condensate flow via the condensate line  13  through the preheating device  15 . Hence in the quick-shutoff situation, the preheating device  15  is completely bypassed, i.e. no condensate K is supplied to the preheating device  15  and hence none is heated. The diversion line  27  that can be activated is used for bypassing and hence protecting the preheating device  15 , in particular the heating surfaces of the heat exchangers  23 A,  23 B. 
     During operation of the steam power installation  1 , service steam D generated in the boiler  3  is fed via the steam line  49  to the turbine  5 , where it expands to produce work. The turbine  5  is here shown simplified, but can consist of a plurality of partial turbines, not shown in greater detail, for example a high-pressure partial turbine, a medium-pressure partial turbine and a low-pressure partial turbine. The steam D expanded to low pressure is fed via the bleeder line  51  to the condenser  7 , and condensed there to condensate K. The condensate K is delivered by means of the first pump  41  via the condensate line  13  into the feed-water container  45  where it is collected. The boiler  3  is fed with preheated condensate K as boiler feed-water S from the feed-water container  45  via the preheating device  15  by means of the second pump  43 , so that a closed water-steam circuit is created. The useful work obtained in the turbine  5  is transferred via the rotating shaft  57  to a generator  39  coupled to the shaft  57 , and converted into electrical energy. 
     In order to increase the power of the turbine  5  according to demand, the condensate K is divided into a first partial flow K 1  and a second partial flow K 2  for condensate preheating, only the first partial flow K 1  being preheated and the second partial flow K 2  being remixed with the preheated first partial flow K 1 . This division of the condensate K into the first partial flow K 1  and the second partial flow K 2  occurs at the separation point  47 , the second partial flow K 2  bypassing the preheating device  15  via the bypass line  17 . The first partial flow K 1  is preheated by means of bleeder steam A 1 , A 2  from the turbine  5 . The first partial flow K 1  is preheated in two stages  9 A,  9 B to a temperature of about 280° C. at a pressure of 300 bar. The first partial flow K 1  is mixed with the second partial flow K 2  at the mixing point  48 , with a mixture temperature of 210° C. to 250° C., in particular 220° C. to 240° C., being established. The partial flows K 1 , K 2  are divided, for example, such that the first partial flow K 1  makes up about 40% of the total condensate flow, and the second partial flow K 2  correspondingly about 60% of the total condensate flow before the separation point  47 . The division of the partial flows K 1 , K 2  is controlled or regulated here via the control or proportioning valve  21 , whose valve position can be set precisely by the motor actuator  33 . This results in proportioned bypassing of the preheating device  15  via the bypass line  17 , with a correspondingly lower requirement for bleeder steam A 1 , A 2  for preheating the first partial flow K 1 . As a result of less bleeder steam A 1 , A 2  being removed compared with traditional installation designs by the selective and proportioned bypassing of the preheating device  15 , a correspondingly greater mass flow of steam D is available for producing work in the turbine  5 . Thus by dividing into two partial flows K 1 , K 2 , the possibility of increasing the power according to demand up to the boiler reserve (not seconds reserves) of the steam power installation  1  is achieved, without needing to raise the live steam pressure above the design value. Moreover, the temperature T S  of the boiler feed-water S fed to the boiler  3  can be set precisely and if necessary varied by the mixing of the first partial flow K 1  and the second partial flow K 2  at the mixing point  48 , with, for example, a boiler feed-water temperature T S  of 210° C. to 250° C. at a pressure of 300 bar being provided as required. The removal of bleeder steam A 1 , A 2  from the turbine  5  advantageously occurs here under self-regulation by the coupling of the first partial flow K 1  with the bleeder steam A 1 , A 2  via the heat exchangers  23 A,  23 B. The greater the first partial flow K 1  that is set, the greater the removal of bleeder steam A 1 , A 2  for preheating, in order to achieve a desired temperature of the first partial flow K 1  after flowing through the preheating device  15 . In thermal equilibrium, the temperature of the first partial flow K 1  after passing through the heat exchangers  23 A,  23 B is normally approximately equal to the temperature of the bleeder steam A 1 , A 2 , i.e. about 280° C. at a pressure of 300 bar for instance. After mixing the non-preheated second partial flow K 2  with the first partial flow K 1  at the mixing point  48 , the mixture temperature establishes itself automatically according to the division ratios of the partial flows K 1 , K 2  and the temperature levels. This mixture temperature is also the preheat temperature T S  of the boiler feed-water S. The preheat temperature T S  is correspondingly lowered compared with traditional steam power installations, yet an increase in power of the turbine  5  is achieved by the lower heat consumption for preheating the condensate K. In particular, the heat consumption is also clearly more efficient than when the preheating device  15  is totally bypassed, which is the usual way to increase power. Using the design of the invention, it becomes possible, by a partial flow through the preheating device  15 , to bring about an increase in the power of the turbine without limiting the lifetime of the components of the preheating device  15 , for example the heating surfaces of the heat exchangers  23 A,  23 B.