Abstract:
The invention relates to a method for operating a steam power plant comprising a steam generator and a combustion chamber associated therewith, wherein pre-warmed combustion air is guided therein in addition to a fossil fuel. According to the invention, said plant can be operated with a particularly high efficiency ratio in various operating conditions. As a result, the combustion air is at least partially released after the pre-warming thereof and before the introduction thereof into the combustion chamber. A steam power plant which is particularly suitable for carrying out said method comprises, in addition to a pre-air warmer, an air turbine which is mounted downstream from said pre-air warmer in a fresh air pipe.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP02/07243, filed Jul. 1, 2002 and claims the benefit thereof. The International Application claims the benefits of European application No. 01117051.1 filed Jul. 12, 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 plant comprising a steam generator and a combustion chamber associated therewith, into which pre-warmed combustion air is fed in addition to a fossil fuel. It also concerns a steam power plant suitable for carrying out the method with a steam generator and a combustion chamber associated therewith for the combustion of a fossil fuel that is connected on the inlet side to both a fuel pipe and a fresh air pipe for combustion air. 
     BACKGROUND OF INVENTION 
     A steam turbine is usually used in a power plant for driving a generator or in an industrial plant for driving a machine. As a result, steam serving as a flow medium is fed to the steam turbine and is released in the steam turbine in an output-producing manner. After its release, the steam usually arrives at one of the condensers mounted downstream of the steam turbine and condenses there. The condensate is then fed as feed water to a steam generator and then again arrives in the steam turbine after its evaporation, so that there is a closed water-steam circuit. A plant with the relevant components, i.e. particularly with a steam turbine and a steam generator is also designated as a steam turbine plant or steam power plant. 
     A combustion chamber in which a fuel is burnt is usually assigned to the steam generator of such a steam power plant. The hot flue gases usually arising during the combustion of fossil fuels are then conveyed through the steam generator, in which case they are used for steam generation by means of heat exchange with the flow medium conveyed in the water-steam circuit. 
     For the combustion of fuel, fresh air or combustion air needs to be fed to the combustion chamber assigned to the steam generator. As a result, the combustion chamber is usually connected on the inlet side not only to a fuel pipe but also to a fresh air pipe for the combustion air. Via the fresh air pipe, the combustion air is fed to the combustion chamber, in which case the comparatively high pressure level required for feeding the combustion air into the combustion chamber is provided via a pneumatic conveyor mounted in the fresh air pipe. 
     When designing such a steam power plant, a customary design goal is a particularly high plant efficiency ratio when converting the energy stored in the fuel to electrical energy or energy that can be used in other operating conditions. Therefore, for example, the plant process of such a steam power plant, particularly its operating parameters, is specified or selected in such a way that for thermodynamic reasons a particularly favorable transfer of energy can be obtained. To this end, among other things, the combustion air to be fed to the combustion chamber is usually pre-warmed to a comparatively high temperature level because this favors the efficiency ratio of the actual combustion process. Such a pre-warming of the combustion air can be carried out, for example, inside the actual steam generator, in which case the energy content is partially used for pre-warming the air during the combustion of the generated flue gas. 
     SUMMARY OF INVENTION 
     The object of the invention is to specify a method for operating a steam power plant of the above-mentioned type, wherein a particularly high plant efficiency ratio can be reached in various operating states and particularly in the partial-load region. 
     Further, a steam power plant particularly suited to carrying out the method is specified. 
     Regarding the method, this object is achieved according to the invention by partially releasing the combustion air in an output-producing manner after it has been pre-warmed and before it is fed to the combustion chamber. 
     The invention then also takes into consideration that a particularly high plant efficiency ratio in various operating states can be obtained by setting as many operating parameters as required. However, pre-warming the combustion air only takes place to a limited extent on the basis of the customary static structure of the air pre-warmer—for example, in the steam generator. On the other hand, the combustion air can also be heated when pre-warming it to a temperature level that, for thermodynamic reasons, clearly exceeds the most favorable temperature level for the specific operating state of the plant. In order to counteract this, provision is made for at least partial utilization of the energy content of the compressed air and pre-warmed combustion air before it enters the combustion chamber. As a result, the combustion air is partially released before it enters the combustion chamber, in which case the combustion air is cooled to a temperature level adapted to the specific operating state of the plant and particularly favorable for thermodynamic reasons. As a result of the release, the combustion air is brought to the operating parameters of the combustion air that are particularly favorable for the specific plant condition, in which case its energy content is still partially utilized. 
     Therefore, a particularly high plant efficiency ratio can be obtained by using, in an advantageous further development, the output gained when releasing the pre-warmed combustion air for driving a pneumatic conveyor provided for compressing the combustion air. Therefore, a comparatively direct provision of the drive energy for the pneumatic conveyor is possible. On the other hand, the drive of the pneumatic conveyor is usually mounted electrically in the fresh air pipe of known plants. The driving power required for this is then provided via the generator assigned to the steam power plant. Therefore, in the case of previous known plants, the driving power required to drive the pneumatic conveyor is in essence provided from the energy content of the fuel, in which case many loss-carrying energy conversion steps are however required through to generating the electrical energy in the generator that is, in the end, again converted into driving energy for the pneumatic conveyor. By way of the comparatively direct drive of the pneumatic conveyor via the output gained when releasing the pre-warmed combustion air, many loss-carrying energy conversion steps can be saved, which in itself already presupposes an increase in the total efficiency of the plant. 
     In order to adhere to, even under varying operating conditions of the steam power plant, a temperature level that is particularly favorable for thermodynamic reasons when feeding the combustion air into the combustion chamber, in an advantageous development an operating parameter for releasing the combustion air on the basis of a characteristic value is set for the temperature of the combustion air flowing toward the combustion chamber. The output extracted during the release can for example be set as an operating parameter for releasing the combustion air, as a function of an actual temperature value. Therefore it can be ensured that the combustion air flowing toward the combustion chamber, also in the case of varying load conditions of the plant, always has a sufficiently high temperature level for a thermodynamically favorable combustion of the fuel. 
     The combustion air can be pre-warmed at many suitable places within the steam power plant. However, the combustion air is advantageously pre-warmed within the steam generator and therefore via flue gas from the combustion chamber. 
     In an alternative or additional advantageous embodiment, the combustion air is pre-warmed via flue gas flowing from an assigned gas turbine. In this way, in the type of steam turbine plant combined with a gas turbine plant, it is possible to utilize the heat still carried in the flue gas flowing from the gas turbine by feeding it into the process of the steam power plant. 
     Advantageously, this heat is on the one hand used for pre-warming air and, on the other hand, by releasing the pre-warmed combustion air, for driving the pneumatic conveyor mounted in the fresh air pipe, so that the energy content of the fuel fed to the combustion chamber of the steam generator can be used more or less exclusively and in a focused manner for generating the electrical energy or output via the assigned steam turbine. 
     In the case of such a steam power plant combined with a gas turbine plant, heat still carried in the flue gas flowing from the gas turbine is further used to pre-warm the feed water for the steam generator in a further advantageous embodiment. 
     With regard to the steam power plant, the above-mentioned object of the invention is achieved by also mounting, in addition to an air pre-warmer, an air turbine which is mounted downstream of said air pre-warmer in the fresh air pipe connected to the combustion chamber assigned to the steam generator. Therefore, the pre-warmed combustion air can be released via the air turbine before it enters the combustion chamber, in a manner that is adapted to the specific operating condition of the plant as required. 
     In an advantageous way, the air turbine then drives a pneumatic conveyor mounted upstream of said air pre-warmer in the fresh air pipe. This can result, for example, in the air turbine and the pneumatic conveyor being arranged on a common shaft by interconnecting a suitable transmission, if required. Particularly in the case of such a circuit, those loss-carrying intermediate stages are omitted for the conversion of energy in which the heat from the combustion air is first of all used for generating steam and then power if the pneumatic conveyor is driven electrically. 
     Therefore, in a further advantageous embodiment, the pneumatic conveyor is also embodied as an air compressor that can generate an output pressure of approximately 4 to 5 bar. Together with the additionally provided warming up of the compressed air, preferably to a temperature of approximately 500° C.–550° C., a particularly effective utilization of the energy content in the compressed and pre-warmed air is made possible. 
     In order to be able to set an operating parameter for the air turbine, particularly of the output extracted from the pre-warmed combustion air during release, to the specific operating state of the plant as required, a regulating device assigned advantageously to the air turbine on the inlet side is connected to a temperature sensor arranged in the fresh air pipe. 
     The air pre-warmer is arranged in an advantageous development within the steam generator so that the combustion air flowing toward the combustion chamber can be heated via flue gas from the combustion chamber. Therefore, the air pre-warmer is advantageously mounted on the primary-side in the flue gas flow flowing from the combustion chamber. 
     The steam power plant is also particularly suitable for a flexible and compact combination with a gas turbine plant. 
     Therefore, as a particularly advantageous embodiment a utilization of the residual heat carried along in the waste gas from the gas turbine to pre-warm the combustion air for the combustion chamber of the steam power plant is provided. Therefore, the air pre-warmer is advantageously mounted on the primary side in a flue gas duct mounted downstream of a gas turbine. 
     In a further advantageous embodiment, in such a combination of a gas turbine plant with a steam power plant, the residual heat in the waste gas from the gas turbine is still used to pre-warm the feed water for the steam generator. Therefore, a feed water pre-warmer assigned to a steam generator is conveniently mounted on the primary side to the flue gas duct mounted downstream of the gas turbine. 
     The advantages achieved with the invention particularly consist in the fact that because the combustion air is at least partially released after it is pre-warmed and before it is introduced into the combustion chamber, particularly favorable operating parameters for the combustion air, particularly a favorable pressure level and a favorable temperature level, can be set for the specific operating state of the plant on the one hand. On the other hand, the energy content carried in the pre-warmed combustion air can be used for the plant process comparatively directly and in a manner which increases the efficiency ratio. Therefore, such a steam power plant is particularly suitable for utilization as a steam power plant, i.e. in conjunction with a steam turbine, because there the efficiency ratio is usually a very important design criterion. 
     In addition, such a concept is particularly advantageous when burning comparatively highly slag-forming fuels such as brown coal, coal or oil shale. As slagging of the combustion chamber increases, the heat transfer is reduced and the heat evolved that can be felt in the steam generator then shifts toward its outlet, so that an air pre-warmer arranged in this area is comparatively exposed to increased heating. Therefore, in such a case, the temperature of the combustion air flowing to the combustion chamber can still be increased, which in turn intensifies the slagging process. Especially in this case, an output-producing release of the pre-warmed combustion air can thus bring the temperature level of said combustion air flowing into the combustion chamber to values adapted in a manner particularly suitable for operation, it being possible to use the output extracted from the combustion air in a manner which increases the efficiency ratio. 
     Such a developed steam power plant can also be designed comparatively compactly and simply, in which case for simple controllability bleeder points on the steam turbine or a pre-warmer section can in particular be dispensed with completely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are explained in greater detail on the basis of drawings. These are as follows: 
         FIG. 1  diagram of a steam power plant, and 
         FIG. 2  diagram of a steam power plant combined with a gas turbine plant. 
     
    
    
     The same parts are provided in both figures with the same reference characters. 
     DETAILED DESCRIPTION OF INVENTION 
     The steam power plant  1  according to  FIG. 1  is provided for generating electrical energy and includes a steam turbine  2  for this purpose that is connected to a generator  6  via a turbine shaft  4 . In the embodiment, the steam turbine  2  includes a high-pressure part  2   a  and a low-pressure part  2   b . Therefore, the steam turbine  2  is embodied in two stages. However as an alternative, the steam turbine  2  can also include only one or several, particularly three, pressure stages. 
     The steam turbine  2  is connected to a condenser  12  on the outlet side via a steam pipe  10 . The condenser  12  is connected to a feed water container  20  via a pipe  14  in which a condensate pump  16  is mounted. The feed water container  20  is connected on the outlet side to a heating surface arrangement  30  arranged in a steam generator  26  via a feed pipe  22  in which a feed water pump  24  is mounted. 
     The heating surface arrangement  30  includes many pre-warming surfaces  32 ,  34  designated as economizers that are connected on the outlet side to an evaporator heating surface  38  via an overflow pipe  36 . The evaporator heating surface  38  can then be embodied as a pass-through evaporator heating surface or a natural circulation evaporator heating surface. To this end the evaporator heating surface  38  can then be connected in a known way to a water-steam drum (not shown in the embodiment) for the formation of a circulation. 
     The evaporator heating surface  38  is also connected to a high-pressure superheater  40  arranged in the steam generator  26 , said high-pressure superheater  40  being connected on the outlet side to the steam inlet  42  of the high-pressure part  2   a  of the steam turbine  2 . The steam outlet  44  of the high-pressure part  2   a  of the steam turbine  2  is connected to the steam inlet  48  of the low-pressure part  2   b  of the steam turbine  2  via an intermediate superheater  46 . Its steam outlet  50  is connected to the condenser  12  via the steam pipe  10  to form a closed water-steam circuit  52 . 
     Therefore, the water-steam circuit  52  shown in  FIG. 1  only consists of two pressure stages. However, it can consist of only one or several, particularly three, pressure stages, in which case further heating surfaces can be arranged in a steam generator  26  in a known way. The low-pressure part  2   b  of the steam turbine  2  is moreover connected to the feed water container  20  via a bleeder steam pipe  54  to release the condensate, as required. 
     A fossil-fired combustion chamber  60  is assigned to a steam generator  26 . A fossil fuel can be applied to the combustion chamber  60  via a fuel supply pipe  62  and combustion air for the combustion can be applied via a fresh air pipe  64 . In order to ensure a pressure level that is sufficient for delivering the combustion air to the combustion chamber  60 , a pneumatic conveyor  66  is mounted in the fresh air pipe  64 . Therefore, the pneumatic conveyor  66  is embodied as an air compressor that on the outlet side generates an operating pressure of compressed air of approximately 4 to 5 bar. In addition, in the case of the steam power plant  1 , a pre-warming of the combustion air before the introduction thereof into the combustion chamber  60  is provided to ensure a temperature level in the combustion chamber  60  which is sufficient for thermodynamic reasons for a high efficiency ratio during combustion. Therefore, an air pre-warmer  68  designed with two stages in the embodiment is mounted in the fresh air pipe  64  and is arranged in the steam generator  26 , and is therefore heated on the primary side by the flue gas from the combustion chamber  60 . 
     The steam power plant  1  is designed for a particularly high efficiency ratio even under changing operating conditions. Therefore, for a high plant efficiency ratio the combustion air is at least partially released before the introduction thereof into the combustion chamber  60  as required. For this, an air turbine  70  is mounted in the fresh air pipe  64  between the air pre-warmer  68  and the combustion chamber  60 . In the same way as the pneumatic conveyor  66 , the air turbine  70  is also arranged together with the high-pressure part  2   a  and the low-pressure part  2   b  on the turbine shaft  4 . The air turbine  70  therefore drives the pneumatic conveyor  66  via the turbine shaft  4 . 
     A regulating device  72  is assigned to the air turbine  70  and gives, as required and depending on the operating point, the manipulated variables S for an operating parameter of the air turbine  70 , for example for that of the output to be extracted from the combustion air during its release into the air turbine  70 . On the inlet side, the regulating device  72  is connected to a temperature sensor  74  arranged in the fresh air pipe  64  via a signal pipe  73 . 
     When operating the steam power plant  1 , the combustion air flowing into the combustion chamber  60  is compressed in the pneumatic conveyor  66  and subsequently pre-warmed in the air pre-warmer  68 . After the pre-warming, the characteristic operating parameters of the combustion air such as the pressure and temperature level then, in essence, depend on the current operating state of the steam generator  26 . For example, the combustion air flowing from the air pre-warmer  68  can have a pressure of approximately 4.8 to 5.2 bar and a temperature of approximately 500° C. However, such operating parameters are not necessarily favorable for combustion in the combustion chamber  60 . In particular, the temperature level of the combustion air can be significantly higher than a temperature level desirable for thermodynamic reasons. 
     In order to use this temperature level, which is possibly too high, to increase efficiency, the combustion air flowing from the air pre-warmer  68  is then at least partially released in the air turbine  70  in an output-producing manner before the introduction thereof into the combustion chamber  60 . As a result, the combustion air is routed via the air turbine  70 , in which case it drives this and in which case the pressure of the combustion air is approximately 1.2 bar and the temperature approximately 300° C. As a result of the partial release into the air turbine  70 , the combustion air is therefore brought to particularly suitable operating parameters for combustion in the combustion chamber  60 , in which case utilization of the output gained during the release is possible. In the embodiment, the air turbine  70  then drives the pneumatic conveyor  66  so that a comparatively direct drive of the pneumatic conveyor  66  is reached by avoiding an unnecessarily high number of loss-carrying intermediate stages during energy conversion. 
     In order to adhere to a particularly favorable parameter level in the combustion air for the combustion in various operating conditions, the regulating device  72 , via the temperature sensor  74 , regularly monitors the operating parameters of the combustion air flowing into the combustion chamber  60 . Therefore, it should in particular be ensured that despite the release of the combustion air in the air turbine  70 , a sufficiently high temperature level of the combustion air is adhered to for favorable combustion in the combustion chamber  60  in each operating state for thermodynamic reasons. As required, the regulating device  72  of the air turbine  70  supplies reference values for influencing the output to be extracted from the combustion air during its release. Thus, throttling members can for example be set to a favorable manipulated variable depending on the operating point. 
     The steam power plant  1 ′ according to  FIG. 2  is also designed for pre-warming the compressed compression air for the combustion chamber  60  with subsequent partial release in an output-producing manner in an air turbine  70 . However, the steam power plant  1 ′ according to  FIG. 2  is designed for a combined arrangement with an assigned gas turbine plant  80 . This includes a gas turbine  82  with a connected air compressor  84  and a combustion chamber  86  mounted upstream of the gas turbine  82 , said combustion chamber  86  being connected to a compressed air pipe  88  of the air compressor  84 . The gas turbine  82  and the air compressor  84  as well as an assigned generator  90  are located on a common shaft  92 . 
     In the embodiment according to  FIG. 2 , the gas turbine plant  80  is connected to the steam power plant  1 ′ in such a way that waste gas flowing from the gas turbine  82  is used to pre-warm the air and the feed water for the steam power plant  1 ′. To this end, a heat exchanger embodied as an air pre-warmer  96  for the combustion air of the steam power plant  1 ′ is mounted on the primary side in a flue gas duct  94  mounted downstream of the gas turbine  82 . The air pre-warmer  96  is then mounted on the secondary side in the fresh air pipe  64  of the steam power plant  1 ′, making possible a heat transfer of waste gas or flue gas flowing from the gas turbine  82  to the combustion air for the combustion chamber  60  compressed in the pneumatic conveyor  66 . The air turbine  70  is then mounted between the air pre-warmer  96  and the combustion chamber  60  in the fresh air pipe  64  so that the fresh air pre-warmed in the air pre-warmer  96  can be at least partially released there. 
     For an ongoing utilization of the residual heat carried along in the flue gas flowing from the gas turbine  82 , a heat exchanger embodied as a feed water pre-warmer  98  is mounted on the primary side in the flue gas duct  94 . The feed-water pre-warmer  98  is then mounted on the secondary side in the supply pipe  22  of the steam power plant  1 ′.