Patent Publication Number: US-2013227958-A1

Title: Method for operating a combined-cycle power plant with cogeneration, and a combined-cycle power plant for carrying out the method

Description:
TECHNICAL FIELD 
     The present invention relates to the field of power plant technology, and relates in particular to a method for operating a combined-cycle power plant with cogeneration according to the precharacterizing clause of claim  1 . It also relates to a combined-cycle power plant for carrying out the method. 
     PRIOR ART 
     Simultaneous generation of electricity and heat is required in certain fields of operation of power plants. The two types of energy, which are generated, are not necessarily subject to the same demand profile from the connected consumers. The electricity production is traditionally governed by the requirements of the electrical grid system or, in some cases, by large local industrial consumers. The demand for heat is typically governed by the requirement of an industrial process or by the daily or seasonal fluctuations in a remote heat network or a drinking-water processing plant. With regard to the last-mentioned drinking water processing, countries with a large number of seawater desalination plants have major fluctuations in the demand for electrical power throughout the year, while the requirement for drinking water is largely constant over time. 
     A large proportion of the heat requirement is in general provided by extraction of steam from the steam turbine or from the main steam lines in a thermal power plant. When the steam is generated in a heat recovery steam generator (HRSG) in a combined-cycle power plant, its generation is linked directly to the load control of the gas turbine, and therefore cannot be completely decoupled from the electricity generation. 
     The design and operation of a gas turbine are normally concentrated on high efficiency during electricity generation. Although partial load operation of the gas turbine is possible within certain limits, it is, however, restricted by the hazardous-substance emissions, which rise when the load is low. During partial load operation, the combustion air and exhaust gas flows through the gas turbine are normally reduced, thus at the same time restricting the steam generation in the heat recovery steam generator which follows downstream. 
     In the past, various operating methods have been proposed for a combined-cycle power plant when the electricity demand is low and there is a high heat requirement (such as that which occurs when sea water is vaporized in sea-water desalination plants which are operated using steam): 
     One known option for controlling low electricity production is to restrict the inlet valves of the steam turbine, or even to close them completely, and to pass the excess steam to a water-cooled or air-cooled condenser, bypassing the turbine. 
     Another known option for maximizing the steam generation independently of the load on the gas turbine is to provide a fan in order to provide the necessary additional air, which is required for large supplementary firing in the heat recovery steam generator. 
     One very simple solution is to provide an auxiliary boiler, which generates the steam for the consumers, in order to allow the gas turbines to be shut down independently thereof. 
     If the power plant has a plurality of gas turbine units, some of the units can be switched off in order to reduce the electricity generation, if the supplementary firing in the heat recovery steam generator is overdesigned such that the steam extraction can be continued at the same level as if all of the gas turbine units were being operated. A comparable solution is disclosed in the document EP 1 701 006 A2. A solution such as this requires additional hardware complexity for the steam lines, the safety valves and the steam bypass lines. 
     The steam, which is extracted from a combined-cycle power plant and is used for seawater desalination plants with multiple-effect distillation units (MED), normally requires a comparatively low temperature and a low pressure, since the seawater vaporization takes place below atmospheric pressure. Therefore, in a typical plant, the steam is extracted at the outlet from the low-pressure turbine. This configuration promotes high electricity production in the combined-cycle power plant, since the low extraction pressure allows better expansion of the steam in the steam turbine. 
     An improved type of desalination is known by the name multiple-effect distillation with thermal vapor compression (MED-TCV) (see, for example, WO 2005/105255 A1 in this context). In this method, the multiple-effect distillation is carried out using a thermal compressor (which operates like a conventional steam-jet ejector), in order to feed the steam back from the vaporization cell at the lowest temperature to that at the highest temperature. The operation of the steam-jet injectors leads to a higher-pressure level in the steam extraction line in the combined-cycle power plant. The advantage of this configuration is that less steam is consumed for the same amount of drinking water produced, compared with simple multiple-effect distillation. On the other hand, the higher steam extraction pressure leads to a slight reduction in the electricity production in the combined-cycle power plant. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a method for operating a combined-cycle power plant with cogeneration and heat output by steam extraction from the steam turbine, which avoids the disadvantages of known methods and, in particular, allows electricity production and drinking-water production to be decoupled in a simple manner during sea-water desalination, and to specify a combined-cycle power plant for carrying out the method. 
     The object is achieved by the totality of the features of the claims. 
     In the method according to the invention, combustion air is inducted in at least one gas turbine, is compressed and is fed to at least one combustion chamber for combustion of a fuel, and the resultant exhaust gas is expanded in at least one turbine producing work, and in which method the exhaust gas which emerges from the at least one turbine is fed through a heat recovery steam generator in order to generate steam, which heat recovery steam generator is part of a water-steam circuit with at least one steam turbine, a condenser, a feedwater tank and a feedwater pump, wherein heat is provided by extraction of steam from the at least one steam turbine. The method is characterized in that the steam can be selectively extracted from the at least one steam turbine as low-pressure steam or intermediate-pressure steam, and in that the steam extraction is switched from low-pressure steam to intermediate-pressure steam in order to restrict the electricity production. 
     A embodiment of the method according to the invention is characterized in that the combined-cycle power plant is associated with a desalination plant, which is operated with the steam extracted from the at least one steam turbine, and in that, the operation of the desalination plant is switched from low-pressure steam to intermediate-pressure steam in order to restrict the electricity production. 
     Another embodiment is characterized in that the desalination plant comprises desalination units, wherein each desalination unit has a multiple-effect distillation device, which is operated with low-pressure steam, and a thermal vapor compression device, which interacts with the multiple-effect distillation device and is operated with intermediate-pressure steam, and in that the operation of the desalination units is switched from operation without a thermal vapor compression device to operation with a thermal vapor compression device in order to restrict the electricity production while keeping, at the same time, the water production at the nominal value. This embodiment allows minimizing the amount of supplementary firing required and hence the fuel consumption at low power-to-water production ratios of the cogeneration plant. 
     A further embodiment of the method according to the invention is distinguished in that in order to restrict the electricity production, a portion of the inducted combustion air is additionally passed through the at least one turbine to the heat recovery steam generator without being involved in the combustion of the fuel in the gas turbine, and in that this portion of the combustion air is used to operate at least one supplementary firing in the heat recovery steam generator. 
     Another embodiment is characterized in that the at least one gas turbine comprises only one combustion chamber and only one turbine for expansion of the exhaust gases, and in that the portion of the compressed combustion air which is not used for combustion of the fuel is passed to the turbine bypassing the combustion chamber. 
     A further embodiment is distinguished in that the at least one gas turbine is designed for sequential combustion and comprises two combustion chambers and two turbines for the expansion of the exhaust gases, and in that that portion of the compressed combustion air which is not used for combustion of the fuel is provided for operation of the supplementary firing by switching off the second combustion chamber. 
     Another embodiment of the method according to the invention is characterized in that the at least one gas turbine is provided with variable inlet guide vanes, and in that the inlet guide vanes are set to the maximum opening at the same time that the second combustion chamber is switched off. 
     Another embodiment is characterized in that a portion of the compressed combustion air additionally bypasses the first combustion chamber. 
     According to another embodiment, the at least one supplementary firing is arranged at the input of the heat recovery steam generator. 
     Yet another embodiment is characterized in that the heat recovery steam generator contains a first superheater, and in that a second additional firing is arranged downstream from the first superheater. 
     The combined-cycle power plant according to the invention for carrying out the method comprises at least one gas turbine with a compressor for compression of inducted combustion air, a combustion chamber for combustion of a fuel using the compressed combustion air, and a turbine for expansion of the exhaust gases created during the combustion, as well as a water-steam circuit with at least one steam turbine and a heat recovery steam generator, through which exhaust gases which emerge from the gas turbine flow, wherein the capability to extract steam is provided in the steam turbine. It is characterized in that the at least one steam turbine is designed for selective extraction of steam as low-pressure steam or intermediate-pressure steam. 
     One embodiment of the combined-cycle power plant according to the invention is characterized in that the combined-cycle power plant is associated with a desalination plant with desalination units, wherein each desalination unit includes a multiple-effect distillation device, which is operated with low-pressure steam, and a thermal vapor compression device which interacts with the multiple-effect distillation device and is operated with intermediate-pressure steam, and in that the desalination units can be selectively switched from operation with low-pressure steam to operation with intermediate-pressure steam. The advantage of coupling the multi-effect distillation device to a thermal vapor compressor lies in the lower consumption of steam at higher pressure necessary in this configuration for producing a given amount of distillate water. Another embodiment is characterized in that a controllable bypass is provided in the at least one gas turbine, via which a portion of the compressed combustion air can be introduced into the turbine bypassing the combustion chamber, and in that a supplementary firing is provided in the heat recovery steam generator, in which supplementary firing fuel can be burnt, in order to heat the exhaust gases as they enter using the combustion air which has been passed via the bypass. 
     According to a further embodiment, a valve is arranged in the bypass. 
     Another embodiment of the combined-cycle power plant according to the invention is characterized in that the at least one gas turbine is designed for sequential combustion, and comprises two combustion chambers and two turbines for expansion of the exhaust gases. 
     Yet another embodiment is characterized in that a first supplementary firing is arranged in the heat recovery steam generator, at the input to the heat recovery steam generator and a second supplementary firing are arranged downstream from a first superheater. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be explained in more detail in the following text using exemplary embodiments in conjunction with the drawing, in which: 
         FIG. 1  shows a simplified plant layout of a combined-cycle power plant with a connected desalination plant, according to one exemplary embodiment of the invention; 
         FIG. 2  shows the exemplary plant layout of a desalination plant, which, within the scope of the invention, can be operated selectively with low-pressure steam or with intermediate-pressure steam; 
         FIG. 3  shows the gas-turbine part of the plant shown in  FIG. 1 , with an additional bypass for partially bypassing the combustion chamber of the gas turbine, according to another exemplary embodiment of the invention; and 
         FIG. 4  shows the gas-turbine part of the plant from  FIG. 1  with sequential combustion, according to a further exemplary embodiment of the invention. 
     
    
    
     BEST MODE OF THE INVENTION 
       FIG. 1  shows a simplified plant layout of a combined-cycle power plant  10  with a connected desalination plant  15  according to one exemplary embodiment of the invention. The illustrated combined-cycle power plant  10  comprises a gas turbine  11 , a water-steam circuit  12  and a desalination plant  15 . The gas turbine  11 , which drives a first generator G 1 , comprises a compressor  18 , a combustion chamber  19  and a turbine  20 . The compressor  18  inducts combustion air via an air inlet  16 , compresses it and then emits it to the combustion chamber  19 , where, together with a fuel which is introduced, it feeds a combustion process which produces hot exhaust gases, which are expanded in the downstream turbine  20 , producing work. The amount of the inducted combustion air can be controlled via variable inlet guide vanes  17 . 
     The hot exhaust gas from the gas turbine  11  flows through a heat recovery steam generator  13 , which is arranged in the water-steam circuit  12 , in order there to convert feed water from a feedwater tank  28  to superheated steam via corresponding economizers  26 ,  27  and superheaters  22 ,  24 . Appropriate pumps P 2  and P 3  are provided in order to convey the feed water. Furthermore, a high-pressure drum  25  is provided in a manner known per se, as well as a valve V 5  by means of which the inlet flow to the high-pressure drum  25  can be controlled. The high-pressure steam which is generated in the heat recovery steam generator  13  is fed via a valve V 3  for the high-pressure turbine  29  to a steam turbine  14 , which is arranged in the water-steam circuit and drives a further generator G 2 , where it is expanded to an intermediate pressure, before entering an intermediate-pressure turbine  30  of the steam turbine  14 . The low-pressure steam, which emerges from the intermediate-pressure turbine  30  is then passed through an appropriate low-pressure turbine  31  in order finally to be condensed in a condenser  32  and to be pumped back to the feedwater tank  28  with a condensate. A cooling medium, which is conveyed by means of a further pump P 4  flows through the condenser  32 . 
     Steam is extracted from the steam turbine  14  via a valve V 4  between the intermediate-pressure turbine  30  and the low-pressure turbine  31 , and is fed to a desalination plant  15 , which, for example, may be designed as shown in  FIG. 2 . The condensate, which is created in the desalination plant  15  is fed back into the circuit via a line, which is shown by dots in  FIG. 1 . Furthermore, low-pressure steam can be extracted at the output of the low-pressure turbine  31  via a further line (see also  FIG. 2 ), or extracted from the low-pressure turbine  31 , and can be fed to the desalination plant  15  (dotted line in  FIG. 1 ). The operation of the desalination plant can be switched between intermediate-pressure steam and low-pressure steam by appropriate operation of the valves V 8 . 
     As can be seen from  FIG. 2 , the desalination plant  15  is designed such that, via separate extraction lines, it can be selectively operated with intermediate-pressure steam from the intermediate-pressure turbine  30  or with low-pressure steam from the low-pressure turbine  31 . This is achieved by using desalination units  15   a - 15   d  in the desalination plant  15 , with so-called multiple-effect distillation devices (MED)  33 , operating alone or in combination with thermal vapor compression devices (TCV)  34 . The multiple-effect distillation devices  33  require low-pressure steam at a pressure 0.5 bar or less, while the thermal vapor compression devices  34  typically require steam at a pressure of 3 bar or higher. 
     The exemplary embodiment in  FIG. 2  shows a steam turbine  14  with a common high-pressure turbine  29  and intermediate-pressure and low-pressure turbines  30  and  31 , which are arranged in two parallel paths, drive a respective generator G 3  and G 4  and receive intermediate-pressure steam from the high-pressure turbine  29  via valves V 6  and V 7 , while the high-pressure turbine  29  receives high-pressure steam  35  via the valve V 3 . The desalination plant  15  is connected to the upper path of the steam turbine  14 , with the operation of the thermal vapor compression devices  34  being controlled via appropriate valves V 8 . The lower path interacts directly with the condenser  32 . 
     As already mentioned, a desalination plant  15  such as this can alternatively be operated in two different operating modes: in one operating mode, the desalination units  15   a - 15   d  are operated without the thermal vapor compression devices  34  (valves V 8  closed), in order to achieve the maximum electricity production. In the other operating modes, the thermal vapor compression devices  34  are likewise operated (valves V 8  open), in order to maintain the production of drinking water while the electricity requirement is low at times or seasonally. 
     One advantage of this configuration is that the supplementary firing in the heat recovery steam generator, which normally has to be designed for partial load operation of the gas turbine, can be reduced in its size because less steam is required for the desalination of the same amount of water, as a result of the combined operation of multiple-effect distillation devices  33  and thermal vapor compression devices  34 . This configuration allows therefore a reduction of the fuel consumption for a given amount of water production at partial load of the turbine. 
     Two supplementary firings  21  and  23  are provided in the heat recovery steam generator  13  in the plant in  FIG. 1 , one ( 21 ) of which is arranged directly at the input of the heat recovery steam generator  13  (so-called “duct firing”), while the second ( 23 ) is arranged between the two superheaters  22  and  24  (so-called “inter-bank firing”). Both supplementary firings  21  and  23  are supplied with suitable fuel via appropriate valves V 1  and V 2 . 
     An operating concept for the gas turbine in a combined-cycle power plant with cogeneration, which results in a large exhaust gas flow from the gas turbine with a low exhaust gas temperature at the same time additionally opens up a wide range for a supplementary firing in the heat recovery steam generator, thus making it possible to ensure a high level of steam production for a connected desalination plant, which is operated using the steam, even when the demand for electricity from the grid is low at the same time. The supplementary firing may in this case be restricted to the input of the heat recovery steam generator (supplementary firing  21 ), although this restricts the steam production. If, in contrast, a further supplementary firing ( 23 ) is provided additionally between the superheaters which are arranged in the heat recovery steam generator, the steam production can be increased considerably, however the steam turbine probably does not need to be switched off, because the temperature level of the steam produced at the output of the heat recovery steam generator is too low. 
     On the basis of the plant illustrated in  FIGS. 1 and 2 , it is now additionally possible to provide (see  FIG. 3 ) for an adjustable portion of the combustion air which is compressed in the compressor  18  to bypass the combustion chamber  19  and turbine  20  via a bypass  36  which is arranged in the gas turbine  11  and can be controlled by means of a valve V 9 , as a result of which it is not involved in the combustion in the gas turbine  11 . This ensures an unchanged high exhaust gas mass flow even when the load on the gas turbine  11  is relatively low. Such operation leads to a lower exhaust gas temperature, which additionally allows independent control of the steam generation by means of the supplementary firings  21  and  23  in the heat recovery steam generator  13 . 
     At the same time, suitable combustion parameters can be maintained in the combustion chamber  19  because of the reduced combustion airflow, with the consequence that the hazardous-substance emission can be kept low, even when the load on the gas turbine is comparatively low. Since the oxygen content in the exhaust gas of the gas turbine is increased considerably in comparison to conventional operation by bypass operation, the supplementary firing in the heat recovery steam generator  13  can be operated on a large scale without additional external air. 
     Another option for implementing a comparable method is provided, as shown in  FIG. 4 , for a combined-cycle power plant, which has a gas turbine  37  with sequential combustion. In the case of sequential combustion such as this, as is known by way of example from document EP 1 914 407 A2, two combustion chambers  19  and  39  are connected one behind the other in the gas turbine  37 , with a respective downstream expansion turbine  38  and  40 . In this case, when the electricity production is restricted, the second combustion chamber  39  is switched off, and the inlet guide vanes  17  are open completely at the same time. This maintains the full exhaust gas mass flow even when the load on the gas turbine  37  is comparatively low, with the first combustion chamber  19  being operated close to its nominal operating point, and the hazardous-substance emissions remaining low. 
     The resultant exhaust gas temperature is low and, even in this case, allows additional independent control of the steam generation by means of the supplementary firings  21 ,  23  in the heat recovery steam generator  13 . Since the oxygen content in the exhaust gas from the gas turbine is increased considerably in comparison to conventional operation in this type of operation as well, the supplementary firing in the heat recovery steam generator  13  can be operated on a large scale without additional external air. 
     However, in addition to switching off the second combustion chamber  39 , it is also possible to provide a bypass  36  for the first combustion chamber  19 , as shown in  FIG. 3 , in order to allow a portion of the compressed air to bypass the first combustion chamber  19 . This makes it possible to ensure that the flame temperature in the first combustion chamber  19  can be operated close to the nominal operating point, and that this results in a low exhaust gas temperature with a high mass flow at the same time, as a result of which, as already described above, additional independent control of the steam generation is possible by the supplementary firings  21  and  23 . 
     The two supplementary firings  21  and  23  illustrated in  FIG. 1  have the advantage that the first supplementary firing  21  can be used to essentially ensure a minimal steam temperature at the output of the heat recovery steam generator  13 , in order to keep the steam turbine  14  within its load limits, while the purpose of the second supplementary firing  23  is essentially to produce and to control the desired amount of steam.