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

Abstract

The invention relates to a method for operating a combined-cycle power plant with cogeneration, in which method 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 produced by extraction of steam from the at least one steam turbine. In a method such as this, simple decoupling of heat and electricity production, which is advantageous for operation, is achieved 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.

1 . A method for operating a combined-cycle power plant for the generation of electrical energy with cogeneration, in which method combustion air is inducted in at least one gas turbine, is compressed and is supplied 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 is exhausted by 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 cycle 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, 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. 
     
     
         2 . The method as claimed in  claim 1 , wherein 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 while keeping the production of distillate water equal or close to the nominal value. 
     
     
         3 . The method as claimed in  claim 2 , wherein 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. 
     
     
         4 . The method as claimed in  claim 1 , wherein, 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. 
     
     
         5 . The method as claimed in  claim 4 , wherein 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. 
     
     
         6 . The method as claimed in  claim 4 , wherein the at least one gas turbine is designed for sequential combustion and comprises two, first and second sequentially arranged 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. 
     
     
         7 . The method as claimed in  claim 6 , wherein 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. 
     
     
         8 . The method as claimed in  claim 6 , wherein a portion of the compressed combustion air additionally bypasses the first combustion chamber. 
     
     
         9 . The method as claimed in  claim 4 , wherein the at least one supplementary firing is arranged at the input of the heat recovery steam generator. 
     
     
         10 . The method as claimed in  claim 9 , wherein the heat recovery steam generator contains a first superheater, and in that a second additional firing is arranged downstream from the first superheater. 
     
     
         11 . A combined-cycle power plant for carrying out the method as claimed in  claim 1 , which combined-cycle power plant 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 are exhausted by the gas turbine flow, wherein the capability to extract steam is provided in the steam turbine, characterized in that the at least one steam turbine is designed for selective extraction of steam as low-pressure steam or intermediate-pressure steam. 
     
     
         12 . The combined-cycle power plant as claimed in  claim 11 , wherein the combined-cycle power plant is associated with a desalination plant with 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 desalination units can be selectively switched from operation with low-pressure steam to operation with intermediate-pressure steam. 
     
     
         13 . The combined-cycle power plant as claimed in  claim 12 , wherein 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. 
     
     
         14 . The combined-cycle power plant as claimed in  claim 13 , wherein a valve is arranged in the bypass. 
     
     
         15 . The combined-cycle power plant as claimed in  claim 13 , wherein 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. 
     
     
         16 . The combined-cycle power plant as claimed in  claim 13 , wherein 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 is arranged downstream from a first superheater.