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

A method for operation of a combined-cycle power plant 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 emerges from the at least one turbine is passed through a heat recovery steam generator in order to generate steam, wherein the heat recovery steam generator is part of a water-steam circuit, wherein heat is provided by extracting steam from the at least one steam turbine, the method comprising:

• passing a portion of the inducted combustion air 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 to restrict electricity production while heat provided by the extracting of steam from the at least one steam turbine remains constant, and
• using the portion of the combustion air to operate at least one supplementary firing in the heat recovery steam generator;
• wherein the at least one combustion chamber is comprised of a first combustion chamber and a second combustion chamber that are sequentially arranged and wherein the at least one gas turbine is designed for sequential combustion and has two sequentially arranged combustion chambers comprising the first and second combustion chambers and wherein the at least one turbine in which the resultant exhaust gas is expanded is comprised of two turbines for expansion of the exhaust gases, and
• wherein the 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.

A combined-cycle power plant comprising:

• at least one gas turbine with a compressor for compression of inducted combustion air, a first combustion chamber for combustion of a fuel using the compressed combustion air, and at least one turbine for expansion of the exhaust gases created during the combustion;
• a water-steam circuit with at least one steam turbine and a heat recovery steam generator through which the exhaust gases which emerge from the gas turbine flow, wherein a capability to extract steam is provided for the steam turbine;
• a controllable bypass provided in the at least one gas turbine, via which a portion of the compressed combustion air is introduceable into the turbine by bypassing the first combustion chamber;
• and
• a supplementary firing is provided in the heat recovery steam generator, in which fuel can be burnt in order to heat the exhaust gases introduced, using the combustion air which is passed via the bypass;
• wherein the controllable bypass is controlled such that a mass flow of the portion of the combustion air that is introduced into the turbine by bypassing the first combustion chamber via the controllable bypass is greater during low load operations as compared to high load operations.