This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Patent Application No. 199 40 174, filed in Germany on Aug. 25, 1999; the entire content of which is hereby incorporated by reference.
The invention relates both to a method for operating a power plant, and to a power plant for employing the method. The power plant in question has at least one gas turbo group with at least one compressor, at least one combustor, and at least one gas turbine, whereby one part of the air compressed in the compressor is branched off, cooled in a cooling air cooler, and used as a coolant for the gas turbo group. The heat removed from the compressed air is used at least in part for generating superheated steam that is introduced at least in part at a suitable place into the gas turbo group.
Many variations of gas turbine power plants and methods for operating such power plant systems are known. In a simple, open gas turbine process, the system essentially consists only of a compressor and a combustor followed by a gas turbine. In order to operate the system, processed (for example filtered, de-iced, heated or cooled), ambient air is introduced into the compressor. The compressed air is then conducted further into the combustor where the enthalpy of the compressed air is increased by combustion. The resulting combustion gases are expanded in the gas turbine, whereby the released energy is returned through a rotor shaft to the compressor and also to a generator.
The exhaust gases of the first gas turbine in principle still contain sufficient oxygen to be usable as preheated air for a second combustion. In order to increase the efficiency of such a system, a second combustor and a second gas turbine are therefore positioned behind the first gas turbine for so-called xe2x80x9csequential combustion.xe2x80x9d The gas enthalpy increases again in this second combustor, and the resulting combustion gases are expanded in the second turbine.
The first turbine is the high pressure turbine and the second turbine a low pressure turbine. Both turbines are usually installed on a common shaft.
In combination systems, the waste gases from one or more gas turbines that have been expanded almost to atmospheric pressure are used in a waste heat steam generator for generating steam. This steam is used in a separate, closed steam cycle for generating additional mechanical or electrical energy to operate a steam turbine. Part of the steam in the steam cycle also can be used as process steam or for a remote heating system, or the like.
One problem in the operation of such power plant systems is cooling. The blades of the shaft and housing of the gas turbines are in constant contact with hot combustion gases from the combustion chambers. Depending on their position and the material of the various parts, cooling is necessary to ensure mechanical integrity during operation. For the cooling of these components, several systems that use coolants such as air, steam, or other coolants are known.
In a method to which the invention at hand is also related, compressed air is removed from the compressor and is supplied to the turbines for cooling. By using part of the compressed air for turbine cooling, the amount of air involved in the thermodynamic working process of the gas turbine is automatically reduced. This results in lower gas turbine output power and efficiency. The cooling air also could result in an increase in gas turbine energy losses, for example because of the so-called dilution effect, i.e., due to the mixing losses caused by the cooling air entering the turbine gas stream.
During the construction of high-performance gas turbines, it is therefore necessary to minimize the amount of cooling air. On the one hand, this can be achieved by using more exotic materials and special temperature protection coatings for the components to be cooled, which, however, is associated with higher investment costs. An alternative for minimizing the amount of cooling air amount is to reduce the temperature of the compressed air externally, before the air is used for turbine cooling. This results in a higher heat exchange since the temperature differential between the coolant and the metal surface of the parts to be cooled is increased. An equivalent cooling therefore requires a smaller amount of cooling air. The gas turbine performance is increased by this since less air bypasses the thermodynamic gas turbine process.
Various methods to cool the compressed cooling air externally are known.
On the one hand, there are so-called quench coolers in which the compressed air is cooled by injection of water. However, this method is associated with a high thermal stress of the air coolers. The gas turbine cooling air also could be contaminated by contaminants in the water, which could lead to catastrophic consequences. To prevent this, large amounts of highly purified water are required for this method. In addition, strict control of the air temperature after mixing is extremely difficult. However, a highly accurate determination of the cooling temperature is necessary in order to prevent damage to the gas turbine.
In another method for cooling the compressed air, cooling elements are used. The removed heat is released into the atmosphere, for example, the coolant used in a heat exchanger is re-cooled by air fans. With this method, the removed heat is lost to the gas turbine process.
DE 195 08 018 A1 furthermore shows another cooling method in which the removed heat can be reused. In the system described in DE 195 08 018 A1, which is hereby incorporated by reference in its entirety, there is a combination system with a gas turbine cycle and a complete, closed water steam cycle. In this method suggested there, the air is cooled in air coolers that are integrated into the water steam cycle. Part of the steam generated in a waste heat steam generator is used as a coolant for the air cooler, whereby the heat removed from the cooling air is used to superheat the steam. The superheated steam then can be returned into the water steam cycle, for example, into the waste heat steam generator, or can be used for injection into the gas turbine. Unfortunately, this method requires a combination cycle with a closed water steam cycle, which again is associated with high investment costs. This method also cannot be used during the times in which the components of the water steam cycle, for example, the steam turbine or waste heat steam generator, are unavailable. It is furthermore not suitable for the phased concept of a system which only functions as a combination system in the last upgrade phase.
EP 0 519 304, which is hereby incorporated by reference in its entirety, furthermore describes how, in a cooling air cooler, steam generated by indirect heat exchange is introduced into a combustor of a gas turbo group and is expanded in a turbine while supplying useful power. However, especially when a gas turbo group is used whose combustion chamber is operated with contemporary premix burners with a lean premixed combustion for minimizing noxious substances, it is not easy to add larger amounts of steam into the combustor. This may lead to a destabilization of the flame in connection with a significant increase in emissions of partially burned and unburned substances and dangerous fluctuations in combustor pressure. In addition, the addition of large amounts of water steam to the hot gas increases the heat transfer to the components to be cooled, which therefore has a counterproductive effect in that it again increases the cooling air requirement. EP 0 519 304 explicitly discloses a further heating of the generated steam in a waste heat steam generator; according to the generally known state of the art, this step in the process certainly could be eliminated.
The invention is therefore based on the task of creating an alternative to known method in which the required cooling air is effectively cooled, and the heat removed hereby used again, while avoiding the above mentioned disadvantages.
According to the invention, this objective is realized with a method for operating a power plant having at least one gas turbo group with at least one compressor, with at least one combustor, and at least one gas turbine, whereby one part of the air compressed in the compressor is branched off, cooled in a cooling air cooler, and used as a coolant for the gas turbo group, and whereby pressurized feed water is added into the cooling air cooler, and heated with the heat removed from the compressed air, evaporated, and the pressurized steam generated in this way is superheated, and whereby the steam generated in this manner is at least in part added to the gas turbo group and is expanded there while supplying useful power, in which in the invention part of the generated steam is branched off before or after the superheating and is added into a cooling air conduit system that conducts the compressed and cooled air.
The basic idea of the invention is that feed water is directly introduced into the cooling air cooler in which the air which was removed from the compressor is cooled, and the heat removed from the compressed air is used to heat and evaporate the feed water and to superheat the generated steam. Part of the steam generated in this way can be introduced, on the one hand, in a known manner into the gas turbo group at a suitable place, preferably upstream from an initial turbine. This partial stream can be introduced directly into the working medium of the gas turbo group or it can be mixed with a quantity of fuel. A combination of these two variations would also be possible. This partial stream is expanded in the turbine while supplying power. Another part is mixed with the cooling air in the cooling system and there displaces cooling air which is then available for combustion, which also results in an increase in performance and efficiency. The steam content in the coolant furthermore increases the heat transfer in the cooling channels of the highly stressed components, which offsets the increased external heat transfer due to the steam content in the working gas in a first approximation. On the other hand, it is extremely disadvantageous for the mass stream of the steam to be introduced into the cooling system in its entirety: the mass stream of the steam generated in the cooling air cooler is highly variable in operation. An addition of this steam to the cooling air line again reduces the steam generation because of the decreasing cooling air mass stream. As a result, serious transient processes may develop. Strong fluctuations of the steam content in the coolant medium result in fluctuations of the heat transfer in the cooling channels of the components to be cooled. On the one hand, this may lead to damage. On the other hand, the cooling configuration then must represent a compromise between air cooling and a coolant with high steam content, which would be far removed from a favorable design for either steam or air cooling. The process according to the invention offers an advantage exactly for this case, in that a quantity of steam, which is undesireable in the cooling system, or transients of the quantity of steam, which have negative effects in the cooling systems, are fed directly into the gas turbine, where this quantity of steam continues to perform useful work. The steam mass stream to be introduced into the cooling channels is regulated by means of various control elements. This helps to a great extent to separate the steam mass stream introduced into the cooling system from the actual steam production.
Since the evaporation of the feed water takes place directly in the cooling air cooler, this method can be used both for simple, open gas cycles as well as for combined cycles. This means that the gas turbines can be used independently from the availability of a waste heat steam generator or other components of the water steam cycle. Overall, this results in a greater availability of the total system.
In principle, standardized evaporators can be used as cooling air coolers so that the construction of such a system has relatively low investment costs.
The method according to the invention furthermore provides the opportunity to very precisely control the cooling air temperature, which is safer for the gas turbine. This is particularly advantageous if the gas turbine is operated with a partial load. The sensitivity of the gas turbine with respect to changes in environmental conditions may also be better taken into account.
It is preferred that the feed water for steam generation and superheating the steam is passed once through the cooling air cooler in counterflow. It is specifically this technique that makes it possible to regulate the temperature of the cooling air exiting the cooling air cooler simply by varying the quantity of the feed water.
In a multi-stage installation of the gas turbo group, it makes sense to branch off the cooling air separately from the compressor for each turbine of the gas turbo group, at a suitable pressure. Naturally, the compressor may also include several sequential compressor stages in which correspondingly compressed air is branched off between the compressor stages. The air with the different pressures is then preferably cooled in separate cooing air coolers and is fed to the turbine or turbine stage working at the corresponding pressure. By using separate cooling air coolers, the optimum temperature of the cooling air can be set independently for each turbine.
The steam generated in the cooling air coolers also can be used to preheat other components or media, for example the feed water or fuel.
In another preferred embodiment, a waste heat steam generator is connected, parallel to the cooling air coolers, to the feed water cycle, which is integrated in the waste stream of the gas turbines. This makes it possible to generate an additional quantity of superheated steam. Although such an arrangement is associated with higher investment costs, it has the advantage of recovering not only the waste heat of the air coolers but also, at least to a major degree, the waste gas heat, i.e., the overall process takes place with a minimum of energy losses. The gas turbine output energy and efficiency are therefore also increased significantly in a simple, open cycle. With this embodiment it should be noted in particular that, in contrast to the methods known from the state of the art, the waste heat steam generator and the cooling air cooler are not positioned one after another, whereby steam is generated in the cooling air cooler which is then superheated in the waste heat boiler, or conversely; rather, both devices are arranged parallel, and superheated steam is thus generated independently, both in the waste heat steam generator and in the cooling air cooler. Because it is possible to work only with one of these two options for steam generation, the system is significantly more flexible, which again results in a higher overall availability of the system on the whole.