The present invention relates to a combined cycle power generation plant, which cools a high temperature section of a gas turbine plant using a steam as a cooling medium and also relates to an operating method thereof.
In a recent thermal power generation plant, in order to improve a plant heat (thermal) efficiency, many combined cycle power generation plants, each of which is composed of a gas turbine plant, a steam turbine plant and an exhaust heat recovery boiler, are operated as a practical equipment (practically usable plant). In the combined cycle power generation plant, there are provided a so-called single-shaft type of making much of a plant operation and a so-called multi-shaft type of making much of efficiency in a rating operation.
The single-shaft type plant is constructed in a manner that one steam turbine is directly connected to one gas turbine through a shaft, and a plurality of shaft systems are provided so as to meet a planned plant power output. On the other hand, the multi-shaft type plant is constructed in a manner that a plurality of shafts of the gas turbines are independently and separately provided with respect to one steam turbine.
The single-shaft type plant is constructed in a manner that one shaft system and the other shaft system do not interfere with each other, so that, in the case where the output power of the plurality of shaft systems is lowered during a partial load operation, the single-shaft type has an advantage of preventing a plant thermal (heat) efficiency from being rapidly lowered. On the other hand, the above multi-shaft type makes much of the steam turbine, and makes capacity (power output) large, so that the multi-shaft type has an advantage that the plant thermal efficiency during a rating operation becomes high by an increase of the capacity as compared with the single-shaft type.
As described above, since both single-shaft type and multi-shaft type have the advantages, respectively, both types are operated as a practical equipment.
In both the single-shaft type and multi-shaft type combined cycle power generation plants, in order to economize fuel consumption and to reduce a unit price of power generation, it has been studied and developed to further improve plant thermal efficiency. The plant thermal efficiency is calculated from a ratio of the total sum of heat output of plants such as a gas turbine plant, a steam turbine plant and an exhaust heat recovery boiler to the total sum of heat input thereof. When reviewing the steam turbine plant, the gas turbine plant and the exhaust heat recovery boiler in the light of improvement of the plant thermal efficiency, the improvement in the steam turbine plant and the exhaust heat recovery boiler has already reached their limits. However, it is expected that an improvement in the thermal efficiency of the gas turbine results in the improvement of a plant thermal efficiency of the entire combined cycle power generation plant.
In the gas turbine plant, the higher an inlet combustion gas temperature of the gas turbine is, the more thermal efficiency can be improved. Further, by recent development of heat resistant material and progress of cooling technique, the inlet combustion gas temperature of the gas turbine is transferring, to 1500xc2x0 C. or more by way of 1000xc2x0 C. to 1300xc2x0 C.
In the case of making 1500xc2x0 C. or more the inlet combustion gas temperature of the gas turbine, although the heat resistant material is developed, a high temperature section of the gas turbine, for example, a gas turbine stationary blade, a gas turbine moving blade, a liner transition piece of a combustor and the like, has already reached their limits of allowable metal temperature. For this reason, during an operation having many start-up and stopping times and a continuous operation of the plant for a long time, there is high possibility that accidents resulting from material breakage and fusion happen. Thus, in the case of increasing the inlet combustion gas temperature of the gas turbine, in order to maintain the gas temperature within the allowable metal temperature of components of the high temperature section of the gas turbine, there has been developed a technique of cooling the high temperature section of the gas turbine using an air, and the practical equipment has been already realized.
However, in the case of cooling the high temperature section of the gas turbine using an air, an air compressor connected directly to the gas turbine is used as an air supply source. Several ten-percent high pressure air supplied from the air compressor to the gas turbine are used for cooling the high temperature section of the gas turbine. Further, the high pressure air after cooling a turbine blade is discharged as a gas turbine driving gas and, for this reason, the temperature of the gas turbine driving gas lowers, and a mixing loss is caused. This is a factor of hindering the improvement of the plant thermal efficiency.
Recently, a steam is reconsidered as a cooling medium for cooling the high temperature section of the gas turbine, for example, the gas turbine stationary blade, a gas turbine moving blade, etc. The technique of using the steam as a cooling medium has been already disclosed in a journal of Machinery Society of America (ASME thesis, 92-GT-240) and Japanese Patent Laid-Open Publication No. HEI 5-163961.
The steam has specific heat twice as much as air, and is excellent in heat conductive performance. Further, in the steam, close loop cooling will be possibly adopted without lowering the temperature of the gas turbine driving gas and causing a mixing loss. Thus, it is expected that the steam is applied to a practical equipment in order to contribute to the improvement of plant efficiency.
However, in the case of using the steam expected as a cooling medium for cooling the high temperature section of the gas turbine, there are several problems because the combined cycle power generation plant is a multi-shaft type.
In general, in the case of cooling the high temperature section of the gas turbine with the use of a steam, a driving steam of the steam turbine plant has been used.
However, in the multi-shaft type combined cycle power generation plant, a plurality of gas turbine plants are combined with respect to one steam turbine plant. For example, during a partial load operation, when only one of the plurality of gas turbine plants is operated, conditions (temperature, pressure, flow rate) of the steam supplied from the exhaust heat recovery boiler to the steam turbine plant greatly vary from design values. If the high temperature section of the gas turbine is cooled under varied steam conditions, the following problem and disadvantage will be caused.
For example, in the case where the temperature of the steam for cooling the high temperature section of the gas turbine is high, it is difficult to maintain a material strength of the gas turbine moving and stationary blades, a gas turbine rotor or the like and, for this reason, the use of the high temperature steam will be a factor of causing breakage and fusion. On the other hand, in the case where the aforesaid temperature of steam is low, an excessive thermal stress, due to the difference in temperatures between the gas turbine driving gas and a cooling steam, is locally generated in the gas turbine moving and stationary blades, a gas turbine rotor or the like. In the case where the steam temperature is further low, the steam is easy to become drain, and a local thermal stress, resulting from excessive cooling, will be generated.
On the other hand, in the case where the pressure of the steam is high, the gas turbine moving and stationary blades are formed to have a thin thickness and, for this reason, there is a possibility that a breakage will be caused due to a so-called ballooning (expansion by internal pressure). Further, in the case where the pressure of the steam is low, there is a possibility that the gas turbine driving gas flows into the moving and stationary blades.
Moreover, in the case where the flow rate of steam decreases, the gas turbine rotating and stationary blades, a gas turbine rotor and the like can not perform preferable cooling function and, for this reason, it is impossible to maintain their material strength. Therefore, it is difficult to cope with a high temperature gas turbine plant.
Furthermore, in the case where the temperature and pressure of steam are high, water is used with the use of temperature and pressure reducing devices so that the temperature and pressure of the steam can be adjusted so as to provide a proper temperature and pressure. In this case, if the water is not sufficiently purified, dust gathers in a passage of the gas turbine moving and stationary blades and, for this reason, the passage is jammed with dust. As a result, the cooling performance lowers, which will constitute a factor of oxidation and corrosion.
As described above, in the multi-shaft type combined cycle power generation plant, in the case of cooling the high temperature section of the gas turbine using a steam, there exists a close relationship between a fluctuation of conditions of the cooling steam supplied to the high temperature section of the gas turbine from the steam turbine plant and an increase and decrease in the number of operating gas turbine plants. For this reason, no fluctuation need to be caused in the steam conditions. In particular, during a start-up operation and a partial load operation, it is difficult to adjust the steam conditions of the cooling steam supplied to the high temperature section of the gas turbine within a design value because a study and development of such adjustment has still not been sufficiently made.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a combined cycle power generation plant and a method of operating the same capable of supplying a cooling steam having proper temperature and pressure to a high temperature section of a gas turbine even if there is a fluctuation in the number of operating gas turbine plants and capable of effectively recovering the steam after cooling to a steam turbine plant so as to restrict an output fluctuation of the steam turbine plant.
This and other objects can be achieved according to the present invention by providing, in one aspect, a combined cycle power generation plant which comprises a gas turbine plant including a high temperature section, a steam turbine plant including high, intermediate and low pressure turbines connected through a common shaft, and an exhaust heat recovery boiler including a reheater, and high, intermediate and low pressure superheaters, the combined cycle power generation plant further comprising:
a low temperature reheat steam system for supplying a turbine exhaust steam from the high pressure turbine of the steam turbine plant to the reheater of the exhaust heat recovery boiler;
a cooling steam supply system which branches from the low temperature reheat steam system and supplies the turbine exhaust steam to the high temperature section of the gas turbine plant as a cooling steam;
an intermediate pressure superheater steam system which is connected to the cooling steam supply system and adapted to join a steam supplied from the intermediate pressure superheater of the exhaust heat recovery boiler together with the turbine exhaust steam;
a reheat steam system connecting the reheater of the exhaust heat recovery boiler and the intermediate pressure turbine of the steam turbine plant;
a cooling steam recovery system which recovers a steam after cooling the gas turbine high temperature section to the reheat steam system;
a bypass system which branches a superheat steam pipe connecting an outlet of a first high pressure superheater of the exhaust heat recovery boiler and an inlet of a second high pressure superheater thereof; and
a main steam system which supplies a steam from the bypass system to the high pressure turbine of the steam turbine plant.
In preferred embodiments, the low temperature reheat steam system includes a low temperature reheat flow control valve for controlling a flow rate of a turbine exhaust steam supplied from the high pressure turbine to the reheater. The bypass system includes a flow control valve for controlling a flow rate of a turbine exhaust steam.
One gas turbine plant is connected to one exhaust heat recovery boiler as one set and a plurality of the sets are disposed and the steam turbine plant has a multi-shaft structure which is separated from a shaft of the gas turbine plant of each of the sets as a single steam turbine plant located independently from the sets.
In a modification, the gas turbine plant may have a single-shaft structure which is directly connected to the shaft of said steam turbine plant.
In another aspect, there is provided a method of operating a combined cycle power generation plant which comprises a gas turbine plant including a high temperature section, a steam turbine plant including high, intermediate and low pressure turbines connected through a common shaft, and an exhaust heat recovery boiler including a reheater, and high, intermediate an low pressure superheaters and in which a turbine exhaust steam of the steam turbine plant is supplied to the high temperature section of the gas turbine plant and the turbine exhaust steam is recovered to the steam turbine plant after cooling the high temperature section of the gas turbine plant, the method comprising the steps of:
supplying a steam from the exhaust heat recovery boiler to the high pressure turbine of the steam turbine plant during a start-up operation or a partial load operation;
carrying out a constant pressure operation in the high pressure turbine while carrying out an inlet pressure control in a reheat steam system for connecting the reheater of the exhaust heat recovery boiler to the intermediate pressure turbine of the steam turbine plant;
joining the turbine exhaust steam after being expanded in the high pressure turbine together with a steam generated from an intermediate pressure drum of the exhaust heat recovery boiler;
supplying the joined steam to the high temperature section of the gas turbine plant so as to cool the high temperature section; and
joining the jointed steam, after cooling, together with a reheated steam from the reheater and recovering the joined steam to the intermediate pressure turbine.
In a preferred embodiment of this aspect, a plurality of gas turbine plants are located and the partial load operation is an operation at a time when the number of the gas turbine plants in a condition of being operated is reduced.
The start-up operation is performed in either one of cases where an air temperature becomes higher than a predetermined temperature and where temperature and pressure of a steam from the high pressure turbine to be supplied to the gas turbine are out of a predetermined range. A constant pressure operation of the high pressure turbine is carried out by throttling down a valve opening of either one of a main steam stop valve and a steam control valve located on a main steam system mutually connecting the exhaust heat recovery boiler and the high pressure turbine.
There is generated a valve signal for throttling down a valve opening of either one of a steam stop valve and a steam control valve, the valve signal being represented by a temperature of either one of a cooling steam supply system and a cooling steam recovery system located to the gas turbine high temperature section.
An inlet pressure control of a reheat steam system is carried out by throttling down a valve opening of a reheat steam combination valve located to the reheat steam system, and a valve signal for throttling down the reheat steam combination valve is represented by a pressure of either one of a cooling steam supply system and a cooling steam recovery system located to the gas turbine high temperature section.
According to the present invention of the characters mentioned above, when supplying the turbine exhaust steam of the high pressure turbine to the high temperature section of the gas turbine as a cooling steam, in the case where the turbine exhaust steam diverges from a proper temperature and pressure operating range, the inlet pressure control (IPC) is carried out in the reheat steam system of the intermediate pressure turbine so as to make high the pressure of intermediate pressure superheated steam generated from the intermediate pressure drum. The intermediate pressure superheated steam is joined together with the turbine exhaust steam, and then, is controlled so as to become a proper pressure, and further, the superheated steam generated from the first high pressure superheater of the exhaust heat recovery boiler is supplied as a low temperature steam to the high pressure turbine with the use of the bypass system. At this time, a constant pressure operation is carried out in the high pressure turbine, and the turbine exhaust steam is controlled so as to become a proper temperature. Therefore, the high temperature section of the gas turbine can be securely cooled, and it is possible to sufficiently cope with the gas turbine which is made high temperature.
Further, in the combined cycle power generation plant according to the present invention, and in the operating method thereof, the steam after cooling the high temperature section of the gas turbine is joined together with the reheat steam supplied from the reheater of the exhaust heat recovery boiler, and then, is recovered into the intermediate pressure turbine. Thus, an output power of the steam turbine plant can be increased, and also, the plant thermal efficiency can be improved.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.