Power plants – Combustion products used as motive fluid
Reexamination Certificate
1998-02-12
2001-01-16
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Reexamination Certificate
active
06173562
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust heat recovery type thermal power plant for recovering heat of a gas turbine exhaust gas and, more particularly, to an exhaust recirculation type combined cycle plant for circulating the exhaust of a gas turbine toward the air inlet thereof.
The exhaust heat recovery type thermal power plant is composed of a gas turbine, an exhaust heat recovery boiler for recovering exhaust heat, and a steam turbine driven by steam generated by the exhaust heat recovery boiler, wherein a generator is driven by the gas turbine and the steam turbine to output electricity.
There are various types of the exhaust heat recovery type thermal power plant. As one example, Japanese Patent Laid-Open No. 45924/1989 discloses an exhaust recirculation type thermal power plant for circulating gas turbine exhaust toward the air inlet thereof.
The exhaust heat recovery type combined cycle plant can provide a rapid load variation and has a high efficiency as compared with a normal thermal electric power plant. Therefore, the exhaust heat recovery type combined cycle plant has been rapidly used in recent years, but has some problems when it is operated with a partial load.
One problem is that, at the time of partial load, the thermal efficiency greatly lowers. For example, assume that the thermal efficiency at the rated load is 1, the thermal efficiency at 50% load lowers to about 0.8, and it lowers to about 0.6 at 30% load. The combined cycle plant has the advantage that the load follow ability is higher than that of a thermal electric power plant using a normal boiler. The combined cycle plant is often operated with a variation in load. In this case, however, the plant is operated at the sacrifice of lowering the thermal efficiency thereof, when operated with a partial load.
A further problem is that an output variation on the side of the exhaust heat recovery boiler with a partial load is so great that it is difficult to operate the plant. For example, assuming that the output power of a steam turbine generator at the rated load is 1, it lowers to about 0.43 at 50% load, and it lowers to about 0.12 at 30% load. In addition, the response time of the gas turbine generator (the time from the increase or decrease of the amount of fuel charged into the gas turbine combustor to the change of the generation output of the gas turbine generator) is in the order of several seconds; whereas the response time of the steam turbine generator (the time from the increase or decrease of the amount of fuel charged into the gas turbine combustor to the change of the generation output of the steam turbine generator) is in the order of several minutes. It is therefore very difficult to provide control for obtaining a desired output when the load is changed and immediately thereafter.
With respect to the foregoing, the above-described prior art discloses an arrangement wherein gas turbine exhaust is circulated toward the air inlet thereof, but suggests neither the presence of the aforementioned problems, when operation is carried out at partial load, nor the specific method for solving the problems.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an exhaust recirculation type combined plant capable of solving the above-described problems occurring at the time of partial load operation.
According to the present invention, there is provided an exhaust recirculation type combined plant for circulating exhaust from a gas turbine to a gas turbine compressor. The quantity of gas to be recirculated is increased as the load lowers. As a result, it is preferable that the quantity of exhaust to be recirculated be regulated so as to make the combustion temperature of the gas. turbine combustor substantially constant.
The intake temperature is raised by the circulation of the gas turbine exhaust to the compressor and the quantity to be recirculated is increased as the load lowers, whereby the combustion temperature (i.e., the temperature of combustion gas at the inlet of the gas turbine) can be maintained substantially constant. In an exhaust heat recovery boiler, since the temperature of the gas turbine exhaust is not changed, the lowering of the evaporation amount when the load is reduced can be minimized.
According to the present invention, the thermal efficiency at partial load can be increased. Further, since the combustion temperature and the exhaust temperature from the gas turbine can be maintained constant irrespective of the load, the operability/controllability is excellent with less thermal damage to the materials constituting the devices. While the conventional record was 30% load per shaft, the operation to the gas turbine zero output operation can be made (to 10%), and the operating range is wide. Low NOx combustion can be achieved. The NOx producing rate within the combustor can be set to ¼ of the prior art. An exhaust taking-out point is set downstream of the exhaust heat recovery boiler (HRSG), whereby the output of the bottoming cycle can be maintained constant at partial load, and the operability/controllability can be further improved. An economizer can be provided in the recirculation pipe to improve the thermal efficiency of the bottoming cycle.
The present invention includes a thermal power plant, which comprises: a compressor for compressing gas; a combustor for mixing said compressed gas with fuel and for burning the mixture; a gas turbine connected to said combustor, and being arranged to be driven by the burned mixture; and a generator connected to said turbine and being arranged to be driven thereby; and means for recirculating at least some of the exhaust gas of the gas turbine to the compressor; characterized in that the thermal power plant is a combined cycle power plant and has a steam turbine driven by heat from an exhaust heat recovery boiler, the exhaust heat recovery boiler being heated by the exhaust gas of the gas turbine.
The present invention is characterized in that the recirculation means has means for controlling the amount of exhaust gas recirculated to the compressor in dependence on at least one of the load of the generator; the rate of flow of fuel to the combustor; and the load demand of an external system powered by the generator.
The present invention of a method includes operating a thermal power plant, which comprises: compressing gas in a compressor; mixing the compressed gas with fuel and burning the mixture; driving a gas turbine with the burned mixture; driving a generator using the gas turbine; and recirculating at least some of the exhaust gas of the turbine to the compressor; characterized in that the method further includes heating an exhaust heat recovery boiler with the exhaust gas of the gas turbine to generate steam, and driving a steam turbine with the steam.
The present invention is characterized in that the amount of exhaust gas recirculated to the compressor is varied in dependence on at least one of the load of the generator; the rate of flow of fuel to the combustor; and the load demand of an external system powered by the generator.
The present invention includes a controller for a thermal power plant, which comprises: means for receiving a load demand signal; means for driving an initial recirculation rate signal from the load demand signal; means for comparing the load demand signal with at least one of a load signal and a fuel flow rate signal to generate a correction signal; and means for correcting the initial recirculation rate signal in dependence on the correction signal to derive a recirculation rate control signal.
The combined plant is comprised of the two power generating equipments. They are a topping cycle comprised of the gas turbine and a bottoming cycle comprised of the boiler or the steam turbine. The former has a faster load responding characteristic and the latter has a slow responding characteristic. The slow responding characteristic results from the fact that a heat transferring passage is complex and it takes much time for energy t
Hoizumi Shinichi
Ikeguchi Takashi
Kirikami Seiichi
Komatsu Hideaki
Sasada Tetsuo
Antonelli Terry Stout & Kraus LLP
Casaregola Louis J.
Hitachi , Ltd.
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