JP6224858B1 - Power plant and operation method thereof - Google Patents

Abstract

A power plant including a boiler feed water heating boiler capable of improving power generation efficiency is provided. A boiler 3 having an economizer 18 and a superheater 13, a steam turbine 5 driven by steam generated in the boiler 3, a generator 37 driven by the steam turbine 5, and a boiler 3 are supplied. A feed water heating boiler 7 for heating the feed water and a control unit 30 for controlling the heating amount of the feed water by the feed water heating boiler 7. The control unit 30 has an outlet water temperature of the economizer 18 that is lower than a saturation temperature and is saturated. The amount of heating is controlled so that the temperature is close to the temperature. [Selection] Figure 1

Description

  The present invention relates to a power plant including a feed water heating boiler that heats boiler feed water, and an operation method thereof.

  In a power plant where steam generated by a boiler using fossil fuels such as coal and oil is supplied to a steam turbine and the steam turbine is driven to rotate, and power is generated by the generator using this rotating drive, power generation efficiency is improved There is a need to reduce fuel consumption and environmental impact. Further reductions in environmental impact include reductions in carbon dioxide emissions that are being addressed globally. For example, Patent Document 1 discloses that power generation efficiency is improved by heating boiler feed water using steam or hot water obtained in a large incinerator.

Japanese Patent No. 3611327

  However, in Patent Document 1, the boiler feedwater is heated with steam or hot water obtained in a large incineration plant to improve power generation efficiency, but there are operational restrictions associated with each equipment specification of the power plant, Even the effective use of the heat of the entire power plant has not been carried out, and further power generation efficiency improvement technology is desired.

  On the other hand, biomass-derived biomass fuels such as woody, agricultural, and sludge are carbon neutral that does not emit carbon dioxide, which is a global warming gas, because carbon dioxide is taken in during the growth of biomass. Various uses have been studied. For example, a power plant that performs co-firing in a boiler using fossil fuel and biomass fuel has been studied.

  However, high-quality biomass fuels are expensive, and there are problems in terms of cost competitiveness with respect to fossil fuels. In addition, since inexpensive biomass fuels contain many corrosive components, only a small amount can be mixed in a boiler. Furthermore, there is an upper limit to the mixed combustion rate due to technical limitations of boiler fuel supply facilities and environmental devices, and when the output of the boiler is low, the amount of biomass fuel input must be further reduced.

This invention is made | formed in view of such a situation, Comprising: It aims at providing the power generation plant provided with the boiler for boiler feed water heating which can improve electric power generation efficiency, and its operating method.
In addition, the present invention can reduce the consumption of fossil fuel by increasing the use ratio of biomass fuel and reduce the emission of carbon dioxide when using carbon-neutral biomass fuel for reducing environmental impact. It aims at providing a power plant and its operating method.

In order to solve the above problems, the power generation plant and the operation method thereof according to the present invention employ the following means.
That is, a power plant according to the present invention includes a boiler having an economizer and a superheater, a steam turbine driven by steam generated by the boiler, a generator driven by the steam turbine, and a supply to the boiler A feed water heating boiler that heats the supplied feed water, and a control unit that controls a heating amount of the feed water by the feed water heating boiler, wherein the control unit has an outlet water temperature of the economizer that is less than a saturation temperature and The heating amount is controlled to be in a range up to a temperature lower by 10 ° C. than the saturation temperature.

  By heating the feed water supplied to the boiler with a feed water heating boiler, the enthalpy (heat amount per unit flow rate) of the feed water supplied to the boiler is increased, and the turbine efficiency is improved and the power generation efficiency for fossil fuel energy ( Hereinafter, it is simply referred to as “power generation efficiency”). Furthermore, the heating amount of the feed water heating boiler is controlled so that the outlet water temperature of the economizer is less than the saturation temperature that does not vaporize and close to the saturation temperature, so that the feed water can be heated as much as possible. it can. Here, the temperature that is lower than the saturation temperature and close to the saturation temperature is a temperature that is lower than the saturation temperature by several degrees C (for example, 2 degrees C) to about 10 degrees C. As a result, the enthalpy of water supply can be increased as much as possible so that the economizer outlet water temperature does not vaporize, turbine efficiency can be improved and power generation efficiency can be improved, and fossil fuel input to the boiler can be reduced. The amount of carbon dioxide that does not become carbon neutral can be reduced.

The power plant according to the present invention further includes an exhaust gas treatment device that treats the exhaust gas discharged from the boiler, and the control unit is configured such that the exhaust gas supplied to the exhaust gas treatment device is an upper limit temperature of an operating temperature of the exhaust gas treatment device. and controlling the heating amount such that less than.

  In an exhaust gas treatment apparatus such as a denitration apparatus, an upper limit temperature of exhaust gas to be supplied is defined. It is possible to control to increase the heating amount of the feed water heating boiler so that the exhaust gas temperature is lower than the upper limit temperature and close to the upper limit temperature. Here, the exhaust gas temperature that is lower than the upper limit temperature and close to the upper limit temperature is, for example, several degrees Celsius (for example, 2 degrees Celsius) to several tens of degrees from the upper limit temperature of the operating temperature of the equipment installed downstream of the exhaust gas flow, such as the catalyst temperature of the denitration device. The temperature is lower by about ℃. Since the temperature is lower than the upper limit temperature of the exhaust gas, the water supply can be heated as much as possible. Thereby, the enthalpy of water supply can be increased, turbine efficiency can be improved and power generation efficiency can be improved.

Furthermore, the power plant according to the present invention includes a feed water heater that extracts steam from the steam turbine and heats the feed water, and the controller controls the amount of steam extracted supplied to the feed water heater to the heating amount. It is characterized by decreasing according to.

Since the feed water is heated by the feed water heating boiler, the steam output from the steam turbine is decreased and the steam flow rate of the steam turbine is increased within the specified range of the mechanical limit of the steam turbine to increase the turbine output. To do. In addition, when comparing the amount before and after reducing the amount of steam extracted, if the total amount of steam supplied from the boiler to the steam turbine is adjusted so as to have the same turbine output, by reducing the amount of extraction, Since the total steam flow can be reduced, turbine efficiency can be improved.
Especially when the boiler has a partial load, even if the amount of steam flowing to the steam turbine is increased, it is still within the specified range of mechanical strength of the steam turbine. Thus, the turbine efficiency can be further improved.

  Furthermore, in the power plant of the present invention, the control unit reduces the amount of steam extracted to the feed water heater when the economizer feed water inlet temperature is higher than the economizer inlet feed water set temperature by a predetermined value or more. It is characterized by.

  Furthermore, in the power plant of the present invention, the control unit controls the amount of steam extracted based on the strength of the turbine blades of the steam turbine.

  There is a turbine blade strength as a specified range of mechanical strength of the steam turbine. The strength of the turbine blade is designed to be durable against the differential pressure across the turbine blade, and this differential pressure is determined by the flow rate and pressure balance of the steam flowing inside the steam turbine. The pressure changes according to the amount of steam extracted. Therefore, the steam extraction amount is controlled in consideration of the strength of the turbine blade. As a result, the amount of extraction can be reduced as much as possible in consideration of the strength of the turbine blades, the total steam flow flowing to the steam turbine can be reduced, and the turbine efficiency can be further improved.

  Furthermore, the power plant according to the present invention includes a superheater spray that controls a temperature of superheated steam supplied from the superheater, and the control unit is configured to change a spray amount of the superheater spray based on a change amount of the heating amount. It is characterized by controlling.

There is a possibility that the temperature of the superheated steam supplied from the superheater may fluctuate due to a change in the heating amount or extraction amount of the feed water heating boiler. The temperature of the superheated steam is set within a specified value that is equal to or lower than the design temperature of the metal material (steam pipe, turbine blade, etc.) used under high temperature and pressure. By quickly suppressing the temperature fluctuation of the superheated steam by the superheater spray, it is possible to suppress the deterioration and damage of the steam pipe and the turbine blade.
In addition, when the boiler is provided with the reheater, it is preferable to also control the flow rate of the reheater spray that controls the temperature of the reheat steam supplied from the reheater.

  Further, in the power plant according to the present invention, the water generated in the feed water heating boiler and the feed water heating heat exchanger for exchanging heat between the feed water supplied to the boiler and the feed water bypassing the feed water heating heat exchanger are provided. A bypass water pipe and a feed water bypass valve provided in the feed water bypass pipe, wherein the controller is configured such that the temperature of the steam generated in the feed water heating boiler is the temperature of the feed water at the inlet of the feed water heating heat exchanger. When the value is higher than a predetermined value, the feed water bypass valve is closed, and the feed water is heated with steam generated in the feed water heating boiler.

  Furthermore, in the power plant of the present invention, the feed water heating boiler is a biomass boiler that uses biomass fuel as a main fuel.

  By making the feed water heating boiler a biomass boiler using biomass fuel as the main fuel, the biomass fuel can be exclusively fired in the feed water heating boiler without co-firing the biomass fuel in the boiler. Therefore, the use ratio of the biomass fuel with respect to the fossil fuel burned with a boiler can be raised. Moreover, inexpensive biomass fuel containing a corrosive component can be used without affecting the combustion in the boiler body, and the amount of fossil fuel used can be reduced and highly efficient power generation can be performed.

The power plant operating method of the present invention includes a boiler having an economizer and a superheater, a steam turbine driven by steam generated by the boiler, a generator driven by the steam turbine, and the boiler A feed water heating boiler for heating the feed water supplied to the power plant, wherein the economizer outlet water temperature is less than the saturation temperature and up to a temperature 10 ° C. lower than the saturation temperature The amount of steam extracted is controlled in accordance with the amount of heating, and the amount of steam extracted from the steam turbine is extracted according to the amount of heating .

Since the feed water is heated to as high a temperature as possible using the feed water heating boiler, the turbine efficiency can be improved, and the amount of fossil fuel input to the boiler into which fossil fuel is input can be reduced. The amount of carbon dioxide that does not become neutral can be reduced.
In addition, by using a biomass water boiler as the feed water heating boiler, the biomass fuel is exclusively fired in the feed water heating boiler without co-firing the biomass fuel in the boiler, so the use ratio of the biomass fuel can be increased.

It is a schematic structure figure showing a power plant concerning one embodiment of the present invention. It is the figure which showed the temperature change by a superheater spray. It is the figure which showed the opening degree of the high pressure extraction valve. It is the flowchart which showed the starting control of the biomass boiler. It is the flowchart which showed the separation control of the biomass boiler. It is the graph which showed the amount of heating before and after applying a biomass boiler. It is the graph which showed the power generation efficiency with respect to the fossil fuel energy before and after applying a biomass boiler. It is the graph which showed the breakdown of the energy before and after applying a biomass boiler.

Hereinafter, an embodiment according to the present invention will be described with reference to the drawings.
FIG. 1 shows a power plant 1 according to an embodiment. The power plant 1 includes a boiler 3, a steam turbine 5, and a biomass boiler (feed water heating boiler) 7.

  The boiler 3 includes a burner 11 that forms a flame using fossil fuel such as coal or oil in the furnace 10 of the boiler body 3a. The furnace wall forming the furnace 10 is a water-cooled wall 12 composed of heat transfer tubes and fins, and water heated by the water-cooled wall 12 is guided to a steam drum 15. A water drum 14 is provided on the vertically lower side of the water cooling wall 12. Water in the steam drum 15 is guided to the water drum 14 by the circulation pump 20 and water is supplied to the water cooling wall 12. In this embodiment, a subcritical pressure boiler having a drum is taken as an example, but the present invention can also be applied to a supercritical pressure boiler having no drum.

  The combustion exhaust gas generated by the flame of the burner 11 flows vertically upward of the furnace 10 and is guided to the superheater 13. A reheater 17 and an economizer 18 are provided in this order on the downstream side of the combustion exhaust gas flow of the superheater 13.

  An economizer outlet pipe 22 through which water after being heated by the economizer 18 circulates is connected between the economizer 18 and the steam drum 15. The economizer outlet pipe 22 is provided with an economizer outlet temperature sensor 22T and an economizer outlet pressure sensor 22P. The measured values of these sensors 22T and 22P are sent to the control unit 30.

  A water supply pipe 24 for supplying water (water) is connected to the economizer 18. As will be described later, the feed water passing through the feed water pipe 24 is heated by steam generated in the biomass boiler 7 and then supplied to the economizer 18. A temperature sensor 58T is provided in the water supply pipe 24 on the inlet side of the economizer 18. The measured value of the temperature sensor 58T is sent to the control unit 30.

  A combustion exhaust gas temperature sensor 25T that measures the temperature of the combustion exhaust gas after passing through the economizer 18 is provided on the downstream side of the combustion exhaust gas flow that has passed through the economizer 18. The measured value of the combustion exhaust gas temperature sensor 25T is sent to the control unit 30. The combustion exhaust gas after passing through the combustion exhaust gas temperature sensor 25T is led to a denitration device (not shown) provided for removing NOx in the combustion gas at a midway position of the exhaust gas duct connected to the boiler body 3a. It is burned.

  The denitration apparatus is a selective catalytic reduction denitration apparatus using ammonia, and an upper limit temperature (for example, about 400 ° C. to 420 ° C.) is determined in relation to the catalytic reaction temperature. Therefore, based on the temperature of the combustion exhaust gas temperature sensor 25T, for example, the control unit 30 adjusts the amount of heat exchange in the boiler 3 so as not to exceed the upper limit temperature of the denitration device, thereby controlling the combustion exhaust gas temperature. The combustion exhaust gas that has passed through the denitration device is, for example, SOx etc. removed by a desulfurization device (not shown) in order to remove SOx in the combustion gas, and further, a dust treatment device and an induction blower not shown on the downstream side Etc. are provided as necessary, and discharged from the chimney at the downstream end of the exhaust gas duct to the atmosphere.

  The superheater 13 is provided with a superheater spray 27. By injecting water or steam from the superheater spray 27, the superheated steam flowing in the superheater 13 is quickly cooled and controlled to a temperature within a predetermined range. For example, a part of the outlet water of the economizer 18 is used as the water used in the superheater spray 27. The water injection amount and the injection timing of the superheater spray 27 are controlled by the control unit 30.

  Although the installation position of the superheater spray 27 is schematically shown in FIG. 1, a more specific installation position will be described with reference to FIG. As shown in FIG. 2, the superheater 13 includes, for example, a primary superheater 13a, a secondary superheater 13b, and a tertiary superheater 13c. The superheater spray 27 is provided between the primary superheater 13a and the secondary superheater 13b, and between the secondary superheater 13b and the tertiary superheater 13c, respectively. Inject to reduce the superheated steam temperature step by step. That is, water spray is performed at the outlet of the primary superheater 13a so as to be the primary superheater outlet set temperature, and water spray is performed at the outlet of the secondary superheater 13b so as to be the secondary superheater outlet set temperature. By controlling the steam temperature by performing water spray in this way, each superheater outlet set temperature is adjusted, and the temperature of the main steam supplied to the steam turbine 5 is controlled to be within a predetermined range.

  Although not shown, a reheater spray may also be provided for the reheater 17. The water injection amount and the injection timing of the reheater spray are also controlled by the control unit 30.

  The superheated steam that has exited the superheater 13 is guided to the steam turbine 5 through the main steam pipe 32 as shown in FIG. In the present embodiment, the steam turbine 5 includes, for example, a high-pressure turbine 34, an intermediate-pressure turbine 35, and a low-pressure turbine 36. The generator 37 is rotationally driven by the rotational power generated in the turbines 34, 35, and 36, and power is generated.

  The main steam pipe 32 is connected to the inlet of the high pressure turbine 34. A high pressure turbine outlet pipe 38 is connected to the exhaust side of the high pressure turbine 34. The downstream end of the high-pressure turbine outlet pipe 38 is connected to the reheater 17 so that steam that has been subjected to predetermined expansion by the high-pressure turbine 34 and rotationally driven the turbine is guided to the reheater 17. It has become.

  A reheat steam supply pipe 40 that supplies reheat steam to the intermediate pressure turbine 35 is provided between the steam outlet side of the reheater 17 and the intermediate pressure turbine 35.

  An intermediate pressure turbine outlet pipe 42 is connected to the exhaust side of the intermediate pressure turbine 35. The downstream end of the intermediate-pressure turbine outlet pipe 42 is connected to the inlet of the low-pressure turbine 36, and steam that has been subjected to predetermined expansion in the intermediate-pressure turbine 35 and rotationally driven the turbine is guided to the low-pressure turbine 36. It is like that.

  A low-pressure turbine outlet pipe 44 is connected to the exhaust side of the low-pressure turbine 36. The downstream end of the low-pressure turbine outlet pipe 44 is connected to a condenser 46. In the condenser 46, the steam is cooled to a vacuum by a cooling water (not shown) and is condensed and liquefied. The condensate liquefied by the condenser 46 becomes feed water and is led to the low pressure feed water heater 50 by the condensate pump 48.

  In the present embodiment, for example, in the low-pressure feed water heater 50, the feed water from the condensate is heated by the low-pressure steam guided from the low-pressure turbine 36. The feed water heated by the low pressure feed water heater 50 is guided to the first intermediate pressure feed water heater 52a and the second intermediate pressure feed water heater 52b through the feed pump 51, and is extracted from the intermediate pressure turbine 35. Heated by steam. The feed water heated by each of the medium pressure feed water heaters 52a and 52b is guided to the first high pressure feed water heater 54a and the second high pressure feed water heater 54b.

The high-pressure steam guided to the first high-pressure feed water heater 54a is guided via the first high-pressure extraction pipe 55a. The first high pressure extraction pipe 55a is provided with a first high pressure extraction valve 56a whose opening degree is controlled by the control unit 30.
The high-pressure steam guided to the second high-pressure feed water heater 54b is guided via the second high-pressure extraction pipe 55b. The second high pressure extraction pipe 55b is provided with a second high pressure extraction valve 56b whose opening degree is controlled by the control unit 30.
In the present embodiment, as an example, the first high-pressure extraction pipe 55a is connected to the upstream side (high-pressure side) of the high-pressure turbine 34 relative to the second high-pressure extraction pipe 55b. The pressure of the high-pressure steam to be applied is higher than the pressure of the high-pressure steam guided by the second high-pressure extraction pipe 55b.

  FIG. 3 shows the opening degrees of the high-pressure extraction valves 56a and 56b with respect to the load of the boiler 3 as an example of the present embodiment. The map (load program) shown in FIG. 3 is stored in the storage unit of the control unit 30, and the valve opening is set in advance according to the load. As shown in the figure, when the load on the boiler 3 is 100%, both the high-pressure extraction valves 56a and 56b are fully closed. When the load is smaller than a predetermined value, both the high pressure bleed valves 56a and 56b are fully opened. However, in this embodiment, the load area for switching between fully closed and fully opened is set to be different between the first high pressure bleed valve 56a and the second high pressure bleed valve 56b, thereby improving the controllability of heating of the feed water. Yes. Specifically, the first high pressure bleed valve 56a operates on the higher load side than the second high pressure bleed valve 56b. As a result, by changing the extraction flow rate of the high-pressure steam with the first high-pressure extraction valve 56a and changing the shortage with the second high-pressure extraction valve 56b, it is possible to arbitrarily control in a wide range. As will be described later, when the biomass boiler 7 is charged, the high pressure extraction valves 56a and 56b are controlled in the closing direction regardless of the load of the boiler 3 instead of the map shown in FIG.

  The steam after the feed water is heated by each feed water heater 50, 52a, 52b, 54a, 54b is sequentially supplied to the low pressure stage feed water heaters 50, 52a, 52b, 54a, as indicated by broken line arrows in FIG. 54b and finally collected by the condenser 46.

  The feed water after being heated by the high-pressure feed water heaters 54a and 54b passes through the feed water pipe 24 and is led to the feed water heating heat exchanger 60. In the feed water heating heat exchanger 60, the feed water is heated by the steam guided from the biomass boiler 7 to the heating heat transfer tube 61. Between the biomass boiler 7 and the feed water heating heat exchanger 60, the steam is supplied to the heating heat transfer pipe 61, and the heating steam supply pipe 62 is passed through the heating heat transfer pipe 61 to perform drainage. A return pipe 64 and a return pump 65 for returning the drain water to the biomass boiler 7 are provided.

  The heating steam supply pipe 62 includes a heating steam temperature sensor 62T that measures the temperature of the heating steam, a heating steam pressure sensor 62P that measures the pressure of the heating steam, and a heating that measures the flow rate of the heating steam. A steam flow rate sensor 62F is provided. The measured values of the sensors 62T, 62P, and 62F are sent to the control unit 30.

  A return pump 65 is provided in the middle of the return pipe 64 so that drain water is sent to the biomass boiler 7 by the return pump 65 and circulates between the biomass boiler 7 and the heat exchanger 60 for heating the feed water. It has become.

  A feed water bypass pipe 67 is provided so as to bypass the feed water heating heat exchanger 60. The water supply bypass pipe 67 is provided with a water supply bypass valve 68 whose opening and closing is controlled by the control unit 30. The feed water bypass valve 68 is fully closed when the feed water heating heat exchanger 60 is used, and is fully opened when the feed water heating heat exchanger 60 is not used. A temperature sensor 57 </ b> T for measuring a feed water heating heat exchanger inlet temperature is provided in the feed water pipe 24 on the upstream side of the feed water bypass pipe 67. The measured value of the temperature sensor 57T is sent to the control unit 30.

  The biomass boiler 7 generates steam by burning biomass fuel. The output of the biomass boiler 7 is controlled by the control unit 30. Specifically, the control unit 30 controls the supply amount of biomass fuel to be introduced into the biomass boiler 7, the flow rate of combustion air, the supply amount of generated steam, and the like. Thus, the heating amount of the feed water in the feed water heating heat exchanger 60 is controlled by controlling the output of the biomass boiler 7 by the control unit 30.

  The biomass boiler 7 is provided with a steam dump valve 72 whose opening degree is controlled by the control unit 30. When the pressure of the steam generated in the biomass boiler 7 becomes excessive, excess steam is discharged to the condenser 46 via the steam dump valve 72. However, the discharge destination of the steam is not limited to the condenser 46, and may be another cooling device or atmospheric discharge, for example. Further, when steam or drain water circulating in the biomass boiler 7 is insufficient, even if a part of the condensate in the condenser 46 is supplied to the upstream side of the return pump 65 of the return pipe 64 by a water supply line (not shown). good.

  The control unit 30 includes, for example, a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), and a computer-readable storage medium. A series of processes for realizing various functions is stored in a storage medium or the like in the form of a program as an example, and the CPU reads the program into a RAM or the like to execute information processing / arithmetic processing. As a result, various functions are realized. The program is preinstalled in a ROM or other storage medium, provided in a state stored in a computer-readable storage medium, or distributed via wired or wireless communication means. Etc. may be applied. The computer-readable storage medium is a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, a semiconductor memory, or the like.

The power plant 1 having the above-described configuration operates as follows.
The superheated steam generated in the superheater 13 of the boiler 3 is guided to the high-pressure turbine 34 of the steam turbine 5, and after being driven to rotate, is guided to the reheater 17. The steam guided to the reheater 17 is reheated by the boiler 3 and guided to the intermediate pressure turbine 35 as reheated steam. The reheat steam rotates the intermediate pressure turbine 35 and then guides it to the low pressure turbine 36 to rotate the low pressure turbine 36. The generator 37 is rotationally driven by the rotational power thus obtained, and power generation is performed.

  The steam that has finished the work of rotating the turbine by performing predetermined expansion in the low-pressure turbine 36 becomes condensate in the condenser 46, and sequentially passes through each of the feed water heaters 50, 52a, 52b, 54a, 54b as feed water. To be heated. Thereafter, the feed water is further heated by the feed water heating heat exchanger 60 and supplied to the economizer 18 of the boiler 3.

  The heating amount of the feed water in the feed water heating heat exchanger 60 is adjusted by controlling the output of the biomass boiler 7 by the control unit 30. Specifically, it is as follows.

<Control based on economizer outlet temperature>
Control is performed so that the outlet temperature of the water heated by the economizer 18 does not evaporate beyond the saturation temperature. Thereby, the water supply temperature as high as possible is realized while preventing the water at the outlet of the economizer 18 from becoming steam.
The control unit 30 obtains the saturation temperature T1 from the measured pressure of the economizer outlet pressure sensor 22P and the measured temperature of the temperature sensor 22T. And the temperature which falls below this saturation temperature T1 in the range of predetermined value (for example, 2 degreeC-10 degreeC) is made into economizer exit temperature setting value T1set, and the control part 30 is the biomass boiler 7 so that it may become this economizer exit temperature setting value T1set. Feedback control. Thereby, the temperature of water supply is made as high as possible so that the outlet water temperature of the economizer 18 is not vaporized, and the enthalpy of water supply is increased as much as possible.

<Control based on combustion exhaust gas temperature>
In consideration of the upper limit temperature of the equipment installed on the downstream side of the combustion exhaust gas, in this embodiment, control is performed so as not to exceed the upper limit temperature T2 determined from the catalytic reaction temperature of the denitration apparatus. Thereby, the feed water temperature as high as possible is realized while maintaining the catalytic reaction of the denitration apparatus.
The control unit 30 obtains the temperature measured by the flue gas temperature sensor 25T and sets the temperature below the upper limit temperature T2 within a predetermined value (for example, 5 ° C. to several tens of ° C. (for example, 50 ° C.)) as the flue gas temperature set value T2set The control unit 30 feedback-controls the output of the biomass boiler 7 so that the combustion exhaust gas temperature set value T2set is obtained. Thereby, the temperature of the feed water is increased as much as possible so that the combustion exhaust gas that has passed through the economizer 18 does not exceed the upper limit temperature, and the enthalpy of the feed water is increased as much as possible.

  The control unit 30 controls the heating amount (for example, steam amount, pressure, temperature, and heat output amount) from the biomass boiler 7 so as to satisfy both the control based on the economizer outlet temperature and the control based on the combustion exhaust gas temperature.

<Control of high pressure bleed valve>
While controlling the heating amount of the biomass boiler 7 as described above, the high-pressure extraction valves 56a and 56b are further controlled to control the extraction amount of the high-pressure steam. Specifically, the control unit 30 controls the high pressure extraction valves 56a and 56b as follows.
When the biomass boiler 7 is not charged, that is, when the feed water is not heated by the feed water heating heat exchanger 60, the high pressure bleed valve according to the load of the boiler 3 according to the load program in the map shown in FIG. The opening degree of 56a, 56b is controlled.

  When the biomass boiler 7 is charged, the high pressure extraction valves 56a and 56b are controlled to be closed, that is, the amount of extraction of high pressure steam is reduced. Specifically, when the feed water is heated by the biomass boiler 7 using the heat exchanger 60 for heating the feed water, the amount of high-pressure steam extracted according to the amount of heat corresponding to the amount of heating is reduced. The heating amount from the biomass boiler 7 is controlled based on the above-described economizer outlet temperature and control based on the combustion exhaust gas temperature from the heating steam temperature sensor 62T, the heating steam pressure sensor 62P, and the heating steam flow rate sensor 62F. The control unit 30 calculates so as to satisfy both.

  When the heating amount by the biomass boiler 7 exceeds the extraction amount of the high-pressure steam, the high-pressure extraction valves 56a and 56b are fully closed, and the extraction amount of the high-pressure steam is set to zero. The timing at which the high-pressure extraction valves 56a and 56b are fully closed at this time is preferably performed based on a control map in which the relationship with the heating amount from the biomass boiler 7 is determined in advance.

  By reducing the amount of high-pressure steam extracted from the high-pressure turbine 34, the flow rate of steam flowing through the high-pressure turbine 34 can be increased within a specified range of mechanical strength, thereby improving turbine efficiency. Here, when controlling the extraction amount of the high-pressure steam supplied to the first high-pressure feed water heater 54a and the second high-pressure feed water heater 54b in the closing direction of the high-pressure extraction valves 56a and 56b, the specified range of mechanical strength is set. The strength of the turbine blades of the high pressure turbine 34 is taken into account. That is, the strength of the turbine blade is designed to have durability against the differential pressure applied before and after the turbine blade. Since this differential pressure is determined by the flow rate / pressure balance of the high-pressure steam passing through the high-pressure turbine 34, the high-pressure bleed valve 56a is considered in consideration of the load of the boiler 3 so that the differential pressure does not exceed the allowable value on the turbine blade strength. , 56b, that is, the amount of decrease in the amount of high-pressure steam extraction is determined. For example, when the high pressure bleed valves 56a and 56b are fully closed, the turbine strength at the MCR point (boiler maximum continuous evaporation amount) is considered. If the turbine strength is sufficient even when the high pressure extraction valves 56a and 56b are fully closed at the MCR point, the high pressure extraction valves 56a and 56b can be fully closed even when the load is 100%. If the turbine strength is not sufficient, the reduction amount of the high-pressure steam extraction amount is determined in consideration of the turbine strength when the load is 100%, and the openings of the high-pressure extraction valves 56a and 56b are set.

<Control of superheater spray>
The control unit 30 controls the superheater spray 27 when the amount of heating of the biomass boiler 7 changes or when the amount of high-pressure steam extracted by the high-pressure extraction valves 56a and 56b changes. When the heating amount of the biomass boiler 7 changes (for example, emergency stop of the biomass boiler 7 or change in the calorific value of the biomass fuel), the temperature of the main steam exiting the superheater 13 may change. Quickly control the main steam temperature within the appropriate range. Moreover, since the balance between the superheater 13 and the reheater 17 may change when the amount of high-pressure steam extracted to the first high-pressure feed water heater 54a and the second high-pressure feed water heater 54b changes, the superheater spray may change. 27, the main steam temperature and the reheat steam temperature are quickly controlled within a range of appropriate values. When a reheater spray is provided, it is preferably controlled in cooperation with the superheater spray 27. Therefore, when heating feed water with the biomass boiler 7, compared with the case where feed water is not heated with the biomass boiler 7, the amount of cooling heat is required, and the control range of the injection amount of the superheater spray 27 and the reheater spray is required. Therefore, it is more preferable that the superheater spray 27 or the reheater spray having a control range of the injection amount corresponding to this is adopted.

<Biomass boiler activation control>
Next, activation control of the biomass boiler 7 will be described with reference to FIG.
First, as shown in step S1, the load of the boiler 3 is equal to or greater than a certain level, the biomass boiler 7 is stopped, and high-pressure extraction for supplying high-pressure steam to the first high-pressure feed water heater 54a and the second high-pressure feed water heater 54b. When the valves 56a and 56b are opened and the feed water bypass valve 68 is opened, the biomass boiler 7 is started as shown in step S2. Thereby, biomass fuel is thrown into the biomass boiler 7, and the production | generation of a vapor | steam is performed.

  In step S3, the biomass boiler outlet steam temperature measured by the heating steam temperature sensor 62T provided in the heating steam supply pipe 62 flows into the feed water heating heat exchanger 60 measured by the temperature sensor 57T. It is judged whether it is more than predetermined value (alpha) rather than the water supply temperature to perform. If the condition of step S3 is satisfied, the process proceeds to step S4. If not satisfied, step S3 is repeated and the activation of the biomass boiler 7 is continued. The predetermined value α is determined from the heat transfer characteristics of the feed water heating heat exchanger 60. To reduce the predetermined value α, it is necessary to increase the heat transfer area of the feed water heating heat exchanger 60, and the heat exchanger is large. Turn into. For example, the predetermined value α is set at 2 ° C. to 10 ° C., more preferably 2 ° C. to 5 ° C.

  In step S4, the feed water bypass valve 68 is closed and the feed water is guided to the feed water heating heat exchanger 60. At this time, the high pressure bleed valves 56a and 56b remain open.

  The water supply pipe 24 is provided with a temperature sensor 58T that measures the water supply inlet temperature of the economizer 18. The measured value of the temperature sensor 58T is sent to the control unit 30. In step S5, it is determined whether the feed water inlet temperature of the economizer 18 measured by the temperature sensor 58T is equal to or higher than a predetermined value β than the set temperature. If the condition of step S5 is satisfied, the process proceeds to step S6. If not satisfied, step S5 is repeated to wait for the temperature of the water supply inlet of the economizer 18 to rise. The predetermined value β allows the opening of the high-pressure extraction valves 56a and 56b to be closed when the feed water inlet temperature of the economizer 18 exceeds the set temperature. When an unstable fluctuation occurs in the heat exchange amount of the exchanger 60, the opening degree control of the high pressure extraction valves 56a and 56b is affected. For example, the predetermined value β is set at 2 ° C to 5 ° C.

  In step S6, the opening degree of the high pressure extraction valves 56a and 56b is controlled in the closing direction. As a result, the extraction amount of the high-pressure steam is decreased, the steam flow rate of the steam turbine is increased within the specified range of the mechanical limit, and the turbine output is increased. In addition, when comparing the amount before and after reducing the amount of steam extracted, if the total amount of steam supplied from the boiler to the steam turbine is adjusted so as to have the same turbine output, by reducing the amount of extraction, Since the total steam flow can be reduced, turbine efficiency can be improved. By improving the turbine efficiency, the power generation efficiency with respect to the load of the boiler 3 is improved. Under the condition that the power generation output after heating to the feed water by the biomass boiler 7 does not change, the amount of fossil fuel input to the boiler 3 can be reduced, and the amount of carbon dioxide emissions that do not become carbon neutral can be reduced. Thus, the introduction of the biomass boiler 7 is completed and the start-up control is finished (step S7).

When the start-up control is finished, as described above, the heating amount of the biomass boiler 7, the extraction amount of the high-pressure steam supplied to the first high-pressure feed water heater 54a and the second high-pressure feed water heater 54b, and the main heater 27 are used. By controlling the temperature of the steam, the appropriate boiler 3, biomass boiler 7 and steam turbine 5 are operated according to the load of the power plant 1.
The heating amount of the biomass boiler 7 is controlled by both the control based on the economizer outlet temperature 22T and the control based on the combustion exhaust gas temperature 25T from the heating steam temperature sensor 62T, the heating steam pressure sensor 62P, and the heating steam flow rate sensor 62F. It is controlled by the control unit 30 so as to satisfy.
Further, the extraction of the high-pressure steam supplied to the first high-pressure feed water heater 54a and the second high-pressure feed water heater 54b is performed so that the differential pressure applied before and after the turbine blade does not exceed the allowable value on the strength of the turbine blade. The bleed valves 56a and 56b are controlled by setting the opening in the closing direction.
Further, when the amount of heating of the biomass boiler 7 and the amount of high-pressure steam extracted by the high-pressure extraction valves 56a and 56b change, the temperature of the main steam is quickly controlled by the superheater spray 27 to a range of appropriate values.

<Biomass boiler separation control>
Next, the separation control for separating the feed water heating by the biomass boiler 7 will be described with reference to FIG.
First, as shown in step S11, the load on the boiler 3 is equal to or greater than a certain level, the biomass boiler 7 is activated, the high pressure extraction valves 56a and 56b are closed, and the feed water bypass valve 68 is closed. Sometimes, the biomass boiler 7 is stopped as shown in step S12.

  In step S13, it is determined whether the feed water inlet temperature of the economizer 18 measured by the temperature sensor 58T is lower than a value obtained by adding a predetermined value β to the set temperature. If the condition of step S13 is satisfied, the process proceeds to step S14. If not satisfied, step S13 is repeated. The predetermined value β makes it possible to adjust the opening degree of the high pressure extraction valves 56a and 56b to be open when the feed water inlet temperature of the economizer 18 exceeds the set temperature. When an unstable fluctuation occurs in the heat exchange amount of the exchanger 60, the opening degree control of the high pressure extraction valves 56a and 56b is affected. For example, the predetermined value β is set at 2 ° C to 5 ° C.

  In step S14, the high pressure extraction valves 56a and 56b are opened while the water supply bypass valve 68 is closed.

  In step S15, the feed water temperature at which the biomass boiler outlet steam temperature measured by the heating steam temperature sensor 62T provided in the heating steam supply pipe 62 flows into the feed water heating heat exchanger 60 measured by the temperature sensor 57T. It is determined whether the value is lower than a value obtained by adding a predetermined value α to the value. If the condition of step S15 is satisfied, the process proceeds to step S16. If not satisfied, step S15 is repeated. The predetermined value α is determined from the heat transfer characteristics of the feed water heating heat exchanger 60. To reduce the predetermined value α, it is necessary to increase the heat transfer area of the feed water heating heat exchanger 60, and the heat exchanger is large. Turn into. For example, the predetermined value α is set at 2 ° C. to 10 ° C., more preferably 2 ° C. to 5 ° C.

  If it progresses to step S16, the feed water bypass valve 68 will be opened, with the opening degree of the high pressure extraction valves 56a and 56b opened. Thereby, feed water bypasses the heat exchanger 60 for feed water heating, and feed water heating by the biomass boiler 7 is stopped. Thus, the separation control of the biomass boiler 7 is completed (step S17).

  In FIG.6 and FIG.7, the feed water heating by the biomass boiler 7 is performed like this embodiment (after biomass boiler additional installation), and the feed water heating by the biomass boiler 7 is not performed as a comparative example (biomass boiler additional installation) An example of the effect of (before) will be described.

  In FIG. 6, the horizontal axis represents the load of the boiler 3, and the vertical axis represents the amount of heating given from the biomass boiler 7 to the feed water. As shown in FIG. 6, the amount of heat is given to the feed water by the feed water heating by the biomass boiler 7. In the figure, the amount of heat given to the water supply when the boiler load is 100% is expressed as a relative value with 1.0.

In FIG. 7, the horizontal axis indicates the output of the power plant, and the power generation efficiency on the vertical axis indicates the power generation efficiency with respect to fossil fuel energy input to the power plant. In the figure, the power generation efficiency before the biomass boiler additional installation when the output is 100% is represented as 1.0 as a relative value. As can be seen from the figure, even if the boiler load changes, the power generation efficiency is higher after the biomass boiler is added than before the biomass boiler is added. In particular, when the boiler load is low, the rate of increase in power generation efficiency is large. In this case, the turbine efficiency of the steam turbine decreases when the power generation output is small, but the effect of heating the feedwater and the effect of reducing the extraction amount of high-pressure steam greatly contribute to the improvement of the turbine efficiency as the power generation output becomes lower. Because it does.
The power generation efficiency in FIG. 7 is defined as (electric energy obtained) / (energy of fossil fuel to be input). The turbine efficiency is defined as (electric energy obtained) ÷ [(steam energy flowing into the turbine) − (steam energy flowing out of the turbine)].

  FIG. 8 shows the relationship and breakdown between the electric energy obtained by the power generation of the power plant 1 and the fuel energy input to the power plant 1 (boiler 3 and biomass boiler 7). Here, a case where the load is, for example, 50% in the middle is compared. 8A shows a state before the biomass boiler 7 is additionally installed, and FIG. 8B shows a state after the biomass boiler 7 is additionally installed. When the electric energy obtained by the power generation before and after the additional installation of the biomass boiler 7 is the same amount, as shown in FIG. 8B (after the additional installation of the biomass boiler 7), after the additional installation of the biomass boiler This fuel energy reduces the amount of fossil fuel input and can reduce carbon dioxide emissions that are not carbon neutral.

  Specifically, the steam generated in the biomass boiler 7 is used for heating the feed water supplied to the boiler 3, the amount of steam extracted from the steam turbine 5 is reduced (the amount of extraction reduced), and further supplied to the boiler 3. Carbon dioxide that does not become carbon-neutral can be further reduced in the amount of fossil fuel charged into the boiler 3 in accordance with the energy change caused by heating the feed water to a temperature lower than the saturation temperature within a predetermined range (increasing the feed water enthalpy). Can be further reduced.

According to embodiment mentioned above, there exist the following effects.
By heating the feed water supplied to the boiler 3 with the biomass boiler 7, the enthalpy of the feed water supplied to the boiler 3 can be increased, turbine efficiency can be improved and power generation efficiency can be improved. Furthermore, since the heating amount of the biomass boiler 7 is controlled so that the outlet water temperature of the economizer 18 is less than the saturation temperature and close to the saturation temperature so as not to be vaporized, the water supply is heated as much as possible. Can do. Thereby, the enthalpy of water supply can be increased, turbine efficiency can be improved and power generation efficiency can be improved. As a result, when generating the same amount of power as before the power generation efficiency is improved, the amount of fossil fuel input to the boiler 3 can be reduced, and the amount of carbon dioxide that does not become carbon neutral can be reduced.

  Since the heating amount of the biomass boiler 7 is controlled so that the exhaust gas temperature is lower than the upper limit temperature of the denitration apparatus and close to the upper limit temperature, the feed water can be heated as much as possible. Thereby, the enthalpy of water supply can be increased, turbine efficiency can be improved and power generation efficiency can be improved.

  Since the feed water is heated by the biomass boiler 7, the amount of steam extracted from the steam turbine 5 can be reduced. If the same power generation output is compared before and after reducing the bleed amount, reducing the bleed amount reduces the total steam flow to be circulated to the steam turbine 5 within the specified range of mechanical strength, thereby improving the turbine efficiency. Can be improved. In particular, when the boiler 3 has a partial load, even if the amount of steam flowing to the steam turbine is increased, it is still within the specified range of mechanical strength, so the amount of extraction of high-pressure steam is set to zero. Thus, the turbine efficiency can be further improved.

  The strength of the turbine blades as the specified range of the mechanical strength of the steam turbine 5 is designed to be durable against the differential pressure applied before and after the turbine blades, and this differential pressure is distributed to the steam turbine 5. It depends on the flow rate and pressure balance of the steam, and this differential pressure changes according to the amount of extraction. Therefore, the steam extraction amount is controlled in consideration of the strength of the turbine blade. As a result, the amount of extraction can be reduced as much as possible in consideration of the strength of the turbine blades, the total steam flow flowing into the steam turbine 5 can be reduced, and the turbine efficiency can be further improved.

  The temperature of the superheated steam supplied from the superheater 13 may fluctuate due to changes in the amount of heating of the biomass boiler 7 and the amount of steam extracted from the steam turbine 5. By suppressing the temperature fluctuation of the superheated steam with the superheater spray 27, the temperature of the steam pipes and turbine blades used under high temperature and high pressure can be quickly set within a specified value, and deterioration and damage can be suppressed. Can do. In addition, since the flow rate of the reheater spray that controls the temperature of the reheat steam supplied from the reheater 17 is also controlled, the temperature of the reheat steam can be easily controlled to a desired value. .

  By using the biomass boiler 7 that uses biomass fuel as the main fuel as an additional boiler for heating the feed water, the biomass fuel can be exclusively burned in the biomass boiler 7 without co-firing the biomass fuel in the boiler 3. Therefore, the usage ratio of biomass fuel to fossil fuel burned in the boiler 3 can be increased, and the amount of carbon dioxide that does not become carbon neutral can be reduced. Moreover, inexpensive biomass fuel containing a corrosive component can be used without affecting the combustion of the boiler 3 main body, and highly efficient power generation can be performed at a low fuel cost.

In the embodiment described above, the heating amount of the biomass boiler 7 is controlled based on the outlet temperature 22T of the economizer 18, but the inlet temperature 58T of the economizer 18 is used instead of the outlet temperature 22T of the economizer 18. Also good.
Moreover, although the biomass boiler 7 was used as an additional steam generator for heating feed water, it is not always necessary to use biomass as fuel. For example, garbage or combustible waste can be used as the fuel.
The feed water heating heat exchanger 60 is arranged in series with respect to the feed water heaters 50, 52a, 52b, 54a, 54b, but at least any of the feed water heaters 50, 52a, 52b, 54a, 54b. It is good also as arrange | positioning in parallel with respect to this.

1 Power Plant 3 Boiler 3a Boiler Body 5 Steam Turbine 7 Biomass Boiler (Boiler for Heating Water Supply)
10 furnace 11 burner 12 water cooling wall 13 superheater 15 steam drum 17 reheater 18 economizer 20 circulating pump 22 economizer outlet pipe 22T economizer outlet temperature sensor 22P economizer outlet pressure sensor 24 water supply pipe 25T combustion exhaust gas temperature sensor 27 superheater spray 30 control Portion 32 Main steam pipe 34 High pressure turbine 35 Medium pressure turbine 36 Low pressure turbine 37 Generator 38 High pressure turbine outlet pipe 40 Reheat steam supply pipe 42 Medium pressure turbine outlet pipe 44 Low pressure turbine outlet pipe 46 Condenser 48 Condensate pump 50 Low pressure Feed water heater 51 Feed water pump 52a First medium pressure feed water heater 52b Second medium pressure feed water heater 54a First high pressure feed water heater 54b Second high pressure feed water heater 55a First high pressure bleed pipe 55b Second high pressure bleed pipe 56a First 1 High pressure extraction valve 56b Second high pressure extraction Valve 57T Temperature sensor 58T Temperature sensor 60 Heat exchanger for feed water heating 61 Heat transfer pipe 62 Heating steam supply pipe 62T Heating steam temperature sensor 62P Heating steam pressure sensor 62F Heating steam flow sensor 64 Return pipe 65 Return pump 67 Water supply bypass piping 68 Water supply bypass valve 72 Steam dump valve

Claims (10)

A boiler equipped with an economizer and a superheater;
A steam turbine driven by steam generated in the boiler;
A generator driven by the steam turbine;
A feed water heating boiler for heating feed water supplied to the boiler;
A control unit for controlling the heating amount of the feed water by the feed water heating boiler;
With
The control unit controls the heating amount so that an outlet water temperature of the economizer is less than a saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature ,
A feed water heater for extracting steam from the steam turbine and heating the feed water;
The said control part reduces the extraction amount of the steam supplied to the said feed water heater according to the said heating amount, The power plant characterized by the above-mentioned . An exhaust gas treatment device for treating the exhaust gas discharged from the boiler;
Wherein the control unit, power plant according to claim 1, characterized in that the exhaust gas supplied to the exhaust gas treatment apparatus for controlling the heating amount such that the maximum temperature less than the operating temperature of the exhaust gas treatment device . Wherein, when the feed water inlet temperature of the economizer is higher than a predetermined value than the economizer inlet feed water setting temperature, claim 1 or 2, characterized in that reducing the extraction of steam supplied to the feed water heater The power plant described in 1. The power plant according to any one of claims 1 to 3, wherein the control unit controls the amount of steam extracted based on the strength of turbine blades of the steam turbine. A boiler equipped with an economizer and a superheater;
A steam turbine driven by steam generated in the boiler;
A generator driven by the steam turbine;
A feed water heating boiler for heating feed water supplied to the boiler;
A control unit for controlling the heating amount of the feed water by the feed water heating boiler;
With
The control unit controls the heating amount so that an outlet water temperature of the economizer is less than a saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature ,
Comprising a superheater spray for controlling the temperature of superheated steam supplied from the superheater,
The control unit may power generation plant you and controlling the spray amount of the superheater spray based on a change amount of the heating amount. A boiler equipped with an economizer and a superheater;
A steam turbine driven by steam generated in the boiler;
A generator driven by the steam turbine;
A feed water heating boiler for heating feed water supplied to the boiler;
A control unit for controlling the heating amount of the feed water by the feed water heating boiler;
With
The control unit controls the heating amount so that an outlet water temperature of the economizer is less than a saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature ,
A heat exchanger for heating the feedwater for exchanging heat between the steam generated in the boiler for heating the feedwater and the feedwater supplied to the boiler;
A feed water bypass pipe that bypasses the heat exchanger for heating the feed water;
A water supply bypass valve provided in the water supply bypass pipe;
With
When the temperature of the steam generated in the feed water heating boiler is higher than a temperature of the feed water at the inlet of the feed water heating heat exchanger by a predetermined value or more, the control unit closes the feed water bypass valve, and the feed water heating power generation plant you characterized by heating the feed water in the steam generated in use the boiler. The power plant according to any one of claims 1 to 6 , wherein the feed water heating boiler is a biomass boiler using biomass fuel as a main fuel. A boiler having an economizer and a superheater;
A steam turbine driven by steam generated in the boiler;
A generator driven by the steam turbine;
A feed water heating boiler for heating feed water supplied to the boiler;
A power plant operating method comprising:
The amount of heating is controlled so that the outlet water temperature of the economizer is less than the saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature ,
A method for operating a power plant, characterized in that a steam extraction amount supplied to a feed water heater for extracting steam from the steam turbine and heating the feed water is reduced according to the heating amount .   A boiler having an economizer and a superheater;
  A steam turbine driven by steam generated in the boiler;
  A generator driven by the steam turbine;
  A feed water heating boiler for heating feed water supplied to the boiler;
  A superheater spray that controls the temperature of superheated steam supplied from the superheater;
A power plant operating method comprising:
  The amount of heating is controlled so that the outlet water temperature of the economizer is less than the saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature,
  A method for operating a power plant, comprising controlling a spray amount of the superheater spray based on a change amount of the heating amount.   A boiler having an economizer and a superheater;
  A steam turbine driven by steam generated in the boiler;
  A generator driven by the steam turbine;
  A feed water heating boiler for heating feed water supplied to the boiler;
  A heat exchanger for heating the feedwater for exchanging heat between the steam generated in the boiler for heating the feedwater and the feedwater supplied to the boiler;
  A feed water bypass pipe that bypasses the heat exchanger for heating the feed water;
  A water supply bypass valve provided in the water supply bypass pipe;
A power plant operating method comprising:
  The amount of heating is controlled so that the outlet water temperature of the economizer is less than the saturation temperature and is in a range up to 10 ° C. lower than the saturation temperature,
  When the temperature of the steam generated in the feed water heating boiler is higher than the temperature of the feed water at the inlet of the feed water heating heat exchanger by a predetermined value or more, the feed water bypass valve is closed and the steam generated in the feed water heating boiler A method for operating a power plant, wherein the feed water is heated with steam.

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