Patent Description:
The present disclosure relates to a vertical type once-through heat recovery steam generator (HRSG) capable of improving operational stability during rapid startup and lifetime of a steam turbine while reducing environmental pollution caused by emissions, and a combined power generation system including the same.

A combined power generation system is a power generation system in which primary and secondary power generation facilities are combined. In general, a combined power generation system operates such that fuel is combusted inside a gas turbine to produce high-temperature combustion gases, which rotate the gas turbine to generate primary power, and a steam turbine is driven to generate secondary power with high-temperature and high-pressure steam generated by using heat of exhaust gases discharged from the primary power generation process.

This combined power generation system has advantages including highly efficient energy utilization and a short time of startup for power generation. The highly efficient energy utilization is achieved by usage of combustion heat of fuel, which is primarily utilized in a gas turbine, and is recycled in a heat recovery steam generator (HRSG). The short time for power generation is achieved by the HRSG system's rapid startup process from startup to power generation. In addition, the combined power generation system can use clean fuel, which is eco-friendly than other fuels.

The heat of exhaust gases that produce high-temperature, high-pressure steam may be recovered by the HRSG. HRSG is a type of heat exchanger.

HRSGs may be broadly categorized into horizontal and vertical types depending on a flow direction of exhaust gases. The horizontal type has the configuration in which exhaust gases flow in a horizontal direction, while the vertical type has the configuration in which exhaust gases flow in a vertical direction. The vertical type HRSG has the advantage of a smaller installation area compared to the horizontal type.

Meanwhile, HRSGs may be broadly categorized into a natural circulation type and a forced circulation type depending on the circulation method utilized in HRSGs. The natural circulation type is a type in which circulation is performed by density difference, and the forced circulation type is a type in which circulation of water or steam inside the HRSG is performed by a separately equipped circulation pump.

On the other hand, when the combined power generation system is started rapidly, an amount of steam generated by the HRSG increases rapidly during the start-up. However, rapidly increase in steam can lead to unstable operation and reduced overall power generation efficiency in the combined power generation system. In addition, there is another problem that a large amount of white smoke is generated due to the difference between the temperature of exhaust gases at an outlet of the HRSG and the ambient temperature.

<CIT> presents a system for operating a gas and steam turbine power plant, comprising: a steam turbine having at least one medium-pressure part and one high-pressure part with an exhaust steam line, a waste heat steam generator through which hot gas flows, having a high-pressure heating surface system connected to said high-pressure part of said steam turbine, and having intermediate superheater heating surfaces connected to said exhaust steam line of said high-pressure part of said steam turbine, said intermediate superheater heating surfaces being connected on the outlet side to said medium-pressure part of said steam turbine, medium-pressure evaporator heating surfaces having an outlet, and said intermediate superheater heating surfaces having an inlet and an outlet, said outlet of said medium-pressure evaporator heating surfaces leading into said inlet of said intermediate superheater heating surfaces, a branching point having a first segment leading into said high-pressure heating surface system and a second segment leading to said medium-pressure evaporator heating surfaces, said waste heat steam generator including a feedwater line leading to said branching point, and means associated with at least one of said first and second segments for varying the distribution ratio therebetween for approximating the temperature of the steam at said outlet of said intermediate superheater heating surfaces to the temperature of the steam at the outlet of said high-pressure superheater heating surfaces.

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an objective of the present invention is to provide a vertical type once-through heat recovery steam generator (HRSG) and a combined power generation system including the same, which is capable of improving operational stability during rapid startup and lifetime of a steam turbine while reducing environmental pollution caused by emissions.

In an aspect of the present invention, there is provided a vertical type once-through heat recovery steam generator (HRSG) including: a low-pressure section including a condensate preheater configured to heat feedwater through heat exchange with combustion gases and supply the heated feedwater to a low-pressure drum, a low-pressure evaporator configured to heat feedwater stored in the low-pressure drum and convert the feedwater to steam, and a low-pressure superheater configured to heat the steam separated from the feedwater in the low-pressure drum and supply the heated steam to a low-pressure steam turbine; a medium-pressure section including a medium-pressure economizer configured to receive a portion of the feedwater heated by the condensate preheater, heat the feedwater through heat exchange with combustion gases, and supply the heated feedwater to a medium-pressure drum, a medium-pressure evaporator configured to heat the feedwater stored in the medium-pressure drum and convert the heated feedwater to steam, a medium-pressure superheater configured to heat the steam separated from the feedwater in the medium-pressure drum and supply the heated steam to a medium-pressure steam turbine, and a medium-pressure side desuperheater installed at the rear of the medium-pressure superheater to lower the temperature of the steam supplied to a medium-pressure steam turbine; and a high-pressure section including a high-pressure economizer configured to receive a portion of the feedwater heated by the condensate preheater and heat the feedwater through heat exchange with combustion gases, a multi-stage once-through evaporator configured to heat the feedwater heated by the high-pressure economizer through heat exchange with the combustion gases, a high-pressure superheater configured to heat the steam heated by the multi-stage once-through evaporator and supply the heated steam to a high-pressure steam turbine, and a high-pressure side desuperheater installed at the rear of the high-pressure superheater to lower the temperature of the steam supplied to the high-pressure steam turbine.

In the vertical type once-through HRSG according to an embodiment of the present invention, the low-pressure section includes a condensate preheater bypass configured to allow a portion of the feedwater to bypass the condensate preheater and flow to the low-pressure drum.

In the vertical type once-through HRSG according to an embodiment of the present invention, the medium-pressure section includes a first medium-pressure reheater, a first medium-pressure desuperheater, a second medium-pressure reheater, and a second medium-pressure desuperheater sequentially between the medium-pressure superheater and the medium-pressure steam turbine such that the steam heated in the medium-pressure superheater is supplied to the medium-pressure steam turbine through the first medium-pressure reheater, the first medium-pressure desuperheater, the second medium-pressure reheater, and the second medium-pressure desuperheater, wherein the first and second desuperheaters form the medium-pressure side desuperheater.

In the vertical type once-through HRSG according to an embodiment of the present invention, a feedwater line between the medium-pressure drum and the medium-pressure superheater is supplied with steam heated by an external heater. Here, the external heater is configured to generate steam through heat exchange of a portion of the feedwater with high-temperature compressed air compressed from a compressor of a gas turbine and supply the generated steam to the feedwater line.

In the vertical type once-through HRSG according to an embodiment of the present invention, the high-pressure superheater includes first and second high-pressure superheaters and the high-pressure side desuperheater includes first high-pressure desuperheater and second high-pressure desuperheater, wherein the first high-pressure superheater, the first high-pressure desuperheater, the second high-pressure superheater, and the second high-pressure desuperheater are disposed sequentially between the multi-stage once-through evaporator and the high-pressure steam turbine such that the steam heated in the multi-stage once-through evaporator is supplied to the high-pressure steam turbine through the first high-pressure superheater, the first high-pressure desuperheater, the second high-pressure superheater, and the second high-pressure desuperheater.

In the vertical type once-through HRSG according to an embodiment of the present invention, a steam separator is disposed between the multi-stage once-through evaporator and the first high-pressure superheater to separate steam contained in the feedwater having flowed through the multi-stage once-through evaporator.

In the vertical type once-through HRSG according to an embodiment of the present invention, a portion of the feedwater heated in the high-pressure economizer is supplied to a fuel preheater of a combustor through a fuel-preheating feedwater line.

In the vertical type once-through HRSG according to an embodiment of the present invention, the multi-stage once-through evaporator includes: an inlet part which receives feedwater; and a heat transfer tube having a front end connected to the inlet part and a rear end out of which steam is discharged such that the feedwater from the inlet part is evaporated through heat exchange with the combustion gases while flowing from the front end to the rear end, wherein the heat transfer tube is formed such that an inner diameter of the rear end is larger than that of the front end.

In the vertical type once-through HRSG according to an embodiment of the present invention, the heat transfer tube includes a plurality of heat transfer tube parts having different inner diameters each increasing stepwise toward the rear end of the heat transfer tube.

In another aspect of the present invention, a combined power generation system includes: a gas turbine including a compressor configured to suck and compress external air, a combustor configured to mix fuel with the compressed air and combust the mixture, and a turbine rotated by combustion gases discharged from the combustor to generate a rotational force; a heat recovery steam generator (HRSG) configured to heat feedwater and generate steam by using combustion gases discharged from the gas turbine; and a steam turbine in which blades are rotated by the steam generated from the HRSG to generate a rotational force. Here, the HRSG includes: a low-pressure section including a condensate preheater configured to heat feedwater through heat exchange with combustion gases and supply the heated feedwater to a low-pressure drum, a low-pressure evaporator configured to heat feedwater stored in the low-pressure drum and convert the feedwater to steam, and a low-pressure superheater configured to heat the steam separated from the feedwater in the low-pressure drum and supply the heated steam to a low-pressure steam turbine; a medium-pressure section including a medium-pressure economizer configured to receive a portion of the feedwater heated by the condensate preheater, heat the feedwater through heat exchange with combustion gases, and supply the heated feedwater to a medium-pressure drum, a medium-pressure evaporator configured to heat the feedwater stored in the medium-pressure drum and convert the heated feedwater to steam, a medium-pressure superheater configured to heat the steam separated from the feedwater in the medium-pressure drum and supply the heated steam to a medium-pressure steam turbine, and a medium-pressure side desuperheater installed at the rear of the medium-pressure superheater to lower the temperature of the steam supplied to a medium-pressure steam turbine; and a high-pressure section including a high-pressure economizer configured to receive a portion of the feedwater heated by the condensate preheater and heat the feedwater through heat exchange with combustion gases, a multi-stage once-through evaporator configured to heat the feedwater heated by the high-pressure economizer through heat exchange with the combustion gases, a high-pressure superheater configured to heat the steam heated by the multi-stage once-through evaporator and supply the heated steam to a high-pressure steam turbine, and a high-pressure side desuperheater installed at the rear of the high-pressure superheater to lower the temperature of the steam supplied to the high-pressure steam turbine.

In the combined power generation system according to an embodiment of the present invention, the low-pressure section includes a condensate preheater bypass configured to allow a portion of the feedwater to bypass the condensate preheater and flow directly to the low-pressure drum.

In the combined power generation system according to an embodiment of the present invention, the medium-pressure section includes a first medium-pressure reheater, a first medium-pressure desuperheater, a second medium-pressure reheater, and a second medium-pressure desuperheater sequentially between the medium-pressure superheater and the medium-pressure steam turbine such that the steam heated in the medium-pressure superheater is supplied to the medium-pressure steam turbine through the first medium-pressure reheater, the first medium-pressure desuperheater, the second medium-pressure reheater, and the second medium-pressure desuperheater, consecutively.

In the combined power generation system according to an embodiment of the present invention, a feedwater line between the medium-pressure drum and the medium-pressure superheater is supplied with steam heated by an external heater. Here, the external heater is configured to generate steam through heat exchange of a portion of the feedwater with high-temperature compressed air compressed from a compressor of a gas turbine and supply the generated steam to the feedwater line.

In the combined power generation system according to an embodiment of the present invention, the high-pressure superheater includes first and second high-pressure superheaters and the high-pressure side desuperheater includes first high-pressure desuperheater and second high-pressure desuperheater, wherein the first high-pressure superheater, the first high-pressure desuperheater, the second high-pressure superheater, and the second high-pressure desuperheater are disposed sequentially between the multi-stage once-through evaporator and the high-pressure steam turbine such that the steam heated in the multi-stage once-through evaporator is supplied to the high-pressure steam turbine through the first high-pressure superheater, the first high-pressure desuperheater, the second high-pressure superheater, and the second high-pressure desuperheater.

In the combined power generation system according to an embodiment of the present invention, a steam separator is disposed between the multi-stage once-through evaporator and the first high-pressure superheater to separate steam contained in the feedwater having flowed through the multi-stage once-through evaporator.

In the combined power generation system according to an embodiment of the present invention, a portion of the feedwater heated in the high-pressure economizer is supplied to a fuel preheater of a combustor through a fuel-preheating feedwater line.

In the combined power generation system according to an embodiment of the present invention, the multi-stage once-through evaporator includes: an inlet part which receives feedwater; and a heat transfer tube having a front end connected to the inlet part and a rear end out of which steam is discharged such that the feedwater from the inlet part is evaporated through heat exchange with combustion gases while flowing from the front end to the rear end, wherein the heat transfer tube is formed such that an inner diameter of the rear end is larger than that of the front end.

In the combined power generation system according to an embodiment of the present invention, the heat transfer tube includes a plurality of heat transfer tube parts having different inner diameters each increasing stepwise toward the rear end of the heat transfer tube.

Details of other implementations of various aspects of the invention are included in the following detailed description.

According to embodiments of the present invention, it is possible to improve the operational stability during rapid startup and lifetime of the steam turbine while reducing environmental pollution caused by emissions.

Terms used herein are used to merely describe specific embodiments, and are not intended to limit the present disclosure. As used herein, an element expressed as a singular form includes a plurality of elements, unless the context clearly indicates otherwise. Further, it will be understood that the term "comprising" or "including" specifies the presence of stated features, numbers, steps, operations, elements, parts, or combinations thereof, but does not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, parts, or combinations thereof. Hereinafter, a vertical type once-through heat recovery steam generator (HRSG) and a combined power generation system including the same according to embodiments of the present invention will be described with reference to the accompanying drawings.

<FIG> is a diagram illustrating a combined power generation system according to an embodiment of the present invention.

Referring to <FIG>, the combined power generation system <NUM> may include a gas turbine <NUM>, a steam turbine <NUM>, a generator <NUM>, and a heat recovery steam generator (HRSG) <NUM>. The steam turbine <NUM> may include a low-pressure steam turbine 120a, a medium-pressure steam turbine 120b, and a high-pressure steam turbine 120c.

The gas turbine <NUM> may serve to suck atmospheric air, compress the air to a high-pressure, burn an air-fuel mixture in static pressure conditions to release thermal energy, expand high-temperature combustion gases for conversion into kinetic energy, and then discharge exhaust gases containing the residual energy to the atmosphere.

The gas turbine <NUM> may include a compressor <NUM>, a combustor <NUM>, and a turbine section <NUM>. The compressor <NUM> of the gas turbine <NUM> may suck and compress external air. The compressor <NUM> may serve both to supply the compressed air generated by compressor blades to the combustor <NUM> and to supply the cooling air to a high temperature region of the gas turbine <NUM>. Here, since the sucked air undergoes an adiabatic compression process in the compressor <NUM>, the air passing through the compressor <NUM> increases in pressure and temperature.

The compressor <NUM> is usually designed as a centrifugal compressor or an axial compressor, wherein the centrifugal compressor is applied to a small-scale gas turbine, whereas a multi-stage axial compressor is applied to a large-scale gas turbine.

The combustor <NUM> may mix compressed air supplied from an outlet of the compressor <NUM> with fuel and combust the air-fuel mixture at a constant pressure to produce high-energy combustion gases.

High temperature, high-pressure combustion gases generated by the combustor <NUM> is supplied to the turbine section <NUM>. A fuel preheater 112a receives fuel from a fuel supply via a fuel supply pipe, heats the fuel, and supplies the heated fuel to the combustor <NUM>.

In the turbine section <NUM>, the combustion gases undergo adiabatic expansion and impact and drive a plurality of blades arranged radially around a rotary shaft of the turbine section <NUM> so that heat energy of the combustion gases is converted into mechanical energy with which the rotary shaft is rotated. A portion of the mechanical energy obtained from the turbine section <NUM> is supplied as the energy required to compress the air in the compressor <NUM>, and the rest is utilized as an available energy to drive the generator <NUM> to produce electric power.

The combustion gases discharged from the turbine section <NUM> is cooled through the HRSG <NUM> and then discharged to the outside. The HRSG <NUM> serves to not only cool the combustion gases, but also generate high-temperature and high-pressure steam using the heat of the combustion gases and deliver same to the steam turbine <NUM>.

The steam generated in the HRSG <NUM> is transmitted to the steam turbine <NUM> through a steam feed line L1, and feedwater cooled in the steam turbine <NUM> is transmitted to the HRSG <NUM> through a turbine feedwater recovery line L2.

The steam turbine <NUM> rotates blades using the steam generated by the HRSG <NUM> and transmits the rotational energy to the generator <NUM>. The steam turbine <NUM> supplies the cooled steam back to the HRSG <NUM>.

Although the gas turbine <NUM> and the steam turbine <NUM> are exemplified in <FIG> as being connected to one generator <NUM>, the present disclosure is not limited thereto. Rather, the gas turbine <NUM> and the steam turbine <NUM> may be disposed in parallel and/or connected to different generators.

The turbine feedwater recovery line L2 is connected with a condenser <NUM> for condensing steam, a condensate storage tank <NUM> for storing condensed feedwater, and a condensate pump <NUM> for supplying the condensed feedwater stored in the condensate storage tank <NUM> to the HRSG <NUM>.

The steam flowing in the HRSG <NUM> may have at least two or three levels of pressure, and accordingly, the feedwater is pressurized to at least two or three pressure levels. In this embodiment, the HRSG <NUM> is exemplified as having three levels of pressure.

<FIG> is a diagram illustrating a vertical type once-through heat recovery steam generator (HRSG) according to an embodiment of the present invention.

Referring to <FIG>, the HRSG <NUM> may include a low-pressure section <NUM> having a relatively low-pressure, a medium-pressure section <NUM> having a medium-pressure, and a high-pressure section <NUM> having a relatively high-pressure.

The high-pressure section <NUM> may be disposed adjacent to an inlet side of the HRSG, through which combustion gases are introduced, and may be heated by high-temperature combustion gases, and the low-pressure section <NUM> may be disposed adjacent to an outlet side of the HRSG, through which the combustion gases are discharged, and may be heated by low-temperature combustion gases. The low-pressure section <NUM> may be disposed vertically upward than the high-pressure section <NUM>.

The low-pressure section <NUM> includes a condensate preheater <NUM>, a low-pressure drum <NUM>, a low-pressure evaporator <NUM>, a low-pressure superheater <NUM>, and a condensate preheater bypass <NUM>.

The condensate stored in the condensate storage tank <NUM> is transferred to the condensate preheater <NUM> by a condensate pump <NUM>, and the condensate preheater <NUM> heats the condensate through heat exchange with combustion gases. The feedwater heated in the condensate preheater <NUM> is supplied to the low-pressure drum <NUM>.

The low-pressure evaporator <NUM> is connected to the low-pressure drum <NUM> to heat the feedwater stored in the low-pressure drum <NUM>. The steam separated in the low-pressure drum <NUM> is heated in the low-pressure superheater <NUM>, and then supplied to the low-pressure steam turbine 120a.

The condensate preheater bypass <NUM> allows a portion of the condensate transferred from the condensate storage tank <NUM> bypass the condensate preheater <NUM> and to flow directly to the low-pressure drum <NUM>. The bypassing portion of the condensate may be diverted from a pipe between the condensate pump <NUM> and the condensate preheater <NUM> to a pipe between the condensate preheater <NUM> and the low-pressure drum <NUM>. By letting a portion of the condensate bypass the condensate preheater <NUM>, the temperature of gases discharged through the outlet of the HRSG may be raised and thereby can prevent a large amount of white smoke from being produced.

The medium-pressure section <NUM> includes a medium-pressure economizer <NUM>, a medium-pressure drum <NUM>, a medium-pressure evaporator <NUM>, a medium-pressure superheater <NUM>, a first reheater <NUM>, a second reheater <NUM>, a first desuperheater <NUM>, and a second desuperheater <NUM>. The medium-pressure section may further include a low-temperature reheating steam line L6 that reheats low-temperature steam discharged from the steam turbine <NUM>. The first reheater <NUM>, a second reheater <NUM>, a first desuperheater <NUM>, and a second desuperheater <NUM> of the medium-pressure section <NUM> may be referred to as the first medium-pressure reheater <NUM>, a second medium-pressure reheater <NUM>, a first medium-pressure desuperheater <NUM>, and a second medium-pressure desuperheater <NUM>.

A portion of feedwater heated by the condensate preheater <NUM>, and/or a portion of the condensate that bypassed the condensate preheater <NUM> through the condensate preheater bypass <NUM> is supplied, by and through the medium-pressure pump P1, to the medium-pressure economizer <NUM>. The medium-pressure economizer <NUM> heats the feedwater or condensate supplied from the medium-pressure pump P1 (hereinafter collectively referred to as "feedwater") through heat exchange with combustion gases. The feedwater heated in the medium-pressure economizer <NUM> is supplied to the medium-pressure drum <NUM>. The medium-pressure evaporator <NUM> is connected to the medium-pressure drum <NUM> to heat the feedwater stored in the medium-pressure drum <NUM>. Specifically, the medium-pressure evaporator <NUM> receives the feedwater from the medium-pressure drum <NUM>, heat the feedwater through heat exchange with combustion gases, and returns the heated feedwater to the medium-pressure drum <NUM>. The heated steam is separated from the medium-pressure drum <NUM> and then heated in the medium-pressure superheater <NUM>. Then, the heated steam is supplied to the medium-pressure steam turbine 120b. While the heated steam is supplied from the medium-pressure superheater <NUM> to the medium-pressure steam turbine 120b, the heated steam may pass through the first reheater <NUM>, the first desuperheater <NUM>, and the second reheater <NUM>, consecutively. The first reheater <NUM>, the first desuperheater <NUM>, and the second reheater <NUM> regulate and adjust the temperature of the steam entering the medium-pressure steam turbine 120b. Between the medium-pressure superheater <NUM> and the first reheater <NUM>, reheated steam from the medium-pressure superheater <NUM> may be joined by the low-temperature reheating steam line L6, which reheats the low-temperature steam discharged from the steam turbine <NUM>.

During rapid startup, the temperature of the steam entering the medium-pressure steam turbine 120b from the second reheater <NUM> may rise rapidly, and this rapid rise is one of the factors that reduces the operational stability and the lifetime of the steam turbine during rapid startup. Thus, according to the present invention, a second desuperheater <NUM> is additionally installed to reduce and adjust the temperature of the steam entering the medium-pressure steam turbine 120b from the second reheater <NUM> during rapid startup. The second desuperheater <NUM> is installed between the second reheater <NUM> and the medium-pressure steam turbine 120b. According to an embodiment, the second desuperheater <NUM> may not need to operate continuously throughout the entire operation of the HRSG, instead, it may be activated solely during rapid startup, and subsequently turned off after the rapid startup process is completed.

In addition, the feedwater line L3 between the medium-pressure drum <NUM> and the medium-pressure superheater <NUM> may be joined by steam heated by an external heater <NUM>. The external heater <NUM> allows to generate steam through heat exchange of a portion of the feedwater supplied provided from the medium-pressure pump P1 using high-temperature compressed air compressed by the compressor <NUM>, and the thereby generated steam is supplied to the feedwater line L3. The compressed air heat-exchanged in the external heater <NUM> is cooled to low temperature and returned to the gas turbine <NUM> to cool at least one of the compressor <NUM> and the turbine section <NUM>. In this sense, the external heater <NUM> may also functions as an external cooler for the gas turbine <NUM>.

The high-pressure section <NUM> includes a high-pressure economizer <NUM>, a multi-stage once-through evaporator <NUM>, a high-pressure superheater <NUM> (<NUM>, <NUM>), a first desuperheater <NUM>, and a second desuperheater <NUM>. The first desuperheater <NUM> and the second desuperheater <NUM> of the high-pressure section <NUM> may be referred to as the first high-pressure desuperheater <NUM>, and the second high-pressure desuperheater <NUM>.

A portion of feedwater heated by the condensate preheater <NUM>, and/or a portion of the condensate that bypassed the condensate preheater <NUM> through the condensate preheater bypass <NUM> is supplied, by and through a high-pressure pump P2, to the high-pressure economizer <NUM>, which heats the feedwater through heat exchange with combustion gases. According to an embodiment, the high-pressure economizer <NUM> may be formed in multiple stages to heat the feedwater sequentially.

The feedwater heated by the high-pressure economizer <NUM> is supplied to the multi-stage once-through evaporator <NUM>. The multi-stage once-through evaporator <NUM> includes an inlet part <NUM> into which feedwater flows in, and a heat transfer tube <NUM> having a front end connected to the inlet part and a rear end out of which steam is discharged such that the feedwater from the inlet part <NUM> is evaporated through heat exchange with combustion gases while flowing from the front end to the rear end. The heat transfer tube <NUM> is formed such that an inner diameter of the rear end is larger than that of the front end. According to an embodiment, the heat transfer tube <NUM> may be formed such that the inner diameter gradually increases from the front end toward the rear end. Such a multi-stage once-through evaporator <NUM> may prevent static instability or Ledinegg instability caused by two-phase flow. The multi-stage once-through evaporator <NUM> will described below furthermore in detail with reference to <FIG>.

Steam heated by the multi-stage once-through evaporator <NUM> is supplied to the high-pressure steam turbine 120c through the first high-pressure superheater <NUM>, the first desuperheater <NUM>, and the second high-pressure superheater <NUM>, consecutively. The first high-pressure superheater <NUM>, the first desuperheater <NUM>, and the second high-pressure superheater <NUM> regulate and adjust the temperature of the steam entering the high-pressure steam turbine 120c.

During rapid startup, the temperature of the steam entering the high-pressure steam turbine 120c from the second high-pressure superheater <NUM> may rise rapidly, and this rapid rise is one of the factors that reduces operational stability and power generation efficiency during rapid startup. Therefore, according to the present invention, a second desuperheater <NUM> may be additionally installed to reduce the temperature of the steam entering the high-pressure steam turbine 120c during rapid startup. The second desuperheater <NUM> is installed between the second high-pressure superheater <NUM> and the high-pressure steam turbine 120c. According to an embodiment, the second desuperheater <NUM> may not need to operate continuously throughout the entire operation of the HRSG, instead, it may be activated solely during rapid startup, and subsequently turned off after the rapid startup process is completed.

During the rapid startup process, the multi-stage once-through evaporator <NUM> may not convert all of the incoming feedwater to steam immediately. Instead, at the beginning of the rapid startup, only a portion of the feedwater may be converted to steam, while the remaining portion may still exist in a liquid form. Therefore, a steam separator <NUM> may be installed at the rear end of the multi-stage once-through evaporator <NUM> and before the first high-pressure superheater <NUM> for steam-water separation at the beginning of the rapid startup.

That is, during a predetermined period of time at the beginning of the rapid startup, the feedwater that has flowed through the multi-stage once-through evaporator <NUM> may flow through the steam separator <NUM> and, after that, into the first high-pressure superheater <NUM>. After a predetermined period of time when the startup process is completed, only steam is supplied to the steam separator <NUM>, so the steam separator <NUM> may function only as a connection channel between the multi-stage once-through evaporator <NUM> and the first high-pressure superheater <NUM>.

A portion of the feedwater heated in the high-pressure economizer <NUM> may be supplied to the fuel preheater 112a of the combustor <NUM> via the fuel preheating feedwater line L4. The high-temperature feedwater supplied to the fuel preheater 112a may be converted to cold feedwater after heating fuel through a heat exchange process, and returned to the condensate preheater <NUM> through a return line L5.

The second desuperheater <NUM> of the medium-pressure section <NUM> and the second desuperheater <NUM> of the high-pressure section <NUM> serve to prevent a rapid temperature increase during rapid startup. This not only enhances operational reliability but also help protect the lifetime of the steam turbine from being reduced.

Next, a vertical type once-through heat recovery steam generator (HRSG) according to another embodiment of the present invention will be described with reference to <FIG> is a diagram illustrating the HRSG according to the embodiment of the present invention.

In the embodiment according to <FIG>, the feedwater supplied to the medium-pressure economizer <NUM> and the feedwater supplied to the high-pressure economizer <NUM> are illustrated as being supplied from the front end of the low-pressure drum <NUM> (in other words, from condensate preheater <NUM> without the feedwater being entering the lower-pressure drum <NUM>) by and through the medium-pressure pump P1 or high-pressure pump P2, respectively.

However, alternatively, according to an embodiment as illustrated in <FIG>, the feedwater supplied to the medium-pressure economizer <NUM> and the feedwater supplied to the high-pressure economizer <NUM> may be supplied from the rear end of the low-pressure drum <NUM> (in other words, after the feedwater passing through the low-pressure drum <NUM>) to the medium-pressure economizer <NUM> or the high-pressure economizer <NUM>, respectively and through by the medium-pressure pump P1 or high-pressure pump P2.

Next, a multi-stage perfusion-type evaporator <NUM> will be described more in detail with reference to <FIG>.

<FIG> is a diagram illustrating a multi-stage once-through evaporator according to an embodiment of the present invention, <FIG> is an enlarged cross-sectional view of the multi-stage once-through evaporator of <FIG>, and <FIG> is a diagram illustrating a variant of the multi-stage once-through evaporator.

Referring to <FIG>, the multi-stage once-through evaporator <NUM> includes an inlet part <NUM> and a heat transfer tube <NUM>, wherein the inlet part <NUM> includes a small-diameter tube portion 1331a.

The inlet part <NUM> is in a form of a tube through which feedwater is introduced from the high-pressure economizer <NUM>. An inlet header 1330a may be disposed at the inlet part <NUM>, which has an inner diameter (D-in). The small-diameter tube portion 1331a may be a slender tube, an inner diameter of which may be the same with the inner diameter (D-in) of the inlet part. As the small-diameter tube portion 1331a is disposed, the pressure characteristic curve of the multi-stage once-through evaporator <NUM> may change, which may improve flow instability within the multi-stage once-through evaporator <NUM>.

The heat transfer tube <NUM> is a tube having a front end connected to the rear end of the inlet part <NUM> and a rear end out of which steam is discharged. An outlet header 1330b may be disposed at the rear end of the heat transfer tube <NUM>. The front end of the heating tube <NUM> is supplied with feedwater from the inlet part <NUM>, and the feedwater is evaporated by heat exchange with a flow of combustion gases stream MF while flowing to the rear end of the heat transfer tube <NUM> and then discharged. The heat transfer tube <NUM> may be formed such that the inner diameter of the rear end is larger than that of the front end.

Further, the inlet-side inner diameter D-in, which is the inner diameter of the small-diameter tube portion 1331a, may be configured to be smaller than the inner diameter of the heat transfer tube <NUM>. In other words, in the multi-stage once-through evaporator <NUM>, the inlet-side inner diameter D-in of the small-diameter tube portion 1331a of the inlet part <NUM> may be smaller than the inner diameter of the front end of the heat transfer tube <NUM>, which in turn may be smaller than the inner diameter of the rear end of the heat transfer tube <NUM>.

The heat transfer tube <NUM> may include a plurality of heat transfer parts having different inner diameters, each of which may be formed to increase stepwise from the front end of the heat transfer tube <NUM> to the rear end of the heat transfer tube <NUM>. The heat transfer tube <NUM> may include a first heat transfer tube part 1332a and a second heat transfer tube part 1332b.

The first heat transfer tube part 1332a is a heat transfer tube that has a first inner diameter D1 and whose front end is connected to the rear end of the inlet part <NUM>. The first inner diameter D1 is formed to be larger than the inlet-side inner diameter D-in. The second heat transfer tube part 1332b is a heat transfer tube that has a second inner diameter D2 and whose front end is connected to the rear end of the first heat transfer tuber part 1332a. The second inner diameter D2 is formed to be larger than the first inner diameter D1. That is, the inner diameter of the multi-stage once-through evaporator <NUM> may progressively increase in sequential steps from the inlet part <NUM> to the first heat transfer tube part 1332a, and further to the second heat transfer tube part 1332b.

A tapered part <NUM> may be disposed between the small-diameter tube portion 1331a and the first heat transfer tube part 1332a, and between the first heat transfer tube part 1332a and the second heat transfer tube part 1332b. Specifically, a first tapered part 1333a may be disposed between the small-diameter tube portion 1331a and the first heat transfer tube part 1332a, and a second tapered part 1333b may be disposed between the first heat transfer tube part 1332a and the second heat transfer tube part 1332b. The tapered part <NUM> has an inner diameter that increases gradually and gently toward a rear end thereof. The inclusion of the tapered part <NUM> helps prevent a sudden and discontinuous increase in the inner diameter, which in turn prevents flow delamination and improve flow stability within the multi-stage once-through evaporator <NUM>. Furthermore, the tapered part <NUM> may be formed in the form of a straight line or a rounded curve when viewed in its cross-section.

Further, according to an embodiment as illustrated in <FIG>, the heat transfer tube <NUM> may further include a third heat transfer tube part 1332c. The third heat transfer tube part 1332c is a heat transfer tube having a front end connected to a rear end of the second heat transfer tube part 1332b and whose inner diameter is a third inner diameter D3. The third inner diameter D3 is formed to be larger than the second inner diameter D2. In this case, the inner diameter of the multi-stage once-through evaporator <NUM> may progressively increase in sequential steps from the inlet part <NUM> to the first heat transfer tube part 1332a and the second heat transfer tube part 133b and further to the third heat transfer tube part 1332c. In addition to the two or three heat transfer tube parts described earlier, the heat transfer tube <NUM> may include four or more heat transfer tube parts, each having a sequentially increasing inner diameter toward the rear end thereof. In other words, a heat transfer tube part disposed more rearward has a diameter larger than the one disposed more forward.

The flow stability of the multi-stage once-through evaporator <NUM> will now be described in detail with reference to <FIG>.

There may occur a pressure difference between the flow at the front end and the flow at the rear end of the multi-stage once-through evaporator <NUM>. This is referred to as a 'system pressure drop' or 'internal pressure drop'. The system pressure drop varies with a mass flow rate (i.e., a mass velocity - W). However, since water evaporates inside the multi-stage once-through evaporator <NUM>, a two-phase flow phenomenon may occur where water and steam flow together. The change in pressure drop of steam, which is a gas phase, is greater than the change in pressure drop of water, which is a liquid phase.

As a result, the system pressure drop curve is formed to have a ridge and a valley where the slope varies from positive to negative value and then to a positive value (see the thin solid line in <FIG>) as the mass flow rate increases.

Meanwhile, a flow rate in the multi-stage once-through evaporator <NUM> is determined by a pressure drop curve of the evaporator outer head. Here, the outer head is the head that provides a flow driving force, which may be determined by a height difference between the inlet header 1330a and the outlet header 1330b, or by a circulation pump or the like (see dashed line A in <FIG>). More specifically, a flow rate in the multi-stage once-through evaporator <NUM> is determined at the intersection between the system pressure drop curve and the pressure drop curve of the outer head.

In the two-phase flow condition, there may be three intersections such as <NUM>, <NUM>, and <NUM>, as illustrated in <FIG>. In this case, a flow rate in the multi-stage once-through evaporator <NUM> may be determined at any of the intersections <NUM> to <NUM>. If a plurality of multi-stage once-through evaporators <NUM> are arranged in parallel, a flow rate in the multi-stage once-through evaporators <NUM> may be arbitrarily determined at any one of the intersection points of <NUM> to <NUM>, rather than determined at a specified one intersection.

Accordingly, a mass flow rate different from an initially designed flow rate (intersection <NUM>) may flow in the multi-stage once-through evaporator <NUM>. This flow instability is referred to as static instability (or Ledinegg instability). If a flow rate is less than an initially designed flow rate, a problem may arise in that steam may overheat excessively at the rear end of the multi-stage once-through evaporator <NUM>.

The multi-stage once-through evaporator <NUM> according to the embodiments of the present invention may have a more gentle system pressure drop curve because the heat transfer tube <NUM> is formed such that the inner diameter of the rear end is larger than that of the front end (see the bold solid line in <FIG>). In this case, only one intersection (intersection <NUM>) is formed between the system pressure drop curve and the pressure drop curve of the outer head, which may prevent static instability. Furthermore, as the inlet part <NUM> includes the small-diameter tube portion 1331a, the system pressure drop curve is formed more gently so that the static instability phenomenon can be more reliably prevented.

Claim 1:
A vertical type once-through heat recovery steam generator (HRSG) comprising:
a low-pressure section (<NUM>) comprising a condensate preheater (<NUM>) configured to heat feedwater through heat exchange with combustion gases and supply the heated feedwater to a low-pressure drum (<NUM>), a low-pressure evaporator (<NUM>) configured to heat feedwater stored in the low-pressure drum (<NUM>) and convert the feedwater to steam, and a low-pressure superheater (<NUM>) configured to heat the steam separated from the feedwater in the low-pressure drum (<NUM>) and supply the heated steam to a low-pressure steam turbine (120a);
a medium-pressure section (<NUM>) comprising a medium-pressure economizer (<NUM>) configured to receive at least a portion of the feedwater heated by the condensate preheater (<NUM>), heat the feedwater through heat exchange with the combustion gases, and supply the heated feedwater to a medium-pressure drum (<NUM>), a medium-pressure evaporator (<NUM>) configured to heat the feedwater stored in the medium-pressure drum (<NUM>) and convert the heated feedwater to steam, and a medium-pressure superheater (<NUM>) configured to heat the steam separated from the feedwater in the medium-pressure drum (<NUM>) and supply the heated steam to a medium-pressure steam turbine (120b); and
a high-pressure section (<NUM>) comprising a high-pressure economizer (<NUM>) configured to receive at least a portion of the feedwater heated by the condensate preheater (<NUM>) and heat the feedwater through heat exchange with the combustion gases, a multi-stage once-through evaporator (<NUM>) configured to heat the feedwater heated by the high-pressure economizer (<NUM>) through heat exchange with the combustion gases, and a high-pressure superheater (<NUM>, <NUM>, <NUM>) configured to heat the steam heated by the multi-stage once-through evaporator (<NUM>) and supply the heated steam to a high-pressure steam turbine (120c);
characterized in that:
the medium-pressure section (<NUM>) further comprises a medium-pressure side desuperheater (<NUM>, <NUM>) installed at the rear of the medium-pressure superheater (<NUM>) to lower the temperature of the steam supplied to the medium-pressure steam turbine (120b); and
the high-pressure section (<NUM>) further comprises a high-pressure side desuperheater (<NUM>) installed at the rear of the high-pressure superheater (<NUM>) to lower the temperature of the steam supplied to the high-pressure steam turbine (120c).