System for reheat steam temperature turndown control in heat recovery steam generators

Abstract

A steam supply system for a power generation system is disclosed. The steam supply system includes a heat recovery steam generator and a combustion reheater. The heat recovery steam generator includes a high-temperature section including a high-pressure superheater and a reheater arranged in series with respect to the exhaust gas flowing through the heat recovery steam generator, and an evaporator located downstream of the high-temperature section. The evaporator extracts heat from the exhaust gas leaving the high-temperature section. The combustion reheater contributes at least part of the energy required to raise the temperature of the low-temperature reheat steam to a target temperature.

Description

Technical Field

[0001] The field of the present disclosure generally relates to gas turbine combined cycle power generation systems, and more particularly to a system for achieving steam temperature turndown control in a gas turbine combined cycle power generation system.

Background Art

[0002] Heat exchangers are used in various industries to transfer heat from one medium to another. A heat recovery steam generator (HRSG) is an example of a heat exchanger and can be used in a combined cycle power plant to generate steam and additional power from exhaust gas. The HRSG can use the gas turbine engine exhaust to heat the fluid flowing through the heat exchanger in the HRSG to convert the water supplied to the steam turbine into steam, for example. In some configurations, the fluid may be steam generated at multiple pressure levels and then flowed to any of the high-pressure, intermediate-pressure, and / or low-pressure sections of the steam turbine. The HRSG generally includes a water spray type superheat reduction device (superheat controller) for controlling the temperature of the high-pressure reheat outlet steam by combining superheated steam with water such that the steam is cooled as the water evaporates.

[0003] Some combined-cycle power generation systems consist of a steam turbine extractor that provides process steam to support industrial, carbon recovery, and / or district heating systems. Such steam extractors are generally pressure-controlled to maintain the supply pressure of process steam across the steam flow or plant load operating range. When the plant load or process steam demand moves outside the permissible operating range for steam extraction from the turbine, steam is typically extracted from a high-pressure steam source and then set to reduce the pressure and temperature to match the process requirements. This fallback mode is very inefficient because the process steam does not initially expand and do work in the steam turbine. Operation is most efficient when the process steam is obtained from the steam extractor after it has done work while expanding to the (lower) pressure required for the process. The minimum steam turbine load at which process steam extraction is technically feasible is limited because, as the load decreases, the extracted steam temperature gradually increases until the steam turbine components at the extraction point reach temperatures close to their operating limits. The operating limit arises as the pressure ratio in steam expansion at the turbine decreases between the steam inlet (where pressure decreases as flow decreases) and the steam extraction section (controlled by a valve to a fixed pressure required to supply process steam users). The ideal location for the steam extraction section is often downstream of the intermediate pressure (IP) turbine inlet. Such extraction may necessitate (reheat) steam temperature turndown control at the intermediate pressure (IP) turbine inlet to manage the steam temperature exiting the steam turbine extraction section. However, conventional steam temperature control methods for reheat steam temperature control can have limitations. Since such steam is more efficient than switching the process steam supply to a source with higher pressure and temperature, it is desirable to extend the operation while steam extraction is active, because such steam no longer has the opportunity to do work when expanding at the steam turbine.

[0004] Conventional reheat steam temperature control systems, which have a water-mist superheat reduction section between the reheat sections, are limited by the amount of water that can be safely injected and evaporated within the HRSG. Furthermore, they are inherently inefficient because a high level of energy is used to evaporate the spray water within the HRSG. A more efficient approach is to use cold steam instead of water to lower the temperature of the steam entering the reheater. However, this approach may also be limited in terms of temperature control capability depending on how much the outlet steam temperature can be lowered by the mixing.

[0005] Therefore, in this field, there is a need to extend the authority for reheat steam temperature turndown in gas turbine combined cycle power generation systems. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] U.S. Patent Application Publication No. 2009 / 0090111 [Overview of the Initiative]

[0007] In one embodiment, a steam supply system for a power generation system is disclosed. The steam supply system includes a heat recovery steam generator, which includes a high-temperature section including a high-pressure superheater and a first reheater arranged in series with respect to exhaust gas flowing through the heat recovery steam generator, and an evaporator located downstream of the high-temperature section that extracts heat from the exhaust gas leaving the high-temperature section. The steam supply system further includes a second reheater that contributes at least a portion of the energy required to raise the temperature of low-temperature reheat steam to a target temperature. In some embodiments, the second reheater is arranged in parallel with the first reheater in the high-temperature section. In some embodiments, the second reheater is a combustion reheater.

[0008] In another embodiment, a combined cycle power generation system is disclosed. The combined cycle power generation system includes a gas turbine for generating electricity and a heat recovery steam generator having a gas inlet communicating with the gas turbine exhaust flow. The heat recovery steam generator includes a high-temperature section including a high-pressure superheater and a first reheater arranged in series with respect to the exhaust gas flowing through the heat recovery steam generator, and an evaporator located downstream of the high-temperature section for extracting heat from the exhaust gas leaving the high-temperature section. The combined cycle power generation system further includes a second reheater which contributes to at least a portion of the energy required to raise the temperature of the low-temperature reheat steam to a target temperature. The combined cycle power generation system further includes at least one of an intermediate-pressure steam turbine and a non-condensable steam turbine configured to receive reheat steam from at least one of the second reheater and the first reheater of the heat recovery steam generator. In some embodiments, the second reheater is arranged in parallel with the first reheater of the high-temperature section. In some embodiments, the second reheater is a combustion reheater. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram illustrating an example of a conventional auxiliary combustion power generation system. [Figure 2] This is a schematic diagram of an exemplary auxiliary combustion power generation system having a steam supply system that includes a heat recovery steam generator (HRSG) and a combustion reheater operating in parallel with an unburned HRSG reheater. [Figure 3] This is a schematic diagram of an exemplary auxiliary combustion power generation system having a steam supply system that includes a heat recovery steam generator and a combustion reheater that performs all steam reheat duties. [Figure 4] Figure 1 is an illustrative graph of the system parameters of the power generation system across the plant load of the auxiliary combustion power generation system. [Figure 5] Figure 2 is an illustrative graph of the system parameters of the power generation system across the plant load of the auxiliary combustion power generation system. [Figure 6] Figure 3 is an example graph of the system parameters of the power generation system across the plant load of the auxiliary combustion power generation system. [Modes for carrying out the invention]

[0010] The reference numerals used in the drawings and their meanings are listed in a summary format in the reference numeral list. As a general rule, identical components are given the same reference numeral in the drawings.

[0011] In the following specification and claims, several terms are used with the following meanings:

[0012] As used herein, the singular forms “a,” “an,” and “the” include plural references unless the context specifically indicates otherwise. The terms “comprising,” “including,” and “having” are intended to be comprehensive, meaning that there may be additional elements other than those listed. The terms “optional” or “optionally” mean that the event or situation described thereafter may or may not occur, and that the description includes both cases in which the event occurs and cases in which it does not.

[0013] Unless otherwise specified, the approximation terms used herein, such as “generally,” “substantially,” and “about,” indicate that the terms thus modified may apply only to an approximate degree as recognized by those skilled in the art, and not to an absolute or complete degree. Therefore, values ​​modified by one or more terms such as “about,” “approximately,” and “substantially” are not limited to the exact values ​​specified. In at least some examples, the approximation terms may correspond to the precision of an instrument used to measure a value. Hereinafter, and throughout this specification and the claims, limits to the scope may be specified. Such scopes are combinable and / or replaceable and include all sub-scopes that fall within that scope unless otherwise indicated by context or wording.

[0014] In addition, unless otherwise specified, terms such as “first,” “second,” etc., are used solely as identifiers in this specification and are not intended to impose any order, position, or hierarchical requirements on the items referred to by these terms. Furthermore, a reference to, for example, an item “second,” does not require or exclude the existence of, for example, an item “first” or a lesser numbered item, or an item “third” or a more numbered item.

[0015] Figure 1 is a block diagram of the high-temperature portion of an exemplary prior art power generation system 10, which includes a gas turbine 20, an HRSG 100, a high-pressure steam turbine 165, and an intermediate-pressure steam turbine 175 (referred to as the "IP steam turbine 175"). Note that the elements of the HRSG 100 shown and labeled in Figure 1 are only the foremost part of interest to this disclosure. The HRSG 100 includes an inlet 102 and an outlet interface 104 that directs the high-temperature gas 40 to downstream components of the HRSG 100, and includes economizers, evaporators, and superheaters for HP, IP, and typically LP steam generation modules (not shown). In some embodiments, the HRSG 100 also includes a downstream emission reduction catalyst (not shown).

[0016] HRSG100 is an exemplary steam supply system for a conventional power generation system 10. Therefore, it should be understood that the illustrated HRSG100 is merely an example of a steam supply system, and other steam supply systems may be included as part of the illustrated conventional power generation system 10.

[0017] In an exemplary prior art embodiment, the HRSG100 receives high-temperature exhaust gas 30 from a gas turbine 20, and the high-temperature exhaust gas 30 flows through a high-pressure superheater and reheater section (called the high-temperature section 110), and an evaporator 150. The HRSG100 is an indirect heat exchanger in which heated water in a fluid conduit 152 is supplied to the evaporator 150 of the HRSG100, and heat is extracted from the high-temperature exhaust gas 30 within the HRSG100.

[0018] The high-temperature section 110 typically includes a configuration of a reheater 130 and a high-pressure superheater 120 arranged in series. High-pressure (HP) steam generated in the evaporator 150 is superheated by exhaust gas 30 flowing through the HRSG 100. The exhaust gas is cooled as heat is transferred to the steam. The superheater 120 may include a steam outlet 128, and the HP steam leaving the superheater 120 is supplied to an HP steam turbine 165 coupled to the steam outlet 128 of the superheater 120. The steam flowing through the HP steam turbine 165 expands to intermediate pressure (IP) and is reheated before the steam enters the IP steam turbine 175. The reheater 130 includes a steam outlet 138 that directs the flow towards the IP steam turbine 175. Additional IP steam 131 may be added to the HP turbine exhaust steam leaving the HP steam turbine 165 before the steam flow enters the reheater 130. The temperature of the HP steam exiting from the steam outlet 128 of the superheater 120 and the temperature of the IP steam exiting from the steam outlet 138 of the reheater 130 are controlled by injecting water into the steam in the superheat reduction device 126 or 136, respectively. The steam 184 exiting the steam outlet of the IP turbine 175 is directed towards a low-pressure turbine (not shown) and / or a process user (not shown).

[0019] The evaporator 150 is located downstream of the high-temperature section 110 and extracts heat from the exhaust gas 30 exiting the high-temperature section 110. In an exemplary embodiment, the evaporator 150 includes a series of fluid tubes (not shown) that extract heat from the exhaust gas 30 exiting the high-temperature section 110. The fluid tubes of the evaporator 150 are connected by fluid conduits 152 to an economizing section (not shown) which is connected to an HP feedwater system (not shown) and to a feedwater pump system (not shown) that circulates fluid within the system. As the exhaust gas 30 flows through the evaporator 150 and downstream HP economizers, IP steam generation circuits, and optionally LP steam generation circuits, the exhaust gas is further cooled before being exhausted into the stack or to downstream processes such as a carbon capture system (CCS).

[0020] Combined cycle power generation systems, such as power generation system 10, have a plant operating range that is operationally dependent on the power demand of the power grid coupled to power generation system 10. Many such systems also serve to supply the steam demand of a process steam system. Operation by process steam supply is most efficient when the steam expands through a steam turbine to perform work before extraction to the process steam system. Process steam is typically required at a substantially constant pressure and temperature over the entire operating range of the combined cycle power plant. When the pressure of the steam for delivery to the process is controlled by a steam turbine, it is said to have automatic pressure control steam turbine extraction.

[0021] The automatic extraction facility meets the steam pressure requirements of the downstream process steam system, but the extraction steam temperature increases with the reduction of the plant load because the expansion ratio from the turbine section inlet to the extraction point decreases. At some point, the steam or turbine hardware becomes too hot for continuous operation of automatic extraction, and an alternative steam supply source must be activated. For example, if the plant load or process steam demand shifts to a demand level that exceeds the operating limit of any component within power generation system 10 for steam extraction from the turbine, instead, the steam may be drawn from a high-pressure steam source such as at least one high-pressure superheater 120. Such a process also requires appropriate pressure and temperature reduction to meet the process requirements and maintain power generation system 10 within an acceptable plant operating range. As used herein, the terms "low load" or "minimum load" refer to the operating state of the plant where power generation system 10 is operating at or near its minimum allowable operating state, and the terms "high load" or "base load" refer to the operating state of the plant where power generation system 10 is operating at or near its maximum allowable operating state.

[0022] FIG. 2 is a schematic diagram of an exemplary power generation system 10 including an HRSG 100, a HP steam turbine 165, an IP steam turbine 175, a combustion reheater 200, a regenerative air heater 210, and a dedicated process steam supply steam turbine 185 (hereinafter referred to as "non - condensing steam turbine 185" and / or "NCST 185"). In some embodiments, the non - condensing steam turbine 185 may alternatively be configured as a pressure - controlled automatic extraction section or, more generally, a condensing section having a dedicated process steam turbine. In an exemplary embodiment, the HRSG 100 includes a high - temperature section 110 typically including a reheater 130 and a high - pressure superheater 120 arranged in series, and a HP evaporator 150 downstream of the high - temperature section 110. The HRSG 100 includes an inlet section 102 and an outlet section interface 104 that directs the high - temperature gas 40 to the HRSG, and includes applicable economizers, evaporators, and superheaters for high - pressure (HP), intermediate - pressure (IP), and / or low - pressure (LP) steam generation modules. In some embodiments, the HRSG 100 also includes an emissions reduction catalyst (not shown).

[0023] The HRSG 100 is an exemplary steam supply system for the power generation system 10. Thus, it is understood that the illustrated HRSG 100 is merely an example of a steam supply system, and other steam supply systems may be included as part of the illustrated power generation system 10. In the illustrated embodiment, the steam supply system includes the HRSG 100 and the combustion reheater 200. In some embodiments, the combustion reheater 200 is a separate component from the HRSG 100. In some embodiments, the combustion reheater 200 is an integral component from the HRSG 100.

[0024] The evaporator 150 downstream of the high - temperature section 110 extracts additional heat from the exhaust gas exiting the high - temperature section 110. In some embodiments, the evaporator 150 is a once - through high - pressure evaporator through which feed water from a fluid conduit 152 (shown in FIG. 1) within a HP feed water system (not shown) flows. The HRSG 100 includes an enclosure wall that defines a heating gas duct through which the exhaust gas 30 from the gas turbine 20 flows (as shown in FIG. 1, the flow is indicated by reference arrow 30).

[0025] In an exemplary embodiment, the high-temperature section 110 includes at least one high-pressure superheater 120 coupled in series orientation with a reheater 130. The superheater 120 includes a gas inlet 122 and a gas outlet 124, and the reheater 130 includes a gas inlet and a gas outlet. Low-temperature reheated steam is distributed between the reheater 130 of the HRSG 100 and a dedicated combustion reheater 200. The steam reheated in the combustion reheater 200 is supplied to the NCST 185 (or a condensed steam turbine with automatic extraction) which provides steam at a controlled pressure to process users. Fuel is supplied to the combustion reheater 200 via line 221. Air is supplied via air line 223 and heated in a flow opposite to the exhaust flow 222 in a regenerative air heater 210. The combustion reheater exhaust flow 222 is exhausted into the atmosphere or sent to a downstream system for carbon recovery.

[0026] In an exemplary embodiment, the combustion reheater 200 heats the intermediate-pressure steam to a predetermined target steam temperature. The combustion reheater fuel supply source 221 and air supply source 223 are adjusted to maintain low excess air for combustion and thus high efficiency of this subsystem over its load range. As the target reheat steam temperature decreases, the fuel 221 and air 223 also decrease. The diversion of low-temperature reheat steam to the combustion reheater 200 also affects the high-pressure steam generation from the HRSG 100, particularly from the steam outlets 128 and 138, and thus the high-pressure and high-temperature reheat steam temperature. As the intermediate-pressure steam flow from low-temperature reheat to the combustion reheater 200 increases, the reheat duty cycle in the reheater 130 of the HRSG 100 decreases, and thus the gas temperature and energy that can be utilized by the evaporator 150 for HP steam generation increases. The control of the fuel combustion rate in the combustion reheater 200 provides complete autonomy over the steam supply temperature from the minimum low-temperature reheat steam temperature in line 141 and the rated steam temperature, typically higher than approximately 300°F (165°C), to the NCST185 via line 142, whereas conventional HRSG configurations do not have a control range beyond approximately 100°F (55°C) for the reheat steam temperature. This allows the exhaust steam (or extracted steam in the case of an automated extraction design) of the NCST185 supplied to the process to be maintained at a minimum load turndown below its limiting temperature, enabling pressure-controlled steam turbine extraction, more so than is possible with conventional designs where the reheat steam temperature control range is far more limited. The control of the fuel combustion rate in the combustion reheater eliminates the need for any other steam temperature control systems (superheat reduction devices) on this steam circuit.

[0027] The combustion reheater 200 can burn fuel at a variable combustion rate to provide temperature control of the steam supplied to the process 186, and the reheater 130 can be sized and positioned within the high-temperature section 110 of the HRSG 100 to provide the rated steam temperature to the IP steam turbine 175 at the base load of the combined cycle power generation system (such as power generation system 10), and the rated low-temperature reheat (IP) steam flow from the combustion reheater 200 is diverted to the process via the steam line 141. The fuel 221 supplied to the combustion reheater 200 can be selectively varied to facilitate control of the reheat steam temperature to the process supply NCST 185 when gas turbine and process steam demands change, in order to facilitate maintaining the steam discharge temperature within the material limits in 186. The steam temperature to the IP steam turbine 175 via flow 138 is controlled conventionally to maximize the thermal performance of the power plant 10, completely independently of the steam leaving the combustion reheater and going to the NCST 185 via flow 142. The minimum reheat steam temperature achieved by zero fuel 221 to the combustion reheater 200 facilitates the activation of pressure-controlled steam turbine exhaust (or extraction) to achieve the target IP turbine exhaust temperature with the minimum possible load turndown.

[0028] The combustion reheater 200 can operate without fuel or selectively with fuel and air supplies in a wide intermediate range between the minimum operating state and the maximum combustion rate. In the non-operating state, the fuel 221 and air 223 are disabled or inoperable so that the combustion reheater 200 is not burning fuel. In contrast, at the maximum combustion rate of fuel 221 and air 223, the combustion reheater 200 operates at a level where it is burning nearly the maximum amount of fuel for which it is rated. During intermediate combustion rates, the combustion reheater 200 operates at a combustion rate between the maximum burner combustion rate and the non-operating state. As will be described in more detail below, the combustion reheater 200 allows for direct control of the reheat vapor temperature by operating at a selectively variable combustion rate.

[0029] During low-load operation of the power generation system 10, it is desirable to lower and maintain the reheat temperature of the steam entering the NCST185 from the combustion reheater 200, limit the extracted steam temperature, and prevent overheating of the components. To achieve a low reheat steam temperature to the NCST185 under minimum load conditions through pressure-controlled extraction, the system can be configured such that the supply of fuel 221 and air 223 to the combustion reheater 200 is not required, and thus the combustion reheater 200 is left in an unburned state. The combustion reheater 200 can also operate selectively at an intermediate burner combustion rate to enable control of the reheat steam temperature from line 142 to the NCST185, and thus facilitate the best possible system efficiency with respect to the reheat steam temperature at plant loads above the minimum.

[0030] As the system load increases, the supply of fuel 221 and air 223 to the combustion reheater 200 can be selectively adjusted within the intermediate combustion range to adapt to the changing load conditions. Under high-load operating conditions of the power generation system 10, the steam flow throughout the system, more specifically through the NCST 185, increases. During such operating conditions, a higher temperature of the reheat steam entering the NCST 185 can be achieved by operating the supply of fuel 221 and air 223 to the combustion reheater 200 to its maximum combustion rate.

[0031] By variably operating the combustion reheater 200 in response to system load demands, it becomes possible to control the temperature of the supply steam 142 entering the process steam supply source NCST185 from the combustion reheater 200. Therefore, in some embodiments, the fuel and air lines 221 and 223 to the combustion reheater 200 can be selectively adjusted in response to process steam demands to enable a stable supply of temperature-controlled process steam from automatic extraction (when NCST185 is a condensing turbine) or exhaust (when NCST185 is a non-condensing turbine). For example, if the steam flow demand for the flow 186 received by the process user decreases, the combustion reheater 200 has direct control over the reheat steam temperature from line 142, so the combustion rate of the combustion reheater 200 can be appropriately reduced without providing additional superheating reductions, thereby keeping the flow 186 below its critical temperature.

[0032] In all embodiments, the combustion reheater 200 may use a dedicated air supply to enable low excess air operation independent of the bulk gas turbine exhaust gas O2 concentration. Such a configuration facilitates maximizing thermal efficiency and exhaust CO2 concentration to the carbon capture and storage (CCS) system, and thus facilitates a reduction in CCS recovery costs. In a preferred embodiment, air is supplied via an air line 223 and heated in a flow opposite to the exhaust flow 222 in a regenerative air heater 210. The exhaust flow 222 of the combustion reheater exhaust 200 is either exhausted into the atmosphere or sent to a downstream system for carbon capture. In some embodiments, the combustion reheater 200 may include a catalytic system for reducing emissions. In some alternative embodiments, the exhaust 222 from the combustion reheater 200 can be mixed with the HRSG 100 before the emission reduction catalytic system of the HRSG itself. In this example, since the opportunity to regenerately heat the combustion air 223 with respect to the exhaust flow 222 is sacrificed, the air supply to the combustion reheater may be regenerated by other means (such as water from the HRSG or steam from a steam turbine) rather than being heated.

[0033] Figure 3 is a schematic diagram of an exemplary power generation system 10, which includes an HRSG 100, an HP steam turbine 165, an IP steam turbine 175, a combustion reheater 200, a regenerative air heater 210, and an NCST 185. Steam supply to the process at a controlled pressure can be provided from either the automatic extraction (or IP exhaust) of the IP steam turbine 175 or the exhaust of the NCST 185. In an exemplary embodiment, the HRSG 100 includes a high-temperature section 110, which includes a high-pressure superheater 120. Downstream of the high-temperature section 110 is an HP evaporator 150. The HRSG 100 includes an inlet section 102 and an outlet interface 104 that directs high-temperature gas 40 into the HRSG, and includes applicable economizers, evaporators, and superheaters for high-pressure (HP), medium-pressure (IP), and / or low-pressure (LP) steam generation modules. In some embodiments, the HRSG 100 also includes an emissions reduction catalyst (not shown).

[0034] A downstream evaporator 150 of the high-temperature section 110 extracts additional heat from the exhaust gases leaving the high-temperature section 110. In some embodiments, the evaporator 150 is a once-through high-pressure evaporator through which feedwater from a fluid conduit 152 (shown in Figure 1) in an HP feedwater system (not shown) flows. The HRSG 100 includes an enclosure wall that defines a heating gas duct through which exhaust gases 30 from the gas turbine 20 flow (the flow is indicated by reference arrow 30, as shown in Figure 1).

[0035] In an exemplary embodiment, the high-temperature section 110 includes at least one high-pressure superheater 120. The superheater 120 includes a gas inlet 122 and a gas outlet 124. Low-temperature reheated steam is directed to a dedicated combustion reheater 200. The steam reheated in the combustion reheater 200 is supplied to either an IP steam turbine 175 equipped with process steam extraction equipment and / or an NCST 185 that exhausts steam at a pressure controlled by the process user. Fuel is supplied to the combustion reheater 200 via line 221. Air is supplied via air line 223 and heated in a flow opposite to the exhaust flow 222 in a regenerative air heater 210. The combustion reheater exhaust flow 222 is exhausted into the atmosphere or sent to a downstream system for carbon recovery.

[0036] In an exemplary embodiment, the combustion reheater 200 heats intermediate-pressure steam to a predetermined target steam temperature. The combustion reheater fuel supply line 221 and air supply line 223 are adjusted to maintain low excess air for combustion, and therefore high efficiency of this subsystem across its load range. As the target reheat steam temperature decreases, the fuel 221 and air 223 also decrease. By diverting all low-temperature reheat steam to the combustion reheater, HP steam generation and plant output are maximized beyond what is achievable with the system shown in Figure 2. Adjusting the fuel combustion rate in the combustion reheater provides complete autonomy over the minimum low-temperature reheat steam temperature of line 141 and the rated steam temperature, which is typically higher than approximately 300°F (165°C), to the steam supply temperature to the IP steam turbine 175 via line 138 and to the steam supply temperature to the NCST 185 via line 142. This enables the extraction (or exhaust) of steam 184 from the IP steam turbine 175 and / or exhaust 186 from the NCST 185, allowing them to be supplied to the process while maintaining their limiting temperatures below their respective threshold values. This capability, in turn, enables pressure-controlled steam turbine extraction to allow for plant operation down to lower minimum load turndowns than is possible in any prior art design where the reheat steam temperature control range is more limited. The supply of reheat steam to the IP steam turbine 175 and NCST 185 and the adjustment of process steam from there add further flexibility to achieve low-load, efficient operation in relation to Figure 2. Adjusting the fuel combustion rate in the combustion reheater eliminates the need for any other steam temperature control systems (superheat reduction devices) on this steam circuit.

[0037] The combustion reheater 200 can burn fuel at a variable combustion rate to provide temperature control for the steam provided for processing via 184 and / or 186. The fuel 221 supplied to the combustion reheater 200 can be selectively varied to facilitate control of the reheat steam temperature to the IP steam turbines 175 and NCST 185, as this facilitates changes in gas turbine and process steam demand and maintains the steam discharge temperature within the material limits of 184 and / or 186. A minimum reheat steam temperature with zero fuel 221 supplied to the combustion reheater 200 facilitates the achievement of a target IP turbine exhaust temperature with pressure-controlled steam turbine exhaust (or extraction) at the minimum possible load turndown. The ability to selectively shut off the steam flow to the IP steam turbines 175 or NCST 185 further extends the range of operation for process steam supply from steam turbine extraction.

[0038] The combustion reheater 200 can operate without fuel or selectively with fuel and air supplies in a wide intermediate range between the minimum operating state and the maximum combustion rate. In the non-operating state, the fuel 221 and air 223 are disabled or inoperable so that the combustion reheater 200 is not burning fuel. In contrast, at the maximum combustion rate of fuel 221 and air 223, the combustion reheater 200 operates at a level where it is burning nearly the maximum amount of fuel for which it is rated. During intermediate combustion rates, the combustion reheater 200 operates at a combustion rate between the maximum burner combustion rate and the non-operating state. As will be described in more detail below, the combustion reheater 200 allows for direct control of the reheat vapor temperature by operating at a selectively variable combustion rate.

[0039] During low-load operation of the power generation system 10, it is desirable to lower and maintain the reheat temperature of the steam entering the IP steam turbine 175 and / or NCST 185 from the combustion reheater 200, limit the extracted steam temperature, and prevent overheating of the components. To achieve a low reheat steam temperature to the IP steam turbine 175 and / or NCST 185 under minimum load conditions through automatic extraction, the system can be configured so that the supply of fuel 221 and air 223 to the combustion reheater 200 is not required, and thus the combustion reheater 200 is left in an unburned state. The combustion reheater 200 can also operate selectively at an intermediate burner combustion rate to enable control of the reheat steam temperature 138 to the IP steam turbine 175 and / or line 142 leading to the NCST 185, thus facilitating the best possible system efficiency with respect to the reheat steam temperature at plant loads above the minimum. The ability to selectively shut off the steam flow to the IP steam turbine 175 or NCST185 further expands the operational range for process steam supply from steam turbine extraction.

[0040] As the system load increases, the supply of fuel 221 and air 223 to the combustion reheater 200 can be selectively adjusted within the intermediate combustion range to adapt to the changing load conditions. Similarly, if either the IP steam turbine 175 or the NCST 185 is offline, this steam turbine can be activated to enable higher load operation. Under high-load operating conditions of the power generation system 10, the steam flow through the entire system, more specifically through the intermediate-pressure steam turbines 175 and NCST 185, increases. During such operating conditions, higher temperatures of reheat steam entering the IP steam turbines 175 and / or NCST 185 can be achieved by operating the supply of fuel 221 and air 223 to the combustion reheater 200 to its maximum combustion rate.

[0041] By variably operating the combustion reheater 200 in response to system load demands, it becomes possible to control the temperature of the supply steam from the combustion reheater 200 to the steam outlet 138 entering the IP steam turbine 175 and / or to the NCST 185 entering 142. Thus, in response to process steam demands, the fuel and air lines 221 and 223 supplied to the combustion reheater 200 can be selectively adjusted to enable a stable supply of temperature-controlled process steam from the automatic extraction of the IP steam turbine 175 or from the exhaust from the NCST 185. For example, if the steam flow demand of the process flow received by the process user decreases, the combustion reheater 200 directly controls the reheat steam temperature from line 142, thereby appropriately reducing the combustion rate of the combustion reheater 200 and maintaining the flows 184 and / or 186 below their limit temperatures without additional overheat protection equipment.

[0042] In all embodiments, the combustion reheater 200 may use a dedicated air supply to enable low excess air operation independent of the bulk gas turbine exhaust gas O2 concentration. Such a configuration facilitates maximizing thermal efficiency and exhaust CO2 concentration to the carbon capture and storage (CCS) system, and thus facilitates a reduction in CCS recovery costs. In a preferred (most efficient) embodiment, air is supplied via an air line 223 and heated in a flow opposite to the exhaust flow 222 in a regenerative air heater 210. The combustion reheater exhaust flow 222 is exhausted into the atmosphere or sent to a downstream system for carbon capture. In some embodiments, the combustion reheater may include a catalytic system for reducing emissions. In some alternative embodiments, the exhaust from the combustion reheater can be mixed with the HRSG before the emission reduction catalytic system of the HRSG itself. In this example, since the opportunity to regenerately heat the combustion air 223 relative to the exhaust flow 222 is sacrificed, the air supply to the combustion reheater may be regenerately heated by other means (water from the HRSG, steam from a steam turbine, etc.) rather than being heated.

[0043] FIG. 4 shows an exemplary graph of system parameters of a power generation system over the plant load of the exemplary prior art auxiliary combustion power generation system of FIG. 1. As shown, the power generation system operates with a range RA corresponding to the load range (>0 and <R0) where the power generation system is operating with backup steam for processing, and a range RB corresponding to the load range (>R0 and <R2) where the power generation system is operating with automatic steam extraction for processing. The load R1 at 100% plant load represents an operation where the gas turbine is at base load and the duct burner is off, and complete combustion in the duct burner increases the plant load at R2 to approximately 116%.

[0044] Throughout range RA, the duct burner 170 is not operating, and burner fuel 172 is not supplied to the duct burner 170. At the lower end of range RB between R0 and R1, the extracted steam temperature is at its highest temperature despite the duct burner 170 not operating; therefore, further load reduction requires a mode change to RA, which involves stopping the automatic extraction of process steam from the steam turbine and starting the backup steam supply for the process steam. This causes step-change losses in plant power and efficiency because, without work extraction by the steam turbine, the process steam here is throttled from high pressure. When the power generation system starts operating above load R0 with automatic steam extraction enabled, the overall pressure ratio of the IP steam turbine 175 increases, causing the extracted steam temperature to decrease. Load R1 and peak plant efficiency are achieved at the base load of the gas turbine with the automatic extraction valve fully open and the duct burner not operating. In this configuration, higher plant loads are available with the start of fuel 172 to the duct burner 170, while steam turbine extraction is still active. As shown in the diagram, the extraction pressure rises with increasing steam flow and is therefore no longer controlled by automatic extraction. This necessitates separately throttling the process steam to process pressure in the extraction steam line supply for the process. Depending on the burner arrangement and HRSG design, the fuel flow 172 to the duct burner 170 can provide an unburned steam flow and up to a 150% power increase for the associated plant. The process steam expands through the steam turbine before being discharged into the process, but the incremental efficiency is very low at this upper end of the operating range RB. The system can be designed so that at the complete combustion point R2, throttling of the steam discharge with complete combustion in the process steam supply line is not required to slightly improve efficiency, but the range RB is reduced and RA is increased.

[0045] FIG. 5 shows an exemplary graph of system parameters of an exemplary power generation system over plant load, which is constituted by a heat recovery steam generator and a combustion reheater operating in parallel with the unburned HRSG reheater of FIG. 2. As shown, the power generation system operates in a range RA corresponding to the load range (>0 and <R0) where the power generation system is operating with backup steam for processing, and in a range RB corresponding to the load range (>R0 and <R2) where the power generation system is operating with fixed pressure steam extraction for processing from NCST185. The plant load R2 represents an operation where the gas turbine is at base load and the combustion reheater is fully combusted in order to bring the reheater steam from line 142 to the rated temperature. This is shown as approximately 102%, which reflects the thermal energy added to the reheater steam from line 142 by the combustion reheater, which increases with respect to the energy available only from the gas turbine exhaust in the HRSG. The output boost from the combustion reheater may be high or low depending on the cycle steam conditions and the split between the steam supply to the IP turbine 175 and NCST185. FIGS. 2 and 5 include both the IP steam turbine 175 and NCST185, but note that this system can also be configured with or without the process steam extraction 184 from the IP steam turbine 175 without NCST185.

[0046] Throughout range RA, the combustion reheater 200 is not operating, burner fuel 221 is not supplied to the combustion reheater 200, and NCST185 is not operating. At the lower end of range RB between R0 and R1, the extracted steam temperature is at the highest temperature controlled by the fuel flow 221 and air flow 223 to the combustion reheater 200. Further load reduction requires a mode change to RA, which involves stopping the pressure-controlled supply of process steam from NCST185 and starting a backup steam supply of process steam. This mode boundary can be managed by fuel and air turndown of the combustion reheater, the throttle control capability of the process steam pressure control valve, or other hardware, sizing, or control constraints. When the power generation system begins operating above load R0 and the pressure-controlled supply of process steam from NCST185 is enabled, the output steam temperature is controlled to its limit by controlling the steam temperature from the combustion reheater 200 and rises as the overall pressure ratio of NCST185 increases with the load. Above load R1, the steam supply temperature in line 142 is controlled to its rated value, and as the overall expansion pressure ratio of NCST185 continues to increase, the steam discharge temperature from NCST185 to process 186 decreases. Peak plant efficiency reaches just before the base load of the gas turbine and decreases slightly as the plant load increases to R2. The incremental efficiency due to combustion reheat operation is about 50% higher than that of the conventional duct burner operation in Figure 1 at the base gas turbine load, and therefore the efficiency decrease for the additional 2% plant power condition at R2 is very gradual. At partial load, the incremental efficiency of the combustion reheater actually improves plant efficiency compared to the unburned operation of the conventional system in Figure 1.

[0047] FIG. 6 shows an exemplary graph of system parameters of an exemplary power generation system over the entire plant load, which is composed of a heat recovery steam generator and a combustion reheater that supply the exhaust of all HP steam turbines 165 and IP steam generated in the HRSG100 of FIG. 3. As shown in the figure, the power generation system operates in range RA corresponding to the load range (>0 and <R0) where the power generation system is operating with backup steam for processing, and in range RB corresponding to the load range (>R0 and <R2) where the power generation system is operating with fixed pressure steam extraction for processing from NCST185. The plant load R2 represents the operation where the gas turbine is at base load and the combustion reheater is fully combusting in order to bring the reheated steam from steam outlets 138 and 142 to the rated temperature. This is shown as approximately 105%, which reflects the thermal energy added to the low temperature reheater steam line 141 by the combustion reheater, which increases with respect to the energy available only from the gas turbine exhaust in the HRSG. The output boost from the combustion reheater may be higher or lower depending on the cycle steam conditions. FIGS. 3 and 6 include both the IP steam turbine 175 and the NCST185, but it should be noted that this system can also be configured without the NCST185, with or without the process steam extraction 184 from the IP steam turbine 175.

[0048] Throughout range RA, the combustion reheater 200 operates to control the inlet temperature of the IP steam turbine 175, while the NCST 185 is not operating. At the lower end of range RB between R0 and R1, the extracted steam temperature is at its highest temperature, controlled by the fuel flow 221 and air flow 223 to the combustion reheater 200. Further load reduction requires a mode change to RA, involving the cessation of the pressure-controlled supply of process steam from the NCST 185 and the commencement of a backup steam supply of process steam. This mode boundary can be controlled by the throttle control capability of the process steam pressure control valve, or by other hardware, sizing, or control constraints. Once the power generation system begins operating above load R0 and the pressure-controlled supply of process steam from the NCST 185 is enabled, the discharge steam temperature is controlled to its limit by controlling the steam temperature from the combustion reheater 200, and increases as the overall pressure ratio of the NCST 185 increases with the load. Since the steam from the steam outlet 138 to the IP steam turbine 175 also comes from the combustion reheater 200, the IP steam turbine exhaust temperature is varied by the fuel 221 and airflow 223 supplied to the combustion reheater. Above load R1, the steam supply temperatures in steam lines 138 and 142 are controlled to their rated values, and as the overall expansion pressure ratio of NCST185 continues to increase, the steam discharge temperature from NCST185 to process 186 decreases. Peak plant efficiency is reached just before the base load of the gas turbine and decreases slightly as the plant load increases to R2. The incremental efficiency due to combustion reheat operation is about 50% higher than that of the conventional duct burner operation in Figure 1 at the base gas turbine load, and therefore the efficiency decrease for the additional 5% plant power condition at R2 is very gradual. At partial load, the incremental efficiency of the combustion reheater actually improves plant efficiency compared to the unburned operation of the conventional system in Figure 1. This improvement is gradual and therefore not visually discernible in these figures.

[0049] The system described herein facilitates enhanced, wide-range reheat steam temperature control during combined cycle turndown conditions. In particular, a combustion reheater operating in parallel with an HRSG that supplies reheat steam to an intermediate-pressure steam turbine, or independently for heating all low-temperature reheat steam. By operating the combustion reheater variably to control the reheat steam temperature to the intermediate-pressure steam turbine, and / or the NCST also supplied by the reheat steam, through automatic extraction, the range of loads to which steam can be supplied for processing after expansion through the steam turbine is greatly expanded. Thus, in response to process steam and power demands, the combustion reheater can be variably adjusted in coordination with the gas turbine load and exhaust flow to provide the necessary supply of process steam from the steam turbine while maintaining operation within hardware operating limits. For example, if equipment in the power generation system requires a lower reheat steam temperature, the combustion rate of the combustion reheater can be appropriately reduced. If all steam reheating is by the combustion reheater, no reheat reduction equipment for the reheat steam is required, as the fuel and air supply to the combustion reheater is directly regulated to control the outlet steam temperature. Even when the combustion reheater operates in parallel with the reheater in the unburned HRSG, reheat steam temperature control within the HRSG can be achieved substantially or completely by managing the steam flow through the combustion reheater. In known systems, the mixing of spray water and steam for steam temperature control lacks the necessary control authority within the HRSG, and substantially extends steam extraction to handle operation down to low loads where pressure-controlled (automatic) steam turbine extraction is effective.

[0050] The methods, systems, and compositions disclosed herein are not limited to the specific embodiments described herein. Rather, the steps of the methods, the elements of the systems, and / or the elements of the compositions can be used independently of the other steps and / or elements described herein. For example, the methods, systems, and compositions are not limited to implementation in rotating machinery as described herein. Rather, the methods, systems, and compositions can be implemented and used in connection with many other applications.

[0051] Specific features of various embodiments are shown in some drawings, and not in others, but this is for convenience only. Furthermore, the reference to “one embodiment” in the above description is not intended to be construed as excluding the existence of additional embodiments that also incorporate the described features. According to the principles of this disclosure, any feature in the drawings may be referenced and / or claimed in combination with any feature in any other drawing.

[0052] This specification uses several embodiments, including best modes, to enable those skilled in the art to carry out the disclosure, including the manufacture and use of any apparatus or system and the execution of any related methods. The patentable scope of this disclosure is defined by the claims and may include other embodiments that a person skilled in the art could conceive. Such other embodiments are intended to be within the claims if they have structural elements that do not differ from the language of the claims, or if they include equivalent structural elements that do not substantially differ from the language of the claims.

[0053] Although the present invention has been described in relation to various specific embodiments, those skilled in the art will recognize that the present invention can be implemented with modifications within the spirit and scope of the claims.

[0054] Further aspects of the present invention are provided by the subject matter of the following clauses.

[0055] According to a first embodiment, a steam supply system for a power generation system, comprising: a heat recovery steam generator, a high-temperature section including a high-pressure superheater and a first reheater arranged in series with respect to exhaust gas flowing through the heat recovery steam generator, and a second reheater located downstream of the high-temperature section and extracting heat from exhaust gas leaving the high-temperature section; and a second reheater that contributes to at least a portion of the energy required to raise the temperature of low-temperature reheat steam to a target temperature.

[0056] The steam supply system according to the preceding embodiment, wherein the second reheater is arranged in parallel with the first reheater in the high-temperature section.

[0057] A steam supply system according to any preceding embodiment, wherein the second reheater is a combustion reheater.

[0058] A steam supply system according to any preceding embodiment, wherein a second reheater burns fuel from a fuel source at a variable combustion rate.

[0059] A steam supply system according to any preceding embodiment, wherein a second reheater is coupled to a dedicated air supply source, and the dedicated air supply source enables low excess air operation.

[0060] A steam supply system according to any preceding embodiment, wherein the second reheater is selectively operable in any of three operating states, including a non-operating state in which the second reheater is not burning fuel, a maximum burner combustion rate in which the second reheater is operating at its maximum energy input, and an intermediate burner combustion rate that is less than the maximum burner combustion rate and greater than the non-operating state.

[0061] A steam supply system according to any prior embodiment, coupled to a gas turbine in a combined cycle power generation system.

[0062] A steam supply system according to any preceding embodiment, wherein the operation of a gas turbine, a second reheater, and a low-temperature reheat steam flow divided between the second reheater and the first reheater is adjusted to control the steam temperature leaving the first reheater and supply it to the intermediate-pressure steam turbine such that the first reheater does not require superheat reduction to regulate the reheat steam temperature.

[0063] A steam supply system according to any preceding embodiment, wherein a low-temperature reheat steam flow is shifted from a second reheater to a first reheater in order to facilitate a reduction in the outlet temperature of the first reheater.

[0064] A steam supply system according to any preceding embodiment, wherein the airflow to the gas turbine is increased to facilitate a decrease in the outlet temperature of the first reheater.

[0065] A steam supply system according to any preceding embodiment, wherein a first reheater provides rated-temperature steam to an intermediate-pressure steam turbine operating at the maximum load of a combined cycle power generation system, and is sized and configured so that a rated portion of the low-temperature reheat steam flow is directed to a second reheater.

[0066] A steam supply system according to any preceding embodiment, wherein the second reheater is selectively operable at or near the maximum burner combustion rate when the combined cycle power generation system is under high load.

[0067] A steam supply system according to any preceding embodiment, wherein a second reheater is selectively operable at an intermediate burner combustion rate, and the reheat steam temperature is selectively adjusted to enable a stable supply of process steam from at least one of an automatically pressure-controlled steam turbine extraction and a non-condensing steam turbine.

[0068] A steam supply system according to any preceding embodiment, wherein the second reheater does not require superheat reduction to regulate the reheat steam temperature.

[0069] A combined cycle power generation system comprising: a gas turbine for generating electricity; a heat recovery steam generator having a gas inlet communicating with the exhaust flow of the gas turbine, the heat recovery steam generator having a high-temperature section including a high-pressure superheater and a first reheater arranged in series with respect to the exhaust gas flowing through the heat recovery steam generator, and a heat recovery steam generator having an evaporator located downstream of the high-temperature section for extracting heat from the exhaust gas leaving the high-temperature section; a second reheater that contributes to at least a portion of the energy required to raise the temperature of low-temperature reheat steam to a target temperature; and at least one of an intermediate-pressure steam turbine and a non-condensable steam turbine configured to receive reheat steam from at least one of the second reheater and the first reheater of the heat recovery steam generator.

[0070] The combined cycle power generation system according to the preceding embodiment, wherein the second reheater is arranged in parallel with the first reheater in the high-temperature section.

[0071] A combined cycle power generation system according to any preceding embodiment, wherein the second reheater is a combustion reheater.

[0072] A combined cycle power generation system according to any preceding embodiment, wherein the second reheater burns fuel from a fuel source at a variable combustion rate.

[0073] A combined cycle power generation system according to any preceding embodiment, wherein a second reheater is coupled to a dedicated air supply source, and the dedicated air supply source enables low excess air operation.

[0074] A combined cycle power generation system according to any prior embodiment, wherein the second reheater is selectively operable in any of three operating states, including a non-operating state in which the second reheater is not burning fuel, a maximum burner combustion rate in which the second reheater is operating at its maximum energy input, and an intermediate burner combustion rate that is less than the maximum burner combustion rate and greater than the non-operating state.

[0075] A combined cycle power generation system according to any preceding embodiment, wherein the first reheater of the heat recovery steam generator provides a rated steam temperature to at least one of an intermediate pressure steam turbine and a non-condensable steam turbine operating at the base load of the combined cycle power generation system, and is sized and configured so that a rated portion of the low-temperature reheat steam flow is directed to the second reheater.

[0076] A combined cycle power generation system according to any preceding embodiment, wherein the second reheater is selectively operable at or near the maximum burner combustion rate when the combined cycle power generation system is under high load.

[0077] A combined cycle power generation system according to any preceding embodiment, wherein a second reheater is selectively operable at an intermediate burner combustion rate, and the reheat steam temperature is selectively adjusted to enable a stable supply of process steam from the exhaust of an automatically pressure-controlled steam turbine and a non-condensing steam turbine.

[0078] A combined cycle power generation system according to any preceding embodiment, wherein the second reheater and heat recovery steam generator do not require superheat reduction to adjust the reheat steam temperature. [Explanation of Symbols]

[0079] 10 Power generation systems, power plants 20 Gas Turbines 30 High-temperature exhaust gas 40 High-temperature gas 100 HRSG 102 Entrance 104 Exit interface 110 High-temperature section 120 High-pressure superheater 122 Gas Inlet 124 Gas outlet 126 Overheating Reduction Device 128 Steam outlet 130 Reheater 131 IP Steam 136 Overheating Reduction Device 138 lines, steam outlet, reheat steam temperature, flow 141 lines 142 lines, steam outlet, supply steam, flow 150 Evaporator 152 Fluid conduit 165 High-pressure steam turbine 170 Duct Burner 172 Burner Fuel 175 Medium-pressure steam turbine 184 Extraction (or exhaust) steam 185 Dedicated process steam supply steam turbine, non-condensable steam turbine 186 Processes, Flows 200 Combustion Reheater 210 Regenerative Air Heater 221 Line, burner fuel, fuel flow 222 Exhaust steam 223 Air lines, airflow RA range RB range R0 load R1 Load R2 Plant Load

Claims

1. A steam supply system for a power generation system (10), A heat recovery steam generator (100), A high-temperature section (110) including a high-pressure superheater and a first reheater arranged in series with respect to the exhaust gas flowing through the heat recovery steam generator (100), and An evaporator (150) located downstream of the high-temperature section (110) extracts heat from the exhaust gas leaving the high-temperature section (110). A heat recovery steam generator (100) is provided, A second reheater that contributes to at least a portion of the energy required to raise the temperature of the low-temperature reheat steam to the target temperature, A steam supply system equipped with the following features.

2. The steam supply system according to claim 1, wherein the second reheater is arranged in parallel with the first reheater in the high-temperature section (110).

3. The steam supply system according to claim 1, wherein the second reheater is a combustion reheater (200).

4. The steam supply system according to claim 3, wherein the second reheater burns fuel from a fuel source at a variable combustion rate.

5. The steam supply system according to claim 4, wherein the second reheater is coupled to a dedicated air supply source, and the dedicated air supply source enables low excess air operation.

6. The steam supply system according to claim 1, wherein the second reheater is selectively operable in any of three operating states, including a non-operating state in which the second reheater is not burning fuel, a maximum burner combustion rate in which the second reheater is operating at its maximum energy input, and an intermediate burner combustion rate that is less than the maximum burner combustion rate and greater than the non-operating state.

7. The steam supply system according to claim 6, which is coupled to a gas turbine (20) of a combined cycle power generation system.

8. The steam supply system according to claim 7, wherein the operation of the gas turbine (20), the second reheater, and the low-temperature reheat steam flow divided between the second reheater and the first reheater is adjusted to control the steam temperature leaving the first reheater and supply it to the intermediate-pressure steam turbine (175) such that the first reheater does not require superheat reduction to regulate the reheat steam temperature.

9. The steam supply system according to claim 8, wherein a low-temperature reheat steam flow is shifted from the second reheater to the first reheater in order to facilitate a decrease in the outlet temperature of the first reheater.

10. The steam supply system according to claim 8, wherein the airflow to the gas turbine (20) is increased to facilitate a decrease in the outlet temperature of the first reheater.

11. The steam supply system according to claim 6, wherein the first reheater provides rated-temperature steam to an intermediate-pressure steam turbine (175) operating at the maximum load of the combined cycle power generation system, and is sized and configured so that the rated portion of the low-temperature reheat steam flow is directed to the second reheater.

12. The steam supply system according to claim 11, wherein the second reheater is selectively operable at or near the maximum burner combustion rate when the combined cycle power generation system is under high load.

13. The steam supply system according to claim 6, wherein the second reheater is selectively operable at the intermediate burner combustion rate, and the reheat steam temperature is selectively adjusted to enable a stable supply of process steam from at least one of an automatically pressure-controlled steam turbine extraction and a non-condensing steam turbine (185).

14. The steam supply system according to claim 6, wherein the second reheater does not require superheat reduction to adjust the reheat steam temperature.

15. A gas turbine (20) that generates electricity, A heat recovery steam generator (100) having a gas inlet (122) that communicates with the exhaust flow of a gas turbine, A high-temperature section (110) including a high-pressure superheater and a first reheater arranged in series with respect to the exhaust gas flowing through the heat recovery steam generator (100), and An evaporator (150) located downstream of the high-temperature section (110) extracts heat from the exhaust gas leaving the high-temperature section (110). A heat recovery steam generator (100) is provided, A second reheater that contributes to at least a portion of the energy required to raise the temperature of the low-temperature reheat steam to the target temperature, The second reheater and at least one of the first reheater of the heat recovery steam generator (100) and the intermediate pressure steam turbine (175) and the non-condensable steam turbine (185) are configured to receive reheat steam from at least one of them. A combined cycle power generation system equipped with the following features.

16. The combined cycle power generation system according to claim 15, wherein the second reheater is arranged in parallel with the first reheater in the high-temperature section (110).

17. The combined cycle power generation system according to claim 15, wherein the second reheater is a combustion reheater (200).

18. The combined cycle power generation system according to claim 17, wherein the second reheater burns fuel from a fuel source at a variable combustion rate.

19. The combined cycle power generation system according to claim 18, wherein the second reheater is coupled to a dedicated air supply source, and the dedicated air supply source enables low excess air operation.

20. The combined cycle power generation system according to claim 17, wherein the second reheater is selectively operable in any of three operating states, including a non-operating state in which the second reheater is not burning fuel, a maximum burner combustion rate in which the second reheater is operating at its maximum energy input, and an intermediate burner combustion rate that is less than the maximum burner combustion rate and greater than the non-operating state.

21. The combined cycle power generation system according to claim 20, wherein the first reheater of the heat recovery steam generator (100) provides a rated steam temperature to at least one of the intermediate pressure steam turbine (175) and the non-condensable steam turbine (185) operating at the base load of the combined cycle power generation system, and is sized and configured so that the rated portion of the low-temperature reheat steam flow is directed to the second reheater.

22. The combined cycle power generation system according to claim 21, wherein the second reheater is selectively operable at or near the maximum burner combustion rate when the combined cycle power generation system is under high load.

23. The combined cycle power generation system according to claim 20, wherein the second reheater is selectively operable at an intermediate burner combustion rate, and the reheat steam temperature is selectively adjusted to enable automatic pressure-controlled steam turbine extraction and a stable supply of process steam from the exhaust of the non-condensable steam turbine (185).

24. The combined cycle power generation system according to claim 23, wherein the second reheater and the first reheater do not require superheat reduction to adjust the reheat steam temperature.

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