TW202417733A - System for readying sub-critical and super-critical steam generator, servicing method of said sub-critical and super-critical steam generator and method of operation of sub-critical and super-critical steam generator - Google Patents

Abstract

A system for readying sub-critical (48, 182, 186) and super-critical steam generator (190), a servicing method of said sub-critical and a super-critical steam generator and a method of operation of sub-critical and super-critical steam generator is provided. The steam generator (48, 182, 186, 190) includes a first auxiliary heating device (55a) disposed on at least one water-steam separator (18, 192) for heating said at least one water-steam separator, and / or a second auxiliary heating device (55b) disposed at least on a part of furnace top-end piping (26) for heating the furnace top-end piping. Said auxiliary heating devices are heating steam producing components of the steam generator (48, 182, 186, 190) and thus allowing to keep them above the temperature in which materials creating said steam producing components are brittle. The method includes recirculation of the water through the steam generator (48, 182, 186, 190).

Description

System for preparing a subcritical and supercritical steam generator, method for maintaining a subcritical and supercritical steam generator, and method for operating a subcritical and supercritical steam generator

Embodiments of the present disclosure generally relate to steam generators for use in steam power plants, and more particularly to systems and methods for getting a steam generator, such as a subcritical or supercritical steam generator, ready for rapid startup with immediate auxiliary heating and deaeration water fill that functions optimally when combined with a steam turbine in hot standby.

A steam generator (such as a subcritical or supercritical steam generator) generally includes a furnace in which a fuel is burned to generate thermal energy or heat to produce steam. The steam produced can be used in a steam turbine to drive a generator to produce electricity or to provide heat for other purposes. Examples of such steam generators include boilers.

The intermittent electricity market, due to renewable energy, has changed the demand for steam generators that were originally built for base load operation. Today operations are with low capacity factors and at very short notice. As a result, start-up time and associated costs have become a significant factor in the profitability of power plants incorporating these steam generators.

Today, power plants with combined cycle gas turbines (CCGT) in warm or hot standby allow significantly shortened start-up times. Standby technology applied to steam turbines operating in boiler power plants is usually of low economic interest, since combined boiler-turbine starting is strongly affected by the time taken for the boiler transition from standstill to steam turbine release (hereinafter also referred to as "boiler transition") and by further boiler delays in providing steam of acceptable quality to the steam turbine.

Due to intermittent electricity prices, the time from standstill to firing mode of operation (i.e., the mode in which sufficient quality steam is produced and ultimately used to generate electricity with a steam turbine) is a major commercial factor in generating revenue for a facility boiler power plant. Many efforts have been made to shorten this time in order to be more responsive to electricity demand. These efforts are also aimed at reducing start-up costs in order to be more competitive in the electricity market.

The boiler transition begins with a furnace purge before the auxiliary fuel system (also called the auxiliary fuel burner) is ignited. The auxiliary fuel system includes the auxiliary fuel. The auxiliary fuel (also called the start-up fuel) produces a controlled and limited amount of heat to produce the first process steam and is used to safely warm up boiler components (such as the boiler drum, separator, or water-cooled wall). The auxiliary fuel system operates until the boiler is ready to safely absorb the heat of the main fuel burner. Safe warm-up and safe absorption involve possible thermal expansion and fatigue of the boiler components. Once the main fuel is ignited, continued firing of the auxiliary fuel system allows for better thermal control while gradually increasing the pressure in the power plant including the boiler. The net use of auxiliary fuel is an essential part of the start-up cost. In addition, reducing the use of auxiliary fuel during startup helps reduce environmental impact.

In addition to the safety component warm-up mentioned above, boiler warm-up also requires prevention of flow-accelerated corrosion (FAC), corrosion fatigue, and other degradations, such as the formation of deposits in the turbine. These degradations originate from the properties of water and contribute to the combined start-up time of the power plant including the steam generator and the steam turbine. The current best technical solutions to these degradations include stand-alone deaerators or condenser-based nozzles to degas the water steam cycle (WSC) and blow down on the boiler drum to desalinate the WSC in the case of subcritical boilers.

The limitation of shortening the time taken to transition from the stop mode to the firing mode originates from the metal blocks, which must be heated within a safe temperature rise benchmark to control thermal stress fatigue on the components of the steam generator. These blocks limit, for example, the rate of temperature increase and, for example, the maximum temperature difference between the material of the components of the boiler and/or steam turbine and the water or steam temperature. The temperature increase rate and difference limits are material grade dependent. In practice, the material thickness is the most relevant limitation. Therefore, thick-walled components determine the time required to safely pass through the transition of the boiler. Well-known examples of thick-walled components include the boiler drum in a subcritical steam generator or the separator in a supercritical steam generator.

Boiler materials can be brittle at low temperatures, i.e. have less durability than in a ductile state. Starting the transition with some brittle components further limits the possible reduction of the transition time. As the temperature of the components increases, larger temperature differences can be tolerated because the metal of the components becomes ductile. In the case of brittle components, the heat-up process must initially be controlled by carefully raising the feed water temperature in the water-cooled walls. This control is achieved by reducing the auxiliary fuel firing rate, which produces a lower fuel flow and a significantly longer firing time. This increases the net use of the auxiliary fuel.

The limitation to shortening the time required to achieve sufficient steam quality to discharge the steam turbine is another factor that prolongs the transition from the standstill mode to the firing mode of the steam generator. This involves the filling of the water circuit of the steam generator, which requires a considerable amount of time. In the case of a subcritical steam generator, the water must pass through the economizer, boiler drum, boiler drum downcomers, and waterwall inlet header before filling the waterwall tubes. In the case of a supercritical steam generator, the incoming water must pass through the economizer, boiler waterwall, separator, separator drain, and boiler feedwater recirculation system.

If contaminated water is used, the water fill can be extended additionally. This includes the time required to remove dissolved gases (by recirculating water through the condenser/deaerator/economizer), and to remove brine components (by blowing through the boiler drum) in order to achieve acceptable water parameters. This is significant when the gases in the drum are recirculated through the downcomers and water walls. The current best technical solutions do not take into account the time required for the steam water circulation purification. This is because the combined start-up time of the steam generator and the steam turbine is longer than the time required for purification. Furthermore, the use of a degassed conductivity measurement system allows for an earlier release of the steam turbine roll off.

Therefore, there is a need to solve the problems of the prior art. Specifically, in order to reduce the time required to transition a steam generator from a stop to a firing mode of operation. In addition, it is necessary to avoid thermal stresses on components of the steam generator, especially on key components of the steam generator. Furthermore, it is necessary to reduce the environmental impact of starting a steam generator by reducing the use of auxiliary fuel during the start-up of the steam generator.

The following presents a simplified summary of the disclosed subject matter to provide a basic understanding of some aspects of the various embodiments described herein. This summary is not an extensive overview of the various embodiments. It is not intended to arbitrarily identify key features or essential features of the claimed subject matter as set forth in the claims, nor is it intended to aid in determining the scope of the claimed subject matter. Its sole purpose is to present some concepts of the present disclosure in a concise form as an introduction to the more detailed description presented later.

Various embodiments of the present invention are directed to reducing the overall "return to service" time of a steam generator, such as a subcritical steam generator or a supercritical steam generator. The "return to service" time includes the transition from a stalled operating mode to an operating mode ready to run a steam turbine, wherein the turbine is preferably maintained in a warm or hot standby state.

Solutions provided by various embodiments involve installing auxiliary heating devices and providing piping that allows for extraction or redirection of heated water and installation/upgrade of auxiliary heating devices. Auxiliary heating devices may include, but are not limited to, nozzles operated with steam from auxiliary boilers, and/or auxiliary electric heaters (such as immersion heaters or heating blankets). The solution also includes methods of maintaining boilers in a standby state and methods of heating start-up stress critical steam generating components or stress critical components within a safe temperature rise baseline. Unless otherwise stated, the embodiments presented herein may be used with subcritical and supercritical steam generators. At the same time, it should be noted that features and components included in the presented embodiments may be transferred between the presented embodiments to produce further embodiments.

In one embodiment, a system for getting a steam generator ready includes a first auxiliary heating device, a second auxiliary heating device, a third auxiliary heating device, a fourth auxiliary heating device, and/or a fifth auxiliary heating device. The first auxiliary heating device is disposed on a water-steam separator for heating the water-steam separator. The water-steam separator may be a boiler drum. Water-cooled wall tubes are connected to the water-steam separator having a furnace top pipe. The second auxiliary heating device is disposed on at least a portion of the furnace top pipe for heating the furnace top pipe. The first and second auxiliary heating devices heat the steam generating components of the steam generator and therefore allow them to be kept at a temperature higher than the material from which the steam generating components are generated is brittle. In particular, when a feedwater starter pump is engaged, the third auxiliary heating device is installed to provide heat to keep a deaerator in the required conditions. The fourth auxiliary heating device is arranged on the economizer for heating the economizer. This allows the economizer to be kept in the required operating conditions and in particular prevents the economizer from freezing. At the same time, this allows the economizer to be kept at a temperature higher than the material from which the economizer is generated is brittle. An economizer outlet feedwater line includes the fifth auxiliary heating device, which heats or (pre)heats the water discharged by the economizer. This allows the heating or insulation of the components of the steam generator downstream of the economizer outlet feedwater line.

In one embodiment, a method of maintaining a steam generator is included. The method comprises installing a system for making the steam generator ready as defined herein on the steam generator. The steam generator may be a subcritical steam generator or a supercritical steam generator.

In one embodiment, a method for operating a steam generator is included. The operating method includes installing a system for getting the steam generator ready as defined herein on the steam generator. The steam generator can be a subcritical steam generator or a supercritical steam generator. The use of a large number of auxiliary devices ensures that the startup procedure is achieved more quickly. At the same time, a defined sequence for switching on specific auxiliary devices ensures a fast startup procedure while keeping the steam generator energy efficient. In a certain embodiment, the startup of the steam generator includes the following sub-steps: heating a deaerator with a third auxiliary heating device to at least partially deaerate the water contained in the deaerator; and using a feed water pump to pump water to an economizer before starting. This allows the startup of the deaerator and the economizer and therefore the steam generator to be speed-regulated. This embodiment may also include: heating water in an economizer outlet feed water line with the fifth auxiliary heating device; and filling a water-steam separator (e.g., a boiler drum) with the water heated in the economizer outlet feed water line. (Pre)heated water separator(s) are filled and used to flush the decontaminated water collected in the circulation within the steam generator. The (pre)heated water entering the boiler drum flows to the water-wall tubes via (multiple) boiler drum downcomers and a water-wall inlet header. In this way, before a furnace is blown, the feed water heats the boiler drum, the boiler drum downcomers, the water-wall inlet header, and the water-wall tubes; and an auxiliary fuel burner can be ignited. This embodiment may also include recirculation of water between the boiler drum, the deaerator, and the economizer in order to speed up the heating and therefore speed up the start of the steam generator.

The operating method may additionally or alternatively include shutting down the steam generator. In one embodiment, in order to shorten the time required to start generating steam with the steam generator, the method includes maintaining the at least one water-steam separator filled with water and the at least one water-steam separator surrounded by nitrogen, or maintaining the deaerator and the third auxiliary heating device sealed together, maintaining the economizer and the fourth auxiliary heating device sealed together, maintaining the at least one water-steam separator and the first auxiliary heating device at least at a temperature at which the material of the at least one water-steam separator is no longer brittle, and maintaining the furnace top pipe and the second auxiliary heating device at least at a temperature at which the material of the furnace top pipe is no longer brittle. Maintaining the deaerator and the economizer sealed includes limiting the diffusion of gas into water stored in the deaerator and the economizer.

In one embodiment of a steam generator, filling the steam generator with feedwater, (pre)heating the at least partially deaerated feedwater with the fifth auxiliary heating device, warming the water-steam separator (e.g., a boiler drum) with the first auxiliary heating device, and the auxiliary fuel burner are controlled based on measurements (e.g., temperature and/or pressure) obtained about the steam generator and the auxiliary heating devices. In this case, a controller can control the inlet temperature of the water-steam feedwater and the outlet temperature of the water-wall to a safe temperature rise rate, and control the temperature difference of components such as the water-steam separator (e.g., boiler drum), the boiler drum downcomers, the water-wall inlet header, and/or the water-wall tubes. With the thick wall components insulated in a ductile condition, the safe temperature rise rate of the thin wall downcomers and water walls is allowed to bring the thin wall components to a ductile condition in a short time. With all components in a ductile condition, these thick wall components will ensure a safe temperature rise rate for the steam generator.

In one embodiment of a primary critical steam generator, water contained in the boiler drum downcomers and water wall inlet headers may be extracted and recycled to a condenser in fluid communication with a deaerator and an auxiliary heating source. After filling the boiler drum, boiler drum downcomers, water wall inlet headers, and water wall tubes, the extraction line is opened to recirculate the cooled and gas-contaminated water back to the condenser while the drum continues to fill with deaerated and heated feed water.

In one embodiment of a primary critical steam generator, once the heating process reaches the boiling point inside the drum, the drum vent valve is closed and the evaporated steam is used to purge the air-steam mixture trapped in the drum and superheater. The contaminated water in the water wall is discharged or recycled.

In one embodiment of a primary critical steam generator, natural circulation in the water wall will begin with the release of the firing of the auxiliary fuel burner. With natural circulation in place, the gas-contaminated water in the water wall will be hotter than the economizer fill water, causing it to cover the drum water surface, while the deaerated water preferentially settles and establishes flow in the downcomers. In this way, the contaminated water in the downcomers, water wall, and its header can be quickly replaced by the deaerated water.

In one embodiment of a supercritical steam generator, filling the steam generator with feed water, (pre)heating the at least partially deaerated feed water with the second auxiliary heating device, heating the water-steam separator with the first auxiliary heating device, the mass flow of the boiler circulation pump, and the auxiliary fuel burner are controlled based on temperature measurements obtained about the steam generator and the heating device. In this case, the controller can control the inlet temperature of the separator feed water and the outlet temperature of the water wall at a safe temperature rise rate, and control the temperature difference of components such as the separator, the water wall inlet pipe header, or the water wall tubes. With thick wall components in a ductile condition, safe temperature rise rates are allowed for thin wall components such as water walls to bring all components to a ductile condition in a short time. By keeping all components in a ductile condition, these thick wall components will ensure safe temperature rise rates.

The use of at least one auxiliary heating device in combination with the recirculation of water will help to accelerate the critical temperature of corrosion through the flow more quickly and at significantly lower dissolved oxygen levels. In this way, a steam generator in extended standby mode can be transitioned more quickly from pre-operational readiness of the steam generator (i.e., filling of water) to the start-up of the main fuel burner. As a result, the need to reheat these components by auxiliary fuel (which is often the case due to safety temperature increases or water chemistry conditions) is reduced to its minimum, and thus the time and cost of the facility can be reduced. Specifically, this is achieved by actively degassing the feed water with a third auxiliary heating device while simultaneously raising the feed water temperature with a fifth auxiliary heating device; and heating the boiler drum/(multiple) water-steam separators with the first and second auxiliary heating devices to heat the steam generating components of the steam generator, together with the recycling of water to the condenser (for subcritical units including a circulation between the water-steam separator/boiler drum, deaerator, and economizer) or to the recycling loop (for supercritical units including a circulation between (multiple) water-steam separators and economizers).

Example embodiments of the present invention will be described more fully below with reference to the accompanying drawings, which show some but not all embodiments. In fact, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the present disclosure will satisfy applicable legal requirements. Like numbers may refer to like elements throughout.

The present disclosure generally relates to a water charge of at least partially degassed and (pre) heated water for rapid startup of a steam generator in a stalled mode of operation in which the steam generator is unfired and the water walls are emptied. In the present disclosure, in some embodiments, preheated water is water that is preheated before the charge, and heated water is water that is heated during the charge. In other embodiments, these terms may be used interchangeably. The present disclosure is primarily described with respect to subcritical steam generators and supercritical steam generators. However, it should be understood that the systems and methods of various embodiments of making subcritical steam generators and supercritical steam generators ready using a water charge of degassed and (pre) heated water for rapid startup may be applied to other types of steam generators. However, as used herein, "keeping warm" (also known as "keeping warm") of a steam generator means maintaining components of the steam generator at an elevated temperature in a stalled mode of operation under unfired conditions, and "warming" of components is by heating the components using hot water, as opposed to "heating" of components achieved under fired conditions, and "deaerated water" or "deaerated feed water" means steam generator feed water having a reduced oxygen content (e.g., less than 8 to 10 mg/l) and other dissolved gases such as carbon dioxide and nitrogen. In this context, "deaerated" may mean "at least partially deaerated." As used herein, "upstream" and "downstream" are terms indicating the direction relative to the flow of a fluid.

In this case, insulation, deaerated water filling, and warming according to various embodiments described herein can place a subcritical steam generator or supercritical steam generator in a more favorable startup condition, which allows the steam generator to transition from a stalled operating mode to a fired operating mode without delay, in a manner that avoids delays due to the fragility of stress-critical steam generating components or stress-critical components and avoids other adverse conditions that can cause excessive thermal stress when the steam generator is fired. As a result, the steam generator (e.g., a subcritical steam generator or a supercritical steam generator) will be more responsive to sudden grid demands.

The insulated and at least partially degassed water fill aspects of various embodiments that facilitate rapid startup of either a subcritical steam generator or a supercritical steam generator without excessive thermal stress of the steam components are achieved by introducing a heat injection point directly into the stress critical steam generating component or stress critical components of the steam generator at the inlet location of the fill feed water to control the fill water temperature directly related to the allowable temperature limit of the critical steam component. As used herein, "stress critical steam generating component" or "stress critical component" means a component of a subcritical steam generator or a supercritical steam generator having a metal mass that needs to be heated within a safe temperature rise baseline to minimize brittle conditions and excessive thermal stress. In one embodiment, the stress critical steam generating components of a subcritical steam generator may include, but are not limited to, a boiler drum and water wall tubes. Other stress critical steam generating components may include boiler drum downcomers and/or water wall inlet headers. In one embodiment, the stress critical steam generating components of a supercritical steam generator may include, but are not limited to (multiple) water-steam separators, water wall tubes, (multiple) downcomers and/or water wall inlet headers. As used herein, the term "in fluid communication" means the presence of a passage that allows fluid to flow. Those skilled in the art understand that the connection between the elements in any steam generator must have a passage that allows fluid to flow, and therefore, this fact will not be emphasized in the present disclosure.

In one embodiment, a plurality of auxiliary heating devices may be used to warm the stress critical steam generating components and/or (pre)heat the at least partially deaerated feed water that flows to or from the components during filling of a water filling circuit. The water filling circuit comprises an economizer, a water-steam separator (which may be a boiler drum), and a furnace. This allows the temperature of the feed water to be controlled in a manner that complies with the permissible temperature limits specified for the stress critical steam generating components. This keeps stress critical steam generation away from brittle conditions or other conditions that may lead to excessive thermal stresses during ignition of the steam generator.

Various embodiments may also require providing a steam generator with an excess water drain line that can be used to recirculate back into the steam generator to keep warm and continuously refill the steam generator with preheated and at least partially deaerated feed water. For example, the excess water drain line can be connected to a steam turbine condenser. The condenser can supply excess water received from the excess water drain line to a deaerator to promote the reduction of gas content in the feed water supplied to an economizer of the steam generator.

Further information about various embodiments is described in more detail below. Comparative prior art examples are also provided. Comparative prior art examples

Turning now to Fig. 1, this figure shows a schematic diagram of a subcritical steam generator 10 according to the prior art, which produces steam that can be used for power generation or heating purposes. This example introduces a description of the various elements of the steam generator associated with the present invention. Furthermore, this example illustrates some of the shortcomings of the prior art solutions.

As shown in FIG. 1 , a subcritical steam generator 10 includes a furnace 12 that burns a mixture 14 of fuel and air provided to the furnace. Fuel, such as coal, which may be a pulverized solid fuel, is provided to the furnace by a pulverizer (not shown). Although pulverized coal is primarily used as the fuel, the furnace 12 may be designed to achieve co-firing of oil, biomass, or byproduct gases. Air may be provided to the furnace 12 via an air source (not shown). The mixture 14 of fuel and air is burned in a combustion chamber of the furnace 12. The combustion of the fuel and air generates thermal energy or heat, which is used to heat a liquid, such as water, in furnace water-cooled wall tubes 16 lining the walls of the furnace 12, which tubes may also be referred to as the evaporator portion of the furnace 12. Heating of the water in the furnace water wall tubes 16 produces saturated water which is separated into water and steam in the boiler drum 18. The steam produced in the subcritical steam generator 10 may be passed to a steam turbine (not shown) to generate electricity or to provide heat for other purposes.

Other components of the subcritical steam generator 10 include one or more boiler drum downcomers 20 (herein, boiler drum downcomers), inlet header(s) 22 (herein, inlet header) to the furnace water wall tubes 16, and economizer 24. The boiler drum 18 is a water/steam storage tank at the top of the furnace water wall tubes 16. In addition to receiving a steam and water mixture from the top of the furnace water wall tubes 16 via the furnace top piping 26, the boiler drum 18 can receive water from the feedwater supplied to the economizer 24 via the economizer outlet feedwater piping 28. The feed water supplied to the economizer 24 comes from a deaerator heating system 30 (deaerator) which includes a tank 32 for water obtained from the condensate system via a condensate pump 34 and a condensate piping 36, an auxiliary steam source 38 for heating the tank 32, and a feed water supply piping 40 or line 40 which supplies heated deaerated feed water to the economizer 24 via a feed water pump 42. It is to be understood that the subcritical steam generator 10 may have other components which are not discussed herein for the purpose of brevity. A non-exhaustive list of such components may include a superheater and/or a reheater.

The furnace water wall tubes 16, the boiler drum 18, the boiler drum downcomers 20, the inlet header 22, the economizer 24, the furnace top piping 26, the economizer outlet feedwater piping 28, the feedwater supply piping 40, the feedwater pump 42, and the deaerator 30, and components of the condensate system (e.g., the condensate pump 34 and the condensate piping 36) are components that may form part of various water filling loops of the subcritical steam generator 10. In these water filling loops, water enters the boiler drum 18 from the economizer 24 and the economizer outlet feedwater piping 28. The water entering the boiler drum 18 will go to the inlet header 22 for use in supplying the furnace water wall tubes 16 at the bottom end of the furnace 12. Water that enters the bottom of the furnace 12 at the furnace water wall tubes 16 rises upward along the tubes lined in the furnace wall. The mixture 14 of burning fuel and air heats the water in the furnace water wall tubes 16 into a steam and water mixture. The boiler drum 18 receives the steam and water mixture from the furnace water wall tubes 16 through the furnace top piping 26 and separates saturated steam from the water and steam mixture in the drum. In this way, the subcritical steam generator 10 can provide saturated steam to other parts in the steam generator for further heating and ultimately to the steam turbine to generate electricity or provide heat for other purposes. The water is mixed with replenished feed water from the economizer outlet feed water piping 28 in the boiler drum 18 and supplied by natural circulation to the boiler drum downcomers 20 and the furnace water wall tubes 16 in the furnace 12. The water in the furnace water wall tubes 16 may then be fed back to the boiler drum 18 for further steam generation.

As mentioned above, a subcritical steam generator, such as the subcritical steam generator 10, slowly transitions from an unfired stall mode of operation to a fired mode of operation upon demand for return to service. For example, getting the subcritical steam generator 10 ready begins by filling the economizer 24, the boiler drum 18, and the water-cooled wall tubes 16 with pre-circulated clean water from the deaerator 30 and the condensate system. Although not shown in FIG. 1 , the condensate system may include a condenser that cools exhaust steam from the turbine, collects latent heat from the steam, and condenses the steam into water. The condensate pump 34 may pressurize the condensate for supply to the deaerator 30 via the condensate piping 36.

The storage tank 32 of the deaerator 30 can receive condensate from the condensate piping 36, or alternatively, as shown by the dotted line in Figure 1, the piping 36 can supply condensate to the economizer of the bypass deaerator. The storage tank 32 can include a tank combination including a feed water storage tank and a connected deaerator tank. Generally, the purpose of this tank combination is to store and deaerate water into at least partially deaerated feed water by preheating the water inventory before feeding the water inventory to the economizer 24 of the subcritical steam generator 10. Preheating the water in the storage tank 32 is achieved by an auxiliary steam source 38, which can be an external heat source. This preheating of the water in the storage tank 32 can occur when the subcritical steam generator 10 is not in use.

This pre-warmed or pre-heated water is then transferred from the tank 32 as heated deaerated feed water (i.e., at least partially deaerated feed water) to the tube bundle section of the economizer 24 of the subcritical steam generator 10 via the feed water supply piping 40 and the feed water pump 42. The feed water passes through the economizer 24 to fill the boiler drum 18 via the economizer outlet feed water piping 28. As the water reaches the boiler drum 18, the boiler drum downcomers 20 will also fill. This establishes a fill connection to the furnace water wall tubes 16 via the inlet tube header 22. As more water is fed through the economizer 24, the furnace water wall tubes 16 will fill to the top plate level or the top of the furnace 12. This allows the drum level to rise to a starting level close to or slightly below the drum centerline 44.

At this point, the pre-operation filling portion of the preparation process of the subcritical steam generator 10 is completed. With the pre-operation filling portion completed, the preparation process of the subcritical steam generator 10 can continue to ignite the fire in the furnace 12 to further heat the water and start the production of steam.

If the subcritical steam generator 10 is subjected to the pre-operational fill portion of the preparation process for the subcritical steam generator 10, the temperature in the components that include metal and are receiving water (e.g., the boiler drum 18, the boiler drum downcomers 20, the inlet tube header 22, and the furnace water-cooled wall tubes 16) decays. Thus, the pre-operational fill reduces the temperature of the components that produce problems such as those mentioned above (as the material of the components becomes brittle). As a result, the pre-operational fill makes the transition from the stop to the firing mode of operation detrimental to the design life and/or operating margin of these components. When the subcritical steam generator 10 is ignited, the resulting increase in tube metal temperature can lead to tube stresses associated with thermal shock. This deteriorates the time and expense incurred by the facility's boiler power plant during the transition from shutdown to firing mode.

FIG2 shows a schematic diagram of a system 46 for readying a subcritical steam generator 48, utilizing an insulated and (optionally) at least partially degassed water fill to place the steam generator in a more favorable starting condition that promotes rapid startup without subjecting stress critical steam generating components to brittle conditions and other adverse conditions that could result in excessive thermal stresses when the steam generator is fired.

In one embodiment, the system 46 includes a subcritical steam generator including a boiler drum 18 and a furnace 12, an economizer 24, an inlet header 22, and water wall tubes 16. The economizer 24 is connected to the boiler drum 18 by an economizer outlet feedwater pipe 52. The water wall tubes 16 are connected to the boiler drum 18 by a furnace top pipe 26. The inlet header 22 is connected to the boiler drum 18 by a boiler drum downcomer 20.

2 , the system 46 may include a heating system 30, such as, for example, a deaerator (hereinafter “deaerator 30”), to provide at least partially deaerated feed water to the economizer 24 via a feed water supply piping 40 and a feed water pump 42. It should be understood that although the deaerator 30 depicted in FIGS. 2 and 3 , 5 , and 6 discloses a steam injection tube, other heating elements may be implemented for use in the deaerator 30 to facilitate the various embodiments described herein.

The system 46 may include one, two, three, four, five or more (plurality) of auxiliary heating devices 55a-e for warming stress critical steam generating components and/or (pre)heating at least partially deaerated feed water flowing to or from the components during filling of the water filling circuit. As shown in FIG2 , a first auxiliary heating device 55a may be disposed on the boiler drum 18 and a second auxiliary heating device 55b may be disposed on the furnace top end piping 26 (i.e., the water wall outlet header). A fifth auxiliary heating device 55e (pre)heats (e.g., at least partially deaerated feed water) water discharged from the economizer 24 before the inlet of the boiler drum 18 (e.g., at the economizer outlet feed water line 52). The third auxiliary heating device 55c may be configured to provide heat to the deaerator 30 to maintain the deaerator at the desired conditions at the mass flow as provided by the feedwater pump 42. The fourth auxiliary heating device 55d may be disposed around the economizer 24 to protect the economizer from temperature stress or even freezing.

The plurality of auxiliary heating devices 55a-e may include any of a number of heating devices. For example, the auxiliary heating devices 55a-e may include, but are not limited to, nozzles operated with steam from an auxiliary boiler, and/or an auxiliary electric heater (such as an immersion heater or a heating blanket). In one embodiment, as shown in FIG. 2 , the aforementioned third auxiliary heating device for providing heat to the deaerator 30 may include an auxiliary steam source 54, which may also be referred to herein as an auxiliary heating source 54.

Using the system 46 with the subcritical steam generator 48 in place, the following method may be applied.

For example, during shutdown of the subcritical steam generator 48, the boiler drum 18 is maintained filled with water and surrounded by nitrogen, and/or the deaerator 30 is sealed with the third auxiliary heating device, and/or the economizer 24 is sealed with the fourth auxiliary heating device, so that gas diffusion is limited to the water-air interface when the sealed water maintains deaeration. Additionally or alternatively, the boiler drum 18 and the water-cooled wall outlet pipe header are insulated above their fragility condition by the first auxiliary heating device and the second auxiliary heating device. Surrounding with nitrogen may involve a separate valving arrangement (not shown in the figure). For this arrangement, possible nitrogen sources may include a bottle rack with steel columns of compressed nitrogen. Typically, containment is accomplished using overflow or vent lines.

At the start-up of the subcritical steam generator 48, the third auxiliary heating device 55c may be used to heat the deaerator 30 to operating conditions to reduce dissolved gases before the feedwater pump 42 is started and begins to provide feedwater to the economizer 24. Such operating conditions include bringing the water to 100°C (212°F) at atmospheric conditions and, optionally, pressurizing the deaerator 30. The fifth auxiliary heating device 55e may be used to (pre-)heat the discharged (e.g., deaerated) feedwater discharged from the economizer 24 before filling the boiler drum 18. The (pre-)heated water entering the boiler drum 18 will flow to the water wall tubes 16 via the boiler drum downcomers 20 and the water wall inlet header 22. In this manner, the feedwater heats the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, and the water wall tubes 16 before the furnace 12 is purged; and the auxiliary fuel burners may be ignited.

In one embodiment, filling the subcritical steam generator 48 with feedwater, (pre)heating the (e.g., at least partially deaerated) feedwater with the fifth auxiliary heating device, heating the boiler drum 18 with the first auxiliary heating device 55a, and the auxiliary fuel burners are controlled by the controller 72 based on temperature measurements obtained with respect to the steam generator and the auxiliary heating devices. In this case, the controller 72 can control the inlet temperature of the drum feedwater and the outlet temperature of the water wall at a safe temperature rise rate and temperature differences of components such as the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, or the water wall tubes 16. With the thick wall components being insulated in a ductile condition, the safe temperature rise rate of the thin wall downcomers 20 and water wall tubes 16 is allowed to bring the thin wall components to a ductile condition in a short time. With all components in a ductile condition, the thick wall components will limit the safe temperature rise rate.

In one embodiment, water contained in the boiler drum downcomers 20 and water wall inlet header 22 may be extracted and recycled to a condenser 96 in fluid communication with the deaerator 30 and the auxiliary heating source 54. After filling the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, and the water wall tubes 16, the extraction line depicted in FIG. 3 (reference element 90) may be opened to recycle the cooled and gas-contaminated water back to the condenser 96 while the drum continues to fill with deaerated and heated feed water.

In one embodiment, once the heating process reaches the boiling point inside the boiler drum 18, the boiler drum vent valve(s) 134 can be closed and the evaporated steam can be used to purge the air-steam mixture trapped in the boiler drum and superheater (not shown). The contaminated water in the water wall tubes 16 can then be discharged or recirculated. For example, the contaminated water in the water wall tubes 16 can be discharged via a discharge line or recirculated via a recirculation line, as shown in FIG. 3 (discharge line 94 and return line 90).

In one embodiment, natural circulation in the water wall will begin with the release of the firing of the auxiliary fuel combustion chamber. With natural circulation in place, the gas-contaminated water in the water wall will be hotter than the fill water of the economizer 24, causing it to cover the water surface of the boiler drum 18, while the deaerated water preferentially settles and establishes flow in the downcomer 20. In this way, the contaminated water in the downcomer 20, the water wall and its header can be quickly replaced by the deaerated water.

FIG3 shows a schematic diagram of a system 180 for use in a subcritical steam generator 182 ready for rapid startup according to another embodiment of the present invention. In this embodiment, the nozzle 50 is used as a fifth auxiliary heating device (55e) for (pre)heating the at least partially deaerated feed water discharged from the economizer 24 before the inlet of the boiler drum 18. As shown in FIG3, the nozzle 50 can be fluidly connected to the economizer 24 and the boiler drum 18 to warm the at least partially deaerated feed water flowing from the economizer 24 to the boiler drum 18 through the economizer outlet feed water line 52.

With the nozzles 50 in this position, the at least partially deaerated feedwater may be warmed prior to entering the boiler drum 18, which in turn will heat the boiler drum, the boiler drum downcomers 20, the waterwall inlet header 22, and the waterwall tubes 16. This eliminates the heat losses associated with utilizing at least partially deaerated feedwater that generally occurs with the subcritical steam generator 10 of FIG. 1 as the feedwater moves from the deaerator 30, through the economizer 24, then to the boiler drum 18, and through the boiler drum downcomers 20 and the waterwall inlet header 22 to fill the waterwall tubes 16. Furthermore, using the nozzle 50 in this manner allows heavy wall components such as the boiler drum 18 to be out of the brittle condition and within the high temperature range elastic window.

Although the embodiment depicted in FIG. 3 (and the embodiment shown in FIG. 5 ) uses nozzles to (pre)heat the water discharged by the economizer 24 before the inlet of the boiler drum 18, it should be understood that the nozzle 50 represents only one type of heating element that can be used to pre-warm the water discharged by the economizer 24. Other types of heating elements (such as, for example, electric heaters, such as immersion heaters or heating blankets) can be used in place of the nozzles in these embodiments. For example, as discussed above in the present disclosure, FIG. 2 shows an embodiment in which auxiliary heating devices 55e and 55a are deployed on the economizer outlet water supply line 52 and the boiler drum 18 and are used in place of the nozzles to pre-heat the water discharged from the economizer 24. Thus, the embodiments described in FIGS. 3 and 5 using nozzles to preheat water discharged from economizer 24 are merely illustrative of one type of heating element that may be deployed and are not intended to be limiting to these embodiments, as well as other embodiments.

The heat losses that may normally occur with the process described in Figure 1 are indicated in Figure 3 and other Figures such as Figures 5 and 6 by the arrows overlying the water wall tubes 16. Such heat losses are eliminated in this embodiment by providing a pressurization boost to the at least partially deaerated feed water provided by the nozzles 50, which heats the steam generating components in the subcritical steam generator 182, such as the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, and the water wall tubes 16.

To facilitate active degassing of the deaerator 30 by producing at least partially deaerated feedwater and heating of temperature critical components by the subcritical steam generator 182 of the nozzle 50, an auxiliary heating source 54 may be placed in fluid communication with the deaerator 30 and the nozzle 50 to heat the deaerator and (via the nozzle 50) the temperature critical components. In one embodiment, as shown in FIG. 3 , the auxiliary heating source 54 may include an auxiliary steam source. In this configuration, the auxiliary steam source 54 may supply steam to the deaerator 30 via a deaerator heating supply line 56 and to the nozzle 50 via a boiler drum heating supply line 58. Because the nozzle 50 heats the circulating water and the deaerator 30 degasses the water, their combined use speeds up the readiness of the steam generator for firing conditions. Although FIG. 3 shows only one auxiliary steam source, it should be understood that more than one steam source may be utilized.

A deaerator warming supply line 56, which may be in fluid communication with the auxiliary steam source 54 and the deaerator 30, supplies steam from the auxiliary steam source 54 to the deaerator 30. In this manner, the deaerator 30 may warm the at least partially deaerated feed water produced in the deaerator before being delivered to the economizer 24 via the feed water supply piping 40 and the feed water pump 42.

In one embodiment, the deaerator heating supply line 56 includes a deaerator heating supply line isolation and control valve assembly for controlling the flow of steam from the auxiliary steam source 54 to the deaerator 30. For example, the deaerator heating supply line isolation and control valve assembly shown in FIG. 3 may include a first isolation valve 60, a second isolation valve 62, and a flow control valve 64 that separates the first isolation valve 60 from the second isolation valve 62. The first isolation valve 60 is fluidly connected to the auxiliary steam source 54, the second isolation valve 62 is fluidly connected to the deaerator 30, and the flow control valve 64 can control the steam supply flow from the auxiliary steam source 54 to the deaerator 30 through the deaerator heating supply line 56 when the first isolation valve and the second isolation valve are in an open state.

A boiler drum heating supply line 58, which may be in fluid communication with an auxiliary steam source 54 and the nozzle 50, supplies steam from the auxiliary steam source 54 to the nozzle 50. In this case, the nozzle 50 will heat the at least partially deaerated feed water in the economizer outlet feed water line 52 before delivering it to the boiler drum 18.

In one embodiment, the boiler drum heating supply line 58 includes a boiler drum heating supply line isolation and control valve assembly for controlling the flow of steam from the auxiliary steam source 54 to the nozzle 50. For example, the drum heating supply line isolation and control valve assembly shown in FIG3 may include a first isolation valve 66, a second isolation valve 68, and a flow control valve 70 that separates the first isolation valve from the second isolation valve. The first isolation valve 66 is in fluid communication with the auxiliary steam source 54, the second isolation valve 68 is in fluid communication with the nozzle 50, and the flow control valve 70 can control the steam supply flow from the auxiliary steam source 54 to the nozzle 50 through the boiler drum heating supply line 58 when the first isolation valve and the second isolation valve are in an open state.

The system 180 of FIG3 may further include a controller 72 operatively coupled to the subcritical steam generator 182, the deaerator 30, the nozzle 50, and the auxiliary steam source 54. In one embodiment, the controller 72 may control filling of the subcritical steam generator 182 with at least partially deaerated feedwater and supply of steam from the auxiliary steam source 54 to the deaerator 30 and the nozzle 50 as a function of a plurality of temperature and, optionally, pressure measurements obtained with respect to the steam generator 182 and the nozzle 50. Specifically, the controller 72 can control the supply of steam from the auxiliary steam source 54 to the deaerator 30 and the nozzle 50 to heat the at least partially deaerated feed water so that the boiler drum 18, the boiler drum downcomer 20, the water-cooled wall inlet tube header 22, and the water-cooled wall tube 16 are heated at a predetermined gradient temperature increase level; keep the operation of the subcritical steam generator away from brittle conditions that cause embrittlement of these steam generating components and away from excessive thermal stress conditions that are harmful to these components.

Temperature and pressure measurements used by controller 72 to control the filling of subcritical steam generator 182 with at least partially deaerated feedwater and the steam supply from auxiliary steam source 54 to deaerator 30 and nozzle 50 can be obtained by a plurality of temperature and pressure sensors located about the steam generator. Examples of sensors that can obtain temperature and pressure measurements and provide such measurements to controller 72 are shown in FIG3. Specifically, boiler drum temperature sensor 74 can obtain the temperature about the outer wall of boiler drum 18. Nozzle temperature sensor 76 can obtain the temperature of at least partially deaerated feedwater discharged from nozzle 50. Water wall temperature sensor 78 can obtain the temperature of the furnace water wall tubes 16. A furnace outlet gas temperature sensor 80 may obtain the exhaust gas temperature in the furnace 12. An air draft temperature sensor 82 may obtain the air draft temperature in the flue gas duct 84 of the subcritical steam generator 182. A boiler drum pressure sensor 88 may obtain the pressure of the boiler drum 18. Details of how the controller 72 may use these measurements to control the steam supply from the auxiliary steam source 54 to the deaerator 30 and the nozzle 50 to warm the at least partially deaerated feed water through the various valves disclosed herein are discussed below.

The above list of temperature and pressure sensors is not intended to be exhaustive, as it is contemplated that other sensors may be utilized. For example, temperature sensors may be located around the boiler drum downcomers 20, the inlet header 22, the economizer 24, the deaerator 30, and one or more of the various piping and lines disclosed herein. In addition to temperature sensors, other types of sensors may be deployed in communication with the controller 72 to detect any of a number of conditions. For example, a non-limiting list of other sensors that may be suitable for use with the system 180 and the subcritical steam generator 182 may include additional pressure sensors, flow sensors, and humidity sensors.

The insulation aspect of the system 180 may be facilitated by extracting the deaerated feedwater to be supplied to the water wall tubes 16 and forwarding the extracted feedwater for recovery and recycling back to the deaerator 30 and subsequently to the economizer 24 of the subcritical steam generator 182, and to the auxiliary steam source 54 for reheating the deaerator 30 and/or the nozzles 50. As shown in FIG3 , the drum warming return line 90 may extract water from one or more of the drum downcomers 20 and the water wall inlet header 22. In one embodiment, excess water from the drum downcomers 20 and the water wall inlet header 22 may be obtained via corresponding pairs of isolation valves 93. Specifically, the pair of isolation valves 93 can be controlled to allow excess at least partially deaerated feedwater in the boiler drum downcomer 20 and the water wall inlet header 22 to flow to the boiler drum warming return line 90 for recovery and recycling. Alternatively, if it is not desired to use the deaerated feedwater for recovery and recycling, the isolation valve 92 can be controlled to allow excess feedwater to be discharged through the discharge line 94.

3 shows that the condenser 96 can receive the recycled at least partially deaerated feed water in the boiler drum warming return line 90. Specifically, a boiler drum warming return line isolation and control valve assembly in fluid communication with the boiler drum warming return line 90 and the condenser 96 can control the flow of the at least partially deaerated feed water to the condenser 96. The boiler drum warming return line isolation and control valve assembly can include a first isolation valve 98, a second isolation valve 100, and a flow control valve 102 separating the first isolation valve from the second isolation valve. As shown in Fig. 3, the first isolation valve 98 is in fluid communication with the boiler drum downcomer 20 and the water wall inlet header 22 via the pair of isolation valves 93 (when in an open state), the second isolation valve 100 is in fluid communication with the condenser 96, and when the first isolation valve 98 and the second isolation valve 100 and the pair of isolation valves 93 are in an open state, the flow control valve 102 controls the flow of the extracted water in the boiler drum warming return line 90 to the condenser 96. It should be noted that the isolation valve 93 in Fig. 3 is redundant with the first isolation valve 98 in the boiler drum warming return line 90 and is used only for the purpose of isolating the return line 90.

The condenser may cool exhaust steam from the turbine 104. As shown in FIG3 , the turbine may include a high pressure section 106, an intermediate pressure section 108, and a low pressure section 110. In operation, the high pressure section 106 may receive superheated steam from a superheater (not shown) in the subcritical steam generator 182 via an inlet stop valve 112. The high pressure section 106 expands and cools the steam to drive a rotating shaft (not shown) that drives a generator (not shown) to generate electricity or provide heat for other purposes. The expanded steam from the high pressure section 106 of the turbine 104 may then be returned to a reheater (not shown) in the subcritical steam generator 182 downstream of the superheater to reheat the steam. The reheated steam is then directed to the intermediate pressure section 108 of the turbine 104 via an inlet stop valve 114. The intermediate pressure section 108 expands and cools the reheated steam and directs it to the low pressure section 110, where the steam is continuously expanded and cooled to further drive the rotating shaft. It should be understood that for the purpose of clarity, this description of the turbine 104 does not contain all operational details. In addition, it should be understood that the turbine 104 depicted and described with reference to FIG. 3 is merely illustrative of one possible turbine configuration and is not intended to be limiting to the various embodiments of the present invention.

Returning the discussion to the system 180, the condenser 96 can cool exhaust steam from the low pressure section 110 of the turbine 104, collect the latent heat of the steam, and condense the steam into water. In one embodiment, the condenser 96 is in fluid communication with the deaerator 30, the auxiliary steam source 54, and the boiler drum warming return line 90. In this way, the condenser 96 can supply the extracted water in the boiler drum warming return line 90 to one or more of the deaerator 30 and the auxiliary steam source 54. In addition, the condenser 96 can supply condensed water from the low pressure section 110 (once the turbine 104 is started and running) to one or more of the deaerator 30 and the auxiliary steam source 54.

3 shows an embodiment in which a condensate supply line 116 may be in fluid communication with the deaerator 30 and the condenser 96. In this manner, the condensate supply line 116 may supply the deaerator 30 with recovered at least partially deaerated feed water received by the condenser 96 from the drum warming return line 90, as well as condensate from the low pressure section 110 (once the turbine 104 is started and running). In one embodiment, the condensate supply line 116 may include a condensate pump 118 for supplying the extracted at least partially deaerated water and condensate from the condenser 96 to the deaerator 30.

In one embodiment, as will be appreciated by those skilled in the art, when the turbine 104 is started and running, the water supply from the condenser 96 to the deaerator 30 may be preheated by the low pressure heater 120 prior to entering the deaerator. For example, the low pressure heater 120 may receive exhaust steam from the low pressure section 110 of the turbine carried by a corresponding low pressure exhaust line 122. In this manner, the low pressure heater 120 may use the low pressure exhaust steam carried along the low pressure exhaust line 122 to heat the low pressure heater 120 so that heat may be applied to the water in the condensate supply line 116 so that the water is preheated prior to deaeration in the deaerator 30.

As will be appreciated by those of ordinary skill in the art, exhaust from other sections of the turbine 104 may be used to heat other components of the system 180 once the turbine is up and running. In one embodiment, the at least partially deaerated feedwater supplied to the economizer 24 of the subcritical steam generator 182 may be heated by the high pressure heater 124. In one embodiment, the high pressure heater 124 may be located about the feedwater supply piping 40 downstream of the feedwater pump 42 and upstream of the economizer 24. The high pressure heater 124 may receive exhaust steam from the medium pressure section 108 of the turbine 104 carried by a corresponding medium pressure exhaust line 126. In this way, exhaust steam carried along the medium pressure exhaust line 126 can heat the high pressure heater 124 so that heat can be applied to the at least partially deaerated feed water in the feed water supply piping 40 so that the deaerated feed water is preheated before entering the economizer 24.

As mentioned above, the condenser 96 can supply the extracted water in the drum warming return line 90 and the condensed water from the turbine 104 to the auxiliary steam source 54. In one embodiment, the drum warming return line condensate pump 128 can transfer one or more of the extracted water and the condensed water in the condenser 96 to the auxiliary steam source 54 along the drum warming return line 90. In this case, the auxiliary steam source 54 can use the fluid provided by the condenser 96 to generate steam. As mentioned above, the auxiliary steam source 54 can be directed to the deaerator 30 along the deaerator heating supply line 56 via the deaerator heating supply line isolation and control valve assembly (valves 60, 62, 64), and/or directed to the nozzle 50 along the boiler drum heating supply line 58 via the drum heating supply line isolation and control valve assembly (valves 66, 68, 70).

The system 180 may include additional components to facilitate the warming and at least partially deaerated water filling operations to facilitate rapid startup of the subcritical steam generator 48. For example, as shown in FIG. 3, the economizer vent valve 130 may be configured to displace any oxygen during the at least partially deaerated water filling of the economizer 24, or to overflow excess at least partially deaerated water from the economizer 24 via the economizer vent line 132. In one embodiment, the economizer vent valve 130 and the economizer vent line 132 may be in fluid communication with the economizer outlet feed water line 52 and the jet 50. In this way, the economizer vent valve 130 and the economizer vent line 132 may displace any oxygen during the at least partially deaerated water filling of the economizer 24, or remove any excess at least partially deaerated feed water overflowing the economizer 24. In an embodiment, the economizer vent valve 130 can be used to prevent a vacuum in the economizer tube caused by the outflow of water.

FIG3 shows that the system 180 may further include a boiler drum vent valve 134 for releasing air from the boiler drum 18 via an air release line 136. In one embodiment, the boiler drum vent valve 134 and the air release line 136 may be in fluid communication with the boiler drum 18. In this case, the boiler drum vent valve 134 may be configured to release air from the boiler drum 18 via the air release line 136 during the filling of the subcritical steam generator 182 with the at least partially deaerated feedwater. In an embodiment, the boiler drum vent valve 134 may be used to prevent a vacuum in the water wall tubes due to the outflow of water.

As will be appreciated by those of ordinary skill in the art, the system 180 may further include a boiler blowdown valve 138 for releasing undesirable deposits from the boiler drum 18 via a blowdown discharge line 140. Undesirable deposits, such as, for example, salts, must be removed from the at least partially deaerated feedwater in order to achieve a predetermined level of steam purity required by the turbine 104 and for operating the steam generator 182. In one embodiment, the boiler drum blowdown valve 138 and the blowdown discharge line 140 are in fluid communication with the boiler drum 18. In this case, the boiler drum blowdown valve 138 and blowdown drain line 140 can drain undesirable deposits in the at least partially deaerated feedwater in the boiler drum 18 during the filling of the subcritical steam generator 182. The rapid startup achieved in various embodiments is facilitated by the desalination process performed around the boiler early in the water filling process in conjunction with the heating of the feedwater by the second auxiliary heating device 55b, specifically the nozzle 50, to remove undesirable deposits such as salt from the at least partially deaerated feedwater.

3 also shows that the system 180 may include an economizer warm-up assist line 142 for accelerating the warming of the at least partially deaerated feedwater during the filling of the subcritical steam generator 182. In one embodiment, the economizer warm-up assist line 142 may be in fluid communication with the boiler drum downcomer 20 and the economizer 24 via an economizer warm-up assist line control valve 144. In this manner, the economizer warm-up assist line 142 and the economizer warm-up assist line control valve 144 may be used to recirculate the at least partially deaerated feedwater in the boiler drum downcomer 20 to warm up the feedwater and the economizer 24 if the feedwater is determined to be too cold.

In order to utilize the system 180 for rapid startup to facilitate insulation and at least partially degassed water filling in the subcritical steam generator 182 without subjecting the critical steam generating components to brittle conditions and other adverse conditions that may result in excessive thermal stress when the steam generator is ignited, the controller 72 must interact with and manage many of the aforementioned components, such as, for example, valves, sensors, pumps, etc. Therefore, the controller 72 may include the necessary electronics, software, memory, storage, databases, firmware, logic/state machines, microprocessors, communication links, displays or other visual or auditory user interfaces, printing devices, and any other input/output interfaces to perform the functions described herein and/or to achieve the results described herein, which may be accomplished in real time. For example, the controller 72 may include at least one processor and a system memory/data storage structure, which may include random access memory (RAM) and read-only memory (ROM). The at least one processor of the controller 72 may include one or more conventional microprocessors and one or more additional co-processors (such as a mathematical co-processor or the like). The data storage structure may include an appropriate combination of magnetic, optical, and/or semiconductor memory and may include, for example, RAM, ROM, a flash drive, an optical disk (such as a CD), and/or a hard disk or hard drive.

In addition, a software application that adapts the controller 72 to perform the operations disclosed herein may be read from a computer-readable medium into the main memory of at least one processor. As used herein, the term "computer-readable medium" refers to any medium that provides or participates in providing instructions to at least one processor of the controller 72 (or any other processor of the device described herein) for execution. Such a medium may take many forms, including but not limited to non-volatile media and volatile media. Non-volatile media include, for example, optical, magnetic, or magneto-optical disks, such as memory. Volatile media include dynamic random-access memory (DRAM), which generally constitutes main memory. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, a magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, a RAM, a PROM, an EPROM or EEPROM (electronically erasable programmable read-only memory), a FLASH-EEPROM, any other memory chip or memory cartridge, or any other medium from which a computer can read.

Although in the embodiment, the execution of instruction sequences in the software application causes at least one processor to perform the method/process described herein, hard-wired circuitry may be used instead of or in combination with software instructions to perform the method/process of the present invention. Therefore, embodiments of the present invention are not limited to any specific combination of hardware and/or software.

As mentioned above, the controller 72 of the system 180 is configured to control the insulation and at least partially degassed water fill to facilitate rapid startup of the subcritical steam generator 182 without delay so that the steam generating components do not operate in a fragility condition or other adverse conditions that could cause excessive thermal stress when the steam generator ignites.

If the water temperature during filling is below a certain critical value, the temperature of the exterior of the boiler drum 18 must exceed the minimum required temperature to remain out of the fragile region and in the unlimited filling rate region. As the temperature of the feed water increases during filling, the temperature of the boiler drum 18 must increase at a gradual rate to remain out of the fragile region and in the unlimited filling rate region.

Utilizing the known temperature relationship of the water fill temperature to the metal surface temperature, the controller 72 can be configured to control the at least partially degassed water fill and insulation of the subcritical steam generator 48 to a rapid start-up that keeps the stress critical steam generating components away from brittle conditions or other adverse conditions that could result in excessive thermal stress when the steam generator is ignited. FIG. 4 illustrates a start-up sequence for a subcritical steam generator 182 using the system 180. Specifically, FIG. 4 shows a flow chart 146 describing the preparatory operations associated with the system 180 depicted in FIG. 3 and the temperature relationships that serve as the basis for controlling such operations to avoid brittle and excessive thermal stress conditions.

Typically, the subcritical steam generator 182 at this time is in a cold standby mode in an unfired state, so that the steam generator is almost at ambient temperature. For example, if the temperature is close to freezing, the temperature of the fill water in the subcritical steam generator 182 in the unfired state can be in the fragile region. The at least partially degassed water filling and holding operation implemented by the preparation process of Figure 4 keeps the subcritical steam generator 182 away from the fragile region and within the allowable thermal stress region as the steam generator transitions from the unfired standby mode to the firing mode.

The start-up operation depicted in the flow chart 146 of Figure 4 begins at 148, where steam purge to the deaerator is initiated. Specifically, the controller 72 can adjust the valves (60, 62, 64) of the deaerator heating supply line isolation and control valve assembly steam to direct steam from the auxiliary steam source 54 to the deaerator 30 via the deaerator heating supply line 56.

To supplement the heating of the deaerator 30 with steam provided by the auxiliary steam source 54, the condensate pump 118 is started at 150. In this way, the condensate pump 118 will supply condensate from the condenser 96 to the deaerator 30 via the condensate supply line 116. The water level of the deaerator 30 can be controlled by leaking the supply of condensate at the condensate pump 118 (leakage not shown).

The at least partially deaerated feedwater produced in the deaerator 30 may be directed to the economizer 24 of the subcritical steam generator 182 at 152. Specifically, the feedwater pump 42 is started and the at least partially deaerated feedwater in the deaerator 30 may be supplied to the economizer 24 via the feedwater supply piping 40. In this case, the at least partially deaerated feedwater, which may be about 21° C. (70° F.) at the economizer 24, will begin to fill and warm the economizer 24. The water level of the economizer 24 may be controlled by leaking the supply of at least partially deaerated feedwater at the feedwater pump 42 (leakage not shown).

Filling of the at least partially deaerated feedwater in the economizer 24 continues until the feedwater overflows the top of the economizer 24, as described in 154. Generally, the economizer 24 is not considered a stress critical component that is subject to fragility conditions and excessive thermal stress conditions, and therefore the at least partially deaerated feedwater can be filled in the economizer at an unlimited filling rate. When the at least partially deaerated feedwater overflows the top of the economizer 24, the excess feedwater will flow toward the economizer vent line 132 through the economizer vent valve 130.

At this point, the temperature of the metal of the steam generating components of the subcritical steam generator 182 is relatively cold. In order for the subcritical steam generator 182 to operate at an unlimited fill rate and away from a brittle condition, an appropriate temperature differential must be maintained between the temperature of the outer wall of the boiler drum 18 and the temperature of the fill water. As a result, the flow chart operation directs warming to the steam generating components of the subcritical steam generator 182, such as the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, and the water wall tubes 16. Specifically, steam from the auxiliary steam source 54 is directed to the nozzle 50 at 156. In this case, the controller 72 may adjust the valves (66, 68, 70) of the drum heating supply line isolation and control valve assembly to direct steam from the auxiliary steam source 54 to the nozzle 50 via the boiler drum heating supply line 58.

To ensure that the subcritical steam generator 182 can operate at an unlimited fill rate, the controller 72 can control the temperature of the stress critical steam generating components (such as the boiler drum 18 and the water wall tubes 16) according to predetermined specified limits, at 158. For example, the controller can obtain the temperature at the discharge from the nozzle 50 via the nozzle temperature sensor 76, the temperature of the outer wall of the boiler drum 18 via the boiler drum temperature sensor 74, and the temperature of the water wall tubes 16 via the water wall temperature sensor 78. By knowing the acceptable temperature difference between the fill water temperature and the boiler drum outer surface temperature, the controller 72 can then control the heating of the at least partially deaerated feed water as it enters the boiler drum 18 through the nozzles 50 or any other similar heating element so as to avoid subjecting stress critical components such as the boiler drum and water wall tubes 16 to excessive stress. Specifically, the controller 72 can adjust the amount of steam directed from the auxiliary steam source 54 to obtain the desired temperature difference.

The flow chart 146 may continue at 160, where the warmed, at least partially deaerated feedwater is passed from the boiler drum 18 to fill the boiler drum downcomer 20. As previously mentioned, although FIG. 3 shows the boiler drum downcomer 20 as a single downcomer, it should be understood that the downcomer may include more than one. Regardless, during the filling of the boiler drum downcomer 20, the controller 72 may monitor the temperature of the outer wall of the boiler drum 18, and if necessary, the flow control valve 70 of the drum warming supply line isolation and control valve assembly may be adjusted to achieve the desired temperature. During the filling of the boiler drum downcomers 20, the water-cooled wall tubes 16 of the subcritical steam generator 182 are filled with warmed, at least partially deaerated feed water via the water-cooled wall inlet header 22 at 162.

The at least partially deaerated feedwater will eventually fill the water wall tubes 16 and downcomers 20. The water level in the boiler drum 18 will then rise and will eventually reach the nominal level as water flows back into the boiler drum via the top piping 26 and the drum warming supply line 58 and economizer outlet feedwater line 52. The controller 72 can then obtain temperature measurements of the outer wall of the boiler drum 18 and the water wall tubes 16 to gain an understanding of how far along in the filling process it is toward getting the subcritical steam generator 48 ready to begin firing for light-off. Once the filling of the water wall tubes 16 is complete and the nominal water level in the boiler drum 18 has been reached, the controller 72 can monitor the pressure of the boiler drum 18 via the boiler drum pressure sensor 88 and limit the air release from the boiler drum 18 to the air release line 136 via the boiler drum vent valve 134. In one embodiment, the controller 72 can monitor the boiler drum pressure to control in a manner so that the pressure does not exceed about 80% of the outlet pressure of the auxiliary steam heat source 54. Additionally, during this time, sediment from the heated, at least partially deaerated feedwater, such as salts, may be removed from the boiler drum 18 on the blowdown discharge line 140 via the boiler drum blowdown valve 138 to achieve a predetermined steam purity in the steam leaving the boiler drum 18.

With feed water at nominal levels in the economizer 24, boiler drum 18, and water wall tubes 16, excess water may be recovered and recycled (at 164), assuming the water is deemed to be in good condition for recovery and recycling. Otherwise, excess water will be discharged to drain line 94 or through blowdown drain line 140. In any case, the recovered and recycled water will be directed to the condenser 96 and auxiliary steam source 54 via the boiler drum warming return line isolation and control valve assembly boiler drum warming return line 90 and valves (98, 100, 102). The auxiliary steam source 54 may use the recovered water to generate steam, which is directed to the nozzle 50 via the boiler drum warming supply line 58, at 166. In this manner, the nozzles 50 will warm the at least partially deaerated feedwater entering the boiler drum 18 and thereby further heat the steam generating components, such as the boiler drum 18, the boiler drum downcomers 20, the water wall inlet header 22, and the water wall tubes 16. Specifically, the controller 72 may obtain the aforementioned temperature and pressure measurements to ensure that the mixed temperature complies with the specified boiler drum temperature limits.

While the mix temperature is being raised and brought into compliance with the specified drum temperature limits, the controller 72 will also control the mix temperature to remain below the saturation temperature calculated by the measured drum pressure obtained from the drum pressure sensor 88. If the water level is stable at the nominal level and it is determined that the discharge temperature at the outlet of the nozzle 50 has reached the saturation temperature calculated from the drum pressure, the controller 72 may close the drum vent valve 134.

Recovery and recirculation of feed water back to the subcritical steam generator 182 continues at 168 until the desired temperature is reached at the boiler drum 18 and water wall tubes 16. Once the boiler drum 18 and water wall tubes 16 have reached the desired temperature, control of the nozzles 50 may be removed and ignition may begin, at 170. In essence, the start-up conditions have been reached and therefore the firing of the subcritical steam generator 182 may be ignited.

With the start-up, the boiler purge will begin. During the boiler purge, the controller 72 will reduce and eventually shut off the recovery and recirculation of feed water, at 172. Specifically, the controller 72 will adjust valves (98, 100, 102) so that there is no flow along the boiler drum warming return line 90. Additionally, during this time, the controller 72 will reduce and eventually shut off the steam supply from the auxiliary steam source 54 to the nozzles, at 174, with the previously achieved or desired boiler drum outer wall temperature. Specifically, the controller 72 will adjust valves (66, 68, 70) so that there is no flow along the boiler drum warming supply line 58. The auxiliary steam source 54 will continue to supply steam to the deaerator along the deaerator warming supply line 56 at 176. In this way, the deaerator 30 can continue to deaerate and warm the feed water to the economizer 24.

During the boiler firing period, the controller 72 will monitor the furnace gas outlet temperature obtained by the furnace outlet gas temperature sensor 80, because there is no flow leaving the superheater and reheater in the subcritical steam generator 182 at this time. If the furnace outlet gas is lower than the predetermined temperature, the controller 72 can allow the temperature of the water-cooled wall tube 16 and the boiler drum pressure to increase by a significant amount to allow the discharge of the steam to be taken to the superheater so that the piping to the turbine 104, and specifically, the high pressure section 106 and the medium pressure section 108 can be heated. In this case, the steam flow can be directed to the high pressure section 106 and brought back to the reheater for subsequent flow to the medium pressure section 108 of the turbine 104. The startup sequence is now complete and all that is necessary is to wait for the metal of all the various components to heat up before allowing steam from the subcritical steam generator 182 to the turbine 104, at 178.

Although for the purpose of easy explanation, the operation shown in Figure 4 is described as a series of actions. It is necessary to understand and appreciate that the innovation of this subject associated with Figure 4 is not limited by the order of actions, because some actions can occur in different orders and/or occur simultaneously with other actions from those shown and described herein. For example, a person with ordinary knowledge in the art will understand and appreciate that the methodology or operation described by Figure 4 can be alternatively represented as a series of related states or events in a state diagram, such as a state diagram. In addition, it is not necessary for all the actions to be drawn to implement the methodology according to this innovation. Furthermore, when different entities formulate different parts of the methodology, (multiple) interaction diagrams can represent the methodology or method according to this disclosure. Further, two or more of the disclosed example methods can be combined with each other to complete one or more features or advantages described herein.

Although flow chart 146 has been described with respect to the aforementioned components of system 180 depicted in FIG. 3 , it should be understood that other configurations may be utilized that provide for at least partially degassed filling and insulation to achieve rapid startup of a subcritical steam generator while ensuring that the steam generating components are not subjected to brittle conditions and other adverse conditions that could result in excessive thermal stress when the steam generator is ignited.

It should be understood that the configuration depicted and described with respect to the subcritical steam generator 182 shown in FIG. 3 is not exhaustive of all possible systems for getting a subcritical steam generator ready and is not intended to be limiting. For example, FIG. 5 shows a schematic diagram of a system 184 for getting a subcritical steam generator 186 ready for rapid startup according to another embodiment of the present invention. In this embodiment, the system 184 does not contain a recovery and recirculation loop as utilized in the system 180 of FIG. 3. In this case, the system 184 would operate in a similar manner to the system 180, except for the recovery and recirculation of feed water from the subcritical steam generator.

As mentioned above, a supercritical steam generator is an example of another type of steam generator, making the aforementioned aspects of getting the steam generator ready with a degassed and (pre) heated water charge for rapid startup applicable. FIG. 6 shows a schematic diagram of a system 188 for getting a supercritical steam generator 190 ready for rapid startup, including a high pressure startup system 191 and a low pressure startup system 193. Instead of a boiler drum, the supercritical steam generator 190 uses one or more separators 192 (hereinafter referred to as "separators 192") to separate the saturated water produced in the water-cooled wall tubes 16 into water and steam. A separator storage tank (SST) 194 stores the water separated by the separator 192. An isolation and control valve assembly in fluid communication with the economizer 24 via a minimum economizer flow return line 196 can supply water from the separator storage tank 194 to the economizer 24. The isolation and control valve assembly can include a first isolation valve 198, a second isolation valve 200, and a minimum economizer flow control valve 202 disposed between the isolation valves 198 and 200.

In one embodiment, a boiler water circulation pump (BWCP) 204 may supply water from the separator storage tank 194 to the economizer 24 via an isolation and control valve assembly formed by valves 198, 200, 202 and a minimum economizer flow return line 196. This water and the at least partially deaerated feed water provided by the deaerator 30 are warmed by the economizer 24. The warmed water and the at least partially deaerated feed water pass from the economizer 24 to the water wall tubes 16 via the economizer outlet feed water line 52, one or more downcomers 206 (collectively referred to as "downcomers 206"), and the water wall inlet header 22.

The system 188 may also include an insulation aspect that involves extracting the deaerated feed water to be supplied to the water wall tubes 16 and transferring the extracted feed water for recovery and recycling back to the deaerator 30 and subsequently to the economizer 24 of the supercritical steam generator 190, and to the auxiliary steam source 54 for reheating the deaerator 30. As shown in Figure 6, the boiler flash tank (BFT) 208 may receive water from the separator storage tank (SST) 194 via an isolation and control valve assembly including an isolation valve 210 and a high water level control valve 212. In one embodiment, excess water from the boiler flash tank 208 may be provided to the flash tank drain tank (FTDT) 214 via a condensate return line 216. A boiler expansion vessel (BFT) can be used as a container in which steam or water at any pressure can expand to a liquid under atmospheric pressure and form condensate. A boiler expansion vessel (BFT) can also be used to store excess water from the SST.

As shown in FIG6 , the boiler condensate pump 218 can supply the extracted feed water in the expansion vessel drain 214 to the condenser 96 along the condensate return line 216. In one embodiment, an isolation and control valve assembly formed by a first isolation valve 220, a second isolation valve 222, and a control valve 224 disposed between the isolation valves can be used to facilitate the extraction of the feed water from the expansion vessel drain 214 to the condenser 96 along the condensate return line 216. The condenser 96 can supply the extracted feed water back to the deaerator 30 via the condensate pump 118 and the low pressure heater 120 or via the auxiliary steam source 54 and the deaerator warming supply line 56.

If it is not desired to use the deaerated feed water for recovery and recycling, excess water in the boiler expansion vessel 208 can be discharged from the supercritical steam generator 190. For example, in one embodiment, the excess water can be directed by the controller 72 to discharge the water through the boiler expansion vessel drain line 226 and one or more boiler expansion vessel drain isolation valves 228.

As an option, Figure 6 shows that the system 188 can divert some or all of the water from the separator 192 to be supplied to the economizer 24 via the minimum economizer flow return line 196 to go directly to the water wall inlet header 22 to be supplied to the water wall tubes 16. For example, the minimum water wall flow return line 230 can provide the diverted water from the minimum economizer flow return line 196 to the water wall inlet header 22 via an isolation and control valve assembly including a first isolation valve 232, a second isolation valve 234, and a bypass flow control valve 236 in fluid communication with the isolation valves. By introducing bypass line 230, warm-up of separator 192 can be accelerated by passing heated liquid directly to the water walls without heat loss and operating the water walls just below the critical point, thereby avoiding the "deadbeat" effect, i.e., avoiding evaporation of water in selected tubes.

The system 188 may include additional components to facilitate the warm-up and at least partially deaerated water filling operations to facilitate rapid startup of the supercritical steam generator 190. For example, as shown in Figure 6, the water wall vent valve 238 and the water wall vent line 240 can release air from the outlet header of the water wall tubes (i.e., the furnace top end pipe 26). The water wall vent valve 238 and the water wall vent line 240 allow the system 188 to release air from the water wall tubes 16 during the filling of the supercritical steam generator 190 with at least partially deaerated feed water.

As with the subcritical steam generators described above, the system 188 associated with the supercritical steam generator 190 may include a plurality of auxiliary heating devices 55 (e.g., 55a-55d) for warming stress critical steam generating components and/or (pre)heating at least partially deaerated feed water that flows to or from the components during the filling period of the water filling circuit. As shown in FIG. 6 , for example, a first auxiliary heating device 55a may be disposed on one or more water-steam separators 192. A second auxiliary heating device 55b may be configured to (pre)heat at least partially deaerated feed water discharged from the water wall prior to the inlet of the separator 192. In one embodiment, the second auxiliary heating device 55b may be disposed around the water wall outlet header (e.g., furnace top piping 26). The third auxiliary heating device 55c may be configured to provide heat to the deaerator 30 to maintain the deaerator at the desired conditions at the mass flow provided by the feedwater pump 42. The fourth auxiliary heating device 55d may be disposed around the economizer 24 to protect the economizer from freezing. The first, second, third and fourth auxiliary heating devices 55 may include any of the aforementioned heating devices.

With the system 188 for preparing the supercritical steam generator 190, the following method may be applied. For example, during the shutdown of the supercritical steam generator 190, the separator 192 is kept filled with water and surrounded by nitrogen, and/or the deaerator 30 is sealed with the third auxiliary heating device 55c, and the economizer 24 is sealed with the fourth auxiliary heating device 55d, so that gas diffusion is limited to the water-air interface when deaeration is maintained by the sealed water. Furthermore, the separator 192 and the water-cooled wall outlet header are insulated above their fragility condition by the first auxiliary heating device 55a and the second auxiliary heating device 55b.

At start-up of the supercritical steam generator 190, the deaerator 30 may be heated to operating conditions to at least partially remove dissolved gases before the feedwater pump 42 is started and begins to provide feedwater to the economizer 24. A second auxiliary heating device 55b may be used to (pre)heat the discharged, deaerated feedwater discharged from the cold water wall before filling the separator 192. The (pre)heated water entering the separator 192 will fill the separator. This (pre)heated water may be used to flush contaminated water in the boiler recirculation system. When the separator 192 is filled with deaerated feedwater, mass flow modulation of the boiler recirculation pump 218 may be initiated to heat the water walls long before the furnace 12 is purged and the auxiliary fuel burners are fired.

In one embodiment, filling the supercritical steam generator 190 with feedwater, (pre)heating the at least partially deaerated feedwater with the fifth auxiliary heating device 55b, heating the separator with the first auxiliary heating device 55a, and the auxiliary fuel burner are controlled by the controller 72 based on temperature measurements obtained with respect to the steam generator and the auxiliary heating device 55. In this case, the controller 72 can control the inlet temperature of the separator feedwater and the outlet temperature of the water wall at a safe temperature rise rate and temperature differences of components such as the separator 192, the water wall inlet header 22, or the water wall tubes 16. With thick wall components insulated in a ductile condition, the safe temperature rise rate of thin wall components such as water walls is allowed to bring all components to a ductile condition in a short time. With all components in a ductile condition, these thick wall components will limit the safe temperature rise rate.

The above method of the supercritical steam generator 190 can be implemented to operate during the startup operation of the generator. Generally, the startup operation of the supercritical steam generator 190 under cold start conditions can be described as follows. Prior to the ignition of the supercritical steam generator 190, the following verification steps are completed: ●      The deaerator 30 is operational and deaerated feed water at a predetermined temperature is available therein. ●      The boiler water recirculation pump (BWCP) 204 has been checked, the isolation valves 198, 200 are open, and all pump instruments are available for use. ●      The supercritical steam generator startup system valve is ready for operation (202, 212), wherein the isolation valves 198, 200, and 210 are open. SST 194 level control using HWL valve 212 is checked and available for use. ●      Start-up system drain transfer system including boiler expansion vessel (BFT) 208, expansion vessel drain tank (FTDT) 214, and drain transfer pump(s) 218. ●      Auxiliary steam 54 may be obtained from another operating boiler or a common system for supplying the deaerator 30, and feedwater tank. ●      Temperature probe 80 or other device for measuring furnace gas outlet temperature is in working order and available for use.

For warm water filling of the supercritical steam generator, the entire economizer 24, water wall tubes 16, and separator 192 must be filled with warm deaerated water (e.g., 104°C (219°F)) and free of air. To ensure that the water system is free of air, the following procedures are performed: ●      Economizer vent valve 130 and water wall vent valve 238 are open. ●      Boiler feed pump (BFP) 42 is started at minimum setting and flow according to the predetermined feed pump operation procedure. ●      The drain transfer pump(s) between the expansion vessel drain tank (FTDT) 214 and the condenser 96 are switched to automatic control in the start position. ●      If the boiler water system is empty (economizer, furnace wall, separator), the system will be filled with approximately 10% BMCR feedwater flow (BMCR = boiler maximum capacity rating). The feedwater flow is preferably controlled using an automatic feedwater control with a set point of 10% BMCR. ●      As soon as clear water vapor is discharged or the level accumulates in the separator storage tank (SST) 194, the economizer vent valve 130 and the water wall vent valve 238 are closed. ●      When the level in the SST 194 reaches the high water level set point, the HWL valve 212 will start to open. Increase boiler feedwater flow to an equivalent of 30% BMCR feedwater flow and ensure HWL valve 212 is >30% open for more than two minutes. The water system is considered intact when: a.     The SST water level remains stable for 2 minutes at a feedwater flow of 30% BMCR feedwater flow; and b.     HWL valve 212 has actively limited the SST 194 level for more than 2 minutes. ●      After filling the water system, the feedwater flow to the boiler can be reduced to 0% (BFP 42 can be maintained on minimum flow recirculation, recirculation line not shown). During the boiler filling period, the water level in the deaerator 30 and the water temperature of 104℃ (219℉) must be maintained.

In one embodiment of the supercritical steam generator 190, pre-boiler water recirculation for the purge phase may occur. Pre-boiler water recirculation for the purge phase involves performing the following procedures: ●      When the feed water quality at the outlet of the deaerator 30 and the high pressure heater 124 is not within the required limits (based on sample analysis), the pre-boiler purge recirculation is necessary. ●      During this time, a constant feed water flow of 10% BMCR feed water flow or more is maintained. ●      The deaerator 30 is pressurized with auxiliary steam from the auxiliary steam source 54, the condenser vacuum is established, and the condensate polisher (not shown in the figure) is placed into service. ●      Water is circulated through the entire pre-boiler system, from the condenser 96 hot well through the final high pressure heater 124, including the condensate polisher (not shown here), and returned to the condenser via a dedicated recirculation line (not shown here). ●      Recirculation continues until the water quality is within specified limits, based on samples taken at the deaerator 30 and high pressure heater 124.

In one embodiment of the supercritical steam generator 190, water recirculation through the boiler for the cleaning phase may occur. Water recirculation through the boiler for the cleaning phase involves performing the following procedures: ●      When the feed water quality at the outlet of the SST 194 is not within the required limits (based on sample analysis), feed water cleaning recirculation through the boiler is required. ●      After the control limit has been met at the outlet of the high pressure heater 124, the pre-boiler recirculation control valve is closed and flow is established through the economizer 24, water wall (evaporator), separator 192, and SST 194 and discharged from the boiler through HWL 212 to the boiler expansion vessel 208, expansion vessel drain tank 214 and condensate drain pump 218 and isolation and control valve assembly (valves 220, 222, 224) back to the condenser 96. ●      Water circulation continues through the entire condensate system, including the condensate polisher (not shown here), feed water system and boiler water system to remove impurities. ●      During this time, a constant feedwater flow of 10% BMCR feedwater flow or more is maintained. ●      Recirculation continues until water quality is within specified limits, based on samples taken at the SST 194 discharge.

In one embodiment of the supercritical steam generator 190, the initiation of the boiler recirculation pump phase may occur. The initiation of the boiler recirculation pump phase involves performing the following procedures: ●      Assume that the following preparations have been completed: a.     Feed water quality is within specified limits. b.     Feed water flow set point is at 10% BMCR feed water flow. c.     SST 194 water level is stable with HWL valve 212 at a stable opening. d.     Isolation valve 198 and drain valve 200 are open. ●      Set the minimum economizer flow control valve (MEFCV) 202 to the minimum (pump start) position, select MEFCV to automatic, and start BWCP 204 via a pump operation command. ●      Once the BWCP 204 is in operation, the SST 194 level will decrease as the upper circuit fills. Maintain 10% BMCR boiler feedwater flow until the SST 194 level shows a sustained increase. Monitor MEFCV 202 for automatic action to establish economizer inlet flow at nominal flow set point (approximately 35% BMCR flow). As the SST 194 level reaches the normal operating set point, reduce feedwater flow to the boiler to zero and select automatic. ●      With the BWCP 204 in operation, flow through the economizer 24 and water wall tubes 16 increases substantially. At this time, the water quality at the separator 192 can be rechecked. If necessary, continue to circulate water through the purge loop including the polishing plant until control limits are met at the separator 192 outlet prior to initial burnout.

The supercritical steam generator 190 is usually ready for start-up at this point. However, to ensure maximum safety margin during start-up, at least 30% of the full load airflow should be maintained to produce the following initial fire prevention conditions: ●      Air-rich furnace atmosphere. This prevents the accumulation of explosive mixtures due to poor or delayed ignition after the fuel is introduced into the furnace. ●      High excess air through the air heater. This minimizes dilution of the combustion air due to inert gases carried by the air heater rotor.

The supercritical steam generator 190 will typically be decoupled from turbine startup using HP and LP bypass systems (not shown) that provide a steam flow path through the superheater, to the reheater, and to the condenser 96. This provides additional flexibility for adjusting the steam temperature to match the turbine demand at startup. The primary methods for controlling steam temperature will be firing rate and airflow adjustments.

With the completion of the aforementioned procedures, the supercritical steam generator 190 is ready for ignition. The ignition of the supercritical steam generator 190 may include the following operations: ●      Start the air heater (not shown here). ●      Start the first airflow group (not shown here). ●      Start the second airflow group (not shown here). ●      Adjust the fan to allow a predetermined amount of purge airflow and a predetermined amount of furnace ventilation. ●      Check that all other purge permissions are met. ●      Put the temperature probe 80 (e.g., a heat probe) into use to measure the furnace outlet gas temperature. ●      The furnace pressure control is automatic. ●      The unit airflow is automatically maintained at 30% of the minimum unit airflow. ●      The start-up system (BWCP 204 and MEFCV valve 202) is automatically maintaining the water wall flow at the minimum flow set point. ●      Start the furnace purge. ●      After the purge cycle is completed, check that all firing prerequisites are met, including the following control settings: a.     The boiler feed water control set point is maintained at 5% to 10% BMCR flow, with the HWL valve 212 acting and limiting the SST 194 level to continuously purge solids that may have accumulated in the separator storage tank 194 during the start-up period, and thereby continuously clean the fluid in the furnace wall. If the water quality has been confirmed, this set point can be reduced to zero to reduce water losses. b.     SST 194 level control is automatic (feedwater flow controls SST 194 level at normal set point) and HWL valve(s) 212 are automatic. ●      When igniting the first level of gas or any other starting fuel, the secondary air damper in the windbox is selected as auxiliary air or fuel air control, automatically based on the fuel in use, and is automatically positioned. Once ignition of the primary fuel is established, the fuel-air damper is opened in proportion to the fuel level burn rate. ●      Place the high pressure bypass valve in automatic operation. ●      Place the low pressure bypass valve in operation. ●      Start a seal air fan. ●      Open the individual seal air valves on each pulverizer. ●      When there is sufficient seal air to the differential pressure below the pulverizer, open the air damper to provide a purge air flow path through the pulverizer. ●      Start at least one fan for pulverizer air flow. When fan(s) start is requested, the pulverizer cold air damper will be positioned to 5% open. ●      After starting the fan(s), open the fan outlet damper. Bring the main hot air duct pressure up to the set point by manually adjusting the fan flow control. Then, transfer to automatic control.

Although the operation of the above-mentioned procedure for the supercritical steam generator 190 is described as a series of actions. It is to be understood and appreciated that the innovation of the subject matter associated with such operations is not limited by the order of the actions, because some actions may occur in different orders and/or simultaneously with other actions from those described herein. For example, a person of ordinary skill in the art will understand and appreciate that the methodology or operation of the supercritical steam generator 190 may alternatively be represented as a series of related states or events. In addition, not all actions are required to implement the methodology of the innovation according to the various embodiments. Further, two or more of the disclosed example methods may be implemented in combination with each other to accomplish one or more features or advantages described herein.

From the description of the illustrative embodiments presented herein, it should be apparent that the present disclosure describes an effective solution for achieving rapid startup of subcritical and supercritical steam generators that provides at least partially degassed water fill along with insulation while ensuring that the steam generating components are not subjected to brittle conditions and other adverse conditions that can cause excessive thermal stress when the steam generator is fired, after being in an unfired standby mode of operation. The embodiments describe many novel and unique features. Such features include, but are not limited to: maintaining the steam generator above the frangibility condition at maximum water wall fill temperature, readiness of the deaerator, initiation of steam generator fill, heating of fill water upstream of the maximum stress critical components of the steam generator, continuous warming flow through closed loop coupling, early combustion of trapped air and steam blow-off in the high pressure superheater section of the steam generator, early reduction of sediment (e.g., salt) content in the feed water through bleed, and displacement of auxiliary steam through the high pressure turbine section bypass system.

These features provide numerous technical and commercial advantages to various embodiments of the present invention. For example, there will be stress-limited start-up of subcritical steam generators and supercritical steam generators. This includes a minimum waiting time to overcome a fragility condition of stress-critical components of a steam generating assembly of a steam generator (e.g., a subcritical steam generator or a supercritical steam generator) that may cause problems as the steam generator transitions from a standby, unfired operating mode to a fired operating mode. Low possible auxiliary fuel consumption and increased life use are associated with stress-limited start-up. Other advantages of various embodiments may include earlier steam turbine release, and therefore less starting fuel consumption. For turbine systems without bypass, there will be reduced sediment/maintenance.

The above description of illustrative embodiments of the present disclosure (including what is described in the Abstract) is not intended to be exhaustive or to limit the disclosed embodiments to the precise form disclosed. Although specific embodiments and examples are described herein for illustrative purposes, various modifications considered to be within the scope of such embodiments and examples are possible as would be appreciated by one of ordinary skill in the art. For example, components, assemblies, steps, and aspects from different embodiments may be combined in other embodiments or adapted for use in other embodiments even if not described in the present disclosure or depicted in the drawings. Therefore, since certain changes may be made to the above invention without departing from the spirit and scope of the invention involved herein, it is intended that all of the subject matter described above shown in the accompanying drawings should be interpreted merely as illustrating examples of the concepts of the invention herein and should not be taken as limitations on the invention.

In this regard, although the disclosed subject matter has been described with respect to various embodiments and corresponding drawings, it is understood that other similar embodiments may be used, or modifications and additions may be made to the described embodiments, to perform the same, similar, alternative, or replacement functions of the disclosed subject matter without departing therefrom, where applicable. Accordingly, the disclosed subject matter should not be limited by any single embodiment described herein, but rather should be construed in accordance with the breadth and scope of the claims appended hereto. For example, reference to "one embodiment" of the invention is not intended to be construed to exclude the existence of additional embodiments that also incorporate the recited features.

In the accompanying claims, the terms "including" and "in which" are used as the plain-English equivalents of the terms "comprising" and "comprise." In addition, in the following claims, terms such as "first," "second," "third," "upper," "lower," "bottom," "top," etc. are used merely as labels and are not intended to impose numerical or positional requirements on such objects. The terms "substantially," "generally," and "about" indicate conditions that are within reasonably achievable manufacturing and assembly tolerances relative to ideally desired conditions applicable to achieve the functional purpose of a component or assembly. Furthermore, the following claim limitations are not written in means-plus-function form and are not intended to be so construed unless and until such claim limitations expressly use the phrase "means for" followed by a recitation of function without further structure.

What has been described above is included to illustrate examples of systems and methods of the disclosed subject matter. Of course, it is not possible to describe every combination of components or methodologies here. One of ordinary skill in the art will recognize that many further combinations and permutations of the claimed subject matter are possible. Furthermore, where the terms "include," "has," "possess," and the like are used in the embodiments, claims, appendices, and drawings, such terms are intended to be inclusive in a manner similar to how the term "comprising" is interpreted when "comprising" is used as a transitional word in the claims. That is, unless explicitly stated to the contrary, embodiments that “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements that do not have that property. Furthermore, the articles “a” and “an” as used in this specification and the accompanying drawings should generally be interpreted as meaning “one or more” unless otherwise specified or clear from context to be directed to a singular form.

This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable a person of ordinary skill in the art to practice the embodiments of the invention, including making and using any device or system and performing any incorporated methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to a person of ordinary skill in the art. If such other examples do not have structural elements that differ from the literal language of the claims, or if such other examples include equivalent structural elements that are not substantially different from the literal language of the claims, then they are intended to be within the scope of the claims.

10: Subcritical steam generator 12: Furnace 14: Mixture 16: Furnace water-cooled wall tubes 18: Boiler drum 20: Boiler drum downcomer 22: Inlet header 24: Economizer 26: Furnace top piping 28: Economizer outlet feedwater piping 30: Deaerator heating system 32: Storage tank 34: Condensate pump 36: Condensate piping 38: Auxiliary steam source 40: Feedwater supply piping 42: Feedwater pump 44: Boiler drum centerline 46, 180, 184, 188: System 48, 182, 186, 190: Steam generator 50: Nozzle 52: Economizer outlet water supply pipe 54: Auxiliary steam source 55a-e: Auxiliary heating device 56: Deaerator heating supply line 58: Boiler drum heating supply line 60: First isolation valve 62: Second isolation valve 64: Flow control valve 66: First isolation valve 68: Second isolation valve 70: Flow control valve 72: Controller 74: Boiler drum temperature sensor 76: Nozzle temperature sensor 78: Water-cooled wall temperature sensor 80: Furnace outlet gas temperature sensor 82: Air flow temperature sensor 84: Flue gas duct 88: Boiler drum pressure sensor 90: Return line 92: Isolation valve 94: Discharge line 96: Condenser 98: First isolation valve 100: Second isolation valve 102: Flow control valve 104: Turbine 106: High pressure section 108: Medium pressure section 110: Low pressure section 112: Inlet stop valve 114: Inlet stop valve 116: Condensate supply line 118: Condensate pump 120: Low pressure heater 122: Low pressure discharge line 124: High pressure heater 126: Medium pressure discharge line 128: Boiler drum heating return line condensate pump 130: Economizer vent valve 132: Economizer vent line 134: Boiler drum vent valve 136: Air release line 138: Boiler blowdown valve 140: Blowdown discharge line 142 Economizer heating auxiliary line: 144: Economizer heating auxiliary line control valve 146: Flow chart 191: High pressure starting system 192: Separator 193: Low pressure starting system 194: Separator storage tank (SST) 196: Minimum economizer flow return line 198: First isolation valve 200: Second isolation valve 202: Minimum economizer flow control valve 204: Boiler water circulation pump (BWCP) 206: Downcomer 208: Boiler expansion vessel 210: Isolation valve 212: High water level control valve 214: Expansion vessel drain tank (FTDT) 216: Condensate return line 218: Boiler condensate drain pump 220: First isolation valve 222: Second isolation valve 224: Control valve 226: Boiler expansion vessel drain line 228: Boiler expansion vessel discharge isolation valve 230: Minimum water wall flow return line 232: First isolation valve 234: Second isolation valve 236: Bypass flow control valve 238: Water wall vent valve 240: Water-cooled wall pipes

The present invention will be better understood by reading the following non-limiting embodiments and referring to the following accompanying drawings herein: [Figure 1] shows a schematic diagram of a subcritical steam generator according to the prior art; [Figure 2] shows a schematic diagram of a system for preparing a subcritical steam generator for rapid startup according to one embodiment of the present invention; [Figure 3] shows a schematic diagram of a system for preparing a subcritical steam generator for rapid startup according to another embodiment of the present invention; [Figure 4] shows a flow chart of a preparatory operation associated with the system depicted in Figure 3 according to another embodiment of the present invention; [Figure 5] shows a schematic diagram of a system for preparing a subcritical steam generator for rapid startup, which is an alternative embodiment of the system depicted in Figure 3; and [Figure 6] shows a schematic diagram of a system for preparing a supercritical steam generator for rapid startup according to an embodiment of the present invention.

12: Furnace

16: Furnace water-cooled wall tubes

18: Boiler drum

20: Boiler drum downflow pipe

22: Inlet header

24: Economizer

26: Furnace top piping

30: Deaerator heating system

40: Water supply piping

42: Water supply pump

46: System

48: Steam generator

52: Economizer outlet water supply pipe

54: Auxiliary steam source

55a-e: Auxiliary heating device

56: Deaerator heating supply line

60: First isolation valve

62: Second isolation valve

64: Flow control valve

72: Controller

74: Boiler drum temperature sensor

76: Nozzle temperature sensor

78: Water-cooled wall temperature sensor

80: Furnace outlet gas temperature sensor

82: Airflow temperature sensor

84: Flue gas duct

88: Boiler drum pressure sensor

96: Condenser

104: Turbine

106: High-pressure section

108: Medium voltage section

110: Low pressure section

112: Inlet stop valve

114: Inlet stop valve

116: Condensate supply line

118: Condensate pump

120: Low pressure heater

122: Low-pressure discharge line

124: High pressure heater

126: Medium pressure discharge line

128: Boiler drum heating return line condensate pump

130:Economizer vent valve

132: Economizer ventilation line

134: Boiler drum vent valve

136: Air release line

138: Boiler drain valve

140: Sewage discharge line

142: Economizer heating auxiliary line

144: Economizer heating auxiliary line control valve

Claims (15)

A system (46, 180, 184, 188) for getting a steam generator (48, 182, 186, 190) ready, wherein the steam generator (48, 182, 186, 190) comprises: ●      at least one water-steam separator (18, 192), ●      an economizer (24), ●      an auxiliary fuel burner for warming up the steam generator (48, 182, 186, 190) using auxiliary fuel, ●      a water-wall inlet header (22), and ●      a furnace (12), wherein the furnace (12) includes water-wall tubes (16), and wherein the water-cooled wall tubes (16) are connected to the at least one water-steam separator (18, 192) by means of a furnace top pipe (26), and wherein the system (46, 180, 184, 188) comprises: ●      a first auxiliary heating device (55a) disposed on the at least one water-steam separator (18, 192) for heating the at least one water-steam separator (18, 192), and/or ●      a second auxiliary heating device (55b) disposed on at least a portion of the furnace top pipe (26) for heating the furnace top pipe (26), and wherein the system (46, 180, 184, 188) further comprises: ●     a controller (72) for controlling the steam generator (48, 182, 186, 190) and the auxiliary heating devices (55), and ●      A measurement system including a pressure and/or temperature sensor for providing pressure and/or temperature measurements, wherein the measurement system is configured to provide pressure and/or temperature measurements of the steam generator (48, 182, 186, 190) and pressure and/or temperature measurements of the system (46, 180, 184, 188) for preparing the steam generator (48, 182, 186, 190) to the controller (72). The system (46, 180, 184, 188) of claim 1, wherein the steam generator (48, 182, 186, 190) further comprises: ●      a heating system (30) for providing at least partially degassed water to the economizer (24), and ●      a condenser (96) connected to the heating system (30) using a condensate supply line (116) for providing condensate to the heating system (30), wherein the heating system (30) is connected to the economizer (24) using a feed water supply piping (40). The system (46, 180, 184, 188) of claim 2, wherein the heating system (30) is a deaerator (30) for providing at least partially deaerated water, wherein the system (46, 180, 184, 188) further comprises: ●      a third auxiliary heating device (55c) disposed on the deaerator (30) for heating the deaerator (30), and/or ●      a fourth auxiliary heating device (55d) disposed on the economizer (24) for heating the economizer (24). The system (46, 180, 184, 188) of claim 3, wherein the steam generator (48, 182, 186, 190) further comprises: ●      At least one auxiliary heating source (54) connected to the deaerator (30) by a deaerator heating supply line (56), and wherein the at least one auxiliary heating source (54) comprises a steam source. The system (46, 180, 184, 188) of claim 4, wherein the steam generator (48, 182, 186, 190) further comprises: ●      A steam turbine (104) comprising at least one low-pressure section (110), wherein the condenser (96) is connected to the at least one auxiliary heat source (54), wherein the condenser (96) is connected to the at least one low-pressure section (110) to condense the steam discharge from the at least one low-pressure section (110) to obtain the condensate, and The at least one low-pressure section (110) is connected to at least one low-pressure heater (120) disposed on the condensate supply line (116) to heat the water in the condensate supply line (116). The system (46, 180, 184, 188) of claim 5, wherein the steam turbine (104) further comprises: ●      At least one medium pressure section (108), wherein the at least one medium pressure section (108) is connected to at least one high pressure heater (124) disposed on the feed water supply piping (40) for heating water in the feed water supply piping (40). A system (46, 180, 184, 188) as claimed in any one of claims 4 to 6, wherein the steam generator (48, 182, 190) is a primary critical steam generator (48, 182, 186), and the at least one water-steam separator (18, 192) is a boiler drum (18), and wherein the steam generator (48, 182, 186) further comprises: ●      an economizer outlet water supply line (52) connecting the economizer (24) to the boiler drum (18), and ●      at least one downcomer (20) connecting the boiler drum (18) to the water wall inlet header (22), and wherein the system (46, 180, 184) comprises: ●      A fifth auxiliary heating device (55e), which is disposed on at least a portion of the economizer outlet water supply line (52) to heat the economizer outlet water supply line (52). A system (46, 180, 184) as claimed in claim 7, wherein the fifth auxiliary heating device (55e) is a nozzle (50), and wherein the nozzle (50) is connected to the at least one auxiliary heat source (54) via a boiler drum heating supply line (58). A system (46, 180, 184) as claimed in any one of claims 7 to 8, wherein the water wall inlet header (22) is connected to the at least one water-steam separator (18) by means of at least one downcomer (20), and wherein the system (46, 180, 184) comprises: a boiler drum heating return line (90) connecting the at least one downcomer (20) to the condenser (96) and/or connecting the water wall inlet header (22) to the condenser (96), and/or an economizer warm-up auxiliary line (142) connecting the at least one downcomer (20) to the economizer (24). A system (46, 180, 184, 188) as claimed in any one of claims 2 to 6, wherein the steam generator (48, 182, 186, 190) is a supercritical steam generator (190), and wherein the at least one water-steam separator (192) comprises: ●      a separator storage tank (194) connected to the outlet of the at least one water-steam separator (192), and wherein the system (188) further comprises: ●      a minimum economizer flow return line (196) connecting the separator storage tank (194) and the economizer (24), and/or ●     A condensate return line (216) connecting the separator storage tank (194) and the condenser (96). The system (188) of claim 10, wherein the condensate return line (216) comprises: ●      a boiler expansion vessel (208) for receiving water from the separator storage tank (194) before transferring the water to the condenser (96), and/or wherein the minimum economizer flow return line (196) comprises: ●      a minimum water wall flow return line (230) connecting the minimum economizer flow return line (196) to the water wall inlet header (22). A method of maintaining a steam generator (48, 182, 186, 190), wherein the method includes installing a system (46, 180, 184, 188) for getting the steam generator (48, 182, 186, 190) ready as described in any of claims 1 to 11 on the steam generator (48, 182, 186, 190). A method of operating a steam generator (48, 182, 186, 190), wherein the steam generator (48, 182, 186, 190) comprises: ●      at least one water-steam separator (18, 192), ●      an economizer (24), ●      an auxiliary fuel burner for warming up the steam generator (48, 182, 186, 190) using auxiliary fuel, ●      a water-wall inlet header (22), ●      a deaerator (30) connected to the economizer (24) for providing at least partially deaerated water to the economizer (24), and ●     A furnace (12), wherein the furnace (12) includes water-cooled wall tubes (16), and wherein the water-cooled wall tubes (16) are connected to the at least one water-steam separator (18, 192) by means of furnace top piping (26), and wherein the method comprises: ●      installing a system for making the steam generator (48, 182, 186, 190) ready as in claim 12, and wherein the system (46, 180, 184, 188) comprises: ○      a first auxiliary heating device (55a) disposed on the at least one water-steam separator (18, 192) for heating the at least one water-steam separator (18, 192), ○     a second auxiliary heating device (55b), which is at least arranged on a portion of the furnace top pipe (26) for heating the furnace top pipe (26), ○      a third auxiliary heating device (55c), which is arranged on the deaerator (30) for heating the deaerator (30), ○      a fourth auxiliary heating device (55d), which is arranged on the economizer (24) for heating the economizer (24), ○      a controller (72) for controlling the steam generator (48, 182, 186, 190) and the auxiliary heating devices (55), and ○     A measurement system comprising a pressure and/or temperature sensor, wherein the measurement system provides pressure and temperature measurements of the steam generator (48, 182, 186, 190) and temperature measurements of the system (46, 180, 184, 188) for preparing the steam generator (48, 182, 186, 190) to the controller (72), and ●      starting the steam generator (48, 182, 186, 190) and/or shutting down the steam generator (48, 182, 186, 190), wherein the step of starting the steam generator (48, 182, 186, 190) comprises the following sub-steps: ○     Using at least one of the first auxiliary heating device (55b), the second auxiliary heating device (55b), the third auxiliary heating device (55c), the third auxiliary heating device (55c), and the fourth auxiliary heating device (55d) to start heating the steam generator (48, 182, 186, 190) while recirculating the water through the steam generator (48, 182, 186, 190), and wherein the step of shutting down the steam generator (48, 182, 186, 190) includes: ○      Keeping the at least one water-steam separator (18, 192) filled with water and surrounding the at least one water-steam separator (18, 192) with nitrogen 192), or keep the deaerator (30) and the third auxiliary heating device (55c) sealed together, ○      keep the economizer (24) and the fourth auxiliary heating device (55d) sealed together, ○      keep the at least one water-steam separator (18, 192) and the first auxiliary heating device (55a) at least at a temperature at which the material of the at least one water-steam separator (18, 192) is no longer brittle, and ○      keep the furnace top end piping (26) and the second auxiliary heating device (55b) at least at a temperature at which the material of the furnace top end piping (26) is no longer brittle, and Wherein maintaining the deaerator (30) and the economizer (24) sealed includes limiting the diffusion of gas into the water stored in the deaerator (30) and the economizer (24). A method of operating the steam generator (48, 182, 186, 190) as claimed in claim 13, wherein the steam generator (48, 182, 186, 190) is a primary critical steam generator (48, 182, 186), and the at least one water-steam separator (192) is a boiler drum (18), and wherein the steam generator (190) further comprises: ●      an economizer outlet water supply line (52) connecting the economizer (24) and the boiler drum (18), ●      at least one auxiliary heating source (54) connected to the deaerator (30) by means of a deaerator heating supply line (56), ●     at least one downcomer (20) connecting the boiler drum (18) and the water wall inlet header (22), ●      a condenser (96) connected to the heating system (30) by means of a condensate supply line (116) for providing condensate to the heating system (30), and wherein the condenser (96) is connected to the at least one auxiliary heat source (54), and wherein the heating system (30) is connected to the economizer (24) by means of a feedwater supply piping (40), wherein the at least one auxiliary heat source (54) comprises a steam source, and wherein the system (188) further comprises: ●     a fifth auxiliary heating device (55e) disposed at least on a portion of the economizer outlet water supply line (52) for heating the economizer outlet water supply line (52), and ●      a boiler drum heating return line (90) connecting the at least one downcomer (20) and the condenser (96) and/or connecting the water-cooled wall inlet header (22) and the condenser (96), and wherein the fifth auxiliary heating device (55e) is connected to the at least one auxiliary heating source (54) by means of a boiler drum heating supply line (58), and wherein the step of starting the steam generator (190) further comprises: ○      heating the water in the economizer outlet water supply line (52) by means of the fifth auxiliary heating device (55e), The sub-step of recycling the water includes recycling the water between the boiler drum (18), the deaerator (30), and the economizer (24) using the boiler drum heating return line (90), the condensate supply line (116), and the feed water supply piping (40). A method of operating the steam generator (48, 182, 186, 190) as claimed in claim 13, wherein the steam generator (48, 182, 186, 190) is a supercritical steam generator (190), and and wherein the at least one water-steam separator (192) comprises: ●      a separator storage tank (194) connected to the outlet of the at least one water-steam separator (192), and wherein the system (188) further comprises: ●      a minimum economizer flow return line (196) connecting the separator storage tank (194) and the economizer (24), Wherein the sub-step of recirculating the water includes recirculating the water between the at least one water-steam separator (192), the separator storage tank (194), and the economizer (24) using the minimum economizer flow return line (196).

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