A vapor recirculation system and method for retarding dry-to-wet transitions

Abstract

The application discloses a steam recirculation system and method for delaying dry-wet state conversion, comprising a steam recirculation module, a water storage flash module and a sewage heat recovery module. Steam at the outlet of a steam-water separator is mixed with a working medium in an intermediate header of a water cooling wall by using a steam ejector to directly control the dryness of the working medium in the vertical water cooling wall, so that the working medium at the outlet of the water cooling wall is still dry steam under low load. Meanwhile, a flash evaporator is used to replace a recirculation water pump, and flash evaporated steam and flash evaporated saturated water are respectively sent into a steam-water separator and a deaerator to realize low-energy-consumption recycling of saturated water in a water storage tank. A sewage cooler is added to heat boiler feed water, heat of sewage discharged from the water storage tank is recycled, the temperature of the feed water is increased, dry-wet state conversion of the boiler is delayed, and the boiler can still be operated in a dry state under low load.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of thermal power generation, and particularly relates to a steam recirculation system and method for delaying dry-wet state conversion. BACKGROUND

[0002] Large-scale grid connection of new energy power requires coal-fired units to undertake the task of peak shaving and frequency modulation. The operating load of the boiler changes according to the requirements of the power grid side, and the once-through boiler faces the problem of dry-wet state conversion. During the peak period of new energy power generation, the boiler load needs to be reduced to a very low level, the coal supply is reduced, and the working medium at the outlet of the water-cooled wall of the once-through boiler changes from superheated steam to saturated wet steam. At this time, the once-through boiler is converted to wet state operation, and the economy is reduced. At the same time, during the process of dry-wet state conversion of the boiler, the properties of the working medium change greatly, and the parameters such as working medium flow fluctuate, which can easily cause problems such as heat transfer deterioration of the water-cooled wall, and the regulation and control is difficult.

[0003] At present, there is little research on delaying the dry-wet state conversion of the once-through boiler. Existing patents and documents propose automatic regulation and control systems for full load of the boiler, but the start-stop operation of the boiler water circulating pump is complex, and frequent start-stop can easily occur, which has the risk of cavitation, affects the reliability of the unit, and increases the amount of auxiliary power, reducing the economy of the power plant. Some patents further increase the steam extraction amount of the steam turbine to increase the working medium temperature at the inlet of the economizer and reduce the working medium enthalpy deficit, which helps the working medium to evaporate into steam, but the safe operation conditions of the high-pressure heater need to be considered, which is limited by the safe temperature at the outlet of the economizer, and the effect of delaying the state conversion is not ideal. SUMMARY

[0004] The present application provides a steam recirculation system and method for delaying dry-wet state conversion, which aims to delay the dry-wet state conversion of the once-through boiler, directly regulates and controls the dryness of the working medium in the water-cooled wall by constructing a steam recirculation loop, mixes the steam at the outlet of the steam-water separator with the working medium in the intermediate header of the water-cooled wall using a steam ejector, realizes that the outlet of the water-cooled wall is still dry steam at low load, and does not need the overloading operation of the high-pressure heater. At the same time, a flash evaporator is used instead of a recirculation water pump to realize the low-energy consumption recycling of the saturated water in the water storage tank.

[0005] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0006] A steam recirculation system for delaying dry-wet state conversion, comprising a spiral water-cooled wall and an intermediate header.

[0007] The spiral water cooling wall is connected with the vertical water cooling wall through an intermediate header; the outlet of the vertical water cooling wall is connected with the first inlet of a steam-water separator; the steam outlet of the steam-water separator is connected with the inlet of a flow dividing valve FL, and the water outlet is connected with the inlet of a water storage tank; the first outlet of the flow dividing valve FL is connected with the inlet of a superheater, and the second outlet is connected with the auxiliary inlet of an ejector YS through valves K1 and K2; an outlet is additionally arranged above the intermediate header and is connected with the main inlet of the ejector YS through a valve K7 and a check valve D2; the outlet of the ejector is connected with the additionally arranged inlet below the intermediate header through a check valve D3; the first outlet of the water storage tank is connected with the inlet of a flash evaporator through a valve K4, and the second outlet is connected with the tube side inlet of a waste water cooler through a valve K8; the steam outlet of the flash evaporator is connected with the second inlet of the steam-water separator through a valve K3 and a check valve D1, and the saturated water outlet is connected with the second inlet of a deaerator through a valve K5; the first inlet of the deaerator receives the feed water from the outlet of a low pressure heater, and the outlet is connected with the inlet of a high pressure heater; the second outlet of the high pressure heater is connected with the third inlet of the deaerator, and the first outlet is connected with the inlet of an economizer through a main feed water line, and is connected with the shell side inlet of the waste water cooler through a valve K6 of a branch feed water line; the shell side outlet of the waste water cooler is connected with the main feed water line through a check valve; and the outlet of the economizer is connected with the inlet of the spiral water cooling wall.

[0008] Further improvement of the present application is that the heat of the waste water is recovered by the waste water cooler to heat the boiler feed water.

[0009] Further improvement of the present application is that the system is divided into a steam recycling module, a water storage and flash evaporation module and a waste water heat recovery module.

[0010] The steam recycling module is responsible for mixing part of the steam and water from the outlet of the steam-water separator with the working medium in the intermediate header of the water cooling wall;

[0011] The water storage and flash evaporation module is responsible for dividing the saturated water in the water storage tank into flash evaporation steam and flash evaporation saturated water through the flash evaporator, and sending them into the steam-water separator and the deaerator respectively for recycling;

[0012] The waste water heat recovery module is responsible for heating part of the feed water by using the waste water of the water storage tank, so as to realize the recycling of the waste water heat.

[0013] The further improvement of the application is that the steam recycling module comprises a flow splitting valve FL, an ejector YS, a water-cooled wall intermediate header, one-way valves D2 and D3, valves K1 and K7, and a safety valve K2; the steam flow extracted is controlled by changing the flow splitting coefficient of the flow splitting valve; the working medium flow entering the main inlet of the ejector YS is adjusted by the opening degree of the valve K7; the ejector YS is responsible for pressure matching between the steam and the working medium in the intermediate header; part of the working medium enters the main inlet of the ejector through the valve K7 and the one-way valve D2, the working medium is accelerated in the contraction section to form a low-pressure area, the extracted steam is sucked into the ejector through the auxiliary inlet, the two fluids are mixed and heat exchanged in the expansion section, the pressure rises, and then enters the intermediate header through the one-way valve D3; the one-way valve functions to prevent backflow.

[0014] The further improvement of the application is that the water storage and flash evaporation module comprises a water storage tank, a flash evaporator, a steam-water separator, a deaerator, valves K3, K4, K5 and a one-way valve D1; the amount of saturated water flowing into the flash evaporator is controlled by the opening degree of the valve K4; the flash evaporator utilizes the sudden pressure drop of the saturated water when entering a large volume space, the saturated temperature is reduced, and the working medium is flashed to obtain flash evaporation steam and flash evaporation saturated water.

[0015] The further improvement of the application is that the sewage heat recovery module comprises a sewage cooler, valves K6, K8 and a one-way valve; the sewage contains many impurities, so the sewage flows quickly in the pipe to delay fouling, part of the feed water flows on the shell side for heat exchange, and finally flows into the main feed water pipeline; the flow is adjusted by the opening degrees of the valves K6 and K8, and the one-way valve is used to prevent backflow.

[0016] A steam recycling method for delaying dry-wet state conversion, which is based on the steam recycling system for delaying dry-wet state conversion, comprises the following steps:

[0017] Step 1: judging whether the boiler needs to run in a wet state according to the load demand of the power grid; if no wet state is needed, the system is not put into operation, and all valves are kept closed, and the flow splitting valve does not function to split flow;

[0018] Step 2, if the boiler load needs to be reduced below the wet load, ensure that the feed water is not less than the minimum working fluid flow of the water-cooled wall, gradually reduce the coal supply, when the superheat degree of the steam at the outlet of the steam-water separator is less than 15℃, open valves K4, K3 and K5 in turn, recover the saturated water in the water storage tank to heat the feed water; divide the water level in the water storage tank into low, medium and high three levels, when the water level in the water storage tank is at the medium or high level, increase the opening degree of valve K4, the working fluid in the flash evaporator increases, the flash evaporated steam mixes with the main steam through the steam-water separator to supplement the steam flow after extraction, the flash evaporated saturated water flows into the deaerator to increase the temperature of the feed water, reduce the sub-enthalpy of the working fluid at the inlet of the spiral pipe under the condition of ensuring safety, reduce the evaporation heat absorption, which helps to delay the dry-wet transition; monitor the temperature of the working fluid at the outlet of the economizer at all times, ensure that the supercooling degree is not less than 10℃, otherwise the opening degree of valve K4 should be reduced;

[0019] Step 3, slowly open valve K7, keep the opening degree at a small value, part of the working fluid in the intermediate header enters the primary inlet of the ejector YS through the valve, open valves K1 and safety valve K2 after the warm pipe is completed, adjust the split ratio of the flow divider FL, for example, set to 1%, that is, 1% of the mass flow of the working fluid at the inlet is extracted to send steam into the auxiliary inlet of the ejector YS, the steam and the working fluid in the intermediate header are fully mixed in the ejector YS to complete pressure matching, and then enter the intermediate header through the check valve D3;

[0020] Step 4, adjust the split ratio of the flow divider and the opening degree of valve K7 according to the dryness of the working fluid at the outlet of the vertical water-cooled wall; if the superheat degree of the working fluid at the outlet of the vertical water-cooled wall is less than or equal to 0, increase the split ratio and the opening degree of valve K7 accordingly to extract more steam into the water-cooled wall to increase the temperature of the working fluid; in this process, the water-cooled wall should be kept from overheating at all times, and the extraction amount should be reduced when the temperature of the water-cooled wall approaches the safety limit; if the superheat degree of the working fluid at the outlet of the vertical water-cooled wall is greater than 0 and does not decrease within 5 minutes, and the temperature of the pipe wall is normal, the settings remain unchanged;

[0021] Step 5, if the water level in the water storage tank is at the low level, close valve K4, further increase the split ratio and the opening degree of valve K7 when the superheat degree of the working fluid at the outlet of the vertical water-cooled wall decreases to 0, increase the amount of recirculated steam to ensure the dry running of the boiler, and keep the water-cooled wall from overheating at all times in this process, and adjust the split ratio to reduce the extraction amount and correspondingly reduce the opening degree of valve K7 when the temperature of the water-cooled wall approaches the safety limit;

[0022] Step 6, when the water storage tank needs to discharge sewage, open valves K8 and K6, the sewage and part of the feed water are heat exchanged in the sewage cooler, the temperature of the feed water increases after absorbing heat on the shell side, and then flows into the main feed water line through the check valve, and the sewage after heat release is directly discharged through the pipeline; if the supercooling degree of the working fluid at the outlet of the economizer is less than 10℃, close valve K6.

[0023] The further improvement of the present application is that the storage water is recycled in the form of flash steam and flash saturated water by means of a flash evaporator in step 2, so that the saturated water in the storage tank is recycled with low energy consumption.

[0024] The pressure matching and mixed heat exchange of the extracted steam and the working medium in the intermediate header are completed by the steam ejector in step 3.

[0025] The further improvement of the present application is that the split ratio of the split valve is adjusted according to the superheat degree of the working medium at the outlet of the water cooling wall in step 4 to control the amount of extracted steam, and the opening degree of the valve K7 is changed accordingly, so that the mixed steam can be smoothly sent into the intermediate header, and the steam recirculation is constructed under the condition of ensuring the safe operation of the water cooling wall, the temperature of the working medium in the vertical water cooling wall is directly adjusted, and the dry running of the boiler under low load is realized.

[0026] The further improvement of the present application is that when the quality of the saturated water in the storage tank does not meet the standard, the valve K6 is first opened to separate part of the feed water into the shell side of the sewage cooler, and then the valve K8 is opened for sewage discharge, and the sewage is discharged into the pipe inlet of the sewage cooler, and the sewage waste heat is used to heat the feed water.

[0027] Compared with the prior art, the present application has at least the following beneficial technical effects:

[0028] The present application takes the once-through boiler as the research object, and researches the problem of dry-wet state conversion under low load, proposes to construct a steam recirculation loop to control the dryness of the working medium in the water cooling wall, and at the same time, aiming at the problem of recycling saturated water by the recirculation water pump, the present application sets a flash evaporator to realize the low-energy-consumption recycling of saturated water. It has the following advantages:

[0029] Firstly, by directly sending part of the steam at the outlet of the steam-water separator into the intermediate header of the water cooling wall, the dryness of the working medium in the vertical water cooling wall can be quickly increased, and only a small amount of heat absorption is needed to keep the working medium at the outlet of the water cooling wall as dry steam, so that the dry running of the once-through boiler under low load is realized without the need for dry-wet state conversion operation.

[0030] Secondly, the saturated water in the storage tank is divided into flash steam and flash saturated water by the flash evaporator, and is sent into the steam-water separator and the deaerator respectively for recycling, without the need for installing a recirculation water pump, reducing energy consumption and simplifying operation, realizing the recycling of saturated water at a lower cost, and at the same time, increasing the feed water temperature, reducing the water cooling wall inlet working medium under-enthalpy, and reducing the evaporation heat absorption, which is helpful to delay the dry-wet state conversion.

[0031] Thirdly, optionally, a sewage cooler is installed to recycle the waste heat of the sewage, and the sewage from the storage tank and part of the feed water are heat exchanged to realize the recycling of waste heat, increase the feed water temperature, and reduce the evaporation heat absorption, which is helpful to delay the dry-wet state conversion.

[0032] Fourth: the mixing of steam and working medium in the intermediate header is realized by using the ejector, pressure loss is small, and the device is simple. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0034] Figure 1 The figure is a system structure diagram of the present application.

[0035] Figure 2 The figure is a logic flow diagram of the steam recirculation and water storage flash module of the present application. DETAILED DESCRIPTION

[0036] In the following, only some exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the present application. Therefore, the drawings and the description are considered to be exemplary in nature rather than limiting.

[0037] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0038] In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0039] In the present application, unless specifically defined otherwise and limited in the specification, the terms "mount", "connected", "connection", "fixed", and the like, should be construed broadly and do not necessarily require a direct connection or attachment between two elements. These terms can include indirect connections between two elements in the form of an indirect connection through one or more intermediate elements. In addition, a connection between two elements can be a mechanical connection, an electrical connection, or a communication connection. It will be apparent to those skilled in the art that these terms can have the same meaning as the corresponding terms used in the art.

[0040] In the present application, unless specifically defined otherwise and limited in the specification, "on" or "under" of a first feature to a second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, "on", "above" and "over" of a first feature to a second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the first feature is higher in horizontal height than the second feature. "Under", "below" and "underneath" of a first feature to a second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the first feature is lower in horizontal height than the second feature.

[0041] It should be understood that the terms "comprise" and "include" as used in the specification and the appended claims indicate the presence of the described features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0042] It should also be understood that the terms used in the present application specification are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the present application specification and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0043] It should be further understood that the term "and / or" as used in the present application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations thereof.

[0044] Various structural diagrams according to the disclosed embodiments of the present application are shown in the accompanying drawings. These drawings are not drawn to scale, in which certain details are exaggerated for the purpose of clarity and certain details can be omitted. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are only exemplary, and in actuality there can be deviations due to manufacturing tolerances or technical limitations, and regions / layers with different shapes, sizes and relative positions can be additionally designed by those skilled in the art according to actual needs.

[0045] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0046] Example 1

[0047] like Figure 1 As shown, this invention provides a delayed dry-wet state transition steam recirculation system that uses a steam ejector to return steam to the water-cooled wall under low load. The system includes: a spiral water-cooled wall and a vertical water-cooled wall connected via an intermediate header; the outlet of the vertical water-cooled wall connected to the first inlet of a steam-water separator; the steam outlet of the steam-water separator connected to the inlet of a diversion valve FL, and the water outlet connected to the inlet of a water storage tank; the first outlet of the diversion valve FL connected to the superheater inlet, and the second outlet connected to the auxiliary inlet of the ejector YS via valves K1 and K2; an additional outlet above the intermediate header, connected to the main inlet of the ejector YS via valve K7 and check valve D2; the ejector outlet connected to the additional inlet below the intermediate header via check valve D3; and the first outlet of the water storage tank connected to the main inlet via valve K4. The flash evaporator is connected to the inlet of the flash evaporator, and the second outlet is connected to the pipe-side inlet of the wastewater cooler via valve K8. The steam outlet of the flash evaporator is connected to the second inlet of the steam-water separator via valve K3 and check valve D1, and the saturated water outlet is connected to the second inlet of the deaerator via valve K5. The first inlet of the deaerator receives feedwater from the outlet of the low-pressure heater, and the outlet is connected to the inlet of the high-pressure heater. The second outlet of the high-pressure heater is connected to the third inlet of the deaerator, and the first outlet is connected to the economizer inlet via the main feedwater line, and simultaneously connected to the shell-side inlet of the wastewater cooler via valve K6 on the feedwater branch line. The pipe outlet of the wastewater cooler directly discharges wastewater, and the shell-side outlet is connected to the main feedwater line via a check valve. The economizer outlet is connected to the inlet of the spiral water-cooled wall.

[0048] The system described in this invention mainly comprises three parts: a steam recirculation module, a water storage flash evaporation module, and a wastewater reheat module. The specific details of each part are as follows.

[0049] 1. Steam recirculation module

[0050] Steam is extracted and fed into the water-cooled wall. The steam flow rate into the ejector is controlled by adjusting the flow coefficient of the diversion valve. Valve K1 controls the on / off state of the control circuit, and safety valve K2 serves as a protection device in case K1 fails. Adjusting the opening of valve K7 controls the flow rate of the working fluid entering the main inlet of the ejector. The working fluid accelerates and entrains the steam at the auxiliary inlet in the expansion section. Subsequently, the mixed fluid undergoes sufficient heat exchange and mixing in the expansion section, decelerates and increases in pressure, and enters the intermediate header through one-way valve D3. This increases the dryness of the working fluid in the water-cooled wall, making it easier for the working fluid in the vertical water-cooled wall to be converted into dry steam, which can delay the dry-wet state transition and achieve dry-state operation of the boiler at low load.

[0051] 2. Water storage flash evaporation module

[0052] The storage water is recycled by flash. The storage water entering the flash evaporator is controlled by the opening of valve K4. The storage water is evaporated by pressure reduction after entering the flash evaporator, and flash steam and flash saturated water are generated. The flash steam is sent to the steam-water separator to complete steam-water separation, supplement the main steam flow, and the flash saturated water enters the deaerator to mix with the feed water, increase the temperature of the feed water, reduce the under-enthalpy of the feed water, and make the working medium in the spiral water-cooled wall absorb less heat to evaporate, which is beneficial to increase the dryness of the working medium in the water-cooled wall. At the same time, due to the increase of the temperature of the feed water, the heat exchange temperature difference between the working medium inside and outside the economizer is reduced, the heat dissipation of the flue gas is reduced, the flue gas temperature at the outlet of the economizer is increased, which is beneficial to low-load denitrification.

[0053] 3. Sewage heat recovery module

[0054] The waste heat of the sewage is recovered. When the water quality in the storage tank does not meet the standard, valves K8 and K6 are opened, and part of the feed water is heated by the discharged sewage to realize the recycling of waste heat. The opening of valve K6 can control the flow of the feed water branch, and the opening is adjusted according to the flow of the sewage. The cooled sewage is directly discharged, and the heated feed water is mixed into the feed water to further increase the temperature of the feed water, so that the evaporation heat absorption of the feed water is reduced, which is helpful to delay the dry-wet state conversion.

[0055] Example 2

[0056] As shown in Figure 2 The present application provides a steam recirculation method for delaying dry-wet state conversion, which comprises the following steps:

[0057] Step 1: According to the load demand of the power grid, it is judged whether the boiler needs to be operated in wet state. If the wet state is not needed, the system is not put into operation, and all valves are kept closed, and the shunt valve does not play a shunt role.

[0058] Step 2: If the load of the boiler needs to be reduced to below the wet state load, the feed water is ensured to be not lower than the minimum working medium flow of the water-cooled wall, the coal supply is gradually reduced, and when the superheat degree of the steam at the outlet of the steam-water separator is less than 15℃, valves K4, K3 and K5 are opened in sequence to heat the feed water by the saturated water in the storage tank. The water level in the storage tank is divided into low, medium and high three levels. When the water level in the storage tank is at medium or high level, the opening of valve K4 is increased, the working medium in the flash evaporator is increased, the flash steam is mixed with the main steam through the steam-water separator, and is used as the supplement of the steam flow after extraction, the flash saturated water flows into the deaerator to increase the temperature of the feed water, and under the condition of ensuring safety, the under-enthalpy of the working medium at the inlet of the spiral pipe is reduced, the evaporation heat absorption is reduced, which is helpful to delay the dry-wet state conversion. The temperature of the working medium at the outlet of the economizer is monitored at any time, and the undercooling degree is ensured to be not less than 10℃, otherwise the opening of valve K4 should be reduced.

[0059] Step 3, slowly open valve K7, keep the valve opening at a small value, part of the working medium in the intermediate header enters the primary inlet of the ejector YS through the valve, open valve K1 and safety valve K2 after the warm-up, adjust the split ratio of the flow divider FL, for example, set to 1%, that is, 1% of the mass flow of the working medium at the extraction inlet is sent into the auxiliary inlet of the ejector YS, and the steam and the working medium in the intermediate header are mixed sufficiently in the ejector YS to complete the pressure matching, and then enter the intermediate header through the check valve D3.

[0060] Step 4, adjust the split ratio of the flow divider and the corresponding opening of valve K7 according to the superheat degree of the working medium at the outlet of the vertical water-cooled wall. If the superheat degree of the working medium at the outlet of the vertical water-cooled wall is less than or equal to 0, increase the split ratio and the corresponding opening of valve K7 to extract more steam into the water-cooled wall to increase the temperature of the working medium. In this process, the water-cooled wall must be kept from overheating at all times, and when the temperature of the water-cooled wall approaches the safety limit, the amount of extracted steam is reduced. If the superheat degree of the working medium at the outlet of the vertical water-cooled wall is greater than 0 and does not decrease within 5 minutes, the tube wall temperature is normal, and the settings are kept unchanged to maintain dry running of the boiler.

[0061] Step 5, if the water level in the water storage tank is at a low level, close valve K4, and when the superheat degree of the working medium at the outlet of the vertical water-cooled wall decreases to 0, further increase the split ratio and the corresponding opening of valve K7 to increase the amount of recirculated steam to ensure dry running of the boiler. In this process, the water-cooled wall must be kept from overheating at all times, and when the temperature of the water-cooled wall approaches the safety limit, the split ratio is adjusted to reduce the amount of extracted steam and the opening of valve K7 is correspondingly reduced.

[0062] Step 6, when the water storage tank needs to discharge sewage, open valves K8 and K6, and the sewage and part of the feedwater are heat exchanged in the sewage cooler. The temperature of the feedwater increases after absorbing heat on the shell side, and then flows into the main feedwater line through the check valve. The sewage after heat release is directly discharged from the pipeline, and the waste heat of the sewage is recovered. If the subcooling degree of the working medium at the outlet of the economizer is less than 10°C, close valve K6.

[0063] Similar to the above example, the position of the flow divider can be changed as needed, including but not limited to adjusting the position of the flow divider between the superheaters to obtain steam with higher parameters.

[0064] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0065] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A vapor recirculation system to retard dry-to-wet transition, characterized by, The spiral water-cooled wall and the intermediate header are connected, the outlet of the vertical water-cooled wall is connected with the first inlet of the steam-water separator, the steam outlet of the steam-water separator is connected with the inlet of the flow dividing valve FL, the water outlet is connected with the inlet of the water storage tank, the first outlet of the flow dividing valve FL is connected with the inlet of the superheater, the second outlet is connected with the auxiliary inlet of the ejector YS through the valves K1 and K2, an outlet is additionally arranged above the intermediate header and is connected with the main inlet of the ejector YS through the valve K7 and the check valve D2, the outlet of the ejector is connected with the inlet additionally arranged below the intermediate header through the check valve D3, the first outlet of the water storage tank is connected with the inlet of the flash evaporator through the valve K4, the second outlet is connected with the tube side inlet of the waste water cooler through the valve K8, the steam outlet of the flash evaporator is connected with the second inlet of the steam-water separator through the valve K3 and the check valve D1, the saturated water outlet is connected with the second inlet of the deaerator through the valve K5, the first inlet of the deaerator receives the feed water from the outlet of the low-pressure heater, and the outlet is connected with the inlet of the high-pressure heater, the second outlet of the high-pressure heater is connected with the third inlet of the deaerator, the first outlet is connected with the inlet of the economizer through the main feed water line, and is connected with the shell side inlet of the waste water cooler through the valve K6 of the branch feed water line, the shell side outlet of the waste water cooler is connected with the main feed water line through a check valve, and the outlet of the economizer is connected with the inlet of the spiral water-cooled wall. The heat of the waste water is recovered to heat the boiler feed water.

2. A vapor recirculation system to delay the dry-out transition according to claim 1, wherein, The system is divided into a steam recycling module, a water storage and flashing module and a waste water heat recovery module.

3. The vapor recirculation system for retarding wet-out according to claim 1, wherein The steam recycling module is responsible for mixing the steam extracted from the outlet of the steam-water separator and the working medium in the intermediate header of the water-cooled wall. The water storage and flashing module is responsible for dividing the saturated water in the water storage tank into flash steam and flash saturated water through the flash evaporator, and sending them into the steam-water separator and the deaerator respectively for recycling. The waste water heat recovery module is responsible for heating part of the feed water by using the waste water from the water storage tank, so as to realize the recycling of the waste water heat. The steam recycling module includes the flow dividing valve FL, the ejector YS, the intermediate header of the water-cooled wall, the check valves D2 and D3, the valves K1 and K7 and the safety valve K2, the steam flow extracted is controlled by changing the flow dividing coefficient of the flow dividing valve, the working medium flow entering the main inlet of the ejector YS is adjusted by the opening degree of the valve K7, the ejector YS is responsible for matching the pressure of the steam and the working medium in the intermediate header, part of the working medium enters the main inlet of the ejector through the valve K7 and the check valve D2, the working medium accelerates in the contraction section to form a low pressure area, the extracted steam is sucked into the ejector through the auxiliary inlet, the two fluids slow down in the expansion section, are fully mixed and heat exchanged, and the pressure rises, and then enters the intermediate header through the check valve D3, and the check valve functions to prevent backflow.

4. A vapor recirculation system for retarding the dry-out transition according to claim 3, wherein The water storage and flashing module includes the water storage tank, the flash evaporator, the steam-water separator, the deaerator, the valves K3, K4, K5 and the check valve D1, the saturated water flow entering the flash evaporator is controlled by the opening degree of the valve K4, and the flash evaporator utilizes the sudden pressure drop of the saturated water when entering a large volume space to reduce the saturated temperature, so that the working medium is flashed to obtain flash steam and flash saturated water.

5. A vapor recirculation system for retarding the dry-out transition according to claim 3, wherein ​ 6. A vapor recirculation system to retard wet-out according to claim 3, wherein, The sewage heat recovery module comprises a sewage cooler, valves K6 and K8, and a check valve; the sewage contains many impurities, so that the sewage flows rapidly in the pipe to delay fouling, part of the feed water flows on the shell side to exchange heat, and finally flows into the main feed water pipeline, the flow is controlled by the opening degree of the valves K6 and K8, and the check valve is used to prevent backflow.

7. A vapor recycle method to retard dry-out, characterized in that, The method is based on the steam recycling system for delaying dry-wet state conversion according to any one of claims 1 to 6, comprising: Step 1, judging whether the boiler needs to run in wet state according to the load demand of the power grid, if not, the system is not put into action, all valves are kept closed, and the shunt valve does not play a shunt role; Step 2, if the boiler load needs to be reduced to below the wet state load, the feed water is ensured to be not lower than the minimum working medium flow of the water cooled wall, the coal supply is gradually reduced, when the superheat degree of the steam at the outlet of the steam-water separator is less than 15℃, the valves K4, K3 and K5 are opened in sequence, the saturated water in the water storage tank is recovered to heat the feed water; the water level in the water storage tank is divided into low, medium and high three levels, when the water level in the water storage tank is at the medium or high level, the opening degree of the valve K4 is increased, the working medium in the flash evaporator is increased, the flash evaporated steam passes through the steam-water separator to mix with the main steam, as a supplement of the steam flow after extraction, the flash evaporated saturated water flows into the deaerator to increase the temperature of the feed water, under the condition of ensuring safety, the subcooling degree of the working medium at the inlet of the spiral pipe is reduced, the evaporation heat absorption is reduced, which is helpful for delaying the dry-wet state conversion; the temperature of the working medium at the outlet of the economizer is monitored at all times, the supercooling degree is ensured to be not less than 10℃, otherwise the opening degree of the valve K4 should be reduced; Step 3, the valve K7 is slowly opened, the opening degree of the valve is kept at a small value, part of the working medium in the intermediate header enters the eductor YS main inlet through the valve, the valve K1 and the safety valve K2 are opened after the warm pipe is completed, and the shunt coefficient of the shunt valve FL is adjusted, for example, if the shunt coefficient is set to 1%, that is, 1% of the mass flow of the steam at the inlet is extracted to enter the eductor YS auxiliary inlet, the steam and the working medium in the intermediate header are fully mixed in the eductor YS to match the pressure, and then enter the intermediate header through the check valve D3; Step 4, the shunt coefficient of the shunt valve and the corresponding opening degree of the valve K7 are adjusted according to the superheat degree of the working medium at the outlet of the vertical water cooled wall; if the superheat degree of the working medium at the outlet of the vertical water cooled wall is less than or equal to 0, the shunt coefficient and the corresponding opening degree of the valve K7 are increased, more steam is extracted to enter the water cooled wall to increase the temperature of the working medium; in this process, the water cooled wall needs to be kept from being overheated at all times, when the temperature of the water cooled wall approaches the safety limit, the extraction amount is reduced; if the superheat degree of the working medium at the outlet of the vertical water cooled wall is greater than 0 and does not decrease within 5 minutes, the temperature of the pipe wall is normal, then the settings are kept unchanged; Step 5, if the water level in the water storage tank is at the low level, the valve K4 is closed, when the superheat degree of the working medium at the outlet of the vertical water cooled wall is reduced to 0, the shunt coefficient and the corresponding opening degree of the valve K7 are further increased, the recycling steam amount is increased to ensure the dry state operation of the boiler, in this process, the water cooled wall needs to be kept from being overheated at all times, when the temperature of the water cooled wall approaches the safety limit, the shunt coefficient is adjusted to reduce the extraction amount and the opening degree of the valve K7 is correspondingly reduced; Step 6, when the water storage tank needs to discharge sewage, open valves K8 and K6, sewage and part of the feed water are heat exchanged in the sewage cooler, the feed water is absorbed heat on the shell side and then flows into the feed water main line through the check valve, and the sewage after heat release is directly discharged from the pipeline, and the waste heat of the sewage is recovered; if the supercooling degree of the working medium at the outlet of the coal economizer is less than 10℃, then close valve K6.

8. The vapor recirculation method of retarding the dry-out transition according to claim 7, wherein In step 2, the storage water is recovered in the form of flash steam and flash saturated water by means of the flash evaporator, the low-energy consumption recovery of the saturated water in the water storage tank is realized, the subcooling degree of the working medium at the inlet of the spiral pipe is reduced under the condition of safety, the evaporation heat absorption is reduced, and the dry-wet state transition is delayed; In step 3, the pressure matching and mixed heat exchange of the extracted steam and the working medium in the intermediate header are completed by the steam ejector.

9. The vapor recirculation method of retarding the dry-out transition according to claim 7, wherein In step 4, the split coefficient of the split valve is adjusted according to the superheat degree of the working medium at the outlet of the water cooling wall to control the amount of extracted steam, and the opening of valve K7 is changed accordingly, so that the mixed steam can be smoothly sent into the intermediate header, the steam recirculation is constructed under the condition of ensuring the safe operation of the water cooling wall, the temperature of the working medium in the vertical water cooling wall is directly adjusted, and the dry state operation of the boiler under low load is realized.

10. The vapor recirculation method of retarding the dry-out transition according to claim 7, wherein In step 6, when the quality of the saturated water in the water storage tank does not meet the standard, first open valve K6, separate part of the feed water into the shell side of the sewage cooler, then open valve K8 for sewage discharge, the sewage enters the pipeline inlet of the sewage cooler, the sewage waste heat is used to heat the feed water, the subcooling degree of the working medium at the inlet of the water cooling wall is reduced, the evaporation heat absorption is reduced, and the dry-wet state transition is delayed.

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