Drainage flash evaporation heat supply system of once-through boiler under low load and operation method of drainage flash evaporation heat supply system

Abstract

The invention discloses a drainage flash heat supply system of a once-through boiler under low load and an operation method of the drainage flash heat supply system. The drainage flash heat supply system comprises a drainage flash control valve, a drainage flash tank, a heat supply steam throttle valve, an electric heater and a condensation drainage recirculation pump which are sequentially arranged behind a water storage tank. When a unit undertakes a peak regulation task and a boiler enters a wet state operation mode under an ultra-low load working condition, drain water generated by a steam-water separator is introduced into a flash evaporation heat supply system, generated high-pressure steam is used for industrial heat supply, the steam meets the requirements of pressure and superheat degree of industrial steam through a throttling valve and an electric heater, and the heat supply efficiency is improved. The drain water produced in the flash tank is re-introduced into the feed line through a condensed drain water recirculation pump. According to the system, power supply and heat supply can be decoupled during low-load operation of a unit, meanwhile, the stable combustion requirement of boiler operation is met, and the hydrodynamic safety of each water cooling wall is protected.

Description

Technical Field

[0001] This invention belongs to the field of safety technology for water-cooled walls of peak-shaving boilers, specifically relating to a once-through boiler condensate flash heating system and its operation method that is adapted to ultra-low load operation of thermal power units. Background Technology

[0002] With the development of new energy power, in order to absorb more and more intermittent energy sources such as wind power and photovoltaics, the power grid requires coal-fired units to undertake deep peak-shaving tasks. The minimum dry-state operating load of once-through boiler units is generally 30%-35% of the rated load. However, the peak-shaving depth requirement is generally much lower than the minimum dry-state operating load (15%-20% of the rated load). Under such ultra-low load, if dry-state operation is maintained, the water entering the evaporation section will instantly vaporize completely, resulting in excessively high superheat of the working fluid at the evaporation section outlet, which can easily cause the metal tube wall to overheat and burn out. To solve this problem, the dry-wet switching operation technology of once-through boilers has been widely studied. By injecting more water into the system than the evaporation rate, the excess unevaporated water is used to cool the heating surfaces. This excess water is in a steam-water mixture state after flowing through the evaporation section. The high-temperature saturated water in this mixture must be discharged from the system, otherwise normal operation cannot be maintained. This part of high-temperature saturated water is also called condensate. This operating condition is called wet operation.

[0003] As the load decreases and the once-through boiler switches to wet operation mode, the discharged high-temperature saturated water enters the condensate storage tank for boiler feedwater recirculation. This condensate is of high quality, and directly entering the economizer and water-cooled walls would result in significant energy loss. To address this energy waste inherent in once-through boilers, energy in the condensate is typically transferred by coupling a thermal storage system or using a heat exchanger to heat the feedwater. However, these two methods do not directly utilize the condensate. The heat exchange process requires meticulous design of heat exchange parameters, resulting in low energy recovery efficiency. Furthermore, both the thermal storage system and the heat exchanger require substantial space for equipment placement and piping. Due to these issues, condensate heat recovery technology has not been widely adopted. One approach to directly recover condensate heat is to convert high-quality condensate into industrial heating steam through flash evaporation.

[0004] The conversion of boiler condensate into industrial heating steam can be achieved through flash evaporation. Boiler condensate is high-temperature, high-pressure saturated water (pressures can reach over 10 MPa, temperatures approaching 300°C), while industrial heating typically requires superheated steam at lower pressures (e.g., 0.8–1.6 MPa). Flash evaporation refers to the process where, when high-pressure, high-temperature saturated water is suddenly introduced into a low-pressure container, due to the sudden pressure drop, some of the water rapidly absorbs its own heat and boils and vaporizes, becoming low-pressure saturated steam, while the remaining water cools and becomes saturated water at the corresponding low pressure. This technology is widely used in seawater desalination, geothermal power generation, food concentration, and other fields, and the process is relatively mature. However, there is currently no research on applying this technology to the heat recovery process of condensate. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a low-load once-through boiler condensate flash evaporation heating system and its operation method, which utilizes boiler condensate flash evaporation under wet conditions to generate steam for industrial steam users. The specific technical approach is as follows: high-temperature, high-pressure condensate from the boiler is introduced into a specially designed flash tank via a valve. The flash tank maintains a low pressure matching the industrial heating network pressure. Inside the flash tank, a portion of the condensate instantly transforms into low-pressure saturated steam. This high-quality steam can be extracted, heated by the plant's power generation to become low-pressure superheated steam, and then directly supplied to the industrial heating network. The remaining low-pressure saturated water, with its still relatively high temperature and pressure, is returned to the feedwater pipeline, achieving cascaded energy utilization. This technology ensures that the once-through boiler unit can undertake deep peak-shaving tasks while simultaneously decoupling power supply and heating, meeting the boiler's stable combustion requirements, protecting the hydrodynamic safety of the water-cooled walls, and achieving multiple objectives through a bypass system design.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] A low-load DC boiler condensate flash heating system is described. This system is a bypass system connected to the outlet of the condensate storage tank 5 on the boiler condensate recirculation pipeline. The system includes a condensate flash control valve 8, a condensate flash tank 9, a heating steam throttling valve 10, an electric heater 11, and a condensate recirculation pump 12. The second outlet of the condensate storage tank 5 is connected to the inlet of the condensate flash control valve 8, the outlet of the condensate flash control valve 8 is connected to the inlet of the condensate flash tank 9, the steam outlet of the condensate flash tank 9 is connected to the inlet of the heating steam throttling valve 10, and the outlet of the heating steam throttling valve 10 is connected to the electric heater 11. The inlet of heater 11 is connected to the industrial heating steam pipeline, and the outlet of electric heater 11 is connected to the industrial heating steam pipeline. The condensate generated by flash evaporation is pressurized by condensate recirculation pump 12 and then re-enters economizer 1. When the once-through boiler unit undertakes peak shaving tasks and the boiler enters wet operation mode, the condensate generated by steam-water separator 3 is introduced into the condensate flash evaporation heating system. The generated high-pressure steam is used for industrial heating. The throttle valve 10 and electric heater 11 are used to ensure that the steam meets the pressure and superheat requirements of industrial heating steam. The condensate generated in flash tank 9 is re-entered into the feedwater pipeline through condensate recirculation pump 12.

[0008] In the boiler condensate recirculation pipeline, feedwater enters the economizer inlet 1, the economizer 1 outlet is connected to the water-cooled wall 2 inlet, the water-cooled wall 2 outlet is connected to the steam-water separator 3 inlet, the steam outlet of the steam-water separator 3 is connected to the superheater 4 inlet, the condensate outlet of the steam-water separator 3 is connected to the condensate storage tank 5 inlet, the condensate storage tank 5 outlet is connected to the condensate recirculation pump 6 inlet, the condensate recirculation pump 6 outlet is connected to the condensate recirculation valve 7 inlet, and the condensate recirculation valve 7 outlet is connected to the economizer 1 inlet; the superheater 4 outlet is connected to the turbine 13, and part of the intermediate steam from the turbine 13 is extracted for industrial heating.

[0009] The condensate flash heating system can provide high-pressure heating steam. The steam-water separator 3 generates high-pressure condensate to ensure that the heating steam pressure meets the requirements. The pressure value is regulated and controlled by the heating steam throttle valve 10. The heating steam is throttled by the heating steam throttle valve 10 and heated by the electric heater 11 to ensure that the superheat of the heating steam meets the requirements.

[0010] Among them, the electric heater 11 consumes the steam generated by the DC boiler unit to heat the steam, thereby increasing the superheat of the heating steam and reducing the power supply load.

[0011] The steam-water flow rate of the hydrophobic flash heating system is regulated by the hydrophobic flash control valve 8.

[0012] The hydrophobic steam flash heating system operates when the once-through boiler is in wet operation mode. When the once-through boiler is in dry operation mode, the industrial heating steam is provided by the steam turbine heating extraction port, which can ensure that the total demand for industrial heating steam load is met under various operating conditions.

[0013] The once-through boiler switches from dry to wet operation mode when the load is below 30%, and switches from wet to dry operation mode when the load is above 30%.

[0014] The operation method of the DC boiler condensate flash heating system under low load is as follows: when the DC boiler unit undertakes peak shaving tasks and the boiler enters wet operation mode, the condensate generated by the steam-water separator 3 is introduced into the condensate flash heating system, and the generated high-pressure steam is used for industrial heating. The heating steam throttle valve 10 and electric heater 11 are used to make the steam meet the pressure and superheat requirements of industrial heating steam. The condensate generated in the flash tank 9 is reintroduced into the feedwater pipeline through the condensate recirculation pump 12.

[0015] The described condensate flash heating system can help once-through boiler units achieve deep peak shaving. When the once-through boiler load is maintained at the minimum stable combustion load, the proportion of heating load is increased by increasing the condensate supply of the condensate flash heating system, while the corresponding electrical load output is reduced to achieve the purpose of peak shaving.

[0016] The aforementioned condensate flash heating system enables rapid load changes during peak-shaving operation of the DC boiler unit. Rapid load increase: With a fixed total heating flow, increasing the inlet water volume of the condensate flash heating system increases its heating load as a percentage of the total heating load, thereby reducing the heating load at the turbine 13's extraction port as a percentage of the total heating load, and reducing the extraction steam volume at the turbine 13's extraction port. The excess steam performs work in the latter half of the turbine 13's extraction port, rapidly increasing the electrical load output. Rapid load decrease: With a fixed total heating flow, reducing the inlet water volume of the condensate flash heating system decreases its heating load as a percentage of the total heating load, thereby increasing the heating load at the turbine 13's extraction port as a percentage of the total heating load, and increasing the extraction steam volume at the turbine 13's extraction port. The amount of steam performing work in the latter half of the turbine 13's extraction port decreases, rapidly reducing the electrical load output.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] The technical solution provided by this invention is a low-load once-through boiler condensate flash steam heating system and its operation method. This invention partially decouples the power generation and heating of the once-through boiler unit by utilizing condensate flash steam for electricity generation and electric heating steam supply. The boiler load is the sum of the power generation load and the heating load. Therefore, while achieving deep peak shaving and industrial heating, it meets the minimum stable combustion load requirements for boiler operation and protects the hydrodynamic safety of each water-cooled wall. The electric heater in the condensate flash steam heating system consumes the steam generated by the once-through boiler unit, thus consuming the power generated by the once-through boiler unit and significantly reducing the peak shaving pressure it bears. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a DC boiler condensate flash heating system under low load. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and examples.

[0021] This invention utilizes the high-quality condensate from the boiler during wet operation to generate high-pressure steam after flash evaporation and throttling heating, which is then supplied to industrial steam users. This enables the once-through boiler unit to simultaneously achieve multiple objectives, including decoupling power supply and heating, meeting the stable combustion requirements of the boiler operation, and protecting the hydrodynamic safety of each water-cooled wall, while undertaking peak shaving and industrial steam supply tasks. This is of great significance for power plants to achieve both economic and safe operation.

[0022] like Figure 1 As shown, this invention discloses a low-load DC boiler condensate flash heating system. The condensate flash heating system is a bypass system connected to the outlet of the condensate storage tank 5 on the boiler condensate recirculation pipeline. The system includes a condensate flash control valve 8, a condensate flash tank 9, a heating steam throttling valve 10, an electric heater 11, and a condensate recirculation pump 12. The second outlet of the condensate storage tank 5 is connected to the inlet of the condensate flash control valve 8, the outlet of the condensate flash control valve 8 is connected to the inlet of the condensate flash tank 9, and the steam outlet of the condensate flash tank 9 is connected to the inlet of the heating steam throttling valve 10. The outlet is connected to the inlet of the electric heater 11, and the outlet of the electric heater 11 is connected to the industrial heating steam pipeline. The condensate generated by flash evaporation is pressurized by the condensate recirculation pump 12 and then re-enters the economizer 1. When the once-through boiler unit undertakes peak shaving tasks and the boiler enters the wet operation mode, the condensate generated by the steam-water separator 3 is introduced into the condensate flash evaporation heating system, and the generated high-pressure steam is used for industrial heating. The throttle valve 10 and the electric heater 11 are used to ensure that the steam meets the pressure and superheat requirements of industrial heating steam. The condensate generated in the flash tank 9 is re-entering the feedwater pipeline through the condensate recirculation pump 12.

[0023] In the boiler condensate recirculation pipeline, feedwater enters the economizer inlet 1, the economizer 1 outlet is connected to the water-cooled wall 2 inlet, the water-cooled wall 2 outlet is connected to the steam-water separator 3 inlet, the steam outlet of the steam-water separator 3 is connected to the superheater 4 inlet, the condensate outlet of the steam-water separator 3 is connected to the condensate storage tank 5 inlet, the condensate storage tank 5 outlet is connected to the condensate recirculation pump 6 inlet, the condensate recirculation pump 6 outlet is connected to the condensate recirculation valve 7 inlet, and the condensate recirculation valve 7 outlet is connected to the economizer 1 inlet.

[0024] The analysis focuses on DC boiler units that undertake industrial heating and power peak shaving tasks.

[0025] When the unit load is high and the boiler is in dry operation mode, the boiler feedwater circulation does not produce condensate, and the condensate flash heating system does not participate in operation. Industrial heating steam is supplied by the turbine heating extraction port. As the unit load decreases, the coal feed rate decreases, the feedwater maintains the minimum boiler feedwater flow rate, the intermediate point temperature decreases, the superheat decreases, and the water level in the condensate storage tank 5 gradually increases. After the water level reaches a certain value, the condensate recirculation pump is started, and the recirculation flow rate is adjusted through the condensate recirculation valve to bring the unit into wet operation, generally at around 30% load. When the boiler enters wet operation mode, the bypass of the condensate flash heating system begins to participate in operation, and its participation in heating is as follows:

[0026] When the boiler switches to wet operation mode, the water in the condensate storage tank 5 is introduced into the condensate flash tank 9 to generate high-pressure steam and condensate. The high-pressure steam is throttled by the heating steam throttle valve 10 and heated by the electric heater 11 to obtain sufficient superheat before being introduced into the industrial heating steam pipeline. The condensate is pressurized by the condensate recirculation pump 12 and then re-enters the economizer 1 for heating.

[0027] This system can help the unit achieve deep peak shaving and rapid load change under peak shaving conditions.

[0028] 1. Help the unit complete deep peak shaving

[0029] When the boiler load is maintained at the minimum stable combustion load, the load provided by the boiler is a certain value. At this time, if the condensate supply of the condensate flash heating system is increased, the heating load will increase and the proportion of the heating load to the boiler load will also increase. Correspondingly, the electrical load output will decrease. The unit achieves the purpose of deeper peak regulation while maintaining the minimum stable combustion requirement of the boiler.

[0030] 2. Rapid load increase and decrease during peak-shaving operation of the unit

[0031] Rapid load increase: Under a fixed heating load, the required steam flow rate at the steam consumption end remains unchanged. At this time, more water from the condensate storage tank is introduced into the condensate flash heating system, increasing the system's water intake and raising its heating load ratio to the total heating load. This reduces the steam extraction rate at the heating extraction port of turbine 13. The saved high-quality steam can continue to perform work in turbine 13 after the extraction port. Due to the turbine's fast response, the electrical load output increases rapidly, achieving rapid load increase in the unit's peak-shaving operation mode. Rapid load decrease: Under a fixed heating load, the required steam flow rate at the steam consumption end remains unchanged. At this time, less water from the condensate storage tank is introduced into the condensate flash heating system, reducing the system's water intake and lowering its heating load ratio to the total heating load. This increases the steam extraction rate at the heating extraction port of turbine 13. The amount of high-quality steam continuing to perform work in turbine 13 after the extraction port decreases. Due to the turbine's fast response, the electrical load output decreases rapidly, achieving rapid load decrease in the unit's peak-shaving operation mode.

Claims

1. A low-load DC boiler condensate flash heating system, characterized in that: The condensate flash heating system is a bypass system connected to the outlet of the condensate storage tank (5) on the boiler condensate recirculation pipeline. It includes a condensate flash control valve (8), a condensate flash tank (9), a heating steam throttle valve (10), an electric heater (11), and a condensate recirculation pump (12). The second outlet of the condensate storage tank (5) is connected to the inlet of the condensate flash control valve (8), the outlet of the condensate flash control valve (8) is connected to the inlet of the condensate flash tank (9), and the steam outlet of the condensate flash tank (9) is connected to the heating steam throttle valve (10). The inlet of the hot steam throttle valve (10) is connected to the outlet of the heating steam throttle valve (10) and the outlet of the electric heater (11) is connected to the industrial heating steam pipeline. The condensate generated by flash evaporation is pressurized by the condensate recirculation pump (12) and then re-enters the economizer (1). When the DC boiler unit undertakes the peak shaving task and the boiler enters the wet operation mode, the condensate generated by the steam-water separator (3) is introduced into the condensate flash evaporation heating system, and the generated high-pressure steam is used for industrial heating.

2. The low-load DC boiler condensate flash heating system according to claim 1, characterized in that: In the boiler condensate recirculation pipeline, feedwater enters the economizer (1), the outlet of the economizer (1) is connected to the inlet of the water-cooled wall (2), the outlet of the water-cooled wall (2) is connected to the inlet of the steam-water separator (3), the steam outlet of the steam-water separator (3) is connected to the inlet of the superheater (4), the condensate outlet of the steam-water separator (3) is connected to the inlet of the condensate storage tank (5), the outlet of the condensate storage tank (5) is connected to the inlet of the condensate recirculation pump (6), the outlet of the condensate recirculation pump (6) is connected to the inlet of the condensate recirculation valve (7), and the outlet of the condensate recirculation valve (7) is connected to the inlet of the economizer (1); the outlet of the superheater (4) is connected to the turbine (13), and part of the intermediate steam of the turbine (13) is extracted for industrial heating.

3. The low-load once-through boiler condensate flash heating system according to claim 1, characterized in that: The condensate flash heating system can provide high-pressure heating steam. The steam-water separator (3) generates high-pressure condensate to ensure that the heating steam pressure meets the requirements. The pressure value is regulated and controlled by the heating steam throttle valve (10). The heating steam is throttled by the heating steam throttle valve (10) and heated by the electric heater (11) to ensure that the superheat of the heating steam meets the requirements.

4. The low-load DC boiler condensate flash heating system according to claim 3, characterized in that: The electric heater (11) consumes steam generated by the DC boiler unit to heat the steam, thereby increasing the superheat of the heating steam and reducing the power supply load.

5. The low-load once-through boiler condensate flash heating system according to claim 1, characterized in that: The water inlet flow rate of the hydrophobic flash heating system is regulated by the hydrophobic flash control valve (8).

6. The low-load once-through boiler condensate flash heating system according to claim 1, characterized in that: The hydrophobic flash heating system operates when the once-through boiler is in wet operation mode. When the once-through boiler is in dry operation mode, industrial heating steam is provided by the steam turbine heating extraction port.

7. The low-load once-through boiler condensate flash heating system according to claim 6, characterized in that: The once-through boiler switches from dry to wet operation mode when the load is below 30%, and switches from wet to dry operation mode when the load is above 30%.

8. The operation method of the once-through boiler condensate flash heating system under low load as described in any one of claims 1 to 7, characterized in that: When the DC boiler unit undertakes peak shaving tasks and the boiler enters wet operation mode, the condensate generated by the steam-water separator (3) is fed into the condensate flash heating system, and the generated high-pressure steam is used for industrial heating. The steam throttle valve (10) and electric heater (11) are used to make the steam meet the pressure and superheat requirements of industrial heating steam. The condensate generated in the flash tank (9) is re-entered into the feedwater pipeline through the condensate recirculation pump (12). The hydrophobic flash heating system can help DC boiler units achieve deep peak shaving. When the load of the DC boiler is maintained at the minimum stable combustion load, the proportion of heating load is increased by increasing the condensate supply of the condensate flash heating system, while the corresponding electrical load output is reduced to achieve the purpose of peak shaving. The condensate flash heating system can achieve rapid load change in the peak-shaving operation mode of the DC boiler unit; rapid load increase: under the condition of a fixed total heating flow, the water intake of the condensate flash heating system is increased, the heating load of the system is increased to the proportion of the total heating load, thereby reducing the proportion of the heating load of the steam extraction port of the turbine (13) to the total heating load, reducing the amount of steam extracted from the steam extraction port of the turbine (13), and the excess steam does work in the latter half of the steam extraction port of the turbine (13) to rapidly increase the electrical load output; rapid load decrease: under the condition of a fixed total heating flow, the water intake of the condensate flash heating system is reduced, the heating load of the system is decreased to the proportion of the total heating load, thereby increasing the proportion of the heating load of the steam extraction port of the turbine (13) to the total heating load, increasing the amount of steam extracted from the steam extraction port of the turbine (13), and the amount of steam doing work in the latter half of the steam extraction port of the turbine (13) is reduced, thus rapidly reducing the electrical load output.

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