Coal-fired power generation unit double-regenerative collaborative energy-saving and flexible operation system and construction method

Abstract

The invention belongs to the technical field of boiler flue gas waste heat recovery, and particularly relates to a coal-fired power generation unit double-heat-regeneration collaborative energy-saving and flexible operation system and a construction method thereof, and the system comprises an economizer and denitration catalytic reactor subsystem which is called the economizer subsystem for short; the boiler flue gas-air heat regenerative system comprises a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem. And a steam turbine steam extraction and heat regeneration subsystem. According to the construction method, through cooperation of airborne heat regeneration and water-borne heat regeneration, the flue gas waste heat recovery effect can be effectively improved, meanwhile, the unit circulation efficiency, the operation flexibility and the environmental protection performance are improved, and the unit coal consumption is reduced by 4-6 g / kWh.

Description

Technical Field

[0001] The present invention belongs to the technical field of boiler flue gas waste heat recovery, and specifically relates to a dual heat recovery synergistic energy-saving and flexible operation system and a construction method for a coal-fired power generation unit. Background Art

[0002] Efficient recovery of flue gas waste heat from coal-fired power generation boilers is an important research direction for improving the power generation efficiency of coal-fired power generation units. The most direct way to recover flue gas waste heat is to use a low-temperature economizer and use condensate to reduce the boiler exhaust temperature from about 130°C to about 90°C. The heated condensate returns to the low-pressure heating system. Since the steam extraction of the steam turbine is reduced, the work of the steam turbine is increased, thereby improving the cycle efficiency of the unit. This method is simple, but the efficiency of waste heat recovery is low. It can usually only reduce the coal consumption of power generation by about 2g / kWh, of which 0.5g / kWh is due to the reduction of flue gas resistance after the flue gas temperature is reduced. The coal consumption directly reduced by recovering waste heat usually does not exceed 1.5g / kWh.

[0003] At present, a boiler flue gas waste heat recovery system using partial flue gas bypass has been launched in China (such as Figure 2 As shown in the figure, it has been widely used in newly built large coal-fired power generation units. This system can recover flue gas waste heat relatively efficiently, and can usually reduce the coal consumption of power supply by about 3g / kWh. However, the boiler flue gas waste heat recovery system still has the following technical problems: (1) The primary air does not participate in the waste heat recovery of low-temperature flue gas. The forced circulation heat pipe system only heats the secondary air, and the primary air directly enters the air preheater. The primary air accounts for about 25% of the total air volume and is not used for low-temperature flue gas cooling and waste heat recovery of flue gas in the low-temperature zone, resulting in a reduced effect of flue gas waste heat recovery; (2) The flue gas temperature level in the bypass flue is not high enough. Since the forced circulation heat pipe system cannot raise the flue gas temperature at the air preheater outlet to a high enough level, the efficiency of the low-pressure heater in the bypass flue to recover the flue gas waste heat is low; (3) The utilization efficiency of the air preheater is reduced. When the flue gas temperature and heating surface area at the inlet of the air preheater remain unchanged, the air temperature at the outlet of the air preheater is lower than the condition without flue gas waste heat recovery. That is, after about 15% of the flue gas bypasses, the air temperature used for combustion is lower than the original design value, which will lead to a decrease in boiler efficiency and an increase in coal consumption. Although the flue gas waste heat recovery effect is improved by setting up a forced circulation heat pipe system to preheat the secondary air and by setting up a flue gas bypass and other technical means, the reduction in the air temperature at the outlet of the air preheater offsets part of the waste heat recovery effect, thereby affecting the overall efficiency of the flue gas waste heat recovery. Although the air preheater used is large in size, the heat transfer temperature difference becomes smaller, the heating surface cannot be fully utilized, and the air temperature cannot be effectively increased; (4) The boiler flue gas-air heat recovery system and the steam turbine extraction heat recovery system do not form a synergistic effect. From the perspective of reducing the heat consumption of the steam turbine, it is hoped that the feed water temperature will be increased, but the increase in feed water temperature will bring difficulties to the boiler design and reduce the boiler efficiency. After the flue gas bypass technology is adopted, although the heat transfer problem in the air preheater area is partially solved, it has no direct effect on increasing the feed water temperature and thus improving the net efficiency of the unit.

[0004] This field is in urgent need of seeking a new technical solution to solve the above problems. Summary of the invention

[0005] In order to solve the problems in the above-mentioned background technology, the present invention provides a method for constructing a dual heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit. The construction method coordinates water-borne heat recovery and gas-borne heat recovery to construct a system that not only efficiently recovers low-temperature flue gas waste heat, but also improves the unit's cycle efficiency, while improving the unit's operating flexibility and safety, economy and environmental protection performance under low load. The unit's power supply coal consumption can be reduced by 4~6g / kWh.

[0006] The first object of the present invention is to provide a method for constructing a double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit. The method is implemented based on the double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit. The system includes: an economizer and a denitration catalytic reactor subsystem, referred to as an economizer subsystem, the economizer subsystem includes a high-temperature economizer, a denitration catalytic reactor and a low-temperature economizer; the system also includes: a boiler flue gas-air heat recovery system, including a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem; the system also includes a steam turbine extraction heat recovery subsystem; The construction method comprises: By setting the low-temperature air preheating subsystem, the exhaust gas temperature of the low-temperature air preheating subsystem is reduced to 85°C, and the flue gas temperature at the flue outlet of the air preheater and the bypass flue subsystem is increased from 120°C to 130°C to 135°C to 220°C; By reducing the heating surface area of ​​the economizer subsystem, the flue gas temperature at the inlet of the air preheater and bypass flue subsystem is increased by 5 to 35K, and the secondary air temperature at the outlet of the air preheater and bypass flue subsystem is increased by 5 to 40K; The steam turbine extraction heat recovery subsystem is used to increase the boiler feed water temperature by 2 to 12K to offset the reduction in heat absorption due to the reduction in the heating surface area of ​​the economizer subsystem, while improving the unit cycle efficiency.

[0007] Furthermore, the construction method includes: The low-temperature economizer is eliminated from the economizer subsystem; or, the heating area of ​​the low-temperature economizer is reduced so that the flue gas temperature at the inlet of the air preheater and the bypass flue subsystem is increased by 5K~35K, and the secondary air hot air temperature is simultaneously increased by 5K~40K.

[0008] Furthermore, in the low-temperature air preheating subsystem, both the primary air and the secondary air participate in the recovery of the waste heat of the low-temperature flue gas, and the heat exchange between the flue gas and the air adopts a direct heat exchange method, an indirect heat exchange method or a partially indirect heat exchange method; When direct heat exchange is adopted, the primary air is sent in by the primary fan, absorbs the flue gas waste heat in the primary air low-temperature preheater, and then enters the primary air preheater to continue absorbing the flue gas heat; the secondary air is sent in by the blower, absorbs the flue gas waste heat in the secondary air low-temperature preheater, and then enters the secondary air preheater to continue absorbing the flue gas heat; When the indirect heat exchange method is adopted, the heat medium water from the steam turbine low-pressure heating system is supplied by the heat medium water supplementary water pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, and enters the heat medium water heater to absorb the flue gas waste heat after being pressurized by the heat medium water circulation pump; a part of the heated heat medium water returns to the steam turbine low-pressure heating system through the heat medium water return valve; the other part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, and after heat exchange with the primary air and the secondary air in the primary air heater and the secondary air heater, passes through the primary air heater inlet regulating valve and the secondary air heater inlet regulating valve and returns to the heat medium water circulation pump inlet, forming a heat medium water cycle; the primary air is supplied by the primary fan, absorbs the flue gas waste heat carried by the heat medium water in the primary air heater, and then enters the primary air preheater to continue to absorb the flue gas heat; the secondary air is supplied by the blower, absorbs the flue gas waste heat carried by the heat medium water in the secondary air heater, and then enters the secondary air preheater to continue to absorb the flue gas heat; When the partial indirect heat exchange method is adopted, the heat medium water from the steam turbine low-pressure heating system is supplied by the heat medium water supplementary water pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, and enters the heat medium water heater to absorb the waste heat of the flue gas after being pressurized by the heat medium water circulation pump; a part of the heated heat medium water returns to the steam turbine low-pressure heating system through the heat medium water return valve; the other part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, and after the primary air heater and the secondary air heater exchange heat with the primary air and the secondary air, the primary air heater and the secondary air heater are used to heat the primary air and the secondary air. The water inlet regulating valve of the blower and the water inlet regulating valve of the secondary air heater return to the inlet of the heat medium water circulation pump to form a heat medium water circulation; the primary air is delivered by the primary fan, absorbs the waste heat of the flue gas carried by the heat medium water in the primary air heater, then enters the primary air low-temperature preheater to continue absorbing the waste heat of the flue gas, and then enters the primary air preheater to absorb the flue gas heat; the secondary air is delivered by the blower, absorbs the waste heat of the flue gas carried by the heat medium water in the secondary air heater, then enters the secondary air low-temperature preheater to continue absorbing the waste heat of the flue gas, and then enters the secondary air preheater to absorb the flue gas heat.

[0009] Furthermore, the air preheater and bypass flue subsystem includes a primary air preheater and a secondary air preheater arranged in the main flue, and a bypass flue; When the flue gas temperature at the bypass flue outlet is higher than the feed water temperature at the feed water pump outlet by more than 10K, a bypass high temperature stage heater is provided in the bypass flue; No bypass low-temperature heater is required, and the flue gas heat of the bypass flue is all used to heat the feed water. In this case, the utilization rate of the water-borne heat recovery is high. When the flue gas temperature at the bypass flue outlet does not exceed the feed water temperature at the feed water pump outlet by 10K, in addition to the bypass high-temperature stage heater, a bypass low-temperature stage heater is also provided in the bypass flue. At this time, the utilization rate of water-carried heat recovery is low; The bypass flue is provided with a bypass flue gas regulating valve, through which the flue gas flow rate discharged from the main flue and the flue gas flow rate discharged from the bypass flue are distributed.

[0010] Further, in the steam turbine extraction and heat recovery subsystem, the boiler feed water pipeline includes: a first feed water pipeline for recovering heat through steam turbine extraction and a second feed water pipeline for recovering heat through flue gas heat in a bypass flue; When the flue gas temperature at the bypass flue outlet is higher than the feed water temperature at the feed water pump outlet by more than 10K, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are provided in the second feed water pipeline; when the flue gas temperature at the bypass flue outlet is lower than the feed water temperature at the feed water pump outlet by +10K, a bypass low-temperature stage heater is also provided in the second feed water pipeline; The construction method also includes: By increasing the steam extraction pressure of the steam turbine high-pressure heater, or providing a feedwater heater (such as a steam cooler), the feedwater temperature at the outlet of the first feedwater pipeline is increased by 2 to 12K, and the specific method is not limited; By bypassing the high temperature stage heater and the high temperature water regulating valve, the outlet water temperature of the second water supply pipeline is increased to the same level as the outlet water temperature of the first water supply pipeline or higher than the outlet water temperature of the first water supply pipeline.

[0011] The second object of the present invention is to provide a dual-reheat synergistic energy-saving and flexible operation system for a coal-fired power generation unit, the system comprising: an economizer and a denitration catalytic reactor subsystem, referred to as an economizer subsystem, the economizer subsystem comprising a high-temperature economizer, a denitration catalytic reactor and a low-temperature economizer; the system further comprises: a boiler flue gas-air heat recovery system, comprising a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem; the system further comprises a steam turbine extraction heat recovery subsystem; The boiler flue gas-air heat recovery system comprises: a boiler air inlet passage for allowing air to enter the boiler, a main flue for discharging the flue gas in the boiler, and a bypass flue for discharging the flue gas in the boiler; The steam turbine extraction steam regeneration subsystem includes: a boiler feed water pipeline that regenerates heat through steam turbine extraction steam and flue gas heat in a bypass flue; The boiler air intake channel includes a primary air intake channel and a secondary air intake channel. A primary air preheater and a primary air low-temperature preheater are arranged in the primary air intake channel. The flue gas discharged through the main flue passes through the primary air preheater and the primary air low-temperature preheater in sequence, and the primary air is reheated by utilizing the waste heat of the flue gas; a secondary air preheater and a secondary air low-temperature preheater are arranged in the secondary air intake channel. The flue gas discharged through the main flue passes through the secondary air preheater and the secondary air low-temperature preheater in sequence, and the secondary air is reheated by utilizing the waste heat of the flue gas.

[0012] Furthermore, in the low-temperature air preheating subsystem, the heat exchange between the flue gas and the air is carried out by a direct heat exchange method, an indirect heat exchange method or a partially indirect heat exchange method; When direct heat exchange is adopted, the low-temperature air preheater subsystem includes: a primary air fan, a blower, a primary air low-temperature preheater and a secondary air low-temperature preheater; When the indirect heat exchange method is adopted, the low-temperature air preheater subsystem includes: a primary air fan, a blower, a primary air heater, a secondary air heater, a primary air heater water inlet regulating valve, a secondary air heater water inlet regulating valve, a heat medium water heater, a heat medium water circulation pump, a heat medium water return valve, a heat medium water flow regulating valve and a heat medium water replenishment pump; When a partial indirect heat exchange method is adopted, the low-temperature air preheater subsystem includes: a primary fan, a blower, a primary air low-temperature preheater, a secondary air low-temperature preheater, a primary air heater, a secondary air heater, a primary air heater water inlet regulating valve, a secondary air heater water inlet regulating valve, a heat medium water heater, a heat medium water circulation pump, a heat medium water return valve, a heat medium water flow regulating valve and a heat medium water replenishment pump.

[0013] Furthermore, the air preheater and bypass flue subsystem includes: a primary air preheater and a secondary air preheater arranged in the main flue, and a bypass flue; a bypass high-temperature stage heater and a bypass flue gas damper regulating valve are arranged in the bypass flue, and when the flue gas temperature at the bypass flue outlet does not exceed the feed water temperature of the feed water pump outlet by 10K, a bypass low-temperature stage heater is also arranged in the bypass flue; and a bypass flue gas regulating valve is arranged in the bypass flue.

[0014] Further, in the steam turbine extraction heat recovery subsystem, the boiler feed water pipeline includes: a first feed water pipeline through the steam turbine extraction heat recovery and a second feed water pipeline through the flue gas heat recovery in the bypass flue; The first feedwater pipeline is provided with a condensate pump, a turbine low-pressure heater, a deaerator, a feedwater pump, a high-pressure heater regulating valve, a turbine high-pressure heater and a feedwater heater; When the flue gas temperature at the bypass flue outlet is higher than the water supply temperature at the water pump outlet by more than 10K, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are provided in the second water supply pipeline; when the flue gas temperature at the bypass flue outlet is lower than the water supply temperature at the water pump outlet + 10K, a bypass low-temperature stage heater is also provided in the second water supply pipeline.

[0015] In summary, the dual heat recovery synergistic energy-saving and flexible operation system and construction method of the coal-fired power generation unit in the disclosed embodiment of the present invention can bring the following beneficial effects: (1) Through the coordination of the two heat recovery systems, air-borne heat recovery and water-borne heat recovery, the waste heat of low-temperature flue gas can be efficiently recovered and the cycle efficiency of the unit can be improved. The coal consumption of the unit can be reduced by 4~6g / kWh; (2) Effectively increase the temperature of combustion air, especially when operating at low load, which is conducive to stable combustion in the furnace and complete burnout of pulverized coal, improving operational stability and economy, and is also conducive to organizing staged combustion, reducing NOx generation, and having good environmental performance; (3) By combining the valves of each system in the unit, the adjustment means are increased and the adjustable range is widened, so the flexibility, economy and reliability of operation are better guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 A schematic diagram of a double heat recovery coordinated energy saving and flexible operation system for a coal-fired power generation unit according to an exemplary embodiment (Example 1); Figure 2 It is the overall structure diagram of the boiler flue gas waste heat recovery system in the prior art; Figure 3 A schematic diagram of a double heat recovery coordinated energy saving and flexible operation system for a coal-fired power generation unit according to an exemplary embodiment (Example 2); Figure 4 A schematic diagram of a double heat recovery coordinated energy saving and flexible operation system for a coal-fired power generation unit according to an exemplary embodiment (Example 3); Figure 5 A schematic diagram of a double heat recovery coordinated energy saving and flexible operation system for a coal-fired power generation unit according to an exemplary embodiment (Example 4); Figure 6The diagram is a structural diagram of a dual-reheat collaborative energy-saving and flexible operation system for a coal-fired power generation unit according to an exemplary embodiment (Example 5).

[0018] 1-high temperature economizer, 2-denitrification catalytic reactor (SCR), 3-low temperature economizer, 4-primary air preheater, 5-secondary air preheater, 6-bypass flue, 7-bypass high temperature stage heater, 8-bypass low temperature stage heater, 9-bypass flue gas damper regulating valve, 10-primary air low temperature preheater, 11-secondary air low temperature preheater, 12-primary fan, 13-air supply fan, 14-primary air heater, 15-secondary air heater, 16-primary air heater water inlet regulating valve, 17-secondary air heater water inlet regulating valve, 18-heat medium water heater, 19-heat medium water circulation pump, 20-heat medium water return valve, 21- Heat medium water flow regulating valve, 22-heat medium water make-up water pump, 23-electrostatic precipitator, 24-induced draft fan, 25-condensate pump, 26-turbine low-pressure heater, 27-bypass low-temperature water pump, 28-bypass low-temperature water regulating valve, 29-deaerator, 30-feed water pump, 31-bypass high-temperature water regulating valve, 32-high-pressure regulating valve, 33-turbine high-pressure heater, 34-feed water heater, 35-boiler air inlet channel, 36-main flue, 38-boiler feed water pipeline, 39-first feed water pipeline, 40-second feed water pipeline, 41-heat medium water circulation pipeline, 42-first heat medium water pipeline, 43-second heat medium water pipeline. DETAILED DESCRIPTION

[0019] The following will be combined with the accompanying drawings in the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention.

[0020] The present invention provides a method for constructing a double heat recovery collaborative energy-saving and flexible operation system for a coal-fired power generation unit. The construction method is based on the double heat recovery collaborative energy-saving and flexible operation system for the coal-fired power generation unit. The system achieves the goal of improving the energy saving and flexible operation performance of the unit through the coordination of the water-borne heat recovery system and the gas-borne heat recovery system.

[0021] For example, before introducing the construction method in the disclosed embodiment of the present invention, the double heat recovery synergistic energy saving and flexible operation system of the coal-fired power generation unit is explained.

[0022] In fact, coal-fired power generation units have two heat recovery systems, namely the boiler flue gas-air heat recovery system (referred to as the air heat recovery system) and the steam turbine extraction heat recovery system. The function of the two heat recovery systems is to recover the waste heat that was originally intended to be discharged into the environment (low-temperature heat source) and improve the energy quality through the high-temperature heat source (furnace) for continued work, which is why it is called heat recovery.

[0023] In the flue gas waste heat utilization scenario, the flue gas waste heat is used to heat the combustion air and returns to the furnace with the combustion air, which is defined as air-borne heat recovery; the flue gas waste heat is used to heat the working fluid water and then returns to the steam turbine heat recovery system with the working fluid water, which is defined as water-borne reflux. The utilization efficiency of air-borne heat recovery is high, which is the cycle efficiency of the unit; the utilization efficiency of water-borne heat recovery is low. The higher the temperature of the returned working fluid water, the higher the utilization efficiency, but it is always lower than the cycle efficiency of the unit. The utilization efficiency of flue gas waste heat is higher when it is carried by combustion air, while the utilization efficiency is lower when it is carried by working fluid water, that is, the efficiency of air-borne heat recovery is always higher than that of water-borne heat recovery.

[0024] For the dual heat recovery synergistic energy saving and flexible operation system of the coal-fired power generation unit described in the present invention, due to the provision of a bypass flue 6, in which a bypass high-temperature stage heater 7 and a bypass low-temperature stage heater 8 are provided, the second water supply pipeline 40 formed is an additional water-carrying heat recovery pipeline; in addition, when the low-temperature air preheating subsystem adopts an indirect or partially indirect heat exchange method, a part of the heated heat medium water is returned to the steam turbine low-heat system through the heat medium water circulation pipeline 41 and the heat medium water return valve 20, forming another water-carrying heat recovery pipeline. Therefore, the water-carrying heat recovery of the system described in the present invention includes three parts: steam turbine extraction heat recovery, water-carrying heat recovery of the bypass flue heater and water-carrying heat recovery of the heat medium water. The water-carrying heat recovery of the three parts constitutes a water-carrying heat recovery system.

[0025] The gas-borne heat recovery system of the dual heat recovery synergistic energy saving and flexible operation system of the coal-fired power generation unit described in the present invention, namely the boiler flue gas-air heat recovery system, includes: a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem.

[0026] The dual heat recovery synergistic energy saving and flexible operation system described in the present invention achieves two goals by coordinating air-borne heat recovery and water-borne heat recovery: first, to increase the share of air-borne heat recovery, and second, to improve the quality of water-borne heat recovery, that is, the temperature of the return water, thereby improving the cycle efficiency and reducing coal consumption.

[0027] When designing the unit, the two heat recovery systems must be optimized in a coordinated manner to maximize the unit's cycle efficiency. An important parameter for the coordinated optimization of the two heat recovery systems is the feed water temperature. From the perspective of reducing the heat consumption of the steam turbine, increasing the feed water temperature will help reduce the heat consumption. However, the increase in feed water temperature also increases the difficulty of boiler design, reduces the cooling capacity of flue gas, reduces boiler efficiency, and thus reduces the unit's cycle efficiency. If the boiler efficiency does not decrease while the feed water temperature is increased, the unit's cycle efficiency will be improved. When the flue gas waste heat at the tail of the boiler is deeply utilized, it is particularly important to coordinate and optimize the two heat recovery systems. In each temperature range of flue gas waste heat utilization, the effective coordination of the two heat recovery systems can ensure efficient heat transfer between flue gas and air, flue gas and water, and make full use of each heating surface, thereby ensuring efficient recovery of waste heat. In addition, if the two heat recovery systems work together effectively, the boiler efficiency can be guaranteed while the feed water temperature is increased, thereby improving the net efficiency of the unit and achieving the goal of deep and efficient coal saving.

[0028] To this end, the construction of double heat recovery synergistic energy saving and flexible operation system should follow the following principles: (1) The waste heat from flue gas should first be used to heat the combustion air as much as possible, especially to heat the secondary air. The waste heat recovery benefit obtained by increasing the temperature of the secondary air is maximized.

[0029] (2) Both primary and secondary air should effectively participate in the flue gas waste heat recovery, especially the waste heat recovery of the low-temperature part of the flue gas, so that the waste heat recovery effect of the flue gas can be fully reflected. If only the secondary air participates in the waste heat recovery of the low-temperature part of the flue gas, the waste heat recovery effect of the flue gas will be partially reduced.

[0030] (3) Raise the flue gas temperature level of the bypass flue as much as possible. The higher the temperature of the steam turbine extraction steam replaced by the outlet water of the bypass flue heater, the higher the efficiency of waste heat recovery. When only a high-pressure heater is installed in the bypass flue and its outlet water is directly used as feed water, the highest waste heat recovery efficiency is achieved.

[0031] (4) Through the coordination of the two heat recovery systems, the feed water temperature can be appropriately increased, which can improve the unit cycle efficiency and improve the low-load operation performance of the unit. Measures to increase the feed water temperature include optimizing the design of the steam turbine heat recovery system and adding a steam cooler or a first-stage high-pressure heater.

[0032] To this end, this patent adopts the following technical measures: 1) Set up a low-temperature air preheating subsystem, and both the primary air and the secondary air must effectively participate in the recovery of low-temperature flue gas waste heat. Setting up a low-temperature air preheating subsystem can effectively recover the waste heat of low-temperature flue gas. Directly using the waste heat of low-temperature flue gas to heat the air can play the role of recovering the waste heat of low-temperature flue gas, which is beneficial to increase the air-borne heat recovery share and has high waste heat utilization efficiency. The temperature of the hot primary air usually does not need to be too high, which mainly depends on the requirements of the coal type for drying. The appropriate hot primary air temperature can be achieved by adjusting the size of the heating surface of the primary air preheater 4, rather than preventing the primary air from participating in the recovery of low-temperature flue gas waste heat.

[0033] When the low-temperature air preheating subsystem adopts the flue gas-air direct heat exchange method, the system and equipment are simple (such as Example 5), but the low-temperature end heating surfaces of the primary air low-temperature preheater 10 and the secondary air low-temperature preheater 11 are in a highly corrosive flue gas atmosphere, and anti-corrosion measures are required; if the forced circulation heat pump (heat medium water circulation) indirect heat exchange method is adopted (such as Example 2), the system is more complex, the equipment is more, the cost is higher, and the corrosion problem is transferred to the low-temperature end of the heat medium water heater 18, and this indirect heat exchange method is not conducive to improving the temperature level of the bypass flue 6, which will affect the quality of water-carried heat recovery, thereby affecting the effect of waste heat utilization. When the outlet temperature of the bypass flue 6 is required to exceed 180°C, it is difficult to directly use condensate or feed water as the working fluid of the forced circulation heat pump (heat medium water circulation) system, and it is difficult to match the steam-water system of the unit. It may be necessary to use other working fluids such as heat transfer oil, which will add uncertain factors to the power plant management and equipment safety. A hybrid of the first two methods, that is, partial indirect heat exchange (such as Examples 1, 3, and 4), is a feasible technical route, and there are more means of adjustment during operation, but the system is complex, with more equipment, and the cost will be increased.

[0034] 2) Increase the flue gas temperature at the outlet of the air preheater and bypass flue subsystem: Increasing the flue gas temperature at the outlet of the air preheater and the bypass flue subsystem, that is, increasing the flue gas temperature level of the bypass flue 6, can improve the quality of the water-borne heat recovery of the bypass flue heater, thereby improving the efficiency of waste heat recovery. The use of a low-temperature air preheating subsystem is an effective means to raise the flue gas temperature at the outlet of the bypass flue 6, which can usually raise the outlet flue gas temperature of the bypass flue 6 from 120°C~130°C to 135°C~220°C. For a 1000MW coal-fired power generation unit, if this temperature is raised to above 210°C, it is no longer necessary to set up a bypass low-temperature stage heater 8 in the bypass flue 6 (as in Examples 3, 4 and 5), and only a bypass high-temperature stage heater 7 is required. At this time, the water-borne heat recovery efficiency of the bypass heater is higher.

[0035] Increasing the flue gas temperature at the outlet of the air preheater and bypass flue subsystem is also beneficial to increasing the temperature of the heat medium water carrier heat recovery, thereby improving the water carrier heat recovery effect.

[0036] 3) Increase the inlet flue gas temperature of the air preheater and bypass flue subsystem by 5K~35K, and the secondary hot air temperature can be increased by 5K~40K simultaneously.

[0037] In the economizer subsystem, the low-temperature economizer 3 is eliminated (such as in Examples 4 and 5) or the heating area of ​​the low-temperature economizer 3 is reduced (such as in Examples 1, 2 and 3), thereby increasing the inlet flue gas temperature of the air preheater and bypass flue duct subsystem by 5K~35K (such as for a 1000MW unit, from the original 360°C to 370~395°C). This not only effectively increases the outlet air temperature of the air preheater, but also increases the heat transfer temperature difference between the flue gas and the air, and can control the heating area of ​​the air preheater within a reasonable range. The increase in the outlet air temperature of the air preheater indicates that the air-borne heat recovery part increases, which directly increases the input heat of the furnace, thereby directly reducing the coal consumption. The increase in the outlet air temperature of the secondary air preheater 5 also helps the stable combustion and burnout of the pulverized coal in the furnace, thereby helping to improve the combustion efficiency of the boiler and the stable operation of the boiler at low load. At the same time, it also helps to better adopt the staged combustion technology, reduce the NOx content at the outlet of the furnace, and help to reduce the burden of the subsequent denitration catalytic reactor (SCR) 2 and reduce the amount of ammonia injection.

[0038] For boilers that were originally designed without a low-temperature economizer 3, the temperature of the flue gas entering the denitrification catalytic reactor (SCR) 2 and the temperature entering the air preheater and bypass flue subsystem can be increased by reducing the heating surface of the economizer (for example, part of the economizer heating surface can be removed in the renovation project).

[0039] Prior art (see Figure 2 ) After the bypass flue bypasses about 15% of the flue gas, the air outlet temperature of the air preheater cannot reach the original design value (the level before the bypass flue is not set), and the air-borne heat recovery does not increase but decreases, which will cause the boiler efficiency and unit cycle efficiency to decrease, and reduce the effect of flue gas waste heat recovery; and after the bypass flue is adopted, although the heat transfer temperature difference is slightly improved, the heating area required by the air preheater is still huge, the manufacturing cost is high and affects the overall layout. This patent solves this problem, and the temperature of the boiler combustion air is also improved, and the air-borne heat recovery is increased.

[0040] 4) Increase boiler feed water temperature by 2K~12K.

[0041] After the low-temperature economizer 3 is removed or the heating surface of the low-temperature economizer 3 is reduced, the feed water temperature needs to be increased, which is equivalent to freeing up space for increasing the feed water temperature, which can increase the steam turbine extraction heat recovery, reduce the steam turbine heat consumption, and improve the unit cycle efficiency. To increase the air preheater inlet flue gas temperature by 5K~35K, the feed water temperature needs to be increased by 2K~12K (for example, for a 1000MW unit, it is increased from the original 318°C to 320~330°C). This can be achieved by increasing the extraction pressure of the steam turbine high-pressure heater 33 or adding a feed water heater 34 (such as adding a steam cooler or a high-pressure heater), etc., and this patent does not make specific restrictions.

[0042] 5) Set the bypass flue gas damper regulating valve 9.

[0043] A bypass flue gas damper regulating valve 9 is provided at the inlet or outlet of the bypass flue 6 to distribute the flue gas flow through the air preheater and the bypass flue 6, thereby adjusting the air temperature at the outlet of the air preheater. At low load, increasing the proportion of flue gas entering the air preheater can increase the outlet air temperature, which is beneficial to the stable combustion of the boiler and the control of the tail flue gas temperature within a reasonable range. The bypass flue gas damper regulating valve 9 is coordinated with the bypass low-temperature water regulating valve 28 and the bypass high-temperature water regulating valve 31, which is more convenient for the stable and economical operation of the unit under low load.

[0044] 6) Increase the means of unit adjustment and increase the adjustable range.

[0045] The technical measures of increasing the flue gas temperature at the air preheater inlet, increasing the feed water temperature, adding flue gas bypass and adjusting dampers, and setting up a low-temperature air preheating subsystem have been combined into one, which not only improves the unit cycle efficiency, but also provides a powerful means for the flexible operation of the boiler. These adjustment measures include: Precise regulation of secondary air temperature - Through the joint regulation of the bypass flue gas damper regulating valve 9, the secondary air heater water inlet regulating valve 17, the bypass low-temperature water regulating valve 28 and the bypass high-temperature water regulating valve 31, the secondary air temperature can be precisely regulated, thereby providing conditions for stable combustion of the boiler and improving boiler efficiency, especially when operating at low load.

[0046] Precise regulation of primary air temperature - Through the joint regulation of the bypass flue gas damper regulating valve 9 and the primary air heater water inlet regulating valve 16, etc., the precise regulation of primary air temperature can be achieved. Usually, the primary air hot air temperature needs to match the coal drying output requirements of the pulverizing system. When the primary air hot air temperature is too high, cold air needs to be added, which will cause irreversible losses. Precise control of the primary air temperature can eliminate or reduce this irreversible loss and improve the unit cycle efficiency.

[0047] Control of flue gas temperature at the inlet of electrostatic precipitator 23 - Through the joint adjustment of the heat medium water flow regulating valve 21, the bypass flue gas damper regulating valve 9 and the heat medium water return valve 20, the flue gas temperature at the inlet of the electrostatic precipitator 23 can be controlled, especially at low load to ensure that this temperature is not lower than 85°C to ensure the safety of subsequent equipment.

[0048] Bypass high temperature stage heater 7 outlet water temperature control - The bypass flue gas damper regulating valve 9 and the bypass high temperature water regulating valve 31 can control the bypass high temperature stage heater 7 outlet water temperature. This temperature is slightly higher than the feed water temperature to improve the circulation efficiency. Too high a temperature may affect the system operation. Therefore, it is very important to achieve accurate and effective control.

[0049] Bypass low-temperature stage heater 8 outlet water temperature control - Through the joint adjustment of the bypass flue gas damper regulating valve 9 and the bypass low-temperature water regulating valve 28, the outlet water temperature of the bypass low-temperature stage heater 8 can be effectively controlled. This temperature needs to match the operating temperature of the deaerator 29 to avoid irreversible losses.

[0050] Full load feed water temperature control - The feed water temperature can be controlled within the full load range through the steam turbine high pressure heater 33, feed water heater 34, bypass flue gas damper regulating valve 9 and bypass high temperature water regulating valve 31, thereby ensuring safe, economical and flexible operation of the unit.

[0051] 7) The dual heat recovery system works together in each flue gas temperature zone to improve the unit's cycle efficiency and operational flexibility.

[0052] A low-temperature air preheating subsystem is set up to raise the flue gas temperature level of bypass flue 6, thereby increasing the outlet water temperature level of the bypass flue heater and improving the quality of water-borne heat recovery. The work capacity of the steam turbine extraction steam replaced by it is increased, which improves the cycle efficiency of the unit and is the first synergy of the unit's double heat recovery system.

[0053] A flue gas bypass 6 is set up to increase its flue gas temperature level (including the flue gas temperature at the outlet and the inlet), and a bypass high-temperature stage heater 7 and a bypass low-temperature stage heater 8 are set therein. The water-borne heat recovery shares part of the flue gas waste heat recovery for the boiler air heat recovery system, sacrificing a little the steam turbine heat consumption index, but providing heat balance and a reasonable heat transfer temperature difference for the heat exchange between the flue gas and the air. Overall, the cycle efficiency of the unit is improved, which is the second synergy of the unit's double heat recovery system.

[0054] Increasing the inlet flue gas temperature of the air preheater increases the heat absorbed by the air from the flue gas and reduces the heat absorbed by the feed water from the flue gas, which frees up space for increasing the feed water temperature and thus reducing the heat consumption of the steam turbine. This is the third synergy of the unit's double heat recovery system, which improves the unit's cycle efficiency as a whole, and the improvement in the unit's cycle efficiency is large, making it the most important synergy among the three synergies. This synergy is a special product under the condition of deep recovery of flue gas waste heat. The addition of a large number of heating surfaces at the tail of the boiler creates conditions for this. At the same time, it can also be said that this synergy fully utilizes the large number of heating surfaces added at the tail of the boiler.

[0055] The three synergies of the unit's dual heat recovery system are an inseparable whole that is interconnected. Its essence is the reconstruction of the boiler tail heat transfer system and the steam turbine heat recovery system. The three synergies of water-borne heat recovery and gas-borne heat recovery work together to improve the economy, flexibility and environmental performance of the unit's operation, and the effect is more obvious when operating at low load.

[0056] The following is combined with Figure 1 , Figure 3-Figure 6 , specifically discuss the embodiments of the present invention and analyze their performance.

[0057] Attached Figure 1 That is, Example 1 has the most complete equipment and system among the examples of the present invention. In this example, the low-temperature air preheating subsystem adopts a partial indirect heat exchange method, and its relatively high temperature zone adopts a direct heat exchange method between flue gas and air, and is provided with a primary air low-temperature preheater 10 and a secondary air low-temperature preheater 11; the low-temperature zone adopts an indirect heat exchange method, and is provided with a heat medium water system. A bypass high-temperature stage heater 7 and a bypass low-temperature stage heater 8 are provided in its bypass flue 6. Its economizer subsystem is provided with a high-temperature economizer 1, a denitrification catalytic reactor (SCR) 2 and a low-temperature economizer 3. The low-temperature economizer 3 is not cancelled, but the heating surface area of ​​the low-temperature economizer 3 is reduced. The example system has good regulation performance and moderate energy-saving effect. There is no corrosion problem in the low-temperature air preheater area, and the low-temperature section of the heat medium water heater 18 requires anti-corrosion measures.

[0058] Attached Figure 3 That is, in Example 2, the low-temperature air preheating subsystem adopts an indirect heat exchange method, and does not have a primary air low-temperature preheater 10 and a secondary air low-temperature preheater 11. The system is relatively simple, but it cannot effectively raise the flue gas temperature at the outlet of the air preheater and the bypass flue subsystem, affecting the quality of water-borne heat recovery. Other subsystems are the same as Example 1. Compared with Example 1, Example 2 is relatively simple, but the energy-saving effect is slightly inferior to Example 1.

[0059] Attached Figure 4That is, Example 3 is a variation of Example 1. The outlet flue gas temperature of the air preheater and the bypass flue subsystem is raised to a sufficiently high temperature through the low-temperature air preheating subsystem. Therefore, only the bypass high-temperature heater 7 is provided in the bypass flue 6, and the bypass low-temperature heater 8 is not required. This will improve the effect of water-borne heat recovery. Under the same conditions of other parameters, the energy-saving effect of Example 3 is better than that of Example 1.

[0060] Attached Figure 5 That is, Example 4 is based on Example 3, but the low-temperature economizer 3 is removed from the economizer subsystem, and the inlet flue gas temperature of the air preheater and bypass flue subsystem is increased by 25-35K; the feed water temperature is increased by 8-12K by increasing the extraction pressure of the steam turbine high-pressure heater and / or adding a feed water heater. Therefore, the energy saving efficiency of Example 4 is better than that of Example 3.

[0061] Attached Figure 6 That is, Example 5 is the most concise system and equipment among the examples of the present invention. The low-temperature air preheating subsystem of this example adopts a flue gas-air direct heat exchange method, and directly sets a primary air low-temperature preheater 10 and a secondary air low-temperature preheater 11, which can effectively raise the outlet flue gas temperature of the air preheater and the bypass flue subsystem; only a bypass high-temperature stage heater 7 is set in the bypass flue 6, and there is no need to set a bypass low-temperature stage heater 8, and the quality of water-borne heat recovery is high; the low-temperature economizer 3 is cancelled in the economizer subsystem, and the inlet flue gas temperature of the air preheater and the bypass flue subsystem is increased by 25 to 35K; by increasing the steam extraction pressure of the steam turbine high-pressure heater and / or adding a feed water heater, etc., the feed water temperature is increased by 8 to 12K. Example 5 has the best energy-saving effect among the examples of the present invention, but its adjustment performance is slightly inferior, and the low-temperature sections of its primary air low-temperature preheater 10 and secondary air low-temperature preheater 11 have the risk of low-temperature corrosion.

[0062] The dual heat recovery synergistic energy saving and flexible operation system of the coal-fired power generation unit provided in the present application is constructed by the above-mentioned construction method, and therefore has at least all the beneficial effects brought by the technical scheme of the embodiment of the above-mentioned construction method, which will not be described one by one here.

[0063] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description here should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in the field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A method for constructing a double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit, characterized in that: The construction method is implemented based on a dual-reheat synergistic energy-saving and flexible operation system for coal-fired power generation units, the system comprising: an economizer subsystem, the economizer subsystem comprising a high-temperature economizer, a denitration catalytic reactor and a low-temperature economizer; the system further comprises: a boiler flue gas-air heat recovery system, comprising a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem; the system further comprises a steam turbine extraction heat recovery subsystem; The construction method comprises: By setting the low-temperature air preheating subsystem, the exhaust gas temperature of the low-temperature air preheating subsystem is reduced to 85°C, and the flue gas temperature at the flue outlet of the air preheater and the bypass flue subsystem is increased from 120°C to 130°C to 135°C to 220°C; By reducing the heating surface area of ​​the economizer subsystem, the flue gas temperature at the inlet of the air preheater and bypass flue subsystem is increased by 5 to 35K, and the secondary air temperature at the outlet of the air preheater and bypass flue subsystem is increased by 5 to 40K; The boiler feed water temperature is increased by 2 to 12K through the steam turbine extraction heat recovery subsystem.

2. The method for constructing a double-reheat synergistic energy-saving and flexible operation system for a coal-fired power generation unit according to claim 1, characterized in that: The construction method comprises: The low-temperature economizer is eliminated from the economizer subsystem; or, the heating area of ​​the low-temperature economizer is reduced so that the flue gas temperature at the inlet of the air preheater and the bypass flue subsystem is increased by 5K~35K, and the secondary air hot air temperature is simultaneously increased by 5K~40K.

3. The method for constructing a double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit according to claim 1, characterized in that: In the low-temperature air preheating subsystem, both the primary air and the secondary air participate in the recovery of the waste heat of the low-temperature flue gas, and the heat exchange between the flue gas and the air adopts a direct heat exchange method, an indirect heat exchange method or a partially indirect heat exchange method; When direct heat exchange is adopted, the primary air is sent in by the primary fan, absorbs the flue gas waste heat in the primary air low-temperature preheater, and then enters the primary air preheater to continue absorbing the flue gas heat; the secondary air is sent in by the blower, absorbs the flue gas waste heat in the secondary air low-temperature preheater, and then enters the secondary air preheater to continue absorbing the flue gas heat; When the indirect heat exchange method is adopted, the heat medium water from the steam turbine low-pressure heating system is supplied by the heat medium water supplementary water pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, and enters the heat medium water heater to absorb the flue gas waste heat after being pressurized by the heat medium water circulation pump; a part of the heated heat medium water returns to the steam turbine low-pressure heating system through the heat medium water return valve; the other part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, and after heat exchange with the primary air and the secondary air in the primary air heater and the secondary air heater, passes through the primary air heater inlet regulating valve and the secondary air heater inlet regulating valve and returns to the heat medium water circulation pump inlet, forming a heat medium water cycle; the primary air is supplied by the primary fan, absorbs the flue gas waste heat carried by the heat medium water in the primary air heater, and then enters the primary air preheater to continue to absorb the flue gas heat; the secondary air is supplied by the blower, absorbs the flue gas waste heat carried by the heat medium water in the secondary air heater, and then enters the secondary air preheater to continue to absorb the flue gas heat; When the partial indirect heat exchange method is adopted, the heat medium water from the steam turbine low-pressure heating system is supplied by the heat medium water supplementary water pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, and enters the heat medium water heater to absorb the waste heat of the flue gas after being pressurized by the heat medium water circulation pump; a part of the heated heat medium water returns to the steam turbine low-pressure heating system through the heat medium water return valve; the other part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, and after the primary air heater and the secondary air heater exchange heat with the primary air and the secondary air, the primary air heater and the secondary air heater are used to heat the primary air and the secondary air. The water inlet regulating valve of the blower and the water inlet regulating valve of the secondary air heater return to the inlet of the heat medium water circulation pump to form a heat medium water circulation; the primary air is delivered by the primary fan, absorbs the waste heat of the flue gas carried by the heat medium water in the primary air heater, then enters the primary air low-temperature preheater to continue absorbing the waste heat of the flue gas, and then enters the primary air preheater to absorb the flue gas heat; the secondary air is delivered by the blower, absorbs the waste heat of the flue gas carried by the heat medium water in the secondary air heater, then enters the secondary air low-temperature preheater to continue absorbing the waste heat of the flue gas, and then enters the secondary air preheater to absorb the flue gas heat.

4. The method for constructing a double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit according to claim 1, characterized in that: The air preheater and bypass flue subsystem includes a primary air preheater and a secondary air preheater arranged in the main flue, and a bypass flue; When the flue gas temperature at the bypass flue outlet is higher than the feed water temperature at the feed water pump outlet by more than 10K, a bypass high temperature stage heater is provided in the bypass flue; When the flue gas temperature at the bypass flue outlet does not exceed the feed water temperature at the feed water pump outlet by 10K, in addition to the bypass high-temperature stage heater, a bypass low-temperature stage heater is also provided in the bypass flue; The bypass flue is provided with a bypass flue gas regulating valve, through which the flue gas flow rate discharged from the main flue and the flue gas flow rate discharged from the bypass flue are distributed.

5. The method for constructing a double heat recovery synergistic energy-saving and flexible operation system for a coal-fired power generation unit according to claim 1, characterized in that: In the steam turbine extraction and heat recovery subsystem, the boiler feed water pipeline includes: a first feed water pipeline for recovering heat through steam turbine extraction and a second feed water pipeline for recovering heat through flue gas heat in a bypass flue; When the flue gas temperature at the bypass flue outlet is higher than the feed water temperature at the feed water pump outlet by more than 10K, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are provided in the second feed water pipeline; when the flue gas temperature at the bypass flue outlet is lower than the feed water temperature at the feed water pump outlet by +10K, a bypass low-temperature stage heater is also provided in the second feed water pipeline; The construction method also includes: By increasing the steam extraction pressure of the steam turbine high-pressure heater or providing a feedwater heater, the feedwater temperature at the outlet of the first feedwater pipeline is increased by 2 to 12K; By bypassing the high temperature stage heater and the high temperature water regulating valve, the outlet water temperature of the second water supply pipeline is increased to the same level as the outlet water temperature of the first water supply pipeline or higher than the outlet water temperature of the first water supply pipeline.

6. A double heat recovery synergistic energy saving and flexible operation system for coal-fired power generation units, characterized in that: The system includes: an economizer subsystem, which includes a high-temperature economizer, a denitration catalytic reactor and a low-temperature economizer; the system also includes: a boiler flue gas-air heat recovery system, which includes a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem; the system also includes a steam turbine extraction heat recovery subsystem; The boiler flue gas-air heat recovery system comprises: a boiler air inlet passage for allowing air to enter the boiler, a main flue for discharging the flue gas in the boiler, and a bypass flue for discharging the flue gas in the boiler; The steam turbine extraction steam regeneration subsystem includes: a boiler feed water pipeline that regenerates heat through steam turbine extraction steam and flue gas heat in a bypass flue; The boiler air intake channel includes a primary air intake channel and a secondary air intake channel. A primary air preheater and a primary air low-temperature preheater are arranged in the primary air intake channel. The flue gas discharged through the main flue passes through the primary air preheater and the primary air low-temperature preheater in sequence, and the primary air is reheated by utilizing the waste heat of the flue gas; a secondary air preheater and a secondary air low-temperature preheater are arranged in the secondary air intake channel. The flue gas discharged through the main flue passes through the secondary air preheater and the secondary air low-temperature preheater in sequence, and the secondary air is reheated by utilizing the waste heat of the flue gas.

7. The double heat recovery synergistic energy saving and flexible operation system of coal-fired power generation unit according to claim 6 is characterized in that: The heat exchange between flue gas and air in the low-temperature air preheating subsystem adopts a direct heat exchange method, an indirect heat exchange method or a partially indirect heat exchange method; When direct heat exchange is adopted, the low-temperature air preheater subsystem includes: a primary air fan, a blower, a primary air low-temperature preheater and a secondary air low-temperature preheater; When the indirect heat exchange method is adopted, the low-temperature air preheater subsystem includes: a primary air fan, a blower, a primary air heater, a secondary air heater, a primary air heater water inlet regulating valve, a secondary air heater water inlet regulating valve, a heat medium water heater, a heat medium water circulation pump, a heat medium water return valve, a heat medium water flow regulating valve and a heat medium water replenishment pump; When a partial indirect heat exchange method is adopted, the low-temperature air preheater subsystem includes: a primary fan, a blower, a primary air low-temperature preheater, a secondary air low-temperature preheater, a primary air heater, a secondary air heater, a primary air heater water inlet regulating valve, a secondary air heater water inlet regulating valve, a heat medium water heater, a heat medium water circulation pump, a heat medium water return valve, a heat medium water flow regulating valve and a heat medium water replenishment pump.

8. The double heat recovery synergistic energy saving and flexible operation system of coal-fired power generation unit according to claim 6 is characterized in that: The air preheater and bypass flue subsystem includes: a primary air preheater and a secondary air preheater arranged in the main flue, and a bypass flue; a bypass high-temperature stage heater and a bypass flue gas damper regulating valve are arranged in the bypass flue, and when the flue gas temperature at the bypass flue outlet does not exceed the feed water temperature of the feed water pump outlet by 10K, a bypass low-temperature stage heater is also arranged in the bypass flue; a bypass flue gas regulating valve is arranged in the bypass flue.

9. The double heat recovery synergistic energy saving and flexible operation system of coal-fired power generation units according to claim 6 is characterized in that: In the steam turbine extraction and heat recovery subsystem, the boiler feed water pipeline includes: a first feed water pipeline for heat recovery through the steam turbine extraction and a second feed water pipeline for heat recovery through the flue gas heat in the bypass flue; The first feedwater pipeline is provided with a condensate pump, a turbine low-pressure heater, a deaerator, a feedwater pump, a high-pressure heater regulating valve, a turbine high-pressure heater and a feedwater heater; When the flue gas temperature at the bypass flue outlet is higher than the water supply temperature at the water pump outlet by more than 10K, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are provided in the second water supply pipeline; when the flue gas temperature at the bypass flue outlet does not exceed the water supply temperature at the water pump outlet by 10K, a bypass low-temperature stage heater is also provided in the second water supply pipeline.

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