Double-regeneration Collaborative Energy-saving and Flexible Operation System and Construction Method for Coal-fired Power Generation Units

Abstract

The present invention belongs to the technical field of waste heat recovery from boiler flue gas, and specifically relates to a double-regeneration collaborative energy-saving and flexible operation system and construction method for a coal-fired power generation unit. The system includes: an economizer and a denitration catalytic reactor subsystem, abbreviated as the economizer subsystem; a boiler flue gas-air regeneration system, including a low-temperature air preheating subsystem, an air preheater, and a bypass flue subsystem; and a steam extraction regeneration subsystem for the steam turbine. Through the collaborative gas-borne regeneration and water-borne regeneration, the construction method can effectively improve the waste heat recovery effect of the flue gas while enhancing the cycle efficiency, operation flexibility, and environmental protection performance of the unit, and reduce the coal consumption of the unit 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 particularly relates to a double-regeneration collaborative energy-saving and flexible operation system and construction method for a coal-fired power generation unit. Background Technique

[0002] Efficient recovery of waste heat from the flue gas of coal-fired power boilers is an important research direction for improving the power generation efficiency of coal-fired power generation units. The most direct way to recover waste heat from flue gas is to use a low-low temperature economizer to reduce the boiler flue gas temperature from about 130 °C to about 90 °C using condensate water. The heated condensate water returns to the low-pressure heating system. Since the extraction steam of the steam turbine is reduced, the work done by the steam turbine increases, thereby improving the cycle efficiency of the unit. This method has a simple system, but the waste heat recovery efficiency is low. Usually, it can only reduce the power supply coal consumption by about 2 g / kWh. Among them, 0.5 g / 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 1.5 g / kWh.

[0003] At present, a boiler flue gas waste heat recovery system using a partial flue gas bypass (as shown in Figure 2 ) has been introduced in China and has been widely applied to newly built large coal-fired power generation units. This system can recover flue gas waste heat more efficiently. Usually, it can reduce the power supply coal consumption by about 3 g / kWh. However, the boiler flue gas waste heat recovery system still has the following technical problems:

[0004] (1) The primary air does not participate in the recovery of low-temperature flue gas waste heat. 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 low-temperature zone flue gas waste heat recovery, resulting in a reduction in the effect of flue gas waste heat recovery;

[0005] (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 outlet of the air preheater to a high enough level, the efficiency of recovering flue gas waste heat by the low-pressure heater in the bypass flue is low;

[0006] (3) The utilization efficiency of the air preheater decreases. When the flue gas temperature at the inlet of the air preheater and the heating surface area remain unchanged, the air temperature at the outlet of the air preheater is lower than that in the condition without flue gas waste heat recovery. That is, after about 15% of the flue gas bypass, the air temperature for combustion is lower than the original design value, which will lead to a decrease in the boiler efficiency and thus an increase in coal consumption. Although the forced circulation heat pipe system is set to preheat the secondary air and technical means such as setting flue gas bypass are adopted to improve the flue gas waste heat recovery effect, since the decrease in the air temperature at the outlet of the air preheater offsets part of the waste heat recovery effect, the overall efficiency of the flue gas waste heat recovery is affected. Although the air preheater adopted has a large volume, the heat transfer temperature difference becomes smaller, the heating surface cannot be fully utilized, and the air temperature cannot be effectively increased;

[0007] (4) The boiler flue gas-air regenerative system and the steam turbine extraction regenerative system do not form a synergistic effect. From the perspective of reducing the heat consumption of the steam turbine, it is desired to increase the feed water temperature, but increasing the feed water temperature will bring difficulties to the boiler design and reduce the boiler efficiency. After adopting the flue gas bypass technology, although part of the heat transfer problem in the air preheater area is solved, it has no direct effect on increasing the feed water temperature and thus improving the net efficiency of the unit.

[0008] There is an urgent need in this field to seek a new technical solution to solve the above problems. Summary of the Invention

[0009] To solve the problems in the above-mentioned background technology, the present invention provides a construction method for a double-regenerative synergistic energy-saving and flexible operation system of a coal-fired power generation unit. The construction method constructs a system that efficiently recovers low-temperature flue gas waste heat, improves the unit cycle efficiency, enhances the flexible operation performance of the unit and the safe, economic and environmental protection performance under low load through the coordination of water-borne regeneration and air-borne regeneration. The coal consumption for power supply of the unit can be reduced by 4 - 6 g / kWh.

[0010] The first object of the present invention is to provide a construction method for a double-regenerative synergistic energy-saving and flexible operation system of a coal-fired power generation unit. The construction method is realized based on the double-regenerative synergistic energy-saving and flexible operation system of a coal-fired power generation unit. The system includes: a economizer and denitration catalytic reactor subsystem, referred to as the economizer subsystem for short. 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 regenerative system, including a low-temperature air preheater subsystem, an air preheater and a bypass flue subsystem; the system also includes a steam turbine extraction regenerative subsystem;

[0011] The construction method includes:

[0012] By setting up the low-temperature air preheating subsystem, the flue gas temperature of the low-temperature air preheating subsystem is reduced to 85°C, and the flue gas temperature at the flue gas outlet of the air preheater and bypass flue subsystem is increased from 120°C - 130°C to 135°C - 220°C;

[0013] 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 - 35K, and the secondary air temperature at the outlet of the air preheater and bypass flue subsystem is increased by 5 - 40K;

[0014] Through the steam extraction and regeneration subsystem of the steam turbine, the feed water temperature of the boiler is increased by 2 - 12K to offset the reduction in heat absorption caused by the reduction in the heating surface area of the economizer subsystem, and at the same time, the cycle efficiency of the unit is improved.

[0015] Further, the construction method includes:

[0016] Cancel the low-temperature economizer in the economizer subsystem; or reduce the heating surface area of the low-temperature economizer to increase the flue gas temperature at the inlet of the air preheater and bypass flue subsystem by 5K - 35K, and the hot air temperature of the secondary air is increased by 5K - 40K synchronously.

[0017] Further, in the low-temperature air preheating subsystem, both the primary air and the secondary air participate in the recovery of low-temperature flue gas waste heat, and the heat exchange between the flue gas and the air adopts direct heat exchange, indirect heat exchange or partial indirect heat exchange methods;

[0018] When adopting the direct heat exchange method, the primary air is sent by the primary air fan, absorbs the waste heat of the flue gas in the primary air low-temperature preheater and then enters the primary air preheater to continue absorbing the heat of the flue gas; the secondary air is sent by the forced draft fan, absorbs the waste heat of the flue gas in the secondary air low-temperature preheater and then enters the secondary air preheater to continue absorbing the heat of the flue gas;

[0019] When adopting the indirect heat exchange method, the heat medium water from the low-pressure heater system of the steam turbine is supplied by the heat medium water make-up pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, is boosted by the heat medium water circulation pump and then enters the heat medium water heater to absorb the waste heat of the flue gas; a part of the heated heat medium water returns to the low-pressure heater system of the steam turbine through the heat medium water return valve; another part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, exchanges heat with the primary air and the secondary air in the primary air air heater and the secondary air air heater, and then returns to the inlet of the heat medium water circulation pump after passing through the primary air air heater inlet regulating valve and the secondary air air heater inlet regulating valve to form a heat medium water circulation; the primary air is sent by the primary air fan, absorbs the waste heat of the flue gas carried by the heat medium water in the primary air air heater and then enters the primary air preheater to continue absorbing the heat of the flue gas; the secondary air is sent by the forced draft fan, absorbs the waste heat of the flue gas carried by the heat medium water in the secondary air air heater and then enters the secondary air preheater to continue absorbing the heat of the flue gas;

[0020] When adopting the partial indirect heat exchange mode, the heat medium water from the low-pressure heater system of the steam turbine is supplied by the heat medium water make-up water pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, is boosted by the heat medium water circulation pump and then enters the heat medium water heater to absorb the waste heat of the flue gas; a part of the heated heat medium water returns to the low-pressure heater system of the steam turbine through the heat medium water return valve; another part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, exchanges heat with the primary air and the secondary air in the primary air preheater and the secondary air preheater, and then returns to the inlet of the heat medium water circulation pump after passing through the primary air preheater water inlet regulating valve and the secondary air preheater water inlet regulating valve, forming a heat medium water circulation; the primary air is sent by the primary air fan, absorbs the waste heat of the flue gas carried by the heat medium water in the primary air preheater and 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 heat of the flue gas; the secondary air is sent by the forced draft fan, absorbs the waste heat of the flue gas carried by the heat medium water in the secondary air preheater and 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 heat of the flue gas.

[0021] Furthermore, the air preheater and bypass flue gas subsystem includes a primary air preheater and a secondary air preheater provided in the main flue, and a bypass flue;

[0022] When the flue gas temperature at the outlet of the bypass flue is more than 10K higher than the feed water temperature at the outlet of the feed water pump, a bypass high-temperature stage heater is provided in the bypass flue;

[0023] The bypass low-temperature stage heater may not be provided, and all the waste heat of the flue gas in the bypass flue is used to heat the feed water. At this time, the utilization rate of the water-carried regenerative heat is relatively high;

[0024] When the flue gas temperature at the outlet of the bypass flue does not exceed the feed water temperature at the outlet of the feed water pump by 10K, in addition to the bypass high-temperature stage heater provided in the bypass flue, a bypass low-temperature stage heater is also provided. At this time, the utilization rate of the water-carried regenerative heat is relatively low;

[0025] A bypass flue gas regulating valve is provided in the bypass flue, and the flue gas flow discharged from the main flue and the flue gas flow discharged from the bypass flue are distributed through the bypass flue gas regulating valve.

[0026] Furthermore, in the steam turbine extraction regenerative subsystem, the boiler feed water pipeline includes: a first feed water pipeline regenerated by the steam turbine extraction and a second feed water pipeline regenerated by the heat of the flue gas in the bypass flue;

[0027] When the flue gas temperature at the outlet of the bypass flue is more than 10K higher than the feed water temperature at the outlet of the feed water pump, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are arranged in the second feed water pipeline; when the flue gas temperature at the outlet of the bypass flue is lower than the feed water temperature at the outlet of the feed water pump +10K, a bypass low-temperature stage heater is also arranged in the second feed water pipeline;

[0028] The construction method further includes:

[0029] By increasing the extraction steam pressure of the high-pressure heater of the steam turbine or arranging a feed water heater (such as a steam cooler), the feed water temperature at the outlet of the first feed water pipeline is increased by 2-12K, and the specific method is not limited;

[0030] By means of the bypass high-temperature stage heater and the bypass high-temperature water regulating valve, the feed water temperature at the outlet of the second feed water pipeline is increased to the same level as the feed water temperature at the outlet of the first feed water pipeline or higher than the feed water temperature at the outlet of the first feed water pipeline.

[0031] The second object of the present invention is to provide a double regenerative collaborative energy-saving and flexible operation system for a coal-fired power generation unit. The system includes: an economizer and a denitration catalytic reactor subsystem, abbreviated as an economizer subsystem, and 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 regenerative heat system, including a low-temperature air preheating subsystem, an air preheater and a bypass flue subsystem; the system also includes a steam turbine extraction steam regenerative heat subsystem;

[0032] The boiler flue gas-air regenerative heat system includes: a boiler air intake 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;

[0033] The steam turbine extraction steam regenerative heat subsystem includes: a boiler feed water pipeline for regenerative heat through steam turbine extraction steam and the heat of the flue gas in the bypass flue;

[0034] The boiler air intake passage includes a primary air intake passage and a secondary air intake passage. A primary air preheater and a primary air low-temperature preheater are arranged in the primary air intake passage. 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 regeneratively heated by using 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 passage. 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 regeneratively heated by using the waste heat of the flue gas.

[0035] Furthermore, in the low-temperature air preheating subsystem, the heat exchange between the flue gas and the air adopts a direct heat exchange method, an indirect heat exchange method or a partial indirect heat exchange method;

[0036] When adopting the direct heat exchange method, the low-temperature air preheater subsystem includes: a primary air fan, a forced draft fan, a primary air low-temperature preheater, and a secondary air low-temperature preheater;

[0037] When adopting the indirect heat exchange method, the low-temperature air preheater subsystem includes: a primary air fan, a forced draft fan, a primary air air preheater, a secondary air air preheater, a primary air air preheater inlet regulating valve, a secondary air air preheater 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 makeup pump;

[0038] When adopting the partial indirect heat exchange method, the low-temperature air preheater subsystem includes: a primary air fan, a forced draft fan, a primary air low-temperature preheater, a secondary air low-temperature preheater, a primary air air preheater, a secondary air air preheater, a primary air air preheater inlet regulating valve, a secondary air air preheater 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 makeup pump.

[0039] 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 baffle regulating valve are arranged in the bypass flue. When the flue gas temperature at the outlet of the bypass flue does not exceed the feed water temperature at the outlet of the feed water pump 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.

[0040] Furthermore, in the steam turbine extraction regenerative subsystem, the boiler feed water pipeline includes: a first feed water pipeline regenerated by steam turbine extraction and a second feed water pipeline regenerated by the heat of flue gas in the bypass flue;

[0041] A condensate pump, a steam turbine low-pressure heater, a deaerator, a feed water pump, a high-pressure heater regulating valve, a steam turbine high-pressure heater, and a feed water heater are arranged in the first feed water pipeline;

[0042] When the flue gas temperature at the outlet of the bypass flue is higher than the feed water temperature at the outlet of the feed water pump by more than 10K, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are arranged in the second feed water pipeline; when the flue gas temperature at the outlet of the bypass flue is lower than the feed water temperature at the outlet of the feed water pump +10K, a bypass low-temperature stage heater is also arranged in the second feed water pipeline.

[0043] In summary, through the dual-regenerative collaborative energy-saving and flexible operation system and construction method of a coal-fired power generation unit in the disclosed embodiments of the present invention, the following beneficial effects can be achieved:

[0044] (1) Through the coordination of two regenerative systems, namely air-borne regeneration and water-borne reflux, the low-temperature flue gas waste heat can be efficiently recovered, and the cycle efficiency of the unit can be improved. The power supply coal consumption of the unit can be reduced by 4 - 6 g / kWh;

[0045] (2) It can effectively increase the temperature of the combustion air, especially during low-load operation, which not only helps to achieve stable combustion in the furnace and complete burnout of pulverized coal, improving operation stability and economy, but also facilitates the organization of staged combustion, reducing the generation of NOx and having good environmental performance;

[0046] (3) Through the valve combination of each system in the unit, the adjustment means are increased and the adjustable range is widened, ensuring better operation flexibility, economy and reliability. Brief Description of the Drawings

[0047] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0048] Figure 1 FIG. is a schematic structural diagram of a double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit shown according to an exemplary embodiment (Example 1);

[0049] Figure 2 FIG. is the overall structural diagram of the boiler flue gas waste heat recovery system in the prior art;

[0050] Figure 3 FIG. is a schematic structural diagram of a double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit shown according to an exemplary embodiment (Example 2);

[0051] Figure 4 FIG. is a schematic structural diagram of a double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit shown according to an exemplary embodiment (Example 3);

[0052] Figure 5 FIG. is a schematic structural diagram of a double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit shown according to an exemplary embodiment (Example 4);

[0053] Figure 6 FIG. is a schematic structural diagram of a double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit shown according to an exemplary embodiment (Example 5).

[0054] 1 - High - temperature economizer, 2 - De - NOx catalytic reactor (SCR), 3 - Low - temperature economizer, 4 - Primary air preheater, 5 - Secondary air preheater, 6 - Bypass flue, 7 - Bypass high - temperature heater, 8 - Bypass low - temperature heater, 9 - Bypass flue gas damper regulating valve, 10 - Primary air low - temperature preheater, 11 - Secondary air low - temperature preheater, 12 - Primary air fan, 13 - Forced draft fan, 14 - Primary air air preheater, 15 - Secondary air air preheater, 16 - Primary air air preheater inlet regulating valve, 17 - Secondary air air preheater 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 pump, 23 - Electrostatic precipitator, 24 - Induced draft fan, 25 - Condensate pump, 26 - Low - pressure heater of steam turbine, 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 heater regulating valve, 33 - High - pressure heater of steam turbine, 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 implementation manners

[0055] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0056] The present invention provides a construction method for a double - regenerative collaborative energy - saving and flexible operation system of a coal - fired power generation unit. This construction method is realized based on the double - regenerative collaborative energy - saving and flexible operation system of the coal - fired power generation unit. This system achieves the goal of improving the energy - saving and flexible operation performance of the unit through the coordination of two systems: water - borne regeneration and air - borne regeneration.

[0057] Exemplarily, before introducing the construction method in the disclosed embodiments of the present invention, the double - regenerative collaborative energy - saving and flexible operation system of the coal - fired power generation unit will be explained.

[0058] In fact, a coal - fired power generation unit itself has two regenerative systems, namely the boiler flue gas - air regenerative system (referred to as the air regenerative system for short) and the steam extraction regenerative system of the steam turbine. The functions of the two regenerative systems are both to recover the waste heat that is originally intended to be discharged into the environment (low - temperature heat source) and to improve the energy quality through the high - temperature heat source (furnace) and then use it for continuous work, so it is called regeneration.

[0059] In the scenario of flue gas waste heat utilization, the flue gas waste heat is used to heat combustion air and return to the furnace with the combustion air, which is defined as airborne heat recovery; the flue gas waste heat is used to heat working medium water and then return to the steam turbine heat recovery system with the working medium water, which is defined as waterborne reflux. The utilization efficiency of airborne heat recovery is high, which is the unit cycle efficiency; the utilization efficiency of waterborne heat recovery is relatively low. The higher the temperature of the returned working medium water, the higher the utilization efficiency, but it is always lower than the unit cycle efficiency. The utilization efficiency is higher when the flue gas waste heat is carried by combustion air, while the utilization efficiency is lower when the flue gas waste heat is carried by working medium water, that is, the efficiency of airborne heat recovery is always higher than that of waterborne heat recovery.

[0060] For the double heat recovery collaborative energy-saving and flexible operation system of the coal-fired power generation unit described in the present invention, since a bypass flue 6 is provided and a bypass high-temperature stage heater 7 and a bypass low-temperature stage heater 8 are arranged therein, the formed second water supply pipeline 40 is an additional waterborne 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 returns to the steam turbine low-pressure heater system through the heat medium water circulation pipeline 41 via the heat medium water return valve 20, forming another waterborne heat recovery pipeline. Therefore, the waterborne heat recovery of the system described in the present invention includes three parts: steam turbine extraction heat recovery, waterborne heat recovery of the bypass flue heater, and waterborne heat recovery of the heat medium water. The waterborne heat recovery of the three parts constitutes the waterborne heat recovery system.

[0061] The airborne heat recovery system of the double heat recovery collaborative energy-saving and flexible operation system of the coal-fired power generation unit described in the present invention is the boiler flue gas-air heat recovery system, including: a low-temperature air preheating subsystem, an air preheater, and a bypass flue subsystem.

[0062] The double heat recovery collaborative energy-saving and flexible operation system described in the present invention achieves two goals by coordinating airborne heat recovery and waterborne heat recovery: firstly, increasing the share of airborne heat recovery, and secondly, improving the quality of waterborne heat recovery, that is, the temperature of the returned water, thereby improving the cycle efficiency and reducing the coal consumption.

[0063] When designing the unit, it is necessary to overall optimize the two regenerative systems to achieve the highest cycle efficiency of the unit. An important parameter for the overall optimization of the two regenerative systems is the feed water temperature. From the perspective of reducing the heat consumption of the steam turbine, increasing the feed water temperature helps to reduce the heat consumption. However, increasing the feed water temperature simultaneously increases the difficulty of boiler design, reduces the cooling capacity of the flue gas, and will reduce the boiler efficiency, thus reducing the cycle efficiency of the unit. If the boiler efficiency does not decrease while the feed water temperature is increased, the cycle efficiency of the unit will increase. When deeply utilizing the waste heat of the flue gas at the tail of the boiler, it is particularly important to overall optimize the two regenerative systems. At each temperature section of the waste heat utilization of the flue gas, effectively coordinating the two regenerative systems can ensure the efficient heat transfer between the flue gas and air, and between the flue gas and water, make full use of each heating surface, and thus ensure the efficient recovery of waste heat. Moreover, if the two regenerative systems are effectively coordinated, the boiler efficiency can be ensured while increasing the feed water temperature, thereby increasing the net efficiency of the unit and achieving the goal of deep and efficient coal saving.

[0064] Therefore, the construction of the double-regenerative collaborative energy-saving and flexible operation system should follow the following principles:

[0065] (1) The waste heat of the flue gas should first be used as much as possible to heat the combustion air, especially the secondary air. The waste heat recovery benefit obtained by increasing the temperature of the secondary air is the largest.

[0066] (2) Both the primary air and the secondary air should effectively participate in the waste heat recovery of the flue gas, especially the waste heat recovery of the low-temperature part of the flue gas, so as to fully reflect the waste heat recovery effect of the flue gas. Only the secondary air participating in the waste heat recovery of the low-temperature part of the flue gas will partially reduce the waste heat recovery effect of the flue gas.

[0067] (3) Try to increase the flue gas temperature level of the bypass flue. The higher the temperature of the water outlet of the bypass flue heater replacing the steam extraction of the steam turbine, the higher the waste heat recovery benefit. When only a high-pressure heater needs to be set in the bypass flue and the water outlet directly serves as the feed water, the highest waste heat recovery efficiency can be obtained.

[0068] (4) Through the coordination of the two regenerative systems, moderately increasing the feed water temperature can increase the cycle efficiency of the unit and at the same time improve the low-load operation performance of the unit. Measures to increase the feed water temperature include optimizing the design of the steam turbine regenerative system and adding a steam cooler or adding a stage of high-pressure heater, etc.

[0069] Therefore, the following technical measures are adopted in this patent:

[0070] 1) Set up a low-temperature air preheating subsystem. Both the primary air and the secondary air must effectively participate in the waste heat recovery of the low-temperature flue gas.

[0071] A low-temperature air preheating subsystem is set up to effectively recover the waste heat of low-temperature flue gas. Directly using the waste heat of low-temperature flue gas to heat air can play a role in recovering the waste heat of low-temperature flue gas, which is beneficial to increasing the air-borne heat recovery share and has a high waste heat utilization efficiency. The temperature of the primary hot air usually does not need to be too high, mainly depending on the drying requirements of the coal type. The appropriate temperature of the primary hot air can be achieved by adjusting the heating surface area of the primary air preheater 4, rather than making the primary air not participate in the recovery of the waste heat of low-temperature flue gas.

[0072] When the low-temperature air preheating subsystem adopts the direct flue gas-air 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 strongly corrosive flue gas atmosphere, and anti-corrosion measures need to be taken; 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, there are more equipment, the cost is higher, and the corrosion problem is transferred to the low-temperature end of the heat medium water heater 18. Moreover, this indirect heat exchange method is not conducive to increasing the temperature level of the bypass flue 6, which will affect the quality of the water-borne heat recovery and thus affect the waste heat utilization effect. When the outlet temperature requirement of the bypass flue 6 exceeds 180 °C, it is difficult for the working medium of the forced circulation heat pump (heat medium water circulation) system to directly adopt condensate water or feed water and it is difficult to match the steam-water system of the unit. It may be necessary to select other working media such as heat-conducting oil, which will add uncertain factors to the power plant management and equipment safety. Adopting a hybrid type of the first two methods, that is, a partial indirect heat exchange method (such as Examples 1, 3 and 4), is a feasible technical route and there are more adjustment means during operation, but the system is complex, there are many equipment, and the cost will increase.

[0073] 2) Increase the outlet flue gas temperature of the air preheater and the bypass flue subsystem:

[0074] Increasing the outlet flue gas temperature 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 and thus improve the waste heat recovery efficiency. Adopting a low-temperature air preheating subsystem is an effective means to raise the outlet flue gas temperature of the bypass flue 6. Usually, the outlet flue gas temperature of the bypass flue 6 can be raised from 120 °C - 130 °C to 135 °C - 220 °C. For a 1000MW coal-fired generating unit, if this temperature is raised above 210 °C, there is no need to set up a bypass low-temperature stage heater 8 (such as Examples 3, 4 and 5) in the bypass flue 6, and only a bypass high-temperature stage heater 7 needs to be set up. At this time, the water-borne heat recovery efficiency of the bypass heater is relatively high.

[0075] Increasing the flue gas temperature level of the air preheater and the bypass flue subsystem is also beneficial to increasing the temperature of the water-borne heat recovery of the heat medium water and thus improving the water-borne heat recovery effect.

[0076] 3) Increase the inlet flue gas temperature of the air preheater and the bypass flue gas duct subsystem by 5K to 35K, and the hot air temperature of the secondary air can be synchronously increased by 5K to 40K.

[0077] In the economizer subsystem, cancel the low-temperature economizer 3 (such as in Examples 4 and 5) or reduce the heating surface area of the low-temperature economizer 3 (such as in Examples 1, 2, and 3), so as to increase the inlet flue gas temperature of the air preheater and the bypass flue gas duct subsystem by 5K to 35K (for a 1000MW unit, from about 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 surface area of the air preheater within a reasonable range. The increase in the outlet air temperature of the air preheater means an increase in the air-borne heat recovery part, directly increasing the input heat of the furnace, thus achieving the effect of directly reducing coal consumption. The increase in the outlet air temperature of the secondary air preheater 5 also helps the stable combustion and complete combustion of pulverized coal in the furnace, thus contributing to improving the boiler combustion efficiency and stable operation at low loads. At the same time, it also helps to better adopt the staged combustion technology, reduce the NOx content at the furnace outlet, and is beneficial to reducing the burden on the subsequent denitration catalytic reactor (SCR) 2 and reducing the ammonia injection volume.

[0078] For boilers without a low-temperature economizer 3 in the original design, the temperature of the flue gas entering the denitration catalytic reactor (SCR) 2 and the temperature entering the air preheater and the bypass flue gas duct subsystem can be increased by reducing the heating surface area of the economizer (such as removing some economizer heating surfaces in the retrofit project).

[0079] The prior art (see Figure 2 ) After bypassing about 15% of the flue gas in the bypass flue, the outlet air temperature of the air preheater cannot reach the original design value (the level before the bypass flue was not installed), and the air-borne heat recovery decreases instead of increasing, which will cause a decrease in the boiler efficiency and the unit cycle efficiency, and reduce the effect of flue gas waste heat recovery; and after adopting the bypass flue, although the heat transfer temperature difference increases slightly, the heating surface area required for the air preheater is still huge, the manufacturing cost is high, and it affects the overall layout. This patent solves this problem, and the temperature of the air for boiler combustion is also increased, and the air-borne heat recovery increases.

[0080] 4) Increase the boiler feed water temperature by 2K to 12K.

[0081] After canceling the low-temperature economizer 3 or reducing the heating surface of the low-temperature economizer 3, it is necessary to increase the feed water temperature, which is equivalent to creating space for increasing the feed water temperature. It is possible to increase the extraction and regeneration of the steam turbine, reduce the heat consumption of the steam turbine, and improve the cycle efficiency of the unit. To increase the flue gas temperature at the inlet of the air preheater by 5K to 35K, it is necessary to increase the feed water temperature by 2K to 12K (for example, for a 1000MW unit, it is increased from the original 318°C to 320 - 330°C). This can be achieved by means such as increasing the extraction pressure of the high-pressure heater 33 of the steam turbine or adding a feed water heater 34 (such as adding a steam cooler or a high-pressure heater), and the present patent does not make specific limitations.

[0082] 5) Set the bypass flue gas baffle regulating valve 9.

[0083] Set the bypass flue gas baffle regulating valve 9 at the inlet or outlet position of the bypass flue 6 to distribute the flue gas flow passing through the air preheater and the bypass flue 6, thereby realizing the regulation of the air temperature at the outlet of the air preheater. At low loads, 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 also beneficial to controlling the flue gas temperature at the tail within a reasonable range. The bypass flue gas baffle regulating valve 9 cooperates with the bypass low-temperature water regulating valve 28 and the bypass high-temperature water regulating valve 31, etc., to make it more convenient for the unit to operate stably and economically at low loads.

[0084] 6) Increase the unit regulation means and increase the adjustable range.

[0085] Combining technical measures such as increasing the flue gas temperature at the inlet of the air preheater, increasing the feed water temperature, adding a flue gas bypass and regulating baffle, and setting a low-temperature air preheater subsystem not only improves the cycle efficiency of the unit, but also provides a powerful means for the flexible operation of the boiler. These regulation means include:

[0086] Precise regulation of the secondary air temperature - Through the combined regulation of the bypass flue gas baffle regulating valve 9, the inlet water regulating valve 17 of the secondary air air heater, the bypass low-temperature water regulating valve 28, and the bypass high-temperature water regulating valve 31, etc., precise regulation of the secondary air temperature can be achieved, thereby providing conditions for the stable combustion of the boiler and improving the boiler efficiency, especially during low-load operation.

[0087] Precise regulation of the primary air temperature - Through the combined regulation of the bypass flue gas baffle regulating valve 9 and the inlet water regulating valve 16 of the primary air air heater, etc., precise regulation of the primary air temperature can be achieved. Usually, the hot air temperature of the primary air needs to match the drying output requirement of the coal pulverizing system for the coal. When the hot air temperature of the primary air is too high, cold air needs to be incorporated, which will cause irreversible losses. Precise control of the primary air temperature can eliminate or reduce this irreversible loss and improve the cycle efficiency of the unit.

[0088] Flue gas temperature control at the inlet of the electrostatic precipitator 23 - By jointly adjusting the heat medium water flow regulating valve 21, the bypass flue gas baffle regulating valve 9, the heat medium water return valve 20, etc., the flue gas temperature at the inlet of the electrostatic precipitator 23 can be controlled. Especially at low loads, this temperature is ensured not to be lower than 85°C to ensure the safety of subsequent equipment.

[0089] Outlet water temperature control of the bypass high - temperature stage heater 7 - By the bypass flue gas baffle regulating valve 9, the bypass high - temperature water regulating valve 31, etc., the outlet water temperature of the bypass high - temperature stage heater 7 can be controlled. A temperature slightly higher than the feed water temperature can improve the circulation efficiency, but too high may affect the system operation. Therefore, it is very important to achieve precise and effective control.

[0090] Outlet water temperature control of the bypass low - temperature stage heater 8 - Through the joint adjustment of the bypass flue gas baffle regulating valve 9, the bypass low - temperature water regulating valve 28, etc., the outlet water temperature of the bypass low - temperature stage heater 8 can be effectively controlled. This temperature needs to match the working temperature of the deaerator 29 to avoid irreversible losses.

[0091] Feed water temperature control at full load - Through the steam turbine high - pressure heater 33, the feed water heater 34, the bypass flue gas baffle regulating valve 9, the bypass high - temperature water regulating valve 31, etc., the feed water temperature can be controlled within the full - load range, thus ensuring the safe, economic and flexible operation of the unit.

[0092] 7) The double - regenerative system coordinates with each other in different flue gas temperature zones, improving the cycle efficiency and operation flexibility of the unit.

[0093] Set up a low - temperature air pre - heating subsystem to raise the flue gas temperature level in the bypass flue 6, thereby increasing the outlet water temperature level of the bypass flue heater, improving the quality of water - borne heat regeneration. The work capacity of the steam turbine extraction steam replaced increases, improving the cycle efficiency of the unit. This is the first coordination of the unit's double - regenerative system.

[0094] Set up a flue gas bypass 6 and raise its flue gas temperature level (including the flue gas temperature at the outlet and inlet), and install a bypass high - temperature stage heater 7 and a bypass low - temperature stage heater 8 in it. The water - borne heat regeneration shares part of the recovery of flue gas waste heat for the boiler air pre - heating system, sacrificing a little steam turbine heat consumption index, but providing a heat balance and reasonable heat transfer temperature difference for the heat exchange between flue gas and air. Generally, the cycle efficiency of the unit is improved. This is the second coordination of the unit's double - regenerative system.

[0095] Increasing the inlet flue gas temperature of the air preheater results in an increase in the heat absorbed by the air from the flue gas and a decrease in the heat absorbed by the feed water from the flue gas, creating room for raising the feed water temperature and thus reducing the heat consumption of the steam turbine. This is the third synergy of the unit's double regenerative system, which overall improves the unit's cycle efficiency, and the increase in the unit's cycle efficiency is relatively large. It is the most important synergy among the three. This synergy is a special product under the condition of deeply recovering the waste heat of the flue gas. Adding a large number of heating surfaces at the boiler tail creates the conditions for this, and it can also be said that this synergy makes full use of the large number of additional heating surfaces at the boiler tail.

[0096] The three synergies of the unit's double regenerative system are interconnected and inseparable as a whole. Its essence is the reconstruction of the heat transfer system at the boiler tail and the regenerative system of the steam turbine. The three synergies of waterborne regeneration and airborne regeneration work together, and the effect is to improve the economy, flexibility, and environmental protection performance of the unit operation, and the effect is more obvious during low-load operation.

[0097] The following combines the appendix Figure 1 , Figures 3 - 6 , and specifically discusses the embodiments of the present invention and analyzes their performance.

[0098] Appendix Figure 1 That is, Example 1, which has the most complete equipment and systems 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 area adopts the direct heat exchange method between flue gas and air, and is equipped with a primary air low-temperature preheater 10 and a secondary air low-temperature preheater 11; the low-temperature area 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 arranged in its bypass flue 6. In its economizer subsystem, a high-temperature economizer 1, a denitration catalytic reactor (SCR) 2, and a low-temperature economizer 3 are provided, and the low-temperature economizer 3 is not cancelled, but only the heating surface area of the low-temperature economizer 3 is reduced. The regulation performance of this example system is good, the energy-saving effect is moderate, there is no corrosion problem in the low-temperature air preheater area, and anti-corrosion measures are required for the low-temperature section of the heat medium water heater 18.

[0099] Appendix Figure 3 That is, Example 2, whose low-temperature air preheating subsystem adopts an indirect heat exchange method, does not have a primary air low-temperature preheater 10 and a secondary air low-temperature preheater 11, and 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 waterborne regeneration. The other subsystems are the same as in Example 1. Compared with Example 1, the system of Example 2 is relatively simple, but the energy-saving effect is slightly inferior to that of Example 1.

[0100] Appendix Figure 4That is, Example 3 is a variant of Example 1. The outlet flue gas temperature of the air preheater and the bypass flue gas duct subsystem is raised to a sufficiently high level by the low-temperature air preheating subsystem. Therefore, only the bypass high-temperature heater 7 needs to be provided in the bypass flue duct 6, and there is no need to provide the bypass low-temperature heater 8, which will improve the effect of waterborne heat regeneration. Under the condition that other parameters are the same, the energy-saving effect of Example 3 is better than that of Example 1.

[0101] Appendix Figure 5 That is, Example 4 is based on Example 3. 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 gas duct subsystem is increased by 25 - 35K; by increasing the extraction steam pressure of the turbine high-pressure heater and / or adding feedwater heaters, etc., the feedwater temperature is increased by 8 - 12K. Therefore, the energy-saving efficiency of Example 4 is better than that of Example 3.

[0102] Appendix Figure 6 That is, Example 5 is the simplest in terms of the system and equipment in the examples of the present invention. The low-temperature air preheating subsystem of this example adopts the direct heat exchange method between flue gas and air, and directly sets the primary air low-temperature preheater 10 and the secondary air low-temperature preheater 11, which can effectively raise the outlet flue gas temperature of the air preheater and the bypass flue gas duct subsystem; only the bypass high-temperature heater 7 is provided in the bypass flue duct 6, and there is no need to provide the bypass low-temperature heater 8, and the quality of waterborne heat regeneration 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 gas duct subsystem is increased by 25 - 35K; by increasing the extraction steam pressure of the turbine high-pressure heater and / or adding feedwater heaters, etc., the feedwater temperature is increased by 8 - 12K. Example 5 has the best energy-saving effect among the examples of the present invention, but its regulation performance is slightly inferior, and there is a risk of low-temperature corrosion in the low-temperature sections of the primary air low-temperature preheater 10 and the secondary air low-temperature preheater 11.

[0103] The dual heat regeneration collaborative energy-saving and flexible operation system for coal-fired power generation units provided by this application is constructed by the above construction method. Therefore, it has at least all the beneficial effects brought by the technical solutions of the embodiments of the above construction method, which will not be elaborated one by one here.

[0104] The above further describes the present invention with the aid of specific embodiments. However, it should be understood that this specific description should not be construed as a limitation on the essence and scope of the present invention. Various modifications made by those of ordinary skill in the art to the above embodiments after reading this specification all fall within the scope protected by the present invention.

Claims

1. A construction method for a double-regeneration collaborative energy-saving and flexible operation system of a coal-fired power generation unit, characterized in that The construction method is realized based on a double regenerative collaborative energy-saving and flexible operation system for a coal-fired power generation unit. The system includes: a 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 regenerative heat system, including a low-temperature air preheater subsystem, an air preheater, and a bypass flue gas subsystem; the system also includes a steam turbine extraction steam regenerative heat subsystem. The construction method includes: By setting the low-temperature air preheater subsystem, the flue gas temperature of the low-temperature air preheater subsystem is reduced to 85°C, and the flue gas temperature at the flue gas outlet of the air preheater and bypass flue gas subsystem is increased from 120°C - 130°C to 135°C - 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 gas subsystem is increased by 5 - 35K, and the secondary air temperature at the outlet of the air preheater and bypass flue gas subsystem is increased by 5 - 40K. By the steam turbine extraction steam regenerative heat subsystem, the boiler feed water temperature is increased by 2 - 12K.

2. The construction method of the double regenerative collaborative energy-saving and flexible operation system for coal-fired power generation units according to claim 1, characterized in that, The construction method includes: In the economizer subsystem, reduce the heating surface area of the low-temperature economizer or cancel the low-temperature economizer, so as to increase the flue gas temperature at the inlet of the air preheater and bypass flue gas subsystem by 5K - 35K, and the hot air temperature of the secondary air is synchronously increased by 5K - 40K.

3. The construction method of the dual regenerative collaborative energy-saving and flexible operation system for coal-fired power generation units according to claim 1, characterized in that In the low-temperature air preheater subsystem, both the primary air and the secondary air participate in the recovery of low-temperature flue gas waste heat, and the heat exchange between the flue gas and the air adopts a direct heat exchange method, an indirect heat exchange method, or a partial indirect heat exchange method. When using the direct heat exchange method, the primary air is sent by the primary air fan, absorbs the waste heat of the flue gas in the primary air low-temperature preheater and then enters the primary air preheater to continue absorbing the heat of the flue gas; the secondary air is sent by the forced draft fan, absorbs the waste heat of the flue gas in the secondary air low-temperature preheater and then enters the secondary air preheater to continue absorbing the heat of the flue gas. When using the indirect heat exchange method, the heat medium water from the steam turbine low-pressure heater system is supplied by the heat medium water make-up pump, enters the heat medium water circulation pipeline after passing through the heat medium water flow regulating valve, is boosted by the heat medium water circulation pump and then enters the heat medium water heater to absorb the waste heat of the flue gas; a part of the heated heat medium water returns to the steam turbine low-pressure heater system through the heat medium water return valve; another part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively, exchanges heat with the primary air and the secondary air in the primary air air heater and the secondary air air heater, and then returns to the inlet of the heat medium water circulation pump after passing through the primary air air heater inlet regulating valve and the secondary air air heater inlet regulating valve to form a heat medium water circulation; the primary air is sent by the primary air fan, absorbs the waste heat of the flue gas carried by the heat medium water in the primary air air heater and then enters the primary air preheater to continue absorbing the heat of the flue gas; the secondary air is sent by the forced draft fan, absorbs the waste heat of the flue gas carried by the heat medium water in the secondary air air heater and then enters the secondary air preheater to continue absorbing the heat of the flue gas. When adopting the partial indirect heat exchange method, the heat medium water from the low-pressure heater system of the steam turbine is supplied by the heat medium water make-up water pump. After passing through the heat medium water flow regulating valve, it enters the heat medium water circulation pipeline. After being boosted by the heat medium water circulation pump, it enters the heat medium water heater to absorb the waste heat of the flue gas. A part of the heated heat medium water returns to the low-pressure heater system of the steam turbine through the heat medium water return valve. Another part of the heated heat medium water enters the first heat medium water pipeline and the second heat medium water pipeline respectively. After exchanging heat with the primary air and secondary air in the primary air preheater and secondary air preheater, it returns to the inlet of the heat medium water circulation pump through the primary air preheater water inlet regulating valve and the secondary air preheater water inlet regulating valve, forming a heat medium water circulation. The primary air is sent by the primary air fan. After absorbing the waste heat of the flue gas carried by the heat medium water in the primary air preheater, it 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 heat of the flue gas. The secondary air is sent by the forced draft fan. After absorbing the waste heat of the flue gas carried by the heat medium water in the secondary air preheater, it 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 heat of the flue gas.

4. The construction method of the double regenerative collaborative energy-saving and flexible operation system for coal-fired power generation units according to claim 1, characterized in that The air preheater and bypass flue subsystem includes a primary air preheater and a secondary air preheater provided in the main flue, and a bypass flue. When the flue gas temperature at the outlet of the bypass flue is higher than the feed water temperature at the outlet of the feed water pump by more than 10K and does not include 10K, a bypass high-temperature stage heater is provided in the bypass flue. When the flue gas temperature at the outlet of the bypass flue does not exceed the feed water temperature at the outlet of the feed water pump by 10K, in addition to the bypass high-temperature stage heater, a bypass low-temperature stage heater is also provided in the bypass flue. A bypass flue gas regulating valve is provided in the bypass flue to distribute the flue gas flow discharged from the main flue and the flue gas flow discharged from the bypass flue.

5. The construction method of the double regenerative collaborative 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 heat regeneration subsystem, the boiler feed water pipeline includes: a first feed water pipeline regenerated by steam turbine extraction heat and a second feed water pipeline regenerated by the heat of the flue gas in the bypass flue. When the flue gas temperature at the outlet of the bypass flue is higher than the feed water temperature at the outlet of the feed water pump 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 outlet of the bypass flue is lower than the feed water temperature at the outlet of the feed water pump +10K, a bypass low-temperature stage heater is also provided in the second feed water pipeline. The construction method further includes: By increasing the extraction pressure of the high-pressure heater of the steam turbine or setting a feed water heater, the feed water temperature at the outlet of the first feed water pipeline is increased by 2-12K. By the bypass high-temperature stage heater and the bypass high-temperature water regulating valve, the feed water temperature at the outlet of the second feed water pipeline is increased to the same level as the feed water temperature at the outlet of the first feed water pipeline or higher than the feed water temperature at the outlet of the first feed water pipeline.

6. A double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit, characterized in that, The system is constructed by the construction method described in any one of claims 1-5. The system includes: an economizer subsystem, which includes a high-temperature economizer, a denitration catalytic reactor, and a low-temperature economizer; the system further includes: a boiler flue gas-air recuperation system, which includes a low-temperature air preheating subsystem, an air preheater, and a bypass flue gas subsystem; the system also includes a steam turbine extraction heat recuperation subsystem. The boiler flue gas-air recuperation system includes: 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 heat recuperation subsystem includes: a boiler feed water pipeline that recuperates heat through steam turbine extraction heat and the heat of the flue gas in the bypass flue. The boiler air inlet passage includes a primary air inlet passage and a secondary air inlet passage. A primary air preheater and a primary air low-temperature preheater are arranged in the primary air inlet passage. The flue gas discharged through the main flue sequentially passes through the primary air preheater and the primary air low-temperature preheater, and the primary air is recuperatively heated using the waste heat of the flue gas. A secondary air preheater and a secondary air low-temperature preheater are arranged in the secondary air inlet passage. The flue gas discharged through the main flue sequentially passes through the secondary air preheater and the secondary air low-temperature preheater, and the secondary air is recuperatively heated using the waste heat of the flue gas.

7. The double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit according to claim 6, wherein In the low-temperature air preheating subsystem, the heat exchange between the flue gas and the air adopts a direct heat exchange method, an indirect heat exchange method, or a partial indirect heat exchange method. When adopting the direct heat exchange method, the low-temperature air preheating subsystem includes: a primary air fan, a forced draft fan, a primary air low-temperature preheater, and a secondary air low-temperature preheater. When adopting the indirect heat exchange method, the low-temperature air preheating subsystem includes: a primary air fan, a forced draft fan, a primary air air heater, a secondary air air heater, a primary air air heater inlet regulating valve, a secondary air air heater 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 make-up pump. When adopting the partial indirect heat exchange method, the low-temperature air preheating subsystem includes: a primary air fan, a forced draft fan, a primary air low-temperature preheater, a secondary air low-temperature preheater, a primary air air heater, a secondary air air heater, a primary air air heater inlet regulating valve, a secondary air air heater 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 make-up pump.

8. The double-regeneration collaborative energy-saving and flexible operation system for a coal-fired power generation unit according to claim 6, characterized in that The air preheater and bypass flue gas 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 baffle regulating valve are arranged in the bypass flue. When the flue gas temperature at the outlet of the bypass flue does not exceed the feed water temperature at the outlet of the feed water pump 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-regeneration collaborative energy-saving and flexible operation system for coal-fired power generation units according to claim 6, wherein, In the steam turbine extraction heat recuperation subsystem, the boiler feed water pipeline includes: a first feed water pipeline that recuperates heat through steam turbine extraction and a second feed water pipeline that recuperates heat through the heat of the flue gas in the bypass flue. The first feed water pipeline is provided with a condensate pump, a low-pressure steam turbine heater, a deaerator, a feed water pump, a high-pressure heater regulating valve, a high-pressure steam turbine heater and a feed water heater; When the flue gas temperature at the outlet of the bypass flue is more than 10K (excluding 10K) higher than the feed water temperature at the outlet of the feed water pump, a bypass high-temperature water regulating valve and a bypass high-temperature stage heater are arranged in the second feed water pipeline; when the flue gas temperature at the outlet of the bypass flue does not exceed 10K of the feed water temperature at the outlet of the feed water pump, a bypass low-temperature stage heater is further arranged in the second feed water pipeline.

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