Flue gas waste heat utilization and steam extraction coupling utilization system

Abstract

The invention relates to the technical field of thermal power plant design, in particular to a flue gas waste heat utilization and steam extraction coupling utilization system. Comprising a flue gas subsystem, a secondary air subsystem and a primary air subsystem, the flue gas subsystem comprises an air preheater, a flue gas cooler, a dust remover and an induced draft fan, the flue gas subsystem enters the flue gas cooler, the dust remover and the induced draft fan through the air preheater, and the air preheater is a two-compartment air preheater; cold secondary air is heated by an air preheater and then enters a boiler, and a secondary air subsystem enters a secondary air heater to be heated after passing through an air feeder, then enters the air preheater and then enters a hearth after passing through the air preheater. The primary air subsystem passes through a primary air fan, a primary air heater and a primary air heater, and an outlet of a primary air heating unit is connected with a coal mill. The flue gas waste heat and the steam extraction superheat degree are fully utilized, so that the boiler efficiency is improved, the steam turbine heat consumption is reduced, meanwhile, the equipment investment is reduced, and the station service power consumption rate is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal power plant design, and in particular to a flue gas waste heat utilization and extraction steam coupled utilization system. Background Art

[0002] Since the exhaust temperature of thermal power plants is relatively high, in order to make full use of the waste heat of flue gas and reduce the exhaust temperature; due to the high pressure head of the primary fan, conventional power plants are designed to use two 50% primary fans, which will cause the problem of rushing wind when the two fans are actually started. If the damper is not closed tightly, it will affect the start of the other fan. At the same time, the arrangement of two primary fans takes up space and increases the initial investment of the primary fan. Since the steam turbine has a high degree of superheat when extracting steam in stages, in order to reasonably utilize the superheat without affecting the extraction of steam; in order to improve the efficiency of the boiler and reduce the heat consumption of the steam turbine, and at the same time avoid the problem of rushing wind when the primary fan is started, how to design a reasonable flue gas waste heat utilization and extraction coupling system is a technical problem to be solved. Summary of the Invention

[0003] The problem to be solved by the present invention is to provide a flue gas waste heat utilization and steam extraction coupling system to improve the efficiency of the boiler and avoid the problem of premature start-up of the primary fan.

[0004] In order to solve the above technical problems, the solution provided by the present invention is a flue gas waste heat utilization and extraction steam coupled utilization system, including a flue gas subsystem, a secondary air subsystem and a primary air subsystem. The flue gas subsystem includes an air preheater, an air preheater preheater bypass, a flue gas cooler, a dust collector and an induced draft fan. The flue gas subsystem is connected to the boiler outlet through the air preheater. The air preheater is a two-bin air preheater. The flue gas side outlet of the air preheater is connected to the flue gas cooler. The flue gas cooler outlet is connected to the induced draft fan through the dust collector. The air side inlet of the air preheater is connected to the secondary air subsystem. The secondary air subsystem includes a connected secondary air heater and a blower.

[0005] The primary air subsystem includes a primary air heating unit and a primary fan. The inlet of the primary air heating unit is connected to the primary fan, and the outlet of the primary air heating unit is connected to the coal mill; the primary air heating unit exchanges heat with the flue gas cooler.

[0006] Preferably, the primary air heating unit includes M heaters.

[0007] Preferably, M heaters are connected in series or partially in series and partially in parallel, M is 3 to 8, the heater close to the primary fan is heater 1, and heater 1 exchanges heat with the flue gas cooler.

[0008] Preferably, the flue gas cooler performs heat exchange with the secondary air heater.

[0009] Preferably, the air preheater preheater bypass includes a feed water heat exchanger and a condensate heat exchanger.

[0010] Preferably, the outlet of heater 1 is connected to heater 2, and heater 2 exchanges heat with the condensate heat exchanger.

[0011] Preferably, the heat source of the flue gas cooler is the waste heat of boiler exhaust gas, and the heat source of the primary air heating unit includes the heat of the flue gas cooler and the air preheater preheater bypass, which are utilized step by step according to grade.

[0012] The beneficial effects of the present invention are as follows:

[0013] 1. Fully reduce the waste heat of flue gas and improve boiler efficiency;

[0014] 2. Rationally utilize the superheat of extraction steam to reduce the heat consumption of steam turbine;

[0015] 3. Reduce the air leakage rate of the air preheater, thereby reasonably reducing the selection of the three major fans, reducing operating power consumption, and thus reducing the power consumption rate of the plant;

[0016] 4. The primary air is heated without passing through the rotary air preheater, thus avoiding the leakage of flue gas into the cold air, thereby reducing wear and tear and increasing the life of the fan;

[0017] 5. The primary air temperature is heated step by step through multi-stage heaters (or bypass heaters), and the end difference is reasonably utilized to achieve full utilization of heat;

[0018] 6. Flexible adjustment of primary air temperature, using heater bypass mode, precise control, adapting to changing coal types, so as to reasonably select primary air outlet temperature, with more adjustment means and more precise air temperature adjustment;

[0019] 7. Rationally utilize the superheat of extraction steam to reduce the heat consumption of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural diagram of a first embodiment of the system of the present invention;

[0021] Figure 2 It is a structural diagram of the second embodiment of the system of the present invention.

[0022] Explanation of the accompanying symbols: 1. Boiler; 2. Air preheater; 22. Flue gas side of air preheater; 21. Wind side of air preheater; 3. Air preheater bypass; 31. Feed water heat exchanger; 311. Feed water outlet; 312. Feed water inlet; 32. Condensate heat exchanger; 321. Condensate outlet; 322. Condensate inlet; 4. Flue gas cooler; 5. Dust collector; 6. Induced draft fan; 7. Forced draft fan; 8. Secondary air heater; 9. Primary air heating unit; 91. Heater one; 92. Heater two; 93. Heater three; 94. Heater four; 95. Heater five; 96. Heater six; 97. Heater seven; 98. Heater eight; 10. Primary fan. DETAILED DESCRIPTION

[0023] The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments. The embodiments of the present invention are provided for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. The embodiments are chosen and described to better illustrate the principles of the invention and its practical application, and to enable those skilled in the art to understand the invention and design various embodiments with various modifications suitable for specific applications.

[0024] Conventional power plants use air preheaters with three or four compartments. These air preheaters heat the cold air in a relatively rough manner. One of the three compartments discharges flue gas, and the other two compartments are connected to fans at the inlet and the outlet of the other two compartments are connected to boilers and coal mills, respectively. This makes it easy for two primary fans to start up and cause a problem of air grabbing. Furthermore, the three-compartment and four-compartment air preheaters have a high air leakage rate. The air preheater 2 of the present invention uses a two-compartment air preheater, which can reduce the air leakage rate of the air preheater. This makes it more advantageous to select the three major fans (induced draft fan, forced draft fan, and primary fan) in the initial design phase, and reduces the power consumption rate of the power plant. In order to more accurately control the air temperature, the present invention flexibly sets a heat exchanger to achieve precise control of the cold air.

[0025] A flue gas waste heat utilization and extraction steam coupling utilization system includes a flue gas subsystem, a secondary air subsystem and a primary air subsystem. The boiler 1 outlet is connected to the flue gas subsystem through an air preheater 2. The flue gas subsystem includes the air preheater 2, an air preheater preheater bypass 3, a flue gas cooler 4, a dust collector 5 and an induced draft fan 6. The air preheater 2 is a two-bin air preheater, including an air preheater flue gas side 22 and an air preheater wind side 21; the air preheater flue gas side 22 inlet is connected to the boiler 1 outlet, the air preheater flue gas side 22 outlet is connected to the flue gas cooler 4, and the flue gas cooler 4 outlet is connected to the induced draft fan 6 through the dust collector 5; the air preheater wind side 21 outlet is connected to the boiler 1, and the air preheater wind side 21 inlet is connected to the secondary air subsystem. The secondary air subsystem includes a blower 7 and a secondary air heater 8. The blower 7 is connected to the air preheater wind side 21 inlet through the secondary air heater 8. The flue gas cooler 4 performs heat exchange with the secondary air heater 8 to achieve heat exchange between the flue gas subsystem and the secondary air subsystem.

[0026] The primary air subsystem includes a primary air heating unit 9 and a primary air fan 10. The inlet of the primary air heating unit 9 is connected to the primary air fan 10, and the outlet of the primary air heating unit 9 is connected to the coal mill; the primary air heating unit 9 exchanges heat with the flue gas cooler 4. The primary air heating unit 9 includes M heaters. The M heaters are connected in series or partially in series and partially in parallel, and M is 3 to 8. The heater close to the primary air fan 10 is heater 1 91, and heater 1 91 exchanges heat with the flue gas cooler 4. The primary air is heated step by step by multi-stage heaters (or bypass heaters), and the end difference is reasonably utilized to achieve full utilization of heat, flexibly adjust the primary air temperature, and adopt a heater bypass method for precise control to adapt to a variety of coal types, so as to reasonably select the primary air outlet temperature, with more adjustment means and more precise air temperature regulation.

[0027] like Figure 1 The figure shows embodiment 1 of the present invention, in which the primary air heating unit 9 includes heater 1 91, heater 2 92, heater 3 93, heater 4 94, heater 5 95 and heater 6 96. Heater 1 91, heater 2 92, heater 3 93, heater 4 94, heater 5 95 and heater 6 96 are connected in series, and heater 1 91 exchanges heat with the flue gas cooler 4 to realize heat exchange between the primary air subsystem and the flue gas subsystem.

[0028] The flue gas subsystem also includes an air preheater preheater bypass 3, which is connected in parallel to the air preheater flue gas side 22. The air preheater preheater bypass 3 includes a feedwater heat exchanger 31 and a condensate heat exchanger 32. The outlet of heater 1 91 is connected to heater 2 92, and heater 2 92 exchanges heat with condensate heat exchanger 32. Feedwater heat exchanger 31 includes a feedwater outlet 311 and a feedwater inlet 312. Feedwater outlet 311 is connected to the main water supply pipeline, and feedwater inlet 312 flows into water from the deaerator outlet. Condensate heat exchanger 32 includes a condensate outlet 321 and a condensate inlet 322. Condensate outlet 321 directs water between the two-stage deaerator, and condensate inlet 322 is connected to the low-pressure heater outlet.

[0029] The exhaust gas temperature of the power plant boiler is relatively high. If it is directly discharged without reasonable utilization, it will cause exhaust gas loss. In order to reasonably utilize the flue gas waste heat, the air preheater preheater bypass 3 and the flue gas cooler 4 are considered to reduce the exhaust gas temperature, thereby improving the efficiency of the boiler. This system design is not a simple utilization of flue gas waste heat, but a coupling of flue gas waste heat and the extraction steam superheat of the heat recovery system to heat the cold air. The present invention proposes a flue gas waste heat utilization and extraction steam coupling utilization system, which improves the efficiency of the boiler 1, reduces the heat consumption of the unit and makes the primary air startup more flexible. (1) The flue gas waste heat utilization system is adopted. (2) The air preheater 2 adopts a two-compartment system. (3) The air preheater preheater bypass 3 is adopted to heat the feed water and condensate. (4) A single primary fan is adopted. (5) The primary air heating unit 9 is adopted to accurately control the primary air temperature; the primary air heating unit 9 is provided with a primary air heater bypass to accurately adjust the primary air temperature and reduce the operating power of the primary fan 10. (6) The primary air heating unit 9 uses the waste heat of flue gas, condensed water to heat the cold primary air, and extraction steam to heat the cold primary air in sequence, and utilizes the waste heat and superheat of each level step by step; reasonably selects the end difference of the heat exchanger; accurately controls the primary air temperature to avoid the problem of two primary fans starting up and competing for air; reasonably utilizes the superheat of extraction steam; thereby improving the power generation efficiency of the unit and reducing the operating power consumption.

[0030] In the existing technology, the two primary fans in operation and startup rush the air, which does not match the operating pressure head. The rush air problem caused by this problem leads to an increase in the power consumption rate of the plant. Due to the high pressure head of the primary fan, conventional power plant designs use two 50% primary fans. This actually causes a rush air problem when starting two fans. If the damper is not closed tightly, it affects the startup of the other fan. At the same time, the layout of the two primary fans takes up space and increases the initial investment of the primary fans. The existing three-compartment and four-compartment air preheaters have a high air leakage rate due to the high primary air pressure head. At the same time, the operating power of the three major fans accounts for a high proportion of the power consumption rate of the plant. Conventional power plant air preheater designs use three-compartment and four-compartment air preheaters. The heating of the cold air in the three-compartment and four-compartment air preheaters is relatively extensive. In order to more accurately control the air temperature, the heat exchanger is flexibly set to achieve precise control of the cold air. In order to reduce operating power consumption, the present invention is designed to adopt a single primary fan system. The primary fan 10 is heated without passing through the air preheater 2, which reduces the air leakage rate of the air preheater 2, thereby reducing the operating power consumption of the induced draft fan 6. This system design does not simply adopt a single primary fan, but combines a two-compartment air preheater 2 and a primary air heating unit 9 to reasonably heat the cold primary air, flexibly adjust, accurately control, and gradually utilize waste heat and heat recovery system to extract steam superheat. The present invention adopts a two-compartment air preheater 2. The primary air is not heated by the air preheater 2, but is heated by a separate primary air heating unit 9. The secondary air can greatly reduce the leakage rate of the secondary air due to the low pressure head, thereby reducing the leakage rate of the air preheater 2, reducing the operating power consumption of the induced draft fan 6, and the operating power consumption of the primary fan 10 is also reduced due to the lower leakage rate. At the same time, the selection of the three major fans can also be appropriately reduced, thereby reducing the investment in the three major fans. This system does not simply adopt a two-compartment air preheater 2, but couples the air preheater preheater bypass 3 and the single primary air heating unit 9, rationally utilizing the flue gas waste heat and steam superheat, and reducing the coal consumption index and plant power consumption rate of the power plant.

[0031] The present invention is based on a flue gas waste heat utilization system. Since the steam turbine has a high degree of superheat when extracting steam step by step, in order to reasonably utilize the superheat without affecting the extraction of steam, a reasonable system of flue gas waste heat utilization and extraction steam coupling utilization is sought to improve the efficiency of the boiler, reduce the heat consumption of the steam turbine, and avoid the problem of the primary fan starting up and rushing to use the wind. The flue gas waste heat and the superheat of the extraction steam are fully utilized to improve the boiler efficiency and reduce the heat consumption of the unit. At the same time, the equipment investment is reduced and the power consumption rate of the plant is reduced, thereby improving the efficiency of the unit. The exhaust temperature of conventional boilers is generally high, reaching about 120°C. In order to make full use of the flue gas waste heat, a flue gas cooler 4 is used to reduce the flue gas temperature and recover heat for heating the cold air. The air leakage rate of a conventional air preheater is generally high, 5% within one year for a three-compartment air preheater and about 6% after one year, and 4.5% within one year for a four-compartment air preheater and about 5% after one year. However, after adopting the two-compartment air preheater 2 of the present invention, the air leakage rate of the air preheater can be appropriately reduced. After adopting the two-compartment air preheater, the air leakage rate can reach 2.5% within one year and 3% after one year.

[0032] Conventional cold air heating uses an air preheater. On the one hand, the high cold primary pressure head causes a higher air leakage rate (the use of a two-compartment air preheater can reduce the primary air volume by 30%). On the other hand, the heat exchange area of the air preheater is fixed, and the regulation of the cold air is not precise. In order to more accurately regulate the cold primary air temperature, a primary air heater is used. By gradually utilizing the waste heat of the flue gas and the superheat of the steam, the heat exchange area can be reasonably selected, and the air temperature can be controlled more accurately without causing heat waste.

[0033] Conventional steam turbine extraction systems utilize energy in a cascaded manner, extracting steam from different stages of the turbine and supplying it to the condensate system and feedwater system to achieve the following goals:

[0034] a) Heating feed water and condensate to improve thermal efficiency of power plants;

[0035] b) Remove oxygen and other non-condensable gases from condensed water;

[0036] c) Provide steam for the feedwater pump turbine.

[0037] Extraction steam is extracted from the steam turbine extraction port to users such as heaters, deaerators, feedwater pump turbines, etc. The system includes extraction steam isolation valves, extraction steam check valves, etc.

[0038] The typical process of this system design is:

[0039] The flue gas subsystem is as follows: the flue gas after combustion in boiler 1 is cooled by air preheater 2 and air preheater preheater bypass 3, and then further cooled by flue gas cooler 4 before entering dust collector 5.

[0040] The secondary air subsystem is as follows: the secondary air is pressurized by the blower 7 and enters the two-compartment air preheater 2 for heating, and then enters the boiler 1 for combustion.

[0041] The primary air subsystem is as follows: after the primary air is pressurized by the primary fan 10, it enters the primary air heating unit 9 (including multi-stage heaters, which can be designed in parallel, in series, or both) for heating, and then enters the coal mill to carry coal powder into the furnace for combustion. The primary air heating absorbs the flue gas waste heat and steam superheat in turn through the heat exchanger. The primary air heat sources are the flue gas cooler 4, the air preheater preheater bypass 3, and various levels of extraction steam. Make full use of lower-grade flue gas, higher-grade flue gas and higher-grade steam, utilize them step by step, and reasonably set the primary air heater to achieve comprehensive utilization of heat. By selecting the area of heaters at each level, precise control of the wind temperature can be achieved. Its equipment composition includes but is not limited to: heater 1 91 as a primary air heater, heater 2 92 as a primary air condensate heater, primary air extraction steam heater (I, II, III...), etc.

[0042] The core idea of this system is: rationally utilize the end difference of the heat exchanger, reasonably select the heat exchange area of the primary air heater, and 1) subdivide the area of the primary air heater and adopt one inlet and multiple outlets to increase or decrease the heat exchange area and adjust the primary air temperature; 2) adopt the parallel connection method of the primary air heat exchanger, and achieve different primary air temperatures by using different heat exchangers; 3) control the primary air temperature by cutting off the heater; 4) control the primary air temperature by increasing the temperature regulating air. This system has more and more flexible control methods and means for the primary air temperature, so as to accurately control the primary air temperature, make full use of the heat, avoid relying on the primary fan to grab the wind, and reduce the operating power consumption of the three major fans.

[0043] The regulation and control means of this system are as follows: the primary air is heated by flue gas waste heat and the superheat of extraction steam in sequence. The heating sources are low-grade flue gas, high-grade flue gas and even higher-grade steam. The heat is utilized step by step, and the composition of the primary air heater is reasonably selected to accurately control the primary air temperature. The primary air heater is coupled with the flue gas cooler through heat medium water to heat the lower cold air. Heater 2 92 is coupled with the air preheater preheater bypass 3 through condensate to heat the hot air after the heater. The primary air extraction steam heater is coupled with the extraction steam of each stage through steam to heat the hot air after heater 2 92 (the regulation measure on the extraction steam side is: by cutting the heaters of each stage into blocks, a one-input and multiple-output method is achieved, and the change of air temperature is achieved by changing the heated area, thereby achieving multi-means and multi-path adjustment of air temperature, and the primary air temperature control air duct can be eliminated). Finally, the cold air is heated to the required temperature. The above heat exchangers are connected in series and can operate independently, thereby achieving precise control of air temperature and adaptability to a wider range of coal types.

[0044] The main technical features of this patent are that by reasonably selecting the end difference and utilizing the heat at each level step by step, the primary air temperature can be reasonably selected and accurately controlled, and it has a wider adaptability to coal types; the air leakage rate of the air preheater is reduced, thereby reducing the selection criteria of the three major fans and reducing investment; the air leakage rate of the air preheater is reduced, thereby reducing the operating power consumption of the three major fans, thereby further reducing the plant power consumption rate; a single primary fan is used to avoid the problem of rushing wind caused by the unbalanced pressure between the two fans, thereby reducing the operating power of the primary air; the superheat of the extraction steam is fully utilized to improve the cascade utilization of energy and reduce the loss of cold source.

[0045] The key point of the present invention is that after adopting this system, the cascade utilization of heat is truly realized, thereby improving the efficiency of the boiler, reducing the heat consumption of the unit, and improving the power generation efficiency.

[0046] Figure 2 This is a system diagram of embodiment 2 of the present invention. It can be seen from the diagram that the primary air heating unit 9 includes heater one 91, heater two 92, heater three 93, heater four 94, heater five 95, heater six 96, heater seven 97 and heater eight 98, among which heater one 91, heater two 92, heater three 93, heater four 94, heater five 95 and heater six 96 are connected in series, and heater seven 97 and heater eight 98 are connected in series and then in parallel at the inlet of heater three 93 and the outlet of heater six 96.

[0047] In summary, the system of the present invention has the following advantages:

[0048] (1) A system for coupling the utilization of flue gas waste heat and extraction steam is proposed. The primary air heat source can be the flue gas cooler 4, the air preheater preheater bypass 3, and the extraction steam at each stage. By making full use of lower-grade flue gas, higher-grade flue gas, and higher-grade steam, and utilizing them step by step, and rationally setting the primary air heater, the comprehensive utilization of heat is achieved.

[0049] (2) A single primary air fan operation mode is proposed. After the primary air heater is set to heat the air without passing through the air preheater, the primary air heating requirements are met, the problem of primary air rushing is avoided, the primary air selection criteria are lowered, and the investment is reduced.

[0050] (3) A two-compartment air preheater operation mode is proposed, which can reduce the selection of air preheaters and reduce the operating power consumption of the three major fans, thereby reducing the plant's power consumption rate.

[0051] (4) A precise control mode of primary air temperature is proposed. Through the reasonable selection of primary air heaters, the investment of different heaters, and the investment of different heater areas, the precise control of primary air temperature can be achieved.

[0052] (5) A broad-sense cascade utilization of energy is proposed, which is not limited to the furnace side. It integrates the machine and furnace, reasonably selects the end difference, and reasonably selects the heat exchanger, so as to achieve the cascade comprehensive utilization of energy.

[0053] (6) A precise adjustment method for primary air temperature is proposed. By adopting different primary air heater bypasses, the precise adjustment of primary air temperature is achieved, and the resistance of the heater is reduced, thereby reducing the operating power of the fan.

[0054] (7) A steam extraction utilization method is proposed: fully utilize the superheat of the extraction steam to couple the primary air heating, and by reasonably selecting the number of extraction steam stages, add primary air extraction steam heater users before the extraction steam users of conventional power plants, thereby improving the cascade utilization of energy and reducing the loss of cold sources.

[0055] The present invention is based on the end difference idea of heat transfer, and rationally utilizes heat, thereby achieving a step-by-step comprehensive utilization of heat. In the system of the present invention, the primary air heat source can be a flue gas cooler 4 or an air preheater preheater bypass 3 or various levels of steam extraction, etc., making full use of lower-grade flue gas, higher-grade flue gas and higher-grade steam, utilizing them step by step, and rationally setting the primary air heating unit 9 to achieve a precise control mode of the primary air temperature. Through the rational selection of the primary air heater, through the investment of different heaters, and the investment of different heater areas, the precise control of the primary air temperature is achieved, thereby achieving comprehensive utilization of heat. By adopting different primary air heater bypasses, the primary air temperature is precisely adjusted, and the resistance of the heater is reduced, thereby achieving a reduction in the operating power of the fan. The system of the present invention is a single primary fan operation mode, without passing through an air preheater. After the primary air heater is set for heating, the primary air heating requirements are met, the problem of primary air grabbing is avoided, the primary air selection benchmark is lowered, and investment is reduced. The two-compartment air preheater operating mode reduces air preheater selection, lowering the power consumption of the three main fans and, consequently, lowering plant power usage. This system represents a broad cascade of energy utilization, extending beyond the boiler side. It integrates the turbine and boiler, rationally selects the end differential, and appropriately chooses the heat exchanger, achieving cascaded energy utilization. By fully leveraging the superheat of the extraction steam coupled with primary air heating and, through the appropriate selection of the extraction steam stages, adding primary air extraction steam heater users before conventional power plant extraction steam users, this system enhances cascade energy utilization and reduces cooling losses.

Claims

1. A flue gas waste heat utilization and extraction steam coupled utilization system, characterized by: The invention comprises a flue gas subsystem, a secondary air subsystem and a primary air subsystem. The flue gas subsystem comprises an air preheater (2), an air preheater bypass (3), a flue gas cooler (4), a dust collector (5) and an induced draft fan (6). The flue gas subsystem is connected to the outlet of the boiler (1) through the air preheater (2). The air preheater (2) is a two-chamber air preheater. The outlet of the flue gas side (22) of the air preheater is connected to the flue gas cooler (4). The outlet of the flue gas cooler (4) is connected to the induced draft fan (6) through the dust collector (5). The inlet of the air side (21) of the air preheater is connected to the secondary air subsystem. The secondary air subsystem comprises a connected secondary air heater (8) and a blower (7). The primary air subsystem comprises a primary air heating unit (9) and a primary air fan (10). The inlet of the primary air heating unit (9) is connected to the primary air fan (10), and the outlet of the primary air heating unit (9) is connected to the coal mill. The primary air heating unit (9) exchanges heat with the flue gas cooler (4).

2. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 1, characterized in that: The primary air heating unit (9) includes M heaters.

3. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 2, characterized in that: M heaters are connected in series or partially in series and partially in parallel, M is 3 to 8, the heater close to the primary fan (10) is heater one (91), and heater one (91) exchanges heat with the flue gas cooler (4).

4. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 1, characterized in that: The flue gas cooler (4) performs heat exchange with the secondary air heater (8).

5. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 3, characterized in that: The air preheater preheater bypass (3) includes a feed water heat exchanger (31) and a condensate water heat exchanger (32).

6. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 5, characterized in that: The outlet of heater 1 (91) is connected to heater 2 (92), and heater 2 (92) exchanges heat with the condensate heat exchanger (32).

7. The flue gas waste heat utilization and extraction steam coupled utilization system according to claim 1, characterized in that: The heat source of the flue gas cooler (4) is the waste heat of the flue gas from the boiler (1), and the heat source of the primary air heating unit (9) includes the heat of the flue gas cooler (4) and the air preheater preheater bypass (3), which are utilized step by step according to their quality.

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