Flue gas recirculation combined low-order regenerative steam extraction energy closed efficiency raising system and method

Abstract

The invention discloses a flue gas recirculation composite low-order regenerative steam extraction energy closed efficiency raising system and method.A low-pressure regenerative steam extraction pipeline is connected to an intermediate-pressure cylinder and / or a low-pressure cylinder of a steam turbine, and the low-pressure regenerative steam extraction pipeline is correspondingly connected with a low-pressure heater; a recirculating flue gas pipeline is led out of the boiler flue gas pipeline between the downstream of the dust remover and the upstream of the desulfurization device, the output end of the recirculating flue gas pipeline is connected to a hearth of the boiler, and one or more flue gas steam heaters which are connected in series are arranged in the recirculating flue gas pipeline; a flue gas heating steam extraction pipeline is led out of the low-pressure regenerative steam extraction pipeline, and the output end of the flue gas heating steam extraction pipeline is connected with the flue gas steam heater. According to the scheme, low-temperature flue gas serves as a carrier for steam extraction latent heat heating and enters the hearth in a circulating mode, the steam extraction latent heat of the steam turbine is fully utilized by arranging the steam flue gas heater, cold source losses are reduced, and the heat economy of low loads and even rated loads in a unit is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal-fired power generation, and in particular relates to an energy closed efficiency improvement system and method for flue gas recirculation combined with low-order heat recovery steam extraction. Background Art

[0002] Traditional coal-fired power generation primarily focuses on efficiency under rated operating conditions, with coal consumption significantly increasing under medium and low load conditions. Under the "dual carbon" goals, coal-fired power generation units will operate at medium and low loads for extended periods, becoming the new norm. Improving the economic efficiency of units at medium and low loads and achieving high efficiency across all operating conditions is a key technology for the new generation of coal-fired power generation.

[0003] Unit efficiency primarily depends on boiler efficiency and turbine absolute internal efficiency. While boiler efficiency generally decreases slightly with decreasing load, turbine absolute internal efficiency shows a significant downward trend. For example, for a 1 million kW wet-cooled unit, boiler efficiency dropped from 95.4% at rated conditions to 94.63% at 30% of rated conditions; turbine absolute internal efficiency dropped from 51.4% at rated conditions to 46.6% at 30% of rated conditions and 44.5% at 20% of rated conditions. This demonstrates that improving unit efficiency at low and medium loads hinges on increasing turbine absolute internal efficiency at these conditions.

[0004] The absolute internal efficiency of a steam turbine reflects the actual thermal efficiency of the turbine unit's thermodynamic cycle. The heat input from the boiler to the turbine, excluding the heat generated during power generation, manifests as cooling losses, which are the heat released to the environment by the exhaust steam from the turbine's low-pressure cylinder through the condensing device. The absolute efficiency of a steam turbine is generally around 50%, with cooling losses accounting for approximately 50%.

[0005] Steam generates power within a steam turbine, primarily utilizing its superheated energy. Latent heat accounts for the majority of the steam's enthalpy, while the superheated portion makes up a smaller portion. For example, for ultra-supercritical main steam at 28 MPa and 600°C, its enthalpy is 3465.7 kJ / kg, while the enthalpy of low-pressure exhaust steam is 2417.9 kJ / kg. The effective enthalpy for work is reduced to 1047.8, accounting for 30% of the total enthalpy. Utilizing steam's latent heat while minimizing exhaust to the low-pressure cylinder is a key efficiency improvement strategy for coal-fired power plants. The turbine extraction heat recovery system, which fully utilizes steam's latent heat to heat condensate and feedwater, is a typical application for reducing cooling losses.

[0006] Regarding how to return the latent heat of extraction steam to the boiler, general considerations include heating the air through extraction steam, heating the feed water at the economizer inlet through extraction steam, and heating the raw coal through extraction steam. However, it is difficult to achieve the goal of reducing the cold end exhaust volume and reducing the loss of the cooling source. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide an energy closed efficiency improvement system and method for flue gas recirculation and composite low-order heat recovery extraction, so as to solve the problem of high coal consumption of existing coal-fired power units under medium and low load conditions. By innovating the system structure and constructing a low-temperature carrier that can recover the latent heat of the unit's extraction steam, the latent heat of the turbine extraction steam is fully utilized, the cold source loss is reduced, and the thermal economy of the unit under medium and low load conditions is improved.

[0008] According to the technical solution of the present invention, the present invention provides an energy-closed efficiency improvement system of flue gas recirculation and composite low-order heat recovery extraction, including a boiler, the boiler is connected to a boiler feed water pipe and a boiler flue gas pipe; a multi-stage low-pressure heater is arranged in the boiler feed water pipe; a low-pressure heat recovery extraction pipe is connected to the intermediate pressure cylinder and / or low pressure cylinder of the steam turbine, and the low-pressure heat recovery extraction pipe is connected to the low-pressure heater accordingly; an air preheater, a dust collector, an induced draft fan, a desulfurization device and a chimney are arranged in sequence in the boiler flue gas pipe along the flue gas conveying direction; a recirculating flue gas pipe is led out from the boiler flue gas pipe between the downstream of the dust collector and the upstream of the desulfurization device, the output end of the recirculating flue gas pipe is connected to the furnace of the boiler, and one or more flue gas steam heaters are arranged in series in the recirculating flue gas pipe; a flue gas heating extraction pipe is led out from the low-pressure heat recovery extraction pipe, and the output end of the flue gas heating extraction pipe is connected to the flue gas steam heater.

[0009] In some embodiments, the input end of the recirculated flue gas duct is located between the induced draft fan and the desulfurization device.

[0010] In some embodiments, the input end of the recirculating flue gas duct is located between the dust collector and the induced draft fan, and a variable frequency recirculating fan is provided in the recirculating flue gas duct.

[0011] In some embodiments, the output end of the recirculated flue gas duct is located on the front and rear walls of the furnace below the lowest burner of the boiler and above the inflection point of the cold ash hopper; or, the output end of the recirculated flue gas duct is located on the front and rear walls of the furnace above the uppermost burner of the boiler and near the overburnt air.

[0012] In some embodiments, an output electric adjustment damper is provided at the output end of the recirculating flue gas duct.

[0013] In some embodiments, an input electric shutoff damper is provided at the input end of the recirculating flue gas duct, and a valve is provided in the flue gas heating steam extraction duct.

[0014] In some embodiments, the number of flue gas steam heaters is N, where N is a positive integer, the low-pressure heat recovery steam extraction pipe is the N-stage steam extraction pipe at the end of the medium-pressure cylinder, and the flue gas steam heaters correspond to the low-pressure heat recovery steam extraction pipe one-to-one; and the N low-pressure heat recovery steam extraction pipes are connected in sequence with the N flue gas steam heaters arranged along the flue gas conveying direction in order of the steam extraction pressure from low to high.

[0015] In some embodiments, the boiler feed water pipeline is provided with multiple stages of low-pressure heaters, deaerators, feed water pumps and multiple stages of high-pressure heaters in sequence along the water delivery direction; the multiple stages of low-pressure heaters include No. 9 low-pressure heater, No. 8 low-pressure heater, No. 7 low-pressure heater, No. 6 low-pressure heater and No. 5 low-pressure heater, which are arranged in sequence along the water delivery direction; the multiple stages of high-pressure heaters include No. 3 high-pressure heater, No. 2 high-pressure heater and No. 1 high-pressure heater, which are arranged in sequence along the water delivery direction; the number of flue gas steam heaters is three; the low-pressure heat recovery steam extraction pipeline is a five-stage steam extraction pipeline and a six-stage steam extraction pipeline of the medium-pressure cylinder. The fifth section of the steam extraction pipeline is connected to the No. 5 low-pressure heater, the sixth section of the steam extraction pipeline is connected to the No. 6 low-pressure heater, and the seventh section of the steam extraction pipeline is connected to the No. 7 low-pressure heater; the flue gas heating steam extraction pipeline led out from the fifth section of the steam extraction pipeline is connected to the flue gas steam heater on the downstream side of the three flue gas steam heaters, the flue gas heating steam extraction pipeline led out from the sixth section of the steam extraction pipeline is connected to the flue gas steam heater located in the middle of the three flue gas steam heaters, and the flue gas heating steam extraction pipeline led out from the seventh section of the steam extraction pipeline is connected to the flue gas steam heater on the upstream side of the three flue gas steam heaters.

[0016] According to the technical solution of the present invention, the present invention also provides an energy closure efficiency improvement method for flue gas recirculation combined with low-order heat recovery extraction, which adopts the energy closure efficiency improvement system for flue gas recirculation combined with low-order heat recovery extraction described in the present invention, and includes the following contents: A low-temperature flue gas recirculation system is formed through the recirculating flue gas duct, flue gas steam heater, and flue gas heating extraction steam duct. The low-temperature flue gas recirculation system is put into operation only when the coal-fired power generation unit is operating below 50% THA conditions; when the coal-fired power generation unit is in other operating conditions, the recirculating flue gas duct and the flue gas heating extraction steam duct are both blocked, and the flue gas steam heater does not work.

[0017] In some embodiments, when the low-temperature flue gas recirculation system is in operation, the amount of recirculated flue gas in the recirculating flue gas duct is 15% of the total flue gas at 40% THA conditions.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows: The energy closed efficiency improvement system and method of the flue gas recirculation composite low-order heat recovery extraction steam of the present invention is based on improving the thermal economy of the medium and low loads of the unit, constructing a low-temperature flue gas recirculation system, and utilizing the extraction steam of the medium and low-pressure cylinder heat recovery system of the turbine to heat the low-temperature flue gas, and the heated flue gas is sent to the furnace to enter the next cycle; this system uses low-temperature flue gas as a carrier for extraction steam latent heat heating, that is, a carrier for heat recovery, and enters the furnace in a circulating manner. By setting one or more steps of steam flue gas heaters, the latent heat of turbine extraction steam is fully utilized, the exhaust volume of the low-pressure cylinder is reduced, the loss of cold source is reduced, and the thermal economy of the medium and low loads and even rated load of the unit is improved. It is an innovation and reconstruction of the boiler system, has strong technical and economic feasibility, and is worthy of development and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the system structure of an embodiment provided by the present invention.

[0020] Figure 2 It is a schematic diagram of the system structure of another embodiment provided by the present invention.

[0021] Description of reference numerals in the accompanying drawings: 1. Boiler; 11. Boiler feed water pipe; 12. Boiler flue gas pipe; 121. Air preheater; 122. Dust collector; 123. Induced draft fan; 124. Desulfurization device; 125. Chimney; 21. High-pressure cylinder; 22. Medium-pressure cylinder; 23. Low-pressure cylinder; 24. Generator; 31. Low-pressure heat recovery steam extraction pipe; 32. Flue gas heating steam extraction pipe; 4. Recirculating flue gas pipe; 41. Flue gas steam heater; 42. Output power Dynamic adjustment damper; 43. Input electric shut-off damper; 5. Variable frequency recirculation fan; 6. Deaerator; 7. Feed water pump; 81. No. 5 low-pressure heater; 82. No. 6 low-pressure heater; 83. No. 7 low-pressure heater; 84. No. 8 low-pressure heater; 85. No. 9 low-pressure heater; 91. No. 1 high-pressure heater; 92. No. 2 high-pressure heater; 93. No. 3 high-pressure heater; 101. Primary fan; 102. Secondary fan. DETAILED DESCRIPTION

[0022] The present invention provides an energy closure efficiency improvement system and method for flue gas recirculation and composite low-order heat recovery extraction, which solves the problem of high coal consumption of existing coal-fired power units under medium and low load conditions. Through system structure innovation and the construction of a low-temperature carrier that can recover the latent heat of unit extraction steam, the latent heat of turbine extraction steam is fully utilized, the cold source loss is reduced, and the thermal economy of the unit under medium and low load conditions is improved.

[0023] See also Figure 1The energy-closed efficiency-enhancing system of the present invention, which combines flue gas recirculation with low-order heat recovery steam extraction, includes a boiler 1 connected to a boiler feedwater pipe 11 and a boiler flue gas pipe 12. A multi-stage low-pressure heater is provided in the boiler feedwater pipe 11. The steam turbine includes a high-pressure cylinder 21, an intermediate-pressure cylinder 22, a low-pressure cylinder 23, and a generator 24. A low-pressure heat recovery steam extraction pipe 31 is connected to the intermediate-pressure cylinder 22 and / or the low-pressure cylinder 23 of the steam turbine. The low-pressure heat recovery steam extraction pipe 31 is correspondingly connected to the low-pressure heater, and the boiler feedwater is heated by the low-pressure heater using appropriate steam.

[0024] An air preheater 121, a dust collector 122, an induced draft fan 123, a desulfurization device 124, and a chimney 125 are sequentially arranged in the boiler flue gas duct 12 along the flue gas conveyance direction. A recirculating flue gas duct 4 extends from the boiler flue gas duct 12 between downstream of the dust collector 122 and upstream of the desulfurization device 124. The output end of the recirculating flue gas duct 4 is connected to the furnace of the boiler 1. One or more flue gas steam heaters 41 are arranged in series in the recirculating flue gas duct 4. A flue gas heating extraction steam duct 32 extends from the low-pressure heat recovery extraction steam duct 31. The output end of the flue gas heating extraction steam duct 32 is connected to the flue gas steam heater 41, thereby drawing a portion of the extraction steam from the low-pressure heat recovery extraction steam duct 31 into the flue gas steam heater 41 to heat the flue gas circulated to the furnace of the boiler 1.

[0025] This solution primarily establishes a low-temperature flue gas recirculation system by adding a recirculation flue gas duct 4, a flue gas steam heater 41, and a flue gas heating extraction duct 32. This system extracts flue gas between the downstream of the dust collector and the upstream of the desulfurization system. Simultaneously, low-pressure steam is extracted from the turbine's intermediate and low-pressure cylinder reheat systems based on flue gas temperature. The first-stage extraction is constrained by the heat exchanger end differential. The lower the extraction pressure, the better the economy. This also facilitates the step-by-step heating of the flue gas steam heater. The number of stages is constrained by achieving coal consumption reduction targets (e.g., a 2g / kWh reduction in power generation coal consumption at 50% operating conditions, or even higher), while also considering investment costs. This allows the low-temperature flue gas to be used as a carrier for recovering steam latent heat and reducing cooling losses. Due to the low initial flue gas temperature, a first-stage extraction pressure above 0.1 MPa.a meets project requirements, and the unit has a suitable extraction point. The flue gas recirculation extraction point, or the input end of the recirculated flue gas duct 4, is located after the dust collector 122. This not only reduces the flue gas temperature and dust content, but also facilitates the use of a low-cost, highly durable recirculation fan. This solution, with flue gas recirculation, increases the flue gas flow from the furnace inlet to the rear heating surface, generally enhancing heat transfer on the heating surface and raising the working fluid outlet temperature, positively impacting boiler operation. Furthermore, this solution does not impact existing boilers or auxiliary systems, including existing capacity and operational adaptability.

[0026] Further, if Figure 1In the illustrated embodiment, the input end of the recirculated flue gas duct 4 (in other words, the flue gas extraction point) is located between the induced draft fan 123 and the desulfurization device 124, thereby utilizing the induced draft fan 123 as the flue gas circulation power. This solution must fully consider the sensitivity of the flue gas recirculation volume to load fluctuations. The advantage is that it eliminates the need for an additional recirculation fan in the recirculated flue gas duct 4, resulting in low cost and reduced investment. Generally, under low to medium loads, the flue gas temperature at the inlet of the induced draft fan 123 is around 85°C, and the flue gas temperature at the outlet of the induced draft fan 123 is around 90°C. After absorbing the latent heat of steam, the flue gas temperature at the output end of the recirculated flue gas duct 4 can reach around 150°C.

[0027] Considering the sensitivity of the low-temperature flue gas recirculation system to load adjustment, it is preferred to set a recirculation fan at the inlet of the induced draft fan as the circulation power of the flue gas, such as Figure 2 In another embodiment shown, the input end of the recirculating flue gas duct 4 is located between the dust collector 122 and the induced draft fan 123, and a variable frequency recirculating fan 5 is provided in the recirculating flue gas duct 4 to track the load adjustment by variable frequency speed regulation.

[0028] Taking into account the ash falling, pulverized coal ignition and stable combustion performance of coal-fired boilers, and combined with the boiler combustion mode, in the embodiment, the output end of the recirculating flue gas duct 4 is located below the lowest burner of the boiler 1 and on the front and rear walls of the furnace above the inflection point of the cold ash hopper. In other words, the recirculating flue gas is injected into the furnace from below the lowest burner and above the inflection point of the cold ash hopper, and the air supply port is arranged on the front and rear walls of the furnace; or, the output end of the recirculating flue gas duct 4 is located above the uppermost burner of the boiler 1 and on the front and rear walls of the furnace near the burnout air. The specific final plan generally needs to be communicated and confirmed with the boiler factory. As a supplementary explanation, burners are provided on the furnace wall of the furnace of the boiler 1, and the burners are generally multiple and distributed in multiple layers at multiple heights; the lower part of the boiler 1 is the cold ash hopper, and the side walls of the cold ash hopper are inclined, and an inflection point of the inclined surface and the vertical surface is formed between the side walls of the cold ash hopper and the furnace wall of the furnace.

[0029] Preferably, an output electric adjustment damper 42 is installed at the output end of the recirculated flue gas duct 4, allowing for flexible adjustment of the flue gas volume entering the boiler 1 or blocking the recirculated flue gas duct 4. Furthermore, preferably, an input electric shutoff damper 43 is installed at the input end of the recirculated flue gas duct 4, and a valve is installed in the flue gas heating extraction steam duct 32. Closing the input electric shutoff damper 43 and the valves in the flue gas heating extraction steam duct 32 blocks the low-temperature flue gas recirculation system. This allows for partial load design as needed, primarily to improve the unit's thermal economy at low and medium loads, while disabling the low-temperature flue gas recirculation system under rated operating conditions and high-load conditions.

[0030] Furthermore, the number of the flue gas steam heaters 41 is N, where N is a positive integer of 1 or greater, such as 1, 2, 3 or 4, and the low-pressure heat recovery extraction steam pipe 31 is the last N-stage extraction steam pipe of the intermediate pressure cylinder 22 (that is, the N pipes with sequentially low extraction steam pressure in the extraction pipes connected to the intermediate pressure cylinder), and the flue gas steam heaters 41 correspond one-to-one to the low-pressure heat recovery extraction steam pipe 31; and (for the case where N is greater than 2) the N low-pressure heat recovery extraction steam pipes 31 are sequentially connected to the N flue gas steam heaters 41 arranged along the flue gas conveying direction in order of extraction steam pressure from low to high, so that the flue gas is gradually heated in a step-by-step manner when flowing through the N flue gas steam heaters 41 in sequence.

[0031] More specifically, if Figure 1 In the illustrated embodiment, the boiler feedwater pipeline 11 is provided with multiple stages of low-pressure heaters, a deaerator 6, a feedwater pump 7, and multiple stages of high-pressure heaters, arranged in sequence along the water flow direction. The multiple stages of low-pressure heaters include low-pressure heater No. 9 85, low-pressure heater No. 8 84, low-pressure heater No. 7 83, low-pressure heater No. 6 82, and low-pressure heater No. 5 81, arranged in sequence along the water flow direction. The multiple stages of high-pressure heaters include high-pressure heater No. 3 93, high-pressure heater No. 2 92, and high-pressure heater No. 1 91, arranged in sequence along the water flow direction.

[0032] There are three flue gas steam heaters 41. The low-pressure regenerative steam extraction pipe 31 is the last three-stage steam extraction pipe of the intermediate pressure cylinder 22, specifically the fifth, sixth, and seventh sections of the intermediate pressure cylinder 22. The fifth section of the steam extraction pipe is connected to the fifth low-pressure heater 81, the sixth section of the steam extraction pipe is connected to the sixth low-pressure heater 82, and the seventh section of the steam extraction pipe is connected to the seventh low-pressure heater 83. The flue gas heating steam extraction pipe 32 leading from the fifth section of the steam extraction pipe is connected to the most downstream flue gas steam heater 41 of the three flue gas steam heaters 41, the flue gas heating steam extraction pipe 32 leading from the sixth section of the steam extraction pipe is connected to the middle flue gas steam heater 41 of the three flue gas steam heaters 41, and the flue gas heating steam extraction pipe 32 leading from the seventh section of the steam extraction pipe is connected to the most upstream flue gas steam heater 41 of the three flue gas steam heaters 41.

[0033] As a supplementary explanation, the specific connection methods and settings of each high-pressure heater, low-pressure heater, and deaerator 6 can be selected to adopt existing technologies or other feasible methods to form a heat recovery system. Generally, the extracted steam enters the high-pressure heater, low-pressure heater, deaerator 6, and flue gas steam heater 41, and the steam side undergoes heat exchange and becomes hydrophobic, and is then output to the condenser and condensate pipe through the pipeline. Common methods include step-by-step gravity flow, but are not limited to this. In addition, the air preheater 121 is connected to the primary air duct and the secondary air duct, and the primary air duct and the secondary air duct are respectively provided with a primary fan 101 and a secondary fan 102. The air preheater 121 is used to heat the primary air and the secondary air, and the primary air duct and the secondary air duct are connected to the boiler furnace. The relevant more specific settings can be selected to adopt existing technologies or other feasible methods. These specific settings are not improvements of the present invention, so they are not described in detail, and are not shown in the figure or are only simple schematic diagrams for example.

[0034] Based on the system of the present invention, the present invention provides an energy closure efficiency improvement method for flue gas recirculation combined with low-order heat recovery extraction, which mainly includes the following contents.

[0035] In this system, a low-temperature flue gas recirculation system is formed by the recirculation flue gas duct 4, the flue gas steam heater 41, the flue gas heating extraction steam duct 32 (and other required components). During operation, the low-temperature flue gas recirculation system is only operational when the coal-fired power generation unit is operating below 50% THA. Specifically, the recirculation flue gas duct 4 and the flue gas heating extraction steam duct 32 are connected, extracting flue gas and steam in a predetermined manner. The flue gas is heated by the heat of the steam through the flue gas steam heater 41 before being fed into the furnace of the boiler 1. In other operating conditions (i.e., above 50% THA), the recirculation flue gas duct 4 and the flue gas heating extraction steam duct 32 are blocked, the flue gas steam heater 41 is inoperative, and the aforementioned flue gas recirculation process does not occur. In a more specific embodiment, when the low-temperature flue gas recirculation system is in operation, the amount of recirculated flue gas in the recirculation flue gas duct 4 is 15% of the total flue gas volume at 40% THA.

[0036] It should be noted that, for example, a 1 million kW double-reheat ultra-supercritical wet-cooled unit has a rated coal consumption of 264 g / kWh, and a 30% rated consumption of 303 g / kWh, representing a 39 g / kWh increase compared to rated operation. At 20% rated operation, the coal consumption reaches 319 g / kWh, a 55 g / kWh increase compared to rated operation. Another example is a 660 MW supercritical single-reheat air-cooled unit, which has a rated coal consumption of 295 g / kWh, and a 30% rated consumption of 335 g / kWh, representing a 40 g / kWh increase compared to rated operation. Existing conventional coal-fired power generation units experience significant increases in coal consumption at low and medium loads, making them difficult to meet the goals and needs of long-term low and medium load operation for new-generation coal-fired power plants. Therefore, improving the thermal economy of these units at low and medium loads is particularly important.

[0037] Regarding the method of returning the latent heat of extraction steam to the boiler, general considerations and problems are as follows. The input heat of the boiler comes from air, raw coal and feed water. Using the extraction steam of the unit to heat the air system before the air preheater, such as the primary cold air and the secondary cold air, will cause the exhaust gas temperature to rise, reduce the thermal efficiency of the boiler, and complicate the problem. For heating the air system after the air preheater, such as the primary hot air and the secondary hot air, engineering practice has found that even under low load conditions, the temperature of the primary hot air and the secondary hot air is generally above 320°C, close to the critical parameters of water vapor. Due to the high temperature of the heated medium, considering the air temperature rise amplitude, the heat exchange end difference, the appropriate extraction parameter matching and other factors, it can be considered that it is difficult to achieve the purpose of reducing the cold end exhaust volume and reducing the loss of the cold source by heating the air by extraction steam. Even under low-load conditions, the economizer inlet feedwater temperature is typically above 240°C. Given the high heating medium temperature, it's difficult to achieve the goal of reducing cold-end exhaust steam and cooling losses by heating the economizer inlet feedwater with extraction steam, considering factors such as the feedwater temperature rise, heat exchange end difference, and appropriate extraction parameters. Due to the gas-solid two-phase nature and flammability of raw coal (pulverized coal gas flow), there's no proven engineering solution for using steam to heat raw coal to reduce cooling losses.

[0038] The energy closed efficiency improvement system and method of the flue gas recirculation composite low-order heat recovery extraction steam of the present invention is based on improving the thermal economy of the medium and low loads of the unit, constructing a low-temperature flue gas recirculation system, and utilizing the extraction steam of the medium and low-pressure cylinder heat recovery system of the turbine to heat the low-temperature flue gas, and the heated flue gas is sent to the furnace to enter the next cycle; this system uses low-temperature flue gas as a carrier for extraction steam latent heat heating, that is, a carrier for heat recovery, and enters the furnace in a circulating manner. By setting one or more steps of steam flue gas heaters, the latent heat of turbine extraction steam is fully utilized, the exhaust volume of the low-pressure cylinder is reduced, the loss of cold source is reduced, and the thermal economy of the medium and low loads and even rated load of the unit is improved. It is an innovation and reconstruction of the boiler system, has strong technical and economic feasibility, and is worthy of development and promotion.

[0039] The system settings and benefits are illustrated by taking a million-kilowatt secondary reheat unit as an example: the low-temperature flue gas recirculation system is only put into operation below 50% THA conditions, with 40% THA conditions as the design point. The recirculated flue gas volume is 15% of the flue gas volume under 40% THA conditions, and the flue gas outlet point is after the induced draft fan; a three-stage flue gas steam heater is set, and the three-stage extraction steam at the end of the unit's intermediate pressure cylinder (8th, 7th, and 6th stages of extraction steam) is used as the heating steam source. The extraction steam pressures are 0.169 MPa.a, 0.314 MPa.a, and 0.478 MPa.a, respectively. The flue gas temperature is heated from 90°C to 140°C, and the power supply coal consumption is reduced by about 3g / kWh, with very significant economic benefits.

[0040] As a supplementary note, this system is suitable not only for low and medium loads, but also for improving efficiency at rated conditions. However, this will increase the capacity of equipment such as the denitrification SCR reactor, air preheater, and dust collector, significantly increasing investment. In engineering applications, it can be flexibly applied based on specific efficiency requirements. Furthermore, in specific projects, this system is rationally determined based on the set energy-saving targets by optimizing multiple factors, including the flue gas recirculation volume, the number of cascade steam heater stages, boiler adaptability, and cost.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; obviously, the embodiments described are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention; for ease of description, only the parts related to the relevant inventions are shown in the accompanying drawings. In the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other; modifying the technical solutions described in the aforementioned embodiments, or making equivalent replacements for some of the technical features therein, does not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An energy closed efficiency improvement system for flue gas recirculation combined with low-order heat recovery steam extraction, characterized in that: The boiler (1) is connected to a boiler water supply pipe (11) and a boiler flue gas pipe (12); A multi-stage low-pressure heater is provided in the boiler feed water pipe (11); a low-pressure heat recovery steam extraction pipe (31) is connected to the intermediate-pressure cylinder (22) and / or the low-pressure cylinder (23) of the steam turbine, and the low-pressure heat recovery steam extraction pipe (31) is connected to the low-pressure heater in a corresponding manner; An air preheater (121), a dust collector (122), an induced draft fan (123), a desulfurization device (124) and a chimney (125) are sequentially arranged in the boiler flue gas duct (12) along the flue gas conveying direction; a recirculating flue gas duct (4) is led out from the boiler flue gas duct (12) between the downstream of the dust collector (122) and the upstream of the desulfurization device (124); the output end of the recirculating flue gas duct (4) is connected to the furnace of the boiler (1); one or more flue gas steam heaters (41) connected in series are arranged in the recirculating flue gas duct (4); a flue gas heating steam extraction duct (32) is led out from the low-pressure heat recovery steam extraction duct (31), and the output end of the flue gas heating steam extraction duct (32) is connected to the flue gas steam heater (41).

2. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 1 is characterized in that: The input end of the recirculating flue gas duct (4) is located between the induced draft fan (123) and the desulfurization device (124).

3. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 1 is characterized in that: The input end of the recirculating flue gas duct (4) is located between the dust collector (122) and the induced draft fan (123), and a variable frequency recirculating fan (5) is provided in the recirculating flue gas duct (4).

4. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 1 is characterized in that: The output end of the recirculating flue gas duct (4) is located on the front and rear walls of the furnace below the lowest burner of the boiler (1) and above the inflection point of the cold ash hopper; or, the output end of the recirculating flue gas duct (4) is located on the front and rear walls of the furnace above the uppermost burner of the boiler (1) and near the burnout air.

5. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 1 is characterized in that: An output electric adjustment baffle (42) is provided at the output end of the recirculating flue gas duct (4).

6. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 1 is characterized in that: An input electric shutoff damper (43) is provided at the input end of the recirculating flue gas duct (4), and a valve is provided in the flue gas heating steam extraction duct (32).

7. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to any one of claims 1 to 6, characterized in that: The number of the flue gas steam heaters (41) is N, where N is a positive integer. The low-pressure heat recovery steam extraction pipe (31) is the last N-stage steam extraction pipe of the medium-pressure cylinder (22). The flue gas steam heaters (41) correspond to the low-pressure heat recovery steam extraction pipe (31) one by one; and the N low-pressure heat recovery steam extraction pipes (31) are connected to the N flue gas steam heaters (41) arranged along the flue gas conveying direction in order of steam extraction pressure from low to high.

8. The energy closed efficiency improvement system of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 7 is characterized in that: A multi-stage low-pressure heater, a deaerator (6), a feedwater pump (7) and a multi-stage high-pressure heater are sequentially arranged in the boiler feedwater pipe (11) along the water conveying direction; The multi-stage low-pressure heater includes a No. 9 low-pressure heater (85), a No. 8 low-pressure heater (84), a No. 7 low-pressure heater (83), a No. 6 low-pressure heater (82), and a No. 5 low-pressure heater (81) which are sequentially arranged along the water conveying direction; the multi-stage high-pressure heater includes a No. 3 high-pressure heater (93), a No. 2 high-pressure heater (92), and a No. 1 high-pressure heater (91) which are sequentially arranged along the water conveying direction; The number of flue gas steam heaters (41) is three; the low-pressure heat recovery steam extraction pipeline (31) is a five-section steam extraction pipeline, a six-section steam extraction pipeline, and a seven-section steam extraction pipeline of the medium-pressure cylinder (22); The fifth section of the steam extraction pipe is connected to the fifth low-pressure heater (81), the sixth section of the steam extraction pipe is connected to the sixth low-pressure heater (82), and the seventh section of the steam extraction pipe is connected to the seventh low-pressure heater (83); the flue gas heating steam extraction pipe (32) drawn out from the fifth section of the steam extraction pipe is connected to the flue gas steam heater (41) on the most downstream side among the three flue gas steam heaters (41), the flue gas heating steam extraction pipe (32) drawn out from the sixth section of the steam extraction pipe is connected to the flue gas steam heater (41) located in the middle among the three flue gas steam heaters (41), and the flue gas heating steam extraction pipe (32) drawn out from the seventh section of the steam extraction pipe is connected to the flue gas steam heater (41) on the most upstream side among the three flue gas steam heaters (41).

9. A flue gas recirculation combined with low-order heat recovery steam extraction energy closure efficiency improvement method, characterized in that: The energy closed efficiency improvement system of the flue gas recirculation combined with low-order heat recovery steam extraction according to any one of claims 1 to 8 is adopted, and includes the following contents: A low-temperature flue gas recirculation system is formed by a recirculating flue gas duct (4), a flue gas steam heater (41), and a flue gas heating extraction steam duct (32). The low-temperature flue gas recirculation system is put into operation only when the coal-fired power generation unit is below 50% THA operating conditions; when the coal-fired power generation unit is in other operating conditions, the recirculating flue gas duct (4) and the flue gas heating extraction steam duct (32) are both in a blocked state, and the flue gas steam heater (41) does not work.

10. The energy closure efficiency improvement method of flue gas recirculation combined with low-order heat recovery steam extraction according to claim 9 is characterized in that: When the low-temperature flue gas recirculation system is in operation, the amount of recirculated flue gas in the recirculating flue gas duct (4) is 15% of the total amount of flue gas under 40% THA conditions.

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