System and method for reconstructing thermodynamic system to improve low-load thermal efficiency in unit

Abstract

The system comprises at least one pressure matcher, and the steam extraction source input end of the pressure matcher is connected with the steam exhaust side of a low-pressure cylinder or a steam exhaust and condensing device. The driving steam source input end of the pressure matcher is connected with the steam exhaust side of the high-pressure cylinder or the regenerative steam extraction of the medium-pressure cylinder, and the steam output end of the pressure matcher is connected with the steam input side of the low-pressure heater corresponding to the high-grade regenerative steam extraction pipeline through a regenerative steam extraction replacement pipeline. The driving steam source input end of the pressure matcher is connected with a driving steam source shut-off door, a normal steam extraction shut-off door is arranged on the high-grade regenerative steam extraction pipeline, and a substitute steam extraction shut-off door is arranged on the regenerative steam extraction substitute pipeline. The low-pressure cylinder is fully used for exhausting steam, cold source losses are reduced, the heat economy of low-load units in the units is improved, the method is suitable for newly-built units and active-service units, and the technical economic feasibility is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal-fired power generation, and particularly relates to a system and method for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads. Background Art

[0002] Traditional coal power mainly considers the high efficiency under rated conditions, and the coal consumption of the unit increases significantly under medium and low load conditions. Taking a million-kilowatt wet-cooled unit as an example, the power supply coal consumption under rated conditions is 275 g / kWh, the power supply coal consumption at 30% of the rated condition is 326 g / kWh, and the power supply coal consumption at 20% of the rated condition reaches 375 g / kWh.

[0003] The efficiency of the unit mainly depends on the boiler efficiency and the absolute internal efficiency of the steam turbine. As the load decreases, generally the boiler efficiency does not decrease much, while the absolute internal efficiency of the steam turbine shows a significant decreasing trend. Taking a million-kilowatt wet-cooled unit as an example, the boiler efficiency drops from 95.4% under rated conditions to 94.63% at 30% of the rated condition; the absolute internal efficiency of the steam turbine drops from 51.4% under rated conditions to 46.6% at 30% of the rated condition and 44.5% at 20% of the rated condition. It can be seen from the above that the key to improving the efficiency of the unit at medium and low loads lies in improving the absolute internal efficiency of the steam turbine at medium and low loads.

[0004] The absolute internal efficiency of the steam turbine reflects the actual thermal efficiency of the steam turbine unit's thermal cycle. The heat input from the boiler to the steam turbine, excluding the part for power generation, is the cold source loss. The cold source loss is the heat released from the exhaust steam of the low-pressure cylinder of the steam turbine to the environment through the condensation device. The low thermal efficiency of the unit at medium and low loads means a large cold source loss. The factors determining the thermal efficiency of the steam turbine generator unit mainly include the initial parameters of the unit, the exhaust back pressure, the thermal system (the number of regenerative system stages, the number of reheats), etc. As the unit load decreases, the initial parameters decrease, the unit efficiency continuously decreases, and the cold source loss continuously increases.

[0005] It will become a new normal for coal-fired power units to operate at medium and low loads for a long time. Improving the economy of the unit at medium and low loads and achieving high efficiency under all operating conditions are one of the key technologies for the new generation of coal power. Therefore, it is necessary to design a new type of thermal system for coal-fired power units to improve the economy of the unit at medium and low loads and achieve high efficiency under all operating conditions. Summary of the Invention

[0006] The technical problem to be solved by the present invention is: to provide a system and method for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads, solve the problems of low efficiency of coal-fired power generation units and large cold source loss under medium and low load conditions, and through innovation and reconstruction of the thermal system, make full use of the exhaust steam heat of the steam turbine, reduce the cold source loss, improve the economy of the unit at medium and low loads, and achieve high efficiency under all operating conditions.

[0007] According to the technical solution of the present invention, the present invention provides a system for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads, including a boiler, a high-pressure cylinder, an intermediate-pressure cylinder, a low-pressure cylinder, an exhaust condensing device, a low-pressure heater group, a deaerator, and a high-pressure heater group that are connected in sequence. The low-pressure heater group includes a plurality of low-pressure heaters connected in series, and at least one of the low-pressure heaters has its steam input side connected to the intermediate-pressure cylinder through a high-grade regenerative extraction steam pipeline. It further includes at least one pressure matcher. The extraction steam source input end of the pressure matcher is connected to the exhaust side of the low-pressure cylinder or the exhaust condensing device, the driving steam source input end of the pressure matcher is connected to the exhaust side of the high-pressure cylinder or the regenerative extraction steam of the intermediate-pressure cylinder, the steam output end of the pressure matcher is connected to the steam input side of the corresponding low-pressure heater on the high-grade regenerative extraction steam pipeline through a regenerative extraction steam replacement pipeline, a driving steam source shut-off valve is connected to the driving steam source input end of the pressure matcher, a normal extraction steam shut-off valve is arranged on the high-grade regenerative extraction steam pipeline, and a replacement extraction steam shut-off valve is arranged on the regenerative extraction steam replacement pipeline.

[0008] Further, the pressure matcher includes a first-stage pressure matcher and a second-stage pressure matcher. The extraction steam source input end of the first-stage pressure matcher is connected to the exhaust side of the low-pressure cylinder or the exhaust condensing device, the driving steam source input end of the first-stage pressure matcher is connected to the exhaust side of the high-pressure cylinder or the regenerative extraction steam of the intermediate-pressure cylinder, and the steam output end of the first-stage pressure matcher is connected to the steam input side of the corresponding low-pressure heater on the high-grade regenerative extraction steam pipeline through a regenerative extraction steam replacement pipeline. The extraction steam source input end of the second-stage pressure matcher is connected to the steam output end of the first-stage pressure matcher, the driving steam source input end of the second-stage pressure matcher is connected to the exhaust side of the high-pressure cylinder or the regenerative extraction steam of the intermediate-pressure cylinder, and the steam output end of the second-stage pressure matcher is connected to the regenerative extraction steam replacement pipeline.

[0009] Further, the low-pressure heater corresponding to the high-grade regenerative extraction steam pipeline is the No. 5 low-pressure heater.

[0010] Further, the driving steam source input end of the pressure matcher is connected to the exhaust side of the high-pressure cylinder.

[0011] Further, the exhaust side of the high-pressure cylinder is connected to the boiler through a steam reheating pipeline, a driving steam source pipeline is led out from the steam reheating pipeline, the driving steam source pipeline is connected to the driving steam source input end of the pressure matcher, and the driving steam source shut-off valve is arranged on the driving steam source pipeline.

[0012] Further, the high-pressure heater group includes a first high-pressure heater, a second high-pressure heater, and a third high-pressure heater that are connected in series; the steam input side of the first high-pressure heater is connected to the high-pressure cylinder, the steam input side of the second high-pressure heater is connected to the steam reheating pipeline, and the steam input side of the third high-pressure heater is connected to the intermediate-pressure cylinder; the low-pressure heater group includes a fifth low-pressure heater, a sixth low-pressure heater, and a seventh low-pressure heater that are connected in series; the steam input side of the fifth low-pressure heater is connected to the exhaust side of the intermediate-pressure cylinder, the steam input side of the sixth low-pressure heater is connected to the low-pressure cylinder, and the steam input side of the seventh low-pressure heater is connected to the low-pressure cylinder; the corresponding low-pressure heater of the high-grade regenerative extraction steam pipeline is the fifth low-pressure heater.

[0013] Further, the steam input side of the deaerator is connected to the intermediate-pressure cylinder, and the water output side of the deaerator is connected to the water input end of the third high-pressure heater through a condensate pump fore-pump and a condensate pump.

[0014] Further, a condensate pump, a condensate water polishing unit, and a gland steam condenser are sequentially arranged between the exhaust condensing device and the low-pressure heater group.

[0015] According to the technical solution of the present invention, the present invention also provides a method for reconstructing a thermal system to improve the thermal efficiency of the unit at medium and low loads. It adopts the thermal system reconstruction system for improving the thermal efficiency of the unit at medium and low loads described in the present invention, and includes the following content: When the load is higher than the set threshold, it operates in the normal mode. The driving steam source shut-off valve and the alternative extraction steam shut-off valve are in the closed state, the normal extraction steam shut-off valve is in the open state, the first-stage pressure matcher does not work, and the steam input side of the corresponding low-pressure heater in the high-grade regenerative extraction steam pipeline receives the normal extraction steam from the corresponding intermediate-pressure cylinder. When the load is equal to or lower than the set threshold, it operates in the medium and low load mode. The driving steam source shut-off valve and the alternative extraction steam shut-off valve are in the open state, the normal extraction steam shut-off valve is in the closed state, the pressure matcher works, extracts the exhaust steam of the low-pressure cylinder and boosts the pressure to obtain generated steam. The steam input side of the corresponding low-pressure heater in the high-grade regenerative extraction steam pipeline receives the generated steam, and the generated steam replaces the normal extraction steam in the normal mode. The part of the steam corresponding to the replaced normal extraction steam will continue to do work and generate electricity in the steam turbine.

[0016] Further, the set threshold is 50%.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows: The present invention is intended to reasonably arrange a pressure matcher, extract the exhaust steam of the low-pressure cylinder with high-pressure steam having appropriate parameters to produce medium-pressure steam with appropriate parameters, and replace the extraction steam of the regenerative system of the unit with appropriate parameters; the regenerative system uses the sensible heat and latent heat of the extraction steam of the unit to heat the condensate or feed water, reducing the cold source loss; the present invention uses the steam produced by extracting the exhaust steam of the low-pressure cylinder by the pressure matcher to heat the condensate or feed water. While utilizing the heat of the exhaust steam of the low-pressure cylinder (mainly latent heat), the replaced inter-stage extraction steam of the unit continues to do work and generate electricity in the steam turbine, improving the thermal efficiency of the unit; during design, the normal extraction steam of the regenerative system is in parallel with the pressure matcher. Considering that the capacity of the pressure matcher is designed according to medium and low loads, the steam produced by the pressure matcher is input to replace the inter-stage extraction steam of the unit under medium and low loads. The present invention makes full use of the exhaust steam of the low-pressure cylinder and reduces the cold source loss, aiming to improve the thermal economy of the medium and low load units of the unit. It is applicable not only to newly built units but also to existing units, without restrictions on unit parameters, capacity, cold end type, etc. It is an innovation and reconstruction of the thermal system, with strong technical and economic feasibility, development value and demonstration significance, and has milestone significance for the progress of the power industry and is worthy of development and promotion. Through preliminary calculation, under 50% load, when the pressure matcher system is input and the 5th extraction steam is cut off, the coal consumption of the unit is reduced by 1.5 g / kWh. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the system provided by the present invention.

[0019] Figure 2 It is a schematic structural diagram of the system of another embodiment provided by the present invention.

[0020] Explanation of the reference numerals in the drawings: 1. Boiler; 2. High-pressure cylinder; 3. Medium-pressure cylinder; 4. Low-pressure cylinder; 5. Exhaust steam condensing device; 6. Deaerator; 7. High-grade regenerative extraction steam pipeline; 8. Pressure matcher; 81. First-stage pressure matcher; 82. Second-stage pressure matcher; 9. Pipeline for replacing regenerative extraction steam; 10. Shut-off valve for driving steam source; 11. Shut-off valve for normal extraction steam; 12. Shut-off valve for replacing extraction steam; 13. Steam reheating pipeline; 14. Driving steam source pipeline; 15. No. 1 high-pressure heater; 16. No. 2 high-pressure heater; 17. No. 3 high-pressure heater; 18. No. 5 low-pressure heater; 19. No. 6 low-pressure heater; 20. No. 7 low-pressure heater; 21. Feed pump booster pump; 22. Feed pump; 23. Condensate pump; 24. Condensate polishing device; 25. Steam seal cooler; 26. Shut-off valve for extraction steam source. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The present invention provides a system and method for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads, solving the problems of low efficiency of coal-fired power generation units and large cold source losses under medium and low load conditions. By innovating and reconstructing the thermal system, the exhaust heat of the steam turbine is fully utilized, the cold source loss is reduced, the economy of the unit at medium and low loads is improved, and high efficiency under all operating conditions is achieved.

[0022] Please refer to Figure 1 、 Figure 2 FIG. 1 and FIG. 2. A system for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads according to the present invention includes a boiler 1, a high-pressure cylinder 2, an intermediate-pressure cylinder 3, a low-pressure cylinder 4, an exhaust condensing device 5, a low-pressure heater group, a deaerator 6, and a high-pressure heater group that are connected in sequence. The high-pressure cylinder 2, the intermediate-pressure cylinder 3, the low-pressure cylinder 4, and the generator are components of the steam turbine. The low-pressure heater group includes a plurality of low-pressure heaters connected in series, and at least one of the low-pressure heaters has its steam input side connected to the intermediate-pressure cylinder 3 through a high-grade regenerative extraction steam pipeline 7. In other words, the steam input side of each low-pressure heater is connected to a regenerative extraction steam pipeline, and at least one of the regenerative extraction steam pipelines is connected to the high-pressure cylinder 2 or the intermediate-pressure cylinder 3 (generally connected to the intermediate-pressure cylinder 3), and thus high-grade extraction steam is used for regeneration, which is called the high-grade regenerative extraction steam pipeline 7.

[0023] The above structure is a common structure (a part) in existing coal-fired power generation units. The main improvement of the present invention lies in Figure 1 、 Figure 2 the pipelines shown in bold in FIG. 3, that is, it further includes at least one pressure matcher 8. The extraction steam source input end of the pressure matcher 8 is connected to the exhaust side of the low-pressure cylinder 4 or the exhaust condensing device 5, and the driving steam source input end of the pressure matcher 8 is connected to the exhaust side of the high-pressure cylinder 2 or the regenerative extraction steam of the intermediate-pressure cylinder 3 (connecting to the regenerative extraction steam of the intermediate-pressure cylinder 3 means connecting to the regenerative extraction steam pipeline on the intermediate-pressure cylinder, such as the 4th stage extraction steam or the 5th stage extraction steam of the intermediate-pressure cylinder). The steam output end of the pressure matcher 8 is connected to the steam input side of the corresponding low-pressure heater of the high-grade regenerative extraction steam pipeline 7 through a regenerative extraction substitution pipeline 9. The pipeline also has a valve group corresponding to the pressure matcher 8 (such as an electric shut-off valve), including a driving steam source shut-off valve 10 connected to the driving steam source input end of the pressure matcher 8, a normal extraction shut-off valve 11 arranged on the high-grade regenerative extraction steam pipeline 7, a substitution extraction shut-off valve 12 arranged on the regenerative extraction substitution pipeline 9, and a extraction steam source shut-off valve 26 connected to the extraction steam source input end of the pressure matcher 8.

[0024] The pressure matcher is a mature industrial equipment in a power station. The water jet air ejector used for vacuum extraction of the unit is an application scenario of the pressure matcher. Its working principle is to use the driving steam with a higher pressure as the power source, form a high-speed jet through the nozzle, generate a low-pressure area in its throat to form a pressure difference with the low-pressure steam to be extracted, and draw in the low-pressure steam; through the mixing and pressure expansion of the high- and low-pressure steams, the output pressure is higher than the pressure of the low-pressure steam to be extracted, so as to achieve the purpose of boosting the pressure of the low-pressure steam. In the present invention, by using the pressure matcher, the high-quality steam of the high-pressure cylinder or the intermediate-pressure cylinder is used as the driving steam source to extract the exhaust steam (waste steam) of the low-pressure cylinder, and the product steam with appropriate parameters is produced to be used as the steam source of the low-pressure heater to replace the original high-quality extraction steam, thereby reducing the cold source loss and improving the thermal efficiency of the unit at medium and low loads.

[0025] The pressure matcher needs to be specifically matched and designed according to the parameters of the driving steam source, the extraction steam source, and the required product steam parameters (pressure, flow rate, etc.). In a specific project, a reasonable pressure matcher system needs to be set according to the requirements of coal saving at medium and low loads, the characteristics of the unit regenerative system, and the investment economy, including the extraction steam grade replaced by the pressure matcher (specific number of regenerative stages, which can be 1 stage or multiple stages), the selection of the driving steam source of the pressure matcher, the number of stages of the pressure matcher, etc.

[0026] For example Figure 1 In the shown embodiment, the pressure matcher 8 includes a first-stage pressure matcher 81 and a second-stage pressure matcher 82. Another example Figure 2 In the shown embodiment, the pressure matcher 8 has only one stage. And in Figure 1 、 Figure 2 only one pressure matcher is shown, corresponding to one low-pressure heater. In some other feasible embodiments, multiple sets of pressure matchers are provided to optimize and replace the steam sources of multiple low-pressure heaters respectively. Each set of pressure matchers can be selected as one stage, two stages or more according to needs; and optionally, the product steam of a set of pressure matchers is output to more than two low-pressure heaters. The basic principle is that if the product parameters of the first-stage pressure matcher cannot meet the requirements, a second-stage pressure matcher can be set; the second-stage pressure matcher uses the product steam of the first-stage pressure matcher as the steam source to be extracted, and the driving steam source can remain unchanged. The following takes the embodiment shown in Figure 1 as an example for more specific description.

[0027] Figure 1In the illustrated embodiment, the pressure matcher 8 includes a primary pressure matcher 81 and a secondary pressure matcher 82. The extraction steam source input end of the primary pressure matcher 81 is connected to the exhaust side of the low-pressure cylinder 4 or the exhaust condensing device 5 (specifically connected to the exhaust condensing device 5 in the illustrated embodiment). The driving steam source input end of the primary pressure matcher 81 is connected to the exhaust side of the high-pressure cylinder 2 or the regenerative extraction steam of the intermediate-pressure cylinder 3. The steam output end of the primary pressure matcher 81 is connected to the steam input side of the corresponding low-pressure heater in the high-grade regenerative extraction steam pipeline 7 through the regenerative extraction steam replacement pipeline 9. The extraction steam source input end of the secondary pressure matcher 82 is connected to the steam output end of the primary pressure matcher 81. The driving steam source input end of the secondary pressure matcher 82 is connected to the exhaust side of the high-pressure cylinder 2 or the regenerative extraction steam of the intermediate-pressure cylinder 3. The steam output end of the secondary pressure matcher 82 is connected to the regenerative extraction steam replacement pipeline 9. For common regenerative system schemes, it can be selected that the low-pressure heater corresponding to the high-grade regenerative extraction steam pipeline 7 is the fifth low-pressure heater 18; in some other embodiments, it can also be selected as the low-pressure heater lower than the fifth low-pressure heater, such as the sixth low-pressure heater or the seventh low-pressure heater, etc.

[0028] More specifically, the high-grade regenerative extraction steam pipeline 7 is connected to the intermediate-pressure cylinder 3. The heating steam originally used by the corresponding low-pressure heater is from the exhaust of the intermediate-pressure cylinder. The exhaust of the intermediate-pressure cylinder still has a lot of available energy. Replacing it with the product steam formed by using the exhaust of the low-pressure cylinder, then that part of the original exhaust of the intermediate-pressure cylinder can continue to enter the low-pressure cylinder for power generation instead of being used for regeneration, thus improving the thermal efficiency; the driving steam source input end of the pressure matcher 8 is connected to the exhaust side of the high-pressure cylinder 2, that is, the exhaust of the high-pressure cylinder is used as the driving steam source of the pressure matcher. For the secondary pressure matcher, both the primary pressure matcher 81 and the secondary pressure matcher 82 can be selected to use the exhaust of the high-pressure cylinder as the driving steam source of the pressure matcher. The specific extraction method of the exhaust of the high-pressure cylinder is, for example, that the exhaust side of the high-pressure cylinder 2 is connected to the boiler 1 through the steam reheating pipeline 13. The steam reheating pipeline is an existing setting. In this scheme, a driving steam source pipeline 14 is led out from the steam reheating pipeline 13. The driving steam source pipeline 14 is connected to the driving steam source input end of the pressure matcher 8. The driving steam source shut-off valve 10 is arranged on the driving steam source pipeline 14.

[0029] The high-pressure heater group includes the first high-pressure heater 15, the second high-pressure heater 16, and the third high-pressure heater 17 which are arranged in series. The steam input side of the first high-pressure heater 15 is connected to the high-pressure cylinder 2 (through the pipeline marked with the number 1 in the figure), the steam input side of the second high-pressure heater 16 is connected to the steam reheating pipeline 13 (through the pipeline marked with the number 2 in the figure), and the steam input side of the third high-pressure heater 17 is connected to the intermediate-pressure cylinder 3 (through the pipeline marked with the number 3 in the figure). The low-pressure heater group includes the fifth low-pressure heater 18, the sixth low-pressure heater 19, and the seventh low-pressure heater 20 which are arranged in series. The steam input side of the fifth low-pressure heater 18 is connected to the exhaust side of the intermediate-pressure cylinder 3 (through the pipeline marked with the number 5 in the figure), the steam input side of the sixth low-pressure heater 19 is connected to the low-pressure cylinder 4 (through the pipeline marked with the number 6 in the figure), and the steam input side of the seventh low-pressure heater 20 is connected to the low-pressure cylinder 4 (through the pipeline marked with the number 7 in the figure). In the illustrated embodiment, the corresponding low-pressure heater of the high-grade regenerative extraction steam pipeline 7 is the fifth low-pressure heater 18, and the pipeline marked with the number 5 in the figure is the high-grade regenerative extraction steam pipeline 7 with an improved design.

[0030] Further, the steam input side of the deaerator 6 is connected to the intermediate-pressure cylinder 3 (through the pipeline marked with the number 4 in the figure), and the water output side of the deaerator 6 is connected to the water input end of the third high-pressure heater 17 through the condensate pump fore-pump 21 and the condensate pump 22. A condensate pump 23, a condensate water polishing device 24, and a gland steam cooler 25 are sequentially arranged between the exhaust condenser device 5 and the low-pressure heater group. The exhaust steam of the low-pressure cylinder 4 is converted from a gaseous state to a liquid state after passing through the exhaust condenser device 5, enters the pipeline of the regenerative system, and sequentially passes through the condensate pump 23, the condensate water polishing device 24, the gland steam cooler 25, the seventh low-pressure heater 20, the sixth low-pressure heater 19, the fifth low-pressure heater 18, the deaerator 6, the condensate pump fore-pump 21, the condensate pump 22, the third high-pressure heater 17, the second high-pressure heater 16, the first high-pressure heater 15, and then returns to the boiler 1 to form a water cycle.

[0031] Based on the above system structure of the present invention, the present invention provides a method for reconstructing a thermal system to improve the thermal efficiency of the unit at medium and low loads, which adopts the thermal system for reconstructing to improve the thermal efficiency of the unit at medium and low loads described in the present invention, and includes the following content: When the load is higher than the set threshold, it operates in the normal mode. The states of the steam source shut-off valve 10, the alternative extraction steam shut-off valve 12 (and the extraction steam source shut-off valve 26) are closed, the state of the normal extraction steam shut-off valve 11 is open, the first-stage pressure matcher 81 does not work, and the steam input side of the corresponding low-pressure heater of the high-grade regenerative extraction steam pipeline 7 receives the normal extraction steam from the corresponding intermediate-pressure cylinder 3. When the load is equal to or lower than the set threshold value, it operates in a medium-low load mode. The driving steam source shut-off valve 10 and the alternative extraction steam shut-off valve 12 (as well as the extraction steam source shut-off valve 26) are in the open state, the normal extraction steam shut-off valve 11 is in the closed state, the pressure matcher 8 operates, extracts the exhaust steam of the low-pressure cylinder 4 and boosts the pressure to obtain generated steam. The steam input side of the corresponding low-pressure heater in the high-grade regenerative extraction steam pipeline 7 receives the generated steam, and uses the generated steam to replace the normal extraction steam in the normal mode. The part of the steam corresponding to the replaced normal extraction steam will continue to do work and generate electricity in the steam turbine.

[0032] Preferably, for example, the set threshold value is 50%.

[0033] In a typical implementation, taking the regenerative system of a certain unit as an example, it is assumed that the pressure matcher system is put into operation at a load of 50% and below of the unit. Using the exhaust steam of the high-pressure cylinder as the driving steam source, extracting the exhaust steam (waste steam) of the low-pressure cylinder, producing product steam with appropriate parameters, and using it as the steam source for the No. 5 low-pressure heater to replace the No. 5 extraction steam. When designing the pressure matcher, the required pressure and mass flow rate of the product steam are designed based on meeting the steam usage parameters of the No. 5 low-pressure heater at 50% load. The pressure matcher system is arranged in parallel with the 5-stage extraction steam system, and a valve group is set for isolation. When the load is above 50%, the regenerative system operates normally; when the load is 50% and below, the 5-stage extraction steam is cut off and the pressure matcher system is put into operation. Through preliminary calculation, at 50% load, when the pressure matcher system is put into operation and the 5-stage extraction steam is cut off, the coal consumption of the unit is reduced by 1.5 g / kWh.

[0034] In summary, the present invention makes full use of the exhaust steam of the low-pressure cylinder and reduces the cold source loss, aiming to use the exhaust steam (waste steam) of the low-pressure cylinder to replace the regenerative extraction steam, reduce the cold source loss, and improve the economy of the unit at medium-low loads; it has a wide range of applications, is applicable not only to newly built units but also to existing units, not only to heating units but also to pure condensing units, and has no restrictions on unit parameters, capacity, cold end type, etc. It is an innovation and reconstruction of the thermal system, with strong technical and economic feasibility, development value and demonstration significance, and is of milestone significance to the progress of the power industry and worthy of development and promotion.

[0035] Finally, it should be noted that the above only explains in detail the main improvement points of the present invention. The parts not described in detail are common sense content or can follow the existing technology or adopt similar technical solutions to achieve the required functions, so there is no need to elaborate. The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Obviously, the described embodiments are some embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention. For the convenience of description, only the parts related to the relevant invention are shown in the drawings. Without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. Modifying the technical solutions recorded in the foregoing embodiments or equivalently replacing some of the technical features does not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A system for reconstructing a thermal system to improve the thermal efficiency of a unit at medium and low loads, comprising a boiler (1), a high-pressure cylinder (2), an intermediate-pressure cylinder (3), a low-pressure cylinder (4), an exhaust condensing device (5), a low-pressure heater group, a deaerator (6) and a high-pressure heater group connected in sequence, wherein the low-pressure heater group comprises a plurality of low-pressure heaters arranged in series, wherein the steam input side of at least one low-pressure heater is connected to the intermediate-pressure cylinder (3) via a high-grade heat recovery extraction pipeline (7); characterized in that: The invention also comprises at least one pressure matcher (8), wherein an extraction steam source input end of the pressure matcher (8) is connected to the exhaust steam side of the low-pressure cylinder (4) or the exhaust steam condensing device (5), and a driving steam source input end of the pressure matcher (8) is connected to the exhaust steam side of the high-pressure cylinder (2) or the heat recovery extraction steam of the medium-pressure cylinder (3). The steam output end of the pressure matcher (8) is connected to the steam input side of the low-pressure heater corresponding to the high-grade heat recovery extraction steam pipeline (7) through the heat recovery extraction steam replacement pipeline (9). The driving steam source input end of the pressure matcher (8) is connected to a driving steam source shutoff valve (10), a normal extraction steam shutoff valve (11) is provided on the high-grade heat recovery extraction steam pipeline (7), and an alternative extraction steam shutoff valve (12) is provided on the heat recovery extraction steam replacement pipeline (9).

2. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 1, characterized in that: The pressure matcher (8) comprises a primary pressure matcher (81) and a secondary pressure matcher (82); The extraction steam source input end of the first-stage pressure matching device (81) is connected to the exhaust steam side of the low-pressure cylinder (4) or the exhaust condensing device (5), the driving steam source input end of the first-stage pressure matching device (81) is connected to the exhaust steam side of the high-pressure cylinder (2) or the heat recovery extraction steam of the medium-pressure cylinder (3), and the steam output end of the first-stage pressure matching device (81) is connected to the steam input side of the low-pressure heater corresponding to the high-grade heat recovery extraction steam pipeline (7) through the heat recovery extraction steam replacement pipeline (9); The extraction steam source input end of the secondary pressure matcher (82) is connected to the steam output end of the primary pressure matcher (81), the driving steam source input end of the secondary pressure matcher (82) is connected to the exhaust side of the high-pressure cylinder (2) or the heat recovery extraction steam of the medium-pressure cylinder (3), and the steam output end of the secondary pressure matcher (82) is connected to the heat recovery extraction steam replacement pipeline (9).

3. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 1, characterized in that: The corresponding low-pressure heater of the high-grade heat recovery extraction steam pipeline (7) is a No. 5 low-pressure heater (18).

4. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 1 or 2, characterized in that: The driving steam source input end of the pressure matcher (8) is connected to the exhaust side of the high-pressure cylinder (2).

5. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 4, characterized in that: The exhaust side of the high-pressure cylinder (2) is connected to the boiler (1) via a steam reheating pipeline (13). A driving steam source pipeline (14) is led out from the steam reheating pipeline (13). The driving steam source pipeline (14) is connected to a driving steam source input end of a pressure matching device (8). A driving steam source shutoff valve (10) is arranged on the driving steam source pipeline (14).

6. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 5, characterized in that: The high-pressure heater group comprises a No. 1 high-pressure heater (15), a No. 2 high-pressure heater (16) and a No. 3 high-pressure heater (17) which are arranged in series; the steam input side of the No. 1 high-pressure heater (15) is connected to the high-pressure cylinder (2), the steam input side of the No. 2 high-pressure heater (16) is connected to the steam reheating pipeline (13), and the steam input side of the No. 3 high-pressure heater (17) is connected to the medium-pressure cylinder (3); The low-pressure heater group includes a No. 5 low-pressure heater (18), a No. 6 low-pressure heater (19) and a No. 7 low-pressure heater (20) which are arranged in series; the steam input side of the No. 5 low-pressure heater (18) is connected to the exhaust side of the medium-pressure cylinder (3), the steam input side of the No. 6 low-pressure heater (19) is connected to the low-pressure cylinder (4), and the steam input side of the No. 7 low-pressure heater (20) is connected to the low-pressure cylinder (4); the low-pressure heater corresponding to the high-grade heat recovery extraction pipeline (7) is the No. 5 low-pressure heater (18).

7. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 6, characterized in that: The steam input side of the deaerator (6) is connected to the medium-pressure cylinder (3), and the water output side of the deaerator (6) is connected to the water input end of the No. 3 high-pressure heater (17) through the feed water pump pre-pump (21) and the feed water pump (22).

8. The system for reconstructing the thermal system to improve the low-load thermal efficiency of the unit according to claim 6, characterized in that: A condensate pump (23), a condensate polishing device (24) and a steam seal cooler (25) are arranged in sequence between the exhaust condensation device (5) and the low-pressure heater group.

9. A method for reconfiguring a thermal system to improve the low-load thermal efficiency of a unit, characterized in that: The system adopts the thermal system reconstruction according to any one of claims 1 to 8 to improve the low-load thermal efficiency of the unit, and includes the following contents: When the load is higher than the set threshold, the system operates in normal mode, the driving steam source shutoff valve (10) and the alternative steam extraction shutoff valve (12) are in the closed state, the normal steam extraction shutoff valve (11) is in the open state, the first-stage pressure matching device (81) does not work, and the steam input side of the corresponding low-pressure heater of the high-grade heat recovery steam extraction pipeline (7) receives the normal steam extraction of the corresponding medium-pressure cylinder (3); When the load is equal to or lower than the set threshold, the system operates in a medium-low load mode, driving the steam source shutoff valve (10) and the replacement steam extraction shutoff valve (12) to be open, and the normal steam extraction shutoff valve (11) to be closed. The pressure matcher (8) operates to extract the exhaust steam of the low-pressure cylinder (4) and increase the pressure to obtain production steam. The steam input side of the corresponding low-pressure heater of the high-grade heat recovery extraction pipeline (7) receives the production steam, and the normal extraction steam in the normal mode is replaced by the production steam. The part of the steam corresponding to the replaced normal extraction steam will continue to generate power in the steam turbine.

10. The method for reconfiguring a thermal system to improve the low-load thermal efficiency of a unit according to claim 9, characterized in that: Set the threshold to 50%.

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