Energy-saving system and method for preventing sulfuric acid dewing of air preheater of heat supply unit

Abstract

The present application belongs to the field of energy saving and control technology of thermal power generation system, and particularly relates to an energy saving system and method for preventing sulfuric acid dewing of an air preheater of a heating unit. The system comprises at least: an air preheater comprising a first flue gas passage and a plurality of hot air passages; a main condensate water pipeline having a water inlet end and a water outlet end; a plurality of warm air heaters, each of which has an air passage and a first hot water passage, each of the air passages is connected to one of the hot air passages, and the water inlets of the first hot water passages are connected to one side of the main condensate water pipeline close to the water outlet end, and the water outlets of the first hot water passages are connected to one side of the main condensate water pipeline close to the water inlet end. The present application is used to ensure that the air preheater inlet air temperature of a boiler is increased all year round and all day long under the wide load operation condition of a thermal power unit, and to meet the requirement that the cold end heat exchange elements of the air preheater are free from corrosion caused by sulfuric acid dewing.

Description

Technical Field

[0001] This invention belongs to the field of energy conservation and control technology for thermal power generation systems, specifically relating to an energy-saving system and method for preventing sulfuric acid condensation in the air preheater of a heating unit. Background Technology

[0002] By increasing the wall temperature of the cold-end components of the air preheater, the overall cold-end temperature is prevented from being too low during winter operation, especially under low and medium load conditions. This fundamentally prevents blockage and corrosion of the cold-end components due to sulfuric acid condensation, ash adhesion, and ammonium bisulfate accumulation. Conventional units use steam heaters, which are theoretically an effective technical measure to increase the average metal temperature of the cold-end heat exchange components of the air preheater. However, the use of steam leads to turbine work capacity loss, affecting the unit's economic efficiency. Furthermore, the phase change during steam cooling can cause strong vibrations during equipment operation, leading to damage and leaks in the heating surfaces, posing a significant safety risk. Therefore, power plants generally only operate steam heaters in winter or during colder periods without other measures. Due to the diversification of coal types used in actual operation, sometimes the increase in sulfur content goes unnoticed. During seasons when steam heaters are not in operation, air preheater equipment frequently suffers from corrosion and blockage. Therefore, power plants selectively operate steam heaters, thus the problem of air preheater corrosion and blockage is not completely resolved. To continuously increase the air preheater inlet temperature, water-based air heaters (hereinafter referred to as "air heaters") have emerged. These air heaters have two sources of heat: one is the waste heat from the boiler exhaust, which is transferred to the air preheater inlet via hot water. In winter, this method cannot accurately increase the air preheater inlet temperature due to insufficient heat from the exhaust. Technically, steam is used to supplement the heating of the hot water to ensure the increase in the air preheater inlet temperature. This, like the aforementioned steam air heaters, suffers from energy consumption and two-phase flow problems. Furthermore, in summer, excessively high temperatures can cause system vaporization, posing a safety hazard. The second source is the turbine condensate system. By directing the turbine condensate to the air preheater inlet, the heat from the condensate is transferred to the air preheater inlet, increasing the air preheater exhaust temperature. The waste heat is then returned to the condensate system. This method balances turbine cooling losses and work capacity, resulting in a positive economic impact.

[0003] In existing technologies, the condensate from the air heater and the low-temperature economizer is typically taken from a fixed water intake point in the regenerative system (e.g., the inlet or outlet of a low-pressure heater) and returned to a fixed return point. However, when the unit operates at low load, especially when the steam extraction rate is high, the exhaust steam from the low-pressure cylinder decreases, leading to a drop in extraction parameters at all stages of the regenerative system and a decrease in the mainstream condensate temperature. At this time, the temperature of the water taken from the fixed point may not meet the requirements of the air heater and the low-temperature economizer, forcing the equipment to shut down or expose it to corrosion risks, thus limiting the unit's peak-shaving capacity and economic efficiency. Therefore, a system that can flexibly adapt to load changes and actively and precisely adjust the supply water temperature is needed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an energy-saving system and method for preventing sulfuric acid condensation in the air preheater of a heating unit. The technical solution of this invention ensures that the inlet air temperature supplied to the boiler air preheater is maintained year-round and around the clock under wide-load (especially low-load) operating conditions of the heating unit, meeting the requirement of protecting the cold-end heat exchange elements of the air preheater from corrosion caused by sulfuric acid condensation. By utilizing the heat of condensate on the turbine side, especially the low-grade steam extracted from the last few stages of low-pressure heaters, and simultaneously coupling the utilization of waste heat from flue gas, the losses from the turbine's cold source and the boiler's flue gas losses are reduced, improving the unit's economic efficiency.

[0005] The technical solution provided by this invention is as follows: An energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit includes: An air preheater, the air preheater including a first flue gas passage and several hot air passages; The main condensate pipeline has an inlet and an outlet. A plurality of air heaters, each having an air channel and a first hot water channel, each air channel being connected to a corresponding hot air channel, the inlet of each first hot water channel being connected to the side of the main condensate pipe near the outlet, and the outlet of each first hot water channel being connected to the side of the main condensate pipe near the inlet.

[0006] Based on the above technical solution, the heat from the condensate on the turbine side can be used to raise the temperature of the cold-end heat exchange elements of the air preheater, preventing sulfuric acid condensation from corroding the equipment. Heat exchange occurs between the air passage and the hot water passage of the air heater.

[0007] Further: The air preheater is also connected to a low-temperature economizer, which has a second flue gas passage and a second hot water passage, the second flue gas passage being connected to the first flue gas passage. At least two first low-pressure heaters are sequentially installed along the flow direction of the condensate on the main condensate pipeline, and the second first low-pressure heater has a first low-pressure outlet pipe section. The inlet of the second hot water channel is connected to the side of the first low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water channel is connected to the side of the first low-pressure heater outlet pipe section near the outlet end.

[0008] Based on the above technical solution, a primary temperature-regulating water circuit can be formed, thus providing a step-by-step temperature regulation method for conventional (primary) thermal power units.

[0009] Further: A first valve is installed on the first low-pressure outlet pipe section; The first low-pressure outlet pipe section is connected to the inlet of the second hot water channel via a second valve near the inlet end; The first low-pressure outlet pipe section is connected to the outlet of the second hot water channel via a third valve near the outlet end.

[0010] Further: The main condensate pipeline is sequentially equipped with two first low-pressure heaters and one second low-pressure heater, the second low-pressure heater having a second low-pressure outlet pipe section; The inlet of the second hot water channel is also connected to the side of the second low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water channel is connected to the side of the second low-pressure heater outlet pipe section near the outlet end.

[0011] Based on the above technical solution, a two-stage temperature-regulating water circuit can be formed, thus providing a stepped temperature regulation method for the large-scale heating (two-stage) of the thermal power unit.

[0012] Further: A fourth valve is installed on the second low-pressure outlet pipe section; The second low-pressure outlet pipe section is connected to the inlet of the second hot water channel via a fifth valve near the inlet end; The second low-pressure outlet pipe section is connected to the outlet of the second hot water channel via a sixth valve near the outlet end.

[0013] Further: The main condensate pipeline is sequentially equipped with two first low-pressure heaters, one second low-pressure heater, and one third low-pressure heater, wherein the third low-pressure heater has a third low-pressure outlet pipe section; The energy-saving system for preventing sulfuric acid condensation in the air preheater of the heating unit is also equipped with a high-temperature condensate and low-efficiency circulating pump set. The third low-temperature outlet pipe section is connected to the inlet of each of the first hot water channels and the inlet of each of the second hot water channels via the high-temperature condensate and the low-temperature circulation pump set; the outlet of the second hot water channel is connected to the inlet of each of the first hot water channels and the inlet of each of the second hot water channels via the high-temperature condensate and the low-temperature circulation pump set.

[0014] Based on the above technical solution, a three-stage temperature-regulating water circuit can be formed, thus providing a stepped temperature regulation method for the thermal power unit to provide a large amount of heat (three-stage) under low load.

[0015] Furthermore, through a three-stage switchable / adjustable water circuit, a stepped temperature regulation method is provided for the thermal power unit from conventional (level 1) to high-volume heating (level 2) and then to low-load and high-volume heating (level 3), perfectly covering the entire operating range of different heating capacities from high load to extremely low load, and completely solving the problem of insufficient water supply temperature for the air heater and low-temperature economizer.

[0016] Further: The third low-temperature outlet pipe section is connected to the high-temperature condensate and the low-temperature circulating pump group through the seventh valve; The outlet of the second hot water channel is connected to the high-temperature condensate and the low-energy circulating pump group through the eighth valve; The high-temperature condensate and the low-temperature circulating pump unit are connected to the inlet of each of the first hot water channels and the inlet of each of the second hot water channels through the ninth valve.

[0017] Further: The low-temperature economizer is also connected in sequence to an electrostatic precipitator, an induced draft fan, and a desulfurization device; The air preheater has a primary air duct and a secondary air duct, and the air heater has a primary air heater and a secondary air heater. The primary air duct is connected to the primary air heater, and the secondary air duct is connected to the secondary air heater. The outlets of the first hot water channels of the primary air heater and the secondary air heater are respectively connected to the main condensate pipeline through the heater booster pump.

[0018] Further: The first valve, the second valve, the fourth valve, the fifth valve, and the ninth valve are all electrically operated regulating valves; The third valve, the sixth valve, the seventh valve, and the eighth valve are all electrically operated on / off valves. A temperature sensor is installed on the main water inlet pipe connecting each air heater and the low-temperature economizer; The energy-saving system is also equipped with a unit distributed control system, which is electrically connected to each valve and the high-temperature condensate and low-energy circulating pump group respectively.

[0019] Based on the above technical solution: The water intake pipe of the primary temperature regulating water circuit and the water intake pipe of the secondary temperature regulating water circuit are connected by a pipeline after the second valve and the fifth valve. The water intake of the primary temperature regulating water circuit and the secondary temperature regulating water circuit can be achieved by controlling the opening of the second valve and the fifth valve to achieve parallel water intake or selective switching. The return water pipe of the primary temperature regulating water circuit and the return water pipe of the secondary temperature regulating water circuit merge through a pipeline before the third valve and the sixth valve. The condensate heated by the low temperature economizer returns to the main unit condensate system through either the third valve or the sixth valve. The operation of the three-stage pressurized and temperature-regulating water circuit can be combined with the first-stage and second-stage temperature-regulating water circuits. The seventh and eighth valves are interlocked during the operation of this water circuit, and only one of them is allowed to be opened during operation to select whether to draw water from the return water of the low-temperature economizer (relatively low water temperature) or from the outlet of the third low-pressure heater (higher water temperature). The distributed control system (DCS) controls the opening and closing status and opening degree of the first to ninth valves based on the deviation between the set temperature value and the measured value, as well as the unit load signal, and controls the start-up, shutdown and operating frequency of the high-temperature condensate and low-energy circulating pump sets.

[0020] This invention also provides an energy-saving method for preventing sulfuric acid condensation in the air preheater of a heating unit. The energy-saving system described above is used for energy saving, and the method includes the following steps: Open the second valve, the third valve, the fourth valve, and the first valve, and close the remaining valves to provide regular heating. Close the third, seventh, eighth, and ninth valves, gradually open the sixth valve according to the temperature sensor value, open the first valve, adjust the second or fifth valve to adjust the water temperature of the main inlet pipe, and adjust the fourth valve to adjust the flow rate of the low-temperature economizer; Close the seventh valve and open the eighth valve, the ninth valve, and the high-temperature condensate and low-efficiency circulation pump set; or close the eighth valve and open the seventh valve, the ninth valve, and the high-temperature condensate and low-efficiency circulation pump set.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. Excellent wide load adaptability: Through a three-stage switchable / adjustable temperature-regulating water circuit, it provides a stepped temperature regulation method for the thermal power unit from conventional (level 1) to high-volume heating (level 2) and then to low load and high-volume heating (level 3), perfectly covering the entire operating range of different heating loads from high load to extremely low load, and completely solving the problem of insufficient water supply temperature for air heaters and low-temperature economizers.

[0022] 2. High operational flexibility and economy: The system can select single-path operation or combined operation mode according to real-time water temperature requirements, avoiding the damage caused by sulfuric acid dew sticking and ash blockage and hydrogen sulfate ammonia corrosion to the cold end heating surface of the air preheater and the heating surface of the low-temperature economizer caused by cold water entering the air heater and low-temperature economizer. It realizes "on-demand temperature adjustment", has flexible operation mode, and always uses low-grade steam to replace high-grade steam for work, realizing energy cascade utilization and optimal economy.

[0023] 3. Precise temperature control and high reliability: Through DCS logic control and precise matching of regulating and switching valves, accurate and rapid adjustment of the inlet water temperature can be achieved. The inlet water temperature is always kept above the acid dew point, fundamentally eliminating the risk of low-temperature corrosion, resulting in high equipment reliability and a long service life.

[0024] 4. High system integration and easy modification: This invention is an optimization and superposition on the existing system. The newly added pipelines and valves are mainly concentrated near the existing pipelines. The amount of modification work is small, it is easy to implement on existing units, and it does not affect the safe operation of the main unit. Attached Figure Description

[0025] Figure 1 This is a system diagram of the energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit, provided by the present invention.

[0026] Appendix Figure 1 The structures represented by each label are listed below: 11. First valve; 12. Second valve; 13. Third valve; 14. Fourth valve; 15. Fifth valve; 16. Sixth valve; 17. Seventh valve; 18. Eighth valve; 19. Ninth valve; 21. Air preheater; 211. First flue gas passage; 212. Primary air duct; 213. Secondary air duct; 22. Low-temperature economizer; 23. Electrostatic precipitator; 24. Exhaust fan; 25. Desulfurization unit; 261. Primary air heater; 262. Secondary air heater; 27. Main condensate pipeline; 28. First low-pressure heater; 29. ​​Second low-pressure heater; 30. Third low-pressure heater; 31. High-temperature condensate and low-pressure economizer circulation pump set; 32. Heater booster pump. Detailed Implementation

[0027] The principles and features of the present invention are described below. The embodiments given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0028] Example 1 like Figure 1 As shown, the energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit includes an air preheater 21, a main condensate pipeline 27, and several air heaters. The air preheater 21 includes a first flue gas passage 211 and several hot air passages. The main condensate pipeline 27 has an inlet end and an outlet end. Each air heater has an air passage and a first hot water passage. Each air passage is connected to one of the hot air passages. The inlet of each first hot water passage is connected to the side of the main condensate pipeline 27 near the outlet end, and the outlet of each first hot water passage is connected to the side of the main condensate pipeline 27 near the inlet end.

[0029] Furthermore, the air preheater 21 is also connected to a low-temperature economizer 22, which has a second flue gas passage and a second hot water passage. The second flue gas passage is connected to the first flue gas passage 211. At least two first low-pressure heaters 28 are sequentially installed on the main condensate pipeline 27, and the second first low-pressure heater 28 has a first low-pressure heater outlet pipe section. The inlet of the second hot water passage is connected to the side of the first low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water passage is connected to the side of the first low-pressure heater outlet pipe section near the outlet end.

[0030] Correspondingly, a first valve 11 is provided on the first low-pressure heater outlet pipe section. The first low-pressure heater outlet pipe section is connected to the inlet of the second hot water channel via a second valve 12 near the inlet end. The first low-pressure heater outlet pipe section is connected to the outlet of the second hot water channel via a third valve 13 near the outlet end.

[0031] Furthermore, the low-temperature economizer 22 is also sequentially connected to an electrostatic precipitator 23, an induced draft fan 24, and a desulfurization device 25. The air preheater 21 has a primary air duct 212 and a secondary air duct 213, and the air heater has a primary air heater 261 and a secondary air heater 262. The primary air duct 212 is connected to the primary air heater 261, and the secondary air duct 213 is connected to the secondary air heater 262. The outlets of the first hot water channels of the primary air heater 261 and the secondary air heater 262 are respectively connected to the main condensate pipeline 27 through the air heater booster pump 32.

[0032] Example 2 like Figure 1 As shown, the main condensate pipeline 27 is sequentially equipped with two first low-pressure heaters 28 and one second low-pressure heater 29, the second low-pressure heater 29 having a second low-pressure heater outlet pipe section. The inlet of the second hot water channel is also connected to the side of the second low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water channel is connected to the side of the second low-pressure heater outlet pipe section near the outlet end.

[0033] Correspondingly, a fourth valve 14 is provided on the second low-pressure heater outlet pipe section. The second low-pressure heater outlet pipe section is connected to the inlet of the second hot water channel via a fifth valve 15 near the inlet end. The second low-pressure heater outlet pipe section is connected to the outlet of the second hot water channel via a sixth valve 16 near the outlet end.

[0034] Example 3 like Figure 1As shown, the main condensate pipeline 27 is sequentially equipped with two first low-pressure heaters 28, one second low-pressure heater 29, and one third low-pressure heater 30. The third low-pressure heater 30 has a third low-pressure heater outlet pipe section. The energy-saving system for preventing sulfuric acid condensation in the air preheater of the heating unit is also equipped with a high-temperature condensate and low-pressure energy-saving circulation pump set 31. The third low-pressure heater outlet pipe section is connected to the inlet of each of the first hot water channels and the inlet of each of the second hot water channels through the high-temperature condensate and low-pressure energy-saving circulation pump set 31; the outlet of the second hot water channel is connected to the inlet of each of the first hot water channels and the inlet of each of the second hot water channels through the high-temperature condensate and low-pressure energy-saving circulation pump set 31.

[0035] Correspondingly, the third low-temperature outlet pipe section is connected to the high-temperature condensate and the low-temperature circulating pump group 31 via the seventh valve 17. The outlet of the second hot water channel is connected to the high-temperature condensate and the low-temperature circulating pump group 31 via the eighth valve 18. The high-temperature condensate and the low-temperature circulating pump group 31 are connected to the inlet of each of the first hot water channels and the inlet of the second hot water channel via the ninth valve 19.

[0036] Example 4 like Figure 1 As shown, the first valve 11, the second valve 12, the fourth valve 14, the fifth valve 15, and the ninth valve 19 are all electrically adjustable valves. The third valve 13, the sixth valve 16, the seventh valve 17, and the eighth valve 18 are all electrically operated on / off valves. A temperature sensor is installed on the main inlet pipe connecting each heater and the low-temperature economizer 22. The energy-saving system also includes a unit distributed control system, which is electrically connected to each valve and the high-temperature condensate and low-temperature economizer circulating pump group 31.

[0037] During normal operation, the main condensate flows sequentially through each low-pressure heater and finally enters the deaerator. The heating water source for the primary air heater, secondary air heater, and low-temperature economizer is the object of regulation in this system.

[0038] Normal operating conditions: When the main condensate temperature is high, open the second valve 12, the third valve 13, the fourth valve 14, and the first valve 11, and close the fifth valve 15 and the sixth valve 16. Use the first-stage temperature regulating water circuit taken from the first low-temperature heater outlet pipe section, and the water temperature will meet the requirements.

[0039] High-volume heating operation: Insufficient water temperature in the primary water circuit. Based on the temperature sensor signal, the DCS gradually opens the sixth valve 16, closes the third valve 13, opens the first valve 11, and uses the second valve 12 and the fifth valve 15 to adjust the inlet water temperature of the low-temperature economizer and the air heater to meet the design requirements. The fourth valve is adjusted to meet the flow requirements of the low-temperature economizer and the air heater.

[0040] Low load and high-volume heating conditions: During high-volume heating conditions, the primary water circuit is completely switched to the secondary water circuit. When the secondary water circuit alone is insufficient to reach the set temperature, or when a rapid temperature increase is required, the tertiary pressurized and temperature-regulating water circuit is activated. At this time, two water source options are available: Mode A (using low-temperature economizer return water): Close the seventh valve 17 and open the eighth valve 18. Part of the condensate from the outlet of the low-temperature economizer (which has been heated by flue gas) is drawn and pressurized by the high-temperature condensate and low-temperature economizer circulation pump group through the eighth valve 18, and injected into the main inlet pipe through the ninth valve 19.

[0041] Mode B (using the highest temperature main condensate): Open valve 17 and close valve 18. High-temperature water is directly drawn from the main condensate pipe at the outlet of the third low-pressure heater with the highest temperature, pressurized by the pump set, and injected into the main inlet pipe through valve 9.

[0042] The DCS selects mode A or B based on temperature requirements, system resistance, and other conditions. The pump unit can use a variable frequency pump, precisely controlling the injection flow rate and pressure according to the required temperature rise. When the pump unit is running at industrial frequency, the injection flow rate and pressure can be adjusted via the ninth valve.

[0043] Control Logic: The DCS uses the inlet main water pipe temperature as the primary controlled variable and the unit load as the feedforward signal. When the temperature falls below a certain set value, it triggers the switching logic from the first to the second stage; if the temperature drops further, it triggers the startup of the third-stage booster pump and the selection of the seventh or eighth valve. All valve opening and closing actions and opening adjustments, as well as pump start-up, shutdown, and frequency conversion, are completed by the DCS program, achieving fully intelligent temperature control.

[0044] The temperature data is as follows: The temperature of the condensate pumped into the system is approximately 80°C. The deaerator water temperature is approximately 120°C; The outlet water temperature after heat exchange with the heater is approximately 30℃. The temperature of the hot air entering the air preheater duct is approximately 60°C. The temperature of the flue gas entering the air preheater is approximately 370°C; The temperature of the flue gas exiting the air preheater is approximately 160°C, which is higher than the condensation point of sulfuric acid, thus preventing condensation from corroding the equipment.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit, characterized in that, At least including: Air preheater (21), the air preheater (21) includes a first flue gas passage (211) and several hot air passages; The main condensate pipeline (27) has an inlet and an outlet; A plurality of air heaters, each of the air heaters having an air channel and a first hot water channel, each of the air channels being connected to a hot air channel, the inlet of each of the first hot water channels being connected to the side of the main condensate pipe (27) near the outlet, and the outlet of each of the first hot water channels being connected to the side of the main condensate pipe (27) near the inlet.

2. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 1, characterized in that: The air preheater (21) is also connected to a low-temperature economizer (22), which has a second flue gas passage and a second hot water passage, and the second flue gas passage is connected to the first flue gas passage (211). At least two first low-pressure heaters (28) are sequentially installed on the main condensate pipeline (27), and the second first low-pressure heater (28) has a first low-pressure outlet pipe section; The inlet of the second hot water channel is connected to the side of the first low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water channel is connected to the side of the first low-pressure heater outlet pipe section near the outlet end.

3. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 2, characterized in that: A first valve (11) is installed on the first low-pressure outlet pipe section; The first low-pressure outlet pipe section is connected to the inlet of the second hot water channel via a second valve (12) near the inlet end; The first low-temperature outlet pipe section is connected to the outlet of the second hot water channel via a third valve (13) near the outlet end.

4. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 3, characterized in that: The main condensate pipeline (27) is provided with two first low-pressure heaters (28) and one second low-pressure heater (29) in sequence, and the second low-pressure heater (29) has a second low-pressure outlet pipe section; The inlet of the second hot water channel is also connected to the side of the second low-pressure heater outlet pipe section near the inlet end, and the outlet of the second hot water channel is connected to the side of the second low-pressure heater outlet pipe section near the outlet end.

5. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 4, characterized in that: A fourth valve (14) is installed on the second low-pressure outlet pipe section; The second low-pressure outlet pipe section is connected to the inlet of the second hot water channel via a fifth valve (15) near the inlet end; The second low-pressure outlet pipe section is connected to the outlet of the second hot water channel via a sixth valve (16) near the outlet end.

6. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 5, characterized in that: The main condensate pipeline (27) is provided with two first low-pressure heaters (28), one second low-pressure heater (29), and one third low-pressure heater (30) in sequence. The third low-pressure heater (30) has a third low-pressure outlet pipe section. The energy-saving system for preventing sulfuric acid condensation in the air preheater of the heating unit is also equipped with a high-temperature condensate and low-efficiency circulating pump set (31). The third low-temperature outlet pipe section is connected to the inlet of each of the first hot water channels and the inlet of the second hot water channel through the high-temperature condensate and the low-temperature circulating pump group (31); the outlet of the second hot water channel is connected to the inlet of each of the first hot water channels and the inlet of the second hot water channel through the high-temperature condensate and the low-temperature circulating pump group (31).

7. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 6, characterized in that: The third low-temperature outlet pipe section is connected to the high-temperature condensate and the low-temperature circulating pump group (31) through the seventh valve (17). The outlet of the second hot water channel is connected to the high-temperature condensate and the low-energy circulating pump group (31) through the eighth valve (18). The high-temperature condensate and the low-temperature circulating pump group (31) are connected to the inlet of each of the first hot water channels and the inlet of the second hot water channel through the ninth valve (19).

8. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 7, characterized in that: The low-temperature economizer (22) is also connected in sequence to an electrostatic precipitator (23), an induced draft fan (24), and a desulfurization device (25). The air preheater (21) has a primary air duct (212) and a secondary air duct (213), and the air heater has a primary air heater (261) and a secondary air heater (262). The primary air duct (212) is connected to the primary air heater (261), and the secondary air duct (213) is connected to the secondary air heater (262). The outlets of the first hot water channels of the primary air heater (261) and the secondary air heater (262) are respectively connected to the main condensate pipeline (27) through the heater booster pump (32).

9. The energy-saving system for preventing sulfuric acid condensation in the air preheater of a heating unit according to claim 8, characterized in that: The first valve (11), the second valve (12), the fourth valve (14), the fifth valve (15) and the ninth valve (19) are all electric regulating valves; The third valve (13), the sixth valve (16), the seventh valve (17) and the eighth valve (18) are all electrically operated valves; A temperature sensor is installed on the main water inlet pipe connecting each air heater and the low-temperature economizer (22); The energy-saving system for preventing sulfuric acid condensation in the air preheater of the heating unit is also equipped with a unit decentralized control system, which is electrically connected to each valve and the high-temperature condensate and the low-energy circulating pump group (31).

10. An energy-saving method for preventing sulfuric acid condensation in the air preheater of a heating unit, characterized in that, Energy saving is achieved using the energy-saving system described in claim 9, wherein the energy-saving method includes the following steps: Open the second valve (12), the third valve (13), the fourth valve (14), and the first valve (11), and close the remaining valves to provide regular heating; Close the third valve (13), the seventh valve (17), the eighth valve (18) and the ninth valve (19), gradually open the sixth valve (16) according to the value of the temperature sensor, open the first valve (11), adjust the second valve (12) or the fifth valve (15) to adjust the water temperature of the main water inlet pipe, and adjust the fourth valve (14) to adjust the flow rate of the low temperature economizer (22); Close the seventh valve (17) and open the eighth valve (18), the ninth valve (19) and the high-temperature condensate and low-energy circulation pump group (31), or close the eighth valve (18) and open the seventh valve (17), the ninth valve (19) and the high-temperature condensate and low-energy circulation pump group (31).

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