Carnot cell system based on coal-fired power generation unit and operation method of Carnot cell system

Abstract

The invention belongs to the technical field of coal-fired power generation, and particularly relates to a Carnot battery system based on a coal-fired power generation unit and an operation method of the Carnot battery system. The Carnot cell system comprises a heat pump system taking supercritical carbon dioxide as a working medium, a heat storage system and a coal-fired power generation system; the heat pump system comprises a first compressor, a second compressor, a first heat regenerator, a second heat regenerator, a third heat regenerator, a first turbine and a second turbine; the heat storage system comprises a hot tank and a cold tank; the coal-fired power generation system comprises a coal-fired boiler, a high-pressure cylinder, an intermediate-pressure cylinder, a low-pressure cylinder, a power generator, a condenser, an eighth low-pressure heater, a seventh low-pressure heater, a sixth low-pressure heater, a fifth low-pressure heater, a deaerator, a first high-pressure heater, a second high-pressure heater and a third high-pressure heater. The split-flow compression process and the split-flow expansion process are matched with the multi-stage heat regenerator and the multi-stage fused salt heater, the energy efficiency coefficient of the Carnot battery in the electricity-to-heat conversion process is improved, and therefore the round-trip efficiency of the Carnot battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal-fired power generation, and in particular relates to a Carnot battery system based on a coal-fired power generation unit and an operation method thereof. Background Art

[0002] As the proportion of renewable energy (such as wind and solar energy) in the power system increases, the stability of the power grid faces challenges. The Carnot battery is a new type of energy storage technology whose working principle is based on the Carnot cycle. It realizes the electric-heat-electric cycle process by converting electrical energy into thermal energy for storage and then converting thermal energy back into electrical energy. Specifically, the Carnot battery includes the following steps: (1) Conversion of electrical energy into thermal energy: When there is excess electricity, the Carnot battery converts electrical energy into high-temperature thermal energy through an electric heater and stores it in a high-temperature heat storage medium. (2) Thermal energy storage: High-temperature thermal energy is stored in a special heat storage device, usually using molten salt or other high-efficiency heat storage materials. (3) Conversion of thermal energy into electrical energy: When electricity demand peaks, the stored thermal energy is converted into electrical energy through a heat engine (such as a steam turbine or Brayton cycle) and resupplied to the power grid. The power generation of renewable energy is volatile and intermittent. When there is excess power generation, the excess electricity can be converted into heat energy and stored in the Carnot battery. When there is insufficient power generation, the Carnot battery can convert the stored heat energy into electricity, helping to stabilize the output of the power grid and reduce dependence on fossil fuel power plants.

[0003] Traditional coal-fired power plants have a slow start and stop speed and cannot respond quickly to changes in grid load. Carnot batteries can be integrated with coal-fired power plants, allowing power plants to convert excess electricity into thermal energy storage when the load is low, and then convert it into electrical energy output when the power demand peaks. This method improves the flexibility of coal-fired units, reduces the idle time of power plants when the load is low, and improves overall operating efficiency. At the same time, it is a general trend to gradually reduce dependence on coal-fired power generation worldwide. However, many existing coal-fired power plants still have a long service life. By introducing Carnot battery technology, the economy and competitiveness of coal-fired power plants can be extended, the economic losses caused by large-scale elimination can be avoided, and more stringent environmental protection requirements can be met. Therefore, in the process of energy structure transformation, Carnot batteries provide an important technical support for the cleanliness and efficiency of coal-fired power plants.

[0004] However, the current electric-to-heat conversion efficiency is still low (the coefficient of efficiency COP is about 1.3), which is much lower than the theoretical Carnot efficiency. Improving the actual efficiency of electric-to-heat conversion is the bottleneck that Carnot batteries need to break through in practical applications, especially in the environment of coal-fired power plants, where high-efficiency electric-to-heat conversion is crucial. Improving thermoelectric materials, technologies and processes is the key to improving overall system efficiency. Summary of the invention

[0005] In view of the shortcomings of the existing methods, the present invention proposes a Carnot battery system based on a coal-fired power generation unit and an operation method thereof. By innovatively transforming the electric-to-heat process of the Carnot battery, the electric-to-heat process has a higher energy efficiency coefficient, thereby improving the round-trip efficiency of the Carnot battery.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A Carnot battery system based on a coal-fired power generation unit, comprising a heat pump system using supercritical carbon dioxide as a working fluid, a heat storage system and a coal-fired power generation system;

[0008] The heat pump system includes a second regenerator, wherein the cold end outlet of the second regenerator is respectively connected to the inlet of the first compressor and the cold end inlet of the first regenerator, the outlet of the first compressor is connected to the hot end inlet of the first molten salt heater, and the hot end outlet of the first molten salt heater is connected to the hot end inlet of the second regenerator; the cold end outlet of the first regenerator is connected to the inlet of the second compressor, the outlet of the second compressor is connected to the hot end inlet of the second molten salt heater, the hot end outlet of the second molten salt heater is connected to the hot end inlet of the first regenerator, and the hot end outlet of the first regenerator is connected to the hot end inlet of the second regenerator; the hot end outlet of the second regenerator is respectively connected to the inlet of the first turbine and the hot end inlet of the third regenerator, the outlet of the first turbine is connected to the cold end inlet of the third regenerator, the hot end outlet of the third regenerator is connected to the inlet of the second turbine, the outlet of the second turbine is connected to the cold end inlet of the working fluid heater, the cold end outlet of the working fluid heater is connected to the cold end inlet of the third regenerator, and the cold end outlet of the third regenerator is connected to the cold end inlet of the second regenerator;

[0009] The heat storage system comprises a hot tank and a cold tank, the outlet of the cold tank is connected to the first molten salt pump, the first molten salt heater, the second molten salt heater and the inlet of the hot tank in sequence, the outlet of the hot tank is connected to the hot end inlets of the reheater and the superheater respectively, the hot end outlets of the reheater and the superheater are both connected to the hot end inlet of the evaporator, and the hot end outlet of the evaporator is connected to the preheater, the second molten salt pump and the inlet of the cold tank in sequence;

[0010] The coal-fired power generation system includes a coal-fired boiler, the main steam of the coal-fired boiler is connected to the high-pressure cylinder inlet, the high-pressure cylinder outlet is connected to the coal-fired boiler reheater inlet and the reheater cold end inlet, the reheater cold end outlet and the coal-fired boiler reheater outlet are both connected to the medium-pressure cylinder inlet, the medium-pressure cylinder outlet is connected to the low-pressure cylinder and the condenser inlet, the condensate at the condenser outlet is connected to the feed water inlet of the first high-pressure heater after passing through the condensate pump, the eighth low-pressure heater, the seventh low-pressure heater, the sixth low-pressure heater, the fifth low-pressure heater, the deaerator, the feed water pump, the third high-pressure heater, the second high-pressure heater in sequence, the feed water outlet of the first high-pressure heater is connected to the economizer and the cold end inlet of the preheater of the coal-fired boiler, and the cold end outlet of the preheater is connected to the evaporator, the superheater and the high-pressure cylinder inlet.

[0011] Preferably, the first compressor, the second compressor, the first turbine, and the second turbine are coaxially connected, and the high-pressure cylinder, the medium-pressure cylinder, and the low-pressure cylinder are coaxially connected to the generator.

[0012] Preferably, the hot end inlet of the working fluid heater is connected to the steam extraction port of the low-pressure cylinder, and the hot end outlet of the working fluid heater merges with the drain of the outlet of the seventh low-pressure heater.

[0013] Preferably, the extraction steam of the first high-pressure heater and the second high-pressure heater comes from the high-pressure cylinder, the extraction steam of the third high-pressure heater and the deaerator comes from the medium-pressure cylinder, and the extraction steam of the eighth low-pressure heater, the seventh low-pressure heater, the sixth low-pressure heater and the fifth low-pressure heater comes from the low-pressure cylinder.

[0014] The operation method of the Carnot battery system based on the coal-fired power generation unit includes an energy storage process and an energy release process: when there is abandoned power that needs to be absorbed and the load of the coal-fired boiler cannot be further reduced, or when there is no abandoned power to be absorbed but the power generation load of the coal-fired power generation unit system needs to be lower than its minimum power generation, the energy storage process is entered. At this time, the external abandoned power or the excess power generated by the coal-fired unit and the first turbine and the second turbine jointly drive the first compressor and the second compressor, and the first molten salt pump is operated to drive the molten salt from the cold tank to the hot tank after being heated by the first molten salt heater and the second molten salt heater in turn, thereby completing the energy storage process of the Carnot battery;

[0015] When there is no abandoned electricity to be absorbed and the required electricity load is higher than the minimum load of the coal-fired boiler, the energy release process begins; at this time, the molten salt in the hot tank enters the reheater and superheater respectively to heat the reheated steam and main steam. After the molten salt releases heat, it converges and is driven by the second molten salt pump. After releasing heat to the feed water through the evaporator and preheater, it enters the cold tank, completing the energy release process of the Carnot battery.

[0016] Preferably, in the energy release process, the steam mass flow rates heated by the reheater and the superheater are the same.

[0017] Preferably, in the energy release process, when the power generation load is low or the coal-fired boiler is shut down due to a fault, all the feed water at the outlet of the first high-pressure heater enters the preheater.

[0018] The positive beneficial effects of the present invention are:

[0019] The present invention discloses a Carnot battery system based on a coal-fired power generation unit and an operation method thereof. Compared with the heat pump cycle with heat recovery in the traditional Carnot battery, the present invention has three improvements: (1) Through the split compression of the first compressor and the second compressor, different working fluid temperatures are obtained, and the first molten salt heater and the second molten salt heater are used to heat the molten salt in stages. The first compressor produces carbon dioxide with a lower temperature, which is used for the first molten salt heater to heat the molten salt, and the second compressor produces carbon dioxide with a higher temperature, which is used for the second molten salt heater to heat the molten salt. Compared with the traditional Carnot battery system that only produces carbon dioxide with a higher temperature and only includes a first-stage molten salt heater, the average heat release temperature of the heat pump cycle is reduced, thereby improving the COP of the system. (2) Through the split expansion of the first turbine and the second turbine, the work done by the turbine is increased, and the characteristic that supercritical carbon dioxide does less work at the critical point is utilized. In part of the working fluid, it does not pass through the third regenerator and directly expands to do work, far away from the critical point. Although this process reduces the heat absorption of carbon dioxide in the working fluid heater, it increases the work done by the working fluid per unit mass flow. (3) The extraction steam of the low-pressure cylinder is used as a heat source to increase the average heat absorption temperature of the heat pump cycle, thereby increasing the cycle COP. At the same time, the increased extraction steam volume can reduce the power generation of the coal-fired unit, which is conducive to its deeper peak regulation. The present invention improves the energy efficiency coefficient of the Carnot battery's electric-to-heat process by innovatively transforming the Carnot battery's electric-to-heat process, utilizing the split-flow compression and split-flow expansion processes, in conjunction with a multi-stage regenerator and a multi-stage molten salt heater, by at least 10%, thereby improving the round-trip efficiency of the Carnot battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of a Carnot battery system based on a coal-fired power generation unit according to the present invention;

[0021] Figure 2 A schematic diagram of a Carnot battery system (energy storage process) based on a coal-fired power generation unit of the present invention;

[0022] Figure 3 A schematic diagram of a Carnot battery system (energy release process) based on a coal-fired power generation unit of the present invention;

[0023] In the figure: 1-first compressor, 2-second compressor, 3-first reheater, 4-second reheater, 5-third reheater, 6-first turbine, 7-second turbine, 8-working fluid heater, 9-second molten salt heater, 10-first molten salt heater, 11-hot tank, 12-cold tank, 13-first molten salt pump, 14-reheater, 15-superheater, 16-evaporator, 17-preheater, 18-second molten salt pump, 19-coal-fired boiler, 20-high pressure cylinder, 21-medium pressure cylinder, 22-low pressure cylinder, 23-generator, 24-condenser, 25-condensate pump, 26-eighth low pressure heater, 27-seventh low pressure heater, 28-sixth low pressure heater, 29-fifth low pressure heater, 30-deaerator, 31-feed water pump, 32-third high pressure heater, 33-second high pressure heater, 34-first high pressure heater. DETAILED DESCRIPTION

[0024] The present invention is further described below in conjunction with some specific embodiments.

[0025] See also Figure 1-3 , a Carnot battery system based on a coal-fired power generation unit, including a heat pump system using supercritical carbon dioxide as a working fluid, a heat storage system and a coal-fired power generation system;

[0026] The heat pump system includes a second regenerator 4, the cold end outlet of the second regenerator 4 is connected to the inlet of the first compressor 1 and the cold end inlet of the first regenerator 3 respectively, the outlet of the first compressor 1 is connected to the hot end inlet of the first molten salt heater 10, and the hot end outlet of the first molten salt heater 10 is connected to the hot end inlet of the second regenerator 4; the cold end outlet of the first regenerator 3 is connected to the inlet of the second compressor 2, the outlet of the second compressor 2 is connected to the hot end inlet of the second molten salt heater 9, and the hot end outlet of the second molten salt heater 9 is connected to the hot end inlet of the first regenerator 3 , the hot end outlet of the first reheater 3 is connected to the hot end inlet of the second reheater 4; the hot end outlet of the second reheater 4 is respectively connected to the inlet of the first turbine 6 and the hot end inlet of the third reheater 5, the outlet of the first turbine 6 is connected to the cold end inlet of the third reheater 5, the hot end outlet of the third reheater 5 is connected to the inlet of the second turbine 7, the outlet of the second turbine 7 is connected to the cold end inlet of the working fluid heater 8, the cold end outlet of the working fluid heater 8 is connected to the cold end inlet of the third reheater 5, and the cold end outlet of the third reheater 5 is connected to the cold end inlet of the second reheater 4;

[0027] The heat storage system includes a hot tank 11 and a cold tank 12, the outlet of the cold tank 12 is connected to the first molten salt pump 13, the first molten salt heater 10, the second molten salt heater 9 and the inlet of the hot tank 11 in sequence, the outlet of the hot tank 11 is connected to the hot end inlets of the reheater 14 and the superheater 15 respectively, the hot end outlets of the reheater 14 and the superheater 15 are both connected to the hot end inlet of the evaporator 16, and the hot end outlet of the evaporator 16 is connected to the preheater 17, the second molten salt pump 18 and the inlet of the cold tank 12 in sequence;

[0028] The coal-fired power generation system includes a coal-fired boiler 19, wherein the main steam of the coal-fired boiler 19 is connected to the inlet of a high-pressure cylinder 20, the outlet of the high-pressure cylinder 20 is connected to the inlet of a reheater of the coal-fired boiler 19 and the inlet of a cold end of a reheater 14, the outlet of the cold end of the reheater 14 and the outlet of the reheater of the coal-fired boiler 19 are both connected to the inlet of an intermediate-pressure cylinder 21, the outlet of the intermediate-pressure cylinder 21 is connected to the inlet of a low-pressure cylinder 22 and a condenser 24, the condensate at the outlet of the condenser 24 is sequentially connected to the feedwater inlet of a first high-pressure heater 34 via a condensate pump 25, an eighth low-pressure heater 26, a seventh low-pressure heater 27, a sixth low-pressure heater 28, a fifth low-pressure heater 29, a deaerator 30, a feedwater pump 31, a third high-pressure heater 32, and a second high-pressure heater 33, the feedwater outlet of the first high-pressure heater 34 is connected to the cold end inlet of an economizer and a preheater 17 of the coal-fired boiler 19, and the cold end outlet of the preheater 17 is connected to the evaporator 16, the superheater 15 and the inlet of the high-pressure cylinder 20.

[0029] Furthermore, in the heat pump system using carbon dioxide as the working fluid, the first compressor 1, the second compressor 2, the first turbine 6, and the second turbine 7 are coaxially connected, and the high-pressure cylinder 20, the medium-pressure cylinder 21, the low-pressure cylinder 22 and the generator 23 are coaxially connected.

[0030] Furthermore, in the heat pump system using carbon dioxide as the working fluid, the hot end inlet of the working fluid heater 8 is connected to the steam extraction port of the low-pressure cylinder 22, and the hot end outlet of the working fluid heater 8 merges with the drain at the outlet of the seventh regenerator 27.

[0031] Furthermore, in the coal-fired power generation system, the extraction steam of the first high-pressure heater 34 and the second high-pressure heater 33 comes from the high-pressure cylinder 20, the extraction steam of the third high-pressure heater 32 and the deaerator 30 comes from the medium-pressure cylinder 21, and the extraction steam of the eighth low-pressure heater 26, the seventh low-pressure heater 27, the sixth low-pressure heater 28 and the fifth low-pressure heater 29 comes from the low-pressure cylinder 22.

[0032] The operation method of the Carnot battery system based on the above-mentioned coal-fired power generation unit includes an energy storage process and an energy release process: when there is abandoned power that needs to be absorbed and the load of the coal-fired boiler 19 cannot be further reduced, or when there is no abandoned power to be absorbed but the power generation load of the coal-fired power generation unit system needs to be lower than its minimum power generation, the energy storage process is entered. At this time, the external abandoned power or the excess power generated by the coal-fired unit and the first turbine 6 and the second turbine 7 jointly drive the first compressor 1 and the second compressor 2, and the first molten salt pump 13 is running, driving the molten salt from the cold tank 12 to be heated by the first molten salt heater 10 and the second molten salt heater 9 in turn and then enter the hot tank 11, completing the energy storage process of the Carnot battery;

[0033] When there is no abandoned electricity to be consumed and the required electricity load is higher than the minimum load of the coal-fired boiler 19, the energy release process begins; at this time, the molten salt in the hot tank 11 enters the reheater 14 and the superheater 15 respectively to heat the reheat steam and the main steam, and the molten salt releases heat and then converges and is driven by the second molten salt pump 18, and then releases heat to the feed water through the evaporator 16 and the preheater 17 and enters the cold tank 12, completing the energy release process of the Carnot battery.

[0034] Furthermore, in the energy release process, the mass flow rate of steam heated by the reheater 14 and the superheater 15 is the same, so as to maintain the heat dissipation safety of the superheater and reheater heating surfaces of the coal-fired boiler 19.

[0035] Furthermore, in the energy release process, when the power generation load is low or the coal-fired boiler 19 fails and shuts down, all the feed water at the outlet of the first high-pressure heater 34 enters the preheater 17.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A Carnot battery system based on a coal-fired generator set, characterized in that: Including heat pump system, heat storage system and coal-fired power generation system using supercritical carbon dioxide as working fluid; The heat pump system comprises a second regenerator (4), wherein the cold end outlet of the second regenerator (4) is respectively connected to the inlet of the first compressor (1) and the cold end inlet of the first regenerator (3), the outlet of the first compressor (1) is connected to the hot end inlet of the first molten salt heater (10), and the hot end outlet of the first molten salt heater (10) is connected to the hot end inlet of the second regenerator (4); the cold end outlet of the first regenerator (3) is connected to the inlet of the second compressor (2), the outlet of the second compressor (2) is connected to the hot end inlet of the second molten salt heater (9), and the hot end outlet of the second molten salt heater (9) is connected to the hot end inlet of the first regenerator (3), The hot end outlet of the first heat exchanger (3) is connected to the hot end inlet of the second heat exchanger (4); the hot end outlet of the second heat exchanger (4) is respectively connected to the inlet of the first turbine (6) and the hot end inlet of the third heat exchanger (5); the outlet of the first turbine (6) is connected to the cold end inlet of the third heat exchanger (5); the hot end outlet of the third heat exchanger (5) is connected to the inlet of the second turbine (7); the outlet of the second turbine (7) is connected to the cold end inlet of the working fluid heater (8); the cold end outlet of the working fluid heater (8) is connected to the cold end inlet of the third heat exchanger (5); and the cold end outlet of the third heat exchanger (5) is connected to the cold end inlet of the second heat exchanger (4); The heat storage system comprises a hot tank (11) and a cold tank (12), wherein the outlet of the cold tank (12) is connected to the inlet of the first molten salt pump (13), the first molten salt heater (10), the second molten salt heater (9) and the hot tank (11) in sequence, and the outlet of the hot tank (11) is connected to the hot end inlets of the reheater (14) and the superheater (15) respectively, and the hot end outlets of the reheater (14) and the superheater (15) are both connected to the hot end inlet of the evaporator (16), and the hot end outlet of the evaporator (16) is connected to the preheater (17), the second molten salt pump (18) and the inlet of the cold tank (12) in sequence; The coal-fired power generation system comprises a coal-fired boiler (19), the main steam of the coal-fired boiler (19) is connected to the inlet of the high-pressure cylinder (20), the outlet of the high-pressure cylinder (20) is connected to the inlet of the reheater of the coal-fired boiler (19) and the inlet of the cold end of the reheater (14), the outlet of the cold end of the reheater (14) and the outlet of the reheater of the coal-fired boiler (19) are both connected to the inlet of the medium-pressure cylinder (21), the outlet of the medium-pressure cylinder (21) is connected to the inlet of the low-pressure cylinder (22) and the condenser (24), and the condensate at the outlet of the condenser (24) is sequentially connected to the condensate pump (25), the eighth low-pressure heater ( 26), the seventh low-pressure heater (27), the sixth low-pressure heater (28), the fifth low-pressure heater (29), the deaerator (30), the feed water pump (31), the third high-pressure heater (32), and the second high-pressure heater (33) are connected to the feed water inlet of the first high-pressure heater (34), the feed water outlet of the first high-pressure heater (34) is connected to the economizer of the coal-fired boiler (19) and the cold end inlet of the preheater (17), and the cold end outlet of the preheater (17) is connected to the evaporator (16), the superheater (15) and the high-pressure cylinder (20) inlet.

2. The Carnot battery system based on a coal-fired power generation unit according to claim 1, characterized in that: The first compressor (1), the second compressor (2), the first turbine (6), and the second turbine (7) are coaxially connected, and the high-pressure cylinder (20), the medium-pressure cylinder (21), the low-pressure cylinder (22) and the generator (23) are coaxially connected.

3. The Carnot battery system based on a coal-fired power generation unit according to claim 1, characterized in that: The hot end inlet of the working fluid heater (8) is connected to the steam extraction port of the low-pressure cylinder (22), and the hot end outlet of the working fluid heater (8) merges with the drain at the outlet of the seventh low-pressure heater (27).

4. The Carnot battery system based on a coal-fired power generation unit according to claim 1, characterized in that: The extraction steam of the first high-pressure heater (34) and the second high-pressure heater (33) comes from the high-pressure cylinder (20), the extraction steam of the third high-pressure heater (32) and the deaerator (30) comes from the medium-pressure cylinder (21), and the extraction steam of the eighth low-pressure heater (26), the seventh low-pressure heater (27), the sixth low-pressure heater (28) and the fifth low-pressure heater (29) comes from the low-pressure cylinder (22).

5. An operating method of a Carnot battery system based on a coal-fired power generation unit according to any one of claims 1 to 4, characterized in that: It includes an energy storage process and an energy release process: when there is abandoned power that needs to be absorbed and the load of the coal-fired boiler (19) cannot be further reduced, or when there is no abandoned power to be absorbed but the power generation load of the coal-fired generator system needs to be lower than its minimum power generation, the energy storage process is entered. At this time, the external abandoned power or the excess power generated by the coal-fired unit and the first turbine (6) and the second turbine (7) jointly drive the first compressor (1) and the second compressor (2), and the first molten salt pump (13) is operated to drive the molten salt from the cold tank (12) to be heated by the first molten salt heater (10) and the second molten salt heater (9) in turn and then enter the hot tank (11), completing the energy storage process of the Carnot battery; When there is no abandoned electricity to be absorbed and the required electricity load is higher than the minimum load of the coal-fired boiler (19), the energy release process begins. At this time, the molten salt in the hot tank (11) enters the reheater (14) and the superheater (15) respectively to heat the reheat steam and the main steam. After the molten salt releases heat, it is combined and driven by the second molten salt pump (18). After releasing heat to the feed water through the evaporator (16) and the preheater (17), it enters the cold tank (12), completing the energy release process of the Carnot battery.

6. The method for operating a Carnot battery system based on a coal-fired power generation unit according to claim 5, characterized in that: In the energy release process, the mass flow rates of steam heated by the reheater (14) and the superheater (15) are the same.

7. The method for operating a Carnot battery system based on a coal-fired power generation unit according to claim 5, characterized in that: In the energy release process, when the power generation load is low or the coal-fired boiler (19) stops working due to a fault, all the feed water at the outlet of the first high-pressure heater (34) enters the preheater (17).

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