Energy-saving system of coal-fired boiler

Abstract

According to the energy-saving system of the coal-fired boiler, when the boiler operates at a low load, part of steam generated during operation of the boiler is used for heating secondary air by arranging the secondary air heater, so that combustion of fuel in a hearth is stabilized, meanwhile, the temperature of flue gas at an outlet of the economizer is increased, and energy is saved. The inlet flue gas temperature of the SCR denitration device can meet the requirement of low-load commissioning; meanwhile, the steam pressure reducer and the sludge drying device are arranged, part of steam generated during operation of the boiler is extracted to be subjected to pressure reduction through the steam pressure reducer to be used for drying the sludge, the dried sludge is input into the boiler to be mixed with pulverized coal for combustion, and the coal combustion cost is saved. According to the device, through cooperative use of the equipment, secondary air can be heated when the boiler is in low load, and the combustion stability of the boiler is improved; and sludge can be blended and burnt under high load, so that the fuel cost is reduced.

Description

Technical Field

[0001] This application relates to the field of thermal power generation technology, and in particular to an energy-saving system for coal-fired boilers. Background Technology

[0002] As the proportion of new energy sources continues to increase in the power industry, the role of traditional coal-fired power units has shifted from a "supporting" power source to a "peak-shaving" power source. This means that coal-fired power plant boilers often need to frequently adjust peak loads between high and low load operating states.

[0003] When operating at low load, the reduced fuel quantity leads to a significant decrease in furnace volumetric heat load, cross-sectional heat load, and furnace center temperature, resulting in difficulty in igniting pulverized coal, poor flame stability, and easy flameout, posing major hidden dangers such as furnace fire extinguishing and deflagration. At the same time, when operating at low load, the flue gas temperature at the inlet of the SCR denitrification system does not meet the requirements for safe operation of the SCR, causing the SCR system to malfunction.

[0004] High-load operation results in high fuel consumption, high coal consumption, and high coal costs. Utility Model Content

[0005] This application provides an energy-saving system for coal-fired boilers to solve the problems mentioned in the background art.

[0006] This application provides an energy-saving system for a coal-fired boiler, including a boiler.

[0007] The primary air pipeline connects to the primary air inlet of the boiler in sequence through the air preheater and the coal mill;

[0008] The secondary air pipeline is connected to the secondary air inlet of the boiler through the air preheater, and a secondary air heater is installed between the air preheater and the secondary air inlet.

[0009] The boiler is connected to the secondary air heater and the sludge drying device respectively through a steam pressure reducer. A first steam flow control device is installed between the steam pressure reducer and the secondary air heater, and a second steam flow control device is installed between the steam pressure reducer and the sludge drying device.

[0010] The sludge drying unit is also connected to a secondary air heater and a coal mill, respectively;

[0011] The secondary air heater is also connected in sequence to the steam condensate heater and the economizer.

[0012] Optionally, the sludge drying apparatus includes a dryer, a condenser, and a vacuum unit connected in series;

[0013] The material output end of the dryer and the gas output end of the vacuum unit are both connected to the coal mill;

[0014] The pipelines connecting the dryer and vacuum unit to the coal mill are also connected to the gas output end of the induced draft fan.

[0015] Optionally, the steam output end of the dryer is connected to the heat exchange medium input end of the heat exchanger, and the heated gas output end of the heat exchanger is connected to the gas input end of the induced draft fan.

[0016] Optionally, the dryer is completely enclosed, including a drying section and a crushing section connected to the drying section;

[0017] The drying section is equipped with a feed hopper at one end and is connected to the crushing section via a connecting pipe at the other end;

[0018] The top of the drying section and the connecting pipe connected to the drying section is equipped with an air extraction port, which is connected to the condenser.

[0019] Optionally, the drying section includes a horizontally arranged drying chamber, which includes an upper drying zone and a lower steam chamber separated from the drying zone;

[0020] The top of one end of the drying zone is connected to the feed hopper, and the other end is connected to the crushing section through a connecting pipe;

[0021] A rotary chain rake is installed at the top of the drying zone to rake the material from one end near the feed hopper to the other end;

[0022] The steam chamber has a steam outlet on one side near the feed hopper and a steam inlet on the other side.

[0023] Optionally, the rotary chain rake includes a pair of chain rollers installed in the upper part of the drying zone, one of the chain rollers being close to the feed hopper and the other being close to the connection between the drying section and the crushing section;

[0024] A pair of chain rollers are connected by a pair of rake chains. Each end of the chain roller is equipped with a sprocket, which meshes with the rake chain.

[0025] A pair of rake chains are connected by multiple parallel rake plates.

[0026] Optionally, the crushing section includes a crushing chamber and a hopper connected to the crushing chamber;

[0027] The inlet of the crushing chamber is connected to the outlet of the drying section;

[0028] At least one pair of crushing rollers shall be installed in the crushing chamber along the vertical direction.

[0029] The system of this application incorporates a secondary air heater, which utilizes a portion of the steam generated during low-load boiler operation to heat the secondary air, thereby stabilizing fuel combustion in the furnace and increasing the flue gas temperature at the economizer outlet. This ensures that the inlet flue gas temperature of the SCR denitrification unit meets the requirements for low-load operation. Simultaneously, a pressure reducer and sludge drying device are installed. During low-load boiler operation, boiler steam is extracted, depressurized by the pressure reducer, and used to dry sludge. The dried sludge is then fed back into the boiler and co-fired with pulverized coal, saving coal costs. Through the combined use of the above equipment, the device of this application heats the secondary air at low boiler load, improving boiler combustion stability. Simultaneously, a portion of the steam is extracted to dry the sludge, which is then co-fired at high load, reducing fuel costs and saving coal consumption. Furthermore, the high-temperature steam after heat exchange in the secondary air heater, when fed back to the steam condensate heater, also heats the steam condensate, thereby increasing the water temperature in the economizer, reducing coal consumption, and decreasing the economizer's absorption of heat from the flue gas, thus increasing the flue gas temperature at the economizer outlet. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of an energy-saving system for a coal-fired boiler provided in an embodiment of this application;

[0032] Figure 2 This is a schematic diagram of a sludge drying apparatus provided in an embodiment of this application;

[0033] Figure 3 A schematic diagram of a sludge drying apparatus provided in another embodiment of this application;

[0034] Figure 4 This is a schematic diagram of the structure of a dryer provided in one embodiment of this application;

[0035] Figure 5 This is a schematic diagram of the main structure of a rotary chain rake provided in an embodiment of this application;

[0036] Figure 6 This is a top view of the rotary chain rake provided in an embodiment of this application;

[0037] Figure 7 This is a schematic diagram of the structure of a rake plate provided in one embodiment of this application.

[0038] Explanation of reference numerals in the attached figures:

[0039] 1. Boiler; 2. Air preheater; 3. Coal mill; 4. Sludge drying unit; 10. Primary air pipeline; 11. Economizer; 20. Secondary air pipeline; 21. Secondary air heater; 30. Steam condensate heater; 40. Steam pressure reducer; 41. Dryer; 42. Condenser; 43. Vacuum unit; 44. Heat exchanger; 100. First steam flow control device; 200. Second steam flow control device; 401. Exhaust port; 410. Exhaust fan; 411. Drying chamber; 412. Feed hopper; 413. Rotary chain rake; 414. Crushing chamber; 415. Collection hopper; 4101. Steam outlet; 4102. Steam inlet; 4111. Drying zone; 4112. Steam chamber; 4113. Chain roller; 4114. Rake chain; 4115. Rake plate; 4141. Crushing roller pair; 41131. Sprocket. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are also within the scope of protection of this application.

[0041] like Figure 1 As shown, this application provides an energy-saving system for a coal-fired boiler, including a boiler 1;

[0042] The primary air duct 10 is connected to the primary air inlet of boiler 1 via air preheater 2 and coal mill 3 in sequence;

[0043] The secondary air pipeline 20 is connected to the secondary air inlet of the boiler 1 through the air preheater 2, and a secondary air heater 21 is installed between the air preheater 2 and the secondary air inlet.

[0044] Boiler 1 is connected to secondary air heater 21 and sludge drying device 4 respectively through steam pressure reducer 40. A first steam flow control device 100 is provided between steam pressure reducer 40 and secondary air heater 21, and a second steam flow control device 200 is provided between steam pressure reducer 40 and sludge drying device 4.

[0045] The sludge drying device 4 is also connected to the secondary air heater 21 and the coal mill 3 respectively;

[0046] The secondary air heater 21 is also connected in sequence to the steam condensate heater 30 and the economizer 11.

[0047] In actual production, boilers are special equipment and are generally not allowed to be shut down. When electricity demand is low, boilers operate at low loads. Under these conditions, it is necessary to maintain normal combustion within the boiler while ensuring a certain amount of power generation. The principle of power generation is "to generate as much as needed." Finding a balance between stable combustion in the furnace and power generation demand is crucial when the boiler is operating at low loads. However, in reality, to maintain combustion in the furnace and avoid boiler shutdowns, the amount of steam generated often exceeds the amount of steam needed for power generation. This excess steam is not effectively utilized and is cooled, resulting in energy loss. In this application, the steam generated during low-load operation is extracted or used to heat secondary air and dry sludge. This allows for the utilization of the excess steam generated by the boiler, reducing energy waste and allowing the boiler unit to avoid reducing furnace combustion to generate more power.

[0048] When the system of this application is in use, when the boiler 1 is operating at a low load, the opening of the second steam flow control device 200 (e.g., a valve) and the opening of the first steam flow control device 100 are turned on and adjusted. The boiler 1 is ignited and started in the furnace. The generated steam is depressurized by the pressure reducer 40, and part of it enters the secondary air heater 21 through the first steam flow control device 100 to reheat the secondary air. Simultaneously, the primary air supplied by the primary air pipeline 10 is heated by heat exchange in the air preheater 2. The heated primary air is then fed into the furnace through the primary air inlet of the furnace along with the pulverized coal from the coal mill 3. Meanwhile, the secondary air supplied by the secondary air pipeline 20 is preheated by the air preheater 2 and then further heated by heat exchange with high-temperature steam from the boiler in the secondary air heater 21 (which isothermally depressurized by the steam pressure reducer 40 and then input from the boiler 1 to the secondary air heater 21 via the first steam flow control device 100). This further increases the temperature of the steam before it enters the furnace through the secondary air inlet of the boiler 1, promoting complete combustion of the fuel (pulverized coal in this application). The steam, after heat exchange with the secondary air in the secondary air heater 21, is output from the secondary air heater 21 to the steam condensate heater 30 for heating, and then sent to the economizer 11 to absorb heat from the flue gas for recycling.

[0049] Another portion of the steam is depressurized by steam pressure reducer 40 (reducing it from 11-12 MPa to 5.8-7 MPa, with the main steam temperature at 566℃). This depressurized steam is then fed into sludge drying unit 4 via a second steam flow control device to dry the sludge. The dried steam, with its temperature lowered, is then fed into secondary air heater 21 to heat the secondary air. Afterward, it is output from secondary air heater 21 to steam condensate heater 30 and returned to the boiler for reuse. When boiler 1 is operating at high load, its coal consumption is high. The sludge dried in sludge drying unit 4 can be fed into coal mill 3, mixed with pulverized coal and primary air, and then fed into boiler 1 for combustion.

[0050] In this application, the main steam extracted from boiler 1 (i.e., the steam supplied to the turbine unit) is used in the secondary air heater 21 to reheat the secondary air. Because the input secondary air is heated to a higher temperature, it absorbs less heat from the furnace, thereby stabilizing fuel combustion within the furnace and increasing the flue gas temperature output from the furnace. Furthermore, since the extracted steam is only used for heat exchange, its temperature is higher than the condensate in the steam condensate heater 30. When the heat-exchanged steam is returned to the steam condensate heater 30, it also heats the condensate. Subsequently, when it is returned to the economizer 11, it increases the water temperature in the economizer 11, reduces the economizer 11's absorption of heat from the flue gas, and thus increases the flue gas temperature at the economizer 11 outlet. In summary, this application uses steam to heat the secondary air, increasing both the flue gas temperature and the water temperature within the economizer 11 to raise the flue gas temperature at the economizer 11 outlet.

[0051] The system of this application uses a secondary air heater 21 to heat the secondary air by using part of the steam generated when the boiler 1 is running at low load, so as to stabilize the combustion of fuel in the furnace and increase the flue gas temperature at the outlet of the economizer 11, so that the inlet flue gas temperature of the SCR denitrification device can meet the requirements of low load operation. At the same time, a steam pressure reducer 40 and a sludge drying device 4 are set up. When the boiler 1 is running at high load, the steam of the boiler 1 is extracted and reduced by the steam pressure reducer 40 and used to dry the sludge. The dried sludge is then fed into the boiler 1 and co-fired with pulverized coal, thus saving coal costs. The device described in this application, through the combined use of the aforementioned equipment, can heat the secondary air when the boiler is under low load, thereby improving the boiler's combustion stability. Simultaneously, it extracts some steam to dry the sludge, and at high load, it co-burns the sludge, reducing fuel costs and saving coal consumption. Furthermore, the steam temperature after heat exchange in the secondary air heater is relatively high, and when it is fed back to the steam condensate heater, it can also heat the steam condensate, thereby increasing the water temperature in the economizer, reducing the economizer's absorption of heat from the flue gas, and thus increasing the flue gas temperature at the economizer outlet.

[0052] like Figure 2 As shown, optionally, the sludge drying device 4 includes a dryer 41, a condenser 42 and a vacuum unit 43 connected in series;

[0053] The material output end of the dryer 41 and the gas output end of the vacuum unit 43 are both connected to the coal mill 3;

[0054] The pipelines connecting the dryer 41 and the vacuum unit 43 to the coal mill 3 are also connected to the gas output end of the induced draft fan 410.

[0055] During operation, the steam, after being reduced in pressure by the steam pressure reducer 40, is fed into the dryer 41 to dry the sludge through heat exchange. The steam after heat exchange is then fed into the secondary air heater 21 to heat the secondary air. The secondary steam generated during the sludge drying process, containing odor molecules and other chemical substances from the sludge, is drawn into the condenser 42 by the vacuum unit 43 for condensation. The uncondensed non-condensable gases (mainly air and some odorous gases from the sludge) mix with the compressed air supplied by the induced draft fan 410 and the dried sludge, and are then discharged into the coal mill 3 to mix with the primary air before being fed into the boiler 1 for combustion. The condensed water, containing a large amount of water-soluble substances, can be sent to the factory's wastewater treatment section for harmless treatment.

[0056] like Figure 3 As shown, optionally, the steam output end of the dryer 41 is connected to the heat exchange medium input end of the heat exchanger 44, and the heated gas output end of the heat exchanger 44 is connected to the gas input end of the induced draft fan 410.

[0057] In this application, a heat exchanger 44 is provided to heat the temperature of the air drawn in by the induced draft fan 410. This is to prevent the temperature of the mixed airflow input to the coal mill 3 from being too low, which would cause the temperature of the primary air to be too low and result in incomplete and unstable combustion in the boiler 1.

[0058] like Figure 4 As shown, optionally, the dryer 41 is entirely enclosed, including a drying section and a crushing section communicating with the drying section;

[0059] The drying section is equipped with a feed hopper 412 at one end and is connected to the crushing section through a connecting pipe at the other end.

[0060] The top of the drying section and the connecting pipe connected to the drying section is provided with an exhaust port 401, which is connected to the condenser 42.

[0061] In this application, since the odor components in sludge are complex and often accompanied by foul odors, the dryer 41 is completely sealed to effectively prevent odorous gases from escaping into the environment and polluting it during the sludge drying process. At the same time, an exhaust port 401 is provided so that the secondary steam generated during the sludge drying process can be centrally treated by the vacuum unit 43 in a timely manner, preventing the secondary steam with foul odors from escaping into the environment and polluting it.

[0062] like Figure 4 As shown, optionally, the drying section includes a horizontally arranged drying chamber 411, which includes an upper drying zone 4111 and a lower steam chamber 4112 separated from the drying zone 4111.

[0063] The top of one end of the drying zone 4111 is connected to the feed hopper 412, and the other end is connected to the crushing section through a connecting pipe;

[0064] A rotary chain rake 413 is installed in the upper part of the drying zone 4111 to rake the material from one end near the feed hopper 412 to the other end.

[0065] The steam chamber 4112 has a steam outlet 4101 on one side near the feed hopper 412 and a steam inlet 4102 on the other side.

[0066] In this application, during use, depressurized steam is input through the steam inlet 4102 of the dryer 41, while sludge is added from the feed hopper 412 at the top of the drying zone 4111. The rotary chain rake 413 is activated, rakes the sludge towards the crushing section, and spreads it on the inner bottom of the steam chamber 4112, where it exchanges heat with the steam input into the steam chamber 4112. The high temperature of the steam heats the sludge and evaporates its moisture, thus drying the sludge. The secondary steam generated during the sludge drying process is drawn into the condenser 42 by the vacuum unit 43 and condensed. The uncondensed non-condensable gases (mainly air and some odorous gases from the sludge) are mixed with the compressed air supplied by the induced draft fan 410 and the dried sludge, and discharged into the coal mill 3 to mix with the primary air, and then fed into the boiler 1 for combustion. The condensed water, containing a large amount of water-soluble substances, can be fed into the factory's wastewater treatment section for harmless treatment.

[0067] like Figures 4-7 As shown, optionally, the rotary chain rake 413 includes a pair of chain rollers 4113 installed in the upper part of the drying zone 4111, one of the chain rollers 4113 being close to the feed hopper 412 and the other being close to the connection between the drying section and the crushing section.

[0068] A pair of chain rollers 4113 are connected by a pair of rake chains 4114. Each end of the chain roller 4113 is provided with a sprocket 41131, which meshes with the rake chain 4114.

[0069] A pair of rake chains 4114 are connected by multiple parallel rake plates 4115.

[0070] In this application, a pair of chain rollers 4113 includes a driving roller and a driven roller. The driving roller is connected to a power source such as an electric motor and drives the driven roller to rotate through the rake chain 4114.

[0071] In this application, depressurized steam is input through the steam inlet 4102 of the dryer 41, while sludge is simultaneously added from the feed hopper 412 at the top of the drying zone 4111. The motor driving the drive roller in the chain roller 4113 is activated, causing the chain roller 4113 to rotate, thereby moving the rake chain 4114. Simultaneously, the rake chain 4114 drives the rake plates 4115 to move towards the output end (near the crushing section). During this movement, the rake plates 4115 rake the sludge input into the drying zone 4111 towards the crushing section and spread it on the inner bottom of the steam chamber 4112, exchanging heat with the steam input into the steam chamber 4112. The high temperature of the steam heats the sludge and evaporates its moisture, thus drying the sludge. In one feasible implementation, to prevent the input sludge from accumulating at the inlet, the inner bottom of the drying zone 4111 can be designed to slope downwards towards the output end, with an inclination angle of, for example, 5~15°.

[0072] like Figure 4 As shown, optionally, the crushing section includes a crushing chamber 414 and a collecting hopper 415 communicating with the crushing chamber 414;

[0073] The inlet of the crushing chamber 414 is connected to the outlet of the drying section;

[0074] At least one pair of crushing rollers 4141 are installed in the crushing chamber 414 along the vertical direction.

[0075] In this application, the dried sludge is discharged from the drying chamber 411 and falls into the crushing chamber 414. The crushing rollers 4141 are started to crush the blocky sludge into small granular sludge (the crushing rollers 4141 can adopt a structure suitable for shear crushing or extrusion crushing, which is the prior art and will not be described in detail here). The crushed small granular sludge falls into the collection hopper 415, and is then fed into the coal mill 3 together with the non-condensable gas discharged from the vacuum unit 43 and the compressed air supplied by the induced draft fan 410. It is then combined with pulverized coal and primary air and fed into the boiler 1 for combustion.

[0076] An energy-saving system for a coal-fired boiler operates as follows:

[0077] When the boiler 1 is operating at a low load, the first steam flow control device 100 and the second steam flow control device 200 (e.g., valves) are turned on and adjusted. The boiler 1 is ignited and started in the furnace. The steam generated by the boiler 1 is depressurized by the pressure reducer 40, and part of it enters the secondary air heater 21 through the first steam flow control device 100 to reheat the secondary air. Simultaneously, the primary air supplied by the primary air pipeline 10 is heated by heat exchange in the air preheater 2. The heated primary air is then fed into the furnace through the primary air inlet of the furnace along with the pulverized coal from the coal mill 3. At the same time, the secondary air supplied by the secondary air pipeline 20 is preheated by the air preheater 2 and then further heated by heat exchange with the high-temperature steam from the boiler in the secondary air heater 21 (which isothermally depressurized by the steam pressure reducer 40 and then input from the boiler 1 to the secondary air heater 21 through the first steam flow control device 100). The steam is then introduced into the furnace through the secondary air inlet of the boiler 1 to promote the complete combustion of the fuel (pulverized coal in this application) in the furnace. The steam after heat exchange with the secondary air in the secondary air heater 21 is output from the secondary air heater 21 to the steam condensate heater 30 for heating, and then sent to the economizer 11 to absorb the heat of the flue gas for recycling.

[0078] Another portion of the steam, after being reduced in pressure by the steam pressure reducer 40, is then fed into the sludge drying device 4 via the second steam flow control device 200 to dry the sludge. The dried steam, with its temperature lowered, is then fed into the secondary air heater 21 to heat the secondary air. Afterward, it is output from the secondary air heater 21 to the steam condensate heater 30 and returned to the boiler for reuse. When the boiler 1 is operating at high load, its coal consumption is high. The sludge dried in the sludge drying device 4 can be fed into the coal mill 3, mixed with pulverized coal and primary air, and then fed into the boiler 1 for combustion.

[0079] The specific process of drying sludge in sludge drying device 4 is as follows: depressurized steam is input from steam inlet 4102 of dryer 41, and sludge is added from feed hopper 412 at the top of drying zone 4111. The motor driving the drive roller in chain roller 4113 is turned on, and chain roller 4113 rotates, thereby driving rake chain 4114 to move. At the same time, rake chain 4114 drives rake plate 4115 to move towards the output end. During the movement, rake plate 4115 rakes the sludge input into drying zone 4111 towards the crushing part and spreads it on the inner bottom of steam chamber 4112 to exchange heat with the steam input into steam chamber 4112. The high temperature of steam is used to heat the sludge and evaporate the moisture in it, thereby drying the sludge. The secondary steam generated during the sludge drying process is drawn into the condenser 42 by the vacuum unit 43 and condensed. The uncondensed non-condensable gases (mainly air and some odorous gases from the sludge) are mixed with the compressed air supplied by the induced draft fan 410 and the dried sludge, and then discharged into the coal mill 3 to mix with the primary air, before being fed into the boiler 1 for combustion. The condensate obtained contains a large amount of water-soluble substances and can be sent to the factory's wastewater treatment section for harmless treatment.

[0080] The dried sludge is discharged from the drying chamber 411 and falls into the crushing chamber 414. The crushing rollers 4141 are started to crush the lumpy sludge into small granular sludge (the crushing rollers 4141 can adopt a structure suitable for shear crushing or extrusion crushing, which is existing technology and will not be described in detail here). The crushed small granular sludge falls into the collection hopper 415, and is then fed into the coal mill 3 together with the non-condensable gas discharged from the vacuum unit 43 and the compressed air supplied by the induced draft fan 410. It is then combined with pulverized coal and primary air and fed into the boiler 1 for combustion.

[0081] The steam after the sludge is dried in the steam chamber 4112 is discharged from the steam outlet 4101 and then fed into the heat exchanger 44 to heat the air drawn in by the induced draft fan 410 before being output to the secondary air heater 21 to heat the secondary air.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. An energy-saving system for a coal-fired boiler, comprising a boiler (1), characterized in that a primary air pipeline (10) is connected to a primary air inlet of the boiler (1) in sequence through an air preheater (2) and a coal mill (3); a secondary air pipeline (20) is connected to a secondary air inlet of the boiler (1) through the air preheater (2), and a secondary air heater (21) is arranged between the air preheater (2) and the secondary air inlet; the boiler (1) is connected to the secondary air heater (21) and a sludge drying device (4) respectively through a steam pressure reducer (40), a first steam flow control device (100) is arranged between the steam pressure reducer (40) and the secondary air heater (21), and a second steam flow control device (200) is arranged between the steam pressure reducer (40) and the sludge drying device (4); the sludge drying device (4) is further connected to the secondary air heater (21) and the coal mill (3) respectively; and the secondary air heater (21) is further connected to a steam condensate heater (30) and an economizer (11) in sequence.

2. The energy-saving system for a coal-fired boiler according to claim 1, characterized in that the sludge drying device (4) comprises a drying machine (41), a condenser (42) and a vacuum unit (43) connected in sequence.

3. The energy-saving system for a coal-fired boiler according to claim 1 or 2, characterized in that a material output end of the drying machine (41) and a gas output end of the vacuum unit (43) are both connected to the coal mill (3).

4. The energy-saving system for a coal-fired boiler according to any one of claims 1-3, characterized in that the drying machine (41) and the vacuum unit (43) are further connected to a gas output end of an induced draft fan (410) on a pipeline connected to the coal mill (3).

5. The energy-saving system for a coal-fired boiler according to any one of claims 1-4, characterized in that a steam output end of the drying machine (41) is connected to a heat exchange medium input end of a heat exchanger (44), and a heated gas output end of the heat exchanger (44) is connected to a gas input end of the induced draft fan (410).

6. The energy-saving system for a coal-fired boiler according to any one of claims 1-5, characterized in that the drying machine (41) is wholly sealed and comprises a drying part and a crushing part in communication with the drying part.

2. The coal-fired boiler energy saving system according to claim 1, characterized in that, 7. The energy-saving system for a coal-fired boiler according to claim 6, characterized in that one end of the drying part is provided with a feeding hopper (412), and the other end is in communication with the crushing part through a connecting pipeline.

8. The energy-saving system for a coal-fired boiler according to claim 6 or 7, characterized in that a top of the drying part and the connecting pipeline in communication with the drying part is provided with an air extraction port (401), and the air extraction port (401) is connected to the condenser (42).

9. The energy-saving system for a coal-fired boiler according to any one of claims 6-8, characterized in that the drying part comprises a horizontally arranged drying bin (411), and the drying bin (411) comprises an upper drying zone (4111) and a lower steam bin (4112) separated from the drying zone (4111).

3. The coal-fired boiler energy saving system according to claim 2, characterized in that, 10. The energy-saving system for a coal-fired boiler according to claim 9, characterized in that a top of one end of the drying zone (4111) is in communication with the feeding hopper (412), and the other end is in communication with the crushing part through a connecting pipeline.

4. The coal-fired boiler energy saving system according to claim 2, characterized in that, 11. The energy-saving system for a coal-fired boiler according to claim 9 or 10, characterized in that a rotary chain rake (413) is arranged in an upper part of the drying zone (4111) for raking materials from one end close to the feeding hopper (412) to the other end.

12. The energy-saving system for a coal-fired boiler according to claim 9-11, characterized in that a steam outlet (4101) is arranged on a side of one end of the steam bin (4112) close to the feeding hopper (412), and a steam inlet (4102) is arranged on a side of the other end. ​ 5. The coal-fired boiler energy saving system according to claim 4, characterized in that, ​ ​ ​ ​ 6. The coal-fired boiler energy saving system according to claim 5, characterized in that, ​ A pair of said chain rollers (4113) are connected by a pair of rakes (4114), and both ends of the chain roller (4113) are provided with a sprocket (41131) which is connected with the rake (4114) in meshing connection; A pair of rakes (4114) are connected by a plurality of parallel arranged rake plates (4115).

7. The coal-fired boiler energy saving system according to claim 4, characterized in that, The crushing part includes a crushing bin (414) and a material collecting hopper (415) communicated with the crushing bin (414); The input port of the crushing bin (414) is connected with the output end of the drying part; At least a pair of crushing counter-rollers (4141) are arranged in the vertical direction in the crushing bin (414).

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