Two-section type heat energy recovery sludge indirect heat drying system and drying method thereof

Abstract

The invention discloses a two-stage heat energy recovery sludge indirect heat drying system and a drying method thereof, the two-stage heat energy recovery sludge indirect heat drying system comprises a high-temperature indirect drying machine, a low-temperature indirect drying machine and a first heat exchanger, a waste steam outlet of the high-temperature indirect drying machine is communicated with a waste steam inlet of the first heat exchanger; a water outlet of the first heat exchanger is communicated with a water inlet header pipe, the water inlet header pipe is communicated with a water inlet of the low-temperature indirect drying machine, a heat exchange water outlet of the low-temperature indirect drying machine is communicated with a water outlet header pipe, and the water outlet header pipe is communicated with a water inlet of the first heat exchanger. Waste heat recovery is carried out on waste steam generated by the high-temperature indirect drying machine, hot water generated by waste heat recovery is provided for the low-temperature indirect drying machine to serve as a heating working medium, and therefore the use amount of energy is saved.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge drying, and in particular relates to a two-stage heat energy recovery sludge indirect thermal drying system and a drying method thereof. Background Art

[0002] At present, the sludge drying methods adopted by domestic sewage treatment plants or sludge disposal centers are usually single-type drying methods, such as low-temperature belt dryers as typical direct drying methods and disc or paddle dryers as typical indirect drying methods.

[0003] Among them, the more commonly used disc sludge indirect thermal drying generally refers to the use of high-temperature saturated hot steam to indirectly contact the sludge, so as to evaporate the water in the sludge. This process requires a large amount of high-temperature and high-pressure steam. The water evaporated from the sludge will vaporize at a higher temperature and enter the tail gas, and be extracted by the induced draft fan. The condensed water produced after dust removal and condensation is sent to wastewater treatment, and the non-condensed steam is sent to deodorization treatment.

[0004] In this process, the heat energy in the high-temperature and high-humidity exhaust gas after drying is directly cooled and discharged, without being effectively recycled. At the same time, the steam required for drying usually needs to be produced by a hot boiler, which consumes a lot of energy and has high operating costs. Summary of the invention

[0005] The main technical problem solved by the present invention is to provide a two-stage heat recovery sludge indirect thermal drying system and a drying method thereof, in which the waste steam generated by a high-temperature indirect dryer is recovered for waste heat, and the hot water generated by the waste heat recovery is provided to a low-temperature indirect dryer as a heating medium, thereby saving energy usage and increasing the output of dried sludge.

[0006] In order to solve the above technical problems, a technical solution adopted by the present invention is: a two-stage heat recovery sludge indirect thermal drying system, comprising a high-temperature indirect dryer, a low-temperature indirect dryer and a first heat exchanger, wherein the waste steam outlet of the high-temperature indirect dryer is connected to the waste steam inlet of the first heat exchanger;

[0007] The water outlet of the first heat exchanger is connected to the water inlet main pipe, the water inlet main pipe is connected to the water inlet of the low-temperature indirect dryer, the heat exchange water outlet of the low-temperature indirect dryer is connected to the water outlet main pipe, and the water outlet main pipe is connected to the water inlet of the first heat exchanger. The waste steam generated by the high-temperature indirect dryer is recovered for waste heat and provided to the low-temperature indirect dryer as a heating medium, thereby saving energy usage, increasing the output of dried sludge, and achieving huge economic benefits.

[0008] Furthermore, the waste steam outlet of the low-temperature indirect dryer is connected to the second dust collector, the second dust collector is connected to the waste steam inlet of the second heat exchanger, and the non-condensable steam outlet of the second heat exchanger is connected to the vacuum system.

[0009] Furthermore, the water outlet main pipe branches into water outlet branch pipes, the water outlet branch pipes are connected to the first water inlet of the heat pump, the first water outlet of the heat pump is connected to the hot water tank, the hot water tank is provided on the water inlet main pipe, the water outlet of the second heat exchanger is connected to the second water inlet of the heat pump, and the second water outlet of the heat pump is connected to the water inlet of the second heat exchanger.

[0010] Furthermore, the condensate outlet of the high-temperature indirect dryer is connected to the hot water tank, the water inlet main pipe branches out into a water inlet branch pipe, the water inlet branch pipe is connected to the boiler, and the boiler is connected to the steam inlet of the high-temperature indirect dryer.

[0011] Furthermore, the waste steam outlet of the high-temperature indirect dryer is connected to the first dust collector, the first dust collector is connected to the waste steam inlet of the first heat exchanger, and the non-condensable steam outlet of the first heat exchanger is connected to the steam induced draft fan.

[0012] Furthermore, the mud outlet of the high-temperature indirect dryer is communicated with the dry mud silo or the mud outlet of the high-temperature indirect dryer is communicated with the mud inlet of the low-temperature indirect dryer.

[0013] Furthermore, the present invention also provides a drying method of a two-stage heat recovery sludge indirect thermal drying system, in which a high-temperature indirect drying machine and a low-temperature indirect drying machine are connected in parallel, comprising the following steps:

[0014] S1: High temperature indirect drying section, a part of the total amount of sludge with a moisture content of 50% to 85% is transported into the high temperature indirect dryer;

[0015] S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%;

[0016] S3: The waste steam at about 100℃ generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100℃ and then enters the hot water tank;

[0017] S4: Low-temperature indirect drying section, the remaining sludge in the total amount of sludge with a moisture content of 50% to 85% is transported to the low-temperature indirect dryer;

[0018] S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating;

[0019] S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

[0020] Furthermore, the present invention also provides a drying method of a two-stage heat recovery sludge indirect thermal drying system, in which a high-temperature indirect drying machine and a low-temperature indirect drying machine are connected in series, comprising the following steps:

[0021] S1: High temperature indirect drying section, transporting sludge with a moisture content of 50% to 85% into the high temperature indirect dryer;

[0022] S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%;

[0023] S3: The waste steam at about 100°C generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside the first heat exchanger. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100°C and then enters the hot water tank;

[0024] S4: Low-temperature indirect drying section, the sludge dried by the high-temperature indirect dryer is transported to the low-temperature indirect dryer;

[0025] S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge is further reduced. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating;

[0026] S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

[0027] It also includes a heat pump. The 50-60°C waste steam generated by the low-temperature indirect dryer undergoes heat exchange in the second heat exchanger. The water at about 50°C obtained by the heat exchange in the second heat exchanger recovers heat energy through the heat pump, and the outlet water is cooled to about 40°C. At the same time, the heat generated by the heat pump reheats the outlet water of the low-temperature indirect dryer to 80-100°C and is discharged to the hot water tank.

[0028] Part of the hot water in the hot water tank is passed into the boiler, and after being heated by the boiler, saturated steam with a temperature of 150-180°C and a pressure of 0.5-1.0MPa is generated and passed into the high-temperature indirect dryer.

[0029] The beneficial effects of the present invention include at least the following points:

[0030] The present invention comprises a high-temperature indirect dryer and a low-temperature indirect dryer. The waste steam outlet of the high-temperature indirect dryer is connected to the waste steam inlet of the first heat exchanger. The water outlet of the first heat exchanger is connected to the water inlet main pipe. The water inlet main pipe is connected to the water inlet of the low-temperature indirect dryer. The heat exchange water outlet of the low-temperature indirect dryer is connected to the water outlet main pipe. The water outlet main pipe is connected to the water inlet of the first heat exchanger. The waste steam generated by the high-temperature indirect dryer is recycled for waste heat. The hot water generated by the waste heat recycling is provided to the low-temperature indirect dryer as a heating medium, thereby saving energy usage and increasing the output of dried sludge. The economic benefits are huge.

[0031] Whether the high-temperature indirect dryer and the low-temperature indirect dryer of the present invention are connected in series or in parallel, the amount of sludge treated by the high-temperature indirect dryer using steam can be reasonably matched with the amount of sludge treated by the low-temperature indirect dryer using hot water recovered by waste heat, ensuring that the moisture content of the final sludge is 10-40%. It is worth mentioning that the two-stage sludge drying process route combining the high-temperature indirect dryer (disc dryer, paddle dryer or thin layer dryer) and the low-temperature indirect dryer (disc dryer, thin layer dryer or paddle dryer) is used, and the heat in the waste steam is recovered by the drying tail gas heat energy, which is provided to the low-temperature indirect dryer as a drying heat source, reducing or eliminating the need for other external heat sources;

[0032] The present invention adopts a combined process of high-temperature indirect drying and low-temperature indirect drying. The dryer does not need to be fed with carrier gas, so the heat loss is small, the amount of dust carried out by the drying tail gas is small, the non-condensable steam output is small, there is no explosion risk, and the moisture content of the dried mud is lower than that of the traditional drying mud.

[0033] The present invention adopts a combination of high-temperature indirect drying and low-temperature indirect drying processes. Compared with the single disc drying method, it is more convenient to achieve maximum heat recovery, reduce the use of hot water, improve the efficiency of waste steam use, and reduce operating costs. Compared with a single high-temperature indirect drying process, the heat source cost required for the entire high-temperature and low-temperature combination drying process can be reduced by 30-40%, and the economic and social benefits are quite considerable. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural schematic diagram of the first embodiment of the present invention (drying machines are connected in parallel);

[0035] Figure 2 This is a schematic diagram of the structure of the second embodiment of the present invention (drying machines are connected in series);

[0036] Figure 3 This is a schematic diagram of the structure of the third embodiment of the present invention (a dryer is connected in parallel with a heat pump);

[0037] Figure 4 This is a schematic diagram of the structure of the fourth embodiment of the present invention (a drying machine is connected in series with a heat pump);

[0038] The parts in the accompanying drawings are marked as follows:

[0039] 1. High-temperature indirect dryer; 2. Low-temperature indirect dryer; 21. Water inlet main pipe; 211. Water inlet branch pipe; 22. Water outlet main pipe; 221. Water outlet branch pipe; 3. Hot water tank; 4. First heat exchanger; 5. Second dust collector; 6. Second heat exchanger; 7. Vacuum system; 8. Heat pump; 9. Boiler; 10. First dust collector; 100. Steam induced draft fan. DETAILED DESCRIPTION

[0040] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0041] Embodiment 1: A two-stage heat recovery sludge indirect thermal drying system, such as Figure 1 As shown, it includes a high-temperature indirect dryer 1, a low-temperature indirect dryer 2 and a first heat exchanger 4, and the waste steam outlet of the high-temperature indirect dryer 1 is connected to the waste steam inlet of the first heat exchanger 4;

[0042] The water outlet of the first heat exchanger 4 is connected to the water inlet main pipe 21, the water inlet main pipe 21 is connected to the water inlet of the low-temperature indirect dryer 2, the heat exchange water outlet of the low-temperature indirect dryer 2 is connected to the water outlet main pipe 22, and the water outlet main pipe 22 is connected to the water inlet of the first heat exchanger 4.

[0043] In this embodiment, the high-temperature indirect dryer is a disc dryer, a paddle dryer or a thin layer dryer, and the low-temperature indirect dryer is a disc dryer, a thin layer dryer or a paddle dryer. The working temperature of the high-temperature indirect dryer is greater than the working temperature of the low-temperature indirect dryer.

[0044] The waste steam outlet of the low-temperature indirect dryer 2 is connected to the second dust collector 5, the second dust collector 5 is connected to the waste steam inlet of the second heat exchanger 6, and the non-condensable steam outlet of the second heat exchanger 6 is connected to the vacuum system 7.

[0045] In this embodiment, the first heat exchanger and the second heat exchanger are both partition wall heat exchangers;

[0046] During specific implementation, the wastewater outlet of the second heat exchanger is connected to the wastewater treatment system.

[0047] The condensate outlet of the high-temperature indirect dryer 1 is connected to the hot water tank 3 , the water inlet main pipe 21 branches out a water inlet branch pipe 211 , the water inlet branch pipe 211 is connected to the boiler 9 , and the boiler 9 is connected to the steam inlet of the high-temperature indirect dryer 1 .

[0048] The waste steam outlet of the high-temperature indirect dryer 1 is connected to the first dust collector 10 , the first dust collector 10 is connected to the waste steam inlet of the first heat exchanger 4 , and the non-condensable steam outlet of the first heat exchanger 4 is connected to the steam induced draft fan 100 .

[0049] During specific implementation, the wastewater outlet of the first heat exchanger is connected to a wastewater treatment system.

[0050] The mud outlet of the high-temperature indirect dryer 1 is communicated with the dry mud silo.

[0051] The present invention also provides a drying method of a two-stage heat recovery sludge indirect thermal drying system, in which a high-temperature indirect drying machine and a low-temperature indirect drying machine are connected in parallel, comprising the following steps:

[0052] S1: High temperature indirect drying section, a part of the total amount of sludge with a moisture content of 50% to 85% is transported into the high temperature indirect dryer;

[0053] S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%;

[0054] S3: The waste steam at about 100℃ generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100℃ and then enters the hot water tank;

[0055] S4: Low-temperature indirect drying section, the remaining sludge in the total amount of incoming sludge with a moisture content of 50% to 85% is transported to the low-temperature indirect dryer;

[0056] S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating;

[0057] S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

[0058] In this embodiment, no carrier gas is introduced during the operation of the high-temperature indirect dryer and the low-temperature indirect dryer;

[0059] In the above process, the proportion of the sludge amounts entering the two dryers can be determined by calculation, ensuring that a portion of the sludge m% entering absorbs heat in the high-temperature indirect drying to produce waste steam, and the amount of high-temperature hot water regenerated through the first heat exchanger is just enough to provide the remaining sludge (1-m%) with the heat required for drying in the low-temperature indirect drying.

[0060] Embodiment 2: Figure 2 As shown, the structure of this embodiment is basically the same as that of the first embodiment, except that the mud outlet of the high-temperature indirect dryer 1 is connected to the mud inlet of the low-temperature indirect dryer 2 .

[0061] The present invention also provides a drying method of a two-stage heat recovery sludge indirect thermal drying system, in which a high-temperature indirect dryer and a low-temperature indirect dryer are connected in series. Since the sludge is first dried by the high-temperature indirect dryer and then by the low-temperature indirect dryer, the moisture content of the sludge is lower, and the method comprises the following steps:

[0062] S1: High temperature indirect drying section, transporting sludge with a moisture content of 50% to 85% into the high temperature indirect dryer;

[0063] S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%;

[0064] S3: The waste steam at about 100°C generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside the first heat exchanger. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100°C and then enters the hot water tank;

[0065] S4: Low-temperature indirect drying section, the sludge dried by the high-temperature indirect dryer is transported to the low-temperature indirect dryer;

[0066] S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge is further reduced. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating;

[0067] S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

[0068] Part of the hot water in the hot water tank is passed into the boiler, and after being heated by the boiler, saturated steam with a temperature of 150-180°C and a pressure of 0.5-1.0MPa is generated and passed into the high-temperature indirect dryer. The steam here can also be purchased commercial steam.

[0069] Embodiment 3 and Embodiment 4: In order to utilize the waste steam generated by the low-temperature indirect drying machine and further save energy, Figure 3 As shown, the third embodiment adds an optional heat pump on the basis of the first embodiment, such as Figure 4 As shown, the fourth embodiment adds an optional heat pump on the basis of the second embodiment.

[0070] The two-stage heat recovery sludge indirect thermal drying system also includes a heat pump, the outlet main pipe 22 branches out a water outlet branch pipe 221, the water outlet branch pipe 221 is connected to the first water inlet of the heat pump 8, the first water outlet of the heat pump 8 is connected to the hot water tank 3, the hot water tank 3 is provided on the water inlet main pipe 21, the water outlet of the second heat exchanger 6 is connected to the second water inlet of the heat pump 8, and the second water outlet of the heat pump 8 is connected to the water inlet of the second heat exchanger 6.

[0071] In this embodiment, the first water inlet of the heat pump is communicated with the first water outlet of the heat pump, and the second water inlet of the heat pump is communicated with the second water outlet of the heat pump.

[0072] When the drying system is running, the 50-60℃ waste steam generated by the low-temperature indirect dryer undergoes heat exchange in the second heat exchanger. The water at about 50℃ obtained by heat exchange in the second heat exchanger recovers heat energy through the heat pump, and the outlet water is cooled to about 40℃. At the same time, the heat generated by the heat pump reheats the outlet water of the low-temperature indirect dryer to 80-100℃ and is discharged to the hot water tank.

[0073] As a better solution, this embodiment provides a solution for recovering heat from the condensed wastewater generated by the first heat exchanger, such as Figure 3 He Ru Figure 4 As shown, in this embodiment, the condensed waste water of the first heat exchanger merges with the water at the water outlet of the second heat exchanger and is discharged into the second water inlet of the heat pump, and the excess water is discharged from the second water outlet of the heat pump. In specific implementation, the temperature of the condensed waste water of the first heat exchanger is 90-100°C, and the temperature of the water at the water outlet of the second heat exchanger is about 50°C. The temperature of the merged water is about 70°C, and the heat of the merged water at about 70°C is recovered by the heat pump.

[0074] The above are only embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A two-stage heat recovery sludge indirect thermal drying system, characterized by: It comprises a high-temperature indirect drying machine (1), a low-temperature indirect drying machine (2) and a first heat exchanger (4), wherein the waste steam outlet of the high-temperature indirect drying machine is connected to the waste steam inlet of the first heat exchanger; The water outlet of the first heat exchanger is connected to a water inlet main pipe (21), the water inlet main pipe is connected to a water inlet of the low-temperature indirect dryer, the heat exchange water outlet of the low-temperature indirect dryer is connected to a water outlet main pipe (22), and the water outlet main pipe is connected to a water inlet of the first heat exchanger.

2. The two-stage heat recovery sludge indirect thermal drying system according to claim 1 is characterized in that: The waste steam outlet of the low-temperature indirect dryer is connected to the second dust collector (5), the second dust collector is connected to the waste steam inlet of the second heat exchanger (6), and the non-condensable steam outlet of the second heat exchanger is connected to the vacuum system (7).

3. The two-stage heat recovery sludge indirect thermal drying system according to claim 2 is characterized in that: The water outlet main pipe branches out into a water outlet branch pipe (221), the water outlet branch pipe is connected to a first water inlet of a heat pump (8), the first water outlet of the heat pump is connected to a hot water tank (3), the hot water tank is provided on the water inlet main pipe, the water outlet of the second heat exchanger is connected to a second water inlet of the heat pump, and the second water outlet of the heat pump is connected to a water inlet of the second heat exchanger.

4. The two-stage heat recovery sludge indirect thermal drying system according to claim 3 is characterized in that: The condensate outlet of the high-temperature indirect dryer is connected to the hot water tank, the main water inlet pipe branches out into a water inlet branch pipe (211), the water inlet branch pipe is connected to the boiler (9), and the boiler is connected to the steam inlet of the high-temperature indirect dryer.

5. The two-stage heat recovery sludge indirect thermal drying system according to claim 1 is characterized in that: The waste steam outlet of the high-temperature indirect dryer is connected to the first dust collector (10), the first dust collector is connected to the waste steam inlet of the first heat exchanger, and the non-condensable steam outlet of the first heat exchanger is connected to the steam induced draft fan (100).

6. The two-stage heat recovery sludge indirect thermal drying system according to claim 1 is characterized in that: The mud outlet of the high-temperature indirect dryer is communicated with a dry mud silo or the mud outlet of the high-temperature indirect dryer is communicated with a mud inlet of the low-temperature indirect dryer.

7. A drying method of a two-stage heat recovery sludge indirect thermal drying system according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1: High temperature indirect drying section, a part of the total amount of sludge with a moisture content of 50% to 85% is transported into the high temperature indirect dryer; S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%; S3: The waste steam at about 100℃ generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100℃ and then enters the hot water tank; S4: Low-temperature indirect drying section, the remaining sludge in the total amount of sludge with a moisture content of 50% to 85% is transported to the low-temperature indirect dryer; S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating; S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

8. A drying method of a two-stage heat recovery sludge indirect thermal drying system according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1: High temperature indirect drying section, transporting sludge with a moisture content of 50% to 85% into the high temperature indirect dryer; S2: Saturated steam at 150-180℃ and 0.5-1.0MPa is introduced into the high-temperature indirect dryer to indirectly contact the sludge, and the water in the sludge is evaporated, and the moisture content of the sludge can be reduced to 10%-40%; S3: The waste steam at about 100°C generated by drying is extracted by the steam induced draft fan, and first passes through the first dust collector to reduce the dust content in the tail gas. The first heat exchanger is set after the first dust collector. Clean hot water flows inside the first heat exchanger, and high-temperature waste steam flows outside the first heat exchanger. The waste steam is cooled and becomes low-temperature waste water and is discharged to the wastewater treatment system. The non-condensable steam is sent to the deodorization system through the steam induced draft fan. The water inside the first heat exchanger comes from the low-temperature indirect dryer in the latter stage. This part of water is heated to 80-100°C and then enters the hot water tank; S4: Low-temperature indirect drying section, the sludge dried by the high-temperature indirect dryer is transported to the low-temperature indirect dryer; S5: The clean hot water in the hot water tank, which has been heated to 80-100℃ by absorbing heat, is passed into the low-temperature indirect dryer to come into indirect contact with the sludge. The water in the sludge is evaporated, and the moisture content of the sludge is further reduced. The hot water cooled by the low-temperature indirect dryer is then passed into the first heat exchanger for reheating; S6: During the low-temperature disc drying process, the vacuum system continuously draws negative pressure to make the vacuum degree in the drying chamber reach about -0.9 bar. The waste steam discharged by vacuum will carry a small amount of dust. It will first pass through the second dust collector for dust removal, and then pass into the second heat exchanger to cool the exhaust gas at 50-60℃ to about 45℃. The condensed wastewater generated is discharged from the system to the wastewater treatment system, and the non-condensed steam is sent to the deodorization system through the vacuum system.

9. The drying method of the two-stage heat recovery sludge indirect thermal drying system according to claim 7 or 8, characterized in that: It also includes a heat pump. The 50-60°C waste steam generated by the low-temperature indirect dryer undergoes heat exchange in the second heat exchanger. The water at about 50°C obtained by the heat exchange in the second heat exchanger recovers heat energy through the heat pump, and the outlet water is cooled to about 40°C. At the same time, the heat generated by the heat pump reheats the outlet water of the low-temperature indirect dryer to 80-100°C and is discharged to the hot water tank.

10. The drying method of the two-stage heat recovery sludge indirect thermal drying system according to claim 7 or 8, characterized in that: Part of the hot water in the hot water tank is passed into the boiler, and after being heated by the boiler, saturated steam with a temperature of 150-180°C and a pressure of 0.5-1.0MPa is generated and passed into the high-temperature indirect dryer.

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