Waste heat resource gradient utilization method and system of sludge drying system

Abstract

The invention provides a sludge drying waste heat resource gradient utilization method and system, and the method mainly comprises the following steps: extracting sludge drying tail gas waste heat by using a circulating water intermediate heat-carrying medium method, carrying out graded utilization, converting first-stage waste heat into high-temperature and high-pressure saturated steam through energy grade improvement, and recycling the high-temperature and high-pressure saturated steam for sludge heat drying; secondary waste heat is used for preheating a sludge conveying pipeline, the conveying resistance of the sludge pipeline is reduced, and the sludge drying energy consumption is reduced; and the third-stage waste heat is recycled in a factory refrigeration / heat supply mode. According to the invention, the energy consumption and carbon emission level of the sludge drying system can be obviously reduced, and the operation stability of the sludge drying system is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge drying tail gas treatment, and in particular relates to a method and system for cascade utilization of waste heat resources of a sludge drying system. Background Art

[0002] As the total amount of urban and industrial sewage discharge in my country continues to increase, and the requirements for sewage treatment become increasingly stringent, sludge discharge is also increasing year by year. At present, the national urban wet sludge production exceeds 60 million tons, and the average daily wet sludge discharge in Shanghai exceeds 3,000 tons. Sludge discharge will continue to rise in the future. Under the background of "dual carbon", the technical route of sludge treatment and disposal is also facing new requirements. In the future, sludge treatment and disposal should aim at energy conservation and consumption reduction and resource and energy recovery to improve the carbon emission reduction level of sludge treatment and disposal in my country.

[0003] Sludge is a semi-solid viscous material with high water content. The traditional mechanical dehydration process can only reduce the water content of sludge to about 80%, which cannot meet the goals of sludge reduction and stabilization, and also greatly limits the energy and resource utilization of sludge. Therefore, sludge drying is particularly necessary. Sludge contains a large amount of organic matter, and the calorific value of dry sludge is close to that of lignite. It can be used as an alternative fuel to achieve environmental protection, economic and social benefits. Sludge thermal drying technology has been widely promoted and applied in the field of sludge drying treatment at home and abroad due to its advantages of high efficiency, small footprint, large processing capacity and high reduction rate. According to statistics, the proportion of sludge thermal drying in China has reached 31%, and the proportion in Shanghai is 39%. According to different heat transfer methods, sludge thermal drying technology can be divided into direct, indirect and mixed drying technology. However, no matter what type of thermal drying technology, high energy consumption is the main problem, and this problem will be more prominent under the background of "dual carbon". Although some emerging low-energy drying technologies have been proposed, such as deep sludge dehydration, heat pump drying, solar drying, and sludge hydrothermal carbonization, these technologies have great limitations in application. Taking deep dehydration technology as an example, the sludge moisture content can only be reduced to about 60%, which cannot meet the requirements of most resource utilization technologies and still needs to be further dehydrated with the help of thermal drying; deep dehydration often consumes a large amount of reagents, resulting in increased treatment costs and a significant increase in the quality of sludge dry base. In comparison, traditional thermal drying technology still has advantages that other technologies cannot match. High energy consumption has become the main factor restricting the sustainable development of sludge thermal drying. Therefore, it is urgent and necessary to carry out research on the comprehensive cascade utilization technology of waste heat resources in sludge drying systems. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for utilizing waste heat resources of a sludge drying system, so as to achieve the purpose of reducing the energy consumption of the sludge drying system and improving the operating stability of the sludge drying system.

[0005] The present invention provides a method for cascade utilization of waste heat resources in a sludge drying system, which utilizes a circulating water intermediate heat carrier method to extract waste heat from sludge drying tail gas and utilizes it in stages, wherein the first-level waste heat is converted into high-temperature and high-pressure saturated steam through energy grade improvement and reused for sludge thermal drying; the second-level waste heat is used for preheating the sludge conveying pipeline, reducing the sludge pipeline conveying resistance and reducing the energy consumption of sludge drying; the third-level waste heat is recovered by a plant area refrigeration / heating method.

[0006] The method comprises: wet sludge passes through a sludge pipeline heater and enters a sludge dryer, a drying heat source of the sludge dryer is high-temperature and high-pressure saturated water vapor, drying tail gas generated by the sludge dryer enters a tail gas heat exchanger, the circulating water entering the tail gas heat exchanger is heated, the heated circulating water enters an evaporator, and performs a primary heat exchange with a low-temperature and low-pressure gas-liquid mixed working medium that simultaneously enters the evaporator, the low-temperature and low-pressure gas-liquid mixed working medium is heated and evaporated into low-temperature and low-pressure steam, which is compressed into high-temperature and high-pressure steam by a compressor and enters a condenser, the low-temperature side of the condenser is steam condensed water discharged by the sludge dryer, the steam condensed water absorbs the heat of the high-temperature and high-pressure steam and evaporates into low-temperature and low-pressure saturated water vapor, which is compressed into high-temperature and high-pressure saturated water vapor by a steam compressor, and returns to the sludge dryer as a sludge drying heat source; after the primary heat exchange, the circulating water in the evaporator is cooled and enters a water tank, and is sent to the sludge pipeline heater by a circulating water pump, and a secondary heat exchange is performed between the circulating water and the sludge; the circulating water after the secondary heat exchange enters a refrigeration / heating device and performs a tertiary heat exchange.

[0007] The high-temperature and high-pressure hot steam in the condenser condenses into a low-temperature and high-pressure liquid working medium, expands into a low-temperature and low-pressure gas-liquid mixed working medium after passing through a throttle valve, and flows into the evaporator, thereby realizing the recycling of the working medium.

[0008] In the method, the circulating water is returned to the tail gas heat exchanger after the three-stage heat exchange, so that the water is circulated and utilized as a heat carrier medium.

[0009] Preferably, in order to achieve good working fluid cycle efficiency, the low-temperature and low-pressure gas-liquid mixed working fluid is R245fa, R245ca, R1234zd (E) or R1234ze (Z).

[0010] More preferably, the low-temperature and low-pressure gas-liquid mixed working fluid is R245ca.

[0011] Furthermore, the refrigeration / heating device is connected to a bypass valve.

[0012] The present invention also provides a cascade utilization system of waste heat resources of a sludge drying system, wherein a sludge dryer is connected to a sludge pipeline heater and a condenser respectively, wet sludge enters the sludge dryer after passing through the sludge pipeline heater, and steam condensed water discharged from the sludge dryer enters the low-temperature side of the condenser;

[0013] The sludge dryer, tail gas heat exchanger, evaporator, compressor, condenser, and steam compressor are connected in sequence to form a circulation loop. The dried tail gas generated by the sludge dryer enters the tail gas heat exchanger to heat up the circulating water. The heated circulating water enters the evaporator, and the low-temperature and low-pressure gas-liquid mixed working medium is heated and evaporated into low-temperature and low-pressure steam, which is compressed into high-temperature and high-pressure working steam by the compressor and enters the condenser. The steam condensed water on the low-temperature side of the condenser is heated and evaporated into low-temperature and low-pressure saturated water vapor, which is compressed into high-temperature and high-pressure saturated water vapor by the steam compressor and returns to the sludge dryer.

[0014] The evaporator is connected to the compressor, condenser, and throttle valve in sequence to form a circulation loop. In the condenser, the high-temperature and high-pressure hot working medium vapor condenses into a low-temperature and high-pressure liquid working medium, and expands into a low-temperature and low-pressure gas-liquid mixed working medium after passing through the throttle valve, and flows into the evaporator;

[0015] The exhaust gas heat exchanger is connected to the evaporator, water tank, circulating water pump, sludge pipe heater, bypass valve, and refrigeration / heating device in sequence to form a circulation loop. The circulating water of the exhaust gas heat exchanger is heated up and enters the evaporator. After the first-level heat exchange, the circulating water is cooled and enters the water tank. It is sent to the sludge pipe heater through the circulating water pump and then enters the refrigeration / heating device, and then returns to the exhaust gas heat exchanger.

[0016] Beneficial Effects

[0017] The present invention utilizes the waste heat of drying tail gas to make the steam condensate from the sludge dryer become high-temperature and high-pressure saturated steam after circulating heating of the working medium and steam compression to enter the sludge dryer, thereby realizing efficient recycling of waste heat of the sludge drying system and greatly reducing the energy consumption of sludge drying.

[0018] The present invention utilizes the waste heat of drying tail gas to preheat the wet sludge in the sludge conveying pipeline, thereby increasing the sludge temperature entering the sludge dryer, reducing the energy consumption of sludge drying, and also greatly reducing the flow resistance of the sludge in the pipeline, thereby improving the stability of the operation of the sludge drying system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the cascade utilization system of waste heat resources in the sludge drying system. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0021] The present invention provides a method for cascade utilization of waste heat resources of a sludge drying system, the method comprising: the sludge passes through a sludge pipeline heater 1 and enters a sludge dryer 2, the drying heat source of the sludge dryer 2 is high-temperature and high-pressure saturated water vapor, the drying tail gas discharged by the sludge dryer 2 enters a tail gas heat exchanger 3, the circulating water entering the tail gas heat exchanger 3 is heated, the heated circulating water enters an evaporator 5, and performs a primary heat exchange with a low-temperature and low-pressure gas-liquid mixed working medium that simultaneously enters the evaporator 5, the working medium absorbs the heat of the circulating water, is heated and evaporated into low-temperature and low-pressure steam, the low-temperature and low-pressure steam is compressed into high-temperature and high-pressure steam by a compressor 6, the high-temperature and high-pressure steam enters a condenser 7, the low-temperature side of the condenser 7 is the sludge The steam condensed water discharged from the dryer 2 absorbs the heat of high-temperature and high-pressure steam and evaporates into low-temperature and low-pressure saturated water vapor. It is compressed into high-temperature and high-pressure saturated water vapor by the steam compressor 9 and returns to the sludge dryer 2 to replace the heat source required for part of the sludge drying. After the primary heat exchange, the circulating water in the evaporator 5 is cooled and enters the water tank 4. It is sent to the sludge pipeline heater 1 through the circulating water pump 11, and the circulating water and the sludge undergo a secondary heat exchange. The water after the secondary heat exchange enters the refrigeration / heating device 12, which is connected to a bypass valve 10 for a three-stage heat exchange. The refrigeration / heating device 12 can meet the needs of winter heating and summer cooling, and realize the cascade utilization of the waste heat from sludge drying. The high-temperature and high-pressure steam in the condenser 7 condenses into a low-temperature and high-pressure liquid working medium, expands into a low-temperature and low-pressure gas-liquid mixed working medium after passing through the throttle valve 8, and flows into the evaporator 5 to realize the working medium recycling.

[0022] like Figure 1 As shown, it is a schematic diagram of the cascade utilization system of waste heat resources of the sludge drying system. The present invention also provides a cascade utilization system of waste heat resources of the sludge drying system. The sludge dryer 2 is connected to the sludge pipeline heater 1 and the condenser 7 respectively. The wet sludge enters the sludge dryer 2 after passing through the sludge pipeline heater 1, and the steam condensed water discharged from the sludge dryer 2 enters the low temperature side of the condenser 7;

[0023] The sludge dryer 2, the tail gas heat exchanger 3, the evaporator 5, the compressor 6, the condenser 7, and the steam compressor 9 are connected in sequence to form a circulation loop. The dried tail gas generated by the sludge dryer 2 enters the tail gas heat exchanger 3 to heat the circulating water. The heated circulating water enters the evaporator 5. The low-temperature and low-pressure gas-liquid mixed working medium is heated and evaporated into low-temperature and low-pressure steam. It is compressed into high-temperature and high-pressure working steam by the compressor 6 and enters the condenser 7. The steam condensed water on the low-temperature side of the condenser 7 is heated and evaporated into low-temperature and low-pressure saturated water vapor. It is compressed into high-temperature and high-pressure saturated water vapor by the steam compressor 9 and returns to the sludge dryer 2.

[0024] The evaporator 5 is connected with the compressor 6, the condenser 7, and the throttle valve 8 in sequence to form a circulation loop. In the condenser 7, the high-temperature and high-pressure hot working medium vapor is condensed into a low-temperature and high-pressure liquid working medium, and after passing through the throttle valve 8, it expands into a low-temperature and low-pressure gas-liquid mixed working medium and flows into the evaporator 5;

[0025] The tail gas heat exchanger 3 is connected with the evaporator 5, the water tank 4, the circulating water pump 11, the sludge pipe heater 1, the bypass valve 10, and the cooling / heating device 12 in sequence to form a circulation loop. The circulating water of the tail gas heat exchanger 3 is heated up and enters the evaporator 5. After the primary heat exchange, the circulating water is cooled and enters the water tank 4. After being sent to the sludge pipe heater 1 through the circulating water pump 11, it enters the cooling / heating device 12 and then returns to the tail gas heat exchanger 3.

[0026] Example

[0027] In this embodiment, the sludge dryer is a wedge-shaped paddle-type sludge dryer, the drying heat source is saturated water vapor at 160°C, the drying tail gas temperature at the outlet of the sludge dryer 2 is 90-100°C, the circulating water temperature is increased to about 85-95°C by the tail gas heat exchanger 3, and the high-temperature circulating water enters the evaporator 5. The circulating working medium R245ca in the evaporator 5 is heated and converted into a low-temperature and low-pressure gaseous working medium at 45-65°C, which is compressed into a high-temperature and high-pressure working medium at 110-120°C by the compressor 6, and enters the condenser 7 to condense the steam at about 90-100°C into water. The saturated water vapor is heated to about 100-110°C, and the saturated water vapor is compressed into saturated water vapor of 150-165°C by the steam compressor 9, which can replace part of the saturated steam of the sludge dryer 2; the high-temperature circulating water is cooled to about 75-85°C after heat exchange in the evaporator 5, enters the water tank 4, and then flows into the sludge pipeline heater 1 through the circulating water pump 11, heating the sludge temperature from room temperature to about 50-60°C, and the circulating water temperature is reduced to about 70-80°C and enters the refrigeration / heating device 12, meeting the heating demand for winter heating and summer cooling. After the wet sludge is preheated, the flow resistance of the sludge in the pipeline is reduced by 40-60%, and the ratio (COP) of the waste heat recovered by the system to the electric energy consumed by the compressor 6 and the steam compressor 9 is 1.7-2.8. The increase in the inlet temperature of the sludge dryer and the steam recovered by the steam compressor can reduce the steam consumption of the dried sludge by about 65%.

Claims

1. A method for cascade utilization of waste heat resources in a sludge drying system, characterized in that: The method comprises: wet sludge passes through a sludge pipeline heater (1) and then enters a sludge dryer (2); the drying heat source of the sludge dryer (2) is high-temperature and high-pressure saturated water vapor; the drying tail gas generated by the sludge dryer (2) enters a tail gas heat exchanger (3), so that the circulating water entering the tail gas heat exchanger (3) is heated; the heated circulating water enters an evaporator (5), and performs a primary heat exchange with a low-temperature and low-pressure gas-liquid mixed working medium entering the evaporator (5) at the same time; the low-temperature and low-pressure gas-liquid mixed working medium is heated and evaporated into low-temperature and low-pressure steam; the low-temperature and low-pressure gas-liquid mixed working medium is compressed into high-temperature and high-pressure steam by a compressor (6) and enters a condenser (7); and the condenser (7) is cooled. The low-temperature side of the condenser (7) is steam condensate discharged from the sludge dryer (2). The steam condensate absorbs the heat of high-temperature and high-pressure steam and evaporates into low-temperature and low-pressure saturated water vapor. It is compressed into high-temperature and high-pressure saturated water vapor by the steam compressor (9) and returns to the sludge dryer (2) as a sludge drying heat source. After the primary heat exchange, the circulating water in the evaporator (5) is cooled and enters the water tank (4). It is sent to the sludge pipe heater (1) through the circulating water pump (11), and a secondary heat exchange is performed between the circulating water and the sludge. After the secondary heat exchange, the circulating water enters the refrigeration / heating device (12) and performs a tertiary heat exchange.

2. The method for cascade utilization of waste heat resources in a sludge drying system according to claim 1 is characterized in that: In the condenser (7), high-temperature and high-pressure steam condenses into low-temperature and high-pressure liquid working fluid, expands into low-temperature and low-pressure gas-liquid mixed working fluid after passing through the throttle valve (8), and flows into the evaporator (5), thereby realizing the recycling of the working fluid.

3. The method for cascade utilization of waste heat resources in a sludge drying system according to claim 1 is characterized in that: The circulating water is returned to the tail gas heat exchanger (3) after the three-stage heat exchange, thereby realizing the recycling of water as a heat carrier medium.

4. The method for cascade utilization of waste heat resources in a sludge drying system according to claim 1 is characterized in that: The low-temperature and low-pressure gas-liquid mixed working fluid is R245fa, R245ca, R1234zd(E) or R1234ze.

5. The method for cascade utilization of waste heat resources in a sludge drying system according to claim 1 is characterized in that: The refrigeration / heating device (12) is connected to a bypass valve (10).

6. A cascade utilization system of waste heat resources in a sludge drying system, characterized in that: The sludge dryer (2) is connected to the sludge pipeline heater (1) and the condenser (7) respectively. The wet sludge enters the sludge dryer (2) after passing through the sludge pipeline heater (1). The steam condensed water discharged from the sludge dryer (2) enters the low-temperature side of the condenser (7). The sludge dryer (2), the tail gas heat exchanger (3), the evaporator (5), the compressor (6), the condenser (7), and the steam compressor (9) are connected in sequence to form a circulation loop. The dried tail gas generated by the sludge dryer (2) enters the tail gas heat exchanger (3) to heat the circulating water. The heated circulating water enters the evaporator (5). The low-temperature and low-pressure gas-liquid mixed working medium is heated and evaporated into low-temperature and low-pressure steam. It is compressed into high-temperature and high-pressure working steam by the compressor (6) and enters the condenser (7). The steam condensed water on the low-temperature side of the condenser (7) is heated and evaporated into low-temperature and low-pressure saturated water vapor. It is compressed into high-temperature and high-pressure saturated water vapor by the steam compressor (9) and returns to the sludge dryer (2). The evaporator (5) is connected to the compressor (6), the condenser (7), and the throttle valve (8) in sequence to form a circulation loop. In the condenser (7), the high-temperature and high-pressure hot working medium vapor is condensed into a low-temperature and high-pressure liquid working medium, and after passing through the throttle valve (8), it expands into a low-temperature and low-pressure gas-liquid mixed working medium and flows into the evaporator (5); The tail gas heat exchanger (3) is connected to the evaporator (5), the water tank (4), the circulating water pump (11), the sludge pipe heater (1), the bypass valve (10), and the cooling / heating device (12) in sequence to form a circulation loop. The circulating water of the tail gas heat exchanger (3) is heated and enters the evaporator (5). After the primary heat exchange, the circulating water is cooled and enters the water tank (4). It is sent to the sludge pipe heater (1) through the circulating water pump (11) and then enters the cooling / heating device (12), and then returns to the tail gas heat exchanger (3).

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