A method for low-energy consumption drying of sludge based on waste heat of power plant and optimal utilization of heat energy

Abstract

The present application belongs to the technical field of sludge drying and power plant waste heat utilization, and discloses a sludge low-energy-consumption drying and heat energy optimal utilization method based on power plant waste heat. Firstly, the exhaust heat of the power plant boiler and the steam turbine extraction driving absorption heat pump are collected, the low-grade heat energy is upgraded into high-grade heat source suitable for sludge low-temperature drying requirement, and is delivered to the drying unit. The belt low-temperature dryer is used to complete the continuous drying of the sludge in cooperation with the fully-closed air circulation system. The drying tail gas is pre-heated with the backflow low-humidity air and then is condensed and dehumidified. The condensation and vaporization latent heat is recovered and delivered to the heat pump input end. The dehumidified air is backflow circulated. The standard sludge is mixed and burned with coal, and the ash is used as building material raw material and is recycled. The method effectively improves the utilization rate of low-grade waste heat of the power plant, the drying energy consumption is far lower than that of the traditional steam drying process, the drying product quality is stable, the harmless reduction and resourceful disposal of the sludge are realized, and the operation requirement of various coal-fired power plants is met.

Description

Technical Field

[0001] This invention belongs to the field of sludge drying and power plant waste heat utilization technology, and in particular to a method for low-energy sludge drying and optimized heat energy utilization based on power plant waste heat. Background Technology

[0002] Municipal sludge is an inevitable byproduct of urban wastewater treatment. With the continuous increase in my country's urbanization rate, the annual output of municipal sludge is growing year by year. Harmless, reduced-volume, and resource-based disposal has become a core and rigid requirement in the solid waste treatment field. Coal-fired power plants, as high-energy-consuming production units, generate a large amount of underutilized waste heat resources during operation, such as boiler flue gas and low-grade steam extracted from turbines. Direct discharge would cause serious energy waste. Utilizing waste heat from power plants for co-processing sludge is currently a recognized key development direction in the industry.

[0003] Currently, the mainstream sludge drying technology is the traditional steam drying process. This process uses high-quality saturated steam generated by power plant boilers as a heat source, and heats the sludge to evaporate moisture through indirect heat exchange. It has a fast drying speed and strong adaptability to feed materials, and currently accounts for more than 40% of the sludge treatment projects already in operation in China. However, this process consumes a large amount of high-quality steam that can be used for power generation, resulting in high energy consumption per unit of sludge drying, which directly reduces the overall power generation efficiency of power plants. In addition, the high-humidity and odorous exhaust gas generated during the drying process requires separate deodorization and dedicated wastewater treatment systems, resulting in high overall operating costs.

[0004] Another existing technology utilizes the waste heat from power plant flue gas directly in a drying process. This involves directly feeding boiler flue gas into the drying unit as a heat source, eliminating the need for high-quality steam and resulting in lower initial system modification costs. However, this type of process does not upgrade the quality of low-grade waste heat, and the heat source temperature fluctuates greatly depending on the power plant's operating conditions. When the power plant is operating at low load, the flue gas temperature cannot meet the drying requirements, leading to large fluctuations and poor stability in the moisture content of the dried sludge. Furthermore, a large amount of latent heat of vaporization carried by the drying exhaust gas is not effectively recovered, resulting in an overall waste heat utilization rate of less than 50%, highlighting the problem of energy waste. In addition, this type of process often uses open drying systems, posing a high risk of odor leakage from the exhaust gas and creating significant environmental control challenges.

[0005] To address the shortcomings of existing technologies, the industry urgently needs to develop a sludge co-treatment technology that can adapt to different operating conditions of power plants, has high waste heat utilization rate, low drying energy consumption, and low exhaust gas treatment cost. This technology can improve the overall energy utilization efficiency of power plants and reduce overall operating costs while achieving efficient and stable sludge treatment. Summary of the Invention

[0006] To address the shortcomings of existing sludge drying methods, such as high energy consumption, low waste heat utilization rate, poor adaptability to operating conditions, and high cost of treating waste gas, wastewater, and solid waste, this invention provides a low-energy sludge drying and heat energy optimization method based on waste heat from power plants.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for low-energy drying and optimized utilization of sludge based on waste heat from power plants includes the following steps: S1. Heat source preparation: Collect waste heat from the flue gas of the power plant boiler and steam extracted from the steam turbine as an energy source to drive the operation of the absorption heat pump, upgrade the low-grade heat energy into a high-grade heat source with a temperature that matches the low-temperature drying requirements of sludge, and deliver it to the sludge drying unit. S2. Sludge drying: A belt-type low-temperature dryer is used to continuously dry municipal sludge. The drying process adopts a fully enclosed air circulation system. The circulating air exchanges heat with a high-temperature heat source and then comes into contact with the wet sludge to realize the evaporation and migration of water in the sludge. S3, exhaust gas treatment and waste heat recovery: pre-exchange heat with the low-humidity air returning to the drying unit, enter the condensation unit for dehumidification, and recover all the latent heat of vaporization released during the condensation process and send it to the low-temperature heat source input end of the absorption heat pump. The dehumidified low-humidity air is returned to the drying unit for recycling. S4. Resource utilization: After drying, the sludge with the required moisture content is directly transported to the raw coal bunker of the power plant boiler. It is then mixed evenly with the coal and sent into the furnace for combustion. The ash produced during combustion is collected and used as raw material for building materials production for resource utilization.

[0008] Preferably, the boiler flue gas collected in step S1 is flue gas with an outlet temperature of 120℃~180℃ from the power plant's air preheater. This flue gas first undergoes dust removal treatment to remove particulate matter, without changing the original boiler flue gas flow path. The turbine extraction steam is extraction steam with a gauge pressure of 0.2MPa~0.5MPa in the low-pressure cylinder of the turbine, which does not affect the normal power generation efficiency of the turbine. After the boiler flue gas is dusted, it is first introduced into a finned tube heat exchange unit as part of the low-temperature heat source to preheat the lithium bromide circulating working fluid of the absorption heat pump. Then, it is introduced into the turbine extraction steam to supplement energy to drive the heat pump to do work, outputting a high-temperature heat source with an output temperature of 60℃~80℃. The finned tube heat exchange unit has an extraction and utilization rate of no less than 90% for the waste heat from the boiler flue gas. After heat exchange, the flue gas with a temperature below 80℃ is directly sent to the power plant's original flue gas desulfurization system for treatment.

[0009] Preferably, in step S2, the wet sludge entering the belt-type low-temperature dryer has a moisture content of 80%~85%, the belt speed of the low-temperature dryer is 0.1m / min~0.5m / min, the internal temperature of the drying chamber is maintained at 60℃~75℃, and there is no open flame contact throughout the process to prevent sludge spontaneous combustion or organic matter decomposition; the circulating air volume of the closed air circulation system is 100m³ per cubic meter of drying chamber volume.3 / h~150m 3 / h, the circulating air entering the drying chamber first passes through a multi-stage filtration unit to remove particulate impurities, and the relative humidity is controlled below 10% to prevent sludge from adhering to the mesh belt or clogging the ventilation holes, ensuring continuous and stable operation of the drying process.

[0010] Preferably, in step S3, the high-humidity exhaust gas first enters the gas-to-gas heat exchange unit, where it undergoes counter-current heat exchange preheating with the low-humidity air returning to the drying chamber. The high-humidity exhaust gas then enters the surface condensation unit, where it is condensed and dehumidified within a temperature range of 10℃ to 15℃. The latent heat of vaporization recovered during the condensation process accounts for more than 30% of the total energy input to the heat pump, and the absolute moisture content of the dehumidified low-humidity air is less than 15 g / m³. 3 The wastewater is recycled back to the drying unit. The suspended solids content of the condensed wastewater is less than 300 mg / L. It is directly transported to the wastewater treatment system of the power plant for treatment and then discharged after meeting the standards. There is no need to set up a separate sludge drying wastewater treatment facility.

[0011] Preferably, in step S4, the moisture content of the dried sludge is 30% to 40%. The dried sludge is first crushed by a crushing unit to a particle size of no more than 20 mm, and then transported to the raw coal bunker of the power plant boiler to be uniformly mixed with coal. The mass ratio of dried sludge to coal is 5% to 15%. The calorific value of the mixed fuel meets the requirements of stable combustion of the power plant boiler. The ash produced by combustion is used as a raw material for building materials production for resource utilization.

[0012] Preferably, it is also equipped with an independent energy scheduling module, which collects various operating parameters in real time, such as boiler flue gas temperature, turbine extraction steam load, sludge treatment volume, drying chamber temperature, and exhaust gas moisture content. Based on a preset energy optimization algorithm, it dynamically adjusts the input energy of the absorption heat pump, the circulating air volume, and the mesh belt running speed. While maintaining stable operation of the drying process and ensuring that the moisture content of the dried sludge meets the standards, it maximizes the thermal energy utilization rate of the entire process and adapts to the operating needs of the power plant under different load conditions.

[0013] Preferably, the piping and drying chamber of the fully enclosed air circulation system adopt an airtight sealing structure. The system maintains a slight negative pressure of 50Pa to 100Pa through the linkage control of the circulating fan and the exhaust regulating valve, so that the amount of external air infiltration is controlled to be less than 1% of the system's circulating air volume. Odor gases such as hydrogen sulfide and ammonia generated during the drying process are sealed in the circulation path under the action of slight negative pressure, eliminating the need for a separate end-of-pipe deodorization treatment device.

[0014] Preferably, a sludge pretreatment process is set before step S2. The municipal sludge first goes through a conditioning unit with added conditioning agent. The conditioning agent uses fly ash from power plants as raw material. The addition mass ratio of the conditioning agent is 2% to 5% of the mass of the wet sludge. No other chemical agents need to be added. The dewatering performance of the conditioned sludge is greatly improved. After being pre-dewatered to a moisture content of 80% to 85% by a belt filter press, it is sent to a belt low-temperature dryer to avoid the problem of excessive energy consumption caused by direct drying of sludge with high moisture content.

[0015] Preferably, the entire process forms a complete closed-loop energy system encompassing flue gas waste heat, turbine steam extraction, heat pump upgrades, drying heat utilization, exhaust gas waste heat recovery, and co-firing for power generation. The drying process does not require additional consumption of high-quality steam or electricity. The energy consumption per unit mass of sludge drying is reduced by more than 60% compared to traditional steam drying processes, and the overall thermal efficiency of the power plant is improved by more than 3%. Simultaneously, it achieves the dual benefits of harmless, reduced, and resource-based sludge disposal and improved energy utilization efficiency of the power plant, making it suitable for the sludge co-processing needs of various coal-fired power plants.

[0016] The present invention has the following beneficial effects: This invention upgrades low-grade heat energy by combining boiler flue gas waste heat with turbine steam extraction to drive an absorption heat pump. Simultaneously, it recovers the latent heat of vaporization of the dried exhaust gas as a low-temperature heat source input for the heat pump, constructing a closed-loop energy system for the entire process. This effectively solves the defects of existing technologies, such as high energy consumption due to the consumption of high-quality steam in traditional steam drying and low waste heat recovery rate in direct flue gas waste heat utilization processes. It reduces the unit energy consumption of sludge drying, improves the utilization rate and overall thermal efficiency of low-grade waste heat in power plants, and avoids energy waste caused by the direct discharge of low-grade waste heat.

[0017] This invention effectively solves the shortcomings of existing technologies by combining a fully enclosed micro-negative pressure air circulation drying system with a dynamic energy dispatch module, which is easily affected by fluctuations in power plant operating conditions, resulting in unstable moisture content of dried products, odor overflow from open systems, and the need for separate treatment facilities for exhaust gas and wastewater. The entire drying process is free of open flame contact and odor gas overflow. The moisture content of the dried products is stable and meets the standards. Condensate wastewater and co-firing flue gas can be directly connected to the power plant's existing waste treatment system without the need for additional dedicated treatment facilities, thus reducing the overall operating cost and environmental control pressure.

[0018] The technical solution of this invention does not require structural modifications to the existing boilers, turbines, and main processes for treating waste in power plants. After drying, the sludge is directly mixed with coal for combustion, and all combustion ash is utilized as a resource. It is suitable for the sludge co-processing function of newly built coal-fired power plants, and can also be applied to the low-cost technical transformation of existing coal-fired power plants. The applicable scenarios cover coal-fired units of various capacity levels, and it has extremely high industry promotion value. It can simultaneously achieve the dual benefits of harmless, reduced, and resource-based sludge treatment and improved power plant energy efficiency. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for low-energy drying and thermal energy optimization of sludge based on waste heat from power plants, as proposed in this invention. Figure 2 This is a bar chart showing the grouping of different core performance indicators of the processes proposed in this invention; Figure 3 This is a line graph showing the fluctuation of moisture content in dried sludge under all operating conditions, as proposed in this invention. Figure 4 This is a stacked bar chart showing the energy composition of the entire process proposed in this invention. Detailed Implementation

[0020] The following will refer to the appendices in the embodiments of the present invention. Figure 1-4 The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1

[0022] The specific steps in this embodiment are as follows: S1. Heat Source Preparation: Boiler flue gas at an outlet temperature of 120℃ from the air preheater of the power plant and steam extraction from the low-pressure cylinder of the turbine at a gauge pressure of 0.2MPa are collected as energy sources. The boiler flue gas first undergoes dust removal treatment to remove particulate matter. After dust removal, it is introduced into the finned tube heat exchange unit to preheat the lithium bromide circulating working fluid of the absorption heat pump. The extraction and utilization rate of waste heat from the boiler flue gas is 92%. Then, steam extraction from the turbine is introduced to supplement energy to drive the heat pump to do work. The high-temperature heat source with an output temperature of 60℃ is delivered to the sludge drying unit. After heat exchange, the flue gas with a temperature of 78℃ is directly sent to the power plant's original flue gas desulfurization system for treatment, without affecting the normal power generation efficiency of the turbine.

[0023] S2. Sludge Drying: Municipal sludge first undergoes conditioning in a unit with 2% (by weight) of power plant fly ash added as a conditioner. It then undergoes pre-dehydration in a belt filter press to a moisture content of 80% before being fed into a low-temperature belt dryer. The belt dryer operates at a speed of 0.1 m / min, maintaining the internal temperature of the drying chamber at 60°C with no open flame contact throughout the process. The closed-loop air circulation system provides a circulating air volume of 100 m³ / m³ for each cubic meter of drying chamber volume. 3 / h, the circulating air entering the drying chamber first passes through a multi-stage filtration unit to remove particulate impurities, and the relative humidity is controlled at 8%. After the circulating air exchanges heat with the high-level heat source and is heated, it comes into contact with the wet sludge to realize the evaporation and migration of water in the sludge.

[0024] S3. Exhaust Gas Treatment and Waste Heat Recovery: The high-humidity exhaust gas generated during drying undergoes counter-current heat exchange preheating with the low-humidity air returning to the drying chamber. The high-humidity exhaust gas then enters a surface-type condenser unit for condensation and dehumidification within a 10°C temperature range. All latent heat of vaporization released during the condensation process is recovered and transferred to the low-temperature heat source input of the absorption heat pump. The recovered latent heat accounts for 32% of the total energy input to the heat pump. The absolute moisture content of the dehumidified low-humidity air is 12 g / m³. 3 The wastewater generated by condensation has a suspended solids content of 220 mg / L and is directly transported to the power plant's wastewater treatment system for treatment before being discharged.

[0025] The fully enclosed air circulation system's piping and drying chamber adopt an airtight sealing structure. The system maintains a slight negative pressure of 50Pa through the linkage control of the circulating fan and the exhaust regulating valve. The amount of external air infiltration is controlled at 0.7% of the system's circulating air volume. Odor gases such as hydrogen sulfide and ammonia generated during the drying process do not overflow, and there is no need to install an independent end-of-pipe deodorization treatment device.

[0026] S4. Resource Utilization: The moisture content of the dried sludge is 40%. The dried sludge is first crushed to a particle size of 18mm by a crushing unit, and then transported to the raw coal bunker of the power plant boiler. It is then uniformly mixed with coal and fed into the furnace for combustion. The mass ratio of dried sludge to coal is 5%, and the calorific value of the mixed fuel meets the requirements for stable combustion of the power plant boiler. The ash produced by combustion in the furnace is utilized as a raw material for building materials production. This embodiment is equipped with an independent energy scheduling module, which collects various operating parameters in real time, such as boiler flue gas temperature, turbine extraction load, sludge treatment volume, drying chamber temperature, and exhaust gas moisture content. Based on a preset energy optimization algorithm, it dynamically adjusts the input energy of the absorption heat pump, the circulating air volume, and the mesh belt operating speed.

[0027] Example 2

[0028] The specific steps in this embodiment are as follows: S1. Heat Source Preparation: Boiler flue gas with an outlet temperature of 150℃ from the air preheater of the power plant and steam extraction from the low-pressure cylinder of the steam turbine with a gauge pressure of 0.35MPa are collected as energy sources. The boiler flue gas first undergoes dust removal treatment to remove particulate matter. After dust removal, it is introduced into the finned tube heat exchange unit to preheat the lithium bromide circulating working fluid of the absorption heat pump. The extraction and utilization rate of waste heat from the boiler flue gas is 94%. Then, steam extraction from the steam turbine is introduced to supplement energy to drive the heat pump to do work. The high-temperature heat source with an output temperature of 70℃ is delivered to the sludge drying unit. After heat exchange, the flue gas with a temperature of 72℃ is directly sent to the power plant's original flue gas desulfurization system for treatment.

[0029] S2. Sludge Drying: Municipal sludge first undergoes conditioning in a unit with 3.5% (by weight) of power plant fly ash added as a conditioner. It then undergoes pre-dehydration in a belt filter press to a moisture content of 82.5% before being fed into a low-temperature belt dryer. The belt speed of the low-temperature belt dryer is 0.3 m / min, the internal temperature of the drying chamber is maintained at 67.5℃, and there is no open flame contact throughout the process. The closed-loop air circulation system provides a circulating air volume of 125 m³ per cubic meter of the drying chamber volume. 3 / h, the circulating air entering the drying chamber first passes through a multi-stage filtration unit to remove particulate impurities, and the relative humidity is controlled at 7%. After the circulating air exchanges heat with the high-level heat source and is heated, it comes into contact with the wet sludge to realize the evaporation and migration of water in the sludge.

[0030] S3. Exhaust Gas Treatment and Waste Heat Recovery: The high-humidity exhaust gas generated during drying undergoes counter-current heat exchange preheating with the low-humidity air returning to the drying chamber. The high-humidity exhaust gas then enters a surface condensation unit for condensation and dehumidification within a temperature range of 12.5℃. All latent heat of vaporization released during the condensation process is recovered and transferred to the low-temperature heat source input of the absorption heat pump. The recovered latent heat accounts for 37% of the total energy input to the heat pump. The absolute moisture content of the dehumidified low-humidity air is 10 g / m³. 3 The wastewater is recycled back to the drying unit. The suspended solids content of the condensed wastewater is 180 mg / L, and it is directly transported to the power plant's wastewater treatment system for treatment before being discharged. The piping and drying chamber of the fully enclosed air circulation system adopt an airtight sealing structure. The system maintains a slight negative pressure of 75 Pa through the linkage between the circulating fan and the exhaust regulating valve, and the amount of external air infiltration is controlled to be 0.5% of the system's circulating air volume.

[0031] S4. Resource Utilization: The moisture content of the dried sludge is 35%. The dried sludge is first crushed to a particle size of 15mm by a crushing unit, and then transported to the raw coal bunker of the power plant boiler. It is then uniformly mixed with coal and sent into the furnace for combustion. The mass ratio of dried sludge to coal is 10%. The ash produced by combustion in the furnace is utilized as a raw material for building materials production. This embodiment is equipped with an independent energy scheduling module.

[0032] Example 3

[0033] The specific steps in this embodiment are as follows: S1. Heat Source Preparation: Boiler flue gas at an outlet temperature of 180℃ from the air preheater of the power plant and steam extraction from the low-pressure cylinder of the turbine at a gauge pressure of 0.5MPa are collected as energy sources. The boiler flue gas first undergoes dust removal treatment to remove particulate matter. After dust removal, it is introduced into the finned tube heat exchange unit to preheat the lithium bromide circulating working fluid of the absorption heat pump. The extraction and utilization rate of waste heat from the boiler flue gas is 95%. Then, steam extraction from the turbine is introduced to supplement energy to drive the heat pump to do work. The high-temperature heat source with an output temperature of 80℃ is delivered to the sludge drying unit. After heat exchange, the flue gas with a temperature of 68℃ is directly sent to the power plant's original flue gas desulfurization system for treatment.

[0034] S2. Sludge Drying: Municipal sludge first undergoes conditioning in a unit with 5% (by weight) of power plant fly ash added as a conditioner. It then undergoes pre-dehydration in a belt filter press to a moisture content of 85% before being fed into a low-temperature belt dryer. The belt dryer operates at a speed of 0.5 m / min, maintaining the internal temperature of the drying chamber at 75°C with no open flame contact throughout the process. The closed-loop air circulation system provides a circulating air volume of 150 m³ per cubic meter of the drying chamber volume. 3 / h, the circulating air entering the drying chamber first passes through a multi-stage filtration unit to remove particulate impurities, and the relative humidity is controlled at 6%. After the circulating air exchanges heat with the high-level heat source and is heated, it comes into contact with the wet sludge to realize the evaporation and migration of water in the sludge.

[0035] S3. Exhaust Gas Treatment and Waste Heat Recovery: The high-humidity exhaust gas generated during drying undergoes counter-current heat exchange preheating with the low-humidity air returning to the drying chamber. The high-humidity exhaust gas then enters a surface condensation unit for condensation and dehumidification within a 15°C temperature range. All latent heat of vaporization released during condensation is recovered and transferred to the low-temperature heat source input of the absorption heat pump. The recovered latent heat accounts for 41% of the total energy input to the heat pump. The absolute moisture content of the dehumidified low-humidity air is 8 g / m³. 3 The wastewater is recycled back to the drying unit. The suspended solids content of the condensed wastewater is 150 mg / L, and it is directly transported to the power plant's wastewater treatment system for treatment before being discharged. The piping and drying chamber of the fully enclosed air circulation system adopt an airtight sealing structure. The system maintains a slight negative pressure of 100 Pa through the linkage control of the circulating fan and the exhaust regulating valve, eliminating the need for a separate end-of-pipe deodorization treatment device.

[0036] S4. Resource Utilization: The moisture content of the dried sludge is 30%. The dried sludge is first crushed to a particle size of 12mm by a crushing unit, and then transported to the raw coal bunker of the power plant boiler. It is then uniformly mixed with coal and fed into the furnace for combustion. The mass ratio of dried sludge to coal is 15%, and the calorific value of the mixed fuel meets the requirements for stable combustion of the power plant boiler. The ash produced during furnace combustion is utilized as a raw material for building material production. This embodiment is equipped with an independent energy dispatching module.

[0037] Comparative Example This comparative example adopts the existing mainstream direct boiler flue gas drying process, and the specific steps are as follows: S1. Heat source preparation: Boiler flue gas at an outlet temperature of 150℃ from the power plant's air preheater is directly collected as the drying heat source. There is no heat pump heat energy upgrade unit, and the heat source is directly sent to the drying unit.

[0038] S2. Sludge Drying: Municipal sludge undergoes the same conditioning and pre-dewatering treatment as in Example 2, i.e., 3.5% of the wet sludge mass of power plant fly ash is added as a conditioning agent, and then it is pre-dewatered to a moisture content of 82.5% by belt filter press before being sent to the dryer. The temperature of the drying chamber fluctuates with the boiler flue gas load. An open air circulation system is used, without a circulating filter unit. There is a risk of open flame contact during the drying process.

[0039] S3. Exhaust gas treatment and waste heat recovery: The high-humidity exhaust gas generated during drying has no latent heat recovery unit. It is directly discharged after being treated by a separately set activated carbon deodorization device. The condensate wastewater is transported to a separately set sludge drying wastewater treatment system for treatment. There is no circulating air loop.

[0040] S4. Resource utilization: The moisture content of the dried sludge fluctuates with the working conditions. After crushing, it is mixed with coal at a ratio of 10% and sent into the furnace for combustion. The ash is disposed of as conventional solid waste.

[0041] Table 1 Comparison of Core Process Parameters

[0042] This table clearly presents the process parameters for the three sets of embodiments. The parameter settings meet the requirements of actual power plant operation conditions. The comparative example and Example 2 use the same sludge pretreatment conditions to ensure the comparability of the drying and exhaust gas treatment processes. The temperature fluctuation range of the drying chamber in the comparative example is 45℃–90℃, which is due to the absence of a heat pump pressure stabilization system and direct impact from boiler load fluctuations (boiler exhaust temperature fluctuation range 120℃–180℃).

[0043] Table 2 Comparison of Core Performance Indicators

[0044] The data in this table shows that the boiler flue gas waste heat utilization rate of the three examples is higher than that of the comparative example, the unit drying energy consumption is reduced by more than 60% compared with the comparative example, the moisture content fluctuation rate of the dried sludge is less than 1%, far lower than the 12.3% of the comparative example, and the cost of treating the three wastes is only about 15% of that of the comparative example. At the same time, it can achieve a stable improvement in the thermal efficiency of the power plant. There is no odor overflow in any of the examples. The calculation scope of unit drying energy consumption includes: the energy input of the drying heat source, which is the equivalent heat corresponding to the steam consumed by the heat pump in the examples, and the heat directly consumed by the boiler flue gas, the power consumption of the circulating fan and mesh belt drive, and the auxiliary power consumption of the tail gas condensation unit in the comparative example. The unit cost of treating the three wastes includes: the cost of condensate wastewater treatment, which is the operating cost of the independent sewage treatment facility in the comparative example; the cost of waste gas treatment; and the cost of waste residue disposal. The above calculation method is uniformly applicable to Examples 1-3 and the comparative example.

[0045] Energy balance calculation verification example The following is a non-limiting example of energy balance calculation verification, which is only used to illustrate the technical feasibility of the present invention and does not constitute any limitation on the scope of protection of the technical claims.

[0046] Taking a 300MW coal-fired power generation unit as an example, the method of this invention extracts a portion of the flue gas from the air preheater outlet and combines it with the steam extracted from the turbine to drive an absorption heat pump. Thermodynamic calculations show that the sum of the waste heat from the flue gas and the heat released by the extracted steam is sufficient to drive the heat pump to generate a high-temperature heat source of 60℃~80℃, meeting the heat demand of a sludge drying system of the corresponding scale. The flue gas mainly plays a preheating role, while the extracted steam serves as a supplementary driving force; the two work together to ensure stable operation of the system under all operating conditions. The latent heat of vaporization recovered from the condensation of the drying tail gas further supplements the low-temperature heat source, forming an energy closed loop. Calculation verification shows that the technical solution of this invention is thermodynamically feasible. For units of different capacities and different sludge treatment scales, those skilled in the art can adjust the proportions using the same method, all of which fall within the protection scope of this invention.

[0047] refer to Figure 2 This figure visually illustrates the performance differences between the present invention and existing direct flue gas drying processes. The boiler flue gas waste heat utilization rate in all three embodiments exceeds 90%, more than double that of the comparative example, improving the efficiency of low-grade waste heat utilization and solving the problem of low waste heat utilization rate in existing technologies. The moisture content fluctuation rate of the dried sludge is less than 1%, far lower than the 12.3% of the comparative example, resulting in stable dried product quality. The unit cost of waste treatment is only 12%-17% of that of the comparative example, eliminating the need for additional dedicated waste treatment facilities and solving the problem of high waste treatment costs in existing technologies, thus verifying the comprehensive performance advantages of this technical solution.

[0048] refer to Figure 3 This figure visually demonstrates the adaptability of this invention to various operating conditions. In the three embodiments, the moisture content of the dried sludge remained consistently within the preset target range across the entire operating range of the power plant, from 60% low load to 110% high load, with fluctuations not exceeding 0.3% and no significant deviation. This is thanks to the dynamic adjustment function of the energy dispatch module, which can match the heat source input and drying requirements in real time, unaffected by fluctuations in the power plant's operating conditions. In contrast, the moisture content of the dried sludge in the comparative embodiment fluctuated with changes in power plant load, ranging from a minimum of 26% to a maximum of 48%, far exceeding the acceptable range of 30%-40%, failing to guarantee the quality of the dried product. This verifies that this invention solves the defect of poor adaptability in existing technologies and can adapt to the sludge co-processing needs of coal-fired power plants with different capacities and operating modes.

[0049] refer to Figure 4This figure visually illustrates the construction effect of the energy closed loop of the present invention. In the three embodiments, the latent heat of vaporization of the exhaust gas recovered accounts for 32% to 41% of the total input energy, all of which is reused as the low-temperature heat source input of the absorption heat pump, avoiding energy waste caused by the direct emission of latent heat. At the same time, the energy loss of the entire process accounts for only 2%, far lower than the 22% of the comparative example. All energy sources are the low-grade waste heat originally discarded by the power plant and the surplus extracted steam that does not affect the power generation efficiency. There is no need to consume high-quality new steam or purchase external energy, which effectively solves the problems of high energy consumption and low waste heat recovery rate of existing technologies, demonstrating the energy-saving advantages of the energy closed loop system.

[0050] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for low-energy drying and optimized utilization of sludge based on waste heat from power plants, characterized in that, Includes the following steps: S1. Heat source preparation: Collect waste heat from the flue gas of the power plant boiler and steam extracted from the steam turbine as an energy source to drive the operation of the absorption heat pump, upgrade the low-grade heat energy into a high-grade heat source with a temperature that matches the low-temperature drying requirements of sludge, and deliver it to the sludge drying unit. S2. Sludge drying: A belt-type low-temperature dryer is used to continuously dry municipal sludge. The drying process uses a fully enclosed air circulation system. The circulating air exchanges heat with a high-temperature heat source and then comes into contact with the wet sludge, causing the water in the sludge to evaporate and migrate. S3, exhaust gas treatment and waste heat recovery: pre-exchange heat with the low-humidity air returning to the drying unit, enter the condensation unit for dehumidification, and recover all the latent heat of vaporization released during the condensation process and send it to the low-temperature heat source input end of the absorption heat pump. The dehumidified low-humidity air is returned to the drying unit for recycling. S4. Resource utilization: After drying, the sludge with the required moisture content is directly transported to the raw coal bunker of the power plant boiler. It is then mixed evenly with the coal and sent into the furnace for combustion. The ash produced during combustion is collected and used as raw material for building materials production for resource utilization.

2. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, The boiler flue gas collected in step S1 is flue gas with an outlet temperature of 120℃~180℃ from the power plant's air preheater. This flue gas first undergoes dust removal treatment without changing the original boiler flue gas flow path. The steam extraction from the turbine is steam extraction with a gauge pressure of 0.2MPa~0.5MPa in the turbine's low-pressure cylinder. After the boiler flue gas is dusted, it is first introduced into a finned tube heat exchange unit as part of the low-temperature heat source, and then introduced into the turbine extraction to supplement energy to drive the heat pump to do work, outputting a high-temperature heat source with an output temperature of 60℃~80℃. After heat exchange, the flue gas with a temperature below 80℃ is directly sent to the power plant's original flue gas desulfurization system for treatment.

3. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1 or 2, characterized in that, In step S2, the wet sludge entering the belt low-temperature dryer has a moisture content of 80%~85%, the belt speed of the belt low-temperature dryer is 0.1m / min~0.5m / min, the internal temperature of the drying chamber is maintained at 60℃~75℃ and there is no open flame contact throughout the process. The circulating air volume of the closed air circulation system is 100 m³ / m³ for the drying chamber volume. 3 / h~150m 3 / h, the circulating air entering the drying chamber first passes through a multi-stage filtration unit to remove particulate impurities, and the relative humidity is controlled below 10%.

4. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, In step S3, the high-humidity exhaust gas first enters the gas-to-gas heat exchange unit, where it undergoes counter-current heat exchange and preheating with the low-humidity air returning to the drying chamber. After heat exchange, the high-humidity exhaust gas then enters the surface condensation unit, where it is condensed and dehumidified within a temperature range of 10°C to 15°C. The absolute moisture content of the dehumidified low-humidity air is then returned to the drying unit for recycling. The suspended solids in the condensed wastewater are directly transported to the power plant's wastewater treatment system for treatment before being discharged.

5. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, In step S4, the moisture content of the dried sludge is 30% to 40%. The dried sludge is first crushed by a crushing unit to a particle size of no more than 20 mm, and then transported to the raw coal bunker of the power plant boiler to be evenly mixed with the coal. The mass ratio of dried sludge to coal is 5% to 15%.

6. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, It is also equipped with an independent energy scheduling module to collect various operating parameters in real time, such as boiler flue gas temperature, turbine extraction steam load, sludge treatment volume, drying chamber temperature, and exhaust gas moisture content.

7. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, The fully enclosed air circulation system's pipelines and drying chamber adopt an airtight sealing structure. The system maintains a slight negative pressure of 50Pa~100Pa through the linkage control of the circulating fan and the exhaust regulating valve. The odorous gases generated during the drying process are sealed within the circulation path under the action of slight negative pressure, eliminating the need for a separate end-of-pipe deodorization treatment device.

8. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, Before step S2, a sludge pretreatment process is set up. The municipal sludge first goes through a conditioning unit with added conditioning agent. The conditioning agent uses fly ash from power plants as raw material. The addition mass ratio of the conditioning agent is 2% to 5% of the mass of the wet sludge. No other chemical agents need to be added. After being pre-dehydrated to a moisture content of 80% to 85% by a belt filter press, it is sent to a belt low-temperature dryer.

9. The method for low-energy drying and optimized utilization of sludge based on waste heat from power plants according to claim 1, characterized in that, The entire process forms a complete closed-loop energy system, including flue gas waste heat, steam turbine extraction, heat pump upgrade, drying heat utilization, exhaust gas waste heat recovery, and co-firing power generation. The drying process does not require additional consumption of high-quality steam or electricity.

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