A device-coupled sludge drying process

Abstract

The application discloses to the technical field of water treatment, and particularly relates to a sludge drying process coupled with equipment, S1, under the condition that no lime and no ferric chloride are added and only PFS and PAM flocculants are added, sludge is dewatered to a water content of 65%-70% by an integrated high-pressure belt dewaterer; S2, the dewatered sludge is sent into a disc dryer, saturated steam at 0.4-0.6 MPa is indirectly heated by the hollow disc piece to dry the sludge, and the gas-carrying ratio R of the disc dryer is controlled to be not greater than 1 kg of dry air per kg of evaporated water. By controlling the gas-carrying ratio at a low level, the adiabatic saturation temperature of the exhaust gas and the outlet temperature of the circulating water of the primary condenser are lifted, the evaporation temperature of the high-temperature heat pump is increased, the heating performance coefficient of the high-temperature heat pump is obviously improved, and the power consumption of the high-temperature heat pump is reduced.

Description

Technical Field

[0001] This application relates to the field of water treatment technology, specifically to a device-coupled sludge drying process. Background Technology

[0002] With the rapid advancement of urbanization in my country, the amount of municipal sludge generated by wastewater treatment plants is increasing year by year. Sludge reduction has become one of the core issues that urgently needs to be addressed in the operation and management of urban wastewater treatment plants. Currently, sludge reduction technologies are mainly divided into two categories: the first is mechanical dewatering, including plate and frame filter presses, centrifugal dewatering, and belt dewatering, but this type of technology can only reduce the sludge moisture content to between 60% and 80%; the second is thermal drying dewatering, including disc drying (indirect thermal drying) and low-temperature thermal drying (direct thermal drying), which can reduce the sludge moisture content to below 40%. Based on 1 ton of sludge with 80% moisture content, a 50% reduction (0.5 tons) is achieved at 60% moisture content, and a 67% reduction (0.33 tons) is achieved at 40% moisture content. This demonstrates that thermal drying technology significantly outperforms simple mechanical dewatering in terms of sludge reduction.

[0003] However, existing sludge thermal drying technologies generally have the following shortcomings: (1) Due to the limited heat source of sewage treatment plants (most sewage treatment plants only have electricity supply), existing indirect thermal drying equipment, represented by disc dryers, usually needs to be built in conjunction with waste incineration plants, thermal power plants, cement plants and other plants with centralized steam heat sources. Therefore, a large amount of concentrated sludge still needs to be transferred to the above-mentioned centralized treatment plants, resulting in high transportation energy consumption and carbon emissions. Although there are some independently constructed indirect thermal drying cases, they mostly use electricity or natural gas as energy to produce steam, and the steam production cost is extremely high. When using natural gas as energy, the cost of steam per ton is about RMB 262; when using electricity as energy, the cost of steam per ton is about RMB 468; both are significantly higher than the market steam price of RMB 130-180 / ton, making it impossible to promote on a large scale.

[0004] (2) Although direct thermal drying technology, represented by low temperature thermal drying, can be built in sewage treatment plants, it suffers from problems such as poor system operation stability, short equipment life and rapid capacity decay due to the characteristics of sludge and the inherent technical defects of low temperature thermal drying. It is difficult to meet the needs of long-term stable operation of large sewage treatment plants.

[0005] (3) The exhaust gas of the dryer in the existing indirect thermal drying system carries a large amount of latent heat and sensible heat of water vapor. Currently, the circulating cooling water provided by the cooling tower is generally used to condense the exhaust gas and then discharge it into the atmosphere, which not only causes serious waste of energy, but also wastes water resources. It is urgent to recycle and reuse it.

[0006] Chinese patent document CN101774743A discloses a latent heat recovery multiphase change sludge drying method and equipment, which uses low-temperature steam as the drying heat source and utilizes a latent heat recovery heat pump to recover the latent heat of the exhaust gas. However, its drying temperature is relatively low (100℃-150℃), and it does not involve mechanical pre-dehydration coupling process and water source heat pump supplementary heating technology. Chinese patent document CN105819646A discloses a system and method for recovering heat from the exhaust gas of dried sludge, mainly focusing on the exhaust gas heat recovery device itself, without involving the coupling process of a high-temperature steam heat pump and a disc dryer. Chinese patent document CN107585996A discloses a highly efficient, energy-saving, and environmentally friendly sludge drying system and method, which utilizes a lithium bromide absorption heat pump to recover the latent heat of vaporization in the sludge drying exhaust gas and transfer the heat to boiler feedwater or heating network return water. This technology uses an absorption heat pump (which requires low-pressure saturated steam or heat transfer oil to drive it), rather than a compression high-temperature heat pump, and does not involve a closed-loop cycle of steam condensate recovery and re-evaporation into dried steam.

[0007] Chinese patent document CN120841810A discloses a sludge drying system and process based on high-temperature heat pump steam technology. It employs a high-temperature heat pump to absorb heat from waste gas at temperatures above 100°C, heating 100°C condensate to generate high-temperature steam exceeding 150°C, thus achieving a closed-loop steam heat source. While this patent shares similarities with the present invention in its high-temperature heat pump steam generation principle, it lacks the high-pressure belt dehydrator pre-dehydration coupling process, multi-heat source synergy technology of water source heat pump, and dual-stage exhaust gas cooling system. This results in a higher moisture content (around 80%) in the sludge entering the dryer, leading to a higher load on the dryer and higher overall energy consumption.

[0008] In summary, there is currently a lack of a technical solution in sludge drying technology that can simultaneously meet the following requirements: independent construction and operation within a wastewater treatment plant, no reliance on external steam heat sources, efficient closed-loop conversion of waste heat from drying exhaust gas to steam heat sources, and the ability to couple pre-dehydration at the front end and coordinate regulation with multiple heat sources. Summary of the Invention

[0009] The purpose of this application is to provide an equipment-coupled sludge drying process to solve the problems mentioned in the background art.

[0010] To achieve the above objectives, this application provides the following technical solution: a device-coupled sludge drying process, comprising the following steps: S1. The sludge is dewatered to a moisture content of 65%-70% using an integrated high-pressure belt dewatering machine under the condition of adding organic flocculant; S2. The dewatered sludge is fed into a disc dryer and dried indirectly by saturated steam at 0.4-0.6 MPa through hollow discs. The carrier gas ratio R of the disc dryer is controlled to be no greater than 1 kg dry air / kg evaporated water, so that the adiabatic saturation temperature Tas of the exhaust gas is no less than 87℃. After drying, the sludge moisture content is reduced to below 40%. The condensate formed after the steam releases heat is returned through the drainage system. S3. The exhaust gas discharged from the disc dryer enters the first-stage exhaust gas condenser after dust removal. S4. In the tail gas primary condenser, the circulating water and tail gas directly contact each other in countercurrent flow for heat exchange. Some of the water vapor in the tail gas condenses and transfers heat to the circulating water. The flow rate of the circulating water is selected based on the ratio of the recovered heat Q1 of the tail gas primary condenser to the heat Qevap required by the high-temperature steam heat pump evaporator, Q1 / Qevap, which is 0.85-1.00, and the temperature rise ΔT of the circulating water is 8-12K. This ensures that the outlet temperature T1 of the circulating water reaches 80-90℃ and the difference Δt between the outlet temperature of the circulating water and the adiabatic saturation temperature Tas of the tail gas is 3-8K. The heated circulating water is sent to the evaporator of the high-temperature steam heat pump, where the refrigerant absorbs its heat and heats and vaporizes the condensate recovered by the condensate drainage system into 0.4-0.6MPa saturated steam. After being distributed by the steam distributor, the steam returns to the disc dryer, forming a closed-loop cycle of the steam heat source. S5. The exhaust gas after primary condensation enters the secondary exhaust gas condenser and is cooled down by indirect heat exchange with low-temperature cold water. S6. The exhaust gas is drawn out by the exhaust gas induced draft fan to maintain the negative pressure operation of the system. After cooling, the exhaust gas is deodorized and then discharged in compliance with the standards. S7. The overflow and forced sewage discharge of the first-stage tail gas condenser, as well as the condensate of the second-stage tail gas condenser, are sent to the wastewater cooler and then sent to the sewage treatment plant for treatment. S8. Wastewater from the wastewater treatment plant is first sent to the secondary condenser of the exhaust gas, where it exchanges heat with the exhaust gas after primary condensation and absorbs the residual heat from the low-temperature section to raise its temperature. The heated wastewater then enters the water source heat pump as its low-grade heat source. The water source heat pump absorbs the residual heat from the wastewater treatment plant to generate 70-80°C high-temperature hot water, which serves as the low-grade heat source and supplementary and regulating heat source for the high-temperature steam heat pump. During system cold start, this high-temperature hot water enters the circulating water loop of the primary condenser of the exhaust gas, serving as the sole low-grade heat source for the high-temperature steam heat pump. The low-temperature chilled water discharged from the water source heat pump is used as a cooling medium for the wastewater cooler, achieving cascade utilization of cooling capacity before discharge.

[0011] Furthermore, the carrier gas ratio R is 0.5-1.0 kg dry air / kg evaporated water, and the exhaust gas adiabatic saturation temperature Tas is 87-93℃; the carrier gas ratio is controlled by adjusting the air volume of the exhaust gas induced draft fan and the opening of the carrier gas replenishment valve. The air volume of the exhaust gas induced draft fan is selected according to the exhaust gas volume corresponding to the upper limit of the carrier gas ratio, with a margin of no more than 1.25 times.

[0012] Furthermore, the flow rate G of the circulating water is determined according to G=η·Qevap / (cp·ΔT), where cp is the specific heat capacity of water at constant pressure, η is the set value of heat recovery rate, which is 0.85-1.00, and ΔT is 8-12K.

[0013] Furthermore, the circulating water outlet temperature is 80-90℃, the circulating water temperature rise ΔT is 9-10K, and the Q1 / Qevap is 0.90-0.98.

[0014] Furthermore, the pressure of the saturated steam is 0.4-0.6 MPa; the condensing temperature of the high-temperature steam heat pump is determined by the required saturated steam pressure, and its evaporation temperature Te is taken as the circulating water temperature of the first-stage condenser of the exhaust gas minus 4K, and the heat transfer end difference of the first-stage condenser of the exhaust gas is taken as 3-8K; the condensate formed by the steam release of the disc dryer is completely returned to the high-temperature steam heat pump through the condensate drainage system, and the condensate flash steam is recovered through the recovery branch. The heating performance coefficient (COP) of the high-temperature steam heat pump is not less than 2.0, and the heating COP of the water source heat pump is not less than 3.5.

[0015] Furthermore, in step S1, no lime or ferric chloride is added, only PFS and PAM flocculants are added, and the amount added is such that the increase in oven-dry solids does not exceed 3%, in order to inhibit the migration of calcium ions and chloride ions with water and their enrichment in the closed circulation of condensate.

[0016] Furthermore, in step S8, the greywater absorbs waste heat from the low-temperature section in the tail gas secondary condenser and is heated to 25-32°C. It then enters the water source heat pump evaporator to extract heat and is cooled to 10-15°C before being discharged. Compared with the conventional sequence where greywater is first cooled by the water source heat pump and then used for condensation, this arrangement increases the evaporation temperature of the water source heat pump by 5-10K and the heating COP by 10%-15%.

[0017] Furthermore, during system cold start, a 70-80°C high-temperature hot water source generated by a water source heat pump enters the circulating water loop of the tail gas primary condenser, serving as the sole low-grade heat source for the high-temperature steam heat pump. The high-temperature steam heat pump generates initial steam for the disc dryer. The disc dryer starts with a partial feed load, gradually increasing to full load as the circulating water temperature rises. At a daily processing capacity of 2.5 tons (80% moisture content wet sludge), the partial feed load is 60%-65%, and it takes 90-120 minutes to cold start to full load.

[0018] A coupled sludge drying system includes an integrated high-pressure belt dewatering machine, a disc dryer, a cyclone dust collector, a primary tail gas condenser, a secondary tail gas condenser, a tail gas induced draft fan, a high-temperature steam heat pump, a steam distribution cylinder, a condensate system, a water source heat pump, a softened water system, a wastewater cooler, a carrier gas replenishment valve, a circulating water pump, and a detection and control unit. The discharge port of the integrated high-pressure belt dewatering machine is connected to the inlet of the disc dryer. The exhaust outlet of the disc dryer is sequentially connected to a cyclone dust collector, a primary exhaust condenser, a secondary exhaust condenser, and an exhaust fan. The steam inlet of the disc dryer is connected to the steam outlet of a high-temperature steam heat pump via a steam distributor. The condensate outlet of the disc dryer is connected to the inlet of the high-temperature steam heat pump via a drainage system, forming a closed-loop steam circulation. A circulating water circuit is provided between the primary exhaust condenser and the high-temperature steam heat pump. The hot water outlet of the water source heat pump is connected to the circulating water circuit of the primary exhaust condenser. The cold water outlet of the water source heat pump is connected to the secondary exhaust condenser and the wastewater cooler, respectively. The outlet of the softened water system is connected to the high-temperature steam heat pump. The drain outlet of the primary exhaust condenser and the condensate outlet of the secondary exhaust condenser are both connected to the wastewater cooler. The detection and control unit includes a temperature transmitter T1 installed on the circulating water outlet pipe of the exhaust gas primary condenser, a pressure transmitter P installed on the steam distribution cylinder, a temperature transmitter Tw installed on the water inlet pipe of the water source heat pump, a humidity or dew point transmitter installed on the exhaust gas outlet pipe of the disc dryer, and a programmable controller; the programmable controller is electrically connected to the exhaust gas induced draft fan, the carrier gas replenishment valve, the circulating water pump, the high-temperature steam heat pump, and the water source heat pump, respectively.

[0019] Compared with the prior art, the beneficial effects of this application are: 1) This invention raises the adiabatic saturation temperature of the exhaust gas and the outlet temperature of the circulating water in the first-stage condenser by controlling the carrier gas ratio at a low level, thereby increasing the evaporation temperature of the high-temperature steam heat pump, significantly improving its heating performance coefficient and reducing its own power consumption. Experiments show that when the carrier gas ratio is reduced from 3.88 to 0.70, the heat pump evaporation temperature increases by 17.8K, the COP increases from 1.95 to 2.38, the total power consumption of the system decreases from 20.62kW to 16.77kW (a reduction of 18.7%), and the power consumption per ton of mud decreases from 189kW·h / t to 158kW·h / t (a reduction of 16.4%). Simultaneously, the circulation is rationally matched according to the heat recovery rate. The water flow rate is optimized, and a reverse arrangement is adopted where greywater first absorbs heat through a two-stage condenser before entering the water source heat pump. This fully recovers the waste heat from the low-temperature section discarded in the conventional process, increasing the evaporation temperature of the water source heat pump by 5-10K and the heating COP by 10%-15%. The entire system can be started from zero using only electricity, without any external preheating steam source. During normal operation, the external heat supplement accounts for 8%-15% of the heat load of the high-temperature heat pump evaporator. Under the combined effect, the electricity consumption per ton of mud is about 158 ​​kWh and the electricity cost per ton of mud is about 103 yuan, which saves about 53% compared to the electric heating steam preparation scheme (about 218 yuan per ton of mud). Furthermore, there is no heat pollution or water consumption due to the elimination of the cooling tower.

[0020] 2) This invention reduces the moisture content of sludge entering the dryer to 65%-70% through a pre-high-pressure belt dewatering process, which reduces the amount of water that needs to be evaporated by about half. The specifications and investment of the dryer and its supporting equipment are reduced accordingly. The amount and volume of sludge after treatment are greatly reduced, and the amount of sludge leaving the plant is reduced from 1 ton to about one-third, which significantly reduces the cost of off-site transportation and disposal. At the same time, the system operates under closed negative pressure throughout the process, and the exhaust gas meets the emission standards after dust removal and deodorization, thus avoiding the unorganized escape of odorous gases.

[0021] 3) This invention limits the addition of only PFS and PAM flocculants to the front-end conditioning process, without adding calcium- or chloride-containing inorganic conditioning agents such as lime and ferric chloride. This reduces the migration of calcium and chloride ions into the closed-loop circulation with the water from the source, ensuring that the condensate returning to the high-temperature steam heat pump maintains a low hardness and low chloride ion level for a long time. This effectively inhibits scaling and pitting on the heat exchange surface and ensures continuous and stable operation of the device. Attached Figure Description

[0022] Figure 1 This is a schematic diagram for the overall application. Figure 2 The curve shows the relationship between the carrier gas ratio R and the adiabatic saturation temperature of the exhaust gas T_as, with R on the horizontal axis and T_as on the vertical axis. The two are approximately monotonically inversely correlated. Figure 3 The graph shows the relationship between the circulating water flow rate G of the first-stage condenser and the total power consumption of the system (left axis) and the heat recovery rate Q1 / Q_evap (right axis). The horizontal axis is G, and the total power consumption of the system changes in a U-shape with G.

[0023] In the diagram: 1. Integrated high-pressure belt dehydrator; 2. Disc dryer; 3. Dust collector; 4. High-temperature steam heat pump; 5. Water source heat pump; 6. Steam separator; 7. Softened water system; 8. Primary exhaust gas condenser; 9. Secondary exhaust gas condenser; 10. Exhaust gas induced draft fan; 11. Drainage system; 12. Wastewater cooler; 13. Detailed Implementation

[0024] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] In the description of this application, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Example

[0026] Please see Figure 1-3 This application provides a technical solution: the specific structure and function of each component are as follows: Integrated high-pressure belt dewatering machine: This machine uses mechanical dewatering to reduce the moisture content of sludge from wastewater treatment plant sludge thickening tanks from approximately 97% to 65%-70%. The equipment includes a conditioning unit, a gravity thickening device, a primary dewatering belt, and a secondary dewatering belt, arranged sequentially along the sludge treatment direction. It employs high-pressure pressing and high-shear force two-dimensional dewatering technology, operates continuously, and features a closed structure design that allows for odor collection and treatment.

[0027] The conditioning unit does not add calcium- or chlorine-containing inorganic conditioners such as lime or ferric chloride, but only PFS and PAM flocculants, ensuring that the increase in oven-dry solids does not exceed 3%. In this application, the condensate formed by steam exothermics needs to be returned to the high-temperature steam heat pump for repeated evaporation, forming a closed loop. If calcium- or chlorine-containing inorganic conditioners such as lime / ferric chloride are used, calcium and chloride ions will enter the drying tail gas condensate and steam condensate along with the sludge moisture and continuously concentrate in the loop, leading to scaling in the heat pump steam generator and chloride stress corrosion. Simultaneously, the increase in oven-dry solids can reach over 15%, directly increasing the drying load. The PFS selected in this invention is a chloride-free polyferric sulfate, whose anion is sulfate and does not contain chloride ions. Furthermore, because the dosage is limited to no more than 3% for the increase in oven-dry solids, the total amount of Fe³⁺ and SO₄²⁻ migrating into the closed loop with the moisture is limited and continuously discharged through the overflow and forced blowdown of the primary condenser, preventing accumulation in the condensate to the point of causing iron scale or sulfate corrosion.

[0028] Disc dryer: This type of dryer uses steam to indirectly heat the sludge, causing the water in the sludge to evaporate and thus achieving the goals of volume reduction and harmlessness. The dryer contains hollow discs, through which steam exchanges heat with the sludge via a hollow shaft and the discs. Approximately 1.4 tons of saturated steam are required to evaporate 1 ton of water.

[0029] Dust collector: A cyclone dust collector is used, located after the exhaust gas outlet of the disc dryer and before the primary condenser, with a dust removal efficiency of no less than 90%. Its function is to remove dust from the dryer's exhaust gas, reducing the harmful impact of dust on the subsequent condenser and circulating water quality. The collected dry dust is discharged through an airlock unloader and incorporated into the dryer's discharge, transported off-site along with the dried sludge, and does not enter the circulating water system.

[0030] High-temperature steam heat pump: This is a steam generating device based on high-temperature steam heat pump technology. The system absorbs heat from the circulating water in the tail gas primary condenser and transfers the heat to the steam condensate collected by the condensate drainage system through a high-temperature heat pump, forming saturated steam that can be used by the disc dryer. The saturated steam pressure is 0.4-0.6 MPa.

[0031] It should be noted that the heat load of a high-temperature steam heat pump should be calculated based on "heating 100°C condensate to its saturation temperature at the corresponding pressure and then vaporizing it," rather than solely based on the latent heat of vaporization. Taking 0.5 MPa as an example, the specific enthalpy of steam is 2748.1 kJ / kg, and the specific enthalpy of saturated water at 100°C is 419.1 kJ / kg, resulting in a unit heat consumption of 2329 kJ / kg, which is approximately 10.5% higher than the calculation based solely on the latent heat of vaporization (2108 kJ / kg). The COP of the high-temperature steam heat pump is not lower than 2.0 under the operating conditions set in this invention.

[0032] Water source heat pumps absorb heat from wastewater (treated effluent) in sewage treatment plants to produce high-temperature hot water at 70-80℃. They serve three main functions: first, as the sole initial heat source during the system's cold start phase, enabling the system to start from scratch without external steam; second, as a supplementary and regulating heat source during normal operation, filling the heat gap when waste heat from the exhaust gas is insufficient or operating conditions fluctuate; and third, the discharged low-temperature chilled water serves as the cooling medium for the secondary condenser of the exhaust gas. The heating COP of a water source heat pump is no less than 3.5. It should be noted that the heating capacity of the water source heat pump is configured for cold start conditions—at which point the system has no waste heat from the exhaust gas to recover, and it must independently provide all the low-grade heat source for the high-temperature steam heat pump. Once the system enters normal operation, the supplementary heating demand decreases significantly, and the water source heat pump operates at approximately 10% partial load. This configuration is necessary to ensure a zero-load start-up under conditions of "only power supply."

[0033] In a preferred embodiment, the wastewater from the wastewater treatment plant can first be sent to a secondary condenser to absorb heat from the low-temperature section of the exhaust gas, raising its temperature to 25-32°C. Then, it is sent to a water source heat pump evaporator to extract heat and cool it to 10-15°C before being discharged. Compared to the conventional sequence of "wastewater first being cooled by a heat pump and then used for condensation," this reverse arrangement can recover the low-temperature heat that would otherwise be discharged with the wastewater, increase the evaporation temperature of the water source heat pump by approximately 5-10K, improve its COP by approximately 10%-15%, and simultaneously improve its heat replenishment capacity under low wastewater temperatures in winter.

[0034] Steam distributor: As a buffer and distribution device for steam generated by high-temperature steam heat pump, it plays a role in stabilizing pressure, buffering and separating steam and water, ensuring stable steam supply pressure for the dryer.

[0035] Softened water system: Provides softened makeup water for high-temperature steam heat pumps, used for initial system filling, replenishment of sewage during operation, and water replenishment during startup.

[0036] The exhaust gas primary condenser is a direct contact condenser. The circulating water forms a water mist at its top and comes into countercurrent contact with the rising exhaust gas. The water vapor in the exhaust gas liquefies and releases heat, which is transferred to the circulating water. After the temperature of the circulating water rises, it is sent to the high-temperature steam heat pump to release heat and cool down, and then returns to the top of the primary condenser for recycling. The condensate produced by the exhaust gas condensation is incorporated into the circulating water, and its overflow and the drainage from the forced sewage discharge are sent to the wastewater cooler.

[0037] The present invention specifies the following operating parameters for the primary condenser: the circulating water flow rate is selected based on the ratio of the heat recovered by the primary condenser to the heat required by the high-temperature heat pump evaporator (hereinafter referred to as "heat recovery rate") being 0.85-1.00, corresponding to a circulating water temperature rise of 8-12K; the circulating water outlet temperature is 80-90℃, and the difference Δt between the circulating water outlet temperature and the adiabatic saturation temperature of the exhaust gas is 3-8K. Based on the scale of this small-scale experiment, the circulating water flow rate is 1.4-1.9 t / h, preferably 1.65 t / h.

[0038] The basis for this limitation is as follows: When the circulating water flow rate is too low, although the exhaust gas is cooled significantly in the primary condenser and a large amount of heat can be recovered, the low return water temperature leads to a decrease in the heat pump evaporation temperature and COP. When the flow rate is too high, although the return water temperature and COP are high, the exhaust gas can only be cooled to near the return water temperature in the primary condenser. A large amount of water vapor cannot be condensed in the primary stage and enters the secondary condenser where it is carried away by the cooling water. The heat recovery rate drops sharply to below 0.6, and the system needs to be supplemented with a large amount of heat by the water source heat pump. There is a clear optimal range between the two.

[0039] The exhaust gas secondary condenser uses a shell-and-tube heat exchanger. Exhaust gas passes through the tube side, while low-temperature chilled water (10-22℃) discharged from the water source heat pump is sent to the shell side. Through indirect heat exchange, the exhaust gas is cooled to 30-40℃, meeting the operating temperature requirements of the subsequent deodorization system. A condensate collection and discharge port is located at the bottom of the tube side, through which the condensate formed by the exhaust gas condensing within the tubes is discharged and sent to the wastewater cooler.

[0040] Exhaust gas induced draft fan: A centrifugal fan is used to discharge the exhaust gas from the dryer and maintain the dryer cavity, dust collector, condenser, and other equipment under negative pressure to prevent odor leakage. The air volume of the induced draft fan is selected according to the exhaust gas volume corresponding to the upper limit of the carrier gas ratio. It should not be too large to avoid excessive air intake causing a drop in the adiabatic saturation temperature of the exhaust gas. The air volume of the induced draft fan is approximately 105 Nm³ / h (standard conditions; based on the upper limit of the carrier gas ratio of 1.0 and the evaporation water volume of 41.7 kg / h, the dry air volume is 41.7 kg / h (32.3 Nm³ / h), and the water vapor volume is 41.7 kg / h (51.9 Nm³ / h), totaling approximately 84 Nm³ / h, which is 105 Nm³ / h after including a 1.25 times margin). The corresponding volumetric flow rate under the working condition of 100℃ and normal pressure is approximately 144 m³ / h. The fan is selected according to the working condition air volume.

[0041] The condensate drainage system separates and reuses the condensate from the steam drying process in a high-temperature steam heat pump, fully utilizing the sensible heat of the condensate and reducing the amount of softened water to be replenished. This invention further includes a flash steam recovery branch: when the 0.5MPa condensate is reduced to atmospheric pressure via a steam trap, approximately 9%-10% of the condensate volume will flash out as low-pressure steam. This flash steam recovers its heat through ejection or heat exchange before being incorporated into the system, avoiding heat and water loss.

[0042] Wastewater cooler: Cools the overflow and sewage wastewater from the primary exhaust gas condenser, as well as the condensate from the secondary exhaust gas condenser, cooling it to below 35°C before sending it to the wastewater treatment plant. The cooling water is low-temperature greywater obtained from heat extraction by a water source heat pump; the greywater first absorbs heat in the secondary condenser, then enters the water source heat pump evaporator, and finally serves as the cooling medium for the wastewater cooler, achieving cascade utilization of cooling capacity.

[0043] The detection and control unit includes a temperature transmitter T1 installed on the circulating water outlet pipe of the primary condenser, a pressure transmitter P installed on the steam distribution cylinder, a temperature transmitter Tw installed on the water inlet pipe of the water source heat pump, a humidity or dew point transmitter installed on the exhaust gas outlet pipe of the dryer, and a programmable controller. This unit adjusts the load of the high-temperature steam heat pump and the water source heat pump according to the following logic, and regulates the induced draft fan airflow and the opening of the carrier gas replenishment valve.

[0044] A device-coupled sludge drying process includes the following steps: S1: Wastewater treatment plant sludge is processed by an integrated high-pressure belt dewatering machine. Without the addition of lime and ferric chloride, only PFS and PAM flocculants are added. After gravity concentration and first and second stage high-pressure pressing dewatering, the sludge moisture content is reduced from about 97% to 65%-70%, forming sludge cakes, which are then sent to a disc dryer.

[0045] S2: The disc dryer uses 0.4-0.6MPa saturated steam as a heat source and indirectly heats the sludge through hollow discs to evaporate the moisture; the air volume (including air leakage) per unit of evaporated water is controlled to be no more than 1.0 kg dry air / kg evaporated water, so that the adiabatic saturation temperature of the exhaust gas is no less than 87℃; the sludge moisture content is reduced to below 40% after drying and then discharged; the condensate formed by the steam exothermic reaction is collected by the condensate drainage system and returned to the high-temperature steam heat pump, and the condensate flash steam is recovered through the recovery branch; the heating performance coefficient (COP) of the high-temperature steam heat pump is no less than 2.0, and the heating COP of the water source heat pump is no less than 3.5.

[0046] S3: The exhaust gas (containing water vapor, volatiles, dust and non-condensable gases) generated by the disc dryer is first removed by a cyclone dust collector before entering the exhaust gas primary condenser.

[0047] S4. In the tail gas primary condenser, the circulating water and tail gas directly contact each other in countercurrent flow for heat exchange. Some of the water vapor in the tail gas condenses and transfers heat to the circulating water. The flow rate of the circulating water is selected based on the ratio of the recovered heat Q1 of the tail gas primary condenser to the heat Qevap required by the high-temperature steam heat pump evaporator, Q1 / Qevap, which is 0.85-1.00, and the temperature rise ΔT of the circulating water is 8-12K. This ensures that the outlet temperature T1 of the circulating water reaches 80-90℃ and the difference Δt between the outlet temperature of the circulating water and the adiabatic saturation temperature Tas of the tail gas is 3-8K. The heated circulating water is sent to the evaporator of the high-temperature steam heat pump, where the refrigerant absorbs its heat and heats and vaporizes the condensate recovered by the condensate drainage system into 0.4-0.6MPa saturated steam. After being distributed by the steam distributor, the steam returns to the disc dryer, forming a closed-loop cycle of the steam heat source.

[0048] S5: The exhaust gas after primary condensation (temperature approximately 77.5℃) enters the secondary exhaust gas condenser, which cools the exhaust gas from approximately 77.5℃ to an outlet temperature of 35℃. It then indirectly exchanges heat with the low-temperature chilled water (10-22℃) discharged from the water source heat pump, reducing the exhaust gas temperature to 30-40℃, thus meeting the operational requirements of the subsequent deodorization system.

[0049] S6: The exhaust gas is drawn out by the exhaust gas induced draft fan, maintaining the system under negative pressure of -200 to -500Pa. After cooling, the exhaust gas is sent to the deodorization system for harmless treatment before being discharged in compliance with standards.

[0050] S7: The circulating water in the primary condenser continuously increases in volume due to the condensation of exhaust gas. Its overflow, along with the forced sewage discharge and the condensate from the secondary condenser (totaling approximately 0.041 t / h), is sent to the wastewater cooler. After being cooled to below 35°C, it is sent to the sewage treatment plant for further treatment.

[0051] S8. Wastewater from the wastewater treatment plant is first sent to the secondary condenser of the exhaust gas, where it exchanges heat with the exhaust gas after primary condensation and absorbs the residual heat from the low-temperature section, thus raising its temperature. The heated wastewater then enters the water source heat pump as its low-grade heat source. The water source heat pump absorbs the residual heat from the wastewater treatment plant to generate 70-80°C high-temperature hot water, which serves as the low-grade heat source and supplementary and regulating heat source for the high-temperature steam heat pump. During system cold start, this high-temperature hot water enters the circulating water loop of the primary condenser of the exhaust gas, serving as the sole low-grade heat source for the high-temperature steam heat pump. A portion of the low-temperature cold water discharged from the water source heat pump is returned to the secondary condenser of the exhaust gas as a cooling medium for recycling, while the remainder is supplied to the wastewater cooler as a cooling medium, achieving cascade utilization of cooling capacity before discharge.

[0052] When directly contacting the condenser to recover waste heat from the exhaust gas of the dryer, the highest temperature that the circulating water can reach has a hard upper limit determined by heat transfer, namely the adiabatic saturation temperature Tas of the exhaust gas (numerically close to the exhaust gas dew point). When the dry-bulb temperature of the exhaust gas and the system pressure are constant, Tas is uniquely determined by the mass ratio of water vapor to non-condensable gases (including leaked air and carrier gas) in the exhaust gas, i.e., the carrier gas ratio R. This forms a chain of constraints: R decreases, Tas increases, the highest temperature T1,max achievable by the circulating water in the first-stage condenser increases (T1,max = Tas − Δt, where Δt is the heat transfer terminal difference, taken as 3-8K), the evaporation temperature Te of the high-temperature heat pump increases, the heat pump temperature rise decreases under constant condensation temperature → COP increases, system power consumption decreases, and heat self-sufficiency is enhanced. Meanwhile, given a flow rate R, the circulating water flow rate G affects the system in two opposite directions: increasing G raises the return water temperature, Te, and COP, but forces the outlet exhaust gas temperature of the first-stage condenser to rise, reduces water vapor condensation, decreases the heat recovery rate Q1 / Qevap, and increases the required heat replenishment; decreasing G has the opposite effect, lowering the return water temperature and COP, but ensuring sufficient exhaust gas cooling and increasing the heat recovery rate. These two factors offset each other, thus an optimal flow rate range exists. The above quantities are defined as follows: Q1 is the heat recovered by the first-stage condenser, Qevap is the heat absorption load of the high-temperature heat pump evaporator, and Qws is the heat replenishment from the water source heat pump.

[0053] Based on the above understanding, the present invention defines the following key parameters: the carrier gas ratio R is 0.5-1.0 kg / kg (0.70 in the example); the exhaust gas adiabatic saturation temperature Tas is 87-93℃ (90.2℃ in the example); the primary condenser circulating water outlet temperature T1 is 80-90℃ and its difference Δt from the exhaust gas adiabatic saturation temperature is 3-8K; the circulating water temperature rise ΔT is 8-12K (preferably 9-11K); the heat recovery rate Q1 / Qevap is 0.85-1.00 (preferably 0.85-0.95); the circulating water flow rate... The flow rate G is 1.4-1.9 t / h (preferably 1.65 t / h in this embodiment, calculated by inversely from Qevap and temperature rise rather than by spray density); the steam pressure is 0.4-0.6 MPa (preferably 0.5 MPa); the high-temperature heat pump COP is ≥2.0 (preferably 2.2-2.5); the induced draft fan air volume is selected according to the exhaust gas volume corresponding to R=1.0 (approximately 105 Nm³ / h in this embodiment); the exhaust gas secondary outlet temperature is 30-40℃ (preferably 35℃); the system negative pressure is −200 to −500 Pa (preferably −300 Pa).

[0054] It's important to note that a lower carrier gas ratio is not always better. The role of the carrier gas is to carry away and remove water vapor evaporated from the sludge surface within the dryer. Its carrying capacity depends on the difference between the partial pressure of water vapor in the exhaust gas and the saturated partial pressure of water vapor at the sludge surface temperature, i.e., the drying driving force. Under conditions where the surface temperature of the wet sludge in the dryer is approximately 100℃ and the surface water vapor partial pressure is close to the saturation value of 101.4 kPa, the partial pressure of water vapor in the exhaust gas increases as the carrier gas ratio decreases, and the drying driving force decreases accordingly: taking the driving force at a carrier gas ratio of 1.00 as a benchmark, it is approximately 79% at a carrier gas ratio of 0.70, approximately 62% at 0.50, and only about 41% at 0.30. When the driving force is too low, the moisture content of the sludge at the dryer outlet cannot be stably reduced to below 40%, and localized over-wetting and wall adhesion are likely to occur within the dryer. Therefore, the lower limit of the carrier air ratio in this invention is set at 0.5 kg dry air / kg evaporated water, which is the result of a trade-off between heat recovery benefits and drying driving force. This lower limit was determined by experiments: when the carrier air ratio drops to 0.4 or below, the moisture content of the sludge at the dryer outlet fluctuates significantly and exceeds 40% multiple times, while it can stably meet the standard when the carrier air ratio is 0.5 or above.

[0055] Furthermore, using the primary condenser circulating water outlet temperature T1, the steam pressure P of the steam distributor, and the wastewater treatment plant's treated water temperature T_w as controlled variables, the following logic is applied for adjustment: Carrier gas ratio control—adjusting the induced draft fan flow rate and the carrier gas replenishment valve opening based on the tail gas dew point transmitter measurement value to ensure that the measured adiabatic saturation temperature is not lower than the set value (87-92℃). If the measured value is lower, the induced draft fan and carrier gas replenishment valve are closed; Circulating water flow rate control—setting the circulating water pump flow rate G=η·Q based on a heat recovery rate η of 0.85-1.00. e ᵥ ap / (c_p·ΔT) (ΔT is taken as 8-12K), the flow rate is increased or decreased accordingly when it deviates from the set range; high-temperature steam heat pump load control, when the steam cylinder pressure P is lower than the set value, the compressor load is increased, and when it is higher than the set value +0.05MPa, the load is reduced; water source heat pump load control, when T1 is lower than the set value, the water source heat pump load is increased to supplement heat, so that the supplementary heat Q_ws is equal to the high-temperature heat pump evaporator heat load Q. e ᵥ ap Satisfying Q_ws≥α·Q e ᵥ ap The design operating condition α = 0.05-0.20, and the fluctuation operating condition α increases to 0.3-0.6; cold start control, the water source heat pump produces 70-80℃ hot water which enters the first-stage condenser circuit as the low-level heat source of the high-temperature heat pump.

[0056] Accordingly, this embodiment also provides an equipment-coupled sludge drying system for implementing the above-mentioned process, including an integrated high-pressure belt dewatering machine, a disc dryer, a cyclone dust collector, a primary tail gas condenser, a secondary tail gas condenser, a tail gas induced draft fan, a high-temperature steam heat pump, a steam distribution cylinder, a drainage system, a water source heat pump, a softened water system, a wastewater cooler, a carrier gas replenishment valve, a circulating water pump, and a detection and control unit; wherein the detection and control unit includes a temperature transmitter T1 installed on the circulating water outlet pipe of the primary tail gas condenser, a pressure transmitter P on the steam distribution cylinder, a temperature transmitter T_w on the water inlet pipe of the water source heat pump, a humidity or dew point transmitter on the tail gas outlet pipe of the dryer, and a programmable controller for executing the above-mentioned multi-heat source collaborative control logic.

[0057] Example 1 (Basic Example): Small-scale experimental setup, material and energy balance, and operational performance. Using dewatered sludge (80% moisture content, 0.5 t / d of oven-dry solids) from a municipal wastewater treatment plant as the subject, a small-scale experimental device with a daily processing capacity of 2.5 tons was built, and continuous drying experiments were conducted using the process of this invention; the device was designed to operate continuously for 20 hours per day.

[0058] Regarding material balance, based on an oven-dry solids of 0.5 t / d: 2.5 t / d of sludge with 80% moisture content is dewatered to 70% by an integrated high-pressure belt dewatering machine, resulting in 1.667 t / d. After being dried to 40% by a disc dryer, it becomes 0.833 t / d. The oven-dry solids remain constant at 0.5 t / d throughout the process. The water evaporation rate of the dryer is 0.833 t / d (equivalent to 41.7 kg / h based on 20 hours of operation), requiring approximately 1.167 t / d of 0.5 MPa saturated steam (steam consumption ratio of 1.4 t steam / t evaporated water, equivalent to 58.4 kg / h based on 20 hours of operation, or approximately 0.058 t / h).

[0059] The main process parameters are as follows: The disc dryer uses 0.5MPa saturated steam (consumption 0.058t / h), and the carrier gas volume per unit of evaporated water is controlled to be no more than 1.0kg dry air / kg evaporated water (0.70 in this example). The dryer exhaust gas temperature is 100℃, and the exhaust gas adiabatic saturation temperature is 90.2℃. The circulating water flow rate of the exhaust gas primary condenser is 1.65t / h, raising the temperature from 75.5℃ to 85.2℃ (temperature rise 9.8K). The primary heat recovery is 18.75kW, and the heat recovery rate is 0.86. The secondary condenser uses 10-22℃ cooling water to cool the exhaust gas from approximately 77.5℃ after primary condensation to an outlet temperature of 35℃, with a heat load of approximately 8.83kW. The high-temperature steam heat pump has a heating capacity of 37.75kW, an input power of 15.88kW, and a COP2. 38 (evaporation temperature 71.5℃, condensation temperature 151.8℃); the water source heat pump is designed to provide 12.25kW of heating capacity (for cold start), with an additional 3.12kW of heating during operation, COP≥3.5, producing 70-80℃ hot water and 10-22℃ cold water; the exhaust gas induced draft fan has an air volume of approximately 105Nm³ / h (corresponding to a volumetric flow rate of approximately 144m³ / h under 100℃ operating conditions, the fan is selected according to the operating conditions air volume), the system maintains a fully enclosed negative pressure operation of −200-−500Pa; the cyclone dust collector has an efficiency ≥90%; the drainage system recovers approximately 0.058t / h of condensate at 100℃ (including approximately 0.0058t / h of flash steam, with a recovery branch); the wastewater cooler cools approximately 0.041t / h of condensate and wastewater to ≤35℃ before sending it to the wastewater treatment plant for treatment.

[0060] The operating results are as follows: 2.5 tons of municipal sludge with 80% moisture content are processed daily, ultimately producing 0.833 tons of dry sludge with 40% moisture content. The total input power of the two heat pumps is approximately 16.77kW (approximately 15.88kW from the high-temperature steam heat pump and approximately 0.89kW from the water source heat pump for supplemental heating), with auxiliary equipment at approximately 3.0kW, for a total system operating power of approximately 19.77kW. The average daily electricity consumption is approximately 395kWh, with a daily operating electricity cost of approximately 257 yuan at a price of 0.65 yuan / (kWh). The electricity consumption per ton of sludge is approximately 158kWh, and the electricity cost per ton of sludge is approximately 103 yuan, saving approximately 53% compared to the electric heating steam preparation scheme (approximately 218 yuan per ton of sludge).

[0061] To further verify the feasibility of this invention under power-only conditions, a system cold start experiment was conducted on the device. During cold start, the high-temperature steam heat pump had no waste heat from the exhaust gas to recover. The water source heat pump absorbed the waste heat from the sewage treatment plant to generate 70-80°C hot water, which entered the first-stage condenser loop as the low-grade heat source for the high-temperature heat pump, producing initial steam for the disc dryer. The water source heat pump was designed to produce 12.25kW of heating capacity, which was improved by the high-temperature heat pump at a COP of approximately 2.0 to output approximately 24.5kW, producing approximately 0.038t / h of 0.5MPa steam (approximately 65% ​​of the rated 0.058t / h). Therefore, the dryer started at approximately 60%-65% load. As exhaust gas was generated, the circulating water temperature gradually rose from room temperature to approximately 85°C, and the steam supply capacity gradually increased, reaching full load in approximately 90-120 minutes. This experiment shows that the invention can start up automatically without an external preheating steam source and tends to be heat self-sufficient, which meets the design goal of this small experimental device: "power only, no external steam supply."

[0062] Example 2 (Key Example): The Impact of Carrier Gas Ratio on System Performance Under the scale and equipment conditions of this small-scale experimental setup, keeping the circulating water flow rate of the first-stage condenser at 1.65 t / h, the exhaust gas temperature of the dryer at 100℃, and the evaporation water volume at 41.7 kg / h constant, only the carrier gas ratio was changed (achieved by adjusting the induced draft volume and the carrier gas replenishment valve). A series of tests were conducted on the device, and the exhaust gas and circulating water parameters under each operating condition were measured and material-heat balance calibration was performed. The results showed that when the carrier gas ratio was 0.50 kg / kg, the adiabatic saturation temperature of the exhaust gas was 92.6℃, the circulating water outlet temperature was 87.6℃ (temperature rise of 10.6 K), the heat pump evaporation temperature was 73.0℃, the COP was 2.43, η=0.91, and the required replenishment was [missing information]. With a heat output of 1.91 kW, a total power consumption of 16.11 kW for the heat pump system, and a power consumption per ton of sludge of 153 kW·h / t; when the carrier gas ratio is 0.70 kg / kg (the value used in this invention), the exhaust gas adiabatic saturation temperature is 90.2℃, the circulating water outlet temperature is 85.2℃ (temperature rise of 9.8 K), the heat pump evaporation temperature is 71.5℃, COP is 2.38, η=0.86, the required supplementary heat is 3.12 kW, the total power consumption of the heat pump system is 16.77 kW, and the power consumption per ton of sludge is 158 kW·h / t; when the carrier gas ratio is 1.00 kg / kg, the exhaust gas adiabatic saturation temperature is 87.1℃, the circulating water outlet temperature is 82.1℃ (temperature rise of 8.9 K), and the heat pump evaporation temperature is 71.5℃, COP is 2.38, η=0.86, the required supplementary heat is 3.12 kW, the total power consumption of the heat pump system is 16.77 kW, and the power consumption per ton of sludge is 158 kW·h / t. At a temperature of 69.2℃, COP 2.31, η = 0.80, required supplemental heat of 4.32kW, total power consumption of the heat pump system 17.56kW, and power consumption per ton of sludge 164kW·h / t; with a carrier gas ratio of 1.50kg / kg (comparative condition), the exhaust gas adiabatic saturation temperature is 82.8℃, the circulating water outlet temperature is 77.8℃ (temperature rise 8.1K), the heat pump evaporation temperature is 65.7℃, COP 2.22, η = 0.74, required supplemental heat of 5.29kW, total power consumption of the heat pump system 18.52kW, and power consumption per ton of sludge 172kW·h / t; with a carrier gas ratio of 2.20kg / kg, the exhaust gas adiabatic saturation temperature is 78.0℃. When the circulating water outlet temperature is 73.0℃ (temperature rise 7.5K), the heat pump evaporation temperature is 61.5℃, COP 2.12, η=0.72, the required supplementary heat is 5.54kW, the total power consumption of the heat pump system is 19.43kW, and the power consumption per ton of sludge is 179kW·h / t; when the carrier gas ratio is 3.88kg / kg (comparative operating condition, comparative example 1), the exhaust gas adiabatic saturation temperature is 70.0℃, the circulating water outlet temperature is 65.0℃ (temperature rise 7.3K), the heat pump evaporation temperature is 53.7℃, COP 1.95, η=0.77, the required supplementary heat is 4.29kW, the total power consumption of the heat pump system is 20.62kW, and the power consumption per ton of sludge is 189kW·h / t.

[0063] The results show that as the carrier gas ratio decreased from 3.88 to 0.70, the exhaust gas adiabatic saturation temperature increased from 70.0℃ to 90.2℃ (an increase of 20.2K), the circulating water outlet temperature increased from 65.0℃ to 85.2℃ (an increase of 20.2K), the heat pump evaporation temperature increased from 53.7℃ to 71.5℃ (an increase of 17.8K), the COP increased from 1.95 to 2.38 (an increase of 22.1%), the total power consumption of the heat pump system decreased from 20.62kW to 16.77kW (a decrease of 18.7%), and the power consumption per ton of sludge decreased from 189kW·h to 158kW·h (a decrease of approximately 16.4%). Further reduction to 0.50 resulted in a power consumption of 16.11kW (a decrease of approximately 21.9% compared to 3.88). This quantitative relationship confirms the constraint chain of carrier gas ratio, adiabatic saturation temperature, upper limit of circulating water temperature, evaporation temperature, COP, and power consumption. Furthermore, R ≤ 1.0 is the limiting threshold of this invention. Beyond this threshold (R ≥ 1.5), the high-temperature heat pump evaporation temperature drops to below approximately 66°C, the COP drops to approximately 2.2, and the total system power consumption rises to over 18.5 kW. It should be noted that since the steam production of the high-temperature steam heat pump is constant and its heating capacity remains essentially unchanged, the evaporator's heat absorption load Qevap increases with the increase of COP. Therefore, under different carrier gas ratios, the heat recovery rate Q1 / Qevap does not have a monotonic relationship with the system's supplementary heat or total power consumption. The indicators for evaluating the technical effect of this invention should be the total system power consumption and the power consumption per ton of sludge, rather than the heat recovery rate itself.

[0064] Comparative Example To highlight the technical advantages of this invention compared to existing conventional solutions, this invention is compared with three comparative examples under the same benchmark scale (daily processing capacity of 2.5 tons of wet sludge with 80% moisture content, evaporation rate of the dryer of 41.7 kg / h, and steam consumption of 0.058 t / h). Comparative example 1 is the original solution that follows the full-scale approach of the project but allows for a relatively high carrier air ratio (only the carrier air ratio is changed to 3.88, and the circulating water flow rate is the same as that of this invention at 1.65 t / h); Comparative example 2 is the original solution that only changes the circulating water flow rate (the carrier air ratio is the same as that of this invention at 0.70, and the circulating water flow rate is the same as the conventional 5 t / h); Comparative example 3 is a conventional dewatering solution that uses plate and frame filter press + FeCl3 + lime conditioning at the front end.

[0065] Comparative Example 3 uses plate and frame filter press + FeCl3 + lime conditioning at the front end, and the increase in oven-dry solids is ≥15%. For ease of comparison, the evaporation water volume and steam consumption in the table have been converted to the same baseline based on the same oven-dry solids processing capacity (0.5t / d) as this invention.

[0066] The comparison results are shown in the table below:

[0067] As shown in the table, compared with Comparative Example 1, this invention reduces the carrier gas ratio from 3.88 to 0.70, thereby increasing the exhaust gas adiabatic saturation temperature from 70.0℃ to 90.2℃ (an increase of 20.2K), the circulating water outlet temperature from 65.0℃ to 85.2℃ (an increase of 20.2K), the high-temperature heat pump evaporation temperature from 53.7℃ to 71.5℃ (an increase of 17.8K), the high-temperature heat pump COP from 1.95 to 2.38 (an increase of 22.1%), the total system power consumption from 20.62kW to 16.77kW (a decrease of 18.7%), the power consumption per ton of sludge from 189kW·h / t to 158kW·h / t, and the electricity cost per ton of sludge from 123 yuan / t to 103 yuan / t. It is evident that the value of reducing the carrier gas ratio lies in raising the exhaust gas adiabatic saturation temperature, thereby increasing the heat pump evaporation temperature and COP, ultimately resulting in a significant decrease in the total system power consumption and the power consumption per ton of sludge. Compared with Comparative Example 2 (circulating water configured at a conventional 5t / h), this invention matches the circulating water flow rate to approximately 1.65t / h with a heat recovery rate of 0.85-1.00. This avoids the problems of insufficient cooling of exhaust gas in the primary condenser and a large amount of water vapor entering the secondary condenser and being carried away by the cooling water under high flow rates. As a result, the heat recovery rate is increased from approximately 48% to 86%, the system's required heat supplement is reduced from 12.23kW to 3.12kW, and the total power consumption is reduced from 17.82kW to 16.77kW.

[0068] The foregoing has shown and described the basic principles, main features, and advantages of this application. It will be apparent to those skilled in the art that this application is not limited to the details of the exemplary embodiments described above, and that this application can be implemented in other specific forms without departing from the spirit or basic characteristics of this application. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects. The scope of this application is defined by the appended claims rather than the foregoing description. Therefore, it is intended to encompass all variations falling within the meaning and scope of equivalents of the claims within this application, and no reference numerals in the claims should be regarded as limiting the scope of the claims.

[0069] Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A coupled sludge drying process, Includes the following steps: Characterized by: S1. The sludge is dewatered to a moisture content of 65%-70% using an integrated high-pressure belt dewatering machine without the addition of lime and ferric chloride, only with the addition of PFS and PAM flocculants. S2. The dewatered sludge is fed into a disc dryer and dried indirectly by saturated steam at 0.4-0.6MPa through hollow discs. The carrier gas ratio R of the disc dryer is controlled to be no more than 1kg dry air / kg evaporated water, so that the adiabatic saturation temperature Tas of the exhaust gas is no less than 87℃. After drying, the sludge moisture content is reduced to below 40%. The condensate formed after the steam releases heat is returned via a drainage system. S3. The exhaust gas discharged from the disc dryer enters the first-stage exhaust gas condenser after dust removal. S4. In the tail gas primary condenser, the circulating water and tail gas directly contact each other in countercurrent flow for heat exchange. Some of the water vapor in the tail gas condenses and transfers heat to the circulating water. The flow rate of the circulating water is selected based on the ratio of the recovered heat Q1 of the tail gas primary condenser to the heat Qevap required by the high-temperature steam heat pump evaporator, Q1 / Qevap, which is 0.85-1.00, and the temperature rise ΔT of the circulating water is 8-12K. This ensures that the outlet temperature T1 of the circulating water reaches 80-90℃ and the difference Δt between the outlet temperature of the circulating water and the adiabatic saturation temperature Tas of the tail gas is 3-8K. The heated circulating water is sent to the evaporator of the high-temperature steam heat pump, where the refrigerant absorbs its heat and heats and vaporizes the condensate recovered by the condensate drainage system into 0.4-0.6MPa saturated steam. After being distributed by the steam distributor, the steam returns to the disc dryer, forming a closed-loop cycle of the steam heat source. S5. The exhaust gas after primary condensation enters the secondary exhaust gas condenser and is cooled down by indirect heat exchange with low-temperature cold water. S6. The exhaust gas is drawn out by the exhaust gas induced draft fan to maintain the negative pressure operation of the system. After cooling, the exhaust gas is deodorized and then discharged in compliance with the standards. S7. The overflow and forced sewage discharge of the first-stage tail gas condenser, as well as the condensate of the second-stage tail gas condenser, are sent to the wastewater cooler and then sent to the sewage treatment plant for treatment. S8. Wastewater from the wastewater treatment plant is first sent to the secondary condenser of the exhaust gas, where it exchanges heat with the exhaust gas after primary condensation and absorbs the residual heat from the low-temperature section, thus raising its temperature. The heated wastewater then enters the water source heat pump as its low-grade heat source. The water source heat pump absorbs the residual heat from the wastewater treatment plant to generate 70-80°C high-temperature hot water, which serves as the low-grade heat source and supplementary and regulating heat source for the high-temperature steam heat pump. During system cold start, this high-temperature hot water enters the circulating water loop of the primary condenser of the exhaust gas, serving as the sole low-grade heat source for the high-temperature steam heat pump. A portion of the low-temperature cold water discharged from the water source heat pump is returned to the secondary condenser of the exhaust gas as a cooling medium for recycling, while the remainder is supplied to the wastewater cooler as a cooling medium, achieving cascade utilization of cooling capacity before discharge.

2. The equipment-coupled sludge drying process according to claim 1, characterized in that: The carrier gas ratio R is 0.5-1.0 kg dry air / kg evaporated water, and the exhaust gas adiabatic saturation temperature Tas is 87-93℃. The carrier gas ratio is controlled by adjusting the air volume of the exhaust gas induced draft fan and the opening of the carrier gas replenishment valve. The air volume of the exhaust gas induced draft fan is selected according to the exhaust gas volume corresponding to the upper limit of the carrier gas ratio, with a margin of no more than 1.25 times.

3. The equipment-coupled sludge drying process according to claim 1, characterized in that: The flow rate G of the circulating water is determined by G=η·Qevap / (cp·ΔT), where cp is the specific heat capacity of water at constant pressure, η is the set value of heat recovery rate, which is 0.85-1.00, and ΔT is 8-12K.

4. The equipment-coupled sludge drying process according to claim 1, characterized in that: The circulating water outlet temperature is 80-90℃, the circulating water temperature rise ΔT is 9-11K, and the Q1 / Qevap is 0.85-0.

95.

5. The equipment-coupled sludge drying process according to claim 1, characterized in that: The pressure of the saturated steam is 0.4-0.6 MPa; the condensing temperature of the high-temperature steam heat pump is determined by the required saturated steam pressure, and its evaporation temperature Te is taken as the circulating water temperature of the first-stage condenser of the exhaust gas minus 4 K, and the heat transfer end difference of the first-stage condenser of the exhaust gas is taken as 3-8 K; the condensate formed by the steam release of the disc dryer is completely returned to the high-temperature steam heat pump through the condensate drainage system, and the condensate flash steam is recovered through the recovery branch. The heating performance coefficient (COP) of the high-temperature steam heat pump is not less than 2.0, and the heating COP of the water source heat pump is not less than 3.

5.

6. The equipment-coupled sludge drying process according to claim 1, characterized in that: In step S1, no lime or ferric chloride is added, only PFS and PAM flocculants are added, and the amount added is such that the increase in oven-dry solids does not exceed 3%, in order to inhibit the migration of calcium ions and chloride ions with water and their enrichment in the closed circulation of condensate.

7. The equipment-coupled sludge drying process according to claim 1, characterized in that: In step S8, the greywater absorbs waste heat from the low-temperature section in the tail gas secondary condenser and is heated to 25-32°C. Then it enters the water source heat pump evaporator to extract heat and is cooled to 10-15°C before being discharged. Compared with the conventional sequence in which greywater is first cooled by the water source heat pump and then used for condensation, this arrangement increases the evaporation temperature of the water source heat pump by 5-10K and the heating COP by 10%-15%.

8. The equipment-coupled sludge drying process according to claim 1, characterized in that: During system cold start, a water source heat pump generates 70-80℃ high-temperature hot water which enters the circulating water loop of the tail gas primary condenser. This serves as the sole low-grade heat source for the high-temperature steam heat pump, which generates initial steam for the disc dryer. The disc dryer starts with a partial feed load, gradually increasing to full load as the circulating water temperature rises. At a daily processing capacity of 2.5 tons (80% moisture content wet sludge), the partial feed load is 60%-65%, and it takes 90-120 minutes to go from cold start to full load.

9. A coupled sludge drying system, as described in claims 1-8, characterized in that: It includes an integrated high-pressure belt dehydrator, disc dryer, cyclone dust collector, tail gas primary condenser, tail gas secondary condenser, tail gas induced draft fan, high-temperature steam heat pump, steam distribution cylinder, condensate system, water source heat pump, softened water system, wastewater cooler, carrier gas replenishment valve, circulating water pump, and detection and control unit. The discharge port of the integrated high-pressure belt dewatering machine is connected to the inlet of the disc dryer. The exhaust outlet of the disc dryer is sequentially connected to a cyclone dust collector, a primary exhaust condenser, a secondary exhaust condenser, and an exhaust fan. The steam inlet of the disc dryer is connected to the steam outlet of a high-temperature steam heat pump via a steam distributor. The condensate outlet of the disc dryer is connected to the inlet of the high-temperature steam heat pump via a drainage system, forming a closed-loop steam circulation. A circulating water circuit is provided between the primary exhaust condenser and the high-temperature steam heat pump. The hot water outlet of the water source heat pump is connected to the circulating water circuit of the primary exhaust condenser. The cold water outlet of the water source heat pump is connected to the secondary exhaust condenser and the wastewater cooler, respectively. The outlet of the softened water system is connected to the high-temperature steam heat pump. The drain outlet of the primary exhaust condenser and the condensate outlet of the secondary exhaust condenser are both connected to the wastewater cooler. The detection and control unit includes a temperature transmitter T1 installed on the circulating water outlet pipe of the exhaust gas primary condenser, a pressure transmitter P installed on the steam distribution cylinder, a temperature transmitter Tw installed on the water inlet pipe of the water source heat pump, a humidity or dew point transmitter installed on the exhaust gas outlet pipe of the disc dryer, and a programmable controller; the programmable controller is electrically connected to the exhaust gas induced draft fan, the carrier gas replenishment valve, the circulating water pump, the high-temperature steam heat pump, and the water source heat pump, respectively.

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