Two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method

Abstract

The invention discloses a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method. Continuous hydrothermal carbonization is innovatively divided into a low-temperature release section and a high-temperature curing section, steam direct mixing is adopted, agricultural cellulose waste is introduced to synergistically improve the mass transfer condition of a system and optimize the hydrothermal carbon quality, and hydrolysis and carbonization function decoupling is achieved through temperature difference subsection operation; and the functional auxiliary agent is added in stages to drive the directional migration and dynamic regulation of nutrients. The obtained liquid phase is subjected to vacuum / membrane distillation concentration and fractional crystallization to recover available nutrients, and solid-phase hydrothermal carbon and crystals are compounded and granulated to prepare the carbon-based slow release fertilizer. According to the system, heat closed-loop circulation is constructed through sleeve heat exchange and waste heat recovery, and efficient closing of energy and substances is achieved in combination with online monitoring. According to the process, the total recovery rate of nitrogen and phosphorus is 70-90%, the solubility of heavy metals is reduced by 70-95%, the external heat compensation requirement is reduced by 30-60%, and the product has a double-peak slow release characteristic. The method is suitable for large-scale high-value resource utilization of the municipal sludge.

Description

Technical Field

[0001] This invention belongs to the field of sludge resource utilization technology, specifically relating to a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method. Background Technology

[0002] Hydrothermal carbonization is a thermochemical treatment technology suitable for wet organic waste. It can convert sludge into a carbon-rich solid phase (hydrothermal carbon) and a liquid phase rich in dissolved nutrients under subcritical water conditions, offering potential for volume reduction, inactivation, and resource recovery. However, existing hydrothermal carbonization processes mostly employ a single-stage isothermal reaction mode, with hydrolysis and carbonization processes highly coupled, making the reaction pathway difficult to control. This results in uncontrollable nutrient distribution between the solid and liquid phases, dispersed nutrient concentration in the liquid phase, high recovery costs, and limited stabilization of some heavy metals and undesirable components in the solid phase. Furthermore, existing processes do not adequately address the cascade recovery of heat energy during the reaction and the improvement of system energy efficiency, making it difficult to form a closed-loop engineering system that integrates matter and energy.

[0003] On the other hand, the large amount of cellulose-based organic waste generated during agricultural production has characteristics such as high carbon content and porous structure, and has the potential to participate in hydrothermal conversion and regulate reaction behavior. However, existing research and patents are mostly limited to simple mixing or independent treatment, and have not yet systematically revealed their synergistic effects on sludge hydrolysis behavior, carbon structure evolution and nutrient migration under hydrothermal conditions. There is also a lack of integrated technical pathways that combine them with continuous hydrothermal processes, nutrient recovery units and heat energy recycling.

[0004] Therefore, there is an urgent need to develop a new hydrothermal carbonization method based on reaction mechanism separation to improve the efficiency of nutrient resource utilization in municipal sludge. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method, which can achieve efficient nutrient recovery from sludge, safe and stable products, and improved system energy efficiency.

[0006] The technical solution is as follows: A two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method Step 1: In the sludge conditioning system, mix the sludge with agricultural organic cellulose waste powder and downstream membrane filtered water in a certain proportion to form a homogeneous slurry. Step 2: After the prepared sludge is preheated by the shell-and-tube heat exchanger, it enters the low-temperature hydrolysis release section of the integrated two-stage reaction system and is directly mixed with high-temperature steam. The sludge is heated to 160-180℃ and hydrothermally reacted for 30-60 minutes to hydrolyze the sludge and promote the migration of nutrients such as nitrogen and phosphorus to the liquid phase. Step 3: The reacted material is transported to the buffer mixing tank of the integrated two-stage reaction system. After the material state is monitored and adjusted, it enters the high-temperature carbonization and curing section. Step 4: After the material enters the high-temperature carbonization and solidification section, it is mixed with steam pipe and heated to 270-300℃ for hydrothermal carbonization reaction for 30-60 min. Nutrients are fixed in situ through carbon skeleton reconstruction and the bioavailability of heavy metals is reduced at the same time to obtain hydrothermal carbon and reaction solution. Step 5: The hydrothermal carbonization product is subjected to two-stage flash evaporation to recover some heat for granulation and drying; the flash evaporated material is preheated by a shell-and-tube heat exchanger and then enters the solid-liquid separation system to obtain solid hydrothermal carbon and nutrient-rich liquid phase, respectively. Step 6: The obtained liquid phase is transported to the selective concentration and fractional recovery system to concentrate the liquid phase by 5-8 times; a precipitation / crystallization inducer is added to the concentrate and the pH is adjusted to fractionate the fast-acting crystalline fertilizer components, and the precipitate is collected to obtain solid fast-acting microparticles; the concentrated filtration water and the remaining mother liquor are returned to the sludge conditioning unit. Step 7: Mix solid hydrothermal carbon and solid fast-acting microparticles in a certain proportion and then transport them to the composite granulation molding system. Use the concentrated liquid from step 7 as a binder, and granulate and dry to obtain carbon-based slow-release compound fertilizer. Furthermore, in step (1), the municipal sludge and agricultural organic cellulose waste powder are mixed at a dry basis mass ratio of 5:1-3:1, and the moisture content of the mixture is adjusted to 83%-85%. The agricultural organic cellulose waste is selected from straw, rice husk, corn cob, fruit shell, wood chips or any combination thereof. It can form a porous carbon precursor structure in the low-temperature hydrolysis release section, which is used to improve the mass transfer conditions of the reaction system and synergistically promote the sludge hydrolysis reaction.

[0007] Furthermore, in the integrated two-stage reaction system in steps (2), (3), and (4), the front and rear ends are tubular hydrothermal carbonization reactors, and the middle is a buffer mixing tank composed of an online buffer and detection unit; a controllable temperature and pressure transfer unit is provided between the two stages to maintain the material temperature drop of no more than 20°C.

[0008] Furthermore, in step (2), a weakly acidic hydrolysis aid or an enzymatic hydrolysis agent is added to the low-temperature hydrolysis release section to enhance the hydrolysis of organophosphorus compounds and the release of nitrogen.

[0009] Furthermore, in step (3), the online buffer and detection unit is equipped with an online ion-selective electrode or a portable spectrophotometer / colorimetric sensor to achieve real-time determination of NH4⁺-N and PO4³⁻-P, and input the determination results into the edge controller to drive the additive dosing pump.

[0010] Furthermore, in step (4), one or more of Mg²⁺, Fe³⁺ or Ca²⁺ are added in the high-temperature carbonization and solidification section to achieve nutrient stabilization and fixation and reduce the risk of heavy metal leaching.

[0011] Furthermore, in step (5), the main equipment of the solid-liquid separation system is a plate and frame filter press, a belt filter press, etc.

[0012] Furthermore, in step (6), the selective concentration of liquid phase is carried out by vacuum distillation, membrane distillation or membrane separation; the precipitation / crystallization inducer is preferably a soluble salt of Mg²⁺ and K⁺ or Ca²⁺, and the fractional crystallization includes: in the first stage, under pH 8-9 control, magnesium potassium phosphate (particle size 1-500 μm) is preferentially precipitated, and in the second stage, under pH 10-11 conditions, calcium phosphate or other slow-release components are precipitated.

[0013] Furthermore, in step (7), solid fast-acting microparticles and hydrothermal carbon are mixed at a mass ratio of 1:10-1:50 and granulated at 50-100℃ to form composite slow-release particles. Optionally, a moisture-sensitive / degradable polymer coating is applied to regulate the release kinetics. The granulation method is disc granulation, roller granulation or extrusion granulation. The resulting fertilizer particles have a particle size of 2-5 mm and a moisture content of ≤15%. The carbon-based slow-release compound fertilizer has the characteristics of fast-acting and slow-release synergistic release, and the slow-release period is not less than 45 days.

[0014] Furthermore, in steps (2), (4), and (7), the thermal closed-loop system reduces the external heat replenishment requirement through heat exchange between the reaction stream and the feed and flash evaporation recovery; the insufficient heat in the low-temperature hot hydrolysis section and the high-temperature hydrothermal carbonization section is supplemented by high-temperature steam with a steam temperature of 320℃-360℃.

[0015] The beneficial effects of this invention are as follows: Compared with existing single-stage isothermal hydrothermal carbonization and conventional sludge resource utilization technologies, this invention constructs a two-stage hydrothermal carbonization process combining a low-temperature hydrolysis release stage and a high-temperature carbonization and solidification stage. This effectively decouples the hydrolysis and carbonization reaction mechanisms, making the release, migration, and fixation of nutrients such as nitrogen and phosphorus in the sludge controllable. Experimental and pilot-scale results show that the total recovery rate of nitrogen and phosphorus under the conditions of this invention can reach 70-90%, with a liquid-phase phosphorus recovery rate of no less than 65%. After introducing agricultural organic cellulosic waste for synergistic reaction, the sludge hydrolysis efficiency is increased by 20-40%, and the specific surface area of ​​the hydrothermal carbon is increased by 30-60%, significantly improving the structural performance of the solid products as fertilizer carriers. In the high-temperature carbonization and solidification stage, functional additives are used to fix nutrients in situ and passivate heavy metals, reducing the solubility of heavy metals by 70-95%, significantly improving the safety of agricultural utilization. By adopting a direct steam mixing heating, two-stage flash evaporation, and heat exchange coupling heat recovery method, the external heat replenishment requirement of the system is reduced by 30-50%, and the overall energy efficiency is significantly improved. The resulting carbon-based slow-release compound fertilizer exhibits synergistic release characteristics of both rapid and slow release, with a cumulative release rate exceeding 80% over 45 days. This invention demonstrates significant advantages in nutrient recovery efficiency, product quality, and system energy efficiency, making it suitable for large-scale, high-value utilization of municipal sludge. Attached Figure Description

[0016] Figure 1 A process flow diagram of a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method provided by the present invention; Figure 2 Physicochemical properties of the intermediate product concentrate of a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method provided by the present invention. Figure 3 The physicochemical properties of carbon-based slow-release compound fertilizer in a two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method provided by this invention; Detailed Implementation

[0017] The purpose, technical solution and advantages of the present invention can be better understood from the following specific embodiments of the invention. The content described in the embodiments is only for illustrating the present invention and should not and will not limit the present invention as described in detail in the claims.

[0018] Example 1 A two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method includes the following steps: (1) In the sludge conditioning system, sludge and agricultural organic cellulose waste powder are mixed at a ratio of 5:1 and mixed evenly with water filtered by the back-end membrane to form a homogeneous slurry with a water content of 83%. (2) After the prepared sludge is preheated by the shell-and-tube heat exchanger, it enters the low-temperature hydrolysis release section of the integrated two-stage reaction system. Steam is directly mixed and heated to 180°C for 30 min for hydrothermal reaction. During this period, a weak acid hydrolysis aid is added to hydrolyze the sludge and promote the migration of nutrients such as nitrogen and phosphorus to the liquid phase. (3) The reacted material is transported to the online buffer and detection unit in the integrated two-stage reaction system. After real-time monitoring and adjustment of the NH4⁺-N and PO4³⁻-P of the material, it enters the high-temperature carbonization and solidification section. A controllable temperature and pressure transfer unit is provided between the two stages to maintain the material temperature drop of no more than 20°C. (4) After the material enters the high-temperature carbonization and solidification section, it is mixed with steam pipe and heated to 300°C for 40 min for hydrothermal carbonization reaction. Mg²⁺ and Fe³⁺ ions are added to achieve in-situ fixation of nutrients and reduce the bioavailability of heavy metals through carbon skeleton reconstruction, so as to obtain hydrothermal carbon and reaction solution. (5) The hydrothermal carbonization products are subjected to two-stage flash evaporation to recover part of the heat for granulation and drying. After flash evaporation, the material is preheated by a shell-and-tube heat exchanger and then enters the solid-liquid separation system to obtain solid hydrothermal carbon and nutrient-rich liquid phase respectively. (6) The obtained liquid phase is transported to the selective concentration and graded recovery system to concentrate the liquid phase by 6 times; Mg²⁺ and K⁺ or Ca²⁺ are added to the concentrate and the pH is adjusted to 8 to precipitate magnesium potassium phosphate. The pH is then adjusted to 10 to precipitate calcium phosphate or other slow-release components. The precipitate is collected to obtain solid fast-acting microparticles; the concentrated filtration water and the remaining mother liquor are returned to the sludge conditioning unit. (7) Solid hydrothermal carbon and solid fast-acting microparticles are mixed in a ratio of 20:1 and then transported to a composite granulation molding system. The concentrated liquid is used as a binder. After granulation and drying at 60°C, carbon-based slow-release compound fertilizer with a particle size of 3 mm and a moisture content of 13% is obtained.

[0019] Example 2 A two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method includes the following steps: (1) In the sludge conditioning system, sludge and agricultural organic cellulose waste powder are mixed in a ratio of 4:1 and mixed evenly with water filtered by the back-end membrane to form a homogeneous slurry with a water content of 84%. (2) After the prepared sludge is preheated by the shell-and-tube heat exchanger, it enters the low-temperature hydrolysis release section of the integrated two-stage reaction system. Steam is directly mixed and heated to 170°C for hydrothermal reaction for 40 min. During this period, a weak acid hydrolysis aid is added to hydrolyze the sludge and promote the migration of nutrients such as nitrogen and phosphorus to the liquid phase. (3) The reacted material is transported to the online buffer and detection unit in the integrated two-stage reaction system. After real-time monitoring and adjustment of the NH4⁺-N and PO4³⁻-P of the material, it enters the high-temperature carbonization and solidification section. A controllable temperature and pressure transfer unit is provided between the two stages to maintain the material temperature drop of no more than 20°C. (4) After the material enters the high-temperature carbonization and solidification section, it is mixed with steam pipe and heated to 290°C for 30 min for hydrothermal carbonization reaction. Mg²⁺ and Fe³⁺ ions are added to achieve in-situ fixation of nutrients and reduce the bioavailability of heavy metals through carbon skeleton reconstruction, so as to obtain hydrothermal carbon and reaction solution. (5) The hydrothermal carbonization products are subjected to two-stage flash evaporation to recover part of the heat for granulation and drying. After flash evaporation, the material is preheated by a shell-and-tube heat exchanger and then enters the solid-liquid separation system to obtain solid hydrothermal carbon and nutrient-rich liquid phase respectively. (6) The obtained liquid phase is transported to the selective concentration and graded recovery system to concentrate the liquid phase by 7 times; Mg²⁺ and K⁺ or Ca²⁺ are added to the concentrate and the pH is adjusted to 9 to precipitate magnesium potassium phosphate. The pH is then adjusted to 10 to precipitate calcium phosphate or other slow-release components. The precipitate is collected to obtain solid fast-acting microparticles; the concentrated filtration water and the remaining mother liquor are returned to the sludge conditioning unit. (7) Solid hydrothermal carbon and solid fast-acting microparticles are mixed in a ratio of 30:1 and then transported to a composite granulation molding system. The concentrated liquid is used as a binder. After granulation and drying at 50°C, carbon-based slow-release compound fertilizer with a particle size of 4 mm and a moisture content of 14% is obtained.

[0020] Example 3 A two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method includes the following steps: (1) In the sludge conditioning system, sludge and agricultural organic cellulose waste powder are mixed in a ratio of 3:1 and mixed evenly with water filtered by the back-end membrane to form a homogeneous slurry with a water content of 85%. (2) After the prepared sludge is preheated by the shell-and-tube heat exchanger, it enters the low-temperature hydrolysis release section of the integrated two-stage reaction system. Steam is directly mixed and heated to 160°C for 60 min for hydrothermal reaction. During this period, a weak acid hydrolysis aid is added to hydrolyze the sludge and promote the migration of nutrients such as nitrogen and phosphorus to the liquid phase. (3) The reacted material is transported to the online buffer and detection unit in the integrated two-stage reaction system. After real-time monitoring and adjustment of the NH4⁺-N and PO4³⁻-P of the material, it enters the high-temperature carbonization and solidification section. A controllable temperature and pressure transfer unit is provided between the two stages to maintain the material temperature drop of no more than 20°C. (4) After the material enters the high-temperature carbonization and solidification section, it is mixed with steam pipe and heated to 270°C for 60 min for hydrothermal carbonization reaction. Mg²⁺ and Fe³⁺ ions are added to achieve in-situ fixation of nutrients and reduce the bioavailability of heavy metals through carbon skeleton reconstruction, and hydrothermal carbon and reaction solution are obtained. (5) The hydrothermal carbonization products are subjected to two-stage flash evaporation to recover part of the heat for granulation and drying. After flash evaporation, the material is preheated by a shell-and-tube heat exchanger and then enters the solid-liquid separation system to obtain solid hydrothermal carbon and nutrient-rich liquid phase respectively. (6) The obtained liquid phase is transported to the selective concentration and graded recovery system to concentrate the liquid phase by 8 times; Mg²⁺ and K⁺ or Ca²⁺ are added to the concentrate and the pH is adjusted to 9 to precipitate magnesium potassium phosphate. Then the pH is adjusted to 11 to precipitate calcium phosphate or other slow-release components. The precipitate is collected to obtain solid fast-acting microparticles; the concentrated filtration water and the remaining mother liquor are returned to the sludge conditioning unit. (7) Solid hydrothermal carbon and solid fast-acting microparticles are mixed at a ratio of 40:1 and then transported to a composite granulation molding system. The concentrated liquid is used as a binder. After granulation and drying at 70°C, carbon-based slow-release compound fertilizer with a particle size of 5 mm and a moisture content of 15% is obtained.

Claims

1. A two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method, characterized in that, It includes the following steps: (1) Mix municipal sludge with crushed agricultural organic fiber solid waste in proportion, reuse the filtered water from the membrane concentration unit to condition the sludge, adjust the moisture content of the mixture, and form a homogeneous sludge. (2) The sludge obtained in step (1) is preheated by a shell-and-tube heat exchanger and then enters the low-temperature hydrolysis release section of the integrated two-stage reaction system. High-temperature steam is directly mixed and heated to 150-180℃ for hot water hydrolysis reaction, which destroys the sludge cell structure, hydrolyzes organic matter, and promotes the migration of nutrients such as nitrogen and phosphorus to the liquid phase. (3) The material after the reaction in step (2) is transported to the online buffer tank in the integrated two-stage reaction system. After the material state is monitored and adjusted, it enters the high-temperature carbonization and curing section. (4) In the high-temperature carbonization and solidification section, the material obtained in step (3) is mixed with steam pipe at 260-300℃ for hydrothermal carbonization reaction. Nutrients are fixed in situ and the bioavailability of heavy metals is reduced simultaneously through carbon skeleton reconstruction, and hydrothermal carbon and reaction liquid are obtained. (5) The reaction product obtained in step (4) is subjected to two-stage flash evaporation to recover part of the heat, which is used for granulation drying in the composite granulation molding system. After flash evaporation, the material is preheated by a shell-and-tube heat exchanger in step (1) and then enters the solid-liquid separation system to obtain solid hydrothermal carbon and nutrient-rich liquid phase respectively. (6) The liquid phase obtained in step (5) is transported to the membrane concentration unit and concentrated by 5-8 times; a precipitation / crystallization inducer is added to the concentrate and the pH is adjusted to grade and precipitate the fast-acting crystalline fertilizer components. The precipitate is collected to obtain solid fast-acting microparticles; the concentrated filtered water is returned to the diluted sludge in step (1). (7) The solid hydrothermal carbon obtained in step (5) and the solid fast-acting microparticles in step (6) are mixed in proportion and transported to the composite granulation molding system. The reserved concentrate in step (6) is used as a binder, and carbon-based slow-release compound fertilizer is obtained by granulation and drying.

2. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, In step (1), municipal sludge and agricultural organic fiber solid waste are mixed in a dry weight ratio of 5:1-3:1, and the moisture content of the mixture is adjusted to 83%-85%.

3. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, The agricultural organic fiber solid waste is selected from straw, rice husks, corn cobs, fruit shells, wood chips, or any combination thereof.

4. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, The integrated two-stage reaction system in steps (2), (3), and (4) has a tubular hydrothermal carbonization reactor at the front and rear ends, and a controllable temperature and pressure transfer unit between adjacent stages to maintain the material temperature drop of no more than 20°C.

5. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, The low-temperature hydrolysis release section is supplemented with a weakly acidic hydrolysis aid or an enzymatic hydrolysis agent to enhance the hydrolysis of organophosphorus compounds and the release of nitrogen. The reaction time is 30-60 min.

6. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, The buffer mixing tank in step (3) is equipped with an online ion-selective electrode or a portable spectrophotometer / colorimetric sensor to achieve real-time determination of NH4⁺-N and PO4³⁻-P, and input the determination results into the edge controller to drive the additive dosing pump.

7. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method according to claim 1, characterized in that, One or more of Mg²⁺, Fe³⁺ or Ca²⁺ are added in the high-temperature carbonization and solidification section to stabilize and fix nutrients and reduce the risk of heavy metal leaching. The reaction time is 30-60 min.

8. The method for two-stage hydrothermal carbonization and solid-liquid coupling nutrient recovery of sludge as described in claim 1, characterized in that, The main equipment of the solid-liquid separation system includes a plate and frame filter press and a belt filter press. The selective concentration of the liquid phase is carried out by vacuum distillation, membrane distillation or membrane separation.

9. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method as described in claim 1, characterized in that, The precipitation / crystallization inducer in step (6) is preferably a soluble salt of Mg²⁺ and K⁺ or Ca²⁺. The graded crystallization includes: in the first stage, under pH 8-9 control, magnesium potassium phosphate (particle size 1-500 μm) is preferentially precipitated; in the second stage, under pH 9-10 conditions, calcium phosphate or other slow-release components are precipitated. In step (7), solid fast-acting microparticles and hydrothermal carbon are mixed at a mass ratio of 1:10-1:50 and composite slow-release particles are formed in a granulation process at 50-100℃. Optionally, a moisture-sensitive / degradable polymer coating treatment is performed to regulate the release kinetics. The granulation method is disc granulation, roller granulation or extrusion granulation. The resulting fertilizer particles have a particle size of 2-5 mm and a moisture content of ≤15%.

10. The two-stage sludge hydrothermal carbonization and solid-liquid coupling nutrient recovery method as described in claim 1, characterized in that, The carbon-based slow-release compound fertilizer in step (7) has the characteristics of rapid and slow release synergistic release, with a slow release period of not less than 45 days.

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