A method for producing clean fuels using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization.

Abstract

This invention discloses a method for producing clean fuels using sludge via piezoelectric catalysis-assisted hydrothermal carbonization. Specifically, a piezoelectric catalyst with a strong piezoelectric effect is prepared using a hydrothermal synthesis method and a heterostructure. The method includes mixing sludge with deionized water, adding the piezoelectric catalyst for piezoelectric catalytic pretreatment, and then subjecting the pretreated sludge to a hydrothermal carbonization reaction at 180°C to 240°C. After the reaction, solid-liquid separation is performed to obtain clean hydrothermal carbon. The piezoelectric catalyst employs a Bi2WO6@SnS2 heterojunction structure, which generates free radicals under ultrasonic treatment, destroying extracellular polymers and cellular structures in the sludge, releasing nitrogen-containing substances, inhibiting Maillard reactions, and reducing the formation of nitrogen oxides. This invention reduces the nitrogen content in the hydrothermal carbon, realizing the resource utilization of sludge and the production of low-nitrogen clean fuels, and has the advantages of improved energy efficiency, reduced operating costs, and reduced environmental pollution.

Description

Technical Field

[0001] This invention relates to the resource utilization of sludge in the field of environmental engineering, and more particularly to a method for producing clean fuels using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization. Background Technology

[0002] With rapid industrialization and urbanization, municipal sewage sludge treatment faces enormous challenges. Sludge is a byproduct of wastewater treatment, and due to its large volume and high polluting potential, its effective management has become an international issue. Wastewater sludge contains abundant organic matter and microorganisms; proper treatment can reduce environmental pollution while effectively utilizing resources, which is of great significance.

[0003] Currently, the most common methods for disposing of sludge are landfill and incineration, which not only occupy a large amount of land resources but also result in low energy utilization. Hydrothermal carbonization, on the other hand, is a technology that uses water as a solvent and reaction medium, and at 180℃-250℃ and autogenous pressure, dehydrates and decarboxylates sludge to obtain carbon-based products. This technology can directly treat sludge with high water content, achieving sludge volume reduction; it can also utilize high temperatures for biological sterilization and decomposition, reducing subsequent environmental pollution and achieving sludge harmlessness; simultaneously, hydrothermal carbonization produces hydrothermal carbon, densifying the energy of the sludge and enabling resource utilization. However, the hydrothermal carbon produced by current sludge hydrothermal carbonization has a high nitrogen content, mainly because the extracellular polymeric substances (EPS) of the sludge cannot be destroyed during the hydrothermal carbonization process, limiting its potential use as a direct substitute for fossil fuels or as a blend with fossil fuels. To address this issue, research often involves pretreating the sludge, such as adding chemical reagents or using advanced oxidation methods. These methods deeply disrupt the sludge's flocs, EPS (extracellular matrix components), and even cellular structure, releasing organic matter and nitrogen-containing precursors. However, achieving better results typically requires the addition of large doses of chemicals, which limits their commercial application.

[0004] Considering the vibration processes involved in sludge disposal, such as transportation, filtration, and pressure dewatering, effectively utilizing and enhancing this vibration energy to achieve initial sludge breakdown would benefit energy conservation and subsequent low-NOx hydrothermal char production. Piezoelectric catalysis is a technology that relies on absorbing vibration energy to generate free radicals and remove organic matter. This technology has gradually attracted attention due to its high energy-to-electricity conversion efficiency and convenient energy utilization. It is reported that the electromechanical energy conversion efficiency based on the piezoelectric effect can reach 78%, far exceeding the 20% energy conversion efficiency of photovoltaics. Currently, commonly used piezoelectric catalysts include Bi₂WO₆, BiTiO₃, and TiO₂. Among them, Bi₂WO₆, with its low cost and large spontaneous polarization (Ps≈50μC cm⁻¹), is particularly valuable. -2 The characteristics of Bi₂WO₆, such as its good piezoelectric response, have gradually attracted attention. However, the low electron-hole pair separation efficiency of Bi₂WO₆ affects its catalytic efficiency and further applications.

[0005] Constructing heterostructures on piezoelectric catalysts to form novel piezoelectric catalytic materials is an effective method to improve electron-hole separation efficiency. Currently, the main method for constructing heterostructures is based on transition metal sulfides, such as CoS... x CdS, MoS2, SnS2, and CuS, among others, have gradually attracted attention and are used due to their excellent catalytic performance and unique band gaps, showing great promise for application. SnS2, in particular, has garnered significant attention for its high conductivity and stability in pollutant removal and chemical catalysis.

[0006] Given the above background, current technologies for producing clean fuels through hydrothermal carbonization of sludge face the following problems and challenges:

[0007] (1) The hydrothermal carbon produced by sludge hydrothermal carbonization has a high nitrogen content and exists in a relatively stable form. These nitrogen-containing substances generate fuel-type NO during combustion. x This affects equipment operation and causes environmental pollution;

[0008] (2) The nitrogen-containing substances in sludge are mainly proteins. Most of these proteins are found in sludge EPS. The stubbornness of EPS structure is the key to the above problems. The strong hydrogen bonding in EPS makes it impossible to be effectively destroyed and release proteins during hydrothermal processes. This further affects the decomposition of these substances under dehydration, decarboxylation and other reactions, ultimately resulting in low carbonization and high nitrogen content.

[0009] Therefore, there is an urgent need to find a method to produce low-nitrogen hydrothermal carbon clean fuel by destroying the extracellular polymers of sludge with low energy consumption and low dosage, releasing organic matter and nitrogen-containing precursors. Summary of the Invention

[0010] Objective of the Invention: To address the shortcomings of existing sludge hydrothermal carbonization technologies, this invention proposes a method for producing clean fuels using piezoelectric catalysis-assisted hydrothermal carbonization of sludge. By applying piezoelectric catalysis technology to the hydrothermal carbonization process of sludge, the extracellular polymers and cell structures of the sludge are destroyed, reducing the nitrogen content in the hydrothermal carbon and decreasing nitrogen oxide emissions during solid fuel combustion. This invention achieves the resource utilization of sludge and the production of low-nitrogen clean fuels, offering advantages such as improved energy efficiency, reduced operating costs, and reduced environmental pollution.

[0011] Technical Solution: To achieve the above objectives, this invention provides a method for producing clean fuels using sludge through piezoelectric catalytic-assisted hydrothermal carbonization, comprising the following steps:

[0012] (1) The sludge was mixed with deionized water, and the prepared bismuth tungstate piezoelectric catalyst was added according to the proportion of the sludge dry weight. The mixture was then placed in an ultrasonic machine for piezoelectric catalytic pretreatment. The preparation process of the bismuth tungstate piezoelectric catalyst is as follows:

[0013] (1.1) Using a hydrothermal synthesis method, bismuth nitrate pentahydrate was dissolved in glacial acetic acid, sodium tungstate dihydrate solution was added, and the mixture was reacted at high temperature, washed and dried to obtain bismuth tungstate nanosheets.

[0014] (1.2) Dissolve tin tetrachloride pentahydrate and thioacetamide in deionized water, react at high temperature, wash and dry to obtain tin sulfide;

[0015] (1.3) Bismuth tungstate and tin sulfide nanosheets were dispersed in anhydrous ethanol, and the ethanol was evaporated by heating after magnetic stirring to obtain a heterojunction piezoelectric catalyst.

[0016] (2) The pretreated sludge is placed in a hydrothermal reactor and subjected to hydrothermal carbonization at 180℃~240℃ for several hours. After cooling to room temperature, solid-liquid separation is performed, and the solid phase is collected to obtain clean hydrothermal carbon.

[0017] In step (1), the sludge is mixed with deionized water, and the prepared piezoelectric catalyst is added at 10% of the dry weight of the sludge. The mixture is then placed in an ultrasonic machine for piezoelectric catalytic pretreatment for 3 to 6 hours.

[0018] In step (1.1), bismuth nitrate pentahydrate was dissolved in glacial acetic acid using a hydrothermal synthesis method; sodium tungstate dihydrate solution was added, and the mixture was transferred to an autoclave and reacted at high temperature for several hours. The white product was then washed with anhydrous ethanol and deionized water and dried at medium temperature to obtain bismuth tungstate nanosheets.

[0019] In step (1.1), the hydrothermal reaction temperature is 180℃ and the drying temperature is 60℃.

[0020] In step (1.1), the volume of glacial acetic acid is 15 mL, the volume of sodium tungstate dihydrate solution is 25 mL, the volume ratio of the two is 3:5, and the concentration of bismuth nitrate pentahydrate glacial acetic acid solution is [missing information]. The concentration of sodium tungstate dihydrate solution is 0.04 mol / L.

[0021] In step (1.2), the molar ratio of tin tetrachloride pentahydrate to thioacetamide is 1:2, and the volume of deionized water is 40 ml.

[0022] In step (1.2), tin tetrachloride pentahydrate and thioacetamide are dissolved in deionized water and reacted at high temperature for several hours; the pale yellow product is washed with anhydrous ethanol and deionized water and dried to obtain tin sulfide.

[0023] In step (1.3), the evaporation temperature is 70°C and the mass ratio of bismuth tungstate to tin sulfide is 20:1.

[0024] In step (2), the nitrogen removal rate formula for hydrothermal carbon is:

[0025]

[0026] In step (2), the pretreated sludge is placed in a hydrothermal reactor and reacted at 180℃~240℃ for several hours. After cooling to room temperature, solid-liquid separation is performed using a filtration device, and the solid phase is collected to obtain clean hydrothermal carbon.

[0027] In step (1), the ratio of sludge to deionized water is 1:1 (g:ml); the processing power of the ultrasonic machine is 180W, and the pretreatment time is 3 / 6 hours.

[0028] In step (2), the hydrothermal reaction temperature is 190℃ and the drying temperature is 60℃.

[0029] Working Principle: This invention utilizes a piezoelectric catalytic-assisted hydrothermal carbonization method to produce low-NOx clean hydrothermal char. A piezoelectric catalyst with a strong piezoelectric effect is prepared through hydrothermal synthesis and heterojunction construction. The method involves mixing sludge with deionized water, adding the piezoelectric catalyst for pretreatment, and then subjecting the pretreated sludge to hydrothermal carbonization at 180°C to 240°C. Solid-liquid separation is performed after the reaction to obtain clean hydrothermal char. The piezoelectric catalyst employs a Bi2WO6@SnS2 heterojunction structure; that is, in the core working principle of this invention, Bi2WO6@SnS2 is used as the piezoelectric catalyst, which generates ·OH and ·O under piezoelectric vibration. 2-Free radicals. These free radicals effectively disrupt the extracellular polymeric substances (EPS) and cell wall structure of sludge, thereby releasing large amounts of nitrogenous substances. During the further depolymerization of these released nitrogenous substances, especially proteins under the high-temperature environment of hydrothermal carbonization, they are broken down into low-molecular-weight organic matter, such as soluble proteins and amino acids. This decomposition not only increases the content of ammonia nitrogen and total nitrogen in the liquid phase, but also inhibits the Maillard reaction by reducing the precursors of the Maillard reaction in the sludge, thereby reducing the formation of nitrogenous heterocyclic compounds and lowering the nitrogen retention in the hydrothermal carbon.

[0030] This invention utilizes a prepared heterojunction bismuth tungstate piezoelectric catalyst, which is then placed in an ultrasonic machine to pretreat sludge under piezoelectric vibration. On one hand, by adjusting the catalyst's structure, the piezoelectric effect of the catalyst is improved, enabling the production of high-quality hydrothermal carbon. On the other hand, the hydrothermal carbonization method treats the sludge, achieving both sludge volume reduction and resource utilization in a one-step process.

[0031] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0032] (1) This invention uses hazardous waste—sludge—as raw material and prepares a novel low-nitrogen hydrothermal carbon through hydrothermal carbonization technology, achieving efficient and clean conversion of solid waste. This method not only helps to reduce the environmental burden caused by solid waste landfill and incineration, but also opens up a new path for the resource utilization of solid waste.

[0033] (2) This invention is the first to apply piezoelectric catalysis technology to the hydrothermal carbonization process of sludge, which destroys the extracellular polymers and cell structure of sludge, reduces the nitrogen content in hydrothermal carbon, and reduces nitrogen oxide emissions during solid fuel combustion. This is of great environmental significance for controlling air pollution, especially reducing the formation of urban smog and acid rain.

[0034] (3) In the hydrothermal carbonization process of this invention, the extracellular polymers and cell structures in the pretreated sludge are destroyed, making the water removal process easier, thereby significantly reducing energy consumption in the dehydration stage of hydrothermal carbonization. This improvement enhances the energy efficiency of the entire production process while reducing operating costs. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the method for producing clean fuels using sludge based on piezoelectric catalysis-assisted hydrothermal carbonization according to the present invention;

[0036] Figure 2 Infrared spectra of hydrothermal carbon prepared under different conditions according to the present invention;

[0037] Figure 3The peak intensity diagram and nitrogen content variation diagram of the hydrothermal carbon produced under different piezoelectric catalytic pretreatment conditions at 240℃ are shown in the present invention.

[0038] Figure 4 These are paradigm diagrams for different conditions of the present invention;

[0039] Figure 5 The graph shows the hydrothermal carbon yield under different conditions according to the present invention.

[0040] Figure 6 Here are the SEM and TEM images of the piezoelectric catalyst synthesized in this invention;

[0041] Figure 7 The image shows the XRD characterization of the piezoelectric catalyst synthesized in this invention.

[0042] Figure 8 This is a fine XPS image of the piezoelectric catalyst synthesized in this invention;

[0043] Figure 9 This is a piezoelectric hysteresis diagram of the piezoelectric catalyst synthesized in this invention. Detailed Implementation

[0044] like Figure 1 As shown, this invention first prepares a bismuth tungstate piezoelectric catalyst. The method for preparing the heterojunction bismuth tungstate piezoelectric catalyst in the examples includes the following steps:

[0045] Step (1): Using a hydrothermal synthesis method, 0.9701 g of bismuth nitrate pentahydrate was dissolved in 15 mL of glacial acetic acid. Then, 25 mL of 0.04 mol / L Na₂WO₄·2H₂O solution was added, and the mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave. After reacting at 180°C for 24 hours, the white product was washed three times with anhydrous ethanol and deionized water, and then dried at a moderate temperature of 60°C for 10 hours to obtain Bi₂WO₆ nanosheets.

[0046] Step (2): 2.1 g of tin tetrachloride pentahydrate and 0.9 g of thioacetamide were dissolved in 40 mL of deionized water. The solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave maintained at 190 °C and reacted for 13 hours. Finally, the pale yellow product was purified by washing with anhydrous ethanol and deionized water, respectively. The SnS2 sample was dried at 60 °C for 12 hours to obtain the final product.

[0047] Step (3): 2 g of Bi2WO6 and 20 mg of SnS2 nanosheets were dispersed in 20 mL of wastewater ethanol and then magnetically stirred for 3 hours. Finally, the suspension was heated to 70 degrees Celsius and the ethanol was evaporated for 12 hours to obtain the heterojunction piezoelectric catalyst.

[0048] like Figure 1 The following section describes a method for producing clean fuels using sludge through piezoelectric catalytic-assisted hydrothermal carbonization, comprising the following steps:

[0049] Step 1: Take 30 grams of sludge and place it in 30 ml of water. Add catalyst at 10% of the dry weight of the sludge. Then, put it into a 180W ultrasonic machine for piezoelectric catalytic pretreatment for 3 hours or 6 hours.

[0050] Step 2: After the reaction is complete, place it in a hydrothermal reactor and react at 180–240 degrees Celsius for 1 hour, then cool to room temperature. Separate the solid and liquid phases by vacuum filtration and collect the hydrothermal liquid. At the same time, dry the hydrothermal carbon in an oven at 60 degrees Celsius for 12 hours, grind and collect it.

[0051] The nitrogen removal rate formula for hydrothermal char is as follows:

[0052]

[0053] Example 1

[0054] Pretreatment time: 3 hours, hydrothermal carbonization temperature: 180℃

[0055] Pretreatment: Sludge and deionized water were mixed at a ratio of 1:1 (g:ml), and 10% of the dry weight of the sludge was added with the synthesized Bi2WO6@SnS2 piezoelectric catalyst. The mixture was placed in an ultrasonic machine with a processing power of 180W for 3 hours.

[0056] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 180°C for 1 hour. After the reaction is complete, it is cooled to room temperature, and solid-liquid separation is performed using a vacuum filtration device. The solid phase is collected to obtain clean hydrothermal carbon.

[0057] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 51.06%.

[0058] Example 2

[0059] Pretreatment time: 3 hours, hydrothermal carbonization temperature: 240℃

[0060] Pretreatment: Same as in Example 1, the pretreatment time is 3 hours.

[0061] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 240°C for 1 hour. The remaining steps are the same as in Example 1.

[0062] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 61.47%.

[0063] Example 3

[0064] Pretreatment time: 6 hours, hydrothermal carbonization temperature: 180℃

[0065] Pretreatment: Same as in Example 1, the pretreatment time is 6 hours.

[0066] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 180°C for 1 hour. The remaining steps are the same as in Example 1.

[0067] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 56.03%.

[0068] Example 4

[0069] Pretreatment time: 6 hours, hydrothermal carbonization temperature: 240℃

[0070] Pretreatment: Same as in Example 1, the pretreatment time is 6 hours.

[0071] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 240°C for 1 hour. The remaining steps are the same as in Example 1.

[0072] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 71.16%.

[0073] Comparative Example 1

[0074] Without pretreatment, the hydrothermal carbonization temperature is 180℃.

[0075] Hydrothermal carbonization: The sludge is directly placed into a hydrothermal reactor and reacted at 180°C for 1 hour. After the reaction is complete, it is naturally cooled to room temperature, and solid-liquid separation is performed using a vacuum filtration device. The solid phase is collected to obtain hydrothermal carbon.

[0076] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 26.24%.

[0077] Comparative Example 2

[0078] Without pretreatment, the hydrothermal carbonization temperature is 240℃.

[0079] Hydrothermal carbonization: Same as Comparative Example 1, the hydrothermal carbonization temperature was 240℃.

[0080] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 43.97%.

[0081] Comparative Example 3

[0082] Pretreatment time: 3 hours, hydrothermal carbonization temperature: 180℃, catalyst: Bi2WO6

[0083] Pretreatment: Mix sludge and deionized water at a ratio of 1:1 (g:ml), and add Bi2WO6 catalyst at 10% of the dry weight of the sludge. Place the mixture in an ultrasonic machine with a processing power of 180W for 3 hours.

[0084] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 180°C for 1 hour. The remaining steps are the same as in Example 1.

[0085] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 43.74%.

[0086] Comparative Example 4

[0087] Pretreatment time: 3 hours, hydrothermal carbonization temperature: 240℃, catalyst: Bi2WO6

[0088] Pretreatment: Same as Comparative Example 3, the pretreatment time was 3 hours.

[0089] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 240°C for 1 hour. The remaining steps are the same as in Example 1.

[0090] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 56.97%.

[0091] Comparative Example 5

[0092] Pretreatment time: 6 hours, hydrothermal carbonization temperature: 180℃, catalyst: Bi2WO6

[0093] Pretreatment: Same as Comparative Example 3, the pretreatment time was 6 hours.

[0094] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 180°C for 1 hour. The remaining steps are the same as in Example 1.

[0095] Results: The nitrogen removal rate of the obtained hydrothermal carbon was 51.06%.

[0096] Comparative Example 6

[0097] Pretreatment time: 6 hours, hydrothermal carbonization temperature: 240℃, catalyst: Bi2WO6

[0098] Pretreatment: Same as Comparative Example 3, the pretreatment time was 6 hours.

[0099] Hydrothermal carbonization: The pretreated sludge is placed in a hydrothermal reactor and reacted at 240°C for 1 hour. The remaining steps are the same as in Example 1.

[0100] The hydrothermal carbon and nitrogen removal rates produced by the present invention using sludge based on piezoelectric catalytic assisted hydrothermal carbonization and those without piezoelectric catalytic pretreatment are shown in Table 1.

[0101] Table 1

[0102]

[0103]

[0104] As shown in Table 1, the nitrogen removal rate of hydrothermal carbon produced by the present invention using sludge based on piezoelectric catalytic assisted hydrothermal carbonization is 51.06% to 71.16%, which can effectively produce low-nitrogen hydrothermal carbon.

Claims

1. A method for producing clean fuel using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization, characterized in that, Includes the following steps: (1) The sludge was mixed with deionized water, and the prepared bismuth tungstate piezoelectric catalyst was added at a ratio of 10% of the dry weight of the sludge. The mixture was then placed in an ultrasonic machine for piezoelectric catalytic pretreatment for 3-6 hours. The preparation process of the bismuth tungstate piezoelectric catalyst is as follows: (1.1) Using a hydrothermal synthesis method, bismuth nitrate pentahydrate was dissolved in glacial acetic acid, sodium tungstate dihydrate solution was added, and the mixture was reacted at high temperature, washed and dried to obtain bismuth tungstate nanosheets. (1.2) Dissolve tin tetrachloride pentahydrate and thioacetamide in deionized water, react at high temperature, wash and dry to obtain tin sulfide; (1.3) Bismuth tungstate and tin sulfide nanosheets were dispersed in anhydrous ethanol, and the ethanol was evaporated by heating after magnetic stirring to obtain a heterojunction piezoelectric catalyst; the evaporation temperature was 70℃, and the mass ratio of bismuth tungstate and tin sulfide was 20:

1. (2) The pretreated sludge is placed in a hydrothermal reactor and subjected to hydrothermal carbonization at 180℃~240℃ for several hours. After cooling to room temperature, solid-liquid separation is performed, and the solid phase is collected to obtain clean hydrothermal carbon.

2. The method for producing clean fuel using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (1.1), bismuth nitrate pentahydrate was dissolved in glacial acetic acid using a hydrothermal synthesis method; sodium tungstate dihydrate solution was added, and the mixture was transferred to an autoclave and reacted at high temperature for several hours. The white product was then washed with anhydrous ethanol and deionized water and dried at medium temperature to obtain bismuth tungstate nanosheets.

3. The method for producing clean fuel using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (1.1), the hydrothermal reaction temperature is 180℃ and the drying temperature is 60℃.

4. The method for producing clean fuels using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (1.1), the volume of glacial acetic acid is 15 mL, the volume of sodium tungstate dihydrate solution is 25 mL, and the concentration of bismuth nitrate pentahydrate glacial acetic acid solution is [missing information]. The concentration of sodium tungstate dihydrate solution is 0.04 mol / L.

5. The method for producing clean fuels using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (1.2), the molar ratio of tin tetrachloride pentahydrate to thioacetamide is 1:

2.

6. The method for producing clean fuels using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (1.2), tin tetrachloride pentahydrate and thioacetamide are dissolved in deionized water and reacted at high temperature for several hours; the pale yellow product is washed with anhydrous ethanol and deionized water and dried to obtain tin sulfide.

7. The method for producing clean fuel using sludge based on piezoelectric catalytic-assisted hydrothermal carbonization according to claim 1, characterized in that: In step (2), the pretreated sludge is placed in a hydrothermal reactor and reacted at 180℃~240℃ for several hours. After cooling to room temperature, solid-liquid separation is performed using a filtration device, and the solid phase is collected to obtain clean hydrothermal carbon.

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