A biochar-based solar evaporator suitable for biogas slurry concentration and a preparation method and application thereof
By activating and pore-forming agricultural and forestry waste and blackening its surface, a biochar-based solar evaporator was prepared, which solved the problems of complex equipment, high cost and nutrient loss in biogas slurry concentration. It achieved efficient biogas slurry concentration and nutrient reflux, and improved the application value of biogas slurry.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HUAZHONG AGRI UNIV
- Filing Date
- 2024-03-26
- Publication Date
- 2026-07-07
AI Technical Summary
Existing biogas slurry concentration technologies suffer from problems such as complex equipment, high cost, severe loss of biogas slurry nutrients, and low evaporation efficiency. In particular, biochar-based solar evaporators are easily affected by nitrogen, phosphorus, and potassium components during biogas slurry concentration, leading to evaporator blockage and a decline in light-to-heat conversion performance.
Using agricultural and forestry waste as raw materials, a hot alkaline solution is prepared by mixing wood ash, saponin, and shell powder to activate and create pores. The pores are then blackened using a high-temperature iron plate to prepare a biochar-based solar evaporator, which enhances the absorption of sunlight and the performance of light-to-heat conversion. At the same time, the natural structure of agricultural and forestry waste is utilized to transport moisture and return nutrients.
It significantly improves the efficiency of biogas slurry concentration, reduces nutrient loss, enhances the transmission smoothness of the evaporator and the light-to-heat conversion efficiency, and achieves a highly efficient biogas slurry concentration effect.
Smart Images
Figure CN118142186B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of biogas slurry resource utilization, specifically relating to a biochar-based solar evaporator suitable for biogas slurry concentration, its preparation method, and its application. Background Technology
[0002] my country has a massive livestock and poultry farming industry, producing up to 4 billion tons of manure annually. Anaerobic fermentation is a green and practical method for treating livestock and poultry manure, generating a large amount of biogas slurry containing nutrients such as nitrogen, phosphorus, potassium, and organic acids. This biogas slurry can be applied to farmland as an organic liquid fertilizer to improve crop yield and quality. Currently, farmland soil's capacity to absorb biogas slurry is far lower than the amount produced, leading to nutrient leaching and runoff, damaging soil structure and microbial ecosystems, and causing serious environmental pollution. Furthermore, biogas slurry is 90% water, resulting in low nutrient quality per unit volume. Its large mass and volume also increase storage and transportation costs, limiting its widespread application as a liquid fertilizer.
[0003] Biogas slurry concentration is the main method for concentrating and enriching nutrients in biogas slurry. Existing biogas slurry concentration technologies mainly include membrane separation, thermal evaporation, and cryoextraction, among which membrane separation and thermal evaporation are suitable for engineering applications. Membrane separation utilizes microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes to separate water and nutrients from biogas slurry. This technology is mature and widely used, but it suffers from drawbacks such as high membrane cost, short lifespan, and susceptibility to fouling. Thermal evaporation uses a heat source to evaporate and separate water from biogas slurry, offering high efficiency and simple operation, making it suitable for engineering applications of biogas slurry concentration. The key to thermal evaporation technology is selecting a suitable heat source. Currently, the mainstream technologies primarily use electrically driven heat pumps, which are costly and do not align with the green, low-carbon, and sustainable development principles.
[0004] Utilizing solar radiation heat to evaporate water and enrich nutrients in biogas slurry is a green and low-carbon biogas slurry concentration technology. Chinese Patent Publication No. CN112742045A discloses a heat storage solar biogas slurry evaporation and concentration experimental device. This invention overcomes the shortcomings of existing solar biogas slurry concentration devices, such as their inconvenience for experimental research and the lack of related experimental equipment in laboratories. However, the high cost of solar evaporator materials and the complex overall structure of the device hinder practical application.
[0005] Developing low-cost, high-efficiency solar evaporators is key to advancing biogas slurry solar evaporation and concentration technology. Among numerous solar energy absorbing materials, carbon materials possess broad-band solar wavelength absorption, good photostability, and wide availability, making them highly promising. Biochar derived from agricultural and forestry crop stems stands out among various carbon materials due to its excellent water absorption, water transport, and strong solar energy absorption properties. Existing inventions and research on biochar-based solar evaporators primarily focus on seawater desalination. For example, Chinese patent CN113979504A discloses a solar interface evaporator based on a pine-shaped biomimetic structure; Chinese patent CN112978826A discloses an algae-based biochar suitable for solar seawater desalination and its preparation method. However, no biochar-based solar evaporators have yet been applied to biogas slurry concentration.
[0006] Biochar-based solar evaporators offer numerous advantages for biogas slurry concentration. As a green and environmentally friendly solar interface water evaporation material, it can efficiently evaporate water from biogas slurry. Furthermore, the concentrated biochar, having absorbed a certain amount of nitrogen, phosphorus, and potassium nutrients from the biogas slurry, can be further used as a biochar-based slow-release fertilizer, applied to the fields along with the concentrated biogas slurry. However, applying biochar-based solar evaporators to biogas slurry concentration also presents some challenges. For example, nitrogen, phosphorus, and potassium components in biogas slurry often accumulate in the solar evaporator along with the water. Excessive absorption of nutrients by the evaporator leads to a decrease in the nutrient content of the biogas slurry, blockage of the evaporator's water transport channels, and limited light absorption and light-to-heat conversion performance, ultimately inhibiting the efficiency of biogas slurry evaporation and concentration. Summary of the Invention
[0007] The purpose of this invention is to overcome the shortcomings and deficiencies of existing technologies and provide a method for preparing a biochar-based solar evaporator suitable for biogas slurry concentration, as well as a biochar-based solar evaporator obtained by this method and its application in biogas slurry concentration, achieving the concentration of nitrogen, phosphorus, and potassium nutrients in biogas slurry. This invention uses agricultural and forestry waste such as corn stalks, sorghum stalks, bamboo, and corn cobs as raw materials. A hot alkaline solution prepared with wood ash, saponin, and shell powder is used to slightly remove lignin from the agricultural and forestry waste, activating and creating pores. This results in richer porosity, which enhances the absorption of water from the biogas slurry by the vascular bundles and sieve tubes of the agricultural and forestry crop stalks, allowing water to be continuously transferred to the photothermal conversion interface. Furthermore, the larger physical cavity reduces the siphon effect of the evaporator on nitrogen, phosphorus, and potassium; when the concentration is too high, nutrients will flow back into the biogas slurry. The surface of the agricultural and forestry waste is then blackened using a high-temperature iron plate to enhance its ability to capture and absorb sunlight, improving the photothermal conversion rate, and thus preparing a biochar-based solar evaporator with excellent biogas slurry concentration performance.
[0008] The objective of this invention is achieved through the following technical solution:
[0009] A method for preparing a biochar-based solar evaporator suitable for biogas slurry concentration includes the following steps:
[0010] (1) Cut agricultural and forestry waste into cylindrical shapes and wash them with water.
[0011] (2) Add plant ash, saponin, and shell powder to water and heat to obtain a hot alkaline solution.
[0012] (3) Soak the agricultural and forestry waste processed in step (1) into the hot alkaline solution obtained in step (2) for activation and pore formation. Take out the agricultural and forestry waste and wash it with water until it is neutral or neutralize it with acid solution and then wash it with water until it is neutral, and then dry it.
[0013] (4) The agricultural and forestry waste processed in step (3) is blackened by high temperature to obtain a biochar-based solar evaporator.
[0014] In step (1), the agricultural and forestry waste includes sorghum stalks, corn stalks, bamboo, corn cobs, etc.; the agricultural and forestry waste is preferably cut into cylinders with a height and a diameter of 2 to 5 cm, and the upper and lower circular cross sections are flattened.
[0015] In step (2), the preferred mass ratio of wood ash, saponin, and shell powder is 10:10:5 to 10:10:1; the preferred temperature of the hot alkaline solution is 70 to 80°C. Further, the hot alkaline solution in step (2) is prepared by a method including the following steps: adding wood ash, saponin, and shell powder to water, heating to 70 to 80°C in a water bath, stirring for 10 to 30 minutes to obtain the hot alkaline solution, and continuing to maintain the temperature at 70 to 80°C for subsequent steps.
[0016] In step (3), the soaking time of agricultural and forestry waste in a hot alkaline solution for activation and pore formation is preferably 10-20 minutes. The drying treatment is preferably carried out at 45-55°C for 18-24 hours.
[0017] In step (4), the conditions for high-temperature blackening are preferably 200℃~400℃ for 5~20min, and it is preferable to use a high-temperature iron plate for blackening. Further, after obtaining the biochar-based solar evaporator in step (4), the process also includes washing away impurities with water and drying, wherein the drying is preferably carried out at 45~55℃ for 18~24h.
[0018] A biochar-based solar evaporator suitable for biogas slurry concentration is obtained by the above preparation method.
[0019] The application of the biochar-based solar evaporator in biogas slurry concentration.
[0020] The method for concentrating biogas slurry using the aforementioned biochar-based solar evaporator includes the following steps: the biochar-based solar evaporator is nested inside a foam board and then added to the biogas slurry, and the biogas slurry is concentrated under sunlight.
[0021] The biogas slurry mentioned above includes biogas slurry obtained from the anaerobic fermentation of livestock and poultry manure such as pig manure, cow manure, and chicken manure.
[0022] The principle of this invention is as follows: Biochar-based solar evaporators are used to concentrate biogas slurry, requiring biochar to possess excellent solar absorption and light-to-heat conversion properties. During the process of scorching agricultural and forestry waste (crop stalks) with a high-temperature iron plate, the flattened circular cross-section of the waste contacts the iron plate. This circular cross-section acts as a light absorption interface, and its surface develops an uneven, rough microstructure after heating, effectively intercepting incident sunlight, reducing sunlight reflection, and increasing multiple refractions in the light path. Furthermore, the organic matter in the agricultural and forestry waste carbonizes and turns black upon heating, producing aromatic substances rich in π bonds. These π bonds can reduce the band gap between the highest occupied orbital (HOMO) and lowest unoccupied orbital (LUMO). When the incident light energy fully absorbed by the biochar equals the band gap value, electrons are excited from the HOMO to the LUMO and then relax to the ground state, releasing heat in the process to increase the temperature of the biochar-based solar evaporator.
[0023] During the evaporation of biogas slurry by a solar evaporator, the water in the biogas slurry needs to be continuously transferred to the photothermal conversion interface of the evaporator material. Agricultural and forestry wastes such as corn stalks, sorghum stalks, bamboo, and corn cobs contain a large number of vascular bundles and sieve tubes. These naturally formed structures can play a role in transporting water and nutrients. By heating an alkaline solution prepared from wood ash, saponin, and shell powder, some lignin in the agricultural and forestry waste raw materials is dissolved and separated. The physical cavities created after the lignin is removed form new channels, ensuring that the biogas slurry water is promptly transported to the photothermal conversion interface to fully absorb heat, while also allowing the highly concentrated nitrogen, phosphorus, and potassium nutrients to flow back into the aqueous solution.
[0024] This invention develops a biochar-based solar interface water evaporation material suitable for biogas slurry concentration using a two-step method of "activation and pore-forming - surface blackening," synergistically realizing the light-heat conversion, water transfer, and nutrient reflux of the evaporator during the concentration process. Using agricultural and forestry waste as raw material, this invention is of great significance for expanding new methods for biogas slurry concentration, opening up new application pathways for biochar, and enhancing the utilization value of agricultural and forestry waste.
[0025] Compared with the prior art, the present invention has the following advantages and beneficial effects:
[0026] (1) This invention overcomes the shortcomings of existing solar biogas evaporation and concentration technologies, such as complex equipment and high cost. The use of biomass solid waste such as agricultural and forestry crop stalks to prepare solar evaporators for the first time for biogas evaporation and concentration not only realizes "using waste to treat waste and turning waste into treasure", but also greatly enhances the application value of biogas slurry.
[0027] (2) The solar evaporator prepared by the present invention is blackened by heating an iron plate. Unlike the existing oxygen-free pyrolysis carbonization using a tubular furnace, the device and method used in the present invention are simpler and more stable. It maximizes the preservation of the well-developed pore structure, rich hydrophilic oxygen groups and a certain amount of carbonaceous aromatic structure inside agricultural and forestry waste, so that the evaporator has good solar light absorption performance and the light-to-heat conversion efficiency is significantly enhanced.
[0028] (3) The solar evaporator prepared by this invention significantly increases the average pore size of natural pore structures such as vascular bundles and sieve tubes in agricultural and forestry waste through a simple and environmentally friendly activation and pore-forming method. The resulting evaporator has optimized mass and heat transfer performance, which is beneficial to increasing the smoothness of the biogas slurry water transport process. The enlarged diameter pores enable water to be continuously transported to the photothermal conversion interface due to capillary coagulation. This ensures that the biogas slurry water is delivered to the photothermal conversion interface in a timely manner to fully absorb heat, and also allows the highly concentrated nitrogen, phosphorus, and potassium nutrients to flow back into the aqueous solution, reducing nutrient loss in the concentrated biogas slurry and preventing nutrients from clogging the water transport channels.
[0029] (4) The biochar-based solar evaporator of the present invention has excellent biogas slurry concentration performance, which is achieved when the average daily light intensity is set at 2 kW / m². 2 Under a light exposure time of 5 hours, the water evaporation rate of the biochar-based solar evaporator was measured to be 4.8 kg / m³. 2 / h, net daily water reduction is 0.27kg, and biogas slurry concentration efficiency is 54%. Attached Figure Description
[0030] Figure 1 These are scanning electron microscope images of the biochar-based solar evaporators prepared in Comparative Example 1(a) and Example 3(b). (a) The number of pores is small, the distribution is disordered, and most of the cavities are blocked; (b) The number of pores is large, the distribution is regular, and the pores are unobstructed. Detailed Implementation
[0031] The present invention will now be clearly and completely described in conjunction with embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0032] Example 1
[0033] First, cut corn stalks into 16 cylinders with a height and a diameter of approximately 3 cm, ensuring the top and bottom surfaces are flat. Wash the processed corn stalks thoroughly with pure water and dry them in a 55℃ oven for 18 hours. Weigh out 10g of wood ash, 10g of saponin, and 5g of shell powder and mix them evenly in a beaker. Add 1L of pure water and place the mixture in an 80℃ water bath, stirring for 20 minutes. Filter out the residue. Place the resulting solution in an 80℃ water bath again, add the corn stalks to the filtered solution, and stir with a glass rod for 20 minutes. After this, remove the corn stalks and soak them in running pure water for 30 minutes. The pH of the solution after the final soaking should be neutral. Then, dry the washed corn stalks in a 55℃ oven for 18 hours to obtain an activated, porous corn stalk solar evaporator. Place the corn stalk solar evaporator on a high-temperature iron plate at 200℃ for 10 minutes to blacken it, remove the corn stalk and let it cool naturally, soak it in running pure water for 30 minutes to wash away impurities, and then put it in a 55℃ oven to dry for 18 hours to obtain a biochar-based solar evaporator with activated pores and surface blackening treatment.
[0034] The obtained biochar-based solar evaporator was nested inside a 24cm square white polyethylene foam core, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation slurry made from cow manure, weighing 0.5kg. The slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0035] Example 2
[0036] First, cut corn stalks into 16 cylinders with a height and a diameter of approximately 3 cm, ensuring the top and bottom surfaces are flat. Wash the processed corn stalks thoroughly with pure water and dry them in a 55℃ oven for 18 hours. Weigh out 20g of wood ash, 20g of saponin, and 10g of shell powder, mix them evenly in a beaker, add 1L of pure water, and stir in an 80℃ water bath for 20 minutes. Filter out the residue, and place the resulting solution in an 80℃ water bath again. Add the corn stalks to the filtered solution and stir with a glass rod for 20 minutes. After this, remove the corn stalks and soak them in running pure water for 30 minutes. The pH of the solution after the final soaking should be neutral. Then, dry the washed corn stalks in a 55℃ oven for 18 hours to obtain an activated, porous corn stalk solar evaporator. Place the corn stalk solar evaporator on a high-temperature iron plate at 200℃ for 10 minutes to blacken it, remove the corn stalk and let it cool naturally, soak it in running pure water for 30 minutes to wash away impurities, and then put it in a 55℃ oven to dry for 18 hours to obtain a biochar-based solar evaporator with activated pores and surface blackening treatment.
[0037] The obtained biochar-based solar evaporator was nested inside a 24cm square white polyethylene foam core, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation slurry made from cow manure, weighing 0.5kg. The slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0038] Example 3
[0039] First, cut corn stalks into 16 cylinders with a height and a diameter of approximately 3 cm, ensuring the top and bottom surfaces are flat. Wash the processed corn stalks thoroughly with pure water and dry them in a 55℃ oven for 18 hours. Weigh out 20g of wood ash, 20g of saponin, and 10g of shell powder, mix them evenly in a beaker, add 1L of pure water, and stir in an 80℃ water bath for 20 minutes. Filter out the residue, and place the resulting solution in an 80℃ water bath again. Add the corn stalks to the filtered solution and stir with a glass rod for 20 minutes. After this, remove the corn stalks and soak them in running pure water for 30 minutes. The pH of the solution after the final soaking should be neutral. Then, dry the washed corn stalks in a 55℃ oven for 18 hours to obtain an activated, porous corn stalk solar evaporator. Place the corn stalk solar evaporator on a high-temperature iron plate at 300℃ for 10 minutes to blacken it. Remove the corn stalk and let it cool naturally. Soak it in running pure water for 30 minutes to wash away impurities. Then, put it in a 55℃ oven to dry for 18 hours to obtain a biochar-based solar evaporator with activated pores and surface blackening treatment.
[0040] The obtained biochar-based solar evaporator was nested inside a 24cm square white polyethylene foam core, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation slurry made from cow manure, weighing 0.5kg. The slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0041] Comparative Example 1
[0042] First, the corn stalks were cut into 16 cylinders with a height and a diameter of about 3 cm, ensuring that the top and bottom circular sections were flat. The processed corn stalks were then washed with pure water and dried in a 55℃ oven for 18 hours to obtain the original control group corn stalk solar evaporator without any treatment.
[0043] The obtained corn stalk solar evaporator was nested inside a 24cm square white polyethylene foam enclosure, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation biogas slurry made from cow manure, weighing 0.5kg. The biogas slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0044] Comparative Example 2
[0045] First, cut corn stalks into 16 cylinders with a height and a cross-sectional diameter of approximately 3 cm, ensuring the top and bottom cross-sections are flattened. Wash the processed corn stalks thoroughly with pure water and dry them in a 55℃ oven for 18 hours. Weigh out 10g of wood ash, 10g of saponin, and 5g of shell powder and mix them evenly in a beaker. Add 1L of pure water and place the mixture in an 80℃ water bath, stirring for 20 minutes. Filter out the residue. Place the resulting solution in an 80℃ water bath again, add the corn stalks to the filtered solution, and stir with a glass rod for 20 minutes. After this, remove the corn stalks and soak them in running pure water for 30 minutes. The pH of the solution after the final soaking should be neutral. Then, dry the washed corn stalks in a 55℃ oven for 18 hours to obtain an activated, porous corn stalk solar evaporator.
[0046] The obtained biochar-based solar evaporator was nested inside a 24cm square white polyethylene foam core, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation slurry made from cow manure, weighing 0.5kg. The slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0047] Comparative Example 3
[0048] First, cut the corn stalks into 16 cylinders with a height and a diameter of approximately 3cm, ensuring the top and bottom surfaces are flat. Clean the processed corn stalks with pure water and dry them in a 55℃ oven for 18 hours. Then, brand the dried corn stalks on a 200℃ hot iron plate for 10 minutes. Remove the corn stalks and allow them to cool naturally. Soak them in running pure water for 30 minutes to remove impurities, and then dry them in a 55℃ oven for 18 hours. This yields a biochar-based solar evaporator with a branded blackened surface.
[0049] The obtained biochar-based solar evaporator was nested inside a 24cm square white polyethylene foam core, which was then placed in an acrylic box containing 0.5L of anaerobic fermentation slurry made from cow manure, weighing 0.5kg. The slurry contained approximately 500mg / L of total nitrogen, 200mg / L of total phosphorus, and 100mg / L of total potassium. The average daily light intensity and duration were set at 2kW / m². 2 5h.
[0050] Test case
[0051] Blank control: 0.5 L of anaerobic fermentation slurry from cow manure, weighing 0.5 kg, with a total nitrogen concentration of approximately 500 mg / L, a total phosphorus concentration of approximately 200 mg / L, and a total potassium concentration of approximately 100 mg / L, was placed in an acrylic box. This acrylic box was then placed under a light intensity of 2 kW / m². 2 Irradiate under light for 5 hours.
[0052] The biochar-based solar evaporators obtained in the above embodiments and comparative examples were tested using BET pore size distribution and UV-Vis-NIR methods to obtain their average pore size (nm) and solar absorptivity (%). The biogas slurry concentrated by the biochar-based solar evaporator was weighed and its nitrogen, phosphorus, and potassium nutrient concentrations were measured to obtain the biogas slurry concentration and evaporation rate (kg / m³). 2 The table shows the following parameters: net reduction in biogas slurry moisture (kg), average loss rate of nitrogen, phosphorus, and potassium nutrients in biogas slurry (%), and biogas slurry concentration efficiency (%). Table 1 shows the average pore size, solar absorption performance, biogas slurry concentration performance, and blank control group of the biochar-based solar evaporators obtained in Examples 1-3 and Comparative Examples 1-3. The data in the table clearly show that the average pore size and solar absorption rate of the prepared biochar-based solar evaporators increase with increasing activation pore formation and surface blackening, respectively, and the biogas slurry concentration efficiency of the evaporator is significantly improved.
[0053] Table 1. Physicochemical properties of solar evaporators and their biogas slurry concentration performance.
[0054]
[0055] Note: Since the blank control group did not have an evaporator, there is no experimental data related to the structure and performance of the evaporator, which is indicated by "-" in Table 1.
Claims
1. A method for preparing a biochar-based solar evaporator suitable for biogas slurry concentration, characterized in that: Includes the following steps: (1) Cut agricultural and forestry waste into cylindrical shapes and wash them with water; (2) Add plant ash, saponin, and shell powder to water and heat to obtain a hot alkaline solution; (3) Soak the agricultural and forestry waste treated in step (1) in the hot alkaline solution obtained in step (2) for activation and pore formation. Take out the agricultural and forestry waste and wash it with water until neutral or neutralize it with acid solution and then wash it with water until neutral. Then dry it. The time for soaking the agricultural and forestry waste in the hot alkaline solution for activation and pore formation is 10~20min. (4) The agricultural and forestry waste processed in step (3) is blackened by high temperature to obtain a biochar-based solar evaporator; the conditions for high temperature blackening are 200℃~400℃ and 5~20min.
2. The method for preparing the biochar-based solar evaporator according to claim 1, characterized in that: In step (1), the agricultural and forestry waste includes sorghum stalks, corn stalks, bamboo, and corn cobs.
3. The method for preparing the biochar-based solar evaporator according to claim 1, characterized in that: In step (1), the agricultural and forestry waste is cut into cylinders with a height and a cross-sectional diameter of 2-5 cm.
4. The method for preparing the biochar-based solar evaporator according to claim 1, characterized in that: In step (2), the mass ratio of wood ash, saponin, and shell powder is 10:10:5 or 20:20:
10.
5. The method for preparing a biochar-based solar evaporator according to claim 1, characterized in that: The hot alkaline solution in step (2) is prepared by the following steps: adding wood ash, saponin, and shell powder to water, heating to 70-80°C in a water bath, and stirring for 10-30 minutes to obtain the hot alkaline solution.
6. A biochar-based solar evaporator suitable for biogas slurry concentration, characterized in that: By claim 1 The preparation method described in any one of the five methods is used to obtain the product.
7. The application of the biochar-based solar evaporator according to claim 6 in the concentration of biogas slurry.
8. A method for concentrating biogas slurry using the biochar-based solar evaporator as described in claim 6, characterized in that: Includes the following steps: The biochar-based solar evaporator is nested inside a foam board and added to the biogas slurry, where it is concentrated under sunlight.