Low-ash biochar and preparation method thereof

Abstract

The invention is applicable to the technical field of biochar preparation, and provides low-ash biochar and a preparation method thereof. According to the invention, lignocellulose agricultural and forestry wastes are used as raw materials, so that the improvement of a waste biomass dealkalization process is realized. By introducing carbon dioxide and utilizing the excellent fluidity of the carbon dioxide, the lignocellulose forestry and agricultural residues and the acid glycosyl natural eutectic solvent are promoted to be fully mixed, the viscosity of a reaction system is reduced, mass transfer of substances is promoted, the reaction activity is improved, and ash separation is promoted; the effect of separating ash is achieved through the dissolution effect of the acidic glycosyl natural eutectic solvent on alkaline earth metal in biomass. Besides, the pressure of the reaction system is increased through carbon dioxide, so that the contact effect of the biomass and the acidic glycosyl natural eutectic solvent is improved, the proton supply capacity of the solvent is improved, the ash removal rate is increased, and the highest ash removal rate of the method reaches 96.8%.

Description

Technical Field

[0001] The invention belongs to the technical field of biochar preparation, and in particular relates to low-ash biochar and a preparation method thereof. Background Art

[0002] As a substitute for fossil energy, biomass energy has become a new direction for global energy development. The preparation of biochar using waste biomass as raw materials has broad development prospects. It can not only make full use of straw waste and solve its processing problems, but also replace fossil fuels such as coal and save energy resources. The preparation of biochar is mainly carried out under oxygen-limited and high-temperature conditions, and goes through stages such as drying, pre-carbonization, carbonization and calcination. Different raw materials and process conditions will affect the performance of biochar, such as biochar yield, carbon content, ash content, combustion value, etc.

[0003] Biomass such as straw contains a variety of mineral components, such as silicon (Si), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), etc. During the cracking process of biochar, these mineral components are enriched in the biochar in the form of ash, resulting in an increase in the ash content. In addition, factors such as carbonization temperature, heating rate, residence time, ash content of biomass raw materials, and carbonization conditions can also increase the ash content of biochar.

[0004] High-ash biochar may require the use of strong acids and alkalis during the modification process, which may lead to secondary pollution. For example, chemical reagents used in the modification process may have negative impacts on the environment, including pollution of water bodies and effects on soil microorganisms. The adsorption capacity of high-ash biochar for pollutants is limited by its characteristics, such as surface functional group content, pore structure and specific surface area, pH value and cation exchange capacity. These factors limit its application in fields such as environmental remediation. In addition, traditional biochar is difficult to separate from the environment after application, which poses an obstacle to its application in agriculture and environmental engineering. Especially in soil improvement, the residue of high-ash biochar may affect soil structure and plant growth. High ash content may also affect the specific surface area and pore structure of biochar, thereby affecting its adsorption performance. Studies have shown that acid modification can reduce the ash content, which may increase the porosity and specific surface area of ​​biochar. The carbon component and ash content of high-ash biochar are significantly positively correlated. The increase in ash content may reduce the carbon content of biochar, affect its fixed carbon content and increase the ash content, reduce the content of volatile acidic substances, weaken the acidity of biochar, and increase the pH value. The total carbon and ash content of biochar vary greatly under different raw materials and preparation conditions. High ash content may affect the effect of biochar in environmental applications, such as soil improvement and pollutant fixation. In summary, the problems of high-ash biochar involve environmental risks, adsorption capacity limitations, separation difficulties after application, the impact of ash on performance, preparation costs and obstacles to large-scale application, lack of dynamic evaluation, etc. These problems limit the further application and development of high-ash biochar. To this end, the present invention proposes a low-ash biochar and a preparation method thereof. Summary of the invention

[0005] The object of the present invention is to provide a low-ash biochar and a preparation method thereof, aiming to solve the problems raised in the above-mentioned background technology.

[0006] The purpose of the present invention is achieved through the following technical solutions: A method for preparing low-ash biochar comprises the following steps: Step 1: preparing an acidic sugar-based natural deep eutectic solvent; Glucose, lactic acid and water were mixed and stirred continuously for 3 h in an oil bath at 60 °C to obtain an acidic sugar-based natural deep eutectic solvent; Step 2: Mixing the acidic sugar-based natural deep eutectic solvent with the biomass for ash separation: The acidic sugar-based natural low eutectic solvent obtained in step 1 and the chemically treated biomass are mixed in a reactor, gas is introduced and the gas pressure in the reactor is controlled to reach 4-7.5 MPa; heating is performed at 70° C. for 30 min to separate the ash; Step 3: treatment of reaction products; The reaction is terminated by using a reaction terminator, and the reaction solution is filtered to separate the acidic sugar-based natural low eutectic solvent, water and ash into the filtrate; the biomass solid obtained by the filtration is vacuum dried, ground and collected to obtain a dealkalized biomass solid residue, i.e., low-ash biochar; Step 4: Ash enrichment product treatment; After filtration, the filtrate was added with water and allowed to stand for precipitation. The precipitate was soaked and stirred in 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural low eutectic solvent, and the ash-enriched sample was collected by centrifugation and drying.

[0007] Furthermore, in step 1, the molar ratio of glucose, lactic acid and water is 1:5:3.

[0008] Furthermore, in step 2, the mass ratio of biomass to acidic sugar-based natural deep eutectic solvent is 1:1-30.

[0009] Furthermore, in step 2, the reactor is a ventilated reactor, the gas is carbon dioxide, and the inflation method is to use carbon dioxide to discharge the air in the reactor through the vent valve multiple times, and then control the air pressure in the reactor to reach 4~7.5 MPa.

[0010] Furthermore, in step 3, a 50% ethanol solution is used as a reaction terminator, and the amount of the reaction terminator added is 10-15 mL.

[0011] Furthermore, in step 3, the vacuum drying temperature is 60° C. and the vacuum drying time is 12 h.

[0012] Furthermore, in step 4, the immersion stirring adopts magnetic stirring, and the stirring speed is 300 rpm.

[0013] Furthermore, in step 4, the centrifugal speed is 8000-11000 rpm; the centrifugal time is 3 min; and the centrifuge tube is a 50 mL centrifuge tube.

[0014] Furthermore, in step 4, a constant temperature oven is used for drying; the drying temperature is 80° C., and the drying time is 18 to 24 h.

[0015] A low-ash biochar prepared according to the method for preparing low-ash biochar described above.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention uses lignocellulosic agricultural and forestry waste as raw materials to achieve an improvement in the process of dealkalization of waste biomass. The present invention introduces carbon dioxide and utilizes its excellent fluidity to promote the full mixing of lignocellulosic agricultural and forestry waste with an acidic sugar-based natural low eutectic solvent, reduce the viscosity of the reaction system, promote mass transfer and increase reaction activity, promote the separation of ash, and utilize the acidic sugar-based natural low eutectic solvent to dissolve alkaline earth metals in biomass to achieve the effect of separating ash. In addition, increasing the pressure of the reaction system by carbon dioxide helps to improve the contact effect between the biomass and the acidic sugar-based natural low eutectic solvent and increase the proton supply capacity of the solvent, thereby improving the ash removal rate. The highest ash removal rate of this method reaches 96.8%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Ash removal rates of corn straw (CS), soybean straw (SS) and rice straw (RS) treated with water, 1% acetic acid (AA) and DES (acidic sugar-based natural deep eutectic solvent). DETAILED DESCRIPTION

[0018] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0019] The present invention provides a method for preparing low-ash biochar, the method comprising the following steps: Step 1: preparing an acidic sugar-based natural deep eutectic solvent; Glucose, lactic acid and water were mixed and stirred continuously for 3 h in an oil bath at 60°C to obtain an acidic sugar-based natural deep eutectic solvent.

[0020] The molar ratio of glucose, lactic acid and water is 1:5:3. The glucose is D-glucose and the water is ultrapure water. The present invention has no particular limitation on the sources of glucose, lactic acid and ultrapure water, and commercial products known to those skilled in the art can be used.

[0021] Use a 250 mL glass container for mixing.

[0022] Step 2: Mixing the acidic sugar-based natural deep eutectic solvent with the biomass for ash separation: The acidic sugar-based natural deep eutectic solvent obtained in step 1 and the chemically treated biomass are mixed in a reactor, gas is introduced and the gas pressure in the reactor is controlled to reach 4-7.5 MPa; heating is performed at 70° C. for 30 min to separate the ash; the present invention separates the ash from the lignocellulose by chelating the metal ions in the ash with the acidic sugar-based natural deep eutectic solvent to form a coordination system.

[0023] Among them, biomass that does not require chemical treatment includes lignocellulosic agricultural and forestry wastes such as rice straw and durian shells.

[0024] The mass ratio of biomass to acidic sugar-based natural deep eutectic solvent is 1: 1 to 30, preferably 1: 1 to 20. In the present invention, the biomass and the acidic sugar-based natural deep eutectic solvent are preferably controlled within the above-mentioned specific ratio, so as to ensure that the ash is fully separated to meet the characterization quality requirements without wasting raw materials.

[0025] The present invention has no special requirements for the reactor, and a heating stirrer for a reactor well known to those skilled in the art can be used. The reactor is a ventilated reactor (with an inner liner, which needs to avoid sample adhesion to avoid incomplete reaction). In the specific implementation, the equipment is preferably provided by Haian Petroleum Research Instrument Co., Ltd.

[0026] The gas is carbon dioxide, and the inflation method is to use carbon dioxide to discharge the air in the reactor through the vent valve multiple times, and then control the gas pressure in the reactor to reach a specific pressure (the pressure is 0.1~8 MPa, preferably 2~8 MPa, and more preferably 4~7.5 MPa).

[0027] The heating temperature is 30-70°C, preferably 60-70°C, most preferably 70°C; the heating time is 5-60 min, preferably 10-30 min, most preferably 30 min.

[0028] The present invention utilizes an acidic sugar-based natural low eutectic solvent to separate ash from biomass, thereby obtaining a dealkalized biomass solid residue and an ash-enriched product. The acidic sugar-based natural low eutectic solvent is prepared by mixing D-glucose, lactic acid, and ultrapure water as a water-soluble metal oxide auxiliary solvent. The present invention uses D-glucose and lactic acid as hydrogen bond acceptors and hydrogen bond donors of the acidic sugar-based natural low eutectic solvent, combined with a pressurized carbon dioxide environment, to separate alkaline earth metals from biomass, while not destroying the chemical structure and properties of the dealkalized lignocellulose.

[0029] Step 3: treatment of reaction products; The reaction is terminated by using a reaction terminator, and the reaction solution is filtered to separate the acidic sugar-based natural low eutectic solvent, water and ash into the filtrate; the biomass solid obtained by the filtration is vacuum dried, ground and collected to obtain a dealkalized biomass solid residue, i.e., low-ash biochar; Among them, 50% ethanol solution is used as a reaction terminator; the added amount of the reaction terminator is 10~15 mL.

[0030] The water used for filtration is preferably ultrapure water. The amount of filtration solvent used is 40-100 mL, preferably 50-75 mL; the number of times the filtration solvent is added is 3-5 times. The vacuum drying temperature is 60-80 °C, preferably 60 °C; the vacuum drying time is 12 h. The present invention preferably controls the temperature of the heat treatment within the above range to avoid the drying temperature being too low, resulting in the biomass residue not being completely dried, and the temperature being too high, resulting in a waste of energy.

[0031] Step 4: Ash enrichment product treatment; After filtration, the filtrate is added with water and allowed to stand for precipitation. The precipitate is soaked and stirred with ethanol washing solution for 12 hours to remove residual acidic sugar-based natural low eutectic solvent. The ash-enriched sample is collected by centrifugal drying and can be used as an inorganic fertilizer.

[0032] The concentration of the ethanol washing liquid is preferably 10%; ethanol and water are mixed to obtain a 10% ethanol washing liquid. The present invention has no special restrictions on the operation of mixing ethanol and water, and a liquid-liquid mixing technical solution well known to those skilled in the art can be used. Water is preferably ultrapure water; ethanol is preferably 99.8% ethanol, and ethanol is only used as a detergent and does not dissolve ash. The present invention has no special restrictions on the source of ethanol, and a commercially available product well known to those skilled in the art can be used.

[0033] Magnetic stirring was used for immersion stirring, and the stirring speed was 300 rpm.

[0034] The centrifugal speed is 5000-12000 rpm, preferably 8000-11000 rpm; the centrifugal time is 3 min; the specific specification of the centrifuge tube is preferably a 50 mL centrifuge tube.

[0035] The drying is carried out in a constant temperature oven; the drying temperature is 60-80°C, preferably 80°C; the drying time is 6-28 hours, preferably 12-28 hours, and more preferably 18-24 hours. The present invention preferably controls the temperature and time of the drying process within the above range to ensure complete drying of the ash enrichment product.

[0036] The present invention uses lignocellulose agricultural and forestry waste as raw materials to achieve the de-alkali value-added transformation of waste biomass. Under the action of an acidic sugar-based natural low eutectic solvent, the ash in the biomass without chemical treatment is separated in one step, so that the alkaline earth metal ions in the ash are chelated by the acidic sugar-based natural low eutectic solvent to form a coordination system, thereby separating the ash from the lignocellulose and retaining the complete structure of the lignocellulose; the glucose and lactic acid in the acidic sugar-based natural low eutectic solvent have a strong oxygen vacancy supply capacity, which promotes the separation of the alkaline earth metals in the ash. During the separation process, the pressurized carbon dioxide atmosphere helps to fully mix the reaction components and promote mass transfer; carbon dioxide dissolves in the reaction solution under pressure, further reduces the pH value of the solution, and creates an acidic reaction environment. In addition, the pressurized carbon dioxide fluid helps to reduce the viscosity of the reaction system with the assistance of continuous stirring, thereby improving the separation efficiency. The dissolved carbon dioxide can protect the chemical structure and properties of the lignin product by combining with the functional groups on the outer side of the separated lignocellulose molecules. The reaction temperature is controlled below 100°C, realizing the green and efficient ash separation reaction.

[0037] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0038] Example 1: Take 1 g of biomass powder (soybean straw) without chemical treatment and mix it with 20 g of 1% acetic acid, introduce carbon dioxide and control the pressure in the kettle to 6 MPa; heat at 70°C for 30 min to separate ash; use 10 mL of 50% ethanol solution to terminate the reaction, and separate the acidic sugar-based natural low eutectic solvent, water and ash into the filtrate by suction filtration; after suction filtration, add water to the filtrate and let it settle, soak and stir the precipitate with 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural low eutectic solvent, centrifuge (3 min, 10000 rpm) and dry (80°C, 20 h) to collect the ash enrichment sample. The biomass solid obtained by suction filtration was vacuum dried at 60°C for 12 h, ground and collected to obtain a dry dealkalized biomass solid residual powder.

[0039] The samples before and after the reaction were calcined at 550° C. for 5 h, and the ash removal rate was calculated using the mass difference method. The ash removal rate of the dealkalized biomass solid residue separated in this embodiment reached 56.0%.

[0040] Example 2: Mix glucose, lactic acid and water, and stir continuously for 3 h in a 60°C oil bath to obtain an acidic sugar-based natural low eutectic solvent; take 1 g of chemically treated biomass powder (soybean straw), mix it evenly with 20 g of acidic sugar-based natural low eutectic solvent, introduce carbon dioxide and control the pressure in the kettle to reach 6 MPa; heat at 70°C for 30 min to separate ash; terminate the reaction with 10 mL of 50% ethanol solution, and separate the acidic sugar-based natural low eutectic solvent, water and ash from the reaction solution by suction filtration; add water to the filtrate after suction filtration and let it settle, soak and stir the precipitate with 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural low eutectic solvent, centrifuge (3 min, 10000 rpm) and dry (80°C, 20 h) to collect the ash enrichment sample. The biomass solid obtained by suction filtration was vacuum dried at 60°C for 12 h, ground and collected to obtain a dry dealkalized biomass solid residual powder.

[0041] The samples before and after the reaction were calcined at 550° C. for 5 h, and the ash removal rate was calculated using the mass difference method. The ash removal rate of the dealkalized biomass solid residue separated in this embodiment reached 83.3%.

[0042] Example 3: The difference between this example and Example 2 is that the biomass powder that does not require chemical treatment is corn stalks, and other operations are the same.

[0043] The ash removal rate of the dealkalized biomass solid residue separated in this example reached 46.5%.

[0044] Example 4: The difference between this example and Example 2 is that the biomass powder that does not require chemical treatment is rice straw, and other operations are the same.

[0045] The ash removal rate of the dealkalized biomass solid residue separated in this example reached 26.3%.

[0046] Example 5: The difference between this example and Example 2 is that the biomass powder that does not require chemical treatment is durian shell, and other operations are the same.

[0047] The ash removal rate of the dealkalized biomass solid residue separated in this example reached 96.8%.

[0048] Figure 1 The ash removal rates of corn straw (CS), soybean straw (SS) and rice straw (RS) under treatment with water (Water), 1% acetic acid (AA) and DES (acidic sugar-based natural deep eutectic solvent) are shown in Table 1. The ash removal rates of different types of biomass under different pretreatment solvents are shown in Table 1.

[0049] Table 1: Ash removal rates of different types of biomass treated with different pretreatment solvents

[0050] Depend on Figure 1 As can be seen from Table 1, the method provided by the present invention exhibits the highest ash removal rate in corn straw, soybean straw, rice straw and durian shell. Affected by the ash component ratio and internal structure of different biomass, the ash removal rate of the same type of solvent varies among different biomass materials.

[0051] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A method for preparing low-ash biochar, characterized in that: The following steps are involved: Step 1: preparing an acidic sugar-based natural deep eutectic solvent; Glucose, lactic acid and water were mixed and stirred continuously for 3 h in an oil bath at 60 °C to obtain an acidic sugar-based natural deep eutectic solvent; Step 2: Mixing the acidic sugar-based natural deep eutectic solvent with the biomass for ash separation: The acidic sugar-based natural low eutectic solvent obtained in step 1 and the chemically treated biomass are mixed in a reactor, gas is introduced and the gas pressure in the reactor is controlled to reach 4-7.5 MPa; heating is performed at 70° C. for 30 min to separate the ash; Step 3: treatment of reaction products; The reaction is terminated by using a reaction terminator, and the reaction solution is filtered to separate the acidic sugar-based natural low eutectic solvent, water and ash into the filtrate; the biomass solid obtained by the filtration is vacuum dried, ground and collected to obtain a dealkalized biomass solid residue, i.e., low-ash biochar; Step 4: Ash enrichment product treatment; After filtration, the filtrate was added with water and allowed to stand for precipitation. The precipitate was soaked and stirred in 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural low eutectic solvent, and the ash-enriched sample was collected by centrifugation and drying.

2. The method for preparing low-ash biochar according to claim 1, characterized in that: In step 1, the molar ratio of glucose, lactic acid and water is 1:5:

3.

3. The method for preparing low-ash biochar according to claim 1, characterized in that: In the step 2, the mass ratio of the biomass to the acidic sugar-based natural deep eutectic solvent is 1:1-30.

4. The method for preparing low-ash biochar according to claim 1, characterized in that: In the step 2, the reactor is a ventilated reactor, the gas is carbon dioxide, and the inflation method is to use carbon dioxide to discharge the air in the reactor through the vent valve multiple times, and then control the air pressure in the reactor to reach 4~7.5 MPa.

5. The method for preparing low-ash biochar according to claim 1, characterized in that: In the step 3, a 50% ethanol solution is used as a reaction terminator, and the amount of the reaction terminator added is 10-15 mL.

6. The method for preparing low-ash biochar according to claim 1, characterized in that: In step 3, the vacuum drying temperature is 60° C. and the vacuum drying time is 12 h.

7. The method for preparing low-ash biochar according to claim 1, characterized in that: In step 4, the immersion stirring is performed by magnetic stirring, and the stirring speed is 300 rpm.

8. The method for preparing low-ash biochar according to claim 1, characterized in that: In step 4, the centrifugal speed is 8000-11000 rpm; the centrifugal time is 3 min; and the centrifuge tube is a 50 mL centrifuge tube.

9. The method for preparing low-ash biochar according to claim 1, characterized in that: In the step 4, a constant temperature oven is used for drying; the drying temperature is 80° C. and the drying time is 18 to 24 h.

10. A low-ash biochar prepared according to the method for preparing low-ash biochar according to any one of claims 1 to 9.

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