A low-ash biochar and its preparation method

Abstract

The present invention is applicable to the technical field of biochar preparation, and provides a low-ash biochar and a preparation method thereof. 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, thereby reducing the viscosity of the reaction system, promoting mass transfer of substances and improving reaction activity, and promoting the separation of ash. The acidic sugar-based natural low eutectic solvent is used 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%.

Description

Technical Field

[0001] The present invention belongs to the technical field of biochar preparation, and particularly relates to a 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. Preparing biochar from waste biomass has broad development prospects. It can not only make full use of straw waste and solve the problem of its treatment, but also replace fossil fuels such as coal and save energy resources. The preparation of biochar is mainly carried out in an oxygen-limited and high-temperature environment, going through stages such as drying, pre-carbonization, carbonization, and calcination. Different raw materials and process conditions will affect the properties of biochar, such as the yield, carbon content, ash content, calorific value, etc. of biochar.

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

[0004] During the modification process of high-ash biochar, strong acids and bases may be required, which may lead to secondary pollution. For example, the chemical reagents used in the modification process may have a negative impact on the environment, including water pollution and the impact on soil microorganisms. The adsorption capacity of high-ash biochar for pollutants is limited by its characteristics, such as the content of surface functional groups, 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, it is difficult to separate traditional biochar 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. The high ash content may also affect the specific surface area and pore structure of biochar, thereby affecting its adsorption performance. Research shows that acid modification can reduce the ash content, which may increase the porosity and specific surface area of biochar. There is a significant positive correlation between the carbon component and ash content of high-ash biochar. The increase in ash content may reduce the carbon content of biochar, affect its fixed carbon content, increase in ash, and decrease in volatile acid content, weakening the acidity of biochar and increasing the pH value. The total carbon and ash content of biochar vary greatly under different raw materials and preparation conditions. High ash may affect the effect of biochar in environmental applications, such as soil improvement and pollutant fixation. To sum up, the problems of high-ash biochar involve multiple aspects such as environmental risks, adsorption capacity limitations, separation difficulties after application, the impact of ash on performance, preparation costs and obstacles to large-scale application, and lack of dynamic evaluation. These problems limit the further application and development of high-ash biochar. Therefore, the present invention proposes a low-ash biochar and its preparation method. Summary of the Invention

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

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A preparation method of low-ash biochar, comprising the following steps:

[0008] Step 1: Prepare an acidic sugar-based natural deep eutectic solvent;

[0009] Mix glucose, lactic acid and water, and continuously stir in an oil bath at 60 °C for 3 h to obtain an acidic sugar-based natural deep eutectic solvent;

[0010] Step 2: Mix the acidic sugar-based natural deep eutectic solvent with biomass for ash separation:

[0011] Mix the acidic sugar-based natural eutectic solvent obtained in Step 1 with the biomass without chemical treatment in a reactor, introduce gas and control the air pressure in the reactor to reach 4 - 7.5 MPa; heat at 70 °C for 30 min and perform ash separation;

[0012] Step 3: Treatment of the reaction product;

[0013] Terminate the reaction using a reaction terminator. The reaction solution is separated by suction filtration to separate the acidic sugar-based natural eutectic solvent, water, and ash into the filtrate; the biomass solid obtained by suction filtration is dried in vacuo, ground, and collected to obtain a de-alkalized biomass solid residue, i.e., low-ash biochar;

[0014] Step 4: Treatment of the ash-enriched product;

[0015] After suction filtration, add water to the filtrate and let it stand for precipitation. The precipitate is soaked and stirred with a 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural eutectic solvent, and the ash-enriched sample is collected by centrifugation and drying.

[0016] Furthermore, in the said Step 1, the molar ratio of glucose, lactic acid, and water is 1:5:3.

[0017] Furthermore, in the said Step 2, the mass ratio of the biomass to the acidic sugar-based natural eutectic solvent is 1:1 - 30.

[0018] Furthermore, in the said Step 2, the reactor is a ventable reaction kettle, the gas is carbon dioxide, and the gas charging method is to use carbon dioxide to discharge the air in the reaction kettle multiple times through the air release valve, and then control the air pressure in the kettle to reach 4 - 7.5 MPa.

[0019] Furthermore, in the said Step 3, a 50% ethanol solution is used as the reaction terminator, and the addition amount of the reaction terminator is 10 - 15 mL.

[0020] Furthermore, in the said Step 3, the temperature for drying in vacuo is 60 °C; the time for drying in vacuo is 12 h.

[0021] Furthermore, in the said Step 4, magnetic stirring is used for soaking and stirring, and the stirring speed is 300 rpm.

[0022] Furthermore, in the said Step 4, the centrifugation speed is 8000 - 11000 rpm; the centrifugation time is 3 min; 50 mL centrifuge tubes are used for centrifugation.

[0023] Furthermore, in the said Step 4, drying is carried out using a constant-temperature oven; the drying temperature is 80 °C, and the drying time is 18 - 24 h.

[0024] A low-ash biochar prepared by the method for preparing low-ash biochar as described above.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 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

[0027] 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

[0028] 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.

[0029] The present invention provides a method for preparing low-ash biochar, which comprises the following steps:

[0030] Step 1: preparing an acidic sugar-based natural deep eutectic solvent;

[0031] 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.

[0032] 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.

[0033] Use a 250 mL glass container for mixing.

[0034] Step 2: Mixing the acidic sugar-based natural deep eutectic solvent with the biomass for ash separation:

[0035] Mix the acidic sugar-based natural eutectic solvent obtained in Step 1 and the biomass without chemical treatment in a reactor, introduce gas and control the air pressure in the reactor to reach 4 - 7.5 MPa; heat at 70 °C for 30 min and perform ash separation; in the present invention, metal ions in the ash are chelated by the acidic sugar-based natural eutectic solvent to form a coordination system, thereby separating the ash from the lignocellulose.

[0036] Among them, the biomass without chemical treatment is lignocellulosic agricultural and forestry waste such as rice straw and durian shells.

[0037] The mass ratio of the biomass to the acidic sugar-based natural eutectic solvent is 1:1 - 30, preferably 1:1 - 20. In the present invention, it is preferably to control the biomass and the acidic sugar-based natural eutectic solvent at the above specific ratio, which can ensure the full separation of the ash and meet the characterization quality requirements while not wasting raw materials.

[0038] The present invention has no special requirements for the reactor, and a heating and stirring instrument for a reaction kettle well-known to those skilled in the art can be used. The reactor is preferably a ventable reaction kettle (equipped with an inner liner, and the inner liner needs to avoid sample adhesion to prevent insufficient reaction). In specific implementation, the equipment is preferably provided by Hai'an Petroleum Scientific Research Instruments Co., Ltd.

[0039] The gas is carbon dioxide, and the inflation method is to use carbon dioxide to discharge the air in the reaction kettle multiple times through the air release valve, and then control the air pressure in the kettle to reach a specific pressure (the pressure is 0.1 - 8 MPa, preferably 2 - 8 MPa, more preferably 4 - 7.5 MPa).

[0040] The heating temperature is 30 - 70 °C, preferably 60 - 70 °C, most preferably 70 °C; the time is 5 - 60 min, preferably 10 - 30 min, most preferably 30 min.

[0041] The present invention uses an acidic sugar-based natural eutectic solvent to separate the ash in the biomass, obtaining a de-alkalized biomass solid residue and an ash-enriched product. The acidic sugar-based natural eutectic solvent is made by mixing D-glucose, lactic acid, and ultrapure water as an auxiliary solvent for water-soluble metal oxides. In the present invention, D-glucose and lactic acid are used as the hydrogen bond acceptor and hydrogen bond donor of the acidic sugar-based natural eutectic solvent, combined with a pressurized carbon dioxide environment, to separate alkaline earth metals in the biomass while not destroying the chemical structure and properties of the lignocellulose after de-alkalization.

[0042] Step 3: Treatment of the reaction product;

[0043] Use a reaction terminator to terminate the reaction, and the reaction solution is separated by suction filtration to separate the acidic sugar-based natural eutectic solvent, water, and ash into the filtrate; the biomass solid obtained by suction filtration is vacuum dried, ground, and collected to obtain a de-alkalized biomass solid residue, that is, low-ash biochar;

[0044] Among them, a 50% ethanol solution is used as the reaction terminator; the addition amount of the reaction terminator is 10 - 15 mL.

[0045] The water used for suction filtration is preferably ultrapure water. The amount of suction filtration solvent is 40 - 100 mL, preferably 50 - 75 mL; the number of times of adding the suction filtration solvent 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 heat treatment temperature within the above range to avoid incomplete drying of the biomass residue caused by too low drying temperature and energy waste caused by too high temperature.

[0046] Step 4: Treatment of the ash enrichment product;

[0047] After suction filtration, water is added to the filtrate and allowed to stand for precipitation. The precipitate is soaked and stirred with an ethanol washing solution for 12 h to remove the residual acidic sugar-based deep eutectic solvent. After centrifugation and drying, the ash enrichment sample is collected and can be used as an inorganic fertilizer.

[0048] Among them, the concentration of the ethanol washing solution is preferably 10%; ethanol and water are mixed to obtain a 10% ethanol washing solution. The present invention has no special limitation on the operation of mixing ethanol and water, and a technical solution for liquid-liquid mixing well-known to those skilled in the art can be adopted. The water is preferably ultrapure water; the ethanol is preferably 99.8% ethanol. Ethanol only acts as a detergent and does not dissolve the ash. The present invention has no special limitation on the source of ethanol, and commercially available products well-known to those skilled in the art can be adopted.

[0049] The soaking and stirring are carried out by magnetic stirring, and the stirring speed is 300 rpm.

[0050] The centrifugation speed is 5000 - 12000 rpm, preferably 8000 - 11000 rpm; the centrifugation time is 3 min; the specific specification of the centrifuge tube is preferably a 50 mL centrifuge tube.

[0051] 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 h, preferably 12 - 28 h, more preferably 18 - 24 h. The present invention preferably controls the temperature and time of the drying treatment within the above range to ensure the complete drying of the ash enrichment product.

[0052] The present invention uses lignocellulosic agricultural and forestry waste as raw materials to achieve the de-alkalization and value-added transformation of waste biomass. Under the action of an acidic sugar-based natural deep eutectic solvent, the ash in the biomass without chemical treatment is separated in one step, and the alkaline earth metal ions in the ash are chelated by the acidic sugar-based natural deep eutectic solvent to form a coordination system, thereby separating the ash from the lignocellulose and retaining the complete structure of the lignocellulose; glucose and lactic acid in the acidic sugar-based natural deep eutectic solvent have strong oxygen vacancy supply capabilities, promoting the separation of various alkaline earth metals in the ash. During the separation process, a pressurized carbon dioxide atmosphere helps the full mixing of reaction components and promotes mass transfer; carbon dioxide dissolves in the reaction solution under pressure, further reducing the pH value of the solution and creating an acidic reaction environment. In addition, the pressurized carbon dioxide fluid, assisted by continuous stirring, helps to reduce the viscosity of the reaction system, thereby improving the separation efficiency. The dissolved carbon dioxide can protect the chemical structure and properties of the lignin product by binding to the functional groups on the outer side of the lignocellulose molecules after separation. The reaction temperature is controlled below 100 °C, realizing the green and efficient progress of the ash separation reaction.

[0053] The following describes the specific implementation of the present invention in detail with specific examples. The described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0054] Example 1: Take 1 g of biomass powder (soybean straw) without chemical treatment and mix it evenly with 20 g of 1% acetic acid. Introduce carbon dioxide and control the pressure in the autoclave to reach 6 MPa; heat at 70 °C for 30 min for ash separation; use 10 mL of 50% ethanol solution to terminate the reaction, and the reaction solution is separated by suction filtration to separate the acidic sugar-based natural deep eutectic solvent, water, and ash into the filtrate; after suction filtration, the filtrate is added with water and allowed to stand for precipitation, and the precipitate is soaked and stirred with a 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural deep eutectic solvent, and then centrifuged (3 min, 10000 rpm) and dried (80 °C, 20 h) to collect the ash-enriched sample. The biomass solid obtained by suction filtration is vacuum dried at 60 °C for 12 h, ground, and collected to obtain a dry de-alkalized biomass solid residue powder.

[0055] The samples before and after the reaction are calcined at 550 °C for 5 h, and the ash removal rate is calculated by the mass difference method. The ash removal rate of the de-alkalized biomass solid residue separated in this example reaches 56.0%.

[0056] Example 2: Glucose, lactic acid and water were mixed and continuously stirred in an oil bath at 60 °C for 3 h to obtain an acidic sugar-based natural eutectic solvent; 1 g of biomass powder without chemical treatment (soybean straw) was taken and mixed evenly with 20 g of the acidic sugar-based natural eutectic solvent, and carbon dioxide was introduced to control the pressure in the autoclave to reach 6 MPa; it was heated at 70 °C for 30 min for ash separation; the reaction was terminated using 10 mL of 50% ethanol solution, and the reaction solution was separated by suction filtration to separate the acidic sugar-based natural eutectic solvent, water and ash into the filtrate; after suction filtration, water was added to the filtrate and allowed to stand for precipitation, and the precipitate was soaked and stirred with 10% ethanol washing solution for 12 h to remove the residual acidic sugar-based natural eutectic solvent, and then centrifuged (3 min, 10000 rpm) and dried (80 °C, 20 h) to collect the ash-enriched sample. The biomass solid obtained by suction filtration was vacuum dried at 60 °C for 12 h, ground and collected to obtain a dry de-alkali biomass solid residue powder.

[0057] 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 de-alkali biomass solid residue separated in this example reached 83.3%.

[0058] Example 3: The difference between this example and Example 2 is that the biomass powder without chemical treatment is corn straw, and other operations are the same.

[0059] The ash removal rate of the de-alkali biomass solid residue separated in this example reached 46.5%.

[0060] Example 4: The difference between this example and Example 2 is that the biomass powder without chemical treatment is rice straw, and other operations are the same.

[0061] The ash removal rate of the de-alkali biomass solid residue separated in this example reached 26.3%.

[0062] Example 5: The difference between this example and Example 2 is that the biomass powder without chemical treatment is durian shell, and other operations are the same.

[0063] The ash removal rate of the de-alkali biomass solid residue separated in this example reached 96.8%.

[0064] Figure 1 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 eutectic solvent). Table 1 shows the ash removal rates of different types of biomass under treatment with different pretreatment solvents.

[0065] Table 1: Ash removal rates of different types of biomass under treatment with different pretreatment solvents

[0066]

[0067] From Figure 1 and Table 1, it can be seen that the method provided by the present invention shows the highest ash removal rate in corn straw, soybean straw, rice straw and durian shells. Affected by the ash component ratio and internal structure of different biomasses, there are differences in the ash removal rate of the same type of solvent among different biomass materials.

[0068] The above are only the preferred embodiments of the present invention. It should be noted that for those skilled in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicability 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; Mix glucose, lactic acid and water, and stir continuously for 3 h in an oil bath at 60°C to obtain an acidic sugar-based natural low eutectic solvent; the molar ratio of glucose, lactic acid and water is 1:5:3; 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 biomass without chemical treatment are mixed in a reactor, and gas is introduced to control the gas pressure in the reactor to reach 4-7.5 MPa; the mixture is heated at 70°C for 30 min to separate the ash; the mass ratio of the biomass to the acidic sugar-based natural low eutectic solvent is 1:1-30; 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 gas pressure in the reactor to reach 4-7.5 MPa; 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 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. A low-ash biochar prepared according to the method for preparing low-ash biochar according to any one of claims 1 to 6.

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