A method for producing biomass porous carbon using casuarina trees

Abstract

This invention discloses a method for producing porous biomass carbon using Casuarina equisetifolia. The method involves reactively extruding Casuarina equisetifolia powder, ammonium polyphosphate, melamine, potassium acetate, urea, and water using a twin-screw extruder to obtain a rheological phase plasticizing precursor. This precursor is then refined through segmented curing and foaming, high-temperature reforming activation, hot water digestion, and ultrasonic acid washing to obtain porous biomass carbon. This invention utilizes a eutectic system to achieve low-temperature continuous processing of the precursor and inhibits equipment corrosion. A well-developed pore structure is constructed through a stepwise cross-linking foaming and mild activation strategy. The porous biomass carbon prepared by this invention has an ash content ≤1.12%, a carbon yield ≥35.7%, an iodine adsorption value ≥1278 mg / g, and a methylene blue adsorption value ≥175 mg / g. Furthermore, the process is continuous, with minimal equipment corrosion, making it suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of biomass carbon material preparation technology, specifically a method for producing porous biomass carbon using Casuarina trees. Background Technology

[0002] Casuarina equisetifolia, a widely planted protective forest species in southeastern coastal China, grows rapidly and generates a large amount of dead branches, leaves, and pruning waste annually. Utilizing these Casuarina equisetifolia wastes to prepare high-value-added biomass porous carbon can not only solve waste disposal problems but also provide high-performance adsorbent or electrode materials, thus having significant economic and environmental implications.

[0003] Currently, the preparation of porous carbon from biomass mainly employs chemical activation methods, with commonly used activators including potassium hydroxide, phosphoric acid, and zinc chloride. However, existing preparation processes still have many limitations in industrial applications. First, the ash content of casuarina-based porous carbon prepared by the traditional phosphoric acid activation method is typically 8%-15%, with a carbon yield of less than 20%; the carbon yield of the strong alkali activation method is only 10%-15%, and the annual equipment corrosion rate exceeds 5%. Simultaneously, traditional production methods are mostly intermittent processes combining liquid-phase impregnation with high-temperature carbonization. This batch production mode makes it difficult to guarantee batch-to-batch product quality stability and has low production efficiency. Furthermore, biomass powder itself lacks thermoplasticity and cannot melt and flow under heating conditions, making it difficult to directly utilize mature twin-screw extrusion equipment in the plastics industry for continuous processing and molding, thus limiting the expansion of production scale.

[0004] Secondly, to improve the processing performance of biomass or impart specific structures, existing technologies attempt to introduce additives such as ammonium polyphosphate for reactive extrusion. However, in the high-temperature shear processing environment, ammonium polyphosphate is prone to hydrolysis, releasing acidic gases or producing acidic oligomers. These acidic substances can chemically react with metal components such as the screw and barrel of the extruder, causing severe equipment corrosion and wear, resulting in high equipment maintenance costs. Furthermore, the introduction of metallic impurities can affect the purity of the final carbon material.

[0005] Furthermore, in terms of activation and pore structure control, existing single-activator processes struggle to balance carbon yield and porosity. While strong base activation can produce activated carbon with high specific surface area, its excessive etching of the carbon framework leads to a significant reduction in carbon yield. Phosphoric acid activation, although yielding relatively high carbon, easily generates pyrophosphates or forms insoluble complex salts with other metal ions within the carbon material, resulting in persistently high ash content. Conventional washing processes are insufficient to completely remove these deep-seated impurities, impacting the electrochemical or adsorption properties of the porous carbon. Therefore, developing a continuous processing method for preparing casuarina-based porous carbon with minimal equipment corrosion and low ash content is a crucial technical challenge in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for producing porous biomass carbon using Casuarina trees. This method solves the problems in existing porous biomass carbon preparation processes, such as the difficulty in achieving continuous and stable processing of carbon precursors, the easy corrosion of processing equipment caused by acidic flame retardants, and the low yield, high ash content, and difficulty in precisely controlling the pore structure caused by conventional activation processes.

[0007] To achieve the above objectives, the present invention provides a method for producing biomass porous carbon using casuarina trees, comprising the following steps:

[0008] Step S1, Precursor Preparation: Raw materials comprising 100 parts by weight of Casuarina equisetifolia branch powder, 35-45 parts by weight of ammonium polyphosphate, 25-35 parts by weight of melamine, 90-110 parts by weight of potassium acetate, 25-35 parts by weight of urea, and 28-35 parts by weight of water are added to a twin-screw extruder and subjected to reactive extrusion at a temperature of 95-105°C to prepare a rheological plasticizing precursor; wherein, the urea forms a eutectic system with potassium acetate and water to achieve low-temperature rheological plasticization;

[0009] Step S2, Curing and Pre-oxidation: The rheological plasticizing precursor obtained in step S1 is subjected to segmented heat treatment in an air atmosphere, successively going through a low-temperature cross-linking curing stage and a medium-temperature confined foaming stage. The gas is confined to escape through the skeleton structure formed by the first cross-linking curing, and foamed carbon precursor is obtained.

[0010] Step S3, High-temperature reforming and activation: The foamed carbon precursor obtained in step S2 is heated under an inert atmosphere to carry out carbonization and activation reaction to obtain porous carbon crude product.

[0011] Step S4, Product Refining: The crude product obtained in step S3 is subjected to a combination of high-temperature hot water digestion and ultrasonic-assisted acid washing for cleaning and drying to obtain the biomass porous carbon.

[0012] This invention utilizes urea, potassium acetate, and water in a specific ratio to construct a eutectic solvent system. This system possesses extremely strong penetrating power, enabling it to enter the microstructure of Casuarina equisetifolia powder, disrupting the hydrogen bond network between lignin and cellulose molecular chains. This imparts fluidity and plasticity to high-filled biomass powder at low temperatures of 95-105℃, thereby preparing a rheological plasticizing precursor and solving the problem that traditional biomass cannot be continuously processed using screw extruders.

[0013] Meanwhile, to address the problem that ammonium polyphosphate is prone to hydrolysis under heating and shearing, producing acidic substances that corrode equipment, the amino group in the urea molecule is used as a proton acceptor to capture and neutralize acidic groups in situ. This fixes the corrosive acid radical ions in the condensed phase, significantly inhibiting the corrosion of metal parts of the twin-screw extruder by acidic components.

[0014] A solidified carbon framework with a certain mechanical strength is pre-constructed through the cross-linking reaction of ammonium polyphosphate decomposition products with biomass hydroxyl groups at a low temperature stage. Subsequently, melamine decomposes to produce gas at a medium temperature stage. At this point, the solidified framework physically restricts the free escape of gas, promoting uniform nucleation and growth of gas within the matrix, forming a dense closed-pore or semi-open-pore structure. This effectively avoids pore collapse or bubble formation caused by insufficient matrix melt strength, ensuring the stability of the precursor's pore structure.

[0015] During the activation stage, potassium acetate is reduced at high temperature to generate metallic potassium vapor which is embedded between carbon layers to create pores and etch. Compared with highly corrosive potassium hydroxide, potassium acetate activation is gentler and retains more carbon skeleton while creating pores, thereby improving carbonization yield. In the refining stage, for the sparingly soluble complex salts such as potassium pyrophosphate and calcium magnesium phosphate formed during activation, a dual cleaning process of high-temperature hot water digestion and ultrasonic acid washing is adopted. The high-temperature hot water environment increases the solubility product constant of the complex salts and removes most of the soluble and slightly soluble salts; the cavitation effect of ultrasound further removes stubborn impurities in deep pores, and combined with acid washing, achieves deep removal of ash.

[0016] Preferably, the weight ratio of the raw materials in step S1 is: 100 parts by weight of Casuarina equisetifolia branch powder, 40 parts by weight of ammonium polyphosphate, 30 parts by weight of melamine, 100 parts by weight of potassium acetate, 30 parts by weight of urea, and 30 parts by weight of water.

[0017] By adopting the above technical solution, the ratio is near the eutectic point, which can provide a suitable liquid phase environment to ensure the smoothness of the extrusion process, while providing sufficient acid neutralization capacity and subsequent foaming gas source.

[0018] Preferably, the Casuarina equisetifolia branch powder in step S1 has a particle size distribution between 180 μm and 250 μm and a moisture content of less than 5 wt%; the ammonium polyphosphate has a type II crystal structure and a degree of polymerization n greater than 1000.

[0019] By adopting the above technical solutions, a specific particle size range is beneficial for the uniform wetting of the eutectic system and avoids uneven plasticization caused by excessively large particle size; high degree of polymerization ammonium polyphosphate has better thermal stability and carbonization, which is beneficial for improving the strength of the skeleton.

[0020] Preferably, the specific process of reactive extrusion in step S1 is as follows: the screw speed of the twin-screw extruder is controlled at 80 to 120 revolutions per minute, the temperature of the barrel homogenization zone is 95°C to 105°C, and the residence time of the material in the barrel is 10 to 15 minutes; the extruded strip is then pelletized into cylindrical particles.

[0021] By adopting the above technical solutions, controlling the appropriate shear rate and thermal process, premature cross-linking or foaming of materials in the extruder is prevented, thus ensuring the consistency of precursor particle quality.

[0022] Preferably, the specific procedure for the segmented heat treatment in step S2 is as follows: In the first stage, the temperature is raised to 200°C at a heating rate of 3°C to 5°C per minute and held at that temperature for 30 minutes, using the polymethic acid produced by the decomposition of ammonium polyphosphate to crosslink and solidify with the lignin hydroxyl groups; In the second stage, the temperature is raised to 300°C at a heating rate of 3°C to 5°C per minute and held at that temperature for 60 minutes, using the decomposition of melamine and the restricted foaming of the precursor to form a porous skeleton.

[0023] By adopting the above technical solution, the heat treatment process is matched with the chemical reaction kinetics of the components, ensuring the structural evolution path of "curing first, then foaming".

[0024] Preferably, the specific parameters for the high-temperature reforming activation in step S3 are as follows: under nitrogen protection, the temperature is raised to 800°C to 850°C at a heating rate of 5°C per minute and held at a constant temperature for 90 minutes; during this process, the potassium vapor generated by the reduction of potassium acetate etches and creates pores in the carbon skeleton.

[0025] Preferably, the specific steps of the cleaning in step S4 include: Step A, immersing the crude product in hot deionized water at 95°C for mechanical stirring and digestion to remove insoluble pyrophosphate and double salt; Step B, placing the digested solid in hydrochloric acid solution and performing ultrasonic-assisted acid washing at 60°C; Step C, washing with deionized water until neutral and drying.

[0026] By adopting the above technical solution, step-by-step cleaning is used to treat inorganic impurities with different solubility characteristics. Thermal digestion is used to remove a large amount of soluble salts, and ultrasonic acid washing is used to remove stubborn insoluble salts, which significantly reduces the ash content of the final product.

[0027] Preferably, in step A, the mechanical stirring digestion time is 45 minutes and the liquid-to-solid ratio is 15:1; in step B, the concentration of the hydrochloric acid solution is 1.0 mol / L, the ultrasonic-assisted acid washing time is 30 minutes, and the liquid-to-solid ratio is 10:1.

[0028] Preferably, in step S1, the twin-screw extruder neutralizes the acidic gas generated by the hydrolysis of ammonium polyphosphate in situ through the decomposition products of urea during the extrusion process, thereby inhibiting equipment corrosion.

[0029] This invention provides a method for producing porous biomass carbon using casuarina trees. It has the following beneficial effects:

[0030] 1. This invention achieves low-temperature rheological plasticization of high-filling-content Casuarina equisetifolia powder by constructing a eutectic system of urea, potassium acetate and water, enabling the precursor to be continuously prepared by a twin-screw extruder; at the same time, by utilizing the urea decomposition products to neutralize the acidic substances released by ammonium polyphosphate in situ, the corrosion of equipment during processing is effectively inhibited, solving the problems of poor continuity and high equipment wear in traditional biomass carbonization processes.

[0031] 2. This invention employs a segmented heat treatment combined with a mild activation strategy. By controlling the cross-linking and curing of ammonium polyphosphate and the foaming and pore-forming of melamine in different temperature ranges, a stable supporting framework is constructed, avoiding pore collapse. Combined with the mild etching effect of potassium acetate, while ensuring that the porous carbon has a well-developed pore structure, a high carbon yield is maintained, which is superior to the traditional strong alkali chemical activation process.

[0032] 3. This invention sets up a refining process that combines high-temperature hot water digestion with ultrasonic-assisted acid washing, which strengthens the removal of pyrophosphate and insoluble complex salts generated during the activation process, significantly reducing the ash content of the final product. This process overcomes the technical defects of phosphoric acid activators, which easily cause high ash residue and low purity in the product, and prepares high-quality biomass porous carbon with low ash content. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the specification of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] This invention provides a method for producing biomass porous carbon using casuarina trees:

[0035] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0036] Casuarina twig powder: As a carbon precursor matrix, it is taken from the dried branches of Casuarina twig, mechanically crushed and passed through an 80-mesh sieve, with a particle size distribution between 180μm and 250μm, and dried to a moisture content of less than 5wt%.

[0037] Ammonium polyphosphate: As an acid source and skeleton reinforcing agent, it adopts a Form II crystal structure, CAS No.: 68333-79-9, degree of polymerization n is greater than 1000.

[0038] Melamine: used as a gas source and intercalating agent, CAS No.: 108-78-1, purity ≥99.5%.

[0039] Potassium acetate: used as a flux and activator, CAS No.: 127-08-2, purity ≥99.0%, deliquescent white powder.

[0040] Urea: Used as a buffer and plasticizer, CAS No.: 57-13-6, purity ≥99.0%.

[0041] Hydrochloric acid: As a post-treatment cleaning agent, it is an analytical grade aqueous solution with a concentration of 37 wt%.

[0042] Deionized water: used as a process medium, with a resistivity ≥15MΩ·cm.

[0043] Preparation Example 1:

[0044] This preparation example provides a method for preparing a casuarina-based rheological plasticizing precursor, including the following steps:

[0045] 100 parts by weight of Casuarina equisetifolia branch powder, 40 parts by weight of ammonium polyphosphate, 30 parts by weight of melamine, 100 parts by weight of potassium acetate, and 30 parts by weight of urea were added to a high-speed mixer and mixed at 400 r / min for 15 minutes at room temperature to obtain a uniformly mixed dry powder material.

[0046] The mixed dry powder material is added to the feed hopper of a co-rotating twin-screw extruder, and 30 parts by weight of deionized water is injected into the feed section of the extruder through a liquid metering pump.

[0047] The screw speed of the twin-screw extruder is set to 100 r / min, and the temperature of each section of the barrel is set as follows:

[0048] The temperature is 80°C in the feeding zone, 100°C in the compression zone, 100°C in the homogenization zone, and 95°C in the die head.

[0049] The residence time of the material in the extruder is controlled to be 12 minutes.

[0050] After being extruded through an extruder die, a black, clay-like strip with a smooth surface, uniform texture, and viscoelasticity is obtained. After naturally cooling to room temperature, it is cut into cylindrical particles with a length of 3 mm to 5 mm to obtain the casuarina-based rheological plasticizing precursor A1.

[0051] Preparation Example 2:

[0052] This preparation example provides a method for preparing a casuarina-based rheological plasticizing precursor, including the following steps:

[0053] 100 parts by weight of Casuarina equisetifolia branch powder, 35 parts by weight of ammonium polyphosphate, 25 parts by weight of melamine, 90 parts by weight of potassium acetate, and 25 parts by weight of urea were added to a high-speed mixer and mixed at 300 r / min for 20 minutes at room temperature to obtain a uniformly mixed dry powder material.

[0054] The mixed dry powder material is added to the feed hopper of a co-rotating twin-screw extruder, while 28 parts by weight of deionized water are injected into the feed section of the extruder via a liquid metering pump. The screw speed of the twin-screw extruder is set to 80 r / min, and the temperatures of each section of the barrel are set as follows: feed zone 75°C, compression zone 95°C, homogenization zone 95°C, and die head 90°C. The residence time of the material in the extruder is controlled to be 15 minutes.

[0055] After being extruded through an extruder die, a black, clay-like strip with a smooth surface, uniform texture, and viscoelasticity is obtained. After naturally cooling to room temperature, it is cut into cylindrical particles with a length of 3 mm to 5 mm to obtain the casuarina-based rheological plasticizing precursor A2.

[0056] Preparation Example 3:

[0057] This preparation example provides a method for preparing a casuarina-based rheological plasticizing precursor, including the following steps:

[0058] 100 parts by weight of Casuarina equisetifolia branch powder, 45 parts by weight of ammonium polyphosphate, 35 parts by weight of melamine, 110 parts by weight of potassium acetate and 35 parts by weight of urea were put into a high-speed mixer and mixed at 500 r / min for 10 minutes at room temperature to obtain a uniformly mixed dry powder material.

[0059] The mixed dry powder material is added to the feed hopper of a co-rotating twin-screw extruder, and 35 parts by weight of deionized water is injected into the feed section of the extruder through a liquid metering pump.

[0060] The screw speed of the twin-screw extruder is set to 120 r / min, and the temperatures of each section of the barrel are set as follows: feed zone 85℃, compression zone 105℃, homogenization zone 105℃, and die head 95℃. The residence time of the material in the extruder is controlled to 10 minutes.

[0061] After being extruded through an extruder die, a black, clay-like strip with a smooth surface, uniform texture, and viscoelasticity is obtained. After naturally cooling to room temperature, it is cut into cylindrical particles with a length of 3 mm to 5 mm to obtain the casuarina-based rheological plasticizing precursor A3.

[0062] Preparation Example 4:

[0063] This preparation example provides a method for preparing a casuarina-based rheological plasticizing precursor, including the following steps:

[0064] The raw material formulation and dosage were exactly the same as in Preparation Example 1. The mixed dry powder material was added to the feed hopper of a co-rotating twin-screw extruder, and 30 parts by weight of deionized water was injected into the feed section of the extruder through a liquid metering pump.

[0065] The screw speed of the twin-screw extruder is set to 90 r / min, and the temperatures of each section of the barrel are set as follows: feed zone 75℃, compression zone 90℃, homogenization zone 90℃, and die head 85℃. The residence time of the material in the extruder is controlled to 14 minutes.

[0066] After being extruded through an extruder die, a dark brown strip with a certain viscosity but a slightly rough surface is obtained. After naturally cooling to room temperature, it is cut into cylindrical particles with a length of 3 mm to 5 mm to obtain the casuarina-based rheological phase plasticizing precursor A4.

[0067] Example 1:

[0068] This embodiment provides a method for preparing casuarina-based hierarchical porous carbon, including the following steps:

[0069] (1) Precursor preparation: 100 kg of Casuarina equisetifolia branch powder with a particle size distribution of 180-250 μm and a moisture content of 4.5% was selected. This wood powder was mixed evenly with 40 kg of ammonium polyphosphate (Type II, degree of polymerization n>1000), 30 kg of melamine, 100 kg of potassium acetate, 30 kg of urea, and 30 kg of water. The mixture was added to a co-rotating twin-screw extruder, with the screw speed set at 100 r / min, the barrel homogenization zone temperature controlled at 100 ± 2℃, and the material residence time in the barrel approximately 12 minutes. During this process, the eutectic system formed by urea, potassium acetate, and water plasticizes the wood powder, and the urea decomposition products neutralize the acidic gases in situ. The extruded strip was air-cooled and pelletized to obtain a cylindrical rheological phase plasticized precursor.

[0070] (2) Curing and Pre-oxidation: The precursor particles were spread evenly in a heat-resistant tray (approximately 2.5 cm thick) and placed in a forced-air drying oven. In the first stage, the temperature was increased to 200°C at a rate of 3°C / min and held for 30 minutes for cross-linking and curing. In the second stage, the temperature was increased to 300°C at a rate of 3°C / min and held for 60 minutes for restricted foaming. After cooling, the foamed carbon precursor was obtained.

[0071] (3) High-temperature reforming and activation: The foamed carbon precursor was placed in a tube furnace and heated to 850°C at a rate of 5°C / min under nitrogen protection, and activated at a constant temperature for 90 minutes. It was then naturally cooled to room temperature to obtain the porous carbon crude product.

[0072] (4) Product refining:

[0073] Step A (Hot water digestion): The crude product is added to hot deionized water at 95°C with a liquid-to-solid ratio of 15:1, mechanically stirred and digested for 45 minutes, and then filtered and separated.

[0074] Step B (Ultrasonic acid washing): Place the digested solids in a 1.0 mol / L hydrochloric acid solution with a liquid-to-solid ratio of 10:1, heat to 60°C and turn on the ultrasonic waves (300W) for 30 minutes to assist in acid washing.

[0075] Step C (washing and drying): Filter and wash the filter cake repeatedly with deionized water until the filtrate is neutral, then dry at 105°C to obtain biomass porous carbon product.

[0076] Example 2:

[0077] This embodiment is used to verify the feasibility of the lower limit of the raw material ratio range in the claims.

[0078] The only difference from Example 1 is the ratio of raw materials, specifically:

[0079] 100kg of casuarina branch powder, 35kg of ammonium polyphosphate, 25kg of melamine, 90kg of potassium acetate, 25kg of urea, and 28kg of water.

[0080] The remaining process steps and parameters are completely consistent with those in Example 1.

[0081] Example 3:

[0082] This embodiment is used to verify the feasibility of the upper limit of the raw material ratio range in the claim.

[0083] The only difference from Example 1 is the ratio of raw materials, specifically:

[0084] 100kg of casuarina branch powder, 45kg of ammonium polyphosphate, 35kg of melamine, 110kg of potassium acetate, 35kg of urea, and 35kg of water.

[0085] The remaining process steps and parameters are completely consistent with those in Example 1.

[0086] Example 4:

[0087] This embodiment is used to verify the feasibility of the lower limit of the activation temperature range in the claims.

[0088] The only difference from Example 1 is the activation temperature in step (3):

[0089] The foamed carbon precursor was heated to 800°C (lower limit of the claims) at a rate of 5°C / min under nitrogen protection and activated at a constant temperature for 90 minutes.

[0090] The raw material ratios and other steps (S1, S2, S4) are completely consistent with those in Example 1.

[0091] Example 5:

[0092] This embodiment is used to verify the feasibility of the boundaries of the twin-screw extrusion process parameters in the claims.

[0093] The only difference from Example 1 is the extrusion process parameters in step (1):

[0094] The screw speed of the twin-screw extruder is controlled at 80 r / min (low shear), the temperature of the barrel homogenization zone is 95℃ (low temperature), and the material residence time is adjusted accordingly to 15 minutes.

[0095] The proportions of the remaining raw materials and subsequent steps are completely consistent with those in Example 1.

[0096] Comparative Example 1:

[0097] This comparative example is used to verify the importance of the amount of ammonium polyphosphate used. The difference from Example 1 is:

[0098] Reduce the amount of ammonium polyphosphate to 30 kg, while keeping other conditions unchanged.

[0099] Comparative Example 2:

[0100] This comparative example is used to verify the importance of casuarina powder particle size. The difference from Example 1 is:

[0101] The particle size distribution of the Casuarina equisetifolia branch powder used was 300-400 μm, and the other conditions remained unchanged.

[0102] Comparative Example 3:

[0103] This comparative example is used to verify the importance of the degree of polymerization of ammonium polyphosphate. The difference from Example 1 is that type I ammonium polyphosphate with a degree of polymerization n=800 is used, while all other conditions remain the same.

[0104] Comparative Example 4:

[0105] This comparative example is used to verify the importance of the product purification process. The difference from Example 1 is:

[0106] In step (4), hot water digestion and ultrasonic acid washing are not performed. Instead, the crude product is placed in room temperature deionized water and mechanically stirred and washed three times. After filtration, it is dried.

[0107] Test Example 1:

[0108] This experimental example aims to quantitatively evaluate the various performance indicators of the biomass porous carbon prepared in Examples 1-5 and Comparative Examples 1-4 of the present invention, in order to verify the influence of the formulation ratio, process parameter boundaries and raw material specifications defined in the claims on the quality of the final product.

[0109] 1. Operating Procedures and Testing Methods: The final products prepared in Examples 1-5 and Comparative Examples 1-4 were selected as the samples to be tested, and the following standards were followed for testing:

[0110] (1) Iodine adsorption value determination (characterizing microporous structure):

[0111] According to GB / T12496.8-2015 "Test Methods for Iodine Adsorption Value of Wood-based Activated Carbon", the test sample was ground and passed through a 325-mesh sieve. After drying, a certain amount of the sample was weighed and mixed with a 0.100 mol / L iodine standard solution and shaken. After filtration, the filtrate was titrated with a 0.100 mol / L sodium thiosulfate standard solution. The residual iodine concentration was calculated, and the iodine adsorption value (mg / g) was obtained.

[0112] (2) Determination of methylene blue adsorption value (characterizing mesoporous structure):

[0113] According to GB / T12496.10-1999 "Test Methods for Wood-based Activated Carbon: Determination of Methylene Blue Adsorption Value".

[0114] Weigh the ground sample and add a 1.5 g / L methylene blue test solution. Shake until adsorption reaches equilibrium. Observe the decolorization of the solution to determine the adsorption endpoint and calculate the mass of methylene blue adsorbed per gram of carbon material (mg / g).

[0115] (3) Ash content determination (characterizing product purity)

[0116] According to GB / T12496.3-1999 "Test Methods for Determination of Ash Content in Wood-based Activated Carbon". Take approximately 1.0 g of the pulverized and dried sample and weigh it precisely in a pre-weighed porcelain crucible. Place the crucible in a high-temperature muffle furnace and slowly heat it to 710℃±10℃ in air atmosphere, then ignite it to constant weight. Calculate the percentage of residual inorganic matter (%).

[0117] (4) Carbon yield calculation (characterizing process economy): Record the mass m0 of the oven-dried Casuarina equisetifolia branch powder input during the precursor preparation stage, and the mass m1 of the dried porous carbon sample obtained after completing the entire carbonization, activation, and purification process. The calculation formula is: Y = (m1 / m0) x 100%;

[0118] 2. Test Results

[0119] The experimental data, after statistical processing, are listed in Table 1.

[0120] Table 1. Comparison of product performance data between each embodiment and the comparative example.

[0121]

[0122] 3. Results Analysis and Conclusions

[0123] Formulation range verification: Data from Examples 2 and 3 show that, within the raw material ratio range defined in the claims (35-45 parts of ammonium polyphosphate, 90-110 parts of potassium acetate, etc.), high-quality porous carbon with an iodine adsorption value >1200 mg / g and a methylene blue adsorption value >160 mg / g can be prepared.

[0124] Temperature range verification: Comparing Example 1 (850℃) and Example 4 (800℃), it can be seen that activation at 800℃ is more conducive to preserving the carbon framework and improving the carbon yield (38.1%); while activation at 850℃ further deepens the pore etching and improves the adsorption performance. This proves that 800-850℃ is an effective process window that balances yield and performance.

[0125] Extrusion parameter verification: Example 5 demonstrates that even under low energy consumption conditions of 95°C and 80 r / min, the eutectic system of the present invention can still achieve effective plasticization, verifying the feasibility of the lower limit of extrusion process parameters.

[0126] Comparative Example 2 shows that when the wood flour particle size (300-400 μm) exceeds the preferred range (180-250 μm), the eutectic solvent has difficulty fully penetrating to the particle core, resulting in uneven plasticization (white spots) and decreased adsorption performance.

[0127] Effect of degree of polymerization: Comparative Example 3 shows that using type I ammonium polyphosphate with a low degree of polymerization (n=800) results in premature decomposition during the high-temperature activation stage due to its poor thermal stability, which fails to maintain a stable three-dimensional cross-linked network, leading to a significant reduction in carbon yield (only 28.5%). This strongly supports the limitation of type II high-polymerization-degree APP in the claims.

[0128] Comparative Example 1 demonstrates that if the amount of ammonium polyphosphate is insufficient, it cannot form a sufficient skeleton support, leading to bubble rupture and structural collapse during the foaming process.

[0129] Comparative Example 4 directly demonstrates that without the high-temperature hot water digestion and ultrasonic acid washing process specific to this invention, the ash content of the product is as high as 8.5%, which cannot meet the requirements for high-purity applications, thus proving the key role of this refining step.

[0130] Test Example 2:

[0131] This experimental example aims to quantitatively evaluate the effect of the urea-potassium acetate-water eutectic system in this invention on improving the processing rheology of precursors, and the effect of urea's in-situ neutralization on suppressing equipment corrosion risk.

[0132] 1. To investigate the effect of urea as a single variable, the following two sets of comparative experiments were set up:

[0133] Experimental Group A (Invention): The complete formula of Example 1 (containing 30 parts of urea) was used.

[0134] Control group B (without urea): Urea was removed from the formula, and the remaining components were the same as in Example 1.

[0135] Since control group B lacked urea and could not form a rheological phase, a wet premixing method was used to mix the material and force-feed it in order to ensure that the material could barely enter the extruder for corrosion testing.

[0136] 2. Operating Procedures and Testing Methods: Two identical co-rotating twin-screw extruders (both with screws made of 38CrMoAlA nitrided steel) were used to conduct the following tests:

[0137] (1) Processing rheology monitoring (characterizing plasticizing effect): When the extruder reaches the steady-state operation stage (set temperature 100℃, speed 100r / min), data is continuously collected for 300 seconds through the industrial control system.

[0138] Main motor current (A): Characterizes the torque borne by the screw. The lower the current, the better the material plasticization and the smaller the internal friction resistance.

[0139] Die head melt pressure (MPa): Characterizes the stability of extrusion.

[0140] (2) Determination of acid gas release (characterizing corrosion source): A gas collection hood was set at the exhaust port of the extruder, and the released gas was introduced into a 0.1 mol / L NaOH absorption solution and sampled for 10 minutes. Then, the release rate of acetic acid (mg / kg·min) was calculated by back titration with standard hydrochloric acid.

[0141] (3) Determination of equipment corrosion (direct characterization of equipment damage): Before the experiment, the metering section assembly of the screw was disassembled, cleaned and dried, and weighed using a high-precision balance (denoted as W). pre ).

[0142] After both sets of materials have been running continuously for 100 hours, the screw section is disassembled again, cleaned to remove surface carbon deposits and attachments, dried, and weighed (recorded as W). post ).

[0143] Calculate the screw weight loss: ΔW = W pre -W post .

[0144] (4) Determination of metal impurities in the product (characterizing product purity): The precursor produced during the 95th to 100th hour of operation was carbonized, activated and purified, and the iron (Fe) content in the final product was detected by inductively coupled plasma mass spectrometry (ICP-MS).

[0145] 2. Test Results

[0146] The test data of each group of samples are shown in Table 2.

[0147] Table 2 Adsorption performance and physical parameter data of each example and comparative sample

[0148]

[0149] 3. Results Analysis and Conclusions

[0150] The main unit current of control group B was as high as 34.7A, and the pressure fluctuation of the die head was violent. This indicates that in the absence of urea, potassium acetate and ammonium polyphosphate cannot form a liquid phase at 100°C. The material is forced through the screw in the form of "solid-solid" friction, which consumes a lot of energy and cannot achieve continuous plasticization.

[0151] The host current of experimental group A dropped to 18.4A, indicating that urea, potassium acetate and water successfully constructed a eutectic system. This system wets the wood flour like a lubricant, achieving efficient plasticization processing at low temperature.

[0152] The acetic acid release rate in control group B was as high as 285.3 mg / kg·min. This was because the strongly acidic groups produced by the hydrolysis of ammonium polyphosphate underwent a metathesis reaction with potassium acetate, displacing volatile acetic acid. This high-temperature acidic environment caused the screw to lose 0.82 g of weight within 100 hours, and the corroded iron filings entered the product, resulting in an Fe content as high as 52.4 ppm.

[0153] Conclusion: The data from Experimental Group A strongly demonstrate that this invention utilizes ammonia (a proton acceptor) generated from urea decomposition to efficiently neutralize acidic gases in situ (acetic acid release rate is only 12.6 mg / kg·min), thereby controlling screw wear to 0.05 g. This protects expensive processing equipment while ensuring high product purity. This technological effect is of decisive significance for the continuous industrial production of porous biomass carbon.

[0154] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing biomass porous carbon using Casuarina trees, characterized in that, Includes the following steps: S1. Precursor preparation: Raw materials comprising 100 parts by weight of Casuarina equisetifolia branch powder, 35-45 parts by weight of ammonium polyphosphate, 25-35 parts by weight of melamine, 90-110 parts by weight of potassium acetate, 25-35 parts by weight of urea, and 28-35 parts by weight of water are added to a twin-screw extruder and reactively extruded at a temperature of 95-105°C to prepare a rheological plasticizing precursor. During the reactive extrusion process, the decomposition products of urea neutralize the acidic gas generated by the hydrolysis of ammonium polyphosphate in situ, inhibiting equipment corrosion and preparing a rheological plasticizing precursor. The urea forms a eutectic system with potassium acetate and water to achieve low-temperature rheological plasticization. The urea forms a eutectic system with potassium acetate and water, thereby achieving low-temperature rheological plasticization. S2. Curing and pre-oxidation: The rheological plasticizing precursor obtained in step S1 is subjected to segmented heat treatment in an air atmosphere, successively going through a low-temperature cross-linking and curing stage and a medium-temperature confined foaming stage. The gas is confined to escape through the skeleton structure formed by the first cross-linking and curing, and foamed carbon precursor is obtained. S3. High-temperature reforming and activation: The foamed carbon precursor obtained in step S2 is heated under an inert atmosphere to carry out carbonization and activation reactions to obtain porous carbon crude product. S4. Product purification: The crude product obtained in step S3 is subjected to a combination of high-temperature hot water digestion and ultrasonic-assisted acid washing for cleaning and drying to obtain the biomass porous carbon.

2. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The weight ratio of the raw materials mentioned in step S1 is as follows: 100 parts of casuarina branch and trunk powder; 40 parts of ammonium polyphosphate; 30 parts of melamine; 100 parts potassium acetate; 30 parts urea; And 30 portions of water.

3. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The raw materials mentioned in step S1 meet the following specifications: The Casuarina equisetifolia branch powder has a particle size distribution between 180 μm and 250 μm, is used for uniform wetting of eutectic systems, and has a moisture content of less than 5 wt%. The ammonium polyphosphate has a type II crystal structure and a degree of polymerization n greater than 1000, which is conducive to the formation of a stable cross-linked network.

4. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The specific process of reactive extrusion described in step S1 is as follows: The screw speed of the twin-screw extruder is controlled at 80-120 r / min, the temperature of the barrel homogenization zone is 95-105℃, and the residence time of the material in the barrel is 10-15 minutes; the extruded strip is then pelletized into cylindrical granules.

5. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The specific procedure for the segmented heat treatment in step S2 is as follows: First stage: Heat to 200℃ at a heating rate of 3-5℃ / min and hold at that temperature for 30 minutes. Use the polymethic acid produced by the decomposition of ammonium polyphosphate to cross-link and solidify with the lignin hydroxyl groups to form a supporting skeleton. Second stage: Continue heating at a rate of 3-5℃ / min to 300℃ and hold at that temperature for 60 minutes. Utilize the gas generated by the decomposition of melamine to create confined foaming within the support framework, forming a porous framework.

6. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The specific parameters for the high-temperature reforming activation in step S3 are as follows: Under nitrogen protection, heat to 800-850℃ at a heating rate of 5℃ / min and maintain the temperature for 90 minutes; In this process, the potassium vapor generated by the reduction of potassium acetate etches pores in the carbon skeleton.

7. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, The specific cleaning steps described in step S4 include: Step A: The crude product is put into hot deionized water at 95°C for mechanical stirring and digestion to remove insoluble pyrophosphate and double salts. Step B: Place the digested solids in hydrochloric acid solution and perform ultrasonic-assisted acid washing at 60°C; Step C: Wash with deionized water until neutral and dry.

8. The method for producing biomass porous carbon using Casuarina trees according to claim 7, characterized in that, In step A, the mechanical stirring digestion time is 45 minutes, and the liquid-to-solid ratio is 15:1; In step B, the concentration of the hydrochloric acid solution is 1.0 mol / L, the ultrasonic-assisted pickling time is 30 minutes, and the liquid-to-solid ratio is 10:

1.

9. The method for producing biomass porous carbon using Casuarina trees according to claim 1, characterized in that, In step S1, the twin-screw extruder neutralizes the acidic gas generated by the hydrolysis of ammonium polyphosphate in situ through the decomposition products of urea during the extrusion process, thereby inhibiting equipment corrosion.

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