A method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon

Abstract

The present invention discloses a method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon. The method comprises the following steps: in a carbon dioxide atmosphere with a slightly positive pressure, the biomass raw material is pyrolyzed and activated by means of staged heating and reaction to obtain high-quality biomass-based activated carbon; specifically, in a carbon dioxide atmosphere, the biomass raw material is heated to the pyrolysis temperature for pyrolysis reaction; then further heated to the activation temperature for activation reaction; and then cooled in a carbon dioxide atmosphere. The production equipment and devices required by the method of the present invention are simpler, avoiding damage to the production equipment and devices caused by frequent heating and cooling, and there is no need to process solid, liquid and gaseous products multiple times during the production process. The method of the present invention has a wide applicability to biomass raw materials, is easy to be popularized and applied industrially, is environmentally friendly, and is suitable for cost reduction and efficiency increase in related production processes.

Description

Technical Field

[0001] The present invention relates to a method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon, belonging to the technical field of biomass-based activated carbon preparation. Background Art

[0002] High-quality activated carbon usually has a high specific surface area, a developed pore structure and a large adsorption capacity, and has been widely used in gas purification, water treatment, energy storage, catalysis, environmental protection, food, medicine, chemical industry, military industry and other fields. Improper disposal of biomass resources will not only pollute the environment, but also cause serious waste. By using innovative technical methods to convert these abundant biomass resources into high-value chemicals, energy products and functional materials with various functions and uses, not only the high-value resource utilization of agricultural waste is realized, but also the "transformation of waste into treasure" is achieved, which is an effective way to control environmental pollution and promote circular economy. In recent years, researchers have developed various methods for preparing high-value products from biomass, which have a wide range of uses in the fields of chemistry, energy, materials and medicine. Among them, biomass pyrolysis is an important thermochemical conversion method of biomass and an important utilization method of biomass energy. Biomass pyrolysis refers to the thermochemical reaction process of converting biomass raw materials into biochar, bio-oil, wood vinegar liquid and combustible gas at high temperature under anaerobic or anoxic conditions (see Figure 1 ). Among the primary products prepared by biomass pyrolysis, biochar is a class of carbon-rich solid products with stable properties, high degree of carbonization and aromatization. Compared with the original biomass, biochar has a significantly increased carbon content and a large specific surface area, and its structure contains rich pore structures and oxygen-containing functional groups. These unique physical and chemical properties make biochar an important activated carbon precursor. By activating biochar through physical activation methods, chemical activation methods or physical and chemical combined activation methods, activated carbon with better performance and higher value can be produced.

[0003] Activated biochar has a large specific surface area, a rich pore structure, and special surface physical and chemical properties. In the physical activation method of biochar, carbon dioxide, high-temperature steam, or a mixture of the two is usually used to open or expand the pores inside the biochar. In the chemical activation method, chemical reagents such as phosphoric acid, hydrochloric acid, nitric acid, potassium hydroxide, potassium carbonate, zinc chloride, and aluminum chloride are used as activators to activate biochar materials. Compared with the chemical activation method, the physical activation method has a relatively lower cost, simpler manufacturing equipment, and easier operation. The preparation of biomass-based activated carbon by the physical activation method usually includes two steps: the pyrolysis carbonization of biomass to prepare biochar and the activation of biochar. After the biomass raw materials are pretreated by crushing, screening, drying, etc., they are first heated to the pyrolysis temperature in an inert atmosphere such as nitrogen or helium or in a vacuum environment for high-temperature pyrolysis to prepare biochar. Subsequently, the biochar is heated to the activation temperature in an atmosphere of carbon dioxide, high-temperature steam, or a mixture of the two and maintained for a period of time. After cooling, washing, and drying, biomass-based activated carbon can be obtained.

[0004] Carbon dioxide has safe physical and chemical properties, wide sources, low prices, and convenient storage and transportation. Moreover, carbon dioxide can react chemically with the carbon-based skeleton of biochar at high temperatures and promote the Boudouard reaction involved in the activation of carbon dioxide by biomass, thereby expanding more pore structures inside the biochar and increasing the specific surface area of the product. As an important activator for biochar, carbon dioxide has received extensive attention from the scientific and industrial communities and has great application prospects in the industrial preparation of biomass-based activated carbon. However, the manufacturing process of biomass-based activated carbon involves two steps: biomass carbonization and biochar activation. During the preparation process, the activation temperature of biochar is usually higher than the carbonization temperature of biomass. In addition, the biomass carbonization step needs to be carried out in an inert atmosphere such as nitrogen or helium or in a vacuum environment, while the physical activation of biochar is carried out in an atmosphere of carbon dioxide, high-temperature steam, or a mixture of the two. Therefore, in the process of manufacturing activated carbon by the two-step method of biomass carbonization and biochar activation, the production equipment needs to be heated and cooled frequently, and the gases in the production process need to be switched (see Figure 2 ). These production steps result in high energy consumption, complex operation, and long production cycles in the relevant production processes. Moreover, in the two-step production process, solid, liquid, and gaseous products need to be processed multiple times, which is likely to cause secondary pollution. In addition, the production equipment for the two-step method of biomass carbonization and biochar activation to produce activated carbon is complex. Frequent heating and cooling and switching of the production atmosphere not only easily cause equipment damage and damage the sealing effect of the equipment, but also reduce production efficiency, increase production energy consumption, and increase production costs. Summary of the Invention

[0005] The object of the present invention is to provide a method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon. Using carbon dioxide as the inert atmosphere required for the biomass pyrolysis process and the activator required for the biochar activation process, and adopting a stepwise temperature increase and reaction method to control the reaction process, high-quality biomass-based activated carbon is directly prepared from biomass raw materials that have been pretreated by crushing, screening, washing, drying, etc. (see Figure 3 , Figure 4 ).

[0006] The method of the present invention can avoid the processes of heating and carbonizing, cooling, switching the reaction atmosphere, reheating and activating, and cooling in the production process of biomass-based activated carbon by the two-step method, thereby reducing production energy consumption and improving production efficiency. The method of the present invention not only avoids damage to production equipment caused by frequent heating and cooling, but also avoids the need to process solid, liquid, and gaseous products multiple times during the production process and the possible secondary pollution caused thereby. In addition, the method of the present invention does not require switching of production gases during the production process, thereby avoiding the processes of pressurization, depressurization, and waiting for the gas pressure to stabilize when switching gases in the two-step production process of carbonization and activation, and further improving production efficiency. The method of the present invention can prepare high-quality biomass-based activated carbon from biomass raw materials in one step on the basis of reducing production costs. The method has the characteristics of wide applicability, easy industrial promotion and application, environmental friendliness, and suitability for cost reduction and efficiency improvement in related production processes

[0007] Specifically, the method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon provided by the present invention includes the following steps:

[0008] In a carbon dioxide atmosphere with a slightly positive pressure, the biomass raw materials are pyrolyzed and activated by adopting a stepwise temperature increase and reaction method to obtain high-quality biomass-based activated carbon.

[0009] In the above method, in the carbon dioxide atmosphere, the biomass raw materials are heated to the pyrolysis temperature for pyrolysis reaction; then further heated to the activation temperature for activation reaction; then cooled in the carbon dioxide atmosphere.

[0010] In the above method, the pressure of the carbon dioxide atmosphere is 0.1 - 0.3 MPa, such as 0.15 - 0.2 MPa.

[0011] In the above method, the heating rate before the pyrolysis reaction is 1 - 20 °C / min, such as 10 - 20 °C / min;

[0012] The pyrolysis temperature is 400 - 700 °C, such as 400 °C, 500 °C, 600 °C, 700 °C or the range value composed of any two numerical values, and the time of the pyrolysis reaction is 1 - 2 h, such as 1 h or 2 h.

[0013] In the above method, the heating rate before the activation reaction is 1-20 °C / min, such as 10-20 °C / min;

[0014] the activation temperature is 800-1200 °C, such as 900 °C, 1000 °C or 900-1000 °C, and the time of the activation reaction is 1-2 h, such as 1 h or 2 h.

[0015] In the above method, the biomass raw material is lignocellulosic biomass, specifically at least one of wheat straw, corn straw, rice straw, bamboo, wood, wood chips, peanut shells, and rice husks.

[0016] In the above method, before the pyrolysis reaction, the biomass raw material is pretreated as follows: washing, crushing, sieving, and drying;

[0017] The steps of the washing are: repeatedly soaking and washing 2-3 times with deionized water;

[0018] After crushing, it is sieved through a 10-100 mesh sieve;

[0019] The steps of the drying are: drying at 90-110 °C until the water content is less than 10%.

[0020] The method of the present invention further includes the following steps:

[0021] The step of washing the obtained solid product with water at 20-60 °C and drying at 90-110 °C.

[0022] In the method of the present invention, carbon dioxide is both an inert atmosphere in the biomass pyrolysis process and an activator for activating biochar in the biochar activation process. By means of staged heating and reaction, high-quality biomass-based activated carbon can be prepared from the pretreated biomass raw material. Straw-like biomasses such as wheat straw, corn straw, and rice straw, as well as agricultural and forestry organic wastes such as fruit shells and tree pruning often contain relatively high ash content. The main components of these ash contents are alkali metals such as potassium and sodium and alkaline earth metals such as magnesium and calcium. During the pretreatment process, most of the free alkali metals and alkaline earth metals can be removed by washing. Compared with inert gases such as nitrogen and helium, carbon dioxide has weak oxidizing properties. By washing to remove alkali metals and alkaline earth metals with certain catalytic effects at high temperatures, the chemical reaction between carbon dioxide and the biomass raw material during the pyrolysis process can be minimized.

[0023] The method of the present invention realizes the regulation of the reaction process by means of staged heating and reaction in a carbon dioxide atmosphere. Generally, the activation temperature of biochar is higher than the pyrolysis temperature of biomass. The method of the present invention first heats the pretreated biomass raw material to a relatively low pyrolysis temperature in a carbon dioxide atmosphere for pyrolysis reaction, and then, without changing the gas atmosphere, further heats it to the activation temperature for activation reaction. After the reaction is completed, it is continuously cooled in a carbon dioxide atmosphere, and the obtained solid is washed and dried to obtain biomass-based activated carbon. Compared with an inert pyrolysis atmosphere or a vacuum environment, carbon dioxide as a pyrolysis atmosphere may undergo a gasification reaction with biochar during high-temperature pyrolysis, reducing the biochar yield. Therefore, the present invention adopts a method of staged heating and reaction to regulate the reaction process (see Figure 4 ). First, the reaction system is heated to a relatively low pyrolysis temperature for pyrolysis reaction. At this stage, the role of carbon dioxide is to provide an inert atmosphere, and the pyrolysis reactions of organic components such as cellulose, hemicellulose, and lignin in the biomass itself are the main reactions. Subsequently, the reaction system is further heated to a relatively high activation temperature for activation reaction. Using carbon dioxide as an activator, the pore structure of biochar is increased and the specific surface area is improved through the chemical reaction between carbon dioxide and biochar at high temperature, thereby preparing high-quality biomass-based activated carbon.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The method of the present invention pyrolyzes and activates biomass raw materials by means of staged heating and reaction in a flowing slightly positive pressure carbon dioxide atmosphere, and prepares high-value biomass-based activated carbon from low-quality biomass raw materials. Using carbon dioxide as the inert atmosphere required for the biomass pyrolysis process and the activator required for the biochar activation process, it avoids multiple heating and cooling steps during the two-step production of biomass-based activated carbon by biomass carbonization and biochar activation, thereby reducing production energy consumption, reducing production time, improving production efficiency, and reducing production costs.

[0026] In addition, the method of the present invention does not require gas switching during the production process, avoiding a series of operation processes such as pressurization, pressure relief, and pressure stabilization during gas switching, simplifying the production process, and further improving production efficiency.

[0027] Compared with traditional production methods, the production equipment and devices required by the method of the present invention are simpler, and it also avoids damage to the production equipment and devices caused by frequent heating and cooling. At the same time, the method of the present invention does not require multiple treatments of solid, liquid, and gaseous products during the production process, avoiding possible secondary pollution caused thereby.

[0028] The method of the present invention has a wide applicability to biomass raw materials, is easy to be industrially promoted and applied, is environmentally friendly, and is suitable for cost reduction and efficiency improvement in related production processes. Description of the Drawings

[0029] Figure 1 The figure shows a schematic diagram of the biomass pyrolysis and the formation process of the main products.

[0030] Figure 2 The figure shows a schematic process diagram for producing biomass-based activated carbon by a two-step method of biomass carbonization and biochar carbon dioxide activation.

[0031] Figure 3 The figure shows a schematic process diagram of the method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare biomass-based activated carbon according to the present invention.

[0032] Figure 4 The figure shows a schematic diagram of the stage heating and reaction regulation of the reaction process in the method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare biomass-based activated carbon according to the present invention.

[0033] Figure 5 The figure shows the yields of the biomass-based activated carbon prepared in Examples 1-12 and Comparative Examples 1-8 provided by the present invention.

[0034] Figure 6 The figure shows the specific surface areas of the biomass-based activated carbon prepared in Examples 1-12 and Comparative Examples 1-8 provided by the present invention. Detailed Description of the Invention

[0035] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0036] Unless otherwise specified, the materials, reagents, etc. used in the following examples can all be obtained from commercial sources.

[0037] Example 1: Continuously pyrolyzing and activating wheat straw in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0038] The wheat straw was crushed and sieved to 30 mesh, then soaked and washed 2-3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, and the drying temperature was 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and kept pyrolyzing for 1 h. After the pyrolysis reaction was completed, it was continuously heated to the activation temperature of 900°C at a heating rate of 10°C / min and kept activating for 1 h. Then it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0039] Example 2: Continuously pyrolyzing and activating Chinese fir wood chips in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0040] The Chinese fir sawdust was crushed and screened to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was cooled down in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0041] Example 3: Continuous pyrolysis and activation of corn straw in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0042] The corn straw was crushed and screened to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was cooled down in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0043] Example 4: Continuous pyrolysis and activation of rice husk in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0044] The rice husk was crushed and screened to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was cooled down in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0045] Example 5: Continuous pyrolysis and activation of peanut shell in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0046] The peanut shells were crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), the temperature was raised to the pyrolysis temperature of 600°C at a heating rate of 10°C / min, and pyrolyzed for 1 h while maintaining the temperature. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 900°C at a heating rate of 10°C / min and activated for 1 h while maintaining the temperature. Then, it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0047] Example 6: Preparation of biomass-based activated carbon by continuous pyrolysis and activation of Chinese fir sawdust in a carbon dioxide atmosphere

[0048] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), the temperature was raised to the pyrolysis temperature of 600°C at a heating rate of 10°C / min, and pyrolyzed for 1 h while maintaining the temperature. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 1000°C at a heating rate of 10°C / min and activated for 1 h while maintaining the temperature. Then, it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0049] Example 7: Preparation of biomass-based activated carbon by continuous pyrolysis and activation of Chinese fir sawdust in a carbon dioxide atmosphere

[0050] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), the temperature was raised to the pyrolysis temperature of 600°C at a heating rate of 20°C / min, and pyrolyzed for 1 h while maintaining the temperature. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 1000°C at a heating rate of 20°C / min and activated for 1 h while maintaining the temperature. Then, it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0051] Example 8: Preparation of biomass-based activated carbon by continuous pyrolysis and activation of Chinese fir sawdust in a carbon dioxide atmosphere

[0052] The Chinese fir wood chips were crushed and screened to 50 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), the temperature was raised to the pyrolysis temperature of 500°C at a heating rate of 10°C / min, and pyrolyzed for 1 h with heat preservation. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 900°C at a heating rate of 10°C / min, and activated for 1 h with heat preservation. Then, it was continuously cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0053] Example 9: Preparation of Biomass-based Activated Carbon by Continuous Pyrolysis and Activation of Chinese Fir Wood Chips in Carbon Dioxide Atmosphere

[0054] The Chinese fir wood chips were crushed and screened to 50 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.2 MPa), the temperature was raised to the pyrolysis temperature of 600°C at a heating rate of 10°C / min, and pyrolyzed for 1 h with heat preservation. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 900°C at a heating rate of 10°C / min, and activated for 1 h with heat preservation. Then, it was continuously cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0055] Example 10: Preparation of Biomass-based Activated Carbon by Continuous Pyrolysis and Activation of Chinese Fir Wood Chips in Carbon Dioxide Atmosphere

[0056] The Chinese fir wood chips were crushed and screened to 50 mesh, then soaked and washed 2 - 3 times in deionized water, and dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), the temperature was raised to the pyrolysis temperature of 600°C at a heating rate of 10°C / min, and pyrolyzed for 2 h with heat preservation. After the pyrolysis reaction was completed, the temperature was continuously raised to the activation temperature of 900°C. Activated for 2 h with heat preservation. Then, it was continuously cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0057] Example 11: Preparation of Biomass-based Activated Carbon by Continuous Pyrolysis and Activation of Chinese Fir Wood Chips in Carbon Dioxide Atmosphere

[0058] The Chinese fir sawdust was crushed and sieved to 50 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 400°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was continued to be cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0059] Example 12: Continuous pyrolysis and activation of Chinese fir sawdust in a carbon dioxide atmosphere to prepare biomass-based activated carbon

[0060] The Chinese fir sawdust was crushed and sieved to 50 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 700°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was continued to be cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0061] Comparative Example 1

[0062] The difference between this comparative example and Example 1 is that: after the pyrolysis reaction in the process of preparing the biomass-based activated carbon from wheat straw, no further carbon dioxide activation was carried out. That is:

[0063] The wheat straw was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be cooled in the carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0064] Comparative Example 2

[0065] The difference between this comparative example and Example 2 is that: after the pyrolysis reaction in the process of preparing the biomass-based activated carbon from Chinese fir sawdust, no further carbon dioxide activation was carried out. That is:

[0066] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0067] Comparative Example 3

[0068] Compared with Example 1, the difference in this comparative example is that: in the process of preparing wheat straw biomass-based activated carbon, inert gas nitrogen was always used as the reaction atmosphere, and carbon dioxide atmosphere was not used for pyrolysis and activation of wheat straw. That is:

[0069] The wheat straw was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure nitrogen atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continuously heated to 900°C at a heating rate of 10°C / min and held for 1 h. Then it was continuously cooled in a nitrogen atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0070] Comparative Example 4

[0071] Compared with Example 2, the difference in this comparative example is that: in the process of preparing Chinese fir sawdust biomass-based activated carbon, inert gas nitrogen was always used as the reaction atmosphere, and carbon dioxide atmosphere was not used for pyrolysis and activation of Chinese fir sawdust. That is:

[0072] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure nitrogen atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continuously heated to 900°C at a heating rate of 10°C / min and held for 1 h. Then it was continuously cooled in a nitrogen atmosphere. The obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0073] Comparative Example 5

[0074] This comparative example is different from Example 2 in that the heating rates during the pyrolysis and activation reactions are different. That is:

[0075] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 50°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was continued to be cooled in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0076] Comparative Example 6

[0077] This comparative example is different from Example 2 in that the heating rates during the pyrolysis and activation reactions are different. That is:

[0078] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 600°C at a heating rate of 50°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 900°C at a heating rate of 50°C / min and held for activation for 1 h. Then it was continued to be cooled in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0079] Comparative Example 7

[0080] This comparative example is different from Example 2 in that the temperatures of the pyrolysis and activation reactions are different during the preparation of the biomass-based activated carbon from Chinese fir sawdust. That is:

[0081] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2 - 3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, with a drying temperature of 105°C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 300°C at a heating rate of 10°C / min and held for pyrolysis for 1 h. After the pyrolysis reaction was completed, it was continued to be heated to the activation temperature of 700°C at a heating rate of 10°C / min and held for activation for 1 h. Then it was continued to be cooled in the carbon dioxide atmosphere, and the obtained solid product was washed with water at about 30°C and dried at 105°C to obtain the biomass-based activated carbon.

[0082] Comparative Example 8

[0083] This comparative example is different from Example 2 in that the temperatures of the pyrolysis and activation reactions are different during the preparation of the Chinese fir sawdust biomass-based activated carbon. That is:

[0084] The Chinese fir sawdust was crushed and sieved to 30 mesh, then soaked and washed 2-3 times in deionized water, and then dried in an oven until the moisture content was less than 10%, and the drying temperature was 105 °C. Subsequently, in a slightly positive pressure carbon dioxide atmosphere (0.15 MPa), it was heated to the pyrolysis temperature of 800 °C at a heating rate of 10 °C / min and kept pyrolyzing for 1 h. After the pyrolysis reaction was completed, it was continuously heated to the activation temperature of 1200 °C at a heating rate of 10 °C / min and kept activating for 1 h. Then it was continuously cooled in a carbon dioxide atmosphere. The obtained solid product was washed with water at about 30 °C and dried at 105 °C to obtain the biomass-based activated carbon.

[0085] The biomass-based activated carbons prepared in Examples 1-12 and Comparative Examples 1-8 were weighed after drying, and the yield of the biomass-based activated carbon was calculated. The calculation formula for the yield of the biomass-based activated carbon is as follows:

[0086]

[0087] In the formula: Y is the yield of the biomass-based activated carbon, m' is the mass of the biomass-based activated carbon after drying, and m is the mass of the raw material after drying.

[0088] In addition, the specific surface area of the obtained biomass-based activated carbon was measured by the BET method using a fully automatic specific surface area and pore analyzer. The test results are as Figure 5 and Figure 6 shown.

[0089] The analysis of the test results shows that the method of the present invention can pyrolyze and activate various biomass raw materials in a carbon dioxide atmosphere by using a stepwise heating and reaction method to obtain different types of high-quality biomass-based activated carbons with a specific surface area similar to that of commercial activated carbon.

[0090] Figure 5 The yields of the biomass-based activated carbons prepared in Examples 1-12 and Comparative Examples 1-8 are shown. Compared with the activated carbons obtained in Comparative Examples 1 and 2 without the carbon dioxide activation step and Comparative Examples 3 and 4 using an inert gas (nitrogen) as the reaction atmosphere all the time, the yields of the wheat straw and Chinese fir sawdust biomass-based activated carbons prepared by pyrolysis and activation in carbon dioxide in Examples 1 and 2 decreased. This result indicates that in the activation step, carbon dioxide undergoes a gasification reaction with part of the carbon formed from the biomass raw material under high temperature and anaerobic conditions, thereby realizing the activation of the biochar generated by the pyrolysis step.

[0091] Figure 6The BET specific surface areas of the biomass-based activated carbons prepared in Examples 1-12 and Comparative Examples 1-8 are shown. The test results indicate that pyrolyzing the biomass raw material to the pyrolysis temperature in a carbon dioxide atmosphere and then directly further heating to the activation temperature for the activation reaction can significantly increase the specific surface area of the obtained biomass-based activated carbons. The specific surface areas of the biomass-based activated carbons in Examples 1 and 2 are significantly increased compared with those of the activated carbons produced in Comparative Examples 1 and 2 without the carbon dioxide activation step, and are also significantly larger than those of the activated carbons prepared using an inert gas (nitrogen) all the time in Comparative Examples 3 and 4.

[0092] In all the examples, the biomass-based activated carbon prepared from Chinese fir sawdust in Example 6 has the largest specific surface area, reaching 1021 m 2 g -1 , while the specific surface areas of the biomass-based activated carbons prepared from Chinese fir sawdust in Comparative Example 2 without carbon dioxide activation and in Comparative Example 4 produced in an inert atmosphere are both less than 60 m 2 g -1 .

[0093] It can be seen from Figure 5 and Figure 6 that reaction parameters such as the type of biomass raw material, pyrolysis reaction temperature, activation reaction temperature, reaction time, and heating rate at different reaction stages will all affect the yield and specific surface area of the obtained biomass-based activated carbons. Comparative Examples 5 and 6 show that when the rates of the pyrolysis reaction and the activation reaction increase, more liquid (wood vinegar, pyrolysis oil) or gaseous (pyrolysis gas) products will be formed during the pyrolysis process, resulting in a significant decrease in the yield of the obtained activated carbon. Moreover, too high a reaction rate is not conducive to the formation of the pore structure of the activated carbon. In Comparative Examples 5 and 6, the specific surface areas of the biomass-based activated carbons prepared from Chinese fir sawdust are both reduced. Comparative Example 7 shows that although a too low pyrolysis temperature (300 °C) will increase the yield of the obtained activated carbon, its specific surface area is reduced. In Comparative Example 8, too high an activation temperature (1200 °C) causes more organic substances in the activated carbon to be gasified, destroying the pore structure in the activated carbon, resulting in a significant reduction in both the yield and the specific surface area of the obtained activated carbon, which is not conducive to the formation of high-quality activated carbon.

[0094] Under the conditions of the examples of the present invention, as the activation reaction time at high temperature decreases, the degree of the gasification reaction between carbon dioxide and biochar decreases, resulting in an increase in the yield of the obtained biomass-based activated carbon, while the specific surface area slightly decreases. Under a slightly positive pressure carbon dioxide atmosphere, the yield of the biomass-based activated carbon slightly decreases with the increase in the carbon dioxide pressure, but does not significantly affect its specific surface area.

[0095] In the method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon according to the present invention, pretreatment of the biomass raw material can reduce the influence of alkali metals and alkaline earth metals in the biomass raw material on the subsequent pyrolysis and carbon dioxide activation processes. In the subsequent reaction, carbon dioxide not only provides an inert atmosphere for the biomass pyrolysis process at a relatively low reaction temperature, but also acts as an activator with weak oxidizing properties to activate biochar at a relatively high activation temperature. During the reaction process, the reaction process is regulated by means of staged temperature increase and reaction, thereby overcoming the shortcomings of the prior art and directly preparing high-quality biomass-based activated carbon materials from various biomass raw materials.

Claims

1. A method for continuously pyrolyzing and activating biomass in a carbon dioxide atmosphere to prepare high-quality activated carbon, comprising the following steps: In a slightly positive pressure carbon dioxide atmosphere, the biomass raw material is pyrolyzed and activated by means of staged heating and reaction to obtain high-quality biomass-based activated carbon; In the carbon dioxide atmosphere, the biomass raw material is heated to the pyrolysis temperature for pyrolysis reaction; then further heated to the activation temperature for activation reaction; then cooled in the carbon dioxide atmosphere; The heating rate before the pyrolysis reaction is 1-20 °C / min; The pyrolysis temperature is 400-700 °C, and the time of the pyrolysis reaction is 1-2 h; The heating rate before the activation reaction is 1-20 °C / min; The activation temperature is 800-1200 °C, and the time of the activation reaction is 1-2 h.

2. The method according to claim 1, wherein: The pressure of the carbon dioxide atmosphere is 0.1 - 0.3 MPa.

3. The method according to claim 1 or 2, wherein: The biomass raw material is lignocellulosic biomass.

4. The method according to claim 3, wherein: The lignocellulosic biomass is at least one of wheat straw, corn straw, rice straw, bamboo, wood, peanut shells, and rice husks.

5. The method according to claim 1 or 2, wherein: Before carrying out the pyrolysis reaction, the following pretreatment is performed on the biomass raw material: cleaning, pulverizing, sieving, and drying.

6. The method according to claim 5, wherein: The step of cleaning is: repeatedly soaking and washing 2 - 3 times with deionized water. After pulverizing, it is sieved through a 10 - 100 mesh sieve. The step of drying is: drying at 90 - 110 °C until the water content is less than 10%.

7. The method according to claim 1 or 2, wherein: The method further includes the following steps: The step of washing the obtained solid product with water at 20 - 60 °C and drying at 90 - 110 °C.

8. Biomass-based activated carbon prepared by the method according to any one of claims 1-7.

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