Smokeless preparation method of tobacco stem biochar

Abstract

The invention relates to the technical field of flue-cured tobacco straw resource utilization and biochar preparation, and particularly provides a smokeless preparation method of tobacco stem biochar, the smokeless preparation method of tobacco stem biochar comprises the following steps: S1, soaking flue-cured tobacco straw; s2, air-drying the flue-cured tobacco straws; s3, the flue-cured tobacco straw is placed in a flame curtain kiln for combustion, and the charcoal is obtained after fire extinguishing. According to the method, the flue-cured tobacco straw is soaked firstly, the tissue structure of the flue-cured tobacco straw is changed, the original hydrophobicity characteristic is lost after the skin is corroded by water, and a large number of hole structures are formed. The hole structure provides a channel for heat exchange, so that heat at the marrow core is not easy to gather; meanwhile, the hole structure destroys the chimney effect of flue-cured tobacco straw, and the evaporation cooling effect on a straw pipeline is reduced. The combustibility of the flue-cured tobacco straw is improved, dense smoke generated in the pyrolysis process is reduced, combustion is more sufficient, and therefore on-site smokeless preparation of the tobacco stem biochar is achieved.

Description

Technical Field

[0001] The present application relates to the technical field of resource utilization of flue-cured tobacco straw and biochar preparation, and in particular to a smokeless preparation method of tobacco straw biochar. Background Art

[0002] Biochar is made from organic biomass by carbonization in anoxic or hypoxic environments. It is an agricultural and environmentally friendly soil conditioner that can not only increase crop yields and reduce fertilizer loss, but also fix carbon and reduce emissions, and improve the soil ecological environment. Biochar has a high specific surface area, porous structure and stability, and is widely used in environmental protection, soil improvement, carbon sequestration and other aspects.

[0003] Common biochar raw materials include pine trees, oil palm leaves, Eupatorium adenophorum, durian wood, grapevines, bamboo chips, etc. Specifically, the article titled "Utilization of pine tree biochar produced by flame-curtain pyrolysis in two non-agricultural applications" (Bioresource Technology Reports, 2020, 9: 1-8) reported that pine trees were used as raw materials, and the article titled "Flame curtain pyrolysis of oil palm fronds for potential acidic soil amelioration and climate change mitigation" (Journal of Environmental Chemical Engineering, 2020, 8(4): 103982(1-39)) reported that oil palm leaves were used as raw materials. Using local materials can reduce transportation costs and carbon footprint.

[0004] In major tobacco producing areas, such as Yunnan, Guizhou, Hunan and other places, tobacco straw, a byproduct of tobacco cultivation, is often treated as waste. The annual output of tobacco straw is large, the raw material is sufficient, and the acquisition cost is low. Tobacco straw is a high-fiber biomass with high lignin, cellulose and hemicellulose. These components can be well converted into stable biochar during the carbonization process, which is suitable for soil improvement and carbon sequestration. During the growth process, tobacco straw also absorbs a large amount of mineral elements such as potassium, calcium, and magnesium. During the carbonization process, some minerals will be enriched in the biochar, and the minerals will be slowly released in the soil, providing long-term nutrient supply for crops and improving crop growth.

[0005] However, since the epidermis and pith of the tobacco straw have different ignition points, after the tobacco stem epidermis is heated, the epidermis has not yet been ignited, and the heat has been transferred from the epidermis to the pith. After the pith is heated, due to lack of oxygen and its own low ignition point, thick smoke is formed under the chimney effect of the straw pipe. When the thick smoke is released from both ends of the tobacco straw, it forms an evaporative cooling effect on the straw pipe, further inhibiting the combustion of the tobacco straw epidermis. When the pith is decomposed, the combustible gas in the combustion pile will still be directly discharged through the tobacco straw pipe. If there is no flame, thick smoke will also be formed. That is, the flammability of the tobacco straw that is directly dried by natural air is poor. Ignition of the tobacco straw that is directly dried by natural air usually burns weakly and is accompanied by a lot of thick smoke. The smoke frequently causes the flame to go out, and the combustion is incomplete, so it is impossible to prepare tobacco stem biochar.

[0006] In summary, the existing method of burning tobacco straw will produce thick smoke during the combustion process, the combustion is incomplete, and tobacco straw biochar cannot be prepared. Summary of the invention

[0007] The purpose of the present invention is to provide a smokeless preparation method of tobacco stem biochar in view of the deficiencies in the above-mentioned prior art, so as to solve the problem that the existing method produces thick smoke during the burning of flue-cured tobacco straw, the combustion is not complete, and the tobacco stem biochar cannot be prepared.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The present application provides a method for preparing smokeless cigarette stem biochar, the method comprising the following steps: S1. Soaking flue-cured tobacco straw; S2, air-dried tobacco straw; S3. placing the flue-cured tobacco straw in a flame curtain kiln for combustion, and obtaining biochar after extinguishing the fire.

[0009] The present application changes the combustion characteristics of the tobacco straw from the internal structure by pre-immersing the tobacco straw, making the combustion more complete, reducing the generation of thick smoke, and improving the quality of biochar. The soaking process destroys the hydrophobicity of the epidermis of the tobacco straw, making it more hydrophilic. At the same time, under the action of water, the cuticle and lignin of the epidermis are partially corroded, thereby forming a large number of tiny pore structures. These holes not only provide channels for heat exchange during the subsequent combustion process, allowing heat to be more evenly transferred to the inside of the straw to avoid local overheating, but also reduce the accumulation of heat in the pith, thereby reducing the risk of local incomplete combustion caused by high temperature.

[0010] The hole structure effectively destroys the chimney effect inside the flue-cured tobacco straw. The chimney effect usually causes the pith to produce a large amount of volatile gases due to rapid temperature rise, and incomplete combustion in an oxygen-deficient environment, eventually forming thick smoke. Through pre-soaking, the formation of holes reduces the pipeline-like evaporation cooling effect during the combustion process, making the overall temperature of the straw more uniform, and improving the stability and thoroughness of combustion. The pretreated flue-cured tobacco straw can quickly enter the stable combustion stage after ignition, avoiding uneven combustion caused by local heat accumulation and reducing smoke emissions.

[0011] During the natural air drying process, the water evaporates more evenly due to the porous structure of the soaked straw, which improves the drying efficiency and ensures that the optimal water content can be achieved during combustion, making the combustion more complete. The soaking process also dissolves and removes some volatile organic matter, such as soluble sugars and low molecular weight lignin, reducing the generation of incomplete combustion products such as tar during the combustion process, making the biochar purer.

[0012] In summary, the method of the present application significantly improves the combustibility of flue-cured tobacco straw, makes the combustion more stable in the flame curtain kiln, significantly reduces flue gas emissions, and makes the pyrolysis process more efficient, ensuring the preparation of high-quality tobacco straw biochar, while improving the carbon yield and environmental friendliness, providing a better solution for the resource utilization of straw.

[0013] Furthermore, in step S1, the soaking time is 120-168 hours.

[0014] Furthermore, in step S1, the flue-cured tobacco straw is completely submerged in water.

[0015] Furthermore, in step S2, both ends of the flue-cured tobacco straw are supported, and the middle part is suspended in the air.

[0016] Furthermore, the natural air drying time in step S2 is 48-72 h.

[0017] Furthermore, the time for preparing biochar in step S3 is 2-4 h.

[0018] Furthermore, the flame curtain kiln includes a bottom wall and a side wall, and the shape of the flame curtain kiln is larger at the top and smaller at the bottom.

[0019] Furthermore, the included angle between the bottom wall and the side wall is 110° to 120°.

[0020] Furthermore, the volume of the flame curtain kiln is 1-2 m 3 .

[0021] Furthermore, in step S3, the fire extinguishing method is natural annealing or quenching.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) The present application first soaks the flue-cured tobacco straw, which changes the tissue structure of the flue-cured tobacco straw. After the epidermis is corroded by water, it loses its original hydrophobic properties and forms a large number of pore structures. The pore structure provides a channel for heat exchange, and the heat in the pith is not easy to accumulate; at the same time, the pore structure destroys the chimney effect of the flue-cured tobacco straw and reduces the evaporation and cooling effect on the straw pipe. The combustibility of the flue-cured tobacco straw is improved, the generation of thick smoke during pyrolysis is reduced, the combustion is more complete, and tobacco stem biochar is prepared.

[0023] (2) The pore structure also provides a channel for water. When soaking, the internal pith is fully soaked; when naturally air-dried, it can better absorb solar radiation to increase temperature and accelerate water evaporation, shortening the air-drying time of the flue-cured tobacco straw, reducing the time occupied by the tobacco-growing land, and improving the processing efficiency of the flue-cured tobacco straw.

[0024] (3) The angle between the bottom wall and the side wall of the flame curtain kiln used in the method of the present application is 110°-120°, presenting a narrowing shape, forming an oxygen-deficient environment near the bottom, and exchanging air with the outside air at the top, so that a certain oxygen concentration is maintained in the kiln, which helps to better form a flame curtain at the top of the kiln and achieve stable combustion; at the same time, such a flame curtain kiln is conducive to concentrating heat, making the pyrolysis process more complete and obtaining tobacco stem biochar with higher stability.

[0025] The present application significantly improves the combustibility of flue-cured tobacco straw through the effects of soaking pretreatment and a windproof flame curtain brick kiln with a narrow angle between the side wall and the bottom wall. The soaking pretreatment improves the combustibility of flue-cured tobacco straw from an internal perspective by destroying the original material organization structure of the flue-cured tobacco straw. The flame curtain kiln with a narrow angle between the side wall and the bottom wall strengthens the formation of the flame curtain, achieves stable combustion, and enhances the combustibility of flue-cured tobacco straw from an external perspective. Ultimately, efficient in-situ smokeless preparation of tobacco stem biochar is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of a smokeless preparation method of cigarette stem biochar provided by the present invention; Figure 2 A schematic diagram of a flame curtain kiln used in a smokeless preparation method of tobacco stem biochar provided by the present invention; Figure 3 This is a comparison diagram of the combustion effects of the flue-cured tobacco straw before and after the soaking pretreatment in Example 2 of the present invention. DETAILED DESCRIPTION

[0027] In order to make the implementation process of the present invention clearer, it will be described in detail below with reference to the accompanying drawings.

[0028] Embodiment 1: The present application provides a method for preparing smokeless tobacco stem biochar, such as Figure 1 As shown, the method comprises the following steps: S1. Soaking flue-cured tobacco straw; Soak the tobacco straw. Soaking can be done on site in the tobacco ditch of the tobacco plantation. A waterproof layer is set at the bottom and side of the ditch. The tobacco straw is placed along the length of the ditch and water is injected into it so that the tobacco straw is completely submerged. Generally, a 30-50 meter long ditch can accommodate about 1,100 tobacco straws of one acre (667 square meters) for soaking. The tobacco straw can also be placed in a ton barrel, with the tobacco straw placed vertically, with the thicker end of the tobacco straw facing down and the thinner end facing up, and water is injected to submerge the top of the tobacco straw for soaking. The thicker end faces down and the thinner end faces down; this helps to fully soak. The lower end of the tobacco straw is thicker, the epidermis is thicker and tougher, and it is difficult to absorb water quickly. This placement method can use the dead weight and pressure of water to make the thicker end absorb more water under the action of higher water pressure, accelerate the corrosion of its epidermis, and form a pore structure faster. The thicker end usually contains more lignin, which is more hydrophobic and difficult to absorb water naturally. The greater water pressure can increase its water absorption rate, causing it to lose its hydrophobicity faster and form a porous structure, thereby improving the uniformity of combustion. In addition, since the water pressure increases with depth, the water pressure at the lower side is greater, which can promote water to penetrate into the pith of the straw more effectively, causing the internal fibers to absorb water and expand, reducing the chimney effect during combustion.

[0029] Soaking can make the water penetrate into the interior of the straw, so that the surface is corroded by water to form a large number of pore structures, thereby greatly improving the uniformity of combustion, destroying the chimney effect, and reducing the generation of dense smoke. The pore structure also causes the internal pith of the flue-cured tobacco straw to be fully soaked, so that it will not release incompletely burned volatile organic matter due to internal overheating during combustion, thereby reducing smoke emissions and improving the completeness of combustion. During the natural air drying process, the pore structure formed by the soaked straw can promote uniform heat distribution, accelerate water evaporation, improve combustion stability, make the straw easier to ignite, and carbonize more stably under high temperature conditions. At the same time, soaking can dissolve and remove some soluble volatiles in advance, such as low-molecular organic matter and soluble sugars, which are prone to produce incomplete combustion products such as smoke and tar during the combustion process. Early removal can further reduce smoke emissions. In general, the main function of soaking is to change the structural characteristics of flue-cured tobacco straw through physical and chemical effects, making it easier to burn and carbonize, reducing dense smoke emissions, and improving the yield and quality of biochar, so as to achieve smokeless preparation. Furthermore, the ton barrel can be vibrated while soaking, for example, an ultrasonic probe is brought into contact with the wall of the ton barrel, so as to better form a hole structure.

[0030] The soaking time is 4-10 days, preferably, the soaking time is 5-7 days, that is, soaking for 120-168 hours. The color of the flue-cured tobacco straw changes from turquoise to brown or dark brown. When the soaking time is too short, the epidermis of the flue-cured tobacco straw is not fully corroded by water, the hydrophobicity is still strong, the pore structure is insufficiently formed, and the heat is difficult to effectively diffuse, resulting in the chimney effect still being obvious during the combustion process, and a large amount of thick smoke is easily generated. If the soaking time is too long, the structure of the flue-cured tobacco straw may be over-softened, the cellulose and hemicellulose may be excessively degraded, and some soluble organic matter may be lost, which reduces the carbonization performance of the straw. Long-term soaking can easily cause the straw to rot, breed anaerobic microorganisms, and may release gases such as hydrogen sulfide and methane, produce odors, and may cause the straw to become brittle during the drying process, affecting the stability of combustion.

[0031] S2, natural air drying of flue-cured tobacco straw; Take out the soaked tobacco straw and air dry it naturally. It can be dried on the roadside or on the ridges of the plantation. The single-layer rack can be placed between two ridges of 30-50 meters long or on the ridges. Soaking and air drying the tobacco straw on site overcomes the problem of long-distance transportation of tobacco straw, greatly reduces the processing cost and carbon footprint, and will enhance the carbon fixation and emission reduction potential of tobacco stem biochar returning to the field. The tobacco straw can also be placed on a support frame, supported at both ends, and suspended in the middle, fully exposed to the air, and dried naturally.

[0032] The natural air drying time is 1-5 days, preferably 2-3 days, i.e. 48h-72h; the moisture content of the flue-cured tobacco straw obtained by air drying is 15-20%, which is equivalent to the water vapor content in the air. If the moisture content is too high, the flue-cured tobacco straw will consume a lot of heat for water evaporation in the early stage of combustion, making it difficult to quickly increase the combustion temperature, thereby prolonging the combustion time, and may cause incomplete combustion and produce a lot of smoke; if the moisture content is too low, the flue-cured tobacco straw is easy to heat up rapidly during the combustion process, which may cause the local temperature to be too high, so that the volatile gas is instantly released in large quantities, and it cannot be fully mixed with oxygen for combustion, and incomplete combustion will still occur, producing thick smoke. If air-dried outdoors, it needs to be done on a sunny day without rain. Rainwater will cause the straw to absorb moisture again, increase its moisture content, prolong the air-drying time, and may even cause the previously partially dried straw to absorb water again, destroying the uniformity of the air-drying process, resulting in uneven distribution of straw moisture, and thus affecting the stability of subsequent combustion. A continuously humid environment can easily cause straw to mold or rot, especially under high humidity conditions, where microorganisms will multiply rapidly, decomposing the cellulose and hemicellulose in the straw, reducing its flammability and possibly causing incomplete combustion and increasing smoke emissions.

[0033] S3. placing the flue-cured tobacco straw in a flame curtain kiln for combustion, and obtaining biochar after extinguishing the fire.

[0034] The air-dried flue-cured tobacco straw is placed in the kiln and ignited for pyrolysis. The fire is extinguished by natural annealing or quenching. After the pyrolysis of the tobacco straw, biochar is obtained. The pyrolysis time of biochar is 2-4 hours to ensure sufficient pyrolysis. In order to reduce transportation costs, a flame curtain kiln can be built in the flue-cured tobacco seed concentration area. One flame curtain kiln can process about 1 hectare of flue-cured tobacco straw by running it twice. The volume of the flame curtain kiln is 1-2 m 3 Preferably, the volume of the flame curtain kiln is 1.5-2 m 3 .

[0035] The flame curtain kiln can be a kiln of any material and shape. Figure 2 As shown, the flame curtain kiln is large at the top and small at the bottom, including the bottom wall and the side wall. The angle between the bottom wall and the side wall (i.e. Figure 2 The angle A) in the kiln is 110° to 120°, and more preferably, the angle between the bottom wall and the side wall is 110° to 113°, which is smaller than that of an ordinary flame curtain kiln. A smaller angle enhances the heat gathering capacity of the kiln body, making the heat more concentrated, reducing heat loss, and improving the temperature stability in the kiln, so that the flue-curtained tobacco straw can be more fully decomposed and carbonized during the pyrolysis process, forming high-quality biochar with a high specific surface area and porous structure. It helps to form a more concentrated flame curtain on the top of the kiln, which effectively covers the combustion area, allowing the combustible gas to decompose and burn more evenly in the kiln, thereby inhibiting the generation of smoke and improving the uniformity and thoroughness of the pyrolysis process. A smaller angle makes the bottom of the kiln body narrower, which helps to form a local oxygen-deficient environment during the combustion process, reduce the direct loss of combustible gas, enable it to be fully cracked and secondary burned in the kiln, improve combustion efficiency, and reduce the generation of thick smoke.

[0036] More preferably, a windproof kiln can be used for pyrolysis. The windproof kiln relies on the wedge-shaped channel to enhance the oxygen combustion effect, effectively improves the windproof performance of the kiln body in an outdoor environment, and makes the pyrolysis process more stable.

[0037] In order to demonstrate the research process in more detail, this application provides the following three embodiments.

[0038] Embodiment 2: Based on Example 1, this example puts the cut flue-cured tobacco straw vertically into a ton barrel, with the thick end at the bottom and the thin end at the top, and adds tap water to cover it. After soaking for 5 days, the original green color turns brown.

[0039] After soaking, the flue-cured tobacco straw was spread on the black asphalt road and air-dried for 2 days, and the moisture content reached 15-20%. For comparison, the flue-cured tobacco straw that was not soaked was also spread on the black asphalt road. After 2.5 months, the moisture content reached 15-20%, and the color changed from cyan to yellow-green. In other words, soaking can also shorten the air-drying time. After soaking pretreatment, a large number of pore structures are formed on the surface of the flue-cured tobacco straw. The presence of the pore structure increases the specific surface area of ​​the straw surface, enabling it to more effectively absorb and store the heat energy of solar radiation, thereby accelerating the gasification process of the water inside the straw. These micropores can enhance the evaporation rate of water, accelerate the speed of water diffusion outward, and shorten the air-drying time.

[0040] The air-dried tobacco straw was put into a 30L flame curtain iron kiln for combustion. The flame was obvious and the sound of combustible gas was emitted during the combustion process, which significantly improved the combustibility of the tobacco straw. However, a small amount of thick smoke occasionally appeared under the interference of wind. Although the tobacco straw directly air-dried for 10 weeks also had a certain degree of combustibility in the 30L flame curtain iron kiln, it was continuously accompanied by thick smoke and the combustion was easily interrupted under the interference of wind. Figure 3 As shown. Compared with directly air-dried tobacco straw, the flammability of soaked tobacco straw is significantly improved, which is related to the pore structure formed during the soaking process; at the same time, the soaking pretreatment time of tobacco straw is only 1 / 10 of that of directly air-dried tobacco straw, which improves the efficiency of tobacco straw processing. It can be seen that the soaking treatment not only improves the flammability of tobacco straw, but also greatly improves the efficiency of tobacco straw processing. The preparation temperature of the 30L flame curtain iron kiln is lower than that of the windproof flame curtain brick kiln, and it is easily disturbed by wind. In the windproof kiln, the pyrolysis process of tobacco straw is more complete.

[0041] This embodiment is carried out in Yunnan Agricultural Vocational and Technical College in Kunming. Kunming has sunny weather with little rain and dry air. It is only introduced as an embodiment and does not limit the area where it is implemented.

[0042] Embodiment 3: On the basis of Example 1, in this example, the flue-cured tobacco straw is soaked in a ton barrel and taken out in batches after soaking for 4 days, 7 days and 10 days respectively. The epidermis loses its original green color and becomes brown or even dark brown. It is spread on a black asphalt road and naturally air-dried for 2 days, and the moisture content reaches 15-20%.

[0043] Cut the flue-cured tobacco straw in the middle to form a thinner upper half and a thicker lower half. Put the two into a 30L flame curtain iron kiln for burning. After the flame is extinguished, seal the kiln top with a stainless steel lid for natural annealing and cooling. When the temperature drops to about 40°C, take out the product, pour it on the shade net, sieve out the ash, and the remaining is the flue-cured tobacco straw biochar. The carbon yield is obtained after weighing. The tobacco straw biochar yield is calculated based on the consumed air-dried flue-cured tobacco straw, as shown in Table 1.

[0044] Table 1: Biochar yields obtained by soaking different parts of flue-cured tobacco straw; The carbon yield of the lower half of the flue-cured tobacco straw was the highest when it was soaked for 4 days, and the difference was small compared with the carbon yield of the corresponding flue-cured tobacco straw soaked for 7 days; the carbon yield of the upper half of the flue-cured tobacco straw was the highest when it was soaked for 10 days; both were higher than the carbon yield of the complete tobacco straw soaked for 10 days. Considering that the volatile solid content of the raw material may decrease if the soaking time is too long, and the lower half of the tobacco straw has a high specific gravity and a high degree of lignification, it is most appropriate to set the soaking treatment for 5-7 days. Reasonable control of the soaking time can not only destroy the hydrophobicity of the epidermis and form an appropriate amount of micropores, but also maintain the carbonization characteristics of the straw. If the soaking time is too short, the epidermis of the flue-cured tobacco straw cannot be fully corroded and still maintains a strong hydrophobicity, the pore structure is insufficiently formed, and the heat is difficult to effectively diffuse, resulting in an obvious chimney effect during combustion, producing a large amount of thick smoke, and affecting the completeness of combustion. On the contrary, soaking for too long will cause the straw structure to soften excessively, cellulose and hemicellulose to degrade excessively, resulting in the loss of some soluble organic matter and reducing the carbonization performance of the straw. If calculated on the basis of the dried weight of the raw material, the yield will be further increased to around 20%. Using windproof kiln pyrolysis will further increase the yield.

[0045] The tobacco stem biochar obtained by soaking for 10 days and air-drying for 2 days in Example 3 was analyzed and compared with the biochar obtained by directly air-drying for 2.5 months in Example 2.

[0046] Compared with the biochar prepared by directly treating the flue-cured tobacco straw with air drying for 10 weeks in a 30L flame curtain iron kiln, the biochar prepared by treating the flue-cured tobacco straw with air drying for 2 days in a 30L flame curtain iron kiln after 10 days of soaking has better carbon stability, as shown in Tables 2 and 3. Table 2 shows the absorbance test results, and the absorbance is smaller after soaking. Table 3 shows the industrial analysis of biochar, and the biochar obtained after soaking has less moisture and higher fixed carbon content. The tobacco stem biochar obtained after 10 days of soaking is qualified for carbonization, with higher fixed carbon and lower ash content, which indicates that it has higher carbon stability and can have a higher proportion of carbon sequestered in the soil environment for a long time.

[0047] Table 4 shows the results of pH, conductivity, and water holding capacity. The pH of the biochar obtained after soaking for 10 days was 9.54, which is greater than 7 and alkaline. It can adjust acidic soil and buffer the changes in soil acidity and alkalinity. Although the electrical conductivity (EC) is slightly higher than 1.8 mS / cm, it is lower than 9.81 mS / cm of the biochar naturally air-dried for 10 weeks. The higher conductivity indicates that the biochar was not successfully prepared; in fact, since the flue-cured tobacco straw directly air-dried for 10 weeks is treated under the conditions of continuous thick smoke and continuous re-ignition, its product can only be strictly speaking a flue-cured tobacco straw carbonized material, which is also proved by the analysis data of the uncarbonized material. The soaked flue-cured tobacco straw will be more fully carbonized in the windproof flame curtain brick kiln, thereby obtaining higher fixed carbon, lower ash content, forming more aromatic carbon, and improving the carbon stability of tobacco stem biochar. The water holding capacity (WHC) is also significantly higher than that of farmland soil. The good water holding performance of biochar is conducive to returning to the field for use to increase the field water holding capacity of the soil. At the same time, it can be used as a building material to enhance the buffering and regulating capacity of humidity.

[0048] Table 2: Analysis results of uncarbonized matter in tobacco stem biochar Table 3: Industrial analysis results of tobacco stem biochar Table 4: pH, EC and WHC test results of tobacco stem biochar Embodiment 4: On the basis of Example 1, the cut flue-cured tobacco straw was bundled and soaked in the ditch beside the field for 5 days. It was taken out and dried on the roadside. Due to the alternation of sunny and rainy days, the surface was dry after 5 days. The next day, it was laid out in a single layer for drying and the tobacco stems that were obviously not soaked were removed. There was still alternation of sunny and rainy days in the next 4 days, so it was not completely air-dried within 2-3 days. The tobacco straw was dried for 60 hours in a tobacco leaf curing room. It was fired for about 2 hours in a windproof flame curtain brick kiln, and tobacco stem biochar was obtained by quenching.

[0049] Since some parts of the tobacco stems were not soaked, the moisture content of the dried tobacco stems was still 35-45% after removing the insufficiently soaked raw materials, and a small amount of smoke was generated during the preparation process; that is, the soaking process will affect both the air-drying and pyrolysis processes. Some products still retain the shape of tobacco stems and are not fully carbonized. Therefore, the soaking treatment requires that the water completely submerges the tobacco stems, and they need to be spread out in a single layer for air-drying. The tobacco stem biochar prepared by the windproof flame curtain brick kiln has a quenching extinguished charcoal yield of 21.86%, and an annealing extinguished charcoal yield of 18.50%.

[0050] This embodiment is implemented in Gengjia Township, Changning County, Baoshan City, Yunnan Province. It is only introduced as an embodiment and does not limit the implementation area.

[0051] The present application pre-soaks the flue-cured tobacco straw to change the hydrophobicity of its epidermis, so that a large number of pore structures are formed, thereby improving the heat transfer efficiency, reducing the local heat accumulation in the pith, and destroying the chimney effect, thereby suppressing the problem of incomplete combustion caused by the cooling effect of pipeline evaporation. A kiln body structure with a large top and a small bottom and a narrow angle between the bottom wall and the side wall is adopted to form an oxygen-deficient environment at the bottom and maintain an appropriate oxygen supply at the top, thereby promoting the secondary combustion of combustible gases and reducing smoke emissions. The flame curtain covers the combustion area to make the heat more concentrated and improve the uniformity and thoroughness of carbonization. At the same time, the heat-collecting characteristics reduce heat loss, improve combustion efficiency, and fully pyrolyze the straw to obtain biochar with high stability and high specific surface area. Finally, smokeless and efficient preparation of flue-cured tobacco straw biochar.

[0052] This application scheme fills the gap in the efficient in-situ smokeless preparation of flue-cured tobacco straw biochar in flame curtain pyrolysis kilns. The applicant also used a flame curtain iron kiln with a volume of more than 200L to process field rapeseed straw (the structure is similar to that of tobacco straw), but the rapeseed straw frequently went out and was accompanied by thick smoke, and even replacing it with a 2000L flame curtain iron kiln with a larger volume did not improve the situation. The same problem exists when using a 30L flame curtain iron kiln to treat tobacco straw. In other words, the combustibility of straw raw materials such as directly air-dried tobacco straw cannot be improved by changing the material or volume of the flame curtain kiln.

[0053] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A smokeless preparation method of cigarette stem biochar, characterized in that: The method comprises the following steps: S1. Soaking flue-cured tobacco straw; S2, air-drying the flue-cured tobacco straw; S3, placing the flue-cured tobacco straw in a flame curtain kiln for combustion, and obtaining biochar after extinguishing the fire.

2. The smokeless preparation method of cigarette stem biochar according to claim 1, characterized in that: In step S1, the soaking time is 120-168 hours.

3. The smokeless preparation method of cigarette stem biochar according to claim 2, characterized in that: In the step S1, the flue-cured tobacco straw is completely submerged in water.

4. The smokeless preparation method of cigarette stem biochar according to claim 3, characterized in that: In step S2, the two ends of the flue-cured tobacco straw are supported, and the middle part is suspended in the air.

5. The smokeless preparation method of cigarette stem biochar according to claim 4, characterized in that: The natural air drying time in step S2 is 48-72 h.

6. The smokeless preparation method of cigarette stem biochar according to claim 5, characterized in that: The time for preparing biochar in step S3 is 2-4 h.

7. The smokeless preparation method of cigarette stem biochar according to claim 6, characterized in that: The flame curtain kiln comprises a bottom wall and a side wall, and the shape of the flame curtain kiln is larger at the top and smaller at the bottom.

8. The smokeless preparation method of cigarette stem biochar according to claim 7, characterized in that: The included angle between the bottom wall and the side wall is 110° to 120°.

9. The smokeless preparation method of cigarette stem biochar according to claim 8, characterized in that: The volume of the flame curtain kiln is 1-2 m 3 .

10. The smokeless preparation method of cigarette stem biochar according to claim 9, characterized in that: In the step S3, the fire extinguishing method is natural annealing or quenching.

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