Biomass charcoal prepared through pyrolysis-water quenching and method for preparing iron ore pellets through biomass charcoal

Abstract

The invention discloses biomass charcoal prepared through pyrolysis-water quenching and a method for preparing iron ore pellets through the biomass charcoal, and belongs to the technical field of iron ore pellet production. Biomass raw materials are pyrolyzed, pyrolytic carbon generated through pyrolysis is rapidly subjected to water quenching at the pyrolysis temperature, then filtering separation, drying and grinding are carried out, and the biomass charcoal is obtained and has the advantages of being low in ash content, low in K and Cl content, high in heat value, low in volatile component content, good in combustion performance and the like, meets the requirement for internal matching of pellets and has the advantages of being environmentally friendly and free of pollution. The coal-based fossil fuel can completely or partially replace fossil fuel for use, and the dependence on the coal-based fossil fuel and the emission of CO2 and pollutants in the iron ore pellet production process are reduced on the premise that normal production of the iron ore pellets is not affected.

Description

Technical Field

[0001] The invention relates to biochar, a method for preparing biochar by pyrolysis-water quenching, and application of biochar as pellet internal fuel in iron ore pellet preparation, belonging to the technical field of iron ore pellet preparation. Background Art

[0002] The steel industry is a major energy consumer and carbon emitter. With the development of society, people's environmental awareness is increasing day by day, and the steel industry is facing tremendous pressure in terms of carbon and pollutant emissions. The reason is that the excessive dependence of steel production on fossil fuels such as coal has led to high CO2 and pollutant emissions. The use of clean raw materials is the key link and the most effective measure to reduce pollution and carbon emissions in steel metallurgy.

[0003] Pellet ore is a high-quality furnace material for long / short processes of iron and steel metallurgy, with the characteristics of high grade, uniform particle size, high cold strength and good metallurgical properties. Magnetite has become a high-quality raw material for pelletizing due to its good balling and roasting properties. In recent years, under the background of "double carbon", pellets have been highly valued due to their low energy consumption and low pollutant emissions. However, the increasing shortage of high-quality magnetite resources has kept its price high. In addition, with the development of "deep grinding and fine selection" technology, the particle size of magnetite powder is too fine, resulting in the structure of its finished pellets being too dense, so its metallurgical properties are poor. These problems seriously restrict the development of the pelletizing industry. Although reducing the proportion of magnetite in the pelletizing raw material will reduce the consumption of magnetite to a certain extent, lowering the proportion of magnetite will reduce the heat released by magnetite oxidation during the pelletizing production process, which has an adverse effect on the consolidation of the pellets and the energy consumption of the process, and will also reduce the compressive strength of the finished pellets. Adding solid fuel to the pelletizing raw material is an effective measure to solve the problem of insufficient self-heating in the production process of low-magnetite pellets. Internal carbon can not only stabilize the heat balance in the pellets and improve the strength of the pellets, but also reduce the preheating-roasting fuel consumption and improve the metallurgical properties of the pellets. At the same time, adding solid fuel to fine-grained magnetite can also increase the porosity of the pellets and improve the metallurgical properties of the pellets. Therefore, internal carbon in the pellets is of great significance for the low-carbon production of pellets and the preparation of high-quality low-carbon smelting furnace materials for steel metallurgy.

[0004] At present, the carbon source for pellets mainly comes from coal-based fossil fuels such as anthracite and coke, which shows that the pellet production process is still heavily dependent on coal, and also means that the production process will emit a large amount of CO2 and pollutants. Biomass energy is the fourth largest energy source after coal, oil and natural gas. The CO2 released by this type of renewable energy material during the combustion process comes from the CO2 absorbed during its growth. Therefore, biomass fuel has the characteristic of net zero emission of CO2. Among the many renewable fuels, biomass is the only energy source that can be used to prepare solid carbon-rich fuel with coal-like characteristics. Preparing biomass as a carbon source for pellets can effectively alleviate the excessive dependence of the pellet process on coal-based fossil fuels and reduce the emission of CO2 and pollutants in pellet production.

[0005] In terms of using biomass to prepare carbon-containing pellets containing biochar, a Chinese patent (publication number: CN118703776A) discloses a method for using biomass to prepare carbon-containing pellets, carbon-containing pellets and metallized pellets, wherein the biomass is hydrothermally carbonized to obtain biochar; the biochar, iron ore powder, binder and solvent are mixed according to a set mass ratio to obtain a premix; and the premix is ​​pelletized and dried to obtain carbon-containing pellets. By optimizing the ratio of biochar, iron ore powder, binder and solvent, the close contact between the iron ore powder and the reducing agent inside the carbon-containing pellets is ensured, thereby promoting the coupling effect of the gasification reaction and the reduction reaction of the carbon-containing pellets, and having excellent self-reduction performance; at the same time, it avoids the carbon particles from hindering the convergence of the iron phase, and improves the strength of the carbon-containing pellets. Thereby solving the technical problem that the carbon-containing pellets in the prior art cannot simultaneously meet the requirements of high strength and high self-reduction. A Chinese patent (publication number: CN115354150A) provides a biomass carbon-containing pellet for blast furnace ironmaking and a preparation method thereof, wherein the biomass is crushed and then hydrothermally carbonized to prepare a biomass hydrothermal charcoal with low alkali metal content and high hot-melt extraction rate; a part of the biomass hydrothermal charcoal is subjected to pyrolysis carbonization to prepare a low-volatile biomass charcoal, and another part of the biomass hydrothermal charcoal is subjected to hot-melt extraction to prepare a biomass high-temperature binder; the biomass charcoal, the biomass high-temperature binder, the iron ore powder and the solvent are crushed and mixed and then hot-pressed to obtain the biomass carbon-containing pellet. In the above manner, the invention can prepare biomass with high volatile matter, high moisture content and high harmful element content into a biomass-based reducing agent and binder with high fixed carbon content, high adhesiveness and low harmful element content, and prepare high-quality biomass carbon-containing pellets that can be used as blast furnace smelting charge by hot-pressing method, effectively reducing the carbon emissions in the ironmaking production process. A Chinese patent (publication number: CN116751971A) discloses a high-strength biomass carbon-containing pellet and its preparation method and application, which relates to the field of blast furnace ironmaking technology. The pellet includes the following mass fractions of raw materials: 70% to 85% iron ore, 4% to 15% binder, 5% to 11% biomass charcoal, and 1% to 3% alkaline flux. The high-strength biomass carbon-containing pellet of the invention has high strength and good wear resistance. The high thermal burst temperature and small thermal burst index enable it to effectively reduce the amount of powder generated after entering the blast furnace. The reducibility, reduction expansion and low-temperature reduction pulverization performance of the high-strength biomass carbon-containing pellet are better than those of traditional sintered ore, pelletized ore and lump ore. It can be directly used as blast furnace charge, reducing the use of coke in blast furnace smelting, thereby reducing ironmaking costs and CO2 emissions, and has significant economic, social and ecological benefits.A Chinese patent (publication number: CN102586529A) discloses a rotary hearth furnace ironmaking method using biomass carbon-containing pellets as raw materials, which relates to a rotary hearth furnace ironmaking process using biomass as ironmaking fuel and reducing agent. The process uses renewable carbon-neutral biomass coke powder instead of coal powder, and mixes the biomass coke powder with iron ore powder at C / O=1.1-1.4, and adds a suitable binder to mix and make pellets, which enter the rotary hearth furnace for heating and reduction to obtain metallized pellets. Biomass coke is carbon neutral, which can fundamentally reduce the emission of pollutants such as CO2 caused by coal consumption; biomass coke has a low impurity content, which can solve the problem of excessive sulfur content in products caused by the use of coal and improve the overall quality of products; the energy in the biomass coke preparation process can be provided by the biomass itself, basically realizing green circular production. The invention broadens the energy source of rotary hearth furnace ironmaking, reduces production costs, improves product quality and production efficiency, and reduces environmental pollution.

[0006] Although the above method has a positive effect on the carbon source of biomass pellet preparation, and theoretically can reduce the dependence of pellet production on fossil fuels, it does not consider the actual situation of biomass resources. Charcoal with low ash content, high calorific value and similar physical and chemical properties to coal is expensive due to its small resources, and it is more luxurious to use it as a fuel in pellets. Although straw resources are abundant and relatively concentrated, the vast majority of agricultural straw pyrolysis charcoal is rich in harmful elements such as K / Cl, and the ash content is high, so its calorific value is low. Adding it to the pellets will affect the consolidation of the pellets, and straw hydrothermal charcoal has problems such as high volatile matter, strong chemical activity, unconcentrated heat release and difficulty in preparation, and is not suitable for pellets. Despite this, agricultural straw resources are abundant, and preparing it as a carbon source for pellets is the best choice to achieve biomass energy to replace coal-based fossil fuels for pellets. Therefore, based on the performance requirements of the carbon source in the pellets, it is of great significance to develop biomass quality improvement technologies including agricultural straw to achieve economical and efficient preparation of high-quality biochar with low ash, low harmful elements, high calorific value, appropriate volatile matter, concentrated heat release, and combustion and gasification characteristics similar to those of coal-free biochar. Summary of the invention

[0007] In order to solve the above problems, the first object of the present invention is to provide a biomass charcoal having the characteristics of low ash content, low K and Cl content, high calorific value, low volatile matter content, good combustion performance, etc., which meets the requirements of pelletizing and can completely or partially replace fossil fuels for the preparation of iron ore pellets.

[0008] The second object of the present invention is to provide a method for preparing biochar, which uses biomass solid waste as raw material, has simple operation, low cost, and meets industrial production requirements.

[0009] The third object of the present invention is to provide a method for preparing iron ore pellets using biochar as an internal fuel. The biochar prepared based on the pyrolysis-water quenching method can partially or completely replace coal-based fuels such as anthracite and coke for use in pellets. On the one hand, it can alleviate the problems of high dependence on fossil energy and high pollutant emissions in the pellet production process. On the other hand, it can alleviate the dependence of pellet production on magnetite, stabilize the heat balance in the pellets, and improve the strength of the pellets. At the same time, it can also reduce the preheating-roasting fuel consumption and improve the metallurgical properties of the pellets.

[0010] In order to achieve the above technical objectives, the present invention provides a method for preparing biochar by pyrolysis-water quenching and its use in iron ore pellets. The method is to pyrolyze the biomass raw material, and the generated pyrolytic char is rapidly water quenched while maintaining the pyrolysis temperature, and then filtered, dried and ground to obtain the biochar.

[0011] The key to the technical solution of the present invention is to prepare biochar by using biomass raw materials through a method combining pyrolysis and water quenching. On the one hand, most of the harmful elements in the pyrolytic charcoal generated by high-temperature pyrolysis are converted into water-soluble ash, and most of the ash can be dissolved out by water. A large number of studies have shown that the higher the temperature, the better the dissolution effect of these ashes in water. Therefore, the present invention quickly places the hot biochar that has just been pyrolyzed in water for water quenching, which can remove the ash in the biochar more thoroughly. On the other hand, during the water quenching process of high-temperature pyrolytic charcoal, due to the rapid cooling of the high-temperature pyrolytic charcoal by water, thermal stress will be generated inside the biochar particles, and the high-temperature steam will corrode the biochar matrix, resulting in a large number of cracks and holes on the surface of the biochar particles after water quenching. The generation of these cracks and holes combined with the temperature field and flow field formed around the biochar during the water quenching process provide favorable thermodynamic and kinetic conditions for the removal of ash from the pyrolytic charcoal. Thirdly, during the water quenching process, the highly chemically active hydrocarbon groups in the biochar are gasified by water vapor, and the content of catalytic alkali metals in the biochar decreases, and the combustion and gasification temperature of the biochar rises, which is closer to the performance of coal-based fossil fuels with high fixed carbon such as anthracite and coke powder. Compared with biochar such as pyrolytic carbon and hydrothermal carbon, it is more advantageous to be added to the pellets. In summary, the present invention directly quenches high-temperature pyrolytic carbon to produce biochar with low ash content, low K and Cl content, high calorific value, low volatile matter content, and good combustion performance.

[0012] In the pyrolysis-water quenching process for preparing biochar of the present invention, water quenching of high-temperature pyrolytic carbon is utilized, which not only accelerates the cooling process of the pyrolytic carbon and improves the production efficiency of the biochar, but also effectively utilizes the carbonization waste heat carried by the biochar for the water immersion deashing process, thereby reducing the energy consumption during the ash immersion process.

[0013] As a preferred solution, the moisture content of the biomass raw material is less than 10wt.%, the ash content is less than 20wt.%, the volatile content is less than 85wt.%, and the combustion calorific value is 13-20MJ / kg. As a preferred solution, the biomass raw material includes at least one of straw waste, forest prunings, and wood processing waste. These biomass raw materials have a high ash content and low calorific value after conventional pyrolysis. Putting them into pellets will affect the consolidation of the pellets, and the method of combining pyrolysis and water quenching can obtain biomass charcoal with low ash content, low harmful elements, high calorific value, suitable volatile matter, concentrated heat release, and combustion and gasification properties similar to those of anthracite and coke powder. The biomass raw materials include but are not limited to straw waste from agricultural production such as corn, rice, wheat, soybeans, cotton, and forestry waste such as forest prunings and wood processing waste. The biomass raw materials of the present invention are pre-treated by shearing and crushing, and the proportion of their length less than 30mm and the maximum cross-sectional diameter less than 10mm is not less than 80wt.%.

[0014] As a preferred solution, the pyrolysis conditions are: temperature between 300 and 900°C, and duration between 30 and 120 minutes. If the pyrolysis temperature is too low or the time is too short, the pyrolysis will be insufficient. Considering the influence of the pyrolysis temperature on the subsequent water quenching process, the preferred pyrolysis temperature is 600 to 800°C. The pyrolysis device of the biomass raw material is preferably carried out in a device similar to a coke oven. After the carbonization is completed, a device similar to a coke pusher can be used to quickly push the high-temperature biochar out of the carbonization chamber and water quench it.

[0015] As a preferred solution, the water quenching process is: pushing the pyrolytic carbon directly into the water, or quickly pouring water on the pyrolytic carbon. The water quenching process is to quickly cool the high-temperature pyrolytic carbon with water, including but not limited to immersing the high-temperature pyrolytic carbon in water and quickly pouring water on the high-temperature pyrolytic carbon. The amount of water used must at least ensure that the biochar can be immersed in it. After the water quenching is completed, according to the dissolution characteristics of the ash content and the degree of deashing of the biochar, it can be immediately filtered to obtain the finished biochar, or it can continue to carry out wet grinding and the introduction of hot flue gas and other water immersion enhanced deashing operations to achieve the preparation of low-ash finished biochar.

[0016] The heat source used in the pyrolysis process of the present invention can be derived entirely or partially from the pyrolysis gas generated in the biomass pyrolysis process, so as to achieve self-exothermic pyrolysis of the biomass as much as possible.

[0017] In the drying process of the pyrolytic carbon of the present invention, the heat source is preferably the hot flue gas of the biomass pyrolysis furnace, and the drying method can be drum drying and exhaust drying.

[0018] The pyrolytic carbon of the present invention can be ground using a coal mill or a ball mill.

[0019] The invention also provides biomass charcoal obtained by the preparation method.

[0020] As a preferred solution, the ash content of the biochar is less than 5wt.%, the K content is less than 1wt.%, the Cl content is less than 0.1wt.%, the calorific value is higher than 25MJ / kg, the volatile matter content is less than 25wt.%, the starting combustion temperature is higher than 350°C, and the starting gasification temperature is higher than 700°C. The physical and chemical properties of the biochar prepared by pyrolysis-water quenching in the present invention are closer to those of coal-based fossil fuels with high fixed carbon such as anthracite and coke powder.

[0021] The present invention also provides a method for preparing iron ore pellets using biomass charcoal. The method comprises adding biomass charcoal to pellet raw materials, and sequentially performing pelletizing and roasting to obtain finished pellets.

[0022] As a preferred solution, the biomass charcoal is added to the pellet raw material as a single fuel, or the biomass charcoal is added to the pellet raw material as a mixed fuel with at least one of coal powder, coke powder, charcoal, straw pyrolysis charcoal, and hydrothermal charcoal.

[0023] As a preferred solution, the proportion of the biochar particle size smaller than 74 microns is not less than 80 wt.%, and the size of the largest particle does not exceed 100 microns.

[0024] As a preferred solution, the biochar is added in an amount between 0.2 and 2 wt.%. Too high an amount of biochar will affect the compressive strength of the pellets.

[0025] The roasting process used in the green pellets prepared by the present invention using biomass charcoal is preferably a belt roasting machine process. Compared with the chain grate machine-rotary kiln process, the pellets in the belt roasting machine can almost achieve static roasting of the pellets, so the pulverization rate of the pellets is low and the yield rate is high.

[0026] The finished pellets prepared by the invention have developed porosity and good reducibility, and are preferably used in the fields of direct reduction and electric furnace steelmaking.

[0027] The iron concentrate in the pelletizing raw material of the present invention comprises at least one of hematite, limonite, magnetite, vanadium-titanium magnetite and laterite-nickel ore.

[0028] The method for preparing biochar by pyrolysis-water quenching of the present invention comprises the following steps:

[0029] S1: Cut agricultural and forestry biomass with moisture, ash and volatile matter less than 10wt.%, 20wt.% and 85wt.% respectively and calorific value between 13 and 20MJ / kg into small biomass pieces with a length of less than 30mm and a maximum cross-sectional diameter of less than 10mm, and place the small biomass pieces in a pyrolysis furnace similar to a coke oven for pyrolysis and carbonization into pyrolytic charcoal. The pyrolysis temperature is 300-900℃ and the pyrolysis time is 30-120min. The carbonization heat can be wholly or partially derived from the biomass pyrolysis gas.

[0030] S2: The hot pyrolytic charcoal is quickly pushed out of the carbonization chamber by a charcoal pushing device, and the hot biomass charcoal is quenched with water, and the biomass charcoal for use in pellets is obtained by water immersion, filtration, drying and grinding.

[0031] S3: adding biomass charcoal into pelletizing raw materials to obtain green pellets containing biomass charcoal, and then subjecting the green pellets to a pelletizing roasting process to obtain finished pellets.

[0032] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0033] 1) In view of the technical problem in the prior art that agricultural biomass waste, which has abundant reserves, is limited in use in the steel industry due to its rich content of harmful substances, the present invention adopts a method combining pyrolysis and water quenching to convert biomass solid waste with high ash and high harmful element content into biomass charcoal with combustion and gasification properties similar to those of anthracite and coke powder. The technical solution of the present invention can efficiently dissolve the ash containing harmful elements in the biochar by directly quenching the high-temperature pyrolytic carbon with water. At the same time, in the rapid cooling process of the high-temperature pyrolytic carbon by water quenching, thermal stress is generated inside the biochar particles and the high-temperature steam has an erosive effect on the biochar matrix, resulting in a large number of cracks and holes on the surface of the biochar particles after water quenching. These cracks and holes are combined with the temperature field and flow field formed around the biochar during the water quenching process, which are more conducive to the removal of ash. In particular, during the water quenching process, the chemically active hydrocarbon groups in the pyrolytic carbon are gasified by water vapor, and the content of catalytic alkali metals in the biochar decreases, and the combustion and gasification temperature of the biochar rises, which is closer to the performance of coal-based fossil fuels with high fixed carbon such as anthracite and coke powder. Compared with pyrolytic carbon, hydrothermal carbon and other biochars, it is more advantageous to be added to pellets.

[0034] 2) In the process of preparing biochar, the hot biochar just pyrolyzed is subjected to water quenching, which not only accelerates the cooling process and improves the production efficiency of biochar, but also effectively transfers the carbonization waste heat carried by the biochar to the water leaching deashing system, thereby reducing the energy consumption in the ash leaching process.

[0035] 3) The present invention uses biochar prepared from biomass solid waste to replace coal-based fossil fuels for pelletization, and processes abundant agricultural biomass into coal-like and coke-like biochar, which effectively solves the problem of limited high-quality charcoal resources and high prices, while pyrolytic charcoal and hydrothermal charcoal cannot be used in large quantities and directly for pelletization without affecting the quality of the finished pellets. The coal-like and coke-like biochar obtained by pyrolysis-water quenching of agricultural and forestry biomass is used for pelletization, and the quality of the produced pellets can be better guaranteed, and the fuel supply stability is good, which effectively reduces the dependence of pellet production on coal-based fuels and carbon emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a process flow chart for preparing biochar by pyrolysis-water quenching and using it in iron ore pellet production.

[0037] Figure 2 This is the combustion weight loss curve of the water-quenched biochar prepared in Example 1.

[0038] Figure 3 This is the combustion weight loss curve of the water-quenched biochar prepared in Example 2. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the scheme of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0041] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0042] Embodiment 1:

[0043] The corn stalks with a moisture content of 8wt.%, ash content of 5wt.%, volatile matter of 75wt.%, and dry basis calorific value of about 17MJ / kg are used as biomass raw materials. They are crushed and sheared into small biomass pieces with a length of less than 30mm and a maximum cross-sectional diameter of 10mm. A carbonization furnace with a structure similar to that of a coke oven is used to carbonize them. The starting gas for pyrolysis is natural gas. The pyrolysis biomass gas is led back to the combustion chamber of the carbonization furnace for heating. As the temperature rises, the amount of biomass pyrolysis gas gradually increases, and the amount of natural gas gradually decreases. Biomass gas is used as much as possible for pyrolysis heating of biochar. The gas injection volume of natural gas and biomass gas in the entire pyrolysis process is controlled by the program. The pyrolysis temperature is raised to 600℃ within 60min and then kept warm for 60min. A carbon pusher is used to push the hot biochar from the charcoal outlet into a cylindrical steel water quenching well while rapidly injecting 5m 3 The quenching of biochar is completed by adding clean water. A spiral stirring paddle is set at the bottom of the cylindrical steel water quenching well to ensure the rapid contact and quenching of biochar with water. Then the water-quenched biochar (referred to as water-quenched carbon) is filtered out, dried by the hot flue gas generated by the pyrolysis furnace, and then placed in a ball mill to grind to -200 mesh to obtain biochar for carbon addition in pellets. At the same time, in order to clarify the advantages of pyrolysis-water quenching method over traditional pyrolysis, corn straw pyrolysis carbon was prepared using the same device and pyrolysis parameters. The industrial analysis, elemental analysis and calorific value of two different biochars are shown in Table 1, the ash composition is shown in Table 2, the specific surface area and pore structure parameters are shown in Table 3, and the combustion weight loss curve is shown in Figure 2 The results all show that the biochar produced by pyrolysis-water quenching has lower ash and harmful element contents, and its high calorific value and combustion temperature are more similar to anthracite and coke.

[0044] Table 1 Industrial analysis, elemental analysis and calorific value of biochar

[0045]

[0046]

[0047] Table 2 Ash composition of biochar

[0048]

[0049] Table 3 Specific surface area and pore structure parameters of biochar

[0050]

[0051] The above two different biochars were used as carbon sources in pellets (the proportion of biochar ground to a particle size less than 74 microns was 85wt.%, and the size of the maximum particles was 100 microns), the carbon content was 2wt.%, the pelletizing raw materials used were shown in Table 4, the amount of bentonite added was 1.80wt.%, and after mixing, a disc pelletizer was used to prepare carbon-containing pellets. Anthracite carbon-containing pellets on the production line of a pelletizing plant were used for comparison. The drop strength test results of the pellets are shown in Table 5. It can be seen that water-quenched carbon has more advantages in the drop strength of the pellets. The two kinds of raw balls were placed in high-temperature resistant alloy cages and placed in the material layer of the belt machine in production. Finished pellets were obtained under the same pellet roasting process conditions. The compressive strength, drum index, wear resistance index and other indicators of the pellets are shown in Table 6. It can be seen that due to the low ash content and harmful elements in the raw water quenched carbon and the high calorific value, the compressive strength, drum index, wear resistance and other indicators of the obtained pellets are better than those of pyrolytic carbon, and are very similar to the indicators of the pellets with anthracite.

[0052] Table 4 Pelletizing Raw Materials

[0053]

[0054] Table 5 Drop strength of carbon-containing green balls

[0055]

[0056] Table 6 Finished ball index

[0057]

[0058] Example 2

[0059] The tobacco straw with a moisture content of 10%, ash content of 4%, volatile content of 73% and dry basis calorific value of about 19MJ / kg is used as the biomass raw material. The biomass pieces with a length of less than 30mm and a maximum cross-sectional diameter of 10mm are crushed and sheared, and a carbonization furnace device with a structure similar to that of a coke oven is used to carbonize the pieces. Natural gas is first used as the carbonization gas source, and the pyrolyzed biomass gas is led back to the combustion chamber of the carbonization furnace for heating. As the temperature rises, the amount of biomass pyrolysis gas gradually increases, and the amount of natural gas is gradually reduced, and biomass gas is used as much as possible for pyrolysis heating of biomass charcoal. The injection amount of natural gas and biomass gas in the whole pyrolysis process is realized by program control. The pyrolysis temperature is raised to 800℃ within 80min and kept warm for 40min. A coke pushing device is used to push the biomass charcoal from the coke outlet into a cylindrical steel water quenching well, and 5m 3Clean water is used to quench the biochar. A spiral stirring paddle is set at the bottom of the cylindrical steel water quenching well to ensure rapid contact between the biochar and water. The water-quenched biochar is then filtered out, dried with the hot flue gas generated by the pyrolysis furnace, and placed in a ball mill to grind to a size below 200 mesh to obtain biochar for carbon addition in pellets (referred to as water-quenched carbon). In order to clarify the advantages of the pyrolysis-water quenching method over traditional pyrolysis, tobacco straw pyrolysis carbon was prepared using the same device and pyrolysis parameters. The industrial analysis, elemental analysis and calorific value of the two different biochars are shown in Table 7, the ash composition is shown in Table 8, the specific surface area and pore structure parameters are shown in Table 9, and the combustion weight loss curve is shown in Table 9. Figure 3 The results all show that the biochar produced by pyrolysis-water quenching has lower ash and harmful element contents, and its high calorific value and combustion temperature are more similar to anthracite and coke.

[0060] Table 7 Industrial analysis, elemental analysis and calorific value of biochar

[0061]

[0062]

[0063] Table 8 Ash composition of biochar

[0064]

[0065] Table 9 Specific surface area and pore structure parameters of biochar

[0066]

[0067] The above two different biochars were used as carbon sources in pellets (the proportion of biochar ground to a particle size less than 74 microns was 85wt.%, and the size of the maximum particles was 100 microns), and the carbon content was 2wt.%. The pelletizing raw materials used are shown in Table 10, and the amount of bentonite added was 1.80wt.%. After mixing, a disc pelletizer was used to prepare carbon-containing pellets, and anthracite carbon-containing pellets on the production line of a pelletizing plant were used for comparison. The drop strength test results of the pellets are shown in Table 11. It can be seen that there is no significant difference in the raw ball strength between the two biochars. The two kinds of raw balls were placed in high-temperature resistant alloy cages respectively and placed in the material layer of a belt roasting machine in production. Finished pellets were obtained under the same pellet roasting process and conditions. The pellet indicators such as compressive strength, drum index, and wear resistance index are shown in Table 12. It can be seen that due to the low ash content and harmful elements in the raw water quenched carbon and the high calorific value, the compressive strength, drum index, wear resistance index and other indicators of the obtained pellets are better than those of pyrolytic carbon, especially in the reduction expansion index, which is most obvious and is very similar to the indicators of the pellets with anthracite.

[0068] Table 10 Pelletizing Raw Materials

[0069]

[0070] Table 11 Drop strength of carbon-containing green balls

[0071]

[0072] Table 12 Finished ball index

[0073]

Claims

1. A method for preparing biochar by pyrolysis-water quenching, characterized in that: The biomass raw material is pyrolyzed, and the generated pyrolytic charcoal is rapidly quenched with water while maintaining the pyrolysis temperature, and then filtered, dried and ground to obtain biomass charcoal.

2. The method for preparing biochar by pyrolysis-water quenching according to claim 1, characterized in that: The biomass raw material has a moisture content lower than 10wt.%, an ash content lower than 20wt.%, a volatile matter content lower than 85wt.%, and a combustion calorific value of 13-20MJ / kg.

3. The method for preparing biochar by pyrolysis-water quenching according to claim 1 or 2, characterized in that: The biomass raw material includes at least one of straw waste, forest prunings, and wood processing waste.

4. The method for preparing biochar by pyrolysis-water quenching according to claim 1 or 2, characterized in that: The pyrolysis conditions are: temperature of 300-900° C. and duration of 30-120 min.

5. The method for preparing biochar by pyrolysis-water quenching according to claim 1, characterized in that: The water quenching process is: pushing the pyrolytic carbon directly into water, or quickly pouring water on the pyrolytic carbon.

6. A biochar, characterized in that: The method is obtained by any one of claims 1 to 5.

7. The biochar according to claim 6, characterized in that: The biochar has an ash content lower than 5wt.%, a K content lower than 1wt.%, a Cl content lower than 0.1wt.%, a calorific value higher than 25MJ / kg, a volatile matter content lower than 25wt.%, a starting combustion temperature higher than 350°C, and a starting gasification temperature higher than 700°C.

8. The method for preparing iron ore pellets using biochar according to claim 6 or 7, characterized in that: The biomass charcoal is added to the pelletizing raw materials, and pelletizing and roasting are carried out in sequence to obtain finished pellets.

9. The method for preparing iron ore pellets using biochar according to claim 8, characterized in that: The biomass charcoal is added to the pellet raw material as a single fuel, or the biomass charcoal is added to the pellet raw material as a mixed fuel with at least one of coal powder, coke powder, charcoal, straw pyrolysis charcoal, and hydrothermal charcoal.

10. The method for preparing iron ore pellets using biochar according to claim 8 or 9, characterized in that: The particle size of the biochar is less than 74 microns, and the proportion is not less than 80 wt.%, and the size of the largest particle does not exceed 100 microns; The proportion of the biomass charcoal in the pellet raw material is between 0.2 and 2 wt.%.

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