Highly active low ash biomass char, method for its production and use in a belt roaster

Abstract

The application discloses a kind of high-activity low-ash biomass carbon and its preparation method and application in belt roaster, belong to iron ore pellet preparation technical field.Biomass raw materials are generated by low-temperature pyrolysis pyrolysis carbon, and the pyrolysis carbon is water quenched at the pyrolysis temperature, and the solid-liquid mixture obtained by water quenching is water leached to remove ash, and then sequentially filtered, dried and ground to obtain biomass carbon.The biomass carbon has low ash content, moderate calorific value volatile matter, fast burning speed and other characteristics.The biomass carbon powder is sprayed into the preheating zone and the roasting zone of the belt roaster, which reduces the dependence of the belt roaster on high-calorific-value fuel gas and the CO2 emission without affecting the normal production of iron ore pellets.

Description

Technical Field

[0001] This invention relates to a biochar, specifically a biochar rich in hydrocarbon volatiles with good combustion performance, low ash content, and high ash melting point. It also relates to a method for preparing biochar through low-temperature pyrolysis-water quenching modification-enhanced hot leaching deashing, and a method for using biochar as a substitute for fuel gas in belt roaster injection during pellet production. This invention belongs to the field of iron ore pellet preparation technology. Background Technology

[0002] Iron pellets are an important raw material for both long-process and short-process iron and steel smelting. They have high iron content, good strength, and excellent metallurgical properties. Moreover, the energy consumption and carbon emissions of the process are only 50% and 65% of those of the sintering process, respectively. They are high-quality, low-carbon raw materials for iron and steel smelting.

[0003] Belt roasters are one of the mainstream technologies for large-scale pellet production. Compared with chain grate rotary kilns, they have advantages such as compact process, small equipment footprint, high single-unit output, and strong raw material adaptability. However, the biggest drawback of belt roasters is their poor fuel adaptability, as they rely entirely on high-calorific-value fossil fuels such as natural gas and coke oven gas.

[0004] The application of solid fuels to belt roasters will significantly broaden their fuel adaptability. The main methods include direct pulverized coal injection in the combustion chamber and pulverized coal injection through the cooling section hood. Chinese patent (publication number: CN115507647A) discloses a heating system and method for a belt roaster using blast furnace gas and pulverized coal as fuel. This system utilizes a mixture of blast furnace gas and pulverized coal as fuel, solving the problem that belt roasters must use high-calorific-value gaseous fuels, thus overcoming fuel limitations. Furthermore, blast furnace gas is a byproduct of the blast furnace ironmaking process in steel plants; its efficient utilization can reduce costs and increase efficiency for steel enterprises. However, due to the limited space in the belt roaster's combustion chamber and the poor combustion performance and long flame of pulverized coal, direct pulverized coal injection is difficult to achieve complete combustion, which not only affects energy efficiency but also causes coal ash and unburned coal powder to affect the permeability of the pellet bed. To expand the combustion space of pulverized coal and improve its combustion rate, Lurgi proposed injecting pulverized coal into the cooling section's fume hood. This utilizes the large space and high temperature of the fume hood to extend the combustion time of the pulverized coal. However, this approach introduces problems such as ash corrosion and wear on the fume hood's refractory materials, and in severe cases, even slagging. Therefore, there is currently no mature technology for injecting pulverized coal into belt conveyors. The poor combustion performance, high ash content, and low ash melting point of pulverized coal are the main reasons restricting its use in belt conveyors. Furthermore, pulverized coal injection into belt conveyors does not align with the trend of low-carbon development in pelletizing.

[0005] Biomass is a carbon-neutral renewable fuel with better combustibility than coal. It is a fuel between natural gas and pulverized coal, and has greater potential for development as a clean fuel for belt conveyors than coal. During combustion, the volatile matter in biomass burns first, igniting the fixed carbon, resulting in a lower combustion temperature and faster combustion rate than coal. However, raw biomass has a low fixed carbon content, low energy density, low ash melting point, and is difficult to pulverize, making it unsuitable for direct use in belt conveyors. In the metallurgical industry, biomass is mainly used as fuel or a reducing agent. Before application, it requires upgrading processing to improve its energy density or reactivity. Processing methods often employ thermal conversion technologies such as pyrolysis, hydrothermal carbonization, steam explosion, and gasification. Its products are mainly used in blast furnace injection, sintering, and direct reduction processes.

[0006] Pyrolysis is the most mature technology for biomass upgrading. Conventional pyrolysis removes unstable oxygen-containing organic structures in the form of volatiles through dry distillation at temperatures above 500°C in an oxygen-deficient atmosphere, thereby increasing carbon content and calorific value and improving grindability. However, because the removal of pyrolysis gas with good combustibility is relatively thorough, the yield of the resulting pyrolysis char is low, its combustibility is relatively poor, and the ash content and alkali metal content such as potassium in the ash are high, making it prone to slagging in the combustion chamber of belt conveyors. Therefore, conventional biomass pyrolysis char is not suitable for belt conveyor injection. Currently, hydrothermal carbonization and steam explosion involve high-pressure operations, resulting in high technical difficulty and limitations in continuous large-scale production, which to some extent restricts their industrial application. Although gasification can produce gaseous fuels usable by belt conveyors, the quality of syngas is closely related to the tar conversion rate and the type of gasifying agent in the production process. Efficiently converting tar into biomass gas is a problem that urgently needs to be solved. Furthermore, even with pure oxygenation, biogas suffers from drawbacks such as high CO2 content and difficulty in removal, resulting in a lower calorific value compared to coke oven gas and natural gas. This makes it unsuitable for processes requiring high calorific values. Currently, there is a lack of biochar preparation technologies adapted to the characteristics of belt conveyors, and research and reports on using biochar as fuel in belt conveyors are even scarcer.

[0007] Chinese patent application (publication number: CN119505940A) discloses a method for preparing biochar through pyrolysis-water quenching. Specifically, biomass raw materials are pyrolyzed (pyrolysis temperature: 300–900℃), and the resulting pyrolyzed char is rapidly quenched in water while maintaining the pyrolysis temperature. After filtration, drying, and grinding, the biochar is obtained. The biochar prepared by this method has an ash content of less than 5 wt.%, a potassium content of less than 1 wt.%, a chloride content of less than 0.1 wt.%, a calorific value of more than 25 MJ / kg, a volatile matter content of less than 25 wt.%, an initial combustion temperature of more than 350℃, and an initial gasification temperature of more than 700℃. This biochar can meet the requirements for carbon blending in iron ore sintering, reducing the dependence on coal-based fossil fuels and the emission of CO2 and pollutants in the iron ore pellet production process. However, the low volatile matter content, high low-melting-point potassium salt content, and high initial combustion temperature of this biochar make it unsuitable for injection applications. Summary of the Invention

[0008] To address the aforementioned technical problems, the first objective of this invention is to provide biochar rich in hydrocarbon volatiles with good combustion performance, low ash content, and high ash melting point. This biochar can partially or completely replace gas as the injection fuel in belt roasters during pellet production, thereby reducing the belt roaster's dependence on high-calorific-value gas and CO2 emissions without affecting the normal production of iron ore pellets.

[0009] The second objective of this invention is to provide a method for preparing biochar. This method uses biomass solid waste as raw material and employs low-temperature pyrolysis to obtain pyrolytic char rich in hydrocarbon volatiles with good combustion performance. Water quenching is then used to further increase the specific surface area and porosity of the pyrolytic char and disrupt the relatively stable CC structure within it, thereby further enhancing its combustion activity. Furthermore, the hot leaching and deashing of the pyrolytic char initiated by water quenching, along with the pores and cracks formed during quenching and the residual heat carried by the pyrolytic char, provides better kinetic and thermodynamic conditions for ash leaching. The resulting biochar exhibits extremely fast combustion speed, extremely low ash content, and a high ash melting point, meeting the fuel requirements for belt roasters. Moreover, this method is simple to operate, low in cost, and meets the requirements of industrial production.

[0010] The third objective of this invention is to provide an application of biochar as a fuel injected into a belt roaster in pellet production. The biochar can partially or completely replace high-cost fuels such as natural gas and coke oven gas in belt roaster injection. On the one hand, this alleviates the dependence of belt roasters in pellet production on high-calorific-value fossil fuels such as natural gas and coke oven gas, breaking through the constraints of the existing energy structure on the development of belt roasters. On the other hand, by using biomass, a renewable and clean energy source, low-carbon production of low-iron ore pellets can be achieved.

[0011] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing biochar. The method involves pyrolyzing biomass raw materials at a low temperature of 180-290℃ for 10-120 minutes to generate pyrolytic char. The pyrolytic char is then water-quenched while maintaining the pyrolysis temperature. The solid-liquid mixture obtained from water quenching is then subjected to water immersion deashing at a temperature of 30-100℃ with a liquid-to-solid ratio of 5-50 L / kg for at least 5 minutes. The mixture is then sequentially filtered, dried, and ground to obtain biochar.

[0012] This invention uses biomass as raw material and employs a coupled process of "pyrolysis upgrading - water quenching activation - water leaching deashing" (see [link to relevant documentation]). Figure 1 This method yields high-yield biochar with low ash content and high combustion activity. More specifically, firstly, because the low-quality oxygen-containing components in biomass feedstock have poor thermal stability, while the highly combustible hydrocarbon volatiles are relatively thermally stable, the low-quality oxygen-containing components can be selectively removed during low-temperature pyrolysis by strictly controlling the pyrolysis temperature. This allows the biochar to complete the endothermic desorption process related to the initial combustion stage during pyrolysis. Simultaneously, the high-combustion-performance hydrocarbon volatiles are retained in the biochar. Therefore, this process not only increases the yield of pyrolyzed char but also makes its combustion reaction rapid and exothermic. Secondly, water quenching increases the porosity and specific surface area of ​​the pyrolyzed char, disrupting the relatively stable C-C structure and enhancing its combustion activity. Thirdly, hot water leaching removes low-melting-point water-soluble ash-forming elements from the pyrolyzed char, resulting in ultra-low ash and highly combustible biochar.

[0013] It is worth noting that pyrolytic char, produced from biomass feedstock using low-temperature pyrolysis, exhibits significantly improved grindability and hydrophobicity compared to the original biomass feedstock. Furthermore, water quenching increases the porosity and cracks in the pyrolytic char, providing favorable conditions for subsequent water leaching deashing, drying dehydration, and grinding. For example, the water leaching deashing process is initiated by water quenching, which increases the porosity and cracks in the pyrolytic char. The heat transferred from the pyrolytic char to the hot leaching system also provides better kinetic and thermodynamic conditions for ash leaching. Additionally, the porosity and cracks formed after water quenching reduce the strength of the pyrolytic char, thus improving its grindability and facilitating the preparation of ultrafine biomass char powder. Moreover, the removal of oxygen-containing components from the pyrolytic char significantly improves its hydrophobicity compared to the original biomass feedstock, making water removal easier.

[0014] As a preferred embodiment, the water leaching deashing process employs mechanical grinding and / or chemical action to enhance ash leaching. The use of mechanical grinding and / or chemical action during water leaching deashing can enhance ash leaching. Mechanical grinding is performed concurrently with the biomass char water leaching process, while chemical action involves adding acidic substances during the biomass char water leaching process to promote the dissolution of alkaline ash components. As a more preferred embodiment, the mechanical grinding method includes stirred milling or ball milling. As a more preferred embodiment, the chemical action includes adding acidic substances. Acidic substances can be organic acids such as oxalic acid, acetic acid, and citric acid, or inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, or acidic gases such as CO2, more preferably organic acids or CO2. CO2 can be replaced by CO2-rich flue gas. Using hydrochloric acid, sulfuric acid, and nitric acid may increase the Cl, S, and N content in the biomass char, which is disadvantageous. The heat source for water leaching deashing can be derived from the combustion of pyrolysis gas generated during the low-temperature pyrolysis of the biomass raw material.

[0015] As a preferred embodiment, the biomass raw material meets the following requirements: length less than 20 mm, maximum cross-sectional diameter less than 10 mm, moisture content less than 15 wt.%, ash content less than 10 wt.%, volatile matter content less than 85 wt.%, and calorific value exceeding 10 MJ / kg. More preferably, the biomass raw material includes at least one of straw waste, orchard trimmings, and wood processing waste. These biomass raw materials, after conventional pyrolysis, have high ash content and slow combustion speed. Using them in a belt calciner for injection can lead to incomplete combustion in a timely manner. Unburned high-ash char powder, along with iron-containing dust in the hot air, easily forms scale in the belt calciner's combustion chamber and can also enter the pellet bed, deteriorating the bed's permeability and affecting normal production. This invention employs low-temperature pyrolysis of the biomass raw material to directionally remove low-quality oxygen-containing components while retaining readily combustible hydrocarbon volatiles. Furthermore, water quenching is used to increase the porosity and specific surface area of ​​the pyrolytic char, thereby improving its combustibility. Furthermore, the use of hot leaching effectively reduces the content of water-soluble, low-melting-point ash-forming substances in the pyrolytic char, thus reducing problems such as ash buildup at the tuyeres during injection, which are detrimental to the normal operation of the belt roaster. Further preferred biomass raw materials include, but are not limited to, straw waste from agricultural production such as corn, rice, wheat, soybeans, and cotton, or forestry waste such as orchard trimmings and wood processing waste. The biomass raw materials of this invention undergo shearing and crushing pretreatment.

[0016] As a preferred embodiment, the low-temperature pyrolysis temperature is 220~280℃, and the time is 40~80min. If the pyrolysis temperature is too low or the time is too short, the removal of low-quality oxygen-containing components will be insufficient, and the improvement in the grindability of the pyrolyzed char and the effect of water quenching will not be significant. If the pyrolysis temperature is too high or the time is too long, a large amount of volatile hydrocarbon gases with good combustibility will be released from the pyrolyzed char, resulting in a low yield of pyrolyzed char and poor combustibility.

[0017] As a preferred embodiment, the low-temperature pyrolysis employs microwave heating or microwave-assisted heating. Given that the oxygen-containing groups in the pyrolyzed char are highly polar, while CH is relatively weakly polar, microwave action can achieve the directional removal of oxygen-containing components. Therefore, the low-temperature pyrolysis method preferably employs microwave heating or microwave-assisted heating to improve the directional removal efficiency of oxygen-containing components from the pyrolyzed char. More preferably, the microwave power used in the microwave heating or microwave-assisted heating method is preferably 100-200 kW.

[0018] As a preferred embodiment, the water quenching process involves: rapidly immersing the hot pyrolytic carbon in water, or instantly submerging the pyrolytic carbon in water, ensuring that the amount of water used during the water quenching process is sufficient to completely submerge the hot pyrolytic carbon. Mechanical stirring can be used during the water quenching process to enhance the contact between the pyrolytic carbon and water, thereby increasing the water quenching rate.

[0019] The present invention also provides a biochar obtained by the preparation method described above.

[0020] As a preferred embodiment, the biochar has an ash content of less than 5 wt.%, a K content of less than 0.5 wt.%, a Cl content of less than 0.1 wt.%, a calorific value of more than 18 MJ / kg, a volatile matter content of 30-70 wt.%, a particle size of less than 74 μm with a powder mass ratio of not less than 95%, and a specific surface area greater than 3 m². 3 / g, starting combustion temperature below 320℃, ash melting point above 1300℃.

[0021] This invention also provides an application of biochar as a fuel injected into a belt roaster during pellet production.

[0022] As a preferred embodiment, the biochar is injected into the preheating section and / or roasting section of the belt roaster. The roasting section and preheating section of the belt roaster are equipped with gas-solid dual-fuel nozzles, which can be used to inject biochar and simultaneously inject gaseous fuel.

[0023] As a preferred embodiment, the amount of biochar injected is based on the principle of replacing the amount of gas injected with equal calorific value. The biochar of this invention can be injected into a belt roaster as a single solid fuel, or it can be mixed and injected into a belt roaster with at least one of natural gas, coke oven gas, or a mixed gas. The amount of biochar injected is based on the principle of replacing the amount of gas injected with equal calorific value, meaning that the calorific value of the injected gas is equal to the calorific value of the injected biochar, and the proportion of biochar replacing the gas is 1% to 100%. When the biochar is injected alone as a single solid fuel, catalytic combustion with Brown gas (HHO) can be used.

[0024] The pyrolysis gas generated from the low-temperature pyrolysis of biomass raw materials of the present invention can be used as a heat source for heating during the low-temperature pyrolysis process of biomass, and the flue gas generated by the combustion of the pyrolysis gas is CO2-rich flue gas, which can be directly added as an acidic substance in the water leaching deashing process to promote water leaching deashing.

[0025] In the biochar preparation process of the present invention, filtration is carried out by pressure filtration, which squeezes out the water in the biochar and presses the biochar into lumps that are easy to dry later.

[0026] In the biochar preparation process of the present invention, the heat source used in the drying process is the hot flue gas generated during the low-temperature pyrolysis process. In order to reduce the amount of powder generated when biochar lumps rub against each other, a static drying chain grate machine is preferred.

[0027] In the biochar preparation process of the present invention, the heat source used in the drying process is the hot flue gas generated during the low-temperature pyrolysis process, and the drying method can be drum drying or exhaust drying.

[0028] In the biochar preparation process of the present invention, the grinding process can be carried out using a high-speed coal mill or a ball mill.

[0029] The method for preparing biochar of the present invention includes the following steps:

[0030] S1: Cut agricultural and forestry biomass raw materials with moisture, ash, and volatile matter content below 15wt.%, 10wt.%, and 85wt.%, respectively, and a calorific value exceeding 10MJ / kg into small biomass blocks with a length less than 20mm and a maximum cross-sectional diameter less than 10mm. Place the biomass blocks in a pyrolysis furnace for pyrolysis and carbonization into pyrolytic char. The pyrolysis temperature is 180~290℃, and the duration is 10~120min. The carbonization heat can be partially derived from biomass pyrolysis gas, or microwave pyrolysis can be used alone or in combination.

[0031] S2: The hot pyrolysis char is pushed from the carbonization chamber into room temperature water that can completely submerge the biomass char for rapid water quenching, followed by deashing. During deashing, the liquid-to-solid ratio can be maintained at the original liquid-to-solid ratio after water quenching, or water can be added to increase the liquid-to-solid ratio. Biomass pyrolysis gas combustion tail gas (CO2-rich flue gas) is introduced into the liquid phase to regulate the water temperature and pH. The liquid-to-solid ratio is controlled at 5~50 L / kg, the temperature at 30~100℃, and the water immersion deashing time at least 5 minutes. Acidic substances are added during the water immersion deashing process to enhance ash content. The leaching process involves simultaneously stirring the deashing system using a ball mill while aerating and grinding. The resulting coarse biochar powder is then obtained through pressure filtration and drying. This coarse powder is further ground using a medium-speed mill or ball mill to achieve fine biochar powder with an ash content below 5 wt.%, a potassium content below 0.5 wt.%, a chloride content below 0.1 wt.%, a calorific value above 18 MJ / kg, a volatile matter content of 30–70 wt.%, and a particle size of at least 95% (powder with a particle size less than 74 μm) and a specific surface area greater than 3 m². 3 / g, starting combustion temperature below 320℃, ash melting point above 1300℃.

[0032] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0033] 1) Addressing the technical challenge of the heavy reliance on high-calorific-value fossil fuels such as natural gas and coke oven gas in pellet belt roasters, this invention utilizes abundant and highly combustible biomass raw materials to prepare solid fuel suitable for injection into pellet belt roasters. The biomass raw materials undergo low-temperature pyrolysis, which directionally removes low-quality oxygen-containing components to increase the biomass' calorific value while retaining highly combustible hydrocarbon volatiles. During combustion, these volatiles rapidly precipitate, burn, and ignite fixed carbon, resulting in fast-burning pyrolyzed char. The pyrolyzed biochar is then rapidly quenched with water to increase its specific surface area and porosity, disrupting its internal CC-stabilized structure, thus further enhancing its combustibility. Based on this, water immersion deashing is employed to obtain ultra-low-ash biochar. The process of "low-temperature pyrolysis upgrading-water quenching activation-water leaching deashing" proposed in this invention can produce high-combustibility, low-ash biochar with high yield. It has the advantages of low energy consumption, high efficiency, and continuous large-scale production, and has great potential for engineering applications and good promotion.

[0034] 2) The biochar preparation process of this invention adopts a "low-temperature pyrolysis-water quenching activation-water leaching deashing" process. The biochar generated by the low-temperature pyrolysis method has good grindability and hydrophobicity. Simultaneously, water quenching increases the porosity and cracks in the biochar, providing favorable conditions for subsequent hot leaching deashing, drying dehydration, and grinding. The increased porosity and cracks in the biochar, on the one hand, reduce its strength, thereby improving its grindability and facilitating the preparation of ultrafine biochar powder. On the other hand, the pores and cracks formed during the rapid cooling process of water quenching serve as channels for ash dissolution in the subsequent deashing process, providing better kinetic conditions for ash dissolution. Furthermore, the residual heat from the water quenching process is used in the subsequent hot leaching deashing process, providing better thermodynamic conditions for ash dissolution and reducing the energy consumption of the hot leaching deashing process. The hydrophobicity of the biochar is significantly improved compared to the biomass raw materials, making water removal easier.

[0035] 3) The biochar of this invention features low ash and high combustion performance, and can partially or completely replace expensive fossil fuels such as natural gas and coke oven gas for belt roasting. It processes abundant agricultural biomass into low-ash, highly active biochar powder with combustion performance similar to coal gas, effectively alleviating the constraints of the existing energy structure on the development of belt roasting machines. Simultaneously, the use of biochar for belt roasting does not exacerbate scaling in the combustion chamber and essentially does not alter the internal temperature field and airflow distribution of the belt roaster, ensuring higher quality pellets and better fuel supply stability, effectively reducing the dependence on fossil fuels and carbon emissions in pellet production. Attached Figure Description

[0036] Figure 1 This is a process flow diagram for the preparation and injection of biochar for belt conveyors.

[0037] Figure 2 The combustion weight loss curve of the biochar powder prepared in Example 1.

[0038] Figure 3 This is an expanded experimental device for a belt roaster.

[0039] Figure 4 Combustion weight loss curves of biomass char powder prepared by pyrolysis under different heating methods.

[0040] Figure 5 A comparison of the combustion characteristic curves of biochar prepared at different pyrolysis temperatures. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] It should also be noted that, in order to avoid obscuring the content of the present invention with unnecessary details, only the structures and / or processing steps closely related to the solution of the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0043] Example 1

[0044] Using 100 kg of corn stalks with a moisture content of 8 wt.%, ash content of 5 wt.%, volatile matter of 75 wt.%, and a dry basis calorific value of approximately 17 MJ / kg as biomass raw material, the biomass is crushed and sheared into small pieces less than 20 mm in length and with a maximum cross-sectional diameter of 9.5 mm. These pieces are then carbonized in a carbonization furnace with a structure similar to a coke oven. Natural gas is used as the start-up gas for pyrolysis. The biomass gas produced during pyrolysis is returned to the combustion chamber of the carbonization furnace for heating. As the temperature increases, the amount of biomass pyrolysis gas gradually increases while the amount of natural gas gradually decreases, with biomass gas being used as much as possible for heating the biochar pyrolysis. The injection rates of natural gas and biomass gas throughout the pyrolysis process are controlled by a program. The pyrolysis temperature is raised to 280℃ at approximately 10℃ / min for 60 minutes. The resulting hot pyrolyzed char is pushed from the outlet into a cylindrical steel water-quenching well using a char pusher, while simultaneously injecting 2 m³ of [unclear text - possibly a specific type of injection] into the well. 3 The biochar was quenched using room temperature water. A spiral agitator was installed at the bottom of the water quenching well to ensure rapid contact and quenching between the biochar and water. The water-quenched solid-liquid mixture was then transferred to a stirred ball mill. The bottom of this mill could be vented with flue gas (containing CO2) from the pyrolysis furnace, and a heating plate was installed under the mill liner to control the internal temperature. The temperature inside the liquid mill was raised to 50°C using a method of simultaneous ventilation, heating, and grinding, and deashing continued at this temperature for 30 minutes. The biochar slurry was then transported to a settling tank for sedimentation. The water on top was skimmed off, and the moist biochar sediment was placed in a plate and frame filter press for dewatering. The filter cake was pressed into biochar lumps approximately 1cm × 1cm × 1cm in size. After drying with hot flue gas from the pyrolysis furnace, the lumps were ground to -200 mesh in a ball mill. The obtained biomass ash has a composition of 3.5 wt.%, a K content of 0.45 wt.%, a Cl content of 0.09 wt.%, a calorific value of 21.06 MJ / kg, a volatile matter content of 54.23 wt.%, and a specific surface area of ​​5.16 m². 3 / g, the softening temperature of the ash is 1361℃, and the combustion characteristic curve is shown in the figure. Figure 2 The initial combustion temperature is below 280℃, and combustion is essentially complete at 500℃. It can be seen that the resulting biochar fine powder has low ash content, low content of harmful elements, and low ignition temperature, while having high calorific value, high volatile matter content, high ash melting point, and high specific surface area. In... Figure 3An experiment was conducted on the expanded experimental device of the belt roaster to replace 30% of natural gas for injection, and the finished iron ore pellets were measured. The results are shown in Table 1. It can be seen that the replacement of 30% of natural gas with biochar for injection does not have a significant impact on the quality of the pellets, and no slagging occurred in the combustion chamber during the experiment.

[0045]

[0046] Example 2

[0047] The only difference between this embodiment and Embodiment 1 is that this embodiment uses microwave heating, employing a 150kW microwave to heat to 280°C and then pyrolyze for 60 minutes. All other operations and conditions remain the same as in Embodiment 1.

[0048] The obtained biochar had an ash content of 3.45 wt.%, a potassium content of 0.42 wt.%, a chloride content of 0.08 wt.%, a calorific value of 23.06 MJ / kg, a volatile matter content of 51.42 wt.%, and a specific surface area of ​​6.16 m². 3 / g, the softening temperature of the ash is 1373℃, and the combustion characteristic curve is shown in the figure. Figure 4 It can be seen that the ash content, harmful element content, ignition temperature and ash melting point of the biomass char fine powder obtained by microwave pyrolysis at the same pyrolysis temperature are comparable to those of conventional pyrolysis. However, due to the higher degree of removal of the more polar oxygen-containing low-quality components, the volatile matter content of microwave pyrolysis char is lower than that of conventional pyrolysis char, which also leads to its higher calorific value and specific surface area than that of conventional pyrolysis char.

[0049] In such Figure 3 An experiment was conducted on the expanded experimental device of the belt roaster shown, in which this type of biochar replaced 30% of natural gas for injection. The finished iron ore pellets were measured. The compressive strength of the pellets was 2953 N / pellet, the drum index (+6.3 mm) was 96.58%, the abrasion resistance index (-0.5 mm) was 4.24%, and the expansion rate was 14.35%. It can be seen that compared with the use of conventional pyrolysis char, the use of microwave pyrolysis char and gas for mixed injection in the belt roaster resulted in better pellet quality. No slag formation occurred in the combustion chamber during the experiment, and the material layer had good permeability.

[0050] Example 3

[0051] The only difference between this embodiment and Embodiment 1 is that the water immersion deashing process does not involve the introduction of hot flue gas (containing carbon dioxide gas) or agitated ball milling to enhance deashing; instead, the temperature is raised to 50°C, and deashing is performed at this temperature for 30 minutes. All other operations and conditions remain the same as in Embodiment 1.

[0052] The obtained biomass ash has a composition of 5.23 wt.%, a K content of 0.86 wt.%, a Cl content of 0.12 wt.%, a calorific value of 18.13 MJ / kg, a volatile matter content of 53.42 wt.%, and a specific surface area of ​​3.49 m². 3 / g, ash melting point is 1289℃. It can be seen that compared with the hot flue gas chemical enhanced deashing and ball mill mechanical enhanced deashing methods used in Example 1, the biochar obtained in this example has higher ash content and harmful elements, and lower volatile matter, ash melting point and specific surface area. Therefore, the biochar obtained in this example is slightly less effective in belt press pulverization than the biochar obtained in Example 1.

[0053] Comparative Example 1

[0054] The only difference between this comparative example and Example 1 is that the pyrolysis temperature is set to 600°C. All other operations and conditions are the same as in Example 1.

[0055] The obtained biomass ash has a composition of 9.65 wt.%, a K content of 1.03 wt.%, a Cl content of 0.32 wt.%, a calorific value of 26.32 MJ / kg, a volatile matter content of 13.43 wt.%, and a specific surface area of ​​16.49 m². 3 / g, the softening temperature of the ash is 1321℃, and its combustion characteristic curve is shown in the figure. Figure 5 It can be seen that compared with Example 1, the higher pyrolysis temperature used in this comparative example resulted in more thorough removal of volatiles, leading to a larger number and angle of pores and thus a larger specific surface area. The thorough removal of volatiles also resulted in ash enrichment and an increased proportion of fixed carbon, thus giving the biochar a higher calorific value. Furthermore, the liquid intermediates produced by lignin decomposition during high-temperature pyrolysis enhance the encapsulation effect on ash, increasing the difficulty of removing ash and harmful elements, resulting in a higher ash content in the biochar. Because the obtained biochar has a lower content of volatiles with good combustion performance, it is less prone to ignition. When this biochar was used to replace 30% of natural gas in a belt roaster expansion test device, the resulting pellets had a compressive strength of 2835 N / pellet, a drum index of +6.3 mm of 89.83%, an abrasion resistance index of -0.5 mm of 6.42%, and an expansion rate of 16.62%. Compared with Example 1, the quality of the pellets was poor when the biochar with a higher pyrolysis temperature was used for belt-driven gas mixing and injection, and slagging occurred in the combustion chamber during the experiment, resulting in poor air permeability of the pellet bed.

Claims

1. A method for preparing biochar, characterized in that: Biomass raw materials are pyrolyzed at low temperature of 180~290℃ for 10~120min to produce pyrolytic char. The pyrolytic char is then water-quenched while maintaining the pyrolysis temperature. The solid-liquid mixture obtained by water quenching is adjusted to a liquid-solid ratio of 5~50L / kg and water-immersed at 30~100℃ for more than 5min to remove ash. Then, it is sequentially filtered, dried and ground to obtain biochar. The low-temperature pyrolysis is performed using microwave heating or microwave-assisted heating. The biochar has an ash content of less than 5 wt.%, a K content of less than 0.5 wt.%, a Cl content of less than 0.1 wt.%, a calorific value of more than 18 MJ / kg, a volatile matter content of 30-70 wt.%, and a particle size of less than 74 μm with a powder mass ratio of not less than 95%, and a specific surface area greater than 3 m². 3 / g, starting combustion temperature below 320℃, ash melting point above 1300℃.

2. The method for preparing biochar according to claim 1, characterized in that: During the water immersion deashing process, mechanical grinding and / or chemical action are used to enhance the water immersion deashing.

3. The method for preparing biochar according to claim 2, characterized in that: The mechanical grinding method includes stirred milling or ball milling; The chemical reaction method includes adding acidic substances.

4. The method for preparing biochar according to claim 1, characterized in that: The biomass raw material meets the following requirements: length less than 20 mm, maximum cross-sectional diameter less than 10 mm, moisture content less than 15 wt.%, ash content less than 10 wt.%, volatile matter content less than 85 wt.%, and calorific value exceeding 10 MJ / kg.

5. The method for preparing biochar according to claim 1, characterized in that: The low-temperature pyrolysis is performed at a temperature of 220~280℃ for a time of 40~80min.

6. The method for preparing biochar according to claim 1, characterized in that: The water quenching process is as follows: the hot pyrolytic carbon is quickly pushed into the water, or the pyrolytic carbon is instantly submerged in water. During the water quenching process, the amount of water used is at least enough to ensure that the hot pyrolytic carbon is completely submerged.

7. A type of biochar, characterized in that: It is obtained by the preparation method according to any one of claims 1 to 6.

8. The application of the biochar according to claim 7, characterized in that: It is used as fuel for belt roasting machines in pellet production.

9. The application of a biochar according to claim 8, characterized in that: The biochar is injected into the preheating section and / or roasting section of the belt roaster. The amount of biochar injected is based on the principle of replacing the amount of gas injection with the amount of biochar with the same calorific value.

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