Method and system for directional regulation of alkali metals in the process of biomass carbonization

Abstract

The application discloses a kind of biomass carbonization process in alkali metal directional regulation method and system, belong to biomass thermochemical conversion and clean metallurgical technology field.The process includes: low-grade steam pretreatment is carried out to biomass raw material, then is separated by solid-liquid, and pretreated biomass solid and alkali-rich condensed water are obtained;Pretreated biomass solid is carbonized by high-pressure superheated steam, and low-alkali biomass carbon is obtained.The application removes water-soluble and ionic alkali metal in it by low-grade steam pretreatment, combined with the "two-stage pressure regulation + phased discharge" mode of high-pressure superheated steam carbonization, forms continuous cooperation between the links of alkali metal form conversion, migration enrichment and discharge, effectively volatilizes and precipitates residual alkali metal, realizes efficient and controllable directional regulation.The application can solve the problem of high alkali metal content in biomass carbon causing blast furnace deterioration, support green transformation of steel industry, and reduce fuel dependence through cascade heat utilization.

Description

Technical Field

[0001] This invention belongs to the field of biomass thermochemical conversion and clean metallurgy technology, specifically relating to a method and system for directional control of alkali metals during biomass carbonization. Background Technology

[0002] Biochar, as a renewable carbon material, offers significant advantages in reducing carbon emissions when used as a substitute for coke in blast furnace injection. However, lignocellulosic raw materials such as agricultural and forestry residues are generally rich in alkali metals (K+Na), with some herbal raw materials containing 2-5% K. The accumulation of alkali metals in the blast furnace can exacerbate furnace lining erosion, catalyze coke pulverization, and affect the quality of molten iron. Therefore, biochar used for blast furnace injection typically requires an ash content of K+Na ≤ 2.0%.

[0003] However, existing biomass carbonization and dealkali removal technologies still have significant shortcomings: low dealkali removal efficiency, high cost, and lack of resource recovery. For example, water washing has limited dealkali removal rate and high water treatment costs; acid washing has high efficiency but complex wastewater treatment, equipment corrosion, and high costs; atmospheric pressure carbonization has limited alkali volatilization, making it difficult to meet the alkali content standards of the products. Therefore, there is an urgent need for an efficient, controllable, and resource-efficient method for targeted regulation of alkali metals. Summary of the Invention

[0004] In view of the technical problems existing in the background art, the present invention provides a method and system for directional control of alkali metals in the biomass carbonization process, aiming to solve the technical problems of low dealkali efficiency, high cost and lack of resource recovery in the existing process.

[0005] In a first aspect, embodiments of this application provide a method for the directional control of alkali metals during biomass carbonization, comprising the following steps: The biomass feedstock is pretreated with low-grade steam and then separated into solid and liquid components to obtain pretreated biomass solids and alkali-rich condensate. Pretreated biomass solids are carbonized by high-pressure superheated steam to obtain low-alkali biochar. The temperature for pretreatment of low-grade steam is 60-100℃; During high-pressure superheated steam carbonization, the pressure of the high-pressure superheated steam is 2.0-4.0 MPa and the temperature is 350-550℃.

[0006] As a further improvement of this application, before pretreating the biomass feedstock with low-grade steam, the biomass feedstock is further pulverized to a particle size of 1-5 mm.

[0007] As a further improvement of this application, the low-grade steam pretreatment time is 30-90 min, and the liquid-solid mass ratio is 5:1-15:1; the heat source for the low-grade steam pretreatment comes from fuel, which includes at least one of the combustible pyrolysis gas produced by the high-pressure superheated steam carbonization and biomass oil.

[0008] As a further improvement of this application, solid-liquid separation is performed by pressure filtration or centrifugation, and the resulting pretreated biomass solids have a water content of 40-60%.

[0009] As a further improvement of this application, the high-pressure superheated steam carbonization includes: performing a first high-pressure superheated steam carbonization treatment at 2.0-3.0 MPa; and performing a second high-pressure superheated steam carbonization treatment at 3.0-4.0 MPa.

[0010] As a further improvement of this application, the method and system for directional control of alkali metals in the process of biomass carbonization is characterized in that the temperature of the first high-pressure superheated steam carbonization treatment is 350-400℃ and the time is 30-40min. The second high-pressure superheated steam carbonization treatment is carried out at a temperature of 400-450℃ for 20-30 minutes.

[0011] As a further improvement of this application, after the high-pressure superheated steam carbonization is completed, a flash explosion treatment is also included. The flash explosion treatment involves depressurizing the material obtained from the high-pressure superheated steam carbonization to atmospheric pressure, and then sequentially entering the jacketed flash explosion tube and the cyclone separator. The inner wall temperature of the jacketed flash explosion tube is 300-350℃.

[0012] As a further improvement of this application, the fixed carbon content of the low-alkali biochar is ≥60%, and the total mass fraction of K and Na in the ash is ≤2.0%.

[0013] As a further improvement of this application, it also includes evaporation and crystallization treatment of alkali-rich condensate, with an evaporation and crystallization pressure of 10-30 kPa and a temperature of 80-100℃, and the resulting alkali metal salt product contains at least one of KCl, K2SO4, NaCl and Na2SO4, and the purity of the alkali metal salt is ≥85%.

[0014] Secondly, embodiments of this application provide a biomass alkali metal targeted regulation system for implementing the method described in the first aspect, comprising: connected sequentially via pipelines: A low-grade steam pretreatment unit is used to perform the low-grade steam pretreatment. A high-pressure superheated steam carbonization unit is used to perform the high-pressure superheated steam carbonization step.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The method and system for targeted control of alkali metals in the biomass carbonization process provided in this application removes 10-30% of water-soluble and ionic alkali metals through low-grade steam pretreatment. In the high-pressure superheated steam carbonization stage, a "two-stage pressure control + staged discharge" method is adopted. First, at 2.0-3.0 MPa, the organic-bound and ion-exchangeable K and Na in the biomass are converted to water-soluble and migratory states and discharged. Then, the pressure is increased to 3.0-4.0 MPa to promote the further migration and discharge of residual alkali metals. This achieves continuous and synergistic transformation, migration, enrichment and discharge of alkali metals. The final product has K+Na ≤2.0% in ash, fixed carbon ≥60%, and particle size D50 of 0.5-3.0 mm. It can directly replace fossil coal for blast furnace injection and meet the strict requirements of green metallurgy for high-quality biochar.

[0016] 2. The method and system for targeted control of alkali metals in the biomass carbonization process provided in this application obtains high-value-added salt products such as KCl and K2SO4 (purity ≥85%) by recovering alkali-rich condensate and evaporating it for crystallization, thus achieving complete resource utilization of alkali metals and eliminating waste liquid discharge. At the same time, it constructs a cascade thermal energy utilization chain of high-temperature steam-high-pressure carbonization-combustion tail gas waste heat-hydrothermal washing, so that the low-grade heat required for pretreatment is supplied by the waste heat of carbonization tail gas. The external fuel dependence of the system is ≤20%, and the modular parallel channel design enables non-stop maintenance and continuous production. The overall thermal efficiency and engineering feasibility are significantly better than the existing technology. Detailed Implementation

[0017] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0019] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more (including two), unless otherwise explicitly defined.

[0020] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0022] To address the problems of low efficiency, high cost, and lack of resource recovery in existing alkali removal technologies, this application provides a method and system for targeted control of alkali metals during biomass carbonization. The method involves targeted removal of 10-30% of water-soluble and ionic alkali metals through low-grade steam pretreatment. During the high-pressure carbonization stage, a "two-stage pressure control + staged discharge" approach is employed. First, at 2.0-3.0 MPa, the organically bound and ion-exchangeable K and Na are converted to water-soluble and migratory states and discharged. Then, the pressure is increased to 3.0-4.0 MPa for stripping to further promote the migration and discharge of residual alkali metals. This is combined with staged steam exhaust... The liquid drainage and instantaneous depressurization operations complete gas-solid separation and directional removal of alkali metals, thereby achieving the technical effect of K+Na ≤ 2.0% in the product ash, fixed carbon content ≥ 60%, particle size D50 of 0.5~3.0mm and uniform particle size. At the same time, by recovering the rich alkali condensate, high-value-added salt products such as KCl and K2SO4 (purity ≥ 85%) are produced by evaporation and crystallization, realizing the complete resource utilization of alkali metals. Furthermore, a cascaded thermal energy utilization chain is constructed to reduce the system's external fuel dependence to ≤ 20%, thereby significantly improving the applicability of the obtained biochar in the field of blast furnace injection and providing technical support for the green and low-carbon transformation of the steel industry.

[0023] Specifically, this application provides a method for the directional control of alkali metals during biomass carbonization, comprising the following steps: The biomass feedstock is pretreated with low-grade steam and then separated into solid and liquid components to obtain pretreated biomass solids and alkali-rich condensate. Pretreated biomass solids are carbonized by high-pressure superheated steam to obtain low-alkali biochar. The temperature for pretreatment of low-grade steam is 60-100℃; During high-pressure superheated steam carbonization, the pressure of the high-pressure superheated steam is 2.0-4.0 MPa and the temperature is 350-550℃.

[0024] In the technical solution of this application embodiment, the above technical solution operates in a step-by-step progressive logic of "low-grade steam pretreatment → high-temperature and high-pressure deep carbonization". First, the biomass raw material undergoes hydrothermal pretreatment in a low-grade steam atmosphere of 60-100℃. Under this temperature condition, the hemicellulose in the biomass undergoes partial hydrolysis, the cell wall structure is destroyed, and the alkali metals (K) that originally existed in the cell lumen and cell wall of the biomass in an ionic or weakly bound state are released. + Na + Under the solvation effect of water, the alkali metals dissolve into the liquid phase and are discharged with the condensate, achieving the initial removal of alkali metals. This step uses low-grade steam (60-100℃) as the pretreatment medium, which has the following advantages: First, the temperature is lower than the severe hydrolysis temperature of hemicellulose, avoiding a large loss of biomass organic matter; second, the hydrothermal environment accelerates the dissolution kinetics of alkali metals, making the dealkali removal efficiency significantly higher than that of room temperature water washing; third, the heat source can be provided by the system's waste heat or low-grade fuel, making the operating cost controllable. Low-grade steam pretreatment involves hydrothermal washing of biomass at 60-100℃ using steam condensate or hot steam as the medium. Alkali metals in biomass are mainly divided into three categories: water-soluble ionic state, exchangeable state, and structural state. Under the hydrothermal environment of 60-100℃, the movement of water molecules intensifies and the permeability increases, which can destroy the binding force between water-soluble ionic and exchangeable alkali metals and the biomass matrix, causing them to dissolve. At the same time, hemicellulose undergoes mild hydrolysis in this temperature range, further destroying the cell wall and releasing the encapsulated alkali metal ions.

[0025] Subsequently, the pretreated biomass solids enter the high-pressure superheated steam carbonization unit, where deep thermochemical conversion is completed under a high-temperature and high-pressure superheated steam atmosphere of 2.0-4.0 MPa and 350-550℃. In this pressure range, the superheated steam density is much higher than that of atmospheric pressure steam, with a high convective heat transfer coefficient and strong thermal penetration, which can quickly and uniformly heat the material. The high-pressure environment prolongs the contact time between steam and carbon particles, enhancing the dissolution and carrying effect of steam on alkali metal chlorides. The temperature of 350-550℃ can ensure that the biomass is fully carbonized, fixing the carbon content to no less than 60%, while preventing excessive graphitization of the carbon skeleton due to excessive temperature, which would hinder the formation of pore structures. The superheated steam in this stage serves a triple purpose: as a heat carrier to complete the deep pyrolysis and carbonization of biomass, increasing the fixed carbon content to over 60%; as an activation medium to react with the carbon matrix in a gasification reaction (C + H₂O → CO + H₂), selectively etching the carbon skeleton to build a porous structure; and as an extractant, leveraging the low polarity of high-temperature, high-pressure water and the steam's ability to dissolve and carry inorganic salts, it dissolves residual alkali metals within the biomass and carries them out with the steam, achieving a synergistic effect of carbonization and deep dealkalization. Furthermore, in some embodiments, before pretreating the biomass raw material with low-grade steam, the biomass raw material is further pulverized to a particle size of 1-5 mm.

[0026] In the technical solution of this application embodiment, by pre-treating the primary biomass by crushing, the particle size of the material can be made uniform, ensuring the leaching efficiency of alkali metals during the subsequent hydrothermal pretreatment. If the particle size is too large, the mass transfer path is long, and the internal alkali metals are difficult to fully dissolve; if the particle size is too small, the crushing energy consumption increases and the material is easily pulverized, which is not conducive to subsequent solid-liquid separation operations. This application controls the crushing particle size to 1-5mm, which can ensure dealkali removal efficiency while taking into account crushing economy and separation operability.

[0027] Furthermore, in some embodiments, the low-grade steam pretreatment time is 30-90 minutes, and the liquid-to-solid mass ratio is 5:1-15:1. The heat source for the low-grade steam pretreatment is fuel, including combustible pyrolysis gas produced by high-pressure superheated steam carbonization and biomass oil.

[0028] In the technical solution of this application embodiment, the pretreatment time, liquid-to-solid ratio, and temperature jointly determine the degree of alkali metal dissolution. When the treatment time is less than 30 minutes, the alkali metal dissolution reaction is insufficient, and the dealkali removal efficiency is inadequate; when it exceeds 90 minutes, further extending the time has a gradual effect on the improvement of the dealkali removal rate, and the production efficiency decreases. When the liquid-to-solid ratio is less than 5:1, the concentration of alkali metal in the liquid phase increases rapidly, the mass transfer driving force decreases, which is not conducive to deep dealkali removal; when the liquid-to-solid ratio is greater than 15:1, although the dealkali removal effect is further improved, a large amount of dilute alkali solution is generated, and the energy consumption for subsequent evaporation, crystallization, and recovery of alkali metal salts increases significantly. Therefore, this application controls the pretreatment time and liquid-to-solid ratio within the above ranges, achieving a balance between dealkali removal efficiency and energy consumption. In addition, the pretreatment heat source preferentially utilizes the combustible cracked gas and biomass oil produced by the system itself, eliminating the need for external fuel supply and further improving the system's energy self-sufficiency rate.

[0029] Furthermore, in some embodiments, solid-liquid separation is performed by pressure filtration or centrifugation, and the moisture content of the pretreated biomass solids is controlled at 40-60%.

[0030] In the technical solution of this application embodiment, solid-liquid separation is a key step connecting the pretreatment and carbonization units. Pressure filtration or centrifugation can achieve solid-liquid separation quickly and efficiently. Controlling the moisture content of the pretreated biomass solids at 40-60% has the following technical considerations: when the moisture content is below 40%, the pressure filtration or centrifugation time needs to be extended, reducing processing efficiency; when the moisture content is above 60%, excessive water entering the subsequent carbonization unit will significantly increase carbonization energy consumption, and at the same time, a large amount of water vapor dilutes the pyrolysis gas, reducing the calorific value of the fuel gas. Controlling the moisture content at 40-60% ensures both the rate of solid-liquid separation and controls the energy consumption of the subsequent carbonization unit, achieving optimal matching between units.

[0031] Furthermore, in some embodiments, the high-pressure superheated steam carbonization step includes: performing a first high-pressure superheated steam carbonization treatment at 2.0-3.0 MPa; and performing a second high-pressure superheated steam carbonization treatment at 3.0-4.0 MPa.

[0032] In the technical solution of this application embodiment, the use of a two-stage pressure-progressive carbonization process has specific technical significance. This process employs two-stage pressure optimization and control: first, a conversion activation treatment is performed at 2.0–3.0 MPa, transforming organically bound and ion-exchangeable K and Na in biomass into water-soluble and migratory states. During this stage, through steam permeation, hydrolysis, and swelling, the binding between alkali metals and the organic structure and ash minerals of biomass is weakened, making it easier for them to enter the liquid phase or migrate with steam. Subsequently, the condensate and tail gas enriched with alkali metals are promptly discharged through steam exhaust, liquid drainage, or instantaneous pressure reduction. Then, the pressure is increased to 3.0–4.0 MPa for enhanced stripping carbonization treatment, further promoting the migration of residual alkali metals to the particle surface and pore channels, and discharging them with steam tail gas and condensate. This achieves a segmented dealkali removal process of "conversion first, discharge first, then enhanced migration," which is more effective than single-pressure carbonization for the directional removal of alkali metals. If a one-step pressurization followed by direct blasting is used, although the instantaneous depressurization can disrupt the pore structure of biomass and promote the expulsion of some migratable components, the lack of a low-pressure conversion and activation process in the early stage means that some K and Na may still exist in organically bound, ion-exchangeable, or stable inorganic salt forms, not yet fully converted into water-soluble and migratable forms. In this case, direct blasting only plays a role in physical fragmentation and pore expansion, and it is difficult to promote the chemical transformation of alkali metals. This easily leads to alkali metals remaining in the carbonized solid phase or inside the ash, resulting in large fluctuations in removal efficiency. Therefore, adopting a "two-stage pressure control + phased discharge" mode can form a continuous synergy between alkali metal transformation, migration and enrichment, and final discharge, achieving efficient and controllable targeted regulation of alkali metals.

[0033] Furthermore, in some embodiments, the temperature of the first high-pressure superheated steam carbonization treatment is 350-400°C and the time is 30-40 min; the temperature of the second high-pressure superheated steam carbonization treatment is 400-450°C and the time is 20-30 min.

[0034] In the technical solutions of this application embodiment, the aforementioned temperatures all refer to the reaction temperature of the materials inside the reactor. The first high-pressure superheated steam carbonization treatment is maintained at 2.0-3.0 MPa and 350-400°C for 30-40 minutes, allowing the material to fully heat and carbonize, forming initial pores, while simultaneously promoting the conversion of weakly bound K and Na into migratable states. After the first stage, the migrated alkali metals are removed through steam and liquid venting. The second high-pressure superheated steam carbonization treatment is maintained at 3.0-4.0 MPa and 400-450°C for 20-30 minutes, utilizing higher pressure and temperature to enhance mass transfer within the particles, and limiting excessive gasification and fixed carbon loss with a shorter residence time.

[0035] Furthermore, in some embodiments, after the high-pressure superheated steam carbonization is completed, a flash explosion treatment is also included. The flash explosion treatment steps include: depressurizing the material obtained from the high-pressure superheated steam carbonization to atmospheric pressure, and then sequentially entering a jacketed flash explosion tube and a cyclone separator; the inner wall temperature of the jacketed flash explosion tube is 300-350°C.

[0036] In the technical solution of this application embodiment, flash explosion treatment is a key unit for achieving ultrafine grinding and gas-solid separation of materials. Its principle is as follows: when the high-temperature, high-pressure carbonization product is instantly depressurized to atmospheric pressure, the high-pressure superheated steam and water vapor in the closed pores inside the carbon particles expand rapidly, generating a powerful explosive impact force that instantly tears and pulverizes the carbon skeleton into ultrafine powder. This process has a grinding efficiency far higher than traditional mechanical grinding, and it does not introduce grinding media, resulting in high product purity. The inner wall temperature of the jacketed flash explosion tube is maintained at 300-350℃ to prevent a sudden temperature drop in the carbon-based skeleton due to adiabatic cooling during the depressurization and expansion process, and to avoid water vapor condensing and adsorbing on the carbon surface at excessively low temperatures, thus blocking the newly developed microporous structure; simultaneously, maintaining a higher temperature is beneficial for the further removal of residual volatiles. The material after flash explosion undergoes gas-solid separation via a cyclone separator to obtain a low-alkali biochar powder product.

[0037] Furthermore, in some embodiments, the fixed carbon content of the low-alkali biochar is ≥60%, and the total mass fraction of K and Na in the ash is ≤2.0%.

[0038] In the technical solutions of this application embodiment, the above-mentioned product indicators reflect the technical advantages of this process. A fixed carbon content of ≥60% indicates that biomass has achieved deep carbonization, and the fuel quality and carbon sequestration stability are significantly improved; the total mass fraction of K and Na in the ash is ≤2.0%, which is a key indicator for meeting the requirements of high-end application scenarios such as blast furnace injection for alkali metal content. Excessive alkali metal content in blast furnace injection carbon powder will lead to accelerated corrosion of refractory materials in the blast furnace lining and a decrease in coke strength. This application controls the alkali metal content below the safe threshold through a two-stage dealkali removal synergy of low-grade steam pretreatment and high-pressure superheated steam carbonization, opening up a path for the high-value application of biomass char.

[0039] Furthermore, in some embodiments, the process further includes evaporation and crystallization of the alkali-rich condensate. The evaporation and crystallization process includes evaporating and crystallizing the alkali-rich condensate under negative pressure and at 80-100°C to obtain an alkali metal salt product and evaporated condensate. The operating pressure of the evaporation and crystallization unit is 10-30 kPa; the alkali metal salt product contains at least one of KCl, K₂SO₄, NaCl, and Na₂SO₄, and the purity of the alkali metal salt in the product is ≥85%.

[0040] In the technical solution of this application embodiment, the evaporation and crystallization treatment of alkali-rich condensate achieves the resource recovery of alkali metals from the alkali removal end. The alkali-rich condensate mainly contains potassium (K) dissolved from biomass. + Na + And with Cl after neutralization - SO4 2- Anions are generated through negative pressure evaporation and crystallization, during which water is evaporated and separated, and alkali metal salts (KCl, K2SO4, NaCl, and Na2SO4, etc.) crystallize out. The negative pressure operation (10-30 kPa) effectively lowers the boiling point of water, controlling the evaporation temperature at 80-100℃. The system's low-grade waste heat can be utilized to drive the evaporation, significantly reducing energy consumption. The purity of the alkali metal salts obtained from evaporation and crystallization is ≥85%, and they can be sold as industrial-grade potassium and sodium salt products or further refined for use. The evaporation condensate can be reused as makeup water for the low-grade steam pretreatment unit, achieving water resource recycling. Through the above treatment, this application achieves low-alkali carbonization of biomass while transforming alkaline condensate, which would otherwise be a burden on wastewater treatment, into valuable alkali metal salt products, realizing the resource utilization of waste.

[0041] In a second aspect, embodiments of this application provide a biomass alkali metal directional control system for implementing the method described in the first aspect, comprising, in sequence, a crushing unit, a low-grade steam pretreatment unit, a high-pressure superheated steam carbonization unit, a gas-solid separation unit, and a salt recovery unit connected by pipelines. The system includes: a pulverizing unit for pulverizing biomass raw materials to 1-5mm; a low-grade steam pretreatment unit with a stirring device and temperature control module, and a solid-liquid separation device for performing low-grade steam pretreatment steps; a high-pressure superheated steam carbonization unit, including a fixed-bed pressure reactor and a connected heat supply unit, which provides superheated steam at 2.0-4.0MPa and 350-550℃ for performing high-pressure superheated steam carbonization steps; a gas-solid separation unit, including a jacketed flash explosion tube connected to the reactor outlet via a release valve and a cyclone separator; and a salt recovery unit, including a negative pressure evaporator crystallizer and a steam compression and condensation assembly for recovering alkali metal salts and circulating condensate.

[0042] In the technical solution of this application embodiment, the units of the above system are connected in series according to the material flow direction of "crushing → pretreatment → carbonization → flash explosion → salt recovery", forming a complete alkali metal directional control and carbonization preparation production line. The low-grade steam pretreatment unit ensures full contact between the material and steam / condensate through an internal stirring device, and the temperature control module achieves precise temperature control of 60-100℃; the supporting solid-liquid separation device (filter press or centrifuge) realizes rapid dehydration of the pretreated material to obtain pretreated biomass solids and alkali-rich condensate, of which the alkali-rich condensate is sent to the salt recovery unit. The high-pressure superheated steam carbonization unit adopts a fixed-bed pressure reactor, which is suitable for batch production. The heat supply unit converts the system waste heat and self-produced cracked gas into superheated steam of 2.0-4.0MPa and 350-550℃ to supply the reactor. After high-pressure superheated steam carbonization is completed, the reactor outlet is connected to the gas-solid separation unit via a release valve. The material is instantly depressurized to atmospheric pressure and then enters a jacketed flash explosion tube. The flash explosion effect is used to tear and pulverize the carbon skeleton into micron-sized powder. The inner wall temperature of the jacketed flash explosion tube is maintained at 300-350℃ to prevent water vapor from condensing and adsorbing on the carbon surface during the depressurization expansion process due to adiabatic cooling, which would block the newly developed microporous structure. The material after flash explosion enters the cyclone separator with the steam to complete the gas-solid separation and obtain low-alkali biochar powder product. The vapor phase product is condensed and sent to the salt recovery unit. The salt recovery unit includes a negative pressure evaporator crystallizer and a vapor compression condenser assembly. Alkali-rich condensate and flash explosion vapor phase condensate enter the negative pressure evaporator crystallizer, where they evaporate and crystallize under conditions of 10-30 kPa and 80-100℃. Alkali metal salts (KCl, K₂SO₄, NaCl, and Na₂SO₄, etc.) crystallize and precipitate, with a purity ≥85%, which can be sold as industrial-grade products or further refined for use. The secondary steam generated during evaporation is condensed by the vapor compression condenser assembly to obtain evaporation condensate, which can be reused as makeup water for the low-grade steam pretreatment unit, achieving water resource recycling. Material transport between units uses closed pipelines, preventing dust escape and loss of alkali metal intermediates, thus achieving clean production.

[0043] It can be noted that in some embodiments, each key unit adopts a modular parallel channel structure with at least two independent branches, and achieves alternating feeding and discharging and continuous production through automatic control valve groups. Any branch can be cut off for maintenance without stopping the machine.

[0044] Example 1 This embodiment provides a method for the targeted regulation of alkali metals during biomass carbonization, which specifically includes the following steps: S1. Select agricultural waste reed stalks (alkali metal K+Na content is 3.26%) and crush them to a particle size of 1-2 mm through the crushing unit.

[0045] S2. The crushed reed stalks are fed into a low-grade steam pretreatment unit at a temperature of 65℃, a liquid-to-solid mass ratio of 10:1, a pretreatment time of 60 min, and a stirring speed of 150 r / min. The low-grade steam is supplied with waste heat from high-pressure carbonization tail gas as the heat source. After treatment, solid-liquid separation is performed by pressure filtration to obtain pretreated reed stalks and alkali-rich condensate. The obtained pretreated reed stalks have a solid water content of 47.8% and a K+Na removal rate of 18.6%. The alkali-containing condensate is collected for later use.

[0046] S3. The pretreated reed straw is transferred to a fixed-bed pressure reactor for high-pressure superheated steam carbonization. Superheated steam at 2.4 MPa and 506℃ is introduced, controlling the reaction temperature at 357℃, the pressure at 2.4 MPa, and the reaction time at 36 minutes. After carbonization, the pressure is rapidly released through a release valve, and the material enters a jacketed flash explosion tube (the jacket is vented with 454℃ combustion exhaust gas, and the inner wall temperature is 307℃) and a cyclone separator to complete gas-solid separation, yielding reed biochar.

[0047] The obtained reed biochar has a fixed carbon content of 62.5%, a total K+Na mass fraction of 1.6% in the ash, a K+Na removal rate of 68.7%, and a particle size D50 of about 1.2 mm, which meets the quality requirements for biochar used in blast furnace injection.

[0048] The alkali-rich condensate obtained in step S2 is fed into a negative pressure evaporator crystallizer at an operating pressure of 18 kPa and a temperature of 88°C. Evaporation and crystallization yield alkali metal salt products mainly composed of KCl and NaCl (purity of 87.4%). The condensate is returned to the hydrothermal elution unit for recycling.

[0049] Example 2 This embodiment provides a method and system for targeted control of alkali metals during biomass carbonization, specifically including the following steps: S1. Select bamboo husks (containing 2.84% alkali metals K and Na) from bamboo processing waste and crush them to a particle size of 2-4 mm through a crushing unit.

[0050] S2. The pulverized bamboo husks are fed into a low-grade steam pretreatment unit at 82℃, a liquid-to-solid mass ratio of 8:1, a time of 75 min, and a stirring speed of 120 r / min. After treatment, solid-liquid separation is performed by pressure filtration to obtain pretreated bamboo husks and alkaline-rich condensate. The resulting pretreated bamboo husks have a moisture content of 52.3% and a K+Na removal rate of 24.7%. The alkaline condensate is collected for later use.

[0051] S3. The pretreated bamboo husks are transferred to a fixed-bed pressure reactor for high-pressure superheated steam carbonization. 2.8 MPa, 520℃ superheated steam is introduced, the reaction temperature is controlled at 375℃, the carbonization time is 40 min, the gas-solid separation jacket temperature is 470℃, and the inner wall temperature is 320℃, ultimately yielding bamboo husk biochar.

[0052] S4, bamboo husk biochar has a fixed carbon content of 65.8%, a total K+Na mass fraction of 1.4% in ash, a K+Na removal rate of 73.9%, and a particle size D50 of approximately 1.8 mm.

[0053] S5. The evaporation and crystallization operation pressure is 22 kPa and the temperature is 92℃. The resulting alkali metal salt product has a high K2SO4 content and a purity of 86.6%.

[0054] Example 3 This embodiment provides a method and system for targeted control of alkali metals during biomass carbonization, specifically including the following steps: S1. Select eucalyptus wood processing residue (alkali metal K+Na content is 1.68%) and crush it to a particle size of 4-5 mm.

[0055] S2. The temperature was controlled at 98℃, the liquid-to-solid mass ratio at 12:1, the pretreatment time at 45 min, and the stirring speed at 100 r / min. After treatment, solid-liquid separation was performed by pressure filtration to obtain pretreated eucalyptus wood processing residue and alkali-rich condensate. The moisture content of the pretreated eucalyptus wood processing residue was 57.2%, and the K+Na removal rate was approximately 13.8%. The alkali-rich condensate was collected for later use.

[0056] S3. Introduce 3.8 MPa, 540℃ superheated steam, control the reactor temperature at 420℃, and maintain pressure for carbonization for 25 minutes. Volatile alkali metals are discharged with the high-pressure carbonization tail gas and recovered by a downstream condensation and collection device. The jacket temperature is 490℃, and the inner wall temperature is 340℃.

[0057] S4. The evaporation and crystallization operation pressure is 12 kPa and the temperature is 83℃. The resulting alkali metal salt is mainly KCl with a purity of ≥88.2%.

[0058] S5, eucalyptus biochar has a fixed carbon content of 68.2%, a total K+Na mass fraction of 1.1% in ash, a K+Na removal rate of 69.4%, and a particle size D50 of approximately 2.5 mm. It is the product with the highest fixed carbon content among the three sets of examples and is suitable as a main alternative carbon source for blast furnace injection.

[0059] Example 4 This embodiment provides a method and system for targeted control of alkali metals during biomass carbonization, specifically including the following steps: S1. Take a mixture of 40 wt% straw powder, 30 wt% sawdust, and 30 wt% rice husks (containing 3.61% alkali metals K and Na) and crush it to 2-3 mm.

[0060] S2. Control temperature 80℃, liquid-to-solid mass ratio 10:1, pretreatment time 60 min. After pressure filtration separation, the moisture content of the pretreated mixed agricultural and forestry residues is 50.6%, and the comprehensive removal rate of K+Na is 21.5%. The system uses parallel channel A for processing, with channel B as backup.

[0061] S3, superheated steam at 3.2 MPa and 520℃ was introduced, and the reaction temperature was controlled at 385℃. Carbonization was carried out under pressure for 40 min. The heat supply unit used recovered pyrolysis gas and biomass oil as fuel, with external natural gas supplementation at 17.3%, verifying the system's heat self-sufficiency.

[0062] S4. The evaporation and crystallization operation pressure is 20 kPa, the temperature is 90℃, and the purity of the alkali metal salt product is 85.7%; the condensate is recycled, and the system water replenishment is about 8% of the evaporation loss.

[0063] S5. During the operation of route A, route B was scheduled for maintenance and put back into operation. The automatic control valve group achieved a smooth switch, and the overall system did not experience any downtime, verifying the reliability of continuous production of modular parallel channels.

[0064] S6. The fixed carbon content of the mixed agricultural and forestry residue biochar is 63.0%, the total mass fraction of K+Na in the ash is 1.8%, the K+Na removal rate is 71.8%, and the particle size D50 is about 1.6 mm. All indicators meet the requirements of blast furnace injection, indicating that the present invention has good adaptability to complex mixed raw materials.

[0065] Example 5 This embodiment provides a method and system for directional control of alkali metals during biomass carbonization. Compared with Embodiment 1, the difference lies in the implementation of a two-stage pressure condition in the high-pressure superheated steam carbonization treatment step. The remaining steps are the same as in Embodiment 1 and will not be described again here.

[0066] In this embodiment, step S3 involves transferring the pretreated reed straw to a fixed-bed pressure reactor for sequential high-pressure superheated steam carbonization treatment and second high-pressure superheated steam carbonization treatment.

[0067] Specifically, in the first stage, superheated steam at 2.4 MPa and 506℃ is introduced to control the reaction temperature of the material inside the reactor at 357℃ and the pressure at 2.4 MPa, and this is maintained for 36 minutes. After the first stage, the steam and liquid vent valve is opened for 90 seconds to reduce the pressure inside the reactor to 2.0 MPa and discharge the alkali-rich condensate. Subsequently, the valve is closed and superheated steam at 3.6 MPa and 535℃ is introduced to control the reaction temperature of the material inside the reactor at 425℃ and the pressure at 3.6 MPa, and this is maintained for 25 minutes, completing the second high-pressure superheated steam carbonization treatment.

[0068] According to the test results, the fixed carbon content of the reed biochar obtained in this embodiment is 67.2%, the total mass fraction of K+Na in the ash is 1.2%, the K+Na removal rate of the whole process calculated based on the absolute mass balance of K+Na in the feed and discharge is 78.6%, and the particle size D50 is about 1.0 mm.

[0069] Compared with Example 1, the two-stage pressure progressive treatment increased the fixed carbon content by 4.7 percentage points, reduced the total K+Na mass fraction in the ash by 0.4 percentage points, and increased the K+Na removal rate of the whole process by 9.9 percentage points, indicating that the dissolution and staged discharge in the first stage can provide a mass transfer channel for the enhanced migration in the second stage.

[0070] Comparative Example 1 This comparative example provides a method and system for directional control of alkali metals during biomass carbonization. Compared with Example 1, the only difference is that the low-grade steam pretreatment under normal pressure in step S2 is omitted. The remaining steps are the same as in Example 1 and will not be repeated here.

[0071] That is, in this comparative example, the crushed reed straw obtained in step S1 of Example 1 is directly fed into the fixed-bed pressure reactor in step S3 of Example 1, and then subjected to high-pressure superheated steam carbonization treatment according to the corresponding parameters in step S3 of Example 1 to obtain reed biochar.

[0072] Testing revealed that the fixed carbon content of the reed biochar obtained in this comparative example was 60.8%, the total K+Na mass fraction in the ash was 2.7%, the overall K+Na removal rate calculated based on the absolute mass balance of K+Na in the feed and discharge was 52.1%, and the particle size D50 was approximately 1.3 mm. Compared with Example 1, the overall K+Na removal rate of this comparative example decreased by 16.6 percentage points, the total K+Na mass fraction in the ash increased by 1.1 percentage points, and the fixed carbon content decreased by 1.7 percentage points. This indicates that the low-grade steam pretreatment not only removed 18.6% of the total K+Na from the raw material, but its wetting, swelling, and preliminary pore-opening effects also improved the mass transfer conditions in the subsequent high-pressure superheated steam carbonization stage.

[0073] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for targeted control of alkali metals during biomass carbonization, characterized in that, Includes the following steps: The biomass feedstock is pretreated with low-grade steam and then separated into solid and liquid components to obtain pretreated biomass solids and alkali-rich condensate. The pretreated biomass solids are subjected to high-pressure superheated steam carbonization to obtain low-alkali biochar; The temperature for the low-grade steam pretreatment is 60-100℃; During the carbonization process using high-pressure superheated steam, the pressure of the high-pressure superheated steam is 2.0-4.0 MPa, and the temperature is 350-550℃.

2. The method for directional control of alkali metals during biomass carbonization according to claim 1, characterized in that, Before pretreating the biomass raw material with low-grade steam, the process also includes crushing the biomass raw material to a particle size of 1-5 mm.

3. The method for targeted regulation of alkali metals during biomass carbonization according to claim 1, characterized in that, The low-grade steam pretreatment time is 30-90 min, and the liquid-solid mass ratio is 5:1-15:1; and / or, The heat source for the low-grade steam pretreatment comes from fuel, which includes combustible pyrolysis gas and biomass oil generated by the high-pressure superheated steam carbonization.

4. The method for targeted regulation of alkali metals during biomass carbonization according to claim 1, characterized in that, The solid-liquid separation is performed by pressure filtration or centrifugation, and the moisture content of the pretreated biomass solids is controlled at 40-60%.

5. The method for directional control of alkali metals during biomass carbonization according to claim 1, characterized in that, The high-pressure superheated steam carbonization step includes: The first high-pressure superheated steam carbonization treatment is carried out at 2.0-3.0 MPa; A second high-pressure superheated steam carbonization treatment is carried out at 3.0-4.0 MPa.

6. The method for targeted regulation of alkali metals during biomass carbonization according to claim 5, characterized in that, The temperature of the first high-pressure superheated steam carbonization treatment is 350-400℃, and the time is 30-40 minutes; The second high-pressure superheated steam carbonization treatment is carried out at a temperature of 400-450℃ for 20-30 minutes.

7. The method for targeted regulation of alkali metals during biomass carbonization according to claim 1, characterized in that, After the high-pressure superheated steam carbonization is completed, a flash explosion treatment is also included; The flash explosion treatment steps include: depressurizing the material obtained by carbonization with high-pressure superheated steam to atmospheric pressure, and then sequentially entering the jacketed flash explosion tube and cyclone separator; the inner wall temperature of the jacketed flash explosion tube is 300-350℃.

8. The method for directional control of alkali metals during biomass carbonization according to claim 1, characterized in that, The low-alkali biochar has a fixed carbon content of ≥60% and a total mass fraction of K and Na in its ash of ≤2.0%.

9. The method for directional control of alkali metals during biomass carbonization according to claim 1, characterized in that, It also includes the evaporation and crystallization treatment of the alkaline-rich condensate; The evaporation crystallization process includes: evaporating and crystallizing the alkali-rich condensate at 80-100℃ to obtain alkali metal salt products and evaporated condensate; The evaporation and crystallization unit operates at a pressure of 10-30 kPa; the alkali metal salt product contains at least one of KCl, K2SO4, NaCl and Na2SO4, and the purity of the alkali metal salt in the alkali metal salt product is ≥85%.

10. A biomass alkali metal targeted regulation system for implementing the method of any one of claims 1-9, characterized in that, Including those connected sequentially via pipes: A low-grade steam pretreatment unit is used to perform the low-grade steam pretreatment. A high-pressure superheated steam carbonization unit is used to perform the high-pressure superheated steam carbonization step.