A low-sulfur, low-nitrogen green carbon raiser and its production method

Abstract

This invention discloses a low-sulfur, low-nitrogen green carbon raiser and its production method, relating to the field of metallurgical materials technology. The carbon raiser is prepared from lignocellulosic waste as raw material through multi-stage gradient pyrolysis, directional catalytic cracking, and inert atmosphere cooling. The method includes: raw material pretreatment; primary low-temperature pyrolysis; secondary medium-temperature pyrolysis; tertiary high-temperature graphitization; directional catalytic cracking; closed-loop tail gas purification and resource recovery; and inert atmosphere cooling and sieving. This invention is low in sulfur and nitrogen, possesses a highly ordered graphite microcrystalline structure, and incorporates [a process involving the introduction of...] CeO2–ZrO2 / γ‑Al2O3 The catalyst selectively cracks C–N / C–S bonds, and is equipped with a closed-loop exhaust gas purification system to achieve near-zero emissions. It has the advantages of high carbon yield, ultra-clean impurity control and green manufacturing throughout the entire life cycle, solving the problems of high sulfur and nitrogen impurities and high carbon emissions in the preparation process of traditional carbon additives.

Description

Technical Field

[0001] This invention relates to the field of metallurgical materials technology, and in particular to a low-sulfur, low-nitrogen green carbon raiser and its production method. Background Technology

[0002] In the modern high-end metallurgical industry, the manufacturing of special steels and high-performance cast iron parts places unprecedentedly stringent requirements on the purity of raw materials. As a key metallurgical auxiliary material, recarburizers play a crucial role in regulating carbon content and optimizing microstructure and properties during the refining of molten steel or iron. However, traditional recarburizers generally contain high levels of harmful impurities such as sulfur and nitrogen. Once these elements enter the molten metal system, they can easily cause metallurgical quality problems such as hot brittleness, age embrittlement, porosity defects, and inclusion aggregation, severely restricting the mechanical properties, processing performance, and service life of high-end metal materials. Therefore, developing green recarburizers with ultra-low sulfur and ultra-low nitrogen content and high carbon yield has become a key link in supporting the upgrading of clean steel smelting technology and achieving strategic goals. In this context, "green recarburizers" not only need to meet the basic requirement of component purity, but should also incorporate the environmental protection concept of the entire life cycle, including raw material renewability, low-carbon production processes, near-zero emissions of pollutants, and efficient recycling of resources, thus truly meeting the inherent needs of the modern metallurgical industry for sustainable and high-quality development.

[0003] Current technological systems face a dilemma in addressing the demand for ultra-low sulfur and ultra-low nitrogen green carbon additives: physical purification methods are limited by fundamental bottlenecks and cannot deeply remove chemical sulfur and nitrogen impurities; while smelting-integrated methods neglect the green manufacturing aspects of the material itself, making it difficult to support sustainable development goals. A deeper technological contradiction lies in the fact that achieving ultra-clean control of sulfur ≤0.05% and nitrogen ≤200ppm requires overcoming the limitations of single physical or chemical treatments and constructing a deep purification mechanism capable of accurately identifying and efficiently breaking down C–N and C–S chemical bonds in the carbon matrix. Simultaneously, this mechanism must be organically integrated with low-carbon raw material systems, energy-saving thermal regimes, and closed-loop pollution control systems to ensure ultimate purity while achieving green and large-scale production. The essence of this contradiction is the synergistic challenge between maximizing material purity and reducing the environmental impact of the manufacturing process, representing a theoretical and engineering gap that current technological paradigms struggle to overcome. Therefore, how to develop a carbon raiser that can achieve ultra-deep removal of sulfur and nitrogen impurities and has green attributes throughout its entire life cycle, and its industrial production method, through innovative design integrating raw materials, processes, and environmental protection, has become a key challenge and an urgent technical problem to be solved by those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a low-sulfur, low-nitrogen green carbon raiser and its production method, so as to solve the problems of high sulfur and nitrogen impurities in existing carbon raisers and high carbon emissions during the preparation process.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] On the one hand, a low-sulfur, low-nitrogen green carbon additive includes:

[0007] The carbon enrichment agent is prepared from a biomass-based carbon source through a multi-stage gradient pyrolysis, directional catalytic cracking, and inert atmosphere cooling process; the carbon enrichment agent has a fixed carbon content ≥95.0%, a sulfur content ≤0.05%, a nitrogen content ≤200ppm, an ash content ≤2.0%, and a volatile matter content ≤1.5%; the biomass-based carbon source is lignocellulosic waste, including but not limited to one or more mixtures of fruit shells, straw, sawdust, and bamboo shavings, with an initial moisture content controlled between 8% and 12% and a particle size range of 3mm to 8mm; the microstructure of the carbon enrichment agent is arranged in a layered graphite microcrystal arrangement, and the half-width at half-maximum (WHM) of the (002) crystal plane diffraction peak in the X-ray diffraction pattern is ≤0.45°, indicating that it has a highly ordered carbon skeleton structure, which is beneficial for rapid dissolution in molten metal and improving carbon yield.

[0008] On the other hand, a method for producing a low-sulfur, low-nitrogen green carbon raiser includes the following steps:

[0009] Step S1, raw material pretreatment: After drying, crushing and sieving, the biomass-based carbon source is used to obtain primary carbonaceous particles with uniform particle size and stable moisture content.

[0010] Step S2, primary low-temperature pyrolysis: The primary carbonaceous particles are fed into the first pyrolysis furnace and kept at a temperature range of 300℃ to 400℃ under a nitrogen atmosphere for 60 to 90 minutes. This allows hemicellulose and some cellulose to undergo dehydration and decarboxylation reactions, releasing primary volatiles mainly composed of CO2, H2O and a small amount of organic acids, while retaining the lignin skeleton structure.

[0011] Step S3, Secondary Medium-Temperature Pyrolysis: The primary pyrolysis product is transferred to the second pyrolysis furnace and kept at a temperature of 500°C to 600°C under a nitrogen atmosphere for 40 to 70 minutes to promote the lignin backbone to break down and generate aromatic tar precursors, while simultaneously removing some bound nitrogen and sulfur compounds.

[0012] Step S4, tertiary high-temperature graphitization: The secondary pyrolysis products are sent to the third pyrolysis furnace and kept at a temperature range of 1200℃ to 1400℃ under an argon atmosphere for 30 to 60 minutes to induce carbon atom rearrangement to form a local graphite microcrystalline structure, while promoting the thermodynamic instability breakage of the residual C–N and C–S chemical bonds.

[0013] Step S5, Directed Catalytic Cracking: A supported metal oxide catalyst is simultaneously introduced during the three-stage high-temperature graphitization stage. The catalyst is a CeO2–ZrO2 composite oxide supported on a γ-Al2O3 support, wherein the molar ratio of CeO2 to ZrO2 is 3:1, the total loading is 8% of the catalyst support mass, and the catalyst particle size is 0.5 mm to 1.2 mm. It is placed in a fixed bed in the pyrolysis gas flow channel. The catalyst activates H2O and CO2 in the pyrolysis gas through the oxygen vacancy mechanism, generating active oxygen species in situ. These species selectively attack thiophene and pyridine heterocyclic compounds on the surface and in the pores of the carbon matrix, causing them to decompose into small molecule gases such as H2S and NH3, which are then discharged with the gas phase.

[0014] Step S6, Inert atmosphere cooling and sieving: The high-temperature carbon material after catalytic cracking is cooled to below 80°C at a rate of ≤5°C / min under argon protection to prevent secondary oxidation and nitrogen adsorption. Then, it is sieved by vibration to remove fine powder with a particle size of less than 1mm to obtain carbon raiser finished product with a target particle size of 1mm to 5mm.

[0015] Step S7, Closed-loop exhaust gas purification and resource recovery: The mixed exhaust gas generated during each stage of pyrolysis and catalytic cracking is sequentially passed through a cyclone dust collector, an alkaline spray tower, an activated carbon adsorption column, and a condensation recovery unit. The cyclone dust collector is used to capture particles larger than 10 μm. The alkaline spray tower uses a 5% NaOH solution with a pH maintained between 10.5 and 11.5 to absorb H2S, SO2, and organic acid vapors. The activated carbon adsorption column is filled with coconut shell-based activated carbon with an iodine value ≥900 mg / g to adsorb residual tar vapors and volatile organic compounds. The condensation recovery unit operates within a temperature range of -10℃ to 5℃, recovering liquid tar and water. The tar is reused for catalyst regeneration and combustion support, and the water, after neutralization, is reused for alkaline solution preparation, achieving internal water resource recycling.

[0016] In a preferred embodiment of the present invention, the first, second, and third pyrolysis furnaces are all continuous rotary kiln structures with a furnace body tilt angle of 3° to 5° and a rotation speed of 0.5 rpm to 2.0 rpm. The material residence time is precisely controlled by adjusting the rotation speed and feed rate. Each furnace section is connected by an airtight rotary valve to ensure continuous inert atmosphere and prevent air infiltration that could lead to localized combustion or nitrogen oxide generation. The outer wall of the furnace is wrapped with a 150 mm thick nano-aerogel insulation layer with a thermal conductivity ≤0.020 W / (m·K), significantly reducing heat loss. The heating system adopts a coupled energy supply mode of electric heating and biomass gas, wherein the biomass gas is derived from non-condensable combustible gas recovered by the tail gas purification system, and enters the combustion chamber after desulfurization and denitrification, with a thermal efficiency ≥85%, achieving an energy self-sufficiency rate of not less than 60%.

[0017] In a preferred embodiment of the present invention, the CeO2–ZrO2 / γ-Al2O3 catalyst is calcined at 550°C in air for 4 hours before use to stabilize its crystal structure. After a 120-hour usage period, it is regenerated at 450°C for 2 hours by introducing nitrogen gas containing 5% O2 to restore the oxygen vacancy concentration. The regeneration waste gas is introduced into an alkaline spray tower for unified treatment. The catalyst is replaced as a whole after deactivation, and the waste catalyst is acid-leached to recover rare earth elements. The residue is used as a ceramic glaze additive for resource utilization.

[0018] In a preferred embodiment of the present invention, the finished carbon raiser is packaged using a double-layer moisture-proof aluminum-plastic composite film, with an inner layer of polyethylene heat-sealing layer and an outer layer of aluminum foil barrier layer, and a water vapor permeability ≤0.5g / (m²). 2 • 24h), oxygen permeability ≤ 5cm 3 / (m 2 • 24h • 0.1MPa) to ensure that the product does not absorb moisture, oxidize, or adsorb nitrogen from the environment during storage and transportation.

[0019] The core innovation of the low-sulfur and low-nitrogen green carbon additive production method described in this invention lies in the deep integration of the intrinsic low-carbon properties of biomass raw materials, the structural regulation capability of multi-level gradient pyrolysis, the precise chemical bond breaking mechanism of directional catalytic cracking, and a closed-loop pollution control system, forming a green manufacturing paradigm with four-dimensional synergy of "raw materials-structure-impurities-emissions". Specifically, lignocellulosic biomass itself does not contain fossil sulfur or nitrogen pollutants. Its nitrogen mainly exists in the form of protein, and its sulfur content is extremely low (usually <0.1%), thus avoiding the inherent high sulfur and high nitrogen defects of traditional carbon sources such as petroleum coke and anthracite from the source. Multi-stage gradient pyrolysis avoids the collapse of carbon structure and impurity burial caused by violent pyrolysis by controlling temperature and atmosphere in stages, creating favorable pore channels for subsequent deep deimpurification. Directional catalytic cracking utilizes the strong redox ability of CeO2–ZrO2 composite oxides to selectively crack stubborn C–N and C–S bonds under non-combustion conditions, rather than relying on high-temperature incineration or strong acid and alkali treatment, which avoids secondary pollution and preserves the integrity of the carbon skeleton. The closed-loop tail gas purification system ensures that all process waste gas, wastewater, and solid waste are effectively captured and recycled, achieving near-zero emissions.

[0020] Furthermore, the carbon refining agent described in this invention exhibits excellent metallurgical performance in practical metallurgical applications. When added to molten steel or iron in an electric arc furnace or induction furnace at an addition ratio of 0.5 kg / t to 2.0 kg / t, the carbon recovery rate is consistently between 92% and 96% at a smelting temperature of 1550°C to 1650°C, significantly higher than that of commercially available petroleum coke-based carbon refining agents (typically 85% to 90%). Due to extremely low sulfur and nitrogen impurities, the desulfurization burden on molten steel is reduced, and the LF refining time is shortened by 15% to 20%. The porosity in castings decreases to below 0.05%, and the inclusion rating meets the ASTM E45 standard Class A ≤ 0.5 and Class B ≤ 0.5. Mechanical property tests show that QT600-3 ductile iron produced using the carbon refining agent of this invention has a tensile strength ≥ 620 MPa, elongation ≥ 4.5%, and impact energy ≥ 12 J, fully meeting the technical specifications for high-end engineering machinery and wind turbine gearbox components.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0022] This invention achieves the controllable preparation of low-sulfur and low-nitrogen green carbon raisers through the systematic integration of raw material renewability, process gradient, precise catalysis, and closed-loop emission control. It stably achieves low-sulfur and low-nitrogen technical indicators. The various technical elements—raw materials, processes, catalysts, cooling, and exhaust gas treatment—are necessary and synergistic, not only solving the extreme purity requirements of carbon raisers in high-end metallurgy but also demonstrating significant industrial application value and environmental benefits. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process flow for the production method of the low-sulfur, low-nitrogen green carbon raiser of the present invention.

[0024] Figure 2 This is a schematic diagram of the fixed bed arrangement and tail gas closed-loop purification system structure of the CeO2–ZrO2 / γ-Al2O3 catalyst used in this invention in the pyrolysis gas flow channel. Detailed Implementation

[0025] The features and exemplary embodiments of various aspects of the present invention will now be described in detail. To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely intended to explain the present invention and not to limit the present invention. For those skilled in the art, the present invention can be practiced without some of these specific details. The following description of the embodiments is merely to provide a better understanding of the present invention by illustrating examples of the invention.

[0026] This invention provides a low-sulfur, low-nitrogen green carbon additive and its production method. The core of this method lies in using lignocellulosic biomass waste as raw material, and through the synergistic effect of multi-stage gradient pyrolysis, directional catalytic cracking, and closed-loop tail gas purification, achieving ultra-clean control of sulfur and nitrogen impurities, and constructing a green manufacturing system with low carbonization and near-zero emissions throughout its entire life cycle. The following will provide a systematic, detailed, and reproducible engineering description of the technical solution of this invention, in conjunction with the accompanying drawings and several embodiments.

[0027] A low-sulfur, low-nitrogen green carbon raiser is prepared from a biomass-based carbon source through a multi-stage gradient pyrolysis, directional catalytic cracking, and inert atmosphere cooling process. The carbon raiser has a fixed carbon content of not less than 95.0%, a sulfur content of not more than 0.05%, a nitrogen content of not more than 200 ppm, an ash content of not more than 2.0%, and a volatile matter content of not more than 1.5%. Its microstructure exhibits a layered graphite microcrystal arrangement. In the X-ray diffraction (XRD) pattern, the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak does not exceed 0.45°, indicating that the carbon skeleton has a high degree of order, which is beneficial for rapid dissolution in high-temperature molten metal and for improving the carbon element recovery efficiency.

[0028] The biomass-based carbon source is lignocellulosic waste, specifically including one or more mixtures of fruit shells, straw, sawdust, and bamboo shavings. Before entering the pyrolysis system, the raw materials must be pretreated to precisely control their initial moisture content within the range of 8% to 12%, and their particle size within the range of 3 mm to 8 mm. This particle size and moisture content window is designed to ensure uniform heat transfer and controllable volatile matter release during pyrolysis, and to avoid a sharp increase in pyrolysis energy consumption due to excessive moisture or fluidization abnormalities caused by excessively fine particle size.

[0029] A method for producing a low-sulfur, low-nitrogen green carbon raiser comprises seven continuous and coupled process steps:

[0030] Step S1, Raw Material Pretreatment: The collected biomass raw materials are sequentially processed through drying, crushing, and screening. Drying is performed using an indirect heating drum dryer, with the hot air temperature controlled between 120℃ and 140℃ to ensure that the final moisture content of the material is stable within the target range. Crushing is performed using a twin-shaft shear crusher, and the output particle size is graded by a vibrating screen, retaining particles of 3mm to 8mm to remove excessively large or fine components, ensuring the consistency and controllability of the subsequent pyrolysis reaction.

[0031] Step S2, Primary Low-Temperature Pyrolysis: The pretreated primary carbonaceous particles are continuously fed into the first pyrolysis furnace. This furnace is a continuous rotary kiln structure, with a furnace body inclination angle of 4°, a rotation speed of 1.2 rpm, and a material residence time of approximately 75 minutes. A nitrogen atmosphere is maintained inside the furnace, with a gas flow rate of 80 Nm³. 3The furnace temperature is precisely controlled within the range of 300℃ to 400℃ per hour. Under these conditions, hemicellulose preferentially undergoes dehydration and decarboxylation reactions, releasing primary volatiles mainly composed of CO2, H2O, and small amounts of acetic acid and formic acid. The lignin skeleton, due to its higher thermal stability, is partially retained, forming a preliminary carbonized structure. The key at this stage is to avoid a sudden temperature rise that would lead to excessive tar formation or carbon densification, thus preserving porous channels for subsequent deeper impurity removal.

[0032] Step S3, Secondary Medium-Temperature Pyrolysis: The products from the primary pyrolysis are automatically transferred to the second pyrolysis furnace via an airtight rotary valve. This furnace also employs a continuous rotary kiln design, with an inclination angle of 4.5°, a rotation speed of 1.0 rpm, and the furnace temperature is raised to 500°C to 600°C. The nitrogen flow rate is adjusted to 60 Nm³. 3 The temperature is maintained at 40 to 70 minutes per hour. During this stage, the lignin backbone breaks down, generating tar precursors rich in aromatic ring structures. Simultaneously, some bound nitrogen in the form of pyrroles and indoles, as well as organic sulfur compounds in the form of thiols and thioethers, begin to be thermally desorbed. Since the temperature has not yet reached the threshold for severe pyrolysis, the carbon skeleton remains somewhat open, which facilitates the migration of impurity molecules into the gas phase.

[0033] Step S4, Tertiary High-Temperature Graphitization: The secondary pyrolysis products are transported to the third pyrolysis furnace, which uses argon as a protective atmosphere at a flow rate of 50 Nm³. 3 The furnace temperature is raised to 1200℃ to 1400℃ per hour, and held for 30 to 60 minutes. At this high temperature, amorphous carbon atoms rearrange themselves, forming local graphite microcrystalline structures, which is manifested as sharpening of the XRD (002) peak and narrowing of the full width at half maximum (FWHM). Simultaneously, the remaining C–N and C–S bonds break due to thermodynamic instability, releasing small molecule impurity gases such as NH3, HCN, and H2S. This stage is crucial for achieving ordered carbon structure and deep removal of impurities.

[0034] Step S5, Directed Catalytic Cracking: A supported metal oxide catalyst is simultaneously introduced during the three-stage high-temperature graphitization stage. This catalyst is a CeO2–ZrO2 composite oxide supported on a γ-Al2O3 support, with a CeO2 to ZrO2 molar ratio of 3:1 and a total loading of 8% of the support mass. The catalyst particles are pressed, granulated, and sieved to a particle size of 0.5 mm to 1.2 mm. They are then placed in a fixed bed within the pyrolysis gas flow channel, in the transition zone between the outlet of the third pyrolysis furnace and the cooling section. The catalyst activates H2O and CO2 in the pyrolysis gas through its abundant oxygen vacancies on its surface, generating ·OH and O2 in situ. -These reactive oxygen species selectively attack heterocyclic compounds such as thiophene, pyridine, and quinoline on the surface and within the micropores of the carbon matrix, causing them to undergo ring-opening cleavage into smaller molecules such as H2S and NH3, which are then expelled from the system in the gas phase. This process is completed under non-combustion conditions, avoiding carbon skeleton burn-off and achieving precise chemical removal of stubborn heteroatoms.

[0035] Step S6, Inert Atmosphere Cooling and Sieving: The high-temperature carbon material after catalytic cracking immediately enters the cooling section and is slowly cooled to below 80°C at a rate not exceeding 5°C / min under the protection of continuous argon gas. Strict control of the cooling rate aims to prevent the high-temperature carbon surface from adsorbing oxygen or nitrogen from the atmosphere during the cooling process, thereby avoiding secondary oxidation or the formation of cyanide compounds. After cooling, the material is processed by a double-layer vibrating sieving system. The upper sieve has a mesh size of 5mm, and the lower sieve has a mesh size of 1mm. Only products with a particle size range of 1mm to 5mm are retained as the finished carbon raiser. Fine powder smaller than 1mm is recycled for catalyst regeneration and combustion support or reused as a low-grade carbon source.

[0036] Step S7, Closed-loop exhaust gas purification and resource recovery: The mixed exhaust gas generated during each stage of pyrolysis and catalytic cracking is collected through a unified pipeline and then passes through a four-stage treatment unit: First, a cyclone dust collector removes solid particles larger than 10μm; then it enters an alkaline spray tower, where a 5% NaOH solution is circulated and sprayed. The pH value of the solution is maintained between 10.5 and 11.5 through online monitoring and an automatic alkali addition system, effectively absorbing H2S, SO2, and organic acid vapors to generate soluble salts such as Na2S and Na2SO3; next, it passes through an activated carbon adsorption column filled with coconut shell-based activated carbon with an iodine value of not less than 900mg / g to adsorb residual tar vapors, benzene compounds, and trace VOCs; finally, it enters a condensation recovery unit, which uses a refrigeration unit to maintain a low-temperature environment of -10℃ to 5℃, allowing condensable components to condense and separate. The condensable components include water and liquid tar. The recovered tar, after being filtered, is used as auxiliary fuel for combustion and heating during the catalyst regeneration stage. The condensate, after passing pH neutralization and trace COD testing, is reused in the alkali preparation system to achieve a closed-loop circulation of water resources.

[0037] The first, second, and third pyrolysis furnaces all employ a continuous rotary kiln structure. The outer wall of the furnace is covered with a 150mm thick nano-aerogel insulation layer with a thermal conductivity not exceeding 0.020 W / (m·K), significantly reducing radiative and convective heat loss. The heating system uses a coupled electric heating and biomass gas supply mode: electric heating is used for the start-up phase and fine-tuning of temperature; the biomass gas originates from non-condensable combustible gas in the exhaust gas purification system. This non-condensable combustible gas mainly consists of CH4, H2, and CO. After desulfurization using ZnO adsorbent and selective non-catalytic reduction (SNCR) denitrification, it is sent to a dedicated combustion chamber, achieving a combustion thermal efficiency of not less than 85%. The overall energy self-sufficiency rate can reach over 60%, significantly reducing dependence on external fossil fuels.

[0038] Before initial use, the CeO2–ZrO2 / γ-Al2O3 catalyst needs to be calcined at 550℃ in air for 4 hours to promote the formation of a solid solution structure between CeO2 and ZrO2 and stabilize the pores of the γ-Al2O3 support. After 120 hours of continuous operation, the oxygen vacancy concentration of the catalyst decreases due to carbon buildup. At this point, a nitrogen mixture containing 5% O2 is introduced, and the catalyst is regenerated at 450℃ for 2 hours to oxidize and remove the carbon buildup, restoring catalytic activity. The regeneration waste gas is then treated in an alkaline spray tower. When the catalyst's activity decreases by more than 15% after three regenerations, it is replaced entirely. The spent catalyst is leached with a 1mol / L HNO3 solution to recover rare earth elements such as Ce and Zr, as well as rare metals. The residue, rich in Al2O3 and silicates, can be used as an additive in ceramic glazes for resource utilization.

[0039] The finished carbon raiser is vacuum-packed using a double-layer moisture-proof aluminum-plastic composite film. The inner layer is a polyethylene heat-sealing layer, 80μm thick, with good heat-sealing strength and puncture resistance; the outer layer is a 9μm thick aluminum foil barrier layer, and the overall water vapor permeability after lamination is no higher than 0.5g / (m²). 2 • 24h), oxygen permeability not higher than 5cm 3 / (m 2 (24h, 0.1MPa). This packaging structure effectively isolates the product from environmental moisture and oxygen, preventing the product from absorbing moisture and clumping, oxidizing and becoming inactive, or adsorbing atmospheric nitrogen during storage and transportation, ensuring long-term stable performance after leaving the factory.

[0040] The technical effects of the present invention are quantitatively verified through four specific embodiments and eight comparative examples. All experiments were conducted on the same pilot-scale apparatus in the same laboratory, with strict uniformity in equipment model, raw material batch, and testing standards. Raw material testing was conducted according to GB / T22878-2020 "Carbonizers". Sulfur content was determined according to GB / T20124-2006, i.e., high-frequency infrared absorption method; nitrogen content was determined according to GB / T20125-2006, i.e., inert gas melting-thermal conductivity method; fixed carbon, ash content, and volatile matter were analyzed according to GB / T212-2008 industrial analysis methods; and carbon yield was calculated through carbon balance before and after steelmaking.

[0041] Example 1

[0042] 500 kg of dried walnut shell granules with a moisture content of 9.2% and a particle size distribution of 4 mm to 6 mm were taken. The material first entered the first pyrolysis furnace at 350℃ and a nitrogen flow rate of 80 Nm³. 3 The mixture was kept at a constant temperature for 75 minutes under the specified conditions; then it was transferred to the second pyrolysis furnace at 550℃ and a nitrogen flow rate of 60 Nm³ / h. 3 Hold at a constant temperature for 60 minutes; then transfer to the third pyrolysis furnace at 1300℃ and an argon flow rate of 50 Nm³ / h. 3 The temperature was maintained at 4℃ / min for 45 minutes, and a CeO2–ZrO2 / γ-Al2O3 catalyst (CeO2:ZrO2=3:1, loading 8%) was introduced simultaneously. The high-temperature carbon material was cooled to 70℃ at a rate of 4℃ / min under argon protection, and the particles were sieved to obtain 1mm to 5mm particles. The tail gas was successively treated by cyclone dust removal, spraying with 5% NaOH at pH=11.0, adsorption by coconut shell activated carbon, and condensation at 0℃. The test results of the obtained carbon additive were: fixed carbon 96.3%, sulfur 0.032%, nitrogen 185ppm, ash 1.7%, and volatile matter 1.2%. It was added to the electric arc furnace of Q235 steel at an addition rate of 1.2kg / t, and the carbon recovery rate reached 94.1%.

[0043] Example 2

[0044] 600 kg of mixed biomass raw materials were used, consisting of 40% rice husks, 30% bamboo chips, and 30% wood chips, with a moisture content of 10.1% and a particle size of 3 mm to 7 mm. The first pyrolysis furnace was set to 400℃ for 60 minutes, the second pyrolysis furnace to 600℃ for 40 minutes, and the third pyrolysis furnace to 1400℃ for 30 minutes. The catalyst was the same as in Example 1. The cooling rate was 5℃ / min, and the product was obtained after sieving. The tail gas treatment process was the same as in Example 1. Product test results: fixed carbon 95.8%, sulfur 0.041%, nitrogen 192 ppm, ash 1.9%, and volatile matter 1.4%. Used in the production of HT250 gray cast iron, the addition amount was 1.8 kg / t, the carbon recovery rate was 93.5%, and the porosity of the castings was only 0.03%.

[0045] Example 3

[0046] 400 kg of almond shell granules with a moisture content of 8.5% and a particle size of 5 mm to 8 mm were used. The first pyrolysis furnace was heated to 300℃ for 90 minutes, the second to 500℃ for 70 minutes, and the third to 1200℃ for 60 minutes. The catalyst loading was increased to 10%, and other conditions were the same as in Example 1. The resulting product contained 95.1% fixed carbon, 0.028% sulfur, 178 ppm nitrogen, 2.0% ash, and 1.5% volatile matter. When used in the electric arc furnace smelting of 45# steel, the addition rate was 1.0 kg / t, the carbon recovery rate was 95.2%, and the LF refining time was shortened by 18% compared to the conventional process.

[0047] Example 4

[0048] 700 kg of corn cob particles with a moisture content of 11.3% and a particle size of 3 mm to 5 mm were used. The first pyrolysis furnace was maintained at 380℃ for 80 minutes, the second at 580℃ for 50 minutes, and the third at 1350℃ for 40 minutes. The CeO2:ZrO2 ratio in the catalyst was adjusted to 2:1, with a loading of 8%. The cooling rate was set to 3℃ / min, and the tail gas condensation temperature was -5℃. Product testing results showed: fixed carbon 96.7%, sulfur 0.035%, nitrogen 189 ppm, ash 1.6%, and volatile matter 1.1%. Used in the production of QT700-2 ductile iron, with an addition amount of 2.0 kg / t, the casting tensile strength reached 635 MPa, and the elongation was 4.8%.

[0049] To verify the necessity of each technical element of the present invention, the following eight comparative examples are provided:

[0050] Comparative Example 1

[0051] A multi-stage physical sorting process (CN111500819B) was used to process petroleum coke raw material with initial sulfur content of 0.8% and nitrogen content of 450 ppm. After crushing, magnetic separation, gravity separation, and color sorting, a carbon raiser was obtained. The product has fixed carbon content of 94.2%, sulfur content of 0.38%, nitrogen content of 380 ppm, ash content of 2.5%, and volatile matter content of 2.0%. When used in Q235 steel smelting, the carbon recovery rate is only 87.3%, and additional desulfurization treatment is required.

[0052] Comparative Example 2

[0053] The preparation method adopted was CN104498805B "low-nitrogen carbon raiser". Low-nitrogen petroleum coke with 0.02% nitrogen was directly crushed and screened without deep desulfurization. The product had a sulfur content of 0.45%, nitrogen of 190 ppm, and fixed carbon of 93.5%. When used in the smelting of 45# steel, the excessive sulfur content caused edge cracking defects in the hot-rolled plate.

[0054] Comparative Example 3

[0055] A single-stage high-temperature pyrolysis process was performed, in which walnut shells were directly pyrolyzed at 1300℃ for 2 hours without gradient temperature control or catalyst. The product contained 92.1% fixed carbon, 0.12% sulfur, 350 ppm nitrogen, and 3.8% volatile matter. The carbon structure was disordered, and the carbon yield was only 82.6%.

[0056] Comparative Example 4

[0057] Under the conditions of Example 1, the CeO2–ZrO2 / γ-Al2O3 catalyst was replaced with pure γ-Al2O3 filler without the active component. The resulting carbon raiser contained 0.085% sulfur and 290 ppm nitrogen, indicating that the C–N and C–S bonds are difficult to break effectively in the absence of catalytic activity.

[0058] Comparative Example 5

[0059] The tail gas alkaline spraying step was omitted, and only cyclone dust removal and activated carbon adsorption were retained. H2S in the tail gas was not absorbed, and some of it flowed back into the cooling section, causing the product sulfur content to rise to 0.072%.

[0060] Comparative Example 6

[0061] Natural air cooling was used instead of inert atmosphere cooling. When the high-temperature carbon material was exposed to air and cooled to room temperature, the nitrogen content of the product increased to 280 ppm. This was due to the adsorption of atmospheric nitrogen on the surface of the high-temperature carbon and the formation of nitrogen-containing functional groups such as cyano (–C≡N).

[0062] Comparative Example 7

[0063] Coal-based activated carbon with an initial sulfur content of 0.6% and nitrogen content of 500 ppm was used to replace the biomass feedstock, and the process was followed according to Example 1. The final product had a sulfur content of 0.15% and a nitrogen content of 320 ppm, which failed to meet the ultra-clean control threshold set by this invention.

[0064] Comparative Example 8

[0065] The CeO2:ZrO2 ratio in the catalyst was changed to 1:3, with other conditions remaining the same as in Example 1. Due to the excessively high ZrO2 content, the oxygen vacancy density was insufficient, resulting in a decrease in catalytic oxidation capacity. The product contained 0.068% sulfur and 260 ppm nitrogen, failing to meet the technical requirements of ≤0.05% sulfur and ≤200 ppm nitrogen.

[0066] The data from the above embodiments and comparative examples are summarized in the table below:

[0067] ;

[0068] As can be seen from the table, all four embodiments consistently achieved the core indicators of sulfur ≤0.05% and nitrogen ≤200ppm, with high fixed carbon content and low volatile matter, demonstrating excellent performance in metallurgical applications. In contrast, the eight comparative examples deviated from the technical solution of this invention in one or more aspects, such as raw material selection, pyrolysis path, catalytic system, cooling method, or tail gas treatment, all resulting in excessive levels of key impurities or a significant decrease in carbon recovery. This fully demonstrates the inseparable integrity and technical necessity of the "raw material-structure-impurities-emission" four-dimensional synergistic process system constructed in this invention.

[0069] This invention achieves large-scale, replicable production of low-sulfur, low-nitrogen green carbon raisers through system integration of renewable biomass raw materials, multi-level gradient pyrolysis structure regulation, directional catalytic chemical bond breaking, and closed-loop resource-based emission control. This technology not only meets the extreme purity requirements of high-end metallurgy for carbon raisers but also aligns with national strategic guidance, providing a practical and feasible technical path for the green transformation of the metallurgical auxiliary materials industry.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A low-sulfur, low-nitrogen green carbon raiser, characterized in that: The carbon raiser is prepared from lignocellulosic waste through a multi-stage gradient pyrolysis, directional catalytic cracking, and inert atmosphere cooling process. The multi-stage gradient pyrolysis includes, in sequence, a first-stage low-temperature pyrolysis at 300°C to 400°C, a second-stage medium-temperature pyrolysis at 500°C to 600°C, and a third-stage high-temperature graphitization at 1200°C to 1400°C. The directed catalytic cracking process involves the simultaneous introduction of a CeO2–ZrO2 / γ-Al2O3 catalyst during the three-stage high-temperature graphitization phase. The inert atmosphere cooling is carried out under argon protection at a cooling rate of ≤5℃ / min; The carbon raiser has a fixed carbon content ≥95.0%, a sulfur content ≤0.05%, a nitrogen content ≤200ppm, an ash content ≤2.0%, and a volatile matter content ≤1.5%. The microstructure of the carbon raiser is arranged in layered graphite microcrystals, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak in the X-ray diffraction pattern is ≤0.45°. The lignocellulosic waste is one or more of the following: fruit shells, straw, wood chips, and bamboo chips, with an initial moisture content of 8% to 12% and a particle size of 3 mm to 8 mm.

2. The low-sulfur, low-nitrogen green carbon raiser according to claim 1, characterized in that: The carbon raiser has a particle size of 1mm to 5mm and is vacuum-packed with a double-layer moisture-proof aluminum-plastic composite film. The inner layer of the composite film is a polyethylene heat-sealing layer, and the outer layer is an aluminum foil barrier layer. The water vapor permeability is ≤0.5g / (m²). 2 • 24h), oxygen permeability ≤ 5cm 3 / (m 2 ·24h·0.1MPa).

3. A method for producing a low-sulfur, low-nitrogen green carbon raiser applicable to any one of the low-sulfur, low-nitrogen green carbon raisers according to claims 1 to 2, characterized in that, Includes the following steps: Step S1, raw material pretreatment: dry the lignocellulosic waste to a moisture content of 8% to 12%, crush and screen it to a particle size of 3mm to 8mm to obtain primary carbonaceous particles; Step S2, primary low-temperature pyrolysis: The primary carbon particles are kept at 300℃ to 400℃ under a nitrogen atmosphere for 60 to 90 minutes to allow hemicellulose and some cellulose to undergo dehydration and decarboxylation reactions. Step S3, Secondary medium-temperature pyrolysis: The primary pyrolysis product is kept at 500℃ to 600℃ under a nitrogen atmosphere for 40 to 70 minutes to promote the lignin backbone to break and remove some bound nitrogen and sulfur compounds; Step S4, tertiary high-temperature graphitization: The secondary pyrolysis products are kept at 1200℃ to 1400℃ in an argon atmosphere for 30 to 60 minutes to induce carbon atom rearrangement to form a local graphite microcrystalline structure and promote the breaking of residual C–N and C–S chemical bonds. Step S5, Directed catalytic cracking: During the three-stage high-temperature graphitization stage, a CeO2–ZrO2 / γ-Al2O3 catalyst is simultaneously introduced. The molar ratio of CeO2 to ZrO2 in the catalyst is 2:1 to 3:1, the total loading is 8% to 10% of the mass of the γ-Al2O3 support, and the catalyst particle size is 0.5 mm to 1.2 mm. The catalyst is placed in a fixed bed in the pyrolysis gas flow channel. H2O and CO2 are activated through the oxygen vacancy mechanism to generate active oxygen species, selectively cracking thiophene and pyridine heterocyclic compounds. Step S6, Inert atmosphere cooling and sieving: The high-temperature carbon material after catalytic cracking is cooled to below 80°C at a rate of ≤5°C / min under argon protection, and then sieved to obtain carbon raiser finished product with a particle size of 1mm to 5mm. Step S7, Closed-loop exhaust gas purification and resource recovery: The mixed exhaust gas generated from each stage of pyrolysis and catalytic cracking is sequentially passed through a cyclone dust collector, an alkaline spray tower, an activated carbon adsorption column, and a condensation recovery unit. The alkaline spray tower uses a 5% NaOH solution with the pH value maintained between 10.5 and 11.

5. The activated carbon adsorption column is filled with coconut shell-based activated carbon with an iodine value ≥900 mg / g. The condensation recovery unit operates at a temperature of -10℃ to 5℃. The recovered tar is reused for catalyst regeneration and combustion support, and the condensate is neutralized and reused for alkaline solution preparation.

4. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 3, characterized in that: The first-stage low-temperature pyrolysis, the second-stage medium-temperature pyrolysis, and the third-stage high-temperature graphitization processes are carried out in the first, second, and third continuous rotary kilns, respectively. The inclination angle of each rotary kiln is 3° to 5°, and the rotation speed is 0.5 rpm to 2.0 rpm. The material residence time is controlled by adjusting the rotation speed and the feed rate. The kilns are connected by airtight rotary valves to ensure continuous flow of inert atmosphere. The outer wall of the kiln is covered with a 150 mm thick nano-aerogel insulation layer with a thermal conductivity ≤0.020 W / (m·K).

5. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 4, characterized in that: The heating systems of the first, second, and third continuous rotary kilns adopt an electric heating and biomass gas coupled energy supply mode. The biomass gas is the recovered gas from the non-condensable combustible gas in the tail gas purification system after desulfurization and denitrification treatment. The combustion thermal efficiency is ≥85%, and the energy self-sufficiency rate is not less than 60%.

6. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 3, characterized in that: The CeO2–ZrO2 / γ-Al2O3 catalyst is calcined in air at 550°C for 4 hours before its first use to stabilize its crystal structure. After a 120-hour service life, it is regenerated at 450°C by passing nitrogen containing 5% O2 for 2 hours to restore the oxygen vacancy concentration. The regeneration waste gas is treated in an alkaline spray tower. The catalyst is replaced as a whole after deactivation. The waste catalyst is acid-leached to recover rare earth elements, and the residue is used as a ceramic glaze additive for resource utilization.

7. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 3, characterized in that: The nitrogen flow rate in the first-stage low-temperature pyrolysis is 60 Nm. 3 / h to 80Nm 3 / h, the nitrogen flow rate in the secondary intermediate-temperature pyrolysis is 50 Nm³ / h. 3 / h to 70Nm 3 / h, the argon flow rate in the third-stage high-temperature graphitization is 40 Nm³. 3 / h to 60Nm 3 / h.

8. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 3, characterized in that: The cooling rate in the inert atmosphere cooling is controlled at 3℃ / min to 5℃ / min, and the final cooling temperature is 70℃ to 80℃.

9. The method for producing low-sulfur, low-nitrogen green carbon raiser according to claim 3, characterized in that: The condensation recovery unit in the closed-loop exhaust gas purification system operates at a temperature of -5℃ to 0℃, and the pH value of the alkaline spray tower is precisely controlled at 11.0±0.5.

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