Heat recovery coke oven and preparation method of low-nitrogen foundry coke

Abstract

The invention discloses a heat recovery coke oven and a preparation method of low-nitrogen foundry coke, and belongs to the technical field of coking chemical industry. The heat recovery coke oven comprises a carbonization chamber, a flue system and a suction and mixing system, a suction facility outlet is connected with a mixer inlet, another inlet of the mixer is connected with a primary air pipeline, and a mixer outlet is communicated with a top air distribution port of the carbonization chamber through a pipeline; the low-nitrogen foundry coke is prepared by adopting the heat recovery coke oven to form flue gas circulation, and meanwhile, the contents of CaO, MgO and Fe2O3 in coking as-fired coal as well as the coking temperature and time are controlled; according to the invention, the low-cost preparation of the low-nitrogen coke with the nitrogen content of less than 0.5% can be realized, and the problem that the low-nitrogen content control of the foundry coke and the controllable process transformation cost cannot be realized synergistically is solved.

Description

Technical Field

[0001] This invention belongs to the field of coking chemical technology, specifically a heat recovery coke oven and a method for preparing low-nitrogen foundry coke. Background Technology

[0002] As a core raw material in metallurgy, casting, and other industrial sectors, the nitrogen content of coke directly affects the quality of end products and environmental emission levels. With increasingly stringent global environmental regulations and the acceleration of green industrial transformation, the demand for low-nitrogen coke, especially low-nitrogen foundry coke, continues to rise, and low-nitrogen foundry coke has become a core industry requirement.

[0003] I. Background of Low-NOx Coke Preparation Technology (I) Application Area Demand Drives Technological Upgrades The core application areas of low-nitrogen coke are concentrated in the foundry, iron and steel metallurgy, and chemical industries. Among them, foundry coke has particularly stringent requirements for nitrogen content control, which constitutes the core driving force for technological research and development. The foundry industry has a rigid demand for coke: foundry coke is a key fuel for cupola furnace molten iron. Its function is to provide a high-temperature heat source for melting the furnace charge and support the permeability of the charge column. It must simultaneously meet mechanical property requirements such as large block size (110-150mm), drum strength >81%, and porosity <28%, as well as low nitrogen emission requirements. Foundry coke produced by traditional processes generally has a nitrogen content of 1.2%-1.8%, and the NO generated during combustion... x This not only leads to excessive emissions from cupola furnace flue gas, but also allows molten iron to seep in, affecting the mechanical properties of castings and causing defects such as porosity and cracks. With downstream enterprises raising their requirements for casting quality and the implementation of regulations such as the "Emission Standard for Air Pollutants in the Foundry Industry," foundry enterprises have an urgent need for low-nitrogen foundry coke.

[0004] Technical positioning of heat recovery coke ovens: Clean heat recovery coke ovens, due to their negative pressure operation, can burn harmful substances such as benzo[a]pyrene to negligible levels, and offer flexible coal selection (up to 30% weakly caking coal can be used). They have become the mainstream production furnace type for foundry coke, and in 2014, the Ministry of Industry and Information Technology included them in the scope of new construction access for the coking industry. This type of furnace achieves coking and waste heat power generation through a negative pressure heat recovery system. However, its existing processes focus on improving waste heat recovery efficiency, lacking targeted design for nitrogen content control. This results in the nitrogen content of the produced foundry coke failing to meet the demands of high-end casting, creating a technological upgrade gap.

[0005] (II) Development and Evolution of Production Technology and Current Status Low-NOx coke production technology has developed around two dimensions: "nitrogen control at the source" and "nitrogen reduction during the process," forming a multi-path exploration pattern. Among them, the heat recovery coke oven process has become a key direction for foundry coke production due to its cleanliness advantages. Raw material pretreatment process: Reducing the nitrogen content of raw materials through coal blending optimization and pretreatment is the most basic means of nitrogen control. Existing technologies mostly use low-nitrogen, weakly caking coal to blend with prime coking coal, or use microwave pretreatment to destroy nitrogen-containing functional groups in coal. However, there are significant bottlenecks: the scarcity of low-nitrogen coal resources leads to a raw material cost increase of more than 40%, and over-reliance on prime coking coal violates the coking coal resource protection policy; although microwave pretreatment can reduce the nitrogen content of raw materials by 15%-20%, it increases equipment investment by 25% and easily leads to a decrease in coal caking properties, affecting the strength of foundry coke.

[0006] Improvements to traditional coking processes: Traditional coke ovens reduce nitrogen conversion by optimizing the heating regime; however, this type of oven focuses on chemical product recovery, resulting in flue gas leakage through the coal charging port and oven door gaps, leading to low nitrogen emission control efficiency. In contrast, clean heat recovery coke ovens achieve low leakage through compacted coal charging and negative pressure combustion; however, their current process uses a single heating curve, resulting in insufficient pyrolysis of nitrogen-containing compounds during the dry distillation stage (500-800℃), and the nitrogen content of coke remains at 1.1%-1.5%.

[0007] II. Core Limitations of Existing Low-Nitrogen Coke Preparation Technologies Although multiple pathways have been explored for low-NOx coke preparation technology, existing technologies still face four major challenges that hinder industry development, considering the performance requirements of foundry coke and the process characteristics of clean heat recovery coke ovens: (i) The contradiction between raw materials and performance is difficult to reconcile. The dual requirements of "low nitrogen" and "high strength" for foundry coke create a natural contradiction with the characteristics of the raw materials: low-nitrogen coals are mostly weakly caking coals, with a caking index (G value) generally <40, making it difficult to form high-strength coke when used alone; while high-caking prime coking coals can increase the drum strength of coke to over 85%, their nitrogen content is usually high, and blending them with a ratio exceeding 40% will result in excessive nitrogen content in the finished product. Existing coal blending processes can only reduce the nitrogen content to a maximum of 0.6%-0.8%, and the drum strength decreases simultaneously, failing to meet both environmental protection requirements and foundry process demands. Furthermore, coking coal resource protection policies restrict the mining volume of prime coking coal, further exacerbating the pressure on raw material supply.

[0008] (ii) Lack of nitrogen control mechanism in heat recovery coke ovens As the mainstream equipment for foundry coke production, the clean heat recovery coke oven has three major shortcomings in its existing process design for nitrogen control: First, the heating regime is unreasonable, with an excessively rapid heating rate (10-15℃ / min) during the dry distillation stage. This causes nitrogen-containing compounds such as pyrrole and pyridine in the coal to decompose rapidly into NH3, which is then re-polymerized in the coke before being discharged with the flue gas, resulting in a nitrogen fixation rate of over 60%. Second, the negative pressure control accuracy is insufficient, with pressure fluctuations within the furnace ranging from ±50Pa. This leads to positive pressure combustion in localized areas, promoting the production of thermal NO. XThe formation and infiltration of nitrogen into coke are twofold: firstly, the synergy between waste heat recovery and nitrogen control is poor, leading to the use of high-temperature and high-pressure boilers to increase power generation per ton of coke, resulting in a final distillation temperature exceeding 1050℃ and exacerbating the formation of nitrogen-containing functional groups on the coke surface; secondly, the fourth-generation heat recovery coke ovens developed by companies such as MCC Coke & Refractory have achieved a furnace life of up to 30 years, but the nitrogen content control index has still not broken through 1.0%.

[0009] (III) High pressure from both energy consumption and environmental protection costs Existing low-nitrogen production technologies generally suffer from an inverse relationship between nitrogen control efficiency and energy consumption costs: microwave denitrification processes in raw material pretreatment increase power consumption by 120 kWh per ton of coke, leading to a 15% increase in production costs. Even with clean heat recovery coke ovens, most domestic enterprises have a burn loss rate exceeding 1.5% per ton of coke, and only 30% of enterprises can achieve power generation greater than 550 kWh per ton of coke. Insufficient energy recovery efficiency limits the cost offsetting space, resulting in low-nitrogen foundry coke being 20%-30% more expensive than ordinary foundry coke in the market, hindering market promotion.

[0010] (iv) Insufficient stability in large-scale production Low-nitrogen preparation technologies validated in the laboratory stage (such as bio-based coke modification) are difficult to scale up: although bio-based coke modification can reduce nitrogen content to 0.8%, the product strength is low (drum strength <75%), and the unstable supply of biomass raw materials leads to product quality fluctuations of ±0.3%. For clean heat recovery coke ovens, existing processes have a furnace temperature unevenness exceeding 15% when the single furnace capacity is >500,000 tons / year, resulting in nitrogen content fluctuations of 0.3%-0.5% in the same batch of coke, which cannot meet the requirements of high-end casting for raw material stability.

[0011] In summary, the foundry industry's rigid demand for low-nitrogen, high-strength foundry coke, and environmental policies' impact on NO... x Strict emission constraints sharply contrast with existing technologies, which suffer from drawbacks such as raw material inefficiencies, low nitrogen control efficiency, high costs, and insufficient stability. Clean heat recovery coke ovens, as the mainstream equipment for foundry coke production, offer the core advantages of negative pressure cleaning and waste heat recovery, providing a foundation for low-nitrogen production at the source. However, the lack of targeted nitrogen control process design remains a technological bottleneck. Therefore, developing a "method for preparing low-nitrogen foundry coke using a clean heat recovery coke oven," by optimizing the furnace structure, temperature control system, and coal blending process to achieve a synergy of low nitrogen (<1.0%), high strength (drum strength >81%), and low cost, has significant technological necessity and industrial value.

[0012] Currently, existing technologies and related solutions have significant drawbacks: Nitrogen source control and the limitations of coal resources: Burchil et al. [BURCHILL P, WELCH L S. Variation of nitrogen content and functionality with rank for some UK bituminous coals[J]. Fuel, 1989, 68(1): 100–104.] and Boudou et al. [BOUDOU J, MARIOTTI A, OUDIN J. Unexpected enrichment of nitrogen during the diagenetic evolution of sedimentary organicmatter[J]. Fuel, 1984, 63(11): 508–1510.] A series of coal samples from lignite to anthracite were studied, and the results showed that in the lignite to sub-bituminous coal stage (carbon mass fraction of 60%~77%), the nitrogen content in the coal increased with the increase of coal grade, reaching the maximum value in coal samples with carbon mass fraction of 80%~85%, and began to decrease after the carbon mass fraction was greater than 85%. This indicates that within the range of coal types suitable for coking, the nitrogen content is generally high, and it is difficult to find coal types that meet the requirements of low nitrogen coke. Shortcomings in nitrogen control of heat recovery coke ovens: The coking process of clean heat recovery coke ovens can realize the recovery of waste heat from flue gas and desulfurization and denitrification, but due to the poor adaptability of coking heating rate and coal blending structure, organic nitrogen such as pyridine nitrogen and quinoline nitrogen in coke is not completely removed. The nitrogen content of finished casting coke is generally higher than 1.0%, which is difficult to meet the technical requirements of high-end precision casting for low nitrogen (≤0.5%). The contradiction between process stability and economy: Some technologies achieve the goal of low nitrogen by adding an external denitrification device (such as CN116123531B, a microwave activated coke co-combustion denitrification device and method), but the equipment modification cost increases by more than 40%, and it is easy to lead to a decrease in the thermal efficiency of the coke oven body, resulting in an imbalance between operating energy consumption and environmental benefits.

[0013] In summary, existing technologies cannot simultaneously achieve the synergistic goals of efficient waste heat recovery from coke ovens, precise control of nitrogen content in foundry coke, and controllable process modification costs. The core challenge lies in how to achieve the directional removal of nitrogen oxides during coal blending and coking under the specific thermal environment of a heat recovery coke oven, as well as the seamless coupling of the waste heat recovery system with the low-nitrogen coking process. This has become a key technological bottleneck restricting the large-scale production of clean low-nitrogen foundry coke. Summary of the Invention

[0014] This invention overcomes the shortcomings of the prior art and proposes a method for preparing heat recovery coke oven and low-nitrogen foundry coke; it solves the problem that the control of low nitrogen content in foundry coke and the controllable cost of process modification cannot be achieved in a coordinated manner.

[0015] This invention is achieved through the following technical solution: A heat recovery coke oven includes a carbonization chamber, a main wall, a bottom flue, and a flue system, as well as a suction and mixing system. The suction and mixing system includes suction facilities and a mixer. The flue system includes a downdraft flue, an updraft flue, a primary air duct, a main flue, and branch flues. The downdraft flue is built into the main wall, and the updraft flues are arranged on both sides of the carbonization chamber. The bottom flue is connected to the updraft flue, and the top of the updraft flue is connected to the main flue and the branch flues, respectively. The branch flues are connected to the suction facilities. The outlet of the suction facilities is connected to the inlet of the mixer, another inlet of the mixer is connected to the primary air duct, and the outlet of the mixer is connected to the air distribution port at the top of the carbonization chamber through a duct.

[0016] Furthermore, the bottom flue is located below the bottom of the carbonization chamber, and the bottom of the downward flue is perpendicularly connected to one end of the bottom flue. The flue gas flows from top to bottom into the bottom flue through the downward flue, and the other end of the bottom flue is connected to the bottom of the upward flue.

[0017] Furthermore, the top of the rising flue connects the main flue and the branch flue via a three-way structure, with an electric regulating valve installed at the three-way joint.

[0018] Furthermore, the suction device is an ejector or a high-temperature fan.

[0019] Furthermore, the flue system also includes secondary air ducts, which are arranged along the bottom of the furnace and connected to secondary air nozzles in the bottom flue through branch pipes to ensure that secondary air is evenly delivered into each bottom flue and fully mixed and burned with the flue gas.

[0020] Furthermore, temperature sensors and oxygen content sensors are installed at the top of the carbonization chamber, the rising flue, and the mixer outlet, and the data is transmitted to the central control system in real time; flow regulating valves are installed in the primary air pipeline, secondary air pipeline, and branch flue, and the central control system automatically adjusts them according to the monitoring data.

[0021] A method for preparing low-nitrogen foundry coke, employing the aforementioned heat recovery coke oven, includes the following steps: S1. Based on the coal blending scheme, calculate and adjust the contents of CaO, MgO, and Fe2O3 in the coking coal fed into the furnace, and control them to achieve the following mass percentage contents: CaO: 0.2-0.5%, MgO: 0.2-0.5%, Fe2O3: 0.5-1.0%; S2. Adjust the heating rate during the coking process so that the time to reach the coking temperature is 70-75 hours, the coking temperature is 1100-1200℃, and the coking time is 180-220 hours. During the coking process, a flue gas recirculation process is adopted, that is, a portion of the high-temperature flue gas exiting the bottom flue 2 is mixed with the primary air at the top of the furnace through the rising flue 4, the suction device 7 and the mixer to form a mixed gas that enters the carbonization chamber 1 together, thus realizing flue gas recirculation.

[0022] Furthermore, the coking temperature is controlled in step S2 using a segmented heating mode: from 0 to 24 hours after loading the furnace, the heating rate is controlled at 10-12℃ / h to slowly preheat the coal cake and initially release volatiles; from 24 to 72 hours, the heating rate is adjusted to 8-10℃ / h to gradually reach the coking temperature, and the temperature is steadily raised to the set coking temperature over a period of 70 to 75 hours. The heating process is monitored in real time by multiple temperature sensors on the furnace body. Furthermore, another portion of the high-temperature flue gas exiting the furnace bottom flue 2 flows into the main flue and is discharged after waste heat recovery, desulfurization, and denitrification.

[0023] Furthermore, the temperature of the gas mixture is 400-500℃, and the oxygen content is 2-3% by mass.

[0024] The beneficial effects of this invention compared to the prior art are as follows: This invention enables the low-cost preparation of low-NOx coke with a nitrogen content of <0.5%, and has the following advantages: First, it requires no new large-scale equipment; it can be achieved simply by optimizing coal blending, adjusting process parameters, and adding a flue gas recirculation branch, resulting in low investment. Second, flue gas recirculation recovers waste heat, which can reduce coking energy consumption by 10-15%. Third, the strong reducing atmosphere reduces NO. X This process reduces the subsequent denitrification load and lowers environmental protection costs.

[0025] The effectiveness can be verified through industrial trials: Select 1-2 carbonization chambers, adjust the coal blending and process parameters according to the above scheme, produce 3 batches of coke continuously, and test the nitrogen content, strength and other indicators of the coke; at the same time, monitor the NOx concentration and energy consumption data in the flue gas to ensure that the goals of low nitrogen, high efficiency and environmental protection are met. Attached Figure Description

[0026] Figure 1 This is a schematic cross-sectional view of a heat recovery coke oven with a flue gas recirculation process structure in this invention. Figure 2 This is a schematic diagram of the cross-section of a conventional heat recovery coke oven (the heat flows from the top of the oven to the main flue). Figure 3 This is a schematic diagram of the cross-section of a conventional heat recovery coke oven (with the flue gas flowing from the bottom of the oven to the main flue). In the diagram, 1 is the carbonization chamber, 2 is the furnace bottom flue, 3 is the downward flue, 4 is the upward flue, 5 is the secondary air duct, 6 is the branch flue, 7 is the suction device, and 8 is the primary air duct. Detailed Implementation

[0027] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail with reference to the embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto.

[0028] This invention proposes a method for preparing low-nitrogen foundry coke, which reduces the nitrogen content of the foundry coke through the following three aspects: 1. Screening the blending coal types to control the content of effective components for catalytic denitrification (such as CaO, MgO, and Fe2O3) in the coal ash composition; achieving in-situ catalytic denitrification of coal ash components; 2. Adjusting the coking process parameters (coking temperature and coking time); 3. Modifying the heat recovery coke oven flue gas process to achieve flue gas recirculation; specifically: (I) Coal type selection and coal blending optimization The core objective of this step is to enhance the efficiency of thermal denitrification during coking by accurately selecting coal types and scientifically adjusting mineral composition, and by leveraging the synergistic catalytic effect of iron-containing and calcium-magnesium minerals, thus laying the raw material foundation for the preparation of low-nitrogen coke.

[0029] 1. Coal Type Collection and Screening Standards: Priority will be given to collecting mainstream coal types suitable for blending and coking, such as gas coal, fat coal, coking coal, and lean coal. The source must be stable and controllable. Each batch of coal must undergo industrial analysis (ash content, volatile matter, fixed carbon, sulfur content) and elemental analysis (nitrogen, carbon, hydrogen, oxygen). Coal types with ash content ≤10% and nitrogen content ≤1.2% will be screened as basic blending raw materials. Coal types with excessively high ash content, excessive nitrogen content, and impurity (such as gangue) content >3% will be excluded.

[0030] 2. Precise Detection and Analysis of Ash Components: For each type of coal that has passed the screening, X-ray fluorescence spectrometry (XRF) is used to precisely detect the content of CaO, MgO, Fe2O3, and other minor components (Al2O3, SiO2, etc.) in the coal ash, with the detection accuracy controlled within ±0.01%. Simultaneously, X-ray diffraction (XRD) is used to analyze the mineral morphology, confirming the presence of iron-bearing minerals (such as hematite and limonite) and calcium-magnesium minerals (such as calcite and dolomite), ensuring their catalytic activity.

[0031] 3. Coal Blending Ratio Calculation and Adjustment: Based on the established coal blending scheme (meeting basic indicators such as coke strength and abrasion resistance), a linear programming model is used to calculate the blending ratio of each type of coal. The core control is to ensure that the weighted average of the target mineral components in the coal fed into the furnace is within a suitable range: CaO 0.2-0.5%, MgO 0.2-0.5%, Fe2O3 0.5-1.0%. During the blending process, the composition can be finely adjusted by adding a small amount of high-purity calcium and magnesium minerals (such as light-burned dolomite powder, purity ≥90%) or iron-containing minerals (such as iron concentrate, Fe2O3 content ≥65%) to ensure uniform mixing. After mixing, multiple sampling tests are conducted, and the deviation is controlled within ±0.05%.

[0032] 4. Synergistic catalytic denitrification mechanism: During pyrolysis, Fe2O3, as an active center, can promote the cracking and migration of nitrogen compounds (pyrrole nitrogen and pyridine nitrogen) in coal, generating gaseous nitrogen-containing products such as NH3 and HCN; CaO and MgO can adsorb gaseous nitrogen-containing products and adjust the pH of the system to reduce the rate of nitrogen-containing products being fixed in coke. The synergistic effect of the two can improve the pyrolysis denitrification efficiency by 15-20%, providing support for subsequent processes to achieve the low-nitrogen target.

[0033] (II) Optimization of coking process parameters in heat recovery coke ovens To meet the needs of low-nitrogen coke preparation, while ensuring the physicochemical properties of coke, the pyrolysis and release of nitrogen-containing products are enhanced by adjusting the heating rate, coking temperature and coking time, while also adapting to the reducing atmosphere requirements of flue gas recirculation. 1. Precise control of heating rate: A segmented heating mode is adopted to avoid nitrogen-containing products being fixed in the coke due to insufficient time to escape during rapid heating. Specifically: for 0-24 hours after loading into the furnace, the heating rate is controlled at 10-12℃ / h to slowly preheat the coal cake and initially release volatile matter; for 24-72 hours, the heating rate is adjusted to 8-10℃ / h to gradually reach the coking temperature (1100-1200℃), and the temperature is steadily raised to the set coking temperature over the entire 72 hours. The heating process is monitored in real time by multiple temperature sensors on the furnace body (arranged on the side walls and top of the carbonization chamber), with the deviation controlled within ±5℃. 2. Coking Temperature Control: The traditional coking temperature (1050-1100℃) is increased to 1100-1200℃. The high-temperature environment promotes the cracking of recalcitrant nitrogen-containing compounds in coal and accelerates the removal of residual nitrogen from coke. Temperature control adopts a zoned control method, with the temperature difference between different areas of the furnace bottom flue ≤10℃. Fine-tuning of the temperature is achieved by adjusting the secondary air supply to ensure the overall temperature uniformity of the carbonization chamber. 3. Coking Time Extension and Control: The conventional coking time (140-180 hours) is extended to 180-220 hours. The extension phase mainly involves a constant-temperature coking period (temperature maintained at 1100-1200℃). Sufficient time allows nitrogen-containing products to fully escape, while simultaneously promoting coke densification and compensating for the slight impact of high temperature on strength. The end of the coking time is determined by monitoring both the composition of the gas escaping from the top of the carbonization chamber (nitrogen content ≤0.03%) and the strength of the coke sample (M40≥85%).

[0034] (III) Flue gas recirculation system and process optimization A strong reducing atmosphere (oxygen content ≤2%) is created in the carbonization chamber by flue gas recirculation, inhibiting the oxidation of nitrogen-containing products to NO. X At the same time, it recovers heat from flue gas and reduces energy consumption. This solution is designed with a special circulation process and supporting facilities for the characteristics of heat recovery coke ovens. 1. Core principle of flue gas recirculation The reducing gases such as CO and H2 contained in the high-temperature flue gas (temperature 800-900℃) exiting the furnace bottom flue are recycled into the carbonization chamber. On the one hand, this dilutes the oxygen concentration in the primary air, creates a reducing atmosphere, and reduces the oxidation of nitrogen-containing products. On the other hand, the high-temperature flue gas can assist in heating the coal cake, optimize the heat distribution, and adsorb some nitrogen-containing products, further reducing the nitrogen content of the coke. 2. Selection and parameters of suction equipment Based on the required flue gas temperature and pressure, the following two types of suction equipment can be selected to suit different operating conditions: (1) Compressed air / high-pressure steam jet: High-pressure steam jet is preferred (suitable for high-temperature flue gas scenarios, avoiding high-temperature corrosion of fan blades). The steam pressure is controlled at 1.2-1.6MPa, the steam temperature is 250-300℃, the suction capacity is 30-50% of the total flue gas volume, and the flow velocity of the jet outlet and the primary air pipeline mixing section is controlled at 15-20m / s to ensure that the flue gas and primary air are fully mixed. (2) High-temperature fan: Made of high-temperature and corrosion-resistant alloy material (such as 310S stainless steel), rated temperature ≤1000℃, air volume adjustment range 2000-5000m³ / h 3 / h, air pressure 1500-2000Pa, equipped with a variable frequency speed control system, which can adjust the suction volume in real time according to the atmosphere of the carbonization chamber, and a filter device is installed at the fan inlet to prevent dust from wearing the impeller. 3. Flue gas flow (1) After the coal is blended, the coal cake is pushed into the carbonization chamber by the charging machine, the furnace door is closed and sealed (the sealing gap is ≤2mm to prevent air leakage), and the coking process is started; (2) The coal cake is heated by the heat conducted by the furnace wall in the carbonization chamber, and undergoes a pyrolysis reaction, releasing pyrolysis gases (including methane, ethane, nitrogen-containing compounds, etc.). (3) The pyrolysis gas mixes with the primary air (air-to-coal ratio 0.3-0.5) drawn in from the top tuyeres of the furnace and undergoes partial combustion in the top area of ​​the carbonization chamber, releasing heat to assist pyrolysis; (4) The unburned products (including CO, H2, N2 and a small amount of organic matter) enter the built-in downdraft flue (300mm×400mm) in the main wall through the smoke inlets at the top of the main wall of the carbonization chamber (4-6 on each side, with a diameter of 80-100mm). (5) The product flows into the bottom four-way flue (or six-way flue, with a single flue cross section of 200mm×300mm and made of high-alumina brick) through the downward flue, mixes with the secondary air (air-fuel ratio 1.2-1.4) introduced into the flue, and complete combustion occurs, with the combustion temperature rising to 1200-1300℃, and heat is transferred to the carbonization chamber through the flue wall; (6) After the combustion products (flue gas, temperature 800-900℃) exit the bottom flue, they enter the rising flue (cross-sectional size 500mm×600mm) on both sides of the carbonization chamber and merge into the main flue (diameter 1200-1500mm) of the heat recovery coke oven. (7) The conventional flue gas process involves the flue gas entering the waste heat recovery system (waste heat boiler) through the main flue, cooling it to 200-250℃, and then undergoing desulfurization (limestone-gypsum method) and denitrification (SCR method, denitrification efficiency ≥90%) treatment to meet emission standards (NOx). X ≤50mg / Nm 3 (After discharge) The optimized flue gas flow path of this invention: The flue gas front-end process (steps 1-6) is completely consistent with the conventional process. The core optimization lies in the branching and recirculation stages after the rising flue outlet, specifically: The flue gas (total Q) exiting the rising flue is divided into two paths: one is the mainstream flue gas (accounting for 50-70%, Q1), which normally flows into the main flue and is discharged after waste heat recovery, desulfurization and denitrification; the other is the circulating flue gas (accounting for 30-50%, Q2), which enters the flue gas distribution pipe (diameter 500-800mm, material is heat-resistant steel). The circulating flue gas is pressurized by the suction device (ejector or high-temperature fan) (pressure rises to 1800-2200Pa) and sent into the primary air mixer (built-in baffle, mixing uniformity ≥90%), where it is fully mixed with fresh primary air; The mixed gas (temperature 400-500℃, oxygen content 2-3%) re-enters the carbonization chamber through the tuyeres at the top of the furnace, participating in the partial combustion and reduction reaction of the pyrolysis gas to complete the flue gas recirculation; During the circulation process, the atmosphere is monitored in real time by an oxygen content sensor at the top of the carbonization chamber. If the oxygen content is >2%, the atmosphere is adjusted by increasing the suction volume and reducing the primary air supply to ensure a stable reducing atmosphere.

[0035] Specifically, in this embodiment, coal types with low nitrogen content and sufficient iron oxide, calcium oxide, and magnesium oxide content to meet coking requirements are selected for blending. Through blending, the key indicators for industrial and elemental analysis of the blended coal are as follows: ; During coking, the temperature of the four-way flue of the coke oven is controlled at 1250℃, and the temperature of the carbonization chamber is 1100℃. That is, the heating regime is 1100±10℃ for the dome and 1250±10℃ for the four-way flue.

[0036] Heating and flue gas recirculation start-up: The coke oven temperature is controlled according to the conventional heating curve. When the temperature of carbonization chamber 1 reaches 500℃, the steam jet is started to introduce part of the flue gas into carbonization chamber 1 to realize flue gas recirculation. The circulation volume is monitored and maintained in real time.

[0037] Recirculation system shutdown and coke discharge: When the temperature in the middle of the carbonization chamber reaches 1100℃ and is kept at that temperature for 20 hours, the flue gas recirculation system is stopped (the steam jet is stopped), and the temperature is kept at that temperature for another 160 hours before coke discharge.

[0038] After coking, the coke is screened. The quality indicators of coke with a diameter >140mm are shown in the table below.

[0039] ; The present invention also proposes a heat recovery coke oven, which is applied to the preparation method of the aforementioned low-nitrogen foundry coke; (I) Overall structural parameters of the coke oven This scheme adopts a single-heat heat recovery coke oven with a brick-concrete structure. The overall dimensions (length × width × height) are 40-60m × 8-10m × 12-15m, and the designed capacity is 500,000-800,000 tons / year. It is compatible with the optimized process mentioned above. The core structure includes the carbonization chamber, main wall, bottom flue, flue system, suction and mixing system, and sealing system. (II) Core Structure and Material Description Carbonization Chamber 1: The core reaction area of ​​the coke oven, with a rectangular structure. The dimensions of a single opening (length × width × height) are 12-15m × 0.45-0.55m × 4-5m. It is made of silica bricks (SiO2 content ≥95%), which have the characteristics of high temperature resistance (≥1400℃) and good thermal conductivity. The top of Carbonization Chamber 1 is equipped with air distribution vents (4-6 per opening, 50-60mm in diameter), and the top of the main wall is equipped with a smoke inlet that connects to the downflow flue 3. The furnace walls on both sides are 300-400mm thick and are constructed with staggered joints to ensure sealing performance. Main wall: The load-bearing structure that separates the adjacent carbonization chamber 1 and the built-in downdraft flue, with a thickness of 600-800mm, and is made of high alumina brick (Al2O3 content ≥70%). The built-in downdraft flue (1-2 carbonization chambers on each side) is coated with wear-resistant castable material to prevent erosion and wear from flue gas. Bottom flue 2: Located at the bottom of carbonization chamber 1, it is arranged in parallel with four or six sections. The length of a single flue is the same as that of carbonization chamber 1, with a cross-sectional size of 200mm×300mm. It is made of high-alumina brick. Secondary air nozzles (spacing 1.5-2m, orifice diameter 30-40mm) are installed in the flue and connected to secondary air pipe 5. Both ends of the flue are connected to the downdraft flue 3 and the updraft flue 4 respectively, forming a flue gas flow channel. The flue system includes a downdraft flue 3, an updraft flue 4, a main flue, and branch flues 6. The downdraft flue 3 is built into the main wall, and the updraft flue 4 is located on both sides of the carbonization chamber 1. Both are made of heat-resistant steel-lined high-alumina bricks. The main flue is located on the top or side of the coke oven, with a diameter of 1200-1500mm, and is made of Q235B steel plate lined with heat-resistant castable. The branch flue 6 connects the updraft flue 4 to the suction system 7, with a diameter of 500-800mm, made of 310S stainless steel, and equipped with an electric regulating valve (adjustment accuracy ±5%) to control the amount of circulating flue gas. Suction and mixing system: including suction device 7 (ejector or high-temperature fan), mixer and connecting pipes. The ejector is made of cast steel with a steam inlet pipe diameter of 100-150mm; the high-temperature fan casing is made of cast steel, and the impeller is made of 310S stainless steel; the mixer is a cylindrical structure (diameter 600-800mm) with built-in guide plates, made of heat-resistant steel, and connected at both ends to the branch flue 6 and the primary air duct 8 respectively. Sealing system: including furnace door seal, pipe joint seal, and flue seal. The furnace door is made of cast iron, and the sealing surface is covered with graphite packing (high temperature resistance ≥800℃); the pipe joints are connected by flanges, and the gaskets are metal spiral wound gaskets; the flue masonry joints are sealed with refractory mortar to prevent air leakage from disrupting the reducing atmosphere. Monitoring and control system: Temperature sensors (thermocouples, measurement range 0-1300℃) and oxygen content sensors (measurement range 0-10%) are installed at the top of carbonization chamber 1, rising flue 4, and mixer outlet. The data is transmitted to the central control system in real time. Flow regulating valves are installed in the primary / secondary air pipelines and branch flues. The central control system automatically adjusts the flow based on the monitoring data to ensure stable process parameters. (III) Connection Relationships of Various Structures Carbonization Chamber 1 and Main Wall: The furnace walls on both sides of the carbonization chamber 1 are integrated with the main wall. The smoke inlet at the top of the main wall is connected to the interior of the carbonization chamber 1. The smoke inlet is connected to the downdraft flue 3 in the main wall, forming a flue gas passage of "carbonization chamber → smoke inlet → downdraft flue". Downward flue 3 and bottom flue 2: The bottom of the downward flue 3 is vertically connected to one end of the bottom flue 2. Flue gas flows from top to bottom into the bottom flue 2 through the downward flue 3. The other end of the bottom flue 2 is connected to the bottom of the rising flue 4, realizing the transportation of flue gas from the bottom flue 2 to the rising flue 4. Ascending flue 4 and main flue, branch flue 6: The top of ascending flue 4 is connected to the main flue and branch flue 6 respectively through a three-way structure. An electric regulating valve is installed at the three-way to switch or adjust the flue gas flow direction and distribution ratio. The main flue is connected to the waste heat recovery system and desulfurization and denitrification device. The branch flue 6 is connected to the suction facility 7. The suction device 7 is connected to the mixer and the primary air duct 8. The outlet of the suction device 7 is connected to the inlet of the mixer, the other inlet of the mixer is connected to the primary air duct 8, and the outlet of the mixer is connected to the air distribution port at the top of the carbonization chamber 1 through the duct, forming a circulation channel of "circulated flue gas + fresh primary air → mixer → air distribution port → carbonization chamber". Furnace bottom fire channel 2 and secondary air pipeline 5: The secondary air pipeline 5 is arranged along the furnace bottom and is connected to the secondary air nozzles in the furnace bottom fire channel 2 through branch pipelines to ensure that the secondary air is evenly delivered into each furnace bottom fire channel 2 and fully mixed and burned with the flue gas.

[0040] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0041] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this invention.

Claims

1. A heat recovery coke oven, comprising a carbonization chamber (1), a main wall, a furnace bottom flue (2), and a flue system, characterized in that, It also includes a suction and mixing system; the suction and mixing system includes a suction device (7) and a mixer; the flue system includes a downdraft flue (3), an updraft flue (4), a primary air duct (8), a main flue and a branch flue (6); the downdraft flue (3) is built into the main wall, and the updraft flue (4) is arranged on both sides of the carbonization chamber (1); the furnace bottom fire channel (2) is connected to the updraft flue (4), the top of the updraft flue (4) is connected to the main flue and the branch flue (6) respectively, and the branch flue (6) is connected to the suction device (7); the outlet of the suction device (7) is connected to the mixer inlet, the other inlet of the mixer is connected to the primary air duct (8), and the mixer outlet is connected to the air distribution port at the top of the carbonization chamber (1) through a pipe.

2. The heat recovery coke oven according to claim 1, characterized in that, The bottom flue (2) is located below the bottom of the carbonization chamber (1). The bottom of the down flue (3) is perpendicular to one end of the bottom flue (2). The flue gas flows from top to bottom into the bottom flue (2) through the down flue (3). The other end of the bottom flue (2) is connected to the bottom of the rising flue (4).

3. A heat recovery coke oven according to claim 1, characterized in that, The top of the rising flue (4) is connected to the main flue and the branch flue (6) respectively through a three-way structure, and an electric regulating valve is installed at the three-way.

4. A heat recovery coke oven according to claim 1, characterized in that, The suction device (7) is an ejector or a high-temperature fan.

5. A heat recovery coke oven according to claim 1, characterized in that, The flue system also includes a secondary air duct (5), which is arranged along the bottom of the furnace and connected to the secondary air nozzles in the bottom flue (2) through branch pipes to ensure that the secondary air is evenly delivered into each bottom flue (2) and fully mixed and burned with the flue gas.

6. A heat recovery coke oven according to claim 5, characterized in that, Temperature sensors and oxygen content sensors are installed at the top of the carbonization chamber (1), the rising flue (4), and the mixer outlet. The data is transmitted to the central control system in real time. Flow regulating valves are installed in the primary air duct (8), secondary air duct (5), and branch flue (6), and are automatically adjusted by the central control system based on the monitoring data.

7. A method for preparing low-nitrogen foundry coke, characterized in that, The method employs a heat recovery coke oven as described in any one of claims 1-6, and includes the following steps: S1. Based on the coal blending scheme, calculate and adjust the contents of CaO, MgO, and Fe2O3 in the coking coal fed into the furnace, and control them to achieve the following mass percentage contents: CaO: 0.2-0.5%, MgO: 0.2-0.5%, Fe2O3: 0.5-1.0%; S2. Adjust the heating rate during the coking process so that the time to reach the coking temperature is 70-75 hours, the coking temperature is 1100-1200℃, and the coking time is 180-220 hours. During the coking process, a flue gas recirculation process is adopted, that is, the high-temperature flue gas exiting the bottom flue (2) is mixed with the primary air at the top of the furnace through the rising flue (4), the suction device (7) and the mixer to form a mixed gas that enters the carbonization chamber (1) together, thus realizing flue gas recirculation.

8. The method for preparing low-nitrogen foundry coke according to claim 7, characterized in that, Coking temperature control: In step S2, a segmented heating mode is adopted: 0-24 hours after loading the furnace, the heating rate is controlled at 10-12℃ / h to slowly preheat the coal cake and initially release volatile matter; 24-72 hours, the heating rate is adjusted to 8-10℃ / h to gradually reach the coking temperature, and the temperature is steadily raised to the set coking temperature over a period of 70-75 hours. The heating process is monitored in real time by multiple temperature sensors on the furnace body.

9. The method for preparing low-nitrogen foundry coke according to claim 7, characterized in that, Another part of the high-temperature flue gas from the bottom flue (2) of the furnace flows into the main flue and is discharged after waste heat recovery, desulfurization and denitrification.

10. The method for preparing low-nitrogen foundry coke according to claim 7, characterized in that, The temperature of the gas mixture is 400-500℃, and the oxygen content is 2-3% by mass.

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