As a core thermal equipment in the glass industry, the operation status of the furnace directly affects the quality of glass products, production energy consumption, and overall operating costs. During the long-term continuous operation of the furnace, the organization and control strategies of the flame, as well as the actual performance of refractory materials, especially fused cast AZS blocks, constitute the two most critical technical elements that determine the furnace's lifespan and operational efficiency. Scientifically regulating the flame length is not only a need for optimizing the thermal process, but also a core technical means to achieve effective protection of fused cast AZS blocks and ensure the safe, stable, and long-term operation of the furnace.
1. Requirements for the optimal flame length in the furnace pool
The core objective is to ensure that the fuel is completely burned before reaching the opposite furnace wall (or the port side slope), while achieving uniform heating of the entire glass liquid surface, and avoiding local scouring and overheating of the furnace refractory materials.
The following are several key requirements for determining and evaluating the optimal flame length:
1.1 Geometric requirements (core constraints)
This is the most direct and basic requirement. "Rigidity" requirement (must be met):
The flame must not scour the opposite furnace wall or the port slope. This is the primary red line. If the flame is too long, it will directly impact the adjacent refractory material, causing a sharp acceleration of local high temperature and erosion of the fused cast AZS-33#/36#/41# blocks, significantly shortening the furnace's lifespan and potentially causing glass liquid contamination.
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Ideal state: The end of the flame (i.e., the tail of the luminous part) should just reach or slightly exceed the front edge of the opposite furnace wall (or the bottom of the port neck outlet slope) to ensure that the fuel is completely burned after entering the melting furnace space and releases heat to the glass liquid instead of the wall.
1.2 Combustion and heat transfer requirements (performance core)
The geometric size of the flame length is a prerequisite, but its combustion state directly determines energy efficiency and melting quality.
►Complete combustion: The optimal flame length must ensure 100% complete combustion of the fuel before reaching the target position. Unburned fuel will continue to burn in the furnace tail or regenerator, which not only causes energy waste but also leads to clogging and erosion of the grid body in the regenerator. Burning in the grid body pores generates a local high temperature, accelerating the erosion and sintering blockage of alkaline dust on the refractory materials. It also increases the content of harmful substances (such as CO) in the flue gas and reduces thermal efficiency.
►Uniform coverage and heat release: The flame is not only a heat source but also a "heating body". Its length and shape should be able to uniformly cover the entire glass liquid surface.
Short flame: The heat is concentrated too much at the front end, resulting in excessively high temperatures in the "hot spot" area and insufficient temperatures at the furnace tail. This causes uneven melting, poor clarity, and local high temperatures, which also accelerate the erosion of the chest wall, arch, and sidewall.
►Long flame (but not hitting the furnace wall): Although it may cover uniformly, the temperature at the tail of the flame is low, the blackness decreases, and the radiation heat transfer efficiency is reduced, which also affects the overall melting efficiency.
►Stability and rigidity: The optimal flame should have a certain "rigidity", that is, it does not drift, does not sway, and has a clear outline. This depends on a good atomization effect, appropriate combustion air speed (momentum matching), and stable furnace pressure. A stable flame is the basis for efficient heat transfer and refractory material protection.
1.3 furnace protection requirements (lifespan key)
Flame characteristics directly affect the furnace's lifespan, especially for expensive fused cast AZS blocks.
►Avoid local overheating and erosion: As mentioned above, scouring the furnace wall is strictly prohibited. At the same time, it is also necessary to avoid the flame sweeping directly over the big arch or pressing directly over the glass liquid surface. The optimal flame should extend along the predetermined spatial path, transferring its energy through radiation to the glass liquid and furnace roof, rather than directly impacting the refractory materials through convection.
►Control of NOx generation: From an environmental perspective, the length and shape of the flame affect the temperature field, which in turn influences the generation of thermal NOx. Generally, a longer and more dispersed flame has a lower peak temperature than a short and rapid flame, which is beneficial for reducing NOx emissions.
2. Application characteristics and technical requirements of fused cast AZS blocks in key parts of the furnace
The doghouse area is one of the most complex parts of the furnace operation. This area simultaneously contains solid raw materials, molten glass liquid, and gaseous combustion products in a three-phase coexistence state, with a temperature fluctuation range of 200-300℃. Fused cast AZS blocks in this area need to withstand mechanical wear from the raw materials, chemical erosion from alkaline components, and rapid temperature changes. Therefore, this area typically uses fused AZS-33 blocks or higher-grade materials with a zirconia content of 33%-35%, which have an appropriate glass phase (about 20%), ensuring good thermal shock resistance and sufficient erosion resistance. The brick structure design adopts a special trapezoidal or wedge-shaped masonry method, combined with air cooling or water cooling systems, effectively controlling the working temperature of the brick.
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2.2 Design of the port and thermal protection system
The port serves as the flame passage, and its structure design directly affects the flame shape and temperature distribution within the furnace. Modern end fired furnaces typically use a multi-layer composite structure: the working surface uses dense fused AZS-41 blocks, with an oxide zirconia content of over 41% and a glass phase content of less than 15%, ensuring excellent high-temperature strength and erosion resistance; the middle layer uses medium-grade AZS materials as a transition; the outer layer uses high-quality sintered zirconia mullite bricks or corundum bricks, forming a gradient refractory structure. At the same time, a dedicated cooling air system is set up at the upper part of the port, precisely controlling the cooling air volume and speed to form a stable condensation protective layer on the hot surface of the brick, significantly extending its service life.
2.3 Optimization of throat structure and anti-erosion measures
The throat is the most demanding part of the furnace in terms of working conditions, with the flow speed of the glass liquid here reaching 2-3 times that of the normal area, and the temperature fluctuation range is small, but the mechanical erosion is intense. This part uses fused AZS-41# material without shrinkage cavities, through a special inclined pouring process, making the material's crystalline orientation consistent with the flow direction of the glass liquid, improving the anti-erosion ability. The structural design adopts a gradually contracting streamline channel to reduce local vortices and erosion. At the outlet of the throat, multiple cooling devices are set up, precisely controlling the temperature to form an appropriate viscosity gradient when the glass liquid flows out, reducing the erosion on the downstream refractory materials.
However, fused cast AZS blocks face multiple severe challenges throughout their service life. The first is chemical erosion, where the molten glass liquid reacts with the alkali metal oxides (such as sodium oxide, potassium oxide, etc.) released from the raw materials, continuously causing chemical corrosion and liquid phase penetration on the brick surface, leading to gradual dissolution and deterioration of the material. The second is mechanical erosion, where the glass liquid in the furnace, driven by temperature differences and the production flow formed by the melting of the raw materials, causes continuous erosion of the sidewall blocks, especially the areas with fluctuating liquid surfaces, gradually thinning the brick. The third is thermal stress damage. If the temperature distribution in the furnace is uneven or undergoes drastic fluctuations, it will cause significant thermal stress in the brick. When this stress exceeds the material's tolerance limit, microcracks will initiate and expand, causing the structural damage process of the brick to accelerate sharply.
3. The direct impact of flame length on the furnace environment and the service life of fused cast AZS blocks
The geometric length of the flame is one of the core parameters in the combustion process, fundamentally determining the spatial position of energy release within the furnace and the overall temperature field distribution, and thus having a decisive impact on the service environment and failure process of each fused cast AZS block.
3.1 Flame length and temperature uniformity
Flame length control directly affects the temperature distribution within the furnace. An excessively short flame will cause the heat to concentrate in the "fire root" area near the port opening, resulting in the chest wall, arch top, and sidewall blocks in this area remaining in an overheated state for a long time, and significantly accelerating the rate of high-temperature creep and erosion. An excessively long flame will cause the high-temperature zone to extend towards the rear end of the furnace, possibly causing the temperature in the liquid throat area to rise abnormally, changing the flow characteristics of the glass liquid, and intensifying the erosion of the refractory materials in this area. The ideal flame length should ensure that the heat is uniformly released along the length of the furnace, forming a stable and appropriately positioned hot spot, avoiding any area of the masonry from experiencing local overheating. When the flame length is controlled at 0.9-1.1 times the length of the furnace, the temperature distribution inside the furnace is the most ideal: the hot spot temperature is stable at 1560-1580℃, the longitudinal temperature difference is controlled within 30℃, and the lateral temperature difference does not exceed 15℃.
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3.2 Flame stiffness and local heat load
The stiffness characteristics of the flame determine the intensity of heat radiation from the flame to the furnace top and chest wall. A flame that is too rigid will directly impact the furnace top and the opposite chest wall, causing the fused cast AZS blocks in these areas to overheat and erode. By adjusting the flame stiffness, the distribution of heat inside the furnace can be optimized, reducing the heat load on the key parts of the refractory materials. The adjustment of flame stiffness is mainly achieved by changing the momentum ratio of fuel and auxiliary combustion air. When the momentum ratio is controlled at 1.05-1.15, the flame has the appropriate stiffness, ensuring sufficient penetration while avoiding direct blowing on the furnace top and chest wall. Through three-dimensional numerical simulation optimization, the highest temperature region of the flame is controlled to be 0.8-1.2 meters above the center line of the furnace pool, maintaining the inner surface temperature of the furnace roof at 1550-1580℃, and the temperature of the chest wall at 1500-1530℃, all within the safe usage temperature range of the fused cast AZS blocks.
3.3 Flame atmosphere and chemical erosion
The oxidation-reduction characteristics of the flame directly affect the volatilization and condensation behavior of volatile components in the raw materials. A reducing flame will accelerate the volatilization of alkali metal oxides, and these volatiles condense at the lower part of the chest wall and the rear part of the furnace roof at lower temperatures, causing severe chemical erosion of the electric-melting bricks. Maintaining an appropriate oxidizing flame can effectively control the generation and migration of volatiles, reducing the chemical erosion on the refractory materials. By precisely controlling the air-fuel ratio within the range of 1.05-1.10, maintaining a micro-oxidizing atmosphere (oxygen content 2%-3%) inside the furnace, effectively inhibiting the formation of a reducing atmosphere. This atmosphere control reduces the volatilization amount of alkali metal oxides in the raw materials by 20%-30%, and the condensation area of volatiles moves towards the rear end of the furnace, avoiding the formation of high-concentration alkali vapor accumulation at the front part of the chest wall and furnace top.
4. The influence of flame length on the melting efficiency and clarification quality of the glass liquid
4.1 The influence of flame length on the melting efficiency
The flame length mainly affects the melting rate and energy consumption by controlling the release position and efficiency of heat in the furnace space.
When the flame length is ideal, its high-temperature zone can precisely cover the center of the glass liquid surface and form a stable high-temperature "hot spot" at the halfway point of the furnace length. This hot spot is the engine of the melting process, which can establish a powerful and stable glass liquid convection circulation: the hot liquid flows upward to the surface and moves forward, heating and melting the newly added raw materials; the cooled glass liquid then sinks along the bottom of the pool and flows back, being re-heated. This efficient and orderly convection enables the maximum utilization of heat energy, significantly accelerating the melting reaction of the raw materials, thereby producing more qualified glass liquid within a unit time and reducing the unit energy consumption.
If the flame is too short, the heat will be overly concentrated in the "fire root" area near the feeding port. A large amount of hot smoke cannot be fully exchanged before entering the heat storage chamber, resulting in significant heat waste. More seriously, the excessive heat at the front end will cause the surface of the raw materials to quickly sinter and form a hard shell, which instead hinders the absorption and melting of the lower layer materials, leading to a decrease in the melting rate and an increase in energy consumption.
If the flame is too long, although it seems to cover a large area, its tail has a low temperature and weak brightness, and the radiation heat transfer efficiency drops sharply. The energy cannot be effectively transferred to the glass liquid, also resulting in low heat utilization. At the same time, the excessively long flame will extend the high-temperature zone backward, possibly disturbing or even suppressing the original hot spot, causing the "engine" of the glass liquid convection to lose stability, and making the overall melting process inefficient and difficult to control.
4.2 The influence of flame length on the clarification quality
Clarification is the process of removing dissolved micro-bubbles from the glass liquid. It relies extremely on the specific and stable temperature gradient and chemical environment within the furnace, and the flame length is the key to shaping this environment.
The ideal flame length helps to form a clear and reasonable temperature curve: the melting zone has a high temperature, the peak temperature of the hot spot reaches its maximum, and then the temperature gradually decreases steadily in the clarification zone. This environment creates perfect conditions for the elimination of bubbles: at the high-temperature hot spot, the viscosity of the glass liquid is the lowest, and micro-bubbles are easy to expand and merge; subsequently, when the glass liquid flows into the slightly lower but extremely stable clarification zone, the viscosity increases moderately, providing sufficient time and a calm, non-boiling thermal environment for the bubbles to float up, ensuring that the glass liquid flowing into the forming stream is clear and uniform.
If the flame is too short, the direct consequence is insufficient temperature in the clarification zone. The viscosity of the glass liquid in this area is too high, and the bubbles are difficult to grow and float out. A large number of micro-bubbles are forced to remain in the glass liquid, resulting in severe poor clarification and product scrapping.
If the flame is too long, the harm is more complex. It may cause the high-temperature zone to extend abnormally to the clarification zone or even the flow hole, causing the refractory materials in these areas to be excessively eroded, and the resulting alteration substances will contaminate the glass liquid, forming streaks and stones. More importantly, the unstable high temperature will disrupt the required thermal balance in the clarification zone, easily triggering "secondary boiling" - that is, the dissolved gases re-evolve due to temperature or atmosphere fluctuations, forming new bubble clusters, causing the entire clarification process to be ruined. In addition, the excessively long flame often accompanies a reducing atmosphere, which will intensify the volatilization of alkali metals, and the alkali vapor condenses on the chest wall and arch top and drips back to contaminate the glass liquid, causing chemical unevenness and also damaging the clarification quality.
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5. Comprehensive measures to achieve the optimal flame length for protecting fused cast AZS blocks
Achieving the optimal flame length is a core technical aspect for ensuring the efficient and stable operation of the rotary furnace with the end-fired furnace and extending the service life of fused cast AZS blocks. Several comprehensive measures need to be taken:
5.1 Scientific design of the combustion system and selection of fused cast AZS blocks
During the design of the combustion system, it is necessary to scientifically design the port structure and the selection of the burner based on the furnace structure, fuel characteristics, and production process requirements. Parameters such as the inclination angle and throat size of the port should match the characteristics of the burner to ensure the complete mixing of fuel and air. Selecting the appropriate performance grade of fused cast AZS blocks for different working environments of the furnace is crucial. In the doghouse area, high-temperature resistant fused AZS-33 blocks with excellent thermal shock resistance should be selected, with a zirconium oxide content of 33%-35% and an appropriate amount of glass phase (about 20%), which can ensure good thermal shock resistance and sufficient erosion resistance. The port outlet area should use dense fused AZS-41 blocks with a zirconium oxide content of over 41% and a glass phase content of less than 15%, ensuring excellent high-temperature strength and erosion resistance. The throat area should use non-crackable fused AZS-41 material, with a special inclined pouring process to ensure that the crystal orientation of the material is consistent with the flow direction of the glass liquid, improving the resistance to erosion.
5.2 Fine operation control and protection of fused cast AZS blocks
During actual operation, precise control of the air-fuel ratio is required to adjust the combustion state, maintaining a slightly oxidizing atmosphere (oxygen content 2%-3%), ensuring complete combustion while avoiding damage to fused cast AZS blocks caused by a reducing atmosphere. The reducing flame will intensify the volatilization of alkali metal oxides, which condense at the lower part of the chest wall and the rear part of the furnace top at lower temperatures, causing severe chemical erosion to the fused cast AZS blocks. It is also necessary to adjust the pressure and flow rate of the fuel and the auxiliary combustion air to control the momentum ratio between them at 1.05-1.15, forming a rigid and moderate flame shape to avoid the soft impact of the flame on the furnace roof and chest wall. At the same time, maintaining a slight positive pressure inside the furnace (5-10 Pa) is necessary to prevent local temperature fluctuations and atmosphere changes caused by the intake of cold air, reducing thermal shock damage to the fused cast AZS blocks.
5.3 Establish a complete monitoring system and early warning mechanism
Deploying thermal thermometers, high-temperature cameras, and infrared thermal imagers is necessary to monitor the temperature field inside the furnace comprehensively. High-precision thermometers should be buried in key areas to monitor the temperature distribution of the bricks and ensure that the fused cast AZS blocks in all parts are operating within the designed temperature range. Using high-temperature infrared thermal imagers to monitor the external surface temperature field of the furnace throughout the day, and monitoring the development of brick cracks through acoustic sensors. Using an atmosphere analyzer to continuously monitor the composition of the exhaust gas to provide data support for combustion optimization. Regularly checking the erosion condition of the fused cast AZS blocks through observation holes and establishing a brick health record to promptly detect and correct operational deviations through data analysis. These monitoring data can help establish a residual life prediction model for fused cast AZS blocks, with a prediction accuracy of over 85%, providing a scientific basis for preventive maintenance.
5.4 Strengthen maintenance management and technical collaboration
Formulate strict shutdown maintenance plans, and during each cold shutdown, detailed inspections of the remaining thickness, crack conditions, and erosion forms of the fused cast AZS blocks in each part should be conducted, establishing a complete "health record". This is not only the basis for replacing the bricks but also an important reference for analyzing the advantages and disadvantages of the previous furnace period's operation and optimizing the operation of the next furnace period. Establishing deep technical cooperation with refractory material suppliers to jointly analyze the root cause of brick damage, whether it is a material issue, thermal operation issue, or mechanical stress issue. Based on the specific operating conditions of the furnace and damage analysis, optimizing the brick type design and masonry scheme, providing professional suggestions on flame adjustment, temperature control, etc. Through this technological collaboration, it is possible to ensure that the fused cast AZS blocks can perform at their best in various working conditions, significantly extending their service life.
By systematically implementing the above measures, a stable working environment can be created for the fused cast AZS blocks, effectively prolonging the service life of the furnace and enhancing production efficiency.
6. Innovation and performance enhancement of fused cast AZS block materials
To cope with increasingly demanding furnace operation environments, the material research and development of fused cast AZS blocks is also continuously advancing. New fused cast AZS materials with high zirconium content and low glass phase have been gradually applied in high-temperature and high-erosion areas. These materials significantly improve erosion resistance and high-temperature strength by optimizing crystalline structure and reducing internal defects. Moreover, the development of composite refractory materials has also expanded the possibilities for the application of fused cast AZS blocks. For example, coating the surface of fused cast AZS blocks with special functional coatings (such as alumina or zirconia-based coatings) can effectively prevent the penetration of glass liquid and alkali vapor, prolonging the service life of the brick.
In terms of manufacturing processes, advanced technologies such as isostatic pressing, high-temperature sintering, and directional crystallization have further improved the density and mechanical properties of electro-melting bricks. Through topological optimization of the brick structure, weight reduction while maintaining its load-bearing capacity and thermal shock resistance provides a material basis for lightweight design and energy-saving operation of furnaces.
7. Importance of systematic management and personnel training
Even the most advanced technologies and materials cannot be achieved without human operation and management. Therefore, establishing systematic furnace operation and management norms and strengthening personnel training are important guarantees for achieving long-term stable operation of end-fired furnaces. Enterprises should formulate detailed standardized operating procedures, covering ignition, temperature rise, normal operation, and furnace shutdown cooling, clearly defining the control range and adjustment methods of key parameters. Regularly organize technical training and exchanges to enhance the ability of operators to judge furnace operation status and handle emergencies.
At the same time, establishing a cross-departmental collaboration mechanism, strengthening communication and collaboration among departments such as process, equipment, refractory material management, and procurement, ensures quality control throughout the process from brick material selection, masonry construction, to operation and maintenance. By introducing performance assessment and incentive mechanisms, enhancing employees' awareness and responsibility for the stable operation of furnaces.
8. Conclusion and outlook
The long-term stable operation of end-fired furnaces is a systematic project that requires the integration of advanced combustion control technology, high-quality fused cast AZS block products, and refined production management. By deeply understanding the relationship between flame characteristics and fused cast AZS block performance, implementing systematic protection strategies can not only significantly extend the service life of furnaces but also improve production efficiency and product quality. Modern glass manufacturing enterprises should incorporate flame optimization and refractory material protection as key technical management contents, establish a complete technical standard and management system, and provide reliable guarantees for achieving efficient and stable production.
In the future, with the deep integration of technologies such as artificial intelligence, big data, and the Internet of Things, the operation management of the end-fired furnace will further move towards intelligence and visualization. Through the analysis and mining of massive operation data, real-time diagnosis of the furnace status and predictive maintenance can be achieved, ultimately reaching the goal of unmanned intelligent operation. At the same time, the continuous breakthroughs in new materials and new processes will also inject new impetus into the performance improvement and application expansion of fused cast AZS blocks.
Practice has proved that only by perfectly combining advanced combustion control technology with high-quality fused cast AZS block products can the production potential of the furnace be maximized, creating continuous economic benefits for the enterprise. In this regard, establishing a long-term technical cooperation relationship with experienced refractory material suppliers often yields twice the result with half the effort.
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