In The Production Of E-glass Fiber in A Tank Furnace, The Control Of Melting Temperature And Flame...
The production of E-glass fiber (alkali-free glass fiber) in a tank furnace is a complex high-temperature melting process. The melting temperature regime is a key process control point, directly affecting the quality of the molten glass, melting efficiency, energy consumption, furnace life, and the final fiber properties. This temperature regime is achieved by adjusting the flame, electric flux, etc.
I. Melting Temperature of E-glass
1. Melting Temperature Range: Complete melting and refining of E-glass typically requires very high temperatures. Its typical melting zone (hot spot) temperature is generally in the range of 1500°C - 1600°C.
Specific target temperatures depend on:
* Batch composition: Specific formulations (such as whether it contains fluorine, the level of boron, whether it contains titanium, etc.) will affect melting characteristics.
* Furnace design: Furnace type, size, insulation effect, burner arrangement, etc.
* Production goals: Melting rate, molten glass quality requirements.
* Refractory materials: The erosion rate of refractory materials at high temperatures limits the upper temperature limit. The temperature in the refining zone is typically slightly lower than the hot spot temperature (approximately 20-50°C lower) to facilitate bubble removal and glass homogenization. The working passage temperature is significantly lower (usually between 1200°C and 1350°C) to bring the glass to the appropriate viscosity and stability for wire drawing.
2. Importance of Temperature Control: Melting Efficiency: Sufficiently high temperatures are crucial to ensuring the complete reaction of the batch materials (quartz sand, pyrophyllite, boric acid/borate, limestone, etc.), complete dissolution of the sand particles, and full release of gases. Insufficient temperature leads to "raw material" residue (unmelted quartz particles), stones, and increased bubbles. Glass Quality: High temperatures promote the refining and homogenization of the glass, reducing defects such as streaks, bubbles, and stones. These defects severely affect fiber strength, breakage rate, and continuity. Viscosity: Temperature directly affects the viscosity of the glass. Wire drawing requires glass within a specific viscosity range. Refractory Material Erosion: Excessively high temperatures drastically accelerate the erosion of kiln refractory materials (especially fused zirconia-corundum bricks, AZS), shortening kiln life and potentially introducing refractory stone formation.
Energy Consumption: Maintaining high temperatures is the primary source of energy consumption in pool kilns (typically accounting for over 60% of total production energy consumption). Precise temperature control to avoid excessive temperatures is crucial for energy conservation.
II. Flame Regulation
Flame regulation is the core method for controlling the melting temperature distribution, achieving efficient melting, and protecting the kiln structure (especially the main arch). Its main goal is to create an ideal temperature field and atmosphere.
1. Key Regulation Parameters: Fuel to Combustion Air Ratio (Air-Fuel Ratio), also known as Oxygen-Fuel Ratio in pure oxygen combustion systems: Goal: To achieve complete combustion. Incomplete combustion wastes fuel, lowers flame temperature, produces black smoke (carbon deposits) that contaminate the molten glass, and clogs heat exchangers; excessive air carries away a large amount of heat, reducing thermal efficiency and potentially exacerbating arch oxidation erosion. Adjustment: The air-fuel ratio is precisely controlled based on flue gas analysis (O₂, CO content).
E-glass furnaces typically control the O₂ content in the flue gas to around 1-3% (slight positive pressure combustion). Impact on Atmosphere:
The air-fuel ratio also affects the furnace atmosphere (oxidizing or reducing), which has a slight impact on the behavior of certain batch components (such as iron) and glass color, but a relatively small impact on E-glass itself (which requires colorless and transparent glass). Flame Length and Shape:
Objective: To form a flame that covers the surface of the melting pool, possessing a certain rigidity and spreading ability. Long Flame vs. Short Flame:
Long Flame: Large coverage area, relatively uniform temperature distribution, less thermal shock to the arch, but local temperature peaks may not be high enough, and penetration into the batch "drilling" zone may be insufficient. Short flame: High rigidity, high local temperature, strong penetration into the batch layer, conducive to rapid melting of raw materials, but uneven coverage, easily causing local overheating (hot spots are more prominent), and significant thermal shock to the arch and breast wall. Adjustment: Achieved by adjusting the burner lance angle, fuel/air injection velocity (momentum ratio), swirl intensity, etc.
Modern tank furnaces commonly use multi-stage adjustable burners. Flame direction (angle): Objective:
To effectively transfer heat to the batch and glass melt surface, avoiding direct flame impact on the arch or breast wall. Adjustment: Adjusting the pitch and horizontal angles of the burner lance. Pitch angle: Affects the flame's effect on the batch ("licking") and coverage of the liquid surface. Too low an angle (flame too downward) may scour the liquid surface or batch, causing flying material to erode the breast wall; too high an angle (flame too upward) results in low thermal efficiency and excessive heating of the arch. Horizontal Angle: Affects the distribution of the flame across the width of the kiln and the location of hot spots.
2. Objectives of Flame Adjustment:
* **Forming Reasonable Hot Spots:** Creating the highest temperature zone (hot spot) at the rear of the melting pool (usually after the sump). This is the key area for clarification and homogenization of the molten glass, and also the "engine" controlling the flow direction of the molten glass (from the hot spot to the feed inlet and working section).
* **Uniform Liquid Surface Heating:** Avoiding local overheating or undercooling, reducing convection unevenness and "dead zones" caused by temperature gradients.
* **Protecting the Kiln Structure:** Preventing the flame from scouring the arch and breast walls, preventing local overheating from causing rapid erosion of refractory materials.
* **Efficient Heat Transfer:** Maximizing the radiation and convection heat transfer efficiency of the flame on the surface of the batch and molten glass.
* **Stable Temperature Field:** Reducing fluctuations and ensuring stable molten glass quality.
III. Comprehensive Control of Melting Temperature and Flame Adjustment
1. Temperature is the goal, flame is the means: Flame adjustment is the main way to control the temperature distribution within the kiln (especially the location and temperature of hot spots). 2. Temperature Measurement and Feedback: Continuous temperature monitoring using thermocouples and infrared thermometers placed at key locations in the kiln (feed inlet, melting zone, hot spots, refining zone, and passageways) serves as the basis for flame adjustment.
3. Automatic Control System: Modern large-scale tank furnaces generally employ DCS/PLC systems. Based on the set temperature curve and measured values, automatic control of the flame and temperature is achieved by adjusting parameters such as fuel quantity, combustion air volume, and burner angle/damper.
4. Process Balance: Finding the optimal balance between ensuring glass melt quality (high-temperature melting, good refining and homogenization), protecting the kiln (avoiding excessively high temperatures and flame erosion), and reducing energy consumption is crucial.
