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Analysis of Defects in Alumina Glass Produced by Full Oxygen Combustion Float Glass Furnace

Full oxygen combustion refers to the modification of the traditional air-fuel combustion system to an oxygen-fuel combustion system. The application of full oxygen combustion technology in glass melting furnaces has numerous advantages: the high flame temperature accelerates the glass melting process, significantly increasing production capacity by over 25%. Since pure oxygen is used as the combustion assistant instead of nitrogen in the air, the emission of nitrogen compounds is reduced by around 80%, contributing to environmental protection. Additionally, it saves more than 25% of the heat energy required for heating nitrogen. After adopting full oxygen combustion technology, there is no need for large regenerators, small furnaces, reversing systems, etc. The one-time investment in the furnace is reduced by one-third. Due to the simplified structure of the furnace, essentially a single component of the melting section, the footprint is significantly reduced, improving the operational environment and maintenance of the furnace. Consequently, full oxygen combustion technology has rapidly developed in glass production. Since the completion of the first full oxygen furnace by Corning, USA, in 1983, there have been 136 full oxygen furnaces in North America, used for melting various types of glass, including glass for television display tubes, container glass, specialty glass, glass fiber, and float glass.

China has only recently begun to focus on full oxygen combustion technology, and there is not yet complete mastery in terms of process control and quality control, especially for large-scale full oxygen glass furnaces. Issues such as excessive foaming, difficulty in resolving bubbles, accelerated erosion of refractory materials, and resulting glass defects have been observed during operation.

Analysis of Alumina Glass Defects

After running a full oxygen combustion float glass furnace for two years, alumina stones and nodules appeared in the glass at a rate of approximately 4-10 per square meter. Eight samples were taken for defect analysis, and their external appearance is shown in Figure 1.

1.Polarized Light Microscope Analysis
Observing the eight defect samples under a polarized light microscope, as shown in Figure 2, revealed that seven of them were corundum stones, often accompanied by periclase. Diffusion stripes with melt erosion reactions were observed on the edges. One sample was a nodule. Thin sections of samples #2 and #7 showed strip-shaped corundum and feather-like periclase.

2. Energy Spectrum Analysis
Using a scanning electron microscope-energy spectrometer, composition analysis was conducted on the eight defects. The results are shown in Table 1 and Figure 3.
The energy spectrum results indicated that the stones contained corundum (Al2O3), spinel (MgO-Al2O3), and periclase (Na2O-Al2O3-2SiO2), while the nodules were rich in aluminum oxide.

Analysis of the Causes of Alumina Glass Defects

From the results of polarized light microscopy and energy spectrum analysis, the glass defects were mainly corundum stones, accompanied by spinel and periclase. Combining the characteristics of refractory materials used in full oxygen combustion glass furnaces and the characteristics of furnace gas, it can be inferred that the glass defects mainly come from the erosion products of the throat flat and adjacent refractory materials. Alumina nodules form when individual corundum stones stay in the glass for a longer time, melting partially without complete diffusion.

The refractory materials used in full oxygen combustion glass furnaces differ from those in ordinary air-assisted combustion furnaces. In the melting section, fused α-β alumina bricks are used. The clearing section chest wall consists of β alumina bricks, and the rear wall of the melting section uses α-β alumina bricks, while the throat flat uses magnesia-containing alumina bricks. The composition of the glass defects is consistent with the chemical composition of the refractory materials used in the throat flat.

Full oxygen combustion glass furnaces do not have regenerators, and the furnace gas is discharged from the front and rear flues without the introduction of cold air. The flame temperature is also higher, resulting in an overall higher temperature in the glass furnace. Compared to ordinary glass furnaces with air-assisted combustion, the temperature of the rear wall of the melting section is approximately 30°C higher. For ordinary furnaces, after running for some time, the lower edge of the rear wall is covered with stalactite-like refractory melt erosion products. In contrast, for full oxygen combustion furnaces, the condensation of furnace gas occurs slightly later, at the throat flat. The high concentration and temperature of steam in the furnace gas of a full oxygen furnace lead to relatively high evaporation of alkali, and without the dilution of nitrogen in the air, the concentration of alkali vapor is significantly higher than in ordinary furnaces. Consequently, the condensation of furnace gas contributes to the erosion of refractory materials.

Conclusion
  1. Full oxygen combustion produces steam concentrations 3.5 times higher than air combustion. Steam is adsorbed on the surface of refractory materials, accelerating the erosion reaction. The high concentration of alkali vapor in the furnace gas of full oxygen combustion glass furnaces accelerates the erosion reaction. The inner surface temperature of the furnace roof increases by 25-50°C compared to air-assisted combustion, and the large gas flow rate exacerbates the erosion of refractory materials.
  2. Alumina bricks have advantages such as strong resistance to alkali vapor erosion, high compressive strength under thermal load, low creep at temperatures up to 1600°C, and good thermal insulation. They are suitable for use in full oxygen combustion glass furnaces. However, with the extension of operating time, glass defects such as corundum stones and alumina nodules may still occur.
  3. Strict control is necessary in the implementation of full oxygen combustion technology to reduce the likelihood of defects. This includes appropriately reducing the pressure in the melting section, allowing high-concentration alkali vapor in the furnace gas to be discharged as much as possible through the flue, and adopting lower temperature control for melting and clarification to reduce the volatility of alkali vapor. Stabilizing the FEF airflow and reducing airflow fluctuations are also essential.
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