With uses in a wide range of applications it’s crystal clear that the glass industry presents a significant drive for industrial gases. Yogender Malik looks at the areas for this demand and the role of gases in the glass industry.
The glass industry is divided into four major segments. The container glass segment produces glass packaging products, such as bottles and jars. The float glass segment produces windows for residential and commercial construction, automobile windshields, mirrors, instrumentation gauges, and furniture, such as tabletops and cabinet doors. The fibre glass segment is composed of two distinct sub industries: building insulation (glass wool); and textile fibres used to reinforce plastics and other materials for the transportation, marine, and construction industries. The specialty glass segment produces handmade glass, tableware and oven-ware, flat panel display glass, light bulbs, television tubes, fiber optics, and scientific and medical equipment.
The glass industry is one of the most energy intensive industries, in fact it is second only to the aluminum industry and consumes the second highest amount of energy to make a single unit or product. During the last several decades, glass manufacturers have worked to combat energy problems caused much in part by an extremely high-energy melting process. The melting and refining stages require around 8.6 million of the 12.98 million Btu (British thermal units) per tonne of glass produced. Glassmakers have come up with a variety of partial solutions, including increased insulation, improved refractory and furnace designs, and more efficient burners. Some manufacturers have even tried changing the fuel source by using oxygen instead of air in furnaces. Industrial gases have helped to ease the energy intensity of the glass industry, in terms of reducing cost, improving processes and enhancing product quality in recent years.
Oxy-fuel melting involves the replacement of the combustion air with oxygen. The elimination of nitrogen from the combustion atmosphere reduces the volume of the waste gases and the use of recuperator systems to preheat the oxygen supply to the burners is usually avoided. The reduction of waste gas volume (typically down to one-third) makes it possible to save furnace energy.
Air fuel combustion has a nitrogen load, whereas in oxy-fuel combustion this waste gas volume is reduced and higher particle pressure of the three atomic gases leads to better heat transfer.
The efficiency arises because the system heats oxygen rather than air (which is approximately 80% nitrogen) to combustion temperatures. Some oxy-fuel furnaces use waste gases to preheat batch materials and cullet. The specific NOx production (kg/kg glass) is significantly reduced, but due to the reduced flue gas flows, NOx concentrations will be much higher than normal. Oxy-fuel furnaces have a basic design similar to unit melters, characterized by multiple lateral burners and one waste gas exhaust port.
Hydrogen is extensively used in the float glass process. An atmosphere of 5-10% hydrogen in nitrogen is used to blanket, and prevent from oxidizing, a tin bath. Molten glass flows from the furnace directly into the float bath. From here, glass is floated onto a bath of molten tin (highly oxidizing) and this process results in glass that has very low distortion and is used for automotive and architectural applications. Hydrogen reacts with oxygen that diffuses into the float bath atmosphere and keeps the tin from reacting with oxygen. If it reacts with oxygen, tin forms an oxide that will cause flaws in the glass surface. Molten glass is held in a controlled atmosphere with a ratio of approximately 90% N2 and 10% H2 and once cooled, the flat glass becomes hard enough to be removed. The hydrogen in the controlled atmosphere acts as a scavenging agent to ensure an oxygen-free environment, because the molten tin is highly sensitive to oxidation, even in trace quantities.
Since nitrogen is virtually inert, it is used to blanket furnace electrodes to reduce safety hazards and prevent oxidation. In float glass processing, nitrogen is used in combination with hydrogen as a protective atmosphere to prevent the tin bath from oxidizing.
Nitrogen boils at -195.8°C, is perfectly suited for the cooling process essential to some glass manufacturing and in container glass manufacturing, liquid nitrogen is injected to cool air to temperatures between 10°C and -30°C. This provides the optimum cooling rates needed to simultaneously cool the inside and the outside of a container.
Cooling both surfaces simultaneously stabilizes the container rapidly without inducing excessive stresses in the glass. Quicker stabilization also means the next container can be formed quicker, increasing production capacity. And, since the nitrogen quantity is controlled automatically, air temperatures remain constant around the clock, ensuring consistent quality and helping to improve the pack rate as well.
Helium in its gaseous form is used to eliminate impurities and also functions as a heat transfer medium during the production of fiber optic strands. Helium also plays an essential role as a heat transfer fluid and inert carrier gas for applying performance coatings to glass substrates.
Enrichment is the most basic form of oxygen use in glass melting applications, typically used in a furnace nearing the end of its campaign that is suffering from regenerator plugging or collapse. This regenerator damage is likely to have resulted in an increase in furnace pressure and reduction in air pre-heat temperature, necessitating more fuel and oxidant, which exacerbates the furnace pressure increase. Ultimately the furnace pressure will be too great for the now required gas and airflow, reducing glass pull rate.
Applying oxygen through enrichment results in a lower volume of exhaust gases. The most noticeable effect of introducing oxygen to replace air is a reduction in furnace pressure, allowing the regenerators to breathe again. The lower exhaust volume will have an increased residence time in the regenerators and will increase the pre-heat temperature, allowing a small reduction in fuel usage. The reduction in furnace pressure will enable additional fuel to be applied to the
furnace, to enable the melting of previously lost, or ‘recovered’ tonnage.
Slow adaptation of Oxy-Fuel technology in Flat glass sector
Though oxy fuel firing is more effective than traditional air-fuel firing, and increasingly new float plants are adapting to this method of firing the batch, it is penetrating at a slow rate in the float glass industry. The conversion to oxygen firing from a conventional regenerative air-fired furnace involves design and refractory changes that can only be implemented at the time of rebuild or cold repair. Rebuilds typically occur only once every 10-15 years, while furnaces cannot be halted mid-campaign for installation of oxygen technology.
Unlike some container glass and fibre glass furnaces that have converted to oxygen firing in recent years, the conversion to oxygen firing to produce float glass for automotive and architectural use has been quite limited.
Fibre Glass Industry
The glass fibre industry was one of the earlier sectors of glass industry to convert to oxy-fuel, partly due to environmental requirements, but also due to improvements in energy efficiency over traditional air fuel recuperative furnaces. Recent improvements in furnace and glass melting technologies, combined with greater awareness of energy use due to higher costs identified that a higher percentage of energy is now being used in the forehearths. In the example of composite glass fibre applications, the energy going into the forehearth is up to 50% of the energy going into the furnace. Traditionally, forehearth systems have not used heat recovery systems or preheating. This scenario offers the highest potential benefit of oxy-fuel, with theoretical energy savings of up to 70%.
The container glass industry is facing stiff competitive pressures from almost all sides. The growing penetration of PET containers is forcing glass manufacturers to reduce production costs and optimize processes. Increasingly strict environmental legislation adds to the challenges. One area offering scope for process efficiencies is surface coating. The use of carbon offers a number of advantages over traditional surface-coating methods. These include improved glass quality and the reduction in workplace concentrations of vapours and mists produced when using oils and emulsions. Burnt under precise conditions, pure acetylene separates into carbon and hydrogen. Almost 100% of the resultant carbon is deposited in a thin layer on the surface to be coated, with this carbon coat applied using an outside/post-mixing burner. Acetylene and oxygen are ignited by an acetylene/oxygen flame that burns permanently and the acetylene is shielded by an oxygen stream.
The installation of an enrichment system is relatively simple: oxygen is injected downstream of the combustion air fan prior to the reversal valve. The oxygen compatibility of the reversal valve materials frequently limits the maximum concentration of the enriched oxygen stream that can be used in the furnace.
Compared with other technologies the enrichment process is less efficient, since the increased percentage oxygen goes to all burners and regenerator ports. Enrichment is typically used on regenerative furnaces firing on either oil or gas and is applicable to all glass types.
Challenges of Oxy-Fuel conversion
Significant technical and economic challenges account for this slow rate of conversion. Strict quality requirements for optical properties, uniformity, and defect inclusions are unique to float glass products.
Oxygen firing affects the finished glass optical and fabricating properties and in-process compatibility with the float process tin bath. Resolving these complex issues requires unique technology and demands time and resources. A facility must manage unique process issues while also sustaining quality, productivity, and profitability critical to a viable operation.