Glass 101: Glass Furnace Types

 

Glass 101: Glass Furnace Types

Posted Krista Grayson on Dec 18, 2019

Humans have been making glass for over three thousand years.1 Despite its age, glass is both ubiquitous and cutting-edge, having found tried-and-tested applications in architecture, transportation, and insulation; as well as more novel applications in electronics, biomaterials, and renewable energy.2,3

Much has changed in three thousand years, and as we continue to designate glass to new applications, the technology that we use to produce various types of glass has improved and diversified. At Mo-Sci, for example, we use a variety of glass production methods to produce glasses with varying hardness, thermal shock resistance, electrical conductivity, optical characteristics, and many other properties. In this article we examine some of the most common glass production methods and furnace types and how they’re used to produce glass for different applications.4

Types of glass production processes

Almost all glass is produced in a furnace, where a precise mixture of raw materials is combined and melted into a homogeneous mixture. Like many industrial manufacturing processes, glass melting can be broadly categorized as either a continuous or discontinuous (batch) process.

Continuous glass production

Continuous glass melting processes work like a production line: a mixture of raw ingredients is added continuously at one end of the furnace, and glass is extracted continuously at the other. Continuous melting processes are high-throughput and preferred wherever a high volume (say 100-500 tons per day) of the same type of glass is required. Continuous processing is typically used for production of architectural glass, fiberglass, screens for consumer electronics, and food and beverage containers.

Discontinuous or batch production

In discontinuous or batch processing, a batch of raw materials is added to a single melting vessel and melted into glass. Once the glass is formed, it can be removed from the vessel and formed into products. Batch-processing glass in this manner trades throughput for flexibility, enabling manufacturers to produce multiple formulations of glass depending on customer demand, without cross-contamination.

Discontinuous glass melting is typically used for smaller production runs and is often required for uncommon glass types with relatively niche applications such as optics, electronics or signal applications.

Types of glass furnaces

The melting point of most glasses lies around 1,400-1,600°C, depending on its composition. As a result, glass production requires a great deal of heat energy, usually provided in the form of natural gas injected into a combustion chamber. Because of this high energy demand, glass furnaces are constructed to minimize heat loss, and often feature some form of waste-heat reclamation system.

Regenerative glass furnaces

Regenerative furnaces are one such example: these furnaces pipe hot exhaust gas out via a regeneration chamber containing a ‘checkerwork’ of refractory bricks – often referred to simply as ‘checkers’. These bricks have a high resistance to thermal shock as well as high specific heat capacity, and act as a thermal energy store.

Regenerative furnaces run in cycles: the direction of gas flow is periodically reversed so that combustion gas is passed over the now-hot refractory bricks on the way to the combustion chamber, absorbing and making use of the waste thermal heat from the previous half-cycle.

Regenerative furnaces always have an even number of regeneration chambers, so that heat regeneration can occur in both directions. Regenerative glass furnaces can be either cross-fired or end-fired depending on application: cross-firing, the use of multiple combustion gas inlets down opposing sides of the furnace, allows more precise control over the location and temperature of hot spots within the furnace; while end-firing (a single inlet of combustion gas at the end of the chamber) generally reduces structural heat losses due to the increased residence time of the combustion gases.5

Recuperative glass furnaces

Recuperative furnaces employ a slightly different approach to reclaim waste heat. In this type of furnace, the chimney and gas inlets are coupled with a radiative heat exchanger.

This enables the continuous transfer of heat from exhaust gas to combustion gas, without the requirement for gas flow reversal as in regenerative furnaces. This enables recuperative furnaces to be employed for continuous glass melting applications, while also offering a relatively low investment cost.

The absence of regeneration chambers also simplifies construction of the furnace and results in a smaller footprint; however, the heat transfer efficiency of recuperative furnaces is generally lower than that of regenerative furnaces.6

Oxygen-fuelled (“oxy-fuel”) glass furnaces

Oxygen-fuelled (“Oxy-fuel”) glass furnaces are a relatively new way of tackling the problem of high heat energy demand: by replacing the air entering the furnace with oxygen (typically at over 90% purity), the total amount of gas entering the chamber can be reduced while maintaining the same combustion energy input.

This means the energy required to heat the input gas is reduced, while also reducing waste heat in exhaust gas. Other furnaces waste a large amount of energy simply heating the nitrogen in the air.

In general, oxy-fuel furnaces have the same basic design as recuperative furnaces, with multiple lateral burners and a limited number of exhaust ports. Most oxygen-fired glass furnaces don’t use heat recovery systems to pre-heat the oxygen supply to the burners, although there are some developments in oxygen and natural gas preheating.

The benefits of oxy-fuel furnaces include cheaper furnace designs, lower NOx emissions per ton of molten glass, smaller flue gas volumes, smaller footprints for furnace systems, and reductions in fuel consumption.7

While oxygen costs may potentially exceed the reduction in fuel costs, research indicates that switching to an oxy-fuel furnace substantially reduces energy costs for both large and small glass manufacturing operations.8

All-electric glass furnaces

Electrically heated furnace technology is nearly as old as regenerative furnace technology.9 These work in a radically different way to conventional furnaces, avoiding combustion altogether and instead imparting heat energy to the glass mixture using high-voltage electrodes. These are typically used for fiberglass production but are also used for specialty glasses.

Typically used for small-batch production, all-electric furnaces offer high thermal efficiency, a high degree of control over temperature, and can yield highly homogeneous glass while minimizing atmospheric pollution and economizing raw materials that volatilize readily.10

References

1. Glass Timeline – Important Dates and Facts. Available at: http://www.historyofglass.com/glass-history/glass-timeline/. (Accessed: 22nd January 2019)
2. Bioactive Glass – Mo-Sci Corporation. Available at: https://mo-sci.com/bioactive-glass. (Accessed: 22nd January 2019)
3. Glass – Mo-Sci Corporation. Available at: https://mo-sci.com/sealing-glass. (Accessed: 22nd January 2019)
4. Lecture 3: Basics of industrial glass melting furnaces IMI-NFG Course on Processing in Glass. Hubert, M. (2015).
5. Regenerative Furnaces | Industrial Efficiency Technology Database Available at: http://ietd.iipnetwork.org/content/regenerative-furnaces. (Accessed: 22nd January 2019)
6. Recuperative Furnaces | Industrial Efficiency Technology Database
   Available at: http://ietd.iipnetwork.org/content/recuperative-furnaces. (Accessed: 17th June 2019)
7. Oxy-Fuel Furnace | Messer Group
   Available at: https://www.messergroup.com/minerals/glass/oxyfuel-furnace. (Accessed 17 June 2019)
8. Energy Efficiency Improvement and Cost Saving Opportunities for the Glass Industry An ENERGY STAR® Guide for Energy and Plant Managers. Worrell, E., Galitsky, C., Masanet, E. & Graus, W. (2008).
9. “The Efficient Future for the Glass Industry Is ‘All-Electric.’” Eurotherm by Schneider Electric, 27 Dec. 2018, https://www.eurotherm.com/en/glass-news/the-efficient-future-for-the-glass-industry-is-all-electric/.
10. Electric melting of glass. Stanek, J. & Matej, J. J. Non. Cryst. Solids 84, 353–362 (1986).
11. Glass Products – Mo-Sci Corporation. Available at: https://mo-sci.com/en/products. (Accessed: 22nd January 2019)

Are Electric Furnaces the Future of Glass Manufacturing?

 

Are Electric Furnaces the Future of Glass Manufacturing?

Posted Krista Grayson on May 16, 2019

Overview of glass production 

Glass production is typically energy-intensive. Glass furnaces may reach 1300-1550 ºC for the melting and refinement of the raw materials, depending on the formulation required. 

Natural gas and electricity are the main energy sources, however historically, the glass industry has favored gas because it is an established technology, with low price, high purity, ease of control and the fact that there is no requirement for storage facilities. Gas furnaces have long life-times, on average over 12 years and sometimes up to 20 years.

Until recently electric glass melting furnaces have been used for specialty glasses, and particularly glasses with significant volatile constituents such as fluoride opal glasses, borosilicates and lead crystal. Interest is growing in extending its use through the industry.

Electric glass furnace production

The most effective method of electric glass production is to use electrodes immersed in the glass either as electric boosting (providing 5-20 % of total energy input) or all-electric melting. The immersed electrodes are connected to a power supply and transformer, to pass an electric current through the glass. 

In all-electric furnaces, the melting energy comes from the electrodes (joule heat), with a gas burner being used for the initial start-up, or as an emergency heat source. These furnaces mainly operate ‘cold top’, where the raw material is distributed evenly over the melting surface of the glass, forming an insulating ‘batch blanket’. Melting and refining take place in one vertical process, with glass being drawn through a throat at the bottom of a deep melting tank.

Advantages of electric melting

Electric furnaces offer several advantages over gas furnaces. For example, they have very low direct emissions of CO2, thermal NOx or SOx emissions. With pressure to reduce emissions coming from both customers and legislation, this is a significant benefit. While it is possible to improve conventional gas furnaces to reduce emissions, this can result in more complex technology that results in additional maintenance, the use of non-environmentally friendly chemicals, and limitations to equipment lifespan. 

Another benefit is that heat losses from electric furnaces are much lower. The thermal efficiency of gas furnaces peaks at around 45%. This means more energy is lost as heat than is used to convert the raw materials to molten glass. Heat losses occur from the superstructure of the furnace and in the residual waste gases, even if heat recovery systems are used. In contrast, the electrical approach means that the melting energy is transferred directly into the glass. Thermal efficiency can be over 70% even in a small electric furnace and can reach 85% in a large electric furnace.

All-electric furnaces are also more energy efficient than gas-fired furnaces; they use around 35% less energy. The difference in energy efficiency is particularly important for small furnaces. As furnace size decreases, the energy efficiency of electric furnaces remains very high, whereas the efficiency of gas furnaces drops dramatically and can be less than 20%. 

Electric boosting can be a highly effective way to reduce overall energy consumption. It also means that energy release can be highly focused, helping to determine conditions in the glass bath. In some cases, a well-designed boost system can improve glass quality homogeneity, seed and stone losses. In contrast, in gas furnaces, where focused energy release is not possible, imprecise temperature profiles can be created in the glass. 

A key advantage of the cold-top electric furnace is that everything that goes into the batch stays in the glass, aside from the gases released from the melting process, which permeate out through the batch blanket. Losses of batch constituents such as fluorine, boron, lead, various volatile refining agents and other constituents are almost eliminated. 

Disadvantages of all-electric melting

While electric furnaces have lower capital costs, they have shorter life-times (2-7 years compared to 10-20 years for conventional furnaces) and higher energy costs. The economic viability of electric furnaces is closely related to the cost of electricity compared with gas. Higher thermal and energy efficiencies can offset this cost for smaller furnaces, but this might not be the case for larger furnaces. 

The low environmental impact is only maintained if the furnace can receive power from renewable energy sources and requires a power grid that is reliable and stable. 

There are also operational considerations. For example, the maintenance of electrodes to limit higher resistance caused by wear. It is not possible to melt higher temperature glasses (>1500C) and there is concern of corrosion/erosion of electrode material from certain glass compositions. Further, recycled glass may be an issue that requires new handling methods. 

Conclusion

In most places, it is still environmentally cleaner to burn fossil fuels in a furnace than to use them to generate electricity for electric melting. However, as renewables increase their contribution to electricity production, this situation will change. It also appears that improvements in energy efficiency of fossil fuel combustion technologies have leveled off. As emissions legislation kicks in and consumers increasingly demand materials and technologies that are environmentally friendly, there may be well a swing in glass manufacture from gas to electric energy. The other advantages of electric melting, such as better thermal efficiency and energy consumption, will also count in its favor.

References

1. https://www.eurotherm.com/efficient-future-for-the-glass-industry-is-all-electric 
2. https://www.glassmanevents.com/content-images/speakers/Andy-Reynolds-Fives.pdf 
3. http://www.electroglass.co.uk/articles/2010-09%20Electric%20Melting%20&%20Boosting%20for%20Glass%20Quality%20Improvement.pdf
4. http://ietd.iipnetwork.org/content/electric-melting