Glass is known for both its fragility and the characteristic firmness of a solid material. The glassmaking process could be compared to a cooking recipe, in that it involves a careful selection of ingredients and a set of steps, such as heating the batch at the right temperature and then correctly cooling the end result.
The ingredients consist of naturally occurring raw materials: sand, silica, limestone, and sodium carbonate or soda ash. Sand is an essential element in glass production and requires a modifying agent such as limestone to reduce the furnace temperature necessary to melt the batch. Limestone makes the glass more durable.
The exact composition of the raw materials may vary depending on the specific requirements of the type of glass that is to be manufactured. Other materials, such as iron oxide, can be added to impart colour to the glass.
Once the ingredients have been selected, they are mixed into a batch and heated at high temperatures in a furnace where they melt at around 1500°C to produce molten glass. At these elevated temperatures, the glass is in liquid form, but it solidifies at room temperature. This makes it possible to mould and work the glass into different shapes.
To reach these high temperatures, the glass-melting furnace requires a significant amount of energy for which it largely depends on natural gas and other fossil fuels. Guaranteeing the quality of the end product also means these high temperatures must be maintained consistently for extended periods of time.
This has two serious drawbacks for the industry: a great dependence on fossil fuels and a significant environmental impact due to the emission of greenhouse gases.
After the melting process, the glass is moulded while still in a liquid state. The molten glass is poured into a mould to obtain the desired shape, in this case, a bottle.
The process concludes with a cooling phase, which is done gradually to release any moulding-related tension in the material and facilitate its transformation into a solid end product such as a glass bottle. This also helps prevent fissures from forming in the glass and strengthens the structural integrity of the bottle.
The glass-melting furnace is unquestionably the most important source of emissions and energy consumption in the manufacturing of glass containers. For this reason, the majority of sustainable initiatives in the sector focus on this phase of the production process. One such initiative – the use of recycled glass – has been gaining momentum in recent years.
Glass can be made from virgin raw materials or waste glass (crushed glass known as cullet). The latter can be further divided into internal and external cullet.
Internal cullet has approximately the same composition as the end glass product, because essentially it consists of glass that failed to pass quality control during the manufacturing process [1]. External cullet is primarily post-consumer waste glass that has been collected for recycling [2].
Adding recycled glass reduces the melting temperature and saves energy, because the chemical reactions between the individual components that comprise the glass have already taken place (direct energy savings: a 10% increase of cullet usage results in energy savings of 2.5–3%) [3].
In addition, the use of recycled materials reduces the need for raw materials (100 kg of recycled materials can substitute around 118–122 kg of primary raw materials), indirectly cutting down on energy consumption and emissions which would otherwise be generated during the extraction and processing of these ingredients.
When it comes to recycled material, it is important to consider the colour of the glass. For instance, green bottles produce undesirable hues during the melting process of clear glass, whereas the use of white cullet to make coloured glass creates defects in the end product.
Likewise, each type of coloured glass has a specific recycling rate, with green glass reaching up to 95%, while the rate for white glass is only 70%.
From an energy source perspective, natural gas and fossil fuels aren’t the only options. There are alternatives today. Electric glass-melting furnaces, for instance, are more efficient and do not generate direct greenhouse gas emissions. However, this option isn’t cost-effective from a financial standpoint due to the high cost of electricity in many countries.
Glass manufacturers are currently seeking effective alternatives to reduce the use of fossil fuels and energy consumption overall. In this spirit, Saverglass announced that it will contribute to the programme FURNACE FOR THE FUTURE. The goal is to reach carbon neutrality through initiatives based on the use of cleaner energy, as well as investments in recycling, improvements to existing technologies, and exploring other solutions such as the use of biomass and hydrogen.
Much like other organizations whose internal operations consume high amounts of energy, the glass manufacturing process puts the industry in a difficult position when it comes to reducing its energy needs, emissions, and the financial costs related to production processes.
Glass plays a vital role in the wine industry and for that reason it is necessary to carry out more research, introduce design adjustments, and improve processes. Furthermore, electricity and hydrogen prices must come down to make these alternatives more competitive vis-à-vis fossil fuels, and the use of renewable energy should be incentivized.
Marta Juega, PhD
References
1. 5 Things to Know about Glass Quality. https//www.linkedin.com/pulse/5- things-know-glass-quality-george-w-dempsey-jr (zugegriffen Sep. 19, 2020).
2. Beerkens R, Kers G, van Santen E. Recycling of post-consumer glass- energy savings, CO2 emission reduction, effects on glass quality and glass melting. In Drummond CH, editor. Ceramic Engineering and Science Proceedings. Hoboken, NJ, USA: John Wiley & Sons Inc 2011. p. 167–94.
3. Michael Zier, M., Stenze, P., Kotzur, L., Stolten, D. 2021. A review of decarbonization options for the glass industry. Energy Conversion and Management 10, 100083
