This course is part of the Glass in Architecture Academy

CULLET, RECYCLING, AND REDUCING EMBODIED CARBON IN GLASS

There are a few other parts of the glass-making process in which greenhouse gas emissions can be reduced, including transportation and the end-of-life phase of glass: recycling.

Transportation

The greenhouse gas emissions associated with the transportation of raw materials to a glass plant are relatively small when compared to raw materials extraction and processing and the float glass manufacturing process, which are more energy intensive.

But that ratio could change over time as the environmental attributes associated with materials extraction and processing and with the manufacturing are reduced. Both the distance and mode of transportation should still be considered when evaluating ways to reduce a product’s embodied carbon.

Reducing Carbon Emissions with Cullet

One of the primary ways float glass manufacturers can minimize the use of virgin raw materials, save energy, and reduce greenhouse gas emissions is the use of cullet, which is broken or rejected glass, in the process. The three primary types of cullet are internal, pre-consumer, and post-consumer.

Internal cullet refers to broken or rejected glass, often from defects discovered in a manufacturing facility through quality control.

Pre-consumer cullet is broken or rejected from the processing of glass before it goes in a building. That means it’s coming from a process other than float, usually from internal processing or an external source such as a glass fabricator.

End-of-life glass from an external source such as project demolition, retrofits, and other sources is called post-consumer cullet.

Benefits of Cullet

In the float glass manufacturing process, the direct emissions associated with the furnace are the largest contributors to a product or product family’s cradle-to-gate embodied carbon. In a traditional gas-fired furnace, approximately two-thirds of the carbon emissions are generated from the combustion of gas and one-third are from the melting of carbonate batch materials. By using glass cullet, a float glass manufacturer reduces embodied carbon in two ways.

The first way is by reducing the energy needed to convert the raw materials into molten glass during the melting reaction. The energy required to melt cullet is lower than the energy required to melt raw batch materials such as sand, soda ash, and dolomite. Lowering the melting energy helps reduce energy consumption and the associated greenhouse gas emissions. According to a 2019 research report, for every 10% of cullet used on average, the melting energy demands are reduced by up to 3%, and the carbon emissions are reduced by approximately 3.6%.¹¹

The second way cullet helps reduce embodied carbon is by minimizing the amount of carbonate materials consumed in the batch. Soda ash and dolomite are in the carbonate form when melted, which means carbon is released during the chemical reactions that occur during the manufacturing process. Reducing the volume of these materials by increasing the percentage of cullet in batch helps reduce the associated carbon emissions. Additionally, having already been through the mineralization process, cullet melts at a much lower temperature than the raw materials, which helps improve the energy efficiency of the process.

As with raw materials, transportation must be considered when working with cullet. Most of the pre-consumer cullet from float glass processing/finishing and insulating glass production (e.g. clear glass offcuts) is currently not sent to float glass recyclers and is instead used in other industries besides float glass. A lot of pre-consumer glass cullet, especially from specialized and well-organized glass processors, can be and is in some cases directly picked up by glass manufacturers when delivering glass and taken directly back to the float glass manufacturer. Increasing direct cullet supply from glass processors to glass manufacturers will help ensure it is reused to produce float glass and can reduce additional transportation.

Photo by Michael Elkan; courtesy of Guardian Glass

The LEED Gold Certified Student Union Building at Simon Fraser University in Burnaby, British Columbia, Canada includes oversized windows with sweeping views to specifically improve a sense of mental and physical well-being for students.

Trade-offs of Cullet

As with the raw materials discussed in section four, choosing a cullet may come with some trade-offs. Incorporating cullet can help enhance the environmental attributes associated with the extraction and processing of raw materials and the manufacturing process, but it’s important to note they could increase the emissions associated with other life cycle stages such as transportation of the raw materials to the plant by increasing the distance traveled or changing the transport mode.

A “what if” analysis in an LCA model can help designers understand how changing different batch materials affects the embodied carbon of a product and other environmental indicators such as acidification potential, eutrophication potential, eco-toxicity potential, and water consumption.

CONCLUSION

If we only look at the numbers, the most efficient passive designs wouldn’t feature glass at all—only solid insulated structures, with no views or natural light. Unsurprisingly, most people wouldn’t want to live or work there which, ironically, means those mythical designs do not consider all aspects of stewardship.

Designers must consider many perspectives: that of the clients, building occupants, neighbors, and community members, as well as the less-visible interested parties like construction workers and those who work in raw material extraction.

To effectively take into account all points of view, both the embodied and operational carbon outputs of construction need to be considered.

Environmental product declarations can help architects make informed decisions about sourcing products, choosing manufacturers, and designing structures that meet their stewardship goals.

SOURCES

Andrew A. Hunt is Vice President of Confluence Communications and specializes in writing, design, and production of articles and presentations related to sustainable design in the built environment. In addition to instructional design, writing, and project management, Andrew is an accomplished musician and voice-over actor, providing score and narration for both the entertainment and education arenas. www.confluencec.com