Over half of the energy consumption of the glass industry is used for melting in order to form the glass. Adding recycled glass to the raw materials reduces energy use and CO2 emissions. Another advantage is that less raw material is needed. Currently, the world-average glass recycling rate is about 50%. Higher recycling rates are possible, especially in regions where the recovery rate is still low.
The virgin raw materials for glass production are mainly silica sand, soda ash and limestone. Glass is produced by melting these materials in glass furnaces (See Figure 1 for a typical process scheme). After the melting process, the glass is formed and annealed. Broken or waste glass (also called cullet) can partly replace the mineral raw materials. Cullet can consist of process losses as well as recycled glass.
Adding cullet reduces energy use and CO2-emissions, because the melting point of cullet is lower than that of the mineral raw materials. As a general rule, 10% extra cullet results in a 2.5 to 3% reduction of furnace energy consumption. Figure 2 shows the relation between the energy consumption and the share of cullet found in a benchmarking study of 130 furnaces (Beerkens, 2001).
Recycling of glass is common. In the manufacturing of container glass (glass used for the production of glass containers such as bottles, jars and cups), cullet use can vary from 10% to over 90%. Even the production of high quality flat glass, which is used for example for windows or transparent walls, allows for use of a large share of recycled glass. The potential depends on the availability of secondary glass of suitable quality. For green glass, for example, cullet of almost any color can be used. White glass production requires white cullet.
Glass recycling requires a well-functioning glass collection system. Consumers and companies have to be prepared to recycle the glass instead of disposing of it as waste. Governments can organize collection systems; and deposit refund schemes can help to increase participation. If there is no sufficient (financial) incentive, the public may be unwilling to recycle glass.
The current world-average glass recycling rate is about 50%. This varies strongly by country and by glass type. In Europe, large scale glass recycling systems have been in place since the 1970s. Globally, higher recycling rates seem possible, as in some regions there is an excess availability of waste glass or recovery rates are low.
The International Energy Agency estimates that 0,5 to 0,8 Exajoule of energy is used for glass production worldwide The production of container and flat glass results in 50 to 60 Megaton of CO2 emissions per year (IEA, 2007).
Increased glass recycling can improve the energy efficiency of the glass industry and reduce energy costs. This can positively affect energy security of supply, and reduce GHG emissions related to energy generation. In addition to energy related GHG emissions, glass manufacturing also causes process related emissions, as carbonates such as soda ash and limestone decompose in the furnace, releasing CO2. These process emissions can also be reduced by using an increased share of recycled glass.
In addition, increased recycling reduces waste streams and diminishes the use of natural resources, and recycling can generate jobs.
A positive effect for the industry can be that, because of decreased melting temperatures and a less corrosive batch, the life of furnaces can be increased by up to 30% (GPI, 2002).
Increasing the recycling rate reduces energy costs and costs for raw materials. However, setting up or expanding a glass recycling system first requires investments and economic incentives. To enable the glass industry to buy recycled glass, a market has to exist. The government could encourage recycling by tax reductions or rebates on levies for companies that recycle glass. Grants and subsidies may be necessary for infrastructure, transportation or initial stimulation of markets. Waste taxes can also help to promote recycling (DANIDA, 2005).
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