To produce 1 ton of formed and finished container glass, the total amount of materials in the furnace batch is higher (1.17 – 1.19 ton) due to decomposition losses. (1)
The average energy consumption is about 5.8 GJ per ton of melted glass, virgin raw material batch (2). CO2 Emissions associated with energy consumption can be calculated using US energy mix composition and carbon conversions data (3), (4)
The decomposition losses (or decarbonation) occur when a portion of the batch materials is converted into gaseous emissions. For example, limestone decomposes into calcium oxide and carbon dioxide when heated. The chemical reactions are:
- CaCO3 -> CaO + CO2 for 56 lb. of CaO there are 44 lb. of CO2 emission
- Na2CO3 -> Na2O + CO2 for 62 lb. of NaO2 there are 44 lb. of CO2 emission
- MgCO3 -> MgO + CO2 for 40 lb. of MgO there are 44 lb. of CO2 emission
Average glass composition (weight %)
Silicon oxide (SiO2) 74.1
Sodium oxide (Na2O) 16.2
Calcium oxide (CaO) 4.5
Magnesium oxide (MgO) 3.1
For the aforementioned glass composition, the raw materials needed weight is 1.184 tons with 370 lb. CO2 emission due to decarbonation. Almost no decarbonation occurs for cullet.
In US is stated that for 6 tons of recycled glass 1 ton of CO2 is offset.
This is equivalent to 330 lb. CO2 offset per 1 ton of glass recycled and seems to take into account only the CO2 emissions associated with the replacement of the raw materials. (5)
Cullet usage in glass production
The container glass industry can use up to 90% cullet. Due to technological and quality constraints, the cullet percent can vary, up to:
- Green glass 90%
- Amber glass 80%
- Flint glass 65%
To quantify the environmental impact of glass bottle production and recycling a Life Cycle Assessment (LCA) must be carried out, including both CO2 offset at the factory and the impact along the supply chain - when the glass is recycled, there is no need to produce, process or transport the virgin raw materials. (1)
CO2 offset at the glass plant
The glass industry is able to quantify the savings in CO2 emissions at the factory when recycled glass (known as cullet) is used in the furnace.
Less CO2 is emitted as less energy is required for melting cullet. On average, a 10% increase of cullet in the furnace decreases its energy use by 3% and CO2 emissions by 5%. (1)
Cullet replaces raw materials which are subject to decarbonation, such as limestone (CaCO3) and soda ash (Na2CO3). These materials release carbon dioxide when they are melted in the furnace while the cullet does not.
A conservative estimate of CO2 offset determined by decarbonation avoidance is 420 lb. CO2 per 1 ton of culets.
Cullet has a lower melting energy than the raw materials (it has been melted previously and there is no endothermal decomposition of carbonates when melting cullet). Cullet can act as a fluxing agent and it decreases the melting energy.
An estimate of CO2 offset determined by energy savings is 240 lb. CO2 per 1 ton of culets.
CO2 offset along the supply chain
Soda ash (Na2CO3) is the raw material with the highest energy consumption but in USA, due to trona based production, the energy required i.e. 4.2 GJ per ton, is half of that necessary with Solvay production technology (based on brine, ammonia and carbon dioxide). (1, III 2.1.3.)
1 ton of culets replace 0.33 ton of natural soda ash from the glass batch, so the CO2 offset can be quantified at 180 lb. CO2 per 1 ton of culets.
For the remaining CO2 offset along the supply chain we can consider it equivalent with the Co2 footprint associated with the activities needed for cullet production, e.g. collection, sorting, processing, transport, etc.
Total CO2 offset aggregated is between 750 - 770 lb. CO2 for each ton of recycled glass used.
1 ton of CO2 is offset for 2.62 ton of recycled glass used (on average)
(1). Life Cycle Assessment of Container Glass in Europe, European Container Glass Federation (FEVE), Frédéric Bettens & Rémi Bagard
(2). Best Available Techniques (BAT) Reference Document for the Manufacture of Glass, European IPPC Bureau
(3). US Energy Information Administration
(4). Energy and carbon conversions, CARBON TRUST
(5). Glass Packaging Institute