Conventional float glass production
To dwell upon our previous post a little… is it possible to create glass with renewable energy sources?
[wikipedia.org] – Float glass (Pilkington process)
Float glass uses common glass-making raw materials, typically consisting of sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate) etc. … The raw materials are mixed in a batch process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace where it is heated to approximately 1500 °C. Common float glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass. Once molten, the temperature of the glass is stabilised to approximately 1200 °C to ensure a homogeneous specific gravity.
The molten glass is fed into a “tin bath”, a bath of molten tin (about 3–4 m wide, 50 m long, 6 cm deep), from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip. The amount of glass allowed to pour onto the molten tin is controlled by a gate called a tweel.
Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and is immiscible with molten glass. Tin, however, oxidises in a natural atmosphere to form tin dioxide (SnO2). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure protective atmosphere of nitrogen and hydrogen.
The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and of even thickness. As the glass flows along the tin bath, the temperature is gradually reduced from 1100 °C until at approximately 600 °C the sheet can be lifted from the tin onto rollers. The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed. Top rollers positioned above the molten tin may be used to control both the thickness and the width of the glass ribbon.
Once off the bath, the glass sheet passes through a lehr kiln for approximately 100 m, where it is cooled gradually so that it anneals without strain and does not crack from the temperature change. On exiting the “cold end” of the kiln, the glass is cut by machines.
– Embodied energy float glass: 15.9 MJ/kg or 4.4 kWh/kg
– Standard glass used in horticulture: 4 mm, unhardened
– 1 m2 glass of 4 mm thick weighs 10.0 kg
– Embodied energy (EE) of 1 m2 glass of 4 mm think is 44 kWh
– EE energy greenhouse glass Sundrop Farm: 200,000 m2 x 44 kWh = 8,800 MWh
– 1 liter of petrol = 10 kWh, 1 m3 petrol = 10 MWh.
– Glass production Sundrop Farm greenhouse = 880 m3 petrol
– Take a factor of 1.5 to account for oblique roof: 1,320 m3 petrol
Do we need fossil fuel for the production of glass? No:
[mo-sci.com] – Are Electric Furnaces the Future of Glass Manufacturing?
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.
[eurotherm.com] – The future for the glass industry is “all-electric”
The burning of fossil fuel as an energy source in the glass melting process results in unavoidable carbon emissions, and improvements to traditional technology have reached their efficiency limits. Moving to electrical heating methods has many benefits including improved energy efficiency, more flexible control and less combustion related emissions. The aim of this paper is to stimulate glass manufacturers into rethinking their existing melting technology and considering “all-electric” melting in the near future.
How much time does it take for a 6 MW offshore wind turbine to generate the energy equivalent of 1,320 m3 petrol or 11,000 barrel of oil? Said wind turbine produces the equivalent of 32,285 barrel/year. So the answer to the question is:
[wikipedia.org] – Glass Production
[wgtn.ac.nz] – Embodied Energy Coefficients