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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 52 TECHNICAL FEATURE ton in a year operating at an efficiency of 30%, it will use approximately 76,800 GJ of natural gas per year and generate around 4,500 ton of CO2 per year in the process. The approximate residence time of this CO2 in the atmosphere is 100 years. It would take a 450 ha (4.5 square kilometres) forest of trees to keep pace with that quantity of CO2 from just this furnace alone . CO2 is regarded as having a global warming potential of 1. Many other gases, including unburned hydrocarbons are much worse meaning that they are much more efficient at absorbing infrared radiation. In the furnace example above producing 4,500 ton of CO2, this is the carbon equivalent of 2,000 cows. Cows do not produce CO2 but methane. As we can see from Table 2, methane is 21 times worse per kilogram. Nitrous Oxide has a warming potential of 310. Spare a thought for that very useful gas SF6. It has a global warming potential 23,900 times worse than CO2. Just one kilogram of SF6 has a global arming potential equivalent of 24 ton of CO2. When we look at environmentally friendly design, we really should take in to consideration all forms of energy input (electricity, natural gas, oil, etc.) and the equivalent global warming effect of all the emissions generated in the process including the Carbon equivalent produced from using electricity, compressed air, etc. MELTING ALUMINIUM In theory, it takes approximately 320 kWh or 1,152 MJ of energy to melt one ton of aluminium from 20oC to a final temperature of 750oC, assuming no losses in conversion of energy. If we are very lucky, we may be able to use hydroelectric power. Then we would not deplete the world's fuel resources or generate CO2. Even around Australia, the states quote different CO2 emission factors for the electrical power used for each state. A software package contained in the Energy and Greenhouse Management Toolkit produced by the State Government of Victoria has useful toolkits for calculating possible savings in the different states by better use of equipment such as furnaces, lighting, compressors, variable speed drives, etc . In order to illustrate the impact the design has on the efficiency and the environment, the examples that follow are based loosely on a recently designed 50 ton aluminium melting furnace for the secondary aluminium industry. Gas Chemical Formula IPPC 1996 global warming potential Carbon dioxide CO2 1 Methane CH4 21 Nitrous oxide N2O 310 Hydrofluorocarbons HFC-23 CHF3 11 700 Sulphur hexafluoride SF6 23 900 Table 2. Greenhouse gas global warming potential examples Figure 2. 50 ton melting furnace 3D model Figure 3. Typical furnace conduction losses