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Metal Casting Technologies : March 2010
24 www.metals.rala.com.au TECHNICAL FEATURE For large liquid aluminum furnaces, a modification in furnace refractory with the purpose of reducing wall loss is limited by the available refractory technology. More backup insulation can move the freeze plane location from within the working lining and into the insulation. Increasing roof backup insulation will increase the roof anchor temperature and creep rate. Optimising refractory insulation value requires extensive design and testing. The potential power reclaimed from hot liquid metal entering the furnace will depend on the metal throughput rate and on the temperature drop between incoming and outgoing metal. The potential fuel saving can be significant, but is highly dependant on operational practices and the casthouse/reduction line communication. One of the most cost effective ways to reduce energy consumption is to ensure good furnace sealing. The example below in Figure 8 is a real time example where the furnace was running at 100l oil/hr when under automatic pressure control. This reduced to 80l/hr when manual damper control was used and the fuel consumption further reduced to 40l/hr when a ceramic fibre block as placed over the poring spout. This shows that a 60% saving was achievable from one small operational change on the existing equipment. Fuel efficiency can be improved with sophisticated and expensive equipment however, the findings from surveys suggest that for aluminium holding furnaces the largest savings can be done with simple improvements to the existing equipment and operations. In most holding furnaces surveyed, the energy was wasted in the form of cold air infiltrating the furnace, getting heated and going out through the flue. To illustrate the above-mentioned statement, consider the following. If we were to retrofit our standard cold air burners with the most fuel-efficient regenerative burners on the market, we would save about 30% of fuel as shown in Figure 7. However, the amount of fuel consumed by cold air infiltration can increase the fuel consumption by 100% to 200%. The largest part of wasted fuel depends on a fragile balance between the air/gas ratio of the combustion system, the temperature of furnace roof, the furnace pressure control and the position and sizes of wall openings. A change to this equilibrium can go unnoticed and results in a furnace doubling its fuel consumption. The problem can originate from mechanical damages, control problems or operational procedures. The fuel consumption of a furnace must therefore be controlled in the same way as any other steady state process. Conclusions In conclusion, although the energy consumption in the casthouse is only a small part of the overall site consumption, it is an area where significant improvements can be made easily as furnaces tend to have the poorest efficiencies. The cost of excess fuel consumed in casthouse varies from $100,000 to $500,000 per furnace per year; and there is the potential for reducing carbon emissions varies from 250 to 1,250 tons per furnace per year. Cold air infiltration in furnaces is responsible for the largest part of the excess fuel consumed. Savings can be made with optimisation of existing equipment. This paper was presented at the 11th Aluminium Cast House Technology conference. The next Cast House conference will be held in Melbourne in September 2011. For further information visit www.aluminiumcasthouse.com ■ Fig 8. Example of improved furnace sealing Fig 7. Energy efficiency by burner type IF WE WERE TO RETROFIT OUR STANDARD COLD AIR BURNERS WITH THE MOST FUEL-EFFICIENT REGENERATIVE BURNERS ON THE MARKET, WE WOULD SAVE ABOUT 30% OF FUEL AS SHOWN IN FIGURE 7.
Media Kit 2010