Aluminium melting furnace saves energy & reduces emissions
Faced with increasing energy prices, in 2007 Alumasc Precision – the Burton Latimer-based aluminium diecaster, made a comprehensive review of its energy use and costs associated with melting and holding aluminium for its high pressure diecasting facilities. A detailed appraisal showed that there would be significant cost savings and reduction of carbon emissions by installing a new high efficiency bulk melting and holding furnace to replace some of the existing older gas-fired crucible furnaces.
The new bulk melting furnace was supplied and installed by the German company ZPF therm GmbH of Siegelsbach. Space available was very limited in the chosen location in Alumasc’s high pressure foundry.
ZPF’s very compact, highly efficient design not only meets Alumasc’s energy reduction targets, but also complies with the latest mandatory emissions limits without the need for filtering systems. Norbert Feth, Marketing Manager of ZPF therm GmbH explained, “The reason for such low emission lies in the unique design of ZPF therm´s furnaces. The core of the system is the combustion chamber which is made of refractory concrete and weighs several tonnes. The thick encasement functions as a protective barrier between the thermal insulation layers and external steel casing on one hand, and the aggressive liquid aluminium on the other. The heavy refractory lining also serves as heat accumulator that helps to maintain a steady state interior temperature of the furnace. A sophisticated system of insulation keeps the thermal energy in the interior of the furnace and contributes significantly to energy saving”.
An unconventional exhaust gas flow from the melting chamber plays a very significant role in the reduction of the furnace’s emissions to air. Rather than being expelled directly to atmosphere from the melting shaft through a chimney as is typical in other furnaces, flue gases in ZPF therm´s furnaces are first held in the interior of the self-contained system. In the process, residues from the charged material – particularly those from returns are subsequently virtually combusted. This means that complex filtration systems are unnecessary.
The measured emissions values of ZPF furnaces fall far below the statutory limits. ZPF claims that the proportion of particulate matter in waste gases is less than three milligrams per cubic meter, for nitrogen oxide, the value is about eleven milligrams and only 0.4 milligrams for hydrogen fluoride. Herr Feth speculates: “Even if the limits become yet stricter in five years time, we will still be able to meet the limit values – without external filtering”.
The longer dwell time of the hot gases in the furnace chamber also contributes greatly to the high energy efficiency of the furnace. In the case of the ZPF therm furnace, an average of 20 to 30 percent more performance capacity is claimed through this method of optimal heat utilization as opposed to other melting furnaces with open exhaust gas systems.
The furnace installed at Alumasc is rated to melt one tonne per hour of aluminium alloy and has a holding bath capacity of two and a quarter tonnes. The unique gas firing system employs two modulating nozzle-mixing gas burners with automatic air/gas ratio control. Both burners are used when the furnace is in the melting mode to give rapid melting which reduces metal loss. Only one of the burners is used when maintaining to give better turn-down and consequently more accurate temperature control.
An integrated charging machine automatically loads aluminium ingots and foundry returns that are pre-loaded in wheeled charge cars. The furnace is mounted on load cells giving the operators accurate information of the weight of metal in the furnace. The weighing system also controls the charging machine to automatically prevent over-filling the furnace. Weight monitoring allows the PLC system to accurately calculate the melting time for each charge according to the weight and regulate the time on ‘high-fire’ which saves energy and prevents over heating and associated oxidation losses.
After a year’s continuous operation, the furnace has met all of Alumasc’s expectations regarding energy cost saving and reduced emissions. An inspection during the recent annual shut-down showed only minimal lining wear; the furnace needed only general cleaning of dross and metal residues.