Afrox is consulting with several key players in the South African aluminium industry with a view to introducing an innovative new optimised oxyfuel melting process developed by its German parent company, The Linde Group, which delivers process improvements leading to significant cost savings.
Following its introduction in Europe in 2005, the new proprietary ‘low-temperature oxyfuel burner process (LTOF®)’ — also known as ‘flameless burner’ technology — is achieving an average 30-50% higher melt rate and in some cases, a 100% improvement in melt rate, compared with conventional airfuel processes. Fuel consumption is reduced by up to 50%, flue gas volumes by up to 80% and dross formation decreased as a result of more uniform heating and melting. Carbon dioxide and sulphur dioxide emissions are also cut by up to 50% and nitrogen oxide emissions are virtually eliminated. The new system requires low maintenance, eliminating the need for a recuperator, electrical air blower or regenerative solution.
The Linde Group has more than 130 conventional oxyfuel installations in the European aluminium sector and has used this expertise to take heating technology to the next level. Sapa Heat Transfer, Sweden, a secondary aluminium melter, converted its conventional oxyfuel burners to LTOF® in mid-2005. The new technology has shown further improvements over the previous conventional oxyfuel system, including a 10% further increase in productivity, a 10% further reduction in fuel requirements, 9% less dross and 90% reduction in NOx emissions.
In February 2009, Stena Aluminium in Sweden, part of the Stena Metal Group, became the latest company in Europe to introduce the new burner technology. Stena produces aluminium alloys used to manufacture cars, trains, aircraft and ships and as raw material for steel mills.
Afrox’s applications engineer, Metallurgical Processes, Michael Hill, says that as in European installations, the new low-temperature oxyfuel burner process, suitable for both rotary and reverberatory furnaces, is capable of delivering real capacity increases in the South African aluminium sector, boosting productivity, saving energy and greatly mitigating environmental impact. It is also suitable for both primary and secondary smelters.
“Increasing the throughput of existing melting furnaces poses a real challenge for the aluminium industry,” says Hill. “Producers must continuously increase process yields, control fuel consumption and reduce emissions of greenhouse gases. In South Africa, the primary drivers are expected to be potential cost savings associated with reduced energy consumption, and reduced dross.”
“In reverberatory furnaces, power must also be controlled so as not to overheat the refractory ‘roof’ — or exceed furnace temperature set point. The new low-temperature oxyfuel burner process makes it possible to run the furnace at full power for longer, without exceeding roof temperatures. This is quite a breakthrough compared to conventional heating which requires the power to be reduced as soon as the maximum set point is reached, to avoid overheating.
“Another key benefit is that the reduced melting, holding time and uniform furnace temperatures prevent hot spots, significantly cutting down the formation of dross, while the improved heating capacity improves energy per ton of heated aluminium.”
Combustion occurs under a diluted oxygen concentration, by mixing the furnace gases into the combustion zone. This slows down oxyfuel combustion reactions and results in lower flame temperatures, comparable to those of airfuel technology, which are below the point at which thermal nitrogen oxide is created. Mixing the furnace gases into the flame also disperses energy throughout the entire furnace for uniform heating and more efficient melting. The dispersed flame contains the same amount of energy, but with a far more effective distribution. The overall result is more homogenous heating and melting, allowing not only for a higher power input and consequent higher melt rates, but also for reduced formation of dross and nitrogen oxide emissions.
Afrox is able to supply a commissioning and support package on a rental basis (including a nominal technology fee) for South African companies who introduce the new process.
“The new low temperature oxyfuel burner process is one of a suite of Linde furnace technologies available through Afrox, which include Universal Rotary Tiltable Furnace (URTF) technology and the proprietary Wastox® oxygen lancing, Airox® — which combines oxygen and air combustion — and the Flexflame™ variable flame oxyfuel burner,” says Hill.