Increasing the energy efficiency of aluminum smelting: Burner air preheating by means of a heat exchanger significantly reduces gas consumption. Increased flame temperature enables process time reduction and further utilization of exhaust heat - a plus for the environment.
In 2016, the prototype of an optimized aluminum smelting system was realized as part of a Federal Ministry of Economics and Technology (BMWi)-funded cooperative project between the industrial and business communities. The EDUSAL II project makes it possible to determine the energy-saving potential of the smelting process while at the same time minimizing the resulting smelting loss. In order to further reduce the consumption values of such a system and thus to increase both energy efficiency and resource conservation, the smelting furnace manufacturer, ZPF GmbH, has now extended its concept with additional components: By incorporating a so-called burner air preheating, in which the warm exhaust gas volume flow is passed through a pipe system to a heat exchanger, the system operator has more room for maneuver - with lower pollutant emissions and lower gas consumption. This new system can – with appropriate adjustments - be used as a retrofit kit for existing ZPF melting systems.
"In last year's project, numerous sustainable improvements, that have optimized the previous furnace system, were achieved" says Sven-Olaf Sauke, head of R & D at ZPF. "The main focus was on the further development of the measurement technology to a sensory detection of the melting shaft, which means that in laboratory operation both the position of the residual material on the smelting link and its quantity can be precisely determined. In addition, a special evaluation algorithm has been developed that has improved the process to the point that an increase in smelting efficiency of up to 15 percent can be achieved."
Another focus of the EDUSAL II system was the testing of burner air preheating. The idea of integrating a burner air preheating, which efficiently uses the heat flow of the system, has been around for a long time. However, since solutions with partly utopian characteristics and promises are offered on the market, an electrical preheating was initially installed. This allows a very precise energy and thus also profitability balance of the measure. The characteristic values obtained were used to design a burner air preheating from the exhaust gas stream of the smelting system. "With this version, the already heated exhaust gas flow is directed to a heat exchanger via a suitable pipe system," continues Sauke.
Ingenious system enables careful handling of gas
The primary air side (hot gas side), which is prefixed to the heat exchanger, requires additional components: for example, a control unit that directs the exhaust gas flow to the heat exchanger according to the requirement, and additional measuring points that ensure safe process management. In the event of failure of the heat exchanger or any of the components, the control unit must direct the exhaust flow through a bypass function, past the heat exchanger, directly into the secondary system to protect the heat exchanger and its components. In the heat exchanger, the energy is released to the secondary air side (burner air) - the cooled exhaust gas is discharged in the other system either directly into the atmosphere or for preheating the melting material in a corresponding chamber.
The burner air fan conveys cold burner air from the environment into the heat exchanger, which is heated there in the ZPF smelting systems to about 200° C. The pipe system after the heat exchanger is insulated and equipped with appropriate butterfly valves, measuring instruments and displays. An air collector installed directly after the heat exchanger helps distribute the heated medium evenly across the piping to the entire burner system. Thus, the flame temperature increases significantly, resulting in a higher energy input and lower gas consumption. "In this way, not only are operating and energy costs reduced, it also allows for a careful handling of the valuable resource of gas," says Sauke. "The efficiency of the entire system is increased, and the pollutant emissions are reduced." This heat exchanger variant is suitable for all ZPF gas-fired aluminum smelting and heating furnaces, which have a connected load of more than 300 kW and a high smelting ratio over the entire operating time of the furnace. However, the specified limit temperatures for the refractory lining of the kilns must be strictly adhered to.
Burner air preheating by means of heat exchanger as retrofit kit for existing melting systems
The extension of the energy-efficient burner air preheating with an integrated heat exchanger can also be carried out with an existing smelting system, but requires some changes, according to Sauke: "When upgrading existing ZPF systems, in addition to the heat exchanger, a hot gas withdrawal point, suitable piping and temperature-resistant control and butterfly valves must be integrated. The adjustments with regard to the control as well as the user interface must also be observed. "All electrically operated components, for example the control valves and the measuring technology, are integrated in the control and in the control cabinet in order to simplify the operation of the system.
All modifications and retrofits of the system are done by ZPF directly at the customer's premises - before this, a comprehensive inventory, consultative discussions and an in-depth conception will take place. In closing, Sauke explains: "The need for pipes, routes and insulation material must be calculated on a project-specific basis. The advantage of the developed system lies in the fact that all components are installed directly at the plant and thus no further space requirement arises in the smelting system. "In addition, ZPF is working intensively on further measures to improve the efficiency of the aluminum smelting systems.