SOLAR PROCESS ENGINEERING

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2 SOLAR FURNACE By Karl-Heinz Funken and Ina Ollenschläger SOLAR PROCESS ENGINEERING SOLAR HEATED ROTARY KILN REACTORS Material-converting and metallurgical industries are the most important users of high-temperature process heat. In order to obtain it in a future, sustainable industrial economy, concentrated solar radiation could take over the role played by burning fossil fuels today. The possibility of recycling aluminium scrap in a temperature range running from 750 to 850 degrees centigrade has been tested in the high-flux-density solar furnace so that reliable statements can be made which are supported experimentally on the basis of a concrete example. 92

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4 SOLAR FURNACE 305. NACH. 109-FUNKEN(E) :10 Uhr Seite 3

5 Fig. previous page right and this page left: Thermal image of the irradiated secondary concentrator: the gas flowing in cools and flushes the front plate of vitreous silica. Fig. above left: The photograph shows the rotary kiln receiver reactor ready for operation, but not irradiated. Fig. above right: Process of pouring solar-molten aluminium. Rotary kilns and rotary drum furnaces are important apparatuses in thermal high-temperature process engineering. They are used, for example, when remelting aluminium scrap. In the growing aluminium market, which is notorious for the huge amount of energy it consumes in primary production, the proportion of recycled metal used has increased to a disproportionate extent in the last few decades. With the support of the Solar Energy Association North Rhine-Westphalia ( a radiant-heated rotary kiln has been developed for testing in the solar furnace. The aim of this work was to demonstrate that, under reproducible conditions, safe and reliable operation is technically feasible on a mini-plant scale. This evidence is a precondition for any scaling up and conceivable industrial use. It was possible to carry out several charging cycles every day. Recycled aluminium produced with solar thermal energy equals the quality of the product made by conventional methods. The concentrated solar radiation is directed through an aperture in the forefront into the rotary kiln, where it first of all heats up a film of melt, which adheres to the refractory lining of the kiln. If the rotary kiln is equipped with a secondary concentrator, the efficiency of the plant increases consi- derably. In a special variant, the aperture in the front is covered by a radiationpermeable plate of vitreous silica. This enables thermal treatment of the material even under an inert-gas atmosphere. As the amount of oxygen in the kiln atmosphere is reduced, the yield of secondary aluminium increases. Major elements of a solar process in addition to the rotary drum furnace, which also functions as the central receiver at the same time, are a heliostat field and a flue gas purification plant. For the Almería location, the total area of mirrors needed would be approximately 6,000 square metres for a rotary drum furnace with a capacity of eight tons of aluminium scrap plus salt. In addition to reducing the consumption of fossil fuels, the solar-powered process has two great advantages: first of all, the amount of waste gas that needs to be cleaned is considerably smaller, because no flue gas is produced by the combustion of natural gas with air as in a conventionally fired furnace. Furthermore, flue gas is in some cases loaded with toxic combustion products from contaminants adhering to the aluminium scrap. Since, moreover, the solar process can also be operated under inert conditions, the aluminium losses on ignition are lower than under the oxidising conditions of a conventional process. The expected cost of constructing and operating a solar plant with a capacity of, for example, 7,300 tons of secondary aluminium a year have been compared with those of a conventional, fossil-fired plant with the same capacity at the Almería location. The investment costs are higher in the case of the solar-fired plant. This disadvantage is, however, partially compensated by savings in the flue gas treatment. In a plant operated solely with solar power, a further point is that the operating costs are lower because of the reduced consumption of natural gas if the natural gas price exceeds 0.02 euros per kilowatt/hour in a location with insolation conditions comparable to Almería. In a scenario based on a doubling or tripling of natural gas prices, a solar-fired process is even superior to a conventionally fired process in terms of cost. When the plant is operated under inert conditions, a further economic benefit is the greater yield of secondary aluminium. This dominates the added value of the process. Dr. Karl-Heinz Funken is the Cologne-Porz Site Manager Solar Research at the DLR Institute of Technical Thermodynamics. Dr. Ina Ollenschläger is Manager Recycling, Hydro Aluminium Cologne.