Using hot slag as a source of waste heat
Research project on the use of slag for energy recovery for the local district heating network.
In collaboration with interdisciplinary partners, the Fraunhofer Institute Umsicht is developing and testing a technology to utilise the unavoidable waste heat from steelworks slag for grid-connected heat supply in future. The project “SchlaGie: Utilising slag for energy recovery in the local district heating network” is being carried out as a pilot scheme at the thyssenkrupp MillServices & Systems site in Duisburg. The aim is to significantly increase the proportion of industrial waste heat in district heating networks.
Contributing to the development of climate-neutral energy systems
The steel industry generates a large amount of unavoidable waste heat – both during the reduction of iron ore and the conversion of pig iron into steel. To date, however, this industrial waste heat has only been used internally to a limited extent or fed into district heating networks. This is primarily due to technical and economic challenges.
A significant, hitherto untapped source of waste heat is the hot slag (up to 1,500°C) produced during steelmaking using the Linz-Donawitz process – both today and following the transition to green steel production. Until now, it has been tipped into pits, where it cools and solidifies. Once sufficiently cooled, it is crushed, processed and used in the construction industry, amongst other applications. The SchlaGie project is developing and testing a process designed to capture, without direct contact, the radiant heat currently released into the environment. By using this LD slag for energy recovery for the local district heating network, CO₂ emissions and the primary energy factor can be reduced.
The project results can thus make an important contribution to the development of new climate-neutral energy systems and support municipal heating planning. The findings can also be applied to other sites in the steel industry as well as to fields of application with comparable waste heat (cooling of molten metal in foundries, hot power station ash, the cement industry).
From the analysis of material properties to a demonstrator for utilising slag waste heat
The project begins with the determination of the chemical and thermophysical properties of LD slag. To this end, several samples are taken, homogenised in the laboratory, ground and examined in detail using established methods. Building on this, laboratory tests are carried out to simulate the actual thermal conditions of the slag bed and to validate the material data determined previously.
Using the material data obtained and taking into account the current slag throughput at the Duisburg site, thermodynamic calculations are carried out to determine the potential heat release from the slag and the heat absorption of a radiant receiver. On this basis, a dynamic simulation model is created to model and investigate the intermittent utilisation of waste heat and its integration into the existing district heating network, as well as various combinations of secondary fluids, heat exchanger geometries and heat storage systems.
Using the findings from the simulation and taking into account safety and regulatory aspects, a demonstrator for the efficient utilisation of slag waste heat is being designed. The radiant receiver developed is intended not to affect either existing process flows or the further valuable utilisation of the slag. On this basis, a demonstrator will be constructed and commissioned. In addition to the actual operating data, the cooled slag will be analysed for its composition and physical properties to rule out any impact on quality caused by the receiver.
Using the data generated from the simulation and the demonstrator, economic feasibility and CO₂ analyses will be carried out and the technical potential assessed. This will enable an estimate of the specific costs of supplying district heating from the previously unused, unavoidable waste heat from slag.
Source: Fraunhofer Umsicht