SchlaGie: Using hot slag as a source of waste heat

In collaboration with interdisciplinary partners, the "SchlaGie: Using slag for energy recovery for the local district heating network" project is developing and testing a technology to use the unavoidable waste heat from steel mill slag for grid-connected heat supply in the future. This is being done on a pilot basis at the thyssenkrupp MillServices & Systems GmbH site in Duisburg. Objective: to significantly increase the proportion of industrial waste heat in heating networks.

The steel industry generates a large amount of unavoidable waste heat – both during the reduction of iron ore and during the conversion of pig iron to steel. Until now, however, only a portion of this industrial waste heat has been used internally or fed into district heating networks. This is mainly due to technical and economic challenges.

A significant, previously unused source of waste heat is the hot slag (up to 1500°C) produced during steel production using the Linz-Donawitz process – both today and after the transition to green steel production. Until now, it has been dumped into beds where it cools and solidifies. Once it has cooled sufficiently, it is crushed, processed and used in the construction industry, among other places. The "SchlaGie" project is developing and testing a process that will absorb the radiant heat currently emitted into the environment without contact. Using this LD slag for energy recovery for the local district heating network will reduce CO₂ emissions and the primary energy factor.

The project results can thus make an important contribution to the development of new climate-neutral energy systems and support municipal heat planning. The knowledge gained can also be transferred to other locations in the steel industry and to other fields of application with comparable waste heat (cooling from melting in foundries, hot power plant ash, cement industry).

The starting point for the project is the determination of the chemical and thermophysical properties of LD slag. To this end, several samples are taken, homogenized in the laboratory, ground and examined in detail using established methods. Based on this, laboratory tests are carried out to simulate the real thermal conditions of the slag bed and validate the previously determined material data.

Using the material data obtained and taking into account the current slag throughput at the Duisburg site, thermodynamic calculations are performed to determine the potential heat release of the slag and the heat absorption of a radiation receiver. On this basis, a dynamic simulation model is created to map and investigate the discontinuous waste heat utilization 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 approval-related aspects, a demonstrator for the efficient use of slag waste heat will be designed. The radiation receiver developed should not influence the existing process flows or further valuable use. A demonstrator will be built and put into operation on this basis. In addition to the real operating data, the cooled slag will be examined for its composition and physical properties in order to rule out any influence on quality by the receiver.

Using the data generated from the simulation and the demonstrator, economic and CO₂analyses will be carried out and the technical potential estimated. This will allow the specific costs for the provision of district heating from the previously unused, unavoidable slag waste heat to be estimated.

• thyssenkrupp MillServices & Systems GmbH
• FEhS – Institute for Building Materials Research
• C-Technik Maschinen- und Anlagenbau GmbH
• Georg Agricola Technical University