Photo Elektroöfen ©Badische Stahlwerke
On the German-French border in the region of Strasbourg-Kehl, a showcase project for cross-border cooperation in the municipal heat transition is emerging. In the future, 7,000 households on both sides of the Rhine will be supplied with waste heat from the local steel mill. Due to its transnational nature, the project faces several challenges. Nevertheless, the high level of commitment of all partners and the active support of governmental institutions combined with financial support from French (ADEME), German (BAFA), and European (INTERREG) funding agencies have made the project possible.
By Dr.-Ing. Volker Kienzlen, KEA-BW, Reiner Hagemann, Badische Stahlwerke GmbH (BSW), Sabine Schimetschek, Calorie-Kehl Strasbourg (CKS)
BSW, the Badische Stahlwerke, runs a steel mill in Kehl. In the harbour area north of Kehl, they produce 2 million tons of steel per year, mainly for the construction sector. Two electrical furnaces use steel scrap to produce reinforcing steel.
The main waste heat source is the exhaust gas from the two electric arc furnaces where the steel scrap is melted. There are also other potential sources of excess heat, e.g., the pusher furnaces, where billets are reheated for further processing.
In the first step, the waste heat from the electric arc furnace will be used. In the future, however, other waste heat potentials could also be used. The exhaust gases, up to 1,300°C hot, will flow through a water-cooled heat exchanger, which pre-cools the gases to approx. 600°C. The cooling water in the heat exchanger runs in a closed circuit and has a flow temperature of approx. 70°C and an outlet temperature of up to 90°C.
Due to the melting process, the water temperature fluctuates greatly since cold scrap is charged at the beginning of the process, and molten steel is tapped at 1,600 °C at the end. In addition, there is a short production downtime in summer and a three-week downtime in winter.
On average, a heat flow of 20 megawatts is cooled to ambient air. Based on the production data of the steel mill, this results in a total available waste heat of 135 gigawatt hours per year.
Schemabild Elektroöfen ©Badische Stahlwerke
Badische Stahlwerke has been looking for a way to use this waste heat for decades. In the nearby industrial area of the port and the town of Kehl, three kilometres away, there are no suitable customers for such large quantities of heat.
On the other side of the Rhine lies Strasbourg – a city that already operates large district heating facilities and would like to expand these even further regarding the climate policy goals.
By 2050, at the latest, Strasbourg’s heating networks are to be entirely climate-neutral. Therefore, Strasbourg not only plans to extend the district heating system but also intends to lower the network’s temperature.
The first idea to use the enormous amount of excess heat dates back to 2014. In 2018, the KEA-BW, the state energy agency in Baden-Württemberg, initiated another attempt to use the heat that cannot be used within the company.
The rising importance of climate protection and the necessity to become climate-neutral was a new baseline for the discussion with BSW and the cities of Kehl and Strasbourg.
Improved funding conditions were also helpful. In July 2018, the Baden-Württemberg Ministry of the Environment commissioned a study to the engineering company EGS-Plan, confirming the project’s technical and economic feasibility.
Since then, the Eurometropolis of Strasbourg, the Badische Stahlwerke, and the city of Kehl have been working with KEA-BW, dena (German Energy Agency), ADEME (French environment and Energie management agency), and several other stakeholders to push forward the German-French cooperation project.
This was followed by further studies to clarify the potential, the heat requirements in Strasbourg and Kehl, as well as the possible layout of the heating grid.
Since the district heating network in Strasbourg is rather old, some areas of the network still require flow temperatures of more than 140 °C when the ambient temperature is very cold. For BSW, this means that the outlet temperature in the heat exchangers has to be raised to 160 °C, which increases the operating pressure significantly.
The existing system cannot meet the safety requirements, which is why the entire primary cooling system must be rebuilt. This also includes replacing the heat exchangers for the secondary cooling tower circuit, which ensures heat dissipation during low district heating demand periods.
Map with the relevant points BSW, Esplanade, and Zollhof area, with the current idea for the route.
The part of Strasbourg’s district heating system relevant to the project consists of the two interconnected heating networks, Esplanade and Elsau. Two gas-fired boilers are used to cover the peak load. The base load is generated by a biomass cogeneration plant and the municipal waste incineration plant.
This means about 60 percent of the heat is already provided by renewable or CO2-neutral energy sources. As the heat from the biomass cogeneration plant is still subject to a multi-year purchase obligation, the waste heat from BSW will only be able to replace a significant part of the heat produced by gas boilers in the winter months.
The planned increase in annual district heating sales from the current 256 GWh to 318 GWh in 2030, however, allows for more and more waste heat use, especially in the transitional months of the year. With the expansion of Strasbourg’s heating system in mind, further sources of excess heat from the steel mill can be used. Furthermore, the operator of Strasbourg’s district heating system intends to lower the operating temperature in the long run.
In the coming years, the new development “Zollhofareal” will also be built in Kehl, on the southern Rhine harbour near the main station, which will also be supplied with waste heat from BSW. Accordingly, at least 70 GWh of the total available waste heat of 135 GWh can be used in the district heating networks of Strasbourg and Kehl.
7,000 households can be supplied with this energy, thus saving almost 20,000 tons of CO2 emissions per year. Kehl’s district heating system is already at a much lower temperature level. With a maximum temperature of 90 degrees, the return flow from Strasbourg can be used today as a supply for Kehl.
This leads to an increased temperature spread in the transport pipe and reduced pumping energy. Should the return temperature from Strasbourg be below Kehl’s operating temperature in the long run, an injection system can raise the temperature level for Kehl.
Implementing district heating projects using industrial waste heat is complex and costly.
The cross-border dimension of the project faces additional challenges. The different languages spoken on the two sides of the Rhine and the different cultures of project management require additional effort to make the project successful.
The coordination effort is very high, given the many players from both countries involved. In the practical implementation, many organisational and legal issues must be solved. This includes a suitable binational organisational structure.
For the construction and operation of the necessary pipeline, a municipal public company was founded under French law, allowing foreign local authorities to participate. In this “Société d’économie mixte locale Calorie Kehl-Strasbourg” (SEML CKS), the Eurometropolis of Strasbourg has the largest share of 47 %.
Smaller shares are also held by the city of Kehl (12.75 %), the region Grand Est (12.75 %), the state of Baden Württemberg (12.75 %), and the French Banque des Territoires (CDC) (15 %). Badische Stahlwerke (BSW) also holds one share and is involved as an observer.
The general director of Calorie Kehl-Strasbourg, Sabine Schimetschek, was born and raised in Germany but worked most of her professional life in France. Thus, she embodies the cross-border character of the project.
Since the common goal is to start operating the system by 2027, CKS pushes the design and planning process forward. The rise in construction costs combined with increasing energy prices due to the energy crisis of 2022 requires an update of the calculation of project economics.
Now, an engineering company analyses the outline of the pipeline with the need to minimize construction costs and the requirements in the approval process at the same time. The initially preferred micro tunnelling minimizes the interference with existing infrastructure but is the most costly solution. Therefore, alternative construction methods are investigated.