Siemens installs heat recovery pilot plant at Stahlwerk Thüringen GmbH – molten salt stores energy from the exhaust gases of the electric arc furnace

A heat recovery pilot plant installed by Siemens Metals Technologies in the electric steel plant of Stahlwerk Thüringen GmbH in Unterwellenborn (Germany) is now in operation since April 2012. The plant uses molten salt as heat transfer medium, and is connected to the existing exhaust gas system of the electric arc furnace. The objective of the pilot project is to validate possible materials for use under high temperatures, and to investigate the conditions required to achieve the highest possible energy recovery efficiency. The duration for operating the pilot plant is preliminary defined for one year.

The heat recovery plant installed by Siemens at Stahlwerk Thüringen uses a molten eutectic ternary salt mixture as heat transfer medium. About 1,500 kg of molten salt are stored in a buffer vessel at an operating temperature of 430 °C. The molten salt can temporarily store and release 65 kWh of energy, which corresponds to a storage capacity of about 90 kWh/m³. The project focuses on investigating a range of materials, especially for the heat exchanger in the hot gas duct, as the materials there are subjected to extreme stresses by the high gas temperatures. Siemens VAI is responsible for the engineering of the heat recovery plant, and for supplying all the components, including the electrical systems and automation, as well as for the installation and commissioning.

The new technology is intended to be used in future to recover energy at a high temperature level, and make it available for high temperature heat consumers or for generating electricity. The knowledge gained from the material investigations is independent of the heat transfer medium, and can therefore also be applied to conventional water/steam systems.

Recovering energy from furnace exhaust gases can make a significant contribution toward reducing both energy costs and the environmental impact. Exhaust gases from electric arc furnaces can reach temperatures of up to 1600 °C. The hot exhaust gas contains around 40 percent of the energy used in the melting process. In the case of an electric arc furnace with a tapping weight of 120 tons, the energy in the off-gas is around 370 kWh per ton of liquid steel, or more than 44 MWh per charge. Up to 24 percent of this energy can be recovered, and used – for example – to generate process steam or electricity for internal use. This considerably improves the energy and environmental balance of a steel works: energy consumption falls by between 60 and 80 kWh per ton of liquid steel, and CO₂ emissions can be reduced by up to 45 kg/ton.

As the stream of hot exhaust gases from an electric arc furnace is not continuous, the recovered energy has to be stored temporarily. Saturated steam is usually used as the heat transfer medium. This requires the installation of pressure vessels and lines, whereas no pressure applies in plants that use molten salt as the transfer medium. Salt also allows media temperatures of over 500°C to be achieved without evaporation starting.

Stahlwerk Thüringen GmbH has been part of the Brazilian corporation CSN (Companhia Siderúrgica Nacional) since February 2012. The company, based in Unterwellenborn, has a range of products that includes beams, profiles and steel ties. It makes more than 200 different profiles according to various national and international standards. The plant produces general structural steels, fine-grain and weather-resistant structural steels, special steel grades for shipbuilding and low temperature applications, as well as offshore grades.

Further information about solutions for steel works, rolling mills and processing lines is available at: http://www.siemens.com/metals

A photo supplements this press release. Please see: http://www.siemens.com/mt-picture/IMT201205194

(Heat recovery plant installed by Siemens in the electric steel plant of Stahlwerk Thüringen GmbH in Unterwellenborn (Germany). The pilot plant runs with molten salt, and is used for material validation and process optimization)

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Based on Siemens (Industry Sector, Metals Technologies) information