Submitted Abstract
Steel production is a significant energy consumer. The iron and steel sector consumes 19% of the total energy consumed in industry and is the main industry consumer in Europe. Energy consumption by steel plants in Luxembourg represented 22% of the final industry energy consumption in 2007. In recent years this industry has undertaken efforts to increase energy efficiency by reducing its consumption and recovering otherwise lost heat, as state of the art research shows. Moreover, the recent European Directive 2012/27/EU on energy efficiency is promoting efficient energy supply, considering the use of waste heat for district heating and cooling.This thesis was framed in this context and its objective was to develop strategies for saving and, in particular, recovering energy in an electric steel plant. Approximately thirty options were technically and economically evaluated. In addition, a risk assessment matrix was developed to assess the technical and economic risk of each option. After a thorough energy audit, the energy consumption of the plant was established and the main waste heat sources available for heat recovery as well as the energy savings possibilities were identified. The potential energy savings in relation to the natural gas consumed in a reheating furnace if the beams are entering hot—which is known as warm charging—were also studied.Moreover, state of the art energy recovery technologies were analyzed as well as new research developments such as slag heat recovery and cooling bed heat recovery. An important part of the work of this thesis was the determination of the potential of heat recovery by radiation in the cooling bed. Numerical simulations and experimental tests were carried out with this aim. The experimental tests were performed using solar absorbers. The results demonstrated that up to 1 kW/m2 could be recovered with a temperature of 70°C at the side of the cooling bed, with a thermal efficiency of approximately 0.4. This shows that the system used has an important efficiency optimization potential, as standard absorbers for solar applications can reach much higher efficiency values. In addition, the simulations showed that panels placed at the top of the cooling bed would receive higher heat flux than those at the side of the cooling bed, thus allowing them to attain higher water temperatures or flows. Three options with different flows and temperatures were economically analyzed, indicating a profitable return over 15 years. Therefore further research in this field is suggested.State of the art technologies applied to different waste heat sources in the plant were analyzed. These technologies were heat exchangers, evaporative cooling systems, Organic Rankine Cycle and steam turbine. The latest heat recovery research into dry slag granulation was also analyzed. The heat recovered could be used for heating the plant or to establish regional synergies with the nearby district heat networks. Electricity generation from the heat recovery was also considered, showing that this option could be convenient if combined with thermal use at the required temperature level. Heat exchangers require relatively low investments and are low risk, thus revealing interesting results for energy recovery and economic profitability if linked to urban districts. If the heat delivery to the districts cannot be achieved, then plant building heating should be targeted and the possibility of installing an evaporative cooling system considered. The advances in research into dry slag granulation should be followed, as this is a heat recovery technology that does not interfere with the steel production and could be added at a later stage.