This thesis will discuss the feasibility of a concept for off-gas waste heat utilization for electric arc furnace steelmaking. Currently waste heat recovery is not implemented in most electric arc furnace steelmaking facilities in Germany. Therefore up to 30 % of the total energy input for secondary steelmaking is lost via off-gas waste heat. The application of off-gas waste heat exchangers is challenging due to the erosive and corrosive behavior of the waste gas stream and batch wise operation of the furnace, which requires the use of a heat storage system. Due to these reasons no economically feasible system for waste heat recovery by means of power generation is available.
Hence a novel process concept for extraction and thermal storage of off-gas waste heat is studied. To avoid the use of waste heat exchangers, thermal energy storage with Ca(OH)2 is applied. The endothermic dehydration of Ca(OH)2 forms lime and steam. The Ca(OH)2 particles are injected inside the post-combustion chamber of the electric arc furnace and dehydrated by the hot off-gas in entrained flow conditions. The existing off-gas system is used as entrained flow reactor. The generated lime particles are separated downstream by a cyclone separator. A storage container is applied the store the hot lime particles and to balance of the fluctuating off-gas waste heat flow. The continuous exothermic lime hydration takes place in a bubbling fluidized bed. The released heat generates superheated steam to power an extraction condensing turbine. The reformed Ca(OH)2-particles are then lead to a second storage container, before they are again injected inside the post-combustion chamber.
The main focus of the thesis is the basic design of the components for the entrained flow concept and their integration within a reference steel mill. The application of heat storage materials in an entrained flow concept was never studied before. Therefore, experiments were conducted to determine the dehydration rate of Ca(OH)2. The results of the experiments are compared to other studies. To evaluate the economic potential of the entrained flow approach and to compare the concept to other technologies, an economic efficiency analysis is performed. The payback time of the novel entrained flow concept is expected to be 6.1 years, if current state incentives are considered. Nevertheless, additional research is necessary before the concept could be used in an industry level.