Process integration in the steel industry : possibilities to analyse energy use and environmental impacts for an integrated steel mill

Sammanfattning: There is a growing awareness of serious problems associated with the use of energy. These problems include local and global environmental degradation associated with energy use and depletion of resources. The awareness of the problems associated with energy use increased as a consequence of the two oil crises in the 70s which led to rising costs for energy. Today, it is environmental issues that are becoming increasingly important, and it is primarily concern for a higher concentration of greenhouse gases in the atmosphere and the effect this has on climate that are highly prioritised. Analysing the potential for improving the specific energy use and environmental performance in a steel mill can be difficult due to the interactions between the many sub-systems. Changes in one unit may lead to a chain of changes throughout the system and the overall effect may not necessarily be improved performance. This thesis addresses the issue of evaluating and analysing an industrial material and energy system with regard to energy use, and economic and environmental performance, through a systematic approach. The main emphasis is on the development of analysis methods and tools for an integrated steel mill. Several applied studies of the integrated steel mill of SSAB Tunnplåt AB have been carried out. The results show that large savings in energy, cost, and emissions to the environment can be achieved using the analysis methods developed. From the analysis of energy use, a potential reduction of up to ~17% could be identified within the existing system, e.g. through more planning and control, less volatile matter in the coking coal mix, and by increasing the scrap rate in the BOF. From a cost perspective, the method showed that significant savings could be achieved through using a wide system boundary when performing the analysis. In the existing steel making system, the CO2 emission proved to be among the lowest compared to similar systems. Improvements could be taken even further using the optimising methodology proposed. A method for analysing the trade-offs between different objectives is also proposed. The process integration model is a good tool providing new insights into the material and energy system. It can serve as a benchmark for different steel making operations and constitute the basis for continued work on improving material and energy efficiency. Implementing such a tool in the industry is a good complement to the existing analysis tools in order to assess the effect of energy saving measures and can be used together with an energy management system.