Primary steelmaking is among the most energy intensive industrial processes in the world and being mainly coal-based it substantially contributes to the global fossil CO 2 emissions. It is therefore important to study potential ways of suppressing the use of fossil reductants and the rate of emissions in the process. This paper analyzes by simulation and optimization the concept of recycling CO 2 -stripped top gas in the blast furnace under massive oxygen enrichment, and its impact on the production economy and emissions of the steel plant. The effect of CO 2 emissions and stripping/storage costs on the optimal states is presented. The system studied is demonstrated to exhibit complex transitions between the optimal states. The findings throw light on the importance of selecting a proper state of operation for achieving a cost-efficient production of steel with reduced environmental impact.KEY WORDS: blast furnace; top gas recycling; economic optimization; CO 2 emissions.
931© 2010 ISIJ concept with tuyere injection of the CO 2 -stripped and heated top gas together with oxygen-enriched blast is evaluated by simulation. The performance of the process is optimized with respect to oxygen injection, top gas recycling rate and temperature, and oil injection rate using a linearized model of the blast furnace. However, since top gas recycling dramatically affects the material and energy flows in the whole plant, 14) the entire system should be studied in order to evaluate the overall benefits. The present study therefore comprises the coke plant, sinter plant, hot stoves, blast furnace, stripping unit, the basic oxygen furnace and power plant, paying attention to the costs and the emissions of the process, and the optimal states of operation of the system are studied with different price settings for CO 2 emissions and stripping/storage.
The SystemWe study the units in an integrated steel works from the raw material processing units through one blast furnace to the basic oxygen furnace (BOF), 5) with liquid steel as the main product (Fig. 1). The models were developed on the basis of process data from a Finnish integrated steel works. The core of the analysis is the blast furnace with oil injection and top gas recycling, which is modeled in more detail. In the next subsection, the models are briefly described. For more details, the reader is referred to Refs. 15, 16).
Process Models and CO 2 EmissionsThe main focus of the study is put on the blast furnace, for which a more detailed description is used while the other unit processes are described in a simplified way (see Appendix). The sinter plant is modelled by expressing the produced sinter, the required coke and limestone as well as the recovered heat as linear functions of the ore inflow. The coke plant is described by linear relations between the mass flow rate of feed coal and the mass flow rate of coke and volume flow rate of (purified) coke oven gas (COG). The (internal) flow rate of coke, available for the blast furnace, is given by the difference of the coke ...