Optimization of Top Gas Recycling Conditions under High Oxygen Enrichment in the Blast Furnace

Abstract

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 ...

Multi-objective Optimization of Ironmaking in the Blast Furnace with Top Gas Recycling

“…The fact that a mixture of air and oxygen was allowed enabled different recycling degrees of the top gas, still keeping the BF operation within the imposed constraints. 11,14) The BF model was run extensively under different inputs uniformly distributed within their admissible regions (Table 1), and the set of resulting feasible solutions was used to construct a linearized model with seven inputs and fourteen outputs (Table 1) ( 1) where the inputs are the volume flow rate of recycled top gas, V rg , the total volume flow rate of blast (i.e., pure oxygen with possible air additions), V bl , and oxygen content, Y O 2 ,bl , the specific oil rate, m oil , the temperature of the injected tuyere gases, T bl , and the specific rates of pellets, m pel , and limestone, m lime,BF . The linear fit was, in general, very good, with R 2 values ranging between 94 % and 100 % for the fourteen variables approximated, with most of the values close to 100 %.…”

“…(4) (5) by contrast to the intuitive approach of including the emission costs in the first and single objective. 14) An advantage of the multi-objective approach is that one need not take a stand on the (future) emission price, but, instead, can consider both goals to be important, leaving it up to the investigator to select a suitable compromise from the solutions evolved. Since these objectives are likely to be conflicting, the solutions that are non-dominated can be represented as a 'Pareto-optimal frontier'.…”

“…A problem with this approach is, however, that it is difficult to provide realistic estimates of the future price of reductants and emissions, and, in particular, of the CO 2 stripping costs. The present paper therefore analyzes the economy and emissions of steelmaking by a multi-objective approach, where the two goals are treated as independent objectives: A similar approach, but for the system with conventional BF operation, was studied by Pettersson et al 15) and Wang et al 16) Like in the earlier analysis, 14) the simulations in the present paper consider the coke plant, sinter plant, hot stoves, blast furnace, stripping unit, the basic oxygen furnace and power plant, with the main attention focused on the blast furnace, for which a more detailed model is applied. The analysis shows how minimizing the conflicting goals results in different optimal states, where, e.g., maximum top gas recycling or maximum externally produced raw materials (coke and pellets) can be preferred despite increased costs due to the low emission rates within the plant, or, vice versa, how the steel production price can be reduced at the expense of increased emissions.…”

“…In a recent study, Helle et al 14) mathematically investigated the conditions under which top gas recycling combined with massive oxy-gen enrichment of the blast would be economically feasible by optimizing the recycling conditions with respect to rate of auxiliary reductant, blast volume, temperature and oxygen content. The investigators concluded that the CO 2 emissions and stripping costs strongly affected the optimal recycling policy.…”

Optimal Resource Allocation in Integrated Steelmaking with Biomass as Auxiliary Reductant in the Blast Furnace

“…New ironmaking technologies 1,2) including HISmelt, FINEX and blast furnaces (BF) with top gas recycling, [3][4][5][6] are likely to play a role in the transition to low-emission concepts, but these technologies are either not mature or do not yet show lower energy demand than (large traditionally operated) blast furnaces. Therefore, it is still important to raise the energy efficiency in existing processes and to investigate options to replace coal, oil and natural gas by more environmental-friendly reductants.…”