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 ...

The potential of using biomass in ironmaking is investigated by simulation. Biomass is used to partially replace fossil reductants in the blast furnace process, which is described mathematically by a thermodynamic model. The model is cast in linear form to facilitate an efficient economic optimization of the production of raw steel, considering costs of raw materials, energy and CO 2 emissions of the unit processes up to the basic oxygen furnace. The high oxygen content and low heating value of biomass makes it necessary to study a possible external pyrolysis of it prior to injection into the furnace. The economy of biomass injection and its dependence on the price structure of raw materials and emissions are investigated. The results throws light on how the prices of biomass and emission rights, compared to the price of coal and coke, affect the optimal biomass pre-processing (pyrolysis) and injection rate.KEY WORDS: ironmaking blast furnace; biomass injection; pyrolysis; optimization; CO 2 emissions. The basic oxygen furnace (BOF) converts the hot metal (hm) produced in the blast furnace into raw steel (rs). Neglecting other additives than scrap, which for the sake of simplicity was assumed to be pure Fe, the mass flow of raw steel and the volume flow rates of oxygen (consumed in) and off-gases (produced in) the BOF can be written where j 1 is obtained from the iron balance equation, and j 2 and j 3 from the carbon balance equation. Biomass Pre-processingWood chips, forest slash or logging residues are potential non-fossil sources of energy and reduction potential in the ironmaking process. 8) However, as pointed out by other investigators 5) and also demonstrated by the present authors in earlier simulation analysis, 7) injection of dry wood chips results in a low coke replacement ratio. A key issue is that the high oxygen content of the biomass, combined with the low heating value, results in low flame temperature and decreased productivity. A possible remedy is to pre-process the biomass into a form that is better suited for injection into the blast furnace 5,6) : A (partial) pyrolysis of the biomass prior to injection would lower its oxygen content, but naturally also decreases the yield (h) due to loss of valuable components, such as carbon and hydrogen. The asterisks ( * ) in Fig. 2 illustrate a typical dependence of yield and composition on the pyrolysis temperature.9) In order to express the variables for arbitrary pyrolysis temperatures, piecewise cubic hermite interpolating functions were adapted to the values within the temperature interval T pyro ϭ(150°C, 800°C), resulting in the curves depicted in the figure. Furthermore, the effective heating value of the biomass (H bio ) was estimated from reported values and approximated as function of the pyrolysis temperature (cf. Fig. 2). For the sake of simplicity, by-products of the pyrolysis units (e.g., tar, gases) were neglected, but so was also the energy needed to maintain the elevated pyrolysis temperatures. ISIJ International, Vol. 49 (2009) Blast ...

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Concern about the growing carbon dioxide content in the atmosphere has induced increasing research activities in the search for means to suppress the emissions of CO 2 in primary steelmaking. Blast furnace top gas recycling, combined with CO 2 stripping, has been proposed as a promising concept. The paper presents a numerical analysis of top gas recycling under massive oxygen enrichment of the blast based on a simulation of the process chain from coal and ore to liquid steel. Because of the conflicting goals of reducing both production costs and emissions, the task is formulated as a multi-objective optimization problem. The optimal states of the system studied were found to vary significantly on the Pareto frontier, which demonstrates that fundamentally different states of operation may be selected to strongly reduce the emissions, still keeping the steelmaking economically feasible. The findings stress the importance of selecting a proper state of operation for achieving a cost-efficient production of steel with reduced environmental impact. The results also show how emissions can be "artificially" reduced by minimizing the arising emissions within the system boundary.KEY WORDS: sustainable steelmaking; CO 2 emissions; top gas recycling; multi-objective optimization.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. 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. The study also throws light on the effect of the steel production rate on the costs and emissions in the optimal states. The System StudiedThe entity comprised in this study is an integrated steel works, considering the main units starting from the raw material processing (sintermaking and cokema...

The economic advantage of using biomass as partial substitute for fossil reductants in the blast furnace (BF) process was studied by simulation. A thermodynamic model of the BF was used in combination with simple models of the coke plant, sinter plant, hot stoves, basic oxygen furnace and power plant. Pretreatment of the biomass before its injection in the BF is considered in a pyrolysis unit where the carbon content and heating value are raised and the oxygen content is lowered, which is beneficial for the BF. The system was optimised with respect to the price of raw steel, considering costs of raw materials, energy and CO 2 emissions of the unit processes. The study demonstrates that biomass in partially pyrolysed form is a potential auxiliary reductant and that the optimal states of operation within certain regions depend strongly on the price structure of the raw materials and emissions.