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Multi-objective Optimization of Ironmaking in the Blast Furnace with Top Gas Recycling
Hannu HELLE, Mikko HELLE, Frank PETTERSSON and Henrik SAXEN
(Thermal and Flow Engineering Laboratory, Finland.)
(June 24, 2010)
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 CO2 in primary steelmaking. Blast furnace top gas recycling, combined with CO2 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.
Concern about the rising levels of carbon dioxide in the atmosphere has brought about intensive research activities, where the options of suppressing or capturing carbon dioxide emissions are being investigated. Thus far, the main effort has been put on investigating the emissions that arise in power plants and industry. Primary steelmaking is known to be one of the most energy intensive sectors. It accounts for about three fourths of the steelmaking (the remaining fourth being scarp based), and almost 90% of the production in the chain from ore to steel goes through the blast furnace (BF) route. The blast furnace, in turn, consumes about 75% of the energy in this chain, giving rise to large quantities of emissions. Therefore, many ways have been suggested for suppressing the emission rates from the blast furnace, e.g., by plastics injection, lowering of the reserve zone temperature, and by partially replacing coal by biomass. There are also active efforts in developing means for carbon dioxide capture and storage in steelmaking.
However, in order to make the latter alternative economically feasible, the nitrogen content of the blast furnace top gas should be decreased. A possible way to achieve this is to apply oxygen blowing combined with top gas recycling:
This concept was originally proposed in the 1980’s as a means for suppressing the coke rate, and several trials in industrial scale at Tulachermet in Russia proved the technical feasibility of the concept. The recent interest in the technique is due to the obvious potential of storing the stripped CO2 and thus suppressing the emissions. Within the European ULCOS research program8 the technique has been investigated in the experimental pilot blast furnace in Lulea, Sweden, and the results have been encouraging.
Extensive simulation studies of the blast furnace with top recycling have also been undertaken in Japan. Austin studied simple recycling of cold gas, recycling with increased blast oxygen enrichment, and recycling of CO2- stripped hot gas after heating together with high oxygen blast. The last concept was found to hold promise due to its beneficial effect on productivity and coke rate. Nogami fixed the main conditions in the high-temperature region and studied plastics injection combined with top gas recycling, with injection of oxygen and the recycled top gas at ambient temperature 25°C. The low “blast” temperature was found to limit the benefits of the concept. The same authors later concluded that BF top gas recycling reduced the energy requirement and that it would be an interesting alternative, in particular if the stripped CO2 could be stored. Murai considered options for minimizing CO2 emissions in steelmaking and proposed top gas recycling in the BF with tuyere and shaft injection and massive plastics injection as a potential solution.
All the above investigators have pointed out the need of an overall analysis of the material and energy flows in the whole plant for assessing the economic advantage of the proposed innovative ways of operation. In a recent study, Helle mathematically investigated the conditions under which top gas recycling combined with massive oxy-gen content. The investigators concluded that the CO2 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 CO2 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 and Wang. Like in the earlier analysis, 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.
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