| Taking advantages of high productivity, high PCR, low fuel rate, low CO2 emission, etc, the top gas recovery oxygen blast furnace (TGR-OBF) process is considered to be one of the promising ironmaking processes in future. However, after adopting high oxygen enriched blast, high PCI and top gas recycling system, the technical and economic indexes usually deteriorate due to the complexity of the whole systems increasing the difficulty in operation technology, and also due to the lack of unsteady simulation of the TGR-OBF systems. Furthermore, because the optimization and overall arrangement of the hot blast stove and TGR-OBF systems are not determined, the industrial operation is usually aimless and uncertain. According to investigation and survey, the thermodynamic data used in the calculation of the oxygen blast furnace process are not consistent and not satisfied to the constraint conditions of the predominance area diagram. Meanwhile, the disputes of the iron oxides reduction mechanisms are usually reported in internal and international academic journals, which makes the fundamental theory of the kinetics are not reliable in simulation of the oxygen blast furnace.In this paper, the accurate predominance area diagram and the corresponding thermodynamic data are obtained according to constraint fitting and differential thermal experiment researches. Taking the parameters of time, atmosphere and pressure into account, the mathematical simulation of the indirect reduction of the constraint step in blast furnace shaft is studied. It is found that the indirect reduction degree is linear to the reducing gas content produced in the bosh. The unsteady mathematical simulation and multi-objective optimization models are established with the hot blast stove and TGR-OBF systems are boundled as a whole research object, considering the kinetic constraint step, chemical equilibrium of the shaft gas combustion and self-consistency of physical properties of the cycled gas. The unsteady simulation of the TGR-OBF process is carried out to illustrate the change of the process parameters and technical & economic indexes. According to researches of the fundamental theory, unsteady mathematical simulation and multi-objective optimization of the TGR-OBF process, four main results are obtained as follows:(1) When iron oxides are reduced by CO and H2, the triple phase point of the reactions is 576℃, the transition temperature of the reducibility of CO and H2 is 819℃; 16 empirical Gibbs free energy equations and 8 enthalpy data at 298K of the iron oxides reduction reactions are obtained; The mechanism of hematite reduction reactions is dependent on the reaction temperature. It is two-step mechanism at low temperatures, while it is three-step mechanism at high temperatures, the transition temperature is 576℃ for isothermal process and it is usually not 576℃ for non-isothermal reactions because of the delay effect induced by different heating rate. The influences of non-stoichiometry on thermodynamic and kinetic of the iron oxides reduction reactioins are theoretically calculated and experimentally confirmed.(2) An application named "Simulation of the Multi-interface Unreacted Core Model" is programmed, and according to the single-interface shrinking core simulation, when the smelting intensity of oxygen blast furnace and traditional blast furnace is the same level, indirect reduction degrees of the blast furnace are 0.53,0.64,0.68,0.73 and 0.81, corresponding to the blast oxygen enrichment of 22.2%,40%,60%,80% and 100%. In other words, when the same indirect reduction degree is adopted by controlling the discharge speed, the more oxygen of the blast furnace is enriched, the less reduction time lasts, and the utilization coefficients are 2.00 t·d-1m’3,2.83 t·d-1·m-3,3.26 t·d-1·m-3,3.84 t·d-1·m-3 and 5.02 t·d-1·m-3 respectively.(3) A program named "Unsteady resolution and multi-objective optimization of the TGR-OBF process" is developed. Taking advantage of the program, the calculation results of the traditional blast furnace process are gained:the coke rate is 337kg/t, the PCR is 198kg/t, the carbon consumption is 425kg/t, dry fuel rate is 523kg/t, the first heat loss is 4.10%, the thermal reservation temperature is 980℃, the theoretical combustion temperature is 2032℃, the two kinds of energy consumptions are 519kgce/t and 377kgce/t respectively, the two kinds of exergy consumptions are 15.63GJ/t and 11.96GJ/t respectively, exergy efficiency is 77%.(4) Influences of six operational parameters on carbon consumption and first exergy consumption of the TGR-OBF process are investigated by three-dimensional optimization. As an example, the three-dimensional function of carbon consumption with respect to blast oxygen content and blast temperature is non-linear, and the direction of cutting down carbon consumption arcs opposite to the origin. When the blast oxygen content is fixed, the carbon consumption decreases with raising blast temperature; when the blast temperature is fixed, the carbon consumption firstly rises to a climax and then goes down non-linearly with increasing blast oxygen content. In the three-dimensional system of "blast oxygen content-blast temperature-first exergy consumption", the profile of the three-dimensional picture is similar to the one mentioned above. The minimum of first exergy consumption is located in the region of high blast temperature and high blast oxygen content.(5) The least carbon consumption of the optimized processes is 289kg/t, which is less than the theoretical least carbon consumption of the conventional blast furnace. The technical and economic indexes of the comprehensive optimized TGR-OBF process are calculated:the coke rate·is 150kg/t, the PCR is 237kg/t, the carbon consumption is 300kg/t which is less than that of the traditional blast furnace by 125kg/t, the two kinds of exergy consumptions are 12.34GJ/t and 11.96GJ/t respectively, where the first exergy consumption is 3.29GJ/t less than that of the traditional blast furnace, and the second exergy consumption is the same as the data of traditional blast furnace, exergy efficiency is 85%. Additionally, because the huge amount of CO is recycled in furnace and the large amount of CO2 is absorbed by VPSA, the CO2 emission of this process is just 71m3/t, which means the new TGR-OBF process cuts down about 90% CO2 emission. That the relevant pollutants are reduced largely because of the less coke usage is simultaneously attractive. |
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