| In modern iron-making throughout the world, sintering process is one of the most important technologies to agglomerate fine particles in iron ore for the subsequent production of pig iron in blast furnace. It is found that compared to the natural ore, the sintered iron ore offers the advantages of more iron content, higher porosity, higher reduction, less harmful impurities, rich in alkaline flux, etc, which can significantly improve the productivity of blast furnace, and reduce the coke consumption.Compared with the conventional sintering (CS) and hot air recirculation sintering process, the flue gas recirculation sintering (FGRS) process is a new technology, would be of dual benefit to save energy and reduce emissions, which can not only save more hot air from sintering cooler for power generation, but also significantly reduce flue gas emissions from the sintering machine itself. FGRS has been received great attention in iron & steel industry all over the world for decades. However, the shifts of input gas conditions brought by FGRS technology can significantly influence the heat and mass transfer, as well as the physical and chemical reactions in the sintering bed, subsequently influence the flue gas emission and the quality and yield of the sintered ore, such as physical heat increase caused by the temperature enhance of the recirculated gas, O2 content reduction and CO2 content increase and the trace existence of CO, reflowed contaminants such as dust impurities and SOx, etc, in the sintering bed. Now, furtherer investigations to reveal the sintering mechanism are needed.To fully demonstrate the advantages of FGRS technology, the thesis makes a 430 m2 FGRS system in China the research target and emphasizes on the study of effects on combustion characteristics brought by recirculated gas conditions. On the basis of experimental studies, including the thermos-gravimetric analysis (TGA) and sintering pot tests, we built an accurate mathematical model and optimized the main thermal parameters in detail, proved it feasible. Above all. the main contents of the research are as follows:(1) Reaction characteristics of three sintering raw materials were studied. Rely on TGA. we fully analyzed the TG curves of coke, limestone and dolomite samples under different reaction conditions, and calculated their reactive kinetic parameters by using the Flynn-Wall-Ozawa method. And then, we quantified the effects of O2/CO2 content in the reaction atmosphere and particle size on the initial reaction temperature Tini, activation energy Er and pre-exponential factor Ar. The specific measurement for above-mentioned chemical reaction characteristic parameters in this paper effectively avoids the shortcomings that parameters come from miscellaneous sources and the empiricism in valuing in the past sintering process simulations. TGA tests provide key basic parameters for the follow-up numerical simulation of mass and heat transfer process in FGRS.(2) Sintering pot experiments based on FGRS were carried out. The pilot experiment platform was utilized to launch thorough and intensive tests, which was built by USTB. Tests were carried out under the premise of keeping initial conditions the same, and focused on two thermodynamic parameters, i.e., temperature and oxygen level of the recirculated gas, which could significantly affect sintering process and quality. Differences between FGRS and conventional sintering process were also studied. The values of parameters in the tests were close to the industrial production, by considering the laboratory limitations. The research results have certain guiding significance for the industrial application of the new technology. Meanwhile, the measurement of the solid bed temperatures and the flue gas composition provides evidence to support the veracity validation of the mathematical model.(3) The quasi-2D mathematical model of FGRS was established. Based on the porous medium thoery, solid multiphase thoery, and unreacted shrinking core model, the mass and heat transmission process inside iron ore sintering was analyzed. And a mathematical model was established, in which physical meanings are clearer, chemical reactions are more complete, and heat transfer modes are more intact. This model is a great improvement and breakthrough to reveal the transmission mechanism of mass and heat coupling and multi reactions coupling in sintering process:1) considering not only the relative integrity of hetero-geneous and homo-geneous reactions, but also the change of the geometrical parameters of the sintering bed due to the above reactions; 2) considering not only the convective and conductive heat transfer between/within gas and solid phases, but also the radiative heat transfer between solid phases; 3) modifying not only the effects of the initial particle size distribution of raw materials, but also the effects of the coating of reacted particle with molten ore, on the reaction rates of key sub-models. Compared with the CS process, the adoption of homo-geneous reactions between gas phases is crucial for accurately simulating FGRS process. The relative errors between the predicted results and the experimental values were basically less than 5%, which proved the correctness of the model.(4) The main thermal parameters of FGRS were optimized. Based on the equipmental and productive parameters of the object, the influences of the temperature of and O2 content in recirculated gas, and the gas supply on the combustion characteristics of the FGRS process were revealed. And then, the best value ranges of the parameters were proposed:the O2 content best between 19-20 vol.%. the temperature best about 200?, the gas supply should be increased by about 3.33% compared with the CS production. After that, the sensitivity coefficient S was proposed to formulate the optimization control strategy of the main thermal parameters. In principle, give priority to adjust O2 content, followed by temperature, and finally for gas supply. In addition, the modeling results indicated that the FGRS technology can significantly reduce coke consumption by 3.34%, which can create economic benefits of CNY 4,430,000 per year.(5) The industrial application of the established sintering model:By using theoretical numerical simulation to guide the production, and a large number of field data to gather statistics and analysis, the dual benefit in production and quality improvement and energy saving brought by FGRS technology was obviously demonstrated, from the point of view of mass and heat balance, quality and production index, waste heat utilization, energy consumption, sinering flue gas and pollutants emission reduction, comprehensive energy-saving and so on. Taking 1000 t/h capacity for example, when setting the circulation rate to 25%, FGRS technology can reduce the fuel consumption by ?4%, while the quality of sintered ore is not destructive affected, and the heat recovery efficiency of system can be improve to 36.65%. Meanwhile, the sintering flue gas emission can be reduced by 22.32%, means 2511.1 billion Nm3 per year, in which the reductions of CO2, CO, NOx and SO2 emission are ?3900,535 1000 and 615 ton per year, respectively. Totally, FGRS technology can create economic benefit of ?8 million yuan per year, equals 11,130 ton standard coal per year. |
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