Experimental Research On Coal-based Direct Reduction Of Iron Ore Powder In Circulating Fluidized Bed

Gas-based direct reduction is the main stream in iron ore direct reduction process in the world. However, in China coal-based direct reduction is more worthy of being reaearched due to the actual condition of lack of gas and oil. Fluidization technology has many advantages, such as full contact of the materials, uniformity of reaction conditions and high capacity utilization rate, etc., which is consequently much concerned in the iron ore direct reduction process. In this paper, experimental research is conducted to investigate the iron ore powder coal-based reduction process in a circulating fluidized bed.Thermogravimetric analyzer and the associated equipments were applied to study the non-isothermal kinetics characteristics of the mixture of different coals and iron oxides, based on which the scheme of coal-based direct reduction of iron ore in circulating fluidized bed was proposed. Then a series of experiments of coal-based direct reduction of iron ore were carried out in a small-scale circulating fluidized bed thermal test rig.The effects of different reactants (ferric oxide powder and iron ore powder were separately mixed with Datong bituminous coal, Shenmu coal and Yangquan anthracite coal) and different reaction atmospheres (nitrogen, carbon dioxide) on the reduction reaction were investigated by thermogravimetric analysis. Comparing the results of taking ferric oxide powder and iron ore powder as reactants respectively, it can be seen that the impurity in iron ore powder has certain effect on the reduction process of the mixture, which will increase the starting reaction temperature, and decrease the reduction fraction in each reaction stage in nitrogen atmosphere. In the comparison of using three types of coal to reduce iron ore powder in nitrogen atmosphere, the mixture of iron ore and Datong coal exhibits better reducibility at relatively low temperatures (800℃～950℃), and at higher temperatures (1000℃～1100℃), the mixture of iron ore and Shenmu coal shows good reducibility. In carbon dioxide atmosphere, comparing the reduction of iron ore powder by three types of coal, it is indicated that the gasification characteristics of coal are determinant factor for the parameters of starting temperature, temperature range and the apparent activation energy. Comparing the atmosphere of nitrogen and carbon dioxide, the reaction stage in carbon dioxide corresponds to the first reaction stage in nitrogen, in which the carbon gasification and the iron oxide reduction were mutually prohibitive. Therefore, in the experiment of coal-based direct reduction of iron ore in the circulating fluidized bed, Datong coal is chosen as the reductant to study the reduction situation at 800℃-950℃.The process of iron ore powder reduced by coal with the fluidization medium of nitrogen was studied in the circulating fluidized bed in different reaction time conditions for the first time, based on which, the effects of reaction temperature on the reduction degree of iron ore powder were investigated with two fluidization media of nitrogen and carbon dioxide. The results demonstrate that with the fluidizing medium of nitrogen and reaction temperature of 950℃, the growth degree of iron ore reduction gradually becomes slow with the increase in reaction time, and reaches the maximum of 87% when the reation time is 60 mimutes, which clearly explaines that increasing reaction time can not make a big contribution to reductive degree within a certain period of reaction time. In the experiment of investigating the effect of reaction temperature on reduced degree of iron ore power, the results present that both the degrees of metallization and the reduction increase with the reaction temperature increasing, and there is obviously larger increase at 800℃-850℃, moderate increase at 850℃-900℃and again larger increase at 900℃-950℃, reaching the metallization degree of 63% and the reduction degree of 87% at 950℃. With the fluidizing medium of carbon dioxide, the results show that with the increase of reaction temperature the reduction degree increases first and then tends to level off, reaching the maximum of 47% at 850℃. The comparison of nitrogen and carbon dioxide as the fluidizing medium respectively tells that the carbon dioxide could not promote the coke gasification which could then promote the reduction reaction, but would inhibit the reduction reactions, leading to the decrease in reduction degree compaired with that in nitrogen before 850℃, and the stop of the mixture reaction after 850℃.