| The development of direct reduction production is an important way in China to get rid of the restraint of coking coal resource, improve structure of energy and products, solve the shortage of steel scrap resource, promote the comprehensive utilization of resource, realize sustainable development of iron and steel industry. Compared with other direct reduction processes, gas based shaft furnace process, with the advantages of fast reduction rate, stable product quality, high automation level, large unit capacity, and low energy consumption, has become one of the main processes of direct reduction in the world. With the continuous innovation of gas based shaft furnace direct reduction process and combined with the mature coal gasification technology, the technology of coal gasification-shaft furnace direct reduction is a predominating direction for the development of direct reduction iron production in China where natural gas is scarce and non-coking coal resource is abundant.Although, as a combined process of developed coal gasification technology and shaft furnace technology, the coal gasification-shaft furnace process is feasible in theory, there are still many key technological problems to be overcome in the course of implementation. Some of those problems can be listed as follows: reasonable selection for coal gasification technology based on the domestic coal resource conditions, preparation for high quality oxidized pellets based on the domestic iron resource conditions, mechanism analysis for pellets reduction swelling and performance improvement, thermodynamic and kinetic analysis for reduction process, and matter flow and energy flow analysis for this technology, and so on. Accordingly, coal gasification-shaft furnace direct reduction process is studied systematically in this paper based on the above problems, and it is applied to the efficient clean comprehensive utilization of ludwigite.Firstly, based on comprehensive grasp of the existing technology features of coal gasification and combined with requirements of shaft furnace reduction process, the four main coal gasification processes, including Lurgi, Ende, Texaco and Shell, are evaluated quantitatively by the methods of single index lateral comparison and multiple index synthetic weighted mark method, taking full consideration of investment costs, oxygen consumption, coal consumption, cold gas efficiency, the ratio of CO and H2 in the gas, oxidizability of coal gas, content of effective reducing gas, net thermal efficiency, carbon conversion efficiency, and production capacity of single chamber. The result shows that, Shell process of entrained-flow bed dry pulverized coal pressure gasification and Ende process of fluidized bed pulverized coal normal pressure gasification are suitable for producing gas used in shaft furnace.Secondly, base on the domestic iron resource conditions, experiment for oxidized pellets preparation and metallurgical performance used for gasification-shaft furnace direct reduction process is carried out. The result shows that, with the high value of colloid index, swelling capacity and water absorption, bentonite is beneficial to improve properties of green balls and crushing strength of finished pellets. Taking crushing strength and grade into consideration, the suitable addition dosage of bentonite is 1%. Comprehensive properties of three kinds of oxidized pellets used in this experiment are good enough to satisfy the needs of gasification-shaft furnace process. In short, based on the domestic iron resources conditions, oxidized pellets with high qualities can be obtained for gasification-shaft furnace direct reduction process through adding suitable adhesive under reasonable technological parameters.Thirdly, given that reduction swellability of oxidized pellets is one of the most important indexes to determine the reasonable distribution of airflow and regular production in the shaft furnace, experiment about the influence of reduction temperature, reduction atmosphere and gangue component on the reduction swelling is carried out, and research on applying boron-magnesium compound additives to improve reduction swellability is also conducted. The result shows that, with increasing of H2 content in gas and decreasing of reducing temperature, reduction swelling of oxidized pellets reduces. Flocculent structure of iron grain, carburization reaction and heat effect of reduction reaction are the main factors for the dramatic increasing reduction swelling of pellets in CO atmosphere. CaO, SiO2 and MgO added to pellets are beneficial to slag bonding, stable crystal structure forming and reduction swelling index decreasing. Appropriate dosage of boron-magnesium compound additives in raw materials can improve the pellets reduction swelling and enhance the crushing strength of reduced pellets.Fourthly, based on thermodynamic principles and unreacted core model, thermodynamics and dynamics mechanism of pellets gas-solid reaction is analyzed systematically. And then calculation equation for theoretical utilization ratio of gas and analytic expression for reaction rate of reduction are obtained. The result shows that, under the condition that the ratio of H2 to CO in the gas is greater than 1 and reducing temperature is higher than 800?, utilization ratio of gas increases with ratio of H2 to CO and reducing temperature increasing, and decreases with carburization quantity and metallization of direct reduction iron increasing. In the temperature range of from 850? to 1050?, with the content of H2 in the gas increasing, activation energy of reduction reaction decreases gradually, resulting to reaction rate accelerating. However, when the content of H2 is higher than 50%, the above effect shows a declining trend gradually. In H2 atmosphere, activation energy of reaction is 27.444 kJ/mol, and interfacial reaction is the limited link in the reduction procedure. In CO atmosphere, activation energy of reaction is 39.907 kJ/mol; in the initial stage, the resistance of interfacial reaction dominates. While with further reduction and thicker products layer, internal diffusion resistance dominates and becomes the main limited link after the reduction ratio is up to 20%.Fifthly, through calculation on material balance and energy balance, material flow and energy flow of gasification-shaft furnace direct reduction are analyzed, and then an exergy model of gas based shaft furnace direct reduction is developed to analyze shifting mechanism of energy in the process. The result shows that, the content of effective reduction gas(H2+CO) in the top gas is greater than 50%, and if put into cyclic utilization, it can reduce half of net coal gas demand for producing per ton direct reduction iron. More than 50% of heat is dissipated with top gas, while only less than 20% is used for reduction process of iron oxides. About 90% of the input exergy of gas based shaft furnace direct reduction is from chemical exergy of reduction gas. Compared with that of blast furnace process, exergy demand for producing per ton direct reduction iron of shaft furnace is less. When top gas is in cyclic utilization, aim exergy efficiency is up to 76%. With the increasing of ratio of H2 to CO in the gas, carburization quantity in direct reduction iron and tapping temperature, and decreasing of volume and temperature of coal gas injected into furnace, the temperature of top gas decreases and energy utilization efficiency increases gradually.At last, the new process of gas based shaft furnace direct reduction-electric furnace melting separation is applied to the efficient and clean comprehensive utilization of ludwigite. The result shows that, the new process realizes the goal of efficient separation for boron and iron, boron yield is up to 95%, and high grade boron-rich slag of activity is 85%. |
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