Fundamental Research On Gas-based Direct Reduction Of Iron Ore With Reformed Coke Oven Gas

Direct reduction iron(DRI)is a high quality raw material of electric arc furnace(EAF)and coolent of converter steelmaking because of its more stable composition,lower harmful impurity contents.Although natural gas resources in China are not enough for the scale development of DRI industry,we can make full use of the hydrocarbon resources in the coal by efficiently reforming the hydrocarbon into hydrogen-rich multiple reduction gas.And then,DRI is produced by the reformed gaes,which achieves the reduction of carbon dioxide(CO2)emission.In this paper,iron ore concentrate was the research object.Thermogravimetric(TG)analysis was used in the basic research on the direct reduction experiments of iron ore by hydrogen-rich gases.Firstly,the conditions of iron oxides reduction and carbon deposition were calculated by thermodynamics method.The thermodynamic conditions of carbon deposition(carbon activity,reaction temperature and composition of carbon precipitate etc.)were investigated.TG experiments were carried out to study the phase transitions of iron ore in reduction processes by pure hydrogen,pure carbon monxoide and hydrogen-rich gas,respectively.Furthermore,the kinetics in the process were discussed in details.Carbon deposition during iron ore reduction process was investigated and the mechanism behind the swelling down of the iron particles during the carbon deposition process was proposed and discussed.The reduction gases composition,gas utilization and the minimal gas demands in the iron oxide reduction were calculated by thermodynamic method.The precipitation line of graphite and the formation line of Fe3C under C-H-O equilibrium system were calculated by the minimum Gibbs free energy method.The results showed that carbon deposition was unavoidable in a DRI process with COG.For iron-based materials,carbon deposition was determined by temperature and pressure in CO-CO2 system and CO-H2-H2O system.According to the gas composition and temperature,carbon deposition area and composition of products could be predicted.Experiments on iron ore isothermal reduction and kinetics studies showed that the reduction reaction rate increased with the increasing temperature.The reduction process of iron ore pellets was carried out step by step.In the whole reduction process,the rduction rate of FeO?Fe was the slowest,which was the limiting step.The reduction rate of hydrogen was significantly faster than carbon monxide.However,with the increase of temperature,a dense iron layer appeared in the reduction products by hydrogen,which retarding the internal gas diffusion.As a result,When the reaction temperature was between 700 ? and 900 ?,the reduction rate of hydrogen was increased slightly or even not changed.The iron produced by carbon monoxide will promote the carbon deposition,because carbon particles dispersed among metal iron particles,which hindering the adhesion of the metal iron particles,and led to the porous structure of the reduction products by carbon monoxide.The porous structure was favorable for the gas internal diffusion of the product layer.The reduction rate of mixture gas with 80%hydrogen and 20%carbon monoxide was almost comparable with pure hydrogen.And the morphology of product was between products which were produce by the hydrogen and carbon monoxide,improving the reduction rate problem in hydrogen reduction process at high temperature.When the temperature was higher than 900 ?,cracks were observed on the surface of reduced pellets.With the risen reduction temperature,the crack became widen.The cracks produced by carbon were more obvious than by hydrogen.The cracks produced by mixture gas with 80%hydrogen and 20%carbon monoxide were between productions by prue hydrogen or by pure carbon monoxide.The kinetic calculation showed that the apparent activation energy of the mixture gas was lower than that of pure hydrogen.As a result,the mixture with 80%hydrogen and 20%carbon monoxide could be used as a substitute for pure hydrogen gas for DRI production.In the reduction process of iron ore in the carbon monoxide atmosphere,iron oxide reduction and carbon deposition were interacted,and carbon deposition was catalyzed by reduced iron.The rate of carbon deposition was fast between 400 ? and 800 ?.When temperature was higher than 900 ?,carbon deposition reaction was almost stopped.The iron particles with smaller size were more susceptible to carbon deposition.The main phases of carbon deposition were cementite and graphite.Carbon deposition was affected by the microstructure of reduced iron,and carbon deposition was much easier for the iron with larger surface areas.Carbon potential was the driving force for carbon deposition.Carbon deposition can only occur when the carbon potential of gas phase was greater than one.Carbon deposition was much easier in the mixtures of hydrogen and carbon monoxide than in pure monoxide.Small amount of hydrogen could promote the disproportionation of carbon monoxide.However,too much hydrogen would inhibit carbon precipitation.When the amount of hydrogen was more than 20%,the amount of carbon deposition increased with increasing concentration of CO in the mixture gas.The study on the pathway of carbon deposition on the surface of reduced iron was carried out.Based on that,a model of iron ore swelling was proposed in the process of carbon deposition,and the procedure of disintegration of iron particles in carbon deposition process was described.Cementite played an intermediate role in the process of carbon deposition which can promote the carbon deposition.As a result,a large amount of carbon deposition came from cementite.The DRI process based on reformed COG was proposed,which included the reformation of the purified COG and the shaft furnace reduction.The research was made on the modification conditions and compositions of the reformed COG which were for shaft reduction.Considering the operation parameters and production feasibility,the optimum reforming temperature and O/CH4 value were given.The numerical simulation of shaft furnace reduction process was carried out.The results of showed that it was feasible to use reformed COG as gas soureces in DRI process with the appropriate process parameters.