Multiscale Study On The Particle Agglomeration And Control Of Fluidized Bed In The Direct Reduction Process

In China,the development of the blast furnace is limited by the rare source of high-grade iron ore and metallurgical coke,smaller size of Chinses iron ore powder,and high iron ore dependence on foreign countries while advantages of fluidized direct reduction process,such as using fine iron ore powder,non-coke iron making,and low pollution emissions,well meets with requires of resource-saving and environment-friendly for iron and steel making plants.The main reasons to prevent the industrial application of fluidized direct reduction process is defluidization and complex composition of reducing gas.Therefore,it is significant to realize the fluidization of iron ore powder reduced by the complex composition of reducing gas.In the paper,scale of research on the mechanism of sticking of iron ore powder reduced by complex reducing gas is from atoms,surface and reactor.The method not only combines with reaction on the gas-solid interface and gas-solid flow together,but also combines with simulation calculations and experiments to build the relationship between different scales of research.The interaction between gas molecules and surface,the correlations between surface and agglomerations and the effects of agglomerations on the gas-solid flow were investigated.The main research contents and results are as follows:（1）The first principle method of density function theory（DFT）gives an explanation for mechanism of growth of iron whiskers.The growth of iron whiskers has to satisfied with directed diffusion of Fe²⁺ion in bulk.The energy released by the reduction of FeO by CO is the driving force for Fe²⁺diffusion and the strong attractive force between CO molecules and Fe²⁺ion makes Fe²⁺ions trend to diffuse along the normal direction of the surface.It shows that the morphology of new metallic iron on the surface is related to the changes of surplus energy and the ratio of CO/H₂ and temperature will have effects on the surplus energy.The simulation results show that when the ratio of CO to H₂ is higher than 82.1%at 900℃the morphology of new metallic iron will form as whiskers,which was proved by the observation using SEM.（2）The molecular dynamics（MD）gives an explanation for competition adsorption behavior between CO and H₂ during iron oxide reduced by CO/H₂ mixtures.The capacity for CO molecular to adsorb on the surface is governed by O^2-ions and Fe²⁺ions,but the capacity for H₂ molecular to adsorb on the surface is only governed by O^2-ions.The ability for CO to adsorb on the iron shell decreases with temperature increasing.The thermogravimetric analysis（TGA）of iron oxide reduction shows that when mole fraction of Fe²⁺ions is higher than 0.57 in bulk,the factor controlling the adsorption of CO changes from O^2-ions to Fe²⁺ions.（3）The growth of iron whiskers was simulated by phase field method.The rate of growth and the numbers of nucleation sites increased with concentration of CO increasing.Moreover,concentration of CO increasing will decrease the time of nucleation incubation.The increasing of gas velocity will also improve the growth rate of iron whiskers and decease time of nucleation incubation,but it has little effects on the numbers of nucleation sites.（4）The surface viscosity of the new metallic iron was measured by the thermal expansion experiments.Analyzing the surface diffusivity of iron atom calculated by surface viscosity,the relationship between surface diffusivity and temperature is well fitted with Arrhenius expression.The chemical energy changes during the reduction reaction will have effects on the activation energy of surface diffusivity.The released energy from the reduction by CO will decrease the activation energy of surface diffusivity.The iron atoms will form as net structure with high surface energy,but the reduction by H₂ will consume the energy to increase the activation energy of surface diffusivity.The iron atoms will form as bridge structure with low surface energy.Based on the diffusion behavior of iron atoms on the surface,the solid bridge force was built.Analyzing the force balance of particles in fluidized bed,the fluidization regions was predicted.It was found that the area of fluidization regions will increase first and the decrease with mole fraction of H₂ increasing.The added H₂ will increase the critical value of metallization to improve the purity of direct reduction iron.（5）The sticking behavior of particles in fluidized bed during direct reduction process was simulated by the CFD-DEM models coupling with mass transfer and heat transfer.The characteristic of gas-solid flow,distribution of particles’temperature and distribution of concentration of gas phase species was analyzed.The state of gas-solid flow was divided into two parts.One is the stable fluidization and another is unstable fluidization.The particles’temperature and concentration of gas phase is more homogeneous with stable fluidization than unstable fluidization.In the state of stable fluidization,the gas-solid flow shows as two circulation flow near the gas inlet hole at the bottom.When the metallization reaches a certain value,the agglomeration of particles happens and the state of gas-solid flow turns from the stable fluidization to unstable fluidization.There are a lot of gas channels in the state of unstable fluidization,which make the pressure drop decrease rapidly.The added H₂ will increase the critical metallization corresponding to the stable fluidized time.At low temperature,the added H₂ will increase the stable fluidized time.However,at high temperature,the added H₂will decrease the stable fluidized time.（6）The first principle method of density function theory used to study on the reaction routes of CO decomposition on the new metallic iron and the mechanism of H₂catalysis effect on the CO decomposition.The TGA was used to measure the adsorption energy of CO on metallic iron surface and the activation energy of CO decomposition reactions.The kinetic model was built to predict the rate of carbon formation with and without H₂ catalysis,which is proved by the experiments.Both calculations and experiments show that the added H₂ will decrease the activation energy of CO decomposition on the iron surface and extend the region of temperature for carbon formation and improve the formation of carbon shell,but the increasing of temperature and H₂ concentration will decrease the adsorption ability of CO on the iron surface.