Meso-scale Simulation Method For The Heterogeneous Reduction Reaction Process In Fluidization State

The gas-solid heterogeneous reaction in a fluidzed bed is a fundamental reaction for new non-blast furnace ironmaking technologies.Studying the characteristics of the iron oxide reduction in fluidization will provide an important theoretical foundation for promoting the development and application of new ironmaking technologies.Noted that the iron oxide reduction has the characteristics of multi-step heterogeneous reaction,and the heterogeneous reaction behavior with complex boundary conditions poses a huge challenge to numerical simulation methods.Thus,we proposed the topic of“Meso-scale simulation method for the heterogeneous reduction reaction process in fluidization state”.This topic aim to realize the cross-level simulation from gas-solid interaction mechanism in the microscopic or mesoscopic level to complex reduction reaction process in the macroscopic level by establishing a novel mesoscale simulation method,and to deepen the understanding of the nature of complex processes in related fields.In this thesis,from the viewpoint of particle scale and intra-particle pore scale,a meso-scale simulation method from cellular automata and lattice gas theory was established for the reduction of iron oxide in fluidization state by analyzing the physical nature of gas-solid heterogeneous reactions and particle motion process.This method allows the use of the simple microscopic local actions and evolutionary rules to describe complex flows and gas-solid heterogeneous reactions.The effectiveness of this method in studying gas-solid reaction system with complex boundary conditions was verified by numerical simulation of basic problems of flow and reaction processes in fluidized systems with heterogeneous reactions.Subsequently,we studied the iron oxide reduction process using a combination of experiments and simulation,and revealed the multi-step reaction characteristics and the law of gas-solid interaction of iron oxide reduction in fluidization state reaction process from the mesoscopic scale.The main research and findings of this thesis are listed as following:(1)A meso-scale simulation method using cellular automata was developed for investigating the heterogeneous reactions in fluidization state.This method had a fully discretized simulation system,including discrete simulation space,gas phase,solid particles,and evolutionary time.Based on the multi-substance and multi-energy state lattice gas cellular automata model,it was proposed to use the motion probability of gas particles in different directions on the grid nodes to describe the diffusion of gas within the solid phase,and to use the reaction probability associated with the solid properties and local reaction conditions to describe the interfacial chemical reaction process;and the accuracy of these two description methods were validated with the diffusion problem and the gas-solid reaction problem within the homogeneous solid particles,respectively.On the basis of the force analysis of solid particles,Newton's second law was applied to describe the particle motion,and the accuracy of this description method was validated by the classical particle settling problem.The coupling of gas-solid reaction and particle motion was achieved by adjusting the evolution time difference between gas particles and solid particles under the limit of the mean free path of the lattice gas particles.(2)To examine the effectiveness of the meso-scale simulation method in studying complex fluidized reaction systems,simulations were performed to investigate processes such as flow and heat transfer over a cylinder,gas-solid reaction of porous media,multi-particle settling,and fluidized heterogeneous reactions.Results showed that the time-averaged drag coefficient of the cylinder decreased with increasing Reynolds number and the initial porosity of the cylinder,while the average Nusselt number on the surface of the cylinder increased with increasing Reynolds.When more than one cylinder was placed in the pipe,the drag on all cylinders were decreased due to mutual interference between cylinders,but the drag increases with increasing the distance between the cylinders.The reaction rate increases with increasing initial porosity of porous media and reaction temperature;if the reaction mechanism was internal gas diffusion,the solid product was concentrated in the periphery of solid reactant with a clear reaction front,while if the reaction mechanism was chemical reaction,solid product was formed in the solid reactant matrix in a dispersed manner.In the process of multi-particle settling,the solid particles settled in the form of agglomerates due to the uneven drag on the upstream and downstream particles;with the increase of the fluid flow rate,the agglomerates gradually changed to a circular shape.As the reaction proceeds,the porosity of the particles in the fluidized state increases,resulting in a significant decrease in the drag on them,so the particles settled after the reaction.(3)Based on the phase transition and oxygen loss rate of the reduction,a multistep analysis method that can divide the reduction of hematite to metallic iron into three stages was developed.The first stage was the single-step reaction of Fe₂O₃?Fe₃O₄ at X<0.11,the second stage was also the single-step reaction of Fe₃O₄?Fe O at 0.11<X<t_p,and the last stage was the simultaneous reactions of Fe₃O₄?Fe O and Fe O?Fe at X>t_p,where t_pdenotes the transfer point between the single-step reaction and the simultaneous reactions.Subsequently,multiple step reduction kinetics of iron oxide was investigated experimentally using the micro fluidized bed reaction analyzer and the multistep analysis method.Results indicated that the reaction rate of the reduction of Fe₃O₄ powder with CO showed a fluctuating trend with increasing temperature,initially increasing then decreasing before increasing again,which was the result of carbon deposition and the bonding of particles.Regarding the hydrogen reduction of Brazilian hematite,the influence of external gas diffusion on the reduction could be ignored when the inlet flow rate was greater than 400 m L/min(at NTP);the reaction mechanism and kinetic parameters of all individual reactions during the reduction process were analyzed systematically in the temperature range of 400–570?and 600–800?,this work will provide simulation parameters for the meso-scale simulation of the hydrogen reduction of iron oxide in fluidization state.(4)On the background of the reduction process of iron oxide in fluidization state,the multistep reaction and the law of gas-solid interaction of the hydrogen reduction of iron oxide were studied.Based on the meso-scale simulation method,we proposed to label the physical states of solid nodes and gas particles to distinguish different iron oxides and different gas phases in the reduction process;the description of each single-step reaction was realized by the description method of chemical reaction according to the physical properties of the solid nodes and gas particles at the nodes.Firstly,the hydrogen reduction of Fe O single particles at rest was investigated,and it was found that the initial porosity and particle size of Fe O were the key factors to determine the transformation of the reaction mechanism,which changed from phase-boundary reaction to internal gas diffusion as the reaction proceeded under large particle size and small porosity.Then,the hydrogen reduction of Fe₂O₃ single particles in fluidization state was studied,and the relationships between the content of different iron oxides and the reaction process under different temperature conditions were obtained.We found that the mass fractions of Fe₃O₄ and Fe O could reach up to 80%and 40%during the reduction,respectively,and the characteristic of multi-step reaction could be revealed by analyzing the content of each intermediate phase.Finally,we investigated the hydrogen reduction of Fe₂O₃ particle clusters in fluidization state,taking into account the effects of reaction temperature,H₂ concentration,and superficial gas velocity on the reduction.It was found that settled Fe₂O₃ particles,which resulted from the increase of porosity,could reach a better fluidization state again when the superficial gas velocity was raised to 6.70 times of the minimum fluidization velocity of Fe₂O₃ particles;moreover,due to the difference of the drag on the upstream and downstream particles in the fluidized bed,Fe₂O₃ particles would be agglomerated during the reduction process.This thesis carried out the study of the meso-scale simulation method for fluidized heterogeneous reduction reactions and their related problems.Findings show that the established simulation method can not only reproduce the complex flow and reaction phenomena in fluidized heterogeneous reaction systems,but also characterize the microscopic interaction mechanism between the gas phase and solid phase,which is difficult to be portrayed by traditional numerical methods.So that the meso-scale method can achieve the crossing from the microscopic(mesoscopic)level to the macroscopic level.Furthermore,this method also has the features of numerical stability and easy handling of complex boundary conditions.Therefore,the meso-scale simulation method will provide a new idea and a new way to study the complex fluidized reaction system,and provide a new theoretical cognitive means for the development of new ironmaking technology.