Structural Evolution Behavior And Muti-phase Reaction Mechanism Of Coke In Blast Furnace

As the coming of low-carbon age,blast furnace?BF?ironmaking process which takes carbon?coke?as its carrier of energy flow and materials flow is facing unprecedented challenge.On the one hand,the coke quality requirement keeps increasing with the promotion of pulverized coal injection technology and enlargement of BF size.On the other hand,producing high quality coke is facing heavy pressure due to the depletion of high quality coking coal resources and the strict limitation control on the pollution and emission of coking process.In addition,because of the widely utilization of both stamping charged coke and top charged coke,the evaluation of coke quality becoming more complication since coke quality will be degraded by factors,especially harmful elements in BF.Therefore,an in-depth understanding about the coke behavior inside BF is required to evaluate the coke function and quality properly,thus improving theory combination between ironmaking industry and cokemaking industry.To uncover the structural evolution and multi-phase reaction mechanism of coke inside BF,simulation experiment,theoretical computation and industrial analyses were adopted to study the coke behavior inside BF aiming at improve the understanding and evaluation of coke properties and quality.Thermodynamic calculation and thermogravity experiment were conducted to investigate the gasification kinetics of coke and graphite in the C-CO₂-Na₂CO₃/K₂CO₃ system.Alkali carbonates were found to poses catalytic effect on the gasification of coke and graphite,their catalytic effect on graphite is stronger than that on coke.A limitation of catalytic effect and kinetic compensation of activation energy and preexponential factor were convinced for both gasification reactions.Due to the influence of disordered carbon structure in coke and the intrinsic catalytic effect of coke minerals,the catalytic limitation on graphite gasification is higher than that on coke gasification.Then the influence of alkali vapors on coke carbon structure,mineral structure and high temperature properties were investigated systematically by alkalization experiment.With the formation of coke fragments and fines,coke structure was found to be seriously destructed by potassium vapor,indicating the strong destruction ability of potassium vapor.This effect was called "peeling effect".Sodium vapor does not possess this kind of effect.X-ray diffraction and element mapping confirmed the new formed phases after potassium alkalization were kalsilite or potassium aluminum silicate?KAlSiO₄?,while that after sodium alkalization was sodium aluminum silicate(Na₆Al₄Si₄O₁₇).The interaction of alkali vapors with carbon matrix leads to the formation of graphite intercalation compound which results in volume expansion of the matrix.The catalytic effect of both alkalis was similar.However,because of the direction destruction of potassium vapor,potassium vapor was found to be more harmful than sodium vapor on coke quality in blast furnace in general.The structure and property evolution of molten coke ash system?SiO₂-Al₂O₃-CaO?with the change of SiO₂ content were investigated in atomic scale by Molecular Dynamics?MD?.Results showed that the stability of[AlO₄]tetrahedral structure is weaker than that of[SiO₄]due to the existence of Al atoms which is coordinated by more than 4 oxygen atoms.The change of SiO₂ contend has little influence on bond length and angle,indicating the stability of local network structure.With the decrease of SiO₂,the amount of bridge oxygen decrease,with that of oxygen tri-clusters increase.The self-diffusion coefficients of Si,Al,Ca and O all increased obviously with the decrease of SiO₂ due to the decrease of Si-O bond in the network.The viscosity calculated with MD is very close to that calculated with MD,and both methods showed the decrease of viscosity with the decrease of SiO₂.Using MD simulation,the influence of K₂O and Na₂O on the structure and fluidity of molten coke ash was investigated.The simulation results can repeat the experiment results and distinguish the key differences of those two kind of alkalis.No obvious changes of bond length and coordination number were observed,indicating the influence of both alkalis on the local network structure is very slight.As expected,the diffusion ability of Na⁺ is higher than that of K⁺,and the diffusion ability of all the atoms in the Na₂O-bearing system is higher than that in the K₂O bearing system.The total diffusion coefficient of system increase with the increase of Na₂O,while it decreases with the increase of K₂O,leading to the opposite change trend of viscosity.Therefore,Na₂O increases viscosity,while K₂O decreases viscosity.This is in accordance with experiment results.By the analysis of tuyere coke and hearth coke,the interface of coke-slag-metal was uncovered,while the interaction between hearth coke and slag was explained.It was found that coke ash/mineral will melt and migrate to the coke surface and wrap the coke.The surface molten ash will exist as a physical barrier to impede the coke reaction with other phases?gas,solid and liquid?.The reduction of FeO of bosh slag occurs in the coke-slag interface,resulting in the formation of a liquid iron layer around coke surface.The reaction between coke and slag will destruct coke structure and texture by erosion.All the heart coke samples,no matter their size,were found to be greatly graphitized.The smaller the hearth coke size,the higher the graphitization degree,indicating that the graphitization process starts from the coke surface and leads to the formation of coke fine.The new formed coke fine will form a complex mixture with slag and metal,and this will influence the gas and liquid permeability in the high temperature zone of BF.In addition,the hearth coke pores were found to be filled with slag,while the original coke ash has been totally melt into the slag.Above all,regarding the coke evolution in blast furnace,based on previous research results,systematic and comprehensive research was conducted to investigate several unclear problems in this thesis.The influence mechanism of alkalis in various state on the coke was studied comprehensively.The structural evolution mechanism of molten coke ash was uncovered in atomic scale.The characteristics of tuyere coke and hearth coke were revealed by analyzing coke extracted from industrial blast furnace.This thesis provide theoretical foundation for proper evaluation and high efficient utilization of coke.