Coke Non-equimolar Diffusion And Pore Wall Solution-Loss Degradation(N-PSD) Model

As one of the important raw materials for blast furnace ironmaking,coke as skeleton is irreplaceable by other fuels.At present,CRI and CSR are used to reflect the effect of solution-loss reaction on coke degradation and skeleton in blast furnace.However,coke with great different CRI and CSR is normally used in blast furnace in China and at abroad.The newly proposed comprehensive thermal property indexes CRR₂₅and CSR₂₅provide a reasonable explanation for the above problem,but there are still two problems to be solved urgently:1)the variation law of CSR₂₅with temperature can not be explained by the existing kinetic model;2)the solution-loss and degradation behavior of coke can not be clearly displayed.In view of this,this paper takes the mechanism of coke solution-loss as the research object,grasps the characteristics of gas non-equimolar diffusion and changing pore structure at any time in the process of coke solution-loss reaction,and integrates Maxwell Stefan equation,porous reaction model and grain model to establish the coke non-equimolar diffusion and pore wall solution-loss degradation model（N-PSD model）.The mechanism of coke solution-loss reaction is studied by N-PSD model,and the CSR₂₅prediction model is established to predict the variation of CSR₂₅with temperature.The main conclusions are as follows:1.Through the analysis of N-PSD model,considering the non-equimolar diffusion in boundary layer,the external diffusion coefficient and mass transfer coefficient of each temperature change from constant to variate and decrease with the reaction process,which leads to the decrease of CO₂concentration on the outer surface of coke,but increases with the increase of coke reactivity.The internal diffusion effective coefficient changes from2D(D_eff,CO2=f（T,x）without non-equimolar diffusion in pores)to 3D(D_effn,CO2=f（T,x,r）with non-equimolar diffusion in pores),and decreases with the increase of coke inner radius.The effective region of CO₂concentration in coke decreases and is close to the coke outer surface.When the non-equimolar diffusion is considered,the apparent reaction rate constant increases and the apparent activation energy decreases.2.The reaction controlling step changes from chemical reaction control to internal diffusion control with the increase of temperature.Without non-equimolar diffusion,the reaction controlling step at each temperature changes with the reaction process,resulting in multiple reaction controlling step at individual temperature.Considering the non-equimolar diffusion,there is no obvious change trend of the reaction controlling step at each temperature with the reaction process,and there is only one reaction controlling step at each temperature.3.Based on the N-PSD model,the 3D diagrams of CO₂concentration distribution in coke(C_CO2n=f（T,x,r）)and porosity distribution（ε=f（T,x,r））are presented by N-PSD model,and the coke degradation behavior is revealed.The coke solution-loss degradation modes of 2 categories and 4 sub-types are summarized,including overall solution-loss degradation model（similar homogeneous solution-loss degradation model and gradient solution-loss degradation model）and regional solution-loss degradation model（regional solution-loss degradation model and surface solution-loss degradation model）.The mechanism of lowest CSR₂₅in gradient solution-loss degradation model is revealed.4.The CSR₂₅prediction model is established by substituting the limit anti degradation porosity of coke into N-PSD model,and the variation law of coke CSR₂₅with temperature is predicted.The results show that the deviation between predicted value and experimental value is approximately 5%,which proves the accuracy of CSR₂₅prediction model.These results provide theoretical support for understanding the mechanism of coke solution-loss degradation in blast furnace and the construction of a new evaluation method.