Multi-scale Relationship Between Coke Gasification Kinetics And Its Microstructure Evolution In The Blast Furnace

Coke is one of the important raw materials in blast furnace ironmaking,and its supporting skeleton function is irreplaceable in blast furnace.In current research,the most widely used indicators for evaluating coke quality are the thermal properties of coke,namely CRI and CSR,to reflect the impact of carbon loss reactions in blast furnaces on the deterioration of coke and its own strength.However,in practical applications,it is found that coke with significant differences in CRI can still be used normally in different blast furnaces,indicating that this evaluation index does not fully reflect the actual deterioration of coke in blast furnaces.During the carbon loss process of coke,the reaction is essentially a competition between the chemical reaction rate and the diffusion mass transfer rate,that is,the chemical properties and physical pore structure properties of coke.The relative size of the two will directly affect the internal structure and deterioration degree of coke.The carbon loss and deterioration of coke is currently the core bottleneck of efficient smelting and coke quality control in blast furnaces.Many scholars have studied its macro dynamic process,while others have conducted in-depth research on the microstructure of coke,such as optical microstructure and pore structure,but have not linked the two above,and have not analyzed the reasons for changes in macroscopic parameters from a microscopic perspective.Therefore,through the carbon loss reaction of coke with different particle sizes,this paper studied the multi-scale changes in the coke matrix and pore structure during the caobon loss process,and characterized the impact of the solution loss reaction on the spatial structure of coke from the macro and micro scales.Quantifying the actual gradient degradation properties of coke provides a basis for achieving multiscale evaluation of coke quality through microstructure and external environment.The results can provide support for coal blending and coking,and further improving the coke quality evaluation method.Firstly,gasification experiments of coke matrix were conducted.The reaction law of fine coke powder was analyzed by thermogravimetric analyzer.The reasons for the change of the reaction rate constant k_reaof coke at different reaction stages and the catalytic principle of alkali metals on the dissolution loss reaction of coke were studied by means of XRD,Raman spectroscopy,coke microstructure,and infrared spectroscopy.The results show that for the same type of coke,due to the catalytic action of surface minerals and the consumption of active components in the early stage of the reaction,the activation energy of the dissolution loss reaction of the coke decreases,and the chemical reaction rate constant k_reaincreases;However,in the late stage of the reaction,due to the large consumption of active components,ash also precipitates more on the surface of the coke,reducing the activity of the solution loss reaction,resulting in an increase in the activation energy of the coke solution loss reaction and a decrease in the chemical reaction rate constant k_rea.By comparing the microcrystalline structure and skeleton structure of original coke and alkali rich coke,it is found that the catalytic effect of alkali metals on the dissolution loss reaction of coke is mainly due to the direct action of alkali metals on the pore wall of coke,resulting in a significant decrease in the energy barrier of C-C bond fracture on the surface of coke,and a significant increase in the chemical reaction rate constant k_rea.Then,a high-temperature carbon loss experiment of the lump coke was conducted.The change of effective internal diffusion coefficient D_effduring the actual dissolution loss process of lump coke was analyzed using a self-made high-temperature dissolution loss device.The changes in pore structure parameters of coke before and after reaction were measured by"mercury intrusion method",and the effective internal diffusion coefficient D_efffor different reaction stages was obtained by combining Maxwell Stephan equation.The results show that the effective internal diffusion coefficient of CO₂in the pores of coke is closely related to the pore structure of coke.As the solution loss reaction proceeds,the pores in coke gradually expand or merge into mesopores and macropores,resulting in a decrease in tortuosity and a decrease in gas molecular diffusion paths,leading to a significant reduction in diffusion activation energy and a gradual increase in diffusion coefficient.It was also found that the effect of alkali metals on the diffusion coefficient was not significant.Finally,by analyzing the actual carbonloss process of lump coke and combining its physical and chemical properties,the local degradation gradient of coke,known as k_rea/D_eff,is obtained.The results show that the chemical reaction rate constant k_rearapidly increases under the influence of alkali metals,but the effective internal diffusion coefficient D_effdoes not change significantly,resulting in a significant increase in the value of k_rea/D_eff.Therefore,alkali metal catalysis not only accelerates the reaction rate of the dissolution loss reaction of coke,but also has a significant kinetic impact on the degradation of the coke structure.That is,alkali metal catalysis can lead to the formation of a larger reaction gradient within the porous structure of coke,and the higher the temperature,the more obvious the impact.The size of k_rea/D_effreflects the gradient degradation characteristics of coke in the blast furnace.When the particle size is constant,it determines the degradation behavior of coke,and the results can provide an important basis for achieving multi-scale control of coke quality.