Inversion Study Of Intrinsic Kinetic Parameters Of Coal Char Oxy-fuel Oxidation Based On TGA

Oxy-fuel combustion technology is regarded as a promising new combustion technology,which can realize large-scale carbon capture in coal-fired power plants.It can be used not only for new generating units,but also for retrofitting existing units.Accurate assessment of the oxidation kinetics of coal char under oxy-fuel combustion conditions is essential for the design of oxy-fuel combustion systems.In this paper,experimental and modeling studies are carried out for the process of analyzing the intrinsic kinetic properties of coal char oxidation based on thermogravimetric analyzer（TGA）.Firstly,the integrated diffusion model of coal char particle layer is established in this paper to analyze in detail the process of reaction gas entering the particle from the outside of the crucible and participating in the reaction.The diffusion process can be divided into three parts:intra-particle diffusion,inter-particle layer diffusion and external diffusion.The physical parameters of coal char involved in the diffusion model can be measured by BET and laser particle size analyzer,etc.The diffusion coefficients are calculated by Chapman-Enskog equation.The results of the oxygen volume fraction calculated by the diffusion model were verified against the results of the Thiele modulus calculation,and it was concluded that the two methods are equivalent.Next,the experimental conditions such as crucible filling method,sample mass,heating rate and oxygen concentration were improved to minimize various diffusion resistance in the crucible,and optimal experimental setting for studying the intrinsic kinetics of coal char oxidation is obtained.Both nonlinear model fitting and linear model-free analysis methods were used to analyze coal char oxidation kinetic parameters,and the accuracy of volumetric reaction model（VM）,grain model（GM）and random pore model（RPM）were compared.The results show that the RPM model has better correlation with the experimental results,and the activation energies of SH coal char in O₂/N₂ and O₂/CO₂atmospheres are 161.8 k J/mol and 165.1 k J/mol,respectively,and the reaction order of coal char oxidation in both atmospheres is close to 1.0.Finally,a one-dimensional oxidation model of coal char in a TGA crucible was developed on the basis of the diffusion model of coal char particle layer,considering that the physical parameters of coal char keep changing with the increase of the burning rate,and the intrinsic kinetic parameters obtained from the experiments were combined to verify the correctness of the one-dimensional oxidation model of coal char at different crucible positions,sample amounts,oxygen concentrations and background atmospheres.Then the model was used to analyze the effects of crucible filling position,particle layer thickness,oxygen concentration,heating rate,oxy-fuel atmosphere and pressure on the coal char burnout process,and it can be found that the higher the stagnation zone,the greater the external diffusion resistance,and the slower the burnout process;the thicker the particle layer,the greater the particle layer diffusion resistance,and the slower the coal char burnout process;appropriately increased oxygen concentration,reduced heating rate and increased reaction gas pressure can all increase the average oxygen concentration inside the crucible and promote the burnout process of coal char;After the effect of diffusion was completely excluded,the difference of coal char oxidation reactivity between O₂/N₂ and O₂/CO₂atmosphere was very small.