Numerical Simulation Of Fixed Bed Coal Gasification Based On CFD-DEM Coupling Method

A fixed bed reactor(FBR)is a common gas-solid heterogeneous reactor,which is usually filled with a particulate phase and the voids of the particles are filled by the gas phase.Proper handling of gas-solid interaction in fixed bed reactor(FBR)is essential for the optimization and design of fixed bed reactor(FBR).The drag force is the most important force in the gas-solid interaction force,and the correct study of drag force can clarify the principle of gas-solid interaction and also predict heat transfer and chemical reaction in the fixed-bed reactor(FBR)more accurately.In this paper,a coupled CFD-DEM(Computational Fluid Dynamics-Discrete Element Method)method is used to numerically simulate a fixed bed reactor(FBR),which treats the particles as discrete phases and can take into account the particle shape and accurately calculate the local porosity,with the aim of exploring new methods to deal with gas-solid two-phase problems.Firstly,a typical spherical particle fixed bed reactor(FBR)was investigated for porosity and pressure drop characteristics.The porosity study revealed that the coupling method can clearly show the trend of average axial porosity and average radial porosity.The pressure drop characteristics were found that the Di Felice drag model can predict the drag force between the spherical particles and the gas phase more accurately than the Gidaspow drag model,and the maximum relative error of the Di Felice drag model simulated predicted pressure drop does not exceed 13%.Cylindrical particles are one of the most representative non-spherical particles,and the study of the drag force between cylindrical particles and the gas phase will lay foundation for the extension of the drag force to non-spherical particles.For cylindrical particles,the trend of average radial porosity and average axial porosity is the same as spherical particles.Based on the Di Felice drag model,a New Di Felice drag model for cylindrical particles was proposed to modify the drag coefficient C_d by introducing two physical parameters of Sault average diameter and shape sphericity.The maximum relative error between the experimental pressure drop and the predicted pressure drop of New Di Felice drag model is less than 10%,so the proposed drag model can be considered as a reliable prediction of the pressure drop of a fixed bed reactor(FBR)with cylindrical particles.In addition,Ganser drag model of non-spherical particles was used to calculate the pressure drop,the drag coefficient C_d’of non-spherical particles was modified,and a New Ganser drag model suitable for the calculation the drag force of cylindrical particles was proposed.The proposed New Di Felice drag model is embedded into Fluent through UDF(Users Defined Functions),and the fixed bed coal gasification process is simulated by combining the gasification reaction kinetics model and the E-E(Euler-Euler)model of particle dynamic theory,with the aim of finding a numerical model applicable to calculate the coal gasification of fixed bed.Finally,the height of the fixed bed gasifier,the inner diameter of the internal components,and the thermal conductivity of the internal components are changed to demonstrate the effects of these geometric mechanisms changes on the distribution of the gas phase temperature field,the distribution of the gas phase pressure field,and the distribution of the exit syngas components.