Numerical Simulation Of Gas-solid Flow Characteristics Of CO Methanation In A Fluidized Bed Reaction Syste

As a kind of clean and efficient energy,the demand of natural gas is increasing year by year.The coal and biomass are transformed into natural gas by using technology of synthetic natural gas,improving resource utilization and reducing pollution emission,which is in accordance with the implementation of"double carbon"strategy.As one of the key technologies of synthetic natural gas,methanation technology has become the current research focus.The reactors of methanation are mainly divided into fixed bed,slurry bed and fluidized bed.The fixed bed reactor has problems like poor heat dissipation and uneven temperature distribution,and the slurry bed has reactor problems such as difficult solid-liquid separation and severe catalyst wear.On the contrary,the good gas-solid heat transfer and uniform temperature distribution characteristics of fluidized bed reactor have been widely focused on in the methanation process.However,it is difficult to fully understand the complex and dense gas-solid flow characteristics and multi-scale coupling in fluidized bed via experimental measurement.Numerical simulation method is widely utilized since it can accurately reflect the multi-physics field characteristics in the system.In current work,the numerical model of CO methanation reaction process is established by using Multiphase Particle-in-Cell Method（MP-PIC）with the laboratory-scale three-dimensional bubbling fluidized bed based on Eulerian-Lagrangian framework,and the accuracy of the model is verified with the consistency between numerical results and experimental data.Based on the numerical model,the performance of CO methanation and characteristics of gas-solid flow are studied under different operating conditions.The main research contents and results are as follows:Firstly,the influence of operating temperature,inlet gas velocity and inlet gas composition on the gas distribution,gas phase thermal properties（density,temperature,viscosity,specific heat capacity and thermal conductivity）,gas products,CO methanation performance and particle thermal properties are discussed in the fluidized bed.The results show that:the CO methanation reaction mainly occurs in the dense region,and enhancing the operating temperature and inlet gas velocity inhibits the CO methanation reaction that decreases the yield of CH4.The operating temperature and inlet gas velocity slightly affect the purity and selectivity of CH₄,while the effect of CO/H₂ ratio is relatively obviously.The distribution trend of gas temperature and density distribution is opposite.Increasing the operating temperature and inlet gas velocity enlarges the gas phase temperature and decreases gas phase density,which impact the spatial distribution of the specific heat capacity,viscosity and thermal conductivity of gas phase.The catalyst particle temperature is the lowest and heat transfer coefficient is the largest at the inlet of the reactor,and the heat transfer coefficient in dense region and freeboard region are respectively 400 W/（m²?K）and 240W/（m²?K）.Secondly,the gas-solid flow characteristics in the circulating fluidized bed are numerically simulated based on the established numerical model.The results show that:increasing the operating pressure and catalyst mass is beneficial to the progress of the methanation reaction that increases the yield of CH₄.The particle velocity mainly ranges from-2 to 4 m/s,and vertical flux of gas-solid is larger than the horizontal flux and the vertical flux is largest in the middle region of the riser.The heat transfer coefficient decreases due to the large particle concentration and increases due to large slip velocity.In conclusion,the numerical simulation results of CO methanation in fluidized bed provide a theoretical reference for the design and operation of this type of reactor and the regulation and optimization of the reaction process,which is of great significance to energy security and the implementation of the“dual carbon”strategy.