Analysis On Characteristics Of Nanocatalysts For Efficient Catalytic CH₄-CO₂ Reforming Based On TEMOM Method

The extensive use of fossil fuels has emitted a large amount of greenhouse gases such as CO₂ and CH₄.The CH₄-CO₂ reforming reaction to produce syngas is a promising carbon capture technology for dealing with greenhouse gases.Nano catalysts have the characteristics of high specific surface area,and fluidized bed technology can effectively improve the dispersion of particle distribution.Combining the two can provide a feasible way to solve the problem of carbon deposition on catalysts.In this paper,the TEMOM（Taylor Expansion Method of Moments）model is established and coupled with the two-fluid model.The flow characteristics of nano catalyst particles in fluidized bed are studied by using particle kinetics theory and reaction kinetics theory,which makes up for the deficiency of TEMOM model in the field of numerical simulation of fluidized bed reaction.The model validation results show that the bed pressure drop and bed expansion ratio are close to the experimental results,which verifies the feasibility and rationality of the model.At the superficial gas velocity of 5.2cm/s,the bed layer generally forms discrete fluidization of distribution,but there is a certain stratification phenomenon,and the particle agglomerate diameter is maintained at 74 to 90 microns.On the whole bed,the axial velocity of particle agglomerations in the middle of the bed and the granular temperature at the wall in the middle of bed are high,while the values of moments of each order are small,and the particle number density is small.It indicates that the effect of Brownian coagulation and diffusion of particles in the middle of the bed are significant,and the degree of molecular pulsation is strong.The numerical simulation results based on different moment methods are compared,and the high accuracy of TEMOM model is verified.Based on the density functional theory,the reaction mechanism of CH₄-CO₂reforming reaction on Co-based nano catalysts was discussed.The vibration frequency calculation and the intrinsic reaction coordinate analysis of each reaction show that the dissociation adsorption of CH₄ and CO₂ gas and the detachment of CO^*and H^*need to overcome the reaction energy barrier above 0.5 e V in order to reach the transition state.The analysis shows that the two paths of CH^*→CHO^*→CO^*and CH^*→C^*→CO^*coexist in the reaction process.From the perspective of the reaction energy barrier,the reaction rate of the former is faster than that of the latter,but the reaction energy barrier of the CH^*→C^*process is 0.03e V smaller than that of the CH^*→CHO^*process.It indicates that there may inevitably be a small amount of carbon deposition on the Co-based catalyst.Based on the TEMOM model and the reaction kinetics model,the characteristics of CH₄-CO₂ reforming reaction in fluidized bed were studied,which laid a certain foundation for the organic combination of computational chemistry and gas-solid two-phase flow.The model validation results show that the conversion ratio of CH₄ and CO₂ are close to the experimental results.It verifies the rationality of the two models.Compared with the cold state simulation,the expansion height of the bed after adding the reaction is lower,and it is easier to generate larger bubbling.The axial velocity difference between the near wall and the wall is large,and the values of moments of each order are increased respectively,and the agglomeration effect of fluidized bed particles is decreased.The temperature of the gas phase and the particle phase are generally lower at the bottom of the bed,higher at the top,lower in the central area,and higher on the wall.The intermediate reactants of the gas phase and the particle phase are mainly distributed in this position.It is found that the probability of carbon deposition in the middle and top of the bed with high dispersion is small.It is further verified from the chemical reaction rate of each reaction in the fluidized bed that a small amount of C^*is inevitably generated and accumulated on the Co-based catalyst.The effects of different CH₄/CO₂ ratios on CH₄ conversion,CO₂ conversion and H₂/CO ratio were analyzed.The results show that increasing the CH₄/CO₂ ratio can increase the CO₂ gas conversion ratio and reduce the CH₄ conversion ratio.When the CH₄/CO₂ ratio is 1.4:1,the H₂/CO ratio is up to 0.95 and close to 1.