Numerical Simulations For CO₂ Adsorption In Fluidized Bed Reactors Based On Shrinking-core Model

Due to the high efficiency of mass-and heat-transfer with highly complex hydrodynamics,circulating fluidized bed reactors（CFBRs）has become a kind of important device for industry-scale CO₂ removal.It is critical to understand and master the adsorption mechanism and performance of the CFBRs.With the development of computer technology,Computational Fluid Dynamics（CFD）has been widely used to study the industrial application of CFBRs.However,because of the fact that the most of theories of current CFD ways are obtained under extreme and ideal experimental conditions,it is difficult for the existing CFD simulation theories to accurately predict the experimental results under the complex working conditions in some applications of CFBRs.Therefore,wholly taking it in account that differences between the CFD simulations and the industrial applications of CFBRs is of significance to improve the accuracy of CFD simulations and further optimize the operation conditions.In the view of the reaction characteristics of CO₂ adsorption process in CFBRs with K₂CO₃-based solid adsorbents and the effects of dimensionality on the numerical CO₂ adsorption runs,this work proposed two-region shrinking-core model、conducted the comparative analysis for CO₂ adsorption in 2D and 3D（2D/3D）simulations and optimization of operating conditions for novel fluidized-bed reactors,mainly including the following three parts:1.Two-region shrinking-core model is proposed based on the classical shrinking-coremodel.By the comparative analysis for the adsorption curves under various flowpatterns in the previous studies,the possible values of reaction radius and pre-exponential factors of reaction rate for the high reaction shell and low reactioncore of the adsorbents were proposed.Two-region shrinking-core model is appliedto the CFD simulation by UDF,and model parameters were verified by CFDsimulations of CO₂ adsorption in fluidized-bed reactors.This not only make theCFD simulation process closer to the gas-slid flow and the CO₂ capture process influidized-bed reactors,but also provides guidance for the following researches fornumerical gas adsorption runs.2.The mass-and heat-transfer and the kinetic characteristics of the CO₂ adsorptionprocess were investigated by CFD simulations with two-region shrinking-coremodel.Besides,the discrepancies between 2D and 3D simulations wereinvestigated by ANOVA analysis and Response Surface Methodology.The resultsshowed that he differences between the 2D and 3D simulations variedsubstantially with the changes of two independent operating conditions,inletvelocity of gas and bed height.It was found that there were obvious errors in theprediction of bed expansion ratio and gas residence time in 2D simulations.Especially while the gas velocity is further increased,the errors in 2D simulationswould be more significant.However,when the bed height is further increased,theeffects of particles mass and the contacts to wall on gas-solid contact and masstransfer resistance cannot be quantified in 2D simulations directly.Therefore,when the quantitative effects of gas velocity and bed height on reactorperformance were studied by CFD simulations,3D simulations should be chosenpreferentially.2D simulations should be chosen in studying the effects oftemperature on the reactor performance and verifying the reaction model.3.In order to improve the CO₂ capture efficiency and the utilization of solidadsorbents,a novel type of CFBR coupled with bubbling fluidized bed andtransport bed was proposed in the previous studies.CFD simulations of adsorbentscirculating in this CFBR and the CO₂ adsorption in the riser were conductedrespectively.By adjusting the central gas velocity,the circulating flow rate ofadsorbents was verified in the full cycle simulation,and the effects of central gasvelocity and adsorbents circulating flow rate on CO₂ capture efficiency andenergy efficiency of this CFBR were studied in the riser simulation.The resultsshowed that the different specific heat capacities of K₂CO₃、Al₂O₃ and KHCO₃and the various amount of KHCO₃ required in the adsorbents regenerated processunder different adsorption efficiency,lead to the inconsistency between the CO₂capture efficiency and optimal energy efficiency in the optimization for operatingconditions of this CFBR.This section provides some theoretical supports andguidance for the further application of this kind of novel CFBRs.