CFD Study Of The Characteristics Of Gas-solid Flow And CO₂ Capture In The Circulating Fluidized Bed Decarburization System

Due to the high efficiency contact and mass-and heat transfer with highly complex hydrodynamics,dual circulating fluidized beds?DCFBs?are widely applied in different fields,such as chemical,petroleum,metallurgy,environmental protection and pharmaceutical industries.Due to the complex structure of the DCFB,the characteristics of the gas-solid flow and reactions are still not clear and need to be further studied.Moreover,computational fluid dynamics?CFD?is believed to be a promising tool for studying the flow regime and industrial processes because of its convenience and efficiency in flow field analysis,which provides the effective information for stable operations.Therefore,it is the key problem that how to combine the CFD simulation with the experiment effectively.To solve the problem mentioned-above,this work focuses on the selection of the optimal CFD model,interaction among reactors and two-dimensional?2D?and three-dimensional?3D?investigation of CO₂ capture characteristics,mainly including the following three parts: 1.An optimal full-loop CFD model has been proposed by comparison of the effects of different turbulent models and drag models on the flow characteristics.Firstly,compared with experimental data,gas and solids volume fraction,pressure gradient and solid velocity are analysed under different turbulent models based on the Euler-Euler model,and thus the optimal turbulent model is obtained.Then,on the basis of the optimal turbulent model,the above parameters are compared with the experimental data under different drag models and thus the optimal drag model is achieved.Finally,the RNG k-? and Gidaspow model are validated to be the optimal CFD models for simulation of the 2D DCFB.2.Based on the optimal CFD models obtained-above,systematic numerical studies of flow characteristics and interaction among reactors in the riser and BFB have been conducted under different operating conditions.The solid volume fraction,pressure and solid velocity are obtained by changing gas velocities of the riser and the BFB.The results indicate that the riser gas velocity has a significant effect on the flow behaviors in the riser,where higher solid velocity and lower solid concentration are obtained with the increased riser gas velocity,causing the larger solid circulating rate and thus more particles enter into the BFB.The pressure drop of the BFB increases due to its higher bed height,which affects the gas-solid two-phase flow.With the increase of the BFB gas velocity,the higher pressure of the top riser and solid concentration are obtained,which means that more resistance needs to be overcome by the particles in the riser in order to enter into the BFB.Therefore,the efficient contact and mixing of the gas and solid in the DCFB can be obtained by adjusting the operating conditions.3.A shrinking core model simultaneously considering physical and chemical adsorption has been proposed for CO₂ capture in the fluidized bed.The flow and reaction characteristics of the CO₂ capture are analysed in 2D and 3D simulations,which is for the first time to reveal the sensitivity of the 2D and 3D simulations to different initial inlet conditions.The results show that the flow and reaction characteristics in the 2D and 3D simulation have the same variation trends but significant numerical differences,indicating that the solid volume fraction and pressure in the 3D systems are larger than 2D simulation results.With the different inlet gas velocity,the similar variation trends and numerical values of the solid volume fraction and CO₂ molar concentration are obtained in the 2D and 3D simulations.Moreover,the same results of the CO₂ molar concentration can be achieved under different initial CO₂ mole fraction while other parameters have significant differences.When studying the influence of inlet gas temperature,it is found that within a certain range,the increase of temperature is contribute to accelerating the adsorption reaction rate.However,when the temperature exceeded a certain value,the reaction was limited.Different from the above two factors,there are significant differences between the 2D and 3D simulation results.Therefore,when studying the influence of phase temperature,3D simulation should be selected for accuracy.