Numerical Simulation Of Gas-Solid Flow,Heat Transfer And Gasification Characteristics In A Bubbling-Spouting Fluidized Bed Biomass Gasifier

The bubbling-spouting fluidized bed has great potential for being widely applied in the field of solid fuels gasification for its high gas-solid phase contact efficiency,fast heat and mass transfer rate,and strong adaptability for large particle fluidization.However,the high degree of coupling among the gas-solid flow,heat transfer and gasification reaction in it has brought great challenges to experimental mearsurement.Hence,the numerical simulation method was adopted by reasearchers to profoundly reveal the interaction mechanisms among gas-solid flow,heat transfer and reaction.In this paper,the Euler-Lagrange and Euler-Eulerian numerical simulation methods have been applied to systematically explore the effects of reactor design parameter?open ratio of fluidization gas inlet?as well as operating parameters?spouting gas velocity,auxiliary gas velocity,particle density,particle diameter,small and light particle mass fraction?on gas-solid flow,heat transfer,and gasification reaction characteristics in bubbling-spouting fluidized bed biomass gasification process,mainly including following three aspects:1. The effects of operating parameters on the time-averaged gas-solid flow,period andmixing characteristics within a pseudo-2D bubbling-spouting fluidized bed were systematically investigated based on CFD-DEM?Computational Fluid Dynamics-Discrete Element Method?.Some conclusions are drawn as follows:Firstly,with the spouting gas velocity increases,the particle circulation rate can significantly increases,and with the fluidizing gas velocity increases the overall fluidity of the particle bed in the annulus region is enhanced.What's more,with particle density or particle size increases,the overall fluidity of the particle bed in the annulus region is restrained.Moreover,it is found that with the mass fraction of small or light particle appropriately increases,the overall particle fluidity of the particle bed is improved.Secondly,power spectrum analysis of particle energy in different regions shows that when the total particle mass is kept unchanged,the main frequency increases indicating that the operating flow regime changes from spouting fluidization to spouting when the spouting gas velocity,particle density,particle size increases,while the main frequency decreases indicating that the operation flow pattern of the reactor changes from spouting to spouting fluidization when the auxiliary gas velocity,the small or light particle mass fraction increases.Thirdly,with the spouting gas velocity,particle density and particle size increases,the overall bed mixing rate rises,while the overall mixing rate of the bed tends to decrease when the fluidizing gas velocity increases.2. The effects of operating parameters on heat transfer behavior of the local and overall regionof a pseudo-2D bubbling-spouting fluidized bed were systematically investigated based on CFD-DEM.Some conclusions are drawn as follows:Firstly,in different regions,the contribution ratio of heat conduction between particles,accounts for the total heat conduction contribution between particles is approximately annulus region?50%?>spouting region?30%?>fountain region?20%?,wihle the contribution ratio of heat conduction among particles-fluid-particles accounted for the total heat conduction among particles-fluid-particles is approximately annulus region?60%?>spouting region?20%?>fountain region?20%?.Moreover,the contribution ratio of particle-fluid convection heat transfer accounted for the total particle-fluid convective heat transfer contribution is approximately spouting region?70%?>annulus region?20%?>fountain region?10%?.Secondly,convection heat transfer is dominant,the particle-fluid-particle heat transfer rate is about 2%of the convection heat transfer rate while the particle-particle contact heat transfer rate is about 0.2%of the convection heat transfer rate which can all be ignored.3. The influence of open ratio?A_m/A?of fluidization gas inlet and auxiliary gas velocity onthe gas-solid flow,heat transfer and reaction characteristics in a bubbling-spouting fluidized bed biomass gasifier?BSFBG?were systematically investigated based on KTGM?Kinetic Theory of Granular Mixture?.Some conclusions are drawn as follows:Firstly,when the total mass flow rate of the inlet gas is kept constant,as A_m/A increases,the internal circulation structure of the reactor becomes complicated,and the distribution of solid phases'concentration and different phases'temperature in the reactor becomes more event.At the same time,the volume fraction and LHV?Lower Heat Value?of the key product gas species at the outlet of the reactor gradually increase,and the gasification efficiency of the biomass gasifier increases.Secondly,compared with the uniform gas distribution condition?A_m/A=100%?,the gas distribution patterns with a higher auxiliary gas velocity at a smaller gas inlet open ratio?A_m/A?50%?or a lower auxiliary gas velocity at a larger gas inlet open ratio?A_m/A?75%?is better help to strengthening the heat transfer among the combustion zone,pyrolysis zone and gasification zone,which promots the uniform distribution of the solid phases'concentration and different phases'temperature in the upper region of the dense-phase bed.Comprehensive analysis shows that under the gas distribution of A_m/A=75%and U_f=1.0U_mf,the volume fraction and LHV of the key product gas species at the outlet of the reactor are the highest,and the performance of the gasification reactor reaches an optimal state.