Multi-Scale Investigation Of Granular Dynamics And Particle Mixing In Biomass Fluidized Bed

Conversion of biomass to high-value products by pyrolysis and gasification is a significant development direction for biomass utilization.Compared with a traditional fixed bed and moving bed,the gas-solid fluidized bed not only enhances the interaction between the gas and solid particles but also promotes the heat and mass transfer efficiencies.Because of these remarkable advantages,the gas-solid fluidized bed is widely used in many industries,including biomass utilization.Particle mixing is an important flow behavior that directly influences the rates of mass,heat and moment transfers among reactants and products.However,due to the difference of fluidization characteristics,good mixing of biomass particles and bed material is very difficult.The multi-scale investigation of particle mixing mechanism is still far from enough.The experimental and simulation method still needs to be improved.By the logical organization which from the micro to the macro,the present work is devoted to revealing complex particle mixing characteristics in gas-solid fluidized bed by experimental and simulated approaches.The force and motion of microscopic single particles are the basis for studying the motion of mesoscopic particle groups and the mass/heat transfer phenomena of macroscopic reactors.The motion of the particles includes not only translation but also rotation.Particle rotation occurs when the particles collide non-centrally or when they are in a heterogeneous flow field.Rotation will consume energy and can not be ignored in some cases.However,particle rotation and Magnus effect are often overlooked in numerical simulation calculations.On one hand,it is difficult to experimentally observe the rotation of the particles;on the other hand,it is difficult to obtain a theoretical relationship that is applicable to actual gas-solid two-phase systems.Therefore,systematic research is still lacking.This study focuses on the effect of particle rotation and the Magnus lift force on the particle motion.Since the microscopic force cannot be measured experimentally,we establishes the CFD-DEM model,studies the particle rotation and the Magnus effect via numerical simulations,and clarifies the application scope of the Magnus lift force.The results show that the influence of Magnus lift force is enhanced with a higher Rer(especially for Rer?102),and might be in the same magnitude as the drag force(Fmag,a/Fdra,a>0.1).That means Magnus lift force might have an apparent influence on the movement of particles in this case.The motions and formation mechanisms of the bubbles are improtant for the study of particle mixing.Although researchers have conducted numerous related studies,more experimental and theoretical support is needed for some specific research subjects,such as the velocity distribution curves of particles around a bubble.Based on PTV technology,an experimental platform suitable for measuring the velocity distribution of dense-phase particles is established.Based on the CFD-DEM model,a numerical model that describes the flow characteristics of a single bubble is established to calculate the velocity distribution of the particles around the bubble.Both experimental and simulation results show that the velocity distribution of particles surrounding a single bubble can be described by tri-peak model which is a linear superposition of three Maxwellian distributions.A tri-peak distribution model based on the fluid and particle control mechanisms is theoretically derived.The error analysis shows that compared with other models,the tri-peak model can profile particle velocity distribution more accurately(R>0.85).At the macroscopic reactor scale,due to the difference of fluidization characteristics,the mixing mechanism is very complex.On the other hand,the imperfection of experimental method restricts the development of simulation method and mixing theory.A novel particle tracing and mixing method based on the microwave heating-particle tracking technique was developed to investigate particle mixing inside the bed.The mixing characteristics of the particle mixtures with different components and different ratios are studied on the gas-solid fluidized bed experimental platform.The results show that for a wide size distribution particle binary system,the discussion of minimum fluidization velocity will become more complex compared with a narrow size distribution binary system.For low gas velocity condition,there is a dead region in the bottom layer.The impact of the dead region depends on the gas velocity and volume ratio for a binary system.The mixing index is changing with time and shows a typical non-linear feature because of the comprehensive impact of dead region and particle separation.