Study On Mixed Flow Characteristics Of Biomass And Bed Material Particles In Fluidized Bed And Numerical Simulation Of Pyrolysis Process Based On DEM-CFD Method

The efficient and clean utilization of biomass energy has been paid more and more attention because of environmental pollution and energy shortage.Biomass pyrolysis technologycan convert biomass into liquid fuel which could be a potential alternative of fossil fuels.In this thesis,a study focues onthe mixing and flow characteristics of biomass and bed particles under fluidization and the establishment of a biomass pyrolysis model in a fluidized bed under the Euler-Lagrangian framework.The mixed fluidization process of biomass and quartz sand is the based part of the biomass pyrolysis process in fluidized bed.Four different biomass,biomass pellets,corn stalks,rice husks and millet particles are studied and the mass ratio of biomass is considered.The results show that biomass pellets and millet particles have better fluidization characteristics after being mixed with quartz sand,and it is extremely difficult for corn straw to achieve the ideal fluidization state even mixed with quartz sand.The fluidization state of rice husk particles varies significantly with the fluidization velocity and the mass ratio of biomass.The pressure fluctuation signals of the fluidization process collected in the experiment are analyzed non-linearly by the Hilbert-Huang transform analysis method.The correlation between the energy ratio of the IMF component and the flow pattern change was summ-arized.With the energy ratio of the IMF component and fluidization number as the sample set,the flow pattern in the fluidization process can be clustered by applying the improved C-means fuzzy clustering method.Four parterns are cluscteredas not-fully bubbling bed,fully bubbling bed,slugging bed and turbulent bed.Then the clustering result is used as training data and detection data,and an intelligent flow pattern recognition system for the mixing process of biomass particles and quartz sand is further established by applying a neural network algorithm.The accuracy of the recognitionsystem can reach 100% for the identification of slugging and turbulent flow patterns.Based on the experiment study,a numerical model of the mixed flow process of biomass particles and quartz sand in a fluidized bed on the Euler-Lagrangian framework is established.The effects of fluidization velocity and the size of biomass particles during the particle mixing process are discussed.On the mesoscopic scale,the local mixing quality of biomass particles and quartz sand particles which have large differences in physical properties is evaluated.Based on the distribution and movement information of biomass and quartz particles,the mixing mechanism of biomass particles and quartz sand is summarized.The soft sphere model is used to describe the collision between particles and between the particles and the wall.The Hertz-Mindlin nonlinear contact algorithm is used to calculate the collision force.Gas-solid drag coupling uses four-way coupling.In the model,the differences in physical properties between biomass particles and quartz sand particles are considered,asthe gas phase fluidization velocities of 1.0m/s,1.5 m/s,and 2.5m/sand the particle size of biomass particles of 1.5 mm,2.0 mm,and 3.0 mm.Based on the distribution of material particles and quartz particles and the change of the average kinetic energy of particles during the movement process,the mixing mechanism of biomass particles and quartz sand was summarized.The simulation results show that increasing the superficial velocity in a certain range can promote the mixing process of the particles.A larger superficial velocity enables the particles to reach a better mixing state faster;biomass particles with smaller diameters correspond to better.Due to the limitation of the mesh size,the mixing quality is not modeled for the larger size biomass particles,and the impact of the three studied biomass particle diameters on the mixing process is not obvious.Coupled with the biomass pyrolysis kinetics model,a numerical model of biomass pyrolysis in a fluidized bed under the Euler-Lagrangian framework was established based on the mixed fluidization model of biomass particles and quartz sand.In the biomass pyrolysis model,the physical properties of the biomass particles and the changes in the gas phase parameters with the pyrolysis reaction process are considered.In the pyrolysis model of biomass particles,the changes in physical properties and gas phase parameters of the biomass particles with the pyrolysis reaction process are considered;in the heat transfer model,gas-solid heat transfer and the radiative transfer of quartz sand particles to the biomass particles are considered.Heat;The shrinkage model is used to calculate the volume change of biomass particles with the pyrolysis reaction.Through simulation,the instantaneous change of biomass particles during pyrolysis was described,and the effects of gas temperature and gas apparent velocity on the yield of pyrolysis products were studied.Studies have shown that the yield of pyrolysis oil increases with increasing temperature.The yield of pyrolysis oil reaches the highest at550 ° C.Due to the progress of the secondary reaction,part of the pyrolysis oil decomposes pyrolysis gas,so when the temperature further increases At 600 ° C,the yield of pyrolysis oil decreased slightly.The effect of apparent gas velocity on the yield of pyrolysis products is reflected in the transport of gas phase components.At higher apparent gas velocity,the yield of pyrolysis oil is higher.