Numerical Simulation Of Flow And Biomass Pyrolysis Characteristics In Gas-solid Fluidized Bed Based On Structure Modulation

Fluidized bed is one of the main reactors for biomass fast pyrolysis at present due to its advantages of good heat and mass transfer performance.Since harsh experimental conditions of fluidized bed biomass fast pyrolysis,where reactor physical-chemical processes are complex,increasingly researchers are using numerical simulation to study patterns of gas-solid flow,heat transfer and pyrolysis reactions in fluidized beds.In this paper,CFD-DEM（Computational Fluid Dynamics-Discrete Element Method）and Euler-Euler model were used to study the effects of reactor structure（biomass and nitrogen inlet modes）and operating parameters（nozzle gas velocity,auxiliary gas velocity）on gas-solid flow,heat transfer and biomass pyrolysis reaction characteristics in fluidized bed,in order to achieve the purpose of improving tar yield.1.A pseudo-2D spouting fluidized bed was established by CFD-DEM coupling method.The effects of gas nozzle numbers,nozzle gas velocity and auxiliary gas velocity on two-component particle flow characteristics were analyzed by two-component particle velocity,near-wall particle velocity,particle dynamic mixing and Lacey index.（1）The main influencing factor of particle mixing process was nozzle numbers.The single nozzle had the highest mixing rate in initial mixing period.With nozzle numbers increasing,particle movement range,motility and mixing degree increased.（2）The effect of nozzle gas velocity on the Lacey index under different nozzle numbers presented three trends.In single nozzle,with nozzle gas velocity increasing,Lacey index growth rate,mixing time and Lacey index at equilibrium increased.In three nozzles,Lacey index growth rate varied less with nozzle gas velocity,and with nozzle gas velocity increasing,Lacey index and mixing time at equilibrium increased.In double nozzle,calculation results were in between.As nozzle numbers increasing,Lacey index varied less with auxiliary gas velocity.The effect of auxiliary gas velocity on particle mixing was mainly reflected final mixing degree.The higher auxiliary gas velocity was,the larger Lacey index was.2.A 2D bubbling fluidized bed biomass pyrolysis model was established by Euler-Euler model.The effects of different drag,heat transfer models on flow,heat transfer and pyrolysis characteristics in reactor were systematically studied.Compared with experimental data,select proper drag and heat transfer model.The effect of nitrogen gas velocity on biomass pyrolysis products was further studied.（1）Different drag models had larger effect on time-average of vertical velocity,temperature and bed material volume fraction.Compared with Huilin-Gidaspow drag model and EMMS drag model,Syamlal-O’Brien drag model was lower in time-averaged of vertical velocity,temperature and bed material bottom volume fraction.Under the same drag model,time-averaged temperature distributions of different heat transfer models were almost the same,and the effect of heat transfer model on temperature distribution was not significant.Compared with experimental data,EMMS drag model with Gunn heat transfer model can predict biomass pyrolysis product distributions more accurately.（2）Under nitrogen gas velocity changing within a certain range,tar yield increased first,then decreased and became flat finally with nitrogen gas velocity increasing,where critical velocity(₂)=0.6 m/s)appeared.Beyond critical velocity,tar yield varied less with nitrogen velocity.3.Based on verified model,the effect of fluidized bed structure（biomass inlet mode and nitrogen inlet mode）on gas-solid flow,heat transfer and pyrolysis characteristics in fluidized bed and the synergistic effects of biomass and nitrogen inlet modes on pyrolysis characteristics were systematically studied.（1）Under constant total biomass feed,when biomass inlet changed from single nozzle（b=1）to double nozzles jetting opposite（b=2）,local circulation numbers increased,resulting flow structure more complex.Particle volume fraction and temperature were more uniformly distributed in dense phase zone,which was beneficial to biomass pyrolysis and tar yield.（2）Under constant total nitrogen gas velocity,nitrogen inlet mode was changed from nozzles numbers（n）and auxiliary gas velocity(₁)).When n=1 and₁)=0.158)/,it was conducive to generate larger local circulation in dense phase zone,which had a positive effect on uniform distribution of particle volume fraction and temperature.And then it was conducive to biomass pyrolysis and tar yield.（3）When biomass inlet changed from b=1 to b=2,change degree of pyrolysis yield was slowed down by nitrogen inlet mode,which weakened the effect of nitrogen inlet mode on biomass pyrolysis reaction.In terms of tar yield,n had the greatest effect on tar yield.When n=1,a higher tar yield can be achieved.Affected by biomass inlet mode,single nozzle inlet should be adopted at small auxiliary gas velocity.Double nozzles jetting opposite should be used at large auxiliary gas velocity.Comprehensive analysis showed that the highest tar yield was achieved when n=1,₁)=0.158)/and b=1.