Study On Biomass Migration Mechanisim During Pyrolysis In Multi-chamber Fluidized Bed

The multi chamber fluidized bed can reduce the particle back mixing and inhibit the secondary pyrolysis of the pyrolysis gas,and become an ideal reactor for biomass pyrolysis and liquefaction.It is the key to optimize the reactor and improve the efficiency of biomass pyrolysis to master the transport mechanism of biomass in multi chamber fluidized bed.At present,the research on gas-solid fluidization in the reactor is only carried out under cold conditions,but due to the complex mechanism of biomass pyrolysis,which is often accompanied by momentum,mass and energy transfer,the research on efficient and stable fluidization of pyrolysis particles in multi compartment fluidized bed is insufficient;On the other hand,it is very important to realize the stable fluidization of char particles at different pyrolysis ages,reduce the backmixing of particles at different pyrolysis stages,and prevent the secondary pyrolysis of pyrolysis gas.Based on this,numerical simulation and experimental measurement were used to study the evolution process of poplar sawdust pyrolysis particles in a multi chamber fluidized bed.(1)Based on the pyrolysis experiment of poplar sawdust particles in tubular furnace,combined with one-step reaction kinetics model and particle shrinkage core model,the mathematical model of particle shrinkage was obtained,which is suitable for pyrolysis temperature of 450 ℃,550℃ and 650 ℃.The mathematical models of particle shrinkage at different pyrolysis temperatures were imported into discrete element software edem through open source API.(2)The particle image velocimetry(PIV)system was used to analyze the flow field of particles in the fluidized bed,and the reliability of the numerical simulation model was verified by particle velocity.The results show that the flow patterns of different chambers are quite different when the bed is stable after the poplar sawdust particles of different pyrolysis ages enter the reactor in sequence,and the material transport effect is good;the experimental data of each chamber are in good agreement with the simulation data,which indicates that the cfd-dem coupling model can well predict the poplar sawdust particle transport law in the multi chamber fluidized bed pyrolysis state.(3)The migration of poplar sawdust particles during pyrolysis in a multi chamber fluidized bed was studied by using cfd-dem coupled simulation method.The volume accumulation,residence time,solid content and radial variation of velocity axis of poplar sawdust particles were studied under the conditions of gas velocity(0.3m/s,0.36m/s,0.5m/s)and temperature(450℃,550℃,650℃).The migration characteristics of pyrolysis particles in the bed were analyzed.The results show that the multi chamber fluidized bed can realize the step-by-step transport of materials.When the inlet gas velocity is 3-5 times of the minimum fluidization velocity,the gas velocity has little effect on the residence time,particle size distribution and solid content of pyrolysis particles in the bed.When the temperature is 550 ℃,the pyrolysis shrinkage is 0.01mm/s,and the inlet gas velocity of each bin is controlled at 3-5 times of the minimum fluidization velocity of the initial material,the stable fluidization can be achieved.The CFD-DEM coupling gas-solid fluidization simulation model established in this paper can accurately simulate the fluidization process of pyrolysis particles in a multi-chamber fluidized bed,which has guiding significance for the optimization and design of the reactor.