Study On Fluidization Characteristics Of Multi-Compartment Fluidized Bed Reactor For Biomass Pyrolysis

Biomass pyrolysis technology is the main way to use biomass energy to produce bio-oil,and the key link is the design of pyrolysis reactor.Therefore,the development of more excellent reactor structure is of great significance for the effective use of biomass energy.Based on the fluidized bed reactor,a multi-compartment fluidized bed reactor equipped with openings and deflectors for biomass pyrolysis was designed.In the numerical simulation,the gas-solid two-phase flow in the reactor under cold state was calculated transiently by software FLUENT 16.0,and the operation effect and fluidization characteristics of the reactor were studied.The multi-phase flow model was based on the Euler two-fluid model,and the turbulence model used the k-?model,and the exchange coefficient between the gas and solid phase was based on Syamlal-O'Brien model.Finite Volume Method was chosen to discretize the governing equations,and the pressure-based separate solver was selected.The momentum,turbulent kinetic energy and turbulent dissipation rate were all MUSCL format,and the volume fraction used QUICK format.Using PC-SIMPLE algorithm to couple pressure with velocity between phases.(1)The effect of feed rates on the materials conveyance time was studied.The results showed that the materials conveyance time in the reactor was about 30 s when the feed rate was 0.04 kg/s,it could ensure the full cracking of the particles;(2)The effect of opening sizes on the materials conveyance time and the variation law of dense bed height were studied.The results indicated that when the opening size was 5 cm,the materials transport was smooth and fast,and the height of each compartment was basically stable at a height of 0.2 m;(3)The influence of the deflectors elevation angles on the materials conveyance time and internal flow field of reactor were studied.The results indicated that the deflectors had obvious drainage effect on the particles at the opening,and the elevation angle of 45°is most favorable for the sequential one-way flow of the particles;(4)The influence of particle size and gas velocity conditions on the fluidization characteristics of the particles,the materials conveyance time,and the variation law of dense bed height were studied.The results showed that when the particle size was 300?m and the gas velocity was 0.3 m/s,the fluidization quality was the best and the operation of reactor was the most stable.After optimized by the simulation,the optimum feed concentration of the reactor was determined to be about 4.4 kg/m~3,the optimum opening size was 1/6 of the compartment height,the optimum elevation angle of the deflector was 45°,and the optimum particle diameter and gas velocity were 300?m and 0.3 m/s.In the experiments under cold state,the reactor was designed and the experimental process was set up,in which wood chips were selected as solid phase and the air was used as gas phase.(1)The pressure difference between the inside of the reactor and the outside atmosphere was measured by the U-tube pressure gauge in the different height positions of the third compartment of the reactor.Compared with the simulation data,it was found that both the data were in good agreement under different particle diameter and gas velocity conditions.The error between the two was within 10%at position below 15 cm(lower edge of the opening);(2)The variation law of bed height in the reactor during continuous operation was measured by the scale on the outer wall of the reactor.Compared with the simulated data,it was found the two trends were basically the same and the numerical error was within 15%.The final stable value of the bed height in the experiment was about 23 cm(simulated value was 20 cm);(3)The particle distribution experiment was designed in which the three groups of materials with different diameter ranges were to approximately replace the particles in three different residence time.The results indicated that about 50%of the particles could enter the expected corresponding compartment during continuous feeding.The experiment verified the correctness of the simulation and the rationality of the structure design of the reactor.Based on the results of simulations and experiments,the multi-compartment fluidized bed reactor ensured that the solid particles had sufficient residence time to be fully cracked while ensuring that the cracked gas was removed from the reactor in time to reduce secondary cracking.The reactor enabled continuous production and the mean residence time of the materials could be controlled by adjusting the feed rate.The multi-compartment design could achieve the effect of materials conveying step by step.The particle pyrolysis process was artificially divided into several stages,which could significantly improve the residence time distribution of materials,inhibiting the particle back-mixing phenomenon.