Numerical Simulation Of Gas-solid Flow And Gasification Characteristics In A Circulating Fluidized Bed Biomass Gasifier By CPFD

Biomass gasification is a clean and efficient biomass thermal conversion technology.Circulating fluidized bed gasifiers have the characteristics of long particle residence time and high fluidization gas velocity.They are suitable for biomass,a highly volatile fuel,and are currently widely used in the field of biomass gasification.However,the circulating fluidized bed gasifier still has problems such as unstable flow field in the furnace and low gasification efficiency,so it is necessary to carry out the gas-solid two-phase flow and biomass gasification reaction in the circulating fluidized bed gasifier.adequate research.Computational particle fluid dynamics（CPFD）methods use the concept of computational particles to describe the motion of a large number of particles and are suitable for simulating gas-solid two-phase flows in larger fluidized beds.Therefore,based on the CPFD method,this thesis conducts a numerical simulation study on the gas-solid two-phase flow and biomass gasification reaction in a circulating fluidized bed gasifier,which provides a theoretical reference value for the application and operation of the circulating fluidized bed gasifier.First,a physical model is established based on the actual size of the circulating fluidized bed gasifier.The motion process of gas and solid phases in a circulating fluidized bed gasifier is described by the Euler-Lagrange method,in which the continuous phase is calculated by the Navier-Stokes equation,and the discrete phase is calculated by the particle computing unit multiphase flow（multiphase particle in-cell,MP-PIC）method.The gasifier is simulated under the same operating conditions as the experiment,and the simulation results are in good agreement with the experimental results in the literature,which proves that the mathematical model used in this thesis has the ability to model the gas-solid flow and chemical reaction in the circulating fluidized bed gasifier.high reliability.Secondly,the proven mathematical model was used to simulate the gas-solid two-phase flow process in the circulating fluidized bed gasifier,and the effects of the operating conditions and physical structure of the circulating fluidized bed gasifier on the velocity field,temperature field,pressure field were studied in detail.and the effect of particle velocity residence time.The results show that the increase of the fluidizing gas velocity leads to an increase in the pressure drop at the inlet and outlet of the gasifier.When the fluidizing gas velocity increases from 3 m/s to 7 m/s,the particle residence time decreases from 6.82 s to3.21 s.The gasification temperature has a great influence on the temperature distribution in the gasifier.When the fluidization gas velocity and gasification temperature are 4m/s and800℃,respectively,the velocity field,temperature field,and pressure field distribution in the circulating fluidized bed are better.to be uniform.The increase of the furnace height leads to an increase in the average residence time of particles.When the furnace height is 11m,the particle residence time interval is more reasonable.Finally,the biomass gasification characteristics of the circulating fluidized bed gasifier under different influencing factors were studied,and the fluidization gas velocity,gasification temperature,initial bed height,air equivalence ratio（ER）,water vapor were quantitatively analyzed.The influence of factors such as ratio（S/B）on the spatial distribution characteristics of gasification products.The research results found that:with the increase of fluidizing gas velocity,the content of combustible gases（H₂,CH₄,CO）in the furnace and gasifier outlet gradually decreased.With the increase of gasification temperature,the content of CH₄ in the furnace and the outlet of the gasifier decreased,and the content of CO and H₂ in the furnace and the outlet of the gasifier increased.With the increase of the bed height,the contents of CH₄ and CO in the furnace and gasifier outlet increased,and the content of H2 was the highest when the bed height was 2200 mm,and then decreased with the increase of the bed height.With the increase of the air equivalence ratio（ER）,the content of combustible gases（H₂,CH₄,CO）in the furnace and the gasifier outlet decreased,while the N₂ and O₂ contents in the furnace increased.When the air equivalence ratio was 0.26,each The distribution of components is relatively uniform.With the increase of water vapor ratio（S/B）,the content of CO in the furnace and gasifier outlet gradually decreased,the increase of H₂ was obvious,the content of CH₄ remained basically unchanged,and the optimal S/B value was 0.4.