Study On The Solids Circulation Rate And Particle Residence Time In A Pressurized Circulating Fluidized Bed

The pressurized circulating fluidized bed（PCFB）has the advantages of excellent fuel flexibility and high heat and mass transfer coefficient.The combination of pressurized fluidized bed with oxy-fuel combustion has been proposed can further reduce the energy consumption of CO₂ capture in flue gas of coal-fired power plants,which is believed to be an attractive technology for CO₂ capture in coal-fired power plants.Although some researchers have conducted related studies on the gas-solid flow characteristics of PCFB,there are relatively few studies on the law of solids circulation rate with pressure.The current understanding of particle residence time distribution under pressurized conditions is also limited.These characteristics are very important for the design of the PCFB furnace.Thus,experiments and numerical simulations are carried out in this work to systematically study the solids circulation rate and particle residence time of PCFB in this work.Main research contents and conclusions of this work are as follows:On a pressurized circulating fluidized bed experimental system with an inner diameter of 50 mm and a height of 3300 mm,the relationships between solids circulation rate and various operating parameters are studied.Based on four different particle sizes of Geldart Class B particles,the experiment is constructed in a wide experimental range（operating pressure=0.1～0.6 MPa,fluidizing gas velocity_g=1.5～8.0 m/s,solids circulation rate_s=20～80 kg/（m²·s））.It can be found that under the same pressure,the fluidization gas velocity increases to achieve the same solids circulation rate as the particle size increases.The gas-solid slip characteristics in the riser are also studied.It is observed that the apparent slip factor increases with the increase of the dimensionless slip velocity.Under different average particle sizes and pressures conditions,the corresponding data points of them are gathered on an approximate exponential function curve.The gas-solid flow diagram which descriptions relationship of_s and(_g-_tr)is summarized.An empirical equation is obtained by fitting the solids circulation rate with operating parameters（particle transport velocity,particle volume fraction,Archimedes number,and Froude number）.Based on the multi-phase particle-in-cell method（MP-PIC）,a three-dimensional model of the full-loop of the circulating fluidized bed is established by tracking the numerical particle.A full-loop numerical simulation of CFB is carried out through this method.The core-annulus flow structure is observed in the riser.It is found that the annulus region gradually disappeared along the bed height,while the core region became larger and expanded toward the wall.The influences of pressure and fluidization gas velocity on solids circulation rate are also studied.It was found that compared with atmospheric pressure,the order of particle movement at elevated pressure is enhanced,and the axial particle volume fraction is more uniform.With the increase of fluidization gas velocity,the probability of collisions between particles increases and becomes more intense.The disorder of particle movement increases.Comparing the solids circulation rate obtained by simulation with experimental empirical equation,the two results match well.The particle movement characteristics of pressurized circulating fluidized bed are further analyzed.The particles form a dynamic balance in the entire full-loop system.The effects of operating pressures,fluidization gas velocity and solids circulation rate on particle residence time distribution and axial diffusion coefficient in the riser are studied.It shows that the increase of fluidization gas velocity would reduce the average residence time of particles,and lead to the axial diffusion coefficient increasing.With solids circulation rate increasing,the average residence time of particles shows a trend of decreasing,while the axial diffusion coefficient increases.The increase of pressure causes the average particle residence time and axial diffusion of particles to increase.The gas-solid flow diagram is applied to analysis particle residence time and axial diffusion,which explains the change law of particle residence time distribution and axial diffusion coefficient from the perspective of flow mode.