Numerical And Experimental Investigation Of Gas-solids Flow Characteristics In Pressurized Fluidized Bed

The pressurized circulating fluidized bed technology(PCFB)has been developed and demonstrated as an advanced gasification technology,which plays an important role in the clean and efficient utilization of coal.In-depth understanding on gas-solids flow characteristics is important for the design,optimization and scaling-up of a pressurized fluidized bed.However,investigation on the multi-scale and non-linear hydrodynamic characteristics in a pressurized fluidized bed is challenging,due to the great difficulty for experiment and complexity in numerical analysis.In this research,the non-intrusive electrical capacitance tomography(ECT)measurement and multi-scale computational fluid dynamic(CFD)simulation were employed,to investigate the gas-solids flow characteristics of a pseudo-2D pressurized bubbling fluidized bed and a pilot-scale PCFB test rig.The effects of pressure on the minimum fluidization velocity and bubble properties,the spatial and temporal distribution of particle concentration and velocity,the flow regimes transition velocities and particle circulation flux were given and discussed.Firstly,CFD-DEM simulation on a pseudo-2D pressurized bubbling fluidized bed was performed based on the open-source software OpenFOAM?LIGGGHTS and CFDEM.The objective is to investigate the effects of operating pressure and fluidization gas velocity on the complex particle motion and bubble dynamic behavior.The simulation results indicate that(?)the minimum fluidization velocity decreases with the increase in operating pressure,and the influence is more obvious on large particles;(?)the particle axial velocity,the standard deviation of pressure fluctuation and the size and number of bubbles increase with the increase in fluidization number;(?)the standard deviation of pressure fluctuation decreases,and the bubbles decrease in size and increase in number with the increase in operating pressure due to the aggravation of bubble breakage;(iv)a "smoother" gas-solids fluidization state is reached as pressure increases,and the particle mixing rate is increased to some degree.Secondly,3D full-loop CPFD simulation on a pilot-scale pressurized circulating bed was carried out based on MP-PIC methodology and BarracudaTM platform.The spatial and temporal distribution of particle concentration and velocity were revealed,the influence of drag model was analyzed,and the effects of operating pressure on full-loop gas-solid flow characteristics were investigated.The results demonstrate that the particles show the bottom dense and upper dilute axial distribution,and "core-annular"radial distribution with dilute center and dense wall in the riser cross-section.Simulation based on EMMS drag model has better performance for gas-solids flow prediction than that by Wen-Yu/Ergun model,including the full-loop pressure distribution,axial and radial particle volume fraction distribution,and particle circulation flux.As pressure increases,the axial and radial distribution of particles become more homogeneous.The CPFD simulation also indicates that the particles axial velocity and the range of particle size participating in the circulation increase as pressure increases.In addition,the pressure drop and the particles tangential and axial velocity in the cyclone separator increase with the increase in operating pressure.Thirdly,electrical capacitance tomography(ECT)and high-frequency pressure measurement were used to investigate the flow regimes transition from bubbling to fast fluidization in a PCFB test rig.Based on ECT image reconstruction and statistical analysis of the measured signals,the effects of pressure on the critical flow regime transition velocities and bubble properties were revealed.The results show that(?)the minimum fluidization velocity,the transition velocity from bubbling to turbulent flow regime,and the transition velocity from turbulent to fast fluidization decrease with the increase in operating pressure;(?)the bubble diameter and bubble rising velocity decrease as pressure increases at a given fluidization number;(?)the bubble diameter and rising velocity first increase with the increase in fluidization number,and then remain almost unchanged.Finally,ECT technique was employed for the real-time gas-solids flow monitoring and particle circulation flux(Gs)measurement in the PCFB dipleg.The traditional accumulation measurement method for Gs was extended to the high-pressure and non-transparent fluidized bed by virtue of dual-plane ECT measurement.In addition,the on-line measurement of Gs was realized,by cross-correlation analysis of the dual-plane ECT signals,eliminating the need to interrupt the steady operation of a PCFB system.This is the first practice of ECT technique in high-pressure fluidization system,and also the first numerical simulation on a full-loop pilot-scale PCFB test rig.The results provide valuable information for the design,optimization and process monitoring for industrial pressurized fluidized beds.