Gas-solid Flow,Flow Regime And Multi-scale Characteristic In Dual Circulating Fluidized Bed Reactor

The new dual circulating fluidized bed reactor is different from the conventional circulating fluidized bed reactor.It is widely applied in fluid catalytic cracking,chemical looping combustion,coal/biomass combustion and gasification,and gas adsorption,etc.From the perspective of dual fluidized bed reactor,the riser is mainly fast fluidization regime and pneumatic conveying.It is capable of gas-solid mixing and heat-and-mass transfer.In a bubbling fluidized bed,the fluidization is mostly of bubbling characteristics,sometimes of turbulent characteristics.The bubbling process leads to the motion of solid particles and the gas-solid interaction enables the particles to be well mixed.This bed can be operated in bubbling and turbulent fluidization regimes.If a pot seal is employed between the bubbling bed and the riser,two separate reaction zones can be created and then different fluidization conditions can be maintained in these zones.In this way,the reaction process can be intensified.This thesis focuses on the control of pressure balance and solids circulation characteristic,flow regime identification in gas-solid two-phase fluidization,the influence of the gas-solid two-phase behavior between the dual bed reactor and multi-scale characteristic of dual circulating fluidized bed reactor,and thus the research work has been carried out from the following three aspects.It is further studied to provide theoretical guidance for control,industrial design and enlargement.1.The study mainly investigates the effects of specific operating parameters on pressure balance and solid circulation rate,including total solids inventory,riser gas velocity,bubbling fluidized bed gas velocity and pot-seal gas velocity.It is found that changes of those parameters lead to variations in pressure balance and riser axial voidage in DCFB system,and further affect solid concentration and solid circulation rate in the system.To be more specific,total solids inventory,riser gas velocity and bubbling fluidized bed gas velocity have a major impact on solid concentration and solid circulation rate,while pot-seal gas velocity only exerts a minor impact.Besides,the changing of overall pressure drop across the bubbling fluidized bed is consistent with variations in pressure drops in the other parts of the system,with the former equaling ?Pr+?Pc+?Pp-t+?Pud1+?Pud2.In this way pressure balance in the system is maintained.Eventually the paper reaches the conclusion that relevant operating parameters can be adjusted to control pressure distribution and solid circulation rate in different parts of the system and further to ensure the safe and stable operation of DCFB reactors.2.To explore flow regime transition processes in gas-solid two-phase fluidization,a non-intrusive,real-time and environment-friendly acoustic emission technique and an auxiliary pressure fluctuation method were applied in this work.Particle activity and collision intensity in the bed are obtained through standard deviation analysis.On that basis,it is found that the acoustic signal measurements can more effectively reflect the transition velocities among major flow regimes,including uc(from bubbling to turbulent fluidization),uk(from turbulent to fast fluidization),and uFD(from fast to dense phase pneumatic conveying).Meanwhile,the multi-scale resolution of acoustic signals during regime transitions can be obtained through Hurst and wavelet analyses.To be specific,micro-scale signals reflect the inter-particle collision and the particle-wall collision,meso-scale signals indicate the interactions and behaviors of particle clusters and gas phase,and macro-scale signals represent the average flow behavior.According to the comparison of experimental results,the experimental values of transition velocities obtained from acoustic signal measurement are closer to corresponding empirical values.In other words,the integration of standard deviation and multi-scale analyses with acoustic signal measurement can effectively identify flow regime transitions in real-time.3.The study mainly investigates the effects of specific operating parameters on gas-solid two-phase behavior and multiscale characteristic in the DCFB system,including total solids inventory,riser gas velocity,bubbling fluidized bed gas velocity and pot-seal gas velocity.The experimental results and analysis show that the disturbance of the gas relative to particles in the bed causes the mixing of the reactor particles,and the degree of particle mixing increases with the increase of the gas phase behavior.Thus,the interaction of the gas phase and particles influence each other.With the change of operating parameters,solid circulation rate changes and then the behavior of particles and bubbles in the riser and bubbling bed change.As a result,the behavior of the particles and bubbles in these two regions influence each other.Combined with Hurst and fractal dimension analyses,the multi-scale resolution of acoustic signals of the upper and lower acoustic signals of the riser can be obtained,reflecting the differences in the behavior of particle clusters and bubbles in the meso-scale between the upper and lower parts of the riser at low superficial gas velocity.Meanwhile,through the establishment of mesoscale structure model,the larger particle cluster size,the more non-uniform flow in the bed,which indicates that the mesoscale effect is more intense.It pointed out that the particle cluster size is related to particle concentration and increases with the increase of the particle concentration.