Study On Flow Characteristics Of Standpipe In Circulating Fluidized Bed Boiler

Circulating flow rate is one of the most important parameters in the standpipe,which has an important influence on maintaining system stability,gas-solid contact,mixing and particle circulation behavior,heat transfer and mass transfer characteristics.And the real-time online measurement of circulating flow rate is helpful to optimize and adjust the flow pattern in CFB boiler and even improve the performance of the whole boiler.The development of the circulating flow rate measurement method depends on the flow characteristics in the standpipe.Therefore,in this paper,the gas-solid two-phase flow characteristics in the CFB standpipe will be studied by numerical simulation,which will provide a detailed theoretical basis for the application of circulating flow rate measurement method.Firstly,in order to choose the most appropriate drag model,the method,CPFD(computational particle fluid dynamics)respectively coupled with Wen&Yu-Ergun,Wen&Yu,Ergun homogeneous drag model and EMMS drag model,are used to simulate the full loop of the CFB apparatus.And the changes of standpipe pressure with the increase aeration air based on different drag models are compared with the experimental data.The results show that the simulation results based on the Wen&Yu-Ergun model are in best agreement with the experimental results,which means the Wen&Yu-Ergun model is the most suitable for the simulation of gas-solid two-phase flow in standpipe.Then,In order to study the gas-solid two-phase flow characteristics in the standpipe of the CFB apparatus,the material accumulation height,pressure gradient,voidage and gas-solid slip velocity under different aeration air are studied.The results show that the material accumulation height decreases with the increase of the aeration air,while the pressure gradient,voidage and gas-solid slip velocity increase.By comparing with the experimental data,the simulation results are basically in agreement with the experimental results,which proves that the CPFD-coupled Wen&Yu-Ergun drag model has a good applicability for the simulation of gas-solid two-phase flow in standpipe.Finally,in order to qualitatively study the gas-solid two-phase flow characteristics in actual CFB boiler standpipe,the method of CPFD coupled with Wen&Yu-Ergun drag model is applied to simulate the standpipe of a 116MW CFB boiler.And the effects of circulating flow rate,aeration air charge on the flow characteristics in the standpipe are analyzed.The results show that the particle circulating flow rate at the inlet of cyclone separator only affects the pressure at the upper part of the standpipe and will not cause the change of flow regime,and the standpipe aeration air has the greatest influence on the flow regime in the standpipe and may cause the change of the flow regime,while the loop seal aeration air mainly affects the flow at the upper part of the standpipe,The flow pattern in the middle and lower sections of the standpipe is relatively stable and will hardly affected by operating conditions,making it suitable for measuring the circulating flow rate.These research results provided theoretical basis for the measurement of circulating flow rate.