Effect Of Cyclone Outlet Connections On Gas-solid Hydrodynamics And Diffusion Of Fuel Particles In A CFB Boiler

The gas-solid hydrodynamics and fuel particle diffusion characteristics of circulating fluidized bed boiler(CFB)are related to the gas-solid concentration distribution,particle flow,fuel distribution and combustion in the furnace,which are important for the design and operation of CFB boiler.This paper utilized experiment method,theoretical models,numerical simulation,combined with field test data,to study the effect of connection between cyclone outlet and backpass on gas-solid hydrodynamics,the solid diffusion effect on gas-solid hydrodynamics,and fuel particle diffusion characteristics in large scale CFB boilers.Firstly,the influence of connection between cyclone outlet and backpass(cyclone outlet)on gas-solid hydrodynamics in CFB boiler was carried out in a four-cyclone cold test bench,and a multi-loop CFB gas-solid flow model was proposed to characterize the cyclone outlet structure using outlet resistance characteristic parameters.With the increase of outlet resistance inhomogeneity coefficient and superficial velocity,the non-uniformity of gas-solid concentration distribution and solid circulation rates increases.In order to study the fuel diffusion behavior in large CFB boiler,the MP-PIC-based three-dimensional diffusion combustion model for large CFB is established.The model is coupled with polydisperse EMMS drag model,coal combustion,heat transfer and particle properties change model.The numerical simulation results are verified by experimental study and and field test data of a 350MW CFB boiler.Based on solid diffusion results,the paper proposed lateral dispersion coefficient correlation and combined to gas-solid flow model,to investigate the effect of lateral solid diffusion behavior on gas-solid hydrodynamics.Based on the 3D diffusion combustion model,the numerical simulation study of fuel diffusion was conducted for 350MW single-distributor CFB boiler and 660MW dual-distributor CFB boiler.The results show that the lateral diffusion coefficients of fuel particles in 350MW CFB boiler and 660MWCFB boiler are in the order of0.1m~2/s～1m~2/s,and the operating velocity and fuel properties both affect the diffusion coefficients.To study the uniform combustion characteristics of fuel particles in diffusion,the combustion rate/diffusion rate ratio(Da number)of fuel particles was calculated.With the increase of superficial velocity,primary air rate,and the fuel particle size,the Da number decreases and the of fuel particle combusts in high uniform.Considering the influence of coal feed and slag discharge arrangement on the combustion process of fuel particles,the fuel combustion characteristics under different coal feed and slag discharge arrangement is calculated.And the results provide important guidance for the optimal arrangement and efficient operation of coal feed and slag discharge in large CFB boiler.