Numerical Simulations Of Solid Circulation Characteristics In An Internally Circulating Fluidized Bed

A baffle type internally circulating fluidized bed(ICFB)is a single vessel reactor appropriately divided into two or more chambers termed as the fast bed or the reaction chamber(RC)and the slow bed or the heat exchange chamber(HEC)by placing a plate called baffle at some distance above the gas distributor forming a slot for the solid circulation between the chambers.Thus the solid circulation process in the reactor is developed by applying unequal gas velocities to these chambers.Such a process is beneficial to several applications and provides the mean for the energy transfer between the chambers in the ICFB.Due to its special layout and features,ICFBs possess several advantages over conventional CFBs such as reduced height,low construction cost,compact size and comparatively less heat loss from the reactor.These advantages enable ICFBs to be implemented in coal/biomass combustion and gasification,solid waste disposal,production of highly pure polysilicon particles for the photovoltaic industry,and flue gas desulfurization.In order to achieve a better understanding of the complex flow process to optimize and improve their design,researchers have performed a lot of experimental and simulations practices on BFBs and CFBs but little attention is given to ICFBs.In this thesis,Eulerian-Eulerian model incorporating the kinetic theory of granular flow is used to simulate internally circulating fluidized bed using commercial code ANSYS-Fluent.The simulated results are quantified in terms of solid circulation rate Gs and the predicted trends are well consistent with the previous studies in literature.Fluidization velocities(UR and UH)to ICFB chambers strongly influence the solid circulation rate and pressure drop across the slot in the internally circulating fluidized beds.When unequal gas velocities(high in one chamber while low in the other)are applied to the ICFB chambers,it causes bed density difference between the chambers resulting in pressure gradient across the slot below the baffle.Formation and rise of bubbles and finally their eruption at the bed surface along with the gas flow diversion,throws the bed particles from the high velocity chamber RC to the low velocity chamber HEC above the baffle.On the other hand,pressure difference across the slot between the chambers is responsible for the solids recirculation from the HEC to RC.Thus,a continuous solid circulation process is established in the ICFB.It is observed that the bed material has a significant impact on the solid circulation rate and pressure drop across the slot in the ICFB.High density bed material exhibits high circulation rate Gs due to high pressure drop across the slot and vice versa.The particles discharge coefficient through the slot varies between 0.5 and 0.6 regardless of the bed material type.The impact of gas distributor design,superficial fluidizing velocities and chambers' elevation in height are also evaluated in the ICFB.It is found that by increasing gas velocity UR in RC at constant HEC gas velocity UH,solid circulation rate is increased from the HEC to RC through the slot regardless of the gas distributor design.Furthermore,solid circulation rate and particles radial velocity through the slot in the tubular gas distributor ICFB are found higher as compared to the other simulated gas distributors.Discharge coefficient of particles through the slot varies in the range of 0.5-0.79 in the ICFB.Moreover,two internally circulating EFB configurations named as RC-elevated and HEC-elevated have also been investigated.Pressure gradient across the slot in the internally circulating EFB is found higher than the conventional un-elevated ICFB,subsequently leading to high solid circulation rate in the internally circulating EFB.The superficial velocities UR and UH strongly influence the solid circulation rate,pressure drop across the slot and granular temperature in the internally circulating EFBs.By increasing UR at constant UH,solid circulation rate from the HEC to RC through the slot is increased in both the HEC-elevated as well as the RC-elevated configurations.On the other hand,solid circulation rate decreases with increase in UH at constant UR as UH approaches UR.Granular temperature increases with increase in UR and decreases with increase in UH.Variation of the chambers' elevation in height and interchange of the elevated and un-elevated chambers considerably affects performance of the internally circulating EFBs.Comparing the two internally circulating EFBs,the HEC-elevated configuration gives high solid circulation rate than the RC-elevated.In addition,both internally circulating EFBs produce high solid circulation rate than the conventional ICFB.Afterwards,ICFB having a binary mixture composed of two particles(GBs and GBb)of different size but same density is also simulated.An increase in the RC gas velocity UR at constant HEC gas velocity UH results in increase in the circulation rate of the binary mixture.Solid particles are flowing upward in RC and coming down in the HEC.The circulation rate of the small particles GBs is found higher than that of the big particles GBb.An increase in the slot size causes decrease in pressure difference between the chambers consequently decreasing the circulation rate of the binary mixture in the ICFB.The solid circulation rate also decreases when the mixture is highly concentrated in big particles GBb than the smaller particles GBs.Finally,solid circulation behavior of binary mixture of particles(G116 and P275)having different size and density is investigated in the ICFB.Here also,an increase in UR at constant UH results in increase in the circulation rate of binary mixture and the particles are found flowing upward in RC and coming down in the HEC.Ratio of the circulation rate of G116 particles to P275 particles is approximately equal to the ratio of their masses in the mixture within ICFB.An increase in UH at constant UR results in decrease in the circulation rate of binary mixture and Gs almost becomes as UH approaches UR.Like UR and UH,the mixture composition also plays a crucial role in controlling the circulation rate of binary mixture in the ICFB.Furthermore,ICFB is found to possess excellent solids mixing capability as compared to conventional bubbling fluidized bed and segregation is greatly minimized.