Gas phase hydrodynamics inside a circulating fluidized bed

Abstract

Circulating Fluidized Beds (CFB's) offer many advantages over traditional pulverized coal burners in the power generation industry. They operate at lower temperatures, have better environmental emissions and better fuel flexibility. The motion of solids inside a CFB has been studied extensively over the previous twenty years. However the motion of gas is less well understood. There has previously only been indirect measurements of gas velocities and fluctuations. The gas phase is important as the motion of the particles is controlled by the gas. Accurate simulations of CFB's are not possible without accurate information on the gas phase. Instrumentation was developed for use in measuring gas phase fluctuations inside a scale model CFB. Results were unexpected in that gas fluctuations were substantially larger than expected. The fluctuation level without particle flow was around 0.15m/s. This was expected to stay constant or decrease with the introduction of particles. However with particle introduction the fluctuation level increased to 0.7m/s, an increase of over 400%. This is more than likely due to the clustering of particles which produces large scale structures with the resulting vortex shedding. A smaller riser was built which allowed the introduction of single individual clusters into the unit. The effect of single clusters on the surrounding gas flow was studied and modeled. These results indicate a mechanism by which, previously unknown, large scale fluctuations are generated inside a CFB.