Three-Dimensional Modelling on the Hydrodynamics of a Circulating Fluidised Bed

Abstract

The rapid depletion of oil and the environmental impact of combustion has motivated the search for clean combustion technologies. Fluidised bed combustion (FBC) technology works by suspending a fuel over a fast air inlet whilst sustaining the required temperatures. Using biomass or a mixture of coal/biomass as the fuel, FBC provides a low-carbon combustion technology whilst operating at low temperatures. Understanding the hydrodynamic processes in fluidised beds is essential as the flow behaviours causing heat distributions and mixing determine the combustion processes. The inlet velocities and different particle sizes influence the flow behaviour significantly, particularly on the transition from bubbling to fast fluidising regimes. Computational modelling has shown great advancement in its predictive capability and reliability over recent years. Whilst 3D modelling is preferred over 2D modelling, the majority of studies use 2D models for multiphase models due to computational cost consideration. In this paper, two-fluid modelling (TFM) is used to model a 3D circulating fluidised bed (CFB) initially focussing on fluid catalytic cracker (FCC) particles. The transition from bubbling to fast fluidisation over a range of velocities is explored, whilst the effects on the bubble diameter, particle distributions and bed expansion for different particle properties including particle sizes are compared. Drag models are also compared to study the effects of particle clustering at the meso-scale.