Time-resolved modeling of gas mixing in fluidized bed units

This work presents a time-resolved model for gas-phase mixing in fluidized beds based on a dynamical description of the gas flow in the bottom bed, which is known to influence strongly the gas mixing throughout the riser. This contrasts with traditional models based on time-averaged parameters which account for the fluctuating gas mixing by fitting the homogeneous reaction rates to experimental data. Assuming that gas mixing governs gas combustion (and thus infinitely fast gas combustion kinetics), the model presented yields results that are in good agreement with measurements in the 12-MWth circulating fluidized bed (CFB) boiler at Chalmers, without the need for any fitting. The model presented here simulates measurements accrued by in-bed gas suction probes, since it divides the gas flow into excess gas and emulsion gas and calculates for each component the fluctuations in composition and velocity. Thus, it is possible to mimic the under-representation of oxygen in the probe measurements made in the bottom region of the bed. In contrast to classical time-averaged modeling, the present model is able to reproduce accurately and explain the complex trends observed in the experimental data derived from the bottom region of a CFB furnace.