Simulation of char and propane combustion in a fluidized bed by extending DEM–CFD approach

Abstract

The simultaneous combustion of char and propane in a fluidized bed is investigated numerically. A comprehensive discrete element method (DEM)–CFD model is developed, which takes into account particle-scale heat transfer, homogeneous and heterogeneous reactions, as well as hydrodynamics of dense gas–solid flow in fluidized beds. The coupling between the gas continuity and heterogeneous reactions is addressed which has been neglected in previous studies for the comprehensive DEM–CFD model. The combustion behaviors of the char particles and combustible gases are discussed with existed experimental findings. The model predicts the gaseous fuel reduces the char combustion rate and the negative effect is more evident with higher bed temperatures or highly reactive chars, which is consistent with the “surprising” experimental finding by Hesketh and Davidson [24]: the char combustion rate decreases as the temperature rises. The calculated profiles of gas species indicate the combustible gases can be burned significantly above the bed surface, in rising bubbles, or inside the emulsion phase, mainly depending on the bed temperature. A further insight into the simulation results shows the local heat source generated from the homogeneous reactions fluctuates with gas volume fraction, indicating the gas reaction is highly related with bubbles, which agrees with the optical measurements by Zukowski et al. [26]. The developed comprehensive DEM–CFD model provides detailed local information as well as macro structures. It can play an important role in a multiscale strategy for fluidized bed combustion.