Coupling CFD-DEM with cohesive force and chemical reaction sub-models for biomass combustion in a fluidized bed

Abstract

The present study employs the CFD-DEM method coupled with cohesive force and chemical reaction sub-models to investigate the hydrodynamic and thermal behaviors in a biomass-fired fluidized bed combustor containing bed particles with cohesive features resulting from the high levels of inorganic alkalis in the biomass. The CFD-DEM simulations effectively reproduce the characteristic phenomena in a cohesive fluidized bed, such as particle agglomeration, gas bubbles with irregular boundaries, rat-holes, and gas channels. The results indicate that the particle kinetic energy continuously decreases with increasing particle cohesion, and the pressure-drop profile flattens suddenly at the onset of complete de-fluidization. Additionally, larger cohesive forces occur at coarse bed-particle conditions, while the particle-averaged cohesive force at the onset of de-fluidization weakens as the bed-particle size increases at the same superficial gas velocity. The regions near the bed bottom and sidewall are more sensitive to the hydrodynamic and thermal alterations resulting from increasing particle cohesion, making them more favorable for early detection. The convection heat and reaction heat dominate the heat transfer of bed particles and biomass particles, respectively. As the particle cohesion becomes more pronounced, the biomass-particle reaction heat flux decreases because biomass trapped in agglomerates experiences oxygen-deficient conditions due to the hindering effects of large agglomerates on the gas and the occurrence of gas channels. Overall, the present study indicates that the particle mixing and inter-phase heat transfer capacity, along with reaction intensity in the biomass-fired fluidized bed, deteriorates as particle cohesion increases. The extended model provides a particle-scale approach to analyze complex cohesive gas–solid reaction flow systems.