Coupling of CFD-DEM and reaction model for 3D fluidized beds

Abstract

In this paper, a three-dimensional (3D) numerical model coupling of CFD-DEM with chemical reaction has been developed to study the combustion process in lab-scale bubbling fluidized beds. The gas phase is modeled with k-ε turbulent model and the solid phase is modeled with discrete element method (DEM). The dense gas-solid flow, heat transfer, and comprehensive chemical reaction are simultaneously considered. The reactive submodel includes the fuel pyrolysis, combustion of char and volatiles (CO, H2, CH4, tar), formation and reduction of gaseous pollutants (NO, N2O, and SO2).The established model is first validated by the simulation of the combustion of single char particle in a quasi-3D fluidized bed. The flow patterns, distributions of particle temperature and combustion gas compositions are comparatively analyzed. The heat released by char combustion raises the temperature of surrounding particles, and meanwhile, CO and CO2 are generated at the same position as char particle. The variations of temperature, char particle diameter, and CO2 concentration with time are compared with experimental data, showing good agreements. Based on model validation, the simulations are performed to study the coal combustion in a 3D cylindrical fluidized bed. The computational domain contains about 110,000 spherical particles. The general flow pattern, distributions of gas and solid temperature, and concentrations of gas compositions inside the reactor are obtained. The effects of excess air coefficients on the particle temperature and gaseous pollutants emissions are also analyzed. The results show that the particle temperature rises rapidly in the initial stage and then tends to be a stable value, accompanied by the oscillation throughout the evolution. An increase in excess air coefficient causes the increase of emissions of NO, N2O, and SO2. The current numerical model presents a promising way to predict the flow behaviors and reactive characteristics for dense particle reaction system at both macro- and meso-scales.