A bubbling fluidized bed (BFB) with a continuous feed/discharge of solids was used as a gasifier in a high-density triple-bed circulating fluidized bed (TBCFB). To carry the heat from the combustor to the pyrolyzer/gasifier effectively, a high-density and high solids-mass flux circulating system is necessary. In this study, the gas-solid flow behavior of a BFB with high solids continuous feeding/discharging mass flux was simulated by an Eulerian-Eulerian model incorporating the Lagrangian (discrete phase model) properties of particles as a tracer to analyze particle motion and determine the optimal feeding tube location and diameter. It was observed that a U-shaped track of particles from the inlet to the outlet and double vortices resulted in particle back-mixing and extended particle residence time. According to the results, some feeding-tube design principles were proposed to achieve smooth solids flow and improve the particle residence time in the BFB in solids mass circulating fluxes ranging from 800 to 1200 kg/m(2) s. (C) 2017 Elsevier B.V. All rights