Solids separation efficiency at the outlet of a circulating fluidized bed riser

In a circulating fluidized bed (CFB) fuel converter the outlet geometry plays a decisive role for the solids flow in the circulating loop, thus affecting the mass and heat balances and the conversion process. A Dense Discrete Particle Model (DDPM) framework is used to investigate the solids flow, with model validation against experimental data from a cold model, scaled to represent the fluid dynamics of a commercial CFB boiler operated under hot conditions. Riser outlets with different configurations (L-shape and T-shape) are studied. The results show that the solids separation efficiency of the outlet (and, hence, of the riser) can be related to the Stokes number (Stk, which varied within 0.034–1.24). Decreased outlet area at a given riser cross-section and/or increased distance of the exit window from the top of the riser yield a higher solids separation efficiency. The smaller the Stk, the lower the solids separation in general, and the less it will be affected by the geometrical configuration. The solids flux value did not show any major impact on the solids separation efficiency attained. Finally, an expression for the solids separation efficiency of the riser outlet is derived, covering a range of efficiencies within 0.3–0.9 and yielding an average error of <20% compared to experimental data from literature.