Solids mixing in the bottom zone of a circulating fluidized bed

Solids mixing in the bottom zone of a circulating fluidized bed was investigated by means of solid carbon dioxide as a tracer in combination with gas concentration and temperature measurements. The experiments were performed in a cold model CFB riser with a rectangular cross-section of 0.3×1.0 m and a height of 8.5 m at operating conditions which are typical of CFB combustors (u=3 m/s, Gs=20 kg/m2 s, dp=150 μm). The measurements show a high extent of mixing in the vertical direction, whereas horizontal mixing in the bottom zone was found to be limited. If the tracer was injected into the solids return line, the observed horizontal temperature gradients were significantly lower, which is due to additional convection caused by externally circulating solids returned into the riser just above the bottom zone. A one-dimensional convection–dispersion model was used to calculate the temperature distribution within the bottom zone. The horizontal dispersion coefficient was found to be much higher in comparison to dispersion coefficients measured in bubbling fluidized beds as well as in the upper dilute zone of circulating fluidized beds.