Solids mixing in the bottom zone of a circulating fluidized bed was investi
gated 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 X 1.0 in
and a height of 8.5 in at operating conditions which are typical of CFB com
bustors (mu = 3 m/s, G(s) = 20 kg/m(2) s, d(p) = 150 mum). 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 injec
ted into the solids return line, the observed horizontal temperature gradie
nts were significantly lower, which is due to additional convection caused
by externally circulating solids returned into the riser just above the bot
tom zone. A one-dimensional convection-dispersion model was used to calcula
te the temperature distribution within the bottom zone. The horizontal disp
ersion coefficient was found to be much higher in comparison to dispersion
coefficients measured in bubbling fluidized beds as well as in the upper di
lute zone of circulating fluidized beds. (C) 2001 Elsevier Science BN. All
rights reserved.