Hydrodynamic behavior of binary mixture of solids in a triple-bed combined circulating fluidized bed with high mass flux

•Flow behaviors of binary mixture of sand and char substitute were examined.•Solids mass flux of of 100–400 kg/m2 s was obtained in a TBCFB cold model.•Apparent solids holdups of silica sand and nylonshot were calculated separately.•A modified pressure balance model successfully predicted solids mass flux.

Flow behaviors of binary mixture of silica sand and nylonshot (coal char substitute) were investigated in a triple-bed circulating fluidized bed (TBCFB) as a cold model of coal gasifier. The TBCFB consisted of a downer (ϕ 0.1 m × 6.5 m), a bubbling fluidized bed (0.75 m × 0.27 m × 1.9 m), a riser (ϕ 0.1 m × 16.6 m) and a gas-sealing bed (GSB, ϕ 0.158 m × 5 m). The initial fraction of the nylonshot in the solid mixture (Xnylon,i) was 15.4 and 30% on mass and volume bases, respectively, or otherwise, 30.7 and 50%. The maximum solids mass flux (Gs) at Xnylon,i of 15.4 and 30.7 wt% were 394 and 349 kg/m2 s, respectively, when the gas velocity in the riser (Ugr) was 10 m/s. Apparent solids holdups of silica sand and nylonshot were calculated separately from the static pressure gradient across the riser and the downer. The results showed possibility of large-mass-flux circulation of char in the gasifier, which plays a significant role in decomposition of tar from pyrolysis as the primary step of gasification. A newly developed pressure balance model successfully predicted Gs of the binary mixtures in TBCFB.

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