Solids inventory and external solids circulation rate in risers of circulating fluidized beds

[Display omitted] •Solids flow rate in riser exit Fs depended on riser solids amount Ms.•Fs depended on elutriation rate constant.•Fs for circulating fluidized bed is similar with that for batch riser.•Fs decreased with increasing riser diameter D.•Ms increased with D. This study was conducted to provide improved relationships on the external solids circulation rate and solids inventory in the riser of the circulating fluidized bed (CFB) through increasing the basic understanding on how the solids flow rate in the riser exit establishes. The relation between the solids inventory in the riser and the solids flow rate in the riser exit was investigated in batch elutriation and CFBs, using air as fluidizing gas at atmospheric pressure and temperature. A batchwise riser (0.1 m-i.d., 2.75 m-height) and the riser of a CFB system (0.1 m-i.d., 2.5 m-height) were used to measure the relation for different gas velocities (1.5–2.4 m/s) and groups of solids (0.064–0.201 mm in diameter, 2045–4080 kg/m3 in apparent density). The batch elutriation indicated that the amount of solids inventory in the riser determined the solids flow rate in the riser exit, increasing with the amount of the solids inventory in the riser. The solids flow rate in the exit of the riser strongly depended on the elutriation rate constant combined with the amount of the solids inventory in the riser. When the riser was same in size, the relationship on the solids flow rate in the riser exit, derived in the batch elutriation, was valid in the CFB. Relationships on the external solids circulation rate and solids inventory in the riser of the CFB were proposed based on the data from this study and the literature. It was found that the external solids circulation rate per cross-sectional area of the riser decreased with increasing the riser diameter, however, the static bed height of solids in the riser (hst,c) increased with the riser diameter. The dependency of hst,c on riser diameter was greater in the fast bed than in the bed of pneumatic conveying.