Hydrodynamic characteristics of a large-scale triple-bed combined circulating fluidized bed

A novel large-scale triple-bed combined circulating fluidized bed (TBCFB), consisting of a riser (16.6 m in height, 0.10 m inner diameter), a downer (6.5 m in height, 0.10 m inner diameter) and a bubbling fluidized bed (BFB; 0.27 × 0.75 × 3.4 m3), was constructed as a cold model for a gasifier. The purpose of the new reactor design was to achieve a high solids mass flux which is required for exergy recuperative steam gasification of coal/biomass. In the TBCFB, a gas-sealing bed (GSB; 5.0 m in height, 0.158 m inner diameter) was installed between the BFB and the riser bottom to increase the pressure head to transport solids to the riser. The hydrodynamic behavior of silica sand particles (arithmetic mean diameter is 128 μm) was investigated by independently controlling the flow rates of air in the riser, downer, BFB and GSB, and varying the bed heights of the BFB (HBFB) and GSB (HGSB) under ambient conditions. When the GSB gas velocity (Ugg) was 0.10 m/s, the solids mass flux (Gs: kg/(m2 s)) substantially increased with increase in riser gas velocity (Ugr). The maximum Gs obtained was 546 kg/(m2 s) at Ugr = 12 m/s and HGSB = 4.6 m. This is due to the fact that the pressure head for transport of solids to the riser bottom increased sufficiently by increase in the GSB height. From the solids holdup (εs) obtained from the apparent pressure difference, a dense phase (solids holdups: 0.070–0.095) was found to be formed at the bottom part of the riser (Hr ≤ 5 m). The flow in the downer was developed in 1.5 m. The solids holdup in the developed area of the downer decreased from 0.0212 to 0.0128 as the downer gas velocity (Ugd) increased from 0 to 1 m/s when Gs was 406 kg/(m2 s).