Experiments on particles flow in a dual circulating fluidized bed with symmetrical structure

The particle flow characteristics in a dual circulating fluidized bed (DCFB) with symmetrical CFB units were experimentally studied. Each CFB unit consists of a riser (H_r = 3 m, D_r = 0.1 m), a downcomer (H_d = 1.73 m, D_d = 0.1 m), a cyclone separator and a loop seal. The solid material applied in the experiments was quartz sand (d = 140 μm, ρ_s = 2600 kg/m³) with a total solid inventory of 70 kg. Four steady flow patterns were formed in each CFB unit, including turbulent flow (TF), cluster flow (CF), annular flow (AF) and suspension flow (SF), which leads to five steady coupled flow patterns in DCFB, i.e. CF-TF, CF-CF, AF-CF, AF-AF and AF-SF. Axial solid holdups were examined at these plow patterns and their transitions. The fluctuation strength and particles flow stability were evaluated with standard deviation (σ) and coefficient of variation (C_V) of time-series solid holdups, respectively. It was found that a growth in the superficial gas velocity of Unit B (U_g,B) would lead to a more uniform Unit B axial solid holdup. Meanwhile, the variation trends of Unit A solid holdup differed under different coupled flow pattern. With U_g,B increasing, σ_B along Unit B riser became more homogeneous while the change in σ_A was roughly linear with the variation of solid circulation rate. Meanwhile, σ and C_V of Unit B solid holdups were found to have a similar variation trend under coupled flow patterns of CF-TF and CF-CF. As for coupled flow patterns of AF-CF, AF-AF and AF-SF, with increasing U_g,B, the C_V of Unit B bottom region kept decreasing while constant growths were found in other regions.