In a circulating fluidized bed (CFB) with a riser 2.6 m in height and 0.009 m in ID, the solid circulation rate and pressure drop of Geldart A and B particles in an L-valve 0.016 m-ID x 0.22 m-horizontal length were investigated as a function of the change in aeration rate through the L-valve. The gas velocity in the riser was maintained at 2.1 m/s to circulate particles transported from the L-valve. The bubbling bed was maintained above the minimum fluidizing velocity, U-mf. As a result, it was found that the solid circulation rate of KR40 (d(p) = 70 mu m, rho(p) = 1730 kg/m(3)), located in the middle region of Geldart A, increased gradually with increasing aeration rate up to U-mf in the L-valve and then increased sharply with increasing aeration rate. Therefore, the solid circulation rate of KR40 could not be controlled at velocities greater than U-mf: On the other hand, in the case of FCC (d(p) = 85 mu m, rho(p) = 1880 kg/m(3)), another set of Geldart A particles that are similar in size and density to KR40 is located at the boundary of Geldart A and B particles. These particles allowed the solid circulation rate to be controlled normally until the aeration rate in the L-valve reached the value of 3.5 U-mf. The bubbling bed of KR40 exhibited a sharp increase in solid circulation rate at U-mf and expanded by a factor of 1.05 as soon as the aeration rate reached the value of U-mf. This phenomenon was not observed for the other particles in this study. At the moment of the sharp increase in the solid circulation rate of KR40, the pressure in the L-valve and standpipe drastically changed, indicating that the solid circulation rate was intimately related to the pressure drop in the L-valve. The correlation equation between the solid circulation rate and gas aeration in the L-valve was derived for Geldart A particles ranging from 70 <= d(p) <= 148 mu m in particle size and 1,350 <= rho(p) <= 1880 kg/m(3) in particle density. (C) 2013 Elsevier B.V. All rights reserved.