Fluidized bed combustors are commonly operating with complex bed materials of feeding solid wastes, sorbent, bottom ash, sinter and particle split. In the present study, the effects of gas velocities to the fluidized (draft tube) and the moving (annulus section) beds and the weight percent of additive (alumina) on the solid circulation rate (G(s)), gas bypassing flow rates (Q(DA), Q(AD), Q(or)) and pressure drop across the orifices (Delta P-or) of binary bed materials have been determined in a square internally circulating fluidized bed (0.28 m width x 2.6 m height) with an orifice-type square draft tube (0.1 m width x 0.9 m height). The gas bypassing flow rate through an orifice (Q(or)), G(s) and Delta P-or increase with increasing weight percent of the additive. The solid circulation rate has been correlated with Delta P-or and the opening area ratio based on the orifice theory. Also, Delta P-or, Q(DA), Q(AD) and Q(or) have been correlated with gas velocities to the fluidized and the moving beds and Archimedes number.