During dry beneficiation of coal using a gas-solid fluidized bed a secondary gas-distribution layer forms at the bottom of the bed. The effect of the particle size and depth of this secondary layer on the fluidization characteristics of the bed was investigated in this work. The experimental results show that the variance in both the bed pressure-drop and the bed density decreased with a decrease in the mean particle size of the secondary layer. This reduced variance indicates an improvement in the fluidization quality. Increasing the depth of the secondary layer causes both of these variances to decrease through a minimum and to then increase. The optimum depth of the secondary layer was determined to be 30 mm. The reason for these experimental results is discussed based on the effect the secondary layer has on the gas distribution region of a bubbling bed. It is pointed out that the secondary layer can divide large bubbles passing through the region into small bubbles and can increase the pressure drop of the gas distributor compared to a normal bubbling bed. Furthermore, the actual gas velocity is decreased by the secondary layer, which leads to a moderated shock from the gas to the bed. The secondary layer is, therefore, beneficial for increasing the bed stability. Mathematical models describing the relationship between the standard deviation in the bed density and the experimental factors are proposed. This work provides a foundation for the design of the discharge structure of the bed and for calculating the capacity of the bed. (C) 2012 Elsevier B.V. All rights reserved.