A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity u(mf) and critical velocity for full fluidization u(mff) were particularly studied. The effects of taper angle alpha, static bed height H, air inlet section width delta and particle size d(p) on the u(mf) and u(mff) were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the u(mf) and u(mff), respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the u(mf) and u(mff) increase with the increase of taper angle alpha, static bed height H and particle size d(p), but decrease with the increase of air inlet section width delta, respectively. Additionally, the predicted values of u(mf) and u(mff) compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the u(mf) of the modified bed. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.