A new type of internal separator for circulating FBB system is described in this paper. The main advantages of the separator are high separation efficiency and low pressure drop. It can be put to use with or without a special circulating passage. Without any sealing device the particles separated from the separator can return easily to the lean phase or to the dense phase through a circulating passage. Experimental results of eighteen sets of model installed in two bench scale facilities at cold condition are described. The cross sections of the bed and the freeboard of one facility are 360 × 360mm2 and 500× 500mm2 respectively. Another facility with a cylindrical combustion chamber of inside diameter φ300mm was also used in the experiments. When the bed material used is flyash from the freeboard of a bubbling bed boiler

the separation efficiency may reach up to 92-97%

while the pressure drop is usually lower than 4mm H2O. Recently

a full scale internal separator has been put into test in a Fluidized Bed Boiler. It has been shown that there is a good prospect for this separator to be used to improve combustion efficiency. Typical axial

tangential and radial velocity profiles measured by a probe and the fields of particle concentration determined by optical

fibre devices above this separator in the freeboard are described in this paper. Computers are used to calculate the critical particle diameter.