PURPOSE: To simplify the separation of pebbles in a boiler by equipping a pebble separation cylinder provided with a fluidized medium reduction port in the top end part, connected with a pebble extraction tube in the intermediate part, provided with a screen for pebble separation across the inner hole in the lower part and provided with a pebble discharge tube above the screen.

CONSTITUTION: A pebble separation cylinder 30 is provided upright along a water-cooled wall on the external side of a combustion chamber 9. A blower is connected to the bottom end air tube 30a of the pebble separation cylinder 30. A fluidized medium reduction port 30b with an opening at the top end of said cylinder 30 is opened to the upper portion 31 of a fluidized bed formed with sand 21 and coal. An inclined-shaped pebble extraction tube 32, is connected to the intermediate part of the pebble separation cylinder 30 and the top end opening 32a of said tube 32 is made to open to the lowest portion 33 of the fluidized bed, which is in the periphery of the protruding parts of distributors 19. A screen 34, which does not allow pebbles to pass through but allows air to permeate, is provided across the inner hole in the lower part of the pebble separation cylinder 30, and a pebble discharge tube 35 is provided over the screen 34 by branching off from the pebble separation cylinder 30.