Circulating fluidized bed boiler

A boiler comprising fuel burning means and means allowing heat exchange between the products of combustion of the fuel and a working fluid, the fuel burning means including a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing, and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said vertical longitudinal plane.