Abstract: A fluid-bed combustion reactor (51) comprising a substantially vertical reactor chamber with a first inlet (9) at the reactor chamber lower portion (52) for the introduction of liquid and/or solid particulate material, and a second inlet (22) at a level below the first inlet for the introduction of gas for fluidization of particulate material within the reactor in order to maintain a primary fluid bed, an exhaust duct (28) at the reactor chamber upper portion for the withdrawal of exhaust gas and particles from the reactor, and a fluid-bed cooler (42) for particular material, formed as an upwards open vessel with generally closed bottom and side walls and arranged so as to collect a portion of particulate material (64, 65) from the reactor chamber upper portion, said cooler comprising heat transfer means (43) such as tubes carrying a heat transfer medium at the inside and having said particulate material flowing at the outside, said cooler comprising at least one conduit (56) for the controlled returning of p