Reactor for producing a product gas from a fuel having a housing (11, 12, 13) with a combustion part accommodating a fluidized bed (7) in operation, a riser (2) extending along a longitudinal direction of the reactor (1), and a downcomer (3) positioned coaxially around the riser (2) and extending into the fluidized bed (7). One or more feed channels (8) for providing the fuel to the riser (2) are provided. The riser (2) is attached to the housing (11, 12, 13) of the reactor (1) in a bottom part (13) of the housing (11, 12, 13), and a part of the riser (2) above the one or more feed channels (8) is moveable with respect to the downcomer (3) in the longitudinal direction of the reactor (1).