A grid nozzle assembly for a fluidized bed reactor that includes a horizontally extending bottom plate, a gas plenum chamber below the bottom plate, and vertical gas pipes having a top end and extending from the gas plenum chamber upwards across the bottom plate. The assembly includes a nozzle head with a gas channel that injects fluidizing gas from one of the vertical gas pipes to the reaction chamber. A tube sleeve is adapted to be fixed by welding around the top end of the vertical gas pipe. The nozzle head and the tube sleeve have a knob and groove portion that form a twist-lock enabling quick connecting and disconnecting of the nozzle head and the tube sleeve.