A grid nozzle assembly for a fluidized bed reactor, a fluidized bed reactor with a grid nozzle assembly, a method of mounting a grid nozzle assembly as a replacement in a fluidized bed reactor, and a method of replacing a grid nozzle assembly in a fluidized bed reactor. The reactor 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 nozzle assembly includes a nozzle head with a gas channel for injecting fluidizing gas from one of the vertical gas pipes to the reaction chamber and a tube sleeve adapted to be firmly fixed by welding around the top end of the vertical gas pipe. The nozzle head and the tube sleeve form a twist-lock enabling quick connecting and disconnecting.