A grate assembly (10) for a gasifier for gasifying carbonaceous material producing ash includes an upper rotatable grate component (11) and a lower rotatable support structure (12) fastened to the upper rotatable grate component (11) by a plurality of removable fasteners (15) acting to prevent vertical displacement of the upper rotatable grate component (11) and the lower rotatable support structure (12). The lower rotatable support structure (12) is configured to be drivingly rotated about a common vertical axis of rotation (21) shared with the upper rotatable grate component (11). The assembly (1) further includes at least two key torque transmitters (16) spaced angularly relative to the common axis of rotation (21) and engaging the upper rotatable grate component (11) and the lower rotatable support structure (12) to transfer torque from the lower rotatable support structure (12) to the upper rotatable grate component (11) when the lower rotatable support structure (12) is driven.