A fixed bed gasifier has a clinker breakup device. The tapping element of the clinker breakup device can be positioned at different points, allowing the tapping element to be small but capable of reaching all regions of the fuel particle bed. The tapping element intermittently enters the fuel particle bed in gasifying operation. Before it re-enters the bed, the drive mechanism adjusts the position of the tapping element so that it does not enter the same tapping channel again. Repeated local spiking at different times and locations moves the gasifier bed and disrupts the sintering of clinker particles that can block the gasifier cross section. Gas continues to flow through individual zones over the entire cross section, and not just in the tapped channel. Lumps of clinker that have sintered together are conveyed onward—toward the support device—or else broken up.