An improved burner for partial oxidation process gas generators is provided which has annular passages formed between coaxially aligned conduits extending from upstream sources to the downstream reaction zone. An outer coolant jacket, internally baffled from optimum coolant flow and sized for minimum downstream area surrounds a recessed and fuel/oxidizer delivery conduit ending in a nozzle. The central delivery conduit is not attached to the coolant jacket and the annular space between them is connected to a high pressure supply of relatively inert gas which can periodically be vented through the annular space to prevent slag build up on the nozzle or coolant jacket.