A partial oxidation process using a swirl burner for producing synthesis gas, fuel gas or reducing gas from slurries of solid carbonaceous fuel and/or liquid or gaseous hydrocarbon fuel. The swirl burner may have pre-mix and/or high turndown features. By means of the burner a first reactant stream is split into a plurality of swirling streams by being passed down through a central bundle of helical tubes. Simultaneously a second reactant stream is split into a plurality of swirling streams by being passed down through a plurality of related passages formed in the cylindrical space that surrounds said central bundle of helical tubes and/or the interstices between the helical tubes. When the two swirling reactant feed streams impinge, either upstream from the face of the burner in a pre-mix zone or downstream from the face of the burner, in the gas generator, they are intimately mixed together. The combustion efficiency of the burner is thereby improved. In another embodiment, the burner includes a coaxial annular bundle of helical tubes surrounding said first conduit and a plurality of related helical passages formed in the annular space occupied by said annular bundle of helical tubes. Flow through the burner may be turned up or down.