A burner and partial oxidation process for producing synthesis gas, fuel gas or reducing gas from slurries of solid carbonaceous fuel and/or liquid or gaseous hydrocarbon fuel. The burner has a high turndown feature and comprises a central conduit; a central bunch of parallel tubes that extend longitudinally through said central conduit; an outer conduit coaxial with said central conduit and forming an annular passage therewith; and an annular bunch of parallel tubes that extend longitudinally through said annular passage; and wherein the downstream ends of said central and annular bunches of parallel tubes are respectively retracted upstream from the burner face a distance of about 0 to 12 i.e. 3 to 10 times the minimum diameter of the central exit orifice and the minimum width of the annular exit orifice. Three ranges of flow through the burner may be obtained by using one or both bunches of tubes and their surrounding conduits. Throughput levels may be rapidly changed--up and down--without sacrificing stable operation. A control system is presented for varying the flow rates of the oxidant and fuel streams and/or for switching the reactant streams through one or both sections of the subject burner, thereby effecting a rapid turndown of the burner and in the production of the effluent gas.