This disclosure pertains to a triple-orifice burner and process for atomizing a stream of normally liquid hydrocarbon with a separate stream of free-oxygen-containing gas a finite distance downstream from the tip of the burner and for simultaneously enveloping the mixed streams with a separate stream of temperature-moderating gas so that none of the three streams comes into contact with any other gas with which it is combustible until it reaches a distance downstream from the tips of the burner orifices beyond that close enough to cause appreciable deterioration of the tips when reaction takes place in a free-flow partial oxidation gas generator at a temperature in the range of about 1,200.degree. to 3500.degree.F. and at a pressure in the range of about 1 to 275 atmospheres to produce a gaseous mixture comprising principally H.sub.2 and CO, e.g., synthesis gas, reducing gas, or fuel gas. While the liquid hydrocarbon is passed through the burner at a velocity in the range of about 10 to 100 feet per second, both the stream of free-oxygen-containing gas and the stream of temperature-moderating gas may be passed through the burner at velocities up to sonic velocity when it is desired to offset the effects of variable back pressure in the gas generator.The burner comprises three coaxial concentric nozzles whose tips are recessed from the downstream tip of the burner and which discharge into a single unobstructed coaxial central outwardly-diverging passage with an annular face of convex configuration at the extremity of the downstream tip of the burner. An imaginary plane tangent to said annular face at its outermost extremity is normal to the burner axis. The three nozzles provide, respectively, a central passage with a circular opening for discharging a stream of liquid hydrocarbon, an intermediate converging annular passage for discharging a stream of free-oxygen-containing gas, and an outer converging annular passage for discharging a stream of temperature-moderating gas, such as steam. An annular coolant chamber is disposed about the outside periphery of said outer discharge passage and downstream burner tip.