A burner apparatus for combustion in a combustion chamber of a furnace a fuel with oxygen contained in an oxidant gas, comprising cavity means including fuel injectors for distributing the fuel in at least two adjacent streams injected in the combustion chamber of the furnace and at least one main oxidant cavity ending with an elongated orifice having a major axis along its largest dimension which is generally parallel to a surface to be heated, the elongated orifice suitable for injecting a major portion of oxidant required for complete combustion of the fuel, wherein the elongated orifice directs the major portion of oxidant to converge toward the fuel streams in order to generate a broad flame substantially parallel to a surface to be heated, wherein the inner geometry of the main oxidant cavity comprises preferably four sections:
- a first section which is generally cylindrical and having an axis;
- a second section which is generally cylindrical, of the same diameter as the first section;
- the second section making an angle (B) with the axis of the first section;
- continuously attached to second section is a third section, generally conical with diverging angle (C) ranging from about 10.degree. to about 120.degree.;
- and a fourth section connecting continuously the third section with the main elongated orifice.