Burner, method for operating burner, and method for melting and refining cold iron source

A burner including a combustion supporting gas supply passage which is configured to supply a combustion supporting gas toward a combustion supporting gas ejection outlet provided at the center of a tip end side;

• a fuel supply passage which is configured to supply a fuel toward a fuel ejection outlet provided around the combustion supporting gas ejection outlet;
• and
• a protective nozzle provided from a position surrounding a periphery of the fuel ejection outlet so as to project forward beyond a tip end surface at which the combustion supporting gas ejection outlet and the fuel ejection outlet are provided;
• wherein the combustion supporting gas supply passage includes a Laval nozzle, and a diameter-enlarged nozzle of which a diameter increases from a tip end of the Laval nozzle toward the combustion supporting gas ejection outlet, and
• the protective nozzle has a shape which is gradually reduced in diameter forward from the tip end surface, and
• wherein when assuming a maximum diameter of the diameter-enlarged nozzle is d3[m], a diameter at a tip end of the protective nozzle is d4[m], and a flow rate of the fuel required to combust at a stoichiometric ratio (oxygen ratio=1) with respect to a flow rate of the combustion supporting gas supplied is Qf[Nm3/h], d3 and d4 are set such that a flow rate V[Nm/s] of the fuel obtained by dividing Qf by an area A, which is obtained by subtracting an area of an outlet of the diameter-enlarged nozzle from an area of an outlet of the protective nozzle obtained by the following formula (1), satisfy 50≤V≤200
• V=(Qf/3600)/A (1)
• A=π/4×(d42−d32).