Method and device for heating a furnace

A method for heating a furnace (100) having a longitudinal direction and a cross plane (C) perpendicular to the longitudinal direction, the furnace (100) having at least one heating zone (120) heated using at least one burner (122) providing a flame which extends in said longitudinal direction, the at least one burner (122) being fed with a fuel and a primary oxidant, the furnace further having a first side and a second side wherein said first and second sides are each parallel to said longitudinal direction, said method comprising:

• operating the burner (122) with a mass relationship between the fed fuel and the primary oxidant permitting less than 90% of the fed fuel to be combusted using the primary oxidant;
• providing at least two pairs of secondary oxidant lances (151,152;153,154) with one pair of the secondary oxidant lances on each of said first and second sides of the furnace (100) pointing into the heating zone (120);
• lancing a secondary oxidant into the heating zone (120) downstream of the burner (122) via said at least two pairs of secondary oxidant lances, the lancing of said secondary oxidant being substantially parallel with the cross plane (C);
• and
• measuring a temperature along a horizontal line parallel to said cross plane downstream of the oxidant lances, wherein each pair of said secondary oxidant lances (151,152;153,154) comprises:
• a respective first lance (152;154) and a respective second lance (151;153), wherein the respective second lance (151,153) is positioned downstream of the respective first lance (152;154),
• operating the respective first lance (152;154) of the secondary oxidant lances (151,152;153,154) at a lower lancing speed than the respective second lance (151;153) of the secondary oxidant lances (151,152;153,154), and
• regulating an amount of the secondary oxidant supplied via the respective first lance (152;154) of each pair of said secondary oxidant lances for achieving a homogenous temperature profile along the horizontal line.