Over fire arrangement and method

[0028]FIG. 1 shows a prior art furnace 1 having a bottom 3 and top 9, and opposing front wall 4 and rear wall 6 as well as opposing first and second side wall 5, 7 (shown in FIG. 2B). The front wall 4 and rear wall 6 as well as opposing first and second side wall 5, 7 form together an enclosure 2 of the furnace 1, as shown in FIG. 2B. The furnace 1 comprises burners 14, 12, 10, 15, 13, 11 provided to the opposing first and second walls 4, 6 of the furnace for combusting fuel. In this embodiment the furnace comprises at least one first primary burner 14 provided to the first wall 4 and at least one second primary burner 15 provided to the second wall 6 for forming a first level of burners and first combustion stage. The furnace 1 further comprises at least one first secondary burner 14 provided to the first wall 4 and at least one second secondary burner 15 provided to the second wall 6, the secondary burners being arranged above the primary burners 14, 15 in vertical direction for stag