A burner includes a plurality of air nozzles open toward a furnace and which are disposed unsymmetrically with respect to the central axis of the burner and so combined that the velocity of combustion air to issue from one or more of the nozzles is higher than the mean air velocity through the total air nozzle opening area and the velocity of combustion air from the rest of the nozzles is lower than the mean velocity. The opening area of the lower-velocity air nozzle or nozzles accounts for from 30 to 60% of the total opening area. A fuel nozzle or nozzles are located within or adjacent the lower-velocity air nozzle or nozzles. The amount of air to issue from the lower-velocity air nozzle or nozzles is not more than 70% of the theoretical air for the fuel to jet out of the burner.
