Abstract: A boiler furnace combustion system typically includes main burners disposed on side walls of or at corners of a square-barrel-shaped boiler furnace having a vertical axis, the burner axes being directed tangentially to an imaginary cylindrical surface coaxial to the furnace. Air nozzles are disposed in the boiler furnace at a level above the main burners, so that unburnt fuel left in a reducing atmosphere or a lower oxygen concentration atmosphere of a main burner combustion region can be perfectly burnt by additional air blown through the air nozzles. The present invention provides two groups of air nozzles disposed at higher and lower levels, respectively. The air nozzles at the lower level are provided at the corners of the boiler furnace with their axes directed tangentially to a second imaginary coaxial cylindrical surface having a larger diameter than the first imaginary coaxial cylindrical surface.