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In the tangential combustion of a pulverized coal boiler, the swirl number is the key design index for evaluating the flue-gas temperature deviation at both sides of the furnace outlet. However, the current calculation method ignores the effect of the distances of the burner nozzles to the furnace outlet, and thus for large-scale utility boilers, the swirl number is significantly large. As such, the calculated result does not coincide with the actual situation. In this study, the calculation method of the swirl number was corrected by considering the effect of the distances of the burner nozzles to the furnace outlet. In addition, the corrected calculation method was verified through actual operation. The obtained results prove that the corrected calculation method is more accurate and reliable. In addition, the study indicates that for the special four-wall tangential combustion with a large tangential circle, the separated over fire (SOFA) air must have a four-corner tangential arrangement for the rotating airflow is still strong at the bottom of the platen and without other ways to reduce it. [ABSTRACT FROM AUTHOR]