Combined effects of yaw and tilt angles of separated overfire air on the combustion characteristics in a 1,000 MW coal-fired boiler: A numerical study

Abstract

Separated overfire air (SOFA) is typically employed in coal-fired boilers with air staged technology to sustain lower NOX emission, and SOFA nozzle angles are crucial adjustment parameters. In this work, the combined effects of SOFA yaw and tilt angles on combustion characteristics were numerically investigated for a 1,000 MW dual circle tangentially coal-fired boiler. Numerical results show that the forward increase of the SOFA yaw angle from 0° brings about the enlarged SOFA tangential circle and the gradual appearance of bimodal high-temperature zones at furnace exit. With further tuning SOFA tilt angles vertically, the bimodal high-temperature zones would separate to the two halves of the furnace, inducing a more severe deviation of gas temperature. Besides, the gas residual rotating momentum is strengthened as SOFA yaw angles forward increase, resulting in the enhancement of traction effect in the upper furnace as well as the rise of flame. However, the gas velocity deviation is somewhat eliminated as SOFA rotates reversely. No matter that the SOFA yaw angle increases forward or reversely, the coal burnout would deteriorate with the overly enlarged SOFA tangential circles. Tuning SOFA yaw and tilt angles, respectively, at 5° and 0° can simultaneously guarantee lower CO and NOX emissions.