Effect of the momentum flux ratio of vertical to horizontal component on coal combustion in an arch-fired furnace with upper furnace over-fire air

Over-fire air is introduced to achieve deep staging conditions for combustion optimization in a 0.7 MW arch-fired furnace. The momentum flux ratio (M) of vertical to horizontal component is determined by variations of air distribution and the inclination angle of the F-layer secondary air. Two coals with relatively large characteristic differences are used in the pilot tests. The results show that M directly affects the arch air penetration length and the position of the flame center. As the inclination angle increases appropriately, the position of high temperature zone moves downward, and the variation amplitude becomes smaller. The distribution of the wall temperature becomes more uniform. An overlarge angle would cause severe dregs on the wall of hopper, however. Increasing the arch air ratio can also delay the mixing of air–fuel flows and stage air and lower downward the flame center. Too small M would cause much higher temperatures in the upper furnace. The unburned carbon in the fly ash and NOx emission both attain their minimum values with an inclination angle of 30° and a momentum flux ratio of 1.35, which is considered as the optimum operation condition in the experimental range.

► Momentum flux ratio of vertical to horizontal component is proposed.
► Momentum flux ratio affects the arch air penetration depth and flame center position.
► Detailed temperature distribution, gas components and unburned carbon are analysed.
► Optimal inclination angle and momentum flux ratio are presented.