Influence of the over-fire air angle on the flow field in a down-fired furnace determined by a cold-flow experiment

It is necessary to introduce over-fire air (OFA) to a down-fired boiler to lower NOx emission. An appropriate OFA nozzle configuration will reduce the adverse effect of unburnt carbon in fly ash. To investigate mixing behaviors for different OFA angles in down-fired boilers, experiments were carried out on a cold-flow test facility using a particle dynamics anemometer with the main body modeled from a furnace of a down-fired pulverized-coal 300 MWe utility boiler. The experiments were carried out for four different OFA nozzle angle settings. For these settings, the distributions of the horizontal and vertical mean velocities and the root mean square velocity fluctuation in the furnace were investigated. The results show that a large OFA nozzle angle results in a shallow horizontal depth reached by the OFA flow in the furnace while a small angle results in a shallow vertical depth reached, both of which are disadvantageous to the final coal burnout. By considering both factors, an optimal angle is selected.