Optimization of Separated Overfire Air System for a Utility Boiler from a 3-MW Pilot-Scale Facility

Abstract

A 3-MW pilot-scale facility was used to study the effect of separated overfire air (SOFA) on NOx emissions from utility boilers. Experiment results showed that NOx emissions decreased as the excess air ratio in the main combustion zone first decreased, but remained unchanged when the excess air ratio was above a certain value; the carbon content in fly ash increased with the decrease of the excess air ratio in the main combustion zone; NOx emissions decreased and the heat loss due to unburned carbon increased as the residence time in the reduction zone (the zone in the furnace from the middle of combustion zone to the middle of the SOFA zone) increased. Based on the experiment results, a residence time of 2.12–2.68 s is recommended for the reduction zone. Besides, numerical simulations were conducted about the pilot-scale facility and a 600-MW boiler firing the blended coal of Shenhua coal (80 wt %) and Baode coal (20 wt %). The simulations, together with the experiments, showed that there was a critical ...