Establishing an appropriate overfire air angle at the furnace throat of a low-NOx W-shaped flame furnace

A cascade-arch-firing low-NOx and high-burnout configuration (CLHC) was developed as a solution for a 600 MWe W-shaped flame furnace suffering from poor burnout under ultra-low NOx combustion conditions. Under the comprehensive low-NOx combustion conditions regulated by the CLHC, overfire air (OFA) was first positioned at the furnace throat in a uniformly flattening OFA port form along the furnace breadth direction. In order to disclose the OFA angle's effect on the furnace performance and meanwhile establish an appropriate angle for the OFA with the above original designs, the in-furnace flow field, coal combustion, and NOx emissions were evaluated at various OFA angles of theta = 10 degrees, 20 degrees, 25 degrees, 30 degrees, and 35 degrees. Increasing theta only changed a little the symmetry of flow field and combustion morphology, i.e., the symmetry generally deteriorating first and then improving. The OFA penetration in the furnace throat zone initially increased but then worsen as theta increased. In the upper furnace strongly affected by OFA, gas temperatures, high-temperature zone area, and levels of O2 and CO all descended first and then increased with theta, while NO generally undergone an increase-to-decrease trend. Trends of furnace outlet's performance indexes with theta showed that the residual O2 and CO emission generally decreased first and then increased, NOx emissions initially increased but then decreased, while carbon in fly ash undergone an increase-to-decrease trend prior to an increase at theta = 35 degrees. In view of the above findings and the gained optimal low-NOx and high-burnout performance (NOx emissions of 667 mg/m3 at 6% O2 and carbon in fly ash of 5.31%), theta = 30 degrees was taken as an appropriate OFA angle for the CLHC furnace. Compared with a currently advanced low-NOx combustion art, the CLHC reduced further NOx emissions by 26.3% while remained a high burnout achievement.