Combustion of bituminous coal blended with NH₃ in an MW-scale pilot Facility: Effects of primary-zone excess air ratio

This study evaluates how the primary-zone excess air ratio (α) dictates in-furnace combustion dynamics and NO_x emissions during coal–ammonia (NH₃) co-firing. Results show that under all tested α conditions, the primary combustion zone exhibits a radially stratified structure, with a fuel-rich core surrounded by an oxygen-rich outer annulus. The oxidation–reduction conditions in the core, however, change significantly with α. Decreasing the primary-zone excess air ratio (α) from 0.97 to 0.70 shifts the core atmosphere from mildly oxidizing to strongly reducing. This causes substantial accumulation of CO and NH₃ both axially and radially, while elongating the flame downstream. NO_x emissions decrease monotonically from 735 mg/m3 (α = 0.97, NH₃ slip 69 ppm), 663 mg/m3 (α = 0.88, 45 ppm), 489 mg/m3 (α = 0.80, 78 ppm), to 350 mg/m3 (α = 0.70, 342 ppm), all with >98.4% coal burnout; α = 0.80-0.88 provides optimal balance. These findings indicate that the excess air ratio in the primary combustion zone is a key parameter for controlling NO_x formation during coal–NH₃ co-firing.