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 NOx 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. NOx 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 NOx formation during coal–NH3 co-firing.