Experimental and numerical investigation of ammonia addition on combustion characteristics and NO formation effects in ammonia/coal co-firing flames

Ammonia, as a carbon-free fuel, offers a promising pathway for achieving emission reduction through co-combustion with pulverized coal. However, the effect of ammonia addition on coal combustion and the intrinsic mechanisms requires further investigation. In this study, an optical diagnostic platform comprising a two-stage flat flame burner, high-speed imaging system and fiber-optic spectroscopy was employed to investigate the combustion characteristics of NH 3 /coal flames at low ammonia blending ratios. CH ⁎ chemiluminescence was utilized to characterize the flame structure and the ignition process. In addition, numerical simulations integrating global mechanisms for coal and ammonia pyrolysis/combustion were conducted to analyze flue gas composition and NO generation. The results indicated that flame height and fuel-rich zone increased significantly with ammonia blending ratios. Coal ignition delay time and the time to the peak intensity exhibit nearly linear growth. Total CO and CO 2 emissions decreased gradually with increasing ammonia concentration, while local CO peak mole fraction increased. It was attributed to the suppression of the critical CO oxidation pathway (CO + OH→CO 2 + H) due to O₂ and OH radical consumption by ammonia. As ammonia blending ratio rises, NO mole concentration increased substantially and coal-N contributed significantly less to NO formation than NH 3 -N. The distribution of the NO core zone was governed by the oxidation reaction R10(NH 3 + O 2 →NO + H 2 O + 0.5H 2 ) and the reduction reaction R11(NH 3 + NO→N 2 + H 2 O + 0.5H 2 ).