Flame structure and reaction diagnostics for ammonia diffusion flame with hydrogen flame stabilizer

An ammonia coaxial jet diffusion flame stabilized with hydrogen was investigated. This type of burner maintains a stable NH3 flame, even when coaxial high-speed air flows at approximately 10 m/s (i.e., turbulent flows). In this study, flame structure, NOx formation, and reduction mechanisms were investigated through experiments and calculations. The novelty of this research is that the flame structure obtained by experiments was compared with the calculation result. Calculations were performed using three schemes and the optimal scheme was selected by comparing the experimental results with the calculations. (1) The results obtained using the CRECKMech scheme were in good agreement with the experimental results of the flame structure and NOx emission level. (2) The formation zone of NOx was determined by separating the thermal and fuel NOx. Thermal NOx formed only near the hydrogen flame and close to the burner rim in the high-temperature region. The amount of thermal NOx was considerably smaller than that of fuel NOx. (3) When OH and H radicals were supplied by turbulence, a large amount of NOx was induced. Concurrently, the mixing of NOx and NH3 and the reduction reaction are promoted, consequently reducing NO to N2. The reduction reaction of NOx was found to be intensely generated on the slightly inner side of the high-temperature region (i.e., the NH3-fuel rich region), thus inhibiting the increase in NOx. Arranging a reduction zone where NO can sufficiently react with N, NH, and NH2 is important in the burner design.