Resolving NOX formation of ammonia-hydrogen flame utilizing PLIF technique collaborated with flame structures and chemical kinetics analysis

Carbon-free fuels such as ammonia and hydrogen have attracted much attention in response to tackling with climate problem of global warming, but their high NOX emissions limit practical applications unavoidably. Currently, very few studies have addressed the inter-relationship between flame structures and NOX formation. In addition, few previous studies have analyzed ammonia-hydrogen combustion with low hydrogen mixing ratio through in-depth NOX formation mechanisms. In this work, NOX formation of ammonia-hydrogen swirl flame with different equivalence ratios and hydrogen mixing ratios (<30 %) has been comprehensively investigated, and the connection between flame structures and NOX has been reflected based on PLIF technique. The analytical results showed that as equivalence ratio (Phi = 0.6-1.2) increased, NOX concentration increased firstly and then decreased subsequently, and peak NOX value was observed between Phi = 0.7-0.8. Besides, NOX increased as the hydrogen mixing ratio increased from 10 % to 25 %, being capable of reaching up to 2795 ppm. Furthermore with flame structure analysis, the flame structure could be classified into single-front flame, transition flame, and double-front flame, in which transition flame featured with the largest decomposition reaction region contributing to NH3 oxidation to form NOX (intensive OH radicals propagation); while double-front flame characterized by smallest decomposition reaction region inhibiting the NOX formation via OH suppression (weak OH radicals propagation). Based on systematically flame surface density and chemical kinetics analysis, lean combustion benefited the NOX pathway, whilst rich combustion favored the N-2 pathway. In addition, as the hydrogen ratio increased, and the reducibility of NH/NH2 to NOX was weakened, which ultimately promoted the production of NOX. The findings achieved suggest that future combustion techniques by the ammonia-hydrogen dual fuel should avoid the occurrence of transition flame, and prone to the generation of double-front flame, which could thus implement effective suppression on NOX formation.