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 NO_X emissions limit practical applications unavoidably. Currently, very few studies have addressed the inter-relationship between flame structures and NO_X formation. In addition, few previous studies have analyzed ammonia-hydrogen combustion with low hydrogen mixing ratio through in-depth NO_X formation mechanisms. In this work, NO_X 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 NO_X has been reflected based on PLIF technique. The analytical results showed that as equivalence ratio (Φ = 0.6–1.2) increased, NO_X concentration increased firstly and then decreased subsequently, and peak NO_X value was observed between Φ = 0.7–0.8. Besides, NO_X 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 NO_X (intensive OH radicals propagation); while double-front flame characterized by smallest decomposition reaction region inhibiting the NO_X formation via OH suppression (weak OH radicals propagation). Based on systematically flame surface density and chemical kinetics analysis, lean combustion benefited the NO_X pathway, whilst rich combustion favored the N₂ pathway. In addition, as the hydrogen ratio increased, and the reducibility of NH/NH₂ to NO_X was weakened, which ultimately promoted the production of NO_X. 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 NO_X formation.