Shock-induced auto-ignition of partially dissociated ammonia mixtures

Ammonia (NH₃) is emerging as a promising zero-carbon fuel, offering vital support for the transition to sustainable energy systems. Among various applications, partially dissociated ammonia mixtures have exhibited great potential in internal combustion engines and gas turbines due to their enhanced reactivity and improved combustion performance. In this study, comprehensive ignition delay times (IDTs) and NH₃ time-history measurements of partially dissociated ammonia mixtures (NH₃/H₂/N₂) were conducted over a wide range of temperatures (1115—1611 K), pressures (1.0—4.0 atm), dissociation proportions, and oxygen concentrations (3.33 %, 7.5 %, and 13.33 %). The results revealed that the reactivity of dissociated ammonia mixtures increases significantly with higher pressures, dissociation degrees, and oxygen contents, while the elevated oxygen concentrations may lead to excessive NO_x emissions. A recently developed NH₃-syngas chemical kinetic model proposed by our group was systematically validated against the experimental data from this work, including IDTs and NH₃ time-histories, as well as laminar flame speeds, speciation data, and NO_x emissions from literature. The model exhibited remarkable predictive accuracy under high-pressure and fuel-lean conditions, filling the gap in current kinetic models for dissociated ammonia combustion. Further rate of production and sensitivity analyses were carried out to unveil the dominant oxidation pathways and identify key elementary reactions controlling the reactivity of dissociated ammonia mixtures. Moreover, the generation and consumption pathways of NO_x were thoroughly elucidated, providing valuable insights into NO_x formation mechanisms under varying dissociation proportions and oxygen contents. This study may enhance the kinetic understanding of partially dissociated ammonia combustion and provides theoretical foundation for the development of two-stage ammonia combustors with optimized performance and reduced NO_x emissions. Novelty and significance statement: Ammonia is a highly promising zero-carbon fuel with considerable potential to support the transition to sustainable energy. However, its inherently low reactivity poses significant challenges to widespread application. Recent studies suggest that two-stage combustors, leveraging partially decomposed ammonia products, can enhance combustion reactivity. In this work, ignition delay times and key species profiles of NH₃/H₂/N₂ mixtures were systematically measured using a shock tube coupled with laser absorption spectroscopy — to the best of our knowledge, this represents the first dataset of its kind in the literature. The NH₃-syngas kinetic model developed by our group was validated against both our experimental results and extensive literature data, demonstrating improved predictive accuracy. Furthermore, rate of production and sensitivity analyses were performed to elucidate NO_x formation, DeNO_x pathways, and key elementary reactions. This study may provide valuable insights into ammonia combustion chemistry and offer guidance for the design and optimization of next-generation two-stage ammonia combustors.