Experimental and chemical kinetic study for the combustion of ammonia-hydrogen mixtures

Chemical kinetic model development and validation against new combustion data for ammonia and ammonia blended hydrogen are still requirements. New ignition delay times of NH₃ and NH₃/H₂ were experimentally measured using a high-pressure shock tube over a temperature range of 1500–1900 K, at pressures ranging from 5 to 30 atm, and at equivalence ratios between 0.5 and 2.0, with hydrogen blended ratios ranging from 0 % to 50 %. Additionally, new experiments were designed to explore extinction limits of NH₃/H₂ (Ra_H2 = 0.25–0.75) using a counterflow burner under normal temperature and pressure (300 K, 1 atm), as well as laminar burning velocities using a high-pressure constant volume combustion chamber. The resulting experimental data, previously published LBVs, and oxidation data were employed for comprehensive validation against the improved NH₃/H₂ reaction kinetic model (referred to as the WUT model), consisting of 35 species and 239 reactions. The WUT model was assembled from the literature models and mainly updated based on the Gotama model, and four reaction rate constants were adjusted through a critical evaluation. To evaluate the updated WUT model, mean absolute errors for predicting combustion characteristics of NH₃ and NH₃/H₂ were calculated. The calculated results demonstrate that mean absolute errors for IDTs, LBVs, and extinction limits are all lower than 12 %. The WUT model can also reasonably predict the profiles of NH₃ and NH₃/H₂ oxidation species in the JSR. Experimental results indicate that an increase in Ra_H2 leads to exponential decreases in IDTs, and the extinction limits of ammonia are broadened, attributed to enhanced ammonia reactivity promoted by hydrogen. Kinetic analysis shows that the conversions of NO₂ to NO and N₂O to N₂ are both over 90 % through one-step pathways. The removal of NO is mainly reaction with ṄH₂ and N¨H via multiple steps, this process plays a crucial role in De-NOx process for NH₃/H₂ combustion. Under hydrogen addition conditions, the process of NH₃ → ṄH₂ was promoted, and the HȮ₂ and ṄH₂ radicals were accumulated rapidly before ignition.