An Experimental and Kinetic Modeling Study of NO2 Sensitization on the Autoignition of Ammonia

An experimental and kinetic modeling study was performed to investigate the influence of NO2 addition on the autoignition characteristics of NH3/O2/Ar mixtures. Ignition delay times were measured using a shock tube at temperatures of 1450–2200 K, pressures of 0.14 and 1 MPa, equivalence ratios of 0.5, 1.0, and 1.5, and NO2 concentrations from 0 to 2000 ppm. The results show that NO2 addition significantly promotes NH3 autoignition, with the enhancement strongly dependent on temperature, pressure, equivalence ratio, and NO2 concentration. The effect is particularly pronounced at intermediate temperatures (1450–1700 K) and higher pressure, and exhibits a nonlinear relationship with NO2 concentration. The updated Shrestha and Glarborg mechanisms accurately reproduced the experimental IDT data. The evolution of key species indicates that NO2 addition significantly promotes NH3 consumption, along with the formation of NO and the radicals OH and NH2. Flux analysis reveals that NO2 addition introduces a novel ammonia consumption pathway via NH3 + NO2 = NH2 + HONO, facilitating rapid NH2 radical production. HONO decomposition further accelerates OH radical formation. Reaction rate analysis confirms that NO2 promotes OH generation through NO2 + H = NO + OH, especially in the early stages of NH3 oxidation. Moreover, NO2 addition significantly alters NH2 consumption pathways, with both NO2 and NO participating in reactions with NH2, not only accelerating its consumption but also suppressing NH2 self-recombination and other radical-consuming channels.