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.