Ammonia oxidation features in a Jet Stirred Flow Reactor. The role of NH2 chemistry

This work focuses on the characterization of the oxidation process of NH3/O2/N2 mixtures in a quartz Jet Stirred Flow Reactor (JSFR). Experimental tests are run as a function of mixture pre-heating temperatures (900–1350 K) and different equivalence ratios (Φ = 0.8, 1, 1.2), while keeping constant pressure, mixture residence time and mixture dilution level (d = 86%). Three different oxidation characteristic regimes are experimentally recognized as a function of the mixture pre-heating temperature (Tin), namely low (Tin < 1100 K), intermediate (1100 < Tin < 1225 K) and high (Tin > 1225 K) temperatures. Ammonia reactivity exhibits no-dependency on the mixture equivalence ratio for low-intermediate temperatures, while it is strongly dependent on it at higher ones. Some effort is devoted to evaluate the heterogeneous surface effects on the homogeneous ammonia oxidation process comparing the previous results with further tests obtained passivating the inner surface of the JSFR with water and comparing experimental results from two tubular Laminar Flow Reactors (LFR) made of different materials (quartz and alumina). Results suggest surface reactions mainly affect NO profiles, with no remarkable influence of key species concentrations. Numerical simulations show that tested models are not able to carefully reproduce the experimental results, specially within the low-intermediate temperature oxidation regimes. The analysis of the Reaction Rates highlights that ammonia oxidation is strongly dependent on the NH2 reaction routes at different temperatures. The discrepancy between the considered kinetic schemes can be ascribed to the different description of NH2 chemistry