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Toward a More Comprehensive Understanding of the Kinetics of a Common Biomass-Derived Impurity: NH 3 Oxidation by N 2 O in a Jet-Stirred Reactor

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Ammonia (NH3) has garnered substantial attention in recent years due to its importance across many scientific and engineering domains—including its potential use as a carbon-free fuel and long-term energy storage option, its use in reducing combustion-generated nitrogen oxide emissions, its role as a decomposition fragment of many energetic materials, and its presence as an important impurity during biofuel and biomass combustion that can affect overall system kinetics, among others. Yet, it is generally recognized that there are still significant gaps in the present understanding of ammonia kinetics—in both experimental data sets and submodels within the overall ammonia kinetic mechanism. For example, most experimental studies of ammonia oxidation have used molecular oxygen as the primary or sole oxidizer. While large mole fractions of molecular oxygen are encountered in many combustion scenarios, there are select systems where ammonia is more likely to be oxidized via nitrogen-containing species (e.g., N2O), and, more generally, there are relatively untested reaction sets that would be accentuated in such conditions. To address one such gap in experimental data sets for the validation of ammonia kinetics submodels, we present results from jet-stirred reactor experiments of an NH3/N2O/N2 mixture over an intermediate temperature range (850–1180 K). In these experiments, the mole fractions of NH3, N2O, and NO are measured through a combination of gas chromatography, chemiluminescence, electrochemical detection, and infrared absorption—where agreement among the different diagnostics (within 3% for N2O and 7% for NO) ensures high confidence in the experimental measurements. Comparison of the experimental results and model predictions suggests deficiencies in commonly used models for nitrogen kinetics. Various modeling analyses point to the central role of the N2O + NH2 = N2H2 + NO reaction, on which recent kinetic models all rely on the same rate constant estimate that appears to have not been tested in previous validation data sets for NH3 kinetics.

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