Ion chemistry in ammonia-hydrogen-oxygen flames

Ammonia is a prospective hydrogen carrier and a carbon-free fuel. Its reactivity can be improved by co-burning with hydrogen. The use of ion-sensitive technologies shows great potential for controlling the combustion of ammonia-hydrogen blends. In this regard, there is a demand for the models reliably predicting ion currents and the behavior of electrified ammonia flames. Understanding the ion chemistry in relevant flames is crucial for developing these models. A flame sampling molecular beam mass spectrometry is used in this work to measure the spatial distributions of positively charged species naturally occurring in the atmospheric-pressure burner-stabilized premixed flames of ammonia/hydrogen/oxygen/argon mixtures with equivalence ratios ϕ = 0.8, 1.0, and 1.2. NH4+, NO+ and H3O+ are detected in the fuel-lean and stoichiometric flames, whereas NH4+ is found to be a dominant cation under the fuel-rich conditions. An ion chemistry mechanism for this carbon-free system, that includes the reactions involving the three cations, three anions and electron, is proposed and validated against the experimental data. The highly accurate W2-F12 quantum chemical calculations are used to obtain the thermodynamic parameters for NH4+ and NO+. The mechanism reproduces properly the measured relative abundance of NH4+ and H3O+ in stoichiometric and fuel-rich flames, however, it underestimates the NO+ relative abundance under fuel-lean and stoichiometric conditions. The major reaction pathways responsible for the production and consumption of the cations are considered to explain the observed tendencies, and the directions for the further mechanism improvement are discussed.