Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH3) is a promising fuel, because
it is carbon-free
and easier to store and transport than hydrogen (H2). However,
an ignition enhancer such as H2 might be needed for technical
applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has
been explored widely. However, for mixtures of both gases, mostly
only global parameters such as ignition delay times or flame speeds
were reported. Studies with extensive experimental species profiles
are scarce. Therefore, we experimentally investigated the interactions
in the oxidation of different NH3/H2 mixtures
in the temperature range of 750–1173 K at 0.97 bar in a plug-flow
reactor (PFR), as well as in the temperature range of 1615–2358
K with an average pressure of 3.16 bar in a shock tube. In the PFR,
temperature-dependent mole fraction profiles of the main species were
obtained via electron ionization molecular-beam mass spectrometry
(EI-MBMS). Additionally, for the first time, tunable diode laser absorption
spectroscopy (TDLAS) with a scanned-wavelength method was adapted
to the PFR for the quantification of nitric oxide (NO). In the shock
tube, time-resolved NO profiles were also measured by TDLAS using
a fixed-wavelength approach. The experimental results both in PFR
and shock tube reveal the reactivity enhancement by H2 on
ammonia oxidation. The extensive sets of results were compared with
predictions by four NH3-related reaction mechanisms. None
of the mechanisms can well predict all experimental results, but the
Stagni et al. [React. Chem. Eng. 2020, 5, 696–711] and Zhu et al. [Combust.
Flame 2022, 246, 115389] mechanisms
perform best for the PFR and shock tube conditions, respectively.
Exploratory kinetic analysis was conducted to identify the effect
of H2 addition on ammonia oxidation and NO formation, as
well as sensitive reactions in different temperature regimes. The
results presented in this study can provide valuable information for
further model development and highlight relevant properties of H2-assisted NH3 combustion.