  Five recently developed chemical kinetics mechanisms for ammonia oxidation
are analysed and compared, in the context of homogeneous adiabatic
autoignition. The analysis focuses on the ignition delay and is based on the
explosive mode that is shown to drive the process. Using algorithmic tools
based on the Computational Singular Perturbation algorithm, the reactions
responsible for the generation of the explosive mode are identified, along with
the variables (species mass fractions and temperature) that associate the most
to this mode. Comparison of these sets of reactions and variables, obtained for
each mechanism, allows to correlate the differences in the predictive outcomes
from the mechanisms with specific reactions. The major differences identified,
which lead to different ignition delay times, relate to (i) the relative
duration of chemical and thermal runaways (a sizeable chemical runaway develops
only in some mechanisms) and (ii) the dominant chemistry during the chemical
runaway (chemistry involving species with two nitrogen atoms is active only in
some mechanisms). The major similarities identified refer to the thermal
runaway and in particular to (i) the chemical activity, which is supported
mainly by OH-producing reactions and by reactions producing their reactants and
(ii) the thermal activity, which is dominated by strongly exothermic
OH-consuming reactions.
