Importance of the Hydrogen Isocyanide Isomer in Modeling Hydrogen Cyanide Oxidation in Combustion

Hydrogen
isocyanide (HNC) has been proposed as an important intermediate in
oxidation of hydrogen cyanide (HCN) in combustion, but details of
its chemistry are still in discussion. At higher temperatures, HCN
and HNC equilibrate rapidly, and being more reactive than HCN, HNC
offers a fast alternative route of oxidation for cyanides. However,
in previous modeling, it has been required to omit the HNC subset
partly or fully in the reaction mechanisms to obtain satisfactory
predictions. In the present work, we re-examine the chemistry of HNC
and its role in combustion nitrogen chemistry. The HNC + O2 reaction is studied by ab initio methods and is
shown to have a high barrier. Consequently, the omission of this reaction
in recent modeling studies is justified. With the present knowledge
of the HNC chemistry, including an accurate value of the heat of formation
for HNC and improved rate constants for HNC + O2 and HNC
+ OH, it is possible to reconcile the modeling issues and provide
a satisfactory prediction of a wide range of experimental results
on HCN oxidation. In the burned gases of fuel-rich flames, HCN and
the CN radical are partially equilibrated and the sequence HCN →+M HNC →+OH HNCO is the major consumption
path for HCN. Under lean conditions, HNC is shown to be less important
than indicated by the early work by Lin and co-workers, but it acts
to accelerate HCN oxidation and promotes the formation of HNCO.