Exploring discrepancies among theoretical and experimental data for NH2 + CH4 ⇌ NH3 + CH3

Ammonia has been of great recent interest as a carbon-free fuel amidst growing concern around greenhouse gas emissions. Ammonia’s poor combustion characteristics have motivated exploration of co-combustion of ammonia and various co-fuels to yield more favorable combustion behavior. When the co-fuel is a hydrocarbon, the co-combustion kinetics can involve a host of reactions between nitrogen-containing species and carbon-containing species that are not otherwise important during combustion of either fuel when pure. Recent studies have highlighted hydrogen abstraction from hydrocarbons by NH${}_2$ as an important class of such C-N interaction mechanisms. However, even for NH${}_2$ + CH${}_4$ $\rightleftharpoons$ NH${}_3$ + CH${}_3$, which is among the simplest and most studied reactions of this reaction class, there is significant disagreement among rate constants from various theoretical and experimental studies. Of particular note, two shock tube studies at high temperatures reported rate constant determinations that differ by a factor of $\sim$4. Interestingly, both studies use thermal decomposition of a precursor following the shock wave to form NH${}_2$ and then monitor NH${}_2$ time profiles, but they use different precursors—raising the possibility that secondary reactions unique to each precursor (methylamine or hydrazine) may contribute to the discrepancies. The disagreement between these experimental studies, along with similar disagreement among theoretical studies, makes this an interesting system for analysis using MultiScale Informatics (MSI), which has previously identified consistent explanations of apparently inconsistent data for other reactions. We find, however, that the data reported in one of the shock tube studies are not internally consistent. An MSI model based on the other experimental and theoretical data is found to be consistent with all other data (including for the methylamine precursor) and essentially upholds the other experimental determinations despite significant revisions to the secondary chemistry since the original analysis, including further insights into methylamine chemistry described herein.