Studies of Small Molecule Reactions Foundational to Combustion Chemistry, Including Experimental Measurements from a Novel High Pressure Flow Reactor

Abstract

A high pressure laminar flow reactor facility (HPLFR) was designed and constructed to serve as a platform for the experimental study of the gas phase chemistry of small molecule species. This facility accommodates pressures from slightly above atmospheric to ~30 atm, temperatures from ambient to ~1000 K, and plug flow residence times on the order of 100 milliseconds to 10 seconds. Quasi-steady state NOx plateau (QSSP) experiments were conducted in the newly-constructed HPLFR to determine rate coefficients for the reaction H+O2(+M)↔HO2(+M) (H.9.M) relative to the reasonably well-known rate coefficient for H+NO2↔OH+NO. Initial experiments for M = Ar and N2, for which a considerable kH.9.M(T,P) database already exists, returned kH.9.M(T,P) determinations in generally good agreement with literature kH.9.M expressions. For the present purpose, these results serve to validate the utility of the HPLFR facility for determining kH.9.M for various bath gases. However, the primary focus of these HPLFR QSSP experiments was to develop recommendations for kH.9.CO2, which is comparatively less well characterized despite its potential importance in high CO2-content combustion applications. QSSP experiments were performed at temperatures of ~800 K and pressures between ~2-8 atm to yield values of kH.9.CO2(T,P), which were then extrapolated to the low pressure limit (LPL) for ease of fusion with other literature results. Other H.9.CO2-sensitive literature experiments were critically reinterpreted to expand the range of conditions considered for kH.9.CO2LPL(T). A final un-weighted least-squares regression fitting to the compilation of all of these experimentally-derived kH.9.CO2LPL determinations yields an absolute, uncertainty-bounded (95% CI) rate coefficient recommendation of

kH.9.CO2LPL = (6.1 +1.2/-1.0)×1015 exp((+1164±306 kcal/mol)/RT) [cm6/mol2/s]

valid over the 633-1305 K range of included data. The present kH.9.CO2LPL recommendation was incorporated into an update of a CO oxidation chemistry submodel also addressed in this work. This update favors the use of available theory-based rate coefficient expressions to avoid the influence of hydrogenous impurities (via CO+OH↔CO2+H, C.1) that tend to skew experimentally-derived rate coefficients for secondary CO oxidation reactions. Additionally, the influence of including HOCO chemistry submodel variants on model predictions was assessed; however, such chemistry appears to contribute little to the predictive abilities of the present CO submodel.