(46c) An Examination of HONO and HNO2 in Low-Temperature Combustion

Current research and engine developments demand additional investigations and research into nitrogen-fuel interactions below the thermal NO_x limit. The use of exhaust gas recirculation as well as the application of cetane enhancers such as 2-ethylhexyl nitrate necessarily result in interaction between fuel molecule fragments and nitrogen-containing species, in particular NO and NO₂, collectively NO_x.

Hydrogen transfer via abstraction or disproportionation in conjunction with NO₂ leads to the formation of HONO and HNO₂. Both HONO and HNO₂ are presently believed to form from the same reactants and ultimately dissociate to the same products, OH and NO. Consequently, there is an open question as to whether HONO and HNO₂ should be treated as independent species with their own sets of reactions and thermodynamic data or whether the two may be lumped into one chemical compound for the purposes of modeling and simulation.

To investigate this problem, the potential energy surfaces of H or CH₃ adding to both HONO and HNO₂ are investigated. Optimization of geometries and saddle point searches were first conducted with Density Functional Theory (DFT) and single-point calculations were then performed using Coupled Cluster theory (CC). The results of these calculations were used to calculate the partition functions and solve the system Master Equation (ME).

Based on the results of the ME solution, the two isomers, HONO and HNO₂ do have different product branching fractions. As considered in the overall reaction scheme, however, these differences are expected to have little discernible effect on combustion parameters of interest, e.g. flamespeed or ignition delay time.

To examine the results of the ME solution and assess the impact of treating HNO₂ as a unique specie, the new rate-constant fits from the ME solution are applied to two current literature mechanisms. Each mechanism is then used in four variants:
(i) the unaltered, published mechanism;
(ii) a revised mechanism that has been updated with rates for HONO and HNO₂ developed in this work;
(iii) the revised mechanism but with the HNO₂ reactions eliminated;
and (iv) the revised mechanism with HNO₂ lumped into HONO to sum the rates and concentrations of the two species.

The intra-mechanism variations for a single set of conditions, either combustion of H₂/O₂/NO₂ or CH₄/O₂/NO₂, are examined by comparing predicted ignition delay times. The approach of modification iii, the removal of HNO₂ from the mechanism, predicts only modest variations in ignition delay time. The lumping of HNO₂ into HONO, modification iv, shows a significant decrease in the ignition delay time as compared with the other three variants.

As a result of this study, it should be acceptable to eliminate HNO₂ from models for lighter fuels. However, given previous work showing substantial growth in the HNO₂ branching fraction as the fuel molecule size increases, it is expected that the differences in pathway will be relevant for heavy fuels and that both isomers, HONO and HNO₂ should be included.