Influence of Trace Nitrogen Oxides on Natural Gas Oxidation: Flow Reactor Measurements and Kinetic Modeling

The reactivity-promoting effect of trace nitrogen oxides (NOₓ) on post-induction oxidation of a synthetic natural gas (2% ethane in methane) has been experimentally studied in a high-pressure laminar flow reactor (HPLFR) at 10 ± 0.1 atm, nominal reaction temperature of 818 ± 5 K, and several equivalence ratios (φ ∼ 0.5, 1.0, and 2.0). Each set of experimental measurements was simulated using several literature C₀–C₂ + NOₓ kinetic models, both recent and legacy, using approaches shown to lead to robust interpretation of present experimental conditions. Coupling between the NOₓ and C₀–C₂ submodel components of these models varies significantly in both qualitative (mechanistic) and quantitative character. A comparison among the experimental measurements and modeling results serves to highlight important kinetic features particular to application-relevant natural gas oxidation in presence of trace (∼25 ppm) NOₓ. Additional insight is offered by a baseline experiment with no NOₓ perturbation, which shows that synthetic natural gas exhibits only incipient reactivity under the present φ ∼ 1.0 experimental condition. A comparison across experimental measurements and simulation results suggests that the reaction CH₃ + NO₂ ↔ CH₃O + NO, often cited as among the most important for NOₓ–natural gas coupling, insufficiently describes the principal net flux of NOₓ species at the relatively high pressures and low temperatures examined by present experiments. Simulation results indicate that accurate kinetics related to CH₃O₂ are necessary to describe a portion of NO ↔ NO₂ cycling driven by fuel fragment chemistry. Modeling suggests that the formation of nitromethane (CH₃NO₂) from the relatively large and long-lived CH₃ pool removes NOₓ from the pool of reactive intermediates, thus altering the reactivity initially imparted by trace NOₓ addition and the total pool of N atoms available as free NOₓ (NO + NO₂). Frequently used kinetic models that lack (accurate) CH₃O₂- and CH₃NO₂-related submodels predict trends in overall reactivity and NOₓ mole fractions that vary from quantitatively distorted to qualitatively incorrect. These disparities have significant implications for combustor design/evaluation computations that rely on several present literature kinetic models, particularly in a “single digit” parts per million of NOₓ regulatory environment.