A Methane Mechanism for Oxy-Fuel Combustion: Extinction Experiments, Model Validation, and Kinetic Analysis

Abstract

While fuel combustion in oxygen-enriched environments provides a number of significant advantages, such as reduced nitrogen oxide emissions and high carbon dioxide purity for carbon sequestration, it is characterized by different physico-chemical oxidation behavior than combustion in air. Compared to nitrogen, carbon dioxide has different specific heat and effective Lewis number, and is chemically more active. Therefore, chemical mechanisms developed for the oxidation of fuel/air mixtures can fail to predict targets of interest for oxy-combustion accurately. In this study, a chemical mechanism of methane, which has been previously validated with data from experiments using air, is evaluated in terms of its prediction accuracy at oxy-conditions by comparing against available literature data. The validation takes various combustion properties into account, including ignition delay times, laminar burning velocities, and extinction strain rates, and covers a wide range of experimental conditions with respect to temperature, pressure, equivalence ratio, and carbon dioxide concentration. As additional targets, extinction strain rates of non-premixed oxy-methane flames are determined in a counterflow burner at conditions, where literature data have not yet been reported. The extensive validation demonstrates that the mechanism is able to describe oxy-methane combustion with reasonable prediction accuracy. For further insights into the underlying kinetics of diffusion flames of methane in oxy-atmosphere compared to its oxidation in air, reaction path and sensitivity analyses are performed using the validated mechanism. Notable differences between both combustion regimes are observed in the branching ratios of H-abstraction reactions by OH and H radicals and in the consumption channels of singlet methylene, which is a key species in the formation of polycyclic aromatic hydrocarbons.