Effect of CO2 Dilution on Methane/Air Flames at Elevated Pressures: An Experimental and Modeling Study

This study reports experimental and kinetic modeling results on the effects of CO2 dilution on laminar premixed methane/air flames, based on spherically propagating flames and one-dimensional adiabatic planar flame simulations, at elevated pressure. The laminar burning velocities of air mixtures with CH4 and CO2 at different dilution ratios were measured. In order to have a comprehensive understanding of the effects of CO2 dilution both chemically and physically, sensitivity analysis, chemical reaction rate analysis, and mole fraction analysis of active radicals were carried out using the Aramco 1.3 kinetic mechanism. The chemical impact of the addition of CO2 was segregated from its physical effects using fictitious species, FCO2 with the same thermochemical and transport characteristics as CO2, but does not participate in any chemical reactions. The CO2 dilution percentage varied was from 0 to 70% (by volume) to measure SLo at 300 K, 1 and 5 bar, and equivalence ratios (Φ) of 0.6 to 1.4. Results show that increasing CO2 dilution ratio decreases the SLo of these CH4/CO2/air mixtures. The results simulated using Aramco 1.3 illustrate that the chemical effect is less important at elevated pressure than at ambient pressure. The H and OH radical reduction rates are remarkably reduced at elevated pressure, where H radicals are primarily controlling the combustion. Elementary reactions with a negative sensitivity coefficient exhibit higher sensitivity toward pressure compared to the dilution effect, while elementary reactions with a positive sensitivity coefficient were equally sensitive to both pressure and dilution effects. The increase in initial pressure suppresses the peak net reaction rates for all the elementary reactions, with increasing CO2 concentration.