Laminar Flame Speeds of Oxy-Methane Mixtures with CO~2~ Dilution at Elevated Pressures

Due in part to increased efforts in the development of carbon capture and sequestration systems in power generation, interest in natural-gas-fueled cycles like the Allam cycle has highlighted the lack of knowledge of the fundamental combustion chemistry on which these systems depend. The Allam cycle relies on carbon dioxide as the working fluid, with oxy-methane combustion occurring with high levels of carbon dioxide. Carbon dioxide is chemically active at these temperatures, and its effect on oxy-methane chemistry at high pressure needs to be further explored.

To remedy this shortcoming, spherically expanding, laminar flame experiments have been conducted at Texas A&M University (TAMU) for oxy-methane mixtures diluted in carbon dioxide. Test conditions consisted of pressures of up to 10 bar, an ambient initial temperature of 298K, and a full range of equivalence ratios from lean to rich. Because of hydrodynamic instabilities, spherically expanding flame fronts tend to transition from laminar to cellular during propagation, especially at higher pressure. It is possible to postpone this transition by doping the parent gas mixture with helium, which has the effect of increasing the effective Lewis number and thereby stabilizing the flame. This stabilization allows access to high-pressure laminar flame speed measurements in mixtures that otherwise would become cellular prohibitively early during flame propagation. The mixtures tested in this study contained helium for this purpose. Helium is chemically inert and therefore has no effect on the chemistry, aside from its behavior as a third-body collider.

The facility used in this work was the Turbulent Flame Speed Vessel (TFSV) at Texas A&M University: a constant-volume, stainless steel cylindrical chamber with two opposing pairs of five-inch diameter, ultraviolet-grade quartz windows. The TFSV is equipped with removable impellers for generating turbulent flow fields inside the chamber for turbulent flame experiments. Herein, the impellers were only used to stir the gas after completion of the filling process to ensure a homogeneous gas mixture, and the gas was allowed to stagnate for several minutes prior to ignition. Ignition was achieved through application of a high-voltage pulse to stainless steel electrodes whose spark gap is positioned at the center of the chamber.

Images of the spherically expanding flame were captured during the constant-pressure region of flame propagation. The imaging system used in this study was a high-speed schlieren imaging diagnostic.

The full manuscript includes new laminar flame speed data for a range of pressures up to 10 bar and equivalence ratios from lean to rich, and comparisons to existing chemical kinetics models, including AramcoMech 2.0 and GRI 3.0.


Laminar Flame Speeds of Oxy-Methane Flames With CO2 Dilution at Elevated Pressures

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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