Investigation of high-pressure methane and syngas autoignition delay times

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Karimi, Miad
Ranjan, Devesh
Sun, Wenting
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This thesis reports methane (CH4) and a syngas mixture (H2/CO=95:5) autoignition delay measurements relevant to operating conditions of supercritical carbon dioxide (sCO2) power cycle (100 to 300 bar) combustors. To acquire data at these conditions as part of this thesis, a new high-pressure shock tube is designed, fabricated and commissioned. The experiments are conducted for diluted carbon dioxide environments at 100 and 200 bar and at temperatures within the range of approximately 1100–1400 K. To investigate the chemical effect of CO2 at supercritical conditions, experiments are conducted at similar pressures and temperatures by substituting CO2 with an inert bath gas, Ar (argon). Obtaining ignition delay times in Ar bath gas allows to systematically study the chemical effect of CO2 on ignition chemistry. Methane ignition delay times are compared to several chemical kinetic models, such as Aramco 2.0, FFCM-1, HP-Mech, USC Mech II and GRI 3.0. For the conditions of this study, predictions of the Aramco 2.0 kinetic model show the overall best agreement with experimental measurements. Following the experimental data, brute-force sensitivity analyses and reaction pathway flux analyses are utilized to gain insight into details of the ignition chemistry of the fuels (CH4 and H2/CO=95:5). These analyses indicate that methyl (CH3) recombination to form ethane (C2H6) and oxidation of CH3 to form methoxide (CH3O) are the most important reactions controlling the ignition behavior of methane at temperatures greater than approximately 1250 K. However, at temperatures below approximately 1250 K, an additional reaction pathway for methyl radicals is found through CH3+O2+M=CH3O2+M, which leads to formation of methyldioxidanyl (CH3O2). This reaction pathway plays a distinct role in dictating the ignition trends at lower temperature conditions. Replacing CO2 with argon as the bath gas reveals that CO2 does not have major effects on ignition chemistry of CH4. A similar approach is taken to obtain experimental data at 100 bar and 200 bar for a syngas fuel mixture of 95% H2 (hydrogen) and 5% CO (carbon monoxide) in CO2 and Ar bath gasses. Aramco 2.0 kinetic model, FFCM-1 kinetic model, HP-Mech and USC Mech II show good agreement with the measured ignition delay times. Detailed sensitivity analyses of these kinetic models highlight the importance of the third-body reaction between hydrogen atoms (H) and oxygen molecules (O2) through H+O2+M=HO2+M to form hydroperoxyl (HO2). In both cases, irrespective of the diluents, this reaction is the most influential reaction to hinder ignition. Ignition delay times obtained from both mixtures not only show a similar trend, but also the same magnitude when compared to the CO2 mixture. While this observation may suggest that CO2 has no chemical effect on ignition chemistry, it is found to play a counterbalancing role on syngas ignition at the elevated pressures and temperatures of this study. CO2 increases the OH (hydroxyl) radical production by colliding with hydrogen peroxide (H2O2) through H2O2+M=OH+OH+M. However, it reduces OH production through HO2+H=OH+OH due to a lower amount of H radical production compared to the Ar mixture. Therefore, these two effects cancel out the change of OH productions, and CO2 does not change the ignition delay time of the syngas mixture considered in this study upon comparison with the mixture with Ar bath gas.
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