Ignition Chemistry of Syngas Highly Diluted in CO2
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AbstractSyngas is a desirable, high-hydrogen fuel source for combustors utilizing the Allam-Fetvedt cycle involving supercritical-CO2 (sCO2). Minimal data are available with high concentrations of CO2, with much of the available data being few in number and reported over very narrow temperature ranges. Considerable model disagreements have been shown at intermediate pressures (10100 atm), while good agreement is seen for higher pressures (< 100 atm). Further examination of literature data highlights that the ignition delay time (IDT) characteristics of syngas at these pressures show little dependence on pressure, mixture composition, and equivalence ratio. To verify these observations, literature experiments were replicated using a high-pressure shock tube facility. Ignition delay time data were collected for syngas mixtures for pressures of 20 and 40 atm with 85% CO2 mixtures at stoichiometric conditions and H2:CO fuel ratios of 1:1 and 1:4. Literature results are limited to IDTs less than 500 s, and data presented herein expand these data to considerably longer IDTs over a wider temperature range. Some disagreement with literature data is seen, and sources of discrepancy from the literature results are discussed. However, similar trends are seen for syngas ignition delay time characteristics and chemical kinetic models do not replicate this behavior. In particular, AramcoMech 2.0 replicates IDT near 40 atm for a H2:CO fuel ratio of 1:1, but is significantly under reactive for the 1:4 fuel ratio. To this end, a detailed sensitivity analysis using the AramcoMech 2.0 chemical kinetics mechanism highlights important chemical reactions. Of these reactions, significant model improvements are shown using the reaction rate suggested by Tsang and Hampson for CO+HO2 CO2+OH.
