Ignition and oxidation of lean CO/H2 fuel blends in air
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Ignition and oxidation characteristics of CO/H2 fuel blends were studied using both experimental and computer simulation methods. Shock-tube experiments were conducted behind reflected shock waves at intermediate temperatures (890 < T < 1300 K) for three pressure regimes of approximately 1, 2.5, and 15 atm Results of this study provide the first undiluted fuel-air ignition-delay-time experiments to cover such a wide range of CO/H2 composition (5-80% H2) over the stated temperature range. Emission in the form of chemiluminescence from the hydroxyl radical (OH*) A2+ X2 transition near 307 nm was used to monitor the reaction progress from which ignition delay tunes were determined. In addition to the experimental analysis, chemical kinetics calculations were completed to compare several chemical kinetics mechanisms with the new experimental results. The models were in excellent agreement with the shock-tube date, especially at higher temperatures and lower pressures, yet there were some differences between the models at the higher pressures and lowest temperatures, in some cases by as much as a factor of 5. Ignition-delay-time and reaction-rate sensitivity analyses were completed at higher and lower temperatures and higher and lower pressures to identify the key reactions responsible for ignition. The results of the sensitivity analysis indicate that the ignition-enhancing reaction H + O2 = O + OH and hydrogen oxidation kinetics in general were most important, regardless of mixture composition, temperature, or pressure. However, lower-temperature, higher-pressure ignition-delay-time results indicate additional influence from HO2- and CO- containing reactions, particularly, the well-known H + O + M = HO2 + M reaction and the CO + O + M = CO2 + M and CO + HO2 = CO2 + OH reactions. Differences in the rates of the CO-related reactions are shown to be the cause of discrepancies among the various models at elevated pressures. Additional calculations were performed to show that the mixtures used are insensitive to small levels of water vapor, and the disagreement between experiment and mode) at the lowest temperatures and higher H2 concentrations cannot be explained by possible impurities.
