Significance of the HO2+CO Reaction during the Combustion of CO+H2 Mixtures at High Pressures
Authors: G. Mittal, C. J. Sung, M. Fairweather, A. S. Tomlin, J. F. Griffiths, and K. J. Hughes
Direct link to the paper: http://dx.doi.org/10.1016/j.proci.2006.07.068
This paper constitutes an experimental and numerical study, using uncertainty analysis of the most important parameters, to evaluate the mechanism for the combustion of CO + H2 mixtures at high pressures in the range 15–50 bar and temperatures from 950 to 1100 K. Experiments were performed in a rapid compression machine. Autoignition delays were measured for stoichiometric compositions of CO + H2 containing between 0 and 80% CO in the total fuel mixture. The experimental results showed an unequivocal monotonic increase as the proportion of CO in the mixture was raised. Comparisons were made also with the measured ignition delays in mixtures of H2 with increasing dilution by N2, corresponding to the proportions of CO present. These times also increased monotonically, albeit with a greater sensitivity to the extent of dilution than those measured in the CO + H2 mixtures. By contrast, numerical simulations for the same mixtures, based on a kinetic model derived by Davis et al. displayed a qualitative discrepancy as there was virtually no sensitivity of the ignition delay to the changing ratio of CO + H2, certainly up to 80% replacement. No exceptions to this trend were found, despite tests being made using seven other kinetic models for CO + H2 combustion. Global uncertainty analyses were then applied to the Davis et al. model in order to trace the origins of this discrepancy. The analyses took into account the uncertainties in all rate parameters in the model, which is a pre-requisite for evaluation against ignition delay data. It is shown that the reaction rate constant recommended by Baulch et al. for the HO2 + CO reaction, at T ~ 1000 K, could be up to a factor of 10 too high and that lowering this rate corrected the qualitative anomaly between experiment and numerical prediction.
Citation: G. Mittal, C. J. Sung, M. Fairweather, A. S. Tomlin, J. F. Griffiths, and K. J. Hughes, “Significance of the HO2+CO Reaction during the Combustion of CO+H2 Mixtures at High Pressures,” Proceedings of the Combustion Institute 31, 419-427 (2007).