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Experiments were performed in a shock-tube facility to examine experimentally the kinetic effect, if any, of excess amounts of CO 2 as part of natural-gas-based fuel−oxidizer mixtures. An important aspect of these experiments was to also observe the role excess amounts of CO 2 play in causing nonidealities, particularly shock bifurcation, in shock-tube experiments using real (nondilute) fuel−air mixtures. Mixtures were composed of methane fuel at an equivalence ratio of 0.5 to represent a typical natural gas in a modified "air" mixture designed to study the effect of large levels of CO 2 dilution. These oxidizer compositions maintained constant levels of O 2 while exchanging N 2 for CO 2 in stages to give oxidizer mixture concentrations ranging from (0.21O 2 + 0.79N 2 ) to (0.21O 2 + 0.79CO 2 ). Low-pressure and high-pressure (near 1 and 10 atm, respectively) experiments were conducted over an approximate temperature range of 1450 to 1900 K. Results showed that the observed effect of CO 2 relating to reflected-shock bifurcation was quite significant, giving stronger bifurcation as amounts of CO 2 increased, as determined by a sidewall pressure transducer. Despite the presence of significant reflected-shock bifurcation in the mixtures containing high levels of CO 2 , the resulting ignition delay times were commensurate with the results expected if one were to assume the test conditions were at the inferred temperature and pressure immediately behind the reflected shock wave. That is, the main ignition events occurred in the gas closest to the endwall, where the effects of the shock bifurcation were minimal for the ignition delay time range of the present study. When the ignition delay times for mixtures with and without CO 2 dilution were compared, the effect of the CO 2 was minimal and within the uncertainty of the data, particularly for the experiments near 1 atm. A small effect of CO 2 addition was seen for the higher pressure near 10 atm, with a general increase in ignition delay time for the largest levels of CO 2 dilution. Predictions from a modern chemical kinetics model also showed a minimal effect of CO 2 addition on the methane ignition delay times, in agreement with the shock-tube data.

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The composition of a representative coal-derived syngas was determined by averaging 40 practical coal syngas compositions from the literature and corresponds to a departure from many recent studies which only focus on syngas blends containing just CO and H 2. Ignition delay times have been measured behind reflected shock waves for this averaged mixture with an equivalence ratio of 0.5 (0.4554% CO / 0.3297% H 2 / 0.1032% CO 2 / 0.0172% CH 4 / 0.2407% H 2 O / 0.8538% O 2 in 98% Ar (mol. %)) at around 1.7, 13, and 32 atm. The same mixture was also investigated with impurities (200 ppm of NH 3 and 50 ppm of H 2 S). Care was taken when working with the blends containing H 2 O and NH 3 to avoid errors in the shock-tube composition; direct measurement of the water vapor mole fractions were performed using a tunable diode laser absorption diagnostic near 1.38 m. The effect of the various constituents on the ignition delay time was also investigated by comparing to results from a baseline mixture (H 2 /CO/O 2 /Ar) and results with this baseline mixture with only one of the other constituents of the syngas (i.e. CO 2 , CH 4 , H 2 S). Experimental data were compared with recent detailed kinetics mechanisms from the literature. Results showed that, under the conditions of this study, extending the mixture composition to include realistic concentrations of species beyond just the CO and H 2 does not have a very large effect on the ignition delay time for a coal-derived syngas. However, a comparison of this coal-derived syngas with a syngas derived from biomass, tested in an earlier study by the authors, exhibited large differences due to the larger CH 4 concentration in the bio-derived syngas. Two chemical kinetic models from the literature were found suitable to reproduce these data over most of the range of mixtures, temperatures, and pressures investigated, namely the mechanisms associated with Galway and with Princeton.

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