Methane Ignition in a Shock Tube with High Levels of CO2 Dilution
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For decades Exhaust Gas Recirculation (EGR) has been used to mitigate several issues related to gas turbine operation: CO2 sequestration; NOx formation and emission; and combustor instabilities. EGR increases CO2 concentrations in turbine exhaust for more efficient CO2 scrubbing, reduces NOx emissions, and reduces combustor instability associated with pressure resonances. As EGR technology has developed, EGR ratios have continued to increase and introduce greater amounts of combustion products, primarily CO2, as part of the oxidizer in gas turbines. The goal of this study was twofold: to observe the role excess amounts of CO2 play in causing non-idealities, bifurcation in particular, in shock-tube experiments using real (non-dilute) fuel-air mixtures, and to experimentally examine the kinetic effect, if any, of excess amounts of CO2 as part of natural gas fuel-oxidizer mixtures. Experiments were performed in a shock-tube facility on the campus of Texas A&M University. Mixtures were composed of a representative natural gas mixture at an equivalence ratio of ϕ = 0.5 using modified oxidizer compositions representative of those used in EGR turbines. These oxidizer compositions maintained constant levels of O2 while exchanging N2 for CO2 in stages to give oxidizer mixture concentrations ranging from (0.21O2+0.79N2) to (0.21O2+0.79CO2) with intermediate combinations of N2/CO2 in between. Low-pressure and high-pressure (near 1 atm and 10 atm, respectively) experiments were conducted over an approximate temperature range of 1450-1900 K for the simulated EGR mixtures. Upon conclusion of all experiments and analyses, the observed effect of CO2 relating to reflected-shock bifurcation was quite significant, giving stronger bifurcation as amounts of CO2 increased, as determined by a sidewall pressure transducer. However, the observed kinetic effect of CO2 on ignition delay time was quite small in comparing ignition delay times with and without CO2. A modern chemical kinetics model also predicts that the effect of CO2 dilution on methane ignition delay times at the conditions herein are very small, within the uncertainty of the experiments. This result helps to confirm the validity of the measured results. One can also conclude that despite the significant bifurcation and proportionately increased uncertainty in the test conditions as a result, the ignition delay time results herein seem to indicate that the test conditions are still at the nominal temperature and pressure as derived from the speed of the incident shock wave in the conventional way.
Hargis, Joshua W. (2015). Methane Ignition in a Shock Tube with High Levels of CO2 Dilution. Master's thesis, Texas A & M University. Available electronically from