Measurement of methane autoignition delays in carbon dioxide and argon diluents at high pressure conditions

Abstract

The directly fired supercritical carbon dioxide (sCO2) power cycle has high efficiency while allowing nearly complete carbon dioxide (CO2) capture. The operating conditions of sCO2 power cycle (100–300 bar) combustors are dramatically different from conventional gas turbine combustors. However, combustion properties such as autoignition delay are not well understood at these conditions. This study reports methane autoignition delay measurements for diluted carbon dioxide environments at 100 and 200 bar and at temperatures within the range of 1139–1433 K using a high pressure shock tube. To study the effect of CO2 on ignition, similar experiments are conducted at 100 and 200 bar by replacing carbon dioxide with argon. The experimental data is then compared with calculations using different chemical kinetics models. For the conditions of this study, predictions of the Aramco Mech 2.0 show the overall best agreement with experimental measurements, while predictions of the GRI 3.0 kinetic model have the largest (by a factor of 3) deviation with experiments. Sensitivity and reaction pathway analyses reveal that methyl (CH3) recombination to form ethane (C2H6) and oxidation of CH3 to form methoxide (CH3O) are the most important reactions controlling the ignition behavior at temperatures greater than approximately 1250 K. However, at temperatures below approximately 1250 K, an additional reaction pathway for methyl radicals is found through CH3+O2+M = CH3O2+M which leads to formation of methyldioxidanyl (CH3O2). This reaction pathway plays a distinct role in dictating the ignition trends at lower temperature conditions.