High pressure ignition delay times of H2/CO mixture in carbon dioxide and argon diluent

Abstract

Ignition Delay Time (IDT) plays a significant role in combustion process of advanced power cycles such as direct-fired supercritical carbon dioxide (sCO2) cycle. In this cycle, fuel and oxidizer are heavily diluted with carbon dioxide (CO2) and autoignite at a combustor inlet pressure range of 10–30 MPa and a temperature range of 900–1500 K. A fuel candidate for sCO2 power cycle applications is syngas (H2/CO mixture); however, its ignition properties at these conditions are not studied. Moreover, the existing chemical kinetics models have not been evaluated for H2/CO mixtures applications relevant to elevated pressure conditions and under large dilution levels of CO2. Therefore, two tasks are performed in this study. First, IDTs of a H2/CO=95:5 mixture at stoichiometric and rich (Φ=2) conditions are measured in a high-pressure shock tube under 95.5% CO2 dilution level and at 10 MPa and 20 MPa for a temperature range of 1161–1365 K. For the experimental conditions considered in this work, Aramco 2.0, FFCM-1, HP-Mech and USC Mech II kinetic models are capable of capturing IDT data. Second, similar experiments are conducted by replacing the CO2 dilute gas with Argon (Ar) to understand the chemical effect of CO2 on IDT globally. Sensitivity analysis results reveal that for both diluents, reaction H + O2(+M)=HO2(+M) is the most important reaction in controlling ignition. Further, a rate of production analysis shows that CO2 has a competing effect on OH radical production. On one hand, CO2 accelerates the consumption of H radicals through H + O2+CO2→HO2+CO2 therefore hindering HO2+HOH+OH reaction for OH production. On the other hand, CO2 is shown to enhance OH production through H2O2+M=OH+OH+M. These kinetic effects from CO2 cancel out, therefore CO2 does not significantly alter the IDT globally when compared to the Ar bath case. This is confirmed by both experimental results and simulation.