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Experimental and Numerical Study of the Effect of CO₂ on the Ignition Delay Times of Methane under Different Pressures and Temperatures
- Liu, Yang, Zou, Chun, Cheng, Jia, Jia, Huiqiao, Zheng, Chuguang
- Energy & fuels 2018 v.32 no.10 pp. 10999-11009
- carbon dioxide, combustion, ethane, fuels, kinetics, methane, models, nitrogen, temperature
- Pressurized oxy-fuel combustion is regarded as a new generation of oxy-fuel technology. The ignition delay times of methane in an O₂/N₂ atmosphere (0.21O₂ + 0.79N₂) and an O₂/CO₂ atmosphere (0.21O₂ + 0.79CO₂) were measured in a shock tube at a pressure of 0.8 atm, an equivalence ratio of 0.5, and within a temperature range of 1501–1847 K. The present experimental data and the experimental data of Hargis and Peterson at 1.75 and 10 atm were adopted to evaluate five representative chemical kinetic models. This paper studied the chemical effects (chaperon effects of CO₂ and the effects of reactions containing CO₂) and physical effects of CO₂ on the ignition of methane at different pressures and temperatures in detail using a modified model. Artificial materials X and Y were employed to analyze the chemical and physical effects. The analysis showed that the physical effects of CO₂ inhibit the ignition of methane and are not sensitive to the temperature. The chemical effects of CO₂ vary greatly with the pressure and temperature. At 0.8 and 1.75 atm, the chemical effects of CO₂ promote the ignition of methane at a high temperature while suppress the ignition of methane at a low temperature. The chaperon effects of CO₂ promote the ignition of methane in O₂/CO₂ atmospheres at a high temperature mainly because of HCO + M ⇔ CO + H + M. The chaperon effects of CO₂ suppress the ignition of methane at a low temperature because of 2CH₃ (+M) ⇔ C₂H₆ (+M). The chemical effects of CO₂ offset half of the physical effects of CO₂ at a high temperature, and those two effects are great at a low temperature, which is the reason for the fact that the effect of CO₂ is subtle at a high temperature and evident at a low temperature. At 10 atm, the chemical effects of CO₂ suppress the ignition of methane at 1350–1700 K. The chaperon effects of CO₂ suppress the ignition of methane mainly as a result of 2CH₃ (+M) ⇔ C₂H₆ (+M) and are strengthened with the decrease of the temperature. The inhibition of reactions involving CO₂ is mainly attributed to CO + OH ⇔ CO₂ + H and weakened with the decrease of the temperature; thus, the chemical effects of CO₂ on the ignition are almost not sensitive to the temperature. The effects of CO₂ have almost not change with the temperature at 10 atm.