Ignition of CH4 intensely diluted with CO2 versus hot O2/CO2 with high oxygen concentration in a counterflow jets

Abstract

Lean combustion has wide range of applications due to very low emissions and very high efficiency. The study of the ignition of the fuel under dilution conditions is crucial for the satisfactory operation and performance of dilution fuel combustion processes. In present study, the ignition temperatures of CH4 diluted with CO2 were measured in a counterflow ignition facility for the mixtures of 9–25% of CH4 in CO2 and 30%, 35%, and 40% oxygen concentration in the O2/CO2 atmospheres, respectively. The experimental results show that for a fixed fuel concentration, the higher the oxygen concentration, the lower the ignition temperature. The differences between ignition temperatures of CH4 for various oxygen concentration cases decrease notably with the decrease of the CH4 concentration, and are not obvious when the fuel is intensively diluted at 9%. A mechanism has been updated based on our previous research work and named as IDIM (Intensive dilution ignition mechanism). The performance of the USC Mech 2.0, Aramco Mech 2.0, FFCM-1 and IDIM has been evaluated. IDIM can predict well the ignition temperatures of three different oxygen concentrations, while FFCM-1 markedly over-predicts the experimental results for all the cases. The effects of the oxidation pathways of CH3, CH3→CH3O, CH3→CO, CH3→CH2O and CH3→CH2(S), on the ignition temperature have been analyzed in detailed. The pathway of CH3→CH3O→CH2O plays an important role on the ignition when the oxygen concentration is high in the oxidant and the fuel is intensely diluted. Weakening the path of CH3→CH3O→CH2O is the main reason for the over-prediction of FFCM-1. Moreover, the effect of the fuel dilution on the ignition temperature is stronger than that of the oxygen concentration at the fuel concentration of 9%, resulting in that the conversion ratios of CH3 to CH3O, CH2O, CH2(S), and CO are almost same for 30% and 40% oxygen concentrations. Therefore, the ignition temperature is almost insensitive of the oxygen concentration within the range of our experiment.