Experimental and numerical study of CH$_4$/CH$_3$Cl/O$_2$/N$_2$ premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH$_3$Cl addition on CH$_4$/O$_2$/N$_2$ premixed flames under oxygen enrichment. The laminar flame speeds of CH$_4$/CH$_3$Cl/O$_2$/N$_2$ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH$_3$Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH$_3$Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH$_3$Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H$_2$ $\to$ H + H$_2$O reaction is reduced because of the reduction of OH concentration. However, the function of CH$_3$Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5.