A numerical study on the effect of CO₂ addition for methane explosion reaction kinetics in confined space

To explore the influence of the CO₂ volume fraction on methane explosion in confined space over wide equivalent ratios, the explosion temperature, the explosion pressure, the concentration of the important free radicals, and the concentration of the catastrophic gas generated after the explosion in confined space were studied. Meanwhile, the elementary reaction steps dominating the gas explosion were identified through the sensitivity analysis. With the increase of the CO₂ volume fraction, the explosion time prolongs, and the explosion pressure and temperature decrease monotonously. Moreover, the concentrations of the investigated free radicals also decrease as the increase of the CO₂ volume fraction. For the catastrophic gas, the concentration of the gas product CO increases and the concentrations of CO₂, NO, and NO₂ decrease as the volume fraction of CO₂ increases. When 7% methane is added with 10% CO₂, the increase rate of CO is 76%, and the decrease rates of CO₂, NO, and NO₂ are 27%, 37%, and 39%, respectively. If the volume fraction of CO₂ is constant, the larger the volume fraction of methane in the blend gas, the greater the mole fraction of radical H and the lower the mole fraction of radical O. For radical OH, its mole fraction first increases, and then decreases with the location of peak value of 9.5%, while the CO concentration increases with the increase of the methane concentration. For all the investigated volume fraction of methane, the addition of CO₂ reduces the sensitivity coefficients of each key elementary reaction step, and the sensitivity coefficient of reaction promoting methane consumption decreases faster than that of the reaction inhibit methane consumption, which indicates that the addition of CO₂ effectively suppresses the methane explosion.