Modeling Study of the Impact of Blending N2, CO2, and H2O on Characteristics of CH4 Laminar Premixed Combustion

N-2, CO2, and H2O are impurity components in the biogas fuel production process, which are added at an appropriate amount to the combustion of hydrocarbon fuel that can effectively reduce the emissions of NOx and soot precursors [polycyclic aromatic hydrocarbons (PAHs)]. In this paper, using N-2, CO2, and H2O as dilution gas, the CHEMKIN-II/PREMIX code with detailed chemical reaction mechanism GRI-Mech 3.0 was chosen to calculate the premixed combustion characteristics and NOx emissions of CH4. At different equivalence ratios (Phi = 0.8, 1.0, and 1.2) and blending ratios (0-40%), the physical and chemical effects of different dilution gases were systematically studied through the introduction of hypothetical substances FN2, FCO2, and FH2O. The results show that the laminar burning velocity and adiabatic flame temperature of CH4 were decreased by adding N-2, CO2, and H2O, and the influence of the three diluents increases with the increase of the blending ratios, following the order of CO2 > H2O > N-2. Moreover, the physical and chemical effects are greatest at stoichiometric conditions, and physical effects are much greater than chemical effects. In particular, at Phi = 1.2, the chemical effect of H2O leads to the adiabatic flame temperatures of CH4 gradually increasing as the dilution ratio (D-r) increases. The sensitivity analysis of the main elementary reactions, which play a leading role in the influence of NO generation, shows that the adiabatic flame temperature of CH4 is reduced after adding N-2, CO2, and H2O, depressing the generation of NO. The generation path of NO is mainly prompt NOx, and the responsible reactions are reactions R38, H + O-2 reversible arrow O + OH; R240, CH + N-2 = HCN + N; and R52, H + CH3 (+M) reversible arrow CH4 (+M).