A detailed numerical analysis of NOx formation and destruction during MILD combustion of CH4/H2 blends using a skeletal mechanism

Moderate and Intense Low-Oxygen Dilution (MILD) combustion is an advanced technology that enhances combustion efficiency while simultaneously reducing NOx emissions. Despite extensive research, the effect of H2 addition in methane on various NOx formation pathways during MILD combustion remain unclear, particularly for methane/hydrogen (CH4/HB) blends. This study investigates the influence of hydrogen addition on NOx formation and destruction pathways in CH4/HB blends using a well-stirred reactor (WSR). Hydrogen concentrations in the fuel range from 0 to 100%, with oxygen mass fractions between 3% and 9% and equivalence ratios spanning 0.5 to 1.5, to ensure a comprehensive analysis of NOx formation. To analyze the key reactions involved in NOx formation and destruction during MILD combustion, the GRI 2.11, GRI 3.0 and PG-mech mechanisms are reduced into a skeletal mechanism. A detailed analysis of direct and indirect effects of H2 addition in methane is carried out based on sensitivity and rate of production analysis of different NOx formation reactions through on a comparison between several mechanisms. The most important reactions for NOx formation and CO reduction are CH3+NO HCN + HBO, H + NO + MHNO + M and OH + CO H + CO2 respectively. The CO emissions are sensitive towards equivalence ratio and increase significantly under fuel-rich conditions. The relative contribution analysis indicates that hydrogen addition and O2 mass fraction has high influence on NNH route. The prompt route is suppressed when hydrogen content exceeds 50%. The hydrogen addition in methane has marginal influence on NBO intermediate route and it more related to oxygen concentration in the oxidizer. Moreover, NO-reburning is significantly reduced with increasing hydrogen content in the fuel and reactor temperature. These findings provide critical insights into the mechanisms governing CO and NOx formation during MILD combustion of CH4/HBblends.