Ammonia combustion enhancement by co-firing with methanol and ethanol: Insight into laminar flame propagation and NO formation

Co-firing NH₃ with renewable e-fuels like methanol (CH₃OH) and ethanol (C₂H₅OH) has become an important strategy to enhance its combustion reactivity. The laminar burning velocities (LBVs) of NH₃/CH₃OH and NH₃/C₂H₅OH up to 10 atm are measured in a combustion vessel. A high-temperature kinetic model for NH₃/CH₃OH and NH₃/C₂H₅OH combustion is built. NO formation characteristics are also investigated using kinetic simulation. Kinetic insights into the impacts of reactive fuel co-firing, equivalence ratio, and pressure on combustion enhancement and NO formation are provided through modeling analyses. Using the modified fictitious diluent gas method, the chemical effect is more dominant than the thermal effect in raising the LBVs of NH₃. The CH₃OH and C₂H₅OH co-firing markedly increases the concentration of reactive radicals (H, O, and OH) and changes the dominant chemistry from NH₃ combustion to CH₃OH and C₂H₅OH combustion. The chemical effect also accounts for the remarkable non-monotonic tendency of NO formation as the co-firing fuel ratios increases. The product of NH and H concentrations exhibits a profile that can closely mirror the non-monotonic tendency of NO, which arises from the synergistic effect of NH and H.