Characterization of moderate or intense low-oxygen dilution (MILD) combustion regimes and NOX emissions in ammonia and hydrogen-enriched ammonia mixtures

This study examines the combustion behavior of ammonia (NH3) and hydrogen (H2) enriched NH3 mixtures under MILD conditions using a Perfectly Stirred Reactor (PSR) model with a validated kinetic mechanism. A parametric analysis identifies the main factors controlling regime transitions and NOX emissions. H2 addition enhances mixture reactivity, shifting autoignition to lower temperatures and reducing the critical oxygen concentration ((Formula presented)) from 12.37% (NH3) to 6.37% (H2). It also lowers the inlet temperature required for MILD combustion and extends the oxygen-dilution range for low-NOX emissions (<100 ppm) from ∼5% to ∼10%. The equivalence ratio (Φ) significantly affects regime and emissions: lean conditions (Φ = 0.5) yield the widest MILD regime (23.2%) but require strong dilution for low-NOX, while rich conditions (Φ = 1.3) re-expand the MILD regime (13.79%) and provide a broader low-NOX window (1 - 12% O2). Higher pressure further compresses the MILD regime, reduces inlet temperature, and suppresses NOX. These findings highlight effective strategies for cleaner NH3-based combustion.