MILD Combustion of Partially Cracked Ammonia: Effects of Fuel Cracking, Steam Dilution, and Oxidizer Preheating

Abstract Ammonia is a promising carbon-free energy carrier, and growing concerns over fossil fuel supply security further motivate its adoption; however, its application in combustion systems remains constrained by low reactivity and high NOx emissions. While ammonia cracking enhances fuel reactivity and Moderate or Intense Low-oxygen Dilution (MILD) combustion enables low-emission operation, the reactive structure and ignition behavior of partially cracked ammonia under MILD conditions remain insufficiently understood, particularly with respect to oxidizer steam dilution and preheating. This study presents a validated, three-dimensional RANS-based CFD investigation of a MILD combustion furnace using detailed chemical kinetics (CEU-NH3 mechanism with dynamic reduction). A total of 54 operating conditions are examined by systematically varying the cracking ratio (γ = 0.0–0.8), steam-to-air ratio (SAR = 0.0–0.3), and oxidizer preheating temperature (300–700 K) at fixed equivalence ratio and thermal power. The results show that no self-sustained combustion is achieved at very low cracking ratios (γ = 0.0–0.1), consistent with ignition delay times reported in the literature that are an order of magnitude longer than at γ = 0.2, due to insufficient fuel reactivity. For ignited cases, increasing γ enhances reactivity and heat release up to γ ≈ 0.4–0.5, leading to upstream ignition, higher peak temperatures, and more compact reaction zones, while further increases cause thermal saturation due to NH3 depletion and dilution effects. Steam dilution broadens the reaction zone, improves temperature uniformity, and promotes distributed MILD-like combustion; however, at high cracking ratios (γ ≥ 0.6), steam is found to promote NO formation, despite its thermal diluent effect. All examined cases fall within the MILD-like regime based on reaction zone inlet conditions, a classification confirmed to be robust across a 6-fold variation in the reaction zone boundary threshold. These findings clarify the coupled roles of cracking, steam dilution, and preheating in controlling ignition, distributed reaction characteristics, and NOx emissions in partially cracked ammonia MILD combustion.