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The influence of MILD-to-flame transition on stabilization, reactive structures, and emissions of NH3/H2 mixtures in a semi-industrial furnace

par Rahmani, Ebrahim ;Cid Rodríguez, Natalia ;Kamal, Muhammad Mustafa ;Coussement, Axel ;Parente, Alessandro ;Lubrano Lavadera, Marco
Référence Combustion and flame, 284, page (114687)
Publication Publié, 2025-12-03

Article révisé par les pairs

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Résumé :

Ammonia (NH3) is a promising carbon-free fuel for decarbonizing energy systems, but its use in practical combustion systems is hindered by low flame stability and NOx emissions. Burning NH3 with hydrogen (H2) has been proposed to improve stability in conventional combustion systems; however, NOx emissions may persist or worsen. Moderate or Intense Low-Oxygen Dilution (MILD) combustion offers a pathway to suppress NOx through distributed reaction zones and reduced peak temperatures. The aim of this study is to stabilize pure NH3 and characterize it with the H2 addition in a semi-industrial reverse-flow furnace under MILD conditions. The ex- periments demonstrated the stabilization of pure NH3 under MILD conditions without reactive enhancers, resulting in negligible NOx emissions but significant NH3 slip. The impact of H2 addition was assessed by analyzing how the transition from MILD to flame influences emissions. A transition from MILD to a lifted flame occurred at ~14 % H2, marked by a sharp rise in NOx and a steep decline in NH3 slip. An optimal trade-off was observed at 12 % H2, where NH3 slip decreased from 2626 to 1336 ppm, accompanied by only a 12 ppm increase in NO, while maintaining MILD conditions. Decreasing the furnace temperature extended MILD combustion to 20 % H2, but compared to the 12 % H2, it caused higher NH3 slip and only a slight reduction in NO, highlighting a trade-off between temperature control and NH3 decomposition. The experimental findings were analyzed from a chemical kinetic viewpoint using a chemical reactor network approach. The results showed that NO reduction at H2≤20 % was dominated by thermal DeNOx, while NO formation at H2≤80 % primarily originated from fuel- bound nitrogen. These findings advance the understanding of NH3-H2 MILD combustion at realistic scales and provide insight into the design of low-emission ammonia-based systems.