Deviation from ideal RQL operation: Diffusion-stabilized secondary flame and its impact on NOx in an axial-staged ammonia-methane combustion

Wang, Ping, Dai, Kailun, Li, Zhuang, Zhang, Zeyu, Valera-Medina, Agustin ORCID: https://orcid.org/0000-0003-1580-7133, Ferrante, Antonio, Qi, Haotian, Li, Ningyi and Qian, Weijia 2026. Deviation from ideal RQL operation: Diffusion-stabilized secondary flame and its impact on NOx in an axial-staged ammonia-methane combustion. Combustion and Flame 289 , 114985. 10.1016/j.combustflame.2026.114985

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Abstract

The implementation of Rich-Quench-Lean (RQL) combustion is a promising strategy to address the challenges of stability and high NOx emissions in ammonia-fired gas turbines. However, the complex flame structure and NOx formation mechanisms in multi-stage ammonia combustors are not yet fully understood. This study combines experimental measurements and Large Eddy Simulations (LES) to investigate the combustion characteristics and NO formation mechanisms in an axially staged ammonia-methane turbulent combustor. Under a fixed primary equivalence ratio of 1.2, the overall equivalence ratio is varied from 0.4 to 0.9. Experiments reveal that lower overall equivalence ratios lead to higher NO emissions. This trend is particularly pronounced at ϕtotal = 0.5, where the increased secondary airflow exerts a stronger upstream influence on the primary flame, compressing its structure and locally shifting the equivalence ratio, thereby elevating NO formation. Analysis of the flame structure identifies two distinct combustion modes: an upstream ammonia-rich premixed flame and a downstream diffusion flame dominated by hydrogen. LES with a dynamic thickened flame combustion model elucidates the critical role of OH radicals, showing they primarily drive NO production in the primary zone, while their influence diminishes significantly in the secondary zone. A key finding is the stabilization of the secondary flame in a diffusion mode, which deviates from the intended RQL concept. Scatter plots of temperature versus mixture fraction further reveal that the secondary combustion zone encompasses not only diffusion flame behavior but also localized quenching and partially premixed combustion. This behavior is attributed to the presence of the highly reactive H2 and CO components in the high temperature primary combustion products, which promotes premature ignition even under high strain rates, thereby preventing sufficient premixing with the secondary air. This work reveals the deviation from the ideal RQL behavior and provides critical insights into the underlying flame modes and NOx chemistry of staged ammonia combustion.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Engineering
Additional Information:	RRS policy applied
Publisher:	Elsevier
ISSN:	0010-2180
Date of First Compliant Deposit:	21 April 2026
Date of Acceptance:	29 March 2026
Last Modified:	21 Apr 2026 14:00
URI:	https://orca.cardiff.ac.uk/id/eprint/186496

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