Combustion radiation in Ammonia-based fuels: Progress and challenges

Ammonia is attracting increasing attention as a carbon-free fuel for high-temperature thermal systems, but its weak radiative heat-transfer capability remains a critical barrier to direct application in industrial furnaces, boilers, and combustors. Unlike hydrocarbon flames, pure ammonia flames contain no carbon dioxide or soot, leading to fundamentally different radiative characteristics that are mainly governed by H₂O, with possible minor contributions from residual NH₃, NO, and other nitrogen-containing species. This review critically synthesizes recent progress in combustion radiation from ammonia-based fuels, including pure NH₃, NH₃/H₂ blends, cracked NH₃, NH₃/hydrocarbon blends, COG–NH₃ flames, and ammonia co-combustion with solid fuels. Particular attention is given to the dominant radiation mechanisms, spectral features, radiative heat-flux trends, measurement reliability, radiation-model applicability, and furnace-scale engineering implications. The reviewed literature shows that pure NH₃ and NH₃/H₂ flames establish a low-radiation baseline because they lack CO₂, soot, and particle radiation; hydrocarbon and COG blending can partially recover radiative output through CO₂ and possible soot-related emission; and ammonia/solid-fuel co-combustion is strongly affected by particle radiation from char, ash, fly ash, and soot. Current measurement data remain difficult to compare because of differences in burner geometry, pressure, equivalence ratio, fuel composition, detector field of view, wavelength range, wall temperature, background subtraction, and optical path length. Radiation modelling also remains challenging because optically thin, WSGG, SNB, FSCK, and LBL approaches differ substantially in their assumptions, spectral accuracy, and computational cost. Key research gaps are identified in high-pressure and furnace-scale radiation data, standardized measurement protocols, high-temperature spectroscopic databases, radiation–combustion–NOₓ coupling, and strategies for compensating weak ammonia flame radiation through both flame-side enhancement and furnace-side heat-transfer adaptation. This review provides a synthesis of current knowledge and practical guidance for the design and optimization of ammonia-based low-carbon combustion systems.