Secondary combustion is central to the rich-burn/quick-mix/lean-burn (RQL) scheme proposed for stable and low-emission combustion of methane-ammonia blends. However, the secondary stage has received relatively little attention, and the formation and reaction characteristics of secondary combustion remain insufficiently understood. In this study, we investigate the structure of laminar premixed fuel-rich methane and methane-ammonia flames and the properties of their secondary front, using a specially designed burner with controllable ventilation conditions. Flame photographs and global chemiluminescence of OH*, CH*, C2* and NH2* are used to characterise the overall flame behaviour. Hyperspectral imaging (HSI), combined with CO2* background-emission removal and Tikhonov-regularisation reconstruction, is then employed to obtain spatial distributions of OH*, CH*, C2*, NH2*, and H2O. The results show that the shape and chemiluminescent signatures of the secondary front in fuel-rich laminar flames exhibit under- and over-ventilated behaviour governed by the flow rate of air ventilation (21% O2 vol%), whereas when the total flow is fixed and O2 vol% is varied this behaviour weakens and the secondary front changes more monotonically. Spatial reconstructions reveal that CH* and C2* are concentrated at the primary premixed front and are almost absent at the secondary front, whereas the secondary front is dominated by OH* and CO2*. NH2* are present only at the primary front and the intermediate region between two fronts. Subsequently, the bluish luminosity of the secondary flame front is predominantly contributed by CO2*. Coupled with kinetic calculations, this study confirms that oxidation of H2 and moist-CO play critical roles in the secondary combustion of methane and methane-ammonia flames up to 40% in ammonia fraction, rather than hydrocarbons or unburnt primary fuels. These findings provide new insights into the formation of secondary combustion under fuel-rich conditions and demonstrate that the employed diagnostic methods are well suited to study the characteristics of the RQL combustion scheme.
Novelty and significance
By applying hyperspectral imaging (HSI) combined with Tikhonov-regularisation reconstruction and background-emission elimination to spontaneous flame emission, this work achieves, for the first time, a species-resolved characterisation of the secondary combustion zone in fuel-rich CH4 and CH4-NH3 flames. The significance lies mainly in two aspects. First, this study experimentally and numerically demonstrates that the luminous secondary front is governed by OH* and CO2*, while CH*, C2* and NH2* remain confined to the primary or intermediate regions, establishing the secondary combustion as CO/H2 oxidation rather than an extension of hydrocarbon or ammonia burning. This provides mechanistic guidance for stabilising and optimising ammonia-containing-fuel-based RQL systems. Second, the HSI-based diagnostic method provides full spectra at each spatial location, achieving combined spatial and spectral resolution and thereby offering a species-resolved tool applicable to investigations of a wide range of stable flames.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)