Plasma-assisted combustion of CH₄/NH₃/H₂ ternary fuel for NO_X formation mechanisms analysis

Under the scope of suppressing greenhouse effectively for approaching aviation carbon neutrality, carbon-free fuels like ammonia and hydrogen serve as crucial alternatives for replacing traditional aviation fossil fuels (e.g. kerosene). This work addresses the dielectric barrier discharge (DBD) plasma-assisted combustion of methane/ammonia/hydrogen (CH₄/NH₃/H₂) under different equivalence ratios, methane blending ratios, and discharge voltages. The reaction mechanisms of nitrogen oxides (NO_X) were analyzed using two techniques, including OH planar laser-induced fluorescence (OH-PLIF) and NH₂*/CH* chemiluminescence concurrently. Based on the analysis of flame morphology, the analytical results indicate that combustion process could be divided into three main stages: the flame development stage (0.7 ≤ ϕ < 0.8), the flame stabilization stage (0.8 ≤ ϕ < 1.0), and the flame lifting stage (1.0 ≤ ϕ ≤ 1.1). Through the analysis of intermediate radicals (OH and NH₂*/CH*), it was confirmed that a competitive mechanism between NO_X formation and reduction existed during the combustion of this ternary fuel. In the flame development stage, NO_X gradually increased, and this was because both OH and NH₂* radicals rose with increasing ϕ , which promoted NO_X formation. In the flame stabilization stage, a maximum NO_X concentration was clearly observed. This phenomenon was mainly attributed to a significant increment of OH while NH₂* remaining nearly unchanged, thereby strongly enhancing NO_X formation. In contrast, the flame lifting stage demonstrated a reverse trend (NO_X decreased), due to the growth of NH₂* outweighing that of OH. Importantly, DBD plasma was applied to assist combustion. When comparing DBD discharge with non-discharge condition, it was found that NO_X formation was maximally enhanced by 10.3 % during lean-burn; this was because the discharge generated abundant OH radicals. While during rich-burn, NO_X formation was maximally inhibited by 13.8 %, primarily due to the large amount of NH₂* produced, which restricted NO_X production. Meanwhile, this stage had the lowest CO emissions, enabling the simultaneous control of both NO_X and CO as DBD applied.