A new low NOx emission technique for NH3/H2 blends in a flameless combustor through offset injection

Abstract

The application of ammonia (NH₃) as a possible future fuel presents a plausible solution for green energy storage. It helps provide a carbon-neutral fuel alternative for industrial power generation and transportation. However, the combustion of NH₃ presents a formidable challenge due to its low reactivity, inadequate flame stability, sluggish flame propagation, and high NO_x emissions. Consequently, its integration into combustion systems necessitates substantial system and strategy modification to enable its deployment to industrial systems. The current study presents a novel fuel/air injection technique, which emphasizes the high recirculation of hot combustion products and the extended residence time of fuel/air mixtures. A comprehensive experimental and numerical investigation is conducted using a swirl air injection and offset fuel injection to achieve the flameless combustion mode for optimized NH₃/H₂ fuel blends. A range of mixture conditions (ϕ = 0.5–1.2) and NH₃/H₂ compositions (50/50–70/30) are experimentally examined. The investigations helped elucidate the effect of residence time and recirculation on NO_x emissions through kinetic simulations using a reactor network model. Subsequently, 3-D numerical simulations helped identify regions of high recirculation, quantified through reactant dilution ratios and uniform temperature distribution. These aspects are determined using a new parameter, the temperature uniformity index along the axial direction of the combustor. The emissions of NO_x, unburnt NH₃, and unburnt H₂ are quantified for different equivalence ratios and NH₃ mole fractions in the fuel mixture. The investigations reveal that NO_x emissions reached their minimum (450–654 ppm) and (344-211 ppm), when the burner operated at lean (ϕ = 0.5–0.8) and rich (ϕ = 1.0–1.2) conditions, respectively, for 70/30 NH₃/H₂ blend. The emissions of unburnt NH₃ and H₂ species remain minimal for lean conditions. Both lean and rich operational regimes demonstrated similar or superior emission characteristics in flameless combustion mode when compared to the conventional combustion mode.